Study On Preparation And Photocatalytic Activities Of Modified Nano-TiO₂ Photocatalyst

Nanosized titanium dioxide is one of most popular environmentally harmonious photocatalysts in this day because of its non-toxicity, physical and chemical stability, high photocatalytic activity and strong antioxide properties. However, the efficiency of photocatalytic reaction is limited by the high recombination rate of photo-induced electron-hole pairs formed in photocatalytic processes and by the absorption capability to visible light of photocatalysts. Numerous studies have been recently performed to improve the efficiency of photocatalytic reactions, this studies include doping metal ion into the TiO₂ lattice, dye photosensitization on the TiO₂ surface and deposition of noble metals and so on. In this paper, the photocatalytic activity and its factors of effect of modified nano-TiO₂ by different modified methods.Firstly, Fe³⁺-doped and Ag-loaded TiO₂ nanoparticles were synthesized using acid-catalyzed sol-gel and photodeposition methods. The nanoparticles were characterized by TEM, XRD, XPS and UV-Vis techniques. The results showed that the average diameter of nanoparticles was 10～15 nm. Fe³⁺doping and Ag loading could all promote the phase transformation of TiO₂ from anatase to rutile. The absorption of TiO₂ nanoparticles with Fe³⁺ doping and Ag loading to light was red-shifted and the absorption intensities laregely increased. The oxidation state of the silver loaded on the TiO₂ was Ag⁰ from X-ray photoelectron spectroscopy (XPS). UV light was used as light source to investigate the photocatalytic activity of the catalyst for the degradation of methyl orange. The results showed that Fe³⁺-doped and Ag-loaded TiO₂ exhibited higher photocatalytic activity compared with pure TiO₂, Fe³⁺-doped TiO₂ as well as Ag-loaded TiO₂, and the optimal amount of doped Fe³⁺ and loaded Ag was 0.4% and 1%(mole fraction), respectively.Secondly, Li⁺-doped nano-TiO₂ photocatalysts were prepared by sol-gel route and by impregnation method, respectively. The catalysts were characterized by TEM and XRD techniques, pH of zero charge (pHpzc) of catalysts were measured by pH drift method. The results show that all catalysts prepared at 500℃are anatase phase. Li⁺-doping restrains the growth of TiO₂ particles, improves the dispersivity of the catalyst. The pHpzc of catalysts is in the range 6.6～8.1, depending on the doping method and concentration of doped Li⁺. UV light and room sunlight were used as light sources to investigate the photocatalytic activity of the catalysts for the degradation of malachite green and methyl orange. It has been found that, for the degradation malachite green, Li⁺-doping both by sol-gel route and by impregnation method can all enhance the catalytic activity of TiO₂ effectively, but the catalysts prepared by the impregnation method are more efficient than those prepared by sol-gel route, and the best photoactivity is obtained for 5% Li⁺-doped TiO₂ prepared by impregnation technology and its photocatalytic efficiency is 7～8 times higher than that of pure TiO₂; For methyl orange degradation, Li⁺-doping reduces the catalytic activity of TiO₂ to some extent, except for 3% Li⁺-doped TiO₂ that is slightly better prepared by sol-gel route. The main reasons of reverse photocatalytic activity of Li⁺-doped TiO₂ in degrading different organic pollutants is that Li⁺-doping changes pHpzc and the state of the charge of TiO₂ surface.And then, S-doped TiO₂ visible-light activated photocatalysts were prepared using titanium isopropoxide and thiourea as starting raw materials. The photocatalysts were characterized by XRD, XPS, TEM and UV-Vis absorption spectra. The photocatalytic degradation of methyl orange in aqueous solution was used as a probe reaction to evaluate the phocatalytic activity of the photocatalysts using UV light and sunlight as light sources. The results show that S-doped TiO₂ calcined at 500℃and 600℃absorbs visible light and ultraviolet light more strongly than pure TiO₂, and photocatalytic activity of them is higher than that of pure TiO₂ under visible and ultraviolet light. The photocatalytic activity of S-doped TiO₂ calcined at about 500℃is the highest in S-doped TiO₂ and pure TiO₂. Compared with pure TiO₂, the photocatalytic degradution rate of methyl orange with S-doped TiO₂ calcined at 500℃under ultraviolet light and solar light is enhanced about 32% and 69% after 40～50 min reaction, respectively. TEM results confirm that the average diameter of S-doped photocatalysts calcined at 500℃is about 15～16 nm. XRD results show that S-doping can promote the phase transformation of TiO₂ from anatase to rutile, reduce the crystallite size of photocatyalysts. XPS results indicate that the oxidation state of the S atom substituted for some of the lattice titanium atoms in S-doped TiO₂ is mainly S6+.In this paper, three different modified nanosized titanium dioxides were prepared, the photocatalytic activities of TiO₂ are all largely improved, so we make instructive research for the applications of TiO₂ to environmental purification and contaminations disposal.