Atomic Structure of Water Monolayer on Anatase TiO₂(101) Surface

The molecular structure and interactions of interfacial water, i.e., the first monolayers of water adsorbed on surfaces, are critical to determine the microscopic properties of solid-liquid interfaces. In particular, interfacial water on TiO₂ anatase (101) is of specific relevance to industrial, environmental, and medical applications. However, owing to the complexity and challenges in sample preparation, direct observation has been limited to individual or short-range ordered water molecules adsorbed on anatase. In this article, we report the direct observation of an unprecedented long-range ordered water monolayer on TiO₂ anatase (101). The adsorption structure and inter/intramolecular interactions of the water molecules have been investigated using scanning tunneling microscopy (STM) at 5 K in conjunction with density functional theory calculations. Additionally, the effect of STM tip-induced electric field was calculated to elucidate the ground-state structure of the water monolayer. The water molecules commensurately adsorb on the anatase (101) substrate; each oxygen atom of water forms a dative bond with one of its lone pair electrons to a surface Ti_5c atom. The remaining lone pair electron and OH bonds are oriented in the surface plane resulting in hydrophobic properties. The herein presented results are important to understand the complex wetting properties of the TiO₂ anatase surface.