Research On Origin Of The High Visible-light Catalytic Activity Of Nano-sized Titanium Dioxide Prepared Via The Solvothermal Method

In this thesis, titanium dioxide products with the form of amorphous or anatase and the visible-light photocatalytic activity were prepared via the solvothermal method with titanium tetra-n-butoxide(TNB) as starting material in acetone. This thesis was primarily focused on elucidating physical and chemical origin of the high visible-light photocatalytic activity in order to make contributions to industrialization of titanium dioxide photocatalysis in both theory and technology.Effect of solvothermal temperature on preparation of titanium dioxide and its photocatalytic activity were investigated. The relation between the surface status and the photocatalytic activity was discussed. In addition, the electron-transfer model under visible-light irradiation was established. Also, the surface status was mediated by heat treatment to confirm further the relation between the surface status and the photocatalytic activity. Futhermore, the operating mechanism of titanium dioxide for phenolic compounds under visible-light irradiation was proposed to explain the high photocatalytic activity. As a result, it was suggested that two surface complexes contribute synergistically to the high visible-light photocatalytic efficiency.The material characterization for the nano-sized titanium dioxide products prepared via hydrothermal method and followed by heat treatment was conducted and analyzed. The results of XRD, TEM and selected area electron diffraction showed that titanium dioxide was transformed to anatase from amorphous as the increase of solvothermal temperature and the anatase began to appear in the product prepared at 120℃. There were about 2.4%～16.2% organic moieties adsorbed onto the surface of titanium dioxide, and the existence of peak 1093cm^-1 from the absorption of Ti-O-C bond in the FT-IR spectrum indicated that some organic compounds were adsorbed in-situ onto the surface during the autoclaving to form the surface complex(≡Ti⁴⁺-OCR), which creates the surface status, traps photogenerated electron, accelerates the electron transfer, and thus improves the photocatalytic activity. The adsorbed H₂O and organic moieties adsorbed physically can be desorbed by heat treatment at 180℃, and that an amount of organic moieties are desorbed at 365℃ results in the lower photocatalytic activity.The interaction between substrates and titanium dioixde during the solution photocatalytic reaction process wad investigated. It was found that the hydroxyl group on the surface of titanium dioxide can react with the hydroxyl group of phenolic compounds to form another surface complex. The surface complexation is essentially a condensation reaction, which is affected by the substitute group on benzene ring. Condensation reaction of hydroxyl group with the higher acidity occurs easier. Due to the stronger electronegativity of Cl, the corresponding hydroxyl group had a higher acidity, which makes 4-chlorophenol higher visible-light photocatalytic activity than phenol and 4-hydroxybenzoic acid. The surface complex can be excited by the visible-light and transfer electrons to the conduction band of titanium dioxide. The photogenerated electrons can be trapped by O₂ to form peroxides such as O₂^·- and HOO^- with the strong oxidation ability, and the hydroxyl radicals are not the primary oxidant in this system.It was concluded that two surface complexes contribute synergistically to the high visible-light photocatalytic activity, and deep trapping of photogenerated electrons is one of reasons for the low visible-light photocatalytic activity of titanium dioxide with the amorphous form.