Combined spectroscopic and microscopic investigation of stability upon atmospheric exposure of Ag nanoparticle solutions produced by sputtering onto rapeseed oil

Abstract

Ag nanoparticle (NP) solutions were produced by magnetron sputtering onto rapeseed oil and their stability under atmospheric exposure up to 35 days was investigated by infrared (IR), ultraviolet-visible (UV-vis), X-Ray photoelectron spectroscopy (XPS), and electron microscopy. The as-synthesized solutions were dark brown, due to the localized surface plasmon resonance (LSPR) of the Ag NPs. Only ~10 % of all NPs were > 2 nm in diameter, with a median size of 1.4 (-0.3/+0.4) nm. Besides Ag NPs, Ag-AgCl Janus-type NPs were detected, whose formation was attributed to the presence of Cl containing triglycerides in the oil. Within 35 days, the color of the solutions gradually changed to light orange and the proportion of NPs > 2 nm increased by almost 20 %. The redshift of ~15 nm and absorbance decay of the LSPR on the UV-vis spectra after 35 days were mainly attributed to the continued formation of Ag-AgCl NPs and the presence of Ag compounds, like Ag-S-O. Furthermore, oil oxidation, revealed, in IR, by the formation of hydroperoxides upon atmospheric exposure, was promoted by the presence of Ag NPs. Finally, XPS investigations on the solutions revealed the interaction, at all exposure times, between carboxylate groups (─COO^-) and Ag, suggesting the opening of triglycerides and/or deprotonation of fatty acids. Besides shedding light experimentally on the interaction between NPs and liquid, this work shows the mutual influence of NP and liquid on each other, and underlines the crucial role of trace contaminants in the host liquid on NP chemistry.