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.