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

• Published in: ACS catalysis Vol. 3; no. 6; pp. 1378 - 1385
• Main Authors: Luan, Yunbo, Jing, Liqiang, Xie, Ying, Sun, Xiaojun, Feng, Yujie, Fu, Honggang
• Format: Journal Article
• Language: English
• Published: American Chemical Society 07-06-2013
• Subjects: exposed 001 facet; TiO2 photocatalysis; residual hydrogen fluoride; charge separation; adsorbed oxygen
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/cs400216a

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 O2-TPD tests, theoretical calculations, and O2 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 O2 so as to promote the photogenerated electrons captured by the adsorbed O2, 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 photocatalytic activity of TiO2 and other oxides.