Optimizing the photocatalytic properties of hydrothermal TiO2 by the control of phase composition and particle morphology. A systematic approach.

The possibility of controlling the photocatalytic activity of TiO2 nanoparticles by tailoring their crystalline structure and morphology is a current topic of great interest. In this study, a broad variety of well-faceted particles with different phase compositions, sizes, and shapes have been obtained from concentrated TiOCl2 solutions by systematically changing temperature, pH, and duration of the hydrothermal treatment. The guide to select the suitable experimental conditions was provided by thermodynamic modeling based on available thermochemical data. By combining the results of TEM, HRTEM, XRD, density, and specific surface area measurements, a complete structural and morphological characterization of the particles was performed. Correlation between the photocatalytic activity in the UV photodegradation of phenol solutions and the particle size was established. Prismatic rutile particles with length/width ratio around 5 and breadth of 60−100 nm showed the highest activity. The surface chemistry of the particles was also investigated. Treatments that decrease the surface acidity, such as washing the powders with ammonia solution and/or calcining at 400 °C, have detrimental effect on photocatalytic activity. The overall results suggest correlation between particle morphology and photocatalytic activity and indicate that both electron−hole recombination and adsorption at the surface can be rate-controlling processes. The systematic approach presented in this study demonstrates that a substantial improvement of the photocatalytic activity of TiO2 can be achieved by a careful design of the particle morphology and the control of the surface chemistry

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 (rw) and HCl:Ti (ra) 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 rw and ra and influence of the copolymer in determining the phase composition was demonstrated. Rutile becomes the main phase by increasing rw. 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 proces