TiO₂ with Tunable Anatase-to-Rutile Nanoparticles Ratios: How Does the Photoactivity Depend on the Phase Composition and the Nature of Photocatalytic Reaction?

We developed low-temperature (up to 200 °C) synthesis of TiO2 nanocomposites, which allows precise tuning of the anatase/rutile ratio. The materials were synthesized hydrothermally from strongly acidic solutions of Ti(III) precursors and hydrogen peroxide. Two series of materials (A and B series) have been synthesized; the A series is characterized by smaller anatase particles, lower crystallinity, and higher specific surface area. The synthesized materials were examined in water oxidation, hydrogen peroxide reduction, photocurrent generation, and methanol-assisted water reduction. The optimal phase composition depends on the reaction type—high contents of rutile or anatase should be considered for H2O2 reduction and water oxidation, respectively, while water reduction requires moderate contents of both polymorphs. The A series appeared more active in oxidation-controlled reactions, while the B series facilitated reduction-controlled processes. The DOS analysis confirmed that rutile is a significantly better reducer, whereas anatase is a better oxidant. Presented data show that a synergistic effect observed usually for anatase/rutile nanocomposites can result from both intrinsic and extrinsic factors. The intrinsic factors are responsible for the improved photocatalytic activity of the material related to its physicochemical properties, e.g., efficiency of charge separation, lifetime of photogenerated charges, absorption properties, redox characteristics, porosity, specific surface area, etc. The extrinsic factors are strongly related to the redox reactions to be performed at the photocatalyst surface, both oxidation and reduction. Therefore, we confirm hereby with presented experimental evidence that the design of the optimal photocatalyst must take into account both intrinsic and extrinsic factors.