Photoelectric Properties And Photocatalytic Activity Of N-doped Tio 2 Nanomaterials

Development and utilization of solar energy is a major issue which is facing the world today, and photocatalysis using solar energy has attracted an enormous amount of research interest. The photocatalytic technology has been widely applied in the field of environmental pollution control because of cheap, environmentally friendly, and high stability advantages. Semiconductor Photocatalysis is one of the most active areas of research in recent years at home and abroad, especially with the rapid development of nanotechnology, photocatalytic nano-materials can break down environmental pollutants, and no secondary pollution. Therefore, it is meaningful to study the nano- photocatalyst for the fundamental solution to environmental problems. Tiypically, the photocatalytic process includes three steps, (1) photocatalytic materials absorb photoenergy (hν> Eg), (2) photogenerated carriers (electron, hole) migration to the catalyst surface under the excitation, (3) redox reactions occurred on the catalyst surface. In this three-step process, the important factors which may affect the quantum efficiency of photocatalytic are: the separation efficiency and recombination efficiency of photogenerated carriers, the carrier mobility of the photogenerated carriers that migrate to the surface and so on. Therefore, it is essential to study the intrinsic relationship between photoinduced charge behavior and photocatalytic activity of nano-materials for the development and application of photocatalytic materials. The behaviors of the photoinduced charges at the surface and interface of the photoactive systems are important to explore the basis of the photocatalytic mechanism, and further optimize the performance of photocatalytic system.TiO₂ is the tradition photocatalytic nanomaterials, but its application is limited by the dissatisfactory quantum efficiency, the neglectable utilization of solar energy and other shortcomings. So it is meaningful to develop the nano-structured photocatalytic materials which can use sunlight efficiently. In recent years, many studies have found that non-metal-doped TiO₂, especially the N-doped TiO₂, could expand the range of the response spectrum, thereby increasing the utilization of solar energy. Therefore, it is essential to study the N-doped TiO₂ photocatalytic material, and we can better understand the new generation of nano-structured materials, and the photocatalytic mechanism.In this thesis, we mainly studied the photoelectric properties of N doped TiO₂ nano-materials, N-TiO₂ / TiO₂ composite materials, TiO₂ with mixed crystal structure by the surface photovoltage technique, transient photovoltaic technology and Kelvin probe technique, and the relationship between the photocatalytic activity and the behaviors of photoinduced charges at the interface and surface of nano-materials was also discussed. Innovative achievements as following:1. Nitrogen-doped TiO₂ (N-TiO₂) photocatalysts with different N doping were successfully synthesized by hydrothermal method using urea as the nitrogen source. The samples were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), X-ray photoelectron spectroscopy (XPS) and photo luminescence spectroscopy (PL). The photodegradation of Rhodamine B (RhB) and Methyl Orange (MO) solution was used to evaluate the photocatalytic activity of the catalysts under UV and visible light irradiation. The surface photovoltage (SPV) and transient photovoltage (TPV) techniques were applied to investigate the separation and transport mechanism of the photo-generated charge carriers of N-doped TiO₂ nanoparticles. The relationship between the photo-generated charge carriers and the photocatalytic activity was also discussed. The results show that the SPV threshold values were shifted to the visible region, and stronger photovoltaic response in the visible region were observed with the increase of N doping. Simultaneously, it is found that the maximum values of TPV response time are different for N-TiO₂. These results indicate that the photo-induced charge carriers are separated efficiently with appropriate amount of N doping, and accordingly the transmission time is extended, which imply the lifetime of photo-induced charge carriers is increased. While an excessive amount of N act as the recombination centers for the photo-induced electrons and holes, they reduce the photocatalytic activity though they contribute to the visible light absorbance.2. Based on N doped TiO₂ nanoparticles, we prepared the N-TiO₂ / TiO₂ composite nanoparticles with different mass ratio of N-TiO₂ and TiO₂. The N-TiO₂ nanoparticles were coated with TiO₂ which had the heterogeneous structure. And we tested the photocatalytic activity by degrading methyl orange and methylene blue. The results showed that under both the UV and visible light irradiation, the photocatalytic activity of the composite powders (the mass ratio of N-TiO₂ and TiO₂ is 8:2) is the best. Kelvin probe results indicated that the surface work function of N-TiO₂ was less than that of TiO₂, the interface between N-TiO₂ and TiO₂ could be formed and the photo-electrons might transfer from TiO₂ to N-TiO₂. The SPV and TPV results revealed the behavior of photo-generated charges, we could observe that the photo-generated charges were easier to separate and difficult to recombination under the hetero-interface in the N-TiO₂ / TiO₂ composite nanoparticles.3. Through the N-TiO₂ / TiO₂ composite nanoparticles study, we found that the nanoparticles with heterogeneous structure have better photocatalytic activity, so we studied the mixed crystal TiO₂ (anatase / rutile)nanopowders. We prepared the mixed crystal TiO₂ (anatase / rutile) nanoparticles via hydrolyze method. The influence of the interface junction between anatase and rutile TiO₂ on the photogenerated charge carriers properties were studied by Kelvin probe (KP), surface photovoltage (SPV) and transient photovoltage (TPV) techniques. The nanopowders of anatase, rutile and mixed-phase TiO₂ were synthesized through hydrolysis method. The prepared nanoparticles were well characterized by XRD, TEM, HRTEM, and UV-vis diffuse reflectance spectroscopy. And the photocatalytic activities of these samples were evaluated on the degradation of RhB under UV-light irradiation. The KP results revealed that the difference of the surface work function between anatase and rutile may cause the built-in field at the interface in the mixed-phase TiO₂. The formation of the interface junction was further revealed through the red shift of the SPV response peak and the time retardation of the TPV response in the mixed-phase TiO₂. The relationship between enhanced photocatalytic activity of mixed-phase TiO₂ and the characteristics of photogenerated charge carriers was also discussed.