Elucidating the role of ZnO in hydrogen evolution reactions: a performance-centric review

Hydrogen production via water splitting has gained significant attention as a sustainable and environmentally friendly approach to meeting global energy demand. Among various materials, ZnO-based photocatalysts offer promising physicochemical properties, such as high electron mobility, large exciton binding energy and structural stability, making them suitable candidates for photocatalytic hydrogen evolution. ZnO-based photocatalysts (PCs), with high electron mobility (~200 cm2/V s), large exciton binding energy (60 meV) and a bandgap of 3.37 eV. ZnO exhibit superior physicochemical properties and chemical stability for photocatalytic hydrogen evolution. However, their efficiency is hindered by rapid charge recombination and limited visible light absorption. Recent advancements, including doping with transition metals (e.g., Ag, Cu, Fe) and rare-earth elements (e.g., Y, Ce), heterostructure formation with g-C3N4 and hybridization with carbon-based materials like reduced graphene oxide, have significantly enhanced performance, achieving hydrogen production rates up to 10.61 mmol/g h for Y/Ce-doped ZnO and ~840 μmol/g in 10 h for AgNPs-ZnO/rGO composites. These modifications reduce the bandgap to as low as 2.40 eV, improving visible light absorption and charge separation efficiency. This review explores the latest progress in ZnO-based materials, emphasizing bandgap engineering, co-catalyst integration and cost-effective synthesis methods (e.g., sol–gel, co-precipitation) for scalable hydrogen production. Challenges such as long-term stability and real-world solar efficiency are addressed, alongside future directions for optimizing ZnO-based photocatalysts to support sustainable hydrogen energy solutions targeting net-zero CO2 emissions by 2050.