Thermally Driven Ag-Au Compositional Changes at the Ligament Surface in Nanoporous Gold: Implications for Electrocatalytic Applications

Nanoporous gold (NPG) is a versatile, nanoporous bulk material with applications in catalysis, sensors, energy materials, and actuation. NPG obtained from dealloying Ag-rich Au-Ag alloys has interesting applications in electrocatalysis, for which methanol oxidation is an interesting test case. Such materials always contain a content of residual silver which may play a role in activating and binding of reaction intermediates. The residual Ag that is contained in NPG due to the dealloying of a Ag-Au alloy was recently found to be present in silver-rich islands, in contrast to being homogeneously distributed. Because of the importance of residual Ag on the surface science and catalytic properties of NPG, we investigated its distribution and chemical nature with the use of X-ray photoelectron spectroscopy. During the in situ annealing, changes of the near surface content of residual Ag were observed, and they were linked to coarsening of the gold ligaments. In addition, a depth-dependent analysis of synchrotron-based XPS results confirmed the surface segregation in the near surface layers of each ligament. The available data neither confirm nor exclude a further systematic enrichment or depletion of silver in the topmost atomic layer for NPG during electrochemical treatments in KOH solution or during thermal coarsening.