Study On The Preparation And Photocatalytic Activity Of Nano-TiO₂ Under Visible Light Irradiation

Nano-TiO₂, as a photocatalyst, is provided with many advantages, such as stable in chemical property, innocuity, no side-effect, excellent photocatalytic activity and light corrosion resistant. It can degrade many trace organic compounds to non-toxic substances and kill bacteria and virus. Therefore, it has great value in the environment protection, biological research, medical and health work and new energy resource developing fields. However, as a semiconductor material, its band gap is so wide that can only make use of ultraviolet light and the utilization of solar rays is too limited and ineffective, which restricts its application. In order to improve the photocatalytic activity under visible light and make full use of solar rays, nano-TiO₂ must be modified. The main purpose of this paper is to modify nano-TiO₂ by the means of heat treating in vacuum condition, and heat treating under hydrogen and nitrogen atmosphere. Through these technologies, oxygen vacancy is introduced into the crystal structure of nano-TiO₂ to change the energy band structure, and consequently the photocatalytic activity under visible light is improved.Anatase, rutile and mixed crystal nano-TiO₂ powders were synthesized by directly hydrolysis of TiCl4 solution. The hydrolysis characteristic of TiCl4 and the mechanism for the forming of TiO₂ particles of different structure were discussed in detail. Then prepared anatase and mixed crystal TiO₂ were calcined in the air. X-ray diffraction (XRD) was used to analyze the effect of temperature on the crystal structure and grain size. The phase transforming temperature was also studied and calculated with the method of thermodynamics. Theoretical results coincided with the results in the experiment.Anatase nano-TiO₂ was modified in the vacuum condition. UV-Vis diffuse reflectance spectra (DRS) revealed that during the heat treatment in vacuum, the light absorption boundary wavelength for the samples modified in 400℃and 1000℃was red shift to 480nm and 600nm. The results of high resolution transmission electron microscope (HRTEM) and X-ray photoemission spectrum (XPS) discovered that atomic content ratio for O/Ti was less than 2, which meant that oxygen vacancy deficiency and Ti3+ were formed in the crystal structure of TiO₂. Thus, the energy band structure of TiO₂ was changed and a new oxygen vacancy state came into being between valence band and conduction band. Different kinds of nano-TiO₂ were heat treated in H2 and N2 atmosphere, and light absorption boundary wavelength for these samples also red shifted with different degree. The factors of atmosphere, temperature and crystal structure were studied. In H2 atmosphere, the optimal temperature was 700℃. The light absorption boundary wavelength for anatase, rutile and mixed crystal TiO₂ were 450nm, 490nm, and 500nm. While in N2 atmosphere, 400℃was the optimal temperature, and the light absorption boundary wavelength for rutile and mixed crystal TiO₂ were 440nm and 420nm. The results of XPS also confirmed the existence of oxygen vacancy.Finally, methyl orange solution was used as a model to observe whether the modified samples own photocatalytic activity under visible light. The relation between illumination time and degradation rate indicated that modified samples had photocatalytic activity under visible light illumination. For different atmosphere, crystal structure and number of deficiency, the activity was a little different.