Article pubs.acs.org/JPCC
Exfoliation of Graphene in Ionic Liquids: Pyridinium versus Pyrrolidinium Vitaly V. Chaban,*,† Eudes Eterno Fileti,*,† and Oleg V. Prezhdo*,‡ †
Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo, 12247-014 São José dos Campos, São Paulo, Brazil Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
‡
S Supporting Information *
ABSTRACT: Exfoliation is an elegant physical technique to directly obtain graphene from graphite. A search for highperformance exfoliation solvents is actively pursued nowadays. We report potentials of mean force and free energies, characterizing steered separation of graphene sheets and their subsequent solvation by room-temperature ionic liquids (RTILs). The exfoliation performance of N-butylpyridinium bis(trifluoromethanesulfonyl)imide [BPY][TFSI] is compared to that of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR][TFSI]. Both RTILs are shown to exhibit comparable exfoliation performance, which is superior to the performance of 1-ethyl-3-methylimidazolium tetrafluoroborate by ∼20 kJ mol−1 nm−1. The sulfur atom of TFSI− contributes an essential portion of the solvation enthalpy due to its dispersive attraction to carbon atoms of graphene. Both RTILs maintain a single, moderately defined shell around the graphene surface. [BPY][TFSI] and [PYR][TFSI] are suitable starting compounds to engineer more advanced solvents. The reported results support development of new higher-performance RTILs for use in solvent-assisted graphene exfoliation.
1. INTRODUCTION Graphene is a carbon allotrope exhibiting many unusual and practically useful mechanical, electrical, and thermal properties.1−7 Both edges and surfaces of graphene are reactive, allowing one to modulate its electronic and physicochemical properties via chemical (covalent) functionalization. Academic researchers proposed numerous and versatile applications of graphene during the recent decade, including electronic and optoelectronic devices, chemical sensors, nanocomposites, and energy storage.8−14 However, the production costs of highquality graphene are still not sufficiently low to foster these applications and, ultimately, produce industry-grade materials. A related engineering problem is to achieve high degrees of graphene purity because graphene’s electronic properties are drastically sensitive to the presence of admixtures.15−19 Synthesis of graphene is performed using the two groups of methods designated by the terms “bottom up” and “top down”. One of the most physically intuitive top down methods of obtaining graphene is to exfoliate graphite, which is abundant in nature and is widely applied in industry.20−22 Graphite can be exfoliated through graphite intercalated compounds followed by rapid thermal treatment.23,24 Preliminary oxidation of graphite simplifies exfoliation by attaching polar oxygencontaining groups to the graphite surface.25 The most frequently observed groups are hydroxyl, carboxyl, and epoxide. The oxidation of graphite can be fulfilled by the same methods, which are applied (during more than a century) to oxidize organic substances. While graphite oxide exfoliation was demonstrated to be feasible and productive, an essential © 2016 American Chemical Society
problem was noticed. The oxygen-containing groups cannot be completely removed from graphene using existing reduction methods after exfoliation. This deteriorates the purity of the resulting product. The ability to produce graphene without having to chemically modify it constitutes an important and urgent goal of modern chemical technology. A pure graphene powder constitutes a nanomaterial with an exceptional set of properties in the context of many emerging applications, such as coatings, polymers, and reinforced composites. Graphite exfoliation is presently considered to be the most promising large-scale method to synthesize high-quality graphene. Solvent plays a paramount role in successful exfoliation.26−28 Graphene exfoliation can be done in both aqueous and organic solvents. The actively employed solvents are ethanol, methanol, N,N-dimethylformamide, acetone, 1-propanol, isopropanol, 1methyl-2-pyrrolidone, 1,2-dichlorobenzene, c-butyrolactone, tetrahydrofuran, naphthalene, and so forth. Use of water as a major solvent requires surfactants, polymers, and organic molecules, which are amphiphilic.29−31 If surfactants are adsorbed on graphite, they improve the stability of colloidal dispersions due to repulsive steric forces between the exfoliated sheets. Mixtures of organic solvents with water are also considered to adjust the surface tension and hydrophobicity. Beyond solvation of graphite, sonication and centrifugation are routinely applied.32 The solvents with surface energy similar to Received: November 2, 2016 Revised: December 16, 2016 Published: December 20, 2016 911
DOI: 10.1021/acs.jpcc.6b11003 J. Phys. Chem. C 2017, 121, 911−917
Article
The Journal of Physical Chemistry C
Figure 1. (Left) Spatial structures of organic ions constituting the investigated RTILs. (Right) Graphene immersed in [BPY][TFSI] at 400 K. Anions are blue, cations are red, and graphene is yellow. Graphene edges were passivated by hydrogen atoms (not shown for simplicity). The depicted molecular configuration corresponds to an equilibrium part of the MD trajectory, after 20 × 106 MD time steps.
that of graphite, 0.07−0.08 J m−2, enhance exfoliation and stabilities of the prepared graphene-containing dispersions. It is essential to prevent chemical reactions of the molecular organic solvents with graphene. Room-temperature ionic liquids (RTILs)33−36 constitute a vigorous research field, in which new solvent combinations designed for specific tasks are reported monthly. RTILs were applied for covalent and noncovalent functionalization of nanoscale carbon.37,38 The surface energy of graphite is similar to that of certain RTILs.39 Furthermore, RTILs exhibit slow ionic dynamics,40,41 impeding reverse aggregation of the exfoliated graphene. Wang and co-workers42 performed direct exfoliation of graphite flakes in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, reporting a suspension with a graphene concentration of 0.95 g L−1. Lu and Zhao43 prepared a few-layer-thick N-doped graphene using controlled electrochemical exfoliation of graphite in ethylammonium nitrate. The proposed method draws interest for obtaining metal-free N-doped graphene in large quantities. Mariani and co-workers44 used 1-hexyl-3-methylimidazolium hexafluorophosphate and recorded exfoliated graphene concentrations of ≤5.33 g L−1 at mild conditions. Yang and co-workers proposed cathodic exfoliation of graphene using a pyrrolidinium-based RTIL as a solvent.45 Computer simulations of the controlled exfoliation of graphene with 1-ethyl-3-methylimidazolium tetrafluoroborate were recently conducted, describing thermodynamics of the overall process.46 Stabilities of the resulting dispersions over macroscopic times represent a central problem of the research.32 Investigations demonstrating exfoliation at the molecular level of detail are scarce,46−48 but they can significantly promote understanding of the phenomenon and suggest systematic methodological improvements. Which ions (cations or anions) interact with graphene most strongly? What is the role of the cation−anion coordination? Does π-stacking in the case of aromatic cations contribute substantially to the success of exfoliation? Baker and Kamath48 performed MD simulations of solvating a 1.5 × 1.5 nm2 graphene bilayer in N-butylpyridinium bis(trifluoromethanesulfonyl)imide ([BPY][TFSI]) and 1butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([PYR][TFSI]). The computed potentials of mean force (PMFs) showed that small graphene sheets prefer to exist in the form of a graphene bilayer in the RTILs rather than in vacuum. Atilhan and Aparicio49 investigated folding of graphene nanostructures driven by numerous RTIL nano-
droplets by MD simulations. Our work complements these simulations by considering explicitly the exfoliation process. We report fully atomistic steered molecular dynamics (MD) simulations to obtain PMFs for graphene exfoliation in [BPY][TFSI] and [PYR][TFSI]. Further, we derive the solvation free energies of a single graphene sheet in both RTILs and discuss site−site interaction energies and key structural motifs. We describe exfoliation as a thermodynamic process of complete spatial separation of the two graphene sheets, in which both the initial state and the final state remain fully solvated. The present simulations mimic experimental practice to the maximum possible extent.
2. METHODOLOGY The nonfunctionalized defect-free single-layer graphene sheet (3.5 × 3.5 nm2) was immersed into [PYR][TFSI] and [BPY][TFSI]. Both RTILs were represented by 600 ion pairs. Figure 1 depicts optimized geometries of the selected ions and a molecular configuration corresponding to the free energy minimum at 400 K and 1 bar, after 2 × 107 time steps of the simulated spontaneous motion. All MD simulations were performed in the isothermal− isobaric ensemble (NPT). We simulated the system at an elevated temperature of 400 K to accelerate the dynamics and to avoid metastable states because viscosities of RTILs and RTIL-based solutions are high.50,51 A constant pressure of 1 bar was maintained by the Parrinello−Rahman barostat52 with a relaxation time of 4.0 ps and compressibility constant of 4.5 × 10−5 bar−1. Constant temperature was maintained by a velocity rescaling thermostat with a relaxation time of 500 fs.53 Graphene was simulated with zero partial charges and an ε = 0.36 kJ mol−1 dispersion attraction constant in a classical 12−6 Lennard-Jones (LJ) equation with σ = 0.34 nm. Bond distance (1.41 Å), bond angle (120°), and dihedral angle (0°) harmonic potentials were used to maintain the integrity of the graphene sheets during MD. These parameters had been successfully employed elsewhere.46 The RTILs were represented in accordance with recently proposed and refined force fields.50,51,54 These force fields provide reliable transport and thermodynamic properties of pyridinium- and pyrrolidiniumbased RTILs. They have been developed using universal and transferrable algorithms. The MD systems were equilibrated for 1.0 ns by monitoring thermodynamic quantities and the MD box dipole moment. An integration time step of 2.0 fs was used. The atomic coordinates and immediate thermodynamic quantities were saved every 0.2 ps. 912
DOI: 10.1021/acs.jpcc.6b11003 J. Phys. Chem. C 2017, 121, 911−917
Article
The Journal of Physical Chemistry C The Bennett Acceptance Ratio method55 was employed to obtain the solvation free energy. Graphene was decoupled from RTILs using 21 consequent states (Δλ = 0.05). At every state λ, the MD systems were equilibrated using stochastic dynamics. In turn, the averaged
∂H(λ) ∂λ λ
was derived from the 10 ns long
production trajectory, generated after the equilibrium was proven. Different parts of the trajectory were tested and subjected to statistical processing to ensure sufficient sampling. The soft-core repulsion term for the LJ interactions was applied to avoid fusion of the atomic nuclei upon decoupling56 VSC(r ) = (1 − λ)V ([ασ 6λ p + r 6]1/6 )
Here, VSC(r) is the conventional hard-core pair potential, α is the LJ size parameter of the atom pair, whereas λ = 0 and 1 correspond to the fully coupled and uncoupled states, respectively. The parameters for the soft-core potential were chosen as follows: α = 0.5, p = 1.0, and σ = 0.3. To increase accuracy, the electrostatic and 12−6 LJ interactions were decoupled separately.57 The PMF was used to describe separation of two graphene sheets immersed in [PYR][TFSI] and [BPY][TFSI]. Such a controlled separation, visualized in Figure 2, is a simplified model of the solvent-assisted exfoliation. Exfoliation was performed by gradually increasing the distance between the centers-of-mass of two graphene layers. No atoms were frozen during steered MD. The umbrella sampling potential58 with a force constant of 3000 kJ mol−1 nm−2 was used to keep graphene in the specified region of space along the reaction coordinate. In the present study, the average distance between the centers-of-mass in two sequential PMF calculation steps was 0.1 nm. Each PMF step was sampled for 20 ns, and the total sampling time per RTIL was over 400 ns. The weighted histogram analysis method59 was used to derive the PMF. MD simulations were performed using the GROMACS 5 molecular simulation engine.60,61 The trajectories were visualized and the molecular artwork was prepared in VMD 1.9.2.62 Figure 2. Three representative configurations visualizing steered MD simulations. The obtained molecular configurations are subsequently used as starting points for umbrella sampling simulations. The ions are omitted in the snapshots for clarity. Flexibility of graphene is an essential simulation detail. Simulation of rigid graphene is expected to provide totally different results in this context.
3. RESULTS AND DISCUSSION The average distances between graphene and various interaction sites of the RTILs are reported in Figure 3. The HC, HA, FC, CF, and OS sites of [BPY][TFSI] are in direct contact with graphene (the first peaks are closer than 0.35 nm). Thus, both the BPY+ cation and the TFSI− anion coordinate graphene. The peak locations in the 0.30−0.35 Å range suggest a moderate affinity of [BPY][TFSI] to graphene. A more favorable binding would lead to smaller interatomic distances, particularly in the case of hydrogen atoms due to a smaller van der Waals radius of this element. The peaks for the NI, SO, CT, and NA sites are located at larger distances, and therefore, these sites do not participate in solvating graphene. Note that the above sites belong to both ions. Thus, neither the cation nor anion plays a leading role in the solvation of graphene. The butyl chain tends to point toward graphene. The second peaks can be explained by availability of more than one symmetrically identical interaction site within the same ion, for example, HC, OS, and FC. By performing an in-depth analysis, we established that there is no second solvation shell of graphene in [BPY][TFSI]. This is in line with the expectations because no strong intermolecular interactions were detected.
The performance of [PYR][TFSI] in solvating graphene is similar to that of [BPY][TFSI] (Figure 3). The HC, FC, and OS sites exhibit peaks below 0.35 nm, whereas other sites prefer larger distances. The anion plays an important role in coordinating graphene. MD simulations employing classical potentials allow for straightforward decomposition of the total potential energy into pairwise components. Such a decomposition demonstrates the importance of different cation and anion parts in binding to graphene (Figure 4). Although the HC sites are closest to graphene, they contribute only −25 kJ mol−1 each. Note that the energies are provided per mole of the corresponding interaction sites, rather than per mole of ion pairs. Fluorinated methyl groups of TFSI− interact with graphene more strongly,
