Novel Preparation Methods, Structural Characterization And Visible Light Photocatalytic Performance Of Non-metal Doped TiO₂ Based Nanomaterials In Gas-phase Toluene Degradation

In recent years, environmental pollution problems caused by volatile organic compounds (VOCs) have received more and more public attention. Visible light induced photocatalytic degradation of VOCs over nonmetal doped TiO2 is a promising and innovative green purification technology. However, the effects of nonmetal doping on the TiO2 visible light photocatalytic mechanism is not clarified. The preparation technology is tedious for nonmetal doped TiO2. The visible light photocatalytic performance needs to be further enhanced. Considering the unsolved sciencetific and technological problems of nonmetal doped TiO2 for future application, valuable explorations have been carried out on visible light photocatalytic mechanism, novel preparation methods of visible light photocatalytic materials, and surface modification of nonmetal doped TiO2 with transition metals to enhance the visible light photocatalytic performance in this thesis. The study provides insights into the visible light photocatalytic mechanism, novel preparation methods for nonmetal doped TiO2 photocatalytic materials and theoretical support for the application of nonmetal doped TiO2.In order to clarify the mechanism of visible light photocatalytic mechanism over nonmetal-doped TiO2, N-doped TiO2 were prepared by facile TiN oxidation method. Based on the evidence obtained from VB-XPS and PL, a physical model was constructed to describe the photo-generated electron transfer process over N-doped TiO2 under visible light irradiation. This process can be described as the following steps:(1) generation of electrons from the N- impurity level to the conduction band under visible light irradiation, (2) trapping of exited electrons by the level of sub-band (OV), (3) recombination of the photo-induced electrons with the holes in the N impurity level to give rise to PL signal. Multi-type N-doped TiO2 was prepared by thermal decomposition of titanium hydroxide and urea mixtures at 400℃. The doped N coexisted in multi-form of substitutional type (N-Ti-O and Ti-O-N) and interstitial type (π* character NO). Substitutional N state (Ns) and interstitial N state (Ni are 0.14 and 0.73 eV above the top of the valence band respectively. A physical model of band structure was constructed to elucidate the role of different types of nitrogen in visible light photocatalysis. The model can be applied to understand visible light induced photocatalysis mechanism over other nonmetal doped TiO2. The visible light induced photocatalytic activity of the as-prepared TiO2-Nw sample was much higher than those of TiO2-Nd and TiO2-Nh samples and Degussa P25. The high activity can be ascribed to strong absorption in the visible light region, good crystallization, large surface hydroxyl group and enhanced separation of photo-induced carriers.A novel one-step green synthetic approach was developed to prepare highly active mesoporous C-doped TiO2 in order solve the problems in the preparation process of nonmetal doped TiO2. Mesoporous C-doped TiO2 with high visible light photocatalytic activity was synthesized by a single hydrothermal process using Ti(SO4)2 and glucose as precursors. This facile method is free of using expensive or unstable precursors and production of undesirable byproducts during synthesis process. Carbon is doped into the TiO2 lattice in the forms of substitutional carbon for oxygen sites. The observed new electronic states above valence band edge are directly responsible for the electronic origin of band gap narrowing and visible light photoactivity of the C-doped TiO2. The green synthesized C-doped TiO2 exhibit efficient photocatalytic activity in degradation of toluene in gas phase under visible light irradiation with respect to P25 and C-doped TiO2 prepared by solid state method. Mesoporous C-doped TiO2 was synthesized by the green process using sucrose as carbon doping precursor. The microstructure and visible light photocatalytic activity were influenced significantly by post thermal treatment. After treatment at 200℃, the content of doped carbon was increased, the intensity of visible light absorption was increased, the separation of photo-induced electron-pairs was enhanced, and thus the activity was improved. The visible light activity of C-TiO2-200 was nearly 3 times higher than that of C-TiO2. This one step green method provides an effective approach for future industrial applications in pollution control and solar energy conversion owing to its low cost and easy scaling up.Considering the special structural properties of one-dimensional titanate or TiO2, a novel preparation method for nonmetal doped TiO2 was developed based on the nano-confinement effect of titanate nanotubes (TNTs). TNTs were prepared using an alkaline hydrothermal process. By utilizing the hollow layered structure of TNTs and ethanol as nonmetal doping source, novel carbon doped TiO2 nanotubes, nanowires and nanorods with visible light activity were fabricated by calcining a complex of ethanol and TNTs in inert N2 atmosphere for the first time. Using thiourea as nonmetal precursor, novel two-path way one-dimensional C, N and S co-doped TiO2 with high visible light photocatalytic activity was prepared by treating the complex of H-titanate nanotubes and thiourea in air. The nanoconfinement of ethanol and thiourea in the inner space of TNTs played a crucial role in the formation of various nonmetal doped TiO2 with 1D nanostructure. This novel fabrication approach is general and can provide new perspective for designing nonmetal doped nanostructured TiO2 photocatalytic materials and modifying various nanotube materials.From the viewpoint of application, it is necessary to further improve the visible light photocatalytic performance of nonmetal doped TiO2. C-doped and N-doped TiO2 was surface modified with transition metals by incipient wetness impregnation method. The mechanisms of the promotion effect of transition metal surface modification on visible light photocatalytic performance of nonmetal doped TiO2 were proposed. N-doped TiO2 was surface modified by Fe3+/Fe2+. The Fe3+/Fe2+ redox cycle process improved the separation rate of electrons/holes pairs effectively and enhanced stabilization of nitrogen in the N-TiO2 lattice, which could improve the photocatalytic performance and photochemical stability of N-doped TiO2 simultaneously. C-doped TiO2 was surface modified with V2O5 based on the principle of composite semiconductor. The formation of heterojunction structure between C-TiO2 and V2O5 could greatly promote the separation of electrons-holes pairs. Thus the composite photocatalyst exhibited enhanced visible light photocatalytic activity. Surface modification of nonmetal doped TiO2 with transition metals could improve the visible light photocatalytic activity significantly, which provided solid basis for practical application of nonmetal doped TiO2.C-doped TiO2 visible light photocatalyst was modified with Pt species by impregnation-calcination method, which resulted in the formation of novel three-component Pt/C-TiO2/PtCl4 nanojunction system. The visible light photocatalytic mechanism over this three-component photocatalyst was proposed. This system increased utilization of visible light and enhanced charge separation and transfer process, thus exhibiting 5-fold higher visible activity than that of naked C-doped TiO2.This result could provide new insight for design and synthesis of highly active multi-component visible light photocatalyst.