Formation and Diffusion of Water Dimers on Rutile ${\mathrm{TiO}}_2(110)$

From an interplay of time-lapsed high resolution scanning tunneling microscopy and density functional theory calculations we reveal the formation and diffusion of water dimers on hydrated rutile ${\mathrm{TiO}}_2(110)\mathrm{\text{−}}(1\times 1)$ surfaces, i.e., surfaces containing ${\mathrm{OH}}_{\mathrm{br}}$ groups. At temperatures between $\sim 150$ and $\sim 210\mathrm{K}$ water monomers diffusing along the Ti troughs were found to form stable water dimers that diffuse faster than the water monomers. An H bond mediated rollover mechanism operating for the water dimers explains the experimental findings.

Figure 1

Snapshots and line profiles from STM movie “Dimer_formation” recorded at $\sim 179\mathrm{K}$ [23]. (a)–(c) Two water monomers in a Ti trough diffuse toward each other (centers marked by blue squares). (d) A water dimer (center marked by a blue circle) has formed from the two monomers. A lattice grid was centered on top of $5f\mathrm{\text{−}}\mathrm{Ti}$ sites using single H adatoms (white small dots) for alignment in the [001] direction.

Figure 2

Diffusion data for water monomers and dimers for two $h\mathrm{\text{−}}{\mathrm{TiO}}_2(110)$ surfaces with different H adatom densities.

Figure 3

Snapshots from STM movie “Dimers-Tetramer” that show the formation and splitting of a water tetramer from and into water dimers at $\sim 161\mathrm{K}$ [26]. Small, blue circles are used to mark some of the water dimers, and the rectangle indicates a water tetramer. Hexagons indicate H adatoms.

Figure 4

Ball models (top and side views) for (a)–(d) water monomer adsorption and diffusion as well as (e)–(j),(l) water dimer diffusion. (k) PW91-based potential energy diagram for the proposed water dimer diffusion mechanism. (m)–(o) Ball models (top views) of alternative, less stable, water dimer configurations. Adsorption potential energies are given in eV, black and gray (red) encoded for PW91 and RPBE, respectively.