Synthesis, Modification And Photocatlytic Properties Of Hollow TiO₂-Based Nanophotocatalyst

Due to its excellent and stable physical/chemical properties, titanium dioxide (TiO2) has many practical applications in a variety of fields including solar cells, photocatalysis, photonic crystals, gas sensors, self-cleaning devices and biomedicine. As to the photocatalytic application of TiO2-based materials, their properties are highly influenced by the crystal phase, band gap, grain size, surface area, and degree of crystallinity. Synthesis of small rutile TiO2 nanocrystals (NCs) with high surface of area and high degree of crystallinity is desirable for their prospective application. At the same time, creation of hollow mesoporous structures is another way to improve the activity of photocatalysts since the diffusion of reactants can be improved. To prepare high-performance hollow mesoporous TiO2 photocatalysts, we developed a ’deposition-etching-redeposition’strategy to synthesize TiO2 hollow spheres (h-TiO2) which imbedded in a thin SiO2 layer; On the basis of this synthesis, we also developed a facile approach to tune the thickness of h-TiO2 by temperature-controlled hydrothermal treatment; Moreover, we have in situ grown Au nanoparticles inside the TiO2 h-TiO2 and obtained a series of Au/TiO2 yolk@shell nanostructures with controllable core size. The content of this dissertation can be divided into three parts:1) In a microemulsion system, we prepared uniform organic-inorganic SiO2 hybrid templates by co-hydrolysis of organic silane and tetraethylorthosilicate. After a SiO2 layer was coated on the surface of the hybrid templates, we found that the templates could be spontaneous etched in solution and a few SiO2 was then re-deposited on the TiO2 surface to form silica-protect h-TiO2. Under the protection of SiO2, amorphous TiO2 shell could be crystallized by calcination at 900 ℃ under an air atmosphere. Since the thin SiO2 layer could also serve as barrier, small TiO2 NCs with a grain size of 5-8 nm and high degree of crystallinity were obtained. At the same time, it is interesting to find that the organic saline doped in the silica templates can simultaneously serve as nitrogen source to be doped in the TiO2 NCs, and the products show obvious light utilization in both UV and visible regions. The samples were characterized by TEM, SEM, XRD, XPS, FTIR spectrum and TGA etc. Excellent photocatalystic performance of the prepared h-TiO2 was justified by the degradation of RhB solution.2) When the silica-protected h-TiO2 (before calcination) were used as precursors, we found that the silica layer can be slowly etched by hot water, leading into the re-organization of the h-TiO2. As such, thin h-TiO2 with different thickness could be obtained. The samples were characterized by TEM, HTEM, XRD, and BET etc. A ’etching-reorganization’formation mechanism has been proposed. Photocatalytic properties of these samples were investigated by degradation of RhB dye and hydrogen production. The relationship between shell thickness and their photocatalytic properties of the h-TiO2were discussed.3) After the etching and re-deposition of organic-inorganic silica templates, the interior of TiO2 shells have plenty of residual amino groups (-NH2). These amino groups can act as reducing agent (e.g., reacting with HAuCl4) and in situ generate gold NCs inside the TiO2 shell. For growing a single Au NC per TiO2 particles, we optimized many influencing factors in the synthesis such as concentration of reagents, cultivation time in dark, ultrasonic treatment, and heating temperatures. Through tuning these parameters, we have obtained a series of Au/TiO2yolk@shell nanostructures with different size of Au core inside. By testing their performance on hydrogen generation, we demonstrated that the photocatalytic activity of Au/TiO2 was much higher than the h-TiO2. At the same time, the yield of hydrogen production was proportionally increased by the size of Au core inside.In summary, we have synthesized nitrogen doped h-TiO2 spheres, ultrathin shelled h-TiO2 spheres and Au/TiO2 yolk@shell nanostructures. These materials all showed improved photocatalytic activities compared with pure h-TiO2 spheres. Such improvement is originated from broadened light response, surface area, and efficient separation of electrons and holes, respectively. These fundamental researches may contribute to the development and practical applications of TiO2-based nanophotacatalyst in the near future.