ABSTRACT
Macroscopic net proton charging curves for powdered rutile and cassiterite specimens with the (110) crystal face predominant, as a function of pH in RbCl and NaCl solutions, trace SrCl(2) in NaCl, and trace ZnCl(2) in NaCl and Na Triflate solutions, are compared to corresponding molecular-level information obtained from static DFT optimizations and classical MD simulations, as well as synchrotron X-ray methods. The similarities and differences in the macroscopic charging behavior of rutile and cassiterite largely reflect the cation binding modes observed at the molecular level. Cation adsorption is primarily inner-sphere on both isostructural (110) surfaces, despite predictions that outer-sphere binding should predominate on low bulk dielectric constant oxides such as cassiterite (ε(bulk) ≈ 11). Inner-sphere adsorption is also significant for Rb(+) and Na(+) on neutral surfaces, whereas Cl(-) binding is predominately outer-sphere. As negative surface charge increases, relatively more Rb(+), Na(+), and especially Sr(2+) are bound in highly desolvated tetradentate fashion on the rutile (110) surface, largely accounting for enhanced negative charge development relative to cassiterite. Charging curves in the presence of Zn(2+) are very steep but similar for both oxides, reflective of Zn(2+) hydrolysis (and accompanying proton release) during the adsorption process, and the similar binding modes for ZnOH(+) on both surfaces. These results suggest that differences in cation adsorption between high and low bulk dielectric constant oxides are more subtly related to the relative degree of cation desolvation accompanying inner-sphere binding (i.e., more tetradentate binding on rutile), rather than distinct inner- and outer-sphere adsorption modes. Cation desolvation may be favored at the rutile (110) surface in part because inner-sphere water molecules are bound further from and less tightly than on the cassiterite (110) surface. Hence, their removal upon inner-sphere cation binding is relatively more favorable.
ABSTRACT
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.
