Role of Steps in the Dissociative Adsorption of Water on Rutile ${\mathrm{TiO}}_2(110)$

The water-${\mathrm{TiO}}_2$ interaction is of paramount importance for many processes occurring on ${\mathrm{TiO}}_2$, and the rutile ${\mathrm{TiO}}_2(110)$-($1\times 1$) surface has often been considered as a test case. Yet, no consensus has been reached whether the well-studied surface O vacancies on the terraces are the only active sites for water dissociation on rutile ${\mathrm{TiO}}_2(110)$-($1\times 1$), or whether another channel for the creation of H adatoms exists. Here we use high-resolution scanning tunneling microscopy and density functional theory calculations to tackle this long-standing question. Evidence is presented that a second water dissociation channel exists on the surfaces of vacuum-annealed ${\mathrm{TiO}}_2(110)$ crystals that is associated with the $⟨1\overline{1}1⟩$ step edges. This second water dissociation channel can be suppressed by blocking of the $⟨1\overline{1}1⟩$ step edges using ethanol.

Figure 1

(a), (b), (c): Selected STM images ($380\AA \times 380\AA$) of a ${\mathrm{TiO}}_2(110)$ surface (a) before and (b) after a 2.5 ML ${\mathrm{H}}_2\mathrm{O}$ exposure at RT, followed by a flash to 368 K [34]. A repeated color scale was used for the presentation of the STM images to show both the step edges and the features on the terraces with high resolution. Roman numerals label the terraces, with “I” being the uppermost and “IV” the lowest ones. In (a), bold black and bold white arrows indicate $⟨1\overline{1}1{⟩}_R$ and $⟨001⟩$ step edges, respectively. Squares indicate ${\mathrm{O}}_{\mathrm{br}}$ vac’s and hexagons ${\mathrm{H}}_{\mathrm{ad}}$ species. Bar graphs in (c) show the densities of ${\mathrm{O}}_{\mathrm{br}}$ vac’s, ${\mathrm{O}}_{\mathrm{S}}$ vac’s ($r\mathrm{\text{−}}{\mathrm{TiO}}_2$) and ${\mathrm{H}}_{\mathrm{ad}}$ species ($h\mathrm{\text{−}}{\mathrm{TiO}}_2$) that were extracted from several STM images. (d), (e): Selected STM images ($380\AA \times 380\AA$) of a $r\mathrm{\text{−}}{\mathrm{TiO}}_2(110)$ surface saturated with EtOH at RT and flashed to 368 K for 120 s (d) before and (e) after 2.5 ML ${\mathrm{H}}_2\mathrm{O}$ exposure. The black arrows indicate ${\mathrm{EtO}}_{\mathrm{S}}$ and the black circles ${\mathrm{EtO}}_{\mathrm{br}}$ species. The bar graphs in (f) show the densities of ${\mathrm{EtO}}_{\mathrm{br}}$ and ${\mathrm{EtO}}_{\mathrm{S}}$ species, the density of bare steps and the density of ${\mathrm{H}}_{\mathrm{ad}}$ species, extracted from several STM images corresponding to the experiments (d) and (e), respectively. In all experiments, the analyzed total areas were $\ge 7000{\mathrm{nm}}^2$.

Figure 2

Computed most stable water adsorption structures at reduced $⟨1\overline{1}1{⟩}_R$ step edges: (a) bare $⟨1\overline{1}1{⟩}_R$ step edge with an ${\mathrm{O}}_{\mathrm{S}}$ vac. (b) Dissociatively adsorbed ${\mathrm{H}}_2\mathrm{O}$ at the step, consisting of ${\mathrm{O}}_{\mathrm{S}}\mathrm{H}$ and ${\mathrm{H}}_{\mathrm{ad}}$ species. (c) ${\mathrm{O}}_{\mathrm{S}}\mathrm{H}$ at the step and a ${\mathrm{H}}_{\mathrm{ad}}$ species on the terrace. (d) ${\mathrm{O}}_{\mathrm{S}}\mathrm{H}$ at the step alone (no ${\mathrm{H}}_{\mathrm{ad}}$ species). (e) ${\mathrm{O}}_{\mathrm{S}}\mathrm{H}$ at the step plus a second dissociatively adsorbed ${\mathrm{H}}_2\mathrm{O}$ at the step. (f) ${\mathrm{O}}_{\mathrm{S}}\mathrm{H}$ and OH at the step plus a ${\mathrm{H}}_{\mathrm{ad}}$ species on the terrace. Computed potential energies (in eV) are given with respect to ${\mathrm{H}}_2\mathrm{O}$ in the gas phase. In (a), ${\mathrm{O}}_{\mathrm{br}}$ atoms, 5f-Ti atoms and ${\mathrm{O}}_{\mathrm{S}}$ vac’s (squares) are indicated. ${\mathrm{O}}_{\mathrm{S}}$ atoms are marked by white crosses, while H’s are shown as small white balls.

Figure 3

Computed most stable EtOH-water coadsorption structures at reduced $⟨1\overline{1}1{⟩}_R$ step edges: (a) bare $⟨1\overline{1}1{⟩}_R$ step edge with an ${\mathrm{O}}_{\mathrm{S}}$ vac. (b) Dissociatively adsorbed EtOH at the step, consisting of ${\mathrm{EtO}}_{\mathrm{S}}$ and ${\mathrm{H}}_{\mathrm{ad}}$ species. (c) ${\mathrm{EtO}}_{\mathrm{S}}$ at the step and a ${\mathrm{H}}_{\mathrm{ad}}$ species on the terrace. (d) ${\mathrm{EtO}}_{\mathrm{S}}$ at the step alone (no ${\mathrm{H}}_{\mathrm{ad}}$ species). (e) ${\mathrm{EtO}}_{\mathrm{S}}$ at the step plus a dissociatively adsorbed ${\mathrm{H}}_2\mathrm{O}$ at the step. (f) ${\mathrm{EtO}}_{\mathrm{S}}$ and OH at the step plus a ${\mathrm{H}}_{\mathrm{ad}}$ species on the terrace. Computed potential energies are given with respect to the molecules in the gas phase. For the ball models, identical colors and symbols were used as in Fig. 2. C atoms of the ${\mathrm{EtO}}_{\mathrm{S}}$ species are shown as medium gray (orange) balls.