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Preparation and visible light catalytic properties of doped titanium dioxide nanoparticle film and Mechanism of

Author: MiZuo
Tutor: WangPeiNan
School: Fudan University
Course: Optics
Keywords: titanium oxide nanoparticulate film nitrogen doping N:H-codoping visible-light photocatalysis pulsed laser deposition ion implantation calcination first-principles
CLC: O484
Type: PhD thesis
Year: 2007
Downloads: 976
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Abstract


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.

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CLC: > Mathematical sciences and chemical > Physics > Solid State Physics > Thin Film Physics
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