3684
J . Phys. Chem. 1985,89, 3684-3689
be fully localized on a single luminactive ligand.
blica Istruzione and by the Swiss National Science Foundation.
Acknowledb“ent. We thank Mr. L. Minghetti and Mr. L. Ventura for technical assistance. This work was supported by the Italian National Research Council and Minister0 della Pub-
Registry No. Ru(bpy)(i-biq)?+, 89340-69-2; Ru(bpy)z(i-biq)2+, 87464-48-0; Ru(bpy)z(biq)2+,75777-90-1; Ru(bpy)(biq)p, 75785-58-9; Ru(bpy)z(DMCH)2+, 75778-00-6; Ru(bpy)(DMCH),2+, 75778-02-8; Ru(bpy),2+, I5 158-62-0; Ru(i-biq)j2+, 82762-29-6.
Photoredox Reactions on Semiconductors at Open Clrcuit. Reduction of Fe3+ on WO, Electrodes and Particle Suspensions Jean Desilvestro* and Michael Neumann-Spallart* Institut de Chimie Physique, Ecole Polytechnique FZdZrale de Lausanne, CH- 101 5 Lausanne, Switzerland (Received: December 14, 1984; In Final Form: April 15, 1985)
The photoreactions at polycrystalline W 0 3 electrodes in Fe3+containing solutions are investigated. Photoreduction rates of Fe3+on unbiased W 0 3 are shown to be quantitatively related to current-potential curves in the dark and under illumination at the same electrode. One reaction product, Fez+, competes with H 2 0 for holes in the valence band of the semiconductor. From the electrochemical results, rates in illuminated W 0 3 particle suspensions can be estimated and compare well to values obtained by product analysis. The influence of Fe3+and Fe2+concentration, light intensity, and mass transfer is discussed.
Introduction The idea of producing “useful” chemical substances by photochemical synthesis on semiconductor materials has shown to be fruitful for the principal understanding of semiconductor electrochemistry. Besides photoelectrochemical cells with solid electrodes, particle suspensions became interesting for the purpose of conversion and chemical storage of energy (for a review see ref 1). However, until very recently the analysis of observed photoproduction rates in particle suspensions has not met quantitatively the insights as yielded from electrochemical research on macroelectrodes. In 1967 Freund et aL2 suggested the investigation of photocatalytic and photosynthetic reactions on suspended semiconductor particles by studying separately cathodic and anodic processes on semiconductor electrodes. Since then astonishingly little work has been performed in this direction. Yoneyama et aL3 and Hada et aL4 found a fairly good correlation between product formation rates and local cell currents for unbiased illuminated TiOz and ZnO electrodes (reduction of methylene blue or Ag’, oxidation of CH30H, H 2 0 , or ZnO). In our work on TiOzSand WO; we have shown that rates of photoredox reactions like reduction of methylviologen (MV2+)and Ag+ can well be related to the internal currents in unbiased electrodes. These currents can be predicted from potentidynamic measurements on the same electrodes with the help of the microelectrode theory of local elements that has already been successfully applied for the understanding of redox catalysis by noble metals’ and corrosion phenomena.* The (1) Kalyanasundaram, K. In ‘Energy Resources through Photochemistry and Catalysis”, GrPtzel, M.,Ed.; Academic Press: New York, 1984. (2) (a) Morrison, S. R.; Freund, T. J. Chem. Phys. 1967,47, 1543. (b) Freund, T.; Gomes, W. P. Catal. Rev. 1969, 3, 1 . (3) Yoneyama, H.; Toyoguchi, Y.; Tamura, H. J . Phys. Chem. 1972, 76, 3460. (4) (a) Hada, H.; Tanemura, H.; Yonezawa, Y . Bull. Chem. SOC.Jpn. 1978, 51, 3154. (b) Hada, H.; Yonezawa, Y.; Ishino, M.; Tanemura, H. J. Chem. SOC.,Faraday Trans. 1 1982, 78, 2671. ( 5 ) Enea, 0.;Neumann-Spallart, M. J. Electrochem. Soc. 1984,131,2767. (6) Erbs, W.; Desilvestro, J.; Borgarello, E.;GrHtzel, M. J . Phys. Chem. 1984,88, 4001. (7) (a) Wagner, C.; Traud, W. 2.Z . Elekrrochem. 1938, 44, 391. (b) Miller, D. S.;McLendon, G . J . Am. Chem. SOC.1981, 103, 6791. (c) Sutcliffe, E.;Neumann-Spallart, M. Helv. Chim. Acta 1981, 64, 2148. (d) Albery, W. J.; Bartlett. P. N.; McMahon, A. J. In ’Photogeneration of Hydrogen”, Harriman, A., West, M. A., Eds.; Academic Press: London, 1982. (8) Evans, U. R. In ‘The Corrosion and Oxidation of Metals”: E.Arnold: London, 1960.
0022-3654/85/2089-3684$01 SO10
performance of a particle suspension of TiOz illuminated through a window of the geometry of a previously investigated electrode (same irradiance) could then be compared to rates on the electr~de.~ In this work we want to continue our studies on the dynamics of photoredox reactions on semiconductor electrodes at open circuit and particle suspensions with the intention of investigating in detail the influence of mass transfer and concentration of electroactive species. This influence will be shown to be exerted on the dark polarization characteristics and to determine the values of the overall production rates on unbiased semiconductor assemblies under illumination. By comparing results obtained on electrodes and particle suspensions we will try to show how the topological differences between the two systems influence their photoresponse. The system W03/Fe3+ was chosen because both reaction products (Fez+, 0,) of the photogenerated charge carriers can easily be followed analytically and the onset potential of Fe3+ reduction is positive enough to allow for significantly high photoproduction rates. This photoreaction has been described previously by Krasnovskii9 and by Mills.lo Bard” studied qualitatively the photoreduction of Fe3+on illuminated TiOz suspensions by in situ polarography. In this paper we will present an improved technique for measuring quantitatively the product formation rates at irradiated electrodes and particle suspensions.
Experimental Section W 0 3 (Johnson & Matthey, 99.998%, 4.1 mZ/g), Fe(N03)3. 9 H 2 0 , FeS04-7Hz0,70% HC104 (Fluka, analytical grade), and K N 0 3 (Merck, analytical grade) were used as supplied. Deionized water was distilled twice. The preparation and characterization of polycrystalline W 0 3 electrodes have been reported previously.6J2 Both sides of a titanium sheet (0.7 cm X 0.7 cm X 0.05 cm) were coated with a W 0 3 layer of 10 pm. For some experiments one side was insulated with plastic paint. Ru0, electrodes were prepared according to Trasatti et al.I3 Finely divided W 0 3 suspensions were obtained by ultrasonification for 10 min. To
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(9) Krasnovskii, A. A.; Brin, G. P. Dokl. Akad. Nauk. 1962, 147, 656. (10) Darwent, J. R.; Mills, A. J . Chem. SOC.,Faraday Tmns. 2 1978, 78, 359. (11) Ward, M. D.; Bard, A. J. J . Phys. Chem. 1982, 86, 3599. (12) Desilvestro, J.; GrHtzel, M. J . Chem. SOC.,Chem. Commun. 1982, 107. ( 1 3) Galizzioli, D.; Tantardini, F.; Trasatti, S. J . Appl. Electrochem. 1974, 4, 57.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3685
Redox Reactions on Semiconductors determine the particle size of the suspensions one drop of a suspension (8 g/L) was put on a glass slide and covered with a thin glass plate. By optical microscopy (magnification: 1OOOX) individual spheric particles moving due to Brownian motion could be distinguished and photographed. An average diameter of 0.9 pm was determined. Some of the particles were aggregated in clusters of up to tens of particles. Usually the solutions were deaerated with Ar before starting the experiments, although the presence of Ozdid not seem to influence mast of the measurements. The electrical resistance of W 0 3 powder contained in a small insulating cavity equipped with two stainless steel pistons was measured under pressure. p decreased monotonically with increasing pressure until a constant value of 5300 (W03as obtained) and 1900 0 cm ( W 0 3 annealed for 10 min under Ar at 760 "C) was reached at >6 X lo8 Pa. a-values of -7000 (400 nm) and -800 cm-I (440 nm) were estimated from transmittance and reflectance measurements of a W 0 3 suspension (0.5 g/L in photographic gelatine, 1 mm cell) using an integrating sphere (Hitachi). The irradiation apparatus consisted of a 450-W Xe lamp (Oriel), a 8-cm water filter, a BG18 blue filter, a 400-nm cutoff filter (in order to suppress direct photolysis of Fe3+by W light14), a Corning C S 5-57 band-pass filter (350-530 nm), a focusing quartz lens, and a shutter. The irradiance (120 mW.cm-,) was measured with a Yellow Springs 65A radiometer. This corresponds to a polychromatic intensity of 4.7 X lo-' einstein.s-'.cm-2. The potentiodynamic measurements as well as the measurements of product formation at electrodes and particle suspensions were performed in the same cell (previously described in ref 5) under identical irradiation geometry. W 0 3 suspensions were irradiated through a mask (0.7 cm X 0.7 cm) placed directly in front of the cell. The liquid volume of the working electrode compartment (1.5 mL) was agitated by a magnetic stirrer spinning at 1100 rpm. The electrochemical setup comprised a Wenking POS73 potentiostat or for the four-electrode arrangement a Pine RDE6 potentiostat. The W 0 3 electrode was placed at a distance of 1 mm from the quartz window. A polarographic detector electrode (RuO,, 1 mm2) was positioned in the middle of the cell with the exposed area pointing away from the light source. The saturated calomel reference electrode (SCE) was placed in between the two working electrodes. A Ti wire counterelectrode was separated from the working electrode compartment by a cellulase membrane. All potentials are quoted vs. SCE. Fez+ and Fe3+ concentrations a t pH 2 were measured polarographically by oxidation or reduction at the Ru02 electrode at 0.9 and 0.1 V, respectively. Calibration was made in situ by coulometric (intensiostatic) release of Fez+(generating electrode W03) at a rate that was close to that observed under illumination, or oxidation of Fe2+ to Fe3+at an additional RuO, electrode (2 cm2). From the rate of current rise a t the RuOz electrode due to Fe3+ reduction or Fez+ oxidation, the calibration factor and the coulometric cell constant, k,, were determined. To avoid consumption of the product, the RuOz detector electrode was made small enough so that the coulometric cell constant was only 10" S-I. Fe2+was also analyzed spectrophotometrically as its phenanthroline complex. The amount of evolved Ozwas measured in a different cell (total volume 13.7 mL). Gaseous samples from the headspace (3.7 mL) were analyzed by GC (molecular sieve 5A, carrier gas Ar). The results were corrected for air leakage. Calibration was performed by injecting known amounts of Oz into the cell. Experiments with a rotating disk electrode (W03 deposited on a Ti rod of 4 mm diameter) were carried out in a 50-mL cell in connection with a "ASR" rotator (Pine). The cell contained a cylindrical Pt gauze counterelectrode (- 150 cm2) which was not separated from the working electrode so that the product was (partially) reoxidized during long runs. The currents were measured quasistationarily at a scan rate of 0.5 mV/s so that no hysteresis was observed upon scan reversal.
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(14) (a) Ross, W. H. J . Am. Chem. Soc. 1906, 28, 790. (b) David, F.; David, P. G. J. Phys. Chem. 1976, 80, 579.
1.0
0. 8
0.6 0. 4 0. 2 c
i
0.0 -0.2 -0. 4
-0.6 -0.8 5
-1.0 0.1
0.2
0.3
0.4
0.5 0.6 0.7
(E vs. SCE)/V
Figure 1. Photocurrent (iph)and dark currents (1-5) for polycrystalline WO,electrodes. Electrolyte: 0.01 M HClO,, 0.1 M KNO,. Two sides (each 0.5 cm2)of the Ti support were covered with W 0 3 , one side was exposed to light (irradiance 120 mW/cm2). [Fe(N03)3]= 4.16 X lo4 M (l), 7.72 X lo4 M (2), 2.08 X lo-, M (3), 5.09 X M (4), and 1.2 X M ( 5 ) . Scan rate 0.5 mV/s, arrows indicate scan direction.
The broken lines represent -id.
Results Figure 1 shows current-potential curves obtained at pH 2 with W 0 3 electrodes in the dark and under illumination with visible light. In the absence of Fe3+ the dark currents, id, are
