Charge separation promoted by phase junctions in photocatalysts

Since the 1980s, photocatalysis research has expanded at an unexpected rate. Fabrication of phase junctions has proved to be an effective method to enhance photocatalytic performance. As a model photocatalyst, titanium dioxide (TiO2) has been extensively studied. This feature article mainly reviews the study on TiO2 surface phase junctions, including the characterization of the surface phases of TiO2, the use of anatase: rutile TiO2 phase junctions in photocatalytic hydrogen production, and the current understanding of how TiO2 phase junctions work in photocatalysis. The surface structure of TiO2 can be well characterized by ultraviolet (UV) Raman spectroscopy, unlike X-ray diffraction and visible Raman spectroscopy. Based on these results, the mechanism of phase transformation processes of TiO2 was clearly identified. The infrared (IR) spectra of probe molecules CO and CO2 on TiO2 further characterized the surface structure of TiO2, strongly supporting the UV Raman results. Furthermore, the typical visible emission of anatase and near-infrared emission of the rutile phase of TiO2 make photoluminescence (PL) a suitable technique to characterize the surface phase structure of TiO2. PL can also provide information about the carrier dynamics of TiO2 in photocatalysis. Because of the surface phase junction formed between anatase and rutile, the mixed-phase structure of TiO2 exhibits a superior 112 production activity to those of pure anatase or rutile phase. The activity of Degussa P25 TiO2 can be further increased by three to five times by tuning the phase structure through thermal treatment. Moreover, the phase transformation of TiO2 from anatase to rutile can be controlled by surface modification with Na2SO4, resulting in catalysts with activity six times higher than that of P25. High-resolution transmission electron microscopy provided a clear phase-junction image of TiO2, which showed atomic contact at the interface of the phase junction. The mechanism of phase junctions improving photocatalytic performance was investigated by time-resolved spectroscopic techniques. The charge transfer process across the anatase: rutile phase junction was confirmed by the results of time-resolved IR measurements, and electron transfer from anatase to rutile phases is proposed to occur in mixed-phase TiO2. These studies on the phase junctions of TiO2 improve our understanding of photocatalysis and may inspire new ideas for the design of promising photocatalytic systems. (C) 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.