V2O5(Ti) thin films prepared by pulsed laser ablation technique

Thin Solid Films 394 Ž2001. 64᎐71

Fabrication and electrochromic properties of double layer WO 3Ž V. rV2 O5Ž Ti. thin films prepared by pulsed laser ablation technique Guo Jia Fang a,b,c,U , Kai-Lun Yao a,b,d , Zu-Li Liu a a

b

Department of Physics, Huazhong Uni¨ ersity of Science & Technology, Wuhan 430074, PR China National Laboratory of Laser Technology, Huazhong Uni¨ ersity of Science & Technology, Wuhan 430074, PR China c Department of Physics, Xiangfan Uni¨ ersity, Xiangfan 441053, PR China d International Center for Materials Physics, The Chinese Academy of Sciences, Shenyang 110015, PR China Received 10 July 2000; received in revised form 3 February 2001; accepted 25 April 2001

Abstract Single and double layers of tungsten oxide and vanadium oxide thin films have been deposited on In 2 O 3:SnO 2 ŽITO. glass using the pulsed laser deposition ŽPLD. technique at a substrate heating temperature of 200⬚C and in rich oxygen pressure. The structural properties and chemical composition of as-deposited WO 3ŽV. and V2 O5ŽTi. thin films were analyzed using X-ray diffraction ŽXRD., Raman spectrum ŽRS., and X-ray photo-electron spectroscopy ŽXPS.. The as-deposited V2 O5 doped WO 3 thin film on ITO glass showed an amorphous structure. Highly oriented growth nano-crystalline TiO 2 doped V2 O5 thin films with polycrystalline orthorhombic structure were successfully synthesized on ITO glass using the scanning laser ablation technique at a deposition temperature as low as 200⬚C. Cyclic voltammograms, at a sweep rate of 50 mV sy1 , show that long-term degradation was noticed for the as-deposited WO 3ŽV. thin film, and no long-term degradation was noticed for the as-deposited double layer WO 3ŽV.rV2 O5ŽTi. thin film. Durability was verified to 8000 cycles in the voltage range between y1.0 and 1.0 V. It is demonstrated that these double layer WO 3ŽV.rV2 O5ŽTi. thin films on ITO are good candidates for cathode thin films for rechargeable batteries and electrochromic devices. As the additional top layer of V2 O5ŽTi. possesses nano-crystalline and c-axis oriented structure, which is suitable for Li-ions transport, the cycle stability and reversibility of WO 3ŽV. films have been improved. Double layer WO 3ŽV.rV2 O5ŽTi. thin films provide a neutral brownish blue electrochromic color. Therefore, this double layer film maybe suitable for most building applications. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: V2 O5 ŽTi.; WO 3 ŽV.; Thin films; Double layers; Pulsed laser deposition technique; Electrochromic properties

1. Introduction Tungsten oxide and vanadium oxide are two impor-

U

Corresponding author. Tel.: q86-27-8755-3981; fax: q86-278754-5438. E-mail address: [email protected] ŽG.J. Fang..

tant opto-electronic materials, which can be used as a catalyst, as a window for solar cells and for electrochromic devices, electronic information displays and color memory devices w1᎐4x. Vanadium oxides are used in optical switching devices, gas and humidity sensors and secondary Li batteries w5᎐7x. Tungsten trioxide are also used in gas and humidity sensors, and low voltage varistors w8᎐10x. It also has been recognized that WO 3

0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 1 3 2 - 4

G.J. Fang et al. r Thin Solid Films 394 (2001) 64᎐71

and V2 O5 can be colored through electro-, photo-, and thermo-chromic processes w11,12x. Research has been focused on electrochromic coloration of these materials, whose transparent amorphous or crystalline thin films can be colored reversibly to a blue state. The process is usually described in terms of the doubleinjection model: V2 O5 q xAqq xeys A xV2 O5 WO 3 q xAqq xeys A xWO 3 where A s H or Li. During coloration in these films W 6q undergoes a reduction to W 5q, V 5q undergoes a reduction to V 4q. A xWO 3 and A xV2 O5 are called metallic ‘bronze’. Thin films of metal oxide doped tungsten trioxide and vanadium pentoxide have been prepared by various methods such as vacuum evaporation w13,14x, sputtering w15,16x and chemical vapor deposition w17,18x. Ozer reported that 5 mol.% titanium doped vanadium pentoxide sol᎐gel films showed marked improvement in the electrochromic behavior and 96.5% WO 3 ᎐3.5% V2 O5 sol᎐gel films had a more neutral color and higher coloration efficiency than that of pure WO 3 w19,20x. These studies indicated the improvement over the basic electrochromic tungsten or vanadium oxides with metal oxide doping. Tungsten trioxide appears to be the best electrochromic compound w21x. Its advantages are high coloration efficiency, reasonable stability and relatively low cost. However, for building window applications the bright blue color of WO 3 thin films, in the reduced state, is not as favorable as neutral gray or bronze. Double layer WO 3 ŽV.rV2 O5 ŽTi. thin films provide a neutral brownish blue electrochromic color. Zhang w22x prepared the V2 O5 thin films with V6 O 13 target by pulsed laser deposition ŽPLD.. Nagashima w23x prepared VO 2 thin films on sapphire single crystal Ž012. with a V2 O5 target by PLD. Bowman w24x prepared VO x thin films on sapphire single crystal substrates with metallic vanadium and ceramic V2 O5 targets at a substrate temperature of 515⬚C by PLD. Haro-Poniatowski w25x prepared the WO 3 thin films with a WO 3 target by PLD and the thin film structure has been studied by Raman spectroscopy. We have carried out a study of the electrochromic performance of the PLD double layer WO 3 Ž3.5 mol.% V2 O5 . and V2 O5 Ž5 mol.% Ti. films. Films made from WO 3 ŽV. and V2 O5 ŽTi. targets are also used for comparison. We report also on microstructure formation, and phase structure in the single layer films deposited on ITO glass substrate with WO 3 ŽV. and V2 O5 ŽTi. targets by PLD.

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2. Experiment

The schematic diagram of the laser scanning ablation system is as described in Song et al. w26x. PLD films were prepared from pressed targets Ž3 cm in diameter, 5 mm in thickness .. The V2 O5 ŽTi. target pressed Ž5 mol.%. was prepared by sintering a mixture of V2 O5 and TiO 2 powders in air at 600⬚C for 20 h. The TirV molar ratio of the target is 2.5%. The pressed WO 3 Ž3.5 mol.%V2 O5 . target was sintered in air at 800⬚C for 20 h. The lens can be rotated with different radii and an excimer laser beam, focused by this lens and passing through the quartz window, can be scanned on the target surface. The target was scanned to avoid depletion of material at any given spot and to obtain uniform thin films. The laser used in these experiments is a XeCl excimer laser ŽLambda Physik EMG201 MSC. with 308 nm wavelength, 36 ns duration, and pulse energy of 200 mJ, which produced a 3 J cmy2 laser power density on the target after focusing. The thin films were deposited on the substrate located opposite to the target in the presence of O 2 gas. The total gas pressure was in the range of 0 ; 20 Pa. The substrates used were ITO glass and Ž111. plane of silicon crystals, and were heated at 100᎐200⬚C during the deposition. The distance from the target to the substrate was 4.3 cm. The angle between the incident laser beam and the perpendicular direction of the target surface was 45⬚. The thickness of the single layer films was measured by an ellipsometer. The growth rate of the films was 0.2᎐0.7 ␮m hy1 and the total thickness was 0.1᎐0.6 ␮m. For the double layer WO 3 ŽV.rV2 O5 ŽTi. thin film deposition, the WO 3 ŽV. layer was deposited first and V2 O5 ŽTi. layer was deposited on top. Thickness uniform WO 3 ŽV. and V2 O5 ŽTi. thin films, with area of 4 = 4 cm2 , were deposited by laser scanning ablation with a lens rotating radius of 9 mm. The deposited single layer films were analyzed using various characterization techniques. The microstructure was determined by X-ray diffraction ŽXRD.. XRD spectra were collected for oxide films deposited onto ITO glass. An ADrMAX-rB diffractometer with a CuK ␣ source and Ni filter was used for XRD. The atomic compositions of deposited thin films were studied by X-ray photoelectron spectroscopy ŽXPS. with a RIBER LAS-3000 MK-2 XPS spectroscopy, using the MgK ␣ line as the X-ray source. The Raman spectrum was recorded at room temperature in the range 50᎐1200 cmy1 using a MK I-1000 double monochromator using the 514.5-nm line of an argon laser at a power level of 100 mW. Electrochemical measurements of films deposited on ITO were made using a CHI660A system. A three-electrode cell with a Pt counter electrode and saturated calomel ŽSCE. as a reference electrode was

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G.J. Fang et al. r Thin Solid Films 394 (2001) 64᎐71

used. The electrolyte was 1 M LiClO4 in anhydrous propylene carbonate ŽPC..

Table 1 X-Ray diffraction data for V2 O5 ŽTi. films deposited at 200⬚C on ITO 2␪

dh k l ˚. ŽA

Identified planes Ž hkl .

Relative intensity Ž%.

15.33 20.28 25.66 31.08 41.58 30.42

5.77 4.38 3.47 2.88 2.17 2.93

200 001 201 400 002 222

0 100 45 37 25 47

3. Results and discussion 3.1. Structural and chemical composition characterization There are two important parameters for the formation of the WO 3 ŽV. and V2 O5 ŽTi. thin films, i.e. the substrate temperature and the gas pressure during the film deposition. Experiments revealed that at substrate temperature 200⬚C: under 7-Pa oxygen pressure, a black and conductive amorphous WO x was produced; under 10-Pa oxygen pressure, a semi-transparent and semiblue conductive amorphous WO x was produced; under 15-Pa oxygen pressure, a total transparent and high resistively WO x was produced even if it was deposited on ITO glass. To get transparent and good electrochromic WO 3 thin films, a total oxygen gas pressure of 20 Pa was selected for the following thin film depositions. The deposition temperatures have an important effect on the structures of the films. Fig. 1 shows diffraction spectra of as-deposited samples of WO 3 ŽV. and V2 O5 ŽTi. on ITO glass substrates. No phase structure of WO 3 thin films on ITO were obtained at substrate temperature 200⬚C Žcurve a.. They show amorphous structure on these films at these temperatures. X-Ray analysis revealed that: in a 7-Pa oxygen pressure, amorphous VO x was produced at substrate temperature below 200⬚C; at 200⬚C c-axis oriented V2 O5 thin film on ITO was produced. The 200⬚C as-deposited thin film on ITO showed orientation with a polycrystalline structure ŽFig. 1b.. The X-ray diffraction data of

Fig. 1. XRD spectra of as-deposited Ž200⬚C. WO 3 ŽV. and V2 O5 ŽTi. thin films on ITO WO 3 ŽV.; Žb. V2 O5 ŽTi..

V2 O5

ITO

as-deposited Ž200⬚C. V2 O5 ŽTi. sample on ITO glass substrate are shown in Table 1. The as-deposited V2 O5 ŽTi. film is polycrystalline. Two main peaks were observed and identified as Ž001. and Ž002. orthorhombic V2 O5 lattices planes. The orthorhombic structure of V2 O5 is described in Benmoussa et al. w27x and is composed of an alternate succession of oxygen atom planes and vanadium᎐oxygen atom planes. Here, the structure of films grown at 200⬚C is such that the c-axis in all crystallites is oriented perpendicular to the substrate, the Ž001.-type planes lying parallel to the substrate. The lattice constants can be calculated using the formula dh k l s

1

(

2

h k2 l2 q 2q 2 2 a b c

where a, b and c are the lattice constants and d h k l is the crystalline surface distance for hkl indices. With the data in Table 1, the average lattice constants of V2 O5 ŽTi. thin films were calculated to be ˚ bs 3.578 A, ˚ c s 4.390 A. ˚ They are alas 11.540 A, most the same as those given in the JCPDS File NR.9-38 for V2 O5 . The grain size of the as-deposited Ž200⬚C. thin film on ITO was calculated to be approximately 5 ; 10 nm, using Scherrer’s formula. Conventionally, a polycrystalline vanadium oxide film can only be obtained by deposition or annealing at ) 180⬚C w8,13x. However, our results indicate that highly nanocrystalline-oriented films of V2 O5 ŽTi. on ITO can be prepared at temperatures as low as 200⬚C by PLD. The oxygen partial pressure also affects the chemical composition of the films. XRD results show that the film deposited at 200⬚C in a rich oxygen partial pressure of 7 Pa is the V2 O5 single phase, but the film deposited at 200⬚C in 4 Pa oxygen partial pressure consists of the mixture of V2 O5 , V4 O 9 and VO 2 . These results show that the films deposited at higher oxygen content have stoichiometric OrW or OrV ratio. It can be explained as that the evaporated tungsten or vanadium particles can combine with more oxygen atoms and the higher pressure of oxygen can retard the revap-

G.J. Fang et al. r Thin Solid Films 394 (2001) 64᎐71

orization of the oxygen adsorbed on the thin film surface during the deposition process. By controlling the parameters described above Žthe substrate temperature and the oxygen partial pressure., single-phase films of WO 3 or V2 O5 can be obtained. The WO 3 ŽV. film deposited at 200⬚C in 15 Pa oxygen partial pressure and the V2 O5 ŽTi. film deposited at 200⬚C in 7 Pa oxygen partial pressure were studied by X-ray photoelectron spectroscopy ŽXPS.. The core level binding energy peaks namely WŽ4f 7r2 ., VŽ2p 3r2 . and OŽ1s. peaks have been used to characterize the chemical states of tungsten and vanadium in the vanadiumdoped tungsten oxide samples. The OŽ1s., WŽ4f 7r2 . and VŽ2p 3r2 . core level area ratios i.e. OŽ1s.:WŽ4f 7r2 ., WŽ4f 7r2 .:VŽ2p 3r2 . have been used to determine the surface chemical composition of these vanadium doped tungsten oxides. The core level binding energy peaks namely VŽ2p 3r2 ., TiŽ2p 3r2 . and OŽ1s. peaks have been used to characterize the chemical states of vanadium and titanium in the titanium doped vanadium oxide samples. The OŽ1s., VŽ2p 3r2 . and TiŽ2p 3r2 . core level area ratios, i.e. OŽ1s.:VŽ2p 3r2 ., VŽ2p 3r2 .:TiŽ2p 3r2 . have been used to determine the surface chemical composition of these titanium doped vanadium oxides. The core level binding energy and the chemical composition of WO 3 ŽV. and V2 O5 ŽTi. thin films are shown in Table 2. From Table 2, we know that oxidation states ŽW 6q, V 5q . in WO 3 ŽV. thin film and ŽV 5q, Ti 4q . in V2 O5 ŽTi. thin film are formed. We conclude that V2 O5-doped WO 3 and TiO 2-doped V2 O5 thin films were obtained in rich oxygen environment. Raman spectroscopy is a very powerful tool for phase analysis of tungsten oxides w10,11,14x. With this technique, it is possible not only to identify different oxide phases but also to detect intercalated H 2 O. This spectrum is the result of the sum of several scans in the region where the Raman peaks are the strongest. The Raman spectrum of WO 3 ŽV. film deposited at 200⬚C is shown in Fig. 2. There are two envelopes at approximately 950 cmy1 and 794 cmy1 respectively, no typical Raman peaks of crystalline WO 3 have been recorded. These two peaks correspond to vibrations of the W 6q s O and W 6q᎐O bonds, respectively w28x. The peak at 950 cm -1 has been assigned to the W 6q s O stretching mode of terminal oxygen atoms possibly on the surface of the cluster and micro-void structures in the film. Since this double bond is stronger than the W 6q᎐O

67

single bond, its vibration frequency is expected to be higher than that of the bond. There is no feature corresponding to this peak in the Raman spectrum of crystalline WO 3 , because a crystal does not have any double bond. V2 O5 crystallizes in the orthorhombic system. The layered-like structure of V2 O5 is built up from distorted trigonal bipyramidal coordination polyhedra of O atoms around V atoms, which share edges to form ŽV2 O4 . n zig-zag double chains along the Ž001. direction and cross-linked along Ž100. through shared corners. It is convenient to compare the spectra in terms of internal and external vibrations with respect to the structural units, though this separation cannot be done in a unique way w29x. Internal modes which are observed in the high-frequency region can be described in terms of stretching and bending of V᎐O bonds, while the external modes can be viewed as relative motions of the units with respect to each other, i.e. translations and vibrations. Some of translations give rise to rigid-layer modes w30x. Normally, they lie in the low-frequency region because each unit is considerably heavier than the atoms building it, while the restoring force is of the same order of magnitude. The RS spectrum of as-grown V2 O5 ŽTi. film on ITO ŽFig. 3. consists of seven obvious bands situated at 146, 281, 405, 482, 526, 700 and 992 cmy1 , respectively. We observe an intense and narrow band at 146 cmy1 which is related to the vibrations of the ᎐V᎐O᎐V᎐ atoms. The peak located at 146 cmy1 appears much stronger and can be viewed as a distorted low frequency mode of V2 O5 . The presence of this peak suggests that films are grown with preferred orientation about the c-axis perpendicular to the substrate plane. An indication of the structure quality of the films is the peak position and line-width of the vanadyl mode located at 992 cmy1 in the crystal. The band at 992 cmy1 is characteristic of V⫽O double chemical bonds and corresponds to the stretching of the shortest bond between vanadium and oxygen Ž0.158 nm in V2 O5 .. Unlike other oxygen atoms, this atom is strongly bonded to only one vanadium atom and is termed terminal oxygen for this reason. The vanadyl mode appears at 992 cmy1 in the RS spectrum of a V2 O5 as-deposited film on ITO showing the relatively good crystallinity. It is worth noting that it is terminal oxygen that is responsible for the stoichiometry of the film. The peaks in mid frequency region between 281 and 700 cmy1 correspond

Table 2 Chemical states and chemical compositions in WO 3 ŽV. and V2 O5 ŽTi. thin films BE ŽeV. WO3 ŽV. film V2 O5 ŽTi. film

WŽ4f7r2 . 35.4 VŽ2p3r2 . 517

Core level area ratio VŽ2p3r2 . 516.9 TiŽ2p3r2 . 459.2

O Ž1s. 530.9 O Ž1s. 530.9

OŽ1s.:WŽ4f7r2 . 2.9" 0.3 OŽ1s.:VŽ2p3r2 . 2.4" 0.2

WŽ4f7r2 .:VŽ2p3r2 . 14 " 2 VŽ2p3r2 .:TiŽ2p3r2 . 40 " 8

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G.J. Fang et al. r Thin Solid Films 394 (2001) 64᎐71

Fig. 2. Raman spectra of as-deposited Ž200⬚C. WO 3 ŽV. thin films on ITO.

to deformation modes w30,31x. There is a good agreement between band frequencies, measured by Raman spectroscopy, in the present work and those previously reported for bulk and amorphous V2 O5 w31x.

Fig. 4. The CV curves for film WO 3 ŽV. deposited at 200⬚C recorded at scan rate of 50 mV sy1 . Solid line for the 4th cycle; dash line for the 30th cycle.

3.2. Electrochromic properties In order to check the electrochemical stability of the as-deposited WO 3 ŽV. film under voltammetric cycling, potentials were cycled 1000 times. Cyclic voltammograms at sweep rate of 50 mV sy1 shows that long-term degradation was noticed, as evident from the difference between the solid and dashed curve in the Fig. 4, for the as-deposited WO 3 ŽV. thin film. The CV curves for as-deposited film recorded at different scan rates are shown in Fig. 5. There is one maximum and minimum in the redox currents suggesting that the film has a single intercalation site for Liq ions in as-deposited film. The charge density is approximately 41 mC cmy2 . Voltammograms show very well-defined peaks that oc-

cur in the films during insertion or extraction of lithium ŽWO 3 m Li xWO 3 .. These peaks were accompanied by change in color of the WO 3 ŽV. film, and thus resulted in clear one-step electrochromism Žtransparent to blue. on the oxide state and one step electrochromism Žblue ª transparent . on the reduced state. In order to check the electrochemical stability of the V2 O5 ŽTi. film under voltammetric cycling, potentials were cycled 1000 times. It was observed that the peak current densities are increased up to 3% for the first three cycles and then no change has been observed up to 1000 cycles. The CV curves for as-deposited film recorded at different scan rates are shown in Fig. 6. First, the sample shows excellent reversibility. The in-

Fig. 3. Raman spectra of as-deposited Ž200⬚C. V2 O5 ŽTi. thin films on ITO.

Fig. 5. The CV curves for as-deposited WO 3 ŽV. film recorded at different scan rates: 1, 100 mV sy1 ; 2, 50 mV sy1 ; 3, 20 mV sy1 .

G.J. Fang et al. r Thin Solid Films 394 (2001) 64᎐71

serted and extracted charges were indistinguishable, to within experimental accuracy, after the first 3 cycles. Second, the charge density is 35 mC cmy2 . It is comparable with that of the film prepared with conventional methods by deposition or annealing at ) 180⬚C w8,13x. Voltammograms show very well-defined peaks that occur in the films during insertion or extraction of lithium ŽV2 O5 m Li xV2 O5 .. These peaks were accompanied by change in color of the film, and thus resulted in clear one-step electrochromism Žyellow to pale blue. on the oxided state and one step electrochromism Žpale blue ª yellow. on the reduced state. The voltammograms evolved during the initial three cycles, no long-term degradation was noted at least up to 1000 cycles, as evident from the similarity between the solid and dashed curve in the Fig. 7. Long term stability was verified to 8000 cycles. Long-term durability prevails only if the voltage sweep is confined to a safe ‘range’ Žbetween y1.5 and 2.0 V. for this PLD film. As a layered structure in which all V2 O5 crystallite is oriented parallel to the substrate is in this PLD thin film, the Li-ions have better mobility between layers than across layers w32x. Therefore, this structure is suitable for Li-ions transport, which induced improved cycle stability and reversibility. In order to check the electrochemical stability of the double layer WO 3 ŽV2 O5 .rV2 O5 ŽTi. film under voltammetric cycling, potentials were cycled 1000 times. It was observed that the peak current densities are increased up to 2% for the first four cycles and then no change has been observed up to 1000 cycles. First, the sample shows excellent reversibility. The inserted and extracted charges were indistinguishable, to within experimental accuracy, after the first 4 cycles. Second, the charge density is 36 mC cmy2 . The film double layer

69

Fig. 7. The CV curves for V2 O5 ŽTi. film deposited at 200⬚C recorded at scan rate of 50 mV sy1 . Dash line for the 4th cycle; solid line for the 1000th cycle.

WO 3 ŽV2 O5 .rV2 O5 ŽTi. deposited at 200⬚C had slight lower current density and charge capacity values compared to WO 3 ŽV2 O5 . films deposited at 200⬚C. However, it shows good neutral yellowish blue electrochromic color. Voltammograms show very welldefined peaks that occur in the films during insertion or extraction of lithium ŽWO 3 q V2 O5 m Li xWO 3 q Li yV2 O5 .. These peaks were accompanied by change in color of the film, and thus resulted in clear one-step electrochromism Žyellow to blue. on the oxided state and one-step electrochromism Žblue ª yellow. on the reduced state. The CV curves of double layer WO 3 ŽV2 O5 . film present the combined CV natures of single layer WO 3 ŽV. and V2 O5 ŽTi. films. The voltammograms evolved during the initial four cycles, no long-term degradation was noted at least up to 1000 cycles, as evident from the similarity between the solid and dashed curve in the Fig. 8. Durability was verified to 8000 cycles. Long-term durability prevails only if the voltage sweep is confined to a safe ‘range’ Žbetween y1.5 and 2.0 V. for this PLD double layer film. As the additional top layer of V2 O5 ŽTi. possesses nano-crystalline and c-axis oriented structure, which is suitable for Li-ions transport, the cycle stability and reversibility of WO 3 ŽV. films have been improved. 4. Conclusions

Fig. 6. The CV curves for as-deposited V2 O5 ŽTi. film recorded at different scan rates: 1, 100 mV sy1 ; 2, 50 mV sy1 .

Single and double layers of tungsten oxide and vanadium oxide thin films have been deposited on ITO glass and Si wafers by a pulsed laser deposition technique at substrate heating temperature 200⬚C and in a rich oxygen atmosphere. The structural characterization shows that the as-deposited WO 3 ŽV. thin films are

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Fig. 8. Cyclic voltammograms of WO 3 ŽV.rV2 O5 ŽTi. double layer thin films deposited at 200⬚C on ITO Žsweep rate: 50 mV sy1 .. Solid line for the 4th cycle; dash line for the 1000th cycle

amorphous. Highly oriented growth nano-crystalline V2 O5 ŽTi. thin films with polycrystalline orthorhombic structure were successfully synthesized on ITO glass at a deposition temperature as low as 200⬚C. XPS results show that oxidation states ŽW 6q, V 5q . in WO 3 ŽV. thin film and ŽV 5q, Ti 4q . in V2 O5 ŽTi. thin film are formed. We conclude that V2 O5 doped WO 3 and TiO 2 doped V2 O5 thin films were obtained in rich oxygen environment. As the additional top layer of V2 O5 ŽTi. possesses nano-crystalline and c-axis oriented structure, which is suitable for Li-ions transport, the cycle stability and reversibility of WO 3 ŽV. films have been improved. It is suggested that these double layer WO 3 ŽV.rV2 O5 ŽTi. thin films on ITO are good candidates as cathode thin films for rechargeable batteries and electrochromic devices. Double layer WO 3 ŽV.rV2 O5 ŽTi. thin films provide a neutral brownish blue electrochromic color. Therefore, this double layer films maybe suitable for window applications as well. Acknowledgements This work was supported by the National Natural Science Foundation of China under the grant No. 19775016 and by the Education Bureau of Hubei province under the grant No. 99B019 for excellent young scholars. References w1x M. Green, Chem. Indust. September Ž1996. 641. w2x M.S. Burdis, R.A. Batchelor, J.M. Gallego, Solar Energy Mater. Solar Cells 54 Ž1998. 93.

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