Shape Controlled Synthesis of Au/Ag/Pd Nanoalloys and Their Oxidation-Induced Self-Assembly into Electrocatalytically Active Aerogel Monoliths

Abstract

The synthesis of size and shape controlled Au/Ag/Pd alloy nanoparticles (NPs) and their self-supported assembly into monolithic aerogels for electro-oxidation of ethanol is reported. Two distinct morphologies of ultrasmall (3–5 nm) Au/Ag/Pd alloy NPs were produced via stepwise galvanic replacement of thiol-coated Ag NPs. The resultant nanoalloys were self-assembled into large, free-standing, aerogel superstructures that exhibit direct NP connectivity, high surface area (269 ± 18.1–76 ± 6.4 m2/g) and mesoporosity (2–50 nm), and high electrocatalytic activity via controlled oxidation of the surfactant ligands. The gelation kinetics have been tuned by varying the oxidant/surfactant molar ratio that governs the acidity of sol–gel reaction and consequently the extent of Ag dealloying with in situ generated HNO3. As-synthesized Au/Ag/Pd aerogels exhibit polymeric or colloidal gel morphology that can be manipulated by varying the shape and composition of precursor NPs. The electrocatalytic activity of ternary alloy aerogels for oxidation of ethanol was investigated using cyclic voltammetry and chronoamperometry. The monolithic aerogels exhibit high catalytic activity and durability, which is ∼20–30 times greater than those of the discrete Au/Ag/Pd alloy NPs. The polymeric morphology of high Pd-containing alloy aerogels resulted in ∼1 order of magnitude higher current density and mass activity in comparison to low Pd-containing colloidal aerogels. The synergistic effect of trimetallic alloy mitigates the catalyst poisoning effects and increases the stability and durability while the self-supported superstructure with direct NP connectivity, high surface area, and mesoporosity offers a facile conduit for both molecular and electron transport, enabling Au/Ag/Pd aerogel as a high-efficiency electrocatalyst.