Atomic-scale computational design of hydrophobic RE surface-doped Al₂O₃ and TiO₂

Intrinsically hydrophobic rare-earth oxides (REOs) have emerged as a robust class of ceramics for a variety of applications. Recently, the hydrophobicity of REOs has been observed experimentally and subsequently scrutinized using electronic structure density functional theory (DFT) calculations. In this work, we applied the DFT method to analyze the possibility of tuning the wettability of commonly used hydrophilic Al₂O₃ and TiO₂ by surface doping with Ce. The calculations indicate that Ce can preferentially segregate to the surface of Al₂O₃ and TiO₂ and form a Ce-rich oxide layer, which is stable under a wide range of oxygen chemical potentials. A remarkable increase in the water contact angle is predicted for Ce-doped Al₂O₃(0001), whereas the water contact angle calculated for Ce-doped TiO₂(110) remains unchanged, regardless of the Ce concentration. The wetting properties of Ce-doped Al₂O₃ are governed by two factors: (1) the unique electronic structure of the rare-earth metal promotes hydrogen bond formation between H₂O and surface oxygen; (2) significant relaxation of the surface Ce and O atoms hampers direct interaction between H₂O and Al cations, preventing dissociative water adsorption. These results provide a valuable opportunity for Al₂O₃ surface modification, in terms of achieving hydrophobicity.