Rb+ and Sr2+ adsorption at the TiO2 (110)-electrolyte interface observed with resonant anomalous X-ray reflectivity

We report the vertical density profiles of Rb(+) and Sr(2+) at the rutile TiO(2)(110)-electrolyte interface for the following bulk electrolyte conditions, [Rb(+)] = 1 mM at pH 11 and [Sr(2+)] = 0.1 mM at pH 10.3, using X-ray reflectivity and resonant anomalous X-ray reflectivity. We find that Rb(+) specifically adsorbs with a coverage of 0.080 +/- 0.003 monolayer (ML) and a height of 3.72 +/- 0.03 A above the surface Ti-O plane. In comparison, Sr(2+) adsorbs with a coverage of 0.40 +/- 0.07 ML and an average height of 3.05 +/- 0.16 A, but with a significant vertical distribution width (0.35 +/- 0.02 A). The Sr(2+) distribution in the presence of a background electrolyte ([Na(+)] = 30 mM) was also investigated, and it is found that, while the ion height and coverage are unchanged within the uncertainties of the measurements, the width of the distribution is apparently increased in the presence of Na(+). Comparison is made with previous results, including XR and X-ray standing waves (XSW) measurements, and molecular dynamics simulations. Our results are in excellent agreement with a recently proposed multisite adsorption mechanism that suggests simultaneous adsorption in two inner-sphere adsorption geometries, the tetradentate and the bidentate sites.