Size Separation and Photochemical Properties of Quantized Semiconductor Microcrystals

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Photochemical and photophysical properties of semiconductor microcrystals depend greatly on their size due to the quantum size effects. Bisides the size effect, properties of microcrystals must be greatly influenced by their surface conditions, i.e.nonstoichiometry in the surface composition and kinds of stabilizers. Therefore, in order to clarify their size-dependent properties, it is desirable to employ microcrystals having almost identical surface conditions except their particle size. A fractionation of CdS microcrystals with the use of gel electrophoresis is one of the promising techniques to allow such conditions. In the present research, effective methods for size-separation of quantized semiconductor microcrystals were developed and photoinduced electron transfer on size-separated CdS microcrystals were examined. Major achievements are as follows ;1. Efficient size-separation of quantized CdS by means of improved electrophoresis.First, the gel electrophoresis of quantized CdS ( … More Q-CdS, stabilized with HMP, diameter=3-9nm)) was carried out employing a polyacrylamide gel column with application of 100V.A thermal diffusion of CdS particles in the gel was inhibited by cooling of the elctrophoresis cell with ice bath. The resulting CdS-gel layr of 6cm length was cut into 6 slices. It was found that the second electrophoresis of each gel slice resulted in very efficient and prompt extraction of CdS particles from the slice of the gel. With the size-separation, the absorption spectrum of Q-CdS blue-shifted and became steep, the degree being more marked with an increase in the fraction number. It was confirmed by TEM observation that the size-separation gave narrow size distribution and particles of 5.02 nm and 3.58 nm average diameter were present in the fraction 1 and 6, respectively.2. Photoinduced electron transfer on size-separated CdS microcrystals.The rate of reduction of methylviologen on CdS microcrystals prepared by fractionation with the use of gel electrophoresis increased with decreasing the particle size. The observed results were successfully analyzed in terms of shifts in the conduction bandedge due to the size-quantization. The potential of the conduction bandedge of the microcrystals of various sizes relative to that of crystalline bulk CdS is derived based on the flat band potential of the bulk CdS and the shifts of the conduction band expected by the size-quantization. By assuming that the potential difference between the derived potential of the conduction bandedge and the redox potential of methylviologen is the overvoltage for the reduction reaction, Tafel relation with the transfer coefficient of ca. 0.3 was observed between the reduction rate and the overvoltage. Less