Materials Letters 65 (2011) 2823–2825

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Materials Letters

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Synthesis of mesoporous tungsten oxide nanofibers usingthe electrospinning method

Tuan-Anh Nguyen, Tae-Sun Jun, Muhammad Rashid, Yong Shin Kim ⁎

Department of Applied Chemistry and Graduate School of Bio-Nano Technology, Hanyang University, Ansan 426-791, Republic of Korea

⁎ Corresponding author. Tel.: +82 31 400 5507; fax:E-mail address: yongshin@hanyang.ac.kr (Y.S. Kim).

0167-577X/$ – see front matter. Crown Copyright © 20doi:10.1016/j.matlet.2011.05.103

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 May 2011Accepted 26 May 2011Available online 2 June 2011

Keywords:Tungsten oxideWO3 nanofiberElectrospinningMesoporous materials

Mesoporous tungsten oxide nanofibers were synthesized via a 500 °C thermal treatment of compositenanofibers prepared by electrospinning an ethanol solution consisting of tungsten ethoxide, P123 triblockcopolymer, and polyvinylpyrrolidone. The as-electrospun composites exhibited unwoven nanofibers with anaverage diameter of 233 nm and a smooth surface morphology. During the calcination process, the compositenanofibers were shrunk to 85 nm in diameter and converted into rough, wormhole-like nanofibers. Thesewere formed by agglomerating polycrystalline WO3 particles of 10–30 nm along the axial direction.Furthermore, a measured pore-size distribution indicated that this nanofiber mat had different types of meso-sized porosities, which may have resulted from their wormhole-like structures and inter-fiber voids. Inaddition, it was observed to have the intra-grain porosity with the diameter of about 1.0 nm.

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

Tungsten oxide has attracted the great attention of many scientistsbecause of its promising electrochromic, photochromic and semicon-ducting properties and its extensive applications in optical devices,gas sensors, electrochromic windows, and photocatalysts [1–5]. Itsone-dimensional nanostructures in the forms of nanorods, nanowiresor nanofibers can effectively facilitate electron transport and enlargean active surface area. These nanostructures have been extensivelystudied by using numerous synthetic approaches including chemicalvapor deposition [6], solvothermal route [7], and electrospinning[8,9]. Among these approaches, electrospinning offers a cost-effectiveand versatile method for producing, in large quantities, very longWO3

nanofibers with diameters of 50–1000 nm.Recently tungsten oxides have been prepared in mesoporous

forms using the soft template method based on amphiphillic blockcopolymers such as Pluronic P123 and F127 [10–12]. These mesopor-ous WO3 materials have exhibited improved electrochromic and gas-sensing properties compared to dense films. These enhancements areusually believed to be the result of a high surface-to-volume ratioresulting from having a large, controllable pore size.

Here, we have synthesized mesoporous WO3 nanofiber mats bymeans of electrospinning and a surfactant-templated sol-gel process.Tungsten ethoxidewas used as a tungsten oxide precursor while P123triblock copolymer was a structure-directional agent. To the best ofour knowledge, there has been no report about mesoporous WO3

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nanofibers even though TiO2 and SiO2 systems had been reportedpreviously [13–15].

2. Experimental

2.1. Materials

WO3 nanofibers were synthesized by using commercial chemicals:tungsten ethoxide (Alfa Aesar), Pluronic P123 triblock copolymer(Mw=5800 g/mol, Sigma-Aldrich), polyvinylpyrrolidone (PVP) poly-mer (Mw=1,300,000 g/mol, Sigma-Aldrich) and anhydrous ethylalcohol (Daejung Chemicals).

2.2. Synthesis of mesoporous WO3 nanofibers

A block copolymer solution was prepared by dissolving 0.2 g P123in 5 ml anhydrous ethyl alcohol. A tungsten precursor of W(OC2H5)6was slowly added into this solution to be 0.8 wt.%. Another ethanolsolution of 4.0 wt.% PVP was prepared in a separate bottle, and thenmixed with the W(OC2H5)6/P123 solution. This was magneticallystirred for 12 h at room temperature to achieve a homogenouscomposite solution. The composite solution was loaded into a 3 mlplastic syringe with a metal needle at the tip. A high-voltage of 5.5 kVwas applied to the needle, and a grounded tube collector was placed10 cm away from the needle tip. Composite WO3 nanofibers werecollected on a piece of aluminum foil at a steady flow rate of 0.4 ml/h.Then, to form WO3 nanofibers, the as-electrospun nanofibers werethermally treated at 120 °C overnight and then calcined at 500 °C inair for 3 h. A slow heating rate of 2 °C/min was used to ensure theformation of mesoporous WO3 nanofibers.

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2.3. Material characterization

An average diameter and morphology of the nanofibers wereobserved by using scanning electron microscopy (SEM; S-4800,Hitachi). More detailed microstructures were observed by transmis-sion electron microscopy (TEM; JEM-2100F, JEOL) under an acceler-ating voltage of 120 kV. X-ray diffraction (XRD) analysis wasperformed with a diffractive meter (D/Max-2500, Rigaku) using CuKα radiation. Fourier transform infrared spectroscopy (FT-IR; Scimitar1000, Varian)was utilized to identify organic functional groupswithinthe nanofibers. A surface area and a pore size distribution were alsoevaluated from nitrogen sorption isotherms measured by an analyzer(Tristar II, Micromeritics) at 77 K.

3. Results and discussion

3.1. Diameter distribution of WO3 nanofibers

Fig. 1A shows a surface SEM image of as-electrospun compositenanofibers prepared from the mixture solution of W(OC2H5)6, PVPand P123. This image exhibits randomly oriented nanofibers eachwith a smooth surface. The average diameter of the compositenanofibers was 233±46 nm as displayed in the inset. A correspond-

Fig. 1. SEM images and diameter distributions (see insets) of (A) as-electrospuncomposite nanofibers and (B) thermally-treated WO3 nanofibers.

ing SEM image after the 500 °C heat treatment of the compositesample is shown in Fig. 1B, clearly indicating distinct changes indiameter and surface morphology. The average diameter decreased to85±37 nm, and the nanofiber surface became extremely rough. Inmore magnified SEM images, the nanofibers were observed to haveformed by agglomerating small-sized particles less than 50 nm alongthe axial direction. This diameter reduction could be due to thecalcination of organic polymer ingredients from the as-preparedcomposite nanofibers. Furthermore, the formation of a rough surfacesuggests that WO3 nanoparticles were effectively produced from thetungsten oxide precursors and located along the 1-D direction duringthe calcination process.

3.2. Material characterization of WO3 nanofibers

Fig. 2A shows FT-IR spectra of the as-electrospun composite andthermally-treated nanofibers. The calcined sample exhibited veryintensive band structures in the region of 500–1000 cm−1 withmaximum absorption at 818 cm−1. These structures could beassigned to the O–W–O stretching modes of monochromic tungstenoxide [16], thus supporting the formation of WO3 nanofibers from the

Fig. 2. (A) FT-IR spectra of the as-electrospun composite and thermally-treatednanofibers, and (B) XRD diffraction pattern of the thermally-treated nanofibers. The barplot corresponds to reference pattern obtained from the orthorhombic WO3 crystal(JCPDS Card No. 20-1324).

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composite nanofibers via the thermal treatment. The broad bands ofthe calcined sample at 1630 and 1392 cm−1 seem to have resultedfrom weakly-bound H2O adsorbates in ambience atmosphere.Furthermore, the characteristic bands observed at 1000–1800 cm−1

in the composite sample could be assigned as followings: ν(C_O)mode for the 1655 cm−1 peak, δ(C_O) for the 1292 cm−1, ν(C\N)for the 1110 cm−1, δ(CH2) for the 1463 cm−1, and δ(CH3) for the1380 cm−1 [17,18]. These results suggest the presence of PVP andP123 polymers within the composite nanofibers and the removal ofthe organic moieties during the 500 °C annealing process.

A typical XRD diffraction pattern of the WO3 nanofibers isdisplayed in Fig. 2B. Many intensive peaks were observed at 2θ valuesof 23.04, 23.49, 24.10, 28.68, 33.98, 41.56, and 55.62. These positionswere well corresponded to the major planes of an orthorhombic WO3

crystal (JCPDS Card No. 20-1324). It confirmed that the nanofiberswere comprised of polycrystalline WO3 crystals. In addition, theaverage crystal sizes were calculated from the FWHM values ofindividual peaks by means of the Debye–Scherrer equation. The sizeswere 10.9 nm for the most intensive (001) plane, and 12.0, 22.4 and16.6 nm for the non-overlapped peaks of (120), (111) and (221),respectively.

Fig. 3. (A) TEM images and (B) nitrogen adsorption/desorption isotherms of our porousWO3 nanofiber sample. The inset displays a pore size distribution of the sample.

3.3. Mesoporous structure in WO3 nanofibers

Detailed structural features of theWO3 nanofiberswere observed byTEM. Fig. 3A shows a TEM image taken for one nanofiber that wasformedby theaxial agglomerationofmanycrystalswith individual sizesof 10–30 nm. These sizes are well consistent with the average crystalsizes calculated using the FWHM in XRD. The nanofiber appears to havea rough surface morphology and wormhole-like porosity with a size ofseveral nanometers. The inset displays a typical high-resolution TEMimage of the WO3 nanofiber. The image clearly exhibits a crystalstructure of (001) planes that were observed as themost intensive peakin the XRD. It demonstrates highly crystalline characteristics of theWO3

nanofiber. In addition, crystal structures with pore diameters of 0.7 and1.0 nm were also observed locally in the circled regions, indicating theexistence of the microporosity within the grain.

Fig. 3B shows nitrogen adsorption and desorption isotherms of theWO3 nanofibers, and the inset displays a corresponding pore sizedistribution calculated using the BJH model. Two maximum peakswere observed at around 4 and 7 nm, and the pore volume wasgradually decreased in the region of 10–100 nm. This result indicatesthat our WO3 nanofibers were mesoporous. The small-sized porosity(less than 10 nm) could be attributed to the inter-gain space withinthe nanofibers as mentioned above in TEM. On the other hand, thelarge-sized porosity could have resulted from the void among thenanofibers. The BET surface area and specific pore volume of thesample were 39.2 m2/g and 0.05 cm3/g, respectively.

4. Conclusion

Unwoven WO3 nanofiber mats were successfully prepared bycombining of electrospinning and sol-gel chemistry of W(OC2H5)6under the soft template by P123 triblock copolymer. Through variousmaterial analyses, the resultant mat has been confirmed to consist ofpolycrystalline WO3 particle-aggregated nanofibers with an averagediameter of 85 nm. This WO3 mat was mesoporous due to wormhole-like fibrous morphology and space between unwoven nanofibers.They could act as an excellent active layer in the electrochromic andgas-sensing devices due to their high surface-to-volume ratio andeffective electron transportation.

Acknowledgments

This work was supported by the research fund of the HanyangUniversity (HY-2010-000-0000-0860).

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