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很大程度上取决于它们的尺寸。除了尺寸效应之外，微晶的性能还必须受到其表面条件的极大影响，即表面组成的非化学计量和稳定剂的种类。因此，为了阐明它们的尺寸依赖性特性，需要使用除了颗粒尺寸之外具有几乎相同的表面条件的微晶。使用凝胶电泳对 CdS 微晶进行分级是实现这种条件的有前景的技术之一。在本研究中，开发了量子化半导体微晶尺寸分离的有效方法，并检查了尺寸分离的 CdS 微晶上的光致电子转移。主要成果如下： 1．通过改进的电泳对量子化 CdS 进行有效的尺寸分离。首先，使用聚丙烯酰胺凝胶柱在施加 100V 电压的情况下进行量子化 CdS（Q-CdS，用 HMP 稳定，直径=3-9nm））的凝胶电泳。用冰浴冷却电泳池可抑制凝胶中 CdS 颗粒的热扩散。将所得的6cm长的CdS-凝胶层切成6片。结果发现，每个凝胶切片的第二次电泳都可以非常有效且迅速地从凝胶切片中提取 CdS 颗粒。随着尺寸分离，Q-CdS的吸收光谱发生蓝移并变得陡峭，并且随着级分数的增加蓝移程度更加明显。通过TEM观察证实，尺寸分离给出了狭窄的尺寸分布，并且级分1和级分6中分别存在平均直径为5.02 nm和3.58 nm的颗粒。2．尺寸分离的CdS微晶上的光诱导电子转移。利用凝胶电泳分级分离制备的CdS微晶上甲基紫精的还原率随着粒径的减小而增加。观察到的结果成功地根据尺寸量子化引起的导带边缘的变化进行了分析。不同尺寸微晶的导带边缘相对于晶体块体 CdS 的导带边缘的电势是基于块体 CdS 的平带电势和尺寸量子化预期的导带移动而得出的。假设导带边缘的导出电势与甲基紫精的氧化还原电势之间的电势差是还原反应的过电压，塔菲尔关系与传输系数为约。观察到还原率与过电压之间相差0.3。较少的

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