Abstract
An inexpensive, fast, selective, and sensitive technique for surface area measurement of metallic nanoporous materials (MNPM) is developed, systematically tested, and validated. The approach employed is based on underpotential deposition (UPD) of metals on foreign substrates. In this work, Pb UPD on Au is chosen to illustrate the applicability of and reveal the advantages and limitations of the proposed method. Experiments are designed for surface area measurement of nanoporous gold (NPG) electrodes with pore sizes in the range of 5−15 nm, prepared by electrochemical dealloying of single phase AupAg1−p (atomic fraction p = 0.1, 0.2, and 0.3). Dealloying is performed galvanostatically at a current density of 1 mA cm−2 in a AgClO4 solution, acidified to pH 1. The experimental results suggest a linearly increasing charge in the Pb UPD layer with NPG thickness. This finding hints at (i) uniformity of the NPG structure and (ii) the general ability of this method to work for analysis of bulk materials. The proposed approach is tested by studying the dependence of the NPG surface area upon the original alloy composition and correlating the results with the NPG structure and morphology imaged by high-resolution scanning electron microscopy. An anomalously high surface area is registered in dealloyed Au0.1Ag0.9 samples and is attributed to the lack of a pre-existing percolation backbone. Unlike the instantaneous Pb UPD process on a flat metal surface, the slow and thickness-dependent kinetics of Pb layer formation on NPG is associated with hindered mass transport through pores. Further validation of the Pb UPD method is made by experimental monitoring of heat treatment-enforced coarsening and the basic modeling of the correlation between surface area and ligament size in NPG. Finally, a critical comparison with Brunauer−Emmett−Teller (BET) analysis reveals important advantages of the developed method for surface area measurement in MNPM specimens.
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