Enhanced <i>Z</i> -scheme photocatalytic water splitting in g-C ₃ N ₄ monolayers via out-of-plane doping and heterostructure design

We present a theoretical investigation of van der Waals heterostructures composed of out-of-plane doped $X@\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ monolayers (X = $\mathrm{S},\phantom{\rule{0.1em}{0ex}}\mathrm{Se}$) coupled with ${\mathrm{WTe}}_{2}$, ${\mathrm{Tc}\mathrm{Te}}_{2}$, ${\mathrm{Al}\mathrm{P}}_{3}$, or ${\mathrm{In}\mathrm{P}}_{3}$ for efficient overall water splitting via a direct Z-scheme mechanism. First-principles calculations reveal that ${\mathrm{WTe}}_{2}/X@\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ and ${\mathrm{Tc}\mathrm{Te}}_{2}/X@\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ exhibit high solar-to-hydrogen (STH) efficiencies, reaching up to 25.88%. However, thermodynamic analysis indicates that both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) remain energetically unfavorable. In contrast, ${\mathrm{Al}\mathrm{P}}_{3}/X@\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ and ${\mathrm{In}\mathrm{P}}_{3}/X@\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ heterostructures demonstrate more balanced photocatalytic behavior, achieving STH efficiencies of up to 31.95%, while the HER can be driven spontaneously. Moreover, cocatalysts are necessary for driving the OER, needing external energies of 0.94 and 1.79 eV for ${\mathrm{In}\mathrm{P}}_{3}/X@\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ heterostructures. Furthermore, nonadiabatic molecular dynamics simulations reveal rapid interfacial electron-hole recombination in the ${\mathrm{Tc}\mathrm{Te}}_{2}/X@\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ heterostructure, suggesting enhanced charge separation and transport pathways in the $Z$-scheme configuration. This study highlights the potential of heterostructure engineering combined with out-of-plane doping as a versatile strategy to optimize $\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$-based photocatalysts, and identifies ${\mathrm{In}\mathrm{P}}_{3}/X@\mathrm{g}$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ as promising candidates for high-efficiency solar-driven water splitting.