60th Annual Report on Research 2015

Introduction

In the last two decades, ionic liquids (ILs) have attracted great interest in various research communities and industries due to their remarkable physicochemical properties, including low melting points, high thermal and electrochemical stability, high ionic conductivity, high tuneability as designer solvents, negligible vapor pressure, etc. Much effort has been made to understand their behaviors in homogeneous liquid and solution phases. Recently, more and more attention has been drawn to the structures and dynamics of ILs in a heterogeneous environment because of IL applications in, e.g., (photo)catalysis, electrochemistry, corrosion inhibition, and thin-film lubrication. To study solid-IL interfaces and obtain the corresponding signals instead of the bulk ones, it is preferred to use experimental methods with a surface-specific probe that has proper spatiotemporal resolutions.

Many of the interfacial studies were conducted using sum frequency generation (SFG), x-ray reflectivity, x-ray photoelectron spectroscopy, atomic force microscopy (AFM), and scanning tunneling microscopy (STM). Essentially all of them were time-averaged measurements, providing a static picture of the solid-IL interfaces in equilibrium states despite that the relevant time scales for molecular motions are many orders of magnitude faster. In recent years, ultrathin films of ILs have be prepared via physical vapor deposition and studied under ultrahigh vacuum conditions, thanks to the nonvolatile property of ILs at room temperature. However, a considerable debate about the ion organization and layered structures still exists in the experimental literature. This is likely due to the different inherent challenges encountered in each technique regarding the probing of interfacial structures: force-dependent observation in AFM, ion adsorption on the tip in STM, beam-induced damage by long exposure of x-ray, and ensemble averaging using optical techniques.

Our method of choice is time-resolved electron diffraction to study the structures and ultrafast dynamics at solid-IL interfaces. Compared to x-ray photons, electrons have orders-of-magnitude higher scattering cross sections with matter and can be easily generated (thermally or via photoemission) and manipulated using electron optics. These advantages make electrons a natural choice of contact-free probe in interfacial studies, with surface specificity and direct probing of the structures of molecular assemblies. Our research plan has two main goals: (1) to examine the structural orders of IL thin films supported on various types of substrate surfaces at different temperatures, and (2) to obtain information of photoinduced energy-transfer and charge-transfer dynamics across solid-IL interfaces. We focused on the first goal during the first year of the project.

Results and Discussion

The representative IL of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [emim][NTf₂], was chosen based on the consideration of thermal stability for vapor deposition and the comparable sizes of cations, [emim]⁺, and anions, [NTf₂]^–, for potentially better packing orders. A Knudsen-type effusion cell holding the IL was heated to around 430 K for the deposition of thin films, whose rate has been calibrated using a quartz crystal microbalance directly under the cell output. By changing the deposition time, IL films of ~1 up to 20 nanometer (nm) thickness were deposited on different substrate surfaces, including highly-oriented pyrolytic graphite (HOPG), hydrogen-terminated Si(111), mica, Cu(111), and Ni(111).

Figure 1 shows the diffraction images of a solid-IL interface with and without IL coverage, using HOPG as an example. An adsorbate-free substrate surface can be recovered after desorption of the IL thin film at an elevated temperature (see Fig. 1, left and right images).

We found that all of the nm-thick films deposited on substrates held at room temperature give a diffuse scattering pattern without clear diffraction spots or rings (the middle image of Fig. 1). Such an observation signifies the poor vertical and horizontal, or local, ordering of cations and anions at the solid-IL interface when no bias voltage is applied. The best description for the structure of these IL thin films is a liquid-like configuration with no apparent nm-sized crystalline order.

Following the deposition, the structures at the solid-IL interface were monitored at different temperatures. It is noted that, among all of the substrates studied so far, only ~3-nm-thick IL films on HOPG repeatedly show clear Bragg diffraction spots as the temperature decreases to ~30 degrees below the IL melting point (Figure 2, the middle image and the intensity-boosted inset). Interestingly, films of much smaller and much larger thickness on HOPG do not produce such temperature-dependent diffraction changes (Fig. 2, left and right images). Based on simulations using the kinematic scattering theory, we confirmed that the Bragg diffractions come from crystals of the IL well-ordered in the surface normal direction but azimuthally rotated in the horizontal two dimensions. Cations and anions in the thin film are packed in the checkboard-type arrangement following the structure of the bulk crystal. Moreover, the vertical order originates from the lattice matching between the IL’s c-axis cell constant and graphite’s interplanar distance. Thus, due to its surface steps and terrace, HOPG serves as a template to promote the vertical order in the 3-nm thin film. It is also consistent that this template effect is not pronounced in a 1-nm-thick film, and ions in a much thicker film farther away from the stepped surface yield a liquid-like arrangement without clear crystalline order.

By following the intensities of the Bragg diffractions, we found that the loss of the crystalline order in the IL thin film starts below, but completes at, the bulk melting temperature of 252 K (see Figure 3). Such an observation signifies that, with the assistance of a template, the IL thin film behaves similarly to the bulk. This is consistent with the picture that the Coulomb interactions between ions are the driving force in the behavior of the IL.

Impact and Future Plans

These new diffraction results show intriguing solid-IL interfacial interactions on the nanometer scale and warrant more studies of structures and dynamics. The proposed research will broaden the applications of ultrafast electron diffraction in chemistry and condensed matter, which is a major focus for us. Three graduate students, Karjini Rajagopal, Chengyi Wu, and Napat Punpongjareorn, has participated in this project and acquired direct experiences working with ILs and advanced experimental techniques. We are preparing a manuscript for this work.