Surface Termination of BaTiO3(1 1 1) Single Crystal: A Combined DFT and XPS Study

The surface termination of tetragonal BaTiO3 is a critical factor that affects the properties of the oxide surface. In this work, we investigated the surface termination of single crystal BaTiO3(1 1 1) using a combined Density Functional Theory (DFT) and X-ray photoelectron spectroscopy (XPS) analysis. The surface stability of the stoichiometric (BaO3 and Ti) and non-stoichiometric (BaO2, O, BaO, O2, Ba, and O3) terminations of BaTiO3(1 1 1) was studied using the DFT + U calculation. The result shows that the BaO2 and O terminations have the lowest cleavage and surface energies, and thus predominate over other terminations in the phase diagrams. Thus, formations of surface defects on the stoichiometric termination effectively stabilize the polar surface of BaTiO3(1 1 1). XPS of the O 1 s and Ti 2p regions of BaTiO3(1 1 1) show the presence of the Ti3+ state and oxygen defects on the surface. Also, the DFT O 1 s chemical shifts of O2*, O*, H2O*, and OH* reveal that OH* species adsorbed on the defect sites of O-terminated BaTiO3(1 1 1) reproduce the experimental XPS chemical shifts. Therefore, a combination of DFT and XPS indicates that BaTiO3(1 1 1) prefers the OH*-covered O termination.