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Ionic liquids (ILs) feature negligibly low vapor pressures and can thus be freely introduced into the high vacuum of a transmission electron microscope. With this extraordinary property, the ILs offer a powerful tool for in situ transmission electron microscopy (TEM) in window-free liquid media at very high resolution. In this work, we use the IL 1-butyl-3-methylimidazolium chloride in order to study nucleation and growth of gold nanoparticles (NPs) in free-standing liquid droplets by scanning TEM (STEM). The results confirm that the used IL allows for generating Au NPs in situ, triggered by electron irradiation and heat. Firstly, the isotropic growth of small, spherical Au NPs was initiated and monitored, whereas different growth mechanisms were observed, i.e. growth by monomer attachment, growth through particle coalescence and possible Ostwald ripening events. After the initial growth phase, a second, anisotropic growth process was induced by a moderate temperature increase and continued electron irradiation. As a result, larger, faceted crystals, such as tetrahedra, octahedra or decahedra, were formed. As all these polymorphs are terminated by {111}-facets, the IL might not only act as liquid medium but in addition as a surfactant which preferentially attaches on the {100}-facets.

Impact Statement

The authors demonstrate the ability to observe ionic liquids in the TEM which withstand high vacuum conditions without evaporating and hence open very attractive possibilities to observe particles formation in very thin, free-standing liquid layers in TEM at high resolution.