Atomic scale X-ray studies of the electrical double layer structure at the rutile titanium dioxide (110)-aqueous interface.

摘要

When a metal oxide surface comes in contact with an aqueous solution, an electrical double layer (EDL) is formed at the interface. The EDL region greatly affects many natural and industrial processes. Efforts for more than a century have been put forward to understand the features of the EDL. However, with little atomic scale structural knowledge, the ability is very limited to test current competing models and further understand or predict EDL properties. In this work, the surface and the adsorbate structure at the rutile TiO 2 (110)-aqueous interface is probed with synchrotron based X-rays.; Combining X-ray standing wave (XSW) imaging, which is direct and model independent, with tradition XSW triangulation, precise atom positions and absolute coverages are achieved. Crystal truncation rod (CTR) measurements yield the interfacial structure.; It has been revealed the rutile (110) surface termination and structure and the specifically adsorbed ion locations while contacting with the bulk water. In the aqueous solution, both the bridging (BO) and the terminal oxygen (TO) rows are present and the surface undergoes minimal relaxations. An additional layer of water molecules with well-defined vertical and lateral positions are formed on top of surface oxygen groups. No more water structure is found farther away from the interface. The metal ions, including mono-, di-, and tri-valent ions, are all found to be 'inner sphere' adsorbates at the rutile (110)-aqueous interface. The adsorption location is primarily determined by the ion sizes. The larger ions, like Rb+, Sr 2+, and Y3+, take the tetradentate positions, which are of equal distances to the two TO and BO atoms. Small ions, like Zn 2+, are at the extended bulk Ti positions.; With monovalent ions as the only background electrolytes at concentrations 1 mol/kg, we found that, the adsorbed divalent ions are independent of the type of the background electrolyte and the solution ionic strength; both Zn2+ and Sr2+ ions adsorbed in the condensed layer saturate around 0.4 monolayer; negligible amount of divalent ions are in the diffuse layer; no evidence of surface ternary complexation is observed; and the Gouy-Chapman-Stem model is more appropriate for describing divalent ion distribution in the EDL.