Role of Ce 4f States in the Electronic Structure and Photoelectrochemical Activity of Brannerite CeTi2O6–X

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 CeTi₂O_6–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, CeTi₂O_6–x thin films exhibit decreased activity compared to the pristine material under one sun illumination. We investigate this behavior through a combination of spectroscopic techniques and theoretical modeling, and we build a detailed electronic band diagram for CeTi₂O₆ and its reduced counterparts. We show that, apart from the appearance of intragap Ce 4f states, the overall band structure remains largely unchanged across different reduction levels. Moreover, density functional theory calculations suggest that the exciton binding energy increased after reduction, leading to inefficient charge separation, which counteracts the benefits of enhanced visible-light absorption. Overall, this study proposes a robust and comprehensive methodology to describe the band structure of photoabsorbers and to decipher the influence of intragap states on their light absorption and conversion properties, well beyond the CeTi₂O₆ system.