Photocatalytic Activity Of Nanostructured TiO₂ Films Deposited By PLD

In today's China and throughout the world, energy and environmental issues in human development has become a major issue. Energy and environmental issues has become an important indication to measure the progress of science and technology, particularly for those promoting dynamical systems with energy consumption. How to improve the efficiency of energy use and reducing the pollution of the environment has become a research priority. And nano-oxide semiconductor embodied in the renewable energy and environmental protection, the outstanding characteristics has become a focus of research in the field of materials. Photocatalytic technology in the environmental governance has played a tremendous role.Titanium dioxide (TiO₂) is a well-known photocatalytic material and has received great attention as a photocatalyst since it is a wide-gap semiconductor and exhibits strong oxidation activity and hydrophilicity under UV irradiation. However, TiO₂ can only be activated by irradiating with ultraviolet (UV) light due to its high band gap energies. Therefore, only a small fraction (<4%) of the solar energy (<390nm) can be utilized in practical application. Therefore, the modification of TiO₂ to render its sensitivity to visible light became one of the most important goals to increase the utility of TiO₂. Many techniques have been proposed to achieve this purpose, such as reducing the particle size, surface photosensitization, metal ion doping, non-metallic ion doped and compound with narrow bandgap semiconductor.In this paper, an advanced and simple technique?Pusled laser deposition was used to deposite films on quzrtz glass substrates. Pusled Nd: YAG laser with a wavelength of 1064 nm was used. Different temperature and different gas pressure were used to deposite TiO₂ films. In order to improve absorption wave length of the solar spectrum and the low utilization rate of carrier, a series of experiments were carried out: under different substrate temperature TiO₂ films have been annealed in oxygen atmosphere mixed structural TiO₂ thin films with different content of rutile phase were obtained; in nitrogen atmosphere nitrogen-doped TiO₂ films with different N concentration were got through the ablation of TiO₂ Target; on the basis of the study of the best deposition process of ZnO thin film the TiO₂/ZnO composite film were deposited. The visible light photocatalytic activity of these films was discussed deeply and the main results are shown as follows:1. The films deposited at somewhat lower temperature (500°C) or lower oxygen pressure (2Pa) showed unclear grained structure and immature crystalline (with some anomalous structure); while the film deposited at high substrate temperature (800°C) or relatively high oxygen pressure (20 Pa) showed anisotropic growth or bigger grain sizes, respectively. The TiO₂ films deposited at 600～700°C and oxygen pressure of 5～10Pa observed homogeneous nanocrystalline grains structure, random or isotropic growth and good crystalline quality, and they therefore exhibited better photocatalytic efficiency of about 21%. 2. The measurement of the band gap energy from the optical transmittance spectra showed that the wavelength response range of TiO₂ films with mixed structures was shifted from UV (388 nm) to visible range (440 nm), which should lead to the improvement of the photocatalytic activity of the films.The difference in band gap energy of anatase and rutile structures of mixed TiO₂ film also decreases the electron and hole recombination rate and thereby increases the absorption of the surface organic or inorganic species of the photocatalytic process.The highest photocatalytic efficiency was obtained from the TiO₂ film with about 33% rutile phase.3. TiO₂-xNx film deposited at lower temperature of 600°C show typical anatase structure and uniform and regular surface morphology. The average grain size is 100 nm. As determined from x-ray photoelectron spectroscopy measurements the x values (nitrogen concentration) of the TiO₂-xNx films were 0, 0.567, 0.421 and 0.185, respectively. Because of more grain diffusion paths the highest nitrogen concentration was obtained from the film deposited at 600°C. All the TiO₂-xNx films have two deep levels at 1.0 eV～1.5 eV and 2.5 eV～2.8 eV below the conduction band estimated by UV-Vis Spectroscopy. The 1.0 eV～1.5 eV level is properly attributable to an O vacancy state. Additionally, the 2.5 eV～2.8 eV level is introduced by mixing the N₂p and O2p states.The TiO₂-xNx film deposited at 600°C with higher crystal quality and higher nitrogen concentration shows better visble-light photocatalytic activity.4. ZnO thin films with c-axis (002) orientation growth were formed on quartz glass substrate through pulsed laser deposition. The ZnO films obtained at different substrate temperatures have nanocrystalline structure with grain size in the range of about 28～35nm. The as-deposited films show ultraviolet emission and accompanied deep-level emission in PL spectra. However, the ZnO film deposited at substrate temperature 200°C exhibits strong UV emission with no deep-level emission due to its low intrinsic defects. While, comparaed with the films deposited at other temperature the film deposited at 200°C shown lower photocatalytic activity which corresponding to the lower defect concentration. The defect state can provide steps for the photocatalytic reaction. The TiO₂/ZnO film shown obvious c-axis orientation and nanostructure with average grain size of 80nm. The best photocatalytic effectivity was got from the film deposited at 150°C. By the analysis of the relationship of photocatalytic activity and photoluminescence spectrum it can be known that the weaker UV emission intensity and higher visible light emission intensity, the higher the separation rate of photo-induced charges, the higher the photocatalytic activity. Compared with the bare ZnO film the TiO₂/ZnO film has weaker photoluminescence property which due to the inport of the crystal defects during the composite process. The energy level for electron injection is decreased after TiO₂ film cover the surface of ZnO film, which increases the driving force for electron injection and hence reducerecombination between electrons and holes further increasing the photocatalytic efficiency.5. To sum up the above arguments, on the basis of the deposition of the anatase TiO₂ thin film, in order to improve the photocatalytic activity of the film three methods were used. Though every method has its own mechanism, all achived the expected aim. Remarkably, the N-doped TiO₂ film has the widest wavelength response range and the TiO₂ film with mixed structure has the higest photocatalytic activity.