Ab initio studies of a water layer at transition metal surfaces

Abstract

This paper presents a detailed study of a water adlayer adsorbed on Pt(111) and Rh(111) surfaces using periodic density functional theory methods. The interaction between the metal surface and the water molecules is assessed from molecular dynamics simulation data and single point electronic structure calculations of selected configurations. It is argued that the electron bands around the Fermi level of the metal substrate extend over the water adlayer. As a consequence in the presence of the water layer the surface as a whole still maintains its metallic conductivity—a result of a crucial importance for understanding the process of electron transfer through the water/metal interface and electrochemical reactions in particular. Our results also indicate that there exists a weak bond between the hydrogen of the water and the Rh metal atoms as opposed to the widespread (classical) models based on purely repulsive interaction. This suggests that the commonly used classical interactions potentials adopted for large scale molecular dynamics simulations of water/metal interfaces may need revision. Two adsorption models of water on transition metals with the OH bonds pointing towards or away of the surface are also examined. It is shown that due to the very close values of their adsorption energies one should consider the real structure of water on the surface as a mixture of these simple “up” and “down” models. A model for the structure of the adsorbed water layer on Rh(111) is proposed in terms of statistical averages from molecular dynamics simulations.

Monte Carlo Study on the Water Meniscus Condensation and Capillary Force in Atomic Force Microscopy

“…58 This was attributed to the hydrogen-down configuration being energetically more stable than any other orientation of molecule. 59,60 Similarly, a relatively small positive peak follows a negative dip located near h = d (near the tip). This suggests that water molecules near the tip orient preferentially so that the hydrogen atoms are closer to the tip (here, the hydrogen up orientation is preferred).…”

Understanding the electrocatalysis of oxygen reduction on platinum and its alloys

“…42,43 The theoretical modelling of electrochemical reactions is equally complex, as it needs to account for the effect of the solvent on the adsorbed intermediates, the highly charged electric field in the double layer, the free energy of the electrons in the solid and the free energy of the solvated reactants as a function of potential. [44][45][46][47][48][49][50][51][52][53][54] However, it turns out that the overall trends can be 7 ‡ We note that a fuel cell would probably be operated at potentials lower than 0.9 V, to maximise the power output. However, catalysts are typically benchmarked at 0.9 V to minimise artefacts from the measurements.…”

Tuning Hydrophobicity of Platinum by Small Changes in Surface Morphology

“…For water adsorbing on undeuterated Pt, STM studies have shown that on (100) step edges, water initially forms one-dimensional chains along the steps, whereas for (110) steps these are not clearly adsorbed [16]. On deuterium-covered Pt, the interaction of water with such surfaces will be driven by the (in)ability of water to form relatively stable two-dimensional hydrogen-bonded networks extending onto the terraces, versus the tendency of water to be structured into solvation shells of the adsorbed deuterium.…”

“…On deuterium-covered Pt, the interaction of water with such surfaces will be driven by the (in)ability of water to form relatively stable two-dimensional hydrogen-bonded networks extending onto the terraces, versus the tendency of water to be structured into solvation shells of the adsorbed deuterium. We note that from density-functional theory (DFT) calculations, the interaction of water layers with Pt(111), i.e., the water-Pt bonding, is known to be very weak and its stability on such surfaces is mainly determined by the two-dimensional hydrogen bonding network [16]. Therefore, we expect that the fact that adsorbed deuterium inhibits the direct interaction of water with the platinum surface atoms, will have only a minor effect on its stability.…”