Toward the Microscopic Identification of Anions and Cations at the Ionic Liquid|Ag(111) Interface: A Combined Experimental and Theoretical Investigation

Abstract

The interaction between an adsorbed 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP][TFSA], ionic liquid (IL) layer and a Ag(111) substrate, under ultrahigh-vacuum conditions, was investigated in a combined experimental and theoretical approach, by high-resolution scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and dispersion-corrected density functional theory calculations (DFT-D). Most importantly, we succeeded in unambiguously identifying cations and anions in the adlayer by comparing experimental images with submolecular resolution and simulated STM images based on DFT calculations, and these findings are in perfect agreement with the 1:1 ratio of anions and cations adsorbed on the metal derived from XPS measurements. Different adlayer phases include a mobile 2D liquid phase at room temperature and two 2D solid phases at around 100 K, i.e., a 2D glass phase with short-range order and some residual, but very limited mobility and a long-range ordered 2D crystalline phase. The mobility in the different adlayer phases, including melting of the 2D crystalline phase, was evaluated by dynamic STM imaging. The DFT-D calculations show that the interaction with the substrate is composed of mainly van der Waals and weak electrostatic (dipole-induced dipole) interactions and that upon adsorption most of the charge remains at the IL, leading to attractive electrostatic interactions between the adsorbed species.

Some challenges in the first-principles modeling of structures and processes in electrochemical energy storage and transfer

“…Both values are also indicative of the absence of a true chemical bond [86,87]. Without dispersion corrections, the ionic liquid pair [EMIM] þ [TFSA] À hardly binds to a metal electrode (adsorption energy À0.05 eV) whereas the inclusion of dispersion corrections leads to an adsorption energy of À1.60 eV, a value close to the one found for a similar ionic liquid pair on Ag(111) [81]. The larger amount of van der Waals attraction compared to ethylene carbonate is mainly due to the larger size of the ionic liquid pair.…”

“…It is important to note that because of the weak interaction between the ion liquid pairs and between ion liquids and any electrode it is important to take the van der Waals interaction into account [81]. Semilocal DFT functionals typically do not reproduce this kind of dispersion interaction.…”

Ionic Liquids in Catalysis

“…Many established UHV-based surface science techniques have successfully contributed to an increasingly detailed understanding not only of the IL/ vacuum interface, but also of IL bulk properties. The applied methods include X-ray photoelectron spectroscopy (XPS) [106,107,, UV photoelectron spectroscopy [138,139,165,166], inverse photoelectron spectroscopy (IPES) [165,166], X-ray absorption spectroscopy (NEXAFS) [165], soft X-ray emission spectroscopy (SXES) [166], low energy ion scattering (LEIS) [141], metastable ion spectroscopy (MIES) [138,139], time-offlight secondary mass spectroscopy (TOF-SIMS) [140], Rutherford backscattering [167][168][169][170][171], high resolution electron energy loss spectroscopy (HREELS) [138], reflection absorption infrared spectroscopy (RAIRS) [172][173][174][175], direct recoil spectroscopy (DRS) [176], and scanning tunneling microscopy [177][178][179][180][181], to name only a few. In addition, an increasing number of theoretical studies and simulations have been conducted [101,[182][183][184][185][186][187][188][189][190].…”

Poly(a)morphic portrait of the electrical double layer in ionic liquids

“…Charge/voltage driven structural transitions in the electrical double layer (EDL) in ionic liquids (ILs) have recently attracted large interest in experimental [1,2,3,4,5,6], theoretical [7] and computational [8,9,10,11,12,13,14] communities due to the importance of this subject for a variety of IL applications [15,16].…”