Structural aspects of transition-metal nanoparticles in imidazolium ionic liquids

Imidazolium-based ionic liquids (ILs) display a pronounced self-organisation in the solid, liquid and even in the gas phase. These pre-organised structures are formed mainly through hydrogen bonds between the cations and anions that induce structural directionality. This structural organisation of ILs can be used as 'entropic drivers' for spontaneous, well-defined, and extended ordering of nanoscale structures. Furthermore, these ILs structures can adapt or are adaptable to many species, as it provides hydrophobic or hydrophilic regions and a high directional polarisability which can be oriented parallel or perpendicular to the included species. The formation and stabilisation of transition-metal nanoparticles in 1-n-butyl-3-methyl imidazolium ILs occurs with the re-organisation of the hydrogen bond network and generate nanostructures with polar and non-polar regions where the nanoparticles are included. The IL forms a protective layer surrounding the transition-metal nanoparticles surface with an extended molecular length of around 2.8-4.0 nm depending on the type of anion suggesting the presence of semi-organised anionic species composed of supramolecular aggregates of the type [(BMI)x-n(X)(x)](n-). This structural organisation is similar to that already observed in solid, liquid, gas phase and in solution of imidazolium ILs. This protective layer is probably composed of imidazolium aggregate anions located immediately adjacent to the nanoparticle surface -providing the Coulombic repulsion - and counter-cations that provide the charge balance.