Stabilization and aggregation of preformed silver clusters in room temperature ionic liquids: a UV/Vis and X-ray absorption spectroscopy study

Abstract

Ionic liquids (ILs) have received considerable interest in various fields of research and promise a vast range of applications; among others, they are promising stabilizers for nanoparticles. ILs consist of a large organic cation and a small inorganic anion. The different charges of the molecules cause a strong attractive interaction, while the large difference of size of the molecules prevents condensation at room temperature. Due to their low vapour pressure ILs can be used under vacuum conditions and - in addition - do not emit volatile organic compounds. Thus their unique physicochemical properties suggest ILs can be used as a "green" replacement for traditional organic solvents. For now, only a few studies of the stability of nanoparticles in RTILs are available, nevertheless the nanoparticle stability is of critical importance in controlling aggregation processes. Further, all sample production processes have in common that cluster growth takes place inside the ionic liquid, which complicates the discussion of sample stability. In the experiments presented here 2 nm silver nanoparticles with a well-characterized size distribution are preformed in a supersonic nozzle expansion and deposited afterwards into the ionic liquid. The cluster plasmon is investigated by-situ by UV/Vis absorption spectroscopy during cluster deposition as well as in temperature dependent measurements after deposition. We observed that the storage temperature as well as the choice of anion and cation influences the cluster stability on timescales of several days. The sample aggregation can be described using the model of diffusion-limited aggregation. All experimental results are compared to calculated spectra using Generalized Mie Theory for several geometric shapes of cluster aggregates. The local interatomic arrangement within the samples is determined in X-ray absorption spectroscopy measurements performed at beamline P64, DESY, Hamburg. Besides conventional EXAFS analysis in order to determine nearest neighbour distances and coordination numbers the experimental results are completed by XANES analysis using artificial neural networks. It could be shown that no coalescence to larger particles takes place during deposition; instead the clusters form chain-like aggregates.