Solvothermal hot injection synthesis of core-shell AgNi nanoparticles

Abstract

Silver-nickel core-shell nanoparticles (NP) were prepared by solvothermal hot injection synthesis by simultaneous thermolysis/reduction of AgNO3 and Ni(acac)2 precursors in the hot mixture of octadecene and oleylamine. Oleylamine decreases decomposition temperature of AgNO3 to that of Ni(acac)2 thus ensuring favorable reaction conditions. The prepared AgNi NPs with different Ag/Ni ratios were completely characterized. Dynamic light scattering (DLS) and small angle X-ray scattering (SAXS) were used for particle size characterization of as-prepared AgNi colloids. There is no dependence of the particle size (13–21 nm by SAXS) on the Ag/Ni stoichiometric ratio, but the ultraviolet–visible spectroscopy (UV–vis) reveals that the intensity of the surface plasmon (SPR) decreases with increasing Ni content. Transmission electron microscopy (TEM) verified the results of DLS and SAXS and showed spherical nanoparticle shape. Distribution of individual elements in the nanoparticles was mapped by high resolution scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM-EDS) and revealed their core-shell structure where an Ag nucleus is covered by a thin amorphous Ni layer. Upon heating to 400 °C, Ni crystallization is substantiated by appearance of diffractions in the high-temperature X-ray powder diffractograms (HT-XRD) and of a magnetic moment. Ultimate phase separation was proven by scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDS) in samples heated to 1000 °C. The reaction course and nanoparticle formation studied by DLS, UV–vis, and Ag and Ni elemental analyses reveal an initial Ag seed formation with subsequent Ni overlayer deposition after 180 s.