(Invited) Development of Newfangled Nanotechnologies By Introducing Ionic Liquid to Vacuum Devices
Authors: Kuwabata Susumu , Tsuda Tetsuya , Uematsu Taro , Torimoto Tsukasa 
Published: February 2020

One of fascinate features that ionic liquid (IL) possesses is extremely low vapor pressure, which enable us to put IL in a vacuum chamber. There are many vacuum devices that have been designed for solid samples, as a matter of course. However, an ability to make wet condition in vacuum system by introducing IL into it allows us to develop newfangled technologies in analytical chemistry and nanomaterial synthesis fields. In this paper, two topics on in situ electron microscope observation of reactions in IL and facile way to synthesis nanoparticles using a sputtering instrument onto IL are described.
Regarding the use of IL for electron microscope observation, we have found the fascinate fact that IL drops give scanning electron microscope (SEM) images without any charging.1 In other words, IL behaves like a conducting material for electron microscope observation. It was also found that IL behaves like a transparent material for SEM observation when its thickness is less than 1 micro meter. Based on these facts, we started to develop ways to conduct in situ electron microscope observation of chemical reactions. Electrically conducting polypyrrole film deposited on an electrode surface was the first sample and changes in its thickness upon redox reaction in IL electrolyte were observed by SEM.2 As another electrochemical reaction, we attempted to observe deposition of Ag metal on an electrode from IL containing Ag+ ions. Nucleation and nuclear growth were dominant when Ag deposition was moderately conducted. Howerver, when the deposition rate was accelerated so as to make the diffusion-controlled condition, growth of Ag metal with dendriform was clearly observed.3

When we observed IL dissolving Na[AuCl4] by SEM, it was found that Au particles appeared in the IL, resulting from reduction of Au ions by electron beam irradiation.4 On the basis of this finding, attempt was made to observe generation and growth of Au nanoparticles by a transmission electron microscope (TEM). Figure 1 shows time course of two Au nanoparticles (diameter = ca. 5 nm). Each particle shows fringes reflecting crystal lattice of Au and it is obvious that coalescence gives a larger Au nanoparticles by production of one Au crystal from two crystals.5

The metal sputtering instrument is useful for depositing a metal thin layer on a substrate surface. The sample was limited to solid materials because the metal sputtering must be conducted under reduced pressure. Our first attempt was to subject IL to Au sputtering and observation of the resulting IL by TEM, giving us a chance to discover synthesis of Au nanoparticles suspended in IL.6 Nanoparticles of several kinds of metals and metal oxides including alloy nanoparticles can be prepared by this method.7

The noble nanoparticles including Au and Pt can be adsorbed on carbon substance surfaces by putting IL containing metal nanoparticles on the substance and heating it. This way is exploitable for preparation of Pt-carbon catalysts for fuel cells.8 Furthermore, adsorption of Pt nanoparticles on surfaces of carbon nanotubes (CNTs) was possible by the similar way as shown in Figure 2.9 Electrochemical measurements of the prepared Pt nanoparticle-adsorbed CNTs revealed that this material possesses distinct catalytic activities toward O2 reduction.

References

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Figure 1