Size Separation and Photochemical Properties of Quantized Semiconductor Microcrystals

由于量子尺寸效应，半导体微晶的光化学和光物理性质在很大程度上依赖于其尺寸。除了尺寸效应外，微晶的性质必然受到其表面条件的很大影响，即表面组成的非化学计量比和稳定剂的种类。因此，为了阐明它们与尺寸相关的性质，需要使用除了它们的颗粒尺寸之外具有几乎相同的表面条件的微晶体。用凝胶电泳法分离硫化镉微晶是一种很有前途的技术。本研究发展了量子化半导体微晶尺寸分离的有效方法，并研究了尺寸分离后的硫化镉微晶的光致电子转移。主要研究成果如下：1.采用改进的凝胶电泳法对量子化镉进行有效的大小分离。首先，量子化镉的凝胶电泳(…用100V的聚丙烯酰胺凝胶柱进行了更多的Q-Cd，用HMP稳定，直径为3-9 nm)。用冰浴冷却电泳池，抑制了Cd颗粒在凝胶中的热扩散。将得到的6 cm长的硫化镉凝胶层切成6片。研究发现，对每个凝胶切片进行二次电泳法，可以非常高效、快速地从凝胶切片中提取出硫化镉颗粒。随着尺寸的分离，Q-CdS的吸收光谱蓝移并变陡，且程度随着分数的增加而更加明显。结果表明：1.通过透射电子显微镜观察，得到的粒径分布较窄，组分1和组分6的平均粒径分别为5.02 nm和3.58 nm。凝胶电泳法分离制备的硫化镉微晶上甲基紫精的还原速率随颗粒尺寸的减小而增加。根据尺寸量子化引起的导带位移，成功地分析了观测结果。根据块体CdS的平带电势和尺寸量子化所预期的导带位移，导出了不同尺寸的微晶导带相对于结晶体的导带电势。通过假定导带电势与甲基紫精氧化还原电势之差为还原反应的过电压，在还原速率与过电压之间观察到转移系数约为0.3的Tafel关系。较少

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