Adsorption Bond Length for ${\mathrm{H}}_2\mathrm{O}$ on ${\mathrm{TiO}}_2(110)$: A Key Parameter for Theoretical Understanding

Scanned-energy mode photoelectron diffraction results show the adsorption site of molecular water on ${\mathrm{TiO}}_2(110)$ to be atop under-coordinated surface Ti atoms, confirming the results of total energy calculations and STM imaging. However, the $\mathrm{Ti}\mathrm{\text{−}}{\mathrm{O}}_{\mathrm{water}}$ bond length is $2.21\pm 0.02\AA$, much longer than Ti-O bond lengths in strongly chemisorbed species on this surface, but significantly shorter than found in most total energy calculations. The need for theory to describe this weak bond effectively may be a key factor in the controversial problem of understanding this important surface reaction system.

Figure 1

Soft x-ray photoelectron spectra in the energy range of the O $1s$ emission recorded at a photon energy of 700 eV from a clean ${\mathrm{TiO}}_2(110)$ surface, and from the same surface following low temperature exposure to water and subsequent heating. The two main component peaks due to emission from the ${\mathrm{TiO}}_2$ substrate, and from the adsorbed water, are labeled.

Figure 2

Comparison of the full set of experimental O $1s$ PhD spectra (full lines) from adsorbed ${\mathrm{H}}_2\mathrm{O}$ on ${\mathrm{TiO}}_2(110)$ used in the structural optimization, with the results of the calculations (dashed lines) for the best-fit surface geometry.

Figure 3

Schematic view of the local adsorption site for ${\mathrm{H}}_2\mathrm{O}$ on ${\mathrm{TiO}}_2(110)$ atop the fivefold coordinated Ti atoms of the surface. The H atoms within the ${\mathrm{H}}_2\mathrm{O}$ molecule are included for illustration only; their location is not known from this study.