4001
J . Phys. Chem. 1984, 88, 4001-4006
Visible-Light-Induced 0, Generation from Aqueous Dispersions of WO, Wilson Erbs, Jean Desilvestro, Enrico Borgarello, and Michael Gratzel* Institut de chimie physique, Ecole Polytechnique Fgddrale, Lausanne. Switzerland (Received: December 21, 1983; In Final Form: April 5, 1984)
Oxygen is produced efficiently under visible light and sunlamp irradiation of aqueous WO, dispersions containing Ag' ions as conduction band electron acceptors. The advantage of this acceptor with respect to Fe3+is the higher quantum yield of O2 formation and the irreversibility of the photoreaction 2H20 4Ag' + hu O2 4H+ 4Ag which allows to sustain 0, generation until reduction of Ag' is completed. Photoelectrochemical experiments served to elucidate the mechanistic details of these redox processes.
+
Introduction
The generation of oxygen from water by visible light is a topic which is currently under intensive investigation.' This process has great importance for the design of photochemical devices that achieve the photocleavage of water by visible light. One successful approach to the solution of this problem involves the use of semiconductor particles as light-harvesting units. Thus, it has been shown in the pioneering work of Krasnovsky, that aqueous dispersions of W 0 3 , when illuminated by visible light, produce O2 as a result of the reaction of valence band holes (h') with water: 4h+(WO3) 2 H 2 0 O2 + 4H+ (1)
-
+
The concomitant conduction band process was reduction of ferric to ferrous ion: ecB- Fe3+ Fe2+ (2)
-
+
-
yielding the overall photoreaction:
+ 4Fe3+
hu
+
+
4Fe2+ O2 4H' (3) wo3 which in acidic solution is energy storing ( A r c o (eV) = +0.46 - 0.059pH a t T = 298 K). Recently, Darwent and Mills3 performed a more detailed investigation of this photoreaction. The initial quantum yield of O2 generation by 405-nm light was determined as >3 X mol/einstein. It was also found that Fe2+ ions inhibit oxygen formation due to competition of Fez' with water for valence band holes. This severely limits the degree of reduction of Fe3' to Fe2+ that can be accomplished through photoreaction 3, a feature that is undesirable for solar energy conversion devices. The present study introduces Ag+ instead of Fe3+ as an acceptor for conduction band electrons in this system. 2H20
Experimental Section
Materials. Puratronic WO, (Johnson-Matthey, Zurich) was used throughout the work. Preliminary experiments showed that the activity of this material is much superior to that of other commercial W 0 3 samples, presumably due to its high purity (99.998%). A surface of 4.1 mz/g was measured by a Micromeritics surface area analyzer, Model 2205. RuO, was obtained from Alfa Inorganics. All other products were at least reagent grade and were used as supplied by the vendor. Apparatus. Investigations were carried out with 25-mL samples contained in Pyrex glass vials. Prior to irradiation the W 0 3 dispersions were freed from 0, by purging with Ar or He. A high-pressure xenon lamp (450 or 600 W) was used as a light source. Infrared and UV radiation was removed by water and a 410-nm cutoff filter, respectively. Additional experiments were performed with a Hanau Suntest lamp (global irradiance 70 mW/cm2) whose emission spectrum mimics closely solar radiation. Irradiations of samples in 1-cm quartz cuvettes with monochro(1) M. Gratzel, Ed., in "Energy Resources through Photochemistry and Catalysis", Academic Press, New York, 1983. (2) A. A. Krasnovsky and G. P. Brin, Dokl. Akad. Nauk., 147, 656 (1962). (3) J. R. Darwent and A. Mills, J . Chem. Soc., Faraday Trans. 2,78, 359 (1978).
0022-3654/84/2088-4001.$01.50/0
-
+
+
matic light employed a Bausch & Lomb monochromator equipped with suitable filters to remove the harmonics Light intensities were measured with the ferrioxalate actinometer. The constancy of the light intensity over long irradiation periods was checked with a Si photodiode using an integrating sphere. Transmittance and reflectance measurements with an integrating sphere showed that a suspension of WO, (8 g/L) in a photographic gelatine of 1 mm thickness absorbs 60% of the incident light at 405 nm. Oxygen was detected by gas chromatography using a carbosieve 5-A column and He as a carrier gas. The results were corrected for air leakage. In some experiments, O2 was monitored in the liquid by means of a Clark type electrode (Yellow Springs Inc.) and electronic backup circuitry kindly provided to us by Dr. A. J. Frank, SERI, CO. In this case, a double wall cylindrical vessel was employed which allowed us to thermostat the solution (20 mL) at 25 O C . Ag' concentrations were determined by potentiometric titration with HCl on a Metrohm Titroprocessor 636 and Dosimat E 635. Fe2+ was analyzed spectrophotometrically as the phenanthroline complex. Electrochemical experiments employed a conventional threeelectrode cell with a flat Pyrex or quartz window. The solution (approximately 20 mL) was agitated by a magnetic stirrer (900 rpm). The saturated calomel reference electrode (SCE) was separated from the electrode compartment by a glass-fritted chloride-free electrode bridge to avoid contamination with C1-. All potentials are quoted vs. SCE. A Ti-wire counterelectrode was separated from the working electrolyte by sintered glass frits. A Wenking POS 76 potentiostat was used in connection with a Hewlett-Packard 7046 B XY recorder. The electrodes were illuminated with the light from a high-pressure xenon lamp (450 W) filtered by a 8-cm H 2 0 filter, a BG 18 blue filter, and a Corning CS 5-57 band-pass filter (360-500 nm). The light intensity was measured with a Yellow Springs Kettering Model 65A radiometer. Photocurrents iphare defined as iph = illght- idark. Catalyst Preparation. A few experiments were performed with Ru0,-loaded W 0 3 particles. Catalysts prepared via decomposition of RuO, yielded best results. An aqueous stock solution of RuO, (ca. 1.2 mg/mL) was made up first and the amount required to obtain 0.5% (w/w) loading was then injected into the WO, dispersion. The RuO, was decomposed by irradiating the solution overnight in the Suntest lamp yielding RuOz according to
+
R ~ OL , ,R ~ O , 0,
(4)
Alternatively, impregnation of W 0 3 with aqueous RuC1, solutions and subsequent calcination at 300 OC was attempted. R u 0 2 deposits prepared this way were, however, rather inactive, leading to no improvement of the photoactivity of the WO, particles. Electrode Preparation. The earlier reported fabrication of W 0 3 electrodes4was slightly modified. A dispersion of W 0 3 was made by sonicating a mixture of 200 mg of W 0 3 in 400 pL of 2propanol. This mixture was applied onto a Ti sheet (0.5 cm2, (4) J. Desilvestro and M. Gratzel, J. Chem. SOC.,Chem. Commun., 107 (1982).
0 1984 American Chemical Society
4002
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
Erbs et al. z
'!
i
3000
El
1
c
< a W 2 W
1 -
0 N
0
.75
-
.s
-
.2s
-
/ O - O
/ O
LL
0 O
W@, (Ru0,O.
U. 0
I
5%) /Fe3*
/
IW
< a W
2 I-
3 J
-I
W
a 0
4
12
8
TIME
log Chg* 1 /M
(Hr)
Figure 1. O2generation from irradiated aqueous solutions of 5 X IOv2 M AgNO, and IO-' M FeC1, in the presence of WO,, W03/Ru02-0.5% or TiO,. The catalyst concentration was 8 g/L, initial pH 4, irradiation with a Suntest lamp.
Siber-Hegner Raw Materials, 99.99%) which previously had been cleaned with CH2Cl2,ethanol, and H20, then etched for 30 s in an aqueous solution of H F (4% wt/wt) and HNO, (30% wt/wt) and rinsed with H20. After the coating was air-dried for 10-15 min, the electrode was heated at 750 OC for 10 min in an Ar stream (flow rate 15 mL/min, 30 ppm of 0,). A thickness of the greenish-yellow W 0 3 layer of 10-20 pm was estimated from the weight and the density of WO, (7.16 g crnW3).The backside of the electrodes was spray insulated. Electron microscopy (SEM) was performed with a Cambridge 250 instrument. A layer of 200 8, of gold was sputtered onto the electrodes. X-ray analysis (EDAX, Tracor Northern N S 880) showed only the presence of W, Ag, and Au, but not Ti.
Results Irradiation of Particle Dispersions. Figure 1 shows the comparison of the two acceptors Ag+ and Fe3+in long-term irradiation experiments where the Suntest lamp was used as a light source to reproduce solar conditions. In this case oxygen was determined by G C analysis of the gas present in the headspace of the glass vials above the WO, (8 g/L) dispersion. With lo-, M Fe3+, 90 pL of 0, is produced during the first hour of irradiation. Thereafter, the rate levels off quickly, a plateau with V(0,) = 630 pL being approached after 13 h. This corresponds to 42% of the stoichiometric yield expected from eq 3. The performance of the Fe3+ based system is significantly improved by depositing Ru02 onto the WO, particles. Here, 0, is generated with a rate of 280 pL/h during the first hour. After 13 h, 1250 pL of O2 was present in the gas phase corresponding to 83% of the maximum yield that can be obtained. A marked improvement in the rate of 0, formation is also obtained when 5 X lo-, M Ag+ instead of Fe3' is used as an electron acceptor. In fact, in the Ag+/WO, system, formation of oxygen bubbles on the WO, particles under sunlamp irradiation becomes readily visible. The average rate of 0, generation during the first 3 h is here 485 pL/h, the ratio of produced 0, and consumed Ag+ always being 1:4 throughout the reaction. When the irradiation was stopped after 13 h, 3.9 mL of 0, had accumulated in the gas phase Corresponding to 52% of the maximum yield expected for the reaction 2 H 2 0 + 4Ag'
0
16
-k0, + 4H+ + 4Ag
(5)
For comparison, we included in Figure 1 an oxygen evolution curve obtained from sunlamp irradiation of TiO, (Degussa, P-25, anatase) dispersions using again 5 X lo-, M Ag+ ions as electron acceptor. The initial rate is here about 5 times lower than that obtained with W03/Ag+, approaching a plateau after 13 h when 700 pL of 0, had accumulated corresponding to ca. 9% of the stoichiometrically expected yield. In order to test whether 0, generation is catalytic with respect to WO,, sunlamp irradiation was performed with 25 mL of solution containing 12 mg of WO,
Figure 2. Initial rate of O2 production as a function of the AgNO, concentration. The catalyst concentration was 8 g/L, initial pH 4, irradiation with a Suntest lamp.
z
1. 25
0
I-
0-0
c < W d
-. 25
3
-1
3
1
5
pHo
Figure 3. Initial rate of O2production as a function of pH. The catalyst concentration was 2 g/L, 5 X 10" M AgNO,, irradiation with a Suntest
lamp. and 5 X lo-, M AgF (pH 4). After 44 h 4.4 mL of O2was produced corresponding to a turnover (n(O,)/n(WO,)) of 3.5. Since no decomposition of W 0 3 was visible after photolysis, the WO, could not be the source of the 0, collected. Sunlamp irradiation of AgN0, solution (pH 4) in the absence of W 0 3 did not yield any oxygen indicating that water oxidation results from band gap excitation of the WO, particles. The effect of Ag+ concentration on the initial rate of 0, generation as measured by gas chromatography is depicted in Figure 2. Even at [Ag'] as low as M there is already significant O2liberation. The rate increases sharply between lo4 and lo-, M attaining a plateau at >5 X lo-, M Ag+. Further increase in silver ion concentration leads to no significant improvement in the 0, output. At lower initial concentration of Ag', practically complete conversion of Ag' to Ag and concomitant formation of stoichiometric amounts of 0, is readily achieved. Thus, when M Ag' solution was used, the reduction was terminated after 3-h sunlamp irradiation and produced 150 pL of O2corresponding to practically 100% yield. The influence of pH on the initial rate for O2 production is shown in Figure 3. At pH 0 no significant amount of O2could be. detected. Above pH 1 the rates increase considerably and reach a maximum of 485 pL/h at pH 4. At pH values higher than 5, the solution turned brown due to precipitation of a silver compound. In most experiments the starting pH was 4. From the formation of 3.9 mL of O2 (Figure l ) , a final pH of 1.6 is calculated for reaction 5. This value compares well with the measured final pH of 1.7. Monochromatic irradiation at 405 nm of a suspension of WO, (8 g/L) in a lo-' M AgN03 solution (initial pH 4) over periods
O2 Generation from Aqueous WO, Dispersions
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4003 i(rnAlcrn2)
TABLE I: Photon and Electron Transfer Efficiencies for W 0 3 Particles and Electrodes system M AgNO,, W 0 3 (8 g/L), pH 4" M AgNO,, WO, (8 g/L), n H Ab Wb3-(8 g/L), M Fe2(S04),, N H2S04" WO, electrode, lo-) M HC104, M AgNO,, M NaC104"
VET
Vph
>0.020
>0.038
>0.024
>0.049c
>0.004
>0.0075
0.058
0.109
'Monochromatic irradiation (405 nm, 1.38 X einstein s-' cm-2). bPolychromatic irradiation (CS 7-59 filter, 10.5 mW cm-2, integrated photon flux = 3.6 X lo-* Einstein s-' cm-'). CCalculated by integration over the action spectrum of the WO, electrode. "Monochromatic ireinstein s-' cmT2). radiation (405 nm, 3.05 X
TABLE 11: Photoreductiono of AgN03 as a Function of Light Intensity light int, m W cm-2
Ag' reduced, fimol
1.23 2.39 10.20 19.60
2.6 0.1 2.2 f 0.2 3.0 0.2 4.0 0.2
* * *
M A g N 0 3 (2 mL), WO, concentration 8 g/L, initial pH 4. The Corning C S 5-57 bandpass filter and neutral density filters were used to adjust light intensities. Irradiated area 1 cm2. Every sample was irradiated with a total of 55.5 W s.
of typically 15 h revealed a photon efficiency vphs for Ag+ consumption of >0.02 (Table I). The same value was obtained with the powder scratched off an annealed W 0 3 electrode. A photon M Fe2(S04)3 efficiency of >0.004 was determined for a solution irradiated for 11 h under the same conditions. In a typical illumination with filtered polychromatic light of a high-pressure Xe lamp, (10.5 mW/cm2, illuminated area = 7.07 cm2), a rate of 130 pL of 0 2 / h was obtained in a M Ag+ solution with 8 g / L of WO,. This O2 generation rate corresponds to a polychromatic photon efficiency of 0.024. The effect of light intensity on the efficiency of Ag+ reduction is shown in Table 11. In these experiments dispersions of WO, (8 g/L) were irradiated with polychromatic light whose intensity was varied by neutral density filters. Exposure time was adjusted such that each sample received the same amount of light energy. Table I1 shows that increasing the light intensity by a factor of 16 leads to a 1.5-fold augmentation of the polychromatic photon efficiency for Ag+ reduction. A few experiments were performed to investigate the effect of R u 0 2 on the performance of the W03/Ag+ system. Rates of O2 formation were determined by a Clark electrode from the initial slopes of the current-time curves. Values of 100 and 300 pL/h were measured for naked W 0 3 and W 0 3 / R u 0 2 4 ) . 5 %in 5 X M Ag'. After 5 min of irradiation, the rates drop due to diffusion of O2 to the gas phase and formation of a silver mirror on the illuminated wall of the cell. Formation of silver mirror may be affected by the stirring rate. Loading the particles with R u 0 2 did not improve the long-term performance of the catalyst at pH 4. However, at pH 2 in 5 X M Ag+ we detected, after 13 h of irradiation, 1400 p L of O2 for W 0 3 (2 g/L) and 2150 pL of O2 for W 0 , / R u 0 2 (2 g/L). Electrochemical Experiments with Polycrystalline Electrodes. WO, electrodes were prepared from the same material which was used in particle experiments. The spectral response of W 0 3 films is known to depend on the preparation m e t h ~ d .We ~ found a similar variation of the photocurrent efficiency vF6 with the (5) The photon efficiency qph is defined as the number of molecules produced per number of incident photons. qphis a lower limit for the quantum yield. (6) The photocurrent efficiency q p is defined as the number of electrons transferred to a collector electrode per number of incident photons. (The semiconductor is held at a potential of + 1.2 V against SCE.)
Figure 4. Photocurrent (I) and dark currents (a-e) for polycrystalline WO, electrodes in lo-) M HC104, IO-* M NaCIO4. [AgNO,] = M (a), lo-, M (b), M (c), 0.05 M (d), 0.2 M (e), and 0 M (I). The scan rate was 0.5 mV s-', arrows indicate scan direction. The light with a Corning CS 5-57 filter. intensity was 21 mW
350light
t
t on
Off
71-:I.1-k - 20-
hght
t
on
t
on
B
t
off
Figure 5. Potential time (A) and current time (B) behavior of a WO, electrode in the dark and under monochromatic irradiation at 405 nm. M AgN03, lo-, M HC104, M NaClO,, light intensity 0.9 mW cm-2, electrode potential (B) 0.403 V vs. SCE.
wavelength as Augustynski et a1.* A plot of (vFhv)'/2vs. hv for our electrodes is linear between 2.55 and 2.85 eV and reveals an indirect interband t r a n s i t i ~ nwith ~ ~ ~an energy of 2.5 eV. The photocurrent efficiency at 405 nm and 1.2 V (SCE) is 0.53. Figure 4 shows the current potential curves obtained with W 0 3 electrodes in the dark and under polychromatic illumination at pH 3. In the absence of Ag+ the dark currents are 0.15 V (photo-onset potential) do not differ from the currents measured in Ar saturated solutions. In the (7) J. M. Berah and M. Sienko, J . Solid State Chem., 2, 109 (1970); G. Hodes, D. Cahen, and J. Manassen, Nature (London),260, 313 (1978); M. A. Butler, R. D. Nasby, and R. K. Quinn, Solid State Commun., 19, 1011 (1976); W. Gissler and R. Memming, J . Electrochem. SOC.,124, 1711 (1977); M. Manfredi, C. Parachini, G. C. Salviati, and G. Schianchi, Thin Solid Films,79, 161 (1981); F. Di Quarto, A. D. Paola, and C. Sunseri, Electrochim. Acta, 26, 1177 (1981); K. Miyake, H. Kaneko, and Y. Teramoto, J . Appl. Phys., 53, 1511 (1982); M. Neumann-Spallart, Abstract, 4th International Conference on Photochemical Conversion and Storage of Solar Energy, Jerusalem, Israel, 1982. (8) M. Spichiger-Ulmann and J. Augustynski, J . Appl. Phys., 54, 6061 (1983). (9) M. A. Butler, J. Appl. Phys., 48, 1914 (1977).
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The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
presence of lo-" M AgN03 the onset of the cathodic dark current occurs at 0.28 V, which is only 40 mV more negative than the Nernst potential. An increase in Ag+ concentration shifts the onset for the reduction currents to more positive potentials and increases the steepness of the current curves. After each scan the deposited Ag layer was dissolved in a saturated KzSzOBsolution at 50 O C . einstein s-' Monochromatic irradiation (405 nm, 3.4 X ern-,) does not change the photo-onset potential or the form of the photocurrent curve presented in Figure 4. The open-circuit potential of a W 0 3 electrode in the presence of lo-, M Ag+ shifts rapidly during illumination from 0.425 V in the dark toward more negative potentials and passes through a minimum at around 0.365 V to rise again slowly (Figure 5A). Under illumination the current density rises rapidly from -17 to > 10 FA cm-, and then falls off to zero within a few seconds (Figure 5B). Upon switching off the light, the current returns to the dark value with a similar current-time transient into the negative direction. At the opencircuit potential under illumination, the cathodic current lz-1 equals the net anodic photocurrent iph+. Assuming that I equals the dark cathodic current id-,l0J1a photon efficiency of 0.058 is calculated for the photoreduction of Ag+ on W 0 3 electrodes under opencircuit conditions (Table I). The potential of a W 0 3 electrode illuminated for 50 min with monochromatic light under zero bias shifts to 0.417 V. During the same time interval, the dark currents increase such that the photon efficiency stays about constant. A grey-black overlayer is formed. From the reduction current, the total amount of Ag deposited is 5 X mol/cm2. The electron micrographs presented in Figure 6 reveal that the silver crystallizes on the W 0 3 surface in the form of thin rectangular plates with a typical length of 5 Fm leaving most of the W 0 3 surface uncovered. By counting over a sufficiently large area, a number of 7 X lo5 Ag crystals per cm2 is estimated. From the total amount of photodeposited Ag, the average volume of one Ag crystallite is 7 X cm3, which compares well with the size of the aggregates shown in Figure 6b. By polychromatic illumination (14.5 mW cm-2) of the electrode under open-circuit conditions, a few grains of metallic Ag of a typical size of 0.5 mm are formed. Still a high percentage of the electrode is covered only with a grey film. The open-circuit potential under the same monochromatic illumination (405 nm) as before was 0.422 V. The lower dark current of -1 1 FA cm-z at this potential indicates that the photon efficiency decreased by approximately 35%. Further polychromatic irradiation for 14 h covers the whole electrode with beautifully glistering silver crystals. The photon efficiency decreased to
Discussion Photoinduced reduction of Ag+ in semiconductor dispersions has been extensively investigated since Tamman12 discovered a darkening of emulsions containing ZnO and Ag+ under UV light illumination. Hada et al." report a quantum yield of 0.385 for Ag+ reduction on ZnO in M AgC104. The valence band process is here dissolution of ZnO to yield Zn2+and oxygen. The coupling of Ag+ photoreduction to catalytic water oxidation according to eq 5 has been achieved with T i 0 2 as semiconductor material. Fleischauer et al.13 report +(Ag) = 0.03 for Ag+ reM) on TiO, single crystals. A higher value, Le., duction ( +(Ag) = 0.19, has been found by Hada et al.14with aqueous TiOz suspensions ( lo-, M Ag+ concentration). To our knowledge, there are no previous reports on the use of W 0 3 to mediate photoinduced water oxidation by Ag+. An advantage of W 0 3 with respect to TiO, or ZnO is its smaller band gap (2.5 vs. 2 3 eV) corresponding to a fundamental absorption edge of 496 nm. Oxygen generation from water can therefore be driven by visible light which is particularly important (10) F. Mallers, J. H. Tolle, and R. Memming, J. Electrochem. Soc., 121, 1160 (1974). (11) H. Hada, H. Tanemura, and Y. Yonezawa, Bull. Chem. SOC.Jpn., 51, 3154 (1978). (12) G . Tamman, Z. Anorg. Chem., 114, 15 (1920). (13) P. D. Fleischauer, H. K. Alan Kan, and J. R. Shepherd, J. Am. Chem. Soc., 94, 183 (1972). (14) H. Hada, Y.Yonezawa, and M. Saikawa, Bull. Chem. SOC.Jpn., 55, 2010 (1982).
Erbs et al.
Figure 6. Electron micrographs of polycrystalline WO, electrodes: (A) no Ag; (B) potentiostatic photodeposition of ca. 5 X lo-' mol of Ag/cmZ. The silver amount was estimated from the reduction currents under chopped light conditions. During illumination the electrode potential was adjusted so that ilight was always zero.
for solar application. Our studies show that the polycrystalline W 0 3 electrode is very efficient at 405 nm under anodic bias (vPc = 0.53). From the fact that the powder taken from the bottle and the one scratched from an electrode gave the same efficiencies for Ag+ reduction, we conclude that the effect of the annealing process is to establish electrical contact between the particles and the Ti substrate rather than to affect the level of doping on the W03. The photon and photocurrent effi~iencies~.~ are related by the equation ?ph
=
(6)
vpc?ET
where vETdenotes the overall electron transfer efficiency. The latter is affected by the relative rate of the processes described by
+ hVB++ D eCBo A
km
A-
+ 2 + + hVB+-k A-
A-
D+
eCR- hVB+
kru
From this sequence, one obtains
kD
kA
A
(7)
D+ A D
recombination
(10) (1 1)
7'he Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4005
O2 Generation from Aqueous W 0 3 Dispersions VET =
~ E T [ A-] ~ A [ A --] kbrA-1 [D'l kETIAl
-k k e c
(12)
Because the recombination rate increases with decreasing band bending, reduction processes which occur close to the flat-band potential should be very inefficient. The current-potential diagrams show that, for example, oxygen is a very poor electron acceptor on WO,. On the contrary, the reduction of Ag' and Fe3+I5occurs at potentials up to 350 mV positive of the onset of photocurrent. The reduction mechanism can be described by a model involving electron transfer through surface states.16 The product of this reaction is an adsorbed Ag atom which is likely" to react with excess Ag+ to form Ag2'. This dimeric intermediate has also been discussed by Whitten et a l l 8 in the context of Ag' reduction sensitized by R ~ ( b p y ) , ~ +In. solution it has strongly reducing properties, but it is probably stabilized when adsorbed onto W 0 3 . Our results indicate that, with the W03/Ag' combination, photoinduced water oxidation is sustained much longer than for both TiOz/Ag+ and W03/Fe3+. Thus, from Figure 1, one notices that in the former system the rate of O2generation is only slightly reduced even after 3.9 mL of O2 had been accumulated. At this time, 70 mg of Ag has been formed by the conduction band process which, theoretically, would give a film thickness of ca. 8 nm if the Ag is deposited entirely onto the W 0 3 particles. However, from the electron microscopy studies, it is expected that Ag is deposited onto WO, in form of large crystallites which cover only a small fraction of the surface. Nevertheless, considerable amounts of Ag are present on the W 0 3 particles, and this does not appear to interfere with water oxidation by valence band holes, eq 1. From this finding, one infers that reoxidation of Ag via reaction 9, though thermodynamically favored, is kinetically slower than oxygen generation from water. In the case of Fe3' as electron acceptor, kA for the process 9 is higher compared to the W03/Ag+ system rendering reaction 3 rapidly autoinhibitive, as was also observed by Darwent and Mills., These authors estimated for process 2 an initial photon efficiency of 0.034. The efficiency over long irradiation periods is, however, much lower as shown in Table I. The behavior of particles has been previously related to the For the first time, photoelectrochemistry of we compare here a particulate photoanode with particles of the same quality. The electrode can be regarded as a huge particle which is electrically contacted by a metallic wire. Thus, potentials and currents can be easily measured and related to the behavior of smaller particles. The potential time characteristics under illumination as presented in Figure 5A are governed by different effects: (i) The initial overvoltage for the formation of Ag adatoms decreases with increasing coverage. In a related experiment, Memming et al.IOestimated from their potential time curves an overvoltage of > 100 mV for the photodeposition of Pd onto Ti02. (ii) Adatoms and metallic Ag act as recombination centers. Thus, the photocurrent decreases upon silver deposition and the potential is shifted to more positive values. Previous studies have shown that also Ru02deposited on WO, by thermal decomposition of RuC1, and Au layers on W 0 3 increase the electron-hole recombination.21 (iii) The silver overlayer formed during longer irradiation absorbs light. The experiments with electrodes showed, however, that the photon efficiency is constant up to 5 X lo-' mol of (15) J. Desilvestro and M. Neumann-Spallart, unpublished results. (16) J. Vandermolen, W. P. Gomes, and F. Cardon, J. Electrochem. Soc., 127, 324 (1980); P. Iwanski, J. S. Curran, W. Gissler, and R. Memming, ibid., 128, 2128 (1981). (17) J. Pukes, W. Roebke, and A. Henglein, Ber. Bunsenges. Phys. Chem., 72, 842 (1968). (18) T. K. Foreman, C. Giannotti, and D. G. Whitten, J . Am. Chem. SOC., 102, 1170 (1980). (19) S. R . Morrison and T. Freund, J . Chem. Phys., 47, 1543 (1967). (20) H. Reiche, W. W. Dum, and A. J. Bard, J . Phys. Chem., 83, 2248 (1979). (21) J. Desilvestro and M. Gratzel, Abstract, 4th International Conference on Photochemical Conversion and Storage of Solar Energy, Jerusalem, Israel, 1982.
Ag/cm2. Electron micrographs revealed that only a small fraction of the surface is covered by Ag crystals. In the case of W 0 3 particles, the highest amount of Ag deposited is 8 X lo-* mol of Ag/cm2. Thus, the absorption by the silver should not decrease the photon efficiency considerably throughout our experiments. The rate decrease observed in this long-term irradiation is rather due to the pH lowering. As the bands of WO, shift with -59 mV/ApH, the photocurrents at a given potential decrease with decreasing pH. On the other hand, the currents for reduction of M Ag' are not influenced by pH changes between 4 and 1. Thus, the surface photopotential shifts with decreasing pH to more positive values and the photon efficiency falls off. Though qETvalues are based on the number of incident photons, the marked difference in vET for the particles (>3.8%, >4.9%, respectively) and for the electrodes (10.9%) cannot be understood completely by systematic errors in determining Vph for the two systems. We present here an explanation based on the currenttime transients observed with electrodes (Figure 5B). The current overshoot was interpreted by Memming et a1.10,22 by charging and discharging of interband surface states. The surface states might be due to imperfections in the crystal structure of the semiconductor and silver adatoms or clusters which act as electron traps as shown by the cathodic transients in the dark. Filling the traps with electrons and reoxidation by valence band holes under illumination does not contribute to the formation of any product. The electron transfer efficiencies of WO, electrodes were estimated from the steady-state currents reached several seconds after opening or closing the shutter. On the other hand, a particle in a stirred solution (900 rpm) is illuminated only for a fraction of a second before it moves to the dark part of the cell. If a simplified model is used where all the light is absorbed in the first half of the cell, Le., a pathlength of 0.5 cm, a particle rotating on a circle with a diameter of 1 cm and a frequency of 900 rpm is periodically 33 ms in the light and 33 ms in the dark. From a representative value of l O I 3 states per cm2 estimated from capacitance measurements on TiOZ2,and from a current density of 16 kA/cm2, a time of
