Pt–Ag cubic nanocages with wall thickness less than 2 nm and their enhanced catalytic activity toward oxygen reduction†
We report a facile synthesis of Pt–Ag nanocages with walls thinner than 2 nm by depositing a few atomic layers of Pt as conformal shells on Ag nanocubes and then selectively removing the Ag template via wet etching. In a typical process, we inject a specific volume of aqueous H2PtCl6 into a mixture of Ag nanocubes, ascorbic acid (H2Asc), NaOH, and poly(vinylpyrrolidone) in water under ambient conditions. At an initial pH of 11.9, the Pt(IV) precursor is quickly reduced by an ascorbate monoanion, a strong reducing agent derived from the neutralization of H2Asc with NaOH. The newly formed Pt atoms are deposited onto the edges and then corners and side faces of Ag nanocubes, leading to the generation of Ag@Pt core–shell nanocubes with a conformal Pt shell of approximately three atomic layers (or, about 0.6 nm in thickness) when 0.4 mL of 0.2 mM H2PtCl6 is involved. After the selective removal of Ag in the core using an etchant based on a mixture of Fe(NO3)3 and HNO3, we transform the core–shell nanocubes into Pt–Ag alloy nanocages with an ultrathin wall thickness of less than 2 nm. We further demonstrate that the as-obtained nanocages with a composition of Pt42Ag58 exhibit an enhanced catalytic activity toward the oxygen reduction reaction, with a mass activity of 0.30 A mg−1 and a specific activity of 0.93 mA cm−2, which are 1.6 and 2.5 times, respectively, greater than those of a commercial Pt/C catalyst.