  Recently, hydrogen generation by water-splitting photocatalysts is attracting
attention as a sustainable and clean energy resource. Photocatalytic
hydrogen-generation systems are much simpler, cheaper, and easier to scale up
than the coupled systems of electrolysis and solar cells, wind-power
generation, etc. However, photocatalytic hydrogen generation is currently
inefficient. This paper proposes the monolayer of boron phosphide as a stable
highly-efficient water-splitting photocatalyst by high-precision
density-functional theory calculations using a HSE06 functional with a solvent
effect. The monolayer of boron phosphide has a direct allowed energy gap of
about 1.4 eV, and functions as a one-step excitation photocatalyst. It absorbs
sunlight with wavelengths below about 890 nm (ultraviolet, visible, and
near-infrared light) and produces both hydrogen gas and oxygen gas from water
at a suitable pH condition. By calculating the overpotentials of hydrogen and
oxygen evolution reactions, its photocatalytic effectiveness was confirmed. The
monolayers of boron phosphide will realize green hydrogen revolution.
