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Applied Surface Science 257 (2011) 5559–5562

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

hape controlled flower-like silicon oxide nanowires and their pH response

i Shaoa,b, Rong-hui Quea, Ming-wang Shaoa,∗, Qing Zhoua, Dorothy Duo Duo Mac, Shuit-Tong Leec

Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou,iangsu 215123, PR ChinaAnhui Key Laboratory of Functional Coordination Compounds, School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, PR ChinaCenter of Super-Diamond and Advanced Films (COSDAF), Department of Biology and Chemistry, City University of Hong Kong, Hong Kong SAR, China

r t i c l e i n f o

rticle history:eceived 9 October 2010

a b s t r a c t

Silicon oxide nanowires were synthesized with high-temperature evaporation using silicon monoxideas starting materials and tin and gallium as catalysts. The products take the shape of flowers with petals

eceived in revised form 10 January 2011ccepted 10 January 2011vailable online 18 January 2011

ey words:ilicon oxide

composed of silicon oxide nanowires. The pH response of the products reveals excellent linear relationdue to their vast surface area.

© 2011 Elsevier B.V. All rights reserved.

anomaterialsH sensor

. Introduction

Silicon oxide nanowires have attracted much interest in theanomaterial field [1–4] because of their special optical [5–7] andlectronics [8] properties. Until now, various methods have beeneveloped to synthesize silicon oxides nanowires, such as laserblation [5], sol–gel [9], thermal evaporation [10], ion bombard-ent [11,12] and chemical vapor deposition [6,13].Owing to nanomaterials’ properties depending greatly on their

rientations of the active constituents, the design of organizedanomaterials with novel structure has been an important activeesearch direction [14]. Here, we developed a method to obtain sil-con oxide nanowires with the organized morphology via Sn-Gai-catalysts. The resulting silicon oxide nanomaterials had a flow-rlike end composed of thousands of silicon oxide nanowires. Thes-prepared nanomaterials are suitable to be used as sensors toetect chemical materials due to their special morphology and largeurface area.

. Experimental details

The typical process for the preparation of silicon oxideanowires is as follows: First, 10 g of SiO powder (Aldrich, 325esh, 99.9%), 0.05 g of Sn powder (Fluka, powder, 99%) and 0.05 g

f Ga (Acros) were mixed on an alumina boat, which was then

∗ Corresponding author.E-mail address: mwshao@suda.edu.cn (M.-w. Shao).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.01.038

placed at the center of a horizontal alumina tube mounted inside ahigh-temperature tube furnace, as shown in Fig. 1. The system wasevacuated to a pressure of 10−2 Torr. A carrier gas of Ar was intro-duced and maintained at a pressure of 10 Torr with a constant flowrate of 150 sccm. The furnace was heated to 1350 ◦C at 40 ◦C/minand was kept at this temperature for 1 h. Then the temperaturewas decreased to 1050 ◦C at the rate of 1 ◦C/min and was kept atthis temperature for 10 h. The products grown on the inside surfaceof alumina tube were collected.

Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were carried out with a FEI Quanta 200FSEM spectrometer; the atomic concentration ratio of elementswas obtained utilizing peak areas and atomic sensitivity factors,which furnished semiquantitative results (within 10–20%). Andthe transmission electron microscopy (TEM) was conducted with aPhilips CM300 transmission electron microscope, with an acceler-ating voltage of 200 kV. Electrical resistances were recorded with apico-ammeter (Keithley 485).

3. Results and discussion

Fig. 2a shows a SEM image of the products that take the shapeof flower with average diameter of 100 �m. Fig. 2b shows a typicalflower consisting of dozens of petals with the average width of 3 �m

and length of 40 �m. And all the petals grow out from a stem asshown in Fig. 2c. Careful observation of one petal (shown in Fig. 2d)indicates that it is composed of tens of nanowires with substantiallyaverage diameter of 60 nm. Many petals share one catalyst ball onthe end of flowers. EDS spectra reveal that petals are silicon oxide

5560 Q. Shao et al. / Applied Surface Scie

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trode and immersed into the solution, coupling with a graphite

Ft

Fig. 1. Schematic diagram of experimental setup.

ith the Si:O atomic concentration ratio of 35:65 (inset, Fig. 2a),owever, catalyst balls contain three elements with the Ga:Si:Otomic concentration ratio of 6:33:61 (inset, Fig. 2d).

Fig. 3a is a TEM image of a small petal. Its SAED pattern shows its amorphous in nature. Fig. 3b is a large-magnification TEM image,

hich reveals that silicon oxide nanowires are 50–60 nm in diam-ter and tens of micrometer in length. Fig. 3c displays a catalyst of80 nm in diameter connecting with a petal. Fig. 3d shows a stemith the diameter of about 1.3 �m.

The Sn-Ga bi-catalysts play vital roles in the formation of thisind of morphology. If only Sn catalysts were employed, hairytructure without flower was produced. This structure had a crys-al silicon wire inside and amorphous silicon oxide hair outside15,16]. If only Ga catalysts were used, the morphology is fish-onelike, gourdlike, carrotlike, badmintonlike or octopuslike [17].nly when Sn-Ga bi-catalysts were employed, may the flowerlikeorphology be obtained.From the SEM observation, the growth process may be as follow:

irst Sn catalysts took the action and silicon wires with a Sn tip wereormed (Fig. 4a, inset is the enlarged image). No gallium peak isbserved in the EDS (Fig. 4a, inset in lower part), which means the

tomic concentration of gallium is below 1% (the detection limitf EDS was ca. 1%). The atomic concentration ratio of Sn:Si:O is8:24:58 (Fig. 4a). The reason why Sn took the action first might behat Sn has larger solubility in silicon than Ga does [18]; the silicon

ig. 2. SEM images: (a) many flowerlike products and EDS (inset), (b) flowerlike productheir tips, and (d) petals composed of tens of nanowires and EDS (inset).

nce 257 (2011) 5559–5562

oxide and tin oxide have the eutectic temperature below 900 ◦Cwhile silicon oxide and gallium oxide are above 1550 ◦C [19]. As theSn exhausted, the Ga was accumulated on the tip and grew into abud (Fig. 4b). Fig. 4c showed a bud connecting with a Ga ball, whichindicated that the bud was caused by Ga catalysts. EDS spectrum(inset, Fig. 4c) shows no Sn peak and the atomic concentration ratioof Ga:Si:O is equal to 7:28:65. With the time going on, the bud gotlarge and blossomed, resulting in the growth of many petals. It isclear to see that the petals origin from one point in Fig. 4d.

According to the above analysis, a growth model was drawnschematically as shown in Fig. 5.

The flower-like morphology has a stem on one end and a floweron the other. And the flower was composed of thousands of sili-con oxide nanowires. The stem can be acted as signal transmissionand the flower with vast amount of surface may be used to detectchemical molecules. It is favorable to be served as chemical or bio-chemical sensors. As we know, on the interface of silica there existtwo types of surface groups: singly coordinated SiOH groups anddoubly coordinated Si2O surface groups. In contact with a solution,only the singly coordinated Si–OH groups can adsorb or desorb aproton [20]. That is.

–SiO− + H+ ⇔ –Si–OH

–SiOH + H+ ⇔ –Si–OH2+

Based on above analysis, this material may have the pH-sensorfunction.

So a bunch of flowerlike silicon oxide was fabricated as an elec-

electrode. This setup was employed to measure the resistance ofthe solution at various pH concentrations. The results show that itsresistance is linear in the pH range from 2 to 12 (Fig. 6) confirmingthe above proposal.

consisting of dozens of petals, (c) all petals connected with a stem, with Ga ball on

Q. Shao et al. / Applied Surface Science 257 (2011) 5559–5562 5561

Fig. 3. TEM images: (a) a silicon oxide petal, the SAED pattern (inset) showing its amorphous nature, (b) high magnification image of silicon oxide wires with average diameterof 55 nm, (c) Ga ball with diameter of 480 nm, and (d) stem image with diameter of 1.3 �m.

Fig. 4. SEM images: (a) wires with silicon core, silicon oxide in outside and Sn on the tip, EDS (inset down) and a large magnification image (inset upper), (b) wires with bud,(c) one bud with a Ga ball on its tip and EDS (inset), and (d) petals from one origin.

5562 Q. Shao et al. / Applied Surface Scie

Fig. 5. Proposed growth model for the flower-like silicon oxide nanowires: (a) sili-con oxide growing on the Sn catalyst; (b) silicon oxide nanowire was obtained; (c)Ga droplet condensing on the tip of the nanowire; (d) a bud was grown on the Gacatalyst; and (e) petals were grown from the bud and a flower was formed.

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Ohio, USA, 2001, p. 1856, 3362.[19] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase Diagrams of Ceramists, The

American Ceramic Society, Inc., Ohio, USA, VI, p. 134, VII, p. 92, 100, VIII, p.126, 1985.

[20] A.P. Legrand, The Surface Properties of Silicas, John-Wiley & Sons Ltd., Chich-ester, England, 1998, p. 368.

Fig. 6. Electrical resistance vs. pH value.

. Conclusion

A moderate morphology of nanomaterials is necessary for theabrication of pH sensor. And this bi-catalyst method might be

nce 257 (2011) 5559–5562

applied in the preparation of desired orientation of materials tomeet the need of nanodevices.

Acknowledgements

The project was supported by the National Natural ScienceFoundation of China (21071106), Specialised Research Fund for theDoctoral Program of Higher Education (20103201110016) and theNational Basic Research Program of China (973 Program) (Grant No.2010CB934502).

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