Unraveling the Formation Mechanism of Nanoparticles Sputtered in Ionic Liquid

The formation of nanoparticles by sputtering on ionic liquids could occur at the surface or in the volume of the liquid. To clarify which process occurs, Cu was sputtered in inert and oxidative plasma onto two different ionic liquids. 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Bmim][(Tf)(2)N] and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Emim][(Tf)(2)N] were selected for their low solubility of oxygen and their different surface tensions to differentiate the influence of the ionic liquid characteristics on the formation process and characteristics of nanoparticles. The chemical state of nanoparticles in the ionic liquids, metallic or oxidized, was analyzed by X-ray photoelectron spectroscopy. Transmission electron microscopy was performed to acquire nanoparticle size distributions and shapes. The results indicate that nanoparticle formation occurs within the ionic liquid volume, contradicting the prevailing assumption that nanoparticle formation begins at the ionic liquid surface. Nanoparticle size distributions indicate that a higher viscosity of the ionic liquid results in higher nanoparticle diameters.