Probing Graphene–Surfactant Interactions in Aqueous Dispersions

Article pubs.acs.org/JPCC

Probing Graphene−Surfactant Interactions in Aqueous Dispersions with Nuclear Overhauser Effect NMR Spectroscopy and Molecular Dynamics Simulations Vaishali Arunachalam and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Sonication-assisted exfoliation of graphite in aqueous solutions of ionic surfactants is an attractive, scalable route for the production of defect-free graphene. The interaction of surfactant chains with graphene is crucial both to the process and to the stability of the dispersion. We use 1H nuclear Overhauser effect (NOE) NMR techniques and classical molecular dynamics (MD) simulations to probe the molecular nature of these interactions in graphene dispersions stabilized by the cationic surfactant cetyltrimethylammonium bromide. We show that the surfactant chains are quasi-bound to the graphene sheets undergoing rapid exchange with the free surfactant ligands in the bulk, but what is surprising is the observation of NOE interactions between groups that are separated by more than 5 Å along the chain. MD simulations provide the key to interpreting these observations; these interactions are a consequence of the arrangement of the quasi-bound surfactant chains that allows segments of different chains to come into spatial proximity on the graphene sheet.

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INTRODUCTION Graphene, the two-dimensional allotrope of carbon consisting of a single sheet of hexagonally arranged sp2-bonded carbon atoms, continues to attract attention ever since its discovery in 2004.1,2 The continuing interest in graphene is a consequence of the impressive range of physical and chemical properties that it exhibits and the prospect of potential applications.3,4 Graphene may be obtained from graphite by procedures that can overcome the van der Waals attractive forces that hold adjacent layers together. Historically this was first achieved by micromechanical cleavage by the deceptively simple procedure of peeling with Scotch tape, and these samples continue to be the benchmark against which samples prepared by other procedures are compared.5 The procedure, unfortunately, is not scalable. A simple and scalable method for the production of defect-free graphene is the liquid-phase exfoliation of graphene by sonication in an appropriate solvent.6,7 The requirements for liquid-phase exfoliation are minimal the starting bulk layered material, a suitable solvent, and equipment that can generate ultrasonic waves. These waves create microsized cavitation bubbles in the solvent that on collapse generate high-pressure jets; the resultant shear forces exfoliate the layers to produce two-dimensional nanosheets.8 The role of the solvent is crucial to the process, as the formation of stable dispersions requires that the exfoliated sheets be prevented from restacking. A suitable or “good” solvent is one that interacts with the nanosheets and stabilizes the dispersion by preventing agglomeration. It has been suggested that the best solvents are those whose surface energy © XXXX American Chemical Society

matches that of the sheet since it can minimize the surface tension between the solvent and the material being exfoliated, hence favoring miscibility.9,10 Solvents such as N-methylpyrrolidine, cyclohexylpyrrolidine, and dimethylformamide, which have surface energies close to that of graphite, 70 mJ m−2, have been successfully used to obtain stable dispersions of defect-free graphene sheets in high concentrations.11−13 These organic solvents are, however, toxic and expensive. Water, on the other hand, is a cheaper and environmentally friendly medium but is unsuitable for graphite exfoliation due to the hydrophobic nature of graphene. Addition of surfactants to water can, however, make it compatible by reducing the surface tension to match that of graphite, making it a suitable medium for sonication-assisted exfoliation.14 Additionally, the charge of the surfactant stabilizes the dispersions through electrostatic repulsions.15 There have been several reports on the use of surfactants (ionic as well as nonionic) to give stable aqueous dispersions of graphene nanosheets via sonication-assisted exfoliation of graphite.16−19 The focus of most of these reports has been on factors that influence the concentration of exfoliated graphene in the dispersion as well as the dimensions of the nanosheets. These factors include the concentration and type of surfactant used, the sonication time, and the initial graphite concentration.16,20 To the best of our knowledge, most efforts Received: June 2, 2017 Revised: June 21, 2017 Published: July 11, 2017 A

DOI: 10.1021/acs.jpcc.7b05404 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) 1H NMR spectra of graphene−CTAB dispersions and the 5 mM CTAB solution in D2O. The assignments of the resonances are indicated. Proton 2D NOESY NMR spectra of (b) CTAB−graphene dispersions and (c) the 5 mM CTAB solution. (d) 1H 1D NOE buildup for the CTAB resonances in the CTAB−graphene dispersion on irradiation of the −N(CH3)3 resonance at 2.93 ppm.

prepared for a wide range of CTAB concentrations ranging from 0.5 to 100 mM. The highest yield of graphene (0.71 mg/ mL) in dispersion was obtained for a CTAB concentration of 5 mM and hence used for all subsequent analyses (details of the estimation are provided as part of the Supporting Information, section S1). The CTAB-exfoliated graphene sheets were characterized by tapping-mode atomic force microscopy (AFM) and scanning electron microscopy (SEM) (Supporting Information, section S1). In both these measurements, the dispersions were diluted, prior to deposition, to avoid overlapping of nanosheets. The AFM images show graphene sheets with thicknesses around 12−180 nm and lateral dimensions of typically ∼200 nm with a fairly monodisperse distribution. Samples for NMR studies were prepared from the residue obtained after centrifugation of 2 mL of the supernatant for another 1 h at 14000 rpm. The resulting residue was dispersed in 500 μL of D2O and sonicated for 10 min. A 5 mM CTAB solution in D2O was also prepared as a control for NMR studies. All NMR experiments were performed on a JEOL ECX II spectrometer operating at a proton resonance frequency of 500 MHz using a pulse length of 6.25 μs, a spectral width of 7500 Hz, and a relaxation time of 3 s at the room temperature of 23 °C. 1H spectra were recorded with water suppression using presaturation pulses. 1H NOESY was recorded with a short mixing time of 200 ms using 4096 data points in the t2 dimension and 256 points in the t1 dimension with a total of eight scans. 1H 1D transient NOE was recorded by selective irradiation of the resonance at 2.93 ppm by a Gauss-shaped pulse at low power for different mixing times ranging from 10 μs to 1 s. The residual water signal at 4.67 ppm from the D2O lock was used as the reference for all spectra.

toward developing a molecular perspective of the surfactant stabilization of graphene dispersions have been limited to molecular dynamics simulations;21,22 experimental studies that probe surfactant−graphene interactions at a molecular level are, unfortunately, not available. Establishing the nature of interaction between the surfactant chains and graphene at a molecular level is important; it is crucial to the development of new methodologies and systems for producing surfactantstabilized aqueous dispersions of graphene at higher concentrations and better yields. Here we have investigated aqueous dispersions of graphene stabilized by the cationic surfactant cetyltrimethylammonium bromide (CTAB) using two-dimensional nuclear Overhauser effect spectroscopy (NOESY) . The potential of NMR spectroscopy to provide a molecular perspective of ligand−nanoparticle interactions in colloidal dispersions of nanostructures has clearly been demonstrated.23−25 This technique offers the advantage of an in situ analysis of the surfactants bound to the graphene sheets even when a large excess of the free surfactant is present in the dispersion. Using the experimental results from NMR spectroscopy, and in conjunction with molecular dynamics simulations, we are able to establish the molecular arrangement of surfactant chains on the graphene sheets in the dispersion.

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EXPERIMENTAL DETAILS Graphene−CTAB dispersions were prepared by sonicating 100 mg/mL graphite powder (CDH, graphite fine powder, 50 μm) in CTAB solution for 45 min using a tip sonicator (Ultrasonics 250 W). The dispersion was allowed to stand overnight and the top 5 mL decanted and centrifuged for 10 min at 5000 rpm to remove the larger aggregates. The supernatant thus obtained was used for further characterization. Dispersions were B

DOI: 10.1021/acs.jpcc.7b05404 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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SIMULATION METHODOLOGY Molecular dynamics (MD) simulations were performed by modeling the dispersion as a single graphene sheet with lateral dimensions of 103.2 Å × 102.1 Å, having 4032 carbon atoms immersed in 112320 water molecules along with 1080 CTA cations and 1080 Br anions in a simulation cell of dimensions 168 Å × 165 Å × 157 Å. The MD simulations were performed using the LAMMPS software26 running on the Xeon-Xeon PHI coprocessor nodes of a Cray XC-40 high-performing supercomputer. The force-field parameters for CTAB and water used in the simulation were derived from OPLS-AA and TIP3P, respectively.27,28 The 1−2 bond and 1−2−3 angle interactions were approximated by harmonic potential energy functions. Dihedral torsions were modeled by the Fourier function ∑im= 1Ki[1.0 + cos(niφ − di)], where Ki is the force constant, ni is the multiplicity for dihedral interaction, di is the phase factor with a value of 0° or 180° to determine the sign of the cosine term for the ith term, and φ is the dihedral angle, while improper torsions were defined using the harmonic cosine function K[1 + d cos(nφ)], where K is the force constant, d is the phase symbol with a value of ±1, n is the multiplicity, and φ is the improper bend angle. The previously reported Lennard-Jones potentials were used for the sp2 carbon atoms in the graphene sheet.29 The sheet was uncharged and held rigid throughout the simulation. The partial charges for the CTAB were obtained from DFT calculations with the B3LYP/6-311++G** basis sets using the Gaussian 03 software package,30 while those water were from the TIP3P model.28 Nonbonded Coulombic interactions were treated using the long-range particle−particle-mesh integration implementation provided by LAMMPS. For van der Waals interactions, a Lennard-Jones potential with a cutoff distance of 8 Å was used. Simulations were performed on an NPT ensemble (P = 1 atm, T = 300 K) for 50 ns. The temperature of the cell was maintained by a Nosé−Hoover thermostat and pressure by a Nosé−Hoover barostat. The equations of motion were integrated using the Verlet algorithm with a time step of 0.5 fs for the first 10 ns, and the time step was subsequently increased to 1 fs. At equilibrium, the theoretical density showed a difference of less than 4% from density values calculated assuming the van der Waals volume of the graphene sheet and that of CTAB and water molecules.

the dispersion and solution showed a single exponential decay as a function of the field gradient strength that resulted in a single DOSY peak with average diffusion coefficients of 0.457 × 10−9 and 1.023 × 10−9 m2/s, respectively (Supporting Information, section S2). The diffusion coefficient values in the dispersion are comparable to that for the solution and much higher than that expected for the bulky nanosheets. The broadening of the CTAB resonances in the dispersion is due to the presence of graphene sheets, which lowers the transverse spin−spin relaxation times, T2, and consequently affects the line width. In the redispersed state, CTAB molecules can either remain tightly bound to the graphene surface and exchange slowly with the bulk or be weakly attached to the sheets and undergo a fast exchange with the bulk. While, for the former case, the 1H spectrum would show two distinct peaks one for the tightly bound surfactant chains and the other for the free surfactant in the bulk solvent, for the latter, a single average peak would appear at a population-averaged value of the chemical shift. The fact that only a single set of surfactant resonances are observed in the dispersion would suggest a scenario where fast exchange occurs with a large excess of free surfactant present in solution. The inference, however, is not conclusive as the absence of a second set of peaks could be a consequence of a tightly bound species at very low concentrations with peaks severely broadened due to very short T2 values. One of the techniques capable of distinguishing bound and free surfactant molecules is transfer NOE (nuclear Overhauser effect) spectroscopy.23−25 In an NOESY experiment, the relaxation of any particular spin nucleus is spatially coupled with other spin nuclei in its proximity (5 Å), when measured along the chain. The 1D transient NOE measurements confirm these observations of apparent very short E

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separation of protons of distal groups of the surfactant chain. These results are a reflection of the arrangement adopted by the surfactant chains in the quasi-bound state in the dispersion. Classical MD simulations of the dispersion, modeled as a single graphene sheet immersed in water molecules along with CTAB surfactant chains, provide a simple interpretation of these observations. The CTA chains on the graphene sheets lie flat, covering most of the sheet, adopting a random arrangement with the “head” of one chain in close proximity to the “tail” of another chain, an arrangement that can give rise to cross-peaks in the NOESY spectrum between groups that are apparently far separated along the chain. In conclusion, we have shown how 2D NOESY NMR aided by MD simulations can provide a molecular perspective of surfactant graphene interactions in aqueous graphene dispersions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05404. Graphene−CTAB dispersions (estimation and characterization procedures), variation of CTAB chemical shifts with concentration and 1H DOSY measurements, and estimation of the number of CTA chains adsorbed on the graphene sheet (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-80-2293-2661. Fax: +91-80-2360-1552/0683. ORCID

Sukumaran Vasudevan: 0000-0002-5059-6098 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of the JEOL-IISc NMR Collaboration Centre for use of the ECX500II NMR spectrometer. We thank the Supercomputer Education and Research Centre at the Indian Institute of Science, Bangalore, for use of the HPC Cray-XC40 facility. S.V. thanks the Department of Science and Technology, Government of India, for the J. C. Bose national fellowship.

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DOI: 10.1021/acs.jpcc.7b05404 J. Phys. Chem. C XXXX, XXX, XXX−XXX