Doped Titanium Dioxide Nanoparticle Membrane And Its Visible Light Photocatalytic Properties And Mechanism

TiO₂ as a photocatalytic material with attractive characteristics of long-term stability and nontoxicity has been widely studied in the past years. However, TiO₂ with its absorption edge below 380 nm has photoactivity only under ultraviolet (UV) light. Thus, only a small portion of the solar energy can be utilized, which increases its application cost seriously. This is the main reason why TiO₂ has not been widely used commercially. How to effectively utilize sunlight as the light source is one of the important subjects for the wide application of TiO₂. In recent years, theoretical and experimental studies have indicated that using non-metal main group dopants, especially nitrogen dopant, can greatly enhance the photoactivity of TiO₂ in the visible spectral range. Hence, great efforts have been made for synthesizing the N-doped TiO₂.In this thesis, N-doped TiO₂ nanoparticulate films with high nitrogen dopant concentrations and high visible-light photocatalytic abilities were prepared by pulsed laser deposition (PLD), ion implantation and calcination, respectively. The films were characterized by atomic force microscope (AFM), surface profiler, Raman Spectroscopy, X-ray photoelectron Spectroscopy (XPS) and ultraviolet-visible (UV-Vis) absorption Spectroscopy. The photoabsorption and photocatalysis activities of these films in the UV and visible region were investigated. The effects of different N-doping and N:H co-doping structures on the photoactivities were studied. Moreover, the mechanism of visible-light photoactivity of N:H co-doped TiO₂ films was discussed on the basis of theoretical calculation. The results are as follows:1. Bare and N-doped TiO₂ nanoparticulate films were prepared by laser ablation of titanium target in O₂, N₂/O₂ and NH₃/N₂/O₂ atmospheres, respectively. It was found by spectral diagnostics that the dissociation of nitrogen molecules was enhanced when adding a small amount of ammonia into the nitrogen gas, which favored the incorporation of nitrogen into the TiO₂ matrix. All the prepared films were in anatase phase, and the size of the particles in the films was 20- 40 nm. The nitrogen concentrations were 2.0 % and 4.4 % for the films deposited in N₂/O₂ and NH₃/N₂/O₂, respectively. They both exhibited enhanced photocatalytic abilities in the visible-light region as compared with the undoped TiO₂, especially the one deposited in NH₃/N₂/O₂.2. The photocatalytic ability declined rapidly with the operating time. The method of mild-heating was applied for the first time for the regeneration of photocatalytic films. It was found that the photocatalytic ability of the used film was fully recovered by this treatment, which is of particular importance for the practical applications.3. N-doped TiO₂ films were prepared by implantation of low energy nitrogen ions into TiO₂ films. It was found that low energy implantation did not change the anatase phase of TiO₂ films. The ion bombardment reduced the particle size and removed some loosely bonded particles. The nitrogen species implanted in the films could be attributed to the interstitial nitrogen, and the dopant concentration was up to 3.4 %. The visible light photoactivities of the N-doped TiO₂ films were greatly improved as compared with the undoped TiO₂, indicating that the interstitial nitrogen could also enhance the visible-light photocatalytic ability.4. Calcination of TiO₂ nanoparticulate films under flowing N₂ and NH₃ were applied to prepare N-doped and N:H co-doped TiO₂ films, respectively. The calcination were carried out under high temperature for a short time and then under lower temperature for a long time to achieve high dopant concentrations and avoid the transformation of anatase phase. Characterizations of the produced films showed that the N-doping was more efficient when calcined in ammonia. With the same nitrogen dopant concentration, the films calcined in ammonia showed remarkable redshifts of the photoabsorption edges and higher visible-light photocatalysis efficiencies.5. Based on the first principles, the band structures and density of states of the bare, N-doped and N:H co-doped TiO₂ were calculated, respectively. The mechanism of the photocatalysis in the visible-light region was discussed. Isolated N 2p states were found just above the valence-band maximum of N-doped TiO₂, which might be the reason for the photocatalytic activity in the visible light region. However, for N:H co-doped TiO₂, hydrogen may contribute to the lowering of the energy levels of nitrogen, bringing the N states closer to the valence band, therefore enhancing the mixing of N 2p states with the O 2p states in the valence band, and leading to a real band gap narrowing and consequently a redshift of the optical absorption edge. The computational results agree with our experimental data.