Adsorption of CO2 at ZnO: A Surface Structure Effect from DFT+U Calculations

A complete understanding of the carbon dioxide (CO2) interaction with zinc oxide (ZnO) is a basis for the development of new ZnO-containing materials for catalytic fixation of CO2 into useful chemicals such as methanol. In this work, the density functional theory plus U (DFT+U) method coupled with periodic boundary conditions has been employed to investigate the adsorption of CO2 molecules on five relevant exposed surfaces of ZnO in an effort to capture the surface chemistry of the adsorbed molecules. Through our calculations, we demonstrated that the CO2 adsorption is sensitive to the oxide surface structure. The activation of CO2 by ZnO via the carbonate-ion formation requires the presence of empty surface Zn-O pairs. It was found that the binding strength of CO2 follows the order of ZnO(000 (1) over bar) <= ZnO(0001) < ZnO(11<(2)over bar>1) < ZnO(11<(2)over bar>0) <= ZnO(10 (1) over bar0), which is, surprisingly, different from the decreasing stability sequence of these surfaces (ZnO(10 (1) over bar0) > ZnO(11 (2) over bar0) > ZnO(000 (1) over bar)/ZnO(0001) > ZnO(11 (2) over bar1)). The counterintuitive difference was discussed in detail on the basis of the adsorption energy decomposition into two antagonistic effects: adsorbate-substrate interaction and their deformation energies. The bonding nature of CO2 to the oxide substrates was analyzed, and the 2p states of CO2 were verified to be able to mix both with the 4s orbital of surface zinc and with the 2p orbitals of surface oxygen. Our calculations indicate that different from the previous viewpoint CO2, as an electronegative adsorbate, may also be positively charged by certain ZnO surfaces, like the stepped (11 (2) over bar1) facet presented here. Interestingly, the stronger the electron transfer between the two interacting moieties, the stronger their interaction, and the stronger the C=O bond activation. Also, it has been observed that the change in the surface work function upon the adsorption of CO2 does not completely depend on the sign of charge transferred.