Theoretical Study of H₂O Adsorption on Zn₂GeO₄ Surfaces: Effects of Surface State and Structure–Activity Relationships

We employed the density functional theory to investigate the interaction of H₂O with Zn₂GeO₄ surfaces, considering both perfect and defective surfaces. The results revealed that the interaction of H₂O with Zn₂GeO₄ surfaces was dependent on the structure of the latter. For perfect surfaces, H₂O adsorbed at the Ge_3c···O_2c site of a (010) surface could spontaneously dissociate into an H atom and an OH group, whereas H₂O tended to adsorb at the O_2c-M_3c-O_3c site of a (001) surface by molecular adsorption. The presence of oxygen defects was found to strongly promote H₂O dissociation on the (010) surface. Analysis of the surface electronic structure showed a large density of Ge states at the top of the valence band for both perfect and defective (010) surfaces, which is an important factor affecting H₂O dissociation. In contrast, perfect and defective (001) surfaces with surface Ge states buried inside the valence band were significantly less reactive, and H₂O was adsorbed on these surfaces in the molecular form. This information about the adsorbate geometries, catalytic activity of various surface sites, specific electronic structure of surface Ge atoms, and their relevance to surface structure will be useful for the future design of the Zn₂GeO₄ photocatalyst, as well as for the atomistic-level understanding of other structure-sensitive reactions