Interfacial electronic coupling in MX-MX (M = Cu, Ag, and Au; X = Cl, Br, and I) van der Waals heterostructures for photocatalytic water splitting and broadband optoelectronic applications.

The structural, electronic and optoelectronic properties of two-dimensional (2D) van der Waals heterostructures (vdWHs) can be effectively modulated for high-performance energy applications. Here, we use first-principles density functional theory to systematically investigate the structural stability, electronic properties, interfacial charge coupling, and optical and photocatalytic performance of MX-MX (M = Cu, Ag, and Au; X = Cl, Br, and I) vdWHs. The optimized vdWHs exhibit favourable energetic stability, with type-I and type-II semiconducting nature and pronounced interfacial electronic coupling, promoting the efficient separation of photogenerated charge carriers. Charge redistribution analysis further confirms intrinsic interlayer charge transfer and the formation of built-in electric fields across the interface of MX-MX vdWHs. The calculated optical spectra show a strong broadband optical absorption extending from the near-infrared to ultraviolet regions, accompanied by an enhanced dielectric response and high absorption coefficients, indicating the excellent light-harvesting capability of MX-MX vdWHs. Band edge alignment in comparison to water redox potentials demonstrates that AgBr-CuI vdWHs possess a suitable energetic alignment for overall photocatalytic water splitting at pH = 0, while the other vdWHs are mainly favourable for photocatalytic oxidation reactions. Our findings highlight MX-MX vdWHs as a promising candidate for solar-driven hydrogen production, photocatalysis, and next-generation broadband optoelectronic devices.