Paper SS+EM+HC+MI-ThM6
Formation of Metastable Water Chains on Anatase TiO₂(101)

Thursday, November 2, 2017, 9:40 am, Room 25

The interaction of water with metal oxide surfaces is of great importance in many diverse areas such as catalysis, electrochemistry, corrosion, atmospheric science, geology, astrophysics, and others. Anatase TiO₂ surface is particularly relevant because it is the most active polymorph of TiO₂ and its commercially employed nanomaterials grow preferentially as anatase. In this study, we employ scanning tunneling microscopy (STM) to study the adsorption of water on a model stoichiometric anatase TiO₂(101) surface. Well-defined anatase TiO₂(101) surface has saw-tooth-like morphology exposing alternating rows of two-fold-coordinated oxygen atoms (O_2c) and five-fold-coordinated Ti atoms (Ti_5c) along the [010] direction. Our STM data show that at 80 K isolated water monomers bind molecularly to the Ti_5c sites. The onset of diffusion is found at ~190 K where water monomers diffuse both along and across the Ti_5c rows. The analysis shows that the along-the-row diffusion is energetically favored by only 0.03 eV. Surprisingly, we find that at 80 K water molecules start to form linear chains along the Ti_5c rows as the coverage is increased. This indicates the presence of transient mobility of water molecules suggesting that the adsorption occurs via a precursor state. When the water chains are annealed at 190 K, they fall apart to monomers that reside on the next-nearest-neighbor Ti_5c sites. These results demonstrate that the water chains are metastable in nature. This is at odds with many other oxide surfaces where hydrogen-bonded water clusters are energetically preferred over the isolated monomers.