Charged Microdroplets Enable Surfactant‐Free Janus Nanoparticles from Homogeneous Precursors

Precise size control of nanoparticle size and uniformity has remained an unmet challenge in microdroplet‐based synthesis, despite recent progress in this area. Here, we demonstrate that electric field–mediated reaction confinement and evaporation dynamics provide a robust handle for size and morphology focusing of nanoparticles in electrosprayed microdroplets. By exploiting this coupling between droplet lifetime, internal electric field gradients, and solute redistribution, we obtained highly monodisperse nanoparticles (8–30 nm) under ambient conditions. Under specific reaction conditions, we observed the formation of fused biphasic (groundnut‐shaped) morphologies via electric field‐driven phase segregation during rapid solvent evaporation in the microdroplets. Building on this mechanistic insight, we extended the strategy to bimetallic systems and, for the first time, achieved direct, surfactant‐free synthesis of Janus Ag–Au nanoparticles from homogeneous precursors, without postsynthetic modification. This work establishes a generalizable principle of tunability based on controlled droplet electrodynamics, enabling both precise size focusing and compositional anisotropy. Such control over nanoparticle architecture opens new directions for scalable design of dual‐site catalysts and multifunctional sensors, demonstrated here in the form of surface‐enhanced Raman spectroscopy.