Formation of Metastable Water Chains on Anatase TiO₂(101)

Anatase TiO₂ is a pivotal material for energy-harvesting applications and catalysis. In this study, we employ scanning tunneling microscopy (STM) and study water adsorption on most stable TiO₂(101) surface of anatase. Well-defined anatase TiO₂(101) surface exposes alternating rows of two-fold-coordinated oxygen atoms (O_2c) and five-fold-coordinated Ti atoms (Ti_5c) along the [010] direction. We 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. 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 show 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.