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 center dot center dot center dot 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 center dot center dot center dot 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 (E-ads = -0.91 to -1.12 eV) or adsorption of CO2 on a surface oxygen defect site (E-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 V 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 delta-/Vo -> COadsorbed + O-surface. This result has an important implication for understanding the photoreduction of CO2 by using Zn2GeO4 nanoribbons.