Exceptional Photocatalytic Activity of 001-Facet-Exposed TiO2 Mainly Depending on Enhanced Adsorbed Oxygen by Residual Hydrogen Fluoride

Is it true that the exceptional photocatalytic activity of 001-facet-exposed TiO2 is attributed to its high-energy surfaces? In this work, nanocrystalline anatase TiO2 with different percentages of the exposed (001) facet has been controllably synthesized with a hydrothermal process using hydrofluoric acid as a morphology-directing agent. It is shown that the percentage of (001)-facet exposure is tuned from 6 to 73% by increasing the amount of used hydrofluoric acid, and meanwhile the amount of residual fluoride in the as-prepared TiO2 is gradually increased. As the percentage of (001) facet is increased, the corresponding TiO2 gradually exhibits much high photocatalytic activity for degrading gas-phase acetaldehyde and liquid-phase phenol. It was unexpected that the photocatalytic activity would obviously decrease when the residual fluoride was washed off with NaOH solution. By comparing F-free 001-facet-exposed TiO2 with the F-residual one, it is concluded that the exceptional photocatalytic activity of the as-prepared 001-facet-exposed TiO2 depends mainly on the residual hydrogen fluoride linked to the surfaces of TiO2 via the coordination bonds between Ti4+ and F-, as well as slightly on the high-energy 001-facet exposure, by means of the temperature-programmed desorption (TPD) measurements, the atmosphere-controlled surface photovoltage spectra, and the isoelectric point change. On the basis of the O-2-TPD tests, theoretical calculations, and O-2 electrochemical reduction behaviors, it is further suggested for the first time that the residual hydrogen fluoride as the form of -Ti:F-H could greatly enhance the adsorption of O-2 so as to promote the photogenerated electrons captured by the adsorbed O-2, leading to the great increase in the charge separation and then in the photocatalytic activity. This work would clarify the high-activity mechanism of widely investigated TiO2 with high-energy 001-facet exposure and also provide feasible routes to further improve photo catalytic activity of TiO2 and other oxides.