Surficial redox reactions play an essential role in photocatalytic water splitting, and are closely related to the surface properties of a specific photocatalyst. In this work, using monoclinic BiVO₄ decahedral single crystals as a model photocatalyst, we report on the interrelationship between the photocatalytic activity and the surficial reaction sites for charge-carrier consumption. By controlled hydrothermal synthesis, the ratio of {010} to {110} facets on BiVO₄, which respectively serve as reductive and oxidative sites, is carefully tailored. Our results show that superior photocatalytic water oxidation could be obtained on BiVO₄ decahedrons with a medium ratio of reductive/oxidative sites and that efficient overall water splitting could be achieved via further modification of appropriate cocatalysts in Z-scheme system. The excellent photocatalytic performance is attributed to the accelerated selective redox reactions by realizing balanced charge-carrier consumption, which provides insightful guidance for prospering photocatalytic reactions in energy conversion.

Charge-carrier separation is regarded as one of the critical issues of photocatalytic water splitting and could be accelerated by constructing microscopic junctions in photocatalysts. Homojunction photocatalysts consisting of different forms of semiconductor with identical compositions could inherit the advantages of heterojunction-based photocatalysts in charge separation due to the built-in electric field, while omitting the potential drawbacks of interfacial lattice distortion by providing continuous band bonding. Therefore, homojunction-based photocatalysts have recently drawn growing attention in water splitting. In this review, the synthetic approaches to preparing photocatalysts with various homojunction structures including p-n junction, phase junction, and facet junction were introduced, together with a comprehensive analysis and discussion on the latest progress in the application of photocatalytic water splitting. This review work is expected to inspire more related work with promoted research on designing efficient homojunction-based photocatalytic systems for water splitting.

Self-passivation in aqueous solution and sluggish surface reaction kinetics significantly limit the photoelectrochemical (PEC) performances of silicon-based photoelectrodes. Herein, a WO₃ thin layer is deposited on the p-Si substrate by pulsed laser deposition (PLD), acting as a photocathode for PEC hydrogen generation. Compared to bare p-Si, the single-junctional p-Si/WO₃ photoelectrodes exhibit excellent and stable PEC performances with significantly increased cathodic photocurrent density and exceptional anodic shift in onset potential for water reduction. It is revealed that the WO₃ layer could reduce the charge transfer resistance across the electrode/electrolyte interface by eliminating the effect of Fermi level pinning on the surface of p-Si. More importantly, by varying the oxygen pressures during PLD, the collaborative modulation of W-O bond covalency and WO₆ octahedral structure symmetry contributes to the promoted charge carrier transport and separation. Meanwhile, a large band bending at the p-Si/WO₃ junction, induced by the optimized O vacancy contents in WO₃, could provide a photovoltage as high as ~ 500 mV to efficiently drive charge transfer to overcome the water reduction overpotential. Synergistically, by manipulating W-O local atomic structures in the deposited WO₃ layer, a great improvement in PEC performance could be achieved over the single- junctional p-Si/WO₃ photocathodes for solar hydrogen generation.

Utilization of visible light is of crucial importance for exploiting efficient semiconductor catalysts for solar water splitting. In this study, an advanced ion implantation method was utilized to dope Cu ions into ZnO nanorod arrays for photoelectrochemical water splitting in visible light. X-ray diffraction (XRD) and X-ray photo-electron spectroscopy (XPS) results revealed that Cu⁺ together with a small amount of Cu²⁺ were highly dispersed within the ZnO nanorod arrays. The Cu ion doped ZnO nanorod arrays displayed extended optical absorption and enhanced photoelectrochemical performance under visible light illumination (λ > 420 nm). A considerable photocurrent density of 18 μA/cm² at 0.8 V (vs. a saturated calomel electrode) was achieved, which was about 11 times higher than that of undoped ZnO nanorod arrays. This study proposes that ion implantation could be an effective approach for developing novel visible-light-driven photocatalytic materials for water splitting.

Hematite (α-Fe₂O₃) nanorod films with their surface tuned by W⁶⁺ doping have been investigated as oxygen-evolving photoanodes in photoelectrochemical cells. X-ray diffraction, field emission scanning electron microscopy, UV-visible absorption spectroscopy, and photoelectrochemical (PEC) measurements have been performed on the undoped and W⁶⁺-doped α-Fe₂O₃ nanorod films. W⁶⁺ doping is found to primarily affect the photoluminescence properties of α-Fe₂O₃ nanorod films. Comparisons are drawn between undoped and W⁶⁺-doped α-Fe₂O₃ nanorod films, WO₃ films, and α-Fe₂O₃-modified WO₃ composite electrodes. A close correlation between dopant concentration, photoluminescence intensity, and anodic photocurrent was observed. It is suggested that W⁶⁺ surface doping promotes charge transfer in α-Fe₂O₃ nanorods, giving rise to the enhanced PEC performance. These results suggest surface tuning via ion doping should represent a viable strategy to further improve the efficiency of α-Fe₂O₃ photoanodes.