An investigation of the structure of water layers at plane and modified metal surfaces

• Main Author: McBride, Fiona
• Format: Dissertation
• Language: English
• Published: University of Liverpool 2012
• Subjects: QD Chemistry
• DOI: 10.17638/00007113

The work presented in this thesis details the structural and chemical flexibility of water layers on a selection of plane and templated metallic surfaces. The water layers are found to adapt their structure to achieve a compromise between optimising its surface and intermolecular interactions differently in each system investigated. This compromise often results in water layers which do not stay in strict registry with the substrate, instead forming complex structures. Modifying the substrate by introducing a secondary metal affects the adsorption of water, the structure and species formed, indicating the sensitivity of water to the exact geometric and electronic structure of the substrate. Initially focussing on plane (non-templated) surfaces, we find an intact water layer on Pd(111), with a (root3 x root3)R30 LEED pattern but a disordered helium atom scattering signal. Using a combination of techniques we propose that the water layer comprises of regions of flat lying water, tightly bound atop Pd(111), separated by anti-phase domain boundaries. Water in the domain boundaries forms from H-bonded rings of water, oriented mostly H-down, interacting weakly with the surface. The disorder in the layer is likely to be in the H-down network and hampers attempts to achieve a complete picture of the detailed water structure. On Ni(110), a preliminary STM study into the structure and dissociation of water reveals that water forms a mixture of diffuse and more rigidly held hexagonal structures at low temperature. We assign the diffuse structures to chains of intact water which are labile under the influence of the tip, with the more rigidly held structures being a mixture of OH/H2O. The proportion of dissociated water increases with dose temperature, and is associated with loss of the labile structure associated with intact water by 200 K. Further study is required in order to establish if water adsorbs intact to Ni(110) at temperatures below 100 K. Creating a Pt skin alloy on a Ni(111) substrate allows us to investigate how a change in the Pt environment perturbs water adsorption. Water dissociates spontaneously on this surface, in a marked departure from its behaviour on the pure Ni or Pt surfaces. Pre-dosing the Pt/Ni(111) surface with oxygen has a negligible effect on the water desorption behaviour, confirming that the mixed OH/H2O phase is less stable on the Pt/Ni(111) surface than on Pt(111). We suggest that the reduced stability of OH(ads) groups on the Pt/Ni surface leads to the improved oxygen reduction reaction efficiency reported for this alloy, making OH less likely to act as a poison, as it is believed to on Pt(111). Based on our understanding of the optimum water adsorption site, we created a r3 Sn/Pt(111) alloy, designed to stabilise a traditional "ice-like" bilayer water structure. The water layer was investigated using HAS, LEED-IV and DFT modelling, which confirm that the water structure is indeed a simple commensurate root3 bilayer. Based on LEED-IV measurements we report the first quantitative structural study of monolayer water adsorbed at a metal surface and compare this to DFT predictions. Maintaining the (root3 x root3)R30 symmetry and altering the host metal from Pt to root3 Sn/M (M = Rh, Cu and Ni) resulted in no stable wetting structures. Alloying Sn with these metals appears to reduce the ability of Sn to accept electron density from the O of water, reducing the water-Sn interaction and leading to water forming clusters.