Single-Step Synthesis and Surface Plasmons of Bismuth-Coated Spherical to Hexagonal Silver Nanoparticles in Dichroic Ag:Bismuth Glass Nanocomposites

Abstract

Here, we report for the first time the synthesis of bismuth-coated silver nanoparticles in dichroic bismuth glass nanocomposites by a novel and simple one-step melt quench technique without using any external reducing agent. The metallic silver nanoparticles (Ag NPs) were generated first, and subsequently, metallic bismuth was deposited on the Ag NPs and formed a thick layer. The reduction of Bi3+ to Bio and subsequently its deposition on the Ag NPs (which were formed earlier than Bio) in the K2O–Bi2O3–B2O3 (KBB) glass system have been explained by their standard reduction potentials. The UV–vis absorption spectra show a prominent surface plasmon resonance (SPR) absorption band at 575 nm at lower concentrations (up to 0.01 wt%); three bands at 569, 624 and 780 nm at medium concentration (0.02–0.03 wt%); and two weak bands at 619 and 817 nm at highest concentration (0.06 wt%) of silver. They have been explained by the electrodynamics theories. TEM images reveal the conversion of spheroidal (5–15 nm) to hexagonal (10–35 nm) shaped Ag NPs with the increase in concentration of silver (up to 0.06 wt%). SAED pattern confirms the crystalline planes of rhombohedral bismuth and cubic silver. Thermal treatment at 360 °C, which is the glass transformation temperature (Tg) of the sample containing lower concentration of silver (0.007 wt%), shows red-shifted SPR band due to increase in size of NPs. Whereas the sample containing higher concentration (0.06 wt%) of silver under similar treatment exhibited changes in SPR spectral profile happened due to conversion to spherical NPs from hexagonal shape and reduction in size (10–20 nm) of NPs after heat treatment for 65 h. HRTEM images corroborate the different orientations of the NPs. FESEM images reveal hexagonal disk like structure having different orientations. Dichroic nature of the nanocomposites has been explained with the size and shape of Ag nanoparticles. We believe that this work will create new avenues in the area of nanometal–glass hybrid nanocomposites and the materials have significant applications in the field of optoelectronics and nanophotonics.