Effect of EDTA on the photocatalytic activities and flatband potentials

Brian A. Korgel and Harold G. Monbouquette. The Journal of Physical Chemistry B 1997 101 ... Fox and Maria T. Dulay. Chemical Reviews 1993 93 (1), 341...
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J . Phys. Chem. 1990, 94, 415-418

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Effect of Ethylenediaminetetraacetic Acid on the Photocatalytic Activities and Flat-Band Potentials of Cadmium Sulfide and Cadmium Selenide Toshio Uchihara,*Michio Matsumura,+Junichi Ono,t and Hiroshi Tsubomura*.+ Laboratory for Chemical Conversion of Solar Energy and Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan, and College of Science, University of the Ryukyus, Nishihara, Okinawa 903-01, Japan (Received: April 5, 1989)

Photocatalyzed hydrogen evolution on Pt-loaded CdS powder from aqueous solutions of sodium sulfite is enhanced by addition of a small amount of ethylenediaminetetraacetic acid (EDTA) to the solution. EDTA is hardly consumed by the reaction. It has been concluded from the measurements of the flat-band potential of CdS electrodes that EDTA and other chelating agents, such as 1,2-~yclohexanediaminetetraacetic acid and nitrilotriacetic acid, are adsorbed strongly on the surface of CdS and shift the conduction band energy toward the negative. The enhancement of the photocatalytic hydrogen evolution by the addition of EDTA is explained as being caused by the upward shift of the conduction band energy of CdS due to the negative charge of the chelating agents. The change of the conduction band energy by the adsorption of EDTA is observed also for CdSe electrodes. Although Pt-loaded CdSe powder is inactive for the hydrogen evolution from aqueous solutions of sodium sulfite, it generates hydrogen when EDTA is added to the solution.

Introduction CdS-photocatalyzed reactions in various kinds of aqueous solutions have been studied by many g r o ~ p s . l - ~We have studied the mechanism of photocatalyzed hydrogen evolution on platinum-loaded CdS powder and clarified the effects of pH of solution,I0*" temperature,ll*l2Pt loading,12 etc.I3 In the course of these studies, we have found that ethylenediaminetetraacetic acid (EDTA) added to the solutions enhances the stability of the p h o t ~ c a t a l y s t and ' ~ the efficiency of hydrogen evolution from aqueous solutions of sodium sulfite." It has been suggested from the measurements of flat-band potential and potential dependence of the photocurrent of CdS electrodes that the enhancement of the reaction efficiency is caused by a negative shift of the conduction band energy of CdS."J4 A marked enhancement of the photocurrent and photovoltage in a photoelectrochemical cell having a semiconductor electrode by the adsorption of EDTA was reported for the case of MoSz by Razzini et al.,ls who attributed the effect mainly to the quenching of the surface states acting as recombination centers and did not consider the change of the conduction band energy, as we have done. The negative shift of the conduction band energy of CdS caused by the adsorption of some kind of chemicals has already been reported. Substances that are known to cause this effect are mostly sulfur compounds1b20and hydroxide ion.17320In this sense EDTA is rather novel for this effect, which might arise from the chelation with surface cadmium atoms. It has been reported that EDTA is oxidized efficiently by the positive holes photogenerated in C d S 3 Interestingly, it is suggested from our previous results that a small amount of EDTA added to an aqueous solution of sodium sulfite is not consumed but enhances the photocatalyzed hydrogen evolution and oxidation of sulfite ions."J4 In this paper, the details of the effect of EDTA on the CdSphotocatalyzed reaction and flat-band potential will be presented. The effect of EDTA on the CdSe photocatalyst will also be presented.

Experimental Section CdS photocatalyst was prepared from commercial CdS powder (Furuuchi Chemical Co.). It was heated at 750 "C for 1 h under a nitrogen stream and loaded with 3 wt % platinum by shaking CdS powder together with platinum powder in a glass flask." Similarly, the CdSe photocatalyst was prepared from CdSe powder (Furuuchi Chemical Co.), heated at 550 OC for 2 h under a nitrogen stream, and loaded with 2 wt % platinum by the shaking method. The photocatalytic reactions were carried out in a 'Osaka University. *University of the Ryukyus.

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100-mL closed flask containing 300 mg of photocatalyst and 50 mL of 1 M sodium sulfite solutions adjusted to pH 8.0 or 10.0 by addition of boric acid and sodium hydroxide. Air dissolved in the solution was removed by repeating the cycle of evacuation at about 2 "C and ultrasonic agitation before the reaction started. A 500-W xenon lamp combined with a UV cut-off glass filter (Toshiba L-39) and a solar simulator (AM 1, 100 mW cm-2, Wacom Co.) were used as the light sources. Experimental con-

(1) (a) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 301. (b) Spanhel, L.; Weller, H.; Henglein, A. J . Am. Chem. SOC.1987, 109, 6632. (c) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (2) (a) Enea, 0.;Bard, A. J. J . Phys. Chem. 1986, 90, 301. (b) Kakuta, N.; Park, K. M.; Finlayson, M. F.; Ueno, A,; Bard, A. J.; Campion, A,; Fox, M. A.; Webber, S. E.; White, J. M. J . Phys. Chem. 1985, 89, 732. (c) Krishnan, M.; White, J. R.; Fox, M. A,; Bard, A. J. J . Am. Chem. SOC.1983, 105, 7002. (3) (a) Darwent, J. R.; Porter, G. J . Chem. Soc., Chem. Commun. 1981, 145. (b) Darwent, J. R. J . Chem. Soc., Faraday Trans. 2 1981, 77, 1703. (c) Darwent, J. R. J . Chem. Soc., Faraday Trans. 1 1984,80, 183. (4) (a) Tricot, Y.-M.; Fendler, J. H. J . Phys. Chem. 1986,90, 3369. (b) Tricot, Y.-M.; Emeren, A.;Fendler, J. H. J . Phys. Chem. 1985, 89, 4721. (5) Sakata, T. Chem. Phys. Lett. 1987, 133, 440. (6) (a) Borgarello, E.; Erbs, W.; Gratzel, M. Nouu. J . Chim. 1983, 7, 195. Serpone, N.; Borgarello, E.; Gratzel, M. J . Chem. Soc., Chem. Commun. 1984, 342. (7) Serpone, N.; Borgarello, E.; Pelizzetti, E. J . Electrochem. SOC.1988, 135, 2760.

(8) (a) Thewissen, D. H. M. W.; Tinnemans, A. H. A,; Eeuwhorst-Reinten, M.; Timmer, K.; Mackor, A. N o w . J . Chim. 1983, 7, 191. (b) Thewissen, D. H. M. W.; Timmer, K.; van der Zouwen-Assink, E. A,; Tinnemans, A. H . A.; Mackor, A. J . Chem. Soc., Chem. Commun. 1985, 1485. (9) Nosaka, Y.; Ishizuka, Y.; Miyama, H. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 1199. (10) Matsumura, M.; Saho, Y.; Tsubomura, H. J . Phys. Chem. 1983,87, 3807. (1 1) Matsumura, M.; Uchihara, T.; Hanafusa, K.; Tsubomura, H. J . Electrochem. SOC.1989, 136, 1704. (12) Uchihara, T.; Matsumura, M.; Yamamoto, A.; Tsubomura, H . J . Phys. Chem. 1989, 93, 5870. (13) Matsumura, M.; Furukawa, S.; Saho, Y.; Tsubomura, H. J . Phys. Chem. 1985.89, 1327. (14) Matsumura, M.; Ohnishi, H.; Hanafusa, K.; Tsubomura, H. Bull. Chem. SOC.Jpn. 1987, 60, 2001. (15) Razzini, G.; Peraldo Bicelli, L.; Pini, G.; Scrosati, B. J . Electrochem. SOC.1981, 128, 2134. (16) Minoura, H.; Tsuiki, M. Electrochim. Acta 1978, 23, 1377. (17) Dewitt, R.; Mesmoeker, K.-D. J. Electrochem. Soc. 1983, 130, 1995. (18) (a) Natan, M. J.; Theckeray, J. W.; Wrighton, M. S. J . Phys. Chem. 1986, 90,4089. (b) Theckeray, J. W.; Natan, M. J.; Ng, P.; Wrighton, M. S. J . A m . Chem. SOC.1986, 108, 3570. (19) Ginley, D. S.; Butler, M. A. J . Electrochem. SOC.1978, 125, 1968. (20) Matsumura, M.; Hiramoto, M.; Iehara, T.; Tsubomura, H. J . Phys. Chem. 1984, 88, 248.

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TABLE I: Material Balance of Pt-Loaded CIS Photocatalyzed Reactions in Various Solutions, Whose pH Was Adjusted to IO by Addition of Boric Acid and Sodium Hydroxide" amount of amount of amount of hydrogen sulfite ion EDTA solution produced, mmol consumed, mmol consumed, mmol 1 M sodium sulfite 3.3 (79.9)b 3.9 1 M sodium sulfite 3.7 (89.2) 5.0 0.001 and 5 mM EDTA 5 m M EDTA 0.1 (2.2) 0.029

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ditions of the photocatalytic reaction and the determination of the amount of evolved hydrogen were described in our previous papers.I1.l3 Quantitative analysis of EDTA and sulfite ions in the solutions before and after the photocatalytic reaction was carried out by chelatometric titration using a zinc standard solution (0.01 M) and by titration using an iodine standard solution ( 1 .O M), respectively. All chemicals were used as purchased without further purification. The CdS electrodes were prepared by using either an undoped single-crystal CdS wafer (Teikokutsushin Co.) or sintered CdS disks made from the same lot of CdS powder as the one used for the photocatalytic reactions. The CdSe electrodes were fabricated from sintered CdSe disks made from the CdSe powder. The CdS and CdSe disks were prepared by sintering at 850 O C under a pressure of 1.25 X IO7 Pa. Both single-crystal wafers and sintered disks of CdS and CdSe were etched in concentrated hydrochloric acid for 60 s before the electrode fabrication in order to remove surface defects caused by the wafer slicing process or by contact with the sintering die. The back contacts were made by using an In-Ga ( I : I ) alloy. The electrochemical measurements were performed by use of a three-electrode cell consisting of a CdS (or CdSe) electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE). The potential of the semiconductor electrode was controlled with a potentiostat (Nikkokeisoku, NPGS-301). Flat-band potentials were determined from the differential capacitance measurements at 1 kHz with an LCR meter (Yokogawa-Hewlett-Packard, 4261A). Before measurements, the electrodes were etched in concentrated hydrochloric acid for a few seconds, rubbed with a cotton swab soaked with 1 M hydrochloric acid, and then rinsed with deionized water. The rubbing treatment with a cotton swab after etching in concentrated hydrochloric acid was very important to obtain flat-band potentials reproducibly. For the case of CdS electrodes, yellow matter, probably sulfur, was removed from the electrode surface by the rubbing treatment.

Results and Discussion Effect of EDTA on the Photocatalytic Hydrogen Evolution from CdS and CdSe Suspensions. Photocatalytic hydrogen evolution occurs efficiently from aqueous solutions of sodium sulfite containing Pt-loaded CdS powder with the concomitant oxidation of sulfite i ~ n s . ' % l ~The * ~ ' material balance of the reaction is shown in Table I . The amount of hydrogen evolved is enhanced by about 12% by addition of 5 mM EDTA to a 1 M sodium sulfite solution as seen by comparing rows 1 and 2 in Table I. It has been observed that such an enhancement in the rate of hydrogen evolution becomes higher in alkaline solutions." Table I also shows that hydrogen evolution from the solution containing only 5 mM EDTA is very small and that EDTA in the mixed solution of 1 M sodium sulfite and 5 m M EDTA is scarcely consumed. These results strongly suggest that the photocatalytic oxidation of EDTA itself is excluded from the main reason for the enhanced hydrogen evolution by addition of EDTA to 1 M sodium sulfite.

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The hydrogen evolved by the photocatalytic reaction was stoichiometrically a little less than sulfite ions consumed as seen in Table I. This can be explained from the fact that 1 mol of hydrogen is produced by sacrificing either 1 mol of sulfite ion oxidized to sulfate ion or two sulfite ions to form one dithionate ion. Thus, the ratio between the amount of sulfite ions consumed and that of hydrogen evolved must be between 1 and 2.1° In previous papers, we analyzed the photocurrent-potential curves of CdS electrodes in the mixed solution of sodium sulfite and EDTAi4and assumed that sulfite ions are solely oxidized by the positive holes photogenerated in CdS, since the photoanodic current for the oxidation of EDTA starts at a potential more positive than that for the oxidation of sulfite ions, and the current-doubling effect due to the oxidation of EDTA disappears when both EDTA and sodium sulfite are present in the solution. The results shown in Table I furnish a convincing evidence that EDTA promotes the photocatalytic reaction of sulfite ions on CdS, EDTA itself being hardly consumed. The photocatalytic activity of Pt-loaded CdSe is much inferior compared with Pt-loaded CdS, and practically no hydrogen evolved from an aqueous solution of 1 M sodium sulfite. By addition of 10 mM EDTA to a 1 M sodium sulfite solution (pH 8.0), however, hydrogen evolution occurred. The rate of the hydrogen evolution under illumination by an AM 1 solar simulator (100 mW was 0.010 mL h-' which was about 70 times lower than that observed by use of Pt-loaded CdS. Although the rate of hydrogen evolution is still very low, this is the first unequivocal example of hydrogen evolution on a CdSe p h o t o c a t a l y ~ t . ~ ~ ~ ~ ~ Effect of EDTA on the Flat-Band Potential of CdS and CdSe. In order to clarify the effect of EDTA on the reactivity of the CdS photocatalyst, we measured the flat-band potential of the CdS electrode in various solutions. The pretreatment of the electrodes was very important for the measurements of the flat-band potential as noted in the Experimental Section. After a proper treatment of the CdS electrodes, the Mott-Schottky plots of the differential capacitance measured at 1 kHz gave straight lines as shown in Figure 1, and the reproducible flat-band potentials were obtained from the intersections of the lines with the abscissa. The flat-band potential thus obtained in solutions of indifferent electrolytes, such as sodium sulfate, is constant at pH lower than 10 and shifts to the negative at pH higher than 10 as shown by curve 1 in Figure 2. The negative shift of the flat-band potential in alkaline solutions is explained by taking account of the adsorption of hydroxide ions on the surface of CdS.i7,20In solutions with pH lower than 10, the flat-band potential is shifted (22) Kambe, S.; Fujii, M.; Kawai, T.; Kawai, S. Chem. Phys. Lett. 1984, 109, 105.

(21) (a) Biihler, N.; Meier, K.; Reber. J.-F.J . Phys. Chem. 1984,88, 3261. (b) Reber, J.-F.: Rusek, M. J . Phys. Chem. 1986, 90,824.

(23) Uchihara, T.; Abe. H.; Matsumura, M.; Tsubomura, H. Bull. Chem. Soc. Jpn. 1989. 62. 1042.

Effect of EDTA on Cadmium Sulfide and Cadmium Selenide

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Figure 3. The flat-band potential of a single-crystal CdS electrode against the concentration of sodium sulfite (curve 1) and that against the concentration of EDTA added to the solution of 0.5 M sodium sulfite (curve 2). The pH of the solutions was adjusted to 8.0. by about 80 mV toward the negative by addition of 0.5 M sodium sulfite (curve 2). This shift of the flat-band potential caused by sodium sulfite is explained by the adsorption of sulfite ions on the surface of CdS as in the case of many sulfur compounds reported by Minoura and Tsuiki.I6 The flat-band potential of a CdS electrode shifts largely toward the negative by addition of a small amount of EDTA to the solution (curve 3). The shift is larger than that caused by sulfite ions, and the effect of sulfite ions is hidden when both EDTA and sulfite ions are present in the solution (closed circles in Figure 2). This suggests that EDTA is strongly adsorbed on the surface of CdS, presumably by chelating with surface cadmium atoms. The negative charge on EDTA causes the shift of the flat-band potential toward the negative. The large pH dependence of the flat-band potential observed in the presence of EDTA is attributable to the protonation equilibria of EDTA molecules adsorbed on CdS. There have been no reports about the shift of the flat-band potential of semiconductor electrodes caused by substances in solution, which have no elements in common with the semiconductors, except for the cases of protons and hydroxide ions. The effect of EDTA is unique in this sense. Presumably, such an effect of EDTA is caused by the surface chelation with the cadmium atoms on the surface of CdS as mentioned above. The negative shift of the flat-band potential of CdS was also observed by addition of other chelating agents, such as 1,2-cyclohexanedi-

Figure 4. I-U curves of a CdSe sinter electrode measured under illumination in solutions of 5 mM EDTA (curve I ) , 1 M sodium sulfite (curve 2), and 5 mM EDTA plus 1 M sodium sulfite (curve 3). All solutions contain 0.56 M boric acid and proper amounts of sodium hydroxide to adjust the pH to 8.0. aminetetraacetic acid and nitrilotriacetic acid, to the solution. Figure 3 shows the flat-band potential of the CdS electrode as functions of the concentration of sodium sulfite and EDTA. For the case of sodium sulfite, the flat-band potential moves toward the negative with the concentration (curve 1). The shift of the flat-band potential becomes larger by addition of a very small amount of EDTA to the solution of sodium sulfite as seen from curve 2. When the concentration of EDTA was lower than loT3 M, the flat-band potential moved toward the negative with time for the initial several minutes after the electrode was immersed in the solution. Therefore, for the solutions containing EDTA M, data were obtained after at the concentrations lower than the electrode was immersed in the solutions for about 30 min. The flat-band potential thus determined reached a constant value at M in the case of EDTA, while it a concentration of about did not reach saturation even at 1 M in the case of sodium sulfite. This also suggests that EDTA is strongly adsorbed on the surface of CdS. In neutral solutions, the position of the conduction band edge of CdS is higher than the thermodynamic hydrogen evolution potential, causing efficient hydrogen evolution on Pt-loaded CdS. In alkaline solutions, the difference between the conduction band edge and the thermodynamic hydrogen evolution potential becomes smaller, and the efficiency of the photocatalytic hydrogen evolution The enhancement of the photocatalytic hydrogen evolution by addition of EDTA to alkaline sulfite solutions can be explained by taking account of the enlargement of the difference in the energy levels between the conduction band edge and the hydrogen evolution potential. It has been reported that some kinds of chemical species attached on the surface of semiconductor electrodes act as mediators for the photoanodic reactions.24 The mediators receive holes (or electrons) from the valence (or conduction) band of a semiconductor electrode and transfer them to reductants (or oxidants) in the solution. When the reaction along this path proceeds faster than the direct reaction between holes (or electrons) in the semiconductor electrodes and reductants (or oxidants) in the solution, the properties of the PEC cells are thought to be improved. Although the role of EDTA observed in the present work seems to be similar to that of a mediator, this role of EDTA as a mediator can be denied because of the fact obtained by us from the photocurrent-potential curves of CdS electrodes showing that sulfite ions are oxidized on the surface of CdS more easily than EDTA.I4 For the case of the Pt-loaded CdSe photocatalyst, hydrogen evolution was observed only when EDTA was added to sodium sulfite solution. This effect of EDTA is also attributable to the shift of the flat-band potential of CdSe. The onset potential of the photocurrent in the CdSe electrode is shifted by addition of (24) Dominey, R. N.; Lewis, N. S.; Bruce, J. A,; Bookbinder, D. C.; Wrighton, M. S. J . Am. Chem. SOC.1982, 104, 461.

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a small amount of EDTA to the sodium sulfite solution as seen in Figure 4. This strongly suggests that the flat-band potential of CdS moves to the negative by the adsorption of EDTA on it. The determination of the accurate flat-band potential of sintered CdSe electrodes from the Mott-Schottky plots was unsuccessful probably because of their porous structure. I n conclusion, we have observed that the position of the conduction band energy of CdS and CdSe can be controlled by addition of chelating agents, EDTA in particular, to the solutions. The photocatalytic activities of the Pt-loaded CdS and CdSe for

the case of sulfite solutions are enhanced by addition of EDTA, and EDTA itself is hardly consumed. Acknowledgment. The authors thank Mr. Koji Hanafusa and Mr. Atsushi Yamamoto for the measurements of the photocur-

rent-potential curves of CdSe electrodes and the photocatalytic activity of the Pt-loaded CdSe powder. Registry No. EDTA, 60-00-4; Pt, 7440-06-4;CdS, 1306-23-6;CdSe, 1306-24-7; hydrogen, 1333-74-0;sodium sulfite, 7757-83-7; 1,2-cyclohexanediaminetetraacetic acid, 482-54-2; nitrilotriacetic acid, 139-13-9.

Role of Defects in Radiation Chemistry of Crystalline Organic Materials. 2. Effective and Selective Formation of Solute Radicals and Its Dependence on the Carbon-Chain Normal Hydrocarbon Mixed Difference in the Radiolysis Products of C, H,, +,/C,,,D,, Crystals As Studied by ESR Spectroscopy Hachizo Muto,* Keichi Nunome, and Mitsuharu Fukaya Government Industrial Research Institute. Nagoya, Hirate-cho, Kita- ku, Nagoya, 462, Japan (Received: April 1 1 1989; In Final Form: June 9, 1989) ~

Radiolysis of the mixed crystals of binary n-alkanes has been studied by ESR spectroscopy to understand the role of defects in the radical formation in crystalline organic compounds. Highly efficient (1O-25%) and selective formation of solute radicals depending on the carbon-chain difference between solute and matrix molecules was found in the CmH2m+2 (1 mol %)/CloD22 mixed crystals ( m = 7-12) irradiated at 77 K. The terminal type of radical C'HzCH2CH2- (I) is formed only in the mixed crystals with shorter solute molecules than the matrix ( m < 10) and is the only solute radical in the case of m 5 8. Penultimate type, CH,C'HCH2- (II), and interior type, -CHzC'HCH2- (III), of radicals are formed selectively in the case of m t 10. These efficient and selective formations were observed neither in the mixed crystals irradiated at 4 K nor in the protiated-alkane matrix (CIOH22) irradiated at 77 or 4 K. These results suggest that the D-atom reaction in the molecular layer boundary regions plays an important role in the solute radical formation. The details of this process are as follows. Deuterium atoms radiolytically produced from the matrix molecules can diffuse at 77 K, and some of them escape to the boundary regions. The escaped D atoms can migrate for some distance in the boundary regions (with looser molecular packings than those in the bulk regions) and efficiently abstract a H atom from a protiated solute molecule via a tunneling reaction with a large isotope effect. In the case of m 5 8, the escaped D atoms can react with only the terminal CH3 groups of the solute molecule in the boundary regions, forming selectively radical I. The selective formation of radicals I1 and 111 in the case of m 1. 10 is due to the exposure of the penultimate- and interior-type secondary C-H bonds (with lower bond strengths than the primary CH3 bond), in addition to the exposure of the CH3 bonds to the boundary regions. These results may support the participation of hydrogen-atom reaction in the neat-alkane radiolysis itself.

Introduction n-Alkanes are the most fundamental nonpolar organic compounds. A large number of studies on the solid-state radiolysis have been reported and the radiation chemistry has been progressively ~ h a r i f i e d . l - ~However, there are still problems in understanding alkyl radical formation.*-' Two mechanisms have been proposed for the radical formation in the primary radiation ( 1 ) Smaller, B.; Matheson, M. S. J . Chem. Phys. 1958, 28, 1169. Gordy, W.; Morehouse, R. Phys. Reu. 1966, 151, 207. (2) Tim, D.; Willard, J. E. J . Phys. Chem. 1969. 73, 2403. Falcorner, W. E.; Salovey, R. J . Chem. Phys. 1966, 44, 3151. Chappas, S. J.; Silverman, J. Radiat. Phys. Chem. 1980, 16, 431, 431. (3) (a) Iwasaki, M.; Ichikawa, T.; Ohmori, T. J . Chem. Phys. 1969, 50, 1991. (b) Nunome, K.; Muto, H.; Toriyama, K.; Iwasaki, M. Chem. Phys. Lett. 1976, 39, 542. (c) Toriyama, K.; Muto, H.; Nunome, K.; Iwasaki, M. Radiat. Phys. Chem. 1981, 18, 1041. (4) Hamanoue, K.; Kamantauskas, V.; Tabata, Y.;Silverman, J. J . Chem. Phys. 1974. 61, 3439. (5) Iwasaki, M.; Toriyama, K.; Muto, H.; Nunome, K. J , Chem. Phys,

1976, 65, 596.

(6) (a) Toriyama, K.; Iwasaki, M.; Fukaya, M. J . Chem. Soc., Commun. 1982, 1293. (b) Iwasaki, M.; Toriyama, K.; Fukaya, M.; Muto, H.; Nunome. K . J . Phys. Chem. 1985,89, 5278. (c) Fukaya, M.; Toriyama, K.; Iwasaki, M.; Ichikawa, T.; Ohta, N. Radiat. Phys. Chem. 1983, 21, 463. ( 7 ) Iwasaki, M.; Toriyama, K.; Nunome, K. J . Am. Chem. SOC.1981, 103, 3591. Nunome. K.; Toriyama. K.; Iwasaki, M. Chem. Phys. Lett. 1984, 105, 414.

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damage process. One involves the C-H bond scission of excited molecules, which are formed by the geminate recombination of the hole and the electron, and the subsequent hydrogen-abstraction The other reaction from a molecule by the H atom involves ion-molecular or decomposition reaction of the hole, which is followed by the subsequent recombination reaction of electrons with the diamagnetic cations f ~ r m e d . ~ ? ~ Defects in crystalline organic compounds are considered to play an important role in radiation chemistry;8-10 however, there are few model systems for investigating the role explicitly. Gilbro and Lund studied the radiolysis of C,oH22/CloD2,mixtures using the single crystals and found selective formation of solute radicals8 The selectivity was explained by an energy-transfer mechanism,* although afterwards it was suggested to originate from H-atom reaction^.^,^-'^ The mixed crystals of binary n-alkanes with different chain lengths are expected to be a good system for investigating the role of defects, since they have preexisting defects defined more clearly than those in glassy They are (8) Gilbro, T.; Lund, A. Chem. Phys. Lett. 1974, 27, 300. Claesson Ola; Lund, A. Chem. Phys. Lett. 1977, 47, 155. (9) Miyazaki, T.; Kasugai, J.; Wada, M.; Kinugawa, K. Bull. Chem. SOC. Jpn. 1978, 51, 1676. (10) Tilquin, B.; Baudson, Th. J . Phys. Chem. 1988, 92, 843. ( 1 1) Maroncelli, M.; Strauss, H. L.; Snyder, R.G. J . Phys. Chem. 1985, 89, 5260.

0 1990 American Chemical Society