Surface area of nanoporous gold: Effect on temperature

Nanoporous Au in two distinctly different surface states, namely, positively charged as-dealloyed and reduced clean metallic were investigated for temperature dependent properties like specific capacitance C_s and specific mass surface area α_m. An electrochemical double layer capacitance ratio method was employed to carry out these studies. It is shown that for reduced clean metallic nanoporous Au sample, the variation in ln (C_s) versus 1/T plot is almost linear, which supports to the negative temperature coefficient predicted by the double layer model. This observation is in direct contrast with positively charged as-dealloyed nanoporous Au that did not reveal any such trend. We compare the specific-mass surface area α_m of nanoporous Au values with the geometrical surface area α_g, obtained by using the "ball-stick geometrical model" coupled with scanning electron microscopy images. Evolution of ultra small nanometric structures during dealloying renders positively charged as-dealloyed nanoporous Au vulnerable to oxidation and ligament coarsening up on its reduction. The validation of the specific-mass surface area obtained from the capacitance ratio method was confirmed by Brunauer-Emmett-Teller analysis and the process of adsorption/desorption of OH adsorbed species on pore surfaces in reduced clean metallic np-Au.