What Are the Mechanisms Controlling the Size of Metal Nanoparticles Precipitated in Ionic Liquids?

Abstract

The chemical synthesis of metallic nanoparticles (NPs) in the range of 1 to 10 nm remains challenging. Specific measures must be taken to control their size and avoid their agglomeration. In traditional solvents, stabilizers are needed such as steric ligands or polymers that coordinate the metal. By contrast, ionic liquids (ILs) readily stabilize NPs without the need of such additives. Indeed, suitable organometallic (OM) precursors can be readily precipitated into metallic NPs in ILs by decomposition under hydrogen.[1-3] In our group, this reaction is routinely performed in 1-alkyl-3-methylimidazoliumbistrifluoro-methylsulphonyl-imide ILs (abbreviated as C1CnImNTf2) and provides suspensions of metallic NPs with quite narrow size distribution and high stability.[2] In the same time, little is known about the exact interaction at play between ILs and metallic NPs.[4] The purpose of the present work is to examine how this interaction modifies the kinetics of Ru NP nucleation and growth in imidazolium-based ILs, in order to gain better control over the final size of these NPs, while preserving narrow size distribution.The Ru(COD)(COT) precursor was decomposed in C1CnImNTf2 ILs. These ILs contain imidazolium cations bearing one alkyl chain of variable length (n=4 to 18). A major outcome of this study is that the length of this alkyl chain influences the size of the Ru NPs. At 100°C, the diameter of Ru NPs decreases when the alkyl chain length increases. This could indicate a more efficient stabilization of the NPs by the larger cations.[4-6] However, this conclusion contrasts with other studies in which the anion was identified as the primary driver for size control.[7]These results also differ from prior observations at 0°C. At this temperature, the size was found to increase with alkyl chain length (n=4 to 8).[8] This effect was correlated with the nanostructure of these ILs in which the alkyl chains segregate into isolated pockets. In the present work, we observe a transition between the two regimes: as the precipitation temperature is decreased from 150°C, the size of Ru NPs increases as expected with a reduced nucleation rate. However, below a critical temperature that decreases for longer alkyl chains, the size decreases with temperature as the “granularity” of the IL becomes stronger and starts to interfere with the nucleation and growth kinetics.The nucleation of Ru NPs in imidazolium-based ILs was monitored in an environmental TEM.[9] Solutions of Ru(COD)(COT) in C1CnImNTf2 (n=4 , 6, 8 and 10) were decomposed under the microscope under 1 mbar hydrogen at room temperature. The formation of Ru NPs with a diameter of 2-3 nm was observed within a few minutes only (see figure). This phenomenon was delayed in ILs with larger cations. For ILs with smaller cations (n<8), the population of Ru NPs in this size range then decreased, due to coalescence. This phenomenon leads to larger size distributions, and gets slower as for the ILs with longer alkyl chains. For C1C10ImNTf2, it was not observed within the duration of the experiment. This demonstrates that in ILs with larger cations, nucleation and coalescence of Ru NPs are slower. This can be ascribed to the higher viscosity of these ILs, but also to a more efficient steric stabilization of the NPs.