Structure of the Rutile ${\mathrm{TiO}}_2(011)$ Surface in an Aqueous Environment

First principles simulations are carried out to investigate the structure and stability of the rutile ${\mathrm{TiO}}_2(011)$ surface in contact with liquid water. Whereas this surface exhibits a ($2\times 1$) reconstruction in vacuo, our results show that the interaction with water leads to an inversion of the stabilities of the reconstructed and unreconstructed surfaces. This indicates that surface structures determined in vacuo or at low water coverages are not generally representative of those occurring in the aqueous environments typical of most photocatalytic applications of ${\mathrm{TiO}}_2$.

Figure 1

Slab models of the rutile ${\mathrm{TiO}}_2(011)$ surface: (a) unreconstructed ($1\times 1$) surface; (b) ($2\times 1$)-reconstructed surface. Color code: $\mathrm{Ti}=\mathrm{\text{light gray}}$ and $\mathrm{O}=\mathrm{\text{red or dark gray}}$. (c) Potential energy profile along the $(1\times 1)\to (2\times 1)$ reconstruction pathway of a single side of the slab, as obtained by a multisegment NEB calculation in a ($2\times 2$) surface supercell. The total energy gain in the reconstruction is $\sim 2.60\mathrm{eV}/(2\times 2)$ cell. The arrows indicate the breaking of Ti-O bonds.

Figure 2

Total and partial densities of states for (a) the clean nonreconstructed surface; (b) the nonreconstructed surface with eight adsorbed water molecules (one monolayer) at the Ti surface sites; (c) the dry reconstructed surface; and (d) the reconstructed surface with eight adsorbed water molecules (only four of them bonded to surface Ti sites). Arrows indicate the Fermi energy. Energy scales have been aligned at the vacuum level (energy zero).

Figure 3

Total potential energy evolution and corresponding probability distribution during 20 ps FPMD simulations for the nonreconstructed (black) and reconstructed (gray) slabs with their surrounding liquid water (64 molecules) environments. The results refer to a ($2\times 2$) surface supercell.

Figure 4

(a) Fully relaxed water monolayer on the nonreconstructed surface, (b) typical snapshot (only first water layer shown), and (c) fraction of dissociated water molecules during the 360 K FPMD trajectory. Panel (d) shows the energy evolution and structures during a single water dissociation NEB calculation. Color code: $\mathrm{Ti}=\mathrm{\text{light gray}}$, $\mathrm{O}=\mathrm{\text{red or dark gray}}$, for clarity in (a) and (b) water O is shown in orange or medium gray and lower ${\mathrm{TiO}}_2$ layers are shown translucent. The results refer to a ($2\times 2$) surface supercell.

Figure 5

Front views (top row) and top views (bottom row) of a water dissociation event at a valley Ti site with subsequent outward relaxation of the Ti atom, shortening of the Ti-OH bond and migration of the H fragment to a ridge oxygen. Water O atoms bound to the Ti site are shown in orange or medium gray. For clarity subsurface layers of the slab are transparent and the water over-layer has been omitted in the top views.