Two-dimensional direct Z-scheme AlN/GaS-SiP heterojunctions enhance photocatalytic hydrogen production from water: a DFT study

Photocatalytic water splitting for hydrogen production offers a feasible solution to the problems of energy shortages and environmental pollution. However, its low photocatalytic efficiency limits the application of this technology in real world scenarios. In this study, a two-dimensional AlN/PSi-GaS-I van der Waals heterojunction is constructed and the properties of water photolysis are studied based on first-principles calculations. The results demonstrate that AlN/PSi-GaS-I exhibits exceptional photocatalytic performance with good stability, a narrow bandgap, appropriate band-edge position, a broader light absorption range and efficient separation of photogenerated electron-hole pairs. Moreover, the Gibbs free energies of different intermediates throughout the entire reaction process are calculated based on type-II and Z-scheme reaction mechanisms. By comparing the free energy barriers of the two pathways, it is observed that the Z-scheme reaction pathway has a lower energy barrier. Consequently, it can be concluded that AlN/PSi-GaS-I belongs to the direct Z-scheme heterojunction. These findings suggest that AlN/PSi-GaS-I exhibits an enhanced redox capacity, efficiently driving the water splitting reaction. More excitingly, the AlN/PSi-GaS-I can undergo spontaneous photocatalytic reactions under acidic conditions when provided with adequate optical driving force. This study not only proves that AlN/PSi-GaS-I is a promising high-efficiency photocatalyst for water splitting, but also describes a method for determining direct Z-scheme heterojunctions, which offers theoretical guidance for the design of efficient and stable photocatalysts. We elucidate the method for determining type-II and direct Z-scheme heterojunctions from the perspective of dynamic simulation.