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

Abstract

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%.