Studies On Photocatalytic Activity Of Powders And Hydrophilicity Property Of Thin Films Of TiO₂ Codoped With Iron And Nitrogen

TiO₂ photocatalytic oxidation is a promising method of sewage treatment and air purification, which, by virtue of its strong oxidation power, can decompose most organic pollutants into CO₂, H₂O and other small inorganic molecules completely. By research on surface structure of TiO₂ thin films, it had been found that ultraviolet (UV) illumination of TiO₂ could create a hydrophilic surface that allows water to spread extensively on the common ground. However TiO₂ shows photocatalytic activity and super-hydrophilic activity only by irradiation of UV light for its wide band gap (Eg = 3.2 eV). Unfortunately the proportion of UV light to the sun light is very small. In addition, easy recombination of photo-induced electrons and holes makes the efficiency of photoquantum lower. These unfavorable factors make the practical application of TiO₂ limited. Having considered the above factors, it is suggested by the investigators that employing iron and nitrogen to codope TiO₂ should improve the photocatalytic performance and hydrophilic activity, and may enable the absorbing wavelength of TiO₂ to move to the visible light region.In this inverstigation, using triethylamine as nitrogen source, TiO₂ powder doped with nitrogen (N-TiO₂) were synthesized by TiCl4. TiO₂ powder codoped with iron and nitrogen (Fe/N-TiO₂) was prepared by adsorption-deposition of Fe3+ on N-TiO₂ powder and heat treatment. The photocatalytic degradation of methylene blue solution was used as model reaction to evaluate the photocatalytic activity under visible light and UV light irradiation of TiO₂ powder prepared under different conditions or by various methods. In addition,TiO₂ doped with iron (Fe-TiO₂) thin films were synthesized by using sol-gel method. TiO₂ codoped with iron and nitrogen (Fe/N-TiO₂) thin films were prepared by heat treatment Fe-TiO₂ thin films in ammonia atmosphere. The influence of doping content and heat treatment temperature on the hydrophilicity and its stability of the thin films was studied by measuring the contact angle of water on the surface of films.SEM, XRD, XPS and UV-Vis absorption spectra were employed to characterize the structure of undoped TiO₂ and doped TiO₂ powders and films. The results have shown as follows: (1) Fe/N-TiO₂ (0.38at.% Fe) powder heat-treated at 400℃has shown a higher photocatalytic activity under visible light irradiation, which is about 2 times as high as that of pure TiO₂ under UV light, and 3 times under sunlight. (2) Fe/N-TiO₂ (0.5at.% Fe) film heat-treated at 500℃has shown a better hydrophilicity, especially for visible light irradiation. (3) The samples prepared in the above two experiments, N1s owns binding energy at 396.6eV and 399.6 eV, suggest that some N atoms substitute O atoms in TiO₂ lattice. (4) Doping of nitrogen in TiO₂ enhances photoresponse in the visible light region and doping of Fe3+ reduces the recombination of electrons and holes. Both of the effects cooperate with each other to help improve the photocatalytic property of the TiO₂ powders and the super-hydrophilicity of the TiO₂ films.