COPYRIGHT AND CITATION CONSIDERATIONS FOR THIS … · v ABSTRACT Titanium dioxide (TiO2) – also known as titania – exists in three common polymorphic crystal phases, i.e. anatase,

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DOPED NANOTITANIUM DIOXIDE FOR PHOTOCATALYTIC APPLICATIONS

by

LANGELIHLE NSIKAYEZWE DLAMINI

Thesis in fulfilment of the requirement for the degree

DOCTOR OF PHILOSOPHY

in

CHEMISTRY

in the

FACULTY OF SCIENCE

of the

UNIVERSITY OF JOHANNESBURG

Supervisor : PROF. RWM KRAUSE Co-supervisors : DR. SH DURBACH : PROF. GU KULKARNI

i

DECLARATION

I hereby declare that this dissertation, which I herewith submit for the research

qualification

DOCTOR’S DEGREE IN CHEMISTRY

to the University of Johannesburg, Department of Applied Chemistry, is, apart from

the recognised assistance of my supervisors, my own work and has not previously

been submitted by me to another institution to obtain a research diploma or

degree.

_______________________________ on this ____ day of _______________

(Candidate)

_______________________________ on this ____ day of _______________

(Supervisor)

_______________________________ on this ____ day of _______________

(Co-supervisor)

_______________________________ on this ____ day of _______________

(Co-supervisor)

ii

DEDICATION

I dedicate this work to the following:

My mother, for she has been there for me unconditionally in all situations. A

blessed son I am to have you as my mother. Ndwadwe.

My Princess, HRH Princess Lomakhetfo Dlamini, your influence to me is without

measure. Forever you shall be in my heart. Nkhosi

Mr Richard, my high school chemistry teacher, for he planted the love and

enthusiasm I have for chemistry. Thank you for paving the way.

For I know that I know that I should know your WORD shall come to pass:

לראייה היא עדיין לזמן שנקבע, אך בסופו

של דבר ידבר ולא לשקר. למרות שזה

מתעכב, לחכות שזה; כי זה בוודאי יבוא

' For the vision is yet for an appointed time, but at the end it shall speak and not lie. Though it

linger, wait for it; because it will certainly come and will not delay.'

Habakkuk 2: 3

iii

ACKNOWLEDGEMENTS

I am indebted to many people and institutions that have helped me complete this

thesis.

My advisors, Prof. RWM Krause, Dr. SH Durbach, and Prof. GU Kulkarni, for their

guidance, motivation and genuine interest in my work and to see me grow in

fullness. I am thankful to Jawaharlal Nehru Centre for Advanced Scientific

Research (JNCASR), especially the unit of Chemistry and Physics Materials, for

warmly welcoming me and shaping my research skills and for hosting me during a

research visit.

I acknowledge the University of Johannesburg, Department of Applied Chemistry

for providing me with the platform to further my studies. I extend special

appreciation to Dr K Pillay who was my mentor when I had classes to teach and

Ms H Oosthuizen for believing in me. Thank you to UJ New Generation

Scholarship (NGS) for the consistent financial support and the UJ Center for

Nanomaterials Science and DST/NRF Centre of Excellence in Strong Materials for

project support. I am grateful to the University of Witwatersrand (microscopy unit)

and Council for Scientific and Industrial Research (CSIR) – nanoscale unit for

allowing me to use their facilities. I am also thankful to Dr Paul Franklyn for his

insight and input on preparation of binary metallic oxides.

In the course of my study, I have met incredible people, who have pulled me up

when the going was non-existent, and showered me with love when I felt totally

pathetic, as well as gave me an ear (to the good/bad moments) when I was either

overwhelmed or overjoyed. I am truly grateful to all, even if I don‟t specifically

mention you. You were a friend when I needed one. However, I am particularly

thankful to Lungile Lukhele, Thabile Ndlovu and Soraya Malinga and I hope and

pray I have been a blessing to you just as you have been to me. Machawe “CFO”

Motsa your sarcasm and rebellious nature was a needed dose. My brother from

another mother, Mnqbobi Dlamini, you are awesome. Nomcebo “Tjotjo” Ngwenya,

your humor and stubbornness is for the history books.

iv

St John‟s College (SJC), an institution that completed my training. I am forever

changed by the experience I had in my stay with the college. I am particularly

thankful to the Department of Science which warmly welcomed me into their

space. Dr. C. Henning an exemplary leader you are, Mr. WR Taylor and Mr. R.

MacDonald for allowing me into their classrooms. Mrs. E. Bosman, the fellowship

we shared was always inspirational. Mr. R. Klement, Mrs. K. van Schoor and

Mr. B. Bradley, I appreciated the warmth you showered upon me. Finally, my

mentor, Mrs. N. Stocks what an incredible person you are. I have learnt so much

from you, be it directly or indirectly. Thank you.

SJC isn‟t SJC without its students. Single-handedly you have changed my

perception about students from private schools. I am truly glad to have met you,

from remove to upper six. I am particularly pleased with lower five (A2-set) as their

inquisitiveness and intelligence made me love chemistry all the more. Lastly, I

would like to thank Mr. C Bossert (Nash housemaster) and Mr. C. Milligan (Deputy

housemaster) for allowing to be a stooge at Nash. I appreciate the friendliness and

brotherhood from the fellow stooges (Jarryd Kuiper, Nick Fouche and Pierre le

Roux). These young (for now) Johannains made my stay worthwhile, Bradley

Tshepo Chauke (kind hearted), Andrew Ian Williamson (true academic), Gary

Francis Jackman (my wingman…) S‟bonakaliso MG Nene (outstanding musician),

Molemo Otsile Ponoane (Sotho patriot) and Matthew Henderson (outstanding

actor).

My colorful family (Friday “Boywizzy” Sihlongonyane, Nelisiwe Mamba, Mangaliso

„Mzomba‟ Ndzinisa, Temangilozi Dlamini, Lindelwa Malidzisa, Tfolele Ndwadwe

and Tengetile Malidzisa) whom I love dearly. Thank you for putting up with me. My

father HRH Prince Mandla Dlamini, you are still the greatest.

Finally, the Lord God Almighty, your limitless grace that overlooks my

shortcomings, hence I can never claim any success as mine but yours always. In

you I have found favor and I am forever in awe. I am glad and grateful that you

honor faithfully our covenant. Amen.

v

ABSTRACT

Titanium dioxide (TiO2) – also known as titania – exists in three common

polymorphic crystal phases, i.e. anatase, brookite, and rutile. The most common

phase is rutile while anatase is regarded as the most photoactive. Titania is used

commercially as a white pigment for cosmetics, plastics, paints and sunscreens.

However, a significantly higher interest in TiO2 lies in its application in the

photocatalytic purification of water and air, the production of hydrogen by water

splitting, and its ability to behave as a disinfectant against biological

microorganisms. Despite the potential applications of titania, its use as a pure

material (pristine) is rather limited by a number of physical properties.

The experiments presented in this thesis show that doped titania composites

containing either anionic or binary metal oxide dopands (and composited with

carbon nanotubes (CNTs)) are photocatalytically superior to pristine TiO2.

The synthetic routes of these nanocomposites were based on facile techniques

such as solventless thermal decomposition followed by the “sol-gel” process for

the preparation of binary metal oxides and nanocomposites. For the anionic doped

nanocomposites a single-step (one-pot) sol-gel process was also used. These

doped TiO2 materials were characterized using microscopic and spectroscopic

techniques, to determine the particle size, polymorphic nature, morphology, band

gap and elemental composition. These characterization tools confirmed the

successful synthesis of doped TiO2 and CNTs nanocomposites.

The catalytic properties of these nanocomposites were evaluated by irradiating

samples with ultra-violet and visible light in the presence of model organic

pollutants such as: bromophenol blue and phenol red. From the experimental data,

a general trend could be observed for the anionic doped nanocomposite, when

exposed to different radiation. The work of this thesis was the first to report the

photocatalytic trend of anionic doped (halide) TiO2.

vi

The nanocomposites containing CNTs degraded the model dyes by over 90%

when both UV and visible light were used. The doped nanocomposites were both

more effective and faster in degrading the dyes. In addition, CNT nanocomposites

containing metal oxides were also effective in degrading over 95% of the model

dyes under visible and UV light. The degradation rates of the pollutants varied

with catalyst and dye, but were at least 80% faster than the commercially available

and widely used Degusa P25. The reaction kinetics for the degradation of the

model dyes for all nanocomposites appeared to follow pseudo-first-order rate laws.

Ultra-violet spectroscopy and ion-chromatography were employed to measure the

decrease in the concentration of the dyes and the resultant by-products

respectively.

The synthesis of TiO2, anionic-doped photocatalysts and binary metal oxide

photocatalysts and composites, all based on TiO2, are presented in this thesis

together with their characterisation and performance against model pollutant dyes.

vii

TABLE OF CONTENTS

Section Page

Declaration .............................................................................................................. i

Dedication ............................................................................................................... ii

Acknowledgements ................................................................................................ iii

Abstract .................................................................................................................. iv

Table of Contents ................................................................................................... v

List of Figures ......................................................................................................... x

List of Tables ........................................................................................................ xiii

List of Abbreviations ............................................................................................. xiv

Presentations and Publications ............................................................................ xvi

CHAPTER 1 : INTRODUCTION .......................................................................... 1

1.1 Problem statement ....................................................................................... 1

1.2 Justification .................................................................................................. 2

1.3 Objectives of the Study ................................................................................ 3

1.4 Outline ......................................................................................................... 4

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

CHAPTER 2 : LITERATURE REVIEW ................................................................ 6

2.1 Introduction .................................................................................................. 6

2.2 Titanium Dioxide .......................................................................................... 8

2.2.1 Surface Perturbation ................................................................... 13

2.2.1.1 Co-deposition of Metals ................................................ 13

2.2.1.2 Anionic Doping .............................................................. 15

2.2.1.3 Composite Semiconductors .......................................... 18

2.2.1.4 Dye Sensitization .......................................................... 20

2.3 Synthesis of Titanium Dioxide Nanoparticles ............................................. 22

2.3.1 Sol-Gel Method ........................................................................... 23

viii

2.3.2 Hydrothermal Method.................................................................. 28

2.3.3 Solvothermal Method .................................................................. 29

2.3.4 Chemical and Physical Vapour Deposition ................................. 30

2.4 Pollutants ................................................................................................... 32

2.5 Dyes........................................................................................................... 36

2.6 Carbon Nanotubes ..................................................................................... 39

REFERENCES: .................................................................................................... 43

CHAPTER 3 : EXPERIMENTAL METHODOLOGY .......................................... 62

3.1 Reagents used ........................................................................................... 62

3.2 Synthesis of Photocatalysts ....................................................................... 63

3.2.1 Anionic-doped Titanium Photocatalyst ........................................ 63

3.2.2 Preparation of Composite Semiconductors ................................. 64

3.2.2.1 CNTs-CoO and CNTs-CoO-TiO2 .................................. 64

3.2.2.2 TiWxOy and CNTs-TiWxOy ............................................. 64

3.3 Characterization ......................................................................................... 65

3.3.1 Transmission Electron Microscopy (TEM) ................................... 65

3.3.2 Scanning Electron Microscopy (SEM) ......................................... 67

3.3.3 Raman Spectroscopy .................................................................. 67

3.3.4 Powder X-ray Diffraction (PXRD) ................................................ 68

3.3.5 Ultraviolet-Vis (UV-Vis) Spectroscopy ......................................... 69

3.3.6 Surface Area Measurements ...................................................... 70

3.3.7 Photocatalytic Measurements ..................................................... 71

REFERENCES ..................................................................................................... 74

CHAPTER 4 Photodegradation of triphenylmethane dyes by anionic doped

titania nanocomposites* .................................................................................... 76

4.1 Introduction ................................................................................................ 76

4.2 Results ....................................................................................................... 77

4.2.1 BET Analysis ............................................................................... 77

4.2.2 Raman Analysis .......................................................................... 80

ix

4.2.3 Optical Studies ............................................................................ 81

4.2.4 Powder X-ray Diffraction (PXRD) ................................................ 83

4.2.5 TEM and SEM ............................................................................. 85

4.3 Photocatalytic Activity of the doped TiO2 and CNTs-TiO2 nanocomposites 87

4.3.1 Photodegradation of Bromophenol Blue ..................................... 88

4.3.2 Photodegradation of Phenol Red (PR) ........................................ 92

4.4 Discussion ................................................................................................. 95

REFERENCES ................................................................................................... 102

CHAPTER 5 Titania-based binary metal oxides and nanocomposites for the

photodegradation of organic pollutants in water* ......................................... 104

5.1 Introduction .............................................................................................. 104

5.2 Experimental ............................................................................................ 105

5.3 Characterization ....................................................................................... 106

5.4 Photocatalytic Activities of pristine TiO2 and various BMO Nanocomposites

115

5.4.1 Photodegradation of Bromophenol Blue (BPB) ......................... 115

5.4.2 Photodegradation of Phenol Red (PR) ...................................... 118

5.5 Discussion ............................................................................................... 120

REFERENCES ................................................................................................... 125

CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS ............................. 127

6.1 Conclusions ............................................................................................. 127

6.2 Future outlook .......................................................................................... 129

APPENDIX ......................................................................................................... 130

x

LIST OF FIGURES

Figure Description Page

Figure 2.1: Representation of the photoexcitation in a semiconductor .................. 7

Figure 2.2: Structures of (a) anatase and (b) rutile.......................................... 9

Figure 2.3: Structure of brookite .................................................................... 10

Figure 2.4: Plausible pathways followed by holes and electrons. ................. 12

Figure 2.5: Absorption of solar spectrum against band gap of titania. .................. 13

Figure 2.6: A representation of the Schottky barrier ............................................. 14

Figure 2.7: Proposed schemes of band gap narrowing of TiO2 by doping with non-

metals. ........................................................................................ 16

Figure 2.8: Plausible mechanism for the fluoride induced enhancement in the

production of free hydroxyl radicals in bulk solution from irradiated

TiO2. ............................................................................................ 17

Figure 2.9: Electron and hole separation in a coupled semiconductor system ..... 19

Figure 2.10 (a) Structure of porphyrin, (b) Structure of metalloporphyrins ........... 21

Figure 2.11: Organic dye (cis – [Ru (dcbH2)LL‟ attached on TiO2. ....................... 22

Figure 2.12: TEM images showing sol-gel prepared titania particles (a) and (b)

uniform distribution of titanium nanoparticles,(c) and (d) reveal

agglomerated nanoparticles. ....................................................... 28

Figure 2.13: SEM images of TiO2 nanoparticles prepared by the hydrothermal

method. ....................................................................................... 29

Figure 2.14: TEM images showing nanocrystals prepared by the solvothermal

method. ....................................................................................... 30

Figure 2.15 (a). Scheme showing the set-up for the synthesis of titanium

nanoparticles by CVD ................................................................ 31

Figure 2.15 (b). Scheme showing the set-up for the synthesis of titanium

nanowires by PVC....................................................................... 32

xi

Figure 2.16: Flow-chat illustrating the different methods used in the removal of dye

from wastewater. ......................................................................... 34

Figure 2.17 Classes and structures of synthetic dye ............................................ 37

Figure 2.17 Classes and structures of synthetic dye ............................................ 38

Figure 2.18: Different allotropes of carbon ........................................................... 39

Figure 2.18: Roll-up of a graphene sheet to form a nanotube. ............................. 40

Figure 2.19: A bottom-up approach of CNTs via CVD route................................. 41

Figure 3.1: Relationship between adsorbed gases compared to pressure ........... 70

Figure 3.2: (a) UV-radiation experimental set-up for the photodegradation of

model pollutants .......................................................................... 71

Figure 3.2: (b) Visible radiation experimental set-up for the photodegradation of

model pollutants .......................................................................... 72

Figure 4.1: Nitrogen sorption isotherm of selected nanocomposites .................... 78

Figure 4.2: Raman spectra of pristineTiO2, dopedTiO2 and doped CNTs-TiO2

nanocomposites .......................................................................... 81

Figure 4.3 Comparative UV absorbance measurements of (a) CNTs-TiO2

nanocomposites (b) doped TiO2.................................................. 82

Figure 4.4: Band gap measurement of pristine TiO2 and iodine doped TiO2 ........ 83

Figure 4.5: PXRD patterns of (A) doped TiO2 , (B) CNT loaded anionic doped

nanocomposites .......................................................................... 84

Figure 4.6: (a, b) TEM images of partly dispersed TiO2 on CNTs (c)

agglomerated TiO2 nanoparticles without CNTs (d) SEM images of

TiO2 nanoparticles without CNTs ................................................ 86

Figure 4.6: ((e, f) EDS spectrum of fluorine and iodine doped CNTs-TiO2

nanocomposites. ......................................................................... 87

Figure 4.7 (a,b) Photodegradation of BPB by anionic doped TiO2 using UV and

visible light .................................................................................. 88

Figure 4.7 (c) reaction kinetics of BPB initiated by visible irradiation .................... 89

xii

Figure 4.8: (a, b) Degradation of BPB by anionic undoped CNTs-TiO2

nanocomposites and CNTs-TiO2-X nanocomposites under UV and

Visible irradiations, ...................................................................... 91

Figure 4.8: (c) Reaction kinetics of CNTs-TiO2-X nanocomposites with visible

light ............................................................................................. 92

Figure 4.9 Photodegradation of PR by doped TiO2 under (a) visible and (b) UV

light (c) reaction kinetics doped TiO2 under UV light. .................. 93

Figure 4.10 (a,b) Photodegradation of PR by CNTs-TiO2-X nanocomposites (c)

reaction kinetics of PR by CNTs-TiO2-X nanocomposites under

visible light. ................................................................................. 94

Figure 4.11: Proposed mechanism of the role played by CNTs upon

photodegradation of dye in water ................................................ 99

Figure 4.12 Generated sulphate ions from the degradation of PR using (a) ultra-

violet and (b) visible light ........................................................... 100

Figure 4.13a Liberated sulphate and bromide ions from the degradation of BPB (a)

CNTs-TiO2, (b) CNTs-TiO2-I, under visible irradiation ............... 101

Figure 4.13b Liberated sulphate and bromide ions from the degradation of (C)

TiO2 (D) TiO2-I under visible irradiation ..................................... 101

Figure 5.1: BET analysis of (a) cobalt oxide titania nanocomposite and (b)

tungsten oxide titania BMOs and nanocomposite ..................... 106

Figure 5.2: Raman spectra of (a) CNTs-CoO-TiO2 (b) TiO2, (c) TiWxOy and (d)

CNTs-TiWxOy ............................................................................ 108

Figure 5.3: (a-b) SEM images of CoO-TiO2 and Ti-WxOy, (c-d) EDS spectra of the

binary oxides ............................................................................. 110

Figure 5.3: (e-h) TEM images of CNTs-CoO-TiO2 and CNTs-Ti-WxOy

nanocomposites, (i) EDS spectra of CNTs-Ti-WxOy nanocomposite

.................................................................................................. 111

Figure 5.4: PXRD pattern of cobalt-titania nanocomposites ............................... 112

Figure 5.5: PXRD patterns of tungsten-titania nanocomposites ......................... 113

xiii

Figure 5.6: Optical studies of the various binary metal oxides, and their

nanocomposites ........................................................................ 114

Figure 5.7: Evaluation of photocatalytic activity of BPB by various TiO2 and BMO

nanocomposites (a, b) under UV irradiation .............................. 115

Figure 5.7: Evaluation of photocatalytic activity of BPB by various TiO2 and BMO

nanocomposites (c,d) under visible irradiation .......................... 116

Figure 5.8: Reaction kinetics of the various (a) TiO2 and (b) BMO nanocomposites

as photocatalysts of BPB using different illumination sources .. 117

Figure 5.9: The photodegradation of PR by TiO2 and BMO nanocomposites (A, B)

under UV irradiation .................................................................. 118

Figure 5.9: The photodegradation of PR by TiO2 and BMO nanocomposites (C,D)

under visible irradiation ............................................................. 119

Figure 5.10: Reaction kinetics of (a) PR photodegradation by various TiO2 and (b)

BMO nanocomposites under UV and visible illumination .......... 119

Figure 5.11: Proposed mechanism for the binary system loaded with CNTs ..... 121

Figure 5.12 Generated sulphate ions from the degradation of PR using (a,c) ultra-

violet and (b,d) visible light ........................................................ 122

Figure 5.12: Generation of bromide and sulphate ions from BPB by TiWxOy and

CNTs- TiWxOy nanocomposites, tested as photocatalysts under

visible and UV light ................................................................... 123

Figure 5.12: Generation of bromide and sulphate ions from BPB by TiWxOy and

CNTs- TiWxOy nanocomposites, tested as photocatalysts under

visible and UV light ................................................................... 124

xiv

LIST OF TABLES

Table Description Page

Table 2.1: Band gaps of semiconductors ....................................................... 7

Table 2.2: Applications of TiO2 ....................................................................... 8

Table 2.3: Crystalline parameters of TiO2 polymorphs ................................. 11

Table 2.4: Physical properties of TiO2 polymorphs ...................................... 11

Table 2.5: Common Alkoxides .............................................................................. 23

Table 3.1 Reagents used ..................................................................................... 62

Table 4.2: Kinetic measurements of the CNTs-TiO2 nanocomposites for each dye

.................................................................................................... 89

Table 5.1 Characterization of photocatalysts ..................................................... 107

Table 5.2 Rate constants of the various TiO2 and BMO nanocomposites as

photocatalysts of BPB using UV and visible light ...................... 117

xv

LIST OF ABBREVIATIONS

AC Activated carbon

AOPs Advanced oxidation processes

BET Brunauer Emmett Teller

BMNPs Bimetallic nanoparticles

BMO Binary metal oxide

BPB Bromophenol blue

CB Conduction band

CCVD Catalytic chemical vapour deposition

CNTs Carbon nanotubes

e- Electron

EDXS Energy dispersive X-ray spectroscopy

Eg Band gap

eV Electron volt

h+ Hole

IC Ion chromatography

JCPDS Joint committee on powder diffraction standards

k Constant

kV Kilovolts

MGDs Millenium development goals

MOs Metal oxides

xvi

nm Nanometer

OECD

Organization for economic cooperation and

development

ppm Parts per million

PR Phenol red

PVCD Physical vapour deposition

PXRDS Powder X-ray diffraction spectroscopy

S(BET) Specific area

sccm Standard cubic centimetre

TEM Transmission electron spectroscopy

UNICEF

United nations international children's

emergency funds

UV Ultra-violet

UV/Vis Ultra-violet spectroscopy

VB Valance band

Vis Visible

Vp Pore volume

xvii

PRESENTATIONS AND PUBLICATIONS

The work presented in this thesis has been submitted to peer-reviewed journals

and presented in conferences as shown below:

Publications

LN Dlamini RW Krause, GU Gulkarni, SH Durbach, Synthesis and characterization

of titania based binary metal oxide nanocomposite as potential environmental

photocatalyst, Materials Chemistry and Physics (2011) 129: 406 – 410

LN Dlamini RW Krause, GU Gulkarni, SH Durbach, Photodegradation of

Bromophenol blue with fluorinated TiO2 composite, Applied Water Science (2011)

1: 19 – 24

LN Dlamini RWM Krause, GU Gulkarni, SH Durbach Photodegradation of

triphenylmethane dyes by anionic doped titania nanocomposites, submitted to the

Journal of Catalysis, under second review.

LN Dlamini RWM Krause, GU Gulkarni, SH Durbach, HI Ezuruike, G Msane,

Improving the efficiencies doped titania nanocomposites, to be submitted to

Materials Chemistry and Physics

Presentations

LN Dlamini, RW Krause, GU Kulkarni, SH Durbach, Fluorinated Titanium dioxide

embedded on carbon nanotubes-co-cyclodextrin polyurethanes for the removal of

organic pollutants in water, Oral Presentation, 1st Young‟s Professional Water

Institute of South Africa (YPWISA), CSIR Pretoria, 19 – 21 January 2010

xviii

LN Dlamini, RW Krause, GU Kulkarni, SH Durbach, Photo-catalytic degradation of

organic pollutants in water with F-TiO2 nanoparticle composites, Poster

presentation, WaterNet symposium, Durban, 12 – 16 April 2010

LN Dlamini, RW Krause, GU Kulkarni, SH Durbach A facile method of preparing

titania based binary metal oxide nanocomposites: CNTs-CoO-TiO2; synthesis and

characterization, Oral presentation, 3rd International Conference for Young

Chemist (ICYC), Penang Malaysia, 23 – 25 June 2010

LN Dlamini, RW Krause, GU Kulkarni, SH Durbach A facile method of preparing

titania based binary metal oxide nanocomposites: CNTs-CoO-TiO2; synthesis and

characterization, poster presentation, 15th international conference on TiO2

photocatalysis: fundamentals and applications, San Diego USA, 15 – 18

November 2010


bromophenol blue with fluorinated TiO2 composite, poster presentation, 2nd

European symposium on photocatalysis, Bordeaux, Cite Mondiale France, 29 – 30

September 2011


bromophenol blue with fluorinated TiO2 composite, oral, 2nd TCS International

chemistry conference, Da es Salaam, Tanzania, 5 – 7 October 2011

1

CHAPTER 1:

INTRODUCTION

1.1 Problem statement

Water is not a renewable resource. The demand for water continues to increase

exponentially, doubling every 20 years.1 Farming makes up to 70% of the global

demand of water. In turn, fresh water is lost either through evaporation or leaks,

never returning to the basin. The demand for water has prompted many

economists to refer to water as the “petroleum for the next century.”1 The United

States alone needs up to US$ 1,000 billion in new pipes and wastewater plants by

2020.1 However, some have also seen the increasing demand for water as a

single, powerful catalyst to bring about world war three, as disputes between

countries of cross-border water basins have increased globally.1

The Organization for Economic Cooperation and Developments (OECD) has

warned that competition for shores of water would increase up to 55% by 2050.2

As nations struggle to provide water for their citizens, ironically, pollution of this

rare natural resource is on the rise. According to the 2009 United Nations

International Children‟s Emergency Funds (UNICEF) report, nearly 4000 children

under the age of five die daily from diseases related to contaminated food and

drinking water around the world.3 World vision, estimates eight million children

annually die from drinking unsafe water, exposure to poor sanitation and hygiene-

related diseases.

The Millennium Development Goals (MGDs) established in 2000 by United

Nations (UN), target 7C, aimed at reducing by halve (by 2015) the portion of global

population without sustainable access to safe drinking water.4 However, in the

past decade, increased agricultural and industrial activities have not only

Chapter 1: Introduction

2

increased the demand for water, but have also contributed to the pollution of

surface and ground water. Hence, there is a need for improved functional, cost-

effective and easy to use technologies that would remove and degrade pollutants

in water. Solutions that include the use of freely available solar radiation are

attractive from an energy perspective.

1.2 Justification

Several water treatment techniques such as activated carbon, reverse osmosis,

coagulation and recently nanoporous polymers have been developed to address

water pollution issues.5,6 In our laboratories we have successfully prepared and

tested novel nanoporous polymers capable of removing and degrading organic

pollutants in water.7 The incorporation of bimetallic nanoparticles (BMNPs) onto

these polymers showed a significant degradation of organic pollutants in water.8

As promising and encouraging as the results were, the toxicity of these BMNPs is

of concern and the technology relies on adsorption- which requires regeneration or

disposal at some point. Furthermore, the degradation of pollutants is largely driven

by the ever presence of the catalytic metal in the BMNPs.

Further improvement of green methods is required that will treat water without the

need of adding extra pollutants. At present, photocatalysis is considered to be a

viable and effective method for degrading organic pollutants in water. In this

regard,TiO2 is considered to be an excellent photocatalyst due to its relative non-

toxicity, low cost, thermodynamic stability and excellent chemical and physical

properties.

Based upon the combined characteristics of TiO2 with CNTs, as reported in the

literature, two significant issues were believed to require further study:

(1) The improvement of the interface between TiO2 and CNTs so as to

separate the photogenerated electron-hole pairs efficiently,

(2) The improvement of the photocatalytic activity of TiO2 to enable the use of

visible light.


3

Many authors in the literature have reported on anionic doped TiO2 and CNTs ,

however, at the time of writing this thesis no report was available (as far as could

be ascertained), that specifically investigated the general pattern of Group 7

elements, when doped with TiO2 or as nanocomposites when CNTs are

incorporated.

1.3 Objectives of the Study

Hence, based upon the above-stated shortcomings the objectives of the thesis

were tailored, for the purpose of tackling issues (1) and (2) mentioned previously,

as follows:

(i) To design, synthesise and characterise novel ((b) & (d)) titania

nanocomposites. These include:

a) †Y-TiO2

b) †Y-TiO2-CNTs

c) ‡MOx-TiO2

d) ‡MOx-TiO2-CNTs

†Y: anionic dopants (F,Cl,Br, I); ‡MOx: metal oxide (CoO, WO3)

(ii) To determine the photocatalytic activity of each of these nanocomposites

on organic pollutants in water, using stable and recalcitrant

triphenylmethane dyes as test cases.

(iii) To deduce whether a relationship for the photocatalytic efficiency of various

doped titania nanocomposites exists.


4

1.4 Outline

The research that is reported in this thesis is broken down in the following

sections:

CHAPTER 2: A literature review of the synthesis of titania as well as the

classification and effects of pollutants in water

CHAPTER 3: The experimental procedures followed in this thesis

CHAPTER 4: Characterization and photodegradation of pollutants by

anionic doped titania nanocomposites by UV and visible light.

CHAPTER 5: Characterization and photodegradation of pollutants by titania

based binary metal oxide nanocomposites under UV and

visible light.

CHAPTER 6: Recommendations and general conclusions which can be

drawn from these investigations.


5

REFERENCES

1 . http://www.telegraph.co.uk/finance/newsbysector/utilities/2791116/Water-

crisis-to-be-biggest-world-risk.html accessed 24 April 2012

2 . http://economictimes.indiatimes.com/news/news-by-industry/et

cetera/water-shortage-a-global- threat-without-urgent-reform-

oecd/articleshow/12178269.cms accessed 24 April 2012

3 . http://www.christianpost.com/news/world-vision-shares-8-million-reasons-

to-keep-working-beyond-world-water-day-71856/ accessed 24 April 2012

4 . http://www.un.org/millenniumgoals/environ.shtml accessed 24 April 2012

5 . Forgas E, Cserhati T, Oros G, Removal of synthetic dyes from

wastewaters; a review, Environmental International, (2004) 30, 953 - 971

6 . Gupta GS, Shukla SP, Prasad G, Singh VN, China clay as adsorbent for

dye house waters, Environmental Technology, (1992) 13, 926 – 936

7 . Mamba BB, Krause RW, Malefetse TJ, Gericke G, Sithole SP, Cyclodextrin

nanosponge in the removal of organic matter for ultrapure water in power

generation, Journal of Water Supply: Research and Technology-Aqua,

(2009) 58, 299 - 304

8 . Krause RW, Mamba BB, Dlamini LN, Durbach SH, Fe-Ni nanoparticles

supported on carbon nanotubes – co-cyclodextrin polyurethanes for the

removal of TCE in water, Journal of Nanoparticle Research, (2010) 12 449 -459

http://www.telegraph.co.uk/finance/newsbysector/utilities/2791116/Water-%20crisis-to-be-biggest-world-risk.html

http://www.telegraph.co.uk/finance/newsbysector/utilities/2791116/Water-%20crisis-to-be-biggest-world-risk.html

http://economictimes.indiatimes.com/news/news-by-industry/et%20%20cetera/water-shortage-a-global-%20threat-without-urgent-reform-oecd/articleshow/12178269.cms



http://www.christianpost.com/news/world-vision-shares-8-million-reasons-%20to-keep-working-beyond-world-water-day-71856/

http://www.christianpost.com/news/world-vision-shares-8-million-reasons-%20to-keep-working-beyond-world-water-day-71856/

http://www.un.org/millenniumgoals/environ.shtml%20accessed%2024%20April%202012

6

CHAPTER 2:

LITERATURE REVIEW

2.1 Introduction

Interest in semiconductor photocatalysts rapidly increased since Fujishima (1972)

reported on the photo-electrochemical splitting of water in Nature.1 Since, various

semiconductors such as metal oxides, sulphides, nitrides and selenides have been

investigated.2,3,4 However, metal oxide photocatalysts have been used more

extensively because they were easier to synthesize, relatively stable and

inexpensive. 5,6 Metal chalcogenides were not stable enough and suffered from

photo corrosion. For instance the metal sulphides or selenides (denoted by Y in

the equation below) were easily oxidized by holes generated in the photocatalytic

process (denoted by h+ in the equation below), which resulted in the degeneration

of the photocatalysts (equation. 1).7

MY + 2h+ Y + M2+ [1]

M (metal), Y (chalcogen)

A semiconductor, unlike a conductor, has a non overlapping valance band (VB)

and conduction band (CB). The energy difference between the two levels is known

as the band gap (Eg). Photocatalytic reactions therefore proceed on a

semiconducting material as illustrated in Figure 2.1. Upon illumination by incident

energy equal to or greater than the band gap of the semiconductor, electrons (e-)

receive enough energy and are excited from the VB to the CB, leaving holes (h+)

in the VB. Without the irradiation, holes and electrons are retained in the VB. Each

semiconductor has a minimum theoretical band gap (Table 2.1), corresponding to

the energy required to promote excitation, as represented by equation 2.

Chapter 2: Literature Review

7

Eg

Figure 2.1: Representation of the photoexcitation in a semiconductor

The photogenerated electrons and holes have strong reducing and oxidizing

capabilities respectively, which can be harnessed into various applications

including the production of hydrogen and the decontamination of water.

Table 2.1: Band gaps of semiconductors

Semiconductor Band Gap (eV) Semiconductor Band Gap (eV)

ZrO2 5.0 SiC 3.0

ZnO 3.2 GaP 2.3

TiO2(rutile/anatase) 3.0/3.2 CdS 2.4

WO3 2.8 MoS2 1.8

In2O3 2.7 ZnS 3.6

Fe2O3 2.2 CdSe 1.7

Si 1.1

Band gap = 1240/λnm [2]

CB

VB

Potential (V) hν ≥Eg


8

2.2 Titanium Dioxide

Titanium dioxide (TiO2) – also known as titania – exists in at least three crystal

modifications: anatase, brookite and rutile.8 The most commonly occurring

polymorph is rutile. Commercially, titania is used as a white pigment for cosmetics,

plastics, paint and sunscreen.8-9 Much of the interest in titania is on its application

in the photocatalytic purification of water and air, its ability to produce hydrogen by

water splitting and its behavior as a disinfectant of biological

microorganisms.10,11,12 Table 2.2, lists some potential applications of TiO2, where

each of these is influenced by one of its properties.13

Table 2.2: Applications of TiO213

Property Category Application

Self-cleaning Buildings and roads Aluminium panels, tiles and windows

Antifogging Optical instruments and vehicles

Tunnel lights, tunnel walls, optical lenses, exterior surfaces windows and headlights

Air cleaning Indoor and outdoor air cleanliness

Room air-cleaner, air conditioner, roadways & concrete for highways

Water purifying Drinking water River water, ground water, lakes and fish feeding tanks

Self-sterilizing Hospitals Hospital garments, ICU walls

Anatase and rutile are the mostly used polymorphs of titania especially in water

and air purification, while brookite is not fully explored.14 Anatase and rutile

structures (Figure 2.2) are described in terms of TiO6 octahedra chains. Each


9

titanium atom is surrounded by six oxygen atoms and three titanium atoms about

each oxygen atom. The octahedron in rutile is not regular and shows a slight

orthorhombic distortion (Figure 2.2 (b)), while a significant distortion is observed in

anatase (Figure 2.2 (a)). 6,15,16

Figure 2.2: Structures of (a) anatase and (b) rutile15

The Ti-Ti bond distances are 3.79 and 3.04 Å in anatase, as compared to 3.57 and

2.96 Å in rutile. On the other hand, the Ti-O bond distances are far shorter for

(a)

(b)


10

anatase (1.934 – 1.980 Å) than rutile (1.949 – 1.980 Å).6,15 In the rutile structure,

each octahedron is in contact with ten neighbor octahedra (two sharing edge

oxygen pairs and eight sharing corner oxygen atoms) while in the anatase

structure each octahedron is in contact with eight neighbors (four sharing an edge

and four sharing a corner).15,16 Brookite (Figure 2.3) the other form of TiO2 is

orthorhombic, which has axial ratios of 0.8416:1:0.9444, when compared to the

tetragonal forms of anatase and rutile.17

Figure 2.3: Structure of brookite17

In the brookite structure, the titanium ion is at the center while the oxygen ions are

at the corners of these octahedra. These octahedra share edges and corners with

other crystals stoichiometrically.15,17 The distance between the Ti-O bond is

constant throughout the crystal (1.95 -1.96 Å), while the edges are shortened to

2.50 Å.15,17 A summary of the polymorphs of TiO2 crystalline and physical

properties are presented in Table 2.3 and 2.4.


11

Table 2.3: Crystalline parameters of TiO2 polymorphs

Parameter Anatase18 Brookite19 Rutile20

Crystal system Tetragonal Orthorhombic Tetragonal

Point group 4/mmm mmm 4/mmm

Space group I41/amd Pbca P42/mmm

Z 4 8 2

Unit cell:

a/Å 3.7845 9.1841 4.5937

b/Å 3.7845 5.4471 4.5937

c/Å 9.5143 5.1450 2.9587

Volume of unit cell/Å3 136.3 257.4 62.4

Table 2.4: Physical properties of TiO2 polymorphs

Physical property @ 298.15 K

Anatase21 Brookite21 Rutile21

∆H°f/kJ.mol-1 -224.6 -941.8 -944.7

∆S°f/kJ.mol-1 49.92 51.56 50.33

∆G°f/kJ.mol-1 -884.5 -886.5 -889.5

C°/J.K-1.mol-1 55.48 53.98 55.02

Hardness/Moh 5.0 – 6.0 5.0 – 6.2 5.0 – 6.5

Density/g.cm-3 3.84 4.26 4.26

Band gap/eV 3.2 2.1-3.54 3.0

Refractive index 2.554 – 2.493 2.583,2.586, 2.791 2.616, 2.903

Mp/K 1820 1825 1830-1850

Bp/K 2500 – 3330 2500 – 3325 2500 – 3000

Solubility H2SO4, alkaline solution

H2SO4, alkaline solution

H2SO4, alkaline solution


12

Anatase and rutile are the only two polymorphic forms of titania known to be

photoactive. Despite the numerous applications of TiO2 as indicated in Table 2.1,

its use in its intrinsic state is rather limited. The limitations of titania are mainly due

to the following reasons:

a) The fast recombination of the generated photoholes and photoelectrons

from the metal oxide. Every recombination results in the release of energy

in the form of unproductive heat and the subsequent loss of the holes and

electrons. The different fate of the produced holes and electrons upon

excitation by sufficient energy is depicted by Figure 2.4. Routes (a) and (b) show the recombination of the photogenerated species on the metal oxide.

Route (c) shows an electron acceptor specie adsorbed on the surface of the

metal oxide, thereby becoming reduced by the migrating electron. Route (d) depicts the oxidation of an adsorbed specie on the surface of the metal

oxide particle by a photogenerated hole.

Figure 2.4: Plausible pathways followed by holes and electrons.6,22

b) The large band gap of TiO2 (3.0/3.2 eV) only allows the ultra violet portion

(< 380 nm) of the solar spectrum to initiate photoexcitation. The solar


13

spectrum is mainly composed of about 95% visible light, which doesn‟t have

enough energy to excite titania to be catalytically active (Figure 2.5).11,23

Figure 2.5: Absorption of solar spectrum against band gap of titania.6

Many research groups have worked on improving the surface of titania, so as to

minimize the above mentioned limitations. Through surface and bulk doping, the

phase transformation of titania (i.e. from anatase to rutile) has been stabilized, the

optical band bap has been altered and the ionic/electric conductivity of titania has

been improved.24,25 Primarily there are four ways currently used to overcome the

limitations of TiO2. These include doping with metal ions,26,27 coupling with another

semiconductor,28,29 sensitizing by dyes,30 and doping with non-metals. 31,32,33

2.2.1 Surface Perturbation

2.2.1.1 Co-deposition of Metals

The surface of TiO2 can be modified by metal deposits such as Au, Ag or

Pt.34,35,36,37 Contact between the semiconductor and metal, involves a redistribution

of electric charges and the formation of a double layer, resulting in a space

charge.6 The barrier formed at the metal and semiconductor interface is called the

Schottky barrier. The metal deposited on the semiconductor acts a sink for


14

photogenerated electrons, mediating the electrons away from the TiO2 surface and

thus minimizing electron-hole recombination.6,11

The deposited metal has a higher work function (Φm) than the semiconductor (Φs),

with which it is in contact, as illustrated by Figure 2.6. Loadings of Pt and Au have

been reported to be more effective than Pd, because of their suitable work function

and electron affinity.38

Figure 2.6: A representation of the Schottky barrier

Care needs to be taken that the metal loading on the surface of titania has to be at

an optimum level, otherwise if excess metal is loaded it in turn acts as a centre for

electron/hole recombination.11 The high costs of noble metals hinders the

practicality of noble-metal doped TiO2, hence low-cost yet effective metals are

being investigated. These include transition metals ions (Fe3+, Cr3+, V3+, Mo5+) and

rare earth metal ions (Y3+, La3+ Os3+).39 Metal ions minimize recombination by

being efficient electron and hole trappers as depicted by the process below:39,40

m

s

Ef

CB

semiconductor - n typemetal

Ef

Ef

metal (eg Pt)Schottkybarrier

hv

TiO2


15

Mn+ + e-cb → M(n-1)+ (electron trap)

Mn+ + h+vb → M(n+1)+ (hole trap)

The Mn+/M(n-1)+ couple lies below the conduction band edge, thereby acting as an

electron sink; while the Mn+/M(n+1)+ couple is above the valance band edge trapping

the holes formed.41 The introduction of such impurities on the metal oxide, also

improves the optical responsiveness to the visible region through a charge transfer

between a dopant and the CB (or VB) or through a d-d transition in the crystal

field.42

2.2.1.2 Anionic Doping

Non-metal doped TiO2 nanomaterials have been regarded as third generation

photocatalysts after pure TiO2 nanomaterials and metal-doped TiO2 nanomaterials

respectively.6,43 A minimum level of TiO2 doping with non-metals such as B,43 C,44

N,45 F,46 ,S,47 Cl,48 Br48 and I49 is required. The advantage of this is that anion

doping results in the absorption edge of TiO2 being red-shifted to wavelengths

longer than 400 nm. Unlike metal dopants, non-metals do not act as charge

carriers thus do not act as recombination centres.

In 2002 Khan et al for the first time reported water splitting of carbon doped TiO2.50

The addition of carbon reduced the band gap from 3.2 eV to 2.32 eV. The authors

used different carbon sources, glycine, hexamethylene tetramine and

oxalydihydrazine and all dopants showed improved visible activity. The visible

activity was due to the carbide ion substituting oxygen, which forms an isolated

electronic state above the O 2p level. In their study the degradation of methylene

blue (MB) was higher for carbon doped TiO2, using glycine as a carbon source

compared to the other sources, under solar irradiation. The increased degradation

of MB was attributed to the dense surface hydroxyl groups, high surface area,

greater acidic sites and high crystallinity.51 In similar work Kisch and co-workers,

used a variety of alcohols as carbon precursors, where they showed that a highly

condensed coke-like carbonaceous species consequently embedded in the TiO2

which was most likely responsible for visible light sensitization. They reported that


16

maximum photocatalytic activity was feasible at a calcination temperature of less

than 250°C, as compared to 400°C where the catalyst was inactive against 4-

chloro phenol (CP).52

Asahi and co-workers reported in Science, that nitrogen doped titania showed

significant catalytic activity under visible light irradiation.53 The authors claimed

that the red-shift could be attributed to the narrowing of the band gap, by the

mixing of the N 2p and the O 2p states. Ihara et al, proposed that the visible light

activity was due to the formation of oxygen vacancies which it is believed nitrogen

atoms stabilized.54,55 Serpone argued that the red-shift in titania, when doped with

non-metals, resulted from the formation of colour centres (Ti3+, F, F+, F++) that

absorbed visible light radiation.56 The doping of TiO2 with non-metals and their

effects on the band gap and electronic structure is depicted by Figure 2.7. In

Figure 2.7, (a) represents pristine TiO2 with a band gap of 3.2 eV, while (b) represents TiO2 doped with localized dopant levels near the VB and CB. On the

other hand (c) represents the band gap narrowing due to the broadening of the

VB, as (d) represents the localized dopant levels and electronic transitions to the

CB. Finally (e) represents the electronic transitions from localised levels near the

VB to their corresponding excited states for Ti3+ and F+ centres.53

Figure 2.7: Proposed schemes of band gap narrowing of TiO2 by doping with non-metals.53

Surface fluorination on TiO2 (F-TiO2) largely depends on the pH being between

two and three to be more successful.57,58 Minero et al, were the first to report

enhanced photoactivity of F-TiO2 for the oxidation of phenol, as compared to

unfluorinated TiO2.57 This was believed to have occurred due to an increased

production of hydroxyl radicals and was then confirmed by using DMPO – spin

trap ESR technique. Furthermore, Yu et al used a photoluminescence technique to

confirm the increased production of hydroxyl radicals through the hydroxylation of

terapthalic acid.59


17

Xu and co-workers have proposed a different mechanism for the excess of free

hydroxyl radicals in F-TiO2.60 They have suggested that the fluoride ions, present

in the Helmholtz layer, promoted the desorption of surface-bound hydroxyl radicals

when TiO2 was irradiated in solution, as depicted in Figure 2.8. Accordingly they

proposed that an increased concentration of fluoride ions in the Helmholtz layer

led to an increase in the desorption of hydroxyl radicals. Likewise their mechanism

proposed that the solution should be maintained at an acidic pH in order for the

desorption of hydroxyl radicals to be highly favoured.60

Figure 2.8: Plausible mechanism for the fluoride induced enhancement in the production of free hydroxyl radicals in bulk solution from irradiated TiO2.60

Tang and co-workers showed that the hydrophilicity of F-TiO2 was improved when

compared to undoped TiO2.61 Here it was suggested that the electronegative

fluorine atom withdrew the electron cloud away from the Ti-O bond and thus

weakened it. This they proposed resulted in the excess availability of oxygen

vacancies under illumination, that in-turn enhanced the hydrophilicity of F-TiO2.

On the other hand, phosphorous doped TiO2 (P-TiO2), using H3PO4 as the source

of dopand, showed enhanced catalytic oxidation of n-pentane in air.62 Here it was

shown that this increased activity was due to the existence of Ti ions that were

tetrahedrally coordinated. These tetrahedrally coordinated Ti ions were thought to


18

enhance greater adsorption of oxygen and water molecules from the air, thereby

having produced more hydroxyl radicals. This tetrahedral configuration was most

likely responsible for having hindered the recombination of electrons and holes via

stabilization, as they are known to act as hole trappers.62

Similarly iodine doped TiO2 or I-TiO2, due the presence of abundant surface

states, has shown enhanced catalytic activity in the degradation of organic

pollutants, as has been confirmed by PL spectral studies.63 However, work carried

by Liu et al has shown that higher concentrations of water molecules and hydroxyl

groups on such catalysts have decreased their activity.63 64 This result was

contrary to that seen in P-TiO2 and has indicated that the presence of surface

hydroxyl groups does not always result in holes having been trapped and hydroxyl

radicals having been generated. When I-TiO2 was prepared by anodization at

20 V, using KI as an electrolyte, it showed greater methylene orange degradation

under visible light due to the presence of iodine in its multivalency state (I7+, I5+

and I-).65 Theoretical calculations indicated that four I 5p bands appeared within

the band gap states for I (cation) –TiO2 while only two I 5p states (deeply trapped)

were present close to the CB for I (anion)–TiO2.

2.2.1.3 Composite Semiconductors

One major limitation of intrinsic TiO2 is the fast recombination of the

photogenerated charges. Another approach of achieving better charge separation,

involves coupling of two semiconductor particles with different energy levels.

Examples include: CdS-TiO2,66 TiO2-SnO2,67 TiO2-SiO2 68, TiO2-ZrO2 and

CNTs-TiO269 For instance, semiconductor particles having a large bandgap and an

energetically low-lying conduction band (e.g. TiO2 or ZnO) can be combined with

particles having a small bandgap and an energetically high lying conduction band

e.g. CdS or WO3.70 When the two types of particles (as mentioned above) are

combined, light absorption at long (>366 nm ) wavelengths results in efficient

electron transfer to the large bandgap particle, while the holes migrate in the

opposite direction from the holes to the small bandgap particle as illustrated by

Figure 2.9.


19

Figure 2.9: Electron and hole separation in a coupled semiconductor system

Photocatalytic activity in coupled systems have been reported to have been

increased when compared to the individual performance of the semiconductor.

This has been attributed to efficient charge separation in these systems.55,57

Yu et al were the first to report CNTs-TiO2 composites in photocatalysis.71 These

CNTs-TiO2 composites were prepared by an ultrasound technique and were

monitored for their physicochemical properties and photocatalytic activity for the

oxidation of acetone in air. Here they reported a significant photodegradation of

acetone by the CNTs-TiO2 composite as compared to P25 and AC-TiO2 (activated

carbon). Furthermore these authors attributed the enhanced photocatalytic activity

of TiO2 by CNTs to an increased migration rate of electrons through CNTs.72,73 PL

spectra of these composites confirmed that an increased electron mobility

suppressed the recombination of the e-/h+ pairs. Similarly they also proposed that

this enhanced photocatalytic activity was due the increased adsorption of hydroxyl

groups (hole trappers) on the surface of CNTs-TiO2, leading to an increased

production of hydroxyl radicals. The production of these hydroxyl radicals was

confirmed by EPR spectra of the DMPO-•OH spin adducts.71,74 However, they also

found that higher loadings of CNTs in such composites hindered their

photocatalytic activity. They suggested that the higher loadings of CNTs hindered

VB

CB CB

VB

hν

CdS TiO2


20

the absorption of UV light, which in turn led to the decrease in the production of

electrons and holes.

Improving on previous approaches, Gao and co-workers developed a novel

surfactant wrapping technique that produced uniformly dispersed TiO2 on the

surface of CNTs via a modified sol-gel method.69 Here the TiO2 film, when

exposed to UV irradiation, significantly decomposed MB under ambient conditions.

The efficiency of this composite was attributed to an improved separation of photo-

generated-electron/hole pairs at the CNTs-TiO2 interface.

2.2.1.4 Dye Sensitization

When a photocurrent is generated with photons less than that of the

semiconductors band gap (Eg ≤ hν), the process is known as sensitization and the

light absorbing dyes are called sensitizers.75 An efficient photosensitizer should

have a) an intense absorption in the visible region, b) be strongly physisorped or

chemisorped on the surface of the semiconductor and c) must efficiently inject

electrons into the conduction band of the metal oxide.6,11,76

Common organic dyes which have been used as sensitizers include, porphyrins

(Figure 2.10a) carboxylated derivatives of anthracene, and transition metal

complexes such as metalloporphyrins (Figure 2.10b) and polypyridine

complexes.30,59,77,78,79,80 The metal centres for the dyes have included: Ru2+, Zn2+,

Mg2+, Fe3+ and Al3+, while the ligands were nitrogen heterocyclics with delocalized

π or aromatic ring systems.60


21

Figure 2.10 (a) Structure of porphyrin, (b) Structure of metalloporphyrins

Organic dyes have been linked to the semiconductor by various interactions which

have included:

electrostatic interactions, via ion exchange, ion pairing or donor-acceptor

interactions

covalent attachment by directly linking groups of interest or via linking

agents

intermolecular forces, such as hydrogen bonding and van der Waals forces.

Figure 2.11, depicts a metal complex that was attached on a TiO2 nanoparticle by

electrostatic carboxyl interaction (-O-(C=O)-).58,59 The complex was attached to the

metal oxide through an anchoring ligand. The auxiliary ligands were those that

were not directly attached to the surface of the metal oxide and could be used to

tune the spectroscopic response of the dyes.59


22

N OH

O

NRu

L'L

N

N

O

-O

-O

O

TiO2

L, L'

N

N

CN

N

hv

e-e-

e-

=

Figure 2.11: Organic dye (cis – [Ru (dcbH2)LL’ attached on TiO2. 59

Organic dyes when attached to TiO2 particles have caused a red-shift in the

adsorption edge of the metal oxide. However, most dyes are toxic and unstable in

aqueous solutions, thus making them unsuitable for application in photocatalytic

reactions.49 Furthermore; the photodegradation of the dye may lead to the

formation of stable intermediates which are more harmful than the parent dye. 45,81

2.3 Synthesis of Titanium Dioxide Nanoparticles

Properties influencing the photocatalytic behaviour of titania nanoparticles are

surface area, crystallite size, morphology and crystal structure (phase).82 All these

physical properties can be manipulated by different synthetic methods of TiO2.

Titania can be prepared by the sol-gel method,65,83,84,85,86,87,88,89,90,91 the

hydrothermal method,92,93,94,95,96 the solvothermal method,97,98,99,100,101 the direct


23

oxidation method,102,103,104,105 by chemical and physical vapour

deposition,106,107,108,109 and by microwave.110,111,112

2.3.1 Sol-Gel Method

A typical sol-gel process involves the hydrolysis and polymerization of a metal

alkoxide (organic) or metal salt (inorganic) resulting in a colloidal suspension.

Metal alkoxide (metalorganic compounds) are often used as precursors in the sol-

gel process because they easily react with water (hydrolysis). Table 2.5 lists some

commonly used alkoxides in the sol-gel method. When the attractive dispersion

forces which are present in the sol during nucleation result in a giant network of

microscopic dimensions, then a gel is formed. Gels are amorphous after drying,

but crystallize when heated.

Table 2.5: Common Alkoxides

Alkoxy Molecular formula

Methoxy OCH3

Ethoxy OCH2CH3

n-propoxy O(CH2)2CH3

iso-propoxy CH3(O)CHCH3

n-butoxy O(CH2)3CH3

sec –butoxy H3C(O)CHCH2CH3

iso-butoxy OCH2CH(CH3)2

tert-butoxy OC(CH3)3

Livage et al, described the sol-gel synthesis using the partial – charge model as

hydrolysis and condensation reactions which can either be oxolation or olation.67


24

Hydrolysis

During hydrolysis the hydroxyl ion becomes attached to the metal atom (M) as

illustrated by equation (3). Depending on the amount of water and catalyst present

the hydrolysis reaction can be partial (3) or go to completion (4).66-67

M(OR)n + nH2O HO M OR (3)n + ROH

M(OR)n + nH2O (4)+ ROHM(OH)n

(OR) : alkoxide

n

Hydrolysis is influenced by the charge (z), the coordination number (N), the

electronegativity (x°m) of the metal and the pH of the solution.

Condensation

Condensation is a reaction in which molecules are joined together through the

intermolecular elimination of water or an alcohol. Condensation can take place

either by oxolation or olation (see below for further description of these two

processes). Furthermore, condensation can be influenced by two nucleophilic

mechanisms: substitution (SN) and addition (AN), both of which are determined by

the coordination number of the metal. The substituent with the largest partial

negative charge (δ-) is the nucleophile, while the substituent with largest partial

positive charge (δ+) is the leaving group. Nucleophilic substitution (SN) is initiated

when the preferred coordination is satisfied (5). However, when the preferred

coordination is not satisfied, condensation occurs by nucleophilic addition (AN) (6).


25

M OH XO M M O M

H

X = R, H

(5)

M OH XO M

M O M

H

OX (6)

OX

Oxolation

Oxolation is a condensation reaction in which an oxo bridge (–O–) is formed

between two metals centres as depicted by (7).68 Oxolation proceeds via

nucleophilic addition (AN) in unsaturated metal coordination.66

H2O M OH OH M OH2 M

O

O

M 2H2O

+2H+ (7)

However, for a coordinatively saturated metal, a SN reaction is possible via a two-

step mechanism as shown in (8) and (9).66


26

M

OH

+ M OH M O M OH

H

(8)

M O M OH

H

M O M + H2O (9)

Olation

Olation condensation is preferred to oxolation when the precursors contain good

water leaving groups. Olation can either be base or acid catalysed.68 In base

catalysis, the base acts as a nucleophile and attacks the partially charged

hydrogen to form water and a metal alkoxide (a stronger nucleophile) (10). This

alkoxide then attacks the partially positively charged end of a second metal

alkoxide, prompting its hydroxide to leave the molecule and depart with its bonding

electrons (11).

M OH + -OH M O- + H2O (10)

M O- + M OH M O M + -OH (11)

In an acid catalyzed reaction, the acid protonates the hydroxide end of the metal

alkoxide the forming an unstable positively charged metal centre (12). Due to the

instability of the complex a water molecule is released, making the hydrogen

highly acidic and prone to nucleophilic attack (by water), forming the hydronium

ion (13), leaving a positive charge behind the metal to propagate the reaction.


27

M O M OH

H

M O M + H2OH3O+

OH2

(12)

H

M O M

OH2H

1. -H2OM O M + H3O+ (13)

2. H3O+

The advantages of the sol-gel method over other techniques is the control it offers

over the particle size (Figure 2.12a –b) and the shape of nanoparticles. It is a

relatively cheap method compared to other techniques and works at low

temperatures. However, because ultra-fine particles are produced, (sol)

agglomeration (Figure 2.12 b- c) is a major limitation. To minimize agglomeration

reverse micelle and micro-emulsion techniques have been used.70

Micro-emulsion is a thermodynamically stable dispersion of two immiscible fluids.

The system is stabilized by adding one or more surfactants. Different kinds of

micro-emulsions are known and these include (a) water-in-oil (w/o) (b) oil-in-water

(o/w) and (c) water-in-super critical (sc)-CO2 (w/sc-CO2).113 Reverse micelles

result from the self assembly of surfactants in apolar solvents. Metal oxide

nanocomposites are prepared by mixing an oil-soluble metal alkoxide M(OR)x, or

an anhydrous reverse micelle solution containing the alkoxide in the oil phase,

which results in the hydrolysis and condensation of the alkoxide nanoparticles of

metal hydroxide and oxides.114 The advantage of the reverse micelle method is

that it acts as an ideal nanoreactor, providing a suitable environment for controlled

nucleation and growth. Steric stabilization provided by the surfactant layer

prevents the nanoparticles from aggregating.


28

Figure 2.12: TEM images showing sol-gel prepared titania particles (a) and (b) uniform distribution of titanium nanoparticles,70 (c) and (d) reveal agglomerated nanoparticles. 71

2.3.2 Hydrothermal Method

Hydrothermal synthesis is another example of a wet-chemical method in which

titania nanoparticles can be prepared in a heated aqueous medium or a heated

non-aqueous medium in which water is added as a reactant.60 This method

represents an alternative route to the sol-gel technique as it requires mild

calcination temperatures to promote crystallization. Indeed, it is during heat-


29

treatment in the sol-gel technique that agglomeration occurs (as previously

mentioned). At low calcination temperatures, through the hydrothermal synthesis,

nanoparticles can be prepared crystalline, uniform in size and with controlled

morphologies.77

Figure 2.13: SEM images of TiO2 nanoparticles prepared by the hydrothermal method.79

2.3.3 Solvothermal Method

The solvothermal method is very similar to the hydrothermal technique of making

nanoparticles. However, one primary difference is that a non-aqueous solvent is

used in this technique.60 Consequently the operating temperatures for such

syntheses can be higher than the hydrothermal method (depending on the organic

solvents used).80 Metal alkoxides readily react with carboxylic acids to form mixed

alkoxy carboxylates (14).81 An oxo bridge could be formed during a condensation

reaction between the two functional groups bonded to the two metal centres, in the

process eliminating an ester (15).81

M OR +H O

O

R' M O C

O

R'

4-x x

xROH+

(14)


30

M OR+M O C

O

R' M O M + ROCR'

O

(15)

The advantages of the solvothermal method are the high crystallinity and near

mono sized/shaped nanoparticles that form, which can easily be dispersed in

organic-solvents (as depicted by Figure 2.14).

Figure 2.14: TEM images showing nanocrystals prepared by the solvothermal method.80

2.3.4 Chemical and Physical Vapour Deposition

Vapour deposition processes takes place in a vacuum chamber. When there is no

chemical reaction, the process is known to be physical vapour deposition (PVD).60


31

An horizontal stainless tube reactor is placed at the central part of the furnace and

is heated by a resistive heater up to 1000°C. In chemical vapour deposition

processes (CVD), the precursor is usually a solution of a metal alkoxide which is

bubbled into a fine vapour using a frequency of between 40 kHz and 300 kHz. The

formed vapour is carried over to the reaction chamber by argon or nitrogen,

controlled by a mass flow controller set to between 20 – 70 standard cubic

centimetres (sccm). Oxygen is also mixed in with the carrier gas to oxidize the as-

formed titanium to its oxide. The scheme for this is shown in Figure 2.15a.

When preparing titanium dioxide nanoparticles by PVC, a pure solid metal is

placed on a quartz boat, in a tube furnace, a few millimetres away from the

substrate. Similar to the set-up in CVD, here the temperature is increased to about

1000°C and the carrier gas (N2 or Ar) and oxygen is infused to the mixture to

oxidize the titanium nanoparticles as depicted by Figure 2.15b.

Figure 2.15 (a). Scheme showing the set-up for the synthesis of titanium nanoparticles by CVD 89

(a)


32

Figure 2.15 (b). Scheme showing the set-up for the synthesis of titanium nanowires by PVC92.

2.4 Pollutants

Water systems are remarkably efficient, for they naturally can purify and renew

themselves, either by sedimentation, or by diluting pollutants to less harmful

concentrations.115 The natural process, however, is lengthy and very difficult to

perform efficiently when larger quantities are continually added to the system.

Pollution sources can be classified as either point sources or non-point sources.96

Point sources are easily identifiable locations of pollution. Examples are factories,

wastewater treatment facilities and sewage leaks. Non-point systems, however,

are not easily identifiable as to their origin. These include run-off sediments,

fertilizers, chemicals and animal wastes from farms.116

Pollutants can be broadly divided into three major categories. The first of these

categories are the inorganic pollutants, such as acids, salts, toxic metals, nutrients

and radioactive materials. The second are organic pollutants, which are pesticides,

plastics, detergents, animal manure and dyes. Lastly, there are microbial

pollutants such as bacteria, parasites and viruses.

(b)

(b)


33

Organic pollutants, particularly dyes, are a concern because of their persistence

and stability in water. Colour is one obvious visible indicator of wastewater

pollution.117 The presence of synthetic dyes in waters even as low as 1 mg.L-1 can

cause visible colouration of water. If synthetic dyes are present in sufficient

quantities in a water system, they shield light penetration, hinder photosynthetic

activity, slow down the growth of biota and have an affinity to chelate with metals

resulting in micro-toxicity of fish and other organisms. 101

It was estimated in 2008 that the worldwide production of dyestuff was over

7.0 x 105 tons, with a market value over US$10 billion.118 There are over 1000

companies and organizations related to textiles and clothing in South Africa

alone.119 Dyes are used in many fields involving numerous branches in the textile

industry,120,121 leather tanning industry,122,123 paper production,124 food

technology,125,126 agricultural research,127,128 light-harvesting arrays,129

photoelectrochemical cells130 and in hair colourings.131 Due to the high demand

and extensive application of synthetic dyes, proper disposal and treatment of dye

infested effluent is necessary. Synthetic dyes are toxic, non-biodegradable and

form recalcitrant compounds.132 However, due to thousands of different organic

dye compounds, research findings on their carcinogenic, mutagenic and

bactericidal properties are not conclusive. Conventional water treatment methods

have limitations dealing with organic dyes and need to be improved in order to

cope with the growing problem that dye effluents pose. Hence, much research

interest is being focused on developing advanced technologies that would

decolourize and degrade dye effluent. A summary of classical and recent

technologies used to remove pollutants in water is illustrated by Figure 2.16.

Traditional physico-chemical treatments include inorganic (alumina, carbon-based

materials – coal, fly ash, activated carbon and wollastonite) and organic (bagasse)

supports.133,134 One drawback of adsorption methods is their lack of sensitivity.

This is in part due to competition amongst the adsorption sites with dye and other

components in the effluent. Another limitation of adsorption methods is that they

only remove and concentrate the dye on the surface, while the structure of the dye


34

remains unchanged. Other physico-chemical methods are coagulation, filtration,

Figure 2.16: Flow-chat illustrating the different methods used in the removal of dye from wastewater.99

ion exchange and adsorptive bubble separation techniques (ion floatation, solvent

sublation and adsorption flotation).135,136,137


35

Chemical methods are more powerful than physical ones and have improved

oxidative and degradative procedures for organic compounds, using ozonation

and oxidation with the hypochlorite ion.138 Combinations of catalytic and

photochemical methods are known as advanced oxidation processes (AOPs) (see

Figure 2.16). AOPs are combinations of: TiO2/H2O2, H2O2/UV, O3/UV, O3/H2O2/UV

and the Fenton reagent (Fe2+/H2O2).98,119,139,140,141 AOPs, primarily rely on the

production of organic radicals and hydroxyl radicals upon photolysis. The

generated radicals are trapped by dissolved oxygen, leading to the production of

very powerful radicals and peroxides.119,120 These radicals improve and hasten the

decolourization and degradation, ultimately mineralizing the dye.122 However,

these methods are generally expensive and might also lead to an incomplete

degradation of the pollutant, producing even more toxic and stable

intermediates.120

The application of microorganisms for the biodegradation and decolourization of

dyes offers significant advantages. The process is economically favourable

compared to the previous methods and the end products are less toxic.113

However, a majority of dye compounds are stable and resistant to microbiological

attack. Microbial biodegeneration and decolourization methods use: mixed

bacterial cultures (either of anaerobic or aerobic kinds),142 pure cultures using

white-rot fungus or bacteria143 or activated sludge which contain both bacteria and

protozoans.144

Lastly, electrochemical (EC) methods have been used to degrade pollutants

(organic and inorganic). EC methods are electrocoagulation, electrochemical

reduction, electrochemical oxidation, indirect electrooxidation with strong oxidants

and photoassisted electrochemical methods. The advantages of EC methods

are99:

rapid separation of organic matter

small amounts of chemicals needed

pH control not necessary except for extreme values


36

reduction in sludge production

relatively inexpensive techniques.

The disadvantages of EC methods are99:

anode passivation, caused by sludge deposition on these electrodes, which

inhibits the electrolytic process

high concentrations of iron and aluminium ions in the effluent need to be

removed.

2.5 Dyes

Dyes are organic molecules that have a chromophore, (an aromatic structure that

absorbs visible light) and an auxochrome (a group that anchors the dye) which

intensifies the dye colour. Wastewater containing dyes may contain other

auxiliaries like salts, heavy metals, dispersing as well as soothing agents and/or

surfactants. This presents an even greater challenge in the treatment of

wastewater.

Dyes can be broadly classified into three categories. The first is anionic dyes.

These are negatively charged, acidic and reactive. Reactive dyes are brightly

coloured and water soluble, while acidic dyes are more problematic as they pass

through conventional treatment techniques unaffected. The second category is

cationic dyes, which are basic and positively charged. Lastly, is the category of

non-ionic dyes, which are also known as disperse dyes. These dyes don‟t ionize in

an aqueous medium and are deemed as potential carcinogens.

The common chromophores in dyes are azo groups, triphenylmethane dyes,

xanthenes, pthalocyanine, anthraquinone and indigoide. Figure 2.17 illustrates

several classes of dyes and their structure.


37

Antraquinone Dyes

Figure 2.17 Classes and structures of synthetic dye

BrBr

Br

O

BrBr

SO3N

CH3H3C

NCH3

CH3

NCH3CH3

S O

O

O

OH

OHPhenol red

Crystal violet

Bromophenol blue

O

O

SO3- Na+

OH

OH

O

O

NH2

NH

SO3H

NH

N

N Cl

N

ClMordant red 3

Reactive blue 4


38

Azo Dyes

Figure 2.17 Classes and structures of synthetic dye

NN

NN

H2N SO-3

NH2

Na+

C

N

N

N(CH3)3

OOH S

N

N

N(CH3)3

OO

OH

p-Methyl red

Congo red

Methyl orange


39

2.6 Carbon Nanotubes

The science of carbon is an ever evolving field, even in this 21st century, new and

fascinating discoveries are made about carbon. Hence, it is still appropriate that

some researchers refer to carbon as an “old but new material”.145 Carbon has

been known to exist in two naturally occurring crystalline forms or allotropes which

are three and two dimensional respectively i.e. diamond and graphite.146 However,

in the mid 80‟s a „third‟ form of closed caged carbon molecules was discovered by

O‟Brien and his co-workers and named fullerenes.147 Figure 2.18 summarises the

allotropic nature of carbon.

Figure 2.18: Different allotropes of carbon127


40

Carbon atoms exist in three hybridization states, sp3, sp2 and sp, corresponding to

3D carbon (i.e. diamond), 2D planar carbon (i.e. planar graphite) and 1D chain-like

carbon (i.e. carbyne) respectively.127 Fullerenes and their derivatives are 0D,

connected by directional bonds only by van der Waals-type interactions. It became

apparent that the discovery of fullerenes paved the way for the discovery and

synthesis of other forms of carbon, specifically tubular nanostructures.

Since their rediscovery in 1991 by Iijima, carbon nanotubes (CNTs) have become

the subject of intense research activity worldwide.148 Increased interest in CNTs

arose because of their unique and superior electronic, chemical, physical and

mechanical properties.149 Carbon nanotubes are nanostructures that arise from

folding 2D graphene sheet into a cylindrical shape with hollow internal cavities

(Figure 2.18).150 Depending on the number of rolled graphene sheets, CNTs can

be classified as single walled (one graphene) nanotubes or multiwalled (two or

more) nanotubes.

Figure 2.18: Roll-up of a graphene sheet to form a nanotube.151

Nanostructured materials can be fabricated by two strategies, either by the top-

down approach which is employed primarily by the nanophysics discipline or by

the bottom-up method which is used by the nanochemistry discipline.152 Synthetic

routes of CNTs can be broadly grouped into the following: arc discharge,153,154

chemical vapour deposition (CVD),155,156 and laser ablation.157. The CVD and arc

discharge techniques are most commonly used in CNT synthesis. Both arc

discharge and laser ablation make use of solid-state carbon precursors, which are

vaporised at very high temperatures, to provide the carbon source that is needed


41

for nanotube growth. The CVD technique represents an example of preparing

CNTs by the bottom-up approach as depicted by Figure 2.19.138 In the CVD

technique, hydrocarbon gases or vaporised organic liquids are used as sources for

carbon atoms, while metal catalysts act as “seeds” for nanotubes growth. The

temperatures employed in this technique are much lower compared to that of the

top-down approach used by arc discharge and laser ablation.158

Figure 2.19: A bottom-up approach of CNTs via CVD route.138

Applications of CNTs arise from their exceptional physical properties. For instance

the electrical properties of CNTs have had a significant impact in field emission in

various electronic devices,132 while their geometrical shape has offered precise

and controlled drug delivery systems.138 Because CNTs have extraordinary

mechanical properties such as high tensile strength and stiffness they have been

used in composites as reinforcements and as cantilever tips in lithographic

techniques.159 Due to the high surface areas of CNTs they have also been used in

hydrogen storage devices,160 in water purification161 as an adsorbent and as

supports for catalysts.162


42

In the work presented in this thesis, CNTs were used to harness their exceptional

electronic properties when composited with TiO2. Here it was found that CNTs

acted as efficient transporters of the species that were generated on titania upon

photo-illumination and hence they increased the photoactivity of the catalyst by

minimizing electron/hole recombination. Likewise the high surface area of CNTs

assisted in the removal of the model organic pollutants chosen. Equally the

structural integrity of the nanocomposites were also improved due to the excellent

mechanical properties of CNTs. However, the results of these studies will be fully

discussed in the chapters which follow.


43

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62

CHAPTER 3:

EXPERIMENTAL METHODOLOGY

3.1 Reagents used

Table 3.1 Reagents used

Reagent Supplier Grade of Reagent

Titanium tert-butoxide Sigma-aldrich Used as received

1-Butanol Merk Used as received

Carbon nanotubes Sigma-aldrich Used as received

Ammonia Merk Used as received

Tungstic acid Sigma-aldrich Used as received

Cobalt acetate Sigma-aldrich Used as received

Sodium bromide Riedel-deHaen Used as received

Argon Afrox scientific UHP

Acids

HF, HCl, HBr,HI

Riedel-deHaen Used as received

Chapter 3: Experimental methodology

63

3.2 Synthesis of Photocatalysts

3.2.1 Anionic-doped Titanium Photocatalyst

Pristine titanium dioxide, anionic doped TiO2 and TiO2-carbon nanotubes with and

without anionic dopants (F, Cl, Br and I) were prepared by the sol-gel method. A

slightly modified method as specified by Zhang et al was followed (so as to suit our

desired product).1 Titanium tert-butoxide (10 mL) was added to 1-butanol (52 mL)

under constant stirring for 30 minutes. In the solution, the respective acid (HF,

HCl, HBr and HI) 10 mL of 20 mM was added, each prepared and made to the

mark with 1-butanol. Formic acid (11 mL) was then added to the solution

dropwisely resulting in the formation of a precipitate. The pH of the solution was

maintained at pH 2 to allow the exchange of the hydroxyl group in the gel and

anionic dopants.

The as-formed precipitate was stirred for 2 hours and aged for another 2 hours. In

order to drive out most of the solvent, the precipitate was then oven dried at 80°C

for 3 hours. The dried precipitate was then ground to fine powder and calcined in

air, in a furnace, for 3 hours at a heating ramp of 5°C/min at 400°C.

CNTs were first functionalised before the preparation of nanocomposites with or

without anionic dopants. This was done because the functionalization of pristine

CNTs (f-CNTs) enhanced their solubility and dispersion in organic and aqueous

solvents. At the same time it also improved the linkage between CNTs and TiO2.

This functionalization was achieved by refluxing CNTs in concentrated nitric acid

for 12 hours. The oxidizing power of nitric acid ensured that the tubes were mostly

functionalized with carboxylic groups; yet it minimized the number of defects

introduced on the CNTs. Carboxylic groups have stronger binding power when

compared to alcohols.2,3

CNTs-TiO2 nanocomposites without anionic dopants were initially prepared by

sonicating 0.2% m/m of f-CNTs in 10 mL of 1-butanol for 15 minutes. This mixture

was then added to tert-butoxide (10 mL). Conversely, for nanocomposites

prepared with anionic dopants the f-CNTs were added to solutions of tert-butoxide


64

(10 mL) which had 10 mL of 20 mM of the corresponding acid (HF, HCl, HBr or

HI). Formic acid was added thereafter so that precipitation could occur in a single-

step process. The drying processes were the same as for pristine titania, however,

the calcination process was performed in an inert atmosphere using argon gas at

a flow rate of 10 mLmin-1.

3.2.2 Preparation of Composite Semiconductors

3.2.2.1 CNTs-CoO and CNTs-CoO-TiO2

Functionalized CNTs (1.0 g) were mechanically mixed together with cobalt (II)

acetate (0.4 g) using a pestle and mortar for almost 30 minutes. The mixture was

then placed in a quartz boat in a furnace at 350°C with a ramp rate of 10°Cmin-1

for 3 hours. The mixture was heated in a flowing stream of argon at a flow rate of

60 mLmin-1. The thermal decomposition of the metal acetate coated the wall of

CNTs with the metal oxide (CNTs-CoO).4

The syntheses of CNTs-CoO-TiO2 nanocomposites were achieved by initially

sonicating (0.2% m/m) of CNTs-CoO for 15 minutes in 1- butanol (10 mL) to

disperse the mixture. The mixture was then added to a solution of

tert-butoxide (10 mL) previously “dissolved” in 42 mL of 1-butanol. This mixture

was stirred for 30 minutes at ambient temperature. Thereafter formic acid (11 mL)

was added to the mixture(s) leading to the formation of a grey precipitate. Drying

and calcination processes were the same as described in section 3.2.1.

3.2.2.2 TiWxOy and CNTs-TiWxOy

Tungstic acid (8.00 x 10-4 mol) was dissolved in a solution of 1–butanol (20 mL)

and ammonia (50:50). Titanium tert-butoxide (2.38 x 10-3 mol) was added in a

dropwise fashion to the solution and the as-formed precipitate was stirred for

2 hours and then aged for another 2 hours.5 This precipitate was dried in an oven

at 80°C to drive most off the moisture, then it was ground into fine powder and

calcined in air for 3 hours at 400°C ramped at 5°Cmin-1. Functionalized CNTs

(0.2% m/m) were sonicated for 15 minutes in 10 mL of an ammonia-butanol


65

solution. The dispersed CNTs were then added to a solution of dissolved tungstic

acid (8.00 x 10-4 mol). Thereafter, under constant stirring, titanium tert-butoxide

(2.38 x 10-3 mol) was added dropwise to the mixture. The mixture was then

vigorously stirred for 2 hours. Finally the precipitate was dried and calcined as

described in section 3.2.1.

3.3 Characterization

All samples were characterized by using various microscopic, spectroscopic and

surface area techniques, to elucidate structure, morphology, size distribution and

confirm the nature of the catalyst.

3.3.1 Transmission Electron Microscopy (TEM)

Imaging can be determined by the wavelength of the radiation, while the best

resolution (r) is determined by Rayleigh criterion equation (Equation 1). In

equation 1, λ is the wavelength of the radiation, n is the refractive index of the

viewing medium and α is the semi-angle of collection. In air, n sinα is ~ 1, making r

approximately equal to 0.61 times the wavelength of radiation, as represented by

equation (Equation 2).6,7

r = 0.61λ/nsinα (1)

r = 0.61λ (2)

For an optical microscope, using light with a wavelength with 510 nm, r would be

311 nm, meaning at this wavelength it would not be possible to observe any

isolated nanoparticles. Thus, an alternative radiation with a smaller wavelength

would have to be used to improve the resolvable spacing.

Using the de Broglie equation and the knowledge that particles of matter can

display wave-like behaviour, it is possible to relate the wavelength of these

particles to their momentum by using equation (Equation 3). Here λ is the

wavelength, h is Planck‟s constant and p is the momentum of these particles.


66

Hence this shows that electrons and other particles of matter can be focussed to

form an image. This can be achieved by accelerating electrons, with a large

voltage, where they attain large momenta and consequently extremely small

wavelengths, thus making it possible to view nanoparticles or even atoms,

according to equation (2).5,6

λ = h/p (3)

Therefore by using electron microscopes it is possible to study nanoparticles.

In the characterisation of the photocatalysts synthesised in these studies, small

quantities of these were placed in a glass vial to which about 5 mL of ethanol was

added. Each sample, produced by the methods described earlier, was sonicated

for 15 minutes in an ultrasonic bath. Thereafter, small volumes of the sonicated

samples were pipetted onto a copper grid coated with a polymer and then with

lacy carbon. Transmission electron microscopy (TEM) images were captured by

TECNAI G2 SPIRIT microscope (FEI Company) and JEOL – 3010 microscope

both at an accelerated voltage of 200 kV. Both of the above-mentioned TEMs

were fitted with energy dispersive X-ray analysis (EDX) facilities which were

operated at 15 kV.

Energy dispersive X-ray spectroscopy (which has abbreviations of EDS, EDX and

EDXS), is predominantly used to determine which elements are present in a given

sample. This can be achieved when electrons, from the focused beam in the

microscope, have sufficient energy to collide with different elements in the sample

to dislodge some of their core-shell electrons. When electrons in the outer-shells

of these elements cascade down to replace the core-shell electrons, they release

their energy in the form of X-rays.5 Since the electronic structure of each element

in a sample is unique to that particular element, the energy that its electrons need

to lose to cascade down to their core-shells is also unique to that element. Hence,

by comparing the X-rays that are emitted from the sample to known values, it is

possible to identify each of the elements present in the sample.5,6


67

3.3.2 Scanning Electron Microscopy (SEM)

Alternatively characterisation of samples can be made by SEM, which like TEM is

also an electron microscope technique. However, in SEM the focused beams of

electrons are accelerated at lower voltages than in TEM and samples are required

to be conductive solids. Like as in TEM, SEM can also be coupled with an EDS.

This non-destructive technique reveals various pieces of information about the

sample including: external morphology (texture), chemical structure and the

orientation of materials.

In the characterisation of the various composites (later tested as photocatalysts)

that were synthesised in these studies, small amounts were first sputtered with a

thin layer of carbon to enhance their conductivity. The samples were then mounted

on double sided tape and placed on a SEM stub. SEM images were taken at an

accelerated voltage of 2 kV using Nova NanoSEM 600 (FEI Co., The Netherlands)

and JEOL JSM – 7500F, both equipped with and EDX which was operated at

15 kV.

3.3.3 Raman Spectroscopy

Raman spectroscopy, named after Prof. Sir CV Raman, was the first to

demonstrate the inelastic scattering of a monochromatic excitation source. Raman

spectroscopy is used to identify phases and crystalline polymorphs.7 It is also used

to measure the stress and symmetries of materials that are molecular or

crystalline. Samples can be analysed in all three states of matter at ambient

temperature.6,7

Raman spectra of the photocatalysts synthesised in these studies were collected

with Horiba Jobin-Yvon spectrometer at room temperature. An argon ion (Ar+)

laser was used as the excitation source of 514.5 nm, with a spectral resolution of

2 cm-1. The laser power at the sample was about 3.0 mW, with a collection time of

120 s. Each spectrum was collected in a backscattering configuration which had a

liquid nitrogen cooled charge-coupled device (CCD) detector. The laser beam was


68

focused onto the sample by 100 X magnification objective lens of an Olympus

microscope.

3.3.4 Powder X-ray Diffraction (PXRD)

X-ray diffraction uses X-rays produced in a cathode ray tube by heating a filament

to produce electrons. The electrons produced in this manner are accelerated at a

high voltage (10 – 100 kV) and bombarded into the target sample (which is Cu,

Mo, Cr or Fe), dislodging their inner-shell electrons which produce X-rays. The

X-rays produced this way are then collimated and focused onto the sample and

detector, where the intensity and reflected rays are recorded.7

This non-destructive technique is primarily used to identify unknown crystalline

materials in the fields of: geology, material sciences, chemistry, biology and the

environmental sciences. It is also used to measure the sample purity, determine

the unit cell dimension and average particle diameter.

In the PXRD characterisation of the photocatalysts synthesised in these studies

X-ray scattering patterns were obtained on X‟Pert PANanalytica and Siemens

Siefert 3000TT diffractometers, using the Cu Kα (0.1540 nm) monochromatic

beam. The power settings were 40 kV and 40 mA. Diffraction patterns were

collected with a scan rate of 5°min-1 over the range 20° ≤ 2θ ≤ 60°. The average

crystal sizes, d, were calculated from Scherrer equation (Equation 5) for a given

phase θ, at full width half maximum (FWHM, β) and wavelength (λ) of the X-ray

source.

d = 0.89λ/β cosθ (5)

The X-ray patterns captured were compared to the international centre for Joint

Committee on Powder Diffraction Standards (JCPDS) database, so as to identify

the polymorphs and crystallinity of the materials so formed.


69

3.3.5 Ultraviolet-Vis (UV-Vis) Spectroscopy

UV-Vis spectroscopy is the measurement of the absorption of ultra-violet and

visible light by a sample. The absorbed light causes electronic transitions in the

sample, prompting light to be absorbed or re-emitted by the sample and hence

elucidating the structure of the sample.

Semiconductor crystallites in the diameter ranges of a few nanometers show a

three dimensional size effect in their electronic structure. These quantum size

effects on the band gap absorption energy can be measured by UV-Vis absorption

spectroscopy.7 Thus the band gap of a semiconductor can be calculated by UV-

Vis absorption spectroscopy. The band gaps of the photocatalysts synthesised in

these studies were calculated by using Tauc‟s equation (Equation(s) 6 and 7),

where α is the absorption coefficient and (hν) is the incident photon energy, and Eg

is the band gap.8,9 The constant n accounts for the type of optical transition. In

Tauc‟s equation the values of n could be: ½, 2, 3/2 and 3, which respectively

corresponds to: the allowed direct, allowed indirect, forbidden direct and forbidden

indirect transitions.

αhν ∞ (hν –Eg)n (6)

(αhν)1/n ∞ (hν – Eg) (7)

TiO2 is an example of an indirect semiconductor, hence the band gap was

measured as a plot of (αhν)2 against hν. The exact value of the band gap was

determined by drawing a line that best fit the slope of the absorption edge as well

as drawing another line that extrapolated the background line before the

absorption edge. The intersection of both lines that corresponded to the x-axis was

taken as the band gap. Band gap measurements were carried out on Shimadzu

UV- 2450.


70

3.3.6 Surface Area Measurements

Brunauer, Emmett, Teller (BET) is a surface area analysis named after the

scientists that studied the relationship between the adsorbent and adsorbate.10

Physical adsorption or physisorption of an adsorbent on an adsorbate is

determined by several factors including the temperature, the pressure of the gas,

the interaction between the surface and the gas as well as the surface area of the

adsorbent. The six different interactions between the adsorbate and adsorbent

have been classified by IUPAC as indicated by Figure 3.1

Figure 3.1: Relationship between adsorbed gases compared to pressure12

Type II, IV and VI can be measured by the BET method as the adsorptive –

adsorbent is greater than the adsorptive-adsorbate.10,1112 Typical gases used are

nitrogen, oxygen, carbon dioxide, helium and methane. BET analyses are routinely

performed on catalysts, pharmaceuticals and cements. Typical analysis would

include specific surface area, pore size distribution (pore diameter, pore volume

and pore surface area) and percentage porosity.

The surface areas and pore volume of the nanocomposites synthesised in these

studies were measured by using a Micrometrics TriStar nitrogen adsorption


71

apparatus. Prior to the analysis, the samples were degassed in vacuum at 150°C

for an hour to reach a final pressure of 1 mmHg.

3.3.7 Photocatalytic Measurements

The photocatalytic efficiency of the nanocomposites synthesised in these studies

were measured against model organic pollutants, bromophenol blue (BPB) and

phenol red (PR). The catalysts (0.1 g) were suspended in a solution of each dye

(100 mL, 10 mgL-1). UV and visible light were used to initiate the photocatalytic

activity of the nanocomposites. Prior to the irradiation, the reaction vessel was

magnetically stirred for 30 min in the dark to ensure an adsorption/desorption

equilibrium. The UV and visible radiation experimental setups are schematically

shown in Figure 3.2 (a and b).

UV - lamps@ 366 nm

5 cm

0.1 g catalyst& 100 mL/10mgL-1

Figure 3.2: (a) UV-radiation experimental set-up for the photodegradation of model pollutants

(a)


72

Xenon lamp

Controller

0.1 g catalyst&100 mL/10mgL-1

Figure 3.2: (b) Visible radiation experimental set-up for the photodegradation of model pollutants

The photodegradation of each model organic pollutant was initiated by the UV

lamp (55Watts) was at 366 nm (2 .0 mWcm-2) and the cabin temperature was kept

constant at 45±5°C for all samples (Figure 3.2 (a)). A solar light simulator

equipped with a xenon lamp (300 nm ≤ λ ≤ 800 nm), with a power output of

2.0 mWcm-2 was used in the case of model organic pollutants irradiated by visible

light (Figure 3.2 (b)). The distance between this lamp and the reactor was 10 cm.

A cut-off filter was also installed so as to allow only visible light to the samples,

The cabin material was constructed from a 2 mm thick rectangular (40 cm x 30

cm) cardboard box. This box had a 5 cm circular hole which allowed the visible

light to pass through. An open and close rectangular shuttle was cut out of the

front side of the box in order to allow regular (5 mL) aliquots to be drawn from the

reaction vessel. These aliquots were taken at intervals of 20 min and filtered

through 0.45 µm teflon membranes.

The degradations of BPB and PR were monitored by measuring the absorbances

at λ = 435 nm and λ = 430 nm respectively, as a function of irradiation time, with a

UV-Vis spectrophotometer (Shimadzu UV- 2450). The resulting [Br-] and [SO42-]

(b)


73

ions were measured with an ion chromatograph (IC –Dionex – IC 2000) equipped

with a Dionex ion pac AS18 (2 X 250 mm) column and conductivity detector. The

eluent solution was 30 mM KOH. The photocatalytic degradation was calculated

with the following formulae:

Amount degraded: Ct/C0

where C0 and Ct are the initial and final concentrations of the dyes at t = 0 min and

t = t min.


74

REFERENCES

1 . Zhu J, Zhang J, Chen F, Ino K, Anpo M, High activity TiO2 photocatalyst

prepared by a modified sol-gel method: Characterization and their

photocatalytic activity for the degradation of XRG and X-GL, Catalysis,

(2005) 35, 261 - 268



removal of TCE in water, Journal. Nanoparticle Research, (2010) 12, 449 -

459

3 . Chen YK, Chu A, Cook J, Green MLH, Harris PJF, Heeson R, Humphries

M, Sloan J, Tsang SC, Turner JFC, Synthesis of carbon nanotubes

containing metal oxides and metals of d-block and f-block transition metals

and related studies, Materials Chemistry, (1997) 7, 545 - 549

4 . Lin Y, Watso KA, Fallach MJ, Ghase S, Smith JG, Delozier DM, Cao W,

Crooks RE, Connel JW, Rapid solventless, bulk preparation of metal

nanoparticles decorated carbon nanotubes, Nano, (2009) 3, 871 - 884

5 . Franklyn PJ, Preparation and characterization of novel titanium-tungsten

oxides, Certificate postgraduate studies in chemistry dissertation, University

of Cambridge, (2005)

6 . Christian GD, O‟Reilly JE, Instrumental analysis, Allyn and Bacon, (1986)

2nd ed., Boston, 412

7 . Skoog DA, Holler JF, Nieman TA, Principles of instrumental analysis,

Harcout Brace College Publishers, (1998) 5th ed, Chicago, 272

8 . Slav A, Optical characterization of TiO2-Ge nanocomposite films obtained

by reactive magnetron sputtering, Digest journal of Nanomaterials and


75

Biostructures, (2011) 6, 915 - 920

9 . Pankove JI, Optical processes in semiconductors, Dower Publications

(1971) 1st New York

10 . Brunauer S, Emmett PH, Teller E, Adsorption of gases in multilayers,

Journal of the. American Chemical Society, (1938) 60, 309 - 319

11 . Peigney A, Laurent Ch, Flahaut E, Basca RR, Rousset A, Specific surface

area of carbon nanotubes and bundles of carbon nanotubes, Carbon,

(2001) 39, 507 -514

12 Sing KSW, Everett DH, Haul RAW, Mascou L, Pierotti RA, Rouquerol KT,

Siemieniewska T, Reporting physisorption data for gas/solid systems with

special reference to the determination of surface area and porosity, Pure

and Applied Chemistry, (1985) 5, 603 - 619

76

CHAPTER 4

Photodegradation of triphenylmethane dyes by anionic doped

titania nanocomposites*

* Part of the work presented in this chapter has been reported and published

in Applied Water Science (2011)1, 19 - 24

4.1 Introduction

The prevalence and repercussions of dyes in water is fully described in detail in

Chapter 2. In this chapter various photocatalysts (pristine TiO2 and anionic doped

TiO2) as well as undoped/anionic doped TiO2-nanocomposites were monitored for

their efficiency of decomposing bromophenol blue (BPB) and phenol red (PR)

when exposed to either UV or visible radiation. Likewise, the results of the

characterizations and the subsequent performances of these various

photocatalysts are also discussed in this chapter. All photocatalysts reported

herein were characterized by the use of various microscopic, spectroscopic and

surface area techniques, which were performed to elucidate their structures,

morphologies, size distributions as well as to establish their nature. Likewise the

photodegradation by-products of these reactions were monitored with ion

chromatography, where the liberated sulphates and bromides were quantitatively

measured.

Chapter 4: Results and Discussion

77

4.2 Results

4.2.1 BET Analysis

The surface areas of the various photocatalysts (pristine TiO2, anionic doped TiO2

and undoped/anionic doped TiO2-nanocomposites) were investigated by nitrogen

adsorption measurements. All the TiO2-nanocomposites were found to be of the

type IV kind according to the classification in the IUPAC system, which has a

typical hysteresis loop (H2) that is associated with capillary condensation with

mesopores as depicted in Figure 4.1.1 The interconnected hysteresis loops for all

photocatalyst samples tested spanned a broad range of relative pressures i.e.

from about 0.4 to 1.0. This is in accordance with a diverse mesopore size

distribution associated with intra and inter-aggregation of nanocrystals with non-

uniform size.2 The estimated values of the BET specific areas (SBET) and pore

volumes were tabulated in Table 4.1. Here it was observed that there was little to

no difference in the surface areas of pure TiO2 and anionic doped TiO2, where

both had an average surface area of 175 m2g-1. Undoped or anionic doped TiO2-

nanocomposites appeared to have a maximum surface area of 188 m2g-1, which

was larger than that of undoped or anionic doped TiO2, even though the surface

area of CNTs in this study was found to be 260 m2g-1. The relatively low loadings

of CNTs (0.2% m/m) in the relevant nanocomposites was used to account for

these observations.


78

0.0 0.2 0.4 0.6 0.8 1.0

0

20

40

60

80

100

120

140

160

180

TiO2-F

TiO2

CNTs-TiO2

CNTs-TiO2-F

Ads

orbe

d ga

ses(

cm3 /g

)

Relative pressure (P/P0)

Figure 4.1: Nitrogen sorption isotherm of selected nanocomposites

The following table summarises the properties of the nanocomposites, and these

results are further explored in the paragraphs following the table.


79

Table 4.1 Physical properties of nanocomposites

Sample S(BET)(m2/g) Vp (cm3/g) Band gap (eV)

Crystallite size (nm)

Raman IG/ID

TiO2 175 0.27 3.30 9.62

TiO2-F 174 0.38 3.27 9.63

TiO2-Cl 175 0.23 3.26 9.62

TiO2-Br 175 0.30 3.27 9.62

TiO2-I 176 0.24 2.97 9.65

CNTs 260 1.09 13.27 0.92

CNTs-TiO2 180 0.17 3.28 15.40 0.88

CNTs-TiO2-F 182 0.14 3.27 15.26 0.68

CNTs-TiO2-Cl 185 0.15 3.27 15.40 0.88

CNTs-TiO2-Br 183 0.14 3.26 15.68 0.70

CNTs-TiO2-I 188 0.15 2.98 15.86 0.83


80

4.2.2 Raman Analysis

When the samples were analysed by Raman spectroscopy, (Figure 4.2) four

bands which were characteristic of anatase, 638, 515, 196 and 143 cm-1 were

observed.42 The (A1 + B1)g2 and Eg(3) bands were associated with the stretching of

bent Ti-O-Ti modes, while the Eg(1), Eg2 and B1g(1) bands corresponded to the

deformation of Ti-O-Ti modes.3 The presence of the CNTs and the anionic dopants

did not appear to affect the structure of the TiO2 as all the Raman bands were

conclusive to the presence of anatase. The nanocomposites with CNTs (insert)

showed two further significant bands, the D and G bands at 1356 and 1600 cm-1

respectively. The G peak corresponded to the tangential stretching (E2g) mode of

the highly orientated pyrolytic graphite (HOPG) which was indicative of crystalline

graphitic carbon in CNTs. The D (A1g) peak indicated disorder induced features.42

The IG/ID ratio (Table 4.1), which measures the defects in/on CNT structures and

determines their level of crystallinity was calculated to be between 0.70 - 0.83,.4

An IG/ID ratio of less than 0.5 indicates severe structural defects on the walls of the

CNTs.4 Crystalline CNTs are crucial, so as they can act as a “high-way” for

electrons transferred by TiO2 upon photoactivity, thus minimising recombination

and improving photocatalytic activity.


81

0 500 1000 1500 20000

10000

20000

1500 20007000

8000

9000

10000

CNTs-TiO2-Br

CNTs-TiO2-I

G-bandD-band

Inte

nsity

Raman shift/cm-1

TiO2-I

TiO2-BrTiO2

CNTs-TiO2-Br

CNTs-TiO2-I

E g(3)

A 1g+ B

1g(2)

B1g(1)

Eg(1)

Inte

nsity

Raman shift/cm-1

Figure 4.2: Raman spectra of pristineTiO2, dopedTiO2 and doped CNTs-TiO2 nanocomposites

4.2.3 Optical Studies

The optical responsiveness of the doped TiO2 and CNTs-TiO2-X-nanocomposites

(where X = F,Cl, Br, I) were studied with UV-Vis spectroscopy. Most of the doped

CNTs-TiO2 nanocomposite samples (Figure 4.3 A) had strong intense absorptions

in the UV region. This could be accounted for by the promotion of the electron

from the valence band of TiO2 to the conduction band (O2p →Ti3d).5 The optical

response to the visible range may have been hindered when the CNTs-TiO2-

nanocomposite was doped with anions partly because the CNTs acted as electron

quenchers and thus hindered the red-shift. The red-shift was appreciably noted

with doped TiO2, and in particular for iodine doped TiO2. (Figure 4.3B)


82

Figure 4.3 Comparative UV absorbance measurements of (a) CNTs-TiO2

nanocomposites (b) doped TiO2

It was noted that the CNTs-TiO2 nanocomposites had almost the same absorption

edges as pristine TiO2 (de-counting the incorporation of carbon in the lattice of

TiO2). However, some authors have reported a red-shift when CNTs were loaded

(95% m/m).164 However, in these studies the loading of CNTs was kept to a

minimum (0.2% m/m), hence the above-mentioned visible response was not

observed. In fact a minimal loading of CNTs was deliberately used in the

nanocomposites that were synthesised, so as to reduce the possibility of dye

removal due to adsorption.

As previously mentioned (Section 4.3.) the band gaps of the doped TiO2 and

CNTs-TiO2 nanocomposites were measured by plots of (αhν)2 against hν. Iodine

doped TiO2 and corresponding CNTs-TiO2 nanocomposites (Figure 4.4.) showed

a narrower band gap than pristine TiO2 and all the other anionic dopants (Table 4.1). The narrower band gap by iodine-doped TiO2 could be attributed to the larger

atomic size of the iodine molecule which could easily be polarized by long

wavelength light (Vis) as compared to the smaller sized dopants which could be

200 300 400 500 6000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60TiO2-ITiO2-BrTiO2-ClTiO2-FTiO2

(A)

Abs

orba

nce

(a.u

.)

Wavelength (nm)200 300 400 500 600-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0(A)

TiO2CNTs-TiO2CNTs-TiO2-ICNTs-TiO2-BrCNTs-TiO2-ClCNTs-TiO2-F

Abs

orba

nce

(a.u

.)

Wavelength (nm)

(B) (a))

(b))


83

polarized by shorter wavelengths (UV). Once polarized, it could more easily

transfer electrons to the conduction band of TiO2.

Figure 4.4: Band gap measurement of pristine TiO2 and iodine doped TiO2

4.2.4 Powder X-ray Diffraction (PXRD)

PXRD patterns of doped TiO2 and CNTs-TiO2 nanocomposites are depicted in

Figure 4.5 (A and B respectively) Intense peaks at 2θ, 25.5, 38.2, 48.6, 54.0 and

55.3° corresponding to crystal planes of TiO2 (101), (004), (200), (105) and (211)

respectively, were confirmed by using the JCPDS card (04047). All of the patterns

and planes were exclusive to the anatase polymorphic form of titania. The CNTs

peak at 26.0° plane (002) was overshadowed by the (101) plane of anatase, which

appeared at the same 2θ. However, the presence of CNTs was confirmed by

Raman analysis as previously discussed in Section 4.3.

TiO2

TiO2-I

EgTiO2: 3.30 eV EgTiO2-I: 2.97eV


84

Figure 4.5: PXRD patterns of (A) doped TiO2 , (B) CNT loaded anionic doped nanocomposites

Crystallite sizes were estimated using the Scherrer equation as shown in Table 4.1 (and mentioned previously in section 3.3.4). To avoid the interference of

CNTs overlapping with (101) reflection, the (200) reflection plane (2θ = 48.6°) was

used for the TiO2 loaded with CNTs, but the (101) plane was used for those

20 30 40 50 60

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

(A)

211

105200

112

004

103

002

101

CNTs-TiO2-F

TiO2-commercial

CNTs-TiO2

Cou

nts

(a.u

.)

220 40 60

0

1000

2000

3000

4000

5000

112

103

(A)TiO2-ITiO2-BrTiO2-ClTiO2-FTiO2 (commercial)

211

10820

0

004

101

Cou

nts

(a.u

.)

2

(B)

20 40 60

0

1000

2000

3000

CNTs-TiO2-I

(B)

211

105200

11200

410

3002

101

CNTs-TiO2-Cl

CNTs-TiO2-BrCou

nts

(a.u

.)

2

(a))

(b))

(c))


85

without CNTs. PXRD patterns showed that the presence of CNTs did not affect the

crystallinity of TiO2 but a slight increase in the crystallite size, caused by the

presence of CNTs in the TiO2-nanocomposites, was observed.

4.2.5 TEM and SEM

The textural morphologies of the doped TiO2 and CNTs-TiO2 nanocomposites

were studied by TEM and SEM. TEM images (Figure 4.6 a-b) showed that close

contact between CNTs and titania was made, which could probably be attributed

to van der Waals forces.6 It was suggested that this close contact allowed proper

electron transfer from the conduction band of titania to CNTs, hence minimizing

electron-hole recombination. Of note was the comparison between the TiO2

nanoparticles with and without CNTs. Here a pronounced agglomeration was

noted with the particles without CNTs (Figure 4.6c) suggesting that CNTs acted

as dispersing supports. Spherical particles were also confirmed by SEM images

(Figure 4.6 d) The crystallographic lattice fringes of the nanoparticles and CNTs

with d-spacings of 0.31 nm and 0.36 nm were assigned to the TiO2 (101) and the

CNTs (002) planes respectively.

The sizes of TiO2 particles were found to be between 5 nm and 20 nm, while the

CNTs had an average inner diameter of 15 nm. Elemental analysis which was

obtained through EDS coupled to a TEM (as previously discussed in section 4.3)

to identify the make-up of the doped TiO2 and the CNTs-TiO2 nanocomposites. An

EDS spectrum of fluorinated and iodated TiO2 is shown in Figures 4.6e,f.


86

Figure 4.6: (a, b) TEM images of partly dispersed TiO2 on CNTs (c) agglomerated TiO2 nanoparticles without CNTs (d) SEM images of TiO2 nanoparticles without CNTs

(a) (b)

(d) (c)


87

Figure 4.6: ((e, f) EDS spectrum of fluorine and iodine doped CNTs-TiO2 nanocomposites.

4.3 Photocatalytic Activity of the doped TiO2 and CNTs-TiO2 nanocomposites

The photocatalytic activities of the doped TiO2 and CNTs-TiO2-X nanocomposites

were evaluated for their ability to photodegrade bromophenol blue and phenol red,

both under ultra-visible and visible irradiation. The experimental setups and

conditions were as described in chapter 3 (section.3.3.7) and data were collected

as a set of three replicates.

0 2 4 6 8

0

500

1000

1500

2000

2500

3000

3500

4000

Ti

Ti

I

IO

TiC

Cou

nts(

a.u.

)

k(eV)0 2 4 6 80

1000

2000

3000

4000

5000

6000

7000Ti

Ti

F

O

Ti

C

Cou

nts

(a.u

.)

keV

(f) (e)


88

4.3.1 Photodegradation of Bromophenol Blue

The photodegradation of a concentration of 10 mgL-1 BPB (Figure 4.7a) was

examined under ultra-visible light by doped TiO2. It was observed that TiO2-F

degraded up to 94% of the 10 mgL-1 of BPB in 140 minutes. TiO2 and TiO2-I were

able to degrade 84% and 80% respectively over the same period of time. Under

visible irradiation, TiO2-I degraded 80% of BPB in 140 minutes. Fluorine doped

titania appeared to be the least effective of these, as it was able to only degrade

77 % of the BPB.

Figure 4.7 (a,b) Photodegradation of BPB by anionic doped TiO2 using UV and visible light

0 20 40 60 80 100 120 140 160 180-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

(A)

C/C

0

Time (mins)

TiO2 TiO2-F TiO2-Cl TiO2-Br TiO2-I

0 20 40 60 80 100 120 140 160 180

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9(B)

C/C

0

Time (mins)



89

Figure 4.7 (c) reaction kinetics of BPB initiated by visible irradiation

The photodegradation of BPB both under visible and UV irradiation appeared to

have followed pseudo-first-order reactions (Figure 4.7 b) and their kinetics were

able to be expressed as:

-lnC/C0 = kt

where k was the apparent reaction constant, C0 and Ct were the initial and final

concentrations of BPB. Amongst the doped TiO2 photocatalysts, those that were

doped with fluorine had the highest reaction rate (0.025 min-1) when illuminated

with UV light, while under visible light iodine doped TiO2 had the highest rate

(0.014 min-1). Table 4.2 shows the reaction kinetics of all the other doped TiO2 and

CNTs-TiO2 nanocomposites.

Table 4.2: Kinetic measurements of the CNTs-TiO2 nanocomposites for each dye

(C) (c))


90

Catalysts Bromophenol blue Phenol red

k(UV)min-1 k(Vis)min-1 k(UV)min-1 k(Vis)min-1

TiO2 0.011 0.010 0.005 0.003

TiO2-F 0.025 0.011 0.012 0.003

TiO2-Cl 0.013 0.010 0.006 0.005

TiO2-Br 0.013 0.010 0.006 0.008

TiO2-I 0.012 0.014 0.005 0.009

CNTs-TiO2 0.015 0.012 0.008 0.005

CNTs-TiO2-F 0.067 0.011 0.016 0.005

CNTs-TiO2-Cl 0.021 0.013 0.009 0.012

CNTs-TiO2-Br 0.023 0.018 0.004 0.013

CNTs-TiO2-I 0.017 0.021 0.003 0.015


91

0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9(B)

C/C 0

Time (mins)

CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-Cl CNTs-TiO2-Br CNTs-TiO2-I

0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

(A)

C/C 0

Time (mins)


When CNTs were incorporated into the doped TiO2, to form CNTs-TiO2-X

nanocomposites, it was observed that the photoactivity was improved under UV

and visible illumination. For instance, about 98% of 10 mgL-1 BPB was degraded

by CNTs-TiO2-F nanocomposites in 20 minutes while undoped CNTs-TiO2

nanocomposites and CNTs-TiO2-I nanocomposites managed to degrade 85% and

80% respectively in 140 minutes (Figure 4.8a). Under visible irradiation CNTs-

TiO2-I nanocomposites degraded 91% of BPB compared to 82% in 140 minutes.

Kinetic studies revealed that the reactions followed a pseudo-first-order rate law as

observed in Figure 4.8b. CNTs-TiO2-F nanocomposites had the highest reaction

rate (0.067 min-1) when UV light was used, while CNTs-TiO2-I nanocomposites

(0.021 min-1) had the highest reaction rate when visible light was used.

Figure 4.8: (a, b) Degradation of BPB by anionic undoped CNTs-TiO2 nanocomposites and CNTs-TiO2-X nanocomposites under UV and Visible irradiations,


92

Figure 4.8: (c) Reaction kinetics of CNTs-TiO2-X nanocomposites with visible light

4.3.2 Photodegradation of Phenol Red (PR)

The photodegradation of 10 mgL-1 PR by doped TiO2 was evaluated under visible

and ultra-visible light. TiO2-F photodegraded PR to 84% while TiO2 only removed

up to 54% in 140 minutes as depicted in Figure 4.9a. Under visible light, TiO2-I

managed to degrade 78% of PR, while pristine TiO2 achieved 45% in 140 minutes.

The reaction kinetics of all the CNTs-TiO2 nanocomposites (doped or undoped)

once again followed pseudo-first-order reaction kinetics, as illustrated by Figure 4.9b. Table 4.2 shows the different reaction kinetics of all the CNTs-TiO2

nanocomposites under the different conditions. CNTs-TiO2-F nanocomposites had

the fastest reaction rate (0.012 min-1) when UV irradiation was used, while CNTs-

TiO2-I nanocomposites had the quickest reaction rate (0.009 min-1) of all the

CNTs-TiO2 nanocomposites under visible light.

(C) (C)


93

Figure 4.9 Photodegradation of PR by doped TiO2 under (a) visible and (b) UV light (c) reaction kinetics doped TiO2 under UV light.

The efficiency of the CNTs-TiO2 nanocomposites was also monitored with PR. The

introduction of CNTs into the matrix to make a nanocomposite improved the

photodegradation of PR, both under UV and visible irradiations. CNTs-TiO2-F

nanocomposites and CNTs-TiO2–I nanocomposites both degraded 90% of PR in

140 minutes under UV and visible irradiation respectively (Figure 4.10a). On the

other hand the undoped CNTs-TiO2 nanocomposites could only degrade 53%

(UV) and 66% ((Vis). The kinetics of the reactions all followed a pseudo-first-order

rate (Figure 4.10b).

0 20 40 60 80 100 120 140 160 1800.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9(A)

C/C

0

Time (mins)


0 20 40 60 80 100 120 140 160 1800.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 (B)

C/C

0

Time (mins)


(C)


94

Figure 4.10 (a,b) Photodegradation of PR by CNTs-TiO2-X nanocomposites (c) reaction kinetics of PR by CNTs-TiO2-X nanocomposites under visible light.

(C)

0 20 40 60 80 100 120 140 160 180-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

(A)

C/C 0

Time (mins)


0 20 40 60 80 100 120 140 160 180-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 (B)

C/C 0

Time (mins)


(B) (A)

(C)


95

4.4 Discussion

Bromophenol blue and phenol red are anionic dyes, making them negatively

charged.7 Undoped TiO2 in a basic medium has a surface which is negatively

charged (equation 1).8 At a higher pH the point of zero charge of TiO2 is negative

and at a lower pH (3) its surface becomes positive according to:

>TiOH + HO- ↔ TiO- (1)

>TiOH + H+ ↔ TiOH2+ (2)

Where: >TiOH = titanol surface group

Based upon this it is suggested that undoped TiO2 was the least photoactive of all

the CNTs-TiO2 nanocomposites due to an increase in the repulsive forces

between the surface of TiO2 and the dye. Photodegradation was only possible

because of the action of a limited amount of hydroxyl radicals which were

produced by the holes from the valence band that reacted with water to neutralize

the OH-, as illustrated by equation (3) and (4) 9

TiO2 hν→ e-CB + h+

VB (3)

{H2O + H+ + OH-} + h+ → OH• + H+ (4)

Another hindrance to the photoactivity of undoped TiO2 was the formation of

surface Ti-O• radicals which were formed by the direct action of holes on the

titanol surface (5) and which acted as hole trappers.

>TiOH + h+ → >Ti-O• (5)

Introducing anionic dopants on titania improved the photocatalytic activity both

under visible and ultra-violet light. Fluorine doped TiO2 performed better than all of

the other halogens when UV light was used. Fluorine being the most

electronegative halogen, partially made the surface of TiO2 positive. It is


96

suggested that this positive surface attracted and bound a negatively charged dye

more strongly than a neutral or negatively charged one. The pH of the solution

was kept at pH 2 during the synthesis of anionic doped nanocomposites, to allow

the exchange of hydroxyl groups with fluorine.

Once the dye (6) was attached on the surface of titania, it was prone to

electrophilic attack by the holes, having produced a positively charged dye surface

R•+(7). The positively charged dye was then attacked by hydroxyl radicals as seen

in (8). This nucleophilic attack mineralized the dye. Morretzi and Selli have also

shown that the number of hydroxyl radicals produced increased when TiO2 was

fluorinated.10 This was suggested to be due to the high potential of fluoride ions

(3.6 V) shielding the surface of TiO2 from being directly attacked by h+, but also

having been able to directly oxidizing water molecules to produce more hydroxyl

radicals (9). Preliminary experiments conducted in our laboratories with

2,2-diphenyl-1-picryhydrazyl (DPPH) have also shown that more hydroxyl radicals

were liberated with fluorine doped TiO2 as compared to pristine TiO2. In that case

it appeared as if an increase in hydroxyl radicals increased the photoactivity of the

nanocomposites. However, in this study it was observed that the photoactivity of

the CNTs-TiO2-X nanocomposites decreased as the electronegativity of the

halogen decreased. Consequently it is proposed that as the polarizing power of

each these halogens decreased, the attraction of the dye to the surface of TiO2

became weaker. With the interfacial distance between the dye and TiO2 having

increased it appears that fewer hydroxyl radicals reached the dye and thus the

photoactivity decreased.


97

R + TiO2 F

R TiO2 F

(6)

TiTi

F F

Ti

F

R R R

+ h+ R + TiO2-F (7)

HO R+ Degradation by-products (8)

TiO2-F + h+ H2O HO + H+ TiO2-F (9)

R

= dye

The data provided from this study suggest that the introduction of TiO2

nanoparticles on CNTs significantly improved the nanocomposites‟s photoactivity,

because these CNTs acted as electron transporters. Since nanoparticles were

perfectly attached on the walls of CNTs (Figure 4. 6a, b) the CNTs acted as

excellent mediators, minimizing electron-hole pair recombination. The trapped

electrons on the CNTs reacted with adsorbed oxygen to form reactive superoxide

radicals (O•2) (10) These superoxide ion radicals were also reactive enough to

degrade the organic dye pollutant (11). However, radicals could also have been


98

neutralized by a proton to form an HO•2 (12) which could have reacted with another

HO•2 to form intermediate hydrogen peroxide H2O2 molecules (13). Hydrogen

peroxide (H2O2) could then have undergone a decomposition (by the reduction of

an electron) to produce more HO• radicals (14) which could have reacted with the

dye molecules to completely mineralize them (15). The increased quantity of OH•

radicals produced by the trapped electrons in CNTs as well as those produced by

the action of h+ from TiO2 (when the holes reacted with the water molecules) most

likely accounted for the significant degradation of the dye. Hence any adsorbed

pollutant on the walls of the CNTs could eventually have been decomposed as

illustrated by Figure 4.11.

TiO2 → e-CB + hVB

+ (3)

O2 + e-/CNTs → O-•2 (10)

O-•2 + dye→ by-products (11)

O-•2 + H+ → HO•

2 (12)

2HO•2 → H2O2 + O2 (13)

H2O2 + e- HO• + HO- (14)

HO• + dye → by-products (15)


99

Figure 4.11: Proposed mechanism of the role played by CNTs upon photodegradation of dye in water

Iodine doped CNTs-TiO2 nanocomposites effectively decomposed the dyes when

visible irradiation was used. Maximum visible light was harnessed by iodine doped

nanocomposites because the band gap of titania was effectively narrowed to

2.97 eV as compared to 3.30 eV in undoped CNTs-TiO2 nanocomposites. The

other anionic dopants were not effective enough in reducing the band gap to make

use of the entire visible spectrum. The formation of Ti3+ surface states have been

reported as colour centres, which assist in the metal oxide to absorb more of the

visible light, have been found to be pronounced in iodine doped TiO2.11 Ti3+ was

able to minimize the re-combination of electron-hole-pair by acting as effective

electron trappers.12 The increased surface area of TiO2-I (176 m2/g) and CNTs-

TiO2-I nanocomposites (188 m2/g) was believed to have enhanced the

photocatalytic activity of the CNTs-TiO2 nanocomposites.

The degradation of BPB and PR was also monitored by the liberation of [SO42-]

and [Br-] using ion chromatography. Only sulphates could be monitored for PR

while both ions were monitored for BPB. The sulphite ion was oxidized to sulphate,

VBh+

e-CB

e-

H+/H2O

HOdye by-products

O2 O-2

dye

H+

by-products

HO2 H2O2 HOe-

CNTs

h dye


100

a more stable (+6) oxidation state of sulphur and hence it was more probable to

measure than the sulphite ion. Fluorine doped TiO2 and corresponding (Figure 4.12a) CNTs-TiO2-F nanocomposites produced the highest amount of the ions

under UV irradiation, while iodine doped (Figure 4.12 b) TiO2 and the

corresponding CNTs-TiO2-I nanocomposites produced the most ions under visible

illumination. Since the ratio of the sulphate to the bromide is 1:4 in the molecule of

BPB, a higher concentration of bromide ions was liberated in BPB (Figure 4.13).

Sulphates have been reported to be irreversibly adsorbed on the surfaces of

titania, which may have also accounted for the low amounts of sulphate ion.8

However, irreversible adsorption of sulphate on the titania doesn‟t affect the

photoactivity of the metal oxide. Devi and co-authors reported enhanced

photocatalytic activity for methyl orange with ammonium persulphate. They

claimed that the enhanced activity that they observed was due to greater

thermostability and the faster kinetic rate of sulphate radicals as compared to

hydroxyl radials.13

Figure 4.12 Generated sulphate ions from the degradation of PR using (a) ultra-violet and (b) visible light

0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

[SO

4]2- p

pm

Time (mins)

TiO2 TiO2-F TiO2-I CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-I

0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8(B)

[SO

4]2- p

pm

Time (mins)

TiO2 TiO2-F TiO2-I CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-I

(A)


101

Figure 4.13a Liberated sulphate and bromide ions from the degradation of BPB (a) CNTs-TiO2, (b) CNTs-TiO2-I, under visible irradiation

Figure 4.13b Liberated sulphate and bromide ions from the degradation of (C) TiO2 (D) TiO2-I under visible irradiation

0 20 40 60 80 100 120 140 160 180

0

2

4

(A)

Br-

[SO4]2-

Time (mins)

[Br- ] p

pm

0.0

0.2

0.4

0.6

0.8

1.0

1.2

[SO4 ] 2- ppm

0 20 40 60 80 100 120 140 160 180

0

2

4(B)

Br-

[SO4]2-

Time (mins)

[Br- ] p

pm

0

2

[SO4 ] 2- ppm

0 20 40 60 80 100 120 140 160 180

0

2

4

(C)

[Br-]

[SO4]2-

Time (mins)

[Br-

] p

pm

0.0

0.2

0.4

0.6

0.8

1.0

1.2

[SO

4 ]2

- pp

m

0 20 40 60 80 100 120 140 160 180

0

2

4

(D)

[Br-]

[SO4]2-

Time (mins)

[Br-

] p

pm

0

2

[SO

4 ]2

- pp

m


102

REFERENCES

1 . Barrett EP, Joyner LG, Halenda PP, The determination of pore volume and

area distributions in porous substances. I. Computations from nitrogen

isotherm, Journal of American Chemical Society, (1957) 73, 373-389

2 . Sing KSW, Everett DH, Haul RAW, Mascou L, Pierotti RA, Rouquerol KT,

Siemieniewska T, Reporting physisorption data for gas/solid systems with

special reference to the determination of surface area and porosity, Pure

Applied Chemistry, (1985) 57, 603 - 619

3 . Hu G, Meng X, Feng X, Ding Y, Zhang S, Yang M, Anatase TiO2

nanoparticles./carbon nanotubes nanofibers: preparation, characterization

and photocatalytic properties, Journal of Materials Science, (2007) 42, 7162

- 7170

4 . Dresselhaus MS, Ekhund PC, Photons in carbon nanotubes, Advances in

carbon nanotubes, Advances in Physics Letters, (2000) 49, 705- 814

5 . Xu YJ, Zhuang Y, Fu X, New insight for enhanced photocatalytic activity of

TiO2 by doping carbon nanotubes: a case study on degradation of benzene

and methyl orange, Journal of Physical Chemistry C, (2010) 114, 2669 -

2676

6 . Eder D, Carbon nanotubes-inorganic hybrids, Chemical Reviews, (2010)

110, 1348 - 1385

7 . Forgas E, Cserhati T, Oros G, Removal of synthetic dyes from

wastewaters: a review, Environmental International, (2004) 30, 953 - 971

8 . Linsebigler AL, Lu G, Yates JT Jr, Photocatalysis on TiO2 surfaces:

Principles, mechanisms and selected results, Chemical Reviews, (1995) 95,

735 – 758


103

9 . Houas A, Lachheb H, Ksibi M, Elaloui E, Guillard C, Hermann J-M,

Photocatalytic degradation pathway of methylene blue in water, Applied

Catalysis B: Environmental, (2001) 31, 145 - 157

10 . Mrowetz M, Selli E, Enhanced photocatalytic formation of hydroxyl radicals

on fluorinated TiO2, Physical Chemistry Chemical Physics, (2005) 7, 1100 -

1102

11 . Serpone N, Is the band gap of pristine TiO2 narrowed by anion – and

cation-doping of TiO2 in second –generation photocatalysts?, Journal of


12 . Fox MA, Dulay MT, Heterogeneous photocatalysis, Chemical Reviews

(1993) 93,341 - 357

13 . Devi LG, Kumar SG,Reddy KM, Munikrishnappa C, Photodegradation of

methyl orange an azo dye by advanced Fenton process using zero valent

metallic iron: influence of various reaction parameters and its degradation

mechanism, Journal of Hazard Materials, (2009) 164, 459 - 467

104

CHAPTER 5

Titania-based binary metal oxides and nanocomposites for the

photodegradation of organic pollutants in water*

* Part of the work presented in this chapter has been reported and published

in Materials Chemistry and Physics (2011)129, 406 – 410, as well as

submitted together with futher work on Ag+ and Ce+ doping of titania.

5.1 Introduction

The use of binary metal oxide (BMOs) systems (i.e. a combination of two metals in

their oxide forms) as photocatalysts has generated much interest because of their

ability to be efficient charge-separators. One major limitation of intrinsic TiO2 is the

fast recombination of the photogenerated charges. One approach of achieving

better charge separation, involves the coupling of two semiconductor particles with

different energy levels. Examples include: CdS-TiO2,1 TiO2-SnO2,2 TiO2-SiO2 3 and

TiO2-ZrO2. For instance, semiconductor particles having a large bandgap and an

energetically low-lying conduction band e.g. TiO2 or ZnO can be combined with

particles having a small bandgap and an energetically high lying conduction band

e.g. CdS or WO3.4 When the two types of particles (as mentioned above) are

combined, light absorption at longer wavelengths (i.e. > 366 nm) result in efficient

electron transfer to the large bandgap particle (i.e. TiO2), while the holes migrate in

the opposite direction from the holes to the small bandgap particle/s (i.e. CdS or

WO3). The minimization of charge recombination increases the photoactivity of

the photocatalysts. Not only do binary systems minimize recombination but they

also improve the photoresponse of the photocatalyst in the visible region as

compared to pristine TiO2.

This chapter of the thesis reports on the efficiency of CoO and WO3 (20% mol)

coupled with TiO2 in the degradation of bromophenol blue and phenol red, under

visible and ultra-visible illumination. Both CoO and WO3 have smaller band gaps


105

i.e. between 2.6 eV and 2.8 eV as compared to TiO2 (3.2 eV), which means that

they would be photoexcited by longer wavelengths while at the same time TiO2

would be comparably poorly excited; yet TiO2 is still a better photocatalyst than

these two metal oxides. To enhance the catalytic activity of the binary system,

CNTs (0.2% m/m) were incorporated into each of the binary metal oxide systems

(i.e. CoO/TiO2 and WO3/TiO2) to make nanocomposites. In addition they served as

efficient electron „bridges‟ between the metal oxides.

5.2 Experimental

Tungsten–titania binary metal oxides were prepared by dissolving tungstic acid

(8.00 x 10-4 mol) in a solution of 1–butanol (20 mL) and ammonia (50:50). Titanium

tert-butoxide (2.38 x 10-3 mol) was added in a dropwise fashion to the solution and

the as-formed precipitate was stirred for 2 hours and then aged for another 2

hours.5 This precipitate was dried in an oven at 80°C to drive most off the

moisture, then it was ground into fine powder and calcined in air for 3 hours at

400°C at a ramp rate of 5°Cmin-1. The tungsten-titania composite was synthesized

by sonicating functionalized CNTs (0.2% m/m) for 15 minutes in 10 mL of an

ammonia-butanol solution. The dispersed CNTs were then added to a solution of

dissolved tungstic acid (8.00 x 10-4 mol). Thereafter, under constant stirring,

titanium tert-butoxide (2.38 x 10-3 mol) was added dropwise to the mixture. The

mixture was then vigorously stirred for 2 hours and the precipitate was dried and

calcined as described in section 3.2.1

Cobalt-titania nanocomposites were also prepared by the sol-gel method, firstly by

thermally decomposing cobalt acetate on CNTs. These decorated CNTs with CoO

were then mixed in a one-step method with titanium tert-butoxide as described in

section 3.2.2.1


106

5.3 Characterization

Cobalt titania nanocomposites and tungsten titania binary oxides and

nanocomposites were studied with nitrogen adsorption BET analysis

(Figure 5.1 a, b). All isotherms represented the type IV classification, which

suggested that the pores followed the “ink bottle” hypothesis.6 The typical

hysteresis loop (H2) arises from the capillary condensation of gases. The

estimated specific areas (SBET) and pore volumes (Vp) were tabulated in Table 5.1.

Cobalt-titania nanocomposites had higher surface areas compared to tungsten

doped nanocomposites.

Figure 5.1: BET analysis of (a) cobalt oxide titania nanocomposite and (b) tungsten oxide titania BMOs and nanocomposite

(a) (b)

0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150

200

Ads

orbe

d ga

ses(

cm3 /g

)

Relative Pressure(P/P0)

CNTs-CoO TiO2 CNTs-CoO-TiO2

0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150

200

Ads

orbe

d ga

ses(

cm3 /g

)

Relative pressure (P/P0)

TiWxOy CNTs-TiWxOy TiO2


107

Table 5.1 Characterization of photocatalysts

Photocatalysts S(BET)m2/g Vp(cm3) Crystallite size (nm)

TiO2 175 0.27 9.62

WO3 1.3 0.34 15.56

CNTs-TiO2 180 1.09 13.27

CoO-TiO2 120 0.54 15.34/12.34

Ti-WxOy 5.7 0.35 12.23/12.93

CNTs-CoO-TiO2 145 0.87 11.34/10.78

CNTs-TiWyOy 6.4 0.56 12.11/12.45

Samples prepared (as discussed in section 5.2.) were analysed by laser Raman

spectroscopy using the equipment described in section 3.3.3. The Raman spectra

of cobalt and tungsten BMOs and their nanocomposites (Figure 5.2 a-d)

confirmed the presence of the anatase phase of titania. All samples displayed

strong bands at 143, 395, 516 and 639 cm-1, which are attributed to the Raman

active modes of TiO2 (with symmetries of Eg, B1g, A1g and Eg respectively).7 The

modes A1 + B1(2g) and Eg3 bands were associated with the stretching of bent

Ti-O-Ti modes, while the Eg(1), Eg(2) and B1g corresponded to the deformation of the

Ti-O-Ti modes.2


108

Figure 5.2: Raman spectra of (a) CNTs-CoO-TiO2 (b) TiO2, (c) TiWxOy and (d) CNTs-TiWxOy

The tungsten BMOs displayed bands at 272, 663 and 809 cm-1 which resulted

from the bending of the O-W-O mode due to the bridging oxygen, the stretching of

O-W-O mode due to the bridging of oxygen and the stretching of W=O bands

respectively.8,9 On the other hand in the nanocomposites (i.e. those that had

CNTs), peaks at around 1345 cm-1 were observed which are known as the D-band

and are associated with disordered sp2 carbon.10 Similarly these nanocomposites

displayed related peaks which are known as the G-band, at around 1600 cm-1, that

arose due to well-ordered graphite. One indication of the extent of the crystallinity

(c (d

CNTs-TiWxOy

TiWxOy

0 500 1000 1500 2000

0

200

400

600

800

1000

1200

G-bandA1gB1g

Eg

Eg CNTs-CoO-TiO2

Inte

nsity

Raman shift/cm-1

0 500 1000

0

200

400

600

800

1000

1200

A1gB1g

Eg

Eg TiO2

Inte

nsity

Raman shift/cm-1

(b)

(a)

0 500 10000

5000

10000

0-W

-O

B(1g)

A1g+ B1g(2)

Eg(3)

O-W-O

W=O

Inte

nsity

Raman shift/cm-1

0 500 1000 1500 20009000

10000

11000

12000

13000

14000

15000

16000

17000

Eg(1)

O-W-O

Eg(3)

W=O

G-b

and

D-b

and

Inte

nsity

Raman shift/cm-1


109

of CNTs is usually a measurement of the intensity ratio or area of the G-band

against the D-band i.e. the IG/ID ratio. A larger ratio (i.e. closer to 1) indicates more

crystalline CNTs, while a smaller ratio (closer to 0) infers highly disordered CNTs.

In this research, the IG/ID ratio of the pristine commercial CNTs that were used was

found to be 0.87. Similarly the IG/ID ratios for these CNTs in the cobalt titania

nanocomposites and in the tungsten nanocomposites were measured to be 0.81

and 0.76 respectively. The slight decrease in these ratios was attributed to the

functionalization process as well as the defects introduced by the attachment of

the BMOs on the surface of these CNTs. However, because this decrease was

relatively small the data suggested that the crystallinity of the CNTs had not been

seriously compromised in either nanocomposite.

The morphological attributes of the nanocomposites were examined by both TEM

and SEM techniques. Figure 5.3 a-b show SEM images of CNTs-CoO and the

TiWxOy nanocomposite and BMOs respectively. In Figure 5.3 b it was observed

that the thermally decomposed cobalt acetate was well decorated on the walls of

the CNTs, while titania nanocomposites were more agglomerated. The elemental

composition of the nanocomposites and BMOs was verified by EDS as seen in

Figure 5.3 c,d, indicating the elemental presence of carbon (from CNTs), oxygen

(oxides and functionalized CNTs) and cobalt and tungsten. Figure 5.3 e-h shows

TEM images of the various nanocomposites. The CoO nanoparticles in CNTs-CoO

showed that close contact between the CNTs and CoO nanoparticles had been

made, which was attributed to van-der Waals forces. The interaction between

titania and tungsten was verified by the d-spacings of 0.31 and 0.38 nm which

were assigned to the TiO2 (101) and the WO3 monoclinic (002) planes

respectively. Elemental composition of the TiWxOy nanocomposite was identified

and confirmed by EDS (Figure 5.3 i).


110

Figure 5.3: (a-b) SEM images of CoO-TiO2 and Ti-WxOy, (c-d) EDS spectra of the binary oxides

(a) (b)

0 5 100

500

1000

1500

2000

2500

3000

3500

CoCo

O

C

Cou

nts

keV0 5 10

0

500

1000

1500

2000

TiTi

Ti

W

O

Inte

nsity

(a.u

.)

keV

(c) (d)


111

Figure 5.3: (e-h) TEM images of CNTs-CoO-TiO2 and CNTs-Ti-WxOy nanocomposites, (i) EDS spectra of CNTs-Ti-WxOy nanocomposite

0.31 nm (101)

0.38 nm (001) WO33

TO2

(e) (f)

(g) (h)

0 5 100

500

1000

1500

2000 Ti

W

Ti

C

O

Inte

nsity

(a.u

.)

keV

(i)


112

PXRD patterns for CoO-TiO2 depicted by Figure 5.4 show intense peaks of

cobalt(II) oxide and anatase. All 2θ peaks at 26.22, 31.70, 34.25, 36.11, 42.11 and

54.10° corresponded to (101), (220), (111), (311), (200) and (411) crystal planes

of CoO (JCPDS 43100).11 While anatase had crystal plane reflections of (101),

(004), (200), (105) and (211) respectively corresponding to 2θ 25.3, 37.8, 48.0,

53.9 and 55.1° (JCPDS 04047).2,12 The composite CNTs-CoO-TiO2 showed a

small decrease in 2θ values for CoO crystal planes, indicating the interaction of

TiO2 and CoO in the nanocomposite. The peak for CNTs (001) was overshadowed

by anatase (101) which appeared around the same 2θ. However, the presence of

CNTs in the nanocomposite was fully confirmed by TEM and Raman analysis.

20 30 40 50 60

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

CoO

CoOCoOCoOC

oO

AAA

A

A

TiO2

CoO-TiO2

CNTs-CoO-TiO2

Cou

nts

2

Figure 5.4: PXRD pattern of cobalt-titania nanocomposites

Figure 5.5 shows the PXRD patterns of tungsten-titania nanocomposites. TiWxOy

and CNTs-TiWxOy both showed reflections due to monoclinic WO3 of (002), (020),

(022) and (202) at 2θ 23.12, 23.58, 24.38, 33.21 and 34.23° respectively (JCPDS-

43-1035).3 Monoclinic WO3 is prominent at temperatures between 320 - 500°C,

while orthorhombic WO3 exists above 500°C but less than 700°C. Above 700°C,

the phase of WO3 changed to hexagonal.3,13 Since the nanocomposites were

calcined at 400°C, the presence of monoclinic WO3 was understandable. Using


113

the Scherer equation, the diameter of the WO3 (002) was found to be 10 nm, while

it was found to be 15 nm for TiO2 (101).

20 30 40 50 600

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

WO3

CNTs-TiWxOyTiWxOyTiO2WO3

WO3

WO3WO3

A

A

WO3

A

Cou

nts

2

Figure 5.5: PXRD patterns of tungsten-titania nanocomposites

Optical studies of the nanocomposites were made by UV-Visible spectroscopy.

Figure 5.6 showed that the absorption of all the photocatalysts, and the tungstate-

titania nanocomposites had absorption bands in the visible region i.e. 440 nm or

2.8 eV. On the other hand the CNTs-TiWxOy nanocomposites showed absorptions

that were blue shifted to 410 nm or 3.0 eV. This indicated that there was an

interfacial interaction between TiO2, WO3 and the CNTs, rather than a mixture

between them.


114

200 300 400 500 600 700-0.08-0.06-0.04-0.020.000.020.040.060.080.100.120.140.160.180.200.220.240.260.280.30

Abs

orba

nce

(a.u

.)

Wavelength (nm)

CNTs-CoO-TiO2 CNTs-CoO TiO2 TiWxOy CNTs-TiWxOy

Figure 5.6: Optical studies of the various binary metal oxides, and their nanocomposites

A blue shift of 380 nm (from the expected 440 nm or 2.8 eV) was observed for

CoO nanoparticles. This can be explained by the effect of size quantization of the

nanoparticles.5 CNTs-CoO-TiO2 were observed to absorb at 390 nm because of

the transfer of electrons from the conduction band of TiO2 to CNTs. A strong

absorption in the UV region 360 nm was due to the interband transition (valence

band to conduction band) which could be attributed to the O2- → Ti4+ charge

transfer. The band at ~410 nm in the CNTs-CoO-TiO2 could be attributed to the

Co2+ → Ti4+ intervalence transition.14 Infra-red and near infra-red bands were not

observed, which confirmed that Co2+ was not oxidized to Co3+ during the

calcination step. Literature indicated that at higher temperatures (greater than

650°C), Co2+ and Ti4+ (anatase) would be transformed into rutile and CoTiO3,

which would have led to an increase in IR absorption bands (780 nm).9,15 Such

transformations were not observed in our samples as they were calcined at lower

temperatures (400°C)


115

5.4 Photocatalytic Activities of pristine TiO2 and various BMO Nanocomposites

The photocatalytic efficiencies of the pristine TiO2 and various BMO

nanocomposites were monitored with model organic pollutants, bromophenol blue

and phenol red, under visible and ultra-violet illumination.

5.4.1 Photodegradation of Bromophenol Blue (BPB)

Figure 5.7 (a,c) shows that the CNTs-CoO-TiO2 and the CNTs-TiWxOy had much

higher activity than pure TiO2, CoO-TiO2, TiWxOy and CNTs-TiO2. Here it was

observed that CNTs-CoO-TiO2 had degraded up to 98.7% and 99% of the dye

using UV and visible light respectively in 180 minutes, while CNTs-TiWxOy

managed to degrade 98 and 93% of the dye under the same conditions.

Figure 5.7: Evaluation of photocatalytic activity of BPB by various TiO2 and BMO nanocomposites (a, b) under UV irradiation

0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 (A)

C/C

0

Time (mins)

TiO2 CNTs-TiO2 TiWxOy CNTs-TiWxOy

0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 (B)

C/C

0

Time (mins)


(b)) (a)

)


116

Figure 5.7: Evaluation of photocatalytic activity of BPB by various TiO2 and BMO nanocomposites (c,d) under visible irradiation

The photodegradation of BPB by various TiO2 and MBO nanocomposites under

UV and visible light followed pseudo-first-order kinetics. Figure 5.8 (a,b) shows a

plot of –lnC/Co vs t for BPB degradation with these photocatalysts. The values of

the apparent reaction rate constant were obtained from the slopes of the linear

curves in the plot and are shown in Table 5.1

0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 (C)

C/C

0

Time (mins)

TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-TiO2

0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 (D)

C/C

0

Time (mins)


(c))

(d))


117

Figure 5.8: Reaction kinetics of the various (a) TiO2 and (b) BMO nanocomposites as photocatalysts of BPB using different illumination sources

Table 5.2 Rate constants of the various TiO2 and BMO nanocomposites as photocatalysts of BPB using UV and visible light

Photocatalyst kapp (min-1) UV kapp(min-1) Vis

TiO2 0.011 0.010

CNTs-TiO2 0.015 0.012

TiO2-CoO 0.014 0.010

CNTs-CoO-TiO2 0.021 0.018

TiWxOy 0.009 0.013

CNTs-TiWxOy 0.019 0.013

(a) (b)


118

5.4.2 Photodegradation of Phenol Red (PR)

Figure 5. 9 (a, - d) depicts kinetic curves for the photodegradation of phenol red

over the vvarious TiO2 and BMO nanocomposites as photocatalysts under both

UV and visible light. Increasing the illumination time resulted in a decrease in the

concentration of PR for all the TiO2 and BMO nanocomposites tested as

photocatalysts. The photocatalysts with CNTs showed the highest photocatalytic

activity when compared to pure titania. The concentration of PR decreased from

10 mg/L to 0.5 and 1 mg/L, respectively, corresponding to 95% and 90%

degradation of PR in 180 minutes using UV and visible light over CNTs-TiWxOy.

CNTs-CoO-TiO2 nanocomposites degraded PR by 98% and 96% in 180 minutes

with UV and visible light respectively. The photodegradation of PR in the presence

of UV and visible light also followed pseudo-first-order reaction kinetics as

depicted by Figure 5.10 (a,b).

Figure 5.9: The photodegradation of PR by TiO2 and BMO nanocomposites (a, b) under UV irradiation

(B)

0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 (A)

C/C

0

Time (mins)


0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 (C)

C/C

0

Time (mins)



119

Figure 5.9: The photodegradation of PR by TiO2 and BMO nanocomposites (c, d) under visible irradiation

Figure 5.10: Reaction kinetics of (a) PR photodegradation by various TiO2 and (b) BMO nanocomposites under UV and visible illumination

(a) (b)

(C)

0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 (D)

C/C

0

Time (mins)


0 20 40 60 80 100 120 140 160 180

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 (B)

C/C

0

Time (mins)



120

5.5 Discussion

The degradation of the dyes (BPB and PR) by the BMOs nanocomposites metal

oxides under the visible and ultra-violet light was over 90% in 180 minutes. All the

nanocomposites with CNTs performed much better than pristine and pure binary

metal oxides. The CoO-TiO2 and TiWxOy BMOs were able to efficiently degrade

the dyes using only visible light because of their smaller band gaps (2.8 eV for -

WO3 and 2.6-2.8 eV for CoO), which meant they could be easily excited by visible

light (for reasons discussed below) and hence transfer this excitation to the TiO2.

Binary metal oxides proved to be efficient charge separators as depicted by

Figure 5.11. Charge separation was achieved as a result of the difference in the

diffusion gradient of the two metal oxides. The conduction band of metal oxide with

the large band gap acted as an electron sink, while the holes produced in its

valence band diffused to the valence band of the metal oxide with the smaller

band gap. The incorporation of CNTs in the composites significantly enhanced

photocatalytic activity because they acted as a “bridge” between the two metal

oxides, and provided a „short-cut‟ as well as an efficient transport of electrons from

(WO3 and CoO) to TiO2.

The holes that were produced were then able to react with water to produce

reactive hydroxyl radicals. These radicals were excellent oxidizing agents and

reacted with the dyes to form their innocuous by-products. The two

photogenerated species were separated efficiently by the system, which lead to

increased photocatalytic activity. On the conduction band, the electrons reacted

with adsorbed oxygen forming superoxide radicals, which were also very reactive

radicals and were able to completely mineralize the dye.


121

Figure 5.11: Proposed mechanism for the binary system loaded with CNTs

The resultant [Br-] and [SO42-] ions were quantified by ion-chromatography to

ascertain the degree of degradation of the dyes. Phenol red degradation (Figure 5.12 a-d), showed a maximum sulphate ion concentration of 2.0 ppm for the

CNTs-CoO-TiO2 nanocomposite under UV light and TiWxOy under visible light in

180 minutes. This signified the complete fragmentation of the sulphate moiety in

the molecule. For bromophenol blue, (Figure 5.12 e- l) the maximum bromide ion

concentration was 4.5 ppm by CNTs-CoO-TiO2 nanocomposites under visible light

while the CNTs-Ti-WxOy nanocomposite liberated 4.1 ppm of bromide ions under

ultra-visible light in 180 minutes. The high concentration of bromide ions compared

to sulphate ions was proportional to the higher atomic ratio of bromide to sulphate

in bromophenol blue.

e- e-

h+ h+

VB VB

CB

hν

OH• dye mineralization

H2O mineralization dye O-•2

O2


122

Figure 5.12 Generated sulphate ions from the degradation of PR using (a,c) ultra-violet and (b,d) visible light

0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(A)

[SO

4]2- p

pm

Time (mins)


0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(B)

[SO

4]2- p

pm

Time (mins)


0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(C)

[SO

4]2- p

pm

Time (mins)

TiO2 CNTs-TiO2 TiWxOy CNTsTiWxOy

0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(D)

[SO

4]2- p

pm

Time (mins)

TiO2 CNTs-TiO2 TiWxOy CNTsTiWxOy


123

Figure 5.12: Generation of bromide and sulphate ions from BPB by TiWxOy and CNTs- TiWxOy nanocomposites, tested as photocatalysts under visible and UV light

0 20 40 60 80 100 120 140 160 1800

2

4

(E)TiWxOy (UV)

Br-

[SO4]2-

Time (mins)

[Br-

] p

pm

0

2

[SO

4 ] 2-p

pm

0 20 40 60 80 100 120 140 160 1800

2

4

(F)TiWxOy (Vis)

Br-

[SO4]2-

Time (mins)

[Br-

] p

pm

0

2

[SO

4 ] 2-p

pm

0 20 40 60 80 100 120 140 160 1800

2

4

(G)CNTsTiWxOy (UV)

Br-

[SO4]2-

Time (mins)

[Br-

] ppm

0

2

[SO

4 ] 2-ppm

0 20 40 60 80 100 120 140 160 1800

2

4

(H)CNTsTiWxOy (Vis)

Br-

[SO4]2-

Time (mins)

[Br-

] ppm

0

2

[SO

4 ] 2-ppm


124

Figure 5.12: Generation of bromide and sulphate ions from BPB by TiWxOy and CNTs- TiWxOy nanocomposites, tested as photocatalysts under visible and UV light

(b) (a)

0 20 40 60 80 100 120 140 160 1800

2

4

(I)CoO-TiO2 (UV)

Br-

[SO4]2-

Time (mins)

[Br-]

ppm

0

2

[SO4 ] 2-ppm

0 20 40 60 80 100 120 140 160 1800

2

4

(J)CoO-TiO2 (Vis)

Br-

[SO4]2-

Time (mins)[B

r-] p

pm

0

2

[SO4 ] 2-ppm

0 20 40 60 80 100 120 140 160 1800

2

4

(K)CNTs-CoO-TiO2 (UV)

Br-

[SO4]2-

Time (mins)

[Br-

] ppm

0

2

[SO

4 ] 2-ppm

0 20 40 60 80 100 120 140 160 1800

2

4

(L)CNTs-CoO-TiO2 (Vis)

Br-

[SO4]2-

Time (mins)

[Br-

] ppm

0

2

[SO

4 ] 2-ppm


125

REFERENCES

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aspects of coupled semiconductors. charge-transfer processes in colloidal

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2 . Vinodgopal K, Kamat PV, Enhanced rates of photocatalytic degradation of

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Morawsky AP, Maradyan VE, Obraztsova ED, Sloan J, Terekhov SV,

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19

127

CHAPTER 6

CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

In this study it has been demonstrated that the photocatalytic activity of doped

titania could be improved when both visible and ultra-violet irradiation were

employed.

Synthetic methods discussed in Chapter 3, showed facile methods of preparing

anionic doped TiO2, binary metal oxides and their corresponding nanocomposites.

All samples were characterized by using various microscopic, spectroscopic and

surface area techniques to elucidate structure, morphology, size distribution and

confirm the nature of the catalyst.

The use of anionic dopants as discussed in Chapter 4, showed that fluorine and

iodine doped titania enhanced the photocatalytic activity in the UV and visible light

region respectively. The continuous generation of hydroxyl radicals in fluorine

doped titania enhanced the high degradation rate of the bromophenol blue (BPB)

and phenol red (PR) when UV light was used. This is due to the high potential of

fluoride ions (3.6 V) to shield the surface of TiO2 from being directly attacked by

photogenerated holes (h+), but at the same time to allow direct oxidation of water

molecules to produce more hydroxyl radicals.

On the other hand the photoactivity of iodine-doped titania increased when

exposed to visible light. The reason for this is that iodine-doped TiO2 was able to

absorb visible light due to its narrower band gap. The narrower band gap of iodine-

doped TiO2 could be attributed to the larger atomic size of the iodine molecule,

which could more easily be polarized by long wavelengths of light (i.e. visible light)

Conclusions and Recommendations

128

as compared to the smaller sized dopants, which could be polarized by shorter

wavelengths (UV). Once polarized, it could more easily transfer electrons to the

conduction band of TiO2.

The photoactivity of the nanocomposites of the doped TiO2 were significantly

improved when CNTs were introduced (over 90% of dyes were degraded by all the

photocatalysts). CNTs acted as efficient electron transporters. Since the doped

TiO2 nanoparticles were perfectly attached on the walls of the CNTs, the CNTs

acted as excellent mediators thereby minimizing electron-hole pair recombination.

In addition, the degradation of the dyes, for all photocatalysts tested, followed

pseudo-first-order reaction kinetics.

In Chapter 5, metal oxides were prepared in conjunction with titania (e.g. TiWxOy

and CoO), so as to minimize the recombination of photogenerated species in the

intrinsic TiO2. Here it was found that combinations of metal oxides and TiO2 also

improved their photoresponse as compared to pristine TiO2. This improved

photoresponse can be attributed to the energetically low-lying conduction band of

TiO2, which was combined with metal oxides that have small bandgaps (CoO and

WxOy) and energetically high lying conduction bands. The difference in the band

gaps of the semiconductors meant the binary metal oxides (BMOs) and their

corresponding nanocomposites were able to be excited by longer wavelengths as

compared to pristine TiO2 which was photoexcited by shorter wavelengths.

The incorporation of CNTs to the BMOs improved the photoactivity (over 95 %

degradation of BPB and PR, both under UV and visible illumination because they

acted as “bridges” between the two metal oxides and provided a „short-cut‟ or a

more efficient way to transport of electrons from (WxOy and CoO) to TiO2. This

minimized the recombination between the photogenerated holes and electrons.


129

6.2 Future outlook

Following the facile synthetic route and successful application of the

photocatalysts (doped TiO2, corresponding nanocomposites, binary metal oxides

and nanocomposites of the BMOs) in the degradation of organic dyes in the

presence of UV and visible irradiation, the following potential studies can be

generated. These include:

Performing extensive degradative pathways of the dyes by using

hyphenated-techniques such as gas chromatography – mass spectrometry

or liquid chromatography – mass spectrometry. These analytical techniques

would yield more information about the degradative fragments of the dyes.

Applying extensive iodine doped TiO2 and their nanocomposites in the

textile industries to efficiently degrade these pollutants under visible light.

This could prove very economical to the sector. Further exploration of other

metal oxides which could be coupled with TiO2 could also be undertaken.

Using these photocatalyst to degrade other selected contaminants (organic

and/or inorganic)

Examining the potential of these photocatalysts for water splitting and the

production of fuel (hydrogen).


130

APPENDIX

Photodegradation of BPB under UV light Time

(min)

CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-

Cl

CNTs-TiO2-

Br

CNTs-TiO2-I

C/C0 Error C/C0 Error C/C0 Error C/C0 Error C/C0 Error

0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01

20 0.5 0.03 0.02 0.02 0.53 0.07 0.56 0.03 0.58 0.02

40 0.44 0.05 0.01 0.03 0.42 0.06 0.43 0.06 0.53 0.03

60 0.36 0.02 0.01 0.04 0.23 0.05 0.31 0.04 0.36 0.04

80 0.19 0.03 0.01 0.05 0.17 0.04 0.17 0.05 0.2 0.05

100 0.16 0.04 0.01 0.06 0.12 0.03 0.13 0.06 0.17 0.08

120 0.09 0.05 0.01 0.07 0.06 0.02 0.05 0.08 0.1 0.07

140 0.08 0.07 0.01 0.05 0.07 0.01 0.06 0.05 0.08 0.06

160 0.07 0.03 0.01 0.04 0.05 0.02 0.03 0.7 0.05 0.05

180 0.06 0.04 0.01 0.03 0.01 0.03 0.01 0.02 0.01 0.04


131

Photodegradation of BPB under visible light Time

(min)

CNTs-TiO2 CNTs-TiO2-F CNTs-TiO2-

Cl

CNTs-TiO2-

Br

CNTs-TiO2-I


0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01

20 0.64 0.03 0.66 0.04 0.65 0.04 0.63 0.04 0.62 0.04

40 0.52 0.02 0.53 0.05 0.52 0.03 0.54 0.03 0.5 0.03

60 0.36 0.04 0.48 0.03 0.46 0.02 0.43 0.02 0.38 0.02

80 0.34 0.05 0.45 0.02 0.42 0.04 0.31 0.04 0.3 0.06

100 0.26 0.08 0.37 0.06 0.33 0.04 0.28 0.04 0.2 0.07

120 0.16 0.05 0.28 0.07 0.23 0.03 0.14 0.03 0.12 0.08

140 0.12 0.03 0.18 0.05 0.16 0.03 0.12 0.03 0.09 0.06

160 0.11 0.02 0.13 0.07 0.12 0.03 0.06 0.03 0.05 0.08

180 0.1 0.04 0.09 0.05 0.06 0.03 0.02 0.03 0.01 0.07


132

Photodegradation of BPB under UV light Time

(min)



0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01

20 0.72 0.01 0.5 0.04 0.64 0.02 0.64 0.03 0.64 0.04

40 0.49 0.03 0.37 0.03 0.32 0.05 0.31 0.02 0.48 0.03

60 0.38 0.02 0.25 0.02 0.26 0.06 0.28 0.04 0.3 0.02

80 0.34 0.01 0.2 0.04 0.21 0.04 0.2 0.05 0.23 0.04

100 0.27 0.02 0.12 0.04 0.19 0.09 0.18 0.08 0.2 0.04

120 0.19 0.03 0.09 0.03 0.15 0.08 0.14 0.05 0.17 0.03

140 0.16 0.03 0.01 0.03 0.13 0.05 0.11 0.03 0.15 0.03

160 0.16 0.03 0.01 0.03 0.1 0.04 0.1 0.02 0.12 0.03

180 0.16 0.13 0.01 0.03 0.05 0.01 0.05 0.04 0.06 0.03


133

Photodegradation of BPB under visible light Time

(min)



0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01

20 0.74 0.04 0.7 0.03 0.72 0.04 0.72 0.04 0.72 0.04

40 0.68 0.05 0.68 0.02 0.65 0.03 0.66 0.03 0.6 0.03

60 0.56 0.03 0.58 0.04 0.59 0.02 0.55 0.02 0.42 0.02

80 0.44 0.02 0.42 0.05 0.41 0.04 0.38 0.06 0.34 0.04

100 0.36 0.06 0.33 0.08 0.32 0.05 0.31 0.07 0.28 0.04

120 0.25 0.07 0.26 0.05 0.27 0.04 0.26 0.08 0.25 0.03

140 0.20 0.05 0.23 0.03 0.22 0.06 0.21 0.06 0.2 0.03

160 0.18 0.07 0.16 0.02 0.17 0.05 0.17 0.08 0.15 0.03

180 0.16 0.05 0.16 0.04 0.16 0.07 0.15 0.07 0.12 0.03


134

Photodegradation of PR under UV light Time

(min)



0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01

20 0.8 0.01 0.67 0.04 0.73 0.02 0.72 0.03 0.8 0.04

40 0.78 0.03 0.53 0.03 0.69 0.05 0.7 0.02 0.74 0.03

60 0.76 0.02 0.44 0.02 0.55 0.06 0.55 0.04 0.69 0.02

80 0.62 0.01 0.31 0.04 0.48 0.04 0.47 0.05 0.61 0.04

100 0.58 0.02 0.28 0.04 0.43 0.09 0.4 0.08 0.54 0.04

120 0.51 0.03 0.25 0.03 0.41 0.08 0.38 0.05 0.5 0.03

140 0.46 0.03 0.17 0.03 0.35 0.05 0.36 0.03 0.44 0.03

160 0.38 0.03 0.11 0.03 0.31 0.04 0.3 0.02 0.39 0.03

180 0.3 0.13 0.09 0.03 0.28 0.01 0.27 0.04 0.26 0.03


135

Photodegradation of PR under visible light Time

(min)



0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01

20 0.81 0.04 0.76 0.03 0.76 0.04 0.75 0.04 0.74 0.04

40 0.8 0.05 0.74 0.02 0.73 0.03 0.7 0.03 0.72 0.03

60 0.76 0.03 0.69 0.04 0.68 0.02 0.65 0.02 0.6 0.02

80 0.69 0.02 0.61 0.05 0.6 0.04 0.58 0.06 0.52 0.04

100 0.6 0.06 0.59 0.08 0.55 0.05 0.47 0.07 0.44 0.04

120 0.58 0.07 0.56 0.05 0.5 0.04 0.39 0.08 0.32 0.03

140 0.55 0.05 0.5 0.03 0.45 0.06 0.31 0.06 0.28 0.03

160 0.5 0.07 0.48 0.02 0.39 0.05 0.27 0.08 0.19 0.03

180 0.41 0.05 0.46 0.04 0.26 0.07 0.18 0.07 0.16 0.03


136

Photodegradation of PR under visible light Time

(min)

CNTS-TiO2 CNTs-TiO2-F CNTs-TiO2-

Cl

CNTs-TiO2-

Br

CNTs-TiO2-I


0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01

20 0.78 0.03 0.76 0.04 0.75 0.04 0.75 0.04 0.72 0.04

40 0.76 0.02 0.7 0.05 0.66 0.03 0.65 0.03 0.62 0.03

60 0.75 0.04 0.69 0.03 0.57 0.02 0.53 0.02 0.43 0.02

80 0.6 0.05 0.65 0.02 0.46 0.04 0.44 0.04 0.33 0.06

100 0.51 0.08 0.58 0.06 0.33 0.04 0.28 0.04 0.21 0.07

120 0.47 0.05 0.51 0.07 0.22 0.03 0.2 0.03 0.18 0.08

140 0.42 0.03 0.46 0.05 0.18 0.03 0.17 0.03 0.15 0.06

160 0.34 0.02 0.38 0.07 0.15 0.03 0.11 0.03 0.09 0.08

180 0.3 0.04 0.31 0.05 0.1 0.03 0.07 0.03 0.04 0.07


137

Photodegradation of PR under UV light Time

(min)

CNTS-TiO2 CNTs-TiO2-F CNTs-TiO2-

Cl

CNTs-TiO2-

Br

CNTs-TiO2-I


0 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01 0.82 0.01

20 0.76 0.03 0.56 0.02 0.78 0.07 0.79 0.03 0.8 0.02

40 0.68 0.05 0.4 0.03 0.75 0.06 0.78 0.06 0.79 0.03

60 0.65 0.02 0.32 0.04 0.62 0.05 0.7 0.04 0.78 0.04

80 0.6 0.03 0.25 0.05 0.59 0.04 0.67 0.05 0.75 0.05

100 0.55 0.04 0.2 0.06 0.53 0.03 0.65 0.06 0.7 0.08

120 0.52 0.05 0.15 0.07 0.48 0.02 0.56 0.08 0.68 0.07

140 0.47 0.07 0.1 0.05 0.31 0.01 0.51 0.05 0.6 0.06

160 0.2 0.03 0.07 0.04 0.19 0.02 0.45 0.7 0.54 0.05

180 0.15 0.04 0.03 0.03 0.13 0.03 0.36 0.02 0.43 0.04


138

Sulphate production during the photodegradation of PR under UV light Tim

e

(mi

n)

TiO2 TiO2-F TiO2-I CNTs-TiO2 CNTs-

TiO2-F

CNTs-

TiO2-I

[SO4

]2-

Err

or

[SO4

]2-

Err

or

[SO4

]2-

Err

or

[SO4

]2-

Err

or

[SO4

]2-

Err

or

[SO4

]2-

Err

or

0 0 0 0 0 0 0 0 0 0 0 0 0

20

0.2

0.0

1 0.3

0.0

4 0.1

0.0

4 0.1

0.0

3 0.2

0.0

2 0.1

0.0

2

40

0.4

0.0

3 0.45

0.0

3 0.2

0.0

3 0.3

0.0

5 0.3

0.0

3 0.2

0.0

3

60

0.5

0.0

2 0.55

0.0

2 0.25

0.0

2 0.4

0.0

2 0.35

0.0

4 0.3

0.0

4

80

0.6

0.0

1 0.6

0.0

4 0.3

0.0

4 0.41

0.0

3 0.38

0.0

5 0.33

0.0

5

100

0.7

0.0

2 0.8

0.0

4 0.33

0.0

4 0.42

0.0

4 0.4

0.0

6 0.42

0.0

8

120

0.8

0.0

3 0.93

0.0

3 0.4

0.0

3 0.45

0.0

5 0.42

0.0

7 0.45

0.0

7

140

0.9

0.0

3 1

0.0

3 0.45

0.0

3 0.5

0.0

7 0.45

0.0

5 0.48

0.0

6

160

0.95

0.0

3 1.3

0.0

3 0.5

0.0

3 0.7

0.0

3 0.5

0.0

4 0.53

0.0

5


139

180

1

0.1

3 1.5

0.0

3 0.6

0.0

3 1

0.0

4 1

0.0

3 0.55

0.0

4

Sulphate production during the photodegradation of PR under visible light Tim

e

(mi

n)

TiO2 TiO2-F TiO2-I CNTs-TiO2 CNTs-

TiO2-F

CNTs-

TiO2-I

[SO4

]2-

Err

or

[SO4

]2-

Err

or

[SO4

]2-

Err

or

[SO4

]2-

Err

or

[SO4

]2-

Err

or

[SO4

]2-

Err

or

0 0 0 0 0 0 0 0 0 0 0 0 0

20

0.1

0.0

4 0.1

0.0

3 0.2

0.0

4 0.1

0.0

3 0.1

0.0

4 0.2

0.0

4

40

0.12

0.0

5 0.13

0.0

2 0.4

0.0

3 0.13

0.0

2 0.15

0.0

5 0.3

0.0

3

60

0.13

0.0

3 0.15

0.0

4 0.6

0.0

2 0.14

0.0

4 0.2

0.0

3 0.5

0.0

2

80

0.14

0.0

2 0.19

0.0

5 0.8

0.0

4 0.15

0.0

5 0.23

0.0

2 0.7

0.0

6

100

0.15

0.0

6 0.21

0.0

8 0.95

0.0

4 0.17

0.0

8 0.3

0.0

6 0.8

0.0

7

120

0.17

0.0

7 0.23

0.0

5 1.25

0.0

3 0.18

0.0

5 0.33

0.0

7 1

0.0

8

140 0.2

0.0

0.25

0.0

1.45

0.0

0.2

0.0

0.36

0.0

1.2

0.0


140

5 3 3 3 5 6

160

0.21

0.0

7 0.27

0.0

2 1.55

0.0

3 0.25

0.0

2 0.38

0.0

7 1.3

0.0

8

180

0.23

0.0

5 0.3

0.0

4 1.6

0.0

3 0.27

0.0

4 0.4

0.0

5 1.6

0.0

7


141

Sulphate and bromide production during the photodegradation of BPB under UV light Ti

m

e

(m

in)

TiO2

TiO2-I CNTs-TiO2

[SO

4]2-

Er

ror

[B

r]

Er

ror

[SO

4]2-

Er

ror

[

B

r]

Er

ror

[SO

4]2-

Er

ror

[B

r]-

Er

ror

[SO

4]2-

E

r

[B

r]-

E

r

0 0 0 0 0 0 0 0 0 0

0.0

1 0

0.0

1 0 0 0 0

20 1.3

0.0

4

0.

1

0.0

3 1.4

0.0

3

0.

3

0.0

4 1.3

0.0

3

0.

2

0.0

7 1.7

0.

04

0.

5

0.

04

40 1.4

0.0

3

0.

2

0.0

2 1.7

0.0

6

0.

5

0.0

3 1.5

0.0

2

0.

25

0.0

6 2.1

0.

03

0.

8

0.

03

60 1.5

0.0

2

0.

3

0.0

4 2

0.0

4

0.

9

0.0

2 1.65

0.0

4

0.

3

0.0

5 2.3

0.

02

1.

2

0.

02

80 1.6

0.0

4

0.

4

0.0

5 2.5

0.0

5

1.

2

0.0

4 1.7

0.0

5

0.

36

0.0

4 2.6

0.

06

1.

3

0.

04

10

0 1.7

0.0

4

0.

5

0.0

8 2.7

0.0

6

1.

4

0.0

5 1.83

0.0

8

0.

42

0.0

3 2.9

0.

07

1.

5

0.

04

12

0 1.9

0.0

3

0.

6

0.0

5 2.9

0.0

8

1.

5

0.0

4 1.95

0.0

5

0.

53

0.0

2 3.2

0.

08

1.

7

0.

03

14

0 2

0.0

3

0.

7

0.0

3 3

0.0

5

1.

7

0.0

6 2.3

0.0

3

0.

67

0.0

1 3.6

0.

06

1.

9

0.

03

16 2.5

0.0 0. 0.0

3.2

0.0 1. 0.0

2.9

0.0 0. 0.0

3.8

0. 2. 0.


142

0 3 95 2 7 8 5 2 8 2 08 1 03

18

0 3

0.0

3 1

0.0

4 3.1

0.0

2 2

0.0

7 3.5

0.0

4 1

0.0

3 3.9

0.

07

2.

3

0.

03

Photodegradation of BPB under UV light by BMOs Time

(min)

TiO2 CNTs-TiO2 TiWXOY CNTs-

TiWxOy

C/C0 Error C/C0 Error C/C0 Error C/C0 Error

0 0.82 0.06 0.82 0.08 0.82 0.07 0.82 0.06

20 0.62 0.05 0.5 0.04 0.6 0.08 0.64 0.05

40 0.51 0.06 0.44 0.09 0.55 0.06 0.49 0.07

60 0.5 0.09 0.32 0.06 0.5 0.08 0.38 0.05

80 0.44 0.08 0.17 0.07 0.37 0.04 0.24 0.08

100 0.41 0.06 0.15 0.05 0.35 0.06 0.16 0.06

120 0.39 0.05 0.12 0.04 0.21 0.07 0.1 0.08

140 0.33 0.07 0.09 0.08 0.19 0.06 0.07 0.05

160 0.25 0.02 0.08 0.03 0.17 0.07 0.05 0.07

180 0.2 0.04 0.07 0.06 0.15 0.09 0.02 0.06


143

Photodegradation of BPB under visible light by BMOs Time

(min)


TiWxOy


0 0.82 0.04 0.82 0.08 0.82 0.05 0.82 0.05

20 0.74 0.05 0.64 0.07 0.62 0.07 0.57 0.04

40 0.64 0.03 0.52 0.09 0.49 0.08 0.43 0.03

60 0.56 0.04 0.36 0.05 0.41 0.04 0.37 0.06

80 0.44 0.03 0.34 0.08 0.33 0.06 0.32 0.07

100 0.36 0.05 0.26 0.03 0.24 0.05 0.22 0.03

120 0.25 0.03 0.16 0.05 0.17 0.08 0.15 0.04

140 0.2 0.06 0.15 0.07 0.14 0.06 0.12 0.06

160 0.18 0.04 0.11 0.06 0.1 0.06 0.08 0.03

180 0.16 0.02 0.09 0.07 0.08 0.05 0.07 0.02


144

Photodegradation of BPB under UV light by BMOs Time

(min)

TiO2 CNTs-TiO2 CoO-TiO2 CNTs-CoO-

TiO2


0 0.82 0.04 0.82 0.08 0.82 0.05 0.82 0.05

20 0.62 0.05 0.5 0.07 0.73 0.07 0.63 0.04

40 0.51 0.03 0.44 0.09 0.62 0.08 0.43 0.03

60 0.5 0.04 0.32 0.05 0.43 0.04 0.25 0.06

80 0.44 0.03 0.17 0.08 0.33 0.06 0.13 0.07

100 0.41 0.05 0.15 0.03 0.32 0.05 1 0.03

120 0.39 0.03 0.12 0.05 0.25 0.08 0.09 0.04

140 0.33 0.06 0.09 0.07 0.17 0.06 0.06 0.06

160 0.25 0.04 0.08 0.06 0.08 0.06 0.03 0.03

180 0.2 0.02 0.07 0.07 0.06 0.05 0.1 0.02


145

Photodegradation of BPB under visible light by BMOs

Time

(min)


TiO2


0 0.82 0.06 0.82 0.08 0.82 0.07 0.82 0.06

20 0.74 0.05 0.64 0.04 0.66 0.08 0.55 0.05

40 0.64 0.06 0.52 0.09 0.5 0.06 0.43 0.07

60 0.56 0.09 0.36 0.06 0.34 0.08 0.33 0.05

80 0.44 0.08 0.34 0.07 0.3 0.04 0.27 0.08

100 0.36 0.06 0.26 0.05 0.23 0.06 0.21 0.06

120 0.25 0.05 0.16 0.04 0.17 0.07 0.18 0.08

140 0.2 0.07 0.15 0.08 0.11 0.06 0.1 0.05

160 0.18 0.02 0.11 0.03 0.09 0.07 0.08 0.07

180 0.16 0.04 0.09 0.06 0.07 0.09 0.01 0.06


146

Photodegradation of PR under UV light by BMOs

Time

(min)


TiO2


0 0.82 0.04 0.82 0.08 0.82 0.05 0.82 0.05

20 0.8 0.05 0.72 0.07 0.72 0.07 0.63 0.04

40 0.78 0.03 0.6 0.09 0.66 0.08 0.54 0.03

60 0.76 0.04 0.42 0.05 0.55 0.04 0.43 0.06

80 0.62 0.03 0.34 0.08 0.38 0.06 0.31 0.07

100 0.58 0.05 0.28 0.03 0.31 0.05 0.28 0.03

120 0.51 0.03 0.25 0.05 0.26 0.08 0.14 0.04

140 0.46 0.06 0.2 0.07 0.21 0.06 0.12 0.06

160 0.38 0.04 0.15 0.06 0.17 0.06 0.06 0.03

180 0.36 0.02 0.12 0.07 0.09 0.05 0.02 0.02


147

Photodegradation of PR under visible light by BMOs

Time

(min)


TiO2


0 0.82 0.06 0.82 0.08 0.82 0.07 0.82 0.06

20 0.81 0.05 0.71 0.04 0.68 0.08 0.69 0.05

40 0.76 0.06 0.68 0.09 0.65 0.06 0.63 0.07

60 0.69 0.09 0.61 0.06 0.6 0.08 0.58 0.05

80 0.6 0.08 0.53 0.07 0.55 0.04 0.52 0.08

100 0.56 0.06 0.42 0.05 0.41 0.06 0.39 0.06

120 0.55 0.05 0.38 0.04 0.32 0.07 0.28 0.08

140 0.5 0.07 0.28 0.08 0.25 0.06 0.2 0.05

160 0.46 0.02 0.2 0.03 0.18 0.07 0.16 0.07

180 0.41 0.04 0.17 0.06 0.12 0.09 0.14 0.06


148

Photodegradation of PR under UV light by BMOs Time

(min)


TiWxOy


0 0.82 0.04 0.82 0.08 0.82 0.05 0.82 0.05

20 0.8 0.05 0.72 0.07 0.66 0.07 0.64 0.04

40 0.78 0.03 0.6 0.09 0.53 0.08 0.31 0.03

60 0.76 0.04 0.42 0.05 0.48 0.04 0.28 0.06

80 0.62 0.03 0.34 0.08 0.45 0.06 0.2 0.07

100 0.58 0.05 0.28 0.03 0.37 0.05 0.18 0.03

120 0.51 0.03 0.25 0.05 0.28 0.08 0.14 0.04

140 0.46 0.06 0.2 0.07 0.18 0.06 0.11 0.06

160 0.38 0.04 0.15 0.06 0.13 0.06 0.1 0.03

180 0.3 0.02 0.12 0.07 0.09 0.05 0.05 0.02


149

Photodegradation of PR under visible light by BMOs Time

(min)


TiWxOy


0 0.82 0.06 0.82 0.08 0.82 0.07 0.82 0.06

20 0.81 0.05 0.71 0.04 0.72 0.08 0.72 0.05

40 0.76 0.06 0.68 0.09 0.66 0.06 0.66 0.07

60 0.69 0.09 0.61 0.06 0.65 0.08 0.55 0.05

80 0.6 0.08 0.53 0.07 0.38 0.04 0.38 0.08

100 0.58 0.06 0.42 0.05 0.31 0.06 0.28 0.06

120 0.55 0.05 0.31 0.04 0.26 0.07 0.25 0.08

140 0.5 0.07 0.28 0.08 0.21 0.06 0.21 0.05

160 0.48 0.02 0.2 0.03 0.17 0.07 0.16 0.07

180 0.46 0.04 0.17 0.06 0.15 0.09 0.1 0.06

Documents

COPYRIGHT AND CITATION CONSIDERATIONS FOR THIS … · v ABSTRACT Titanium dioxide (TiO2) – also known as titania – exists in three common polymorphic crystal phases, i.e. anatase,