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a stronger oxidative force as compared to the Ru02/Ti02 system and thus oxidizes carbon completely. Kawai and Sakata13 have tested the activities of several kinds of metals (Ru, Ni, and Pt) mixed with Ti02 and carbon and found that the initial H2formation is 7-15 times faster for Pt than the Ru02/Ti02system. Although they ascribed this effect to the oxidation of metals, Pt is not easily oxidized, and we find no evidence for such in our systems. Their Ru02/Ti02system produced -4 X lo4 mol of H2 during 5-h illumination by a 500-W Hg lamp a t room temperature, while, in our Pt/Ti02 system, 9 X lo* mol of H2 was produced during 2-h illumination by a 200-W Hg lamp. This suggests that the Pt/Ti02 system has higher activity. The H2/C02ratio in Kawai and Sakata’s experiment13 was 1-1.5, much less than than the stoichiometric ratio, suggesting the reduction of Ru02. In preliminary experiments, we find some evidence that Ni or Co mixed with Ti02is oxidized in the presence of H20and UV irradiation, while the oxides of metals such as Pt and Rh are reduced under the same experimental conditions.’* The kinetics of the present reaction are similar to those of the photoassisted water-gas shift reaction over Pt/ TiOz.l0 The almost zero-order dependence of the rate on H20pressure is the same as in the latter, and the activation energy (-5 kcal/mol) is close to 7.5 kcal/mol of the latter. As for the wavelength dependence, the present reaction shows a little shorter onset than the shift reaction, but the reason is not clear. The decline of the H2 formation rate in a given run is probably due to the accumulation of H2 which competes with carbon for oxygen species. The long-term decline
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arises from the loss of a good contact between the catalyst and carbon since the initial reaction rate can be reproduced by remixing the sample. The formation of O2 increases with time but, in any run, its maximum amount is less than that observed in the reaction with lignite.12 Although the reaction of H2with O2 occurs rapidly on a clean Pt/Ti02 even in the presence of gas-phase H2019CO inhibits this reaction to some extent as observed in the water-gas shift reaction.1° Since CO was not detected in the gas phase of the present reaction, its inhibitory effect is not established. The result of Figure 6 shows that the decrease in the amount of adsorbed H20 on the sample results in a decrease in the O2 pressure. Therefore, it is reasonable to assume that, since active carbon adsorbs a large amount of H20, the H20layer on the Pt that is in contact with carbon is thicker than in the absence of carbon. When the H20layer is thick, O2does not react readily with HD As the contact area between the catalyst and carbon decreases with the consumption of carbon, oxygen species would be required to migrate longer distances to react with carbon, and consequently they would have an increasing chance to desorb as 02.If one assumes, as an upper limit, a flux of lo1’ photons/s (ref 25) with energy greater than the band-gap energy of Ti02 (-3.0 eV), the quantum yield of the H2production is 2% at the beginning of the reaction at room temperature and increases with increasing temperature.
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Acknowledgment. This work was supported in part by the Office of Naval Research. (25) J. M. White, Ph.D. Thesis, University of Illinois, Urbana, IL, 1966.
Laser-Induced Photoelectrochemistry. Time-Resolved Coulostatic-Flash Studies of Cadmium Sulfide Electrodes J. H. Richardson,* S. P. Perone,+ and S. B. Deutscher General Chemistry Division, University of California, Lawrence Livermore Laboratory, Livermore, California 94550 (Received: August 15, 1980)
Coulostatic-flashirradiation of semiconductor-liquid-junctioncells with a pulsed laser source permits time-resolved measurements of photopotential transients in the nanosecond time domain. The transients observed with CdS electrodes are unusual in that they exhibit wavelength and solution dependence. Both fast (10 ns) and slow (- 100 ns) transients are observed in various aqueous electrolytes with irradiation near the band gap. Similar results in nonaqueous solution and with CdSe suggest that these transient photopotentials are related to processes within the semiconductor itself. A much slower (-1 hs) transient is observed in polysulfide solution with irradiation in the ultraviolet; it is suggested that this transient photopotential is due to light absorption by polysulfide.
Introduction Currently there is considerable interest in the conversion of solar energy to either electricity or fuels.lV2 The best artificial chemical systems for solar energy conversion involve interfacial systems, specifically solid semiconductor-liquid junction electrochemical cells. The first work which served to catalyze the recent activity was the photodissociation of water using a n-Ti02electrode to produce Department of Chemistry, Purdue University, Lafayette, IN 47907.
oxygen and h y d r ~ g e n . ~Since that initial work a wide variety of other electrode materials have been studied in an attempt to achieve improved perf~rrnance.~ However, overall solar energy conversion efficiency has rarely exceeded 1%. Invariably semiconductor electrodes which are stable to photodissolution appear to have too large a
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(1) M. S. Wrighton, Ed., Adu. Chem. Ser., No.184 (1980). (2) A. Heller, Ed., “Semiconductor Liquid-Junction Solar Cells”, The Electrochemical Society, Princeton, NJ, 1977. (3) A Fujishima and K. Honda, Nature (London), 238, 37 (1972). ( 4 ) A. J. Nozik, Annu. Rev. Phys. Chern., 29, 189 (1978).
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band gap be to be efficient. Electrode materials which have a band gap corresponding to absorption in the visible or near-infrared (i-e.,the maximum of the solar spectrum) have been found to be unstable; for example, photoanodes undergo electrode oxidation rather than water oxidation. I t has been shown that unstable electrodes could be stabilized by the addition of appropriate redox couples in s01ution.~~~ While such systems usually no longer produce a fuel, they can be used to generate electricity quite efficiently; solar-to-electrical conversion efficiency of 12% has been demonstrated with n-GaAs electrodes using a regenerative Se,2-/Se2- redox couple for photo~xidation.~ Other stabilizing redox couples include S2-/S2-,6 Te2-/ Te,2-,5 Fe(CN)$-/Fe(CN):-,B and I-/I There were several orders of magnitude difference in both the doping densities and the mobilities between the CdS and Ti02 electrodes. Alternate temporal dependences have been observed in photoemission coulostatic-flash experiments, but a different mechanism is involved.% The dependence on time with CdS did diminish as the potential was made more positive or the amount of charge injected increased, similar to that seen with n-Ti02.
Conclusions The two most striking observations with the CdS electrodes in the coulostatic-flash experiment were the twocomponent response and the slow response with UV irradiation in polysulfide solution. The former observation was independent of solution, and consequently must represent the dynamics of charge separation in the depletion layer and/or charge transfer at the interface. This twocomponent risetime was observed only with high laser intensity. An interpretation based on a variation of drift time through a depletion layer, which is varying as a function of laser power and wavelength, qualitatively accounts for the change in response time and magnitude as a function of the laser-induced potential change. Such a variation in drift time would affect the net population of holes at the surface, which would be manifested by a change in the apparent charge transfer rate. Because this phenomenon is seen in LiC104/acetonitrile solutions, the initial charge transfer reaction probably involves the production of S/S- (extrinsic holes) from S2-at the CdS electrode surface. We do not attribute this two-component response to slow charge transfer into solution because it has such a well-defined wavelength dependence and it also occurs in nonaqueous LiC04 solutions. One caveat with respect to these qualitative interpretations is that a nonequilibrium distribution of charge can give rise to a complex energy band diagram for the semiconductor within the depletion region.39 Also, nonuniform illumination of (38) G. C. Barker, B. Stringer, and M. J. Williams, J . Electroanal, Chem., 51, 305 (1974).
(39) A. Many, Y. Goldstein, and N. B. Grover, “Semiconductor Surfaces”, Elsevier, New York, 1971, pp 75-89. (40) D. Laser and A. J. Bard, J . Electrochem. SOC.,123, 1828 (1976). (41) D. Laser and A. J. Bard, J. Electrochem. SOC.,123, 1837 (1976).
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J. Phys. Chem. 1981, 85,350-354
Acknowledgment. We thank Jackson Harrar, Lloyd Steinmetz, and Charles Stevens for their contributions to this study, and Mark Wrighton for his critical reading of the manuscript. This work was performed under the
auspices of the U.S. Department of Energy by the Lawrence Livermore Laboratory under contract No. W-7405Eng-48 and No. DE-AC02-77ER04263.AO02, Purdue University.
Vibrational Spectra, Jahn-Teller Distortlon, and the Structure of the Benzene Radical Anion Jesse C. Moore, Cynthla Thornton, Wllliam B. Collier, and J. Paul Devlln' Department of Chemktty, Oklahoma State lJn;vers& Stillwater, Oklahoma 74078 (Received: August 25, 1080)
Dilute thin-film codeposits of potassium or rubidium with benzene are light green or blue-green and, for the organic-rich samples, have an electronic absorption spectrum that can be identified with the ion pair M+B;. These films have a characteristic infrared spectrum but in a laser beam produce no intense discrete Raman spectra until bleached for several minutes. The resonant Raman spectrum that emerges during bleaching has many properties anticipated for that of the isolated benzene radical anion so the dominant features have been assigned to the stretching modes for that species. This assignment requires a Jahn-Teller splitting of the ea ring stretching mode of 111and 136 cm-I for C6H6-and C&-, respectively, values that are shown to be within a very few wavenumbers of the expected splitting based on the best theoretical estimate of the magnitude of the static Jahn-Teller distortion. A weaker subset of resonant Raman bands has been assigned to the ion pair M'B;, and it is suggested that, within the laser radiation field, the samples experience a steady-state dynamic distribution of electrons between isolated B; and M'B; and a mobile electron state.
Introduction The benzene anion radical (B;) has been the sourke of theoretical interest and speculation for many years.' In recent decades considerable structural information has been won from ESR2 and UV-~isible~?~ measurements. The ESR data for the ion-paired anion indicate that, on that particular time scale, the cation oscillates about a symmetric position over the ring with the ring carbon atoms assuming magnetically equivalent positions. The UV-visible measurements each show two principal broad anion absorptions of comparable intensity centered near 420 and 290 nm. However, Gardner's study, wherein the anion was generated by reduction in dimethoxyethane solution at -80 "C, also revealed an absorption band in the 600-700-nm range that was attributed to solvated potassium atoms.3 The solution studies have further shown that the more electropositive potassium or rubidium, rather than sodium, is required for benzene reductions2 This observation is consistent with the report by McCollough and Duly that a benzene-sodium complex, but no ion pair, is formed in a dilute low-temperature deposit of sodium in b e n ~ e n e . ~ As part of a program to elucidate the structures of conjugated radical anions, such as the anions of TCNE,6 TCNQ, and anthracene, we were interested in the preparation of the benzene anion in a form convenient for in(1)See, for example: (a) A. L. Hinde, D. Poppinger, and L. Radom,
J.Am. Chem. SOC.,100,4681(1978);(b) A. D. Liehr, Rev. Mod. Phys.,
32,436 (1960). (2)M. T.Jones and J. C. Kuechler, J. Phys. Chem., 81,360 (1977). (3)C. L.Gardiner, J . Chem. Phys., 45,572 (1966). (4)T.Shida and S. Iwata, J. Am. Chem. SOC.,95,3473 (1973). (5)J. D. McCollough and W. W. Duly, Chem. Phys. Lett., 15, 240 (1972). (6)See, for example, J. J. Hinkel and J. P. Devlin, J. Chem. Phys., 58, 4750 (1973). 0022-3654/81l2085-0350$01 .OO/O
frared and Raman measurements. It seemed likely that potassium and rubidium would be sufficiently electropositive to produce the anion in a low-temperature codeposition with benzene. This paper reports the spectroscopic characterization of such codeposits with particular emphasis on photoionization processes induced in the matrices by 4880-A photons from an argon laser. The effect of the addition of a radical electron on the bonding of the conjugated molecules TCNE and TCNQ is best characterized as a strengthening of single bonds and a weakening of multiple bonds with a net reduction in molecular bond energie~.~ Both theory and an empirical force constant analysis indicate that the TCNQ quinonoid ring becomes progressively more benzenoid with the addition of each electron to the system, up to a maximum of three.7b,8 However, when static Jahn-Teller distortion is considered, quite an opposite effect is expected for benzene. The ground state of the undistorted anion is degenerate so the Jahn-Teller distortion, which in principle must occur, will produce either a quinonoidlike structure (I) or an elongated structure with two weakened
0 0 I I1 ring bonds and four that are relatively unaffected (11). Both of the possible structures are of D 2 h symmetry, and, for comparable distortions from D6h symmetry,one expects a comparable magnitude of splitting of the e2gand el, vibrational modes. (7)(a) M. S. Khatkale and J. P. Devlin, J. Phys. Chem. 83, 1636 (1979);(b) M. S. Khatkale and J. P. Devlin, J. Chem. Phys., 70,1851 (1979). (8)D. A. Dixon, H. Simmons, and W. N. Lipscomb, J. Mol. Struct., 50, 155 (1978).
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