Strain-Tuned Z-Scheme Heterostructures of GeH/GeCH ₃ , GeH/InSe, and GeCH ₃ /InSe for Efficient Photocatalysis

Germanane (GeH) and methyl germanane (GeCH3) are promising two-dimensional (2D) materials with sub-2 eV direct band gaps, high electron mobilities, and strong optical responses. Stacking these materials with other 2D systems enables the design of heterostructures (HSs) with tailored electronic structures for optoelectronic and photocatalytic applications. Here, we employ density functional theory (DFT) to investigate van der Waals (vdW) HSs formed by stacking GeH, GeCH3, and InSe monolayers (MLs), focusing on their structural, electronic, and optical properties. The GeH/InSe, GeCH3/InSe, and GeH/GeCH3 HSs exhibit thermodynamic and thermal stability, with direct band gaps of 1.15, 0.14, and 0.79 eV, respectively. While the HSs display type-II band alignments, analysis of interfacial polarization and built-in electric fields suggests that a Z-scheme charge separation mechanism may be favored, facilitating efficient photogenerated carrier separation. Combined with large redox potentials, sufficient charge carrier mobilities, and low exciton binding energies, these features render the HSs promising candidates for photocatalytic water splitting across a wide pH range. Furthermore, strain engineering enables effective band gap tuning while preserving the Z-scheme character. Notably, under compressive strain, GeH/InSe and GeCH3/InSe HSs exhibit band alignments that enable overall water splitting across the entire pH spectrum, underscoring their potential as practical, pH-universal Z-scheme photocatalysts.