On the stabilisation and surface properties of soluble transition-metal nanoparticles in non-functionalised imidazolium-based ionic liquids

Doubtless ionic liquids (ILs), particularly those based on the 1,3-dialkyl imidazolium cation, provide a flexible liquid platform to prepare, soluble and stable transition metal nanoparticles (TMNPs). ILs can act as a “solvent”, stabiliser, ligand and support for TMNPs. Soluble and stable TMNPs for specific applications can be easily prepared in ILs using a bottom-up or top-down approach. The stability of TMNPs in non-functionalised ILs is mainly related to the surface electronic stabilisation provided by protective layers of discrete supramolecular imidazolium aggregates, non-polar imidazolium alkyl side chains and NHC carbene species as well as surface hydrogen species, together with an oxide layer when present on the metal surface. The IL provides a template-like effect and does not exist as a pure double layer, rather, the IL interacts directly with the TMNP surface through both cationic and anionic species, and the non-polar groups are preferentially directed away from the surface, forming a protective layer at the interface of at least one layer thick. The main aspects involving the stabilisation, in particular the interface of TMNP surface with the ionic liquids and other species present in the media, will be presented and discussed in light of the recent experimental and theoretical results reported.