Formation of Ultrasmall Noble Metal Alloy Nanocrystals in Microdroplets

Alloy nanocrystals (NCs) have attracted considerable attention due to their broad applications. Herein, we present examples of ultrasmall (<5 nm) noble metal alloy NCs, i.e., AuPd and PdPt NCs, synthesized by sprayed water microdroplets. Electrons generated at the air-water interface of microdroplets serve as the primary reducing species, enabling metal ion reduction in the absence of added reductants. Through physical characterizations and mass spectrometry analysis, we elucidate the unique formation mechanism of NCs: the air-water interface of microdroplets simultaneously promotes the reduction and polymerization of precursor ions, followed by efficient surface reduction of the polymerized metal clusters, forming nuclei that undergo subsequent coalescence. The submillisecond reaction time prevents excessive crystal growth, yielding ultrasmall NCs featuring high-energy facets. This work provides insights into nonclassical nucleation and reduction of metal ions, and demonstrates the unique capabilities of microdroplet chemistry in the manipulation of nanomaterial growth dynamics and structures.