First-Principles Study of the Auxetic and Photocatalytic Properties of Rippled Ge₉C₁₅ Monolayers: Implications for Photocatalytic Water Splitting

Two-dimensional (2D) materials comprising Group-IV elements have garnered significant attention owing to their captivating properties and immense potential for application in nanotechnology. Based on first-principles calculations, we propose a stable configuration of a 2D germanium carbide material, namely, Ge 9 C 15 monolayer, which exhibits a unique rippled geometry. Our calculations reveal that this Ge 9 C 15 monolayer exhibits an anisotropic Young’s modulus ranging from 25.3 to 70.4 GPa·nm, as well as auxeticity characterized by a negative Poisson’s ratio of up to −0.6. The coexistence of sp 2 and sp 3 hybridization, along with mixed binding characteristics, results in a direct bandgap of 2.06 eV. Remarkably, the electronic properties of the rippled Ge 9 C 15 monolayer, including bandgaps, band edges, and work function, remain robust even under extensile strains of up to 6%. Additionally, it exhibits high sunlight absorption and an appropriate band edge, rendering it highly promising for photocatalytic water splitting. The analysis of Gibbs free energy reveals that the rippled Ge 9 C 15 monolayer possesses photogenerated electrons with a highly favorable redox potential; multiple sites throughout the material fulfill the criteria of hydrogen reduction reaction. These findings expand the application scope of 2D Group-IV materials to diverse fields such as photocatalysis, electronic devices, and nanomechanics.