Langmuir 1994,10,17-22
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Articles Electrochromic and Photoelectrochromic Behavior of Thin WO3 Films Prepared from Quantum Size Colloidal Particles Swat Hotchandani,*JJ Idriss Bedja,tJ Richard W. Fessenden,t and Prashant V. Kamat**t Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556,and Centre de Recherche en Photobiophysique, Universite du Qukbec h Trois Rivihres, Trois Rivihres, Qubbec, Canada G9A 5H7 Received January 22,1993. I n Final Form: October 22,199P Thin films of WOs are cast from quantum size colloids onto an optically transparent electrode. These thin particulate films are found to exhibit reversible electrochromic and photoelectrochromic effects. A
blue coloration of the WOS particulate fim is seen when electrons are injected into the conduction band of WOs by electrochemical or UV-excitation methods. The onset potential of the electrochromic effect is dependent on the pH and corresponds to the flat band potential of WOS. Spectroelectrochemical and microwave absorption experimenta suggest that trapped electrons are the major species responsible for the blue coloration of the WOS particulate film.
Introduction The approach of using semiconductor colloids for the design of optically transparent thin semiconductor films has attracted considerable interest in recent years.'-lOThis technique is relatively simple and inexpensive compared to other commonlyemployed techniques such as molecular beam epitaxy or chemical vapor deposition (CVD). Moreover, by contr+olling the preparative conditions it is possible to tailor the properties of semiconductorparticulate films. Recent investigations of ZnOl-8 and Ti0ZC1O semiconductor films prepared from quantized semiconductor colloidal suspensions have shown that these films exhibit excellent optical and photoelectrochemical properties. These f i b s have also been coupled with chemically or electrochemically deposited short band gap semiconductor films (CdS298, CdSe,lo and F e S P to extend the photoelectrochemical response further into the visible region. Thin films of transition-metal oxides (e.g., WO3, MOOS) are considered to be important because of their electrochromic behavior. Several efforts have been made in the past to investigate electrochemically induced chromic Notre Dame. Quebec A Trois RiviBres. Abatract published in Advance ACS Abstracts, December 16,
7 Univereity of
t Univereite du
1993. (1) (a) Hotchandani, S.; Kamat, P. V. Chem. Phys. Lett. 1992, 191, 320. (b)Hotchandani, S.; Kamat, P. V.J. Electrochem. SOC.1992,139,
1630. (2) Ho-dani, 5.;Kamat, P. V. J. Phys. Chem. 1992,96,96. (3) Sakohara, S.; Tickanen, L. D.; Andereon, M. A. J. Phys. Chem. 1992, M,11086. (4) (a)O'Regan,B.; Moeer, J.;Anderson,M.; Grlitzel, M. J. Phys. Chem. 1990,91,8720. (5) O'Fbgan, B.; Moeer, J.; Grlitzel, M.; Fitzmaurice, D. Chem. Phys. Lett. 1991, 183, 89. (6) O'Regm,B.; Moeer, J.; Grlitzel,M.; Fitzmaurice, D. J.Phys. Chem. 1991,96,10525. (7) Rothenberger, G.; Fitzmaurice, D.; Grlitzel, M. J.Phys. Chem. 1992, 96,5989. (8) O'Regan, B.;Grlitzel, M.Nature 1991,363,737. (9) Vogel, R.; Pohl,K.; Weller, H. Chem. Phys. Lett. 1990,174,241. (10) (a) Liu, D.; Kamat, P. V. J. Electroanal. Chem. 1993,347,451466. (b) Liu, D.; Kamat, P. V. J. Phys. Chem.1993,97,10769. (11) Ennaoui, E.;Fiechter, S.; Tributech, H.; Giereig, M.; Vogel, R.; Weller, H. J. Electrochem. SOC.1992, 139, 2514.
effects in WO3 films (see for example, refs 12-15). Efforts have also been made to employ mixed TiOz-WOa systems for enhancing the efficiency of electrochromic effects.16J7 A variety of techniques, such as vacuum evaporation, evaporation sputtering, anodization, and pressed powder, have been employed in the past to synthesize W 0 3films.12 For the first time we are able to employ quantized WOs colloids for preparing optically transparent thin films on glass plates. Controversy still exists regarding the mechanism of coloration proposed to explain electrochromic effects in WO3 films.l2 Some of these proposed mechanisms include formation of a blue-colored oxide product,'* simultaneous injection of electrons and cations into interstitial sites in the WOa atomic lattice,ls intervalence transfer absorption,20and polaron absorption?' Both intervalence transfer and polaron models attribute coloration to the tight localization of conduction band electrons to W6+ sites, although a recent electron resonance study has argued against such a strong localization.22 As indicated in the transient absorption study of Ti02 and ZnO colloidsz3and spectroelectrochemical study of Ti02 particulate films,67 the inherent semiconductor properties such as trapping (12) Lampert, C. M. Sol. Energy Mater. 1984,11,1. (13) Oi, T. Annu. Reo. Mater. Sci. 1986, 16, 185. (14) Nguyen, M. T.; Dao, L.H. J.Electrochem. Soc. 1989,136,2131. (15) (a) Paseerhi, S.; Scrosati, B.; Gorenstein, A. J.Electrochem. SOC. 1989,136,3394. (b) Yamanaka, K. Jpn. J. Appl. Phys. 1987,26,1884. (c) Akhtar,M.; Paiete,R. M.; Weakliem, H. A. J. Electrochem. SOC.1988, 135,1597. (16) Ohtani, B.; Ataumi, T.; Nishimoto, S.; Kagiya, T. Chem. Lett. 1988,295. (17) Hashimoto, S.; Matauoka,H.J. Electrochem. SOC.1991,138,2403. (18) Howe,A.; Sheffeld, S.; Childs, P.; Shilton, M. Thin Solid Films 1980, 67, 365. (19) Hurditch, R. Electron. Lett. 1975, 11, 142. (20) Faughnan, R.; Crandall, R. S.; Heyman, P.M. RCA Reo. 1976,36, 177. (21) (a) Denuville, A.; Gerard, P. J.Electron. Mater. 1978,7,559. (b) Schirmer, 0. F.; Blazey, K. W.; Berlinger, W. Phys. Reo. 1978,11,4201. (22) Pfier, J. H.;Sichel, E. K. J. Electron. Mater. 1980, 9, 129. (23) (a) Rothenberger, G.; Moser, J.;Griitzel,M.; Serpone,N.; Sharma, D. K. J. Am. Chem. SOC.1985,107,8054. (b) Kamat, P. V.; Gopidas, K.
R. In Picosecond and Femtosecond Spectroscopy from Laboratory to Real World. Proc. SPZE's Tech. Symp. Laser Spectrosc. 1990,115-122. ( c ) Kamat, P. V.; Patrick, B. J. Phys. Chem. 1992, 96, 6829.
0743-7463/94/2410-0017$04.50/0 Q 1994 American Chemical Society
18 Langmuir, Vol. 10,No. 1, 1994 of electrons at the defect sites or formation of an accumulation layer at the semiconductor/electrolyte interface may also be responsible for the coloration effects. We have considered both electrochemical and photoelectrochemical approaches to probe the mechanism of coloration in W03 films. Spectroelectrochemicalexperiments which describe the electrochromic and photoelectrochromic behavior of WO3 particulate films cast on conducting glass plates are described here.
Experimental Section Materials. Optically transparent electrodes (OTE)were cut from a conducting glass plate obtained from Donnelley Corporation, Holland, MI. Sodium tungstate was obtained from Aldrich. All other chemicalswere analytical reagents of highest available purity. Preparation of WOS Particulate Films. A transparent colloidal suspension of W03 was prepared by the method described by Nenadovic et alSzrThe suspensions of W03were prepared in both water and ethanol. Tungstic acid precipitate was dissolved in respective solvents by adding oxalic acid at elevated temperatures. The diameter of WOs particles as determined from the transmission electron microscopy was in the range of 20-50 A. They were sphericallyshapedwith a nearly symmetric distribution. A small aliquot (usually 0.1 mL) of the W03sol (0.15 M) was applied to a conducting surface of 0.8 X 5 cm2of OTE and was dried in air on a warm plate. The WOs-coated glass plates were then heated in an oven at 673 K for 1 h. Thin semiconductor particulate films treated at 673 K adhered strongly to the glass surface and were stable in the pH range 1-10. The films disintegrated in strong alkaline solutions. Typical thickness of these films as measured from gravimetry is -10 pm. The area of the film exposed to solution or UV light was approximately 3 cmz. Absorption spectra were recorded with a Perkin-Elmer 3840 diode array spectrophotometer. Electrochemical and Spectroelectrochemical Measurements. Electrochemicalmeasurements were carried out with a standard three-compartment cell consisting of a Pt wire counter electrode and a saturated calomel electrode (SCE) as reference. Spectroelectrochemical measurements were carried out in a 1 cm cuvette which was modified to accommodate the OTE/W03 working electrode, Pt wire counter electrode, and Ag/AgCl reference electrode. APrinceton Applied Research(PAR)Model 173potentiostat and Model 175universal programmer or a BAS 100 electrochemical analyzer was used in electrochemical and spectroelectrochemicalmeasurements. Photoelectrochromicexperiments were carried out by irradiating the WOa particulate films with UV light from a 150-W xenon lamp (A > 300 nm). Time-Resolved Microwave Absorption Studies. Microwave absorption measurementa were made using an apparatus described previouslyzSdwith modifications to improve time response and to allow the phase of the microwave signal to be determined.%ba The samples of WOa-coated glass plates (approximately 0.5 X 1 cm2) were contained in a fused silica cell. Excitation of the sample was carried out with a third harmonic laser pulse (355 nm) from a Quanta-Ray Nd-YAG laser system (pulse width 6 ne). Relative dose measurements were made by reflecting part of the incident light from a silica plate onto a pyroelectric sensor (Laser Precision Corp. RJP-735). Results Absorption Characteristics. The absorption spectra of W 0 3 colloidal suspension in water and the film coated on OTE are shown in Figure 1 (spectra a and b). The colloidal suspension exhibits strong absorption in the UV region with an absorption onset around 390 nm. This (24) Nenadovic, M. T.; Rajh, T.; Micic, 0. I.; Nozik, A. J. J. Phys. Chem. 1984,88, 5827. (25) (a) Fessenden, R. W.; Carton, P. M.; Shimamori, H.; Scaiano, J. C. J. Phys. Chem. 1982,86,3803. (b) Fessenden, R. W.; Scaiano, J. C. Chem. Phys. Lett. 1986,117,103. (c) Fessenden, R. W.; Hitachi, A. J. Phys. Chem. 1987,91,3456.
Hotchandani et al. I. " I
0.8
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Wavelength (nm) Figure 1. Absorption spectra (a) of colloidal W03in water, (b) of WOa particulate film on OTE plate, (c) after charging OTE/ WOSat 4.9 V vs Ag/AgCl at pH 6, (d) after chargingOTE/WOs at -1.4 V vs Ag/AgCl at pH 6. The electrolyta was 0.1 M LiClO, in aqueous solution.
n
T IOOpA
I
E ( V o l t vs. SCE) Figure 2. Cyclic voltammogram of W03 particulate film in
aqueous solution containiqg 0.1 M LiClO, (pH 6): WE, OTE/ W03; CE, Pt wire; RE, SCE; scan rate, 5 mV/s.
onset of absorption which is blue-shifted compared to its bulk band gap of 2.8 eV indicates size quantization effects in these particles. The WOs film exhibits similar absorption features, but the onset of absorption is red-shifted indicating a growth of particles dying the annealing process. Some scattering effects are also expected to contribute to the tail absorption in the visible region. Nevertheless the W03 particulate film is transparent enough to carry out the absorption studies in the visible region. The changes in the color of the film observed upon charging at negative potentials are shown in Figure 1 (spectra c and d). The blue color was easily noticeable at low applied negative potentials. This change in the color is completelyreversible when the electrode was discharged by reversing the applied potentials. However,at negative potentials beyond -1.2 V, the film turned brown (spectrum d in Figure 1). The overall electrochromism of WOa particulate films can be described by the expression (1).
-
oto -1.0
W03(colorless)
v
-1.4 v WO,(blue) * W03(brown)
(1) Cyclic Voltammetry of OTE/WO,. The cyclic voltammogram of OTE/W03 recorded in aqueous LiClOr solution is shown in Figure 2. When the wO3 particulate film was charged at cathodicpotentials more negative than -0.3 V vs SCE,a change in the color from colorless to blue was observed. This electrochromic effect is characteristic of wO3 material as reported in earlier electrochemical
Behavior of Thin W03 Films
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.
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500
0
Langmuir, Vol. 10,No. 1, 1994 19
1
600
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Wavelength (nm)
Figure 3. Differenceabsorption spectraOf WOs particulate film recorded after charging at different levels (chargein coulombs): (a) 0; (b) 0.01; (c) 0.025; (d) 0.043;(e) 1.96; ( f ) 2.0; (9) 2.06. Potentialsmaintained at -1.2 V (spectraa-d) and -1.5 V (spectra e-g) vs Ag/AgCl reference. The electrolyte in all these experiments was 0.1 M LiC104 (pH 6) in aqueous solution. The spectrum recorded at 0.0 V served as reference. Spectra b, c, and dcorrespond to blue coloration and spectra e, f, and g correspond to brown coloration.
studies of WO3 electrodes.12 Although no characteristic peak was observed in the cathodic scan, a broad anodic peak is observed in the reverse scan around 0.0 V. The anodic peak observed in the reverse scan indicated discharging of the W03 particulate film. SpectroelectrochemicalStudies. Inorder to evaluate the electrochromic behavior of WO3 particulate films, in situ absorption measurements were made during the electrochemicaloperation of the OTE/ W03 electrode. The absorption spectra recorded after charging at different levels are shown in Figure 3. The potential was maintained constant at -1.2 V vs Ag/AgCl. It is evident from these spectra that at low charging levels the absorption in the red-infrared region increases with increasing amount of charge (spectra a-d). This absorption change corresponds to the blue coloration of the electrode. But at higher charging levels a new absorption band develops around 400nm. These results suggest that two kinds of chemical events must be occurring when WO3 particulate film is subjected to negative potentials. The change in the brown coloration was resistant to reversal with a recovery seen only when discharged at positive potentials P 1 . 0 V). The reversible electrochromism (colorless-blue) observed in the OTE/W03 film was further investigated by recording the absorption spectra at applied potentials between 0.0 and -0.8 V (Figure 4). A t positive potentials (10 V), no change in the absorption is seen, but with increasing negative potentials an increase in the IR absorption band is seen. After a potential of -0.8 V was attained, the electrode was discharged by reversing the potentials. The absorption spectra recorded during the charging cycle could easily be reproduced during discharge with a &5%error. The changes in the absorbance at 850 nm recorded during charging and discharging cycles are shown as an insert of Figure 4. This dependence of the absorbance on the applied potential clearly highlights the reversibility of electrochromic behavior of the wo3 particulate film. The pH of the medium plays an important role in altering the onset of the electrochromic effect. The experiment in Figure 4 was repeated at different pHvalues. The overall features of the absorption band remained the same at all pH values, but the onset of the blue color is dependent on the pH. Figure 5 shows the absorption change (850 nm) versus applied potential at different pH
- A
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500
600
700
So0
900
Figure 4. Dependence of electrochromic effect of WOs particulate f i i on the chargingpotentials. Absorption spectraof OTE/ WOs were recorded at various applied potentials: WE, OTE/ WO,; CE, Pt wire; RE, Ag wire; electrolyte, 0.1 M LiClO4 (pH 6). Insert shows the absorption change at 850 nm during charqng and discharging of the WOs electrode.
u.u
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Potential (V vs Ag/AgCI) Figure 5. pH effect on the electrochromic effect of WOa film. The dependence of the absorption at 850 nm on the applied potential was monitored at pH 1.9and 9.0 WE, OTE/WOs; CE, Pt wire; RE, Ag/AgC1; electrolyte, 0.1 M LiClO,.
values of the medium. With increasing pH, the onset of electrochromic effect shifts to more negative potentials. Similar dependence of electrochromic onset on the pH has also been observed for Ti02 particulate films.s PhotoelectrochromicEffects. When irradiated with UV light, W03 particulate films were found to exhibit photoelectrochromic effects. (Since the photoelectrochemical process is responsible for the blue coloration of the semiconductor films, we refer to this phenomenon as the photoelectrochrornic effect.) The W03 particulate films turned blue when irradiated with UV light in the absence of any applied potential. The photolysis of W03 film was performed in air and no additional wetting of the surface was necessary to observe the photoelectrochromic effect. The difference absorption spectra of WOs particulate film recorded before and after UV irradiation are shown in Figure 6. A broad absorption in the red-IR region and a bleaching below 600nm are seen upon UV irradiation (spectra b-d). This broad absorption in the red-IR region is very similar to that observed in electrochemical experiments as described in Figure 4. The bleaching effects are more pronounced in the photolysis experiments. Such a bleaching, which represents a shift in the absorption edge of the semiconductor, arises as a result of population of
Hotchandani et al.
20 Langmuir, Vol. 10, No. 1, 1994
-0.101
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Figure 6. Effect of band-gap excitation on the absorption of W 0 3 particulate film cast on an OTE glass plate. Difference
absorption spectra were recorded (a) before photolysis and (b) 5, (c) 10, and (d) 30 8 after UV photolysis. The film was preannealed at a temperature of 423 K. lower-lying energy levels in the conduction band. Similar interesting property of the Ti02 and CdS semiconductor colloids and films has been the topic of discussion in earlier investigations.7~2'Q7 The mechanism of this photoelectrochromic effect is very similar to that of the electrochromic effect observed with W03 particulate films. The band-gap excitation of W03 particulate film leads to charge separation followed by trapping of charge carriers (reaction 2)
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Time (s) Figure 7. Effect of annealing temperature on the photoelectrochromic effect. The WO8 films were annealed at (a) 383, (b) 423, and (c) 473 K. The absorption changes at 850 nm were monitored following the UV irradiation of the W 0 3 particulate film.
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