Hydrothermal Preparation and Optical Characterization of Silver Nanoparticles with Surface Plasmon Resonance Analysis

Abstract

 In this work, the silver nanoparticles (AgNPs) were synthesized by hydrothermal method and the effect of thermal treatment on structure, morphology and plasmonic properties of the prepared AgNPs were studied. The synthesis process was designed to precisely control the growth of the particles and crystallinity to form nanostructures that can be used for plasmonic applications. The prepared AgNPs were then calcined at 200 °C and 400 °C to check the effect of temperature on the morphology and optical properties of the prepared AgNPs. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) confirmed the formation of a face-centred cubic (FCC) metallic silver phase, with high crystallinity and purity, at 400 °C, where the nanorods had an anisotropic shape with an average diameter of ~7 nm and a length of ~70 nm. The elemental purity and homogeneity of the distribution of Ag in the samples was confirmed by energy-dispersive X-ray spectroscopy (EDX) analysis. The surface plasmon resonance (SPR) feature was seen in the UV–Vis absorption spectra, which is the collective oscillation of the conduction electrons in resonance with the incident absorption light. AgNPs had a well-defined plasmonic absorption peak (~ 410 nm) characteristic of LSPR, being more pronounced for the spherical AgNPs. The nanorod-shaped particles, however, exhibited two plasmonic modes: One transverse mode with a peak around 405 nm (electron oscillations in the direction of the nanrods' length) and a second longitudinal mode with a peak at 850-900 nm (electron oscillations at the cross axis of the nanrods). This resonance splitting and resonance red-shift is a spectral evidence of tunable optical response with temperature, which allows a direct correlation between the particle shape, aspect ratio and the plasmonic behavior. The results demonstrate that the hydrothermal synthesis coupled with thermal calcination under controlled conditions is a suitable approach for the tuning of the plasmonic properties of Ag nanostructures. The modulation of SPR observed here has great potential applications in areas such as plasmonic sensing, photocatalytic and optoelectronic applications where the localization of the electromagnetic field and the tunability of the SPR is crucial for high functional performance.