Anatase, Brookite, and Rutile Nanocrystals via Redox Reactions under Mild Hydrothermal Conditions:  Phase-Selective Synthesis and Physicochemical Properties

Anatase, rutile, and especially brookite nanocrystals have been selectively synthesized in this work via a redox route under mild hydrothermal conditions (180 °C, 3 h), employing trichloride as the titanium source and ammonium peroxodisulfate (APS), hydrogen peroxide, nitric acid, or perchloric acid as the oxidant. Characterizations of the three pure phases were achieved by XRD, Raman spectroscopy, FTIR, TG, HRTEM, UV−vis, and BET. The use of APS consistently yields anatase, but the particle morphology can be tuned from wormhole-structured agglomerates to more dispersed nanocrystallites. The use of other oxidants yields almost identical results, and phase selection can be attained in this case by controlling the reactant concentration and solution pH. The three phases show their distinctive crystal shapes: rounded nanocrystals for anatase, nanoplates for brookite, and nanorods for rutile. Both the optical band gap (3.11 eV) and the indirect band gap (2.85 eV) of brookite were found to lie in between those of anatase and rutile. Under the same surface area of loaded TiO2, the brookite nanoplates exhibit the highest efficiency in the beaching of methyl orange solution under UV irradiation. The mechanism of phase selection was discussed based upon a systematic investigation into the effects of synthetic parameters on phase constituents of the hydrothermal products.