SYNTHESIS AND CHARACTERISATION OF METAL ...eprints.qut.edu.au/45712/1/Jing_Yang_Thesis.pdfSYNTHESIS AND CHARACTERISATION OF METAL OXYHYDROXIDE AND OXIDE NANOMATERIALS by JING (JEANNE)

SYNTHESIS AND

CHARACTERISATION OF METAL

OXYHYDROXIDE AND OXIDE

NANOMATERIALS

by

JING (JEANNE) YANG

Bachelor of Science, Beijing Normal University, China

Chemistry Discipline

Faculty of Science and Technology

Queensland University of Technology

A thesis submitted to the Queensland University of Technology, in

fulfillment of the requirements of the degree of Doctor of Philosophy

November 2010

谨以此论文

献给我挚爱的父母：

杨百良先生和许再美女士

PREFACE

i

ABSTRACT

In this work, a range of nanomaterials have been synthesised based on metal

oxyhydroxides MO(OH), where M=Al, Co, Cr, etc. Through a self-assembly

hydrothermal route, metal oxyhydroxide nanomaterials with various morphologies

were successfully synthesised: one dimensional boehmite (AlO(OH)) nanofibres,

zero dimensional indium hydroxide (In(OH)3) nanocubes and chromium

oxyhydroxide (CrO(OH)) nanoparticles, as well as two dimensional cobalt hydroxide

and oxyhydroxide (Co(OH)2 & CoO(OH)) nanodiscs. In order to control the synthetic

nanomaterial morphology and growth, several factors were investigated including

cation concentration, temperature, hydrothermal treatment time, and pH.

Metal ion doping is a promising technique to modify and control the properties of

materials by intentionally introducing impurities or defects into the material.

Chromium was successfully applied as a dopant for fabricating doped boehmite

nanofibres. The thermal stability of the boehmite nanofibres was enhanced by

chromium doping, and the photoluminescence property was introduced to the

chromium doped alumina nanofibres. Doping proved to be an efficient method to

modify and functionalize nanomaterials.

The synthesised nanomaterials were fully characterised by X-ray diffraction (XRD),

transmission electron microscopy (TEM) combined with selected area electron

diffraction (SAED), scanning electron microscopy (SEM), BET specific surface area

analysis, X-ray photoelectron spectroscopy (XPS) and thermo gravimetric analysis

(TGA). Hot-stage Raman and infrared emission spectroscopy were applied to study

the chemical reactions during dehydration and dehydroxylation. The advantage of

these techniques is that the changes in molecular structure can be followed in situ

and at the elevated temperatures.

PREFACE

ii

KEYWORDS

Aluminium oxyhydroxide; Chromium oxyhydroxide; Cobalt oxyhydroxide; Cobalt

hydroxide; Indium hydroxide; Chromium doped; Dopant; Alumina; Aluminium

oxide; Indium oxide; Chromium oxide; Chromium oxide gel; Cobalt oxide;

Boehmite; Grimaldiite; Eskolaite; Heterogenite; Nanostructure; Nanofibres;

Nanorods; Nanoparticles; Nanodiscs; Nanocubes; Nanotubes; Nanosheets;

Nanomaterials; Self-assembly; Hydrothermal treatment; X-ray diffraction;

Transmission electron microscopy; Scanning electron microscopy; X-ray

photoelectron spectroscopy; N2 adsorption/desorption; Thermal decomposition;

Thermogravimetric analysis; Raman spectroscopy; Hot-stage Raman; Infrared

spectroscopy; Infrared emission spectroscopy.

PREFACE

iii

TABLE OF CONTENTS

Abstract ........................................................................................................................ i

Keywords .................................................................................................................... ii

Table of contents ....................................................................................................... iii

List of tables ............................................................................................................. viii

List of figures ............................................................................................................. ix

Publication and conference presentations ............................................................ xiv

List of publications as a result of this project ........................................................ xiv

List of other publications in which I was a co-author ............................................ xv

List of conferences attended in this course of study ............................................ xvii

Abbreviations ........................................................................................................ xviii

Statement of originality .......................................................................................... xix

Acknowledgements ................................................................................................... xx

CHAPTER 1

INTRODUCTION ...................................................................................................... 1

1. INTRODUCTION ........................................................................................... 2

2. DESCRIPTION OF SCIENTIFIC PROBLEMS INVESTIGATED ............... 2

3. RESEARCH OBJECTIVES OF THE STUDY ............................................... 3

4. ACCOUNT OF SCIENTIFIC PROGRESS LINKING THE SCIENTIFIC

PAPERS ................................................................................................................... 3

REFERENCES ......................................................................................................... 5

CHAPTER 2

LITERATURE REVIEW .......................................................................................... 7

1. INTRODUCTION ........................................................................................... 8

2. NANOTECHNOLOGY AND NANOMATERIALS ...................................... 8

3. METAL OXYHYDROXIDE AND OXIDE NANOMATERIALS .............. 10

4. DOPED NANOMATERIALS ....................................................................... 20

5. CONCLUSIONS ............................................................................................ 23

REFERENCES ....................................................................................................... 24

PREFACE

iv

CHAPTER 3

SYNTHESIS AND CHARACTERISATION OF BOEHMITE

NANOFIBRES .......................................................................................................... 31

SYNOPSIS ............................................................................................................. 32

STATEMENT OF CONTRIBUTION OF CO-AUTHORS .................................. 33

RESEARCH HIGHLIGHTS .................................................................................. 34

ABSTRACT ........................................................................................................... 35

KEYWORDS.......................................................................................................... 35

1. INTRODUCTION .......................................................................................... 35

2. EXPERIMENTAL ......................................................................................... 36

3. RESULTS AND DISCUSSION .................................................................... 37

4. CONCLUSIONS ............................................................................................ 42

ACKNOWLEDGMENTS ...................................................................................... 42

REFERENCES ....................................................................................................... 43

CHAPTER 4

SYNTHESIS AND CHARACTERISATION OF INDIUM HYDROXIDE

IN(OH)3 NANOCUBES ........................................................................................... 45

SYNOPSIS ............................................................................................................. 46

CHAPTER 4.1

THERMOGRAVIMETRIC ANALYSIS AND HOT-STAGE RAMAN

SPECTROSCOPY OF CUBIC INDIUM HYDROXIDE ..................................... 47



ABSTRACT ........................................................................................................... 50

KEYWORDS.......................................................................................................... 50

1. INTRODUCTION .......................................................................................... 51

2. EXPERIMENTAL ......................................................................................... 52


4. CONCLUSIONS ............................................................................................ 63

ACKNOWLEDGMENTS ...................................................................................... 63

REFERENCES ....................................................................................................... 64

PREFACE

v

CHAPTER 4.2

APPLICATION OF INFRARED EMISSION SPECTROSCOPY TO THE

THERMAL TRANSITION OF INDIUM HYDROXIDE TO INDIUM

OXIDE NANOCUBES ............................................................................................ 67



ABSTRACT ........................................................................................................... 70

KEYWORDS ......................................................................................................... 71

1. INTRODUCTION ......................................................................................... 71

2. EXPERIMENTAL ......................................................................................... 72


4. CONCLUSIONS ............................................................................................ 84

ACKNOWLEDGEMENTS ................................................................................... 84

REFERENCES ....................................................................................................... 85

CHAPTER 5

SYNTHESIS AND CHARACTERISATION OF CHROMIUM DOPED

BOEHMITE NANOFIBRES .................................................................................. 89

SYNOPSIS ............................................................................................................. 90



ABSTRACT ........................................................................................................... 93

KEYWORDS ......................................................................................................... 93

1. INTRODUCTION ......................................................................................... 94

2. EXPERIMENTAL ......................................................................................... 95


4. CONCLUSIONS .......................................................................................... 106

ACKNOWLEDGEMENTS ................................................................................. 106

REFERENCES ..................................................................................................... 107

CHAPTER 6

SYNTHESIS AND CHARACTERISATION OF CHROMIUM

OXYHYDROXIDE CRO(OH) NANOPARTICLES ......................................... 111

SYNOPSIS ........................................................................................................... 112

PREFACE

vi

CHAPTER 6.1

SIZE-CONTROLLABLE SYNTHESIS OF CHROMIUM

OXYHYDROXIDE NANOMATERIALS USING A SOFT CHEMICAL

HYDROTHERMAL ROUTE ............................................................................... 113

STATEMENT OF CONTRIBUTION OF CO-AUTHORS ................................ 114

RESEARCH HIGHLIGHTS ................................................................................ 115

ABSTRACT ......................................................................................................... 116

KEYWORDS........................................................................................................ 117

1. INTRODUCTION ........................................................................................ 117

2. EXPERIMENTAL ....................................................................................... 118

3. RESULTS AND DISCUSSION .................................................................. 121

4. CONCLUSIONS .......................................................................................... 139

ACKNOWLEDGEMENTS.................................................................................. 139

REFERENCES ..................................................................................................... 140

SUPPORTING INFORMATION......................................................................... 144

CHAPTER 6.2

TRANSITION OF SYNTHETIC CHROMIUM OXIDE GEL TO

CRYSTALLINE CHROMIUM OXIDE: A HOT-STAGE RAMAN

SPECTROSCOPIC STUDY ................................................................................. 145



ABSTRACT ......................................................................................................... 148

KEYWORDS........................................................................................................ 149

1. INTRODUCTION ........................................................................................ 149

2. EXPERIMENTAL ....................................................................................... 150


4. CONCLUSIONS .......................................................................................... 161

ACKNOWLEDGMENTS .................................................................................... 161

REFERENCES ..................................................................................................... 162

CHAPTER 6.3

TRANSITION OF CHROMIUM OXYHYDROXIDE NANOMATERIALS

TO CHROMIUM OXIDE: A HOT-STAGE RAMAN SPECTROSCOPIC

STUDY .................................................................................................................... 165


PREFACE

vii


ABSTRACT ......................................................................................................... 168

KEYWORDS ....................................................................................................... 169

1. INTRODUCTION ....................................................................................... 169

2. EXPERIMENTAL ....................................................................................... 170


4. CONCLUSIONS .......................................................................................... 179

ACKNOWLEDGMENTS .................................................................................... 179

REFERENCES ..................................................................................................... 180

CHAPTER 7

SYNTHESIS AND CHARACTERISATION OF COBALT HYDROXIDE,

COBALT OXYHYDROXIDE AND COBALT OXIDE NANODISCS ............ 183

SYNOPSIS ........................................................................................................... 184



ABSTRACT ......................................................................................................... 187

KEYWORDS ....................................................................................................... 188

1. INTRODUCTION ....................................................................................... 188

2. EXPERIMENTAL ....................................................................................... 190


4. CONCLUSIONS .......................................................................................... 208

ACKNOWLEDGEMENTS ................................................................................. 208

REFERENCES ..................................................................................................... 209

SUPPORTING INFORMATION ........................................................................ 213

CHAPTER 8

GENERAL DISCUSSION & FUTURE WORK ................................................. 215

1. GENERAL DISCUSSION........................................................................... 216

2. FUTURE WORK ......................................................................................... 222

REFERENCES ..................................................................................................... 224

PREFACE

viii

LIST OF TABLES

CHAPTER 5

Table 1 Results of the thermogravimetric analyses of the undoped and various % Cr

doped boehmite nanofibres. ....................................................................................... 98

CHAPTER 6.1

Table 1 Samples ID used in this paper and their preparation conditions. ............... 119

Table 2 BET specific surface area (SBET), pore volume (Vp), and pore diameter for

synthesised CrO(OH) nanomaterials. ....................................................................... 128

Table 3 Results for curve-fitted binding energies and their atomic contents (at.%) of

highly resolved Cr 2p3/2 and O 1s XPS spectra shown in Fig. 7 for sample ppt-5.0,

CrO(OH)-5.0, CrO(OH)-7.5and CrO(OH)-10.0. ..................................................... 129

Table 4 Summary of peaks shown in dTG curves for synthesised chromium

materials. .................................................................................................................. 137

CHAPTER 6.2

Table 1 Summary of Raman shifts (cm-1

) and their assignment for synthetic Cr-gel

and its thermal-decomposed product Cr2O3 in the hot-stage Raman spectroscopic

study. ........................................................................................................................ 157

CHAPTER 6.3


) and their assignment for -CrO(OH) and

Cr2O3 nanomaterials in the hot-stage Raman spectroscopic study. ......................... 178

CHAPTER 7

Table 1 Curve-fitted XPS binding energies of as-prepared cobalt hydroxide, cobalt

oxyhydroxide and cobalt oxide nanomaterials. ........................................................ 202

Table S1 Results of Raman spectra of Co(OH)2, CoO(OH) and Co3O4 in comparison

with published work. ................................................................................................ 213

PREFACE

ix

LIST OF FIGURES

CHAPTER 3

Fig. 1 XRD pattern of the synthetic boehmite nanofibres, after hydrothermal

treatment at 170 ºC for 2 days under pH 5. ................................................................ 38

Fig. 2 SEM image of the synthetic boehmite nanofibres, after hydrothermal treatment

at 170 ºC for 2 days under pH 5. ................................................................................ 38

Fig. 3 IES spectra of the synthetic boehmite nanofibres, collected at an interval of

50 ºC, over the range 100 ºC – 850 ºC. ...................................................................... 40

Fig. 4 Raman spectra of the synthetic boehmite nanofibres, after hydrothermal

treatment at 170 ºC for 2 days under pH 5. ................................................................ 41

CHAPTER 4.1

Fig. 1 Thermo gravimetric analyses of synthetic In(OH)3 nanocubes. ...................... 54

Fig. 2 XRD patterns of synthetic In(OH)3 (a) and its thermally treated products In2O3

(b). The peaks are labeled with their Miller indices................................................... 55

Fig. 3 (a) Image of In(OH)3 synthesised at 180 °C and (b) image of In2O3, product of

as-synthetic In(OH)3 calcined at 500 ºC for 4 h. ........................................................ 56

Fig. 4 Raman spectrum of In(OH)3 in the 100 – 700 cm-1

region. ............................ 57


region. .......................... 58


region. ........................ 59

Fig. 7 Hot-stage Raman spectra of In(OH)3 in the 100 – 800 cm-1

region. ............... 60


region. ........... 61

Fig. 9 Hot-stage Raman spectra of In(OH)3 in (a) 3900 – 2400 cm-1

region and (b)

800 – 1800 cm-1

region. ............................................................................................. 62

PREFACE

x

CHAPTER 4.2

Fig.1 XRD pattern of the synthetic indium hydroxide nanomaterials with a reference

pattern: JCPDS card No. 01-076-1463 In(OH)3 ......................................................... 74

Fig. 2 Schematic of In(OH)3 in cubic structure (space group Im 3 ) ........................ 75

Fig.3 Infrared absorption spectrum (curve-fitted) of the synthetic In(OH)3 nanocubes

in the region of 3600 – 600 cm-1

. ............................................................................... 76

Fig. 4 (a) Infrared emission spectra of the synthetic In(OH)3 nanocubes in the region

of 4000 – 650 cm-1

and (b)curve-fitted Infrared emission spectra from 100 – 300 °C.78

Fig. 5 Curve-fitted Infrared emission spectra in the region of 1200 – 650 cm-1

. ....... 80


. ..... 81

Fig. 7 XRD patterns of thermal products at 300 and 500 °C from In(OH)3 nanocubes

with a reference pattern: JCPDS card No. 01-071-2195 In2O3. ................................. 82

CHAPTER 5

Fig. 1 (a) XRD patterns of undoped boehmite and 1% Cr-doped boehmite nanofibres

with different hydrothermal treatment time at 170 ºC. (b) XRD patterns of undoped

boehmite and various Cr % doped boehmite nanofibres, after hydrothermal treatment

at 170 ºC for 3 days. ................................................................................................... 97

Fig. 2 TEM images of the synthetic nanofibres with 3-day hydrothermal treatment: (a)

undoped boehmite, (b) 3% Cr-doped and (c)5% Cr-doped. ...................................... 97

Fig. 3 Thermogravimetric analyses patterns of (a) undoped boehmite nanofibres and

1% Cr doped boehmite with different hydrothermal treatment time: (b) 1 day, (c) 3

days, (d) 5 days, and (e) 10 days. ............................................................................. 101

Fig. 4 Dehydroxylation temperature of the dTG peak as a function of added Cr

content and with the hydrothermal treatment time. .................................................. 102

Fig. 5 Thermo gravimetric analyses patterns of various % Cr-doped boehmite

nanofibres with 3-day hydrothermal treatment:(a) 3% , (b) 5%, (c)10%, and (d) 20%.104

PREFACE

xi

Fig. 6 Temperature of the main dTG peak and the total mass loss percentage as a

function of added Cr content. ................................................................................... 105


function of the hydrothermal treatment time. .......................................................... 105

CHAPTER 6.1

Fig .1 XRD patterns of ppt-5.0, ppt-7.5, ppt-10.0, CrO(OH)-5.0, gel-7.5-a and

gel-10.0-a. XRD pattern from literature: JCPDS card No. 01-070-0621 Grimaldiite.122

Fig. 2 SEM images of (a) ppt-5.0 and (b) CrO(OH)-5.0. ........................................ 123

Fig. 3 XRD patterns of CrO(OH)-10.0 and CrO(OH)-7.5. XRD pattern from

literature: JCPDS card No. 01-070-0621 Grimaldiite. ............................................. 124

Fig. 4 (a) TEM image of CrO(OH)-5.0; (b) SAED result of the corresponding area

in Fig.4 (a); (c) TEM image of CrO(OH)-7.5 and its SAED result (inset); (d)

TEM image of CrO(OH)-10.0 and its SAED result (inset). .................................... 125

Fig. 5 N2 adsorption/desorption isotherms for CrO(OH)-5.0, CrO(OH)-7.5 and

CrO(OH)-10.0. ......................................................................................................... 126

Fig. 6 Pore size distribution study for CrO(OH)-5.0, CrO(OH)-7.5 and

CrO(OH)-10.0. ......................................................................................................... 127

Fig. 7 XPS high resolution spectra of Cr 2p3/2 and O 1s for ppt-5.0, CrO(OH)-5.0,

CrO(OH)-7.5 and CrO(OH)-10.0 ............................................................................. 131

Fig.8 Thermogravimetric analyses (TGA) of (a) ppt-5.0, (b) CrO(OH)-5.0, (c)

CrO(OH)-7.5 and (d) CrO(OH)- 10.0. ..................................................................... 134

Fig. 9 Mass spectrometric analysis associated with the thermal decomposition

process for sample CrO(OH)-10.0. .......................................................................... 136

Fig. 10 XRD patterns of products after TGA from ppt-5.0, CrO(OH)-5.0,

CrO(OH)-7.5 and CrO(OH)-10.0. XRD patterns from literature: JCPDS card No.

01-084-1616 Cr2O3. ................................................................................................. 138

PREFACE

xii

CHAPTER 6.2

Fig. 1 (a) XRD pattern of the precipitated Cr-gel material and a reference pattern:

JCPDS card No. 01-070-0621 CrO(OH) and (b) SEM image of the precipitated

Cr-gel material .......................................................................................................... 152

Fig. 2 Hot-stage Raman spectrum of the precipitated Cr-gel material in the region of

100 to 2000 cm-1

at 25 °C ........................................................................................ 153

Fig. 3 Hot-stage Raman spectra of the precipitated Cr-gel material in the region of

100 to 2000 cm-1

from 130 to 350 °C ...................................................................... 156

Fig. 4 XRD pattern of thermal product at 350 °C from the precipitated Cr-gel

material and a reference pattern: JCPDS card No. 01-084-0314 (Cr2O3). ............... 158


100 to 2000 cm-1

at 550 °C ...................................................................................... 159


material and a reference pattern: JCPDS card No. 01-084-0314 (Cr2O3). ............... 160

Fig. 7 Schematic of Cr2O3 in rhombohedral structure (space group R-3c) .............. 160

CHAPTER 6.3

Fig. 1 XRD pattern for the synthetic -CrO(OH) and a reference pattern: JCPDS

card No. 01-085-1374 (Grimaldiite). ....................................................................... 172

Fig. 2 Schematic of the synthetic -CrO(OH) in rhombohedral structure (space

group R3m) observed from different directions. The hexagonal unit cells are shown.173

Fig. 3 TEM image of the synthetic -CrO(OH) nanomaterial ................................ 174

Fig. 4 Hot-stage Raman spectra of the synthetic -CrO(OH) nanomaterial in the 200

to 1800 cm-1

region at 25 and 100 °C....................................................................... 175

Fig. 5 Raman spectra of the synthetic -CrO(OH) nanomaterial in the 200 to 1800

cm-1

region at 350 and 550 °C .................................................................................. 177

PREFACE

xiii

CHAPTER 7

Fig. 1 XRD patterns of (a) synthesised Co(OH)2, (b) synthesised CoO(OH), (c)

synthesised Co3O4, (d) JCPDS card No. 00-030-0443, (d‟) -Co(OH)2 pattern

synthesised according to literature, (e) JCPDS card No. 01-073-1213 and (f) JCPDS

card No. 01-071-0816. ............................................................................................. 193

Fig. 2 SEM images of (a, b) Co(OH)2, (c, d) CoO(OH) (e, f) Co3O4 at two

magnifications. ......................................................................................................... 195

Fig. 3 TEM images of (a) Co(OH)2, (c) CoO(OH) and (d) Co3O4; SAED patterns of

the corresponding particles showing in TEM images: (b) Co(OH)2 and (e) Co3O4 197

Fig. 4 XPS survey spectra of (a) Co(OH)2, (b) CoO(OH) and (c) Co3O4................ 199

Fig. 5 XPS high resolution Co 2p3/2 spectra of (a) Co(OH)2, (c) CoO(OH) and (e)

Co3O4; O 1s spectra of (b) Co(OH)2, (d) CoO(OH) and (f) Co3O4.......................... 201

Fig. 6 Raman spectra of (a) Co(OH)2, (b) CoO(OH), and (c) Co3O4 in the 100 – 1200

cm-1

region ............................................................................................................... 204

Fig. 7 Thermo-gravimetric analyses of (a) Co(OH)2, (b) CoO(OH) nanomaterials. 206

Fig. 8 XRD patterns of the thermal products after TG study: (a) from Co(OH)2, (b)

from CoO(OH). XRD patterns from literature: (c) JCPDS card No. 01-080-1533 (d)

JCPDS card No. 00-043-1004. ................................................................................. 207

PREFACE

xiv

PUBLICATION AND CONFERENCE

PRESENTATIONS

LIST OF PUBLICATIONS AS A RESULT OF THIS PROJECT

1. Yang, J.; Frost, R. L., "Synthesis and characterization of boehmite nanofibres."

Research Letters in Inorganic Chemistry 2008. DOI:10.1155/2008/602198.

2. Yang, J.; Frost, R. L.; Yuan, Y., "Synthesis and characterization of chromium

doped boehmite nanofibres." Thermochimica Acta 2009, 483 (1-2), 29-35.

(SCI) Impact Factor: 1.742. Citation: 3.

3. Yang, J.; Zhao, Y.; Frost, R. L., "Surface analysis, TEM, dynamic and

controlled rate thermal analysis, and infrared emission spectroscopy of gallium

doped boehmite nanofibres and nanosheets." Applied Surface Science 2009, 255

(18), 7925-7936. (SCI) Impact Factor: 1.616

4. Yang, J.; Zhao, Y.; Frost, R. L., "Infrared and infrared emission spectroscopy of

gallium oxide α-GaO(OH) nanostructures." Spectrochimica Acta Part A:

Molecular and Biomolecular Spectroscopy 2009, 74A (2), 398-403. (SCI)

Impact Factor: 1.566

5. Yang, J.; Liu, H.; Martens, W. N.; Frost, R. L., "Synthesis and characterization

of cobalt hydroxide, cobalt oxyhydroxide, and cobalt oxide nanodiscs." The

Journal of Physical Chemistry C 2010, 114 (1), 111-119. (SCI) Impact Factor:

4.224. Citation: 6.

6. Yang, J.; Baker, A. G.; Liu, H.; Martens, W. N.; Frost, R. L., "Size-controllable

synthesis of chromium oxyhydroxide nanomaterials using a soft chemical

hydrothermal route." Journal of Materials Science 2010, 45 (24), 6574-6585.

(SCI) Impact Factor: 1.471

7. Yang, J.; Martens, W. N.; Frost, R. L., "Transition of chromium oxyhydroxide

nanomaterials to chromium oxide: a hot-stage Raman spectroscopic study."

Journal of Raman Spectroscopy 2010, in press. DOI: 10.1002/jrs.2773. (SCI)


8. Yang, J.; Martens, W. N.; Frost, R. L., "Transition of chromium oxide gel to

crystalline chromium oxide: a hot-stage Raman spectroscopic study." Journal of

Raman Spectroscopy 2010, in press. DOI:10.1002/jrs.2794. (SCI) Impact Factor:

3.147

PREFACE

xv

9. Yang, J.; Frost, R. L.; Martens, W. N., "Thermogravimetric analysis and

hot-stage Raman spectroscopy of cubic indium hydroxide." Journal of Thermal

Analysis and Calorimetry 2010, 100 (1), 109-116. (SCI) Impact Factor: 1. 587.

Citation: 1.

10. Yang, J.; Cheng, H.F.; Martens, W.N.; Frost, R.L. "Application of infrared

emission spectroscopy to the thermal transition of indium hydroxide to indium

oxide nanocubes." Applied Spectroscopy 2011, 65 (1). In press. (SCI) Impact

Factor: 1.564

11. Yang, J.; Martens, W. N.; Frost, R. L., "Synthesis and characterisation of cobalt

hydroxyl carbonate Co2CO3(OH)2-relationship to the rosasite mineral group."

Spectrochimica Acta, Part A: Molecular Spectroscopy 2010, in press. DOI:

10.1016/j.saa.2010.11.004. (SCI) Impact Factor: 1.566

12. Zhao, Y.; Yang, J.; Frost, R. L.; Martens, W. N., "Size and morphology control

of gallium oxide hydroxide GaO(OH), nano- to micro-sized particles by

soft-chemistry route without surfactant." The Journal of Physical Chemistry C

2008, 112 (10), 3568-3579. (SCI) Impact Factor: 4.224. Citation: 6.

13. Zhao, Y.; Yang, J.; Frost, R. L., "Raman spectroscopy of the transition of

α-gallium oxyhydroxide to β-gallium oxide nanorods." Journal of Raman

Spectroscopy 2008, 39 (10), 1327-1331. (SCI) Impact Factor: 3.147

14. Zhao, Y.; Yang, J.; Frost, R. L.; Kristof, J.; Horvath, E., "Synthesis,

Characterisation and thermal analysis of Fe-doped boehmite nanofibres and

nanosheets." Journal of Materials Science 2009, 44 (14), 3662-3673. (SCI)


LIST OF OTHER PUBLICATIONS IN WHICH I WAS A CO-AUTHOR

1. Cheng, H.F.; Yang, J.; Frost, R. L.; Liu, Q.F.; Zhang, J.S., "A spectroscopic

comparion of kaolinite, coal bearing kaolinite and halloysite: a mid-infrared and

near-infrared study." Spectrochimica Acta part A: Molecular and Biomolecular

Spectroscopy 2010, 77 (4), 856-861. (SCI) Impact Factor: 1.566

2. Cheng, H.F.; Yang, J.; Frost, R. L.; Liu, Q.F.; Zhang, Z.L., "Thermal analysis

and Infrared emission spectroscopic study of kaolinite-potassium acetate

intercalate complex." Journal of Thermal Analysis and Calorimetry 2010, in

press. DOI: 10.1007/s10973-010-0917-3. (SCI) Impact Factor: 1. 587

PREFACE

xvi

3. Cheng, H.F.; Liu, Q.F.; Zhang, J.; Yang, J.; Frost, R. L.,"Delamination of

kaolinite–potassium acetate intercalates by ball-milling." Journal of Colloid and

Interface Science 2010, 348 (2),355-359. (SCI) Impact Factor: 3.019

4. Cheng, H.F.; Yang, J.; Liu, Q.F.; He, J.K.; Frost, R. L., "Thermogravimetric

analysis – mass spectrometry (TG – MS) of selected Chinese kaolinites."

Thermochimica Acta 2010, 507-508 (C), 106-114. (SCI) Impact Factor: 1.742

5. Cheng, H.F.; Liu, Q.F.; Yang, J.; Zhang, Q.; Frost, R. L., "Thermal behavior

and decomposition of kaolinite-potassium acetate intercalation composite."

Thermochimica Acta 2010, 503-504 (C), 16-20. (SCI) Impact Factor: 1.742

6. Cheng, H.F.; Liu, Q.F.; Yang, J.; Frost, R. L.,"Thermogravimetric analysis of

selected coal-bearing strata kaolinite." Thermochimica Acta 2010, 507-508 (C),

84-90. (SCI) Impact Factor: 1.742

7. Cheng, H.F.; Frost, R. L.; Yang, J.; Liu, Q.F.; He, J.K., "Infrared and infrared

emission spectroscopic study of typical Chinese kaolinite and halloysite."

Spectrochimica Acta part A: Molecular and Biomolecular Spectroscopy 2010,

77(5), 1013-1019. (SCI) Impact Factor: 1.566

8. Cheng, H.F.; Liu, Q.F.; Yang, J.; Frost, R. L., "Thermal analysis and infrared

emission spectroscopy study of halloysite-potassium acetate intercalation

complex." Thermochimica Acta 2010, in press. DOI:10.1016/j.tca.2010.08.003.

(SCI) Impact Factor: 1.742

9. Cheng, H.F.; Yang, J.; Frost, R. L., "Thermogravimetric analysis-mass

spectrometry (TG-MS) of selected Chinese palygorskites." Thermochimica Acta

2010, in press. DOI:10.1016/j.tca.2010.10.008. (SCI) Impact Factor: 1.742

10. Cheng, H.F.; Liu, Q.F.; Yang, J.; Du, X.M.; Frost, R. L., "Influencing factors

on kaolinite-potassium acetate intercalation complexes." Applied Clay Science

2010, in press. DOI: 10.1016/j.clay.2010.09.011. (SCI) Impact factor: 2.784

PREFACE

xvii

LIST OF CONFERENCES ATTENDED IN THIS COURSE OF STUDY

1. GeoRaman Conference

Sydney, Australia 28th

June – 2nd

July, 2010

Oral presentation: "Raman spectroscopy of the transition of chromium

oxyhydroxide to chromium oxide nanoplates", Jing Yang, Llew Rintoul, Wayde

N. Martens and Ray L. Frost.

2. 21st Australian Clay Minerals Society Conference

Brisbane, Australia 7th

– 8th

August, 2010

Oral presentation: "Facile self-assembly synthesis of high-quality disc-like

heterogenite (CoO(OH)) nanomaterials ", Jing Yang, Wayde N. Martens and

Ray L. Frost.

3. 5th

International Conference on Advanced Vibrational Spectroscopy

Melbourne, Australia 12th

– 17th

July, 2009

Poster: "Hot-stage Raman spectroscopic study of cubic indium hydroxide", Jing

Yang and Ray L. Frost.

4. 21st International Conference of Raman Spectroscopy (ICORS)

London, United Kingdom 17th

– 22nd

August, 2008

Posters:

"Thermo-Raman spectroscopy study of the transition of alpha GaO(OH) to

beta Ga2O3 nanorods", Jing Yang, Yanyan Zhao and Ray L. Frost.

"Raman spectroscopy of the uranyl phosphate minerals yingjiangite and

phosphuranylite", Ray L. Frost and Jing Yang.

PREFACE

xviii

ABBREVIATIONS

1D One dimensional

nm Nanometer

XRD X-ray diffraction

TEM Transmission electron microscopy

SAED Selected area electron diffraction

EDX Energy dispersive X-ray spectroscopy

SEM Scanning electron microscopy

XPS X-ray photoelectron spectroscopy

BET Brunauer-Emmett-Teller

TGA Thermogravimetric analysis

MS Mass spectrometry

IR Infrared spectroscopy

IES Infrared emission spectroscopy

UV-vis Ultraviolet-visible spectroscopy

PL Photoluminescence spectroscopy

ICP Inductively coupled plasma

VOC Volatile organic compounds

PREFACE

xix

STATEMENT OF ORIGINALITY

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the

best of my knowledge and belief, the thesis contains no material previously

published or written by another person except where due reference is made.

Signature:

Jing (Jeanne) Yang

Date:

PREFACE

xx

ACKNOWLEDGEMENTS

Thanks a lot to all the people and organisations who made this thesis a possibility.

They include:

My supervisors: Prof. Ray L. Frost, Dr. Wayde N. Martens and Prof.

Huaiyong Zhu for their patience, guidance, and continuous encouragement

and help. I cannot thank you enough. It was my huge fortune having you with

me throughout this PhD journey. You are the best supervisors in the world.

Dr. Xuebin Ke and Dr. Dongjiang Yang for their extensive support.

Dr. Llew Rintoul for all his assistance and help with the vibrational

spectroscopy instruments.

AsPro. Godwin Ayoko, AsPro. Peter Fredericks, Prof. Graeme Millar, Dr.

Eric Waclawik, Dr. Geoffrey Will and other QUT SPCS staff for your great

help and encouragement.

Mr. Anthony (Tony) Raftery for advice and technical support with XRD,

crystallography and software.

Dr. Thor E. Bostrom, Prof. Hongwei Liu, Mr. Lambert Bekessy, Dr. Loc

Doung, Dr. Deborah Stenzel, Mr. Ashley Locke, Dr. Christina

Theodoropoulos and Dr. Marek Zbik for their patience, advice and technical

support with the operation of the electron microscope. And my thanks also go

to the other QUT AEMF staffs for the training and allowing me access to

your instruments.

Dr. Barry Wood from UQ for your help with XPS.

My fellow postgraduate students who have helped and encouraged me, and

provided an interesting and supportive work environment and friendship.

Lovely thanks to Milica Ivkovic from Croatia, the office cleaner for our H

block. I will miss your sunshine smile in the early morning and massage.

Last but not least, I would like to acknowledge my family and friends for their love,

supports and encouragement, especially my dearest parents (Bailiang Yang and

Zaimei Xu) and my boyfriend Jinming Zeng. Thank you so much for your company,

without which I could not complete this work.

Chapter 1 Introduction

1

CHAPTER 1

INTRODUCTION


2

1. INTRODUCTION

This thesis, entitled “Synthesis and characterisation of metal oxyhydroxide and oxide

nanomaterials”, reports the effect of various synthesis conditions on the formation of

metal (Al, Co, Cr and In) hydroxide, oxyhydroxide and oxide nanomaterials which

synthesised by the hydrothermal method. This chapter details the scientific problems

investigated, the research objectives of the study and the links between the

manuscripts in this study, in order to build the framework for the thesis as a whole.

2. DESCRIPTION OF SCIENTIFIC PROBLEMS

INVESTIGATED

Because of their high surface area, chemical and thermally stability and their high

porosity, nanostructured metal oxides have been extensively used as catalysts or

catalytic supports for a variety of industrial reactions.1-3

There are many literature

reports on the formation of metal oxides through the thermal dehydration of their

corresponding hydroxides or oxyhydroxides, which preserve the morphology of their

precursors.4,5

To apply these nanostructured metal oxides in industry, their phases,

morphology, properties and formation mechanism must to be fully understood.

Moreover, it is known that metal ion doping is a promising technique to modify and

control the properties of materials by intentionally introducing impurities or defects

into a material.6 Without a comprehensive study of the synthesis of these metal

oxides and their doped counterparts, the application of these materials to industrial

problems, would not be fully achieved. This study investigated the effect of synthesis

conditions on the formation and properties of nanomaterials, with particularly focus

on the hydrothermally treatment procedure, and on the effect of metal ion dopants.

Vibrational spectroscopy, especially Raman and infrared, has proven useful

technique for the non-destructive identification of various minerals and

nanomaterials through its use as an easy finger printing technique.7,8

However,

limited studies have been reported on the detailed interpretation of the spectra of

metal oxyhydroxide and oxide nanomaterials. This study aims to address this issue

through the detailed investigation of the effect of doping on the spectra of these

materials.


3

3. RESEARCH OBJECTIVES OF THE STUDY

The research objectives of this study were:

To examine the synthesis conditions of selected metal hydroxide and oxyhydroxides

through hydrothermal treatment, and to determine the influencing factors during the

formation of the nanomaterials. It is intended to realise the control of the size, phase

and morphology of the nanomaterials during synthesis and to discover their

formation mechanism.

To investigate the effect of metal ion dopants on the morphology, size and properties

of hydroxide and oxyhydroxide nanomaterials. Trivalent metal elements such as

chromium and cobalt will be employed as dopants into boehmite structure.

To determine the properties of the synthesised nanomaterials using various

characterisation techniques. Phase identification, element analysis, pore,

morphological studies, surface area analysis, thermal stability and vibrational spectra

of these nanomaterials will be investigated in detail. Their potential application will

also be proposed.

4. ACCOUNT OF SCIENTIFIC PROGRESS LINKING

THE SCIENTIFIC PAPERS

The following flow chart, presents the structure of this thesis, indicating the linkage

between the objectives and the outcomes reported in the published papers which

were written in this course of study.


4


Chapter 2 Literature review

Chapter 3

AlO(OH)

nanofibres

Chapter 6

CrO(OH)

nanoparticles

Chapter 8 General discussion & future work

Chapter 7

CoO(OH)

nanodiscs

Chapter 5

Cr-AlO(OH)

nanofibres

Modification

of AlO(OH)

by Cr doping

Chapter 4

In(OH)3

nanocubes

(In progress)

Co-AlO(OH)

nanofibres

Modification

of AlO(OH)

by Co doping

Group IIIA elements (Al & In)


5

REFERENCES

[1] J. C. W. Chien, J. Am. Chem. Soc. 2002, 93, 4675-4684.

[2] Y.-Y. Lyu, Seung Hwan Yi, Jeong Kuk Shon, Seok Chang, Lyong Sun Pu,

Sang-Yun Lee, Jae Eui Yie, Kookheon Char, Galen D. Stucky, J. M. Kim, J.

Am. Chem. Soc. 2004, 126, 2310-2311.

[3] P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Nature

(London) 1998, 396, 152-155.

[4] X. B. Ke, Z. F. Zheng, H. W. Liu, H. Y. Zhu, X. P. Gao, L. X. Zhang, N. P.

Xu, H. Wang, H. J. Zhao, J. Shi, K. R. Ratinac, J. Phys. Chem. B 2008, 112,

5000-5006.

[5] S. C. Shen, Q. Chen, P. S. Chow, G. H. Tan, X. T. Zeng, Z. Wang, R. B. H.

Tan, J. Phys. Chem. C 2007, 111, 700-707.

[6] S. C. Erwin, L. Zu, M. I. Haftel, A. L. Efros, T. A. Kennedy, D. J. Norris,

Nature (London, United Kingdom) 2005, 436, 91-94.

[7] X. Yang, H. Zhu, J. Liu, X. Gao, W. N. Martens, R. L. Frost, Y. Shen, Z.

Yuan, Micropor. Mesopor. Mater. 2008, 112, 32-44.

[8] R. L. Frost, W. N. Martens, K. L. Erickson, J. Therm. Anal. Calorim. 2005,

82, 603-608.

6


7

CHAPTER 2

LITERATURE REVIEW


8

1. INTRODUCTION

Nanoscience involves a study of nanotechnology and nanomaterials, of which at least

one of the dimensions is in 1 – 100 nm range. Compared to the bulk materials, the

nanoscale size of materials brings the materials new quantum mechanical effects,

like “quantum size effect”.1-3

And the size dependent behaviour of nanomaterials

enables researchers to design and produce devices with advanced properties by

employing nanostructures as building blocks. Interest has increased significantly in

the inorganic nanomaterials with particular morphology for their novel chemical,

electrical, optical, magnetic, mechanical and other properties.4-8

There is no doubt

that the synthesis and characterisation of nanomaterials has great significance.

This chapter will introduce the concepts of nanotechnology and nanomaterials first.

Then focus on the discussion of the controllable synthesis of desired nanomaterials.

Techniques for characterising nanomaterials properties will be reviewed.

2. NANOTECHNOLOGY AND NANOMATERIALS

2.1 Nanotechnology

Nanotechnology refers to a highly multidisciplinary field of applied science whose

theme is the fabrication with dimensions of less than 100 nm (1 nm is a billionth of a

metre) on an atomic and molecular scale. The involved fields include nutritional

sciences, pharmaceutical science, applied physics, materials science, interface and

colloid science, device physics, chemical engineering, mechanical engineering,

biological engineering, and electrical engineering. Nanotechnology is making a

further significant contribution to the fields of computer storage, semiconductors,

biotechnology, manufacturing and energy.

The concept of nano-technology was first introduced by physicist Richard P.

Feynman, who gave a talk titled “There‟s Plenty of Room at the Bottom” at an

American Physical Society meeting at Caltech in 1959.9 The possibility was

proposed by Feynman that one can manipulate individual atoms and molecules, by

using one set of precise tools to build and operate another proportionally smaller set,

so on down to the needed scale. In the system, instead of gravity, surface tension and

Van der Waals attraction would become much more important.


9

Until 1974, the term “nanotechnology” was firstly defined by Professor Norio

Taniguchi10

from Tokyo Science University: “„Nano-technology mainly consists of

the processing of, separation, consolidation, and deformation of materials by one

atom or by one molecule.” Then Dr. K. Eric Drexler, in 1980s, developed and

promoted the technological significance of nano-scale phenomena and devices. So

the term of “nanotechnology” acquired its current sense.11,12

2.2 Nanomaterials

As a crucial aspect of nanotechnology research, synthesis of nanomaterials and their

characterisation and property studies are attracting more and more attention.

Nanomaterials present specific functionality, such as field emission, gas sensing,

medical diagnostics, transistor action, lasing behaviour, photovoltaic properties and

catalytic properties. It becomes a challenging issue to explore ways to control and

manipulate the desired physical and chemical properties of nanomaterials. Mainly,

the unique properties and potential application of nanomaterials are greatly

determined by two important nanoscale geometrical parameters: size and shape.13

2.2.1 Size effect

For nanoscale materials, it is found that most of their behaviour cannot be described

by classical mechanics, but can only be explained by quantum mechanics.14,15

As

materials are reduced to the nanoscale, they can suddenly show very different

properties compared to what they exhibit on a macroscale, enabling unique

applications.16,17

“Size effect” refers to the strong influence in physical properties

of nanocrystals caused by the size of the nanocrystals.18-25

In nanoscale, particles are

small enough to confine their electrons and often obtain unexpected visual

properties. For instance, when the gold particle size was reduced in nanoscale, the

material appears red, deep violet and blue, instead the colour of gold.20

A large

ratio of surface area to volume is found in nanomaterials, which enables the increase

of surface activities of the materials.26

Take gold as an example again, which is

chemically inert as a bulk metal; however gold nanoparticles are reported to be

excellent catalyst for the oxidation of secondary alcohols into ketones.27

Most

important application of nanomaterials has been in catalysis and gas sensing because

of their unique size dependent properties.


10

2.2.2 Morphology control

The term, morphology, is used to describe the study of form comprising shape, size

and structure.28

Unlike bulk materials, the functional characteristics of nanomaterials

or nanodevices are strongly correlated to their shapes.28,29

It is reported that the

most distinct shape effects are observed in the density of energy states, and the

band-gap energy of nanomaterials is also influenced by their shapes.13

Nanomaterials can be simply classified by their dimensionalities: zero – dimensional

(0 D) nanomaterials including particles, isotropic spheres, cubes, and polyhedrons;

one – dimensional (1 D) nano-rods, fibres, tubes, ribbons and wires; two –

dimensional (2 D) nanodiscs, prisms and plates; and other advanced three –

dimensional (3 D) hetero-structural shapes such as rod-based multi-pods and

nano-stars. Low dimensional nanocrystals and nanostructures have become important

building blocks for assembling and patterning future nanodevices. Therefore, the

control of nano-building blocks is crucial for the success of future nanodevices.

3. METAL OXYHYDROXIDE AND OXIDE

NANOMATERIALS

3.1 Metal hydroxide, oxyhydroxide and oxide nanomaterials

Metal oxide nanomaterials have been widely studied because of their high surface

area, porosity, and catalytic activities.13,30,31

Properties of materials can be controlled

not only by the morphology of individual nanocrystals, but also by the nature of the

chemicals. The nanomaterials involved in this project are mainly trivalent metal

hydroxides and their oxides. In nature, these metal hydroxide minerals are important

sources for the corresponding metal. Whereas, their synthetic materials with unique

nanostructures have great potential for application in industry as catalysts, gas

sensors, etc.

Metal hydroxides and their oxyhydroxides are crucial precursors for metal oxides.

This research focuses on the synthesis of metal oxyhydroxides nanomaterials.

Transformation into the corresponding metal oxides is usually found to be a

topological process. The oxides obtained by the dehydroxylation normally preserve

the morphology of metal oxyhydroxide nanomaterials.


11

3.1.1 Group IIIA metal oxides nanomaterials

The series of elements in Group IIIA (“boron group”, or “group 13” by IUPAC,

International Union of Pure and Applied Chemistry) are characterised by having

three electrons in their outer energy levels (valence layers), and form trivalent

compounds. Their oxides share the same formula as M2O3, where M = aluminium

(Al), gallium (Ga) and indium (In).

Aluminium oxides, known as alumina Al2O3, exist in many forms

etc, which arise during the heat treatment of aluminium

hydroxide or aluminium oxyhydroxide. Among these oxides, the most

thermodynamically stable form is Al2O3. Because of their high specific surface

areas and the large number of defects in their crystalline structure, most alumina

phases are important for ceramic catalysts,32

membranes,33

coatings,34,35

and

adsorbents.36,37

Especially, -Al2O3 is widely used in catalysis as active phase and is

characterised by having acidic sites which determines the activity and selectivity of

the catalyst for specific catalytic reactions.38

Furthermore, due to its excellent

thermal stability and chemically properties, 1D -Al2O3 nanomaterials have been

extensively used as carrier and supports for a variety of industrial catalysts in many

chemical processes including cracking, hydrocracking, and hydrodesulfurization of

petroleum feedstock.39-41

Similar to alumina, gallium oxide (Ga2O3) has several polymorphs and

among which, Ga2O3 is the only thermodynamically stable oxide. Ga2O3 is also

an important semiconductor having a wide direct bad gap (Eg = 4.9 eV), and known

for its conduction and luminescence properties.42-45

Ga2O3 nanobelts are46

reported

to be efficient gas sensor for NO2 detection, and it can also be used combined with

other oxides or metal as sensor nanodevices for hydrogen detection.47

It can be

applied to dye-sensitized solar cells, which are based on GaN/gallium oxide

core-shell structures.48

Indium hydroxides and oxides are a series of important semiconductor materials,

which have attracted much attention in the past decade. In(OH)3 is a wide-gap

semiconductor with Eg = 5.15 eV,49

which has potential applications in

photocatalystic, electronic, and solar energy fields.50-52


12

While In2O3 is known as n-type semiconductor with a direct band gap of 3.6 eV

(which is close to that of GaN53

) and an indirect band gap of 2.6 eV.54

In2O3 has

been used widely as solar cells, transparent conductors and sensors.55-59

In2O3 is

mostly used in combination with tin dioxide, forming tin doped indium oxide (ITO).

ITO is well known for application of transparent conducting devices.60,61

3.1.2 Transition metal hydroxide, oxyhydroxides and oxide

Transition metal elements are those whose atoms have an incomplete d sub-shell, or

which can give rise to cations with an incomplete d sub-shell, according to IUPAC

definition. Because of this electron filling status, transition metal compounds exhibit

unique properties which are not found in other elements. Firstly, they can form

compounds in many oxidation states. Take chromium as an example, it has a very

wide range of oxidation states, from Cr (0) to Cr (VI). The oxidation state Cr (III) is

the most stable; and chromia (Cr2O3) is important in specific applied applications

such as in high-temperature resistant materials,62-64

solar energy collectors,65-67

liquid

crystal displays,68,69

and catalysts.70-73

Cr (VI) compounds are toxic and can be very

powerful oxidants, which are commonly found to be CrO42-

and Cr2O72-

anions.

Secondly, the transition metal compounds are normally colorful, which is due to the

electronic transitions. Cr (III) is normally a green color; and because of its stability,

chromia (Cr2O3) can be used as green pigment. However, Cr (VI) appears the color

of orange or yellow. Co(OH)2 shows a pink color, while CoO(OH) is brown and

Co3O4 is black. Thirdly, the transition metal compounds show magnetism when the

element in the compound have one or more unpaired d electrons. For instance, CrO2

is the magnetic substance once widely used in magnetic tape industry. Because of

these unique properties, transition metal oxides are received more and more attention

and become important materials in industry in heterogenous catalysis, which support

materials as well as active components.

3.2 Synthesis approaches

As discussed above, the size and shape of nanomaterials are key factors for the

determination of materials properties. If the precise control of size and shape of

nanocrystals is possible, their chemical and physical properties can be manipulated

as desired.8 Various procedures for controllable inorganic nanomaterials synthesis


13

have been reported, and can be classified into two methods, according to the present

of morphology directing reagents or not. Here in this section, the synthesis of

boehmite (AlO(OH)) nanofibres was discussed as an example to present various

synthesis routes for nanomaterials.

A common approach to direct the size and morphology of crystals formed under

hydrothermal conditions is the use of surfactant templates or directing agents that

control the nucleation process or growth in a specific crystallographic direction.74,75

The template-assisted approaches were already widely employed in the synthesis of

boehmite nanomaterials.

In the condition of adding no surfactants as template, sol-gel and the neutralisation of

acidic aluminium salts by direct addition of a base such as NaOH, ammonia

(NH4OH) or thermal decomposition of urea are the most common employed

synthesis methods of boehmite nanoparticles. Through a self-assembling process, the

size and morphology of metal oxyhydroxides formed depend on the conditions such

as pH, temperature, pressure, nature of salts and bases, and also the ageing of the

hydrolysates.76

The first report on synthesis of boehmite nanofibres was published by

Bugosh in 1961.77

In this study, the procedure includes the addition of Al powder to

an aqueous solution of AlCl3 with an Al/Cl ratio of 2:3. The obtained aluminium

chloride solution was diluted and heated at 160 ˚C for 40 hours. The resulting

boehmite consists of stable colloidal fibrils with a length of 100-200 nm and a width

of 5 nm, which are physically analogous to linear, high molecular weight organic

molecules. This route was further investigated by Brusasco et al.78

Soda was used as a precipitant reacted with aluminium nitrate to produce boehmite in

Hochepied‟s research.79,80

In the studies, experimental conditions such as pH,

temperature and mixing procedure on the morphology and porosity of boehmite

particles synthesized were investigated. Through a one-step synthesis at 60 ˚C and

pH=9, a crystallized boehmite phase with fibre shape was observed. In the other

two-step synthesis an isolable amorphous hydroxide was firstly found, and then the

irregular boehmite nanocrystals were aggregated and transformed into porous

boehmite.80

The mixing procedure was also crucial for the formation of products: a)

when soda was added into nitrate aluminium solution, micro-porous bayerite was

formed; b) adding nitrate aluminium solution into soda resulted micro-porous


14

boehmite; whereas c) boehmite nanofibres can be recovered from a double jet

procedure.79

Bunches of aligned boehmite nanowires were produced with widths of 700 to 800

nm and lengths of about 1 m by Zhang et al. with Na2B4O7 as the mineralizer.81

AlCl3 and Na2B4O7 were used as precursors. High quality boehmite nanowires with

an average diameter of ~20 nm were obtained through a hydrothermal treatment at

200 ˚C for 24 hours. The synthesised boehmite nanowires were arrayed in an ordered

fashion and are closely packed together. It was proposed that the addition of

Na2B4O7 in the reaction increased the chemical potential of the chemicals for 1D

nanostructure growth.82

Besides, NH4OH is a common precipitant in the formation of boehmite as well. For

example, a neutralisation of aluminium nitrate solution with NH4OH was taken place

by Morgado et al.,83

in the investigation of the relationship between the synthesis

conditions and the peptisation ability of the resulting boehmite gel.

In the assistant of NH4OH, Shen et al.84

synthesized boehmite nanorods with

clear-cut edge via a steam-assisted solid wet-gel conversion process. Al(NO3)3 and

NH4OH were the precursors and hydrothermally treated at 200 ˚C for up to 48 hours.

After five hours calcination at 600 ˚C, -Al2O3 nanorods were obtained and the 1D

nanostructures of boehmite was well preserved. The influence of pH values during

precipitation to the morphology control of resulting was investigated, and it was

found that under pH 5.0 or 7.0, boehmite nanorods with lengths in the range of

100-400 nm and uniform diameters (20-30 nm) were gained, whereas irregular

boehmite particles were obtained under alkaline condition (pH=10.0). This process

was unique in the simplicity of preparation and the high efficiency of crystal growth,

which can be operated at a large scale. In addition to the high yield and high purity of

uniform 1D boehmite nanomaterials, this method proved superior because of the

negligible formation of other phase by-products.

As mentioned above, numerous synthesis methods have been developed for boehmite

nanomaterials, especially for 1D nanomaterials with various structures. Different

method could be used depending on the different purposes. Since the main task of

my project is to achieve doped boehmite nanomaterials, especially with 1D

nanostructures, the first step to produce undoped boehmite nanomaterials, simplicity


15

of the experimental operation and high efficiency of the crystal growth are important

for the further study of doping. Moreover, in this synthesis process, no organic

surfactants or additives need to be used and the main by-product NH4NO3 from the

reaction of precipitant NH4OH and nitrate slats can be easily removed by annealing,

which provides high purity of the product. Considering this, it is worthwhile to apply

Shen‟s method84

into my research to investigate the synthesis of doped boehmite.

Furthermore, in Shen‟s study,84

hydrothermal treatment was used. It is considered

that hydrothermal synthesis is an important way to yield inorganic materials with

large good-quality crystals at high vapor pressures from high-temperature aqueous

solutions. The crystal growth is performed in an apparatus consisting of a steel

pressure vessel called an autoclave. Synthesis under hydrothermal conditions offers

some significant advantages over other chemical synthesis techniques.85

First, it is

easy to control particle size and morphology by varying the synthesis conditions.

Secondly, many materials can be synthesised directly in the desired crystalline phase

at low temperature. Finally, a substance with an elemental oxidation state can be

produced and since the particles are produced in a sol form, the resultant sol can be

used directly in the production of green bodies using pressure filtration or

extrusion.85

Most of the formation reported of boehmite nanofibres was carried out

using the hydrothermal method.

After reviewing most of the boehmite synthesis approaches, we prefer Shen‟s84

steam-assisted solid wet-gel idea as the basic method for my project to facilitate

large-scale manufacturing of 1D boehmite nanomaterials, even 1D doped boehmite

nanomaterials.

3.3 Characterisation of nanomaterials

To study the obtained products, conventional characterisation techniques will be

applied, such as: Raman spectroscopy, X-ray diffraction (XRD), selected area

electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), scanning

electron microscopy (SEM), transmission electron microscopy (TEM), N2

adsorption/desorption and BET, thermal gravimetric analysis (TGA/dTA) – mass

spectrometer (MS), IR spectroscopy, UV-Vis, and photoluminescence studies, etc.


16

3.3.1 Phase, structure and morphology studies

X-ray diffraction (XRD) and Raman spectroscopy are basically used as the common

methods to determine the purity of the products.

Powder X-ray diffraction (XRD) is a most useful technique used to characterise the

crystallographic structure, crystallite size (grain size), and preferred orientation in

polycrystalline or powdered solid samples. X-ray methods of characterisation

represent a powerful approach to study of nanophase materials. By characterizing the

sample as a whole, they are an essential complement to other high-resolution

methods, which provide rather detailed information on only a few particles.

Moreover, based on XRD data, much information on crystals, such as lattice

parameters and crystal size etc., can be provided. For example, crystal sizes can be

calculated using the Scherrer Equation: D = Kλ/βcosθ, where D is the crystalline size

(nm), K presents as Scherrer, which has a value of 0.89, λ is the wavelength (nm), β

is the observed peak width, and θ is the diffraction angle. With these crystal

parameters, one can analysis the influence of dopants to boehmite crystal lattice.

Scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS)

and transmission electron microscopy (TEM) with selected area electron diffraction

(SAED), as well as high-resolution transmission electron microscopy (HRTEM) are

important and direct ways to study not only morphology of the samples, but also

their composition and crystalline properties.

Scanning electron microscope (SEM) is a type of electron microscope that images

the sample surface by scanning it with a high-energy beam of electrons in a raster

scan pattern. The electrons interact with the atoms that make up the sample

producing signals that contain information about the sample's surface topography,

composition and other properties such as electrical conductivity. SEM has a large

depth of field, which allows more of a specimen to be in focus at one time than

optical microscopic methods. SEM also has much higher resolution, so closely

spaced specimens can be magnified at much higher levels than with an optical

microscope. Because SEM uses electromagnets rather than lenses, the researcher

has easily much more control in the degree of magnification. During the SEM

study, the EDS spectrum can be obtained and roughly reveal the presence of

aluminium, oxygen and dopant metals.

http://en.wikipedia.org/wiki/Powder_diffraction

http://en.wikipedia.org/wiki/Crystallite

http://en.wikipedia.org/wiki/Electron_microscope

http://en.wikipedia.org/wiki/Topography


17

Transmission electron microscopy (TEM) is a microscopy technique whereby a

beam of electrons is transmitted through an ultra thin specimen, interacting with the

specimen as it passes through it. It is also possible to produce an image from

electrons deflected by a particular crystal plane. By either moving the aperture to the

position of the deflected electrons, or tilting the electron beam so that the deflected

electrons pass through the centred aperture, an image can be formed of only

deflected electrons, known as a Dark Field image. In the most powerful diffraction

contrast TEM instruments, crystal structure can also be investigated by High

Resolution Transmission Electron Microscopy (HRTEM), also known as phase

contrast imaging as the images are formed due to differences in phase of electron

waves scattered through a thin specimen. Images taken form TEM or HRTEM are in

much higher magnification than that from SEM, telling much more details of sample

morphology in nano scale. Furthermore, the SAED patterns detected by TEM or

HRTEM can indicate the crystal structure of resulting doped boehmite

nanomaterials, which patterns are match with the XRD patterns and can be compared

with the JCPDS powder diffraction files to defied materials. The SAED patterns tell

the sample is polycrystalline by appearing in rings or monocrystalline by showing

separated dots.

XPS analyses the binding energy of elements within a compound and can be used to

study their chemical environment. Through the binding energies bonding of the

atoms can be determined (i.e. Al-O, O-H, etc). Al3+

, O2-

and the metal dopant ions

are in different environments due to different symmetries and bonding environments

in their unit cells and are therefore expected to have different binding energies. With

the characterisation and comparison of the binding energies of the undoped and

doped nanomaterials, XPS can be used to evaluate the doping efficiency.

As mentioned above, boehmite and alumina are important porous materials, whose

pore prosperities are important for further applications in industry, such as catalysis

applications. Brunauer-Emmett-Teller (BET) method is the most widely used

procedure for the determination of specific surface area of porous solids. The

gradual changes in the texture of the doped boehmite nanofibres can also be reflected

in the nitrogen adsorption isotherms. The N2 adsorption isotherm curve presents the

relationship, at constant temperature, between the amount of gas adsorbed and the

http://en.wikipedia.org/wiki/Microscope

http://en.wikipedia.org/wiki/Electron


18

pressure, or relative pressure, respectively.86

The data of the isotherm are used to

detect BET surface area and porosity of solids.

3.3.2 Vibrational spectroscopy

Raman spectroscopy is the measurement of the wavelength and intensity of

inelastically scattered light from molecules. The Raman scattered light occurs at

wavelengths that are shifted from the incident light by the energies of molecular

vibrations. Indeed, Raman spectroscopy has proven most useful for the study of

diagenetically related minerals as often occurs with oxyhydroxide and carbonate

minerals. Some previous studies 87-90

have been undertaken using Raman

spectroscopy to study complex secondary minerals formed by crystallisation from

concentrated solutions. Limited spectroscopic studies of nanomaterials have been

forthcoming. The mechanism of Raman scattering is different from that of infrared

absorption, and Raman and IR spectra provide complementary information.

Vibrational information is very specific for the chemical bonds in molecules, Raman

spectroscopy therefore provides a fingerprint by which the molecule can be

identified. The fingerprint region of organic molecules is in the range 500-2000 cm-1

.

Another way that the technique is used is to study changes in chemical bonding of

the boehmite nanomaterials after doping.

3.3.3 Thermal analysis techniques

As mentioned above, for a further application of metal oxides, metal hydroxides and

oxyhydroxides are required to be calcined through the thermal decomposition

process. For a better understanding of these nanomaterials, it is important to monitor

the dehydration and dehydroxylation process during heating by thermal analysis

techniques. Three instruments for the study on decomposition procedure were

employed: thermogravimetric analysis coupled to a mass spectrometer for gas

products analysis, Raman spectroscopy combined with a hot-stage and the infrared

emission spectroscopy.

The thermogravimetric analysis (TGA/dTG) is a method to determine changes in

mass with change in temperature. TGA is commonly employed in research of the

dehydration and decomposition point and then to determine the thermal stability for

samples.

http://elchem.kaist.ac.kr/vt/chem-ed/spec/spectros.htm#scattering


19

In this study, the thermal stability of the synthetic nanomaterials can be easily

investigated by comparing their main decomposition temperatures and the

corresponding mass loss. The mass spectrometer (MS) is set up for gas analysis, and

only water vapour, nitric oxide, carbon dioxide, and oxygen were analysed. TG-MS

proves to be very useful to monitor the reasons for every mass loss step.

The combination of Raman spectroscopy with a hot-stage lends itself as the

technique of choice for studying the chemical reactions during dehydration and

dehydroxylation. The advantage of this technique is that the changes in molecular

structure can be followed in situ and at the elevated temperatures. Spectroscopic

studies with a hot-stage to reveal the thermal transition from metal hydroxides and

oxyhydroxides nanomaterials to metal oxides are limited.91-93

Hot-stage Raman

spectroscopy allows us to monitor the changes occurring in the molecular structure

during the thermal transition procedure. Spectra at elevated temperatures were

obtained using a Linkam thermal stage (Scientific Instruments Ltd., Waterford

Surrey, England). Spectra were taken from room temperature (25 ºC) at certain

temperature intervals up to a desired high temperature (limit to 600 ºC).

Infrared emission spectroscopy (IES) has proven to be a very useful tool that can be

applied in situ. IES measures discrete vibrational frequencies emitted by thermally

excited molecules.94

Frost and Kloprogge have developed extensive work on

minerals at the molecular level using infrared emission spectroscopy.95-99

They

applied infrared emission spectroscopy in the studies on the dehydration,

dehydroxylation, decarbonization behavior and other changes of minerals, especially

clay minerals.100-103

However, limited work has been published on inorganic

nanomaterial using this in-situ thermal analysis technique. Interpretation of the

changes in the obtained infrared emission spectra with temperature increase can yield

information concerning the thermal reaction and thermal stability of materials.

Changes in the molecular structure, particularly in the hydroxyl groups and water, as

a function of temperature of the synthetic nanomaterials can be determined using

infrared emission spectroscopy. This will enable the further studies on applications

of the synthetic nanomaterials in industry.


20

4. DOPED NANOMATERIALS

4.1 Doping

Doping refers to the intentional introduction of impurities into a material, and is

applied in many semiconductor industry fields. Introducing impurities is fundamental

to controlling the properties of bulk semiconductors,104

and this has stimulated

similar efforts to dope semiconductor nanocrystals.4,8,23,105

Doping is critical for

semiconductors, which would otherwise be electrically insulating. For this reason,

much work has been done to explore how dopants can influence semiconductor

nanocrystals, crystallites a few nanometres in scale with unusual and size-specific

optical and electronic behaviour.23

With the development of modern industry, it is

exciting to find out that the promise of nanocrystals as a technological material, for

applications including solar cells,106

bioimaging,107

and wavelength-tunable lasers,108

may ultimately depend on tailoring their behaviour through doping.

Impurities can strongly modify electronic, optical and magnetic properties of bulk

semiconductors.104,108,109

For instance, it is reported109

that a substitutional impurity

with one more valence electron than the host atom it replaces can be ionized by

thermal energy and donate its extra electron to the semiconductor (n-type doping).

These electrons or holes are then available as carriers of electrical current. For

nanocrystals, where applications often require thin conducting films, the ability to

introduce these carriers is essential. Besides, dopants can also strongly influence

optical behaviour of materials. Take CdSe nanocrystals are an example, the lasing

threshold in these quantum dots can be reduced threefold by adding extra

electrons.110

As for the bio-imaging applications, fluorescent dopants may mitigate

toxicity problems by producing visible or infrared emission in nanocrystals made

from less-harmful elements than those currently used.111

Moreover, carriers whose

spins are aligned one way are conducted preferentially, while others are blocked, an

effect that could be used in future spintronic devices.109

4.2 Metal ions doped AlO(OH) (boehmite)

To a large extent, the key for fabrication of functional catalysts, ceramics and

nanostructures is manipulation of the surface and interfacial energies. As for

alumina, the transformation from -alumina (defect spinel structures) into the


21

-alumina (corundum structure) leads to a dramatic decrease in surface area.

Therefore, the major challenge for alumina in applications as catalyst supports and

ceramic precursors is to retain large surface areas during high-temperature annealing.

In order to manipulate phase transformation temperature and surface area of alumina,

Castro et al.112

investigated the relationship between dopants and surface energy and

thermodynamic stability of -alumina by using Zr and Mg as additives. Results

showed that dopants changed the pattern of phase transformation and densification.

Zr doped -alumina showed a higher energy of the hydroxylated surface than that of

pure -alumina but showed a lower energy of the anhydrous surface. Mg addition did

not significantly change surface energies but decreases the energetic instability of the

-alumina phase. Djuričić et al.113

also studied the morphology and thermal stability

of Zr doped alumina prepared by homogeneous precipitation from an aqueous salt

solution followed by calcination in air. Boehmite nanofibres and nanosheets with

irregular shape were formed after hydrothermal treatment. Results showed that

zirconia was insoluble in -Al2O3 so that phase transformation to -Al2O3 was

accompanied by a phase separation to form an alumina-zirconia nanocomposite. The

thermal stability of the transition phases was increased both by the dopant and by

hydrothermal treatment.

Besides the application in high-temperature catalytic industry, doped boehmite

nanomaterials have potential to exhibit novel properties compared with undoped

boehmite nanomaterials. In recent years, a great deal of research on rare-earth (RE)

and transition metal (TM) ions doped nanostructure materials has been focused to

find their potential applications in photonic applications.114

For transition metal (TM)

ions, the electronic d-d transitions involve electrons which are localised in atomic

orbitals of the ions. In this case, no size-dependent quantization effect (due to

confinement of delocalised electrons) is found in these transitions.114,115

However,

confinement effects may be induced by inter-ionic electronic interaction and

particularly, through electron-phonon interaction,116

which has crucial manifestations

in influencing the optical properties. Take chromium as an example. As mentioned

before,117

-Al2O3 containing chromium is basically naturally occurring ruby, the

crystal of which is known as ruby laser. And since nanoparticles have recently been

recognised to hold tremendous potential in the area of photonic applications,115

it is

expected that ruby nanomaterials will offer interesting possibilities for novel laser


22

applications using powdered media.118

As we know, boehmite is a vital precursor for

-Al2O3, therefore, chromium doped -Al2O3 nanomaterials can be expected to be

obtained by annealing chromium doped boehmite with nanostructures. It is

predicted that the luminescence properties of ruby nanocrystals will strongly depend

on the phase of the nanocrystals, the concentration of Cr3+

ions and the sintering

temperature.

As for magnetic properties, Xue et al.119

successfully synthesized iron doped Al2O3

nanocomposite by reducing the mixture of FeO(OH) and AlO(OH) dry gel at

different temperatures in a hydrogen atmosphere and then studied the magnetic

properties of Fe doped Al2O3 nanocomposite. The results showed that the magnetic

properties of the nanocomposite can be controlled by the concentration and size of

the dopant Fe nanoparticles.

To our knowledge, there have been several reports on doped alumina with dopants

such as chromium114,120-122

and iron,119

however most of these studies are about bulk

materials, less work has been done on doped boehmite nanocrystals. One of the main

aims of my project is to produce doped boehmite nanomaterials.. As mentioned

above, undoped alumina is a promising material in that it is stable at high

temperatures, an electrical insulator and chemically inert, which makes it suitable for

several technical applications, for example, diffusion barrier,32,33

or hard coating.34,35

The introduction of dopants, such as chromium and iron, would bring additional

interesting properties of the aluminas.


23

5. CONCLUSIONS

Nanomaterials science is a significant area of research and more efforts are needed to

produce novel and functional materials and devices. For a better application in

industry, it is important to develop controllable synthesis methods for nanomaterials

(building blocks) with desired morphologies, to functionalise the materials such as

by introducing a dopant, and to study and characterise the synthetic materials

comprehensively. Therefore, this study was aimed to produce the nanostructures

with desired properties, to establish controllable synthesis methods, to generate new

classes of good performance nanomaterials with or without introducing dopants, and

to investigate the detailed properties of the synthetic materials, including thermal

stability, porous and surface properties, etc.


24

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(122) Pflitsch, C.; Siddiqui, R. A.; Atakan, B. Appl. Phys. A-mater. 2008, 90, 527.

30

Chapter 3 Boehmite AlO(OH) nanofibres

31

CHAPTER 3

SYNTHESIS AND

CHARACTERISATION OF

BOEHMITE NANOFIBRES

This paper was originally published:

Yang, J.; Frost, R. L., "Synthesis and characterization of boehmite nanofibers."

Research Letters in Inorganic Chemistry 2008. DOI:10.1155/2008/602198.


32

SYNOPSIS

The aluminium oxyhydroxide is well known as boehmite, AlO(OH), which is a

crucial precursor for alumina. Boehmite nanomaterials have attracted great attention

in catalyst science, especially their one dimensional structures, such as, nanofibres,

nanotubes, nanorods, etc. In Chapter 3, hydrothermal synthesis of uniform

high-quality boehmite nanofibres was present. This synthesis route without use of

any surfactants or structure directing agents showed its advantages in manufacturing

1D boehmite nanofibres in large quantities environmental friendly, facilely and

economically. 500 nm long and 4-6 nm wide boehmite nanofibres were fabricated.

The synthesised boehmite nanofibres were characterised by X-ray diffraction (XRD),

scanning electron microscopy (SEM), infrared emission spectroscopy (IES) and

Raman spectroscopy.


33

STATEMENT OF CONTRIBUTION OF CO-AUTHORS

The authors listed below have certified* that:

1. they meet the criteria for authorship in that they have participated in the conception, execution, or interpretation, of at least that part of the publication in their field of expertise;

2. they take public responsibility for their part of the publication, except for the responsible author who accepts overall responsibility for the publication;

3. there are no other authors of the publication according to these criteria;

4. potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor or publisher of journals or other publications, and (c) the head of the responsible academic unit, and

5. they agree to the use of the publication in the student’s thesis and its publication on the Australasian Digital Thesis database consistent with any limitations set by publisher requirements.

In the case of this chapter:

Synthesis and Characterisation of Boehmite Nanofibres

Yang, J.; Frost, R. L., Research Letters in Inorganic Chemistry 2008. DOI:10.1155/2008/602198.

Contributor Statement of contribution*

Jing Yang Developed experimental design, conducted experiments and data analysis, and wrote the manuscript.

Ray L. Frost * Overall supervisor of the project, aided in experimental design, data analysis, manuscript writing and editing.

Principal Supervisor Confirmation

I have sighted email or other correspondence from all Co-authors confirming their certifying authorship.

____________________ ________________________ ______________________

Name Signature Date


34

RESEARCH HIGHLIGHTS

The article reports a facile hydrothermal synthesis of boehmite nanofibres. The

reaction was carried out at low temperature (170 ºC) without any assistance of

surfactants templated or directing agents. The synthesised boehmite nanofibres were

uniform in size and shape with an average length of 500 nm and width of 4 – 6 nm.

In their Raman spectra, bands for hydroxyl stretching vibrations were found at 3216,

3077 and 2989 cm-1

; hydroxyl deformation modes at 448 and 340 cm-1

; and doubly

degenerate mode of AlO6 octahedron at 495 and 675 cm-1

. The thermal stability of

the resultant boehmite nanomaterials were studied by infrared emission

spectroscopy, which confirms that their dehydroxylation starts at 250 ºC and is

completed by 450 ºC.


35

SYNTHESIS AND CHARACTERISATION OF

BOEHMITE NANOFIBRES

Jing Yang and Ray L. Frost*

Inorganic Materials Research Program, School of Physical and Chemical Sciences,

Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001,

Australia. E-mail: [email protected]

Received: June 18, 2008

Accepted: August 5, 2008

ABSTRACT

Boehmite nanofibres of high quality were synthesised through a wet-gel conversion

process without the use of a surfactant. The long nanofibres of boehmite with

clear-cut edges were obtained by steaming the wet-gel precipitate at 170 ºC for 2

days under a pH 5. Hydrothermal treatment of the boehmite gels enabled

self-assembly through directed crystal growth. Detailed Characterisation using

X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared emission

spectroscopy (IES) and Raman spectroscopy is presented.

KEYWORDS

Boehmite; Nanofibres; Hydrothermal Treatment; Nanomaterials

1. INTRODUCTION

The - and -Al2O3 polymorphs have found numerous applications in technical

ceramics, thin solid films, catalysis, and many others. In many cases, boehmite

-AlO(OH) is the starting material in the applications of alumina phases. As a typical

oxyhydroxy compound, boehmite is also extensively used as absorbents,[1, 2]

catalysts,[3] and optical materials.[4] Nanosized materials are well known for their


36

quantum size effects [5] and are expected to gain novel physical and chemical

properties, with many more potential applications in a wide range of areas. In recent

years, boehmite with nanoscale dimensions and morphological specificity has

attracted enormous interest from both fundamental and practical viewpoints.

Boehmite nanofibres were reported to be assembled with the assistance of

poly(ethylene oxide) (PEO) surfactant [6] and tubular γ-Al2O3 was fabricated via soft

solution route using N-cetyl-N,N,N-trimethylammonium bromide surfactant.[7] It

has been reported a solution-based chemical synthesis of boehmite nanofibres and

alumina nanorods by a modified sol-gel process in the presence of organic

solvents.[8] Shen et al. reported that a steam-assisted solid-phase conversion of

amorphous aluminium hydroxides wet gel to well crystallized 1D nanostructure of

boehmite nanorods without using surfactants and solvents.[9] The process is unique

in the simplicity of preparation and the high efficiency of crystal growth, which can

be operated on a large scale. In this work, long, large boehmite nano/microfibres

with high crystallinity and purity were synthesised based on Shen‟s methodology,[9]

and the characterisation of these the nano/microfibres are reported.

2. EXPERIMENTAL

2.1 Preparation procedures

15 g of Al(NO3)3•9H2O was dissolved in 25 mL ultrapure water, and 28% ammonia

was diluted into 10% solution. At room temperature 10% ammonia solution was

added dropwise into Al(NO3)3 solution at a constant rate of 5 mL/min while stirring

vigorously. Ammonia was ceased to be added when the pH value of the reaction

mixture reached pH 5, which was measured at room temperature. (Please note that at

high temperature the pH value of the reaction was probably slightly different.) The

reaction mixture was stirred constantly in air at room temperature for 1 hour. The

obtained white gel was filtrated to obtain the gel-cake, which was then transferred

into a glass beaker (25 mL). Before adding the gel-cake to a Teflon vessel (200 mL),

2 mL of ultrapure water was added. The Teflon vessel was sealed and heated at 170

ºC for 2 days. The resulting white material was washed with ultrapure water,

centrifuged and dried at 35 ºC for 2 days.


37

2.2 Characterisation

X-ray diffraction analyses were performed on a PANalytical X‟Pert PRO X-ray

diffractometer, with a Cu X-ray tube ( = 1.54 Å), operating at 40 kV and 40 mA.

The scanning electron microscopy (SEM) images were taken with a FEI Quanta 200

operating at 25 kV. The specimens were mounted on SEM mounts with carbon tape

and sputter-coated with a thin layer of gold. The infrared emission spectroscopy

(IES) was carried out on a Nicolet Nexus 870 FTIR spectrometer. The emission

spectra were collected at an interval of 50 ºC, over the range 100 ºC – 850 ºC.

Further details have been published.[10-13] Raman spectra were obtained using a

Renishaw 1000 Raman microscope system, which also includes a monochromator, a

filter system and a Charge Coupled Device (CCD). 64 Raman spectra were

collected using 5 mW of power at the sample. Further details of the Raman

technique have been published [14-19].

3. RESULTS AND DISCUSSION

3.1 X-ray diffraction

X-ray diffraction is normally used to determine the phase and phase purity of the

synthesised boehmite. Fig. 1 displays well-defined XRD patterns observed and all

diffraction peaks were perfectly indexed to the XRD pattern of pure boehmite

(JCPDS card 01-083-2384). No XRD peaks representing other crystalline phases

were detected, indicating that the nanofibres of the synthetic boehmite exhibited

excellent crystallinity and a high purity.

3.2 Scanning electron microscope

Whilst TEM images are normally used to show the morphology of the boehmite

nanomaterials, if the nanofibres are large enough in length then scanning electron

microscopy (SEM) can be used to prove the nature of the nano/micro-material.

Fig. 2 presents the SEM images of the synthetic boehmite nanofibres. It is clearly

seen the long nanofibres in bundles. Zhu [20] et al. reported that through a

soft-templated process with PEO surfactant micelles at 100 ºC, the nanofibres could

be assembled, the maximum length of which was 100 nm.


38

In this study, the nanofibres were synthesised in the absence of surfactant, and in a

supersaturated hydrothermal condition, which was reported by Shen et al. [9]. This

steam-assisted self-assembly fabrication of boehmite yielded high-quality 1D

nanostructures with clear-cut edges and high purity. The structure was confirmed by

the XRD patterns.

Fig. 1 XRD pattern of the synthetic boehmite nanofibres, after hydrothermal

treatment at 170 ºC for 2 days under pH 5.

Fig. 2 SEM image of the synthetic boehmite nanofibres, after hydrothermal treatment

at 170 ºC for 2 days under pH 5.


39

3.3 Infrared emission spectroscopy

Infrared emission spectroscopy (IES) is known as a measurement of discrete

vibrational frequencies emitted by thermally excited molecules. The major

advantages of IES are that the samples are measured in situ at the elevated

temperature and IES requires no sample treatment other than making the sample of

submicron particle size.

Fig. 3 clearly shows such a suite of the dehydroxylation of the synthetic boehmite

nanofibres. The dehydroxylation is followed by the decrease of intensity of the

hydroxyl deformation modes and the loss of intensity of the hydroxyl stretching

frequencies. The hydroxyl deformation frequencies are observed at 840 cm-1

and 757

cm-1

. The spectral changes in these low- frequency bands show that dehydroxylation

commenced at 250 ºC. The three bands displayed at 3671 cm-1

, 3360 cm-1

, 3140 cm-1

are recognized in the hydroxyl stretching region. Above 450 °C the hydroxyl

stretching bands are no longer observed. These spectral changes confirm that

dehydroxylation starts at 250 ºC and is completed by 450 ºC.

Each of the hydroxyl stretching bands shows an increase in bandwidth with

temperature increase, and this indicates that the molecular structure of the boehmite

nanofibres was becoming more disordered during the dehydroxylation process.


40

Fig. 3 IES spectra of the synthetic boehmite nanofibres, collected at an interval of

50 ºC, over the range 100 ºC – 850 ºC.


41

3.4 Raman spectroscopy

Many minerals both natural and synthetic lend themselves to analysis by Raman

spectroscopy. The great advantage of Raman spectroscopy is that just so long as the

materials are 1 micron in size or larger than individual nanofibres can be analysed as

is the case for the boehmite fibres shown above.

Fig. 4 Raman spectra of the synthetic boehmite nanofibres, after hydrothermal

treatment at 170 ºC for 2 days under pH 5.

Fig. 4 depicts the Raman spectra of the synthetic boehmite nanofibres. The bands at

3216, 3077 and 2989 cm-1

are in the region of the hydroxyl stretching vibrations.

Frost et al. [21] characterised hydroxyl stretching bands in the Raman spectrum of

boehmite and reported bands at 3413, 3283, 3096 and 2997 cm-1

. The

low-wavenumber region (1100-200 cm-1

) of boehmite consists of hydroxyl

deformation modes (1050-900 cm-1

) and hydroxyl translation modes (800-400 cm-1

).

The doubly degenerate mode of the AlO6 octahedron caused the observation of the

bands at 495 and 675 cm-1

. [22] The band at 362 cm-1

was attributed to the

vibration of fully symmetric Ag mode, in which all aluminium and oxygen atoms

move parallel to the b-axis. [23] The weak bands at 732, 448 and 340 cm-1

were

attributed to OH- deformation modes. [24]

(a) (b)


42

4. CONCLUSIONS

In this research, long boehmite nanofibres with high crystallinity and purity were

formed under steam-assisted hydrothermal treatment at 170 ºC for 48 hours with pH

5. The structure and morphology of the nanofibres were detected by XRD and SEM,

while the IES spectra illustrated their thermal properties. Raman spectroscopy was

applied to characterize the 1D-nanostructured boehmite.

ACKNOWLEDGMENTS

The financial and infrastructure support of the Queensland University of Technology

Inorganic Materials Research Program of the School of Physical and Chemical

Sciences is gratefully acknowledged.


43

REFERENCES

[1] V.S. Burkat, V.S. Dudorova, V.S. Smola, T.S. Chagina, Light Metals (1985)

1443-1448.

[2] C. Nedez, J.-P. Boitiaux, C.J. Cameron, B. Didillon, Langmuir 12 (1996)

3927-3931.

[3] J.-L. Le Loarer, H. Nussbaum, D. Bortzmeyer, Alumina extrudates, methods

for preparing and use as catalysts or catalyst supports. (Rhodia Chimie,

Fr.). Application: WO, 1998, p. 44.

[4] D. Mishra, S. Anand, R.K. Panda, R.P. Das, Materials Letters 42 (2000)

38-45.

[5] G.D. Stucky, J.E. Mac Dougall, Science 247 (1990) 669-678.

[6] H.Y. Zhu, X.P. Gao, D.Y. Song, Y.Q. Bai, S.P. Ringer, Z. Gao, Y.X. Xi, W.

Martens, J.D. Riches, R.L. Frost, Journal of Physical Chemistry B 108

(2004) 4245-4247.

[7] D. Kuang, Y. Fang, H. Liu, C. Frommen, D. Fenske, Journal of Materials

Chemistry 13 (2003) 660-662.

[8] S.C. Kuiry, E. Megen, S.D. Patil, S.A. Deshpande, S. Seal, Journal of

Physical Chemistry B 109 (2005) 3868-3872.

[9] S.C. Shen, Q. Chen, P.S. Chow, G.H. Tan, X.T. Zeng, Z. Wang, R.B.H. Tan,

Journal of Physical Chemistry C 111 (2007) 700-707.

[10] R.L. Frost, G.A. Cash, J.T. Kloprogge, Vibrational Spectroscopy 16 (1998)

173-184.

[11] R.L. Frost, B.M. Collins, K. Finnie, A.J. Vassallo, Clays Controlling the

Environment, Proceedings of the International Clay Conference, 10th,

Adelaide, July 18-23, 1993 (1995) 219-224.

[12] R.L. Frost, J.T. Kloprogge, Spectrochimica Acta, Part A: Molecular and

Biomolecular Spectroscopy 55A (1999) 2195-2205.

[13] R.L. Frost, J.T. Kloprogge, Tijdschrift voor Klei, Glas en Keramiek 19

(1998) 11-15.


44

[14] R.L. Frost, J. Cejka, Journal of Raman Spectroscopy 38 (2007) 1488-1493.

[15] R.L. Frost, J. Cejka, G.A. Ayoko, M.L. Weier, Journal of Raman

Spectroscopy 38 (2007) 1311-1319.

[16] R.L. Frost, J.M. Bouzaid, Journal of Raman Spectroscopy 38 (2007)

873-879.

[17] R.L. Frost, M.L. Weier, P.A. Williams, P. Leverett, J.T. Kloprogge, Journal

of Raman Spectroscopy 38 (2007) 574-583.

[18] R.L. Frost, J.M. Bouzaid, W.N. Martens, B.J. Reddy, Journal of Raman

Spectroscopy 38 (2007) 135-141.

[19] R.L. Frost, S.J. Palmer, J.M. Bouzaid, B.J. Reddy, Journal of Raman

Spectroscopy 38 (2007) 68-77.

[20] H.Y. Zhu, J.D. Riches, J.C. Barry, Chemistry of Materials 14 (2002)

2086-2093.

[21] R.L. Frost, J.T. Kloprogge, S.C. Russell, J. Szetu, Applied Spectroscopy 53

(1999) 572-582.

[22] A.B. Kiss, G. Keresztury, L. Farkas, Spectrochimica Acta, Part A:

Molecular and Biomolecular Spectroscopy 36A (1980) 653-658.

[23] C.J. Doss, R. Zallen, Physical Review B: Condensed Matter and Materials

Physics 48 (1993) 15626-15637.

[24] T. Assih, A. Ayral, M. Abenoza, J. Phalippou, Journal of Materials Science

23 (1988) 3326-3331.

Chapter 4 Synthesis and characterisation of In(OH)3 nanocubes

45

CHAPTER 4

SYNTHESIS AND

CHARACTERISATION OF

INDIUM HYDROXIDE In(OH)3

NANOCUBES


46

SYNOPSIS

Since high-quality one dimensional AlO(OH) nanofibres can be successfully

achieved through hydrothermal treatment (see Chapter 3), it is of great interest to see

if this hydrothermal synthesis route works on indium, which is also an important

metal element in Group IIIA. However, in this chapter, it is found that instead of

indium oxyhydroxide InO(OH), In(OH)3 was achieved. The synthetic In(OH)3

nanocubes were 350 nm in size. A topotactical relationship was observed between

In(OH)3 and its thermal product In2O3.

Hot-stage Raman and infrared emission spectroscopy prove to be useful tools to

study the thermal decomposition behaviour of nanomaterials in situ. Two published

articles compose this chapter, one (Chapter 4.1) is on hot-stage Raman study and the

other (Chapter 4.2) is on the infrared emission spectroscopy of the synthetic indium

hydroxide nanocubes. It is investigated that changes happened in the molecular

structure as a function of temperature of the indium hydroxide to indium oxide

nanocubes. Interpretation of the changes in the obtained spectra with temperature

increase can yield important information concerning the thermal reaction and thermal

stability of materials.


47

CHAPTER 4.1

THERMOGRAVIMETRIC

ANALYSIS AND HOT-STAGE

RAMAN SPECTROSCOPY OF

CUBIC INDIUM HYDROXIDE


Yang, J.; Frost, R. L.; Martens, W. N., "Thermogravimetric analysis and hot-stage

Raman spectroscopy of cubic indium hydroxide." Journal of Thermal Analysis and

Calorimetry 2010, 100 (1), 109-116.


48









Thermogravimetric Analysis and hot-stage Raman Spectroscopy of Cubic Indium Hydroxide

Yang, J.; Frost, R. L.and Martens, W. N., Journal of Thermal Analysis and Calorimetry 2010, 100 (1), 109-116.



Ray L. Frost* Overall supervisor of the project, aided in experimental design,

data analysis, manuscript writing and editing.

Wayde N. Martens Aided in experimental design, data analysis, manuscript writing and editing.



____________________ _______________________ ______________________

Name Signature Date


49

RESEARCH HIGHLIGHTS

The article reports that 400 nm In(OH)3 nanocubes were obtained after a 2 days

hydrothermal treatment at 180 ºC. Thermo-gravimetric analysis revealed that the

dehydroxylation of the synthesised In(OH)3 nanocubes happed at 219 ºC, which was

also confirmed by the hot-stage Raman spectroscopic study. The cubic morphology

was retained even after a thermal treatment at 500 ºC for 4 h, which was known as a

topotactical relationship between and In(OH)3 its thermal product In2O3.


50

THERMOGRAVIMETRIC ANALYSIS AND

HOT-STAGE RAMAN SPECTROSCOPY OF

CUBIC INDIUM HYDROXIDE

Jing Yang, Ray L. Frost* and Wayde N. Martens




Received: March 24, 2009

Accepted: October 09, 2009

Published Online: November 01, 2009

ABSTRACT

The transition of cubic indium hydroxide to cubic indium oxide has been studied by

thermogravimetric analysis complimented with hot-stage Raman spectroscopy.

Thermo gravimetric analyses showed the transition of In(OH)3 to In2O3 occurred

mainly at 219 °C. The structure and morphology of In(OH)3 synthesised using a soft

chemical route at low temperatures was confirmed by X-ray diffraction and scanning

electron microscopy. A topotactical relationship existed between the nanocubes of

In(OH)3 and In2O3. The Raman spectrum of In(OH)3 was characterised by an intense

sharp band at 309 cm-1

attributed to ν1(In-O) symmetric stretching mode, bands at 1137

and 1155 cm-1 attributed to δ(In-OH) deformation modes, and bands at 3083, 3215, 3123

and 3262 cm-1 assigned to the OH stretching vibrations. Upon thermal treatment of

In(OH)3 new Raman bands were observed at 125, 295, 488 and 615 cm-1 attributed to

In2O3. Changes in the structure of In(OH)3 with thermal treatment were readily

followed by hot-stage Raman spectroscopy.

KEYWORDS

Thermo-gravimetric Analysis; Hot-Stage Raman Spectroscopy; Indium Hydroxide;

Indium Oxide


51

1. INTRODUCTION

Indium hydroxides and oxides are a series of important semiconductor materials,

which have attracted much attention in the past decade. In(OH)3 is a wide-gap

semiconductor with Eg = 5.15 eV, [1] which has potential applications in

photocatalytic, electronic, and solar energy fields. [2-4] While In2O3 is known as

n-type semiconductor with a direct band gap of 3.6 eV (which is close to that of GaN

[5] ) and an indirect band gap of 2.6 eV. [6] In2O3 has been used widely as solar

cells, transparent conductors and sensors. [7-11]

In recent years, In(OH)3 and In2O3 with various morphologies (e.g. nanowires, [12]

nanobelts, [13] nanorods, [14] nanotubes [15] and nanospheres, [16] etc.) have been

synthesised via different methods, such as chemical vapour deposition,

hot-injection techniques, organic solution synthetic routes, hydrothermal methods,

and solvothermal and others. It is known that cubic particles expose a specific

surface, which provides an ideal model for the study of surface related properties,

[17] and connecting these cubic particles into microscale devices may provide future

applications. In particular, the production of indium hydroxide and indium oxide

microcubes has been realized up to now. There are several reports on synthesis of

indium hydroxide microcubes via hydrothermal routes. [6, 17] However the

characterisation on indium hydroxide and oxide microcubes is not fully recorded yet,

or appreciated, especially the thermo gravimetric analyses and spectroscopic studies.

The underlying objective of this research is to synthesise an adequate indium oxide

semiconductor [18-20]. The properties of these semiconductors depend very

heavily on the synthesis method and preparation of the indium oxide [1, 21].

Therefore the aim of this research is to demonstrate the use of thermo gravimetric

analyses and hot-stage Raman spectroscopy to assess the thermal stability of indium

hydroxide, and to determine the changes in the molecular structure of the material as

the indium hydroxide is thermally treated. Such research compliments the thermo

gravimetric analyses and differential thermogravimetric analysis of materials

[22-36]. It is reported that many factors, such as disorder, size and shape

distribution, can all influence the thermal analytical properties and vibrational

properties. Moreover, the vibrational properties of semiconductors are also strongly

affected by temperature.


52

An increase in temperature introduces perturbations in the harmonic potential term,

which changes the vibrational properties. There are only limited thermal Raman

spectroscopic studies of synthetic nanomaterials or micromaterials have been

forthcoming, for example GaO(OH) nanorods. [37] In the present work, we report

the thermo gravimetric analyses and hot-stage Raman spectroscopy of cubic indium

hydroxide, and study the transition of cubic In(OH)3 to cubic In2O3, relating the

spectra to the structure and morphology of the synthesised materials.

2. EXPERIMENTAL

2.1 Synthesis of In(OH)3 cubic materials

Analytical grade In(NO3)3•5H2O and ammonia solution (wt 28%) were used as

precursor to prepare the indium hydrate precipitate. 3 g of In(NO3)3•5H2O was

dissolved in 15 mL ultrapure water, and 28% ammonia was diluted into 10%

solution. At room temperature, 10% ammonia solution was added dropwise into the

indium ions solution while stirring vigorously. Stop adding ammonia when the pH

value of the reaction mixture reached 8, and then kept the mixture stirring in the air

at room temperature for 0.5 hour.

The obtained mixture was filtrated, and then transferred into a Teflon vessel (125

mL) together with 2 mL ultrapure water. Sealed the Teflon vessels and heated it at

180 ºC for 2 days. The resulting material was washed with ultrapure water by

centrifuging, and dried at 35 ºC for 3 days. Finally, the obtained In(OH)3 sample was

calcined in a furnace at 500 ºC for 4 h to form In2O3 product.


X-ray diffraction (XRD) analyses were performed on a PANalytical X‟Pert PRO

X-ray diffractometer (radius: 240.0 mm). Incident X-ray radiation was produced

from a line focused PW3373/10 Cu X-ray tube, operating at 40 kV and 40 mA,

wavelength of 1.540596 Å.


53

2.3 Scanning electron microscopy



and sputter-coated with a thin layer of gold.

2.4 Raman microprobe spectroscopy

The crystals of In(OH)3 were placed and oriented on the stage of an Olympus BHSM

microscope, equipped with 10x and 50x objectives and part of a Renishaw 1000

Raman microscope system, which also includes a monochromator, a filter system

and a Charge Coupled Device (CCD). Raman spectra were excited by a 633 nm laser

at a resolution of 2 cm-1

in the range between 100 and 4000 cm-1

. Repeated

acquisition using the highest magnification was accumulated to improve the

signal-to-noise ratio. Spectra were calibrated using the 520.5 cm-1

line of a silicon

wafer. Details of the technique have been published by the authors. Spectra at

elevated temperatures were obtained using a Linkam thermal stage (Scientific

Instruments Ltd., Waterford Surrey, England). Spectra were taken from room

temperature (25 ºC) at 50 ºC intervals up to a temperature of 550 ºC in order replicate

the acquisition of data in the TGA-MS plots. Spectral Manipulation such as

baseline adjustment, smoothing and normalisation was performed using GRAMS®

software package (Galactic Industries Corporation Salem, NH, USA).

Band component analysis was undertaken using the Jandel „Peakfit‟ software

package, which enabled the type of fitting function to be selected and allows specific

parameters to be fixed or varied accordingly. Band fitting was done using a

Lorentz-Gauss cross-product function with the minimum number of component

bands used for the fitting process. The Lorentz-Gauss ratio was maintained at values

greater than 0.7 and fitting was undertaken until reproducible results were obtained

with squared correlations of r2 greater than 0.996.

2.5 Thermal analysis

Thermal decomposition of the indium hydroxide sample was carried out in a TA®

Instruments incorporated high-resolution thermo gravimetric analyser (series Q500)

in a flowing nitrogen atmosphere (60 cm3 min

–1).


54

Approximately 35 mg of sample underwent thermo gravimetric analyses, with a

heating rate of 5 °C/min, with resolution of 6 from 25 to 1000°C. With the

isothermal, isobaric heating program of the instrument the furnace temperature was

regulated precisely to provide a uniform rate of decomposition in the main

decomposition stage.



In order to confirm the results obtained from the hot-stage Raman spectroscopy,

thermo gravimetric analysis was undertaken. Fig. 1 shows typical the TG-dTG

curve of the synthesised indium hydroxide In(OH)3. A large mass loss is observed at

219 ºC. The total mass loss of 15.76% is in good agreement with the value of 16.28%

calculated by assuming the following thermal decomposition:

2In(OH)3→In2O3+3H2O. A small mass loss at 197 °C of 0.35% is observed. This

mass loss step is attributed to the thermal decomposition of InO(OH), some of which

is formed during the synthesis of the In(OH)3. The reaction is 2InO(OH)

→In2O3+H2O. The results of the gravimetric analyses are in harmony with the

results inferred from hot-stage Raman spectroscopic results (see below). Both

techniques show the transition occurs between 200 and 225 °C.

Fig. 1 Thermo gravimetric analyses of synthetic In(OH)3 nanocubes.


55


X-ray diffraction (XRD) was used to determine the phase structure of the

as-synthesised In(OH)3 and its thermally manufactured products (In2O3). Fig. 2

shows the typical XRD patterns of the synthetic In(OH)3 and In2O3 and all the

diffraction peaks of these XRD patterns could be perfectly indexed to those of

body-centred cubic In(OH)3 with a lattice constant a = 7.9743 Å (JCPDS Card No.

01-076-1463) and In2O3 (JCPDS Card No. 01-071-2195). The miller indices are used

to label the diffraction peaks. No XRD peaks representing other crystalline phases

were detected, indicating that the final product exhibited excellent crystallinity and

high purity. Fig. 2 also shows the calcined product of In(OH)3 is purely In2O3. No

other materials were found. Thermal treatment of In(OH)3 results in the formation of

In2O3.

Fig. 2 XRD patterns of synthetic In(OH)3 (a) and its thermally treated products

In2O3 (b). The peaks are labeled with their Miller indices.


56


Scanning electron microscopy (SEM) is a well-known microscopy technique heavily

used in material sciences for the study of the morphology of materials. Fig. 3a and 3b

compare the morphology of the In(OH)3 and its thermally treated product In2O3 after

treatment at 500 ºC for 4 h. Fig. 3a clearly shows the cubic nature of the In(OH)3.

The cubes are between 300 and 400 nm. Fig. 3b displays the morphology of the

thermally treated In(OH)3. It is noted that the cubic morphology is retained in the

In2O3. It is apparent there is a topotactical relationship between the In(OH)3 and the

In2O3 microcubes. Some shrinkage of the micro-nanocubes is observed upon thermal

treatment.

Fig. 3 (a) Image of In(OH)3 synthesised at 180 °C and (b) image of In2O3, product of

as-synthetic In(OH)3 calcined at 500 ºC for 4 h.

3.4 Raman spectroscopy

Many minerals both natural and synthetic lend themselves to analysis by Raman

spectroscopy. The great advantage of Raman spectroscopy is that just so long as the

materials are 1 micron in size or larger than individual cubes can be analysed as is

the case for In(OH)3 as shown above. In order to study the changes in the spectra of

In(OH)3 as the nanomaterial is thermally treated it is necessary to describe the

spectra collected at room temperature.

(b) (a)


57


region.

Fig. 4 depicts the Raman spectra of cubic In(OH)3 in the 100 to 700 cm-1

region. The

spectrum is dominated by an intense band at 309 cm-1

. This band is attributed to the

ν1 In-O symmetric stretching vibration. Two additional bands are found at 356 and

391 cm-1

. These bands are attributed to the ν3 In-O antisymmetric stretching

vibrations. Low intensity bands are observed at 142, 186, 208 and 227 cm-1

. The

three bands at 186, 208 and 227 cm-1

are assigned to the O-In-O bending modes.

Two low intensity Raman bands are found at 660 and 670 cm-1

.


58


region.

Fig. 5 shows the Raman spectrum of cubic In(OH)3 in the 950 to 1200 cm-1

region.

The spectrum is composed of a broad band centred upon 1044 cm-1

together with two

overlapping bands at 1137 and 1155 cm-1

. These bands are assigned to In-OH δ

deformation modes. The Raman spectrum in the hydroxyl stretching region is shown

in Fig. 6. A complex set of overlapping bands are observed which may be

decomposed into selected component bands at 3083, 3215, 3123 and 3262 cm-1

.

These bands are attributed to the OH stretching vibrations.


59

The other bands fitted in Fig. 6 were understood to be the combinations or overtones

of bands at lower wavenumbers.


region.

3.5 Hot-stage Raman spectroscopy

Hot-stage Raman spectra of the transition of In(OH)3 to In2O3 in the 100 to 800 cm-1

region over the temperature range ambient to 400 °C are displayed in Fig. 7. The

Raman spectrum at 25 °C shows the typical Raman bands of In(OH)3 at 137, 204,

307, 356, 390 and 659 cm-1

. Raman can be used as a semi-quantitative technique and

the decrease of Raman peak intensity reveals the loss of the corresponding sample. As

the temperature increases, the intensity of these bands decreases obviously,

indicating that the amount of In(OH)3 reduces.


60

After 200 ºC, the Raman spectrum of In(OH)3 were replaced by a totally different

spectrum, indicating the completion of dehydroxylation of In(OH)3. No Raman bands

or bands of very low intensity are observed in the spectra collected at 300 to 350 °C.

It is proposed the thermally decomposed indium hydroxide goes through a

recrystallisation stage over this temperature range. At temperatures of 400 °C and

above new Raman peaks are observed at 125, 295, 488 and 615 cm-1

. These bands

are attributed to the new phase formed by the thermally decomposed In(OH)3 that is

In2O3.


region.

The base-lined and curve-resolved Raman spectra at 150 and 400 °C are reported in

Fig. 8, which displays the phase changes over temperature increase. In the spectrum

of 150 °C, typical Raman bands of In(OH)3 were found at 306, 318, 353, 387 and

652 cm-1

.


61

A huge massif can be observed between 100 and 200 cm-1

and was fitted using 9

band components, which were assigned to be the metal-oxygen vibrations.

According to group theory the bands are assigned as follows 615 cm-1

(E2g), 488 cm-1

(A1g), 295 cm-1

(E1g).

According to Wang et al., Raman bands at 630, 497, 366 and 307 cm-1

belong to the

vibrational modes of the bcc-In2O3. The position of the bands differs from those

reported in this research. Liu et al. [38] studied the effect of pressure on the Raman

spectra of In2O3 in the 100 to 700 cm-1

region up to a pressure of 26 GPa.


region.

The Raman spectra in the OH stretching region are shown in Fig. 9a. Clearly the

intensity of the bands in the OH stretching region at 2844, 3079 and 3240 cm-1

diminish in intensity. No intensity remains in these bands at 300 °C. At 400 °C, a

very weak band is observed at 3637 cm-1

.


62

The band may be due to the OH stretching vibration of OH units from the

intermediate compound InO(OH). No intensity remains in this band at 550 °C.

Evidence for these changes is also observed in Fig. 9b, in which the Raman spectra

of the 800 to 1800 cm-1

region are displayed.

Raman bands are observed at 1037and 1135 cm-1

with other low intensity bands

observed at 1288, 1453 and 1593 cm-1

. The intensity of these bands approaches

zero at 200 °C. Intensity is observed only in the band at 1120 cm-1

. New Raman

bands appear in the 300 and 350 °C spectrum at 1333 and 1569 cm-1

. It is thought

that these bands are associated with InO(OH).

Fig. 9 Hot-stage Raman spectra of In(OH)3 in (a) 3900 – 2400 cm-1

region and (b)

1800 – 800 cm-1

region.

(b) (a)


63

4. CONCLUSIONS

Micro and nanocubes of In(OH)3 were synthesised by using soft chemical techniques

without surfactants at low temperatures. The conversion of In(OH)3 to In2O3 cubes

was achieved by thermal treatment. The phase composition was proven by X-ray

diffraction and SEM showed there was a topotactical relationship between the micro

and nanocubes of In(OH)3 and micro and nanocubes of In2O3. The transition of

In(OH)3 to In2O3 was studied by hot-stage Raman spectroscopy and thermo

gravimetric analysis.

The In(OH)3 nanocubes are characterised by Raman spectroscopy. An Intense

Raman band at 309 cm-1

is assigned to the ν1 symmetric stretching mode of In(OH)3.

The intensity of this band decreases as the temperature increases to 300 °C after

temperature which no intensity remains. A new band at 295 cm-1

is observed above

this temperature and is attributed to the ν1 symmetric stretching mode of In2O3. The

intensity of the two Raman bands at 3079 and 3240 cm-1

attributed to the OH

stretching bands of In(OH)3 decrease in intensity until at temperatures above 200 °C

no intensity remains. At the same time the intensity of the Raman band at 1135 cm-1

assigned to the δ(In-OH) deformation modes decrease in intensity such that at 250 °C

no intensity remains. Raman spectroscopy shows that the transition of In(OH)3 to

In2O3 occurs in the 200 to 225 °C temperature range. Such a transition temperature

was confirmed by thermo gravimetric analyses.

ACKNOWLEDGMENTS

The financial and infra-structure support of the Queensland University of

Technology Inorganic Materials Research Program is gratefully acknowledged. The

Australian Research Council (ARC) is thanked for funding the instrumentation.

One of the authors (JY) is grateful to the Queensland University of Technology

Inorganic Materials Research Program for the award of an international doctoral

scholarship.


64

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Zhou, J. Amer. Chem. Soc. 127 (2005) 6922-6923.

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5179-5187.

[12] C. Li, D.H. Zhang, S. Han, X.L. Liu, T. Tang, C.W. Zhou, Adv. Mater. 15

(2003) 143-148.

[13] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947-1949.

[14] J. Yang, C.K. Lin, Z.L. Wang, J. Lin, Inorg. Chem. 45 (2006) 8973-8979.

[15] C.L. Chen, D.R. Chen, X.L. Jiao, C.Q. Wang, Chem. Commun. (2006)

4632-4634.


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[16] B.X. Li, Y. Xie, M. Jing, G.X. Rong, Y.C. Tang, G.Z. Zhang, Langmuir 22

(2006) 9380-9385.

[17] X.H. Liu, L.B. Zhou, R. Yi, N. Zhang, R.R. Shi, G.H. Gao, G.Z. Qiu, J.

Phys. Chem. C 112 (2008) 18426-18430.

[18] H.-X. Dong, H.-Q. Yang, W.-Y. Yin, W.-Y. Yang, L.-F. Wang, Huaxue

Xuebao 65 (2007) 2611-2617.

[19] J. Du, M. Yang, S.N. Cha, D. Rhen, M. Kang, D.J. Kang, Cryst. Growth

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[20] C. Wang, D. Chen, X. Jiao, C. Chen, J. Phys. Chem. C 111 (2007)

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999-1005.

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66

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Therm. Anal. Calorim. 92 (2008) 589-594.

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[37] Y. Zhao, J. Yang, R.L. Frost, J. Raman Spectrosc. 39 (2008) 1327-1331.

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G.T. Zou, J. Appl. Phys. 104 (2008) 083506/083501.


67

CHAPTER 4.2

APPLICATION OF INFRARED

EMISSION SPECTROSCOPY TO

THE THERMAL TRANSITION OF

INDIUM HYDROXIDE TO

INDIUM OXIDE NANOCUBES


Yang, J.; Cheng, H.F.; Martens, W.N.; Frost, R.L. "Application of infrared emission

spectroscopy to the thermal transition of indium hydroxide to indium oxide

nanocubes" Applied Spectroscopy 2011, 65 (1). In press.


68









Application of infrared emission spectroscopy to the thermal transition of indium hydroxide to indium oxide nanocubes

Yang, J.; Cheng, H.F.; Martens, W.N.; Frost, R.L. Applied Spectroscopy 2011, 65 (1). In press.


Jing (Jeanne) Yang Developed experimental design, conducted experiments and data analysis, and wrote the manuscript.

Hongfei Cheng Aided in data presentation and manuscript revising.





____________________ _______________________ ____________________

Name Signature Date


69

RESEARCH HIGHLIGHTS

The article reports the application of infrared emission spectroscopy (IES) in the

study on phase change of In(OH)3 nanocubes during a thermal treatment from room

temperature to 600 ºC. A typical infrared spectrum at room temperature of In(OH)3

nanocubes was characterised by an intense OH deformation band at 1150 cm-1

and

two OH stretching bands at 3107 and 3221 cm-1

. These bands diminished

dramatically upon heating and no intensity remained after 200 ºC, which indicated

the completion of dehydroxylation. IES also detected that new InOH bonds formed

during calcination of nanomaterials because of the release and transfer of protons. In

this study, IES spectra were well related to the structure of the materials and this

technique has proven to be a very useful tool to investigate changes in the material

structure upon thermal treatment.


70

APPLICATION OF INFRARED EMISSION

SPECTROSCOPY TO THE THERMAL

TRANSITION OF INDIUM HYDROXIDE TO

INDIUM OXIDE NANOCUBES

Jing (Jeanne) Yang, Hongfei Cheng, Wayde N. Martens and Ray L.

Frost*

Chemistry Discipline, Faculty of Science and Technology, Queensland University of

Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. E-mail:

[email protected]

Received: July 29, 2010

Accepted: September 27, 2010

ABSTRACT

Cubic indium hydroxide nanomaterials were obtained by a low-temperature

soft-chemical method without any surfactants. The transition of nano-cubic indium

hydroxide to cubic indium oxide during dehydroxylation has been studied by infrared

emission spectroscopy. The spectra are related to the structure of the materials and

the changes in the structure upon thermal treatment. The infrared absorption

spectrum of In(OH)3 is characterised by an intense OH deformation band at 1150

cm-1

and two O–H stretching bands at 3107 and 3221 cm-1

. In the infrared emission

spectra, the hydroxyl-stretching and hydroxyl-bending bands diminish dramatically

upon heating, and no intensity remains after 200 ºC. However, new low intensity

bands are found in the OH deformation region at 915 cm-1

and in the OH stretching

region at 3437 cm-1

. These bands are attributed to the vibrations of newly formed

InOH bonds because of the release and transfer of protons during calcination of the

nanomaterial. The use of infrared emission spectroscopy enables the low-temperature

phase transition brought about through dehydration of In(OH)3 nanocubes to be

studied.


71

KEYWORDS

Infrared Absorption Spectroscopy; Infrared Emission Spectroscopy; Nanomaterials;

Indium Hydroxide; Indium Oxide

1. INTRODUCTION

Special attention has been focused on indium hydroxides and oxides as a series of

important semiconductor materials. As a wide-gap semiconductor (Eg = 5.15 eV),1

In(OH)3 has potential applications in the photocatalytic, electronic, and solar energy

fields.2-4

Also, In2O3 is known as an n-type semiconductor with a direct band gap of

3.6 eV (which is close to that of GaN5) and has been used widely as solar cells,

transparent conductors, and sensors.6-10

By controlling the synthesis techniques

and/or conditions, In(OH)3 and In2O3 can be produced with various morphologies

(e.g., nanowires,11

nanotubes,12

and nanospheres,13

etc.). We have successfully

prepared indium hydroxide nanocubes without any organic surfactants through a

low-temperature hydrothermal process.14

The average size of In(OH)3 nanocubes was

350 nm, and a topotactical relationship was observed between the synthesised

In(OH)3 and In2O3. Thermal decomposition of In(OH)3 is one of the synthetic routes

of In2O3. Studies are important on the thermal transformation of In(OH)3 to In2O3 for

further application of these nanomaterials to industry.

Infrared emission spectroscopy (IES) has proven to be a very useful tool that can be

applied in situ. IES determines vibrational wavenumbers emitted by thermally

excited molecules at elevated temperatures.15

Frost and Kloprogge have developed

extensive work on minerals at the molecular level using infrared emission

spectroscopy.16-20

They applied infrared emission spectroscopy in studies on the

dehydration, dehydroxylation, decarbonization behavior, and other changes of

minerals, especially clay minerals.21-24

However, limited work has been published on

inorganic nanomaterial using this in situ thermal analysis technique. Interpretation of

the changes in the obtained infrared emission spectra with temperature increase can

yield information concerning the thermal reaction and thermal stability of materials.

Our interest in nanomaterials for industrial applications led our motivation for this

research.


72

In this paper, we report changes in the molecular structure as a function of

temperature of the synthetic indium hydroxide nanocubes as determined using

infrared emission spectroscopy.

2. EXPERIMENTAL

2.1 Synthesis of cubic Indium hydroxide nanomaterial

Analytical grade In(NO3)3·5H2O and ammonia solution (28 wt%) were used as

precursors to prepare the indium hydroxide precipitate. Three grams of

In(NO3)3·5H2O was dissolved in 15 mL ultrapure water, and 28% ammonia was

diluted into 10% solution. At room temperature, the diluted ammonia solution (10%)

was added at a rate of 1 mL min-1

into the indium ion solution with vigorous stirring.

Ammonia solution addition ceased when the pH of the reaction mixture reached 8.

The reaction mixture was kept stirring constantly in the air at room temperature for

0.5 h. The obtained mixture was centrifuged and washed at 13,000 rpm for 10 mins,

3 times. The resultant precipitate was transferred into a vessel (125 mL) together

with 2 mL ultrapure water. The vessel was then sealed and placed in a 180 ºC oven.

After a 2-day hydrothermal treatment, the ultrapure resultant product was washed

and collected by centrifugation (at 13,000 rpm for 10 min, repeated 3 times). Sample

was dried at 65 ºC overnight.

2.2 X-ray diffraction analyses



from a line-focused PW3373/10 Cu X-ray tube, operating at 40 kV and 40 mA, with

Cu Kα radiation of 1.540596 Å. The incident beam passed through a 0.04 rad soller

slit, a 1/2º divergence slit, a 15 mm fixed mask, and a 1° fixed anti-scatter slit.

2.3 Infrared spectra

Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a

smart endurance single-reflection diamond attenuated total reflection (ATR) cell

(Thermo Scientific). Spectra over the 4000-525 cm-1

range were obtained by the

co-addition of 64 scans with a resolution of 4 cm-1

.


73

2.4 Infrared emission spectroscopy

Infrared emission spectroscopy (IES) was carried out on a Nicolet Nexus 870 FTIR

spectrometer equipped with a MCT detector, which was modified by replacing the IR

source with an emission cell. A description of the cell and the principles of the

emission experiment have been published elsewhere.25-28

Approximately 0.2 mg of

synthesised indium hydroxide was spread as a thin layer (approximately 0.2 μm) on a

6 mm diameter platinum surface and held in an inert atmosphere within a

nitrogen-purged cell during heating. The emission spectra were collected at intervals

of 50 ºC over the range 100-600 ºC. The spectra were acquired by co-addition of 64

scans for the whole temperature range, with a nominal resolution of 4 cm-1

. Three

sets of spectra were obtained: (1) the blackbody radiation at selected temperatures,

(2) the platinum plate radiation at the same temperature, and (3) the spectra from the

platinum plate covered with the sample. Only one set of blackbody and platinum

radiation is required for each temperature. The emission spectrum at a particular

temperature was calculated by subtraction of the single-beam spectrum of the

platinum backplate from that of the platinum + sample, with the result ratioed to the

single beam spectrum of an approximate blackbody (C-graphite). This spectral

manipulation is carried out after all the spectral data have been collected.

2.5 Spectral manipulation

Spectral manipulation such as baseline adjustment, smoothing, and normalization

was performed using the GRAMS® software package (Galactic Industries

Corporation, Salem, NH). Band component analysis was undertaken using the Jandel

„Peakfit‟ (Erkrath, Germany) software package, which enabled the type of fitting

function to be selected and allows specific parameters to be fixed or varied

accordingly. Band fitting was done using a Lorentz–Gauss cross-product function

with the minimum number of component bands used for the fitting process. The

Lorentz–Gauss ratio was maintained at values greater than 0.7 and fitting was

undertaken until reproducible results were obtained with squared correlations (r2)

greater than 0.999. Band fitting of the spectra is quite reliable providing there is

some band separation or changes in the spectral profile.


74


3.1 Phase identification

X-ray diffraction (XRD) is normally used to determine the phase and structure of the

as-prepared samples. Fig. 1 displays a typical XRD pattern of the synthetic indium

hydroxide nanocubes, which is in excellent agreement with the pattern reported in

the literature (JCPDS Card No. 01-076-1463, cubic In(OH)3 with a lattice constant a

= 7.9743 Å). All the diffraction peaks were well indexed by Miller indices. It is also

reported that the (200) peak is particularly strong, which results from the regular

cubic shape and ordered assembly of as-prepared In(OH)3.10

No other crystalline

impurities, such as InO(OH) or In2O3, were detected in the sample, which indicates

high purity of the synthetic cubic In(OH)3 nanomaterial. The cubic morphology of

the synthetic material was confirmed by scanning electron microscopy (SEM)

published in our previous work.14

Fig.1 XRD pattern of the synthetic indium hydroxide nanomaterials with a reference

pattern: JCPDS card No. 01-076-1463 In(OH)3


75

Fig. 2 Schematic of In(OH)3 in cubic structure (space group Im 3 )

3.2 Infrared spectroscopy of the synthetic cubic In(OH)3

It is important to understand that the structure of cubic In(OH)3 adopts a polyhedral

framework consisting of two distorted pentagonal dodecahedra and twelve 7-hedra

per cell in a body-centered cubic arrangement.29

By X-ray and neutron diffraction

methods, Mullica et al. found that six oxygen atoms were octahedrally coordinated

about each indium atom, and each oxygen atom was coordinated by two indium

atoms, while the hydrogen atoms were disordered.29,30

Indium hydroxide nanocubes

crystallize in space group Im 3 ( 5

hT ). The cubic unit cell of In(OH)3 is shown in Fig.

2. There are eight formula units in a unit cell. This means there are 81 degrees of

freedom. The irreducible representation is given by Γ = 3Au + 4Ag + 3Eu + 4Eg + 8Fg

+ 12Fu. All the Au and Fu modes are infrared active and all the Ag and Eg modes are

Raman active.

The infrared absorption spectrum of the synthetic indium hydroxide nanocubes in the

region of 3600-600 cm-1

is presented in Fig. 3. Two intense bands at 3221 and 3107

cm-1

are observed, which was attributed to OH stretching modes. Bands were

observed in similar positions in our previous work on Raman spectra of In(OH)3,14

in

which bands are found at 3215 cm-1

and 3083 cm-1

.


76

These results are in good agreement with the infrared absorption at 3240 cm-1

and

3120 cm-1

for In(OH)3 reported by Cao et al,31

who synthesised In(OH)3 nanocubes

using a amino acid-assisted hydrothermal process. Likewise, Wang et al. 32

found a

OH stretching band for lotus-root-like In(OH)3 at 3200 cm-1

. Bands observed in

region of 2600-1300 cm-1

are attributed to overtones and combination bands.

Two OH deformation bands were reported by Chen et al.33

at 1154 and 1067 cm-1

.

However, curve fitting of the IR spectrum shows 3 bands at 1150 (sharp), 1129

(shoulder) and 1066 cm-1

. The Raman spectrum14

in the hydroxyl-bending region

showed bands at 1155, 1137 (sharp), 1132, 1072 and 1044 cm-1

. Cao et al.31

published bands at 1160, 783 and 498 cm-1

to OH deformation modes. Instead, for

the lower wavenumber region, we assign the infrared absorption bands at 852 and

772 cm-1

found in this work to OInO vibrational modes.

Fig.3 Infrared absorption spectrum (curve-fitted) of the synthetic In(OH)3 nanocubes

in the region of 3600 – 600 cm-1

.

3.3 Infrared emission spectroscopic study

One method of studying the thermal decomposition of indium hydroxide is through

the changes in the structure caused by dehydration of the material.


77

The use of infrared emission spectroscopy enables the molecular structural changes,

especially of the hydroxyl units, to be observed. The infrared emission spectra (IES)

of the synthetic In(OH)3 nanocubes as a function of temperature are shown in Fig.

4a. The infrared emission spectra below 250 °C are similar to the correlated

room-temperature infrared absorption spectrum (Fig. 3). This observation is in

accordance with Kirchoff‟s law of thermal radiation, which states that for a body (or

surface) in thermal equilibrium with its surroundings the absorbed and emitted

energies are equal.15

(a)


78

Fig. 4 (a) Infrared emission spectra of the synthetic In(OH)3 nanocubes in the region

of 4000 – 650 cm-1

and (b)curve-fitted Infrared emission spectra from 100 – 300 °C.

As shown in Fig. 4a, the intensity of bands in the hydroxyl-stretching and

hydroxyl-bending regions decreases dramatically after 200 °C, which indicates the

obvious loss of hydroxyl groups happened between 200 and 250 °C. This finding is

in harmony with the results of the thermo gravimetric analyses,14

which showed the

dehydration of In(OH)3 happened from 200 to 225 °C.

(b)


79

Bands in the hydroxyl-stretching region approach zero intensity by 450 °C upon

dehydroxylation of the indium hydroxide nanomaterial. For a further study on

structural change in the thermal decomposition process, the infrared emission spectra

below 300 °C were curve-fitted and displayed in Fig. 4b.

In the lower wavenumber region as shown in Fig. 5, bands at 1156, 1139 and 1063

cm-1

are attributed to the OH deformation modes. These bands display a shift to

lower wavenumbers with temperature increase. Such a shift indicates a lessening of

the bond strength of the hydroxyl units upon thermal treatment. Moreover, these

bands diminish rapidly in intensity upon heating, and disappeared above 250 °C.

This observation records the loss of hydroxyl units in the structure of indium

hydroxide according to the work of Martens et al.34

It is published that various

hydroxyl-deformation bands represent different energy levels of the

hydroxyl-deformation modes and the intensity of the bands is a population

measurement of the hydroxyl units at any of these energy levels.34

Furthermore, a

weak band is observed at 915 cm-1

with a shoulder at 972 cm-1

in the spectrum at 250

°C, which are assumed to be OH deformation modes without hydrogen-bonds.

Associated with the hydroxyl-bending vibrations are the hydroxyl-stretching bands,

as shown in Fig. 6. Before 250 °C, the infrared emission spectra show complex

hydroxyl-stretching vibrations, which present various energy levels for the OH

stretching modes in the molecular. It still can be observed that these

hydroxyl-stretching bands shift to higher wavenumbers with the temperature

increase. This shift is caused by the destruction of hydrogen bonds with the loss of

hydroxyl groups. A new weak band is found at 3437 cm-1

at 250 °C, which is

assigned to be OH stretching modes without hydrogen bonding.


80


.


81


.

In this infrared emission spectroscopic study, small bands which may be assigned to

OH groups is observed both in the hydroxyl stretching and deformation regions from

250 to 400 °C. One possible explanation for this observation is the formation of new

transitional hydroxyl groups upon calcination of indium hydroxide nanomaterial.

This shows the correlation with the previous thermo gravimetric analyses, which

indicated that after the major mass loss between 200 and 225 °C, there was a very

slow mass loss observed with the temperature increase.


82

It is assumed that during the later stage of dehydroxylation, calcination of indium

hydroxide results in the release of protons which are able to diffuse through the

molecular structure which interact with InOIn groups forming a new In(OH) bond.

These protonated In(OH)In linkages can be characterised by infrared emission

spectroscopy which show a stretching band at 3437 cm-1

and a bending band at 915

cm-1

. This phenomenon has been observed previously in the calcination of clay

minerals using infrared emission spectroscopy.21, 35

In this experiment the advantage

of infrared emission spectroscopy as a sensitive tool for detecting the OH

environments in minerals and nanomaterials is easily seen.

3.4 Indium oxide converted from indium hydroxide

For a better understanding to the phase transition, the resultant In(OH)3 sample was

calcined in furnace at 300 ºC and 500 ºC separately for 4 hours. The thermal products

were characterised by X-ray diffraction, and the results shown in Fig. 7. The Only

crystalline phase was found in these thermal products to be cubic In2O3 (JCPDS No.

01-071-2195, cubic In2O3 with a lattice constant a =10.1170 Å). This suggests that

the nucleation and growth of In2O3 crystals occurred before 300 ºC. The product

from 500 ºC calcination shows a better crystallinity than the 300 ºC heated product,

indicating that the increase of temperature improves the crystallinity of In2O3

nanocubes.

Fig. 7 XRD patterns of thermal products at 300 and 500 °C from In(OH)3 nanocubes

with a reference pattern: JCPDS card No. 01-071-2195 In2O3.


83

According to the crystallography data, the In2O3 material is body centred cubic and

its crystal structure belongs to the space group Ia3 , with the point group Th. The

structure contains two types of cations: 8 In3+

cations with site symmetry S6.

Secondly there are 24 In3+

cations with point symmetry C2. The 48 oxygens in the

body centred cubic structure have site symmetry C1. Thus the irreducible

representation for the vibrational spectroscopy of In2O3 is given by: 4Ag + 4Eg + 14

Fg + 5Au + 5Eu + 16Fu. The modes with symmetry Ag, Eg and Fg are Raman

active/infrared inactive. The vibrational modes Fu are infrared active/Raman inactive.

The modes Au + Eu are both Raman and infrared inactive. However, due to the

detecting limit of our instrument, no infrared bands of significant intensity for In2O3

are reported in this study.


84

4. CONCLUSIONS

Infrared absorption spectroscopy and infrared emission spectroscopy were applied to

follow the changes in the structure during the dehydration and dehydroxylation of the

material. The infrared emission spectra of the synthetic indium hydroxide nanocubes

at low temperatures are similar to the correlated room-temperature infrared

absorption spectrum. In the infrared emission spectrum at 100 °C, OInO vibrational

modes are observed at 771 and 853 cm-1

. Bands at 1063, 1139 and 1156 (sharp) cm-1

are assigned to be In(OH) bending modes, and the OH stretching modes are observed

at 3123 and 3247 cm-1

.

The intensity of these bands nearly reaches zero above 200 °C, which indicates that

almost all the OH units are lost in the structure. However, new weak bands were

found at 915 and 3437 cm-1

in infrared emission spectra from 250 to 450 °C, which

are assumed to due to OH bending and stretching modes separately. This observation

shows the releasing and transfer of protons and formation of transition new OH

bonds upon calcination. The results presented in this paper clearly show the strength

of infrared emission spectroscopy to study in situ the transformation taking place

during the thermal treatment of indium hydroxide nanocubes. Cubic In2O3 has been

confirmed by XRD to be the thermal product from indium hydroxide nanocubes after

heated at 300 °C. The crystallinity of the thermal product is improved by increase the

calcination temperature to 500 °C.

ACKNOWLEDGEMENTS


Technology Chemistry Discipline is gratefully acknowledged. The Australian

Research Council (ARC) is thanked for funding the instrumentation. One of the

authors (JY) is grateful to the Queensland University of Technology Chemistry

Discipline for the award of an international doctoral scholarship.


85

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88

Chapter 5 Cr doped AlO(OH) nanofibres

89

CHAPTER 5

SYNTHESIS AND

CHARACTERISATION OF

CHROMIUM DOPED BOEHMITE

NANOFIBRES


Yang, J.; Frost, R. L.; Yuan, Y., "Synthesis and characterization of chromium doped

boehmite nanofibres." Thermochimica Acta 2009, 483 (1-2), 29-35.


90

SYNOPSIS

To achieve multi-functional boehmite nanomaterials, the technique doping was

applied. In this chapter, chromium doped boehmite nanofibres were fabricated

through hydrothermal treatment based on the synthesis method reported in Chapter 3.

The effect of the adding dopant amount and hydrothermal treatment period to the

material properties was investigated. Thermal stability of these doped and undoped

boehmite nanofibres was studied by thermogravimetric analysis and infrared

emission spectroscopy. It is found that doping of chromium resulted in an increase of

the main dehydroxylation temperature of boehmite nanofibres from 406.5 °C to

436.5 °C.


91









Synthesis and Characterization of Chromium Doped Boehmite Nanofibres

Yang, J.; Frost, R. L.; Yuan, Y., Thermochimica Acta 2009, 483 (1-2), 29-35.



Yong Yuan Conducted TEM and data analysis, manuscript editing.




____________________ _____________________ ____________________

Name Signature Date


92

RESEARCH HIGHLIGHTS

The article is the first published paper on chromium doped boehmite nanofibres

using a hydrothermal method without any assistance of surfactants. The synthesised

Cr-doped boehmite nanofibres were uniform in shape and size, up to 500 nm long

and 6 nm wide. It is also found that doping with chromium resulted in an increase in

the dehydroxylation temperature of boehmite from 406.5 ºC to 436.5 ºC. Besides, as

the dehydroxylation temperature of the Cr-doped boehmite nanofibres increased, the

mass loss in the dehydroxylation step decreased.


93

SYNTHESIS AND CHARACTERIZATION OF

CHROMIUM DOPED BOEHMITE NANOFIBRES

Jing (Jeanne) Yang, Yong Yuan and Ray L. Frost*




Received: September 8, 2008

Revised Manuscript Received: October 27, 2008

Accepted: October 28, 2008

Published Online: November 6, 2008

ABSTRACT

Chromium doped boehmite nanofibres have been synthesised and characterised by

X-ray diffraction, transmission electron microscopy and thermal analytical

techniques. Hydrothermal treatment of doped boehmite with chromium resulted in

the formation of nanofibres over a wide dopant range. Nanofibres up to 500 nm

and between 4-6 nm wide were synthesised. Doping with chromium resulted in an

increase in the dehydroxylation temperature of boehmite from around 406.5 °C to

436.5 °C. The temperature of dehydroxylation increases with time of hydrothermal

treatment. As the dehydroxylation temperature increases the mass loss from the

dehydroxylation step decreases. The dehydroxylation temperature increases

significantly from 0% to 5% doping, after which the dehydroxylation temperature

shows a small steady increase up to the 20% doping level.

KEYWORDS

Boehmite; Chromium Doping; Nanofibres; Nanomaterial; Transmission Electron

Microscopy; Thermo-gravimetric Analysis


94

1. INTRODUCTION

Compared to their micro and macro counterparts, nanosized materials have been

received wider attention because of their intrinsic properties, which are determined

by their composition, size, shape, and structure. [1] Such as nanofibres, nanotubes,

nanoribbons and nanorods, one dimensional (1D) nanoscale inorganic materials have

attracted intensive interest due to their distinctive geometries, novel physical and

chemical properties, and the potential applications in many fields. [2] Boehmite (γ

-AlO(OH)) and its oxide derivatives such as α-Al2O3 and γ -Al2O3 have been studied

extensively because they can be used as catalysts, adsorbents, flame retardants and

optical materials. [3-6]

Synthesis forms an essential component of nanoscience and nanotechnology. While

nanomaterials have been generated by physical methods such as laser ablation,

arc-discharge and evaporation, chemical methods have proved to be more effective, as

they provide better control as well as enable different sizes, shapes and

functionalization. Among these methods, hydrolysis and precipitation are the most

common. John Bugosh first synthesised the boehmite nanofibres by a hydrothermal

method in 1961. [7] Since then, numerous studies on boehmite nanofibres have been

undertaken, for example, boehmite (AlO(OH)) nanofibres were reported to be

assembled with the assistance of poly(ethylene oxide) (PEO) surfactant [8] and

tubular γ -Al2O3 was fabricated via soft solution route using

N-cetyl-N,N,N-trimethylammonium bromide surfactant. [9] Shen et al. [10] reported

that a steam-assisted solid-phase conversion of amorphous aluminium hydroxides wet

gel to well crystallized 1D nanostructure of boehmite AlO(OH) nanorods without

using surfactants and solvents. The process is unique in the simplicity of preparation

and the high efficiency of crystal growth, which can be operated at a large scale.

As for doping clays, the addition of other metal ions into boehmite, especially into

nanostructured boehmite would have great potential to contribute the further

application of these inorganic nanomaterials due to the enhancement of its properties,

and there have been reports on boehmite doped by Fe, Ga, and Eu. [11-13] It is also

reported that materials doped with Chromium could obtain special electric, magnetic

or optical properties and gain more application. [14-17] This paper would present our

work on Cr-doped boehmite, which has not been seen before.


95

Thermal analysis has been proved as the most useful method for the analysis of

minerals and related materials. [18-27] In this work, boehmite nanofibres based on

Shen‟s methodology were synthesised by introducing Chromium as dopant and a

series of Chromium doped boehmite nanofibres with different iron content

percentage and varying hydrothermal treatment time have been systematically

studied with the thermo gravimetric techniques.

2. EXPERIMENTAL

2.1 Synthesis of chromium doped boehmite nanofibres

A total amount of 0.02 mol aluminium nitrate and chromium nitrate were mixed

before being dissolved in ultra-pure water. To make a comparison, mixtures with

chromium molar percentage of 0, 1, 3, 5, 10 and 20% were prepared separately and

then dissolved in ultra-pure water to form solutions with a metal ion to H2O molar

ratio of 1:35. At room temperature, 10% ammonia solution was added dropwise into

the metal ions solution while stirring vigorously. Stop adding ammonia when the pH

value of the reaction mixture reaches 5, and then keep stirring in the air at room

temperature for 1 hour. The obtained gel was filtrated to obtain the gel-cake, which

was then transferred into a glass beaker (25 mL). Before putting the beaker with

gel-cake into a Teflon vessel (200 mL), 2 mL ultrapure water was poured to the

bottom of each vessel separately. Sealed the Teflon vessels and heated them at 170

ºC for 1, 3, 5 and10 days. The resulting materials were washed with ultrapure water by

centrifuging, and dried at 35 ºC for 2 days.


XRD analyses were performed on a PANalytical X‟Pert PRO X-ray diffractometer

(radius: 240.0 mm). Incident X-ray radiation was produced from a line focused

PW3373/10 Cu X-ray tube, operating at 40 kV, 40 mA, wavelength of 1.540596 Å.

2.3 TEM analysis

A Philips CM 200 transmission electron microscopy (TEM) at 200 kV was used to

investigate the morphology of the boehmite nanofibres. All samples were dispersed

in absolute ethanol solution and then dropped on copper grids.


96


Thermal decomposition of the Cr-doped boehmite was carried out in a TA®

Instruments incorporated high-resolution thermo gravimetric analyser (series Q500)

in a flowing nitrogen atmosphere (60 cm3 min

–1). Approximately 20 mg of each

sample underwent thermogravimetric analyses, with a heating rate of 5°C/min, with

resolution of 6 from 25 to 1000°C. With the isothermal, isobaric heating program of

the instrument the furnace temperature was regulated precisely to provide a uniform

rate of decomposition in the main decomposition stage.



X-ray diffraction is normally used to determine the phase and purity of the

synthesised boehmite. Fig. 1a and 1b display well-defined XRD patterns observed

and all diffraction peaks were perfectly indexed to the XRD pattern of pure boehmite

(JCPDS card 01-083-2384). No XRD peaks representing other crystalline phases

were detected, indicating that the chromium doped-nanofibres of the synthetic

boehmite exhibited excellent crystallinity and a high purity. Fig. 1a shows that the

peaks are higher and narrower with the increase of the hydrothermal treatment time

to 10 days, which means the crystals are growing better as synthesis time getting

longer.

3.2 Transmission electron microscopy

The transmission electron microscopy images of the synthesised Cr doped boehmite

are shown in Fig 2a, b, c. The figures show the TEM images of un-doped boehmite

(a), 3% doped (b) and 5% doped (c).

The figures clearly show that the boehmite is fibrous with very long narrow fibres

often exceeding 500 nm in length and with widths of between 2 and 6 nm. Many of

the fibres are curved or bent as may be observed in Fig. 2c for the 5% doped

boehmite.


97

Fig. 1 (a) XRD patterns of undoped boehmite and 1% Cr-doped boehmite nanofibres

with different hydrothermal treatment time at 170 ºC. (b) XRD patterns of undoped

boehmite and various Cr % doped boehmite nanofibres, after hydrothermal

treatment at 170 ºC for 3 days.

Fig. 2 TEM images of the synthetic nanofibres with 3-day hydrothermal treatment:

(a) undoped boehmite, (b) 3% Cr-doped and (c)5% Cr-doped.

(a)

un

do

pe

d

bo

eh

mit

e

(b)

(c) (b) (a)


98

3.3 Thermo gravimetric analysis

The thermo gravimetric analysis and the differential thermo gravimetric analyses of

boehmite and Cr doped boehmite and their nanostructures with varying amounts of

Cr from 0 to 20% are shown in Figs 3a–3e. The results of the thermal analyses are

reported in Table 1.

Table 1 Results of the thermogravimetric analyses of the undoped and various % Cr

doped boehmite nanofibres.

The TG of the pure boehmite shows a strongly asymmetric curve with a peak

temperature of 406.5 °C and a mass loss of 15.8%. The thermal decomposition

occurs as follows: 2AlO(OH) → Al2O3 + H2O. This major decomposition step is

attributed to the dehydroxylation of the boehmite. Two low mass loss steps at 45 and

260 °C with mass losses of 1.5 and 1.7% are also observed. The first mass loss step

is assigned to the dehydration of boehmite (Column 1 in Table 1).

The thermal decomposition of 1% doped boehmite with 1 day hydrothermal

treatment shows three mass loss steps at 46, 311 and 403 °C with mass losses of 1.7,

6.6 and 11.0% (Fig. 3b). The asymmetry observed in Fig. 3a is no longer observed

but a second peak at 311 °C is found.


99

(b)

(a)


100

(c)

(d)


101

Fig. 3 Thermogravimetric analyses patterns of (a) undoped boehmite nanofibres and

1% Cr doped boehmite with different hydrothermal treatment time: (b) 1 day, (c) 3

days, (d) 5 days, and (e) 10 days.

The thermal decomposition of 1% doped boehmite with 3 day hydrothermal

treatment shows three mass loss steps at 50, 321 and 419.5 °C with mass losses of

1.0, 4.8 and 11.5% (Fig. 3c). The thermal decomposition of 1% doped boehmite with

5 day hydrothermal treatment shows three mass loss steps at 46, 311 and 403 °C with

mass losses of 0.8, 3.9 and 12.1% (Fig. 3d). With the 10 day hydrothermal

treatment, mass loss decomposition steps are observed at 54, 342 and 435 °C. The

mass losses at these temperatures are 0.7, 3.1 and 12.2%. The results of the 1% Cr

doped boehmite hydrothermally treated for 10 days (Fig. 3e) shows a large mass loss

step at 435 °C with a mass loss of 12.2%.

In addition two smaller mass loss steps at 54 and 342 °C with mass losses of 0.7 and

3.1% are observed. It is apparent that thermal decomposition temperature of the Cr

doped boehmite varies with the hydrothermal treatment time. This variation is

reported in Fig. 4.

(e)


102

Fig. 4 Dehydroxylation temperature of the dTG peak as a function of added Cr

content and with the hydrothermal treatment time.

The variation in the % Cr doping on the thermo gravimetric analyses patterns and

decomposition of boehmite is explored in Fig 5a to 5d. The thermo gravimetric

analyses patterns of 3% Cr doped boehmite hydrothermally treatment for 3 days

shows three mass loss steps at 46, 315 and 427 °C with mass losses of 1.1, 2.9, and

12.4%. The dehydroxylation peak at 427 °C is very narrow indication the

dehydroxylation occurs over a very narrow temperature range. For the 5% Cr doped

boehmite the dehydroxylation step is observed at 430.5 °C with a mass loss of

13.1%. For the 10% Cr doped boehmite hydrothermally treated for 3 days results in

a sharp mass loss peak at 433 °C with minor mass loss steps at 50 and 380 °C. The

temperature for the Cr 20% doped boehmite is 436.5 °C. The variation in the

temperature of the decomposition of boehmite as a function of % doping is reported

in Fig. 4. It is apparent that as the 5 of Cr is increased in the boehmite the

dehydroxylation temperature is increased and shifts from 406.5 °C to 436.5 °C.


103

(b)

(a)


104

Fig. 5 Thermo gravimetric analyses patterns of various % Cr-doped boehmite

nanofibres with 3-day hydrothermal treatment:(a) 3% , (b) 5%, (c)10%, and (d)

20%.

(c)

(d)


105


function of added Cr content.


function of the hydrothermal treatment time.

The variation of the dehydroxylation temperature and associated mass loss with the

% of Cr doping is shown in Fig. 6. As the dehydroxylation temperature increases

the mass loss from the dehydroxylation step decreases. The dehydroxylation

temperature increases significantly from 0% to 5% doping, after which the

dehydroxylation temperature shows a small steady increase up to the 20% doping

level. The associated mass loss decreases and then shows a constant mass loss.

The variation of mass loss and dehydroxylation temperature with hydrothermal

treatment time is illustrated in Fig. 7. The temperature of dehydroxylation increases

with time of hydrothermal treatment.


106

4. CONCLUSIONS

Boehmite and chromium doped boehmite were synthesised by low temperature

precipitation from aqueous solution and hydrothermally treated for differing time

intervals. Very long nanofibres were produced often exceeding 500 nm in length.

Normally at above the 5% doping level a mixture of nanofibres and nano-sheets are

produced.

ACKNOWLEDGEMENTS


Technology Inorganic Materials Research Program of the School of Physical and

Chemical Sciences is gratefully acknowledged. The Australian Research Council

(ARC) is thanked for funding the instrumentation.


107

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iowaite. Journal of Thermal Analysis and Calorimetry, 2006. 86(2): p.

437-441.

20. Frost, R.L., et al., Thermal decomposition of sabugalite. Journal of Thermal

Analysis and Calorimetry, 2006. 83(3): p. 675-679.

21. Frost, R.L., et al., Thermal decomposition of metatorbernite - A controlled

rate thermal analysis study. Journal of Thermal Analysis and Calorimetry,

2005. 79(3): p. 721-725.


109

22. Frost, R.L., et al., Thermal decomposition of hydrotalcite with

hexacyanoferrate(II) and hexacyanoferrate(III) anions in the interlayer.

Journal of Thermal Analysis and Calorimetry, 2006. 86(1): p. 205-209.

23. Frost, R.L., M.L. Weier, and W. Martens, Thermal decomposition of

jarosites of potassium, sodium and lead. Journal of Thermal Analysis and

Calorimetry, 2005. 82(1): p. 115-118.

24. Frost, R.L., M.L. Weier, and W. Martens, Thermal decomposition of

liebigite: A high resolution thermogravimetric and hot-stage Raman

spectroscopic study. Journal of Thermal Analysis and Calorimetry, 2005.

82(2): p. 373-381.

25. Frost, R.L., et al., Thermal decomposition of ammonium jarosite

(NH4)Fe3(SO4)2(OH)6. Journal of Thermal Analysis and Calorimetry, 2006.

84(2): p. 489-496.

26. Frost, R.L., et al., Thermal decomposition of hydronium jarosite

(H3O)Fe3(SO4)2(OH)6. Journal of Thermal Analysis and Calorimetry,

2006. 83(1): p. 213-218.

27. Zhao, Y., et al., XRD, TEM and thermal analysis of Fe doped boehmite

nanofibres and nanosheets. Journal of Thermal Analysis and Calorimetry,

2007. 90(3): p. 755-760.

110

Chapter 6 Synthesis and characterisation of CrO(OH) nanoparticles

111

CHAPTER 6

SYNTHESIS AND

CHARACTERISATION OF

CHROMIUM OXYHYDROXIDE

CrO(OH) NANOPARTICLES


112

SYNOPSIS

Chromium doped aluminium oxyhydroxide nanofibres were successfully achieved as

mentioned in last chapter (Chapter 5). It would be interesting to take a further

investigation on the hydrothermal synthesis of pure chromium oxyhydroxide

nanomaterials. In this chapter, a similar hydrothermal synthesis procedure was

applied to fabricate chromium oxyhydroxide. This chapter consists of three published

articles.

Chromium oxyhydroxide has important application in catalyst industry; however, its

nanomaterials receive limited reports so far. In Chapter 6.1, it is reported that

well-crystalline chromium oxide hydroxide nano-plates with an average size of 11

nm were successfully synthesised. The synthetic CrO(OH) nanoparticles are uniform

in morphology and size. Detailed discussion is presented, particularly on the

controllable synthesis, as well as material structure, surface properties and their

thermal stabilities of the synthetic CrO(OH) nanomaterials. The aim of this research

is to investigate the influence of experimental procedures on the synthesis and

properties of chromium oxyhydroxide nanomaterials with controlled size and

morphology.

Raman spectroscopy allows the possibility of studying the structure at the molecular

level. The combination of Raman spectroscopy with a hot-stage lends itself as the

technique of choice for studying the chemical reactions during dehydration and

dehydroxylation. In Chapter 6.2 and 6.3, detailed hot-stage Raman spectroscopic

studies were presented on the thermal transition process of chromium oxide gel and

chromium oxyhydroxide nanomaterials to crystalline chromium oxide materials.


113

CHAPTER 6.1

SIZE-CONTROLLABLE

SYNTHESIS OF CHROMIUM

OXYHYDROXIDE

NANOMATERIALS USING A

SOFT CHEMICAL

HYDROTHERMAL ROUTE


Yang, J.; Baker, A. G.; Liu, H.; Martens, W. N.; Frost, R. L., " Size-controllable

synthesis of chromium oxyhydroxide nanomaterials using a soft chemical

hydrothermal route." Journal of Materials Science 2010, 45 (24), 6574-6585.


114









Size-controllable Synthesis of Chromium Oxyhydroxide Nanomaterials Using A Soft Chemical Hydrothermal Route

Yang, J.; Baker, A. G.; Liu, H.; Martens, W. N.; Frost, R. L., Journal of Materials Science 2010, 45 (24), 6574-6585.



Adrian G. Baker Helped with experimental work and data presentation.

Hongwei Liu Conducted TEM and data analysis, manuscript editing.





____________________ ______________________ ____________________ Name Signature Date


115

RESEARCH HIGHLIGHTS

The article focuses on the synthesis and characterisation of chromium oxyhydroxide

nanomaterials. The best experimental method to control the size of the nanoparticles

was investigated, and CrO(OH) nanomaterials were found to form most easily with

an acidic precipitate process. The structure, morphology, specific surface analysis

were well characterised as a function of the experimental conditions of synthesis. A

detailed thermogravimetric study on samples synthesis in different conditions was

presented, which showed that the dehydroxylation of the dehydroxylation of the

synthetic chromium oxyhydroxides occurred at ~460 °C, while chromium hydroxide

impurities decomposed at ~400 °C.


116

SIZE-CONTROLLABLE SYNTHESIS OF

CHROMIUM OXYHYDROXIDE

NANOMATERIALS USING A SOFT CHEMICAL

HYDROTHERMAL ROUTE

Jing Yang, Adrian G. Baker, Hongwei Liu,

Wayde N. Martens and Ray L. Frost*



[email protected]

Received: March 14, 2010

Accepted: June 28, 2010

Published Online: July 15, 2010

ABSTRACT

Chromium oxyhydroxide nanomaterials were synthesised through a simple soft

chemistry hydrothermal method. The chromium oxyhydroxide materials display

platelet morphology with clear edges, ~11 nm in diameter. CrO(OH) nanomaterials

synthesised under different conditions were fully characterised by X-ray diffraction

(XRD), transmission electron microscopy (TEM) combined with selected area

electron diffraction (SAED), scanning electron microscopy (SEM), BET specific

surface area analysis, X-ray photoelectron spectroscopy (XPS) and thermal

gravimetric analysis (TGA). Bonding of the trivalent chromium from the

oxyhydroxide nanomaterials was defined through the analysis of their high resolution

XPS spectra for Cr 2p3/2 and O 1s. The thermal stability of the nanomaterials

CrO(OH) were established. This research has developed methodology for the

synthesis of chromium oxyhydroxide nanoplates.


117

KEYWORDS

Chromium Oxyhydroxide; Grimaldiite; Nanomaterials; Nano-plates; XRD; SEM;

TEM; BET; XPS; Thermal Analysis

1. INTRODUCTION

Transition metal oxides are important materials in industry as heterogenous catalysis,

which support materials as well as active components. Special attention has been

focused on the formation and properties of chromia (Cr2O3), which is important in

specific applied applications such as in high-temperature resistant materials, [1-3]

solar energy collectors, [4-6] liquid crystal displays, [7,8] and catalysts. [9-12] Rust

formed by corrosion of Cr-containing steels, Cr2O3 and CrO(OH) were also detected

and extensively studied. [13-16] Similar with the relationship of cobalt oxyhydroxide

to oxides, [17] chromium oxyhydroxide is a principal precursor for synthesis of

chromium oxides. The morphology and nanosize of the oxyhydroxide can be

retained through to the oxides through a topotactical relationship. [17-20] Therefore,

studies on controllable synthesis and characterisation for CrO(OH) nanomaterials are

important for the synthesis and application of Cr2O3 nanomaterials in industry.

It is reported that there are three currently known synthetic polymorphs of CrO(OH):

trigonal -CrO(OH) with a space group of R3m, orthorhombic -CrO(OH) with a

space group of Pnnm, and -CrO(OH) with a space group of Cmcm.[21] Approved in

1977 by the Commission on New Minerals and Mineral Names of the International

Mineralogical Association (IMA-CNMMN), the naturally occurring chromium

oxyhydrates include three species: bracewellite (orthorhombic CrO(OH)), guyanite

(orthorhombic β-CrO(OH)), and grimaldiite (trigonal CrO(OH)). [21] So far, limited

work has been devoted on the synthesis and industrial application of CrO(OH) and

Cr2O3 nanomaterials.

By reducing potassium chromate with hydrogen and then calcining the obtained

intermediate, which consists of Cr(OH)3·nH2O and CrO(OH), the ultra-fine chromia

(Cr2O3) powder with a mean diameter of 0.3m has been prepared. [22] The

preparation of chromium oxide is of considerable interest since this compound is a

component of many oxide-type catalysts.


118

Cr(III)-Fe(III)-(oxy)hydroxides have been reported by Tang et al. [23] It is found

that the Cr end member sizing around 1 nm is likely to be amorphous with aspects of

local structure similar to -CrO(OH). Landau [24] and Rotter[9] reported catalytic

materials based on chromia aerogel, which consists of 1-2 nm CrO(OH) nanocrystals

with very high surface area. CrO(OH)·2H2O nanocrystals of 3-5 nm in diameter were

prepared by critical CO2 extraction of the urea-assisted wet chromia gel mixture at

373 K in vacuum. [25,26] As reported, metal oxides can be formed through the

thermal dehydration of their corresponding oxyhydroxides, which can preserve the

texture of their precursors. [27,28,20,29] It is reported that hydrated chromium

oxides are mostly used as initial compounds for the synthesis of chromium oxides.

[30] As the important precursor for chromium oxides through thermal treating, the

thermal stability and decomposition process of chromium hydroxide or chromium

oxide hydroxide has been investigated. [30-35] However, not much work has been

done on the thermal study of nanoscaled CrO(OH) materials.

Possible industrial applications of CrO(OH) and Cr2O3 materials depend on their

chemical, physical and micro-/nano-structure properties. Generally the properties of

materials can be modified by their synthesis conditions. Therefore, researchers have

tried to establish the relation between the properties of chromium oxide materials and

synthesis conditions. In this study, a simple soft-chemical method is used and

well-crystalline chromium oxide hydroxide nano-plates with an average size of 11

nm were successfully synthesised. The synthetic CrO(OH) nanoparticles are very

uniform in morphology and size. Detailed discussion is presented in the text,

particularly on the controllable synthesis, as well as material structure, surface

properties and their thermal stabilities of the synthetic CrO(OH) nanomaterials. The

aim of this research is to investigate the influence of experimental procedures on the

synthesis and properties of chromium oxyhydroxide nanomaterials.

2. EXPERIMENTAL

2.1 Material synthesis

Typically, 15 g of Cr(NO3)3•9H2O was dissolved in 75 mL ultra pure water, and 28%

ammonia was diluted to 10% solution. At room temperature 10% ammonia solution

was added at a rate of 1 mL/min into Cr(NO3)3 solution with vigorous stirring.


119

Ammonia solution addition ceased when the pH of the reaction mixture reached the

desired values (5.0, 7.5 or 10.0). The reaction mixture was then kept stirring

constantly in air at room temperature for 0.5 h. The obtained gel was centrifuged and

washed at 13000 rpm for 10 mins, 3 times. The washed wet gel was transferred into a

glass beaker (25 mL). The beaker was placed into a Teflon vessel, 2 mL ultra pure

water was poured into the button of the Teflon vessel. The Teflon vessel was then

sealed and hydrothermally treated at 170 ºC for 12 h to 4 days. Ultra pure water was

added to the resultant product and then collected by centrifugation (at 13000 rpm for

10 mins). The washing process was repeated 3 times. Samples were dried at 65 ºC

overnight. Samples ID used in this paper and their preparation conditions are shown

in Table 1.

2.2 X-ray diffraction (XRD)

X-ray diffraction patterns were collected using a PANalytical X‟Pert PRO X-ray

diffractometer (radius: 240.0 mm). Incident X-ray radiation was produced from a line

focused PW3373/10 Cu X-ray tube, operating at 40 kV and 40 mA, with Cu K

radiation of 1.540596 Å. The incident beam passed through a 0.04 rad soller slit, a

1/2 ° divergence slit, a 15 mm fixed mask, and a 1 ° fixed antiscatter slit.

Table 1 Samples ID used in this paper and their preparation conditions.

Sample ID pH for wet gel

precipitate HT temp. HT duration

TGA

products

ppt-5.0 5.0 - - -

ppt-7.5 7.5 - - -

ppt-10.0 10.0 - - -

CrO(OH)-5.0 5.0 170 °C 12 h -

gel-7.5-a 7.5 170 °C 12 h -

gel-10.0-a 10.0 170 °C 12 h -

CrO(OH)-7.5 7.5 170 °C 4 days -

CrO(OH)-10.0 10.0 170 °C 4 days -

ppt-5.0-TGA 5.0 - - Yes

CrO(OH)-5.0-TGA 5.0 170 °C 12 h Yes

CrO(OH)-7.5-TGA 7.5 170 °C 4 days Yes

CrO(OH)-10.0-TGA 10.0 170 °C 4 days Yes


120

2.3 Scanning electron microscopy (SEM)

The specimens were mounted on SEM mounts with carbon tape and coated with a

thin layer of evaporated gold. The secondary electron images were obtained using a

scanning electron microscope (FEI Quanta 200 SEM, FEI Company, Hillsboro,

Oregon, USA), operating at 30 kV.

2.4 Transmission electron microscopy (TEM)

A Philips CM20 transmission electron microscope (TEM) at 160 kV was used to

investigate the morphology of the as-prepared samples. All samples were

ultrasonically dispersed in absolute ethanol solution, and then dropped on copper

grids. Selected area electron diffraction (SAED) was performed via the same TEM.

2.5 Specific surface area analysis

Surface area analyses based upon N2 adsorption/desorption techniques were analysed

on a Micrometrics Tristar 3000 automated gas adsorption analyser. Before the

analysis, samples were pre-treated at 110 °C under the flow of N2 on a Micrometrics

Flowprep 060 degasser.

2.6 X-ray photoelectron spectroscopy (XPS)

Data was acquired using a Kratos Axis ULTRA X-ray Photoelectron Spectrometer

incorporating a 165 mm hemispherical electron energy analyser. The incident

radiation was Monochromatic Al Kα X-rays (1486.6 eV) at 150 W (15 kV, 10 mA)

and at 45 degrees to the sample surface. Photoelectron data was collected at take off

angle of theta = 90 o. Narrow high-resolution scans were run with 0.05 eV steps and

250 ms dwell time. Base pressure in the analysis chamber was 1.0×10-9

torr and

during sample analysis 1.0×10-8

torr. A small amount of each finely-powdered

sample was carefully applied to double sided adhesive tape on a standard Kratos

Axis Ultra sample bar. This was attached to the sample rod of the Load Lock system

for initial evacuation to ~1 × 10-6

torr. The sample bar was then transferred to the

UHV Sample Analysis Chamber (SAC) for collection of X-ray Photoemission

spectra. Spectra were subjected to a Shirley baseline. Various data handling

procedures were carried out using the CasaXPS version 2.3.14 software.


121

2.7 Thermal gravimetric analysis

Thermal decomposition of the samples was carried out in a TA® Instruments

incorporated high-resolution thermo gravimetric analyser (series Q500) in a flowing

nitrogen atmosphere (60 cm3 min

–1). Approximately 50 mg of each sample

underwent thermo gravimetric analyses, with a heating rate of 5 °C/min, with

resolution of 6 from 25 °C to 1000 °C. With the isothermal, isobaric heating program

of the instrument the furnace temperature was regulated precisely to provide a

uniform rate of decomposition in the main decomposition stage. The TGA

instrument was coupled to a Balzers (Pfeiffer) mass spectrometer for gas analysis.

Only water vapour, nitric oxide, carbon dioxide, and oxygen were analysed. In the

MS figures, e.g. Fig. 9, a background of broad peaks may be observed. This

background occurs for all the ion current curves. The background becomes more

prominent as the scale expansion is increased. It is considered that this background

may be due to the loss of chemicals which have deposited in the capillary which

connects the TA instrument to the MS.


3.1 Structure and morphology

The XRD patterns of the colloidal precipitation obtained under different pH values

and of the samples after being hydrothermally treated at 170 °C for 12 h are shown in

Fig. 1. Prior to the hydrothermal treatment, the colloidal chromium hydroxide

materials resulting from reaction of Cr(NO3)3 and NH4OH solutions, noted as

ppt-5.0, ppt-7.5 and ppt-10.0 separately, were amorphous. No XRD peaks were

observed for these precipitated samples. This agrees with literature, which reported

that the hydrogel obtained from the neutralisation of Cr3+

ions is usually amorphous

or poorly crystalline. [36,37] The conversion process from colloidal chromium

precipitate to crystalline CrO(OH) was found to be significantly affected by the

precipitation conditions. After a hydrothermal treatment at 170 °C for 12 h, only the

sample precipitated at pH = 5.0 was transformed to well crystalline phase, noted as

CrO(OH)-5.0; while the other two samples, which precipitated at pH 7.5 and 10.0,

remain amorphous, as shown in Fig. 1.


122

Fig .1 XRD patterns of ppt-5.0, ppt-7.5, ppt-10.0, CrO(OH)-5.0, gel-7.5-a and

gel-10.0-a. XRD pattern from literature: JCPDS card No. 01-070-0621 Grimaldiite.

A well defined XRD pattern for CrO(OH)-5.0 was observed and all diffraction peaks

were perfectly indexed to rhombohedral CrO(OH), which has the mineral name

Grimaldiite, with a space group of R3m (JCPDS card No. 01-070-0621).

The parameters of the rhombohedral unit cell of Grimaldiite are: a = 2.979 Å, c =

13.370 Å. [38] No XRD peaks representing other crystalline phases were detected,

indicating a high purity of the resultant crystalline solid by a hydrothermal

conversion process. Based on XRD results, the average crystallite size of sample

CrO(OH)-5.0 was 11 nm by application of the Scherrer equation. Scherrer equation

is D = Kλ/βcosθ, where D is the crystalline size (nm), K presents as Scherrer, which

has a value of 0.89, λ is the wavelength (nm), β is the observed peak width, and θ is

the diffraction angle.


123

Fig. 2 SEM images of (a) ppt-5.0 and (b) CrO(OH)-5.0.

Fig. 2 compares the SEM images of materials synthesised at pH = 5.0 before and

after the hydrothermal treatment at 170 °C. As shown in Fig. 2a, the colloidal

precipitate appears to be non-crystallized lumpy aggregates. A similar SEM

morphology of natural amorphous -CrO(OH) was reported by Shpachenko et al.

when they studied the genesis and compositional characteristics of natural

-CrO(OH). [21] After 12-hour hydrothermal treatment, the morphology of the

crystallized nanostructures of CrO(OH) was observed to be nano-particles (Fig. 2b).

The average diameter of these spherical nanoparticles was 50 nm.

As discussed above, after a hydrothermal treatment for 12h, only the precipitate

obtained at acidic conditions (pH = 5.0) transformed into crystalline CrO(OH)

material. In comparison, the precipitates obtained at pH = 7.5 and 10.0 were

hydrothermally treated for a longer period of time. The products after 4-day

treatment were characterised and shown to be CrO(OH) by XRD, Fig. 3. Peaks of

both XRD patterns can be well indexed and matched with the peaks of the

grimaldiite standard pattern (JCPDS card No. 01-070-0621). This indicates that a

much longer hydrothermal duration time is required to promote the conversion to

crystalline CrO(OH) from the precipitates obtained in neutral (pH = 7.5) and alkaline

(pH = 10) conditions. However the CrO(OH) product from alkaline precipitation

condition shows a much better crystallinity. On the basis of the broadening of

diffraction lines, it was estimated that the mean crystallite size of sample

CrO(OH)-10.0 was 12 nm.


124

Fig. 3 XRD patterns of CrO(OH)-10.0 and CrO(OH)-7.5. XRD pattern from

literature: JCPDS card No. 01-070-0621 Grimaldiite.

In the XRD pattern of CrO(OH)-7.5, low intensity of XRD reflections were found at

positions exactly corresponding to grimaldiite structure. However, diffraction peaks

were observed with lager full width at half-maximum (FWHM), which indicated that

the crystalline size of CrO(OH)-7.5 was much smaller than that of CrO(OH)-10.0,

even after 4-days hydrothermal treatment.

We note that particle size differs from crystallite size by definition; however, in the

case of very small particles their size is often comparable with the crystallite size, as

estimated by the Scherrer formula. Typical TEM images of the three crystalline

CrO(OH) materials synthesised under different conditions are shown in Fig. 4. The

resulting samples from acidic and alkaline conditions appear as nano-plates. The

average size in diameter for sample CrO(OH)-5.0 is measured to be 10 nm, while for

CrO(OH)-10.0 the average measured size is 12 nm. This slight increase in size is

considered to be due to longer hydrothermal crystal growth duration. It is noted that

size of the crystals observed under transmission electron microscope is in excellent

agreement with the results calculated by Scherrer equation determined by XRD data.

For a further study on the morphology of these CrO(OH) nanocrystals, high

resolution TEM (HRTEM) with a double tilt holder was utilised. Detailed high

resolution imaging micrographs were included in the supporting information.


125

Fig. 4c presents one HRTEM image for CrO(OH)-7.5 and the SAED of the

corresponding area has been inserted. It is revealed that no extra dimensionality of

the nano-platelets can be seen, which meant that the nanoparticles were in a plate

shape, instead of being bulk crystals. The particles size of CrO(OH)-7.5 derived from

this HRTEM image was measured to be 4 nm, which was in excellent agreement

with the results based on the Scherrer equation with presented XRD pattern.

Moreover, area was found showing no interference patterns under the HRTEM

detection, indicating there was an amorphous fraction in the sample. For

CrO(OH)-5.0 the selected area electron diffraction (SAED) pattern of the

corresponding area in Fig. 4a appears in rings, which indicates that the hydrothermal

synthesis yielded polycrystalline nanomaterials of CrO(OH).

Fig. 4 (a) TEM image of CrO(OH)-5.0; (b) SAED result of the corresponding area in

Fig.4 (a); (c) TEM image of CrO(OH)-7.5 and its SAED result (inset); (d) TEM

image of CrO(OH)-10.0 and its SAED result (inset).


126

The nano-plates of synthesised CrO(OH) exhibited excellent crystallinity, which is

confirmed by the results of the XRD investigation. The SAED pattern (Fig. 4b)

matched well with that in the XRD study, and the indexing of the ring pattern

indicated that the nano-plates possessed a crystal structure consistent with the

rhombohedral form of CrO(OH). It is observed that there are individual bright spots

in the rings, which indicate large and well formed crystals in the sample. This

observation is in harmony with the corresponding TEM image.

3.2 Surface analysis

3.2.1 N2 adsorption studies

Fig. 5 N2 adsorption/desorption isotherms for CrO(OH)-5.0, CrO(OH)-7.5 and

CrO(OH)-10.0.


127

The N2 adsorption-desorption measurement at a liquid N2 temperature of 77K was

applied to study the porosity and textural properties of the CrO(OH) nanomaterials

precipitated under different pH conditions. Fig. 5 depicts the N2

adsorption-desorption isotherms of all the samples, all of which exhibit a type IV

sorption behaviour, indicating the presence of mesoporous structural characteristic

according to the classification of IUPAC. [39] Type IV isotherm possesses a

hysteresis loop, the lower branch of which represents measurements obtained by

progressive addition of vapour to the system and the upper branch by progressive

withdrawal. [40] The apparent step in the adsorption branch combined with the sharp

decline in the desorption branch is an obvious verification of the present of

mesoporosity.

Fig. 6 Pore size distribution study for CrO(OH)-5.0, CrO(OH)-7.5 and

CrO(OH)-10.0.


128

As shown in Fig. 5, the adsorption of N2 of CrO(OH)-7.5 is much higher than that of

the other two samples, which indicates that CrO(OH)-7.5 possesses a higher specific

surface area. It is known that the specific surface area, pore volume and pore size

distribution can be calculated and compared from the N2 adsorption isotherms of

materials. [40] A pore size distribution is presented in Fig. 6, which shows that the

distribution of pore size is mostly in the mesopore region (pore size between 2 and

50 nm. The textural properties of CrO(OH)-7.5 synthesised under a neutral condition

are given in Table 2 as follows: a BET specific surface area of 384 m2∙g

-1 with a BJH

mean pore diameter of 3.5 nm (within mesopore region) and desorption cumulative

pore volume of 0.33 cm3∙g

-1.

Table 2 BET specific surface area (SBET), pore volume (Vp), and pore diameter for

synthesised CrO(OH) nanomaterials.

Sample ID SBET (m2 g

-1) VP,

a (cm

3 g

-1)

Mean D (nm)

BETb BJH

c

CrO(OH)-5.0 119.09 0.25 8.5 6.7

CrO(OH)-10.0 114.63 0.26 8.9 6.9

CrO(OH)-7.5 384.14 0.33 3.8 3.5

a BJH desorption cumulative pore volume of pores between 1.7 and 300 nm in diameter.

b

Adsorption average pore diameter (4V/A by BET). c Barrett-Joyner-Halenda (BJH)

desorption average pore diameter (4V/A).

CrO(OH)-5.0 and CrO(OH)-10.0, precipitated respectively under the acidic and

alkaline conditions, present similar patterns of N2 adsorption isotherm and pore size

distribution to those of sample CrO(OH)-7.5. However, their textural properties show

the following changes: CrO(OH)-5.0 has a BET specific surface area of 119 m2∙g

-1

with a BJH mean pore diameter of 6.7 nm (within mesopore region) and desorption

cumulative pore volume of 0.25 cm3∙g

-1, while CrO(OH)-10.0 has a BET specific

surface are of 115 m2∙g

-1 with a BJH mean pore diameter of 6.9 nm (within mesopore

region) and desorption cumulative pore volume of 0.26 cm3∙g

-1. These changes can

be attributed to the difference of the crystalline size for the nanomaterials: the

increase of the crystals/particle size leads to decrease in the specific surface area and


129

cumulative pore volume, but increase in the mean pore diameter. By comparison, the

calculated mean pore size of materials is found to be slightly smaller than the

crystalline size observed from XRD and TEM studies; since it is the inter crystalline

porosity that is studied by this N2 adsorption-desorption measurement.

3.2.2 X-ray photoelectron spectroscopy (XPS)

To further determine the surface chemical composition and the chemical states of the

prepared nanomaterials, X-ray photoelectron spectroscopy (XPS) analysis was

carried out. XPS has been widely applied for the investigation of the top few layers

of material surfaces with partially filled valence band. Spectral results from the

interaction with metal valence electrons can provide chemical environment

information about the metal ions. For the transition metal element chromium, high

resolution Cr 2p spectrum shows spin-orbit splitting into 2p1/2 and 2p3/2 components,

[41,42] and both components qualitatively contain the same chemical information.

Therefore, in this study only the higher intensity Cr 2p3/2 bands were curve-fitted and

discussed.

The high-resolution XPS spectra of Cr 2p3/2 and O1s of the chromium nanomaterials

synthesised under various conditions are compared in Fig. 7, and their curve-fitted

results are summarised in Table 3. All the spectra were referenced to the adventitious

carbon of binding energy (BE) 285.0 eV.

Table 3 Results for curve-fitted binding energies and their atomic contents (at.%) of

highly resolved Cr 2p3/2 and O 1s XPS spectra shown in Fig. 7 for sample ppt-5.0,

CrO(OH)-5.0, CrO(OH)-7.5and CrO(OH)-10.0.

Samples Cr 2p3/2 (eV) O 1s (eV)

Cr1 Cr2 Cr3 O1 O2 O3

ppt-5.0 576.9

(40.8%)

578.1

(44.7%)

579.4

(14.5%)

530.3

(7.0%)

531.7

(45.6%)

533.0

(47.4%)

CrO(OH)-5.0 576.2

(41.0%)

577.4

(48.3%)

578.6

(10.7%)

530.2

(40.4%)

531.4

(40.1%)

532.4

(19.5%)

CrO(OH)-7.5 576.6

(38.5%)

577.8

(48.7%)

579.1

(12.8%)

530.6

(28.7%)

531.7

(42.9%)

532.8

(28.4%)

CrO(OH)-10.0 576.4

(41.7%)

577.6

(48.2%)

579.1

(10.1%)

530.4

(39.7%)

531.6

(39.6%)

532.6

(20.7%)


130

High resolution XPS spectra for Cr 2p are often involved because of the complex

multiplet and spin-orbit splitting; however their decomposition can reveal all the

component peaks. XPS spectra may be fitted in different ways and this may affect

the interpretation of the XPS data. Studies were performed on simple compounds to

obtain reference data for the proper peak assignment. [28]

In this study, as shown in Fig. 7, the precipitate sample ppt-5.0 and the three

hydrothermally treated samples: CrO(OH)-5,0, CrO(OH)-7.5 and CrO(OH)-10.0

exhibit XPS Cr 2p3/2 spectra that are very similar in nature to each other with

virtually identical peak profiles. Three peaks are required to curve fit the Cr 2p3/2

XPS spectra of all the 4 samples, which indicates that there were three chemical

states for chromium.

The first component (Cr 1) at around 576.4 eV used for curve fitting is assigned to

Cr3+

trivalent oxide state, a second one (Cr 2) at around 577.6 eV is associated to the

trivalent hydroxide state, which has been well documented. [43-45] The third fitted

binding energy with atomic content of ~10% is found at round 579.0 eV, which is

slightly higher than the reported binding energy for normal Cr (III) from chromium

hydroxide, and much lower than that for Cr (VI). [46,47] However, it is reported that

the binding energy for chromium could be slightly higher when the element has a

more complex load in the compound. [48,49] Therefore, the third fitted component

(Cr 3) observed in this study is considered to be trivalent chromium combined with

additional ligands, such as OH- and H2O ligands present in the surface of the

synthesised materials.

The observed slight increase in the Cr 2p3/2 binding energies for ppt-5.0 can also be

explained by the more complex load to chromium in this precipitate sample before

hydrothermal treatment. The atomic ratios for three kinds of fitting Cr components

are very close, where Cr 2 has the biggest portion.


131

Fig. 7 XPS high resolution spectra of Cr 2p3/2 and O 1s for ppt-5.0, CrO(OH)-5.0,

CrO(OH)-7.5 and CrO(OH)-10.0

As shown in Fig. 7, the O 1s spectrum for CrO(OH)-5.0 could also be fitted with

three peaks: O1 at 530.2 eV corresponding to O2-

in CrO(OH), O2 at 531.4 eV

corresponding to OH- in oxyhydroxide or hydroxide, and a third peak, O3 at 532.4

eV corresponding to oxygen in water, which are in excellent agreement with the

values reported in literature. [50,47,51]

The qualitative analysis shows that O1 and O2 obtained similar ratios (40.0% and

40.1%) in the sample, which is consistent with their atom ratio of 1:1 in CrO(OH).

This illustrates the main composition in CrO(OH)-5.0 is chromium oxyhydroxide,

with minimal chromium hydroxide.

The O 1s spectrum for CrO(OH)-10.0 shows similar shape and curve fitted results as

CrO(OH)-5.0. The absence of significant binding energy shifts between these two

chromium nanomaterials suggests that the chemical nature of the elements in the

top-most surface layers is very similar, and that CrO(OH)-10.0 mainly contains

CrO(OH) as well.


132

In the O 1s spectra for ppt-5.0 and CrO(OH)-7.5, three peaks can also be fitted and

assigned to O2-

, OH- and H2O, respectively. Their binding energies are slightly

higher than that of CrO(OH)-5.0.

However, a decrease of atomic contents for O1 and an increase of atomic contents

for O2 are observed in CrO(OH)-7.5, which suggests a lower content of CrO(OH) in

the sample. One possible interpretation of this CrO(OH) content variation is that the

amorphous precipitates obtained at neutral condition (pH = 7.5) require longer

hydrothermal treatment to form a better CrO(OH) phase. This finding is consistent

with the XRD and TEM observations, which reflect the fact that CrO(OH)-7.5

contains non-CrO(OH) amorphous phase which is not very well crystalline.

Moreover, in the O 1s spectrum for ppt-5.0, a low atomic content of O1 is observed

(7%), while the atomic percentage for O2 is 45.6%. Therefore, without hydrothermal

treatment, at pH=5.0, the formula of the amorphous materials is represented by

CrO0.13(OH)1.87. High atomic contents for O2 from OH- and O3 from H2O are

observed, which confirm that the precipitate contains large amounts of hydroxide and

water in the structure.

3.3 Thermo gravimetric analysis

The thermogravimetric and differential thermogravimetric analysis of the synthesised

chromium materials are presented in Fig. 8, which shows the TG and dTG curves for

samples obtained in different conditions: (a) pH = 5.0, precipitate without

hydrothermal treatment (HT), noted as sample A; (b) pH = 5.0, HT for 12h, noted as

sample B; (c) pH = 7.5, HT for 4 days, noted as sample C; (d) pH = 10.0, HT for 4

days, noted as sample D. Thermal decomposition properties of the synthesised

materials are discussed in details as follows.

Fig. 8a shows the thermal decomposition of amorphous chromium hydroxide gel

precipitated under pH = 5.0 at room temperature (A). Two sharp dTG peaks are

observed at 149 °C and 177 °C with mass losses of 6.94% and 21.54%, respectively.

These two mass loss steps are attributed to the loss of water in the structure.


133

(a)

(b)


134

Fig.8 Thermogravimetric analyses (TGA) of (a) ppt-5.0, (b) CrO(OH)-5.0, (c)

CrO(OH)-7.5 and (d) CrO(OH)- 10.0.

(c)

(d)


135

A second mass loss step is observed at 439 °C with a mass loss of 5.75%, which is

attributed to the dehydroxylation of the compound. A minor mass loss observed at 77

°C is attributed to the desorption of water. Ratnasamy and Leonard [36] reported that

below 200 °C water molecules were eliminated from the amorphous hydroxide

precursor precipitated from Cr(NO3)3 solution with NH4OH, and a crystalline Cr2O3

phase was produced at 400 °C. This finding is in excellent agreement with the results

in this study. In the dTG curve of this experiment, the shoulder at 401 °C can be

assigned to the dehydration of the small amount of amorphous chromium hydroxide

to chromium oxide.

After the hydrothermal treatment of the chromium hydroxide gel, the product (noted

as sample B) shows a totally different thermal decomposition pattern in Fig. 8b. As

confirmed from XRD (Fig. 1), this hydrothermal product is crystalline CrO(OH)

nanomaterial. In the thermal study, the major mass loss steps occur at 444 °C and

463 °C with a total mass loss of 10.52%. These mass loss steps are attributed to the

dehydroxylation of the compound according to the reaction 2CrO(OH) → Cr2O3 +

H2O. The theoretical mass loss for this reaction is 10.59%, which is consistent with

our experimental mass loss of 10.52%. The minor mass losses observed below 110

°C are possible attributed to desorption of water.

Fig. 8c presents the thermo gravimetric analyses of CrO(OH) obtained after 4-days

of hydrothermal treatment from precipitate at pH = 7.5, which is noted as sample C.

The dTG curve shows a large band at 396 °C with two broad shoulders. The main

mass loss of sample C at 396 °C is attributed to the dehydroxylation of amorphous

chromium hydroxide to crystalline chromium oxide, as discussed above. The

shoulder at 276 °C is attributed to the formation of CrO(OH) from chromium

hydroxide which was X-ray amorphous.

It is suggested that surface formation of CrO(OH) during the heating of chromium

hydroxide at lower temperatures occur. [30] Another shoulder peak in the dTG curve

of sample C appears at 438 °C. This mass loss is assigned to the dehydroxylation of a

small amount of chromium oxy-hydroxide to crystalline chromium oxide.


136

Fig. 9 Mass spectrometric analysis associated with the thermal decomposition

process for sample CrO(OH)-10.0.

Sample D synthesised at pH 10.0 shows a similar dTG pattern as sample B. XRD

showed these samples had a more crystalline CrO(OH) phase. The dTG curve of

sample D shown in Fig. 8d, and the associated mass spectrometric analysis is

reported in Fig. 9. The main mass losses at 436 °C and 458 °C are attributed to the

dehydroxylation of CrO(OH) to Cr2O3, which is confirmed by the maxima for ion

current curves for H2O, OH, and O (Fig. 9). A small mass loss of 0.91% at 238 °C is

observed, which is attributed to the decomposition of nitrate impurity. Such

decomposition is confirmed by the ion current curves of NO where a maximum at

238 °C is observed. The mass loss step at 318 °C is assigned to the release of CO2,

confirmed by the ion current curves of CO2 and C.


137

Table 4 Summary of peaks shown in dTG curves for synthesised chromium materials.

ppt-5.0 CrO(OH)-5.0 CrO(OH)-7.5 CrO(OH)-10.0

77 °C 45 °C, 84 °C, 109 °C 40 °C 50 °C, 80 °C

149 °C, 177 °C

276 °C 238 °C

315 °C 318 °C

401 °C 396 °C

439 °C 444 °C, 463 °C 438 °C 436 °C, 458 °C

643 °C

Peaks shown in dTG curves of the four samples are summarised in Table 4. The four

samples all present a dTG peak at around 440 °C, at which CrO(OH) losses hydroxyl

groups which form Cr2O3.

Sample B and D, are more crystalline (XRD), show an additional dehydroxylation

peak at around 460 °C associated with CrO(OH). Sample A and C show peaks at

around 400 °C, which are assigned to the dehydroxylation of amorphous chromium

hydroxide.

These samples were obtained by precipitation at room temperature from chromium

nitrate and ammonia. Similar gel materials have been reported by Milligan [37] and

Ratnasamy [36]. Milligan‟s study found that there was a certain amount of chromium

trihydroxide (Cr(OH)3) in the sample. A mass loss at around 315 °C for sample B

and D, but not for sample C, is assigned to the release of CO2 (identified by MS)

from an impurity.


138

Fig. 10 XRD patterns of products after TGA from ppt-5.0, CrO(OH)-5.0,

CrO(OH)-7.5 and CrO(OH)-10.0. XRD patterns from literature: JCPDS card No.

01-084-1616 Cr2O3.

The thermo gravimetric analyses of the samples up to 1000 °C all were found to

transform to a well crystalline Cr2O3 phase (XRD), as shown in Fig. 10. All the

diffraction peaks of products were well indexed using a rhombohedral system of

lattice for Cr2O3, with space group of R-3c (JCPDS card, No. 01-084-1616).

It is noticed that all these thermal products have very high crystallinity and purity,

even from the colloidal precipitate (sample A). It is reported that when chromium

hydroxide materials are heated, the amorphous material can be converted into

crystalline Cr2O3 of around 400 °C. [36,52]

However, the thermal product from sample C has a slightly lower crystallinity. It is

suggested that the pH value that the sample precipitates at can affect the crystallinity

of the synthesised material.


139

4. CONCLUSIONS

Chromium oxyhydroxide nanomaterials were obtained by soft-chemistry methods

under various conditions. A combination of techniques were used to study

morphology and structure of the as-prepared nanomaterials, including XRD, SEM,

TEM, BET specific surface area analysis, XPS and thermo gravimetric analyses. The

pH value in precipitate process was proven to be critical for the formation and

crystallinity of the synthesised nanomaterials.

The CrO(OH) nanomaterials were found to form most easily with an acidic

precipitate process. The synthetic chromium oxyhydroxide was identified with a

plate-like morphology of ~11 nm in size. N2 adsorption study is proven as a facile

way to provide information on textural properties of the synthesised compounds.

XPS measurements for the as-prepared chromium oxyhydroxide nanomaterials allow

us to develop a view to determine spectral characteristics and to identify element

chemical environment in the compounds. It is also reported that the dehydroxylation

of the synthetic chromium oxyhydroxides occurs at ~460 °C, while chromium

hydroxide impurities decomposed at ~400 °C.

ACKNOWLEDGEMENTS


Technology Inorganic Materials Research Program of the School of Physical and

Chemical Sciences is gratefully acknowledged. The Australian Research Council

(ARC) is thanked for funding the instrumentation. One of the authors (JY) thanks

the Queensland University of Technology for a postgraduate doctoral scholarship.

Thanks are also given to Dr. B. Wood for technical and computing support in the

XPS study.


140

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144

SUPPORTING INFORMATION

A double tilt holder was utilised to take the HRTEM images of sample

CrO(OH)-10.0. The holder can be tilted in two directions: and . The following

images of one area were taken from the positions of different tilted angles:

(a) = 0°, = 0°

(b) = 0°, = 10°

(c) = -20°, = 0°

(d) = -60°, = 0°


145

CHAPTER 6.2

TRANSITION OF SYNTHETIC

CHROMIUM OXIDE GEL TO

CRYSTALLINE CHROMIUM

OXIDE: A HOT-STAGE RAMAN

SPECTROSCOPIC STUDY


Yang, J.; Martens, W. N.; Frost, R. L., "Transition of synthetic chromium oxide gel to

crystalline chromium oxide: a hot-stage Raman spectroscopic study." Journal of

Raman Spectroscopy 2010, in press. DOI:10.1002/jrs.2794.


146









Transition of Synthetic Chromium Oxide Gel to Crystalline Chromium Oxide: A Hot-stage Raman Spectroscopic Study

Yang, J.; Cheng, H. F.; Martens, W. N.; Frost, R. L., Journal of Raman Spectroscopy 2010, in press. DOI:10.1002/jrs.2794.



Hongfei Cheng Aided in data presentation and manuscript revising.





____________________ ______________________ ____________________

Name Signature Date


147

RESEARCH HIGHLIGHTS

The article studied the transition of synthetic chromium oxide gel to crystalline

chromium oxide using Raman spectra at different temperatures based on the results

of thermogravimetric analysis. The evolution of the Raman spectra versus

temperature was carefully described as well as band assignments. Two bands were

observed at 849 and 735 cm-1

, which were separately assigned to symmetric

stretching modes of O-CrIII

-OH and O-CrIII

-O. With the temperature increased, the

intensity of these bands changed, which was attributed to the loss of OH groups and

formation of O-CrIII

-O units in the material structure. The thermal product of the

synthetic chromium oxide gel in N2 atmosphere was found to be crystalline Cr2O3.


148

TRANSITION OF SYNTHETIC CHROMIUM

OXIDE GEL TO CRYSTALLINE CHROMIUM

OXIDE: A HOT-STAGE RAMAN

SPECTROSCOPIC STUDY

Jing Yang, Hongfei Cheng, Wayde N. Martens and Ray L. Frost*



[email protected]

Received: July 6, 2010

Accepted: August 02, 2010

Published Online: September 21, 2010

ABSTRACT

Chromium oxide gel material was synthesised and appeared to be X-ray amorphous.

The changes in the structure of the synthetic chromium oxide gel were investigated

using hot-stage Raman spectroscopy based upon the results of thermogravimetric

analysis. The thermally decomposed product of the synthetic chromium oxide gel in

nitrogen atmosphere was confirmed to be crystalline Cr2O3 as determined by the

hot-stage Raman spectra. Two bands were observed at 849 and 735 cm-1

in the

Raman spectrum at 25 °C, which were attributed to the symmetric stretching modes

of O-CrIII

-OH and O-CrIII

-O. With temperature increase, the intensity of the band at

849 cm-1

decreased, while the band at 735 cm-1

increased. These changes in intensity

are attributed to the loss of OH groups and formation of O-CrIII

-O units in the

structure. A strongly hydrogen bonded water H-O-H bending band was found at

1704 cm-1

in the Raman spectrum of the chromium oxide gel, however this band

shifted to around 1590 cm-1

due to destruction of the hydrogen bonds upon thermal

treatment. Six new Raman bands were observed at 578, 540, 513, 390, 342 and 303

cm-1

attributed to the thermal decomposed product Cr2O3.


149

The use of the hot-stage Raman microscope enabled low-temperature phase changes

brought about through dehydration and dehydroxylation to be studied.

KEYWORDS

Hot-stage; Raman Spectroscopy; Chromium Oxide Gel; Grimaldiite; Chromium

Oxide; Eskolaite

1. INTRODUCTION

Because of their high surface area, chemical and thermally stable properties and

mesoporous properties, transition metal oxides have been extensively used as carriers

and support for a variety of industrial catalysts at high temperature as well as low

temperature.1-3

Special attention has been focused on the formation and properties

of chromium oxide (Cr2O3), which is a mineral known as eskolaite. Chromium oxide

is important in high-temperature resistant materials,4, 5

liquid crystal displays,6, 7

catalysts,1, 3

and so on. A solid catalyst usually requires thermal treatment for

attributing the desired specific structural and textural character to be finished catalyst

which is also related to be activity and stability.8 Therefore it is important to

understand the thermal transformation for precipitated chromium gel material, which

is a precursor for Cr2O3 catalyst materials.

Raman spectroscopy has proven to be a very useful tool for studying materials.9-11

Raman spectroscopy allows the possibility of studying the structure at the molecular

level. Many minerals both natural and synthetic lend themselves to analysis by

Raman spectroscopy.10, 12-14

The combination of Raman spectroscopy with a

hot-stage lends itself as the technique of choice for studying the chemical reactions

during dehydration and dehydroxylation. The advantage of this technique is that the

changes in molecular structure can be followed in situ and at the elevated

temperatures. Spectroscopic studies with a hot-stage to reveal the thermal transition

from chromium oxide gel material to chromium oxides are limited. Our interest in

materials for industrial applications led our motivation for this research, as did the

search for fundamental knowledge of thermal stability of materials using hot-stage

Raman spectroscopy.15-17


150

This paper is a part of a vibrational spectroscopic investigation on synthetic

transition metal catalyst materials. The aim of this paper is to study the thermal

transition process of amorphous chromium oxide gel to crystalline chromium oxide

materials. Hot-stage Raman spectroscopy allows us to monitor the changes occurring

in the molecular structure during the thermal transition procedure. In the present

work, the authors report the hot-stage Raman spectra of a synthetised chromium

oxide gel as a function of temperature, and determine the transition to chromium

oxide, thus relating the spectra to the structure of the materials.

2. EXPERIMENTAL

2.1 Synthesis of chromium oxide gel

15 g of Cr(NO3)3•9H2O (laboratory reagent) was dissolved in 75 mL ultrapure water,

and 28% ammonia (analytical reagent) was diluted into 10% solution. At room

temperature, the ammonia solution (10%) was added at a rate of 1 mL min-1

into the

chromium ion solution with vigorous stirring. Ammonia solution addition ceased

when the pH of the reaction mixture reached 5.0. The reaction mixture was kept

stirring constantly in the air at room temperature for 0.5 h. The obtained gel-like

mixture was centrifuged and washed at 13000 rpm for 10 mins, 3 times. The

washed gel was dried at 65 ºC overnight, and denoted as “chromium oxide gel”.




from a line focused PW3373/10 Cu X-ray tube, operating at 40 kV and 40 mA, with

Cu K radiation of 1.540596 Å. The incident beam passed through a 0.04 rad soller

slit, a 1/2 ° divergence slit, a 15 mm fixed mask, and a 1 ° fixed antiscatter slit.




and sputter-coated with a thin layer of gold.


151


The synthetic chromium oxide gel was placed on the stage of an Olympus BHSM

microscope, equipped with 10x and 50x objectives and part of a Renishaw 1000

Raman microscope system, which also includes a monochromator, a filter system

and a Charge Coupled Device (CCD). Raman spectra were excited by a 633 nm laser

at a resolution of 4 cm-1


. Repeated

acquisition using the highest magnification was accumulated to improve the

signal-to-noise ratio. Spectra were calibrated using the 520.5 cm-1

line of a silicon

wafer. Details of the technique have been published by the authors.18-23

Spectra at elevated temperatures were obtained using a Linkam thermal stage

(Scientific Instruments Ltd., Waterford Surrey, England). Spectra were taken from

room temperature (25 ºC) up to a temperature of 550 ºC in a flowing nitrogen

atmosphere. Temperatures for the Raman spectroscopic study were selected based

on our previous studies on the thermogravimetric analysis of the synthetic chromium

oxide gel.

Spectral Manipulation such as baseline adjustment, smoothing and normalisation was

performed using GRAMS® software package (Galactic Industries Corporation

Salem, NH, USA).





bands used for the fitting process.

The Lorentz-Gauss ratio was maintained at values greater than 0.7 and fitting was

undertaken until reproducible results were obtained with squared correlations of r2

greater than 0.997.


152


3.1 Phase and Raman studies of the precipitated Cr-gel

The chromium oxide gel material was obtained by recovering the precipitate from

the reaction of Cr(NO3)3 and NH4OH solutions at pH 5.0.

No peaks were observed for this precipitated material in its XRD study (Fig. 1a),

which indicated that the precipitated chromium oxide gel was X-ray non-diffracting.

This agreed with published results, which reported that the material obtained from

the neutralisation of Cr3+

ions was usually amorphous or poorly crystalline.24, 25

Fig. 1b displays the SEM image of the Cr oxide-gel material, which appeared to be a

non-crystalline lumpy aggregate.

Fig. 1 (a) XRD pattern of the precipitated Cr-gel material and a reference pattern:

JCPDS card No. 01-070-0621 CrO(OH) and (b) SEM image of the precipitated

Cr-gel material

In the region between 2000 and 100 cm-1

shown as Fig. 2, the Raman spectrum taken

at room temperature (25 °C) of the synthetic chromium oxide gel was characterised

by a sharp intense band at 849 cm-1

. The band was assigned to be the symmetric

stretching mode of O-CrIII

-OH bonds.

(a) (b)


153


100 to 2000 cm-1

at 25 °C

The presence of a shoulder at 735 cm-1

was due to the symmetric stretching modes of

O-CrIII

-O. These two bands were too low in wavenumber to be associated with

[CrVI

O4] units, which were reported at around 899 cm-1

by Maslar et al.26-28

In the lower wavenumber region, a band at 439 cm-1

was observed and assigned to be

antisymmetric stretching modes of O-CrIII

-O. The band at 258 cm-1

observed for this

Cr-gel material was probably attributable to the bending/deformation modes of

[CrIII

O6] units in the structure. A Raman band was reported at 269 cm-1

for [CrIII

O6]

units in a study on selected chromate minerals.

29

It is well known that gel materials are highly hydrated and as such both water and

hydroxyls play a significant role in the structure of gels. It is common for water to

play a major role in the degree of polymerization because of the asymmetric nature

of hydrogen bonding systems.30-33

Water may bond to the interstitial cation or may

simply be held in the structure through hydrogen bonding in the chromium oxide gel

net-work.

A broad band was found at 1704 cm-1

in the Raman spectrum at 25 °C of the

synthetic Cr-gel material. This band is attributed to be H-O-H bending mode of the

very strongly hydrogen-bonded water.


154

The band shifted to lower wavenumbers in the Raman spectra taken at higher

temperatures, which indicated that the hydrogen bonds were destroyed as the

temperature increased.

Another band was observed at 1006 cm-1

, which is considered to be due to O-H

deformation modes according to the Raman study on minerals as reported in the

previous work.34, 35

A weak band at 1309 cm-1

is understood to be a combination or

overtone of band at lower wavenumbers.

3.2 Hot-stage Raman spectroscopy for the thermal transition

process

One method of studying the thermal decomposition of precipitated chromium oxide

gel is through the changes in the structure caused by dehydration of the material. The

use of Raman spectroscopy in conjunction with a hot-stage enables the molecular

structural changes to be observed.

Fig. 3 shows the spectra of the Cr-gel material in the region of 2000 to 100 cm-1

as a

function of temperature. The results of the band component analysis of these spectra,

comparing with the spectrum taken at room temperature, are reported in Table 1.

The band at 1704 cm-1

, at 25 °C assigned to be very strongly hydrogen-bonded water

H-O-H bending mode, could not be observed in the Raman spectra above 100 °C,

which indicated that after 100 °C a significant amount of water was removed from

the structure as water vapour. Instead, a new band was found at 1579 cm-1

at 130 °C.

This band was proposed to be water H-O-H bending mode of non-hydrogen-bonded

water.

The band intensity decreased significantly when temperature above 130 °C and it is

understood to be due to the escape of H2O trapped in the pores and vacant sites of the

chromium oxide gel structural network when the temperature increased. This band

shifted to higher wavenumbers as the temperature increased, to 1597 cm-1

at 350 °C.

The position of non-hydrogen-bonded H-O-H bending mode in Raman spectrum

reported in this study was in good agreement with that published by Frost and his

co-workers36

in the study of the Zn-Al hydrotalcite.


155

The band attributed to the O-H deformation modes at around 1008 cm-1

can be found

in all the Raman spectra at 25 to 350 °C, before the chromium oxide gel material

completely thermal decomposed to be metal oxide.

Bands at around 845 and 740 cm-1

assigned to be symmetric stretching of O-CrIII

-OH

and O-CrIII

-O modes showed obvious changes in the Raman spectra at 25 °C to 350

°C. The band at 845 cm-1

decreased, while the band at 740 cm-1

increased in their

relative intensity with the increase of temperature. It is proposed to due that the

O-CrIII

-OH vibration modes were transformed to O-CrIII

-O vibration modes because

of the dehydroxylation in the structure. This is in accordance with the results from

the thermo gravimetric analyses of this chromium oxide gel, which showed that there

were two large mass loss steps observed at 150 °C and 180 °C due to the dehydration

of the material.

A band at around 435 cm-1

, ascribed to the antisymmetric stretching modes of

O-CrIII

-O, can also be found in similar positions in all hot-stage Raman spectra from

130 to 350 °C.

The weak, broad band remained in the region of 260-240 cm-1

in all the Raman

spectra at 25 to 350 °C, which is proposed to be associated with the bending modes

of [CrIII

O6] units in the chromium oxide gel structural network. The band increased

in intensity with temperature, which meant that more [CrIII

O6] units in the structure

formed resulted from the dehydroxylation of the Cr-gel material as temperature

increased. More overtone and combination bands were found at 1969, 1858, 1792

and 1195 cm-1

in the Raman spectrum at 350 °C.


156

Fig. 3 Hot-stage Raman spectra of the precipitated Cr-gel material in the region of

100 to 2000 cm-1

from 130 to 350 °C


157


) and their assignment for synthetic Cr-gel

and its thermal-decomposed product Cr2O3 in the hot-stage Raman spectroscopic

study.

25 °C 130 °C 160 °C 250 °C 350 °C Assignment 550 °C

(Cr2O3)

1969,

1858,

1792

Combinations

& overtones

1704

Water H-O-H

bending (strongly

hydrogen bonded)

1579 1590 1594 1597 Water H-O-H

bending

1309 1504,

1335

1530,

1288

1544,

1330

1551,

1195

Combinations

& overtones

1006 1009 1007 1009 1008 O-H

deformation

578 (Eg)

540 (A1g)

849 841 843 845 842 s (O-CrIII

-OH) 513 (Eg)

735 734 740 749 747 s (O-CrIII

-O) 390 (Eg)

439 448 435 434 430 as (O-CrIII

-O) 342 (Eg)

258 246 249 239 241 ( [CrIII

O6] ) 303 (A1g)

s (symmetric stretching);

as (anti-symmetric stretching);

(bending/deformation).


158

For a better understanding to the phase identification after heating to this

temperature, XRD was applied to study the samples at this transition stage. Fig. 4

shows the XRD pattern of product from Cr-gel material heated at 350 °C for 4 h. A

crystalline phase was found with the diffraction pattern indexed to rhombohedral

Cr2O3 (JCPDS No. 01-084-0314). This suggests that the nucleation and growth of

Cr2O3 crystals occurred at this stage.

Yet no typical bands of Cr2O3 were observed in the obtained hot-stage Raman

spectrum at 350 °C (Fig. 3), which indicating the material at 350 °C is a mixed phase

of Cr-gel thermal transition species and crystalline Cr2O3 at an early stage of

crystallisation. However the transition species is amorphous or in low concentration

and could not be detected by XRD.


material and a reference pattern: JCPDS card No. 01-084-0314 (Cr2O3).

3.3 Phase, structure and morphology of Cr2O3

The Raman spectrum of the Cr-gel material heated at 550 °C in nitrogen atmosphere

is presented in Fig. 5, which was identical to a typical Raman spectrum of Cr2O3.37, 38

It is understood that thermally treated product in N2 from the Cr-gel was proved to be

crystalline Cr2O3, as confirmed by XRD (Fig. 6).


159

It is important to understand that rhombohedral Cr2O3 has the corundum-type

structure of a space group R-3c, with hexagonal close-packed layers of oxygen atoms

and two thirds of the octahedral holes in between filled by chromium atoms.39, 40

As

shown in the molecular model of Cr2O3 (Fig. 7), there are 6 formula units in the

hexagonal unit cell. It is reported38

that the chromium oxide belongs to the D6

3d space

group, and the irreducible representations for the optical modes in the crystal are

2A1g + 2A1u+ 3A2g + 2A2u + 5Eg + 4Eu.

Vibrations with symmetry A1g and Eg are Raman active, which means there are seven

Raman-active modes. Only five bands were found in most of the previous works of

Raman studies on chromium oxide37, 38, 41

However, six bands were revealed in this

study, and another weak mode42

below 250 cm-1

was not observed. The most intense

peak at 540 cm-1

was corresponding to the 1 (A1g) vibration mode of chromia.43

Another two sharp peaks at 578 and 342 cm-1

, as well as two weak peaks at 513 and

342 cm-1

were due to the Eg vibration modes. A sharp peak at 303 cm-1

was attributed

to A1g mode.


100 to 2000 cm-1

at 550 °C


160


material and a reference pattern: JCPDS card No. 01-084-0314 (Cr2O3).

Fig. 7 Schematic of Cr2O3 in rhombohedral structure (space group R-3c)


161

4. CONCLUSIONS

The chromium oxide gel material was obtained by precipitation from the reaction of

Cr(NO3)3 and NH4OH solutions at pH 5.0. The transition of the synthetic chromium

oxide gel to crystalline Cr2O3 was followed by hot-stage Raman spectroscopy up to

550 °C. The changes in the structure of the material in the dehydration and

dehydroxylation processes were determined in detail by hot-stage Raman

spectroscopy.

The Raman spectrum of the synthetic Cr oxide-gel material was characterised by an

intense peak at 849 cm-1

with a shoulder at 735 cm-1

. These two bands were

separately assigned to be O-CrIII

-OH and O-CrIII

-O symmetric stretching modes.

These bands changed in their relative intensity with the temperature increase and the

loss of hydroxyl units. Part of the water in the synthetic Cr-gel network structure

was released before 100 °C with the Raman band shift for water H-O-H bending

modes from 1704 to round 1590 cm-1

. In the low-wavenumber region, a band at 439

cm-1

was ascribed to the anti-symmetric stretching modes of O-CrIII

-O, and a band at

258 cm-1

was due to the bending modes of [CrIII

O6] units of the chromium oxide gel

structural network. These bands were observed up to 350 °C.

Raman spectroscopy confirmed that the synthetic Cr-gel material was converted to

be crystalline Cr2O3 after thermally treated up to 550 °C. Six typical bands were

observed for the thermal product Cr2O3 at Raman shifts (cm-1

) of 578 (Eg), 540 (A1g),

513 (Eg), 390 (Eg), 342 (Eg) and 303 (A1g).

ACKNOWLEDGMENTS


Technology Chemistry Discipline is gratefully acknowledged. The Australian

Research Council (ARC) is thanked for funding the instrumentation. One of the

authors (JY) is grateful to the Queensland University of Technology Chemistry

Discipline for the award of an international doctoral scholarship.


162

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165

CHAPTER 6.3

TRANSITION OF CHROMIUM

OXYHYDROXIDE

NANOMATERIALS TO

CHROMIUM OXIDE: A

HOT-STAGE RAMAN

SPECTROSCOPIC STUDY


Yang, J.; Martens, W. N.; Frost, R. L., "Transition of chromium oxyhydroxide

nanomaterials to chromium oxide: a hot-stage Raman spectroscopic study." Journal

of Raman Spectroscopy 2010, in press. DOI: 10.1002/jrs.2773.


166









Transition of Chromium Oxyhydroxide Nanomaterials to Chromium Oxide: A Hot-stage Raman Spectroscopic Study

Yang, J.; Martens, W. N.; Frost, R. L., Journal of Raman Spectroscopy 2010, in press. DOI: 10.1002/jrs.2773.







____________________ _____________________ ____________________

Name Signature Date


167

RESEARCH HIGHLIGHTS

The article reports the transition of chromium hydroxide nanomaterials to chromium

oxide using XRD, TEM and hot-stage Raman spectroscopy. Good quality Raman

spectra displayed the structure revolution of the synthetic CrO(OH) nanomaterials

during heating from room temperature to 550 ºC. Raman spectrum of CrO(OH) was

characterised by two intense bands at 823 and 630 cm-1

attributed to CrIII

-O symmetric

stretching mode and the band at 1179 cm-1

due to CrIII

-OH deformation mode. Upon

heating to 350 ºC, new Raman bands were observed at 599, 542, 513, 396, 344 and 304

cm-1

, which were typical bands of Cr2O3.


168

TRANSITION OF CHROMIUM

OXYHYDROXIDE NANOMATERIALS TO

CHROMIUM OXIDE: A HOT-STAGE RAMAN

SPECTROSCOPIC STUDY

Jing (Jeanne) Yang, Wayde N. Martens and Ray L. Frost*



[email protected]

Received: May 31, 2010

Accepted: July 08, 2010

Published Online: August 28, 2010

ABSTRACT

The transition of disc-like chromium hydroxide nanomaterials to chromium oxide

nanomaterials has been studied by hot-stage Raman spectroscopy. The structure

and morphology of -CrO(OH) synthesised using hydrothermal treatment was

confirmed by X-ray diffraction and transmission electron microscopy. The Raman

spectrum of -CrO(OH) is characterised by two intense bands at 823 and 630 cm-1

attributed to ν1 CrIII

-O symmetric stretching mode, bands at 1179 cm-1

attributed to

CrIII

-OH δ deformation modes. No bands are observed above 3000 cm-1

. The absence

of characteristic OH vibrational bands may be due to short hydrogen bonds in the

-CrO(OH) structure. Upon thermal treatment of -CrO(OH), new Raman bands

are observed at 599, 542, 513, 396, 344 and 304 cm-1

, which are attributed to Cr2O3.

This hot-stage Raman study shows that the transition of -CrO(OH) to Cr2O3

occurs before 350 °C.


169

KEYWORDS

Raman Spectroscopy; Hot-Stage Raman; Chromium Oxyhydroxide; Chromium

Oxide; Nanomaterials

1. INTRODUCTION

In recent years, nano-scaled inorganic materials, particularly transition metal based

materials, have received more and more attention in industry, in heterogenous

catalysis, as support materials as well as active components. Because of their high

surface area, chemical and thermally stable properties and mesoporous properties,

metal oxides have been extensively used as carriers and support for a variety of

industrial catalysts at high temperature as well as low temperature. Novel

nanomaterials may be based upon boehmite,1, 2

titania,3 gallium oxyhydroxide

4, 5

and designed clay minerals.6, 7

Special attention has been focused on the formation

and properties of chromium oxyhydroxides (CrO(OH)) and chromia (Cr2O3), which

are important in specific applied applications such as in high-temperature resistant

materials,8, 9

liquid crystal displays,10, 11

catalysts,12, 13

and so on. It is well known

that intrinsic properties of inorganic materials are mainly determined by their

composition, structure, crystallinity, size and morphology; great efforts have been

devoted to the investigation of different chromium oxide materials synthesis.14-16

3-5

nm CrO(OH) nanocrystals have been prepared by critical CO2 extraction of the

urea-assisted wet chromia gel mixture,17

and smaller CrO(OH) crystals with size of

1-2 nm were obtained in Chromia aerogel as well.13

To our best knowledge, 10 nm

CrO(OH) synthesised in this work is the biggest hydrothermally synthetic

CrO(OH) nanocrystal size so far.

As the development of all kinds of nanomaterials, technologies meet big challenge to

achieve perfect control on nanoscale-related properties. Modern Raman instruments

offer great advantages to assess some properties that are characteristic of the

nanoscale. Many minerals both natural and synthetic lend themselves to analysis by

Raman spectroscopy.18-21

The combination of Raman spectroscopy and a hot-stage

22-24 has been proved to be a powerful tool for studying the chemical reactions during

dehydration and dehydroxylation. The advantage of this technique is that the changes

in molecular structure can be followed in situ and at the elevated temperatures.


170

The purpose of this study is to elucidate the use of hot-stage Raman spectroscopy to

assess the thermal stability of chromium oxyhydroxide nanomaterials, and to

determine the changes in the molecular structure of the nano-scaled materials as the

chromium oxyhydroxide is thermally treated. In the present work, the authors report

the hot-stage Raman spectra of synthetic chromium oxyhydroxide nanomaterial, and

the transition of CrO(OH) to Cr2O3 relating the spectra to the structure of the

synthesised materials.

2. EXPERIMENTAL

2.1 Synthesis of CrO(OH) nanomaterial

15 g of Cr(NO3)3•9H2O was dissolved in 75 mL ultrapure water, and 28% ammonia

was diluted into 10% solution. At room temperature, the ammonia solution (10%)

was added at a rate of 1 mL/min into the chromium ion solution with vigorous

stirring. Ammonia solution addition ceased when the pH of the reaction mixture

reached 5.0. The reaction mixture was kept stirring constantly in the air at room

temperature for 0.5 h. The obtained gel-like mixture was centrifuged and washed at

13000 rpm for 10 mins, 3 times. The washed wet gel was transferred into a glass

beaker (25 mL). The beaker was placed into a Teflon vessel; 2 mL pure water was

poured into the button of the Teflon. The Teflon vessel was then sealed and placed in

a 170 ºC oven to process a 12 h steam-assisted hydrothermal treatment. Ultra pure

water was added to the resultant product and then collected by centrifugation (at

13000 rpm for 10 mins). The washing process was repeated for 3 times. Sample was

dried at 65 ºC overnight.




from a line focused PW3373/10 Cu X-ray tube, operating at 40 kV and 40 mA, with

Cu K radiation of 1.540596 Å. The incident beam passed through a 0.04 rad soller

slit, a 1/2 ° divergence slit, a 15 mm fixed mask, and a 1 ° fixed antiscatter slit.


171

2.3 Transmission electron microscopy

A Philips CM20 transmission electron microscope (TEM) at 160 kV was used to

investigate the morphology of the as-prepared sample. The sample was ultrasonically

dispersed in absolute ethanol solution, and then dropped on copper grid, which dried

in the air.


The crystals of CrO(OH) were placed and oriented on the stage of an Olympus

BHSM microscope, equipped with 10x and 50x objectives and part of a Renishaw

1000 Raman microscope system, which also includes a monochromator, a filter

system and a Charge Coupled Device (CCD). Raman spectra were excited by a 633

nm laser at a resolution of 2 cm-1


.

Repeated acquisition using the highest magnification was accumulated to improve

the signal-to-noise ratio. Spectra were calibrated using the 520.5 cm-1

line of a silicon

wafer. Details of the technique have been published by the authors. 22-27

Spectra at

elevated temperatures were obtained using a Linkam thermal stage (Scientific

Instruments Ltd., Waterford Surrey, England). Spectra were taken from room

temperature (25 ºC) up to a temperature of 550 ºC in a flowing nitrogen atmosphere.

Spectral Manipulation such as baseline adjustment, smoothing and normalisation was

performed using GRAMS® software package (Galactic Industries Corporation

Salem, NH, USA).





bands used for the fitting process. The Lorentz-Gauss ratio was maintained at values

greater than 0.7 and fitting was undertaken until reproducible results were obtained

with squared correlations of r2 greater than 0.996.


172


3.1 Phase structure and morphology of the synthetic -CrO(OH)

nanomaterial

X-ray diffraction (XRD) was used to determine the phase structure of the

hydrothermally synthesised material. It is reported that chromium oxyhydroxides can

crystallised in three polymorphs, , and .28

The -CrO(OH) phase has a layer

crystal structure with trigonal symmetry (space group R-3m or R3m).29

-CrO(OH)

has a different structure from -CrO(OH) and consist of a distorted rutile-type

structure with orthorhombic symmetry (space group Pnnm or P21nm).30

-CrO(OH)

was reported to be X-ray amorphous28

and assumed to have the same structure as

boehmite, -AlO(OH).31, 32

-CrO(OH) is found as grimaldiite a naturally occurring

mineral, while -CrO(OH) is found as guyanite, another mineral phase.33

Fig. 1 presents the typical XRD pattern of the resultant material. All diffraction peaks

in this pattern are well indexed and in good agreement with the standard JCPDS card

No. 01-085-1374 (Grimaldiite). No impurity peaks are observed, indicating that the

resultant material was a single crystalline phase, -CrO(OH).

Fig. 1 XRD pattern for the synthetic -CrO(OH) and a reference pattern: JCPDS

card No. 01-085-1374 (Grimaldiite).


173

The rhombohedral unit cell of -CrO(OH) (space group R3m) is shown in Fig. 2a,

presenting a three-layered structure. The parameters for this unit cell are: a = b =

2.979 Å, c = 13.70 Å.34

As reported by Christensen34

and Fujihara35

, in the structure

of -CrO(OH) , layers of Cr atoms perpendicular to the trigonal axis are sandwiched

between two parallel sheets of oxygen atoms, which are joined by short hydrogen

bonds aligned along the trigonal axis (Fig. 2a). It is easily observed in the structure

(Fig. 2b) that 6 oxygen atoms are octahedrally coordinated about each chromium

atom, and each oxygen atom was coordinated by three chromium atoms. Hydrogen

atoms are assumed to be involved in a disordered structure.

Fig. 2 Schematic of the synthetic -CrO(OH) in rhombohedral structure (space

group R3m) observed from different directions. The hexagonal unit cells are shown.

(a)

(b)


174

Fig. 3 TEM image of the synthetic -CrO(OH) nanomaterial

Morphology of the synthetic -CrO(OH) material was examined using transmission

electron microscopy (TEM), as shown in Fig. 3. Because of the high surface energy

of nano-particles, nanocrystals in the TEM image are aggregated. However we can

still observe that the synthetic -CrO(OH) crystals have an overall shape of discs

with average size of 10 nm in diameter. The size of synthetic -CrO(OH)

nanocrystals observed from TEM is in accordance with the result estimated by

Scherrer equation from the XRD data, which was 11 nm.

3.2 Hot-stage Raman spectroscopy

3.2.1 Raman spectra of -CrO(OH)

In order to study the changes in the spectra of -CrO(OH) as the nanomaterial is

thermally treated, it is necessary to describe the spectra collected at low temperature.

According to the previous thermogravimetric study published by the authors, the

dehydration of -CrO(OH) was observed from 120 °C. Therefore, this study reported

the spectra before and after 120 °C and will discussion the structure change of the

material during the thermal decomposition process.


175

Fig. 4 Hot-stage Raman spectra of the synthetic -CrO(OH) nanomaterial in the 200

to 1800 cm-1

region at 25 and 100 °C

Fig. 4 depicts the Raman spectra at 25 and 100 °C of -CrO(OH) nanomaterial in the

region of 200 to 1800 cm-1

. The spectra are characterised by two intense bands at 823

and 630 cm-1

.

As discussed above, -CrO(OH) adopts the sheet structure built from [CrIII

O6]

octahedra. Bands at 823 and 630 cm-1

are attributed to the ν1 CrIII

-O symmetric

stretching vibration. An additional band is found at 558 cm-1

. This band is

attributed to the ν3 CrIII

-O anti-symmetric stretching vibration. A low intensity band

is observed at 452 cm-1

, which is assigned to the O-CrIII

-O bending modes. This is in

good agreement with the reports by Maslar et al.36

Low intensity Raman bands are found at the 985-889 cm-1 region, however this

wavenumber range is too high to due to CrIII-O vibrational modes, and is assumed to be

the CrVI-O stretching modes or mixed CrIII/CrVI –O vibrational modes as reported by

Maslar et al..37, 38 Maslar published Raman studies on chromium coupons, and

-CrO(OH) was identified as a corrosion product. It is possible to detected trace of

Cr (VI) in the resultant sample due to the hydrothermal treatment, where little


176

portion of Cr (III) material can be oxidised to Cr (IV) compounds in the high

temperature and high pressure conditions. Raman spectroscopy once again shows its

advantage as a powerful tool to examine the phases trace in the samples, when the

content of materials is too little to be detected by bulk technique, such as XRD.

In the region of 1000 to 1200 cm-1

, the Raman spectra at 25 and 100 °C are

composed of broad low intensity bands. These bands are assigned to CrIII

-OH δ

deformation modes. Raman spectra of some crystalline oxyhydroxides exhibit OH

stretching modes in the region above 3000 cm-1

. However, no obvious bands are

observed in this wavenumber range. This lack of characteristic OH vibrational bands

is possible due to the short hydrogen bonds in the -CrO(OH) structure. This

observation is accordant with the reports made by Christensen28

, which revealed no

OH absorption was found in an infrared study.

An intense band at 1607 cm-1

is presented in the Raman spectrum of 25 °C, which

shifts to 1593 cm-1

in the 100 °C spectrum. These bands are assumed to the bending

modes of absorbed water in the -CrO(OH) layered structure.

3.2.2 Thermal transition from -CrO(OH) to Cr2O3 nanomaterials

The hot-stage Raman spectroscopy of the transition of -CrO(OH) to Cr2O3 in the

region of 200 to 1800 cm-1

over the temperature range ambient to 550 °C is studied

in this work.

The Raman spectrum at 350 °C shows different features from that at low

temperatures (25 and 100 °C) in Fig. 5. Bands at 1607, 1171, 823, 630, 556 and 425

cm-1

are not observed anymore. New Raman bands at 602, 544, 518, 389, 348 and

304 cm-1

are found, which are attributed to the new phase (Cr2O3) formed by the

thermal decomposition of chromium oxyhydroxide. It is reported that Cr2O3 adopts

the corundum (Al2O3) structure consisting of vertex-, edge-, and face-sharing

[CrIII

O6] octahedral.36

Kemdehoundja et al. discussed that there are five vibrational

modes for chromia, four Eg modes and one A1g mode.39

Chen et al. reported as well

that Cr2O3 presented the most intense A1g band at 540 cm-1

, with another two lower

intensity bands at 291 and 335 cm-1

.40

A sharp band was observed at 1009 cm-1

with

a shoulder at 997 cm-1

. These bands are assumed to due to O-H deformation modes.


177

Fig. 5 Raman spectra of the synthetic -CrO(OH) nanomaterial in the 200 to 1800

cm-1

region at 350 and 550 °C

Broad bands are observed in the region of 620-830 cm-1

, which are assigned to the

vibrations of CrVI

-O bridging bonds. It is reported that CrVI

is probably present in

bridging bonds but not in polychromate structures.36

In such structures, CrVI

is

incorporated into the CrIII

-O surface network rather than being present as a

monochromate or polychromate. There is a small band found at 1363 cm-1

at the

temperature of 350 °C, which is assigned to be the combination band.

The Raman spectrum at 550 °C is very similar with what that at 350 °C, which

indicated the thermal decomposition product from -CrO(OH) nanomaterial is

Cr2O3. Bands in 620-830 cm-1

region show the loss of intensity at 550 °C. The

combination band at 1363 cm-1

disappeared at 550 °C. No intensity of OH stretching

bands remained at 550 °C, which reveals that -CrO(OH) had totally transformed to

Cr2O3. This agrees with the results of thermal gravimetric analysis reported in

somewhere else by the authors, which showed no sample mass loss above 550 °C.

All the peaks and their assignment are summarised in Table 1.


178


) and their assignment for -CrO(OH) and

Cr2O3 nanomaterials in the hot-stage Raman spectroscopic study.

-CrO(OH) nanomaterial Cr2O3 nanomaterial

25 °C 100 °C Assignment 350 °C 550 °C Assignment

1634

1593 1607 Water H-O-H

bending

1537 1537

1504 1387 Combinations

& overtones

1363 Combinations

& overtones

1179, 1153 1171, 1129 O-H

deformation

1009, 997 1007, 997 O-H

deformation

981 985, 938 Cr

VI-O

889 889

817

CrVI

-O 771 770

729 719

823 823 1 (O-Cr

III-O)

683 684

630 630

558 556 3 (O-CrIII

-O)

602 599 Cr2O3

544, 518 542, 513 Cr2O3

452 452 2 (O-CrIII

-O) 477 CrVI

-O

389 396 Cr2O3

348 344 Cr2O3

304 304 Cr2O3

1 (O-CrIII

-O symmetric stretching);

2 (O-CrIII

-O symmetric bending);

3 (O-CrIII

-O anti-symmetric stretching)


179

4. CONCLUSIONS

Disc-like of -CrO(OH) nanomaterial was synthesised by using hydrothermal

techniques without surfactants at low temperatures. The phase composition was

proven by X-ray diffraction and TEM showed a10 nm size of the nanocrystals. The

conversion of -CrO(OH) to Cr2O3 nanomaterial was achieved by thermal

treatment up to 350 °C. The transition of -CrO(OH) to Cr2O3 was studied by

hot-stage Raman spectroscopy; upon thermally treating the synthetic -CrO(OH)

nanomaterial at 550 °C.

The structure of synthetic nanomaterials, -CrO(OH) and Cr2O3, are deduced

from their Raman spectra. Intense bands at 823 and 630 cm-1

, as well as relatively

weaker peaks at 558 and 452 cm-1

were observed in the Raman spectra of

rhombohedral-CrO(OH). These bands are attributed to O-CrIII

-O vibrations. Bands

at 1179 cm-1

are assigned to CrIII-OH δ deformation modes.

Upon thermal treatment of -CrO(OH) at 350 °C, new Raman bands at 599, 542, 513,

396, 344 and 304 cm-1

are found. The Raman spectrum of resultant Cr2O3 is

characterised by an intense sharp peak at 542 cm-1

, which was due to the A1g band.

ACKNOWLEDGMENTS


Technology Inorganic Materials Research Program is gratefully acknowledged. The

Australian Research Council (ARC) is thanked for funding the instrumentation.

One of the authors (JY) is grateful to the Queensland University of Technology

Inorganic Materials Research Program for the award of an international doctoral

scholarship.


180

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Chapter 7 Synthesis and characterisation of CoO(OH) nanodiscs

183

CHAPTER 7

SYNTHESIS AND

CHARACTERISATION OF

COBALT HYDROXIDE, COBALT

OXYHYDROXIDE AND COBALT

OXIDE NANODISCS


Yang, J.; Liu, H.; Martens, W. N.; Frost, R. L.,"Synthesis and characterization of

cobalt hydroxide, cobalt oxyhydroxide, and cobalt oxide nanodiscs." The Journal of

Physical Chemistry C 2010, 114 (1), 111-119.


184

SYNOPSIS

CoO(OH) has more oxidation state (Co3+

) than Co3O4, and is considered to be a

potential better nanosensors than Co3O4. CoO(OH) and Co3O4 materials can be

prepared from the precursor Co(OH)2, which itself is well known as additives of

alkaline secondary batteries and also important starting materials of heterogeneous

catalysts.

Hydrothermal treatment proves to be a facile method to fabricate desired

nanomaterials as mentioned in the previous chapters. A similar synthesis procedure

was applied to produce cobalt oxyhydroxide nanomaterials in this chapter. Co(OH)2

nanomaterial with hexagonal nanostructure was synthesised. CoO(OH) and Co3O4

nanomaterials were produced from the as-prepared Co(OH)2 nanomaterial. The size

and disc-like morphology was successfully kept between these materials. The

resultant Co(OH)2, CoO(OH) and Co3O4 materials were systematically studied by

XRD, TEM, SEM, XPS, Raman and thermogravimetric techniques. The aim of this

research is to controllably synthesise a series of cobalt hydroxide, oxyhydroxide and

oxide nanomaterials with disc-like morphology by simple route and to investigate

their intrinsic properties for potential applications.


185









Synthesis and Characterisation of Cobalt Hydroxide, Cobalt Oxyhydroxide, and Cobalt Oxide Nanodiscs

Yang, J.; Liu, H. W.; Martens, W. N.; Frost, R. L., The Journal of Physical Chemistry C 2010, 114 (1), 111-119.



Hongwei Liu Conducted TEM and data analysis, manuscript editing.





____________________ ______________________ ____________________

Name Signature Date


186

RESEARCH HIGHLIGHTS

The article presents a soft chemistry method for synthesis of cobalt hydroxide, cobalt

oxyhydroxide and cobalt oxide nanodiscs. A detailed survey of the morphology,

structure and properties of the synthesised materials was given, combining XRD,

SEM, TEM, XPS, etc. The synthesised Co(OH)2 displayed hexagonal morphology

with clear edges of 20 nm long, and this morphology and nanosize were retained to

Co3O4 through a topotactical relationship. Analysis of the high resolution XPS

spectra on pure components allows the identification of the chemical environment

change of cobalt element during the synthesis process. Raman spectra were

compared to provide information on structure of the synthesised nanomaterials. It

was also found that the dehydroxylation of Co(OH)2 happened at 130 ºC and

CoO(OH) decomposed at 252 ºC.

halla

Due to copyright restrictions, this article is not available here. Please consult the hardcopy thesis available from QUT Library or view the published version online at: http://dx.doi.org/10.1021/jp908548f


215

CHAPTER 8

GENERAL DISCUSSION &

FUTURE WORK


216

1. GENERAL DISCUSSION

This thesis has increased our fundamental knowledge of the synthesis and

characteristic properties of metal oxyhydroxides, oxide and hydroxide nanomaterials.

Facile soft chemistry routes, including hydrothermal techniques, were utilised to

synthesise the desired nanomaterials without using any surface directing agents.

Hydrothermal treatment offers the advantages of high purity, good dispersion, and

high crystallinity, as well as uniform and controllable morphology. All the

synthesised nanomaterials were fully characterised using XRD, TEM/SAED, SEM,

N2 adsorption/desorption study, XPS, TGA-MS, Raman, infrared, as well as

hot-stage Raman and infrared emission spectroscopy.

The main conclusions of this thesis are list below. More detailed discussion is

presented in the following paragraphs.

(1) Oxyhydroxide and hydroxide nanomaterials of Group IIIA elements (Al &

In). Well-crystalline and uniform boehmite (aluminium oxyhydroxide)

nanofibres of 500 nm long were achieved under hydrothermal treatment without

the assistance of any surfactants or structure directing agents. Under the similar

hydrothermal synthesis route, instead of indium oxyhydroxide, indium hydroxide

was obtained, which appeared to be nanocubes of 350 nm in size. Infrared

emission spectroscopy has proven to be very useful technique to study in situ the

dehydration, dehydroxylation behavior, and other thermal structure changes of

nanomaterials.

(2) Transition elements doped boehmite nanomaterials. Chromium (III) ions were

successfully doped into boehmite nanostructures through facile hydrothermal

treatment. By controlling the reaction pH and reaction duration, it was possible to

obtain desired Cr dopant amounts and Cr doped boehmite nanostructures. It was

found that doping with Cr enhanced the thermal stability of boehmite nanofibres

(~500 nm long) by increasing their dehydroxylation temperatures.

(3) Chromium oxyhydroxides (CrO(OH)) nanomaterials. Through hydrothermal

treatment, amorphous chromium oxide gels were easily converted to CrO(OH)

nanomaterials (4-12 nm) with high surface area. The particle size of CrO(OH)

nanomaterials could be controlled by adjusting the pH and hydrothermal treatment


217

duration. Hot-stage Raman spectroscopy has shown its advantages in monitoring

material structural changes during the thermal decomposition process of Cr oxide

gel and CrO(OH) nanoparticles.

(4) Cobalt-based nanomaterials. Well-crystalline cobalt oxyhydroxide, cobalt

hydroxide and cobalt oxide nanodiscs (40 nm) were achieved and a “topological

relationship” was found between these nanomaterials. Morphology of other

cobalt-based nano-/micro-materials can be controlled to be nano-needles,

nano-rings, nano-flowers, and other nano-morphologies.

1.1 Aluminum oxyhydroxide (boehmite) nanofibres and indium

hydroxide nanocubes

One dimensional (1D) nanomaterials with high aspect ratio show wide applications

in catalysis, adsorption, ceramic separation membranes and other applications.1-5

1D

boehmite nanomaterials, such as nanofibres, nanotubes and nanorods have attracted

extensive interest because of their particular morphology and high surface areas. In

Chapter 3, boehmite nanofibres with very surface areas 133 m2 g

-1 were reported.

The nanofibres appear in bundles under the SEM, and were found to be 500 nm long

and between 4-6 nm wide. The IES spectra indicated the dehydroxylation of boehmite

nanofibres started from 250 ºC and was completed by 450 ºC. The synthesis of

boehmite nanofibres was achieved under hydrothermal conditions at 170 ºC for 48

hours at pH 5, based on Shen‟s work.6 This synthesis route without use of any

surfactants or structure directing agents showed its advantages in manufacturing 1D

boehmite nanofibres in large quantities environmental friendly, facilely and

economically.

Since the elements aluminium and indium are both in Group IIIA, which have 3

valence electrons and are in the main trivalence compounds. Indium is expected to

show similar chemistry with aluminium, therefore, I am interested in fabricating

indium oxyhydroxide nanomaterials through the similar hydrothermal synthesis route

for boehmite (aluminium oxyhydroxide) nanofibres. As reported in Chapter 4,

surprisingly, instead of indium oxyhydroxide (InO(OH)), indium hydroxide (In(OH)3)

nanocubes (300-400 nm) were found after 2-day hydrothermal treatment at both 100

ºC and 180 ºC.


218

This may be explained by the Ralph Pearson‟s HSAB theory, which assigns the terms

“hard” or “soft”, and “acid” or “base” to chemical species. Aluminium ions are small,

have high charge states, and can be classified as “hard” species, while indium presents

to be “soft” because of its large ionic radium. The “hard” aluminium species were

more easily to form oxyhydroxides through hydrothermal treatment. However, under

the similar hydrothermal synthesis conditions, the “soft” indium species tended to

form its hydroxides instead of oxyhydroxides. Interestingly, a trace of InO(OH) specie

was found in the product synthesised at 100 ºC, which indicates that it is possible to

obtain InO(OH) nanomaterials by hydrothermal methods. This needs further

investigation.

Thermal analysis showed the transition of the synthetic In(OH)3 to In2O3 nanocubes

occurred mainly at 219 °C. SEM showed there was a topotactical relationship

between the of In(OH)3 nanocubes and its oxide thermal decomposition product

In2O3.

1.2 Transition metals doped boehmite nanomaterials

Transition metal doping is a promising method to modify properties and increase the

applications of nanomaterials. Since long and uniform boehmite nanofibres were

successfully obtained through a facile hydrothermal route, chromium and cobalt were

used as dopants to modify boehmite nanofibres. The project of cobalt doped

boehmite nanofibres proved more difficult and requires further research, because that

the starting Co (II) species are required to be oxidised to Co (III) first before they can

be applied to substitute the Al ions in the boehmite structure to form Co doped

materials. However, in the case of chromium doped boehmite nanofibres, Cr (III)

species were utilised directly as dopants. In Chapter 5, I demonstrated the first report

of chromium doped boehmite nanofibres prepared by a hydrothermal method under a

range of conditions. Chromium was successfully doped into boehmite nanostructure,

and the nanofibre (~500 nm) morphology was retained with a low amount of Cr

doping. In this chapter, the thermal stability of the synthetic Cr doped boehmite

nanofibres was detailed. It is found that doping with chromium resulted in an

increase in the main dehydroxylation temperature of boehmite nanofibres from 406.5

°C to 436.5 °C.


219

The dehydroxylation temperature rose with the increased hydrothermal treatment

period. It was also found that as the dehydroxylation temperature increased the

mass loss from the dehydroxylation step decreases.

More detailed studies have been undertaken on (a) optimizing synthesis parameters to

enhance the efficiency of doping into boehmite crystal structures and (b) how doping

affected the morphology of nanomaterials. However these results are not published yet

or included as a chapter in this thesis. The main findings are: (i) 1% Cr was doped in

the time dependent series study, and the hydrothermal duration varied from 1 day to 10

days. The increase of hydrothermal treatment duration resulted in better crystallinity

and larger nanofibres. this was attributed to the Ostwald ripening process. (ii) 1% -

20% Cr was applied in the added Cr amount dependent series. Nanofibres were formed

when the added Cr was less than 10%. When 20% Cr was added, nanorods were found.

(iii) Detected Cr amount in the resultant samples was less than 2%. When 10% Cr was

added, 2% Cr was detected, which is more than that in the 20% Cr sample. However, it

was noticed that there was large Cr loss under these synthesis conditions. The Cr loss

was assumed to be due to the oxidisation of Cr(III) to Cr(IV) in hydrothermal

conditions by NO3-, which was induced by the starting materials. After washing NO3

-

off carefully before hydrothermal treatment, one can detect much more Cr doped into

the boehmite nanofibres. (iv) It was found that alkaline hydrothermal conditions can

increase Cr doped amount. 9.5% Cr was detected in the 10% Cr added sample when

certain amount of ammonia solution was applied during the hydrothermal treatment.

However, the increase of detected Cr in doped samples resulted morphology change

from uniform nanofibres to very short rods and broken nanoparticles.

1.3 Chromium oxyhydroxide nanoparticles

As found in the system of chromium doped boehmite nanofibres, when 2% Cr was

doped into the boehmite structure, the morphology was nanofibres; while 9.5% Cr

was detected in the boehmite material, the morphology of the material converted to

be nanorods, even nanoparticles. Obviously, the Cr dopant amount into the boehmite

structure can affect the morphology of doped boehmite nanomaterials. Therefore, it

is worthwhile to investigate further the properties of pure chromium oxyhydroxide

nanomaterials.


220

In Chapter 6, chromium oxyhydroxide nanomaterials were achieved after a

hydrothermal treatment to chromium oxide gel precipitations obtained from the

reaction of Cr(NO3)3 and NH4OH solutions at various pHs. The pH value during

precipitation was critical for the growth and crystallinity of the synthesised

nanomaterials: CrO(OH) nanomaterials were formed most easily under an acidic

precipitate process. The CrO(OH) nanomaterials synthesised both from acidic and

alkaline precipitates were identified with a plate-like morphology of ~11 nm in

diameter under TEM. Round particles with 50 nm in size were observed by SEM,

which were considered to be the aggregates of the CrO(OH) nanoplatelets. The

CrO(OH) nanomaterials possess BET specific surface area of ~120 m2 g

-1.

However, the CrO(OH) nanomaterial obtained from neutral precipitate show smaller

size as 4 nm and larger surface area of 380 m2 g

-1. XPS measurements for the

as-prepared chromium oxyhydroxide nanomaterials allow us to develop a view to

determine spectral characteristics and to identify element chemical environment in

the compounds. It is also observed that the dehydroxylation of the synthetic

chromium oxyhydroxides occurs at ~460 °C.

1.4 Cobalt-based nanomaterials

Cobalt hydroxide, oxyhydroxide and their oxides are important transition metal

compounds because of their wide applications as semiconductors, catalysts, gas

sensor, and so on. In Chapter 7, well-crystalline Co(OH)2 nanodiscs (~40 nm) were

prepared through a soft chemistry method. CoO(OH) nanodiscs were easily

achieved through the oxidisation of the synthetic Co(OH)2 with the assistance of

H2O2; while Co3O4 nanodiscs can be obtained by calcination of Co(OH)2 and

CoO(OH). There was a so-called topological relationship among these as-prepared

cobalt compounds, which means their size and shape remain similar. Raman

spectroscopy proves as a facile way to provide information on structure of the

synthesized compounds. The dehydroxylation of the synthetic cobalt hydroxide

happened at 130 °C, while cobalt oxyhydroxide decomposed at 252 °C.

Several morphology control studies on cobalt-based nanomaterials were undertaken

during the course of PhD research. The Co(OH) 2 nanodiscs was precipitated in 45

°C sand bath from the reaction between cobalt nitrate and sodium hydroxide

solutions, and it is found that the sand bath temperature can affect growth of


221

nanomaterials. With low concentrations of the starting solutions, very uniform

Co(OH)2 singlets were prepared. However as reported in Chapter 7, a higher

concentration results Co(OH)2 doublets along with some broken particles. The discs

appear in pairs, which may be due to a surface-edge charge as is found in kaolinite

platelets. An alternative explanation is ascribed in terms of magnetism. The Co(OH)2

platelets are magnetic and the platelets may have a +/- relationship. Co(OH)2

nanorings were found after a much longer hydrothermal duration or with the

assistance of PEO surfactants. Cobalt hydroxy carbonate Co2CO3(OH)2

nano-needles and nano-flowers were achieved with the assistance of urea and

Na3PO4. These nanomaterials can all be the precursor of cobalt oxides, which are

widely applied transition metal oxides. Because of the topological relationship

happened during calcination, one can control the morphology of cobalt oxides by

controlling the desired cobalt-based nanomaterial precursors.

1.5 Hot-stage Raman and infrared emission spectroscopic studies

on the synthesised nanomaterials

Hot-stage Raman and infrared emission spectroscopy (IES) enabled in situ

monitoring of the structural changes at the molecular level through the thermal

decomposition process. Extensive studies on the application of hot-stage Raman and

IES to synthetic nanomaterials were included in this thesis.

The transition of the precipitated chromium oxide gel (before hydrothermal

treatment) to crystalline Cr2O3 was followed by hot-stage Raman spectroscopy up to

550 °C. The Raman spectrum was characterised by an intense peak at 849 cm-1

,

s(O-CrIII

-OH), with a shoulder at 735 cm-1

, s(O-CrIII

-O). These two bands changed

in relative intensity with the temperature increase because of the loss of hydroxyl

units. Part of the water in the synthetic Cr-gel network structure was released

before 100 °C with the Raman band of the water H-O-H bending modes shifting

from 1704 to round 1590 cm-1

. The conversion of CrO(OH) nanoparticles to Cr2O3

was studied by hot-stage Raman spectroscopy upon 550 °C. Intense bands at 823 and

630 cm-1

, as well as relatively weaker peaks at 558 and 452 cm-1

were observed in

the Raman spectra of rhombohedralCrO(OH). Upon thermal treatment of CrO(OH)

at 350 °C, new Raman bands appear characterised to Cr2O3.


222

The transition of In(OH)3 to In2O3 was studied by hot-stage Raman spectroscopy and

thermogravimetric analysis. An Intense Raman band at 309 cm-1

is assigned to the

ν1(In(OH)3), and two bands at 3079 and 3240 cm-1

are attributed to OH stretching

band. Raman spectroscopy shows that the transition of In(OH)3 to In2O3 occurs in the

200 to 225 °C temperature range.

In the infrared emission spectrum of the synthetic In(OH)3 nanocubes at 100 °C,

OInO vibrational modes are observed at 771 and 853 cm-1

. Bands at 1063, 1139 and

1156 (sharp) cm-1

are assigned to be In(OH) bending modes, and the OH stretching

modes are observed at 3123 and 3247 cm-1

. The intensity of these bands nearly

reaches zero above 200 °C, which indicates that almost all the OH units are lost in

the structure. However, new weak bands were found at 915 and 3437 cm-1

in infrared

emission spectra from 250 to 450 °C, which are assumed to due to OH bending and

stretching modes separately. This observation shows the releasing and transfer of

protons and formation of transition new OH bonds upon calcination.

2. FUTURE WORK

2.1 Doping with other elements for new functional materials

In this study, I have successfully synthesised chromium doped boehmite, and

introduced a better thermal stability for boehmite nanofibres and created the

photoluminescence properties of chromium doped alumina nanofibres. Therefore, it

is viable to create novel properties of boehmite nanofibres by doping elements into

its structure. New nanomaterials will be designed using other transition metal

elements, such as cobalt, cerium and lanthanides.

2.2 Doping mechanism

Further investigation into the chromium doped boehmite nanomaterials formation

mechanism is needed. This involves the examination of samples produced over a

matrix of hydrothermal conditions by systematically altering many parameters, such

pH value, temperature, aging, washing, etc. In the study of chromium

oxyhydroxide nanoparticles, it is found the pH value of the precipitation before

hydrothermal treatment crucially influenced the particle size of products.


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Therefore, it is worthwhile to take a detailed study if the pH values for precipitation

affect the chromium doping into boehmite nanofibre structure. Under the help of

different analysis techniques, evidence would be collected and mechanism of doping

and nanostructure formation will be proposed.

2.3 Application studies: catalyst, gas sensors

As discussed previously, nanomaterials exhibit large surface area and high surface

activity because of the size effect of nanoparticles. This shows great potential for the

synthetic nanomaterials to be used as catalysts or gas sensors. The reaction efficiency

needs to be studied for the further modification of the nanomaterials.

2.4 Environment friendly study

Finally, to be on the environment safe side, further studies on the leaching of

elements of the nanomaterials need to be researched, especially for the element

chromium, cobalt, etc.


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REFERENCES

[1] M. Sumetsky, Y. Dulashko, A. Hale, Optics Express 2004, 12, 3521-3531.

[2] G. S. Wu, T. Xie, X. Y. Yuan, B. C. Cheng, L. D. Zhang, Materials Research

Bulletin 2004, 39, 1023-1028.

[3] E. A. Stach, P. J. Pauzauskie, T. Kuykendall, J. Goldberger, R. He, a. P. Yang,

Nano Letters 2003, 3, 867-869.

[4] C. Mao, D. J. Solis, B. D. Reiss, S. T. Kottmann, R. Y. Sweeney, A. Hayhurst,

G. Georgiou, B. Iverson, A. M. Belcher, Science 2004, 303, 213.

[5] S. Sharma, M. K. Sunkara, Jounal of the American Chemical Society 2002,

124, 12288-12293.

[6] S. C. Shen, Q. Chen, P. S. Chow, G. H. Tan, X. T. Zeng, Z. Wang, R. B. H.

Tan, the Journal of Physical Chemistry C 2007, 111, 700-707.

Documents

SYNTHESIS AND CHARACTERISATION OF METAL ...eprints.qut.edu.au/45712/1/Jing_Yang_Thesis.pdfSYNTHESIS AND CHARACTERISATION OF METAL OXYHYDROXIDE AND OXIDE NANOMATERIALS by JING (JEANNE)