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Physica B 396 (2007) 164–168

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Optical, electrochemical and structural properties of long-term cycledtungsten oxide films prepared by sol–gel

Kai Huang, Jianfeng Jia, Qingtao Pan, Feng Yang, Deyan He�

School of Physics Science and Technology, Lanzhou University, Lanzhou 730000, China

Received 12 February 2007; received in revised form 23 March 2007; accepted 27 March 2007

Abstract

A peroxopolytungstic acid sol has been employed for preparing tungsten oxide (WO3) films by sol–gel dip-coating technique. Three-electrode

cells with 0.5mol/L H2SO4 electrolyte were fabricated using the prepared WO3 films as active working electrode. Optical, electrochemical and

structural properties of the films as a function of coloration–bleaching cycle were characterized by ultraviolet–visible spectrophotometer, cyclic

voltammetry (CV), X-ray diffraction (XRD), scanning electron microscopy (SEM). The films showed a stable behavior of reversible and

reproducible electrochemical switching up to 1000 coloration–bleaching cycles. XRD peaks were found for the films being cycled more than

2000, and the irregular cracks, uniform nano-particles and novel nano-blades were observed on the long-term cycled film surface by SEM. The

degradation of the electrochromic behavior is due to the film change to loose and crystallization after the long-term cycling.

r 2007 Elsevier B.V. All rights reserved.

PACS: 81.20.Fw; 68.55.�a; 68.37.Hk

Keywords: Sol–gel technique; Dip coating; WO3 film; Electrochromism; Long-term cycling

1. Introduction

Electrochromism (EC) is the phenomenon displayed bysome chemical species of reversibly changing color under anapplied voltage [1–3]. The varied devices based on electro-chromic material have been developed for their outstandingelectro-optic properties, such as ‘‘smart windows’’, wide-angle high contrast displays, architectural glazing and anti-glare rearview mirrors. As we known, tungsten oxide (WO3)continues to be the favorite candidate as the primarycathodic material in electrochromic windows applications[4,5]. Much of the existing works in the literatures contain-ing the influence of the substrate on the electrochromiccharacteristics [6,7], synthesis active electrochromic materi-als [8–11] and their efficiency with H+ or Li+ intercalation[12,13], ionic conductor layer to enhance the electrochromicstability [14,15], and the properties of the as-prepared WO3

films [16–18] can be easily found. However, the detaileddiscussions on the electrical, optical and structural proper-

front matter r 2007 Elsevier B.V. All rights reserved.

ysb.2007.03.031

ng author. Fax: +86 931 891 3554.

ss: hedy@lzu.edu.cn (D.Y. He).

ties of WO3 films after long-term cycling are rarely studied.It is obvious that the application of the films in devicesrequires a well understanding of the properties of thematerial after long-term cycling.Recent years, extensive and exhaustive efforts were

devoted to the preparation of electrochromic films byvarious techniques, like pulsed spray pyrolysis, electronbeam evaporation, electrodeposition, and chemical vapordeposition, etc [19–22]. In this work, we prepared WO3

films by sol–gel technique. Sol–gel technique has beendemonstrated to be a potential method to prepare ECmaterials. We report some interested experimental resultsobtained by characterizing the long-term cycled WO3 filmsprepared by sol–gel technique.

2. Experimental

2.1. Sample preparation

The WO3 solution was prepared by dissolving metallictungsten powder (99.9% W powder, China Medical(Group) Shanghai Chemical Reagent Corporation) into

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Fig. 2. Transmittance changes for the 10- and 1000-cycled WO3 films in

0.5mol/L H2SO4 electrolyte under the voltage-off state.

Fig. 1. Transmittance spectra in bleached state for WO3 films after 10 (a),

1000 (b), 2000 (c) and 3000 (d) coloration–bleaching cycles.

K. Huang et al. / Physica B 396 (2007) 164–168 165

30% hydrogen peroxide (30% H2O2, Tianjin Guangfu FineChemical Research Institute) [23,24]. The aqueous solutionwas kept in an ice water bath under stirring. ExcessiveH2O2 was removed catalytically using Pt foil. After thesolution was dried in an oven at 60 1C for 30 h, a kind ofyellowish glassy solid was obtained. By dissolving theobtained yellowish glassy solid (2 g) in anhydrous ethanol(A.R. C2H5OH, Chengdu Union Chemical IndustryReagent Research Institute) (15ml) and adding a smallquantity of oxalic acid dehydrate (A.R. H2C2O4 � 2H2O,Tianjin Guangfu Fine Chemical Research Institute) as acatalyst [25], the peroxopolytungstic acid sol was synthe-sized. The sol was aged for several days at roomtemperature before being coated.

Indium tin oxide coated glass (Shenzhen Hivac VacuumPhoto-Electronics Co., Lt) (1� 2 cm2 in area) was used as sub-strate. The sheet resistance of the ITO layer was 35O/&,with a visible light transmittance higher than 90%. WO3

films were prepared on the ITO substrates by dip-coatingwith a rate of 5 cm/min. The samples were left inatmosphere so that hydrolysis and condensation progressspontaneously. Three layers were successively coatedfollowing the same procedure to obtain better electro-chromic behaviors. All the prepared films were annealed at100 1C in air for 1 h.

2.2. Characterization

The structure of the films was determined by X-raydiffraction (XRD) (using a Rigaku RINT2400 X-raydiffractometer with Cu Ka radiation). The surfacemorphology of the films was obtained by a scanningelectron microscope (S-4800, HITACH). Electrochemicalmeasurements were performed on a three-electrode glasscell using an electrochemistry station (Model CHI660, CHCorporation, USA), Pt grid was used as a counter electrodeand a saturated calomel electrode (SCE) as the referenceelectrode. The electrolyte was 0.5mol/L H2SO4 (A.R.H2SO4, Tianjin Guangfu Fine Chemical Research Insti-tute). Cyclic voltammograms were recorded at a step of20mV/s over the voltage range from �0.9 to 0.6V vs. SCE.Ultraviolet–visible spectrometry (UV–VIS) was taken by aUV-260 spectrophotometer.

3. Results and discussion

3.1. Optical and electrochemical characterization

The prepared WO3 films were long-term tested toanalyze their durability. Coloration–bleaching cycles werepreformed by switching a pulse potential with a durationtime of 30 s. Fig. 1 shows the transmittance spectra inbleached state obtained after the potential were switchedfor 10, 1000, 2000 and 3000 cycles over the wavelengthrange of 300–900 nm, respectively. The transmittance of theITO/glass was assumed to be the reference. For the 1000-cycled film, the transmittance in bleached state is still 80%.

The transmittance in the bleached state has decreased to65% and 45% for the 2000- and 3000-cycled films,respectively, which are beyond the device application. Itis possible that, after a long-term cycling, the irreversibleincorporation of H ion and hydroxyl radicals in WO3 filmleads to the lower light transmittance in the bleaching state.A memory effect refers to a situation where the optical

properties remain the same for extended periods of timeunder the voltage-off state [15]. Fig. 2 shows the memoryeffect for the 10- and 1000-cycled WO3 films in 0.5mol/LH2SO4 electrolyte after a cathodic potential was applied for30 s. It can be seen that, for the 1000-cycled film, thetransmittance was rapidly increased when the voltagesource was removed and recovered to the bleached statewithin 10min. The fact that the self-bleached process of the1000-cycled film is faster than that of the 10-cycled film

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Fig. 4. XRD patterns of the cycled WO3 films coated on ITO substrate.

(a): 10 cycles; (b): 1000 cycles; (c): 2000 cycles; (d): 3000 cycles.

Fig. 3. Cyclic voltammetry recorded for WO3 films after 10, 1000, 2000

and 3000 cycles in a 0.5mol/L H2SO4 electrolyte, respectively.

K. Huang et al. / Physica B 396 (2007) 164–168166

indicates that the intercalated H ions in the 1000-cycledfilm can easily diffuse into the electrolytes. Under voltage-off conditions, an electrical leakage occurs so thatprotons spontaneously diffuse out due to balance beingchanged. It is possibly due to the structural deformation ofthe WO3 film for a long-time exposure to the electrolyticsolution.

Typical CV curves for the cycled WO3 films recorded at asweep rate of 20mV/s were shown in Fig. 3, with potentialfrom �0.9 to +0.6V vs. SCE. It can be seen that the CVprofile does not change much up to 1000 cycles. The resultsconfirm to those shown in Fig. 1. Furthermore, the currentdensity decreased after long-term cycling. It indicated that,due to the trapping of charge as a result of irreversiblechemical reactions between the film matrix and the insertedH ions, the amount of incorporated charge decreasedduring the cycling. The peak current density gives a roughevaluation of the electrochemical activity of the workingelectrode. For the present WO3 films, the larger electro-chemical activity (6–8mA/cm2) was found to be thosecolored–bleached less than 1000 cycles. The peak currentdensity is about 2mA/cm2 for the 3000-cycled film. Theelectrochromic behaviors have almost lost for the filmsbeing colored–bleached more than 3000 cycles.

3.2. Structural and morphological characteristics

XRD patterns of the cycled WO3 films and the ITOsubstrate in Fig. 4. As we can see, all the XRD peaks ofspectra (a) and (b) shown belong to ITO substrates,indicating that the structures of the 10- and 1000-cycledWO3 films are amorphous, and the structure of WO3 filmhas no change during less than 1000 cycles. However, it canbe seen from Fig. 4(c) that two weak peaks correspond to[1 3 1] and [3 2 1] of the WO3 �H2O crystal orientation[JCPDS 83-0951]. And the [1 1 1] peak can be found in

spectrum (d) for the 3000-cycled film. The relevant latticeparameter values are estimated to be 4.31, 2.27, and 1.97 A,respectively, It is suggested that the films were crystallizedas they were colored and beached up to 2000 cycles.Fig. 5 (a) shows SEM image for an as-prepared WO3

film. A compactly and crack-free surface morphology canbe seen, and the micrographs remain featureless even undera magnification of 100K. The observation is similar to thatreported in literature [26]. SEM images for the 3000-cycledfilm are shown in Fig. 5 (c) and (d) with differentmagnifications. Irregular cracks, uniform nano-particlesand novel nano-blades with thickness of 10 nm and widthup to 200 nm can be seen. We supposed that the irregularcracks were formed due to stresses imposed by the insertedH ions, and the film was dispersed into nano-particles byintercalation and deintercalation of H ions. The formationof nano-blades is not well understood yet. It was possiblethat WO3 dissolved in the electrolytic solution was re-grown from the surface of ITO during the long-termcycling. Such a surface morphology was observed for the2000-cycled films too, but the cracks and nano-blades weremuch smaller than those for the 3000-cycled films. Asshown in Fig. 5 (b), only nano-particles, without cracksand nano-blades can be seen for the 1000-cycled films.Based on the above results, we supposed that the XRDpeaks shown in Fig. 4 are due to the formation of nano-blades during the long-term cycling. The WO3 films afterlong-term cycled have been changed to loose and crystal-lization.

4. Conclusions

Electrochromic WO3 films were prepared by sol–geltechnique using metallic tungsten and hydrogen peroxide.The films were long-term tested in 0.5mol/L H2SO4

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Fig. 5. SEM images of the WO3 films as-deposited (a), 1000-cycled (b) and 3000-cycled (c), (d) at different magnification.

K. Huang et al. / Physica B 396 (2007) 164–168 167

electrolyte to analyze their degradation. Optical, electro-chemical and structural properties of the films werecharacterized as a function of coloration–bleaching cycle.The films showed a stable behavior of reversible andreproducible electrochemical switching up to 1000 color-ation–bleaching cycles. Structural crystallization of thefilms were confirmed for the films being cycled more than2000 cycles, and the irregular cracks, uniform nano-particles and novel nano-blades were observed on filmsurface. The degradation of the electrochromic behaviorsfor the long-term cycled films is attributed to the loosesurface and structural crystallization.

Acknowledgments

The authors appreciate the financial support of theSpecialized Research Fund for the Doctoral Program ofHigher Education (No. 20040730029) and the Teachingand Research Award Program for OutstandingYoung Teachers in High Education Institutions of MOE,China.

References

[1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials,

Elsevier, Amsterdam, 1995.

[2] P.J. Gellings, H.J.M. Bouwmeester, in: C.G. Granqvist (Ed.), The

CRC Handbook of Solid State Electrochemistry, CRC Press, Boca

Raton, 1997.

[3] C.G. Granqvist, J. Eur. Ceram. Soc. 25 (2005) 2907.

[4] G.A. Niklasson, L. Berggren, A.L. Larsson, Sol. Energy Mater. Sol.

Cells 84 (2004) 315.

[5] M. Vasilopoulou, I. Raptis, P. Argitis, I. Aspiotis, D. Davazoglou,

Microelectron. Eng. 83 (2006) 1414.

[6] A. Siokou, S. Ntais, S. Papaefthimiou, G. Leftheriotis, P. Yianoulis,

Surf. Sci. 566–568 (2004) 1168.

[7] R. Sivakumar, M. Jayachandran, C. Sanjeeviraja, Mater. Chem.

Phys. 87 (2004) 439.

[8] L.H.M. Krings, W. Talen, Sol. Energy Mater. Sol. Cells 54 (1998) 27.

[9] A. Patra, K. Auddy, D. Ganguli, J. Livage, P.K. Biswas, Mater. Lett.

58 (2004) 1059.

[10] C.G. Granqvist, Sol. Energy Mater. Sol. Cells 60 (2000) 201.

[11] R. Azimirad, O. Akhavan, A.Z. Moshfegh, Thin Solid Films 515

(2006) 644.

[12] I. Porqueras, E. Bertran, Thin Solid Films 377–378 (2000) 129.

[13] M. Stromme-Mattsson, Solid State Ionics 131 (2000) 261.

[14] K.S. Ahn, Y.C. Nah, Y.E. Sung, K.Y. Cho, S.S. Shin, J.K. Park,

Appl. Phys. Lett. 81 (2002) 3930.

ARTICLE IN PRESSK. Huang et al. / Physica B 396 (2007) 164–168168

[15] Y.C. Nah, K.S. Ahn, Y.E. Sung, Solid State Ionics 165 (2003) 229.

[16] P.S. Patil, S.H. Mujawar, A.I. Inamdar, P.S. Shire, H.P. Deshmukh,

S.B. Sadale, Appl. Surf. Sci. 252 (2005) 1643.

[17] R. Vijayalakshmi, M. Jayachandran, C. Sanjeeviraja, Curr. Appl.

Phys. 3 (2003) 171.

[18] Y.S. Krasnov, S.V. Volkov, G.Ya. Kolbasov, J. Non-Cryst. Solids

352 (2006) 3995.

[19] J.M. Ortega, A.I. Martınez, D.R. Acosta, C.R. Magana, Sol. Energy

Mater. Sol. Cells 90 (2006) 2471.

[20] R. Sivakumar, R. Gopalakrishnan, M. Jayachandran, C. Sanjeevir-

aja, Opt. Mater. 29 (2006) 679.

[21] Y.S. Krasnov, S.V. Volkov, G.Y. Kolbasov, J. Non-Cryst. Solids 352

(2006) 3995.

[22] L. Meda, R.C. Breitkopf, T.E. Haas, R.U. Kriss, Thin Solid Films

402 (2002) 126.

[23] J.P. Cromin, A. Agrawal, D.J. Tarico, J.C.L. tonazzi, US Patent No.

5,277,986, 11 January 1994.

[24] J.P. Cromin, A. Agrawal, D.J. Tarico, J.C.L. tonazzi, US Patent No.

5525,264, 11 Junuary 1996.

[25] E.O. Zayim, Sol. Energy Mater. Sol. Cells 87 (2005) 695.

[26] M. Deepa, T.K. Saxena, D.P. Singh, K.N. Sood, S.A. Agnihotry,

Electrochim. Acta 51 (2006) 1974.