Size Separation and Photochemical Properties of Quantized Semiconductor Microcrystals

由于量子尺寸效应，半导体微晶体的光化学和光物理性质在很大程度上取决于它们的尺寸。除了尺寸效应外，微晶体的性能还受到其表面条件的影响，即表面组成的非化学计量性和稳定剂的种类。因此，为了阐明它们与尺寸相关的性质，除了它们的粒度外，需要采用具有几乎相同表面条件的微晶体。利用凝胶电泳分离CdS微晶是一种很有前途的技术。本研究开发了量子化半导体微晶体尺寸分离的有效方法，并对尺寸分离的CdS微晶体上的光致电子转移进行了研究。主要工作成果如下：改进电泳技术用于定量CdS的高效粒度分离。首先，采用聚丙烯酰胺凝胶柱，在100V电压下，对量子化CdS（更多Q-CdS，用HMP稳定，直径=3-9nm）进行凝胶电泳。用冰浴冷却电泳池可以抑制CdS颗粒在凝胶中的热扩散。将得到的cd -凝胶层切成6片，长度为6cm。结果发现，对每个凝胶切片进行第二次电泳，可以非常有效和迅速地从凝胶切片中提取CdS颗粒。随着粒径的分离，Q-CdS的吸收光谱发生蓝移并变得陡峭，且随着分数的增加，蓝移的程度更加明显。通过TEM观察证实，分选过程粒径分布较窄，分数1和分数6的平均粒径分别为5.02 nm和3.58 nm。尺寸分离的CdS微晶体的光致电子转移。凝胶电泳分离制备的CdS微晶上甲基紫素的还原速率随着粒径的减小而增大。观察结果成功地分析了由于尺寸量化导致的导带移位。基于块体CdS的平带电位和尺寸量化所期望的导带位移，推导了不同尺寸的微晶体相对于晶体块体CdS的导带电位。假设导带衍生电位与甲基紫素氧化还原电位之间的电位差为还原反应的过电压，则还原速率与过电压之间的传递系数为0.3左右的Tafel关系。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