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 = $\mathrm{S},\mathrm{Se}$) 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.

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• Energy conversion technologies
• Energy materials
• Hydrogen energy
• Photocatalysis
• Heterostructures
• Semiconductor compounds
• Water
• van der Waals systems