hydrothermal synthesis of tungsten oxide nanobelts

China Received 29 January 2006; accepted 5 March 2006 Available online 3 April 2006 Abstract Tungsten oxide nanobelts have been hydrothermally fabricated at 180 °C for 12 h with sodium s

Hydrothermal synthesis of tungsten oxide nanobelts

Xuchun Songa,⁎ , Yang Zhao b

, Yifan Zhengc

a

Department of Chemistry, Fujian Normal University, Fuzhou 350007, P R China

b

Department of Chemistry, Henan Normal University, Xinxiang, 453002, P R China

c

Coll Chem Engn and Mat Sci, Zhejiang Univ Technol, Hangzhou, 310014, P R China

Received 29 January 2006; accepted 5 March 2006

Available online 3 April 2006

Abstract

Tungsten oxide nanobelts have been hydrothermally fabricated at 180 °C for 12 h with sodium sulfite and cetyltrimethylammonium bromide (CTAB) as assisted, respectively X-ray diffraction (XRD) pattern indicates that the as-prepared samples are the pure orthorhombic phase WO3 EDS spectra show that the ratio of W/O is about 1:3 The morphology was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques Based on a series of comparative experiments under different reaction conditions, the probable formation mechanism of tungsten oxide nanobelts is proposed

© 2006 Elsevier B.V All rights reserved

Keywords: Hydrothermal; Tungsten oxide; Nanobelts

1 Introduction

As a new family of one-dimensional (1D) nanostructures,

nanobelts have attracted increasing attention because of their

unique physical properties and potential applications [1–5]

Although some nanobelts made of semiconductors, metals, and

other materials have been already synthesized in various ways

[6–10], the preparation of nanobelts under mild conditions

remains a considerable challenge

WO3is a versatile wide band-gap semiconductor for many

valuable applications To date, WO3has been one of the most

extensively studied materials for electrochromic devices,

in-formation displays, sensor devices and smart windows[11–13]

In addition, WO3 has a high potential for the use in

elec-trochemical devices, such as rechargeable lithium batteries,

based upon its rich chemical intercalation reactivity However,

studies of tungsten oxides nanobelts were comparatively rare

because of lack of preparation methods for such materials Very

recently, Li et al met the challenge and developed a simple

method to prepare tungsten oxide nanobelts via physical vapor

deposition process[14] In this work, we successfully

synthe-sized tungsten oxide nanobelts via a Na2S and CTAB as assisted

by a hydrothermal method at 180 °C This strategy may offer an opportunity for the further investigation of some fundamental properties of 1D tungsten oxides and may also serve as a general method for the synthesis of 1D nanostructure

2 Experimental 2.1 Sample preparation CTAB (0.8 g) was dissolved in 35 ml deionized water to form a transparent solution Then 0.3 g H2WO4 powder and

5 ml HCl (3 M) were added to the above solution under con-tinuous stirring The resulting suspension was transferred into a

50 ml Teflon-lined stainless steel autoclave Then 1.2 g Na2S powder was added to Teflon-lined stainless steel autoclave and sealed tightly Hydrothermal treatments were carried out at

180 °C for 12 h After that, the autoclave was allowed to cool down naturally Precipitates were collected, and washed with deionized water several times and dried in air at 80 °C 2.2 Characterization

The morphologies were characterized using scanning electron microscopy (SEM, Hitachi S-4700 II, 25 kV) and transmission electron microscopy (TEM, JEM200CX, 120 kV)

Materials Letters 60 (2006) 3405 –3408

www.elsevier.com/locate/matlet

⁎ Corresponding author.

E-mail address: songxuchunfj@163.com (X Song).

0167-577X/$ - see front matter © 2006 Elsevier B.V All rights reserved.

doi: 10.1016/j.matlet.2006.03.022

The composition of the product was analyzed by energy

dis-persive X-ray detector (EDX, Thermo Noran VANTAG-ESI,

120 kV) The X-ray diffraction (XRD, Thermo ARL SCINTAG

X'TRA with CuKα irradiation, λ=0.154056 nm.) was used to

analyze the crystallinity

3 Results and discussion

The morphologies of as-prepared WO3nanobelts were determined

by SEM and TEM (Fig 1).Fig 1a and b shows the typical patterns of

WO3nanobelts prepared via the hydrothermal route by using CTAB

and Na2S as assisted; these WO3 nanobelts have a typically curly

morphology with a length from 1μm to over 3 μm and a width of 30–

100 nm; the thickness of the belts is about 15 nm The product was

further characterized by TEM accompanied by selected area electron

diffraction (SAED) and shown inFig 1c and d The result also shows

that the WO3nanobelts assemble together with some curly

morphol-ogy The corresponding SAED pattern displays its single crystalline

nature and could be indexed to the pure orthorhombic WO3and shows

the growth direction along the [200]

The electron-induced X-ray fluorescence (EDS) analysis was em-ployed to determine the composition of the tungsten oxide nanobelts As shown inFig 2, only oxygen and tungsten elements existed in the nanobelts with the molar ratio of about 3 (O/W) InFig 3, a represen-tative XRD pattern for our assynthesized tungsten oxide nanobelts is displayed All the main peaks can be indexed undisputedly to ortho-rhombic WO3(JCPDS card 20-1324) This agrees well with the SADE results No impurities could be detected in this pattern, which implies that pure WO3could be obtained under the current synthetic route The morphologies of WO3 nanocrystals synthesized in different reaction conditions were shown inFig 4a–d When 1.2 g Na2S and

5 ml HCl (3 M) were added into the reaction systems without addition

of CTAB, the prepared WO3agglomerated together severely (shown in

Fig 4a) However, if we just added CTAB not Na2S, the nanolamellars and short nanobelts in the sample were shown in theFig 4b The result indicated that nanocrystals dispersed well in the presence of CTAB and the morphologies of WO3were independent of the HCl When 1.2 g

Na2S and 0.8 g CTAB were added at the same time, the morphologies

Fig 1 (a) SEM, (b) SEM, (c) TEM, (d) TEM images of WO 3 nanobelts.

Fig 2 EDS patterns of WO nanobelts Fig 3 XRD patterns of WO nanobelts.

of WO3synthesized with 7 and 10 ml HCl (3 M) were shown inFig 4c

and d individually Compared withFig 1c (5 ml HCl), it can be known

that with the content of HCl decreased, the length and outcome of WO3

nanobelts in the products increased accordingly But when the content

of HCl decreased further, the products were the Na2WS4solution not

the WO3precipitates during the hydrothermal reaction From the

re-sults, it can be concluded that addition of Na2S and CTAB at the same

time is important and both of them affect corporately on the formation

of WO3nanobelts

The growth mechanism of crystal is determined by both the internal

structure and external conditions such as temperature, pressure, and

composition of the solution [15] A possible mechanism has been

proposed to explain the nanobelts growth in thermal evaporation Since

our situation is quite different from the dry method, the explanation of

the nanobelts growth mechanism in hydrothermal condition remains

speculative It is well known that the shape of the nanocrystals can be

controlled by adding chemical capping reagents into the solution The

selective interaction of the capping molecules on the facets of the

first-formed nanoparticles is crucial to the anisotropic growth of

nano-structures For example, we have chosen a CTAB as the capping

molecules in our experiments, and found that the CTAB has a great

influence on the morphology of WO3nanostructures In addition, it has

been known that the superposition of growth units on crystal surfaces

strongly affects the growth speed and orientation of crystals The size

and structure for growth units depend on the hydrothermal reaction

conditions The addition Na2S and proper content of HCl produced

different kinds of stable growth units (H2WO4 −xSx) The growth units

were more stable, the probability for which existed in the systems was

bigger [16] The stable growth units grew preferentially along the

crystal face (200) and produced the nanobelts in the end Moreover, it

could be found that the sulfur element was removed as the stable

growth units (H2WO4 −xSx) superposed on the crystal surfaces The

result predicted that the site for sulfur in the growth units (H2WO4−xSx)

would be the active site for the growth of crystal along the crystal face

(200) In a word, both Na2S and CTAB acted corporately during the

formation of WO nanobelts

4 Conclusion

In conclusion, large quantity and single crystalline WO3

nanobelts have been synthesized through a hydrothermal method Structural characterization by SEM and TEM revealed that the WO3 nanobelts possess orthorhombic with [200] growth orientation A possible mechanism was proposed that

Na2S and CTAB acted corporately on the formation of WO3

nanobelts The effect should be able to be extended to the synthesis of other 1D nanomaterial

Acknowledgments

We wish to acknowledge the financial support from the Foundation of Educational Committee of Fujian Province (No: K04027)

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