Technical note

Microstructural properties of an electrochromic WO3 thin film

Y.A. Yang, J.N. Yao*

Institute of Photographic Chemistry, CAS, Beijing 100101, People’s Republic of China

Received 10 March 1999; accepted 12 June 1999

Abstract

This is the first report of extended X-ray absorption of fine structure (EXAFS) study on vacuum-evaporated electrochromicWO3 thin film. When monoclinic WO3 powder, the precursor, was evaporated, its long-range order was vigorously destroyedand decomposed into many highly-dispersed clusters. The formed WO3 film, compared to WO3 powder, contracted in W–Odistance from 0.138 to 0.136 nm (without phase-shift revised). Its lattice cell further decreased if it was cathodically colored inLiClO4/propylene carbonate solution. Raman measurement during this change process also strongly supported that conclusion.q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: A. Thin films; Electrochromic WO3 thin film; D. Microstructure; C. EXAFS; C. Raman spectroscopy

1. Introduction

In the past two decades, a world-wide interest into theinvestigation of electro-photochromic materials has resultedfrom the first work [1] of Deb that reported the photo-chromic (PC) and electrochromic (EC) properties of WO3

at room temperature. Till now, many kinds of electrochro-mic materials have been discovered, such as inorganiccompounds including MoO3 [2], V2O5 [3], NiO [4], andorganic compounds [5] including polyaniline, polypyrroleand DMP. Among the inorganic compounds, WO3 thin filmhas significant advantages over others on reversibility, stabi-lity and color efficiency. Because of those qualities, it hasbecome one of the most promising EC materials, whichpossesses attractive application potential in large-areadisplaying and light-modifying materials [6,7].

The EC WO3 thin film can be prepared by various meth-ods, such as vacuum evaporation [8], electron-beam sputter-ing [9], sol–gel [10], electrodeposition [11]. Difference ofcoloration properties exists among these films by differentmeans, which is mainly attributed to the difference betweentheir microstructures. Though scientists have contributed alot of work to improve the coloration capacity of the film,the natural microstructure and micro-properties of various

WO3 thin films have not been entirely elucidated till now.However, the resolution of this problem is rather importantfor ameliorating the coloration properties, probing thecoloration mechanism, seeking more profitable materials,etc.

X-ray diffraction (XRD) is a powerful tool to reveal thestructure of crystalline substances. However, it is not veryuseful for non-crystalline materials. Raman spectroscopycan provide some valuable information, though not verymuch, for amorphous materials. In contrast, extended X-ray absorption of fine structure (EXAFS) is able to reflectthe detailed chemical environment of a central atom (gener-ally metal atom), such as the type, number and distance ofthe coordinated atoms, since it utilizes the synchronic lightto substitute the general X-rays. In some cases, it can deter-mine the orientation and bond angle of the adsorbed mole-cule [12]. EXAFS is based on short-range effect, not relatedto crystal structure (long-range). It thereby demonstratesunique capacity to determine the microstructure of amor-phous materials. It has strong atom-selectivity, since theabsorption refers to specific absorption type (K, L, …) ofa certain kind of atom. The inside impurity or superficialcontaminant does not have a detrimental effect on themeasurement. In the past decades, EXAFS has greatlycontributed to the structural investigation of the condensedphases. We first employed EXAFS for the investigation ofmicrostructural changes in the EC WO3 thin film, andRaman spectroscopy was utilized as well.

Journal of Physics and Chemistry of Solids 61 (2000) 647–650

0022-3697/00/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S0022-3697(99)00257-7

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* Corresponding author. Tel.:186-10-6488-154; fax:186-10-6487-9395.

E-mail address:jnyao@ipc.ac.cn (J.N. Yao).

2. Experimental

The EC WO3 thin film was prepared by vacuum evapora-tion of WO3 powder onto a conductive glass substrate of0.2 mm thickness. A 0.1 mol/l LiClO4/propylene carbonate(Plc) was used as the electrolyte solution during the ECprocess. The detailed description of the evaporation andEC process is given in our previous report [8]. A double-beam photoespectrometer (Shimadzu UV-1601) wasemployed to measure the coloration state of WO3 thinfilm. The Raman spectra were recorded on a Raman spectro-meter (Renishaw model 2000). The exciting light used was

514.5 nm supported by Ar1 laser with the power on samplesless than 5 mW. The spectral resolution was no more than2 cm21. EXAFS experiments were performed at XAFSstation of BSRF (Beijing Synchronic Radiation Faculty).The energy of storage ring was about 2.2 GeV, and theaverage current was about 50 mA. The spectra weremeasured on the LIII-absorption edge of W atom by trans-mittance mode at room temperature. The detector was anionization cell full of argon gas and a double-sides singlecrystal Si(111) was used as monochromator.

3. Results

The freshly prepared WO3 thin film was transparent in thevisible range, whose absorption spectrum is shown in Fig.1a. It became blue (see Fig. 1(b)) when it was cathodicallypolarized in a 0.1 mol/l LiClO4/propylene carbonate solu-tion. If a reverse potential was employed, the blue colorbleached to the initial state nearly same as Fig. 1(a) (notshown). This cycle could be performed many times and

Y.A. Yang, J.N. Yao / Journal of Physics and Chemistry of Solids 61 (2000) 647–650648

Fig. 1. Absorption spectra of: (a) vacuum-evaporated initial WO3

thin film; and (b) its electrochromic colored state.

Fig. 2. Fourier transform spectra of: (a) WO3 powder; (b) initialstate; and (c) EC-colored state of WO3 thin film corresponding toFig. 1(a) and (b). Weightk � 3:

Fig. 3. Raman spectra of: (a) WO3 powder; (b,b0) initial state; and(c,c0) EC-colored state of WO3 thin film. Curves (b) and (c) arefitted with three Lorentzian functions, whereas curves (b0) and(c0) were fitted with two Lorentzian functions.

may be described by the following equation [13].

WO3Transparent

1 xLi 1 1 xe2 $ Li xWO3Blue

0 , x , 2 �1�

The EXAFS spectra of the W atom correlated to Fig. 1(a)and (b) were measured for the WO3 powder as well. Theircorresponding Fourier transform spectra (without phase-shift revised, and no longer noted in the following), aftercomplicated processing, were obtained and are shown inFig. 2(c), (b) and (a), respectively. The individual maximumpeak standing for the nearest distance of the neighboring Oatoms from the central W atom is: (a) 0.138 nm; (b)0.136 nm; or (c) 0.134 nm. A weak peak at about0.348 nm in Fig. 2(a) might be attributed to the nearestW–W distance [14].

The Raman spectra of WO3 powder, initial state and EC-colored state of the vacuum-evaporated (VE) WO3 film areshown in forms of wave-like lines in Fig. 3(a)–(c), respec-tively. Two sharp peaks showing up in Fig. 3(a) at 715 and804 cm21 indicate that the WO3 powder used was monocli-nic crystal [11]. However, Fig. 3(b) and (c) shows two ratherbroad bands at about 750 and 950 cm21, which stronglyimply that the initial state and the EC-colored state of theWO3 film are both amorphous [15]. To get more actualinformation, their Raman spectra were curve-fitted withLorentzian functions denoted as dashed line in Fig. 3. Ifthe band at ca. 750 cm21 was taken as one peak, the fittedcurves could not match the experimental ones well, as isshown Fig. 3(b0) and (c0). When, however, taking thatband as two peaks, we got quite well-fitted results shownin Fig. 3(b) and (c) as well as in Table 1.

4. Discussion

Analyzing Fig. 2(b) and (a), a conclusion that the W–Odistance in the initial WO3 thin film decreased was drawnsince the peak center (0.136 nm) in Fig. 2(b) was less thanthat (0.138 nm) in Fig. 2(a), i.e. its lattice cell contracted.Besides, the full-width at half-maximum (FWHM) of Fig.2(b) is larger than that of Fig. 2(a), implying that the differ-ence between various W–O bond lengths increased. Ramanspectra supported this point as well. In Fig. 3(b), threeclasses of bonds can be seen whose lengths are significantlydifferent corresponding to three peaks at ca 691, 789 and949 cm21, respectively, which is quite consistent with theincrease of FWHM in Fig. 2(b). Comparing Fig. 3(b) withFig. 3(a), the newly emerged strong peak at ca. 949 cm21

should be attributed to the newly formed WyO double bondduring the vacuum evaporation processes. This indicatesthat the original long-ordered crystal structure of monoclinicWO3 powder is destroyed. Hahshimoto [9] reported thathexagonal phase mainly in the form of W3O9 was producedwhen monoclinic WO3 was sublimated [16]. We also found,using time-of-flight mass spectroscopy, that the VE WO3

film mainly consisted of (WO3)3–4 units. Arnoldussen [17]recognized that trimeric W3O9 containing a quantity ofWyO bonds bound weakly to each other through waterbridge, hydrogen and van der Waals bonding. The otherpeaks at 691 and 789 cm21 in Fig. 3(b) are both shifted tolower wavenumbers relative to peaks at 715 and 804 cm21

in Fig. 3(a), which means that the bond lengths of both thesetwo types of W–O bonds increased [2,18]. However,because of the existence of WyO double bond, it shouldnot be concluded that the lattice cell necessarily enlarged.

This paradox is comprehensible: When WO3 powder wasevaporated, its long-range order was vigorously destroyed,producing many highly-dispersed WO3 clusters, to depositinto WO3 film. Though the W–O bonds might be slightlylengthened due to the influence of the newly produced WyOdouble bonds, the WyO bond itself reflects the decrease ofdistance between O and W atoms. Additionally and substan-tially, rather high surface free energy of the small clusterswould result in more significant contraction of the lattice ofWO3. Hongwei Xiang [19] using EXAFS reported a similarphenomenon in the highly dispersed Co/ZrO2 catalyst.

When the initial WO3 thin film was EC-colored with Li1/PC, the W–O distance shown as its radial distance function,Fig. 2(c), decreased further from 0.136 to 0.134 nm and alsothe FWHM of Fig. 2(c) became smaller. This suggests thatthe lattice cell of the EC-colored WO3 thin film contractsfurther and the difference between the various W–O bondlengths decreased. Correspondingly, all three peaks in itsRaman spectrum shifted to higher wavenumbers, relativeto those of initial state, by 17, 19 and 2 cm21, respectively.The standard deviation (SD) of peak positions in Fig. 3(c) issmaller than that in Fig. 3(b), though not very significantly,which is consistent with decrease of FWHM of Fig. 2(c). Onthe basis of above results, it may be concluded that theinjected Li1 ions were not possible to bond to WyO formingW–O–Li as reported in an earlier study [20] since the peakof WyO bond did not disappear or significantly weakenafter coloration. Though we now are not sure about thisactual reason, we put forward a possible explanation.Under the force of electric field, Li1 ions were injectedinto the interstice of the WO3 framework that is much bigger

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Table 1Peak positions of the fitted curves for Fig. 3(b) (initial film) and Fig. 3(c) (colored film)

Peak 1 (cm21) Peak 2 (cm21) Peak 3 (cm21) Standard deviation (cm21)

Fig. 3(b) 678 782 951 138Fig. 3(c) 695 801 953 130

than the size of Li1 (0.059 nm) [21]. The electrostatic attrac-tion between oxygen atoms and Li1 ions caused the furthercontraction of the WO3 lattice. Berera [22] also reported thatthe crystal lattice of WO3 contracted when Li1 ions wereinjected.

Acknowledgements

This work was supported by National Natural ScienceFoundation and Chinese Academy of Sciences, and mainlyfulfilled at BSRF. The authors are grateful to Dr Tiandou Huand Dr Tao Liu for EXAFS experimental help. Thanks arealso owed to Prof. Kunquan Lu and Dr Yuren Wang for dataprocessing.

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