Plasma-Assisted Trimetallic Nanoparticles and Nanoalloys: A Review of Synthesis Routes, a Reaction–Transport Framework, and Evidence Standards with an Au–Ag–Pd Case Study

Abstract

Trimetallic nanoalloys combine three metals within a nanoscale object and thereby offer a larger compositional, structural, and electronic design space than mono- or bimetallic particles. Their performance depends less on the nominal three-metal label than on atomic distribution, ordering, surface segregation, oxidation state, particle size, support interaction, and the dynamic structure under reaction conditions. This review evaluates trimetallic nanoparticles produced by chemical co-reduction, sequential and seed-mediated growth, galvanic replacement, photochemical and radiolytic routes, microwave and sonochemical synthesis, biological reduction, laser ablation and deposition, gas-phase sputtering, and plasma-assisted processing. Particular emphasis is placed on plasma–liquid interactions as a rapid, reagent-lean route for generating solvated electrons, hydrogen radicals, peroxide, ultraviolet radiation, shock waves, and transient interfacial fields. Direct evidence establishes plasma formation of several mono-, bi-, and multimetallic nanostructures, while atomic-scale proof of fully alloyed trimetallic products remains less common. The review therefore distinguishes demonstrated mechanisms from transferable process hypotheses. It presents a reaction–transport framework, including electron-kinetic parameters, a Damköhler-number treatment of the reaction/transport balance, and a reduction-potential analysis of the three precursor couples, for controlling asynchronous precursor reduction, nucleation, alloying, and ligand-lean stabilization; defines the characterization required to prove a trimetallic nanoalloy; compares catalytic, electrocatalytic, optical, sensing, antimicrobial, biomedical, and environmental applications; and proposes measurable criteria for scale-up, safety, energy efficiency, and circular use of critical metals. Au–Ag–Pd is reserved for the final future-perspectives case study, where the conflicting reduction and oxidation chemistry of its three precursors is used to define decisive experiments for the next generation of plasma–liquid synthesis.