2D materials and heterostructures for photocatalytic water-splitting: a theoretical perspective

Photocatalytic water-splitting for hydrogen generation by sunlight provides a new route to address energy and environmental problems. In recent years, tremendous efforts have been devoted to designing highly efficient water-splitting photocatalysts (PCs). Adequate light absorption, effective photogenerated carrier separation, and sufficiently large overpotentials for water redox are crucial in achieving high solar-to-hydrogen (STH) efficiency. These parameters thus strongly influence the design of novel photocatalytic materials. Two-dimensional (2D) PCs have flourished because of their large specific surface area ratio, short carrier migration distance compared to bulk PCs, enormous design flexibility via van der Waals heterostructure (HS) engineering and many other unique capabilities that meet the criteria for high-efficiency STH conversion. In this review, we summarize the recent developments of 2D materials and HSs for water-splitting applications from a theoretical perspective. Specifically, we first discuss a number of 2D materials and HSs employed for water-splitting. We review various strategies of material design to modulate and enhance the photocatalytic performance via improving light harvesting and carrier separation, such as the introduction of defects and dopants, and the application of strain, external electric field, rotation angles and ferroelectric switching. We then discuss the methods to evaluate hydrogen evolution reaction, oxygen evolution reaction and STH efficiency. Finally, the opportunities and challenges of designing 2D materials and HSs for water-splitting are presented.