Controlled Dealloying of Alloy Nanoparticles toward Optimization of Electrocatalysis on Spongy Metallic Nanoframes

Abstract

Atomic-level understanding of the structural transformations of multimetallic nanoparticles triggered by external stimuli is of vital importance to the enhancement of our capabilities to fine-tailor the key structural parameters and thereby to precisely tune the properties of the nanoparticles. Here, we show that, upon thermal annealing in a reducing atmosphere, Au@Cu2O core-shell nanoparticles transform into Au-Cu alloy nanoparticles with tunable compositional stoichiometries that are predetermined by the relative core and shell dimensions of their parental core-shell nanoparticle precursors. The Au-Cu alloy nanoparticles exhibit distinct dealloying behaviors that are dependent upon their Cu/Au stoichiometric ratios. For Au-Cu alloy nanoparticles with Cu atomic fractions above the parting limit, nanoporosity-evolving percolation dealloying occurs upon exposure of the alloy nanoparticles to appropriate chemical etchants, resulting in the formation of particulate spongy nanoframes with solid/void bicontinuous morphology composed of hierarchically interconnected nanoligaments. The nanoporosity evolution during percolation dealloying is synergistically guided by two intertwining structural rearrangement processes, ligament domain coarsening driven by thermodynamics and framework expansion driven by Kirkendall effects, both of which can be maneuvered by controlling the Cu leaching rates during the percolation dealloying. The dealloyed nanoframes possess large open surface areas accessible by the reactant molecules and high abundance of catalytically active undercoordinated atoms on the ligament surfaces, two unique structural features highly desirable for high-performance electrocatalysis. Using the room temperature electro-oxidation of methanol as a model reaction, we further demonstrate that, through controlled percolation dealloying of Au-Cu alloy nanoparticles, both the electrochemically active surface areas and the specific activity of the dealloyed metallic nanoframes can be systematically tuned to achieve the optimal electrocatalytic activities.

Dealloyed Nanoporous Gold Catalysts: From Macroscopic Foams to Nanoparticulate Architectures

“…Under certain dealloying conditions, almost all bimetallic alloys feature a characteristic threshold p known as the parting limit, above which percolation dealloying occurs. The parting limits of Au−Ag and Au−Cu alloys were determined to be ∼55–60 atomic% of Ag,, and ∼70 atomic% of Cu, respectively, in acidic etching environments at room temperature.…”

“…For chemoselective hydrogenation reactions under low pressures, the catalytic activity decreases with the increase in the fraction of residual Ag, which exhibits an opposite trend to the aerobic oxidation reactions . For electrocatalytic oxidation reactions that does not involve O 2 , no clear correlation between the residual less noble elements and catalytic activity has been observed so far ,. The exact nature of the active sites on the dealloyed nanoporous Au for various reactions is still a fundamentally intriguing topic under intense debate and well‐worthy of further investigations.…”

“…A general design criterion for structural optimization of catalysts is to maximize both the surface area‐to‐volume ratio and the density of surface active sites. Using the electrocatalytic methanol oxidation reaction (MOR) as a model reaction, we recently demonstrated that both the mass‐specific surface area and the density of surface active sites of spongy nanoparticles could be systematically fine‐tuned through kinetically controlled percolation dealloying of Au−Cu alloy nanoparticles ,. Analogous to that of the macroscopic Au−Ag alloy membranes, the percolation dealloying of Au−Cu alloy nanoparticles is essentially dictated by two interplaying structure‐transforming processes, leaching of Cu atoms from the alloy and coarsening of the nanoligaments.…”

“…We started from Au@Cu 2 O core‐shell nanoparticles whose core and shell dimensions could be precisely tuned over a broad size range using a seed‐mediated growth method . The Au@Cu 2 O core‐shell nanoparticles first transformed into Au−Cu bimetallic heteronanostructures through chemical reduction, and then underwent intraparticle alloying to form Au−Cu alloy nanoparticles upon thermal treatment in either a reducing atmosphere, such as H 2 , or in a high boiling point‐polyol solvent, such as tetraethylene glycol. The sizes and Cu/Au stoichiometric ratios of the alloy nanoparticles were essentially predetermined by the core and shell dimensions of their parental Au@Cu 2 O core‐shell nanoparticles and thereby could be systematically tuned over a broad range.…”

“…The relative rate of Cu leaching with respect to that of the ligament coarsening can be tuned by choosing different etchants or varying the etchant concentrations . By kinetically trapping the partially dealloyed spongy nanoframes (NFs) at various dealloying stages, both the mass‐specific surface area and the density of surface active sites can be controlled.…”