synthesis of bundled tungsten oxide nanowires with controllable morphology

Accompanied by an apparent drop of specific surface area from 151 m2g− 1for the longer nanowires synthesized using a lower concentration of WCl6to 106 m2g− 1for the shorter nanowires syn

Synthesis of bundled tungsten oxide nanowires with

controllable morphology

a

School of Materials Science and Engineering, Shandong University, Jinan 250061, China

b

Nanotubes Laboratory, Advanced Materials Research Group, School of Mechanical, Materials and Manufacturing Engineering,

the University of Nottingham, Nottingham NG7 2RD, UK

A R T I C L E D A T A A B S T R A C T

Article history:

Received 18 September 2008

Received in revised form

12 November 2008

Accepted 12 November 2008

Bundled tungsten oxide nanowires with controllable morphology were synthesized by a simple solvothermal method with tungsten hexachloride (WCl6) as precursor and cyclohexanol as solvent The as-synthesized products were systematically characterized

by using scanning electron microscopy, X-ray diffraction and transition electron microscopy Brunauer–Emmett–Teller gas-sorption measurements were also employed Accompanied by an apparent drop of specific surface area from 151 m2g− 1for the longer nanowires synthesized using a lower concentration of WCl6to 106 m2g− 1for the shorter nanowires synthesized using a higher concentration of WCl6, a dramatically morphological evolution was also observed With increasing concentration of tungsten hexachloride (WCl6)

in cyclohexanol, the nanostructured bundles became larger, shorter and straighter, and finally a block-shape product occurred

© 2008 Elsevier Inc All rights reserved

Keywords:

Tungsten oxide nanowires

Solvothermal

Brunauer–Emmett–Teller

1 Introduction

One-dimensional (1−D) nanostructured materials, such as

nanorods, nanowires and nanotubes have attracted

tremen-dous attention owing to their unique physical, chemical and

optical properties[1–3] Among them, tungsten oxides WOx

(x = 2–3) are of much importance due to their potential

application in electrochromic display, semiconductor gas

sensors and photocatalysts [4–6] Particularly, 1-D W18O49

nanomaterials which exhibit unusual structural defects have

received special attention in recent years[7] Following the

first synthesis of W18O49nanowires by breaking the

mircro-trees, a variety of techniques including thermal treatments,

vapor phase growth, etc have been employed in the

produc-tion of W18O49 nanostructures, and their structure and

property characterizations have also been investigated

thor-oughly[8–10] Nonetheless, most of the synthetic methods

involved high temperature or complicated procedures,

mak-ing them difficult for production in large quantity Thus, wet chemical methods, which are relative simple and promising in large-scale production, need to be further developed Very recently, W18O49nanowires have been successfully synthesized by soft-chemistry methods with WCl6or W (CO)6

as the main precursor in different solvents that are usually alcohol, water and cyclohexanol[11, 12] Following this facile route, we have reported the successful synthesis of ultra-thin bundled tungsten oxide nanowires via a solvothermal method In addition, the morphology and phase transforma-tion behaviour of the ultra-thin bundled nanowires under thermal processing were also characterized in detail[13] In this paper, further expanding from our previous work, we demonstrate the synthesis of bundled tungsten oxide nano-wires with various morphologies by simply changing the concentration of tungsten chloride (WCl6) in cyclohexanol The as-synthesized bundles were systematically character-ized and influences of the morphology on their specific

⁎ Corresponding author School of Materials Science and Engineering, Shandong University, Jinan 250061, China Tel.: +86 531 88396145; fax: +86 531 82616431

E-mail address:sunshibin1982@yahoo.com.cn(S Sun)

1044-5803/$– see front matter © 2008 Elsevier Inc All rights reserved

doi:10.1016/j.matchar.2008.11.009

surface areas were also investigated Possible mechanisms

were proposed

2 Experimental

Typically, WCl6was dissolved in 2 ml of ethanol in a beaker to

obtain a solution Cyclohexanol was then added to the

solution, which was subsequently transferred to and sealed

in a PTFE-line 120 ml autoclave The concentration of WCl6in

cyclohexanol varied from 0.003M to 0.007M in our

experi-ments The autoclave was heated in a furnace at 200 °C for 6 h,

to realize the synthesis After thorough washing with water,

ethanol and acetone several times, the centrifuged products

were used for further examination

The structure, morphology and phase composition of the

as-synthesized products were then characterized by using a

scanning electron microscope (SEM, Philips XL 30, operated at

20 kV) equipped with energy-dispersive X-ray spectroscopy

(EDX), a X-ray diffractometer (XRD, Siemens D500, Cu

radia-tion) and a transmission electron microscope (TEM, JEOL

2000FX, 200 kV) Selected area electron diffraction (SAED)

investigation was also performed during the TEM experiment

Brunauer–Emmett–Teller (BET) gas-sorption measurements

were conducted by using an Autosorb-1 sorptometer The

surface area was calculated using the BET method based on

adsorption data in the partial pressure (P/Po) range 0.02–0.22,

and total pore volume was determined from the amount of

nitrogen adsorbed at P/Po= 0.99

3 Results and discussions

Fig 1 shows SEM images of the as-synthesized products at concentrations ranging from 0.003M to 0.007M As shown in

Fig 1a, the as-synthesized product at a concentration of 0.003M exhibits ultrathin features with length up to few microns Further TEM investigation can identify the bundled feature, giving evidence that each 1-D nanostructured bundles consist of nanowires with diameters of about 2-10 nm, as shown inFig 2a This bundled feature was often observed in thin and long 1-D nanostructured materials due to their large surface areas[14] The SAED pattern (top left inset,Fig 2b) of the bundles exhibits broadened and strand spots, suggesting that individual nanowires within the bundle all adopted the same growth direction This is a typical characteristic of bundled 1-D nanostructured materials Increasing the con-centration to 0.004M, there is no evidence of major changes in morphology except that the bundles become shorter When the concentration increased to 0.005M, apparent evolution can

be observed As displayed in Fig 1c, thicker and shorter bundles with uniform size occurred, and they appeared to be composed of small bundles.Fig 2c is a corresponding TEM image of the bundles synthesized at the concentration of 0.005 M In comparison with the TEM image of the bundles synthesized at the concentration of 0.003 M (Fig 2a), major change in the morphology can be seen The diameter of the bundles increased to about 150 nm and individual nanowires cannot be discriminated At a high concentration of 0.007 M, the as-synthesized product exhibits block-like structure rather

Fig 1– SEM images of bundled tungsten oxide nanowires synthesized at different concentrations of (a) 0.003 M, (b) 0.004 M, (c) 0.005M and (d) 0.007 M Inset inFig 1d is a high magnification image

than bundled feature The irregular blocks are of about 8μm in

length and 3μm in diameter By careful examination (inset in

Fig 1d), it can be found that some bundles are randomly

distributed on the surface of these blocks

XRD analysis was carried out to identify the crystalline

structure of the as-synthesized products As shown inFig 3a,

the main diffraction peaks of the products synthesized at

concentrations of 0.003 M and 0.004 M match well with the

monoclinic W18O49phase (JCPDS No.71-2450) It should be noted

that the overall intensities of the diffraction spots are weak;

whilst the strongest peak intensity of (010) plane indicates that

theb010N is the dominant growth direction and that the

close-packed plane (010) is roughly perpendicular to the nanowire axis

in this monoclinic regime The HRTEM image (Fig 2b) indicates

that the lattice fringe separations are measured ca 0.38 nm and

0.37 nm, respectively, which can be indexed as (010) and (103) of

monoclinic W18O49and in agreement with the election

diffrac-tion results (inset inFig 2b)[15] At concentrations of 0.005M and

0.007M, some new peaks can be observed from the patterns of

the as-prepared products We cannot identify the new peaks

exactly, but they are believed to correspond to WO3 − xbecause all

the as-synthesized products consist only of tungsten and

oxygen elements based on EDX results (Fig 3b) However, the

major peaks can still be assigned to W18O49with the strongest

intensity of (010) plane, indicating that the growth direction of

nanowires remained unchanged with increasing concentration

It has been reported that the concentration of the

precursors greatly influences the morphology of the

hydro-thermal products[16] The shape of a crystal is determined by

the difference in the relative growth rates of the individual

crystal planes and the resulting particles are anisotropic in

shape under certain supersaturation In our work, at lower

precursor concentration, ultrathin and long bundles

com-posed of numbers of nanowires can be obtained, while larger

and shorter bundles or even block-shape products were

produced with increasing concentration Here, we believe

that low solution concentration contributed to the lower

supersaturation of the tungsten source, promoting the growth

of tungsten oxide nanowires[11] At higher concentration, the

highly saturated WCl could prohibit the growth of tungsten

oxide nanowires along theb010N direction, leading to short nanowires, and finally resulting in shorter and thicker bundles due to agglomeration As indicated in Fig 2, the resulting

Fig 3– (a) XRD patterns of bundled tungsten oxide nanowires synthesized at different concentrations ranging from 0.003 M

to 0.007 M Peaks of monoclinic W18O49are marked by + and Unassigned peaks are marked by *; (b) EDX profile of bundles synthesized at concentration of 0.007M

Fig 2– (a) TEM and (b) HRTEM images of bundled tungsten oxide nanowires synthesized at a concentration of 0.003 M; (c) TEM of bundled tungsten oxide nanowires synthesized at a concentration of 0.005 M Inset in Fig 2b is the corresponding SAED patterns of the bundled nanowires inFig 2a

products synthesized at high concentrations are a mixture of

different types of WO3− x The formation of the tungsten oxide

mixture may be related to the oxygen content in the reaction

system As the precursor concentration increases, the mole

fraction of oxygen decreases, which will inevitably lead to

oxygen vacancies in the as-synthesized tungsten oxides In

addition, EDX result of the product prepared at concentration

of 0.007M indicates that the O/W atomic ratio is about 2.4

(Fig 3b) Therefore, it is suggested that the as-synthesized

products at high concentration should be a mixture of W18O49

(WO2.72) and WO2

BET gas-sorption measurements were employed to

evalu-ate the specific surface area and porous features of the

bundled tungsten oxide nanowires The calculated specific

surface area and pore volume of the longer bundles

synthe-sized using concentration of 0.003 M are 151 m2/g and 0.51 cc/g

[12], whilst the shorter bundles synthesized using

concentra-tion of 0.005 M are 106 m2/g and 0.21 cc/g, respectively The

high specific surface area of the bundled nanowires can be

ascribed to a combination of the ultra-thin feature of

individual nanowires and the unique packing characteristic

of the bundles themselves The high specific surface area is

also associated with the sizes and distributions of the pores,

which arise from the solvothermal process at low temperature

and inter-nanowire spaces with bundles[12] The shorter and

thicker bundles will consequently lead to decreased pore

volumes (e.g from 0.51 cc/g to 0.21 cc/g) with increasing

concentration, which in turn resulted in the decreased specific

surface area

4 Conclusion

In summary, bundled tungsten oxide nanowires with

con-trollable morphology were prepared by a simple solvothermal

method with tungsten hexachloride (WCl6) as precursor and

cyclohexanol as solvent With increasing concentration of

tungsten hexachloride (WCl6) in cyclohexanol, dramatically

morphological evolution can be observed The bundles

became larger, shorter and straighter, and finally a

block-shape product occurred The resulting longer tungsten oxide

bundles exhibit a high specific surface of 151 m2g− 1, which

decreased to 106 m2g− 1for shorter tungsten oxide bundles

Acknowledgement

We thank the China Scholarship Council (CSC) of the Ministry

of Education for sponsoring the study of SBS in the UK, and the

EPSRC (UK) for financial support

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