The interface between electrode and electrolyte is notoriously difficult to probe. The structure of water at these surfaces affects reactions for many naturally occurring phenomena as well as influencing the performance of electrode materials. Now researchers led by Jian-Feng Li and Jun Cheng have combined Raman spectroscopy and ab initio molecular dynamics simulations to study the orientation of water molecules at electrochemical surfaces. Using their methods, the researchers were able to identify three characteristic water configurations at the interface while sweeping a potential difference across the electrodes.
The researchers used a technique that enhances the Raman signal – scattered light that provides highly detailed information of the structure under study, in this case interfacial water. They placed gold nanoparticles with ultrathin silica shells (to give an inert interface) on a single-crystal Au (111) surface and applied a potential bias to create a “hotspot” between the single-crystal surface and the nanoparticle. At this “hotspot”, the Raman signal is six orders of magnitude larger than the surrounding bulk. From the spectroscopic shifts recorded as they modified the potential across the electrodes the researchers uncovered three distinct regimes of water ordering at the surface.
Simulations point at water orientation
Using ab-initio molecular dynamics simulations (AIMD), the researchers were able to further study the orientation of water molecules at the Au(111) electrode surface. Their simulations showed good agreement with the in situ Raman experiments indicating three different orientations of water molecules as a function of negative potential bias.
Water proves to be electrically dead at interfaces
At a potential bias of -1.3 V below the potential of zero charge (PZC, when the electrode surface is uncharged), the water molecules oriented “parallel” to the electrode surface. Between -1.3 and -1.85 V, the water molecules tilt slightly so that one hydrogen bond tilts to be almost parallel to the surface and the other tilts towards the negatively charged electrode, an orientation the researchers describe as “one-H-down”. At lower voltages, a fraction of the water molecules orient themselves to a “two-H-down” configuration.
These fundamental results greatly improve our understanding of the interfacial water at electrochemical surfaces, which may help efforts to optimize electrode behaviour. Furthermore, this study shows the power of coupling spectroscopic measurements with computational methods.
Full details are reported in Nature Materials.
- This article was updated 13th May 2019 with the article link
