Interaction mechanism between different facet TiO2 and U(VI): Experimental and density-functional theory investigation

TiO2 with different exposed facets often shows different adsorption and photoreduction performances in environmental pollutant treatment, but the latent mechanisms are not fully understood. In this work, {0 0 1}, {1 0 0} and {1 0 1} facet-dominated anatase TiO2 was prepared and studied for its effectiveness in U(VI) removal, and the molecular level surface chemistry was studied by density functional theory (DFT) calculations. The experimental results indicated that {0 0 1} facet TiO2 exhibited the best adsorption capacity and photoreduction ability compared to {1 0 0} and {1 0 1} facet TiO2. DFT calculation results showed that the adsorption of U(VI) on the three surfaces leaded to the formation of inner-sphere complexes, in which the monodentate complex was most favored for {0 0 1} facet TiO2, whereas bidentate complexes were most favored for {1 0 0} and {1 0 1} facet TiO2. The results from experimental techniques, such as steady fluorescence emission spectra, time-resolved photoluminescence spectra, photocurrent density, electro-chemical impedance spectroscopy, Mott-Schottky plots suggested that the higher photocatalytic activity could be ascribed to higher electron-hole separation efficiency for three facets TiO2. Furthermore, the dissolved oxygen played different roles in U(VI) photoreduction on the three types of TiO2 in the experiments due to different surface structure arrangement. This work provides knowledge of toxic metal ion removal by facet-dependent metal oxides and a basis for the design and synthesis of other reactive facet-dependent materials for environmental management.