Spontaneous Silver Reduction and Nanostructure Formation at Water–Hydrophobic Interfaces

Abstract Water–hydrophobic interfaces (WHIs) are ubiquitous in natural and synthetic systems, yet their role as chemically active environments remains poorly understood. Here, we report the spontaneous formation of monodisperse silver nanostructures at WHIs in the absence of added reducing agents, surfactants, or light. Across a variety of silver salts and hydrophobic media, a free-standing interfacial film reproducibly develops and can be transferred onto diverse substrates. Electron microscopy reveals extended domains composed of monodisperse silver nanoparticles arranged in ordered assemblies over micrometer length scales despite the absence of conventional directing agents. Transmission electron microscopy further uncovers a continuous silver-rich amorphous phase containing ultrasmall nanoparticles (∼2 to 3 nm) distributed throughout the interfacial film. Time-resolved observations reveal a progressive structural evolution from this amorphous phase through defect-rich and non-fcc intermediates toward crystalline fcc silver, suggesting a non-classical pathway for nanoparticle formation. Optical spectroscopy of the aqueous phase indicates the presence of silver clusters and nanoparticle growth products, while gas analysis reveals concomitant oxygen generation during the process. Taken together, these observations indicate that the WHI behaves as a persistent thin-layer reactor (TLR) capable of driving spontaneous silver reduction and directing nanoparticle assembly under interfacial water confinement. We propose that the unique physicochemical environment of the WHI also promotes the accumulation and stabilization of intermediate silver species, enabling the emergence of ordered nanostructures without external reducing or capping agents. These findings provide insight into nanoparticle formation resulting from the unique properties of WHIs and establish the role of interfacial water as an active component in nanoscale self-organization.