Structure of water layers on hydrogen-covered Pt electrodes

The structure of water layers above hydrogen-covered Pt(1 1 1) surfaces at room temperature has been studied by ab initio molecular dynamics simulations based on periodic density functional theory calculations. Fully hydrogen-covered Pt(1 1 1) with additionally either a hydrogen vacancy or another hydrogen adatom have been considered. The resulting structures have been analyzed in detail as a function of the hydrogen coverage. In particular, the thermal disorder in the water layer is examined in terms of deviations from the ice lattice, orientational inhomogeneity within a water bilayer, as well as the onset of proton transfer. On hydrogen-covered Pt(1 1 1), the water layer is located at a much larger distance from the Pt atoms than on the pure metal surfaces. Surprisingly, the more weakly bound water layer on the hydrogen-covered Pt(1 1 1) electrode exhibits a greater order than the water layer on clean Pt(1 1 1) which is attributed to the stronger water-water interaction above hydrogen-covered Pt(1 1 1). The relevance of our findings for the understanding of electrochemical electrode/water interfaces is discussed.