Manipulating solar absorption and electron transport properties of rutile TiO₂ photocatalysts via highly n-type F-doping

In this work, we report a facile and nontoxic one-pot hydrothermal method for synthesizing F-doped rutile single crystalline TiO₂ with tuneable solar absorption. The optical band gap of the catalyst can be easily manipulated from 3.05 to 2.58 eV via altering the initial F:Ti molar ratio of reaction precursors. The photoanodes made of rutile TiO₂ single crystals with appropriate F-doping concentration show excellent photoelectrocatalytic activity towards water oxidation under ultraviolet and visible light illumination. The best photoelectrocatalytic performance under UV irradiation can be obtained by F-doped TiO₂ with an initial F:Ti molar ratio of 0.1, which is almost 15 times higher than that of un-doped TiO₂. Further, the F-doped TiO₂ photoanodes also exhibit superior photoelectrocatalytic activity under visible irradiation, and the best performance can be achieved by F-doped TiO₂ photoanode with an initial F:Ti molar ratio of 0.05. The superior photoelectrocatalytic activity could be attributed to the highly n-type dopant introduced by fluorine, which significantly tunes the electrical conductivities and band structures of the resulting TiO₂ photoanodes, and thus the photoelectrocatalytic activities under both UV and visible irradiation. Different techniques have been employed to characterize the electrical conductivity, charge carrier density and band structures of the F-doped rutile TiO₂ films, such as photoelectrochemical method, electrical impedance spectroscopy (EIS) measurements, Mott-Schottky plots and XPS valence band spectra.