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.

Size-quantized CdS particles (Q-CdS) capped with 2-aminoethanethiol and 2-mercaptoethanesulfonate were prepared by means of the AOT/heptane inverse micelles method. The resulting CdS particles were covalently immobilized in a high dispersion on an Au(111) surface coated previously with a self-assembled monolayer of 3,3‘-dithiobis(succinimidyl propionate). The immobilized Q-CdS particles were stable against tip scanning in scanning tunneling microscopy (STM). Tunneling spectroscopy (TS) of a single particle whose size was determined from a STM image allowed successfully the determination of the band-gap energy of the size-quantized particle, and the band-gap value obtained was in agreement with that predicted from the tight-binding approximation.

Two-dimensionally organized CdS nanoparticle films were prepared with the use of the Langmuir−Blodgett (LB) technique. 2-Aminoethanethiol-modified CdS nanoparticles were spread on a water subphase which contained glutaraldehyde as a cross-linking agent for binding the surface-modified CdS nanoparticles with each other, and the cross-linking reaction was undertaken under several different compressions of the CdS nanoparticle layer spread on the water subphase. Surface pressure−area isotherms of the CdS monoparticulate layer taken after the cross-linking were largely different depending on the area used for the cross-linking. By transfer of the CdS monoparticulate film prepared at the air−water interface to the 2-aminoethanethiol-modified gold substrate using the LB technique, CdS monoparticulate films containing different surface concentrations were prepared on the gold substrate. It was easily done to cumulate the monoparticulate film on the gold substrate using the same transfer technique. The prepared CdS nanoparticles films showed n-type photosensitivities which were enhanced by increasing the number of cumulation of monoparticulate films.

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.

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