Investigating the M(hkl)| ionic liquid interface by using laser induced temperature jump technique

Abstract

The interface between several Room Temperature Ionic Liquids (RTILs) in contact with both Au(hkl) basal planes and Pt(111) was studied by using cyclic voltammetry and Laser Induced Temperature Jump Technique (LITJT). Three RTILs, based on the imidazolium cation and the [Tf2N] anion were investigated: [Emmim][Tf2N], [Emim][Tf2N] and [Bmmim][Tf2N]. These three RTILs were selected with the aim to analyse how the balance between the different ion-ion interactions influences the interfacial properties of the M(hkl)|RTIL interface. It was found that the voltammetric response of the Au(hkl)|[Emmim][Tf2N] was highly sensitive to the geometry of the active surface sites, displaying sharp spikes superimposed to a capacitive voltammetric current. Conversely, these sharp spikes disappeared when [Bmmim][Tf2N] replaced [Emmim][Tf2N], although the capacitive voltammetric current profile was essentially maintained. This result is most likely related to the increase of the van der Waals interactions in the [Bmmim][Tf2N]. When [Emim][Tf2N] was analysed, the increase of the hydrogen bond interactions due to the hydrogenation of C2 (second carbon at the imidazolium ring) resulted also in the disappearance of the voltammetric spikes. The laser measurements showed that the highest values of the potential of maximum entropy (pme) in RTIL media correspond to the atomically closest packet surface structures, following the order: Au(111)>Au(100)>Au(110), in agreement with work function values. The measurement with Pt(111) revealed that the voltammetric profiles for this surface are featureless in all cases. However, the laser experiments revealed that solvent restructuration, as a function of both value and direction of the applied potential, is dependent on the type of cation. Finally, the interface Au(hkl)|Choline chloride:urea Deep Eutectic Solvent (DES) was also investigated by using cyclic voltammetry and LITJT. The voltammetric response of DES was also sensitive to the orientation of the Au single crystal, and the cyclic voltammograms displayed distinct sharp and characteristic features. Nevertheless, the laser response could not provide a value of the pme for the Au(hkl)|DES interface, likely due to the complex chemical structure of the DES which, in addition, strongly adsorbs on Au(hkl).