Modification Of TiO₂ By Doping And Study On Its Photocatalysis

Since Fujishma found out that the hydrogen can be generated from water by TiO₂ electrode in 1972, many efforts have been done to improve the photocatalytic performance of TiO₂. Comparing to other semiconductor photocatalyst, TiO₂ shows many advantages, such as high photocatalytic activity, chemical inertness, strong oxidizing power and producting only CO₂ and H₂O. In the recent decade years, TiO₂ have been studied extensively in the photocatalytic field. However, it is still not applied in practice due to its low light quantum yield, responding only to UV light thus can not utilize solar energy, difficulty to recycle photocatalyst. Many efforts have been done to improve the light quantum yield of TiO₂ and extent its response to visible light and found doping is a good method. People have doped TiO₂ with Fe and W and found its photocatalytic performance can be improved, while doping with N and F extent its response to visible light. Recent years, investigator found one-dimension perovskite oxide, such as titanate nanowires, had high photocatalytic activity and light quantum yield. Moreover, it is easy to improve its photocatalytic activity by embellishing other ions due to its distinct spatial structure. People develop many methods to recycle TiO₂ catalyst, such as loading TiO₂ on mineral, preparing TiO₂ thin film and so on. Though these methods have solved the problem, its photocatalytic activity is still not high enough to apply in practice. Moreover, it spends too much time on recycling TiO₂ catalyst. In this thesis, to resolve the problems stated above, works were focused on improving the photocatalytic activity of TiO₂ under visible light irradiation, studying the performance of new catalyst with one-dimension structure and immobilization of TiO₂.In part 1, TiO₂ catalyst has been reviewed, including the principles and mechanisms of photocatalysis, environmental applications, the lattice and electronic structure of TiO₂, its surface modification and immobilization of photocatalylsts.In part 2, based on a great deal references, we put forward a method to improve the photocatalytic activity of TiO₂ and realize its response to visible light. We prepare F-, LiF- and NaF-doped TiO₂ by sol-gel method and characterize with XRD, XPS, TG-DTA, UV-Vis, PL and so on. The effects of concentration of dopant, annealing temperature, pH value of initial feed liquid and different light source on photocatalytic performance of TiO₂ are investigated. The roles of F and Li ions in the co-doped system were also studied carefully. The conclusions can be drawn as the follows:(1) In the LiF-, NaF- and F-doped systems, the photocatalytic activity of LiF-doped TiO₂ is the highest, which is 6 times higher than that of undoped TiO₂. The second is NaF-doped system, and the third is F-doped system, which improve the photocatalytic activity of TiO₂ about 3 and 1.5 times, respectively. The experiment of degradation methenyl chloride (CHCl₃) under visible light irradiation indicates that the absorption of visible light by catalyst cause the catalytic reaction, while sensibilized reaction of methylene blue (MB) make the assistant action. The experiment of degradation MB under UV light irradiation indicates that doping TiO₂ with LiF doesn't decrease the photocatalytic activity under UV light irradiation.(2) In doping system, F ions make the roles as follows: (a) F ions can react with Ti⁴⁺ and form Ti-F ligand, which will hinder the transformation from helix-chain of antase TiO₆ octahedra to linear-chain of rutile TiO₆ octahedra thus inhibit the formation of rutile TiO₂. (b) F ions can break the Ti-OH bond and form Ti-F bond, which decrease the concentration of O_oh on the surface of TiO₂. (c) F ions substitute the lattice oxygen and form Ti³⁺ defect, which can effectively inhibit the recombination of electrons and holes. Li ions also make the roles as follows: (a) Because the radius of Li ions is small (about 0.68 A), it can easily enter into the interspaces of TiO₂ crystal lattice and augment the lattice constant thus accelerate the formation of rutile TiO₂. (b) The presence of Li ions in the TiO₂ lattice causes the increase of concentration of O_oh. (c) Li ions can act as the recombination center of electrons and holes. In doping system, the roles of Na ions is similar to that of Li ions, but the action effect of the former is worse than that of the latter due to its large ionic radius (the radius of Na ions is about 0.97(?)).(3) In the F and Li co-doped system, the photocatalytic activity of TiO₂ is improved greatly due to the Synergistic reaction of two doping elements. (a) Co-doped system accelerates the formation of O_oh on the surface of TiO₂. As we know, the more the surface OH is, the higher the photocatalytic activity is. So the co-doped system has the higher activity. (b) Co-doped system improves the transition probability of photo generation electron of TiO₂ from valance band to conduction band and extent the absorption region of TiO₂ to visible light. (c) Co-doped system improves the surface oxygen vacancy of TiO₂. The more the surface oxygen vacancy is, the more the chemical reaction points are, and thus the higher the photocatalytic active is. (d) Co-doped system affects the crystal structure obviously. Li ions act as the main roles, thus co-doped system accelerate the phase transition of TiO₂ from anatase to rutile. There is similar Synergistic reaction in the F and Na co-doped system. But the effect is worse than that of the F and Li co-doped system due to the large radius of Na ions.(4) When the content of F ions is less than that of Li ions in the F/Li co-doped system, Li ions will act as the recombination center of photo generation electrons and holes thus decrease the photocatalytic activity greatly. When the content of F ions is higher than that of Li ions, the photocatalytic activity of catalyst will be improved greatly. However, if the content of F ions is too high in system, it will excessively consume surface O_oh thus decrease the photocatalytic activity. Obviously, there is an optimal proportion about the two elements in system. In our case the-optimal proportion is 8:8 to 18:8.In part 3, considering the distinct spatial structure of one-dimension structure material and the photocatalytic activity itself, which is easy to improve the photocatalytic activity by ions embellishment, we prepare the TiO₂ nanowires by alumina template method and hydrothermal method, respectively. The crystal phase and the pattern of nanowires are characterized by XRD and FE-SEM. Because the yield of nanowires prepared by alumina template method is too low, the photocatalytic experiments are performed with the TiO₂ nanowires prepared by hydrothermal method. The results indicate that the photocatalytic activity of TiO₂ nanowires can be improved by WO3 embellishment. We also discuss the formation mechanism of TiO₂ nanowires prepared by two methods, and prepared the high monodisperse TiO₂ nano-particle by hydrothermal method and characterize its performance.In part 4, we combine the photocatalysis technique and membrane separation technique and solve the problem of recycling catalyst. The photocatalytic mineral membrane has many advantages, such as high photocatalytic activity, no membrane pollution; no need of sedimentation and filtration and operation continuously, thus it is practical method in water treatment. In this part we study mainly focusing on the catalytic performance of photocatalytic mineral membrane. The results indicate that the photocatalytic activity of membrane is higher than that of TiO₂ suspension due to the porosity, large specific surface area and strong adsorbability of mineral. The recycle experiments suggest that the photocatalytic mineral membrane can be used continuously without any treatment.In part 5, we make conclusions about this thesis and propose the application prospect of TiO₂. We believe, in the future, that the application of TiO₂ catalysts will become a new technical industry in environmental sciences.