Abstract
The rutile (110)-aqueous solution interface structure was measured in deionized water
(DIW) and 1 m RbCl + RbOH solution (pH 12) at 25°C with the X-ray crystal truncation rod
method. The rutile surface in both solutions consists of a stoichiometric (1×1) surface unit
mesh with the surface terminated by bridging oxygen (BO) and terminal oxygen (TO) sites.
An additional hydration layer is observed above the TO site, with three distinct water
adsorption sites each having well-defined vertical and lateral locations. Rb+ specifically
adsorbs at the tetradentate site between the TO and BO sites, replacing one of the adsorbed
water molecules at the interface. There is no further ordered water structure observed above
the hydration layer. Structural displacements of atoms at the oxide surface are observed to be
sensitive to the solution composition. Ti atom displacements from their bulk lattice positions,
as large as 0.05 Å at the rutile (110)-DIW interface, decay in magnitude into the crystal with
significant relaxations that are observable down to the fourth Ti-layer below the surface. A
systematic outward shift was observed for Ti atom locations below the BO rows, while a
systematic inward displacement was found for Ti atoms below the TO rows. The Ti
displacements were mostly reduced in contact with the RbCl solution at pH 12, with no
statistically significant relaxations in the fourth layer Ti atoms. The distance between the
surface 5-fold Ti atoms and the oxygen atoms of the TO site is 2.13 ± 0.03 Å in DIW and
2.05 ± 0.03 Å in the Rb+ solution, suggesting that water adsorbs mainly in molecular form to
the rutile (110) surface at the TO site in DIW, while it is primarily in the form of an adsorbed
hydroxyl group at pH 12.