Sol-gel pure and mixed-phase titanium dioxide for photocatalytic purposes: Relations between phase composition, catalytic activity, and charge-trapped sites

The sol-gel synthesis of TiO₂ from TiCl₄ assisted by the triblock copolymer EO₂₀-PO₇₀ -EO₂₀ (EO = -CH₂CH₂O-, PO = -CH₂ (CH₃)CHO-) as templating agent was carried out by systematically changing H₂O:Ti (r_w) and HCl:Ti (r_a) molar ratios. Mesoporous and nanocrystalline TiO₂ samples with well-defined and controlled phase composition (anatase, rutile, and mixed phase) were obtained after calcination at 400 °C and characterized for the morphology, particle size, and shape using TEM, HRTEM, XRD, and surface area measurements. The role of r_w and r_a and influence of the copolymer in determining the phase composition was demonstrated. Rutile becomes the main phase by increasing r_w. In fact, the decrease of Ti concentration slows down the condensation rate, favoring formation of rutile seeds in the gel. The photocatalytic activity of TiO₂ in the UV photomineralization of phenol depends on the phase composition and oxidizing agent, H₂O₂ or O₂. When the oxidation is performed by H2O₂, rutile, formed by large crystalline rods with high aspect ratios (size 15-20 × 100-120 nm), shows higher catalytic activity with respect to the small, almost cubic, anatase particles (5-15 nm). If O₂ is used, the catalytic activity generally decreases and the behavior of polymorphous species is reverse. EPR investigation of the paramagnetic charge carriers, formed under UV irradiation at 10 K, showed the resonance lines of holes trapped at O^- lattice sites and electrons trapped at Ti³⁺ and O₂^- sites. The rutile crystalline rods present the largest quantity of O^- and Ti³⁺ centers. The overall results suggest correlation between TiO₂ particle size and shape and the photocatalytic activity and indicate that electron-hole recombination is the most probable rate-controlling process. © 2008 American Chemical Society.