First-principles investigation of a promising type-II SiN/SiCH van der Waals heterojunction with superior optical absorption, electronic properties and photocatalytic potential for overall water-splitting

This study presents a van der Waals heterojunction (vdWH) that shows great promise as a photocatalyst, with performance improvements driven by its unique electronic properties. Through first-principles density functional theory calculations, we examined its electronic property, optical behaviour, photoactivity, and structural stability. Interface binding energy calculations identified the AB1 stacking configuration as energetically stable among all configurations tested. The strong interlayer interactions between SiN and SiCH result in a type-II band alignment with an indirect band gap of 1.64 eV. This arrangement drives effective charge separation while limiting electron-hole recombination. The built-in interfacial electric field enhances carrier transport by localizing electrons within the SiN layer and directing holes toward the SiCH layer, thereby enhancing surface redox activity. Our mechanistic analysis reveals a substantially reduced energy barrier for overall water splitting, coupled with favourable water adsorption over the pH 0–5 range. The H-bottom and N-top sites function as the primary active centres for the oxygen and hydrogen evolution reactions. Gibbs free energy calculations show that the hydrogen evolution reaction (HER) has a ΔG of 0.469 eV. This indicates that the heterostructure can drive the HER spontaneously under light irradiation, whereas the oxygen evolution reaction requires an applied potential of 2.33 eV to proceed. These results establish the SiN/SiCH vdWH as a compelling candidate for high-efficiency photocatalytic water splitting. The findings provide practical theoretical guidance for developing advanced photocatalysts for sustainable energy conversion applications.