Novel Analytical Techniques for Smart Ionic Liquid Materials

TETSUYA TSUDA, CHIH-YAO CHEN AND CHARLES L. HUSSEY

Introduction

As described elsewhere, an ionic liquid (IL), which is sometimes called a room-temperature ionic liquid (RTIL), a room-temperature molten salt (RTMS), or an ambient-temperature molten salt (ATMS), has many unique properties. But, the most important point is that one IL combines nearly all of these features. For this reason, many scientists and engineers are keeping an eye on ILs as liquid materials and functional reaction media for supporting the development of future technologies, e.g., electrolytes for next generation secondary batteries and PEM fuel cells, functional solvents for organic synthesis and nanoparticle preparation, extraction solvents for rare metal ions and CO2, and lubricants for precision instruments. Some scientists have attempted to establish novel analytical techniques that combine ILs with analysis equipment operating under vacuum conditions, e.g., scanning electron microscopes (SEM), transmission electron microscopes (TEM), energy dispersive X-ray analysis (EDX), electron diffraction (ED), and X-ray photoemission spectroscopy (XPS). The negligible vapor pressure and antistatic properties of ILs enables analytical techniques that were previously considered to be impossible. Now these techniques have become widely recognized and accepted as powerful tools to reveal various chemical reaction processes in IL and ionic conformation change at the interface between IL and other phases. In this chapter, we divide these cutting-edge techniques into roughly three categories, SEM, TEM, and XPS, as described below.

1.2 SEM Observations with ILs

SEM is firmly established as a powerful tool for obtaining a three-dimensional surface image of specimens, and it is suitable for observing relatively large objects on a micrometer scale. The entire microscope has to be held at high vacuum for several reasons: First, gas in the gun assembly would interfere with electron emission and degrade the electron source. Second, electron beams will be scattered by any gas in the chamber, degrading column performance. Finally, the ionization of gas could cause electrical discharge and destroy the detector. These conditions imply that specimens must be vacuum tolerant, so they are traditionally dry solids. Wet specimens are typically frozen or desiccated before observation. This means that it is not easy for us to directly observe variations in the sample during dynamic experiments. For non-conductive/insulating specimens, creating a thin conductive coating (tens of nanometres) of a metal or carbon, although not mandatory, can prevent charging and improve the secondary electron signal. The most common vacuum coating methods are sputtering and thermal evaporation.

The extremely low vapor pressure (and thermal stability) of ILs match up well to the requirements of electron microscopy. The introduction of ILs into SEM chambers was first suggested in 2006 by Kuwabata et al. (Figure 1.1). It has been reported that Ils act as electrically conductive materials with high fluidity and can be directly observed by SEM without the accumulation of electron charges. Since this milestone work, many publications have followed. It is important to note that other solvents that possess negligible volatility such as silicone oil are also compatible with the vacuum conditions. However, the images for samples coated with these oils are distorted and fluctuate greatly owing to the build-up of charge on the sample. The distinct behavior observed for ILs could be attributed to their ability to solvate electrons and allow them to move in the liquid. It has been reported that even if the accelerating voltage is reduced to 1 kV, clear images are obtained for ILs, suggesting that such low energies are sufficient for injecting electrons into ILs. This unique property of ILs is useful as an alternative way to afford electrical conductivity to non-conductive materials. Over the past decade, the application of ILs for SEM observation can be divided into the following two categories: (i) as pre-treatment reagents to make the sample vacuum tolerant and electrically conductive and (ii) as media for (electro)chemical reactions proceeding under vacuum conditions.

1.2.1 ILs as Pre-treatment Reagents

Specimen preparation is decisive with any microscopic technique, the basic concern being that the specimen prepared is truly representative of the sample of interest. Although the preparation of SEM specimens is easy in comparison to TEM because there is no strict requirement to make the specimen exceedingly thin, it could still be challenging for specific samples. For instance, most biological specimens are made up largely of water and non-dense tissue materials. Thus, the sample must first be chemically fixed with aldehyde, dehydrated through an acetone or alcohol series, and then dried at the critical point (in order to mitigate specimen deformation due to tension during drying). Finally it must be coated with a conductive film prior to SEM observation....