Top-down and Bottom-up Reconstruction of Matter in Charged Water Microdroplets

Abstract

The synthesis of metal nanoparticles is conventionally achieved either by top-down fragmentation of bulk metals by mechanical milling or by bottom-up chemical reduction of metal salts, with each route typically relying on specific methods, distinct infrastructure, and conditions. In this work, we show that charged water microdroplets provide a unified platform that supports both pathways, and the results are presented for noble metals. Micron-sized powders of silver and copper undergo efficient top-down fragmentation in charged microdroplets, yielding stable crystalline nanoparticles under mild, reagent-free conditions, whereas gold powders remain largely resistant to fragmentation under identical electrospray parameters. In parallel, bottom-up electrospray of acetate solutions of silver, copper, and gold generates nanoparticles of all three metals via reduction and nucleation within the same microdroplet environment. The contrasting top-down responses of Ag and Cu versus Au, together with the universal bottom-up formation for all of them, reveal a metal-dependent fragmentation behaviour that is independent of initial particle morphology, yet governed by intrinsic material properties. These results establish charged water microdroplets as a versatile, green, and scalable medium in which complementary top-down and bottom-up mechanisms for noble metal nanoparticle formation coexist, expanding the scope of microdroplet chemistry in nanomaterial synthesis.