Formation of Metal Nanoparticles in Ionic Liquids By Low-Energy Quantum Beam Irradiation Technique
Authors: Imanishi Akihito 
Published: February 2020

In our previous study, we reported a novel method on synthesizing Au nanoparticles via a reductive reaction in an ionic liquid droplet containing Au3+ ions using a low-energy electron beam irradiation technique1,2). The formed Au nanoparticles are dispersed well and crystallized. This new technique can be applied to many industrial fields. For example, this could be applicable to electron-beam lithography if we could firmly anchor the formed particles to the substrate surface. Besides, we can easily prepare the metal alloy particles (which is very important in the field of catalysis) without considering the redox potential of metal ions. Of course, this technique is attractive from the point of view of pure chemistry, because there is a possibility that we will find novel types of reductive reaction in the ionic liquids.
Development of techniques for controlling the size and morphology of the metal particles is very important for adapting those particles to the actual devices or the catalyst. To achieve this aim, control of the formation manner of metal particles in ionic liquids is needed. Thus, we investigated how electron beam conditions (acceleration energy, beam current and irradiation time) and the combination of cations and anions of ionic liquids affect the size and shape of Au particles. Fig. 1 shows the averaged sizes of the particles plotted against the viscosity of ionic liquids. We can see that the anion of the ionic liquid strongly affected the size and shape of the primary particles, which was due to the different local structure of the ionic liquid around the Au particles. Besides, it was found that the sizes of the primary particles increased with higher acceleration energy of the electron beam, whereas they did not depend so much on the beam current and irradiation time.
On the other hand, it is known that ILs confined in small spaces exhibit characteristic properties which are different from those of bulk ILs. Therefore, it is an interesting subject to study how Au particles are formed in ILs confined in the nano-micro sized space. We should also note that such confined space can be used as structural template with which the size and shape of metal particles can be controlled. Nanopore is good template for controlling the formation process of Au particle. Fig. 2 shows a TEM image of Au nanoparticles formed in nanopores of mesoporous silica by X-ray irradiation. Highly dispersed Au particles whose size was limited by the pore diameter were observed. The particles were stable even after annealing at 373K, indicating that the IL molecules surrounding the Au nanoparticles prevented the aggregation. The chemical states of IL and Au particles confined in nanopores were characterized by XPS measurements. Au 4f spectra revealed that the Au particles formed in nanopore was positively charged. Interestingly, the result of N 1s spectra revealed that cation and anion of IL were organized in the nanopores. Such characteristic local structure in nanospace may affect not only the formation process of Au particles but also the catalytic reaction in the pore.
References:
1) A. Imanishi, M. Tamura, S. Kuwabata, Chem. Commun. (2009) 1775.
2) A. Imanishi, S. Gonsui, T. Tsuda, S. Kuwabata, K. Fukui, Phys. Chem. Chem. Phys., 13 (2011) 14823.