Langmuir 1999, 15, 1503-1507
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Photoelectrochemical Characterization of Nearly Monodisperse CdS Nanoparticles-Immobilized Gold Electrodes Masahide Miyake,† Tsukasa Torimoto,† Takao Sakata,‡ Hirotaro Mori,‡ and Hiroshi Yoneyama*,† Department of Applied Chemistry, Faculty of Engineering, and Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan Received August 4, 1998. In Final Form: November 24, 1998 Several kinds of nearly monodisperse size-quantized CdS particles (Q-CdS) capped with 2-aminoethanethiol were prepared by means of size selective photoetching and were covalently immobilized onto a gold electrode substrate coated previously with a self-assembled monolayer of 2-aminoethanethiol. The potential of the conduction band edge of Q-CdS estimated from the onset potential of photocurrents was negatively shifted with a decrease in the diameter of CdS nanoparticles, the results being in good accord with the theoretical prediction.
Introduction Size-quantized semiconductor nanoparticles (Q-particles) possess unique features in its photochemical properties1-20 as compared to the corresponding bulk particles. If they are assembled in films, they might be useful in novel photovoltaic13,14,21 and/or light-emitting devices22,23 and other numerous optoelectronic applications. Several techniques have been employed to prepare size-quantized semiconductor particle films for the use as photoelectrodes: chemical24-26 and electrochemical7,27-29 † ‡
Faculty of Engineering. Research Center for Ultra-High Voltage Electron Microscopy.
(1) Steigerwald, M. L.; Brus, L. E. Annu. Rev. Mater. Sci. 1988, 19, 471. (2) Henglein, A. Chem. Rev. 1989, 89, 1861. (3) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (4) Colvin, V. L.; Goldstein, A. N.; Alivisatos. A. P. J. Am. Chem. Soc. 1992, 114, 5221. (5) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (6) Kamat, P. V. Chem. Rev. 1993, 93, 267. (7) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136. (8) Aleperson, B.; Cohen, S.; Hodes, G. Phys. Rev. B 1995, 52, 7017. (9) Ogawa, S.; Fan, F. F.; Bard, A. J. J. Phys. Chem. 1995, 99, 11182. (10) Ogawa, S.; Hu, K.; Fan, F. F.; Bard, A. J. J. Phys. Chem. B 1997, 101, 5707. (11) Nosaka, Y.; Ohta, N.; Miyama, H. J. Phys. Chem. 1990, 94, 3752. (12) Nosaka, Y.; Nakaoka, Y. Langmuir 1995, 11, 1170. (13) Mansur, H. S.; Grieser, F.; Marychurch, M. S.; Biggs, S.; Urquhart, R. S.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1995, 91, 665. (14) Mansur, H. S.; Grieser, F.; Urquhart, R. S.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1995, 91, 3399. (15) Kotov, N. A.; Dekany, I. Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (16) Zhao, X. K.; McCormick, L. D.; Fendler, J. H. Langmuir 1991, 7, 1255. (17) Tian, Y.; Fendler, J. H. Chem. Mater. 1996, 8, 969. (18) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 11064. (19) Hotchandani, S.; Bedja, I.; Fessenden, R. W.; Kamat, P. V. Langmuir 1994, 10, 17. (20) Nakanishi, T.; Ohtani, B.; Uosaki, K. J. Phys. Chem. B 1998, 102, 1571. (21) Kamat, P. V. CHEMTECH 1995, 22. (22) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (23) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature, 1994, 370, 354. (24) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834. (25) Gorer, S.; Hodes, G. J. Phys. Chem. 1994, 98, 5338.
deposition on conducting electrode substrates and the immobilization of surface-modified semiconductor nanoparticles to electrode substrates.4,9,10,16,17,20 Among these, the last technique allows the preparation of nanoparticle electrodes that guarantee the appearance of size quantization effects, because in that case the intimate contacts between the particles, which may result in disappearance of the size quantization effect more or less, are prevented by surface modification of semiconductor nanoparticles. Since chemical and physical properties of Q-particles are greatly influenced by their size, it is desired to investigate photoelectrochemical properties of Q-particle films by preparing the films using monodisperse Qparticles, which can in principle be obtained by applying several technique such as chromatography,30-32 electrophoresis,33,34 size-selective precipitation,35,36 and size selective photoetching37 to polydisperse Q-particle colloids. Recently, we have reported that the size selective photoetching allows easy preparation of nearly monodisperse Q-CdS particles.37 In the present study, therefore, Q-CdS particle film electrodes were prepared using nearly monodisperse Q-CdS particles of various sizes prepared by this technique, and photoelectrochemical properties of the prepared film electrodes were investigated. (26) Gorer, S.; Alubu-Yaron, A.; Hodes, G. J. Phys. Chem. 1995, 99, 16442. (27) Golan, Y.; Margulis, L.; Hodes, G.; Rubinstein, I.; Hutchison, L. Surf. Sci. 1994, 311, L633. (28) Golan, Y.; Hodes, G.; Rubinstein, I. J. Phys. Chem. 1996, 100, 2220. (29) Mastai, Y.; Hodes, G. J. Phys. Chem. B 1997, 101, 2685. (30) Fisher, Ch.-H.; Weller, H.; Fojtik, A.; Lume-Pereira, C.; Janata, E.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 46. (31) Fisher, Ch.-H.; Weller, H.; Katsikas, L.; Henglein, A. Langmuir 1989, 5, 429. (32) Fisher, Ch.-H.; Weller, H.; Katsikas, L.; Henglein, A. Ber. BunsenGes. Phys. Chem. 1989, 93, 61. (33) Fisher, Ch.-H.; Giersig, M. Langmuir 1992, 8, 1475. (34) Wang, Y.; Harmaer, M.; Herron, N. Isr. J. Chem. 1993, 33, 31. (35) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (36) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (37) Matsumoto, H.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem. 1996, 100, 13781.
10.1021/la980975l CCC: $18.00 © 1999 American Chemical Society Published on Web 01/06/1999
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Experimental Section Cadmium perchlorate (Kishida Chemicals), 2-aminoethanethiol (Tokyo Kasei Organic Chemicals), and other chemicals (Wako Pure Chemical Industries) were reagent grade and used without further purification. Water purified by a Millipore Milli-Q purification system was used to prepare aqueous solutions. The preparation procedures of nearly monodisperse CdS nanoparticles adopted in the present study were essentially the same as those described previously.37 CdS nanoparticles were prepared by injecting H2S gas into a nitrogen-bubbled aqueous solution containing 1.0 × 10-3 mol dm-3 Cd(ClO4)2 and 5.0 × 10-4 mol dm-3 sodium hexametaphosphate (HMP) at pH 10.3. A 500 W Xe lamp was used as a light source, and monochromatic light of the desired wavelength was obtained using interference filters. The width at half of the intensity maximum of the monochromatic light was about 10 nm. The monochromatic light was irradiated onto 500 cm3 of air-saturated CdS colloid in a flask. Absorption spectra of the colloid were measured intermittently during the course of irradiation using a photodiode array spectrophotometer (Hewlett-Packard, HP8452), and the irradiation was continued until no change in absorption spectra was observed (it usually took more than 100 h to attain such situation). Then 10 cm3 of 1.0 × 10-2 mol 2-aminoethanethiol(2-AET) was added to 500 cm3 of the photoetched CdS colloids as a surface modifier, followed by agitation for overnight, resulting in precipitation of 2-AET-modified CdS nanoparticles (2-AET/QCdS). The surface-modified CdS nanoparticles prepared in this way were then isolated by a centrifugation and washed successively with water and methanol. Elemental analysis and atomic absorption spectroscopic measurements were performed using a Perkin-Elmer 240C CHN-corder and a NJA AA-8500 Mark II spectrometer. Prior to immobilization of the prepared 2-AET/Q-CdS onto Au electrodes, the Au electrodes were modified with 2-aminoethanethiol. The Au electrode substrate (7 mm-diameter disk) were polished with 0.3 µm alumina. Before use, the gold surfaces were cleaned with piranha solution (H2SO4:H2O2 ) 3:1) for 60 s and then rinsed with water. The 2-AET/CdS was soluble in acidic aqueous solutions due to protonation of the amino group present on the surface. Then the 2-AET/CdS nanoparticles were dissolved in dilute hydrochloric acid (pH ) 1.5) so as to give absorbance of 1.0 at 350 nm. Then 3.0 cm3 of the resulting CdS colloids was added to 2.6 cm3 of a phosphate buffer solution of pH ) 7 in which the 2-AET-modified Au substrate was immersed. After pH adjustments of the CdS colloids to pH ) 7, which was necessary to induce chemical reactions between glutaraldehyde and the amino group of 2-AET, 0.4 cm3 of 70 wt % glutaraldehyde was added to bind CdS nanoparticles to 2-AET/Au. Pictures of CdS nanoparticles were taken using a Hitachi H-9000 transmission electron microscope (TEM) at an operating voltage of 300 kV for samples prepared by putting colloidal particles onto amorphous carbon overlayered on a Cu grid. Electrochemical measurements were carried out using the lockin technique. A lock-in amplifier (Nichia keisoku, P-51A), light chopper (NF, CH-353), a potentiostat (Hokuto Denko, HA-301), a potential sweeper (Hokuto Denko, HB-104), and a recorder (Kobayashi Keisokukiki, WX4000) served for this purpose. As a light source for irradiation of the electrodes, a 500 W Xe lamp was used, and light of wavelengths shorter than 400 nm was cut off by a filter. The irradiation intensity at the electrode surface position was 0.22 W cm-2. Action spectra measurements were carried out by irradiating the monochromatic light obtained by passing lights from the Xe lamp through an interference filter. Electrolyte solutions used were an aqueous solutions containing 0.1 mol dm-3 KCl and 0.02 mol dm-3 triethanolamine, and the pH of solutions was adjusted to pH ) 12 by the use of sodium hydroxide to remove the influence of surface states of CdS nanoparticles.
Figure 1. Absorption spectra of CdS colloid before (A) and after (B) capping with 2-AET. Original CdS particles (a) and CdS nanoparticles prepared by irradiating the monochromatic lights at 480 (b), 450 (c), 430 (d) and 410 nm (e). Spectra A were taken in aqueous solution containing 5.0 × 10-4 mol dm-3 HMP, and spectra B were taken in dilute hydrochloric acid (pH ) 1.5). The arrows show the first exciton peak.
Results and Discussion Characterization of Q-CdS Particles Capped with 2-Aminoethanethiol. We reported previously37,38 that HMP-stabilized CdS particles were photoetched with monochromatic light, and the sizes of the resulting nanoparticles were easily controlled by choosing the wavelength of monochromatic light used for irradiation. Figure 1A shows steady-state absorption spectra obtained before and after the photoetching with monochromatic irradiation at 480, 450, 430, and 410 nm. The spectra were normalized at their first exciton peaks. The exciton peaks were defined by a position of the absorption maximum for spectra d and e and by a position of the shoulder for spectra a, b, and c. With a decrease of irradiation wavelength, the absorption onset was blueshifted, and furthermore the absorption spectra became more structured with clearer exciton peaks. The size distribution profiles of the Q-CdS nanoparticles determined by TEM observations are listed in Table 1. The experimentally obtained average diameter of CdS nanoparticles was in good agreement with that obtained theoretically from tight-binding approximations39 with the assumption that the exciton peak energy in Figure 1A gives the band gap energy. With a decrease in the wavelength of irradiation light, both the average diameter of CdS nanoparticles and its standard deviation decreased. The standard deviation became smaller than about 8% of the average diameter for Q-CdS particles prepared by monochromatic light irradiation with light of wavelengths shorter than 450 nm. Figure 1B shows normalized absorption spectra of the Q-CdS colloid in dilute HCl solution (pH ) 1.5) after capping the particle surface with 2-AET. When the photoetched Q-CdS particles were modified with 2-AET, the resulting 2-AET/Q-CdS particles were precipitated (38) Torimoto, T.; Nishiyama, H.; Yoneyama, H. J. Electrochem. Soc. 1998, 145, 1964. (39) Lippens, L. E.; Lannoo, M. Phys. Rev. B 1989, 39, 1035.
Monodisperse CdS Nanoparticles
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Table 1. Average Diameter, the Standard Deviation and the Surface Coverage of 2-AET/Q-CdS Determined from Data Given in Figure 1a composition λirrad/nm Dav/nm ∆d/nm Eexc/nm Eg/ev Dtheor/nm Rexp a b c d e
No 480 450 430 410
6.5 3.5 2.9 2.6 2.4
1.5 0.49 0.24 0.17 0.17
485 450 430 410 390
2.6 2.8 2.9 3.0 3.2
4.8 3.2 2.8 2.4 2.2
0.11 0.32 0.92 0.72 1.0
Rtheor 0.35 0.60 0.80 0.88 0.92
a Key: λ irrad, photoirradiation wavelength; Dav, experimentally determined average diameter; ∆d, standard deviation; Eexc, exciton peak wavelength; Eg, band gap energy; Dtheor, theoretically predicted average diameter; Rexp, experimentally determined molar ratio of the surface-bound 2-AET to Cd; Rtheor, theoretically predicted molar ratio of the surface-bound thiolate to Cd.41,42
Table 2. Amount of Immobilized CdS Nanoparticles onto Electrode Substrate and the Photonic Yield Obtained under Irradiation of Monochromatic Light of 350 nm at 0 V vs SCE amount of immobilized CdS/ nmol cm-2 photonic yield/10-3 %
a
b
c
d
e
190
97
75
57
56
24
19
6.5
1.9
1.0
in the preparation bath, as described in the Experimental Section. However, since the 2-AET/Q-CdS particles whose spectra are given by curves c-e in Figure 1B were soluble in acidic aqueous solutions due to protonation of the amino group (pKa)8.240) of the CdS particle surface, the spectra measurements were carried out in very dilute HCl solution (pH ) 1.5). CdS particles whose spectra are given by curves a and b were not dissolved for the reason that the amount of surface bound 2-AET was small in relative sense, as shown in Rexp values in Table 1 which give the experimentally determined molar ratio of the surface-bound 2-AET to Cd atoms. In the table, Rtheor values which are obtained theoretically for fully capped CdS particles using the previously reported procedures41,42 are also included for comparative purpose. Then the spectra measurements were carried out using particle-dispersed colloids. By comparing spectra A with B in Figure 1, it is noticed that the absorption onsets and exciton peaks were unchanged by the surface modification of Q-CdS with 2-AET. The absorption spectra of 2-AET/Q-CdS colloids were little changed for several hours, indicating that corrosion of 2-AET/Q-CdS particles hardly occurred even in the aqueous solution of pH ) 1.5, though naked Q-CdS colloids are corroded in acidic solution. Photoelectrochemical Characterization of the Q-CdS Particle Immobilized Electrodes. 2-AET/QCdS particles shown in Table 1 were immobilized onto Au electrode substrate using the procedure described above. Table 2 shows the amount of immobilized CdS nanoparticles onto the electrode substrate, determined by using atomic absorption spectroscopy. The amount of the immobilized particles was different and seems to decrease with a decrease of the particle size. Figure 2 shows photocurrent-potential curves of CdS nanoparticlesimmobilized electrodes. The curves have a similar shape to those of n-type semiconductor electrodes; with an increase of anodic polarization of the electrodes from the (40) Fasman, G. D., Ed. Handbook of Biochemistry and Molecular Biology, 3rd ed.; Physical and Chemical Data; CRC Press: Cleveland, OH, 1976; Vol. I, pp 305-351. (41) Torimoto, T.; Uchida, H.; Sakata, T.; Mori, H.; Yoneyama, H. J. Am. Chem. Soc. 1993, 115, 1874. (42) Inoue, H.; Ichiroku, N.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. Langmuir 1994, 10, 4517.
Figure 2. Photocurrent-potential curves of CdS nanoparticles-immobilized electrodes taken using the lock-in-technique. Electrolyte solution was an aqueous solution at pH ) 12 containing 0.1 mol dm-3 KCl and 0.02 mol dm-3 triethanolamine. Potential scan rate was 5 mV s-1 and illumination intensity was 0.22 W cm-2. The chopping frequency of light was 80 Hz and the time constant was 1s. Irradiation wavelength (nm) for the preparation of nearly monodisperse CdS: (a) none; (b) 480; (c) 450; (d) 430; (e) 410. Table 3. Onset Potential of Anodic Photocurrent Obtained in Aqueous Solution and Acetonitrile Solution Determined from the Results Given in Figure 2 a
b
c
d
e
diameter (TEM)/nm 6.5 3.5 2.9 2.6 2.4 aqueous solution/V vs SCE -0.9 -1.15 -1.2 -1.25 -1.35 acetonitrile solution/V vs SCE -0.91 -1.12 -1.18 -1.25 -1.33
onset potential, anodic photocurrents increase, and then show saturating tendencies. The magnitude of photocurrents was largely different among the five kinds of electrodes. This is partly because the amount of immobilized CdS was different. If the apparent photonic yield, which is defined as the ratio of the number of electrons injected in the electrode to that of the irradiated photons, was obtained under irradiation of monochromatic light of 350 nm at 0 V vs SCE in the same electrolyte solutions as shown in Figure 2, the results given in Table 2 were obtained. It is noticed that with an increase of the number of moles of the immobilized CdS, the photonic yield was increased, suggesting that the photonic yield was determined by the number of photons absorbed in the immobilized CdS particles. Furthermore, the photocurrent-potential curves of CdS nanoparticles-immobilized electrodes were little changed after scanning the electrode potential for the potential range shown in Figure 2 more than three times under irradiation. Table 3 shows the onset potential of anodic photocurrents obtained at various kinds of CdS nanoparticlesimmobilized electrodes in the same aqueous solution as that shown in the caption of Figure 2 and that in an acetonitrile solution containing 0.1 mol dm-3 lithium perchlorate and 0.02 mol dm-3 triethylamine. In both
1506 Langmuir, Vol. 15, No. 4, 1999
Figure 3. Absorption spectra and photocurrent action spectra of CdS nanoparticles-immobilized electrodes taken at 0 V vs SCE. Electrolyte solution was an aqueous solution at pH ) 12 containing 0.1 mol dm-3 KCl and 0.02 mol dm-3 triethanolamine. The symbol given to CdS nanoparticles was the same as that given in Figure 2. Solid curves give absorption spectra of photoetched CdS nanoparticles taken before being immobilized onto the electrodes substrates.
Miyake et al.
Figure 4. Potential of the conduction band edge (O) and the valence band edge (4) of CdS nanoparticles determined as a function of the size of the particles. The solid curve is theoretically derived one using the finite depth potential well model.48 (Details, see text.) Electrolyte solutions were an aqueous solution at pH ) 12 containing 0.1 mol dm-3 KCl and 0.02 mol dm-3 triethanolamine (a) and an acetonitrile solution containing 0.1 mol dm-3 lithium perchrolate and 0.02 mol dm-3 triethylamine (b).
solutions, the onset potential was negatively shifted with a decrease of the diameter of CdS nanoparticles, being in accordance with the results predicted from the operation of the size quantization effects. It should be noted that the onset potential values obtained were almost the same between the aqueous solution and acetonitrile solutions if the electrode was the same. The results seem reasonable, because the flat band potential of the bulk CdS electrodes is almost the same between in aqueous solution43,44 at pH ) 12 and in acetonitrile solution.45-47 Figure 3 shows action spectra of CdS nanoparticles-immobilized electrodes taken at 0 V vs SCE in the same solution as that given in Figure 2. Absorption spectra of CdS nanoparticle colloids taken before the surface modification was made (Figure 1A) are also included. The action spectra were in good accord with the absorption spectra for all cases, indicating that the photocurrents resulted from photoexcitation of the CdS nanoparticles immobilized onto the Au electrode substrate and also that no coalescence of CdS nanoparticle occurred in the course of the immobilization. It seems interesting to compare the potential of the conduction band edge and valence band edge of the CdS particle obtained experimentally with that of theoretically predicted one. It seems not unreasonable to assume that the photoinduced electron transfer from CdS nanoparticles to the electrode substrate can occur unless the electrode potential is more negative than the conduction band edge (ECB). Then ECB should be equal to the onset potential of anodic photocurrents, and the potential of the valence band edge (EVB) is obtained by using the following equation48
Figure 4 as a function of the diameter of the CdS nanoparticle. Both ECB and EVB are obviously shifted toward negative and positive potentials, respectively, with a decrease of the diameter of CdS nanoparticles. This figure also includes the theoretically predicted ECB and EVB with the use of the finite depth potential well model.48 The theoretical prediction allows the estimation of the difference in the magnitude of ECB and EVB from those of the bulk material, but the estimated shifts are not directly related to their potentials in electrolyte solutions. To correlate them with each other, ECB of the theoretically predicted one for CdS nanoparticles of 6.5 nm is assumed to be equal to that determined experimentally. In this way, ECB of other particles was estimated theoretically as given by solid curves in the figure. According to the obtained curves, ECB of bulk CdS in the aqueous solution is -0.78 V vs SCE, which accords with the published results.43,44 The theoretically derived curve of EVB was obtained first by applying 2.4 eV as the bulk band gap48 to ECB of bulk material to determine EVB of bulk material and then by applying the theoretically predicted shifts of EVB to that of bulk material. It is recognized that the experimentally obtained ECB and EVB are in good accord with the theoretically derived ones both for aqueous solutions and acetonitrile solutions.
Eex ) EVB - ECB + ECoulomb
Summary
Here Eex is the lowest exciton energy which was determined from the exciton peak in absorption spectra shown in Figure 1A, ECoulomb is the Coulomb interaction energy which is given by -3.6e2/d,48,49 where is 5.6 for CdS,48 and symbols d and e represent the particle diameter and electronic charge. The obtained ECB and EVB are given in (43) White, J. R.; Bard, A. J. J. Phys. Chem. 1985, 89, 1947. (44) Matsumoto, H.; Matsunaga, T.; Sakata, T.; Mori, H.; Yoneyama, H. Langmuir 1995, 11, 4283. (45) Nagasubramanian, G.; Wheeler, B. L.; Bard, A. J. J. Electrochem. Soc. 1983, 130, 1680. (46) Kohl, P. A.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 7531. (47) Nakatani, K.; Tsubomura, H. Bull. Chem. Soc. Jpn. 1977, 50, 783. (48) Nosaka, Y. J. Phys. Chem. 1991, 95, 5054. (49) Brus, L. E. J. Chem. Phys. 1984, 80, 4403.
To investigate photoelectrochemical properties of nearly monodisperse CdS nanoparticles, we attempted to immobilize them to gold electrode substrates using the crosslinking reaction of the surface modifiers of the particles with electrode substrate with assistance of glutaraldehyde. The immobilization technique used in this study is said to be simple and we believe that the results obtained in photoelectrochemical studies on the prepared photoelectrodes demonstrated the utilities of the immobilization technique. This is one important aspect of this paper. The other important thing is to have shown that size-quantized photoelectrodes can be prepared using nearly monodisperse CdS particles. The obtained photoelectrochemical properties were those predicted theoretically from the size
Monodisperse CdS Nanoparticles
quantization. The electrodes prepared in the present study, however, gave too small photocurrents to be used as photoelectrodes for a variety of applications. This problem may be solved if the immobilization techniques are improved so as to increase the surface concentration of immobilized particles.
Langmuir, Vol. 15, No. 4, 1999 1507
Acknowledgment. This research was supported by a Grant-in-Aid for Priority Area of “Electrochemistry of Ordered Interface”, No. 09237104, from the Ministry of Education, Science, Culture, and Sports, Japan. LA980975L
