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CO2 Adlayers on the Mixed Terminated ZnO(10-10) Surface Studied by He Atom Scattering, Photoelectron Spectroscopy and Ab Initio Electronic Structure Calculations

He-atom scattering (HAS), X-ray photoelectron spectroscopy (XPS and NEXAFS), periodic density functional theory and quantum chemical ab initio cluster calculations have been used for characterizing the adsorption of CO2 on the mixed terminated ZnO(10-10) surface. The HAS data allow to identify three different structural phases of CO2 in the monolayer regime: A high-density (1×1) phase at low temperature, a lower density (2×1) phase at higher temperatures, and in addition an incommensurate phase (which has not been reported before) with a slightly lower coverage and a 2.3-fold periodicity at surface temperatures above 270 K. XPS and NEXAFS measurements performed for the two low-coverage phases show that CO2 is adsorbed in form of a carbonate species. This is supported by the ab initio calculations for CO2 adsorbed on ZnO(10-10). The calculated XP and NEXAFS spectra agree with the experimental data and are only compatible with an adsorbed carbonate species, the presence of physisorbed CO2 can be excluded. In addition, the calculations demonstrate that X-ray emission spectroscopy (XES) is a sensitive method to distinguish between linear CO2 and carbonate.