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AFM and STM Studies on the Surface Interaction of [BMP]TFSA and [EMIm]TFSA Ionic Liquids with Au(111) Rob Atkin,*,† Sherif Zein El Abedin,‡,§ Robert Hayes,† Luiz H. S. Gasparotto,‡ Natalia Borisenko,‡ and Frank Endres*,‡ Centre for Organic Electronics, Chemistry Building, The UniVersity of Newcastle, Callaghan, New South Wales 2308, Australia, and Institute of Particle Technology, Clausthal UniVersity of Technology, Robert-Koch-Strasse 42, 38678 Clausthal-Zellerfeld, Germany ReceiVed: March 24, 2009; ReVised Manuscript ReceiVed: May 15, 2009
The surface interaction of two air- and water-stable ionic liquids (ILs), 1-ethyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]amide ([EMIm]TFSA) and 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]amide ([BMP]TFSA), with Au(111) has been investigated using atomic force microscopy (AFM), cyclic voltammetry, and scanning tunneling microscopy (STM) measurements. AFM experiments reveal that multiple solvation layers are present at the gold interface for both ILs and that the strength of the interaction between the innermost layer and the substrate is dependent on the cation type; the force required to rupture the innermost solvation layer is greater for [BMP]TFSA than for [EMIm]TFSA, attributed to stronger cation surface interactions. In situ STM elucidates the influence of IL species on restructuring of the Au(111) interface. In the presence of [BMP]TFSA, the Au(111) surface restructures to produce a wormlike pattern, but this unusual morphology is not observed for the [EMIm]TFSA-Au(111) system. This remarkable difference in electrochemical behavior is ascribed to the greater strength of the interaction of [BMP]+ compared to [EMIm]+ with the Au(111) surface. These results demonstrate that such interfacial effects have to be considered for all electrochemical reactions and provide insight into the electrical double-layer structure in IL systems. Introduction Room temperature ionic liquids (ILs) are currently the focus of intense research activity on several fronts. Much of this interest stems from their “tunable” nature, whereby important physical parameters (polarity, viscosity, Lewis acidity, etc.) may be controlled by selection of the appropriate cation and anion types in the first instance, with fine control facilitated by subtle variation in the molecular structure. These “designer” characteristics, combined with typically low vapor pressures and the ability to solubilize unusual combinations of chemical species, have led to a vast number of research articles in which ILs are employed as solvents for chemical synthesis.1-3 Comparatively little is known about the relationship between cation and anion molecular structure and physical properties. Studies that have emerged have dealt with molecular organization,4 interfacial structure,5-8 bulk liquid properties9-11 dissolved polymers,12 block copolymer aggregation13 and surfactant assembly into micelles,14-19 liquid-crystal phases,15 microemulsions,20,21 and surface aggregates,22 among others. Similarly, only a few papers have examined the potential-dependent restructuring/reconstruction of metal surfaces in the presence of ILs.23-25 Notably, [BMP]TFSA induced restructuring/reconstruction of Au(111) most likely because of adsorbed IL. Prior to the cathodic decomposition of the [BMP] cation, the TFSA anion is subject to cathodic breakdown, leading to surface films that can be probed by the in situ STM.23,24 Cathodic breakdown of the TFSA anion was previously reported,26 although more recent results * Corresponding authors. † The University of Newcastle. ‡ Clausthal University of Technology. § Permanent address: Electrochemistry and Corrosion Laboratory, National Research Centre, Dokki, Cairo, Egypt.
show that recrystallized IL is almost inert against cathodic breakdown.27 The Clausthal group has shown that nano- and microcrystalline aluminum can be electrodeposited from both [EMIm]TFSA and [BMP]TFSA; cf. Table 1. These ILs are attractive solvents for electrochemical applications because of their wide electrochemical windows (down to -2.5 and -3 V vs NHE) and thermal stability over a wide temperature range (about -50 to +250 °C). Furthermore, these ILs can be easily dried to water contents below 3 ppm because of their low vapor pressure. [BMP]TFSA and [EMIm]TFSA form biphasic mixtures when AlCl3 is present at concentrations between 1.6 and 2.5 mol/L and between 2.5 and 6 mol/L, respectively.28,29 Electrodeposition of aluminum at room temperature occurs only from the upper phase at AlCl3 concentrations g1.6 mol/L for [BMP]TFSA and g5 mol/L for [EMIm]TFSA. This demonstrates that the reducible aluminum-containing species exists only in the upper phase of the biphasic mixture. Shiny, dense, and adherent aluminum deposits with nanosized crystallites are obtained in [BMP]TFSA without the addition of organic brighteners, as shown in the scanning electron microscopy (SEM) micrograph of Figure 1a. In contrast, coarse, cubic-shaped, micrometer-sized aluminum particles are obtained using [EMIm]TFSA (Figure 1b). Variation of the temperature and electrochemical parameters suggested that it is unlikely that this observation is due to viscosity effects alone. To test whether aluminum speciation or complexing with the IL cation produced the brightening effect, a comprehensive study of the species present in [BMP]TFSA/AlCl3 and [EMIm]TFSA/AlCl3 as a function of the AlCl3 concentration was completed using Raman/ IR, 27Al and 19F NMR, and density functional theory calculations.30 It was found that the same electrochemically active aluminum species, [AlCl2(TFSA)2]-, was present in both
10.1021/jp9026755 CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
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TABLE 1: Abbreviation, Molecular Structure, Molecular Weight (MW), Density (G), Molecular Volume (MV), and Ion-Pair Diameter (D) of the ILs Used in This Studya
a D is determined from F assuming a cubic packing geometry according to the method described by Horn et al.47 Carbon atoms are shaded gray, nitrogen are blue, fluorine are yellow, sulfer are orange, and oxygen are red.
Figure 1. (a) SEM micrograph of an electrodeposited aluminum film on gold formed in the upper phase of the mixture AlCl3/[BMP]TFSA after potentiostatic polarization at -0.45 V (vs Al) for 2 h at 100 °C. (b) SEM micrograph of an electrodeposited aluminum film on gold made in the upper phase of the mixture AlCl3/[EMIm]TFSA after potentiostatic polarization at -0.05 V (vs Al) for 2 h at 100 °C.
[BMP]TFSA/AlCl3 and [EMIm]TFSA/AlCl3 mixtures. In light of this, it is hypothesized that the [BMP]+ cation acts as a grain refiner by adsorbing to the substrates and growing nuclei, hindering the crystallite growth, and nanocrystalline deposits result. To test this theory, interaction of the ultrapure and dry ILs [BMP]TFSA and [EMIm]TFSA with a metal substrate, Au(111), has been investigated using atomic force microscopy (AFM), cyclic voltammetry, and in situ scanning tunneling microscopy (STM) measurements. AFM reveals that several solvation layers form at the gold-IL interface and that the strength of the interaction between the innermost layer and the negatively charged metal substrate is dependent on the cation type. STM shows that key electrochemical properties are controlled by the strength of this interaction. Solvation layers were first detected for octamethylcyclotetrasiloxane confined between two mica surfaces using the surface force apparatus (SFA).31 It was found that the force measured normal to the interface oscillated with a period equal to the
size of the solvent molecule and that the oscillation amplitude decreased with separation.31,32 These results were attributed to variation in the liquid density profile due to the arrangement of solvent molecules into discrete liquid layers near the surface, with order decaying into the bulk. Further SFA studies with other nonpolar liquids showed that the number of detectable layers decreased with molecular flexibility because the molecules could spacefill effectively without layering.33 In polar solvents, oscillating force profiles were superimposed onto an electrostatic double-layer force.34 The capacity of the solvent to form hydrogen bonds did not play a role; the period of the molecular layers depended only on the molecular size.35,36 Surface roughness, however, destroys the order in the liquid layers, and a purely monotonic solvation force results.37 Compared to SFA, the interaction area is much smaller and less well-defined in AFM. This leads to much higher signalto-noise ratios, and generally solvation layers may only be detected using AFMs modified with lock-in amplifiers in frequency modulation or tapping mode, which allows the
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tip-sample interaction stiffness and tip deflection to be monitored simultaneously.38-44 The primary advantage of AFM over SFA is that it can be used with a wider variety of substrates, and AFM has been used to measure solvation forces for a wide range of liquid-surface combinations.39-43,45,46 The AFM mechanism of operation means that the force curves obtained consist of a series of steps or “push throughs” rather than the oscillating profiles measured using SFA. These push throughs result from the AFM tip rupturing successive solvation layers upon approaching the surface. Given that only modified AFMs are able to measure solvation layers in traditional solvents, it is somewhat surprising that IL solvation layers can be measured using a standard AFM in the deflection mode.5 A recent article by one of the authors described solvation layers for ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and N-ethyl-N′-methylimidazolium acetate confined between Si3N4 AFM tips and mica, silica, and graphite. The measurements revealed stepped force curves in all cases, with the step size corresponding to the physical dimension of the ion pair. The surface charge, surface roughness, and orientation of the cations in the interfacial layer all influenced solvation layer formation in ILs. The greatest number of solvation layers was measured for EAN adjacent to mica, which is highly charged and atomically smooth, and the measured force profiles were in excellent agreement with those of a prior SFA investigation of the same system.47 Subsequent SANS experiments revealed that both EAN and PAN possess bulk nanostructure, with the alkyl groups associated and segregated from the ionic moieties.48 These results explain the clarity of the IL solvation layer data. In contrast to traditional solvents, in which the liquid becomes ordered purely because of the presence of the surface, in ILs the surface acts to align the inherent bulk solution structure, which allows the detection of many solvation layers using a standard AFM, although factors such as molecular flexibility and surface roughness still play important roles. The magnitude of the large repulsive force associated with the solvent layer nearest the surface was a function of both the cation and surface types. This suggests a specific attractive interaction (electrostatic or solvophobic in nature, dependent on the surface) between the cation and the surface. The bulk nanostructure has also been elucidated in imidazolium-type ILs using X-ray diffraction,9 with the bulk order increasing as the length of the alkyl group is increased. As the alkyl group length is increased from C2 for [EMIm]TFSA to C4 for [BMP]TFSA and given the relationship between the bulk and interfacial structures described above, one may reasonably expect that more and better formed solvation layers will be present for [BMP]TFSA than for [EMIm]TFSA. The work conducted in this study allows this hypothesis to be tested. Because the anion is the same in both ILs, any differences noted must be due to the cation. Materials and Methods [BMP]TFSA and [EMIm]TFSA (ultrapure quality) were purchased from Merck/EMD (chloride
