|First Principles Investigations on the Interface between Electrode and Electrolyte |
Laboratory for Surface Modification
|Timo Jacob, Fritz-Haber-Institut der MPG, Germany|
FRIDAY, 10:30 AM, Chem. 260
Highly-disperse nanoparticles are often used to catalyze (electro-)chemical reactions. Unfortunately not all nanoparticles have the same size and shape, but rather show a relatively large distribution. Since the electronic properties of the nanoparticles are correlated with their morphology, experimental measurements usually represent the averaged behavior of (almost) the entire ensemble of particles. This limits our understanding of the ongoing processes and makes direct comparison with theoretical studies difficult.
In the group of Ted Madey it was recently found that on particular rough surfaces certain adsorbates are able to induce the formation of well-defined nanostructures after annealing the system to elevated temperatures.1,2 Motivated by these observations we used density functional theory and the extended ab initio atomistic thermodynamics approach2 to study the adsorption of oxygen and nitrogen on the different surface faces, which are involved in the nanostructuring of Ir(210) and Re(11-21). Constructing the (p,T)-surface phase diagrams of the corresponding surfaces in contact with an oxygen, respectively nitrogen, atmosphere, we find that at high temperatures the planar surfaces are stable, while lowering the temperature stabilizes those nanofacets found experimentally. Afterwards we constructed the (p,T,)-phase diagrams for Ir(210) and Re(11-21) in contact with an aqueous electrolyte and found that the same nanofacets should be stable under electrochemical conditions. Quite recently the group of Kolb at University of Ulm was able confirm this theoretical prediction by cyclic voltammetry and in-situ scanning tunneling microscopy. The presence of nanofacets for Ir(210) gives rise to a characteristic current-peak in the hydrogen adsorption region for sulfuric acid solution. In addition, for Ir(210) we already started to investigate the electrocatalytic behavior of the nanofacets and found that their activity for typical surface reactions is considerably lower than that of the planar Ir(210) surface.
Since understanding electrochemical systems requires an atomistic picture of the electrode/electrolyte-interface, afterwards DFT-calculations are presented on the experimentally well-known Au(111)/H2SO4 double-layer. At potentials of E > +0.8 V vs. SCE various surface sensitive techniques found evidence for a (√3×√7)R19.1° (bi)sulfate structure, but the nature of coadsorbates remains still unclear. Focusing on a sulfate adlayer, the coadsorption of H3O+ and/or H2O has been studied, showing the highest binding energy for a single H3O+ per sulfate. In addition, the charge density distribution within the resulting adlayer well agrees with effective barrier heights deduced from recent distance tunnelling spectroscopy measurements . Similarly we studied the interfacial structure that forms at negative electrode potentials and found that water arranges near the electrode in an ice-like hexagonal structure with hydronium ions being located in the second water layer and non-specifically adsorbed.
 I. Ermanoski, C. Kim, S. P. Kelty, T. E. Madey, Surf. Sci. 596, 89 (2005).
 H. Wang, W. Chen, T. E. Madey, Phys. Rev. B 74, 205426 (2006).
 T. Jacob, J. Electroanal. Chem. 607, 158 (2007).
 S. Venkatachalam, F. C. Simeone, D. M. Kolb, T. Jacob, Angew. Chem. Int. Ed. 46, 8903 (2007).