The surface area of nanoporous gold films of thickness up to 930 nm and pores tens of nanometers in size was characterized by various electrochemical methods, including Cu underpotential deposition (UPD), surface Au oxidation/reduction reaction, and electrochemical impedance spectroscopy (EIS). The different approaches provide surface area values that are comparable and, in all cases, linearly increasing with thickness. However, whereas Cu UPD and Au oxidation/reduction methods yielded surface area values in good quantitative agreement, estimates based on EIS measurements were found to be 1.5-1.75 times larger. This discrepancy is tentatively attributed to a higher specific capacitance of nanoporous gold, possibly due both to the presence of residual surface Ag and to the high curvature of the nanoporous structures, that may have different interfacial properties as compared to planar surfaces. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.097204jes] All rights reserved.

We report on the synthesis of nanoporous Ni (NP Ni) materials, discuss their morphology, and investigate their electrocatalytic properties in alkaline electrolytes. NP Ni was obtained by the electrochemical dealloying of co-sputtered, X-ray amorphous Ni-Al alloy films in diluted KOH. The solution pH and applied potential were chosen so that Al would selectively dissolve while Ni would remain immune and would not form a passive layer. Upon dealloying, NP Ni develops a nanofibrous morphology, different from the 3-D network of interconnected ligaments observed for nanoporous Au. This is likely due to the inhomogeneity of the Ni-Al precursor, which may tend to form Al-enriched grain boundaries. Relative to smooth Ni, NP Ni showed little or no catalytic enhancement towards the hydrogen evolution and oxygen evolution reactions. We attribute this effect to either i) the pore occlusion caused by nascent gas, and/or ii) the presence of oxides on the surface which may passivate potential active sites.

The electrodeposition of Au-Ni alloys from near-neutral, sulfite-based electrolytes derived from a commercial bath for soft gold plating is investigated. Alloy compositions ranging from 0 to 90 atom % Ni were obtained by varying the deposition potential, with Ni content increasing with overpotential. Cathodic efficiency was lower than 50% due to concurrent parasitic reactions, including the reduction of products from the decomposition of sulfites and the hydrogen evolution reaction. As-deposited films form a continuous series of metastable solid solutions and exhibit a nanocrystalline morphology, with grain size decreasing with increasing Ni content and a possible Ni enrichment at the grain boundaries. Thermal annealing at 200 degrees C was sufficient to start the relaxation of the metastable solid solution toward the thermodynamically stable biphasic configuration of pure Au and Ni phases; however, 400 degrees C was necessary to complete the phase separation process within similar to 1 h. The formation of a metastable structure is interpreted in terms of the limited surface diffusivities of adatoms at the growing interface and atomic volume differences. The excess free energy of the as-deposited alloys with respect to the stable, phase separated configuration is estimated between 6 and 18 kJ/mol, consistent with what can be expected in electrochemical processing. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3421749] All rights reserved.

We investigate the stress evolution in situ during the potentiostatic electrodeposition of metastable, nanostructured Au-Ni alloy films and develop a correlation between alloy composition, deposition rate, growth mode, and the observed stress state. We find that the internal stresses during Au-Ni deposition can be explained, at least for Ni-rich films, assuming a three-dimensional Volmer-Weber growth mode, where the stress is initially compressive, then transitions to tension, and finally remains tensile over longer times. The observed trends in maximum compressive stress, compressive-to-tensile transition thickness, and net steady-state tensile stress (SSTS) with alloy composition can all be related to the observed decrease in grain size with increasing Ni content. In particular, the SSTS is hypothesized to be the result of a dynamic competition between the stresses generated from nanoscale roughening and adatom insertion into grain boundaries. The Au-rich films, however, do not follow the above trends, suggesting that other stress mechanisms may be operative in this compositional range. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3477933] All rights reserved.