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 O1s and C1s core levels with a binding-energy separation ΔE_(O1s-C1s)=242.1±0.2eV . 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 (Θ<0.2) , CO₂ is predicted to strongly bond (E_ads∼-1eV) 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.