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Langmuir 1999, 15, 2714-2718
Effects of Surface Charges and Surface States of Chemically Modified Cadmium Sulfide Nanoparticles Immobilized to Gold Electrode Substrate on Photoinduced Charge Transfers Masahide Miyake,† Tsukasa Torimoto,† Matsuhiko Nishizawa,† 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 June 29, 1998. In Final Form: February 2, 1999
Cadmium sulfide nanoparticles (Q-CdS) modified with 2-mercaptoethanesulfonate and 2-aminoethanethiol in a molar ratio of 2:1 were covalently immobilized onto an Au surface covered with a self-assembled monolayer of 3,3′-dithiobis(succinimidylpropionate), and the resulting electrodes were further immobilized with Q-CdS using glutaraldehyde as a binding agent. The degree of anodic photocurrents was greatly influenced by charged conditions of hole scavengers used because of the presence of sulfonate groups on the Q-CdS surfaces; triethylamine having positive charges gave large photocurrents, triethanolamine medium photocurrents, and formate small photocurrents. If Q-CdS having a large emission from their surface trap states was used, anodic photocurrents were depressed with increasing anodic polarization from the onset potentials which were ca. -1.1 V vs SCE for the use of any kinds of hole scavengers, and the greatest depression appeared at -0.25 V, beyond which a steep increase in anodic photocurrents was seen. In contrast, no significant depression in photocurrents was observed and anodic photocurrents were monotonically increased, in the case of using Q-CdS having an intense band-gap emission. When the energetic position at the emission maximum is correlated to the potential at which the greatest photocurrent depression appeared, photocurrent-potential characteristics are discussed in terms of involvements of surface states in the photoelectrode reactions.
Introduction Photoelectrochemical properties of size-quantized semiconductor electrodes have been reported in recent years.1-15 Several techniques have been employed to prepare the electrodes such as the chemical deposition of semiconductor nanoparticles on conducting electrode substrates,16-18 * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Applied Chemistry, Faculty of Engineering. ‡ Research Center for Ultra-High Voltage Electron Microscopy. (1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Kamat, P. V. Chem. Rev. 1993, 93, 267. (3) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136. (4) Aleperson, B.; Cohen, S.; Hodes, G. Phys. Rev. B 1995, 52, 7017. (5) Ogawa, S.; Fan, F. F.; Bard, A. J. J. Phys. Chem. 1995, 99, 11182. (6) Ogawa, S.; Hu, K.; Fan, F. F.; Bard, A. J. J. Phys. Chem. B 1997, 101, 5707. (7) Mansur, H. S.; Grieser, F.; Urquhart, R. S.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1995, 91, 3399. (8) Mansur, H. S.; Grieser, F.; Marychurch, M. S.; Biggs, S.; Urquhart, R. S.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1995, 91, 665. (9) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (10) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (11) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (12) Zhao, X. K.; McCormick, L. D.; Fendler, J. H. Langmuir 1991, 7, 1255. (13) Tian, Y.; Fendler, J. H. Chem. Mater. 1996, 8, 969. (14) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 11064. (15) Hotchandani, S.; Bedja, I.; Fessenden, R. W.; Kamat, P. V. Langmuir 1994, 10, 17. (16) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834. (17) Gorer, S.; Hodes, G. J. Phys. Chem. 1994, 98, 5338.
the electrodeposition of semiconductor nanoparticles,3,19-21 the coating of conducting electrode substrates with semiconductor nanoparticle colloids followed by evaporation of the solvents,10,22-25 and the immobilization of the surface-modified nanoparticles to conducting substrates with use of physical interactions11-13,26 and chemical reactions.5,6,27-30 Of these, the former three do not guarantee the appearance of the size-quantized effects at the prepared electrodes, because the size-quantization effects may disappear if the coagulation of the particles occurs with high extents, though several electrodes prepared by using these techniques have been reported to exhibit the size-quantization effects. On the other hand, the last (18) Gorer, S.; Alubu-Yaron, A.; Hodes, G. J. Phys. Chem. 1995, 99, 16442. (19) Golan, Y.; Margulis, L.; Hodes, G.; Rubinstein, I.; Hutchison, L. Surf. Sci. 1994, 311, L633. (20) Golan, Y.; Hodes, G.; Rubinstein, I. J. Phys. Chem. 1996, 100, 2220. (21) Mastai, Y.; Hodes, G. J. Phys. Chem. B 1997, 101, 2685. (22) Enright, B.; Fitzmaurice, D. J. Phys. Chem. 1996, 100, 1027. (23) Chemseddine, A.; Fearheilcy, M. L. Thin Solid Films 1994, 247, 3. (24) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (25) Kamat, P. V.; Bedja, I.; Hotchandani, S.; Patterson, L. K. J. Phys. Chem. 1996, 100, 4900. (26) Dabbousi, B. O.; Murray, C. B.; Rubner, M. F.; Bawendi, M. G. Chem. Mater. 1994, 6, 216. (27) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (28) Miyake, M.; Matsumoto, H.; Nishizawa, M.; Sakata, T.; Mori, H.; Kuwabata, S.; Yoneyama, H. Langmuir 1997, 13, 742. (29) Nakanishi, T.; Ohtani, B.; Shimazu, K.; Uosaki, K. Chem. Phys. Lett. 1997, 278, 233. (30) Nakanishi, T.; Ohtani, B.; Uosaki, K. J. Phys. Chem. B 1998, 102, 1571.
10.1021/la9807762 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
Chemically Modified Cadmium Sulfide Nanoparticles
Langmuir, Vol. 15, No. 8, 1999 2715
Table 1. Characterization of Q-CdS Prepared and the Amount of Q-CdS Immobilized onto Gold Electrode Substrates
sample A sample B
average diameter/nm
standard deviation/nm
Ega/eV
predictedb diameter/nm
R-NH2/ R-SO3- c
(thiol/Cd)expd
(thiol/Cd)theore
amount f of immobilized CdS/(mol cm-2)
2.6 2.6
0.43 0.44
3.06 3.00
2.3 2.4
0.38 0.32
0.80 0.84
0.86 0.85
6.9 × 10-8 6.7 × 10-8
a A band-gap energy determined from an exciton peak of an absorption spectrum. b The theoretically predicted diameter by applying Eg to the reported energy gap vs particle diameter relation.37 c An experimentally determined molar ratio of modified 2-aminoethanethiol to modified 2-mercaptoethanesulfonate on CdS nanoparticles. d An experimentally determined molar ratio of the surface-bound thiolate to Cd. e A theoretically predicted molar ratio of the surface-bound thiolate to Cd. f The amount of Q-CdS immobilized on the gold electrode substrate.
technique in which surface-modified semiconductor nanoparticles were used ensures the preparation of sizequantized photoelectrodes, because intimate contacts between the particles are prevented by surface modification of semiconductor nanoparticles. It is then believed that electrodes prepared by immobilizing surface-modified semiconductor nanoparticles must exhibit photoelectrochemical properties intrinsic to the particles. The last technique has another merit of allowing functionalization of semiconductor nanoparticles by surface modification.28,31,32 In this paper, it is described how photoelectrode properties of surface-modified CdS nanoparticle electrodes are influenced by charged conditions of the surface modifier. Furthermore, significant influences of surface states on photoelectrode behaviors were discovered at the surface-modified Q-CdS particles, and the results are discussed based on energy level diagrams derived from photocurrent-potential characteristics and emission spectra. Experimental Section Chemical and Materials. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) of reagent grade (Tokyo Kasei), cadmium perchlorate (Mitsuwa Kagaku), 3,3′-dithiobis(succinimidylpropionate) (Nacalai Tesque), and other chemicals of reagent grade (Wako Pure Chemical Industry) were used in the present study. Preparation of Q-CdS Particles. Two types of Q-CdS were prepared in AOT/heptane inverse micelles.28 The inverse micelles used were prepared by adding 2.0 cm3 of distilled water containing 7.0 g of AOT to 100 cm3 of heptane. All procedures were performed under a N2 atmosphere. Sample A. A 0.24 cm3 aliquot of a 1.0 mol dm-3 Cd(ClO4)2 aqueous solution and 0.16 cm3 of a 1.0 mol dm-3 Na2S aqueous solution were respectively added to 60 and 40 cm3 aliquots of the prepared inverse micelle solution. After the solution was stirred individually for 1 h, these were mixed together and stirred for another 1 h, resulting in the formation of Q-CdS in the inverse micelles. In this study a 50% excess amount of Cd2+ to S2- in the molar ratio was used to produce a large amount of Cd2+ sites on the particle surface which could be modified with thiol as described below. The surfaces of the resulting Q-CdS were modified both with 2-aminoethanethiol and with 2-mercaptoethanesulfonate. The modification with the latter compound was essential to dissolve the resulting particles into water in which immobilization of Q-CdS particles to a self-assembled monolayer (SAM) of 3,3′dithiobis(succinimidylpropionate) was conducted. Both 0.17 cm3 of a 0.32 mol dm-3 2-aminoethanethiol aqueous solution and 0.33 cm3 of a 0.32 mol dm-3 2-mercaptoethanesulfonate solution were added to 100 cm3 of the inverse micelles solution containing Q-CdS and stirred for 1 day, resulting in thiol-capped Q-CdS. Pyridine was then added to destroy the AOT inverse micelles. After drying under vacuum, the thiol-capped Q-CdS was washed successively with pyridine, n-heptane, petroleum ether, 1-butanol, acetone, and methanol. (31) Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem. 1994, 98, 3036. (32) Torimoto, T.; Maeda, K.; Maenaka, J.; Yoneyama, H. J. Phys. Chem. 1994, 98, 13658.
Sample B. The preparation procedures were essentially the same as those of sample A. A different point was only that H2S instead of Na2S was used as a sulfur source. After 0.24 cm3 of a 1.0 mol dm-3 Cd(ClO4)2 aqueous solution was added to 100 cm3 of the prepared inverse micelles solution and the resulting mixture was stirred for 1 h, 0.16 mmol of H2S gas was injected into the solution and the resulting solution was stirred for 1 h. A composition of the resulting Q-CdS was determined by elemental analysis (Perkin-Elmer 240C CHN-corder) and atomic absorption spectroscopy (NJA AA-8500 Mark II spectrometer). Absorption spectra were measured using a Hewlett-Packard HP8452A UV/vis photodiode array spectrophotometer. Aqueous solutions of 0.10 mg cm-3 of the surface-modified Q-CdS particles were used in the absorption measurements. Static emission spectra were measured with a Hitachi F3010 fluorescence spectrometer. The measured Q-CdS colloids were previously adjusted to 0.1 at 350 nm in the absorption spectra. Size distributions of Q-CdS were determined by observations with 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 overlayers on a Cu grid. Preparation of Gold Electrode Substrate Immobilized Q-CdS Particles. To immobilize the prepared Q-CdS onto Au electrodes, the Au electrodes were first immersed in a DMSO solution containing 5.0 mmol dm-3 3,3′-dithiobis(succinimidylpropionate) to form its self-assembled monolayer (SAM) on the gold electrodes. The prepared electrodes were immersed in an aqueous solution containing dissolved Q-CdS (5.0 g dm-3), resulting in chemical bond formation between the 2-aminoethanethiol on Q-CdS and the SAM on the Au substrate.33,34 To increase the amount of Q-CdS particles on the electrode surface, further immobilization of the CdS particles was attempted by immersing overnight the Q-CdS-bound gold electrode in a 1.0 g dm-3 surface-modified Q-CdS solution containing 10 wt % glutaraldehyde as a binding agent. Glutaraldehyde reacts with the amino group35,36 of Q-CdS, resulting in a three-dimensional linkage between the CdS nanoparticles on the Au substrate. The amount of immobilized Q-CdS determined by applying atomic absorption spectroscopy to solutions prepared by dissolving the immobilized Q-CdS is shown in Table 1. Photoelectrochemical Measurements. The electrolyte solution was a 0.10 mol dm-3 KCl aqueous solution containing a 20 mmol dm-3 hole scavenger. In this study, triethylamine, triethanolamine, or formate was used as a hole scavenger. A Pt flag was used as the counter electrode. The potential was determined against a SCE reference electrode, and a potential sweeper (Hokuto Denko, HB-104) was employed to sweep applied potentials. Photocurrents were measured under N2 with a commercially available potentiostat (Hokuto Denko, HA-301) and were amplified with a lock-in amplifier (Nichia Keisoku, P-51A) by extracting the signal which was synchronized with irradiation. A 500 W xenon lamp was used as a light source, and lights of wavelengths shorter than 400 nm were cut off by a colored glass filter. The irradiation light was chopped at 80 Hz by a light chopper (NF, CH-353) before irradiating the electrode. The irradiation intensity at the electrode surface was 0.22 W cm-2. (33) Ohtani, M.; Kuwabata, S.; Yoneyama, H. J. Electroanal. Chem. 1997, 422, 45. (34) Adir, N.; Ohad, I. Biochim. Biophys. Acta 1986, 850, 264. (35) Calvo, E. J.; Danilowicz, C.; Diaz, L. J. Chem. Soc., Faraday Trans. 1993, 89, 377. (36) Kuwabata, S.; Okamoto, T.; Kajiya, Y.; Yoneyama, H. Anal. Chem. 1995, 34, 1684.
2716 Langmuir, Vol. 15, No. 8, 1999
Figure 1. Absorption spectra of CdS nanoparticles in an aqueous solution (a) and their size distribution (b). The solid line and filled bar are for sample A, and the dashed line and open bar are for sample B. The arrows show the first exciton peak.
Results and Discussion Photoelectrochemical Properties of Q-CdS Particles Immobilized on Gold Electrode Substrates. Almost the same absorption spectra were observed between samples A and B, as shown in Figure 1a; the absorption onset was 460 nm in both samples A and B, and the first exciton peak was 405 nm in sample A and 413 nm in sample B. TEM observations revealed that the size distribution of CdS nanoparticles of sample A ranged from 1.6 to 3.8 nm and the average diameter was 2.6 nm with its standard deviation of 0.43 nm, which was almost the same as that of sample B, as shown in Figure 1b. When the band-gap value estimated from the position of the exciton peak of absorption spectra is applied to the reported energy gap vs particle diameter relation,37 theoretically predicted diameters of samples A and B were obtained (Table 1), which were found to be in good agreement with the experimentally determined average diameters despite the presence of a large size distribution of the prepared CdS nanoparticles. From the elemental analyses of CdS nanoparticles, the molar ratio of the surface-bound thiolate to Cd atoms in the CdS nanoparticles was obtained and is listed in Table 1 with those expected theoretically for CdS particles fully covered with thiol compounds. In the theoretical derivations, the published procedures were employed.38,39 In the case of sample A, the experimentally obtained molar ratio of surface-bound thiolate to Cd (0.80) was a little smaller than the theoretically predicted one (0.86), and the observed difference suggests that 7% Cd sites on the CdS surface were free from the chemical modification. As for sample B, as small as 1% of Cd sites remained unmodified. Figure 2 shows emission spectra of samples A and B. Sample A gave an emission peak at around 565 nm, while sample B gave two emission peaks at around 470 and 565 (37) Lippens, L. E.; Lannoo, M. Phys. Rev. B 1989, 39, 1035. (38) Torimoto, T.; Uchida, H.; Sakata, T.; Mori, H.; Yoneyama, H. J. Am. Chem. Soc. 1993, 115, 1874. (39) Inoue, H.; Ichiroku, N.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. Langmuir 1994, 10, 4517.
Miyake et al.
Figure 2. Effect of solution pH on the emission intensity of Q-CdS colloid solutions of sample A (a) and sample B (b) at room temperature. Solid lines are spectra measured in an aqueous solution of pH 9.5 and dash lines those in pH 12. The excitation wavelength was 350 nm. The absorbance of the colloids was adjusted to 0.1 at 350 nm.
nm. The emission peak at around 565 nm is believed to result from the involvement of surface states which originated from sulfur vacancies,40,41 while that at around 470 nm must result from band-gap transitions or the involvement of shallow trap sites. Because both samples A and B were synthesized with an excess of Cd2+ to S2-, the presence of sulfur vacancies in the prepared Q-CdS is not unreasonable. The intensity of the emission peak at 565 nm became small with an increase in the solution pH from 9.5 to 12, as already reported.42 Adsorption of OH- onto unmodified Cd sites and/or sulfur vacancies seems responsible for the intensity suppression observed. The difference in the emission intensity at around 565 nm between samples A and B and the lack of the emission at around 470 nm at sample A suggest that the amount of sulfur vacancies was much greater at sample A than at sample B. However, why the difference appeared is not known at present. Figure 3 shows photocurrent-potential curves of the CdS immobilized electrodes in the presence of various hole scavengers at pH 9.5. A noticeable difference is seen between the electrodes prepared using samples A and B. In the case of using sample B, the photocurrents showed a monotonically increasing tendency with increasing anodic polarization, while sample A gave first an increase in anodic photocurrents with increasing anodic polarization, but the anodic photocurrents decayed with further anodic polarization from a certain potential, with the degree depending on the kinds of hole scavengers used. When the electrode was anodically polarized at a more positive value than -0.25 V vs SCE, the anodic photocurrents showed again a steep increase for all cases. Figure 4 shows action spectra of anodic photocurrents taken in the presence of triethanolamine as the hole (40) Ramsden, J. J.; Gra¨tzel, M. J. Chem. Soc., Faraday Trans. 1 1984, 80, 919. (41) Ramsden, J. J.; Webber, S. E.; Gra¨tzel, M. J. Phys. Chem. 1985, 89, 2740. (42) Resch, U.; Eychmu¨ller, A.; Haase, M.; Weller, H. Langmuir 1992, 8, 2215.
Chemically Modified Cadmium Sulfide Nanoparticles
Figure 3. Photocurrent-potential curves of Q-CdS immobilized electrodes in a 0.10 mol dm-3 KCl aqueous solution containing 20 mmol dm-3 of triethylamine (a and d), triethanolamine (b and e), and formate (c and f). Solution pHs were 9.5 for all cases. The potential scan rate was 5.0 mV s-1, and the illumination intensity was 0.22 W cm-2. Parts a-c are for sample A, and parts d-f are for sample B.
Figure 4. Absorption spectra (solid line) of Q-CdS colloids taken before immobilization to the self-assembled monolayer on Au and photocurrent action spectra (open circle) of Q-CdS immobilized electrodes taken in a 0.10 mol dm-3 KCl aqueous solution containing 20 mmol dm-3 triethanolamine at pH 9.5. The applied potential was 0 V vs SCE. Part a is for sample A, and part b is for sample B.
scavenger at pH 9.5. Also shown in this figure are absorption spectra of the surface-modified Q-CdS colloids taken before the particles were immobilized onto the gold substrate. The action spectra accord very well with
Langmuir, Vol. 15, No. 8, 1999 2717
Figure 5. Photocurrent-potential curves of Q-CdS immobilized electrodes in a 0.10 mol dm-3 KCl solution containing 20 mmol dm-3 triethanolamine. The potential scan rate was 5.0 mV s-1, and the illumination intensity was 0.22 W cm-2. Solid lines are spectra measured in an aqueous solution of pH 9.5 and dashed lines those obtained in pH 12. Part a is for sample A, and part b is for sample B.
the absorption spectra, indicating that the immobilization of the Q-CdS particles on the gold electrode substrate did not cause any appreciable changes in the particle size. The magnitude of anodic photocurrents shown in Figure 3 increased in the order of formate < triethanolamine < triethylamine, at both kinds of Q-CdS used. The results obtained can be explained well in terms of charged conditions of the hole scavengers used in the measurement solutions (pH of 9.5); triethylamine (pKa ) 10.7) is positively charged, triethanolamine (pKa ) 7.8) has no net charges, and formate (pKa ) 3.6) is negatively charged.43 The immobilized Q-CdS has negative surface charges due to the presence of a larger fraction of the immobilized 2-mercaptoethanesulfonate on Q-CdS surfaces. The negatively charged formate cannot then be accumulated at the Q-CdS surfaces. It is rather repelled from the particle surfaces because of electrostatic repulsion, resulting in small anodic photocurrents. With changing the charged condition of the electroactive species from negative to neutral and then to positive, the anodic photocurrents became large. However, as described above and as shown in Figure 3, samples A and B showed quite different dependences of anodic photocurrents on potentials. The observed difference cannot be explained in terms of the charged conditions of hole scavengers alone. There is a common feature in photocurrent-potential curves that the highest depression appeared at -0.25 V vs SCE for sample A regardless of the kinds of hole scavengers (Figure 3a-c). In contrast, no significant depression of photocurrents was seen at sample B. Figure 5 shows photocurrent-potential curves taken in two different pHs of electrolyte solutions containing triethanolamine as a hole scavenger. In both cases, the photocurrents were enhanced with an increase of pH from 9.5 to 12, but the effect was more remarkable at sample A. (43) Fasman, G. D., Ed. Handbook of Biochemistry and Molecular Biology, 3rd ed.; Physical and Chemical Data, Vol. I; CRC Press: Cleveland, OH, 1976; Vol. I, pp 305-351.
2718 Langmuir, Vol. 15, No. 8, 1999
Figure 6. Schematic energy diagrams of the interface between the gold electrode and the Q-CdS particle. EC, EV, and EF are the potentials of the conduction band edge, the valence band edge, and the Fermi level, respectively. R is the hole scavenger, and SS is the surface state: (a) at photocurrent onset, (b) under small anodic polarizations, (c) at potentials where anodic photocurrents were depressed at the highest degree, and (d) with anodic polarizations above -0.25 V vs SCE.
As already described above, an increase of the solution pH from 9.5 to 12 deactivates the surface states, and with the deactivation, the photocurrent depression at sample A was remarkably relaxed, suggesting that the surface states are responsible for the photocurrent depression, as already discussed by Bard et al.5 Discussion on the Energy Structures of Q-CdS Particles Immobilized on Gold Electrode Substrates. The band gap of Q-CdS of sample A used was 3.06 eV, as estimated from the position of the exciton peak of the absorption spectrum shown in Figure 1. As shown in Figure 2a, its emission spectrum ranged from 450 nm (2.8 eV) to 690 nm (1.8 eV) with its peak at 565 nm (2.2 eV) in the case of using sample A. When it is assumed that the emission arises from the recombination of electrons trapped at the surface states with holes in the valence bands and the band-gap value of 3.06 eV is taken into consideration, the top of the surface state energy level is deduced to be 0.26 eV below the conduction band edge and its bottom to be 1.26 eV below the conduction band edge. The energy diagram of Q-CdS relative to the gold substrate is illustrated as shown in Figure 6a for the case of electrode polarization at the onset potential of anodic photocurrents (-1.1 V vs SCE) given in Figure 3a-c. When the potential at which the largest photocurrent depression appeared (-0.25 V vs SCE) is correlated to the energy position which gives the maximum emission intensity (0.86 eV below the conduction band edge), the potential of the conduction band edge of Q-CdS is estimated to -1.11 V vs SCE. The finding that the current-potential curves gave the photocurrent onset at -1.15 V vs SCE in the case of sample A at pH 12 in which a small amount of surface states was available (Figure 5a) also supports the validity of our estimation of the conduction band potential. With increasing anodic polarizations, the Fermi level of the gold electrode substrate becomes lowered, resulting in an increase in energy differences between the gold substrate and Q-CdS, as shown in Figure 6b. Then, the anodic photocurrents should be increased, but because the degree of overlaps of the gold electrode with the surface states also become great, the photocurrents show complicated dependences on the electrode potentials, as shown in Figure 3a-c. The magnitudes of anodic photocurrents are determined by a canceling action of anodic photocurrents due to electron flow from Q-CdS to the gold electrode substrate with cathodic currents in the dark which are originated from electron motion from the gold substrates to surface states of Q-CdS, as already discussed by Bard et al.5
Miyake et al.
The highest depression of anodic photocurrents occurs when the Fermi level of the gold electrode substrate overlaps with the surface state energy levels having the highest density which gives the emission intensity maximum. With a further increase in anodic polarizations of the gold electrode substrate, the degree of overlaps of the gold electrode substrate with surface states decreases, and then we may expect to have a gentle increase in anodic photocurrents with increasing anodic polarizations. According to the photocurrent-potential curves shown in Figure 3a-c, however, the increase in the photocurrents occurs with a very steep dependence on the electrode potential for polarizations greater than -0.25 V vs SCE. Because the electron transfer from the gold substrate to the surface states of Q-CdS does not come to an end with anodic polarizations more positive than -0.25 V vs SCE, the observed steep increase in the anodic photocurrents cannot be explained in terms of such canceling action of the anodic photocurrents as those described above and shown in Figure 6c. One plausible explanation of the steep increase in the anodic photocurrents may assume participations of anodic photocurrents arising first from electron motion from the conduction band to the surface states and then to the gold electrode substrates, as shown in Figure 6d. The direction of this electron flow is opposite to the electron transfer from the gold substrate to the surface states of Q-CdS which we have discussed above. With an increase in anodic polarizations, the surface state energy levels become less occupied with electrons, resulting in an increase in dropping probabilities for electrons from the conduction band to the surface states and then to the Au substrate. With contributions of anodic photocurrents with such mechanisms, the electron flow in the opposite direction (from Au to the surface states of Q-CdS) is suppressed, resulting in the steep increase in anodic photocurrents, as observed in Figure 3a-c. Conclusion The present study showed one successful approach to the preparation of size-quantized photoelectrodes by covalently immobilizing CdS nanoparticles to a gold electrode substrate. The immobilization technique used in this study allowed the preparation of the photoelectrode of high electrochemical stability that possessed original properties of the CdS nanoparticles arising from surface states and surface charges, and in this sense, sizequantized photoelectrodes of desired size-quantization effects would be prepared using the present technique. Other interesting features obtained in this study are that photocurrent-potential curves of the prepared photoelectrodes were greatly influenced by the presence of the surface states in the CdS nanoparticles used in the immobilization and the effect appeared most remarkably in the form of depression of photocurrents in a certain potential range which were seen when electrostatic repulsion operated between the immobilized particles and the reactants. It is then concluded that the preparation of CdS nanoparticles free from surface states is important to the preparation of the size-quantized photoelectrodes of high photoelectrochemical activities. Acknowledgment. This research was supported by Grant-in-Aid for Priority Area “Electrochemistry of Ordered Interfaces” (09237104) from the Ministry of Education, Science, Culture and Sports. LA9807762
