Atomic-scale topography of rutile TiO₂(110) in aqueous solutions: A study involving frequency-modulation atomic force...

The interfaces between metal oxides and liquids represent the next frontier in the study of oxide chemistry. In this work, (110)-oriented rutile TiO₂ wafers were annealed in oxidative atmospheres and immersed in aqueous KCl solutions of pH 3, 6, and 11. Topographic imaging of the TiO₂ wafers was carried out in solution via atomic force microscopy using the frequency-modulation force detection technique. Crystalline terraces of 100 nm in width were observed with no sign of solution-induced etching. In a pH-6 solution, ridges parallel to the [001] axis with trenches in between were observed and assigned to the rows of oxygen anions protruding from the surface plane to the solution. Individual anions were further resolved in the ridges, revealing atomic-size protrusions located on the (1 × 1) meshes of the (110) truncation. The topography in an acidic solution (pH 3) was similar to that observed in a neutral solution and could be interpreted as protruding oxygen anions covered partially by protons. In a basic solution with pH 11, qualitatively different features were observed; atomic-size swellings formed a p(2 × 1) superstructure covering the surface, which was hypothesized to be Ti-OH^- on five-fold coordinated Ti cations in the surface plane. These results show the feasibility of advanced atomic force microscopy for probing metal-oxide surfaces submerged in liquids.