One-Step Synthesis and Characterization of Silica Nano

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2013 Article ID 843570 4 pageshttpdxdoiorg1011552013843570

Research ArticleOne-Step Synthesis and Characterization ofSilica Nano-Submicron Spheres by Catalyst-AssistedPyrolysis of a Preceramic Polymer

Feng Gao12 Zhijian Peng1 and Xiuli Fu2

1 School of Engineering and Technology China University of Geosciences Beijing 100083 China2 School of Science Beijing University of Posts and Telecommunications Beijing 100876 China

Correspondence should be addressed to Zhijian Peng pengzhijiancugbeducn and Xiuli Fu xiulifubupteducn

Received 2 May 2013 Accepted 17 June 2013

Academic Editor Renzhi Ma

Copyright copy 2013 Feng Gao et alThis is an open access article distributed under theCreative CommonsAttribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Silica nanospheres have attracted tremendous interest due to their importance in extensive applications However the direct large-scale fabrication of silica nanospheres with controlled morphology and high purity remains a significant challenge In this worksilica nano-submicron spheres were successfully synthesized by a simple method through pyrolysis of an amorphous polysilazanepreceramic powder with catalyst FeCl

2 The synthesized spheres possess well-designed shape with diameter of 600ndash800 nm and

high purity The surfaces of the spheres are smooth and clean without any flaws Besides the spheres are identified as amorphoussilica and their growth mechanism was also proposed

1 Introduction

In the past few years silica nanostructure materials haveattracted tremendous interest due to their importance inbasic scientific research and unique applications in nanoscaledevices [1 2] In particular extensive efforts have beenfocused on silica nanospheres because they can be widelyused in electronic substrates thermal and electrical insula-tors humidity sensors optoelectronic devices photographicemulsions the protection of environmentally sensitive mate-rials and so on [3ndash8] So new and versatile routes tosynthesize silica nanospheres which have well-defined shapeand high purity are highly desirable [9]

Current strategies for preparing silica nanospheresrely heavily on hydrolysis and hydrolytic condensationof tetraethyl orthosilicate [9] by which however theprepared silica nanospheres are of relatively low purity andthe experiment procedures need several steps [10 11] Todate the large-scale fabrication of silica nanospheres withcontrolled morphology and high purity remains a significantchallenge In the newly reported methods catalyst-assistedpyrolysis of polymeric precursors for nanostructures issimple and easy controlling and the resultant products are

of high purity Theoretically it can be applied in fabricatingvarious forms of Si-based nanostructures by adjustingthe composition of polymeric precursors atmospherecatalyst and so on [12 13] For example Peng et al [12]reported the preparation of clustered one-dimensional andapproximately stoichiometrical SiO

2amorphous nanowires

with high productivity via catalytic pyrolysis of a polymerprecursor onto alumina wafers And Li et al [14] preparedSiO2straight nanowires spring-shaped nanowires twinborn

nanospring fishbone-shaped nanowires and braided-likehelical nanowires In this paper we report the preparation ofsilica nano-submicron spheres of high purity and sphericityusing catalyst-assisted pyrolysis of a preceramic polysilazanewhich has never been reported before in the literature

2 Experimental

Polysilazanes are chain-like polymers with Si backboneand organic side-chain groups They can be called perhy-dropolysilazanes when the side groups are all hydrogenA perhydropolysilazane was used as the starting precursorwhich is a liquid with average molecular weight Mn = 900ndash1200 at room temperature The precursor was first solidified

2 Journal of Nanomaterials

by heat treatment at 260∘C for 05 h in N2 After that the

resultant amorphous SiCN chunk was crushed into powderwith an agatemortar andmixedwith 20wt FeCl

2powder as

catalystThen the powdermixturewas placed in a high-purityalumina crucible in a conventional furnace under flowinghigh-purity nitrogen (999 vol) of 120 sccm (standard cubiccentimeter per minute) The powder mixture was heated to1250∘C with a speed of 10∘Cmin and pyrolyzed there for2 h Finally the furnace was cooled naturally to room tem-perature Experiments were also performed without FeCl

2as

catalyst in order to investigate the growth mechanismThe obtained products are characterized by a field emis-

sion scanning electron microscope (SEM LEO-1530) high-resolution transmission electronmicroscope (HRTEM JEOLJEM 2011) energy dispersive X-ray (EDX) spectrometerattached to the TEM and Fourier transformation infrared(FT-IR) spectroscope

3 Results

The morphologies of the as-prepared products were firstexamined by SEM Figure 1 shows two typical SEM imagesof the as-produced silica nano-submicron spheres underdifferent magnifications From Figure 1(a) it can be seen thatthe products are of uniform morphology with high densityindicating that the present method can produce sphericmaterials in large yield A closer examination of the spheres athighermagnification as shown in Figure 1(b) reveals that theyown diameter less than 1 120583m high sphericity and smoothclean surface

The structure of the spheres was further analyzed byTEM TEM images confirm the conclusions about the mor-phology of the products drawn from SEM From Figure 2(a)it can be seen that the products possess uniformhigh spheric-ity with diameter of 600ndash800 nmHighermagnification TEMimage as shown in Figure 2(b) reveals that the surface ofthe spheres is smooth without any cracks pores or holesindicating that the present method can produce solid sphericmaterials of high quality

Figure 2(c) illustrates a typical EDX spectrum of thesphere as shown in Figure 2(a) revealing that the chemicalcompositions of the as-produced materials are of mainly Siand O and a tiny Al absorbed from the crucibleThe detectedCu element in the spectrum comes from the grid whichsupports the samples during TEM observation In additionthe quantitative analysis on the spheres shows that the atomicratio of Si O is about 1 19 revealing that the spheres arealmost stoichiometric silica

Moreover typicalHRTEM image as shown in Figure 2(d)and the highly diffusive ring pattern in the correspondingselected area electron diffraction as shown in the inset ofthis figure display that the silica nano-submicron spheres areamorphous

In order to confirm the chemical composition of theas-produced nano-submicron spheres FT-IR spectra wererecorded The collected complex set of infrared vibrationbands as illustrated in Figure 3 can be assigned separatelyas follows the band at 79760 cmminus1 is associated with

the SindashOndashSi symmetric stretch while the sharp one located at47527 cmminus1 corresponds to the SindashOndashSi or OndashSindashO bendingmode and the peak at 109392 cmminus1 to the SindashOndashSi asym-metric stretching vibrations The strong peaks at 344519and 163171 cmminus1 agree with OndashH stretching and bendingof absorbed H

2O respectively From IR analysis it can be

concluded that the as-produced nano-submicron spheres areof silica

4 Discussion

In our experiments without FeCl2 no nanostructures were

observed indicating that FeCl2directed the growth of the

silica nano-submicron spheres and the most likely sourceof oxygen that contributed to the formation of the spheresmight come from the atmosphere the low content of O

2

in the carrier gas of N2 which could supply a constant

oxygen source during the growth of nanomaterials In ourprevious work [15] Fe-rich droplets were observed on thetips of nanowires frompyrolysis of polymeric precursors with20wt FeCl

2as catalyst The only difference between the

current study and our previous one is that the oxygen partialpressure in the current one is higher than that in the previouswhich can attribute to the oxygen content in the N

2 On the

basis of the evidence mentioned above it was proposed thatFe-rich droplets were also formed and played a significantrole in the nucleation and growth of the present nanospheresSo it can be inferred that the synthesis process goes as followsThe SiCN derived from the polymer precursor reacted withoxygen survival in N

2to produce silica and CO

2via Reaction

(1) CO2reacted with C from pyrolytic product to produce

CO through Reaction (2) which reduced Fe2+ ions into Featoms during reaction After atomic Fe was produced itformed tiny eutectic liquid droplets of Fe Si and O as nucleiat above 1200∘C [16] The nanospheres gradually developedfrom the nuclei when elemental Si andO saturated in the alloyvia Reaction (3)2 SiCN (g) + 8O (g) 997888rarr 2 silica (s) + 2N (g) + 2CO

2(g)(1)

CO2(g) + C (s) 997888rarr 2CO (g) (2)

Si (g) +O2(g) 997888rarr silica (s) (3)

So the last question is why the silica grew intonanospheres instead of other morphologies such asnanowires In the present experiment the speed of flowingN2reached 120 sccm much higher than that conducted

in our previous experiment for producing nanowires in[15] which will lead to an increase in gas pressure in thesynthesis system and solubility of the corresponding gasesin the droplets at high temperature and thus a decreasein their supersaturation there And the growth rate of thenanostructures is determined by the supersaturation in thecatalyst droplet [17] Therefore the nanostructures grewslowly from the droplets and the high surface tension of thealloyed particles at high temperature promotes them to growinto nanospheres instead of nanowires

Journal of Nanomaterials 3

(a) (b)

Figure 1 Typical SEM images of the as-produced nanospheres (a) in low magnification and (b) in high magnification

(a) (b)

7000

6000

5000

4000

3000

2000

1000

00 2 4 6 8 10

Energy (keV)

Cou

nts

C

O

AlCu

Si

CuCu

(c) (d)

Figure 2 TEM results of the as-produced nanospheres (a) low-magnification TEM image (b) high-magnification TEM image (c) EDXspectrum of the sphere as-illustrated in Figure 2(a) and (d) HRTEM image of a nanosphere and its corresponding selected-area electrondiffraction pattern

5 Conclusions

Silica nano-submicron sphereswere synthesized via catalyst-assisted pyrolysis of a perhydropolysilazane precursor

The spheres possess well-designed shape with diameterof 600ndash800 nm high purity and smooth clean surfacewithout any flaws The spheres were identified as amorphoussilica The growth of the nanospheres might be controlled


105

100

095

090

085

080

075

070

Tran

smitt

ance

()

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Wavenumber (cmminus1)

79760SindashOndashSi symmetric stretching vibration

109392SindashOndashSi asymmetric stretching vibration

163171OndashH bending vibration

47527SindashO

bendingvibration 344519

OndashH stretching vibration

Figure 3 Typical FT-IR spectrum of the as-produced sphericmaterials

by the high pressure in the synthesis system and highsurface tension of the alloyed particles at high temperatureThe reported technique would provide a facile method tofabricate high-quality nanospheres

Acknowledgments

The authors would like to thank the financial support forthis work from the National Natural Science Foundation ofChina (Grants nos 61274015 11274052 and 51172030) PhDPrograms Foundation by Ministry of Education of China(Grant no 20100022110002) Excellent Adviser Foundationin China University of Geosciences from the FundamentalResearch Funds for the Central Universities and NationalBasic Research ProgramofChina (Grant no 2010CB923200)

References

[1] W Y Yang Z P Xie J J Li et al ldquoUltra-long single-crystalline120572-Si3N4nanowires derived from a polymeric precursorrdquo Jour-

nal of the American Ceramic Society vol 88 no 6 pp 1647ndash1650 2005

[2] Y N Xia P D Yang Y G Sun et al ldquoOne-dimensionalnanostructures synthesis characterization and applicationsrdquoAdvanced Materials vol 15 no 5 pp 353ndash389 2003

[3] J Zygmunt F Krumeich andRNesper ldquoNovel silica nanotubeswith a high aspect ratio synthesis and structural characteriza-tionrdquo Advanced Materials vol 15 no 18 pp 1538ndash1541 2003

[4] J Jang and H Yoon ldquoA top-down approach to fullerenefabrication using a polymer nanoparticle precursorrdquo AdvancedMaterials vol 16 no 18 pp 1650ndash1653 2004

[5] N I Kovtyukhova T E Mallouk and T S Mayer ldquoTemplatedsurface sol-gel synthesis of SiO

2nanotubes and SiO

2-insulated

metal nanowiresrdquo Advanced Materials vol 15 no 10 pp 780ndash785 2003

[6] MMHessienMM Rashad R R Zaky et al ldquoControlling thesynthesis conditions for silica nanosphere from semi-burnedrice strawrdquo Materials Science and Engineering B vol 162 no 1pp 14ndash21 2009

[7] N Venkatathri and S Nanjundan ldquoSynthesis and characteri-zation of a mesoporous silica microsphere from polystyrenerdquoMaterials Chemistry and Physics vol 113 no 2-3 pp 933ndash9362009

[8] F Caruso ldquoHollow capsule processing through colloidal tem-plating and self-assemblyrdquo Chemistry vol 6 no 3 pp 413ndash4162000

[9] CA Bradley BDYuhasM JMcMurdo et al ldquoFunctionalizedsılıcone nanospheres synthesis transition metal immobiliza-tion and catalytic applicationsrdquo Chemistry of Materials vol 21no 1 pp 174ndash185 2009

[10] N Venkatathri ldquoSynthesis of mesoporous silica nanosphereusing different templatesrdquo Solid State Communications vol 143no 10 pp 493ndash497 2007

[11] G Gnana Kumar S Senthilarasu D N Lee et al ldquoSynthesisand characterization of aligned SiO

2nanosphere arrays spray

methodrdquo Synthetic Metals vol 158 no 17 pp 684ndash687 2008[12] Z J Peng X L Fu N Zhu X Guo C Wang and Z

Fu ldquoPreparation and growth mechanism of clustered one-dimensional SiO

119909amorphous nanowires by catalytic pyrolysis

of a polymer precursorrdquo Journal of Non-Crystalline Solids vol135 no 43 pp 2156ndash2159 2009

[13] N Zhu Z J Peng C BWang Z Fu andHMiao ldquoPreparationand characterization of bundled one-dimensional Si

3N4single-

crystalline nanowires by catalytic pyrolysis of a polymer precur-sorrdquo Solid State Sciences vol 11 no 6 pp 1094ndash1097 2009

[14] J P Li Z J Zhang YM Luo L Guo and Z Xie ldquoSynthesis andcharacterization of amorphous SiO

2nanowires derived from a

polymeric precursorrdquo Journal of Nanoscience and Nanotechnol-ogy vol 8 no 2 pp 997ndash1002 2008

[15] X L Fu N Zhu and Z J Peng ldquoOne-step synthesis and charac-terization of tree-like branched alpha-Si

3N4nanosubmicron-

structures by pyrolysis of a polymer precursorrdquo Solid StateScience vol 14 no 9 pp 1267ndash1272 2012

[16] Z J Peng N Zhu X L Fu et al ldquoGrowth and mechanismof network-like branched Si

3N4nanostructuresrdquo Journal of the

American Ceramic Society vol 93 no 8 pp 2264ndash2267 2010[17] N Wang Y Cai and R Q Zhang ldquoGrowth of nanowiresrdquo

Materials Science and Engineering R vol 60 no 1ndash6 pp 1ndash512008

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Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawi Publishing Corporation httpwwwhindawicom Volume 2013

The Scientific World Journal

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NanoparticlesJournal of


Smart Materials Research



MetallurgyJournal of

BioMed Research International


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Na

nom

ate

ria

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Journal ofNanomaterials


by heat treatment at 260∘C for 05 h in N2 After that the

resultant amorphous SiCN chunk was crushed into powderwith an agatemortar andmixedwith 20wt FeCl

2powder as

catalystThen the powdermixturewas placed in a high-purityalumina crucible in a conventional furnace under flowinghigh-purity nitrogen (999 vol) of 120 sccm (standard cubiccentimeter per minute) The powder mixture was heated to1250∘C with a speed of 10∘Cmin and pyrolyzed there for2 h Finally the furnace was cooled naturally to room tem-perature Experiments were also performed without FeCl

2as

catalyst in order to investigate the growth mechanismThe obtained products are characterized by a field emis-

sion scanning electron microscope (SEM LEO-1530) high-resolution transmission electronmicroscope (HRTEM JEOLJEM 2011) energy dispersive X-ray (EDX) spectrometerattached to the TEM and Fourier transformation infrared(FT-IR) spectroscope

3 Results

The morphologies of the as-prepared products were firstexamined by SEM Figure 1 shows two typical SEM imagesof the as-produced silica nano-submicron spheres underdifferent magnifications From Figure 1(a) it can be seen thatthe products are of uniform morphology with high densityindicating that the present method can produce sphericmaterials in large yield A closer examination of the spheres athighermagnification as shown in Figure 1(b) reveals that theyown diameter less than 1 120583m high sphericity and smoothclean surface

The structure of the spheres was further analyzed byTEM TEM images confirm the conclusions about the mor-phology of the products drawn from SEM From Figure 2(a)it can be seen that the products possess uniformhigh spheric-ity with diameter of 600ndash800 nmHighermagnification TEMimage as shown in Figure 2(b) reveals that the surface ofthe spheres is smooth without any cracks pores or holesindicating that the present method can produce solid sphericmaterials of high quality

Figure 2(c) illustrates a typical EDX spectrum of thesphere as shown in Figure 2(a) revealing that the chemicalcompositions of the as-produced materials are of mainly Siand O and a tiny Al absorbed from the crucibleThe detectedCu element in the spectrum comes from the grid whichsupports the samples during TEM observation In additionthe quantitative analysis on the spheres shows that the atomicratio of Si O is about 1 19 revealing that the spheres arealmost stoichiometric silica

Moreover typicalHRTEM image as shown in Figure 2(d)and the highly diffusive ring pattern in the correspondingselected area electron diffraction as shown in the inset ofthis figure display that the silica nano-submicron spheres areamorphous

In order to confirm the chemical composition of theas-produced nano-submicron spheres FT-IR spectra wererecorded The collected complex set of infrared vibrationbands as illustrated in Figure 3 can be assigned separatelyas follows the band at 79760 cmminus1 is associated with

the SindashOndashSi symmetric stretch while the sharp one located at47527 cmminus1 corresponds to the SindashOndashSi or OndashSindashO bendingmode and the peak at 109392 cmminus1 to the SindashOndashSi asym-metric stretching vibrations The strong peaks at 344519and 163171 cmminus1 agree with OndashH stretching and bendingof absorbed H

2O respectively From IR analysis it can be

concluded that the as-produced nano-submicron spheres areof silica

4 Discussion

In our experiments without FeCl2 no nanostructures were

observed indicating that FeCl2directed the growth of the

silica nano-submicron spheres and the most likely sourceof oxygen that contributed to the formation of the spheresmight come from the atmosphere the low content of O

2

in the carrier gas of N2 which could supply a constant

oxygen source during the growth of nanomaterials In ourprevious work [15] Fe-rich droplets were observed on thetips of nanowires frompyrolysis of polymeric precursors with20wt FeCl

2as catalyst The only difference between the

current study and our previous one is that the oxygen partialpressure in the current one is higher than that in the previouswhich can attribute to the oxygen content in the N

2 On the

basis of the evidence mentioned above it was proposed thatFe-rich droplets were also formed and played a significantrole in the nucleation and growth of the present nanospheresSo it can be inferred that the synthesis process goes as followsThe SiCN derived from the polymer precursor reacted withoxygen survival in N

2to produce silica and CO

2via Reaction

(1) CO2reacted with C from pyrolytic product to produce

CO through Reaction (2) which reduced Fe2+ ions into Featoms during reaction After atomic Fe was produced itformed tiny eutectic liquid droplets of Fe Si and O as nucleiat above 1200∘C [16] The nanospheres gradually developedfrom the nuclei when elemental Si andO saturated in the alloyvia Reaction (3)2 SiCN (g) + 8O (g) 997888rarr 2 silica (s) + 2N (g) + 2CO

2(g)(1)

CO2(g) + C (s) 997888rarr 2CO (g) (2)

Si (g) +O2(g) 997888rarr silica (s) (3)

So the last question is why the silica grew intonanospheres instead of other morphologies such asnanowires In the present experiment the speed of flowingN2reached 120 sccm much higher than that conducted

in our previous experiment for producing nanowires in[15] which will lead to an increase in gas pressure in thesynthesis system and solubility of the corresponding gasesin the droplets at high temperature and thus a decreasein their supersaturation there And the growth rate of thenanostructures is determined by the supersaturation in thecatalyst droplet [17] Therefore the nanostructures grewslowly from the droplets and the high surface tension of thealloyed particles at high temperature promotes them to growinto nanospheres instead of nanowires


(a) (b)


(a) (b)

7000

6000

5000

4000

3000

2000

1000

00 2 4 6 8 10

Energy (keV)

Cou

nts

C

O

AlCu

Si

CuCu

(c) (d)


5 Conclusions




105

100

095

090

085

080

075

070

Tran

smitt

ance

()

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000





47527SindashO





Acknowledgments


References







2nanotubes and SiO

2-insulated














3N4single-






3N4nanosubmicron-












ISRN Corrosion




Advances in




ISRN Ceramics



Volume 2013

MaterialsJournal of


Journal of





ISRN Nanotechnology












Na

nom

ate

ria

ls




(a) (b)


(a) (b)

7000

6000

5000

4000

3000

2000

1000

00 2 4 6 8 10

Energy (keV)

Cou

nts

C

O

AlCu

Si

CuCu

(c) (d)


5 Conclusions




105

100

095

090

085

080

075

070

Tran

smitt

ance

()

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000





47527SindashO





Acknowledgments


References







2nanotubes and SiO

2-insulated














3N4single-






3N4nanosubmicron-












ISRN Corrosion




Advances in




ISRN Ceramics



Volume 2013

MaterialsJournal of


Journal of





ISRN Nanotechnology












Na

nom

ate

ria

ls




105

100

095

090

085

080

075

070

Tran

smitt

ance

()

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000





47527SindashO





Acknowledgments


References







2nanotubes and SiO

2-insulated














3N4single-






3N4nanosubmicron-












ISRN Corrosion




Advances in




ISRN Ceramics



Volume 2013

MaterialsJournal of


Journal of





ISRN Nanotechnology












Na

nom

ate

ria

ls









ISRN Corrosion




Advances in




ISRN Ceramics



Volume 2013

MaterialsJournal of


Journal of





ISRN Nanotechnology












Na

nom

ate

ria

ls



Documents

One-Step Synthesis and Characterization of Silica Nano