Nanoscale mechanism of the stabilization of nanoporous gold by alloyed platinum

Abstract

Nanoporous gold (NPG) is usually made by electrochemical dealloying of Ag from binary AgAu alloys. The resulting nanoscale ligaments are not very stable, and tend to coarsen with time by surface self-diffusion, especially in electrolyte, which may lead to inferior electrocatalytic properties. Addition of a small amount of Pt to the precursor alloy is known to refine and stabilize the nanoporous product (NPG-Pt). However, the mechanisms by which Pt serves to refine the microstructure remain poorly understood. The present study aims to expand our knowledge of the role of Pt by examining NPG-Pt at atomic resolution with Atom Probe Tomography (APT), as well as by aberration-corrected Transmission Electron Microscopy. Atomic level observation of Pt enrichment on ligament surfaces sheds light on the underlying mechanisms that give rise to Pt's refining effect. Owing to improved Ag retention with higher Pt content, NPG-Pt (made by dealloying AgAuPt) was shown to have the highest surface area-to-volume ratio, compared to NPG-Pt (made by dealloying AgAuPt). Quantitative estimates reveal up to 5-fold enrichment of Pt at nanoligament surfaces, compared to the precursor content, in NPG-Pt. The interface between the dealloyed layer and the substrate was captured by APT, for the first time. The findings of this investigation add insight into the functionality of NPG-Pt and its prospective catalytic performance.

Synthesis of Nanoporous Au−Cu−Pt Alloy as a Superior Catalyst for the Methanol Oxidation Reaction

“…In addition, the pore size and the ligament width are quite uniform for both np structures. The EDS composition analysis conducted right after the electrochemical stripping process suggests retention of the least noble metal, Cu, in the np structure (SI Figure S1‐C & D) seen and discussed elsewhere as well . The compositions of the de‐alloyed samples are summarized in the SI (Table S1).…”

“…The retention of less noble metal is most likely due to the slow diffusivity of the Pt as well as to the strong alloying tendency of Cu with both Au and Pt. The effect that Pt atoms could act to pin mobile step edges was reported by Newman and Erlebacher in the Au−Ag‐Pt ternary metal system.…”

“…The ECSA Pb UPD of the np Au−Cu−Pt (4 %, 8 % and 20 %) is determined to be 121 m 2 ⋅ g Pt −1 , 87 m 2 /g Pt and 61 m 2 ⋅ g Pt −1 , respectively. Overall, in accordance with earlier results of R. Newman et al, the ECSA decreases with the increase of Pt content.…”

“…Recent studies of np Au−Ag‐Pt ternary alloys by R. Newman's group and np Au−Cu−Pt ternary alloys by our group reveal that the limited mobility of Pt enables the size refining of Au np structures upon less‐noble metal (Ag or Cu) de‐alloying. At the same time, the Pt becomes atomically dispersed on the Au surface layers which in turn entrap a certain amount of Ag in the core of np ligaments .…”

“…Recent studies of np Au−Ag‐Pt ternary alloys by R. Newman's group and np Au−Cu−Pt ternary alloys by our group reveal that the limited mobility of Pt enables the size refining of Au np structures upon less‐noble metal (Ag or Cu) de‐alloying. At the same time, the Pt becomes atomically dispersed on the Au surface layers which in turn entrap a certain amount of Ag in the core of np ligaments . Inspired by research on Pt‐based alloy catalysts, and considering the relatively low cost of Cu along with the finer structure achievable by the use of Cu as de‐alloyed element, we herein report on the synthesis of np Au−Cu−Pt catalysts by facile electrochemical tri‐metal co‐deposition and selective Cu anodic stripping.…”