Hussain, H; (2014) Quantitative structural investigations of the water/metal oxide interface. Doctoral thesis , UCL (University College London).

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Abstract

This thesis presents new findings that provide insight into the structural features of the adsorption of water on the TiO2(110), ZnO(10-10) and SrTiO3(001) surfaces. After exposing the TiO2(110)(1 x 1) surface to 1E6 mbar partial pressure of H2O for 3 h a (2 x 1) overlayer is produced as seen with STM. This overlayer was shown to lie in registry with the Ti5c rows. SXRD measurements show that every other surface Ti atom is occupied with an OH in atop position. The same average structure was found after exposing the surface to partial pressures of H2O and after the dipping the surface in 20 ml liquid H2O for 15 s. SXRD measurements were also collected for in-situ immersion of liquid. The results revealed the presence of a hydration layer. We found that adsorption of water from the residual produces a (1 x 1) structure consisting of a slightly shifted H2O/OH molecule on the ZnO (10-10) surface. After exposing the surface to a constant partial pressure of 5E7 mbar, a second water layer was detected with partial occupancy. Significant changes occurred when exposing the surface to 8 mbar partial pressure of H2O. The results revealed that the slightly shifted H2O/OH molecule displaces to a position that is atop of the surface Zn and the second water layer is now fully occupied. SXRD results demonstrate that the SrTiO3 (001) surface is mixed terminated with SrO and TiO2 layers and cover equally large areas. These layers are only partially occupied and leads to a surface coverage TiO2:SrO ratio of 68:32. When contacting this surface with a drop of water, the adsorption mode for the TiO2 terminated terrace is molecular in nature. On the other hand, for the SrO terminated terrace it appears that dissociation is the favoured adsorption mode.

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