Size-tunable synthesis of Ag@Pt core-shell nanoparticles as highly active electrocatalysts using ultrafine bubbles and ultrasound.

In this study, core-shell nanoparticles (NPs) composed of an Ag core and a Pt shell (Ag@Pt NPs) were synthesized. The Ag core was first synthesized using a combination of ultrasound and chemical reduction, followed by the deposition of the Pt shell by chemical reduction in the presence of a non-ionic surfactant and ultrafine bubbles (UFBs). The Pt shell thickness increased in the presence of UFBs. During the shell formation, Pt NPs adsorbed onto the surface of UFBs through electrostatic and hydrophobic interactions. Subsequently, UFBs approached the surface of the Ag core and provided additional Pt NPs onto the core surface, thereby increasing the Pt shell thickness. This demonstrates that the Pt shell thickness can be controlled via UFBs, which enables the efficient utilization of Pt NPs as shell materials. The catalytic activity of Ag@Pt NPs for the oxidation of methanol was evaluated. At Pt/Ag molar ratios of 1.0 and 2.0, the catalytic activity was improved as the Pt shell thickness decreased. For Pt/Ag molar ratios of 0.50 and 0.66, however, Ag@Pt NPs synthesized with UFBs exhibited better catalytic performance than those without UFBs. Morphological analysis suggests that at a Pt/Ag molar ratio of 0.66, UFB-assisted delivery of Pt NPs onto the Ag core contributed to the formation of a thin Pt shell with enhanced Pt coverage on the Ag core, thereby maximizing the catalytic performance. This work establishes a basis for the efficient and environmentally friendly synthesis of highly active metallic NP electrocatalysts.