Effect of the Potentiostatic and Potentiodynamic Dealloying on the Structure and Chemical Distribution of Ag Atoms in Au-Enriched Nanoparticles

Abstract

Recently, nanoporous gold (np-Au) prepared by dealloying has emerged as an efficient and durable catalyst for electrochemical oxidation of small alcohols. [1] An additional increase of surface-to-volume ratio by nano-sizing of AuxAg1-x master alloy might improve the catalytic methanol oxidation reaction (MOR). Only few research groups have recently shown that the morphology, structure and elemental distribution of dealloyed nanoparticles (NPs) depend on the various parameters such as initial particle size and composition.[2-4] Starting from AuxAg1-x alloy NPs, it is, however, unclear how the different dealloyed Au-rich NP motifs such as core-shell, hollow or porous NPs and the Ag distribution influence the catalytic MOR performance.In this work, we have investigated the effects of potentiodynamic and potentiostatic dealloying methods on the structure and chemical distribution of the residual Ag atoms for dealloyed Au-rich NPs. First, two size types of Ag-rich AuxAg1-x alloy NPs were prepared by one pot colloidal route and wet impregnation route. Larger Au23Ag77 NPs (size of 77 ± 26 nm) form a homogeneous disordered alloy, while the Au14Ag86 NPs with a size of 12 ± 5 nm show a strong enrichment of Ag atoms at the particle surface, referred to as core-shell arrangement. Subsequently, the as-prepared NPs with two sizes (12 nm and 77 nm) were electrochemically dealloyed using a potentiodynamic (cyclic voltammetry, CV) or potentiostatic (chronoamperometry, CA) method in 0.1 M HClO4. We observed that the CV method leads to the formation of pores inside the initial 77 nm NPs. Very interestingly, high-resolution STEM-EDX shows the appearance of Ag-rich regions near to the pore network of these dealloyed NPs. Unlike, after the dealloying by CV method the initial 12 nm NPs are still dense and solid, forming a thin Au-rich particle shell. After the exposure to air, the internal Ag atoms tend to segregate to the particle surface. In the CA method, we pointed out that the applied anodic potential has a strong influence of the surface diffusion behavior of the remaining Au atoms. Below a critical potential of 1.3 V vs. RHE, the dealloying process is kinetically hindered by the passivation of Au surface atoms. This critical potential in the CA method is strongly different to the critical dissolution potential of Ag atoms for AgxAu1-x NPs obtained from the CV profile (around 1.05 V vs. RHE). However, when the applied potential is larger than the critical potential, e.g. 1.6 V vs. RHE, the difference in the obtained structure and composition of dealloyed 77 nm NPs is negligible between potentiodynamic and potentiostatic approaches. On the other hand, the potentiostatic dealloying at 1.6 V vs. RHE leads to higher surface content of residual Ag for dealloyed NPs with 12 nm compared to potentiodynamic. This increased enrichment of the residual Ag atoms near the particle surface is also supported by XPS.In summary, we correlate the structural parameters (initial particle size, composition/distribution) of dealloyed AgxAu1-x NPs with the electrochemical dealloying methods (potentiostatic vs. potentiodynamic). Tailoring the surface diffusion rate of the Au atoms and the Ag distribution helps to improve the catalytic performance for MOR.