The Preparation And Photocatalytic Performance Of Highly Active TiO₂ Based Photocatalysts With Nano-micro Structure

TiO₂ photocatalysis has attracted extensive attention as a promising technique for the degradation of inorganic and organic pollutants in water and air. However, TiO₂ photocatalysts have two inherent and significant drawbacks; the one is, the TiO₂ photocatalysts owned wide band-gap can not use solar irradiation or interior lighting efficiently, the other is, the photogenerated charge carriers (hole-electron pairs) can recombine. Therefore, to achieve the practical application of TiO₂, it is important to enhance the absorption of visible light and decrease the recombination of photogenerated carriers. Furthermore, the mechanisms of photocatalysis under visible light are still not clear, there is no suitable theoretical guidance for synthesis of high efficient photocatalyst under visible light.The high efficient TiO₂ photocatalysts with nano-micro structures were fabricated by combining the traditional doping and coupling methods with the novel nanotechnology. The photodegradation of 4-chlorophenol was employed to evaluate the photocatalytic activities of the photocatalysts. The reasons of the enhanced photocatalytic activities had been investigated with the help of X-ray diffraction (XRD), Diffuse reflectance UV-Vis spectra, X-ray photoelectron spectra (XPS), surface photovoltage spectra (SPS) and so on.The summary of the work are as follows: 1. Samples of pure TiO₂ and TiO₂-xBx were obtained with Ti (OC4H9)4 by sol-gel methods. Compared with the pure TiO₂, the samples of TiO₂-xBx exhibited higher photocatalytic activity under both ultraviolet and visible light irradiation. The reasons were discussed, based on the characterization with XRD, RAMAN, XPS, UV-Vis and PL spectra. The lattice O is substituted by B, and the orbital of B2p is mixed with O₂p orbital, which is responsible for the band gap narrowing. The raise of the photocatalytic activity is chiefly because B doping enhanced the absorption of visible light, and promoted the separation of photogenerated carriers.2. In this work, sol-gel process was employed to prepare TiO₂ and Nickel-doped TiO₂ powder .The photodegradation of 4-chlorophenol was employed to evaluate the photocatalytic activities of the catalysts. Compared to TiO₂, Nickel -doped TiO₂ exhibited improved activity for degradation of 4-chlorophenol under both ultraviolet and visible light irradiation. The properties of these catalysts were investigated with the help of XRD, UV-Vis, IR, Raman spectra, SPS and other analytic methods. We found that Ni2+ was chemisorbed on the surface of TiO₂ and formed the ONiOO which introduced the energy level of surface states 2.84 eV above the valence band. This energy level not only generated response to visible light but also promoted the separation of photogenerated carriers. Thus, the activity under both ultraviolet and visible light irradiation was increased.3. Indium doped TiO₂ nanotubes were fabricated by a two-step pre-doping method. It was found that the TiO₂ nanotubes with indium doped content at 3% exhibited the best photocatalytic activity being over twice as much as that of pure TiO₂ nanotubes on the photocatalytic degradation of 4-chlorophenol under visible light. Based on XRD, XPS and SPS, it can be inferred that when the doped content is low, the indium ion substitutes Ti into the TiO₂ lattice forming the InxTi1-xO₂ structure and the In doped energy-band narrows the band gap by mixing with Ti 3d states. With increasing the doped content, In2O3 comes up on the surface of InxTi1-xO₂ nanotubes to form the InxTi1-xO₂/In2O3 composite structure. This composite structure efficiently enhances the visible light response, promotes photogenerated carriers separation and increases the utilization of photogenerated carriers in photocatalytic reactions at the solid/liquid interface, resulting in the higher photocatalytic activity under visible light.4. A composite photocatalyst with novel nanostructure was prepared by deposition of Au nanoparticles on needles of sea-urchin-like TiO₂ (s-TiO₂). The Au/s-TiO₂ composite photocatalyst presented higher photocatalytic activity than pure s-TiO₂ under ultraviolet irradiation. IR spectra and XPS analysis suggested that the two components, Au and TiO₂, in the composite might be linked by–O–C(=O)– groups. The transfer process of photogenerated carriers at the interface of s-TiO₂ and Au nanoparticles and the shift of Fermi level of the composite were inferred by SPS. The improved photocatalytic activity of the composite photocatalyst was attributed to the promoted utilization efficiency of the photogenerated carriers upon the deposition of the Au nanoparticles on the needles of s-TiO₂.5. The SnO₂/TiO₂-N composite photocatalysts were prepared by depositing SnO₂ onto nitrogen doped TiO₂ nanoparticles. The composite catalysts present much higher photocatalytic activity than TiO₂ and nitrogen-doped TiO₂ under both ultraviolet and visible light irradiation. Diffuse reflectance UV-Vis spectra, XPS analysis and IR spectra show that the nitrogen species existed with two states: the doped N with responsibility for visible light response, the other N specie linking to Sn atom without contribution to visible light response. The enhanced visible light response caused by doped N and the promoted separation of photogenerated carrier driven by coupling with SnO₂ will be attributed to the higher light photocatalytic activity.