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

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

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

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

Surface Dependence of CO₂ Adsorption on Zn₂GeO₄

An understanding of the interaction between Zn₂GeO₄ and the CO₂ molecule is vital for developing its role in the photocatalytic reduction of CO₂. In this study, we present the structure and energetics of CO₂ adsorbed onto the stoichiometric perfectly and the oxygen vacancy defect of Zn₂GeO₄ (010) and (001) surfaces using density functional theory slab calculations. The major finding is that the surface structure of the Zn₂GeO₄ is important for CO₂ adsorption and activation, i.e., the interaction of CO₂ with Zn₂GeO₄ surfaces is structure-dependent. The ability of CO₂ adsorption on (001) is higher than that of CO₂ adsorption on (010). For the (010) surface, the active sites O_2c···Ge_3c and Ge_3c–O_3c interact with the CO₂ molecule leading to a bidentate carbonate species. The presence of Ge_3c–O_2c···Ge_3c bonds on the (001) surface strengthens the interaction of CO₂ with the (001) surface, and results in a bridged carbonate-like species. Furthermore, a comparison of the calculated adsorption energies of CO₂ adsorption on perfect and defective Zn₂GeO₄ (010) and (001) surfaces shows that CO₂ has the strongest adsorption near a surface oxygen vacancy site, with an adsorption energy −1.05 to −2.17 eV, stronger than adsorption of CO₂ on perfect Zn₂GeO₄ surfaces (<i>E</i>_ads = −0.91 to −1.12 eV) or adsorption of CO₂ on a surface oxygen defect site (<i>E</i>_ads = −0.24 to −0.95 eV). Additionally, for the defective Zn₂GeO₄ surfaces, the oxygen vacancies are the active sites. CO₂ that adsorbs directly at the Vo site can be dissociated into CO and O and the Vo defect can be healed by the oxygen atom released during the dissociation process. On further analysis of the dissociative adsorption mechanism of CO₂ on the surface oxygen defect site, we concluded that dissociative adsorption of CO₂ favors the stepwise dissociation mechanism and the dissociation process can be described as CO₂ + Vo → CO₂^δ−/Vo → CO_adsorbed + O_surface. This result has an important implication for understanding the photoreduction of CO₂ by using Zn₂GeO₄ nanoribbons

A Theoretical Study of Water Adsorption and Decomposition on Low-Index Spinel ZnGa₂O₄ Surfaces: Correlation between Surface Structure and Photocatalytic Properties

Water adsorption and decomposition on stoichiometrically perfect and oxygen vacancy containing ZnGa₂O₄ (100), (110), and (111) surfaces were investigated through periodic density functional theory (DFT) calculations. The results demonstrated that water adsorption and decomposition are surface-structure-sensitive processes. On a stoichiometrically perfect surface, the most stable molecular adsorption that could take place involved the generation of hydrogen bonds. For dissociative adsorption, the adsorption energy of the (111) surface was more than 4 times the energies of the other two surfaces, indicating it to be the best surface for water decomposition. A detailed comparison of these three surfaces showed that the primary reason for this observation was the special electronic state of the (111) surface. When water dissociated on the (111) surface, the special Ga_3c-4s and 4p hybridization states at the Fermi level had an obvious downshift to the lower energies. This large energy gain greatly promoted the dissociation of water. Because the generation of O_3c vacancy defects on the (100) and (110) surfaces could increase the stability of the dissociative adsorption states with few changes to the energy barrier, this type of defect would make the decomposition of water molecules more favorable. However, for the (111) surface, the generation of vacancy defects could decrease the stability of the dissociative adsorption states and significantly increase their energy barriers. Therefore, the decomposition of water molecules on the oxygen vacancy defective (111) surface would be less favorable than the perfect (111) surface. These findings on the decomposition of H₂O on the ZnGa₂O₄ surfaces can be used toward the synthesis of water-splitting catalysts