【作者】 邹志军；

【导师】 谢长生；

【作者基本信息】 华中科技大学 ， 材料学， 2013， 博士

【摘要】 能源短缺与环境污染已经成为全球发展所面临的重大难题,因而,开发新能源与保护环境成为当前科学研究的热点。在太阳光的照射下,利用半导体进行光催化降解污染物,是一条既能利用新能源又能实现环境净化的重要途径。半导体光催化的基本原理是,半导体光催化剂吸收太阳能,产生具有氧化还原能力的电子空穴对,进而将有害污染物降解为无毒无害的CO2和H2O。半导体的光电响应过程是指半导体材料在一定光源的照射下,产生电子与空穴对,电子在外加电场的驱动下定向移动,进而在外电路中形成光电流的过程。因而,考虑到在实验室进行光催化实验耗时长、费用高等特点,也考虑到光催化过程涉及半导体内部载流子的产生与转移等基本过程,所以,研究半导体材料的光电响应过程,对于指导光催化材料的设计与探讨光催化反应的机理有重要的意义。因此,本文致力于研究金属氧化物及其复合体系的气相光电性能,进而探讨载流子的产生、分离、复合、捕获等基本过程。本文首先从金属氧化物复合体系出发,基于组合材料学的思想,设计了由66个成分点组成的Ti02/WO3/MnO2三元复合体系的材料库。并通过球磨和丝网印刷的方法并行制备了相应的66个光电器件。借助自行搭建的光电性能综合测试平台,我们高通量表征了所有器件的光电性能。结果表明,在66个成分点中,TiO2/WO3/MnO2摩尔比例为2：8：0的成分点,在白光、紫外、蓝光与绿光照射下均有最好的光电性能,这要归因于W03本身的特性与TiO2/WO3复合体系的能带匹配模式,错开型的能带匹配有效地促进了载流子的分离。靠近TiO2角没有明显光电响应的原因要归结于所施加的偏压0.2V过低,因为当偏压为10V时,TiO2表现出相对较为明显的光电响应。而靠近MnO2角没有明显光电响应的原因,可能是由于MnO2相在烧结过程中向Mn2O3相转变,当Mn2O3的量过多时,Mn2O3可能会加剧载流子的复合,从而导致载流子的复合率大大的增加,进而导致外电路中光电流不明显。鉴于TiO2在过低的偏压条件下没有明显的光电响应,因而,为了进一步研究TiO2及其复合体系的光电性能,我们引入了敏化剂CdS对TiO2进行敏化改性。我们首先制备了纯TiO2器件,并通过连续离子层吸附反应法制备了CdS/TiO2复合体系的器件。之后,我们研究了CdS/TiO2复合体系的气相光电性能,并与纯的Ti02进行了对比。结果表明,CdS/TiO2复合体系无论是在紫外还是在白光照射的条件下,均表现出更加优异的光电性能。在研究了由敏化剂CdS与金属氧化物Ti02组成的复合体系的气相光电性能的基础上,我们扩展了复合体系的形式,制备了CdS/ZnO复合体系,并测试了CdS/ZnO复合体系与纯ZnO分别在紫外与白光照射下的气相光电性能。结果表明,在白光照射的条件下,纯ZnO也有较为明显的光电响应；同时,特别的是,在仅仅施加0.01V的偏压时,CdS/ZnO复合体系就能展现出非常明显的光电流响应,且其光电流幅值相对于纯ZnO而言提高了153倍。由于很多文献报道,ZnO基材料的光电性能的获取都是在几十伏的偏压下进行的,因此,该研究结果对于节约能源和实际应用有着非常重要的意义。因为纯ZnO在白光照射下,也有光电响应,且断光后,光电流不能较快地恢复到基线水平,所以这说明了断光后,ZnO内部还残余了大量的电子；通过分析,我们知道这是由于ZnO内部存在缺陷所导致的。那么,为了研究残余电子对光电流曲线的影响,最后,我们还分别测试了ZnO在紫外与白光照射下的光电流循环曲线。结果表明,紫外条件下,断光后每个循环对应的光电流都能够恢复到基线附近,且每个循环的光电流幅值能基本保持一致；而在白光条件下,出现了截然不同的现象,随着循环次数的增加,光电流不仅不能恢复到基线附近,而且光电流幅值随着循环次数的增加,也逐步增加。为了说明测试结果的差异性,我们定义了三个与光电流有关的参数,这些参数很好的说明了光电流产生量、被捕获的光电流量以及残余光电流量之间的关系。更多还原

【Abstract】 Energy shortage and environmental pollution have become the significant challenge for the global development, thus, the exploitation of new energy and protection of the environmental has become one of the most active research directions. With the irradiation of sunlight, using photocatalytic technology to degrade pollutants is an important pathway for solving both of the problems of energy shortage and environmental pollution. The basic principle of photocatalytic technology is as follows, the semiconductor as a photocatalyst can absorb solar energy to generate electron-hole pairs, which are with powerful redox ability, then, through a series of degradation reactions initiated by the electrons and holes, the harmful pollutants can be decomposed into harmless carbon dioxide and water.The photoelectric response process refers to the fundamental process that the semiconductor as photoelectric material can generate electrons and holes with the irradiation of a light source, and by application of a bias voltage, the photogenerated electrons can move directionally, which induced the forming of photocurrent in the external circuit. Taking into account the carrying out of the degradation experiments in laboratory always consumes tremendous amounts time and project cost but without substantial progress, and also taking into account that the photocatalytic process involves the generation and recombination of carriers within the semiconductor, thus, study of the photoelectric response process is of great significance for guiding of photocatalysts designment and exploring of the photocatalytic reaction mechanism. Therefore, in this paper, the research focuses on the photoelectric response in gas phase based on the metal oxide semiconductor materials and their composites, and then to investigate the fundamental process of the generation, separation, recombination and trapping of the carriers in semiconductor materials.This paper started from the mental oxide composite. On the basis of the equilateral ingredient triangle, a material library of the TiO2/WO3/MnO2composite material system was designed, which consisted of66ingredient points. To fabricate the66devices, the ball milling and the technology of screen printing were used. The photocurrent of each device was measured using a self-designed high-throughput screening system. The testing results showed that the largest photocurrent of the device under the irradiation of white light, ultraviolet, blue and green is the one when the mole ratio of TiO2/WO3/MnO2is2/8/0in the66ingredient points, this might be ascribed to the characteristic of WO3itself and the energy band matching type between TiO2and WO3, the staggered type of matched potentials effectively enhanced the charge carriers separation. For these devices in the TiO2corner, no obvious photoelectric response was observed under the irradiation of the four kinds of light source, it might be attributed to the small application of the bias voltage (0.2V), as we increased the bias voltage to10V, pure TiO2showed relatively obvious photoelectric response. While for these devices in the MnO2corner, no distinct photoelectric response was observed under any light source or at any bias voltage. Then this might be attributed to the phase transition from the MnO2phase to the Mn2O3phase during the sintering process. When the amount of Mn2O3is excessive, it might accelerate the recombination of the carriers, then, it resulted in the largely increased recombination rate and in turn it leaded to the photocurrent in the external circuit was not obvious.In view of TiO2showed no obvious photoelectric response by application of the low bias, thus, in order to further study the photoelectric properties of TiO2and its composite, we introduced CdS sensitizer to modify TiO2. Firstly, the pure TiO2device was fabricated by the technology of screen printing, and then the porous CdS/TiO2composite device was prepared by the successive ionic layer adsorption and reaction process. After that, we studied the photoelectric properties of TiO2device and CdS/TiO2composite device in the gas phase, the final results displayed that the CdS/TiO2composite showed an enhanced and excellent photoelectric properties either under the irradiation of ultraviolet or under the irradiation of white light in comparison with that of the pure TiO2device.After the study of the gas phase photoelectric properties of the composite system composed by the CdS sensitizer and the metal oxide semiconductor TiO2, the form of the composite system was extended, we prepared the CdS/ZnO composite system. The photoelectric properties of ZnO device and CdS/ZnO composite device were then tested under the irradiation of ultraviolet and white light in the gas phase, respectively. The results showed that the pure ZnO device displayed obvious photoelectric response under the irradiation of white light, in the meanwhile, the CdS/ZnO composite device exhibited an obvious photoelectric response and a typical photocurrent curve when applying a very low bias voltage of just0.01V, and the photocurrent amplitude of it is153times higher than that of the pure ZnO device. As a lot of literature reported that the obtaining of the photoelectric response of ZnO-based materials were often tested by application of tens of bias voltages, therefore, these results of this chapter has a very great significance for energy-saving and practical application.Owing to the pure ZnO device showed obvious photoelectric response under the irradiation of white light, and the photocurrent of it could not rapidly recover to the baseline levels after the light source was turned off, so all of these results illustrated that a large number of electrons was remained within ZnO when the light off. Through analysis, we knew that this phenomenon was caused by the defects within ZnO. Thus, to study the impact of the remained electrons to the photocurrent testing curves, we tested the photoelectric responses of ZnO under the irradiation of ultraviolet and white light in gas phase via the light-on and light-off measurement, respectively. The results showed that under the irradiation of ultraviolet, the photocurrent with almost the same amplitude can recover to the baseline levels. While for white light, distinctively different photocurrent curves were obtained, with the increasing of the cycle number, the photocurrent just partially recovered and it could not recover to the baseline levels when the light was off, what is more, the photocurrent amplitude also gradually increased with the cycle number increased. To illustrate the differences of the testing results, three different parameters related to photocurrent were defined. These defined parameters well illustrated the relationship among the amount of the generated photocurrent, the amount of the trapped photocurrent and the amount of the remained photocurrent.更多还原