Nanostructured Silver Microparticles from Microdroplet Reduction and Evaporative Assembly for Plasmon-Driven Photocatalysis and SERS

Abstract

Plasmonic nanoparticle aggregates are excellent plasmonic photocatalysts and substrates for surface-enhanced Raman spectroscopy (SERS) due to localized surface plasmon resonance (LSPR) coupling and hot-spot generation. Yet facile but scalable synthesis methods for such morphology remain limited. Herein, we introduce a green, one-step spray-drying process involving the reduction of silver nitrate by ascorbic acid in evaporating aqueous microdroplets, facilitating in situ reduction and subsequent evaporation-induced self-assembly (EISA) of Ag nanocrystals into hierarchical microparticle aggregates. Reduction to metallic Ag was confirmed by X-ray diffraction (XRD). Electron microscopy and small-angle X-ray scattering (SAXS) revealed aggregate morphology with constituents of polydisperse primary particles with sizes of a few tens of nanometers. Diffuse-reflectance UV–vis spectroscopy demonstrated broad LSPR absorption arising from interparticle plasmonic coupling. When used as a plasmonic photocatalyst for the photocatalytic reduction of p-nitrothiophenol (p-NTP), time-resolved SERS revealed complete dimerization of p-NTP to 4,4′-dimercaptoazobenzene (DMAB) under 514 nm illumination with stretched exponential kinetics, whereas partial conversion was observed at 633 nm and no conversion at 830 nm. When used for the oxidation of p-aminothiophenol (p-ATP), complete conversion was observed for 633 nm illumination, and partial conversion to DMAB occurred at 830 nm. When applied as a SERS substrate, analytical SERS enhancement factors of 103 were determined using 4-methylbenzenethiol. The Ag microparticles (MPs) were able to produce a strong SERS signal for the detection of organophosphate pesticides at parts-per-billion levels. This droplet-accelerated, scalable approach yielded hierarchical Ag MPs for efficient plasmonic photocatalysis and SERS-based environmental sensing.