Role of Ce 4f States in the Electronic Structure and Photoelectrochemical Activity of Brannerite CeTi₂O_6–<i>X</i>

Summary

Cerium has been proposed as an attractive element in photoconversion systems, but the role of the Ce3+/Ce4+ redox couple in light-driven processes remains a subject of debate due to the localized nature of Ce 4f electrons. Moreover, in state-of-the-art TiO2 photoabsorbers, Ce doping generally introduces various defects, complicating the interpretation of the contribution of cerium. In this study, we address this question by synthesizing brannerite CeTi2O6 thin films and powders and treating them with H2 to form isostructural CeTi2O6–x phases, in which Ce4+ cations are selectively reduced to Ce3+. The reduction introduces deep intragap Ce 4f states, which greatly enhance visible light absorption for the material. However, when studied as photoelectrodes, CeTi2O6–x thin films exhibit decreas