Exploiting Z-scheme charge-carrier dynamics in MA2Z4-based van der Waals heterostructures for bias-assisted water-splitting applications

Abstract

The MA₂Z₄ (M = transition metal; A = Si, Ge; Z = N, P, As) monolayers, characterized by exceptional stability and tailorable electronic structures, offer a versatile platform for constructing van der Waals heterostructures for water-splitting-related applications. Here, we systematically investigate six MA₂Z₄-based heterostructures—specifically MoSi₂P₄/MoSi₂As₄, MoSi₂P₄/WSi₂P₄, WSi₂P₄/WSi₂As₄, CrSi₂P₄/CrSi₂As₄, CrSi₂P₄/WSi₂P₄ and CrSi₂P₄/MoSi₂P₄—using first-principles calculations. All candidates exhibit intrinsic direct bandgaps and robust type-II band alignment. The resultant interfacial built-in electric fields facilitate a Z-scheme migration pathway, which ensures the spatial separation of photogenerated carriers while preserving potent redox potentials essential for overall water splitting. Model-based theoretical solar-to-hydrogen efficiency estimations give a maximum value of 38.65%. However, Gibbs free energy analyses for the hydrogen and oxygen evolution reactions reveal that, among the six systems, WSi₂P₄/WSi₂As₄, CrSi₂P₄/WSi₂P₄, and CrSi₂P₄/MoSi₂P₄ exhibit greater thermodynamic feasibility for photocatalytic water splitting, though the reactions are not spontaneous without bias. Furthermore, by integrating strain engineering with non-adiabatic molecular dynamics simulations, we delineate a competitive preservation of redox capacities: WSi₂P₄/WSi₂As₄ excels in reduction protection, whereas CrSi₂P₄/WSi₂P₄ prioritizes oxidation potency. Notably, the CrSi₂P₄/WSi₂P₄ heterostructure demonstrates ultrafast carrier dynamics with an interfacial electron–hole recombination time of 0.2 ps, significantly enhancing Z-scheme efficiency. Under 4% tensile strain, the of CrSi₂P₄/WSi₂P₄ reaches 36.21% among the thermodynamically more favorable candidates, highlighting the tunability of MA₂Z₄-based heterostructures for bias-assisted water-splitting applications.