Doping, Composite Of TiO₂ Thin Films And Their Structure And Properties

Titanium dioxide (TiO₂) is one of the most promising photocatalysts due to its chemical inertness, non-toxic nature, low cost, and has been studied for decades. Potential applications, such as anti-fouling, deodorizing, sterilizing, antifogging, self-cleaning, etc. are in prospect.Recently, TiO₂ has received a great deal of attention since a series of papers were published in Science journal.Using atmospheric pressure chemical vapor deposition (APCVD) and dielectric barrier discharge enhanced chemical vapor deposition (DBD-CVD), we fabricated TiO₂ thin films. APCVD method can coat a moving float ribbon yet withstand the harsh environments associated with manufacturing float ribbon. Hence, fabrication of TiO₂ films with large area, low cost, high efficiency and good quality can be expected with APCVD compatible with the float glass product line. DBD-CVD can produce plasma with high energy under high pressure without fear of forming an electric arc, and produce different kinds of discharges to control the microstructure of films. In this case, by using DBD-CVD method, the photocatalysis of TiO₂ film can be further improved.We systematically studied the effect of fabrication parameters, doping and composition on the structure and properties of TiO₂ thin films. By microstructure controlling, N doping and two semiconductor layers compositing, we modified photocatalysis and hydrophilicity properties of TiO₂. The main works and results are as following:1. Studies of structure and properties of intrinsic TiO₂Using DBD-CVD technique, we fabricated the intrinsic TiO₂, and systematically studied the effect of substrate temperature, applied voltage and gas pressure on the structure and properties of TiO₂.With the increase in substrate temperature, photocatalytic activity and hydrophilicity of TiO₂ thin films increase due to the increase of anatase phase in the films.With the increase of applied voltage, the energy of plasma increases, and photocatalytic activity and hydrophilicity of TiO₂ thin films improve due to the increase in anatase phase and surface area.With the increase in gas pressure, TiO₂ films show increase in degree of orientation, TiO₂ films deposited under low gas pressure show no preferred orientation, while TiO₂ films deposited under atmosphere pressure show preferred orientation, and exhibit columnar-like structure. The columnar-like structure with preferred orientation exhibits higher photocatalytic efficiency, since the columnar structure has larger surface area. However, it contributes little to the improvement of hydrophilicity.2. Studies of structure and properties of N-doped TiO₂Using NH₃ or N₂O as N-doping source, we fabricated N-doped TiO₂ films both by APCVD and DBD-CVD, and systematically studied the effect of N-doping on the structure and properties of TiO₂.Compared with TiO₂ films deposited by APCVD, those deposited by DBD-CVD have higher degree of crystallinity due to plasma with high energy, and flux of N-doping sources change the growth orientation, and affect the surface microstructure of thin films. Using APCVD, flux of N₂O accelerates anatase-rutile transformation through O vacancies formation; while flux of NH₃ in the processing introduces a new phase of Ti₄O₇ into TiO₂ thin films, which inhibits the anatase-rutile transformation.Compared with TiO₂ films deposited by APCVD, those deposited by DBD-CVD have more Ti-N concentration and narrower band gap. Using NH₃ (25 sccm) as N-doping source, TiO₂ film deposited by DBD-CVD has the narrowest band gap (2.64eV).Compared to the pure TiO₂, the N-doped TiO₂ films deposited both by APCVD and DBD-CVD show an improvement in visible-light-induced photocatalysis and similar photocatalytic ability in UV light. Using NH₃ (25 sccm) as N-doping source, TiO₂ film deposited by DBD-CVD has the best photocatalysis both in visible and UV light due to its high degree of crystallinity, orientation in (200) plane, tetragonal crystal shape and narrow band gap. 3. Studies of structure and properties of composite systemSnO₂:F/TiO₂ composite film system, which combines low-e and self-cleaning properties, was designed and prepared. It consists of a two-layer coating of SnO₂:F as an inner layer and TiO₂ as an outer layer. Compared to the pure TiO₂, composite films show higher photocatalytic activity and hydrophilicity, which may be due to the interfacial electron transfer from TiO₂ to SnO₂. With the thickness of TiO₂ films increasing from 25 to 590 nm, the photocatalytic ability and hydrophilicity of the samples increase initially and then show a slight decrease. The composite film with 150-nm-thick TiO₂ shows the best photocatalytic activity. On the other hand, the reflectance in far-range infrared irradiation of composite films decreases with the increase in the thickness of TiO₂ films. The reflectance in far-range infrared region is mostly due to the low-e property of SnO₂:F inner layer.a-Si: H/TiO₂ composite film system was designed and fabricated. Firstly, a-Si: H films were prepared by DBD-CVD at room temperature. It is found that applied voltage is the key influence factor. The increase of applied voltage (U_p), firstly, can increase the deposition rate of the thin films. When U_p =12kV, the deposition rate can reach to 0.34nm/s, and the adherence between the films and the substrates is good. Secondly, the increase of Up may increase the Si-Si network order in DBD-CVD deposited a-Si: H films. Finally, the increase of Up can indroduce the hydrogen bonding configurations into the films, which are di-hydrogen (SiH₂) bonded species or (SiH₂)_n complexes.One the base of fabrication of a-Si: H films, we fabricated a-Si: H/TiO₂ composite film system of a-Si: H as an inner layer and TiO₂ as an outer layer. Single-layer TiO₂ shows orientation in (200) plane with tetragonal crystal shape, while TiO₂ deposited on a-Si:H shows orientation in (211) plane with flake crystal shape. Compared to thesingle-layer TiO₂ film, a-Si: H/TiO₂ composite film shows superior photocatalysis and hydrophiliciy in visible light due to the increase in absorptance of visible light, and shows inferior photocatlytic activity under UV light. 4. Studies of mechanism of influence factors on photocatalysisMechanism of influence factors on photocatalsis was systematically studied, including surface microstructure, doping and two semiconductor layers composition.Crystal cell structrue models were proposed to explain the formation of crystal grains with different shape. When the surfaces of TiO₂ anatase crystal cell are dominated by {211} facets, the shape of crystal cell is dominated by octahedron; when the surfaces of crystal cell are dominated by {211} and {200}, the shape of crystal cell is dominated by truncated octahedron; when the surfaces of crystal cell are dominated by {200} facets, the shape of crystal is dominated by tetragonal. The shape of crystal grains is depended on the shape of crystal cell, and affects the surface microstructure and photocatalysis of TiO₂ films. With the increase in crystal grain size, the photocatalysis of TiO₂ increases. When films show orientation growth, the TiO₂ films have better photocatalysis, especially in plane (200). Furthermore, TiO₂ films with columnar-like structure, have large surface area, show further improve in photocatalytic activity.We calculated the density of states of substitutional doping of N in the TiO₂ anatase crystal. The doped nitrogen species give rise to a midgap level slightly above the top of the valence band, which can utilize the photo with lower energy, and enhance the photocatalysis under visible light.The mechanism of free carrier separation model in SnO₂:F/TiO₂ and a-Si: H/TiO₂ composite films was studied. The separation of holes and electrons in composite films explain the higher photocatalytic activity and boarder photo-response region.