Core-Level Binding Energy Reveals Hydrogen Bonding Configurations of Water Adsorbed on ${\mathrm{TiO}}_2(110)$ Surface

Using x-ray photoelectron spectroscopy of the oxygen $1s$ core level, the ratio between intact (${\mathrm{D}}_2\mathrm{O}$) and dissociated (OD) water in the hydrated stoichiometric ${\mathrm{TiO}}_2(110)$ surface is determined at varying coverage and temperature. In the submonolayer regime, both the ${\mathrm{D}}_2\mathrm{O}:\mathrm{OD}$ ratio and the core-level binding energy of ${\mathrm{D}}_2\mathrm{O}$ ($\mathrm{\Delta }\mathrm{BE}$) decrease with temperature. The observed variations in $\mathrm{\Delta }\mathrm{BE}$ are shown with density functional theory to be governed crucially and solely by the local hydrogen bonding environment, revealing a generally applicable classification and details about adsorption motifs.

Figure 1

(a) Structural model and (b) O $1s$ spectra for water molecules adsorbed on ${\mathrm{TiO}}_2(110)$ surface, at two different temperatures, 210 and 97 K, for an uptake of about 0.2 ML. The BE of O $1s$ for the clean $\mathrm{s}\text{−}{\mathrm{TiO}}_2$ surface has been taken as reference.

Figure 2

Real-time monitoring of coverage upon water adsorption on $\mathrm{s}\text{−}{\mathrm{TiO}}_2$ surface at three temperatures, 97, 160, and 210 K.

Figure 3

(a) ${\mathrm{D}}_2\mathrm{O}/\mathrm{OD}$ ratio and (b) difference in BE ($\mathrm{\Delta }\mathrm{BE}$) of O $1s$ peaks between ${\mathrm{D}}_2\mathrm{O}$ and ${\mathrm{TiO}}_2$ versus total coverage at three temperatures, 97, 160, and 210 K.

Figure 4

(a) Top view of three possible geometric configurations ($\mathbb{M}\to \mathbb{D}$, $\mathbb{M}\to \mathbb{M}$, and $\mathbb{D}\to \mathbb{M}$) for 30% coverage of water molecules on ${\mathrm{TiO}}_2(110)$. (b) Adsorption energy versus coverage.