Pt–Ag cubic nanocages with wall thickness less than 2 nm and their enhanced catalytic activity toward oxygen reduction

Abstract

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.