Photoemission and quantum chemical study of SrTiO3(001) surfaces and their interaction with CO2

The electronic structure of SrTiO3 single-crystal surfaces and their interaction with CO2 at room temperature is studied by angle-resolved x-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) and density-functional theory (DFT). CO2 exposure results in spectral features in the O 1s and C 1s core levels with a binding-energy separation Delta E(O 1s-C 1s)=242.1 +/- 0.2 eV. No measurable influence of CO2 exposure on Sr and Ti core level spectra is observed. Adsorbate induced changes in XPS core levels and UPS valence-band spectra do not support SrCO3 formation. Surface sites and bonding geometry of chemisorbed CO2 on SrTiO3(001) surfaces are investigated using DFT-adsorbate-slab calculations and the calculated surface density of states compared to UPS spectra. On defect free surfaces and at lower coverages (Theta < 0.2), CO2 is predicted to strongly bond (E-ads similar to-1 eV) to both SrO and TiO2 terminated surfaces in a monodentate structure with the C atom above a lattice oxygen. Adsorption energies, electron transfer to adsorbate, and bonding geometry are found to be strongly coverage dependent with smaller adsorption energies on TiO2 terminated surfaces at higher coverages. These results have important implications for the identification of metal-carbonate layers on perovskite-structure metal titanate materials by photoemission spectroscopies.