Multifaceted review of TiO2 modifications for photoelectrochemical water splitting: Bandgap tuning to charge transport

This review explores the potential of TiO<inf>2</inf> and its various forms as photoanodes for the process of photoelectrochemical water splitting (PEC-WS). Due to its outstanding reactivity, advantageous band locations, and stability, TiO<inf>2</inf> is the best material for this purpose. However, the elevated recombination rate and substantial bandgap limit its applicability. Dye sensitization, non-metal doping (C, N, S), metal doping (Au, Fe, Cu, Ag, Pd, Cr, Sn, W), defect engineering, and heterojunction formation have been investigated for PEC-WS, as these modifications may enhance TiO<inf>2</inf> performance. Non-metal doping leads to a reduction in the bandgap, while dye sensitization enhances the absorption spectrum. The introduction of metal elements increases the absorption of visible light and encourages charge separation. Defective TiO<inf>2</inf>, specifically hydrogenated black TiO<inf>2</inf>, has improved solar absorption. Efficient charge transfer is facilitated by heterojunctions such as TiO<inf>2</inf>/Si, TiO<inf>2</inf>/ZnO, and TiO<inf>2</inf>/CeO<inf>2</inf>, among others. Additional reactive sites are supplied by cocatalysts, such as transition metal phosphides, gold, and platinum. The review aims to discuss the synthesis procedures, morphology, and PEC performance of several modified TiO<inf>2</inf> photoanodes. This study provides a detailed examination of progress in TiO<inf>2</inf>-based photoelectrodes designed to improve the performance, along with approaches to increase the adaptability of TiO<inf>2</inf> for PEC-WS. © 2025 International Solar Energy Society