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, Ge9C15 monolayer, which exhibits a unique rippled geometry. Our calculations reveal that this Ge9C15 monolayer exhibits an anisotropic Young's modulus ranging from 25.3 to 70.4 GPanm, 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 Ge9C15 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 Ge9C15 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.