Photocatalytic water-splitting for hydrogen generation by sunlight provides a new route to address the energy and environmental problems. In recent years, tremendous efforts have been devoted to design highly efficient water-splitting photocatalysts. 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) photocatalysts have flourished because of the large specific surface area ratio, short carrier migration distance compared to bulk photocatalysts, enormous design flexibility via van der Waals heterostrucutre (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 designs to modulate and enhance the photocatalytic performance via improving light harvesting and carrier separation, such as the introduction of defects, dopants and the application of strain, external electric field, rotation angles, 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.

The ZnO/MoS2 (ZnO/MoSe2) heterostructures with the strain of –2% (+2%) have suitable bandgap and band edge position for hydrogen production via visible light photocatalytic water splitting.

Organic-inorganic hybrid perovskites as new emerging functional materials stand out from numerous photovoltaic materials thanks to the unprecedentedly rapid improvement of their power conversion efficiency within a short period. To explore potentially more efficient photovoltaic candidates, the structural and electronic properties of FAMAPbI based on prototype MAPbI are investigated for superior performance. Specifically, structural relaxation is performed at the PBE+D2 level and the electronic and optical properties are investigated at the HSE + SOC level. Optical simulations show that significantly improved performance can be successfully achieved by means of the injection of FA cations. Moreover, the calculations of defect formation energies imply that MA-poor ambient conditions are energetically favorable to synthesize a variety of FA-doped pervoskite compounds FAMAPbI of different ratios. It is interesting to find that compared with the prototype MAPbI, the optical performance of the perovskite series FAMAPbI is effectively improved with an increase in FA content; meanwhile the relative stability of the perovskite series is also efficiently enhanced. Our study not only sheds new light on further understanding perovskite absorbers but also provides the basic rationale for designing new functional materials used for photovoltaics.