Synthesis of silver nanoparticles by sputtering onto non-ionic liquids

With nanoparticles (NPs) finding increasing use in various fields such as the biomedical industry or catalysis, sputtering onto liquids (SoL) has attracted interest over the last decade as a single step method for NP production, requiring only a target and a host liquid. Nonetheless, the current description and understanding of the SoL process is still limited. This work investigates the role of sputtering parameters and liquid chemistry on the NP formation, NP characteristics, and solution stability, specifically in Ag/non-ionic liquid systems, using electron microscopy, as well as optical and X-Ray photoelectron spectroscopy (XPS). First, the influence of flux and kinetic energy of magnetron sputtered Ag arriving at the surface of castor oil and polyethylene glycol (PEG600), on the NP size distributions, was investigated. Over ~60 % of all NPs were in the size range of 1-2.5 nm and 1-2 nm in castor oil and PEG600, respectively. The samples further differed in terms of NP stabilization, with the castor oil solutions appearing orange due to the localized surface plasmon resonance (LSPR) of Ag NPs, whereas PEG600 solutions were grey, indicating strong NP aggregation. While neither flux (from ~80 to 300 µg/cm²/min) nor kinetic energy (from 3-6 eV to 12-18 eV) had a significant influence on the NP size distributions in castor oil, NP growth seemed primarily triggered by an increase in kinetic energy (~30 % more NPs > 2 nm for 12-18 eV than for 3-6 eV) in the case of PEG600. These differences in behavior between liquids were attributed to their chemical structures, governing the NP coordination and thereby mobility. Another important parameter of NP solutions is their stability over time. Herein, the evolution of Ag NPs/rapeseed oil solutions, upon atmospheric exposure, was investigated, and the interface between NP and liquid was probed by XPS. The NPs in the dark brown as-synthesized solutions exhibited a median size of 1.4 (-0.3/+0.4) nm, with ~10 % of the NPs > 2 nm. In addition to metallic Ag NPs, Ag-AgCl Janus-type NPs were present, due to the presence of Cl containing triglycerides in the refined oil. Within 35 days, the gradual colour change to light orange was accompanied by NP growth, with ~20 % more NPs > 2 nm. The evolution of the LSPR suggested the continued formation of Ag-AgCl NPs and the presence of Ag compounds, such as Ag-S-O. Moreover, oil oxidation, signaled by the presence of hydroperoxides, was promoted by the presence of Ag NPs. Lastly, regardless of the exposure time, Ag interacted with carboxylate groups (─COO⁻), suggesting the opening of the triglyceride molecules and/or deprotonation of fatty acids. Finally, to gain insights in NP formation, the deposition process was monitored in situ by optical spectroscopy during ion beam sputtering (IBS). Ag NP solutions were produced by IBS onto castor oil, for Ag fluxes of 8 and 34 µg/cm²/min. The absorption spectra revealed the presence of Ag NPs of a few nanometers in diameter, and Ag clusters already in the first stages of deposition. Janus-type Ag-AgCl NPs were detected after deposition of ~2-5 µg/cm² of Ag, due to sufficient exposure of Ag NPs to the Cl-containing oil. After the end of the deposition, Ag-AgCl NPs were still forming and growing, whilst all species diffused from the highly concentrated (sub)surface region to the oil volume. While this observation was made for both deposition rates, flux-dependent accumulation of Ag below the liquid surface led to different homogenization rates. Overall, this work aims to describe the interplay between the SoL process and NP formation, while providing a methodological toolbox for investigations of SoL, towards a knowledge-based choice of NP/liquid system.