Study On Photoelectrical Properties In Gas-phase And Characterization Of Metal Oxide Semiconductor And Their Composites

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.