We present a study of the chemical and atomic properties of 5 nm TiO2-terminated BaTiO3 (0 0 1) epitaxial films on Nb-doped SrTiO3, as a function of the atomic oxygen plasma power for film growth. Lower plasma power produces non-stoichiometric films with oxygen vacancies and Ti3+ ions. The larger Ti3+ ion radius and the in-plane clamping gives rise to an increase in the out-of-plane lattice parameter. XPS measures the Ti3+ concentration and the concomitant increase in dissociative water uptake in the film, giving rise to on-top OH- adsorption on surface Ti, proton adsorption on surface oxygen, and a near surface Ba-OH environment. (C) 2013 Elsevier B.V. All rights reserved.