Photoelectrochemical Water Splitting With TiO 2 Thin Films: Effects of Thickness, Porosity, and Illumination Configuration

Nanostructured TiO 2 photoanodes have attracted great attention for solar fuel generation via photoelectrochemical water splitting. However, to date, their application remains limited due to an incomplete understanding of the influence of morphology and thickness on the separation, transport, and transfer of photogenerated charge carriers. In addition, little attention has been paid to the critical role of illumination direction, which determines the spatial distribution of charge carriers. We conducted a systematic investigation into the synergistic impact of thin film morphology (mesoporous vs. dense), thickness (thin vs. thick), and illumination configuration (back vs. front) on the photoelectrochemical performance of nanostructured TiO 2 photoelectrodes in the oxygen evolution reaction. Detailed structural, optical, photoelectrochemical, and ultrafast charge carrier dynamic characterization of the photoanodes reveal that mesoporous photoanodes are globally less efficient than dense ones, regardless of thickness and illumination direction. Electrochemical and ultrafast techniques indicate that this is due to the increased density of surface defects, a weaker electric field and slower charge transfer and transport in the mesoporous films, which promote higher recombination losses, mainly under front‐side illumination. These findings indicate that optimizing not only the illumination mode but also film thickness, morphology, and surface quality is essential to achieve high photoelectrocatalytic activity.