Novel C4P2 monolayers: forming Z-scheme heterojunction and Janus structure for high-efficiency metal-free photocatalytic water splitting.

Metal-free two-dimensional (2D) semiconductors have garnered significant attention in the realm of photocatalytic water splitting, primarily owing to their inherent clean, stable, and efficient photoresponsive properties. Motivated by it, we have proposed two types of stable C4P2 monolayers with indirect band gaps, mediocre carrier mobility and excellent optical absorption in visible-light and ultraviolet regions. Although the too-low work function of monolayer α-C4P2 and the too-high work function of monolayer β-C4P2 make them only suitable for single-side redox reaction in photocatalytic water splitting, the creation of an α-C4P2/β-C4P2 Z-scheme heterojunction, combined with the Janus monolayer γ-C4P2 that integrates features of both α and β structures, effectively addresses this limitation, fulfilling the prerequisites for comprehensive photocatalytic water splitting. Furthermore, the calculations indicate that the α-C4P2/β-C4P2 Z-scheme heterojunction and Janus monolayer γ-C4P2 not only demonstrate improved carrier mobility and optical absorption but also feature internal electric fields that effectively enhance driving energy and photo-induced charge separation. Notably, Janus monolayer γ-C4P2 achieves a high electron mobility of ∼105 cm2 V-1 s-1 and an impressive solar-to-hydrogen conversion efficiency of 25.62%.