Limiting and achievable efficiencies of solar-to-hydrogen in heterojunction photocatalysts

Although two-dimensional heterojunctions are regarded as potential photocatalysts for water splitting due to their high solar-to-hydrogen (STH) efficiency, it remains unclear whether there is a theoretical limit to the STH efficiency of such materials. This uncertainty significantly hinders the design and optimization of these photocatalytic materials. Here, the limiting efficiencies of different types of heterojunctions are investigated by developing a user-friendly computational program and by creatively utilizing mapping to visualize the changes in STH efficiency as a function of overpotentials for water redox reactions or bandgap widths. From the efficiency maps, it is clear that the STH efficiencies of type-I, type-II, and Z-scheme heterojunctions all exhibit a limiting value. Specifically, the limiting efficiencies of type-I and type-II heterojunctions are the same, both at 17.12%, while the efficiency of Z-scheme heterojunctions is higher, reaching 37.83%. Taking type-I γ-SnS/hBN heterojunction as a case study, first-principles calculations show that the limiting efficiency of type-I heterojunctions is readily achievable. Through a literature survey and data analysis, it is found that the limiting efficiency of type-II heterojunctions is also attainable, whereas that of Z-scheme heterojunctions is quite challenging. These results are consistent with the predictions from the STH efficiency maps. This work not only confirms the existence of an STH efficiency limit in different types of heterojunctions but also provides theoretical guidance for the rational design of heterojunction photocatalysts.