11064
J. Phys. Chem. 1993,97, 1106611070
Photoelectrochemistry of Quantized WOj Colloids. Electron Storage, Electrocbromic, and Photoelectrochromic Effects Idriss Bedja,t*#Swat Hotchandani,t**and Prashant V. Kamat'J Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, and Centre de Recherche en Photobiophysique, Universite du Quebec h Trois Rivikres, Trois Rivikres, Quebec, Canada G9A 5H7 Received: May 21, 1993; In Final Form: July 27, 1993"
Electron storage effects in quantized wo3 colloids have been investigated by spectroelectrochemical and photochemical methods. Electrons trapped within the colloidal particles exhibit blue coloration with absorption in the red-IR region. From picosecond laser flash photolysis experiments, we estimate the rate constant for electron trapping to be 1Olo s-l. These trapped electrons are stable in an inert atmosphere and can be utilized to reduce substrates such as thiazine and oxazine dyes which have reduction potentials less negative than the conduction band of W03. The rate constants for the heterogeneous electron transfer a t the semiconductor/ electrolyte interface are in the range (0.7-2.4) X lo9 M-l s-l.
Introduction Photoelectrochemical conversion and storage of solar energy using semiconductor colloids have attracted considerable interest in recent years.'-3 Photophysical and photochemical properties of several metal oxides and metal chalcogenides have been studied in this context. Semiconductor films prepared from quantized semiconductor colloidal suspensions have been shown to exhibit excellent photoelectrochemical properties."O Recently, we employed quantized W 0 3colloids to prepare optically transparent thin films on glass plates.11 These films were found to exhibit blue coloration when electrons were injected by either electrochemical or UV-irradiation methods. In photoelectrochemical studies which examine the interfacial charge-transfer processes in semiconductor colloid systems, it is usually assumed that free charge carriers from conduction and valence bands are the major participants in the semiconductormediated photocatalytic reactions (see, for example, refs 1-3). So far, little effort has been made to utilize the trapped charge carriers for redox processes in semiconductor systems. W 0 3 is an excellent candidate for this purpose since electrons can be trapped by electrochemical or photolysis methods (Scheme I). Efforts have been made in the past to investigate the electrochemically induced chromic effects in W 0 3 films (for example, see refs 12-17). Several mechanisms have been proposed to explain electrochromic effects in W 0 3 and other transition metal oxide films. These include formation of a blue oxide product,'* simultaneous injection of electrons and cations into interstitial sites in the W 0 3atomic lattice,19 intervalence transfer absorption,ZOand polaron absorption.21 Both intervalence transfer and polaron models attribute coloration to the tight localization of conduction band electrons to W5+ sites although a recent electron resonance study argued against such a strong localization.22 In a radiolytic study of W 0 3 colloids as well as in our spectroelectrochemical study of W 0 3particulate films,11-23it has been shown that the blue coloration occurs as a result of electron trapping at the defect sites. Such stored electrons can conveniently be utilized to reduce other substrate^.^^*^^ The electron-transfer kinetics and energetics of illuminated W 0 3 colloids have been investigated by Leland and Bard.25 We have now probed this process further by means of transient absorption spectroscopy t University of Notre Dame. Universite du Qu6bec t l Trois Rivibres. *Abstract published in Aduance ACS Abstracts, September 15, 1993.
*
0022-3654/93/2097- 11064$04.00/0
SCHEME I: Electron Trapping in W 0 3 Colloids by (a) Electrochemical and (b) Photolysis Methods'
(a)
(b)
CB, VB, and q indicaterelativeenergy levels of the conduction band, valence band, and defect site for electron trapping, respectively. OTE refers to an optically transparent electrode.
using transparent wo3 colloids. The ability of W 0 3 colloids to store photogenerated electrons and reduce oxazine and thiazine dyes in the dark has also been demonstrated with laser flash photolysis experiments.
Experimental Section Materials. Sodium tungstate was obtained from Aldrich, and oxazine dyes (laser grade) were obtained from Exciton. Thionine, methylene blue, and phenosafranin were purified by column chromatography. All other chemicals were analytical reagents of highest available purity. Preparation of W 0 3 Colloids. A transparent colloidal suspension of WO3 was prepared by a method described earlier.*3.*6 The suspensions of W03 were prepared in both water and ethanol. Tungstic acid (W03.2H20) precipitate was dissolved in respective solvents by adding oxalic acid at elevated temperatures. The concentration of oxalic acid was varied (0.16-0.3 1 M) to obtain colloids of different sizes. We expect the diameter of these particles to be in the quantum size regime ( 300 nm). Laser Flash Photolysis Experiments. Nanosecond laser flash photolysis experiments were performed with laser pulses from a Quanta-Ray CDR-1 Nd:YAG laser system (-64s pulse width). The photomultiplier output was digitized with a Tektronix 79 12 AD progammable digitizer. A typical experiment consisted of a series of three to six replicate shots per single measurement. The average signal was processed with an LSI- 11 microprocessor interfaced with a VAX 11/780 ~omputer.2~a Picosecond laser flash photolysis experiments were performed with 532-nm laser pulses from a mode-locked, Q-switched Quantel YG-501 DP Nd:YAG laser system (output 2-3 mJ/pulse, pulse width -18 ps). The white continuum picosecond probe pulse was generated by passing the fundamentaloutput through a D20/ H2O solution. The excitation and the probe pulse were incident on the sample cell at right angles. The output was fed to a spectrograph (HR-320, ISDA Instruments, Inc.) with fiber optic cables and was analyzed with a dual diode array detector (Princeton Instruments, Inc.) interfaced with an IBM-AT computer. The details of the experimental setup and its operation are described el~ewhere.2~bThe zero time (At = 0) in these experiments corresponds to the end of the excitation pulse. All the lifetimes and rate constants reported in this study carry an experimental error of 5%.
Results and Discussion Absorption Characteristics of WOs Colloids. The colorless transparent colloidal suspension prepared as per the method of
Nenadovie et al.23 consists of the dihydrate form of tungsten oxide, W03-2H20. The colloids were stable, especially at lower pH. Some growth was seen during overnight storage of colloids prepared at higher pH. All measurements were carried out with freshly prepared sols. The absorption spectra of W 0 3 colloids prepared in aqueous solution are shown in Figure 1. The absorption onset for all these samples of W 0 3colloids is less than 380 nm. In W 0 3particulate films and in colloids of larger particle size,11*23the absorption onset was at wavelengths greater than 400 nm, corresponding to the bulk bandgap energy of 2.8 eV. As indicated in earlier studies:* the blue-shift observed in the absorption onset of W03 colloids represents an increase in bandgap which arises as a result of size quantization effects. Similar size-dependent optical properties have been reported for other metal oxides such as Ti02 and ZnO.29930 The diameters of these particles as determined from the transmission electron microscopy were in the range 2050 A. They were spherically shaped with a nearly symmetric distribution. The concentration of oxalic acid which was employed for the dissolution of the tunstic acid precipitate influenced the particle size. Smaller particles (Dp 20 A) are formed at higher oxalic acid concentrations. Spectroelectrochemistry of WO3 Colloids. Recent efforts to study the optical properties of Ti026 and W03l1particulate films with the application of a cathodic bias have opened up a new avenue to probe the behavior of trapped charge carriers by spectroelectrochemiclmethods. To the best of our knowledge, no such effort has been made to investigate similar spectroelectrochemical properties of semiconductor colloids in suspensions. In the present study, we were able to carry out spectroelectrochemical measurements of aqueous suspensions of W 0 3 colloids by employinga thin-layer cell consistingof an opticallytransparent working electrode (OTE), Pt counter electrode, and Ag/AgCl reference electrode. When a potential of -0.8 V was applied to the working electrode, the colloidal suspension turned blue. The absorption spectra recorded at different levels of charging are shown in Figure 2. It is evident from these spectra that the absorption in the redinfrared region increases with increasing amount of charge. When the OTE is held at potentials more negative than the flatband potential of W03, the electrons are injected into the colloidal particles (reaction 1).
-
WO,
+e
-
-0.8 V
WO, (e,)
(1) These injected electrons are quickly trapped at the surface, resulting in an increased absorption in the red-IR region. When the applied potential was reversed to 0.0 V, the blue coloration disappeared, thus indicating the reversibility of chromic effects
11066 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
0.8
Bedja et al.
-
F
E 0.6 c E
300
400
Figure 2. Spectroelectrochemical investigation of W 0 3 colloids. Absorption spectra of deaerated aqueous WOOcolloidal suspension (0.4 M) recorded after the application of potential at -0.8 V vs Ag/AgCI. The spectra were recorded after the passage of cathodic charge of (a) 0.018, (b) 0.03, (c) 0.05, and (d) 0.09 C. (The absorption spectrum of aqueous WOOcolloidal suspension without any applied potential was used as a reference.)
in W 0 3colloids. Similar blue coloration has also been observed for W 0 3films at applied potentials of 0 to-1.2 Val1The flatband potential of W 0 3colloids is reported to be + 0.33 V vs NHE (pH The reversible electrochromism (colorless-blue) observed in the W 0 3 colloids was further investigated by recording the absorption spectra at applied potentials between 0.0 and -0.8 V (Figure 3). At positive potentials (0 V), no change in the absorption was seen, but with increasing negative potentials an increase in the IR absorption band was seen. After a potential of -0.8 V was attained, the electrode was discharged stepwise by decreasing the applied negative potentials. The increased absorption observed during the charging cycle decreased during the discharging cycle. The changes in the absorbance at 850 nm recorded during the charging and discharging cycles are shown in the inset of Figure 3. The dependence of the difference absorbance (PA) on the applied potential highlights the reversibility of electrochromic behavior of the W 0 3 colloids. The hysteresis or asymmetry observed in PA during charging and discharging cycles disappears if longer time intervals (more than 2 min) are allowed for equilibration a t each applied potential. PhotoelectrochromicEffect in WOj Colloidswith Steady-State UV Photolysis. When W 0 3 colloids were subjected to bandgap excitation, a blue coloration similar to the one observed in electrochemical experiments was observed. The absorption spectra recorded 6 min after the UV photolysis of three different colloidal W 0 3suspension are shown in Figure 4. The absorbance changes at 630 nm versus the duration of photolysis are also plotted in the inset of Figure 4. It is clear from these experiments that smaller colloids present maximum color changes. Bandgap excitation of WO3 colloids leads to charge separation followed by trapping of electrons (reactions 2-4)
+ hv
-
WO, (e...h)
WOde) -wo, WO,(h)
+ oxalic acid
-
(et)
WO,
+ products
600
700
800
900
Figure 3. Dependenceof spectralchange in WO3 colloids on the applied
potential. The absorption spectra of deaerated aqueous W03 colloidal suspension(0.4 M) recorded at variouscathodicpotentials. Thcahrption spectrum of aqueous WO3 colloidal suspension without any applied potential (spectrum a) was used as a reference spectrum. The applied potential was controlled at (b) -0.3, (c) -0.4, (d)-0.5, (e) -0.6,V, -0.7, and (g) -0.8 V vs Ag/AgCl for a duration of 40 s. The inset shows the absorption change at 850 nm during the charging (0 to -0.8 V) and discharge (-0.8 to 0 V) cycles. Arrow indicates the beginning of the discharge cycle.
""1
0).233
WO,
500
Wavelength, nm
Wavelength, nm
(2) (3) (4)
where e and h refer to electrons and holes, respectively. Efficient scavenging of holes by oxalic acid facilitates trapping of electrons in the W 0 3colloids. Bard and his co-workers have demonstrated that oxidation of carboxylates by valence band holes proceeds via a decarboxylation process.31 Higher particle concentration and larger surface areas in smaller size particle suspensions facilitated increased blue coloration (spectrum c in Figure 4).
0.4,
20 0.6 f C
I
o 0300
400
500 Wavelength, . 600
nm700
I
800
z9 1 0
Figure 4. Absorption spectra of deaerated aqueous suspension of WO3 colloids (0.4 M) following the UV photolysis (A > 300 nm) for 6 min.
The oxalic acid concentration in the wo3 colloidal suspensionwas varied to controlthe particle size of the colloids: (a) 0.16,(b) 0.23, and (c) 0.31 M. Inset shows the absorbance change monitored at 630 nm during the course of photolysis. The pH of the medium also plays an important role in affecting the photoelectrochromic effect. The experiment in Figure 4 was repeated at different pH values. The overall features of the absorption band remained the same a t all pH, but the intensity of blue coloration increased with decreasing pH. Figure 5 shows the absorption change (630 nm) versus time of photolysis a t different pH. It has been demonstrated in the case of Ti0+ and W0311 particulate films that the onset of electrochromic effect is dependent on the flatband potential of the semiconductor. Since decreasing pH shifts the flatband potential to more positive potentials, the electron trapping processes occur with greater efficiency in acidic solutions. Charge Trapping As Monitored by Picosecond Laser Flash Photolysis. In earlier studies, it has been shown that transient absorption spectroscopy is a convenient technique for detecting trapped charge carriers in metal oxides.32 Bandgap excitation of Ti02 and ZnO colloids with a laser pulse resulted in the appearance of transient absorption in the red-IR region. In the present study, we excited W 0 3 colloids with 355-nm laser pulse (pulse width 18 ps) excitation. The transient spectra recorded at different time intervals are shown in Figure 6. A growth in the transient absorption with increasing time could be seen in the spectra
-
Photoelectrochemistry of Quantized W 0 3 Colloids
E
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 11067
h
0.3
0
m
s 0.2
3
a n
a
U
a
0.01
1
I
'
0.1
d
U 0.0 0
40 300
2
4
6
8
Time
400
500
10
Time, min F i p e 5. Effect of pH on the photoelectrochromiceffcct in W03 colloids.
The absorption changes at 630 nm at different photolysis time were recorded for three differentdeaeratedaqueous suspension Of WO3 colloids maintained at pH of (a) 1.5, (b) 4.0, and (c) 6.4.
Time
700
600
Wavelength, nm
Figure 7. Electron trapping in W 0 3 colloids as probed by nanosecond laser flash photolysis. The transient absorption spectrum was recorded immediately after the 3374111 laser pulse (pulse width 6 ns) excitation of a 0.06 M deaerated,aqueous suspension of WO3 colloids in the absence of electron acceptors. The inset show the absorption-time profiles at 325 and 620 nm. 50 P
0.12 0. IO
500 pa U
-500
300
550
600
650
700
750
Wavelength, nm Figure 6. Time-resolved transient absorption measurements of wo3 colloids in ethanol following 355-nm laser pulse (pulse width 18 p) excitation. The difference absorption spectra were recorded at time intervals (A?)of (a) 0, (b) 100, (c) 300, and (d) lo00 p after laser pulse excitation. The deaerated suspension flowed through the cell during the measurements. The inset shows the growth of transient absorbance at 630 nm.
recorded at delay times less than 1 ns (see inset in Figure 6).The spectra recorded at longer times did not exhibit any significant decay, thereby indicating stabilization of electrons. Since the photogenerated holes are quickly scavenged by oxalic acid, one does not expect to see decay of electrons in the recombination process. A similar effect of hole scavenger on the stabilization of trapped electrons has been demonstrated in the case of Ti02 colloids.32 The absorptionfeatures of the transient spectra shown in Figure 6 are similar to those observed in electrochemical and steadystate photolysis experiments (Figures 2-4). This shows that the blue coloration of W 0 3colloids observed in both electrochemical and photoelectrochemicalexperiments originates from electron trapping at the surface. Metal ions such as W6+ at the surface are likely to trap electrons and get converted into W5+. In the case of Ti02and ZnO colloids, similar metal ion sites act as traps for electrons, and the trapping process is completed within the laser pulse duration of 20 ps.32 However, in the case of W 0 3 colloids the electron trapping is relatively a slow process. The
400
500
600
700
800
Wavelength, nm
N p e 8. Photoelectrochemical reduction of thionine in W03 colloidal suspension. Timeresolved transientabsorption spectra recorded following 337-nm laser pulse (pulse width 6 ns) excitation of deaerated aqueous wo3 colloidal suspension (0.06 M) containingthionine (20 pM). The absorption-time prfiles at 590 and 740 nm (shown as insets) represent, respectively, the depletion of ground-state thionine at 590 nm as a result of TH' formation and the decay of trapped charge carriers at 740 nm.
first-order rate constant obtained from the growth of absorption at 630 nm (inset of Figure 6) is -lolo s-'. HeterogeneousElectron Transfer between Photoexcited WO3 Colloids and Dyes. ( i ) Transient Absorption Studies. Semiconductor colloids under bandgap excitation are capable of reducing substrates whose redox potentials are less negative than the conduction band of the semiconductor colloid. Oxazine and thiazine dyes which have reduction potentials in the range -0.1 to 0.25 V vs NHE are considered to be excellent probes for the interfacial electron transfer in colloidal semiconductor^.^^^^ The flatband potential of W 0 3 colloids (En = -0.1 V vs NHE at pH 7) is 0.15 V more negative than that of the bulk electrode material.23.25 Thus, the electron transfer from W03 colloids to these dyes is energetically favored. We employed a nanosecond laser flash photolysis technique to investigate the interfacial electron-transferprocess in W 0 3colloidal systems. The transient absorption spectra recorded following 337-nm laser excitation of W 0 3 colloids in the absence and in the presence of an electron acceptor are shown in Figures 7 and 8. The transient absorption spectrum recorded in the absence of any electronacceptor shows broad absorptionin the red-IR region. As indicated in the picosecond laser flash photolysis experiments,
Bedja et al.
11068 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
this spectral feature arises as a result of electron trapping in WO3 colloids. The absorption-time profiles shown in the insets of Figure 7 show that the trapped charge carriers have relatively long life and do not exhibit any decay. Another interesting feature in this difference absorption spectrum is the bleaching at wavelengths below 350 nm. Such a transient bleaching arises as a result of a blue shift in the absorption of W03colloids following the laser pulse excitation. The origin of this photoinduced blue shift has been attributed to mechanisms such as a decrease in oscillator strength of the excitonic transitions due to trapping of charge carriers34 and the dynamic Burstein-Moss effect.35 The transient absorption in the red as well as the bleaching in the UV region can conveniently be used to probe the kinetics of electron transfer in W 0 3 colloids. In the presence of an electron acceptor such as thionine (TH+), the IR absorption decays rapidly with simultaneous bleaching in the 600-nm region. This bleaching corresponds to the disappearance of ground-state thionine as it is converted into semireduced dye, TH' (reaction 5).
+
-
phenosafranin nile blue oxazine 725 methylene blue cresyl violet thionine a
-0.25 -0.119 -0.02 0.01 1 0.048 0.064
0.73 0.70 2.12 1.44 2.02 1.55
Flatband potential of WO3 colloids is around 4 . 1 V vs NHE (pH
7) (refs 23, 25). *Versus NHE at pH 7 (ref 43).
+
WO,(e) TH+ WO, TH' (5) This semireduced dye absorbs in the 380- and 720-nm regions. No such long-lived transients were detected when thioninesolution alone was excited at 337 nm. It has been shown that thiazine and oxazine dyes do not absorb significantly at this excitation wavelength, and their interference with the selective excitation of the semiconductor colloids is considered to be negligible.33 The time-resolved transient absorption spectra recorded in Figure 8 show the disappearance of trapped electrons with simultaneous formation of semireduced dye. The absorptiontime profiles recorded in the insets of Figure 8 further highlight the diffusion-controlledkinetics of the interfacial chargetransfer process. Dynamic electron transfer is not so commonly observed in other colloidal semiconductorsystemssince the photogenerated charge carriers are short-lived. For example,in our earlier studies with Ti02 and CdS colloids we have observed such an interfacial charge transfer to occur when the dye is strongly adsorbed on the semiconductor surface and the charge transfer is completed within the laser pulse duration. The adsorption of cationic dye, TH+, on the positively charged W 0 3 surface is rather weak, and we do not expect to see such a fast electron transfer. However, in the present case the trapped electrons are sufficiently long-lived so that the dye from the solution phase is capable of interacting with W 0 3colloid by diffusionand thereby participating in the chargetransfer process. Similar diffusion-controlled heterogeneous electron transfer between nonadsorbing dye, triplet excited thionine, and ZnO colloid has also been demonstrated by us in an earlier study.36 (ii) Kinetics of Electron Transfer. Although a variety of approaches have been considered to monitor the kinetics of heterogeneous electron transfer at the semiconductor interface, there is still a controversy in accepting a general kinetic model (see ref 37 for an overview). Albery et al. have proposed a general model which considered a Gaussian distribution of the logarithm of the rate constants about some mean, and the width of distribution as an additional parameter.38 However, Darwent and his co-workers,39 and Meisel and his co-workers,m showed that the potential distribution around colloidal particles coupled with the charge of the reducing species could significantly affect the rate of electron transfer to and from the colloid. The dynamic interaction between the dye and W 0 3in the present experiments provides a convenient method to determine the rate constant for the heterogeneous electron transfer (k&)by monitoring the pseudofirst-order growth of semireduced dye (kd) at various dye concentrations, by the expression
k, = ko + ke,[dyeI
TABLE I: Rate Constants for the Heterogeneous Electron Transfer in WOp Colloids'
(6) The rate constants for the heterogeneous electron transfer (kc,)
".""
~~
0.1
0.2
0.4
0.3
0.5
0.6
[Dye],104M Figure 9. Dependence of pseudo-firstsrder rate constant on the dye concentration. The slope of the straight line plot (cq 5) gave the rate constant for the heterogeneous electrontransfer in W03 colloidal system: ( a ) oxazine 725, (b) thionine, and (c) nile blue. The concentration of aqueous suspension of W03 colloids was kept constant at 0.06 M.
as obtained from the slope of a straight line plot, kd vs [dye], are summarized in Table I. Some representative plots of kd versus the concentrations of dyes are shown in Figure 9. The rate constants kct, which vary from 0.7 X 109 to 2.4 X lo9 M-1 s-l, are close to the value for a diffusion-controlledelectron-transfer reaction. For the dyes whose reduction potentials are positive compared to the flatband potential of WO3, we expect AG to be sufficiently negative so that the electron transfer occurs by a diffusion-controlledprocess. For nile blue and phenosafranin, the observed rate constants are lower as their reduction potentials are slightly negative compared to the flatband potential of WO,. Energy Storage in WOJ Colloids. Both laser flash photolysis and pulse radiolysis studies have shown that metal oxide colloids such as Ti02 and ZnO are capable of trapping electrons with high efficiency (for example, see refs 23,31, and 41). However, no effort has been made so far to consider this process for storage of light energy in the form of trapped electrons. The experiments described in the earlier subsections indicate that photogenerated electrons in WOOcolloids can easily be trapped and stored for a long time. For example, an Nz-saturated and UV-photolyzed W03 colloidal suspension can retain blue coloration for 2-3 weeks. The usefulness of W03 colloids for energy storage is further demonstrated by carrying out the reduction of thionine in the dark. Known amounts of deaerated thionine solution were injected into a previously irradiated W03 solution, and absorption changes were recorded (Figure 10). Electron scavenging by thionine resulted in the reduction of thionine. The semireduced dye (TH') formed in the initial reduction step (reaction 5) further undergoes disproportionation to generate the leuco form of the dye, T H (reaction 7).
-
+
2TH' TH- TH' (7) This leuco form of the dye ( T W is stable in an inert atmosphere.
Photoelectrochemistry of Quantized W 0 3 Colloids
The Journal of Physical Chemistry, Vol. 97, No. 42, I993
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Acknowledgment. The work described herein was supported by the Office of the Basic Energy Sciencesof the US. Department of Energy. This is Contribution No. 3606 from the Notre Dame Radiation Laboratory.
0.6 0.4
0.2 0
E o
References and Notes
2
g -0.2
n
-0.4 -0.6
-OA
\I
t
300
400
500
600
700
800
900
Wavelength (nm)
Figure 10. Dark reduction of thionine by utilizing electrons stored in WO3 colloids. Spectrum a shows the absorption spectrum of deaerated aqueous suspension of W 0 3 colloids (0.2M) recordedaftcr UV photolysis. The spectra b-f were recorded following the successive addition of a concentrated solution of thionine to the previously irradiated WO3 suspension (spectrum a). The concentrations of thionine added in these sampleswere(b) lS,(c)26,(d) 36,(e)58,andV) 180pM. Theincreased bleaching at 590 nm shows the depletion of thionine as a result of leucothionine formation. The contents in the cell were maintained under N2 atmosphere throughout.
Since TH- does not absorb in the visible region, it is possible to monitor the reduction process in the dark from the disappearance of the parent dye (TH+). The UV-irradiated W 0 3 suspension is shown as spectrum a in Figure 10. This spectrum has absorption in the red-IR region which is characteristic of trapped electrons. Upon addition of thionine solution, this absorption decreased and an intense bleaching was seen at the ground-state dye absorption band at 590 nm. (Please note that these absorption spectra (b-f) were recorded with corresponding concentration of thionine solutionas reference. Thus, spectra b-f in Figure 10 represent difference absorption spectra.) With increasing thionineconcentration, the bleaching increases, thus indicating the ability of the dye to scavengequantitatively the trapped electrons. Once all the stored electrons are consumed, no further increase in the dye reduction could be seen. These experiments show that one can retrieve all the stored electrons in the dark with a suitable electron acceptor. A similar ability of W 0 3 colloids and polycrystalline electrodes in carrying out reduction of Cu2+ and Fe3+has been reported earlier.23924 The high efficiency of electron storage (0= 0.5) makes W 0 3 colloids an attractive choice for storage of solar energy. The stored energy in the form of trapped electrons can be utilized to reduce other substrates in the dark at a later time. It should be noted that the oxalic acid present here acts as a sacrificial donor by scavenging photogenerated holes. It would be useful if one could initiate water oxidation to generate oxygen instead of utilizing a sacrificial donor. The ability of W 0 3colloids in water oxidation has already been d e m ~ n s t r a t e d . ~ ~
Conclusions W 0 3 colloids exhibit interesting electrochromic and photoelectrochromic effects. Electron injection into W 0 3 colloids by electrochemical or photoelectrochemical methods leads to the blue coloration. Charge carriers which are trapped at W6+ sites in W 0 3 colloids are responsible for blue coloration. Such an electron trapping process is potentially useful in the storage of light energy since the trapped electrons can be utilized to carry out reduction processes in the d a r k . Thus, it should be possible to employ W 0 3 colloids as an electron-relay system in the direct conversion and storage of solar energy.
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