Enhancing Hydrophilicity of Anatase TiO2 Surfaces by Deposition of Alkaline Earths: The Case of Ca

We show with DFT calculations within the COSMO framework that the wetting energy of clean or hydroxylated anatase TiO2 surfaces is strongly enhanced by Ca deposition at low coverage. Two mechanisms contribute to this behavior: the ionic character of Ca adsorption and the inhomogeneity of the transferred charge density, leading together to a strong polarization of the embedding medium. In turn, this polarization is responsible for a large wetting energy and the predicted hydrophilicity. By increasing coverage, Ca adsorption becomes more and more neutral and the transferred charge density is progressively more homogeneous: the effects of the two mechanisms are gradually reduced, leading eventually to surface hydrophobicity at full coverage. In order to support this theory, investigations of the wetting properties of TiO2 slabs in the presence of methane are presented.