Research On Titanium-based And Niobium-based Semiconductor Photocatalysts: Synthesis And Photocatalytic Properties

With the excessive exploitation of natural resources and grave pollution of environment, energy problem and environment problem have become the world's major problems. New ideas and techniques came up and researchers tried to solve these problems from many ways.The mechanism of photocatalysis is that after excitated by light irradiating the photo-generated electrons and holes can be formed in the semiconductor conduction and valence band, respectively. The formed electron-hole pairs, after being separated and transmitted to the surface area of the photocatalyst, will participate in the photodegradtion process of water or organic compounds. The photocatalytic process can break water up to oxygen and hydrogen, or mineralize the hazardous organic chemicals to carbon dioxide, water and simple mineral acids. thus one of the major advantages of the photocatalytic process over existing technologies is that there is no further requirement for secondary disposal methods. Moreover, photocatalysts can be reused or recycled, which means the process can be operated in a low cost. Photocatalysis, as one of the novel green process to solve both of energy problem and environment problem, has gain more and more attentions these days.The preparation of photocatalysts with high photocatalytic properties is most important in photocatalysis technology. An ideal photocataylst should be stable inexpensive, non-toxic, and highly photoactive and good visible light absorptive. TiO₂ is known as the most extensively and most effective photocatalysts. However, because of the size of its band gap, TiO₂ is effective only under ultraviolet irradiation. Sunlight is comprised of less than 4% ultraviolet light, so it can not make full use of the solar energy, which confined its commercial application. Thus it is important to develop new photocatalysts to extend the absorbed wavelength range into the visible region, where less expensive light sources exist.In this dissertation, we discussed the microwave hydrothermal synthesis of BaTiO₃ and it's Nb doping, niobates and niobate-titanates synthesis by citrate sol-gel combustion method, and their photocatalytic properties by photodegration of methylene blue. The following is the content of this paper and the results we've got.In chapter 1 we briefly introduced the research background, the context of photocatalysis, the type and principle of photocatalytic reaction, appliance and changing quality of photocatalysts and research review of titanates and niobates. On the base of these theories, we brought forward our research ideas and research plan.In chapter 2 we introduced the raw material, instruments charaterzation methods and photocatalysis testing device used in this dissert.In chapter 3 we synthesized BaTiO₃ powers through microwave assisted hydrothermal method. We studied the crystal type of the as-made BaTiC>3 power, proposed that the BaTiO₃ synthesized by microwave assisted hydrothermal method hase the structure of pseudo-tetragonal phase. And because of its large energy gap and high dielectric constant, pure BaTiO₃ showed low photocatalytic property. After doping Nb element into BaTiO₃ nanocrystal, we saw a lifting of photocatlatic efficiency. An apparent reaction constant- doping constration curve was drawn and the best doping constration was about 0.1% mole ratio.In chapter 4 we synthesized MNb₂O₆ (M=Zn, Mg, Ni, Co, Fe) photocatalyst by citrate sol-gel combustion method. The basic shape of them were laminated structure agglomerated by smaller particles with 30⁴0nm in diameter. We studied the photoabsorption and photocatlytic efficiency of them and found that NiNb₂O₆ showed the best efficency with MgNb₂O₆ and ZnNb₂O₆ showed less efficiency. Fe and Co niobate showed good photoabsorption but low photocatlytic efficiency, we considered that complicated energy level structure affected their performance.In chapter 5 we prepared some divalent metal and trivalent mental niobate-titanate through citrate sol-gel combustion method. We studied the relationship between their photoabsorbance and photocatalytic efficiency, and a preliminary explanation was given.In the last chapter, a concise summary of the contents was given and some advices were proposed.