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transfer complexes or association dimers are formed between the two comonomer species. Frequently, a significant tendency toward equimolar incorporation of the two monomers in the copo:lymer is observed, which may result from involvement of the comonomer complex in the copolymerization. The patterned search procedure outlined above enables best estimates of reactivity ratios and the equilibrium constant to be obtained for a general complex-participation mechanism based on copolymer compositions. The equations for sequence distributions can be utilized to give discriminatory tests between possible mechanisms, which may be evaluated by recently developed experimental techniques such as 13C NMR. The material properties of copolymers depend on the overall composition, lbut are also quite sensitive to the comonomer sequence distribution. Future developments in this field are likely to include (i) compilation of more extensive data on copolymerizations to enable reliable evaluation of different mechanisms, (ii) utilization of 13C NMR and other techniques to determine comonomer sequence distributions in many more copolymers, and (iii) selection of polymerization conditions, including temperature and pressure, to maximize the participation of co-
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monomer complexes and hence control the sequence distributions in copolymers.
Acknowledgment The authors wish to thank the Australian Research Grants Committee and the Australian Institute of Nuclear Science and Engineering for supporting their research.
Literature Cited Alfrey, T., Goldfinger, G., J . Chem. fhys., 12, 205 (1944). Booth, D., Dainton, F. S., Ivin, K. J., Trans. Faraday SOC.,55, 1293 (1959). Cais, R . E., O'Donnell, J. H., Eur. fo/ym. J., 11, 749 (1975). Cais, R. E., Farmer, R. G., Hill, D. J. T., O'Donnell, J. H., Macromolecules, 12, 835 (1979). Chandler, J. P., "Quantum Chemical Program Exchange", 1976, No. 11, p 307. Dodgson, K., Ebdon, R., Eur. folym. J . , 13, 791 (1977). Farmer, R . G., Hill, D. J. T., O'Donnell, J. H., J. Macromol. Sci. Chem., A14, 51 (1980). Ham, G. E., in "Copolymerlzation", G. E. Ham Ed., Interscience, New York, 1984, p 10. Kellou, M., Jenner, G..Makromol. Chem., 180, 1687 (1979). Mayo, F. R., Lewis, F. M.. J . Am. Chem. Soc., 86, 1594 (1944). Seiner, J. A., Li, M., Macromolecules, 4, 308 (1971). Seymour, R . B., Garner, D. P., Po/ym. News, 4, 209 (1978).
Received for review April 21, 1980 Accepted April 28, 1980
Dye Sensitization and Surface Structures of Semiconductor Electrodes Mlchio Matsumura, Shigeyuki Matsudaira, and Hiroshi Tsubomura' Department of Chemistty, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, 560, Japan
Masasuke Takata and Hiroaki Yanagida Department of Chemistry, Faculty of Engineering, University of Tokyo, Tokyo, 113, Japan
The dye-sensitization effects on the ZnO, CdS, and TiO, electrodes in electrochemical photocells were investigated for anionic, cationic, and zwitterionic dyes. The most efficient dye-sensitized photocell was achieved by using an aluminurndoped porous ZnO sinter electrode dyed with rose bengal (an anionic dye), the energy conversion efficiency being i!.5% for incident light of 562 nm. The effect of aluminum doping was attributed to the increase of the porosity (or surface area) of the sinter and the decrease of the electrical resistance. The effect of pH and the added salts in the solution as well as the effect of crystal face were extensively investigated. It turned out that these effects mainly influence the adsorptivity of the electrode surfaces for the sensitizing dyes, not the intrinsic current quantum efficiency. From these results, the structures of the dyes on the semiconductor surfaces were discussed in relation with the mechanism of photoinjection of electrons. The merits of dye-sensitized photoelectrodes as a photoenergy converter were discussed.
Introduction The electrochemicad photocells composed of semiconductor photoelectrodes, metal counterelectrodes, and aqueous solutions of redox systems and supporting electrolytes have been attracting wide attention in recent years from the viewpoint of solar energy conversion. Functionally, the electrochemical photocells are divided into two types-those producing electrical power in the presence of regenerative redox systems and those producing fuels, e.g., hydrogen from water. In either of these two types, however, the semiconductor electrode of an efficient solar energy converter should have 0196-4321/80/1219-0415$01 .OO/O
the following properties, in addition to the fundamental requisite of a good semiconducting material: (1)a band gap (I$)small enough to absorb the main part of the solar radiation lying in the visible region, and (2) sufficient resistivity against corrosion or dissolution. These two requirements seem to be somewhat dilemmatic, for it has been recognized that most materials having narrow band gaps are rather corrosive, e.g., in the case of silicon, gallium phosphide, and cadmium chalcogenides, while relatively stable semiconductors, e.g., titanium oxide and some metal titanates, have band gaps higher than 3 eV and work only with ultraviolet light which
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Figure 1. A model for the electron injection from an excited dye into an n-type semiconductor.
is rather scanty in the solar spectrum. There are a number of possible approaches to realize efficient photoelectrochemical solar cells: search for a new semiconductor material having all the requisite properties, use of suitable redox agents causing protective electron transfer toward the electrodes, e.g., selenide or sulfide ions to cadmium chalcogenides (Inoue et al., 1977; Heller et al., 1977; Gerischer and Gobrecht, 1978;Bard and Wrighton, 1977), surface treatments on corrosive semiconductors to make them more stable (Nakato et al., l975,1976a,b),and use of dye sensitization to expand the spectral response of the electrodes to a longer wavelength region. The feasibility of dye-sensitization is one of the important and interesting characteristics of the electrochemical photocells. The phenomena of dye-sensitized photocurrents in semiconductor photoelectrodes have been known for many years by Tributsch and Gerischer (1968, 1969a,b), Hauffe and Bode (1974), and Fujishima et al. (1975a). However, there has not been much work done with a positive intention to utilize this phenomenon as a technique of solar energy conversion. One of the reasons for this might have been that the dye-sensitized photocurrents of semiconductor electrodes as recognized were one order of magnitude smaller than those caused by the intrinsic excitation of the semiconductors. We have been undertaking a systematic study of the dye sensitization effect of various dyes on semiconductor electrodes, especially on sintered zinc oxide (Matsumura et al., 1976a,b, 1977, 1979a,b; Tsubomura et al., 1976, 1977a,b, 1978; Yamada et al., 1978). There have been two mechanisms often employed to explain the dye sensitized photocurrents: (1)the electron injection from excited dye to the conduction band of the semiconductor (Figure l),and (2) the energy transfer from the excited dye to the semiconductor causing electron transitions from an intermediate state of the semiconductor to its conduction band followed by electron transfer from the dye to the intermediate state. Both theories imply the final electron injection from dye to electrode and hence it is hard to conclude which one is true by experiments. However, the latter theory is based on a presence of an intermediate state which has not been experimentally verified, and seems less likely. As far as we are aware, all experimental observations do not contradict the former theory. In either case, the photocurrent is caused by the electron removal from dyes on the surface of the semiconductor, and the current should decay if an electron supply mechanism does not exist. We studied the decay of photocurrent kinetically and have found that the photocurrent decays quickly following first-order kinetics if there is no suitable reductant, e.g., hydroquinone or iodide ion, in the electrolyte solution (Matsumura et al., 1977). We have also found that in such a case, the dye is quickly deteriorated,
forming a material which has a different color and is insoluble in water (Matsumura et al., 1977; Yamada et al., 1978). The decay as well as chemical change of the dye is prevented if we add a reductant in solution. We have also confirmed from various experiments that only those dyes that are directly adsorbed on the surface of the semiconductor are relevant to the photocurrent (Matsumura et al., 1977). The clearest evidence for this is that the action spectra for the photocurrent are definitely shifted from the absorption spectra of dyes in solutions. The small photocurrent efficiency as observed by some authors as well as in a primary stage of our own work in systems where the dye was dissolved in the electrolyte solution was therefore attributed to a very weak absorption of light by the adsorbed dye layers on the surface of the semiconductor. We then tried to increase the photocurrent efficiency by applying insoluble dyes thickly on the surface of the semiconductor so as to increase the absorbance of the dye, but such a method did not lead to a substantial improvement of the efficiency. The reason for the failure was probably the energy deactivation by the dye layer or the formation of an insulation layer of the dye. Then, we found that by use of a porous zinc oxide sinter electrode which adsorbed some dyes-e.g., rose bengal-a marked increase of the photocurrent was obtained (Tsubomura et al., 1976). The apparent quantum efficiency q (see Appendix) of the photocurrent obtained reached ca. 20% for a sintered ZnO electrode dyed with rose bengal in an electrolyte solution containing the I-/&- couple, about one order of magnitude higher than the previous data, and the power conversion efficiency 4 for the monochromatic light of 1.5% was obtained. In the present paper we describe our further studies on the mechanism of dye-sensitized photocurrent and the results of improving the power conversion efficiencies of such cells.
Experimental Section The photocurrent between a semiconductor electrode and a platinum counterelectrode was measured in a cell containing an electrolyte solution at constant potentials between the semiconductor electrode and a reference saturated calomel electrode (SCE) using a Hokutodenko HA-101 potentiostat. Most of the measurements were made using a Ushio 500-W Xe lamp and a Japan-Jarrell Ash 0.25-m Ebert type monochromator. The light intensity was measured with an Eppley bismuth-silver thermopile. The photocurrent was measured with a Yokogawa-Hewlett-Packard 4304B electrometer. For the measurements of the quantities of dyes adsorbed on the surface, a Shimadzu MPS-50L multipurpose spectrophotometer was used with an accessory for diffuse reflectance measurements. Most of the work was done for sintered zinc oxide (ZnO) electrodes which were made by compressing zinc oxide powder into a disk and sintering. As described later, the properties of the ZnO sinter are largely affected by the temperature and time of sintering, as well as impurities included in the ZnO powder. Most of the work was done with electrodes made of ZnO from Kanto Chemical Co. without further purification, sintered at 1300 OC for 1 h in air. The sinter has a density almost equal to that of a single crystal. In the case where we wanted to make use of a porous sinter, some materials were added in the ZnO powder, as will be described later in detail. The sinter thus prepared was ground with a silicon carbide abrasive (No. 2000), etched in 2 M HC1, washed with water, and dried. The ohmic contact was taken by coating the back side of
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Table I. The Degree of Sintering of the ZnO Specimens Sintered at Various Temperatures and Periods
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