ZnO–TiO2-Based Nanocomposite Photocatalysts: Advances Toward Solar-Driven Hydrogen Evolution

Photocatalytic water splitting driven by solar energy offers a sustainable route for hydrogen production as a clean, carbon-free energy carrier. Among the semiconductor photocatalysts investigated, zinc oxide (ZnO) and titanium dioxide (TiO 2 ) have attracted significant attention due to their chemical stability, low toxicity, favorable band-edge positions, and cost-effectiveness. However, their wide band gaps limit light absorption mainly to the ultraviolet region, leading to suboptimal utilization of the solar spectrum and rapid recombination of photogenerated charge carriers. To address these limitations, this mini-review focuses on the rational design and analysis of ZnO–TiO 2 -based heterojunction photocatalysts aimed at enhancing charge separation and extending visible-light absorption. Particular emphasis is placed on interfacial interactions and charge transfer pathways in ZnO–TiO 2 heterojunctions that govern solar-driven hydrogen evolution. Recent advances in nanocomposite structures are critically discussed, along with key challenges related to photocatalytic performance, operational stability, and scalability, providing insights for the development of efficient, practical, and sustainable oxide-based photocatalysts.