Electrochemically Triggered Pore Expansion in Nanoporous Gold Thin Films

Nanoporous electrode coatings have played a significant role in enhancing the performance of catalysts and sensors. For optimal performance in these applications, the fundamental requirement is a large effective surface area (site at which catalysis and sensing events occur) with unhindered transport of reactants and products to/from the active surface. This necessitates low-density porous electrodes with an interconnected 3D network of thin conductive ligaments to maintain high effective surface area. While the logical approach to create such electrodes is to etch the ligaments uniformly through the entire porous network, accomplishing this has not been trivial. Here, we use nanoporous gold (np-Au) as a model material system to demonstrate an electrochemically triggered etching method for restructuring sputter-deposited submicron np-Au thin films for enlarging the pores with minimal decrease in the effective surface area. We systematically employ time-varying potential waveforms to electrochemically modify morphologies and reveal underlying mechanisms of the etching process. The results suggest that the etch cycle at positive potentials plays a dual role of electrophoretic attraction of chloride ions and initiating the electrochemical etch. The final nanoporous morphology is dictated by a competition between ligament coarsening and ligament thinning, which provide a means to generate a wide range of electrode morphologies.