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 degrees 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 O-2. When the oxidation is performed by H2O2, rutile, formed by large crystalline rods with high aspect ratios (size 15-20 x 100-120 nm), shows higher catalytic activity with respect to the small, almost cubic, anatase particles (5-15 nm). If O-2 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 O-2(-) 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.