Article pubs.acs.org/Langmuir
Distinguishing Self-Assembled Pyrene Structures from Exfoliated Graphene Maxim Varenik,*,† Micah J. Green,§ and Oren Regev†,‡ †
Department of Chemical Engineering and ‡Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel § Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, United States S Supporting Information *
ABSTRACT: Sonication-assisted graphene production from graphite is a popular lab-scale approach in which ultrasound energy breaks down graphite sheets into graphene flakes in aqueous medium. Dispersants (surfactant molecules) are incorporated into the solution to prevent individual graphene flakes from reaggregating. However, in solution these dispersants self-assemble into various structures, which can interfere with the characterization of the graphene produced. In this study, we characterized graphene dispersions stabilized by a family of pyrene-based surfactants that facilitate a high exfoliation yield. These surfactants self-assembled to form flakes and ribbonsshapes very similar to those of graphene structures. The dispersant structures were present both in the graphene dispersion and in the precipitate after the solvent had been evaporated and could therefore have been mistakenly identified as graphene by electron microscopy techniques and other characterization techniques, such as Raman and X-ray photoelectron spectroscopy. Contrary to previous reports, we showedby removing the dispersants by filtration and washingthat the surfactants did not affect the shape of the graphene prepared by sonication.
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layered structure.21−24 In all analytical methods, except cryogenic transmission electron microscopy (cryo-TEM),25 the characterization is carried out on the dry material after the solvent has been removed, e.g., by filtration or evaporation.26 As the solvent is removed, the concentration of the dispersant rises dramatically, which results in its phase transition to various aggregated structures or even to its precipitation in crystalline form. In some cases, these evaporation-induced structures can be ordered and very large (micrometers in the case of mesoporous materials).27,28 The dispersants used in the sonication of graphite are usually amphiphilic molecules or polymers that bind to the graphene surface, thereby preventing itby steric or electrostatic repulsionfrom reaggregating.29 The amphiphiles are physically bound to the graphene by weak van der Waals interactions or by stronger π−π bonding.30 The structure of dispersant selfassemblies on the surface of the graphene structures remains controversial,31−33 with some studies providing evidence of either hemicylinders or monolayers.34−37 Above the critical aggregation concentration, the dispersants will self-assemble either in solution or on the graphene surfaces, with the most common structures in solution being spherical or elongated
onication is a widely used method for preparing pristine graphene from graphite,1,2 for exfoliating boron nitride,3 or for separating individual carbon nanotubes from bundles or aggregates.4 Sonicating an aqueous dispersion of graphite generates shear forces5 that exfoliate graphene layers off the graphite (but may also fracture the graphite into smaller pieces). In most cases, the sonication process is conducted in the presence of surfactant moleculestermed dispersantsto enhance the exfoliation and the colloidal stability of the nanomaterials produced in the dispersion. The lateral dimensions of graphene flakes produced by sonication usually range between hundreds of nanometers and several micrometers, with a high aspect ratio (width/thickness).1,2,6−11 The shape of these graphene structures is mostly isotropic, although some recent reports have shown that with certain dispersants12,13 (that self-assemble on the surface of the graphite) or under particular sonication conditions14,15 elongated, ribbon-like structures are favored. The importance of the different preparation methods for graphene lies in the fact that various sizes and shapes of graphene are required for different applications; e.g., larger, symmetrical graphene flakes are more suitable for thermal conductivity enhancement,16 while graphene nanoribbons are more suitable for enhancement of mechanical17,18 and electrical19,20 properties. Shape and size analyses of graphene are usually performed by atomic force microscopy21 or electron microscopy.22 In the latter, graphene is identified by its diffraction pattern and its © XXXX American Chemical Society
Received: September 13, 2016 Revised: September 25, 2016
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DOI: 10.1021/acs.langmuir.6b03379 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
Figure 1. (a) SEM of as-received Asbury Carbons expanded graphite (grade 3805), bar = 10 μm. (b) Pyrene derivatives: 1-pyrenesulfonic acid sodium salt (PySASS), 1-pyrenecarboxylic acid (PCA), and 1-pyrenebutyric acid (PBA). (c) Scheme of the preparation process: graphite mixed in water with a pyrene derivative. After sonication and centrifugation, graphene flakes (gray) and self-assembled structures (flakes or ribbons, blue) remain in the solution. added to the solution, which was then tip sonicated for 1 h with a Qsonica sonicator (Q-700) operated at an output wattage of 15 W at room temperature with a water bath for temperature control. The dispersion was then centrifuged (Centrific Centrifuge 225, Fisher Scientific) at 1500g (5000 rpm) for 4 h to remove larger aggregates, and the supernatant was collected. Supernatant was either drop-cast as is or filtered to remove the dispersant. Dispersant Removal. Each supernatant was vacuum filtered through a cellulose acetate filter (Sertorius) with a pore size of 0.2 μm several times until the filtrate was clear, in order to build up a layer of graphene + dispersant. The graphene + dispersant cake on top of the filter was then rinsed under vacuum with a large excess (3 L) of solvent to remove the surfactant (DI water for PySASS and ammonia solution, pH 10, for PCA and PBA). The filter with the particles was dried at 80 °C overnight. Graphene powder was then gently scraped off the filter. Scanning Electron Microscopy (SEM). Solution-cast samples were prepared by drop-casting a drop of graphene dispersion (supernatant) on an aluminum disk, followed by drying at 80 °C for 3 h. Some of the powder was then spread on carbon tape. Samples were imaged (high-resolution cold FEG scanning electron microscopy SEM, JSM-7400F, JEOL, in secondary electron mode) directly on the aluminum disk or of after being transferred to a sticky carbon tape, both sputter-coated with Pt. X-ray Photoelectron Spectroscopy (XPS). XPS data were collected using an X-ray photoelectron spectrometer, fitted with an ESCALAB 250 ultrahigh vacuum (10−9 bar) apparatus with an Al Kα X-ray source and a monochromator. Powder samples were pressed into indium foil. Aqueous samples were solution cast on a silicon wafer. The spectra were recorded with pass energies of 150 or 20 eV (high-energy resolution). The spectral components of the C signals were found by fitting a sum of single component lines to the experimental data by means of nonlinear least-squares curve fitting. To correct for charging effects, all the spectra were calibrated relative to a carbon 1s peak, positioned at 285.0 eV. Cryogenic Transmission Electron Microscopy (Cryo-TEM). Specimens for cryo-TEM were prepared using a Leica EM GP cryopreparation station operated at 100% relative humidity. A drop of 4 μL of graphene dispersion solution was applied onto a holey carbon TEM grid (Lacey substrate, 300 mesh, Ted Pella, Inc.), automatically blotted with a filter paper, and plunged into liquid ethane at its freezing point. The vitrified samples were stored under liquid nitrogen before being transferred to a FEI-Tecnai T12 TEM equipped with a Gatan workstation and cryo-holder for imaging at 98 K. The microscope was operated at 120 kV in low electron dose mode (to reduce radiation damage) and at a few micrometers under focus to increase the phase contrast. Images were recorded on a Gatan 794 CCD camera. The cryo-TEM micrographs were analyzed manually by Digital Micrograph software using the line profile tool. Raman Spectroscopy. Raman spectroscopy of the dried supernatant or graphene powder was performed on a Jobin Yvon HR
micelles. The type of structure is affected by the geometry and concentration of the dispersant.38 Of recent interest are pyrene-based dispersants (Figure 1b), which bind strongly to the aromatic surface of graphene by virtue of the pyrene’s polyaromatic structure. Sonication of graphite together with these dispersants yields dispersions with rather high graphene concentrations at low graphite-todispersant ratios.39−43 The dispersant molecules, which consist of a large hydrophobic polyaromatic group attached to a hydrophilic group (Figure 1b), tend to self-assemble into ribbon-like aggregates,44,45 possibly due to π-stacking.46 The ability of pyrene derivatives to self-assemble may also be utilized in the formation of self-assembled films47 or for directing the self-assembly of other nanoparticles 48 or molecules.49 Despite recent interest in such pyrene derivatives due to their use in graphene dispersion, the effect of their self-assembly on the exfoliation process has not been explored. A particular problem is that these dispersants are usually used at42 or above40,41,43 the critical aggregation concentration (e.g., 0.1 mM for 1-pyrenebutyric acid),50 at which the dispersant molecules not only tend to crowd at the air−solution interface and on the graphene surface but also to aggregate in solution. We therefore set out to study the self-assembly of a family of pyrene-based dispersants that are used in sonication-assisted graphene production, both in solution and after solvent evaporation. We found that in solution these compounds selfassemble into aggregates whose structure remains unchanged even after the solvent is removed. Importantly, in both cases (in solution and after solvent removal) such structures may easily be misidentified as graphene because of their similarities to grapheneboth their morphology (as viewed by electron microscopy) and their Raman and X-ray photoelectron spectra.
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EXPERIMENTAL SECTION
Materials. 1-Pyrenecarboxylic acid (PCA), 1-pyrenebutyric acid (PBA), 1-pyrenesulfonic acid sodium salt (PySASS), hydrochloric acid (ACS reagent, 37%), and ammonia (anhydrous, >99.99%) were purchased from Sigma-Aldrich. Expanded graphite was provided by Asbury Carbons (CAS# 7782-42-5, grade 3806). All the chemicals were used as received. Graphite Sonication. Graphene dispersions were prepared as previously described41 with minor modifications: In a typical experiment, 2 mg/mL of PBA, PCA, or PySASS were added to 20 mL of deionized water (DI). For PBA and PCA, the pH of the solution was adjusted to 10 by addition of ammonium hydroxide in order to dissolve the dispersants. Expanded graphite, 20 mg/mL, was B
DOI: 10.1021/acs.langmuir.6b03379 Langmuir XXXX, XXX, XXX−XXX
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Langmuir LabRAM micro-Raman device operated at 514 nm (1 μm spot size) on a quartz slide.
morphologies to those formed by graphene, in both size and shape, and their presence could therefore lead to misinterpretation of the micrographs and spectra. Ribbon structures formed by PBA and PCA with or without graphite were found to be several micrometers in length, as analyzed in solution by cryo-TEM and in the dry state by SEM (Figure 2a−f). In solution, ribbons formed by PCA alone were narrower (∼50 nm, Figure 2e) than those formed by PBA (∼500 nm, Figure 2a). PBA also formed flakes (micrometer size) with characteristic outlines and surface features (crisscross patterns with straight cut edges, Figure 2b). In cryo-TEM, these structures did not show a diffraction pattern (unlike graphene). Upon solvent evaporation, larger and longer ribbons were formed. PCA ribbons (Figure 2f) did not aggregate upon solvent evaporation, in contrast to PBA ribbons (Figure 2e), which precipitated into bow-tie-shaped structures (Figure 2d). Importantly, we found that graphene nanoribbons obtained by chemically unzipping carbon nanotubes51 demonstrated a similar morphology to that of the PCA or PBA aggregates (Figure 2g). PySASS molecules (Figure 1b) self-aggregated into flake-like structures. Although these structures were small (hundreds of nanometers in diameter, Figure 3a) and not ordered in solution, when the solvent was removed, they assembled into very large (>10 um, Figure 3b) individual flakes. Unlike the ribbons, the flakes were isotropic in shape. Again, a comparison of these flakes with graphene (Figure 3b vs Figure 4) showed
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RESULTS AND DISCUSSION Our graphene suspensions were prepared by sonicating expanded graphite (Figure 1a) in the presence of a pyrene derivative as a dispersant41 (Figure 1b). We showed (Figure 1c) that upon mixing graphite with such a dispersant (see Experimental Section) we could obtain a stable dispersion of graphene at a concentration of 0.1−1 mg/mL and with a zeta potential of −20 to −30 mV. However, by using a wide spectrum of electron microscopy techniques, we also detected other structures in addition to grapheneboth in solution and in the dry state. Graphene dispersions prepared using PBA or PCA as dispersants contained ribbon-like structures (Figure 2),
Figure 2. Cryo-TEM images (bars = 200 nm) of graphene-free PBA ribbons (a) and crisscrossed ribbon aggregates (b). White arrows point at the perpendicular aggregation of the ribbons. SEM (bars = 1 μm) of PBA ribbons (c) and bow ties (d) formed by evaporation of the solvent. Graphene-free PCA ribbons in cryo-TEM (e, bar 100 nm), and SEM (f, bar = 1 μm). SEM of graphene nanoribbons prepared by chemical unzipping18 of carbon nanotubes (g, bar = 1 μm).
Figure 4. SEM of graphene prepared by sonication with PCA (a), PBA (b), and PySASS (c) after the solvents and surfactants had been removed by filtration and washing. Bar = 100 nm.
many structural similarities. An important finding was that while the assembly of pyrene derivatives into ribbon-like structures has been reported previously44,45,47−49the assembly of pyrene into flake-like morphologies is unique to PySASS. The similarity between the self-assembled structure of PySASS and 2D graphene flakes implies a conformal coating of the dispersant over the graphene and could explain why PySASS yields higher graphene concentrations (0.2 mg/mL, at 2 mg/ mL of dispersant) in dispersions than PBA or PCA (0.1 mg/ mL).41 Removal of the dispersant from the graphene−dispersant suspension after sonication (by filtration and washing) provided us with graphene flakes. These isotropic structures are hundreds of nanometers in lateral size (Figure 4). To examine the effect of the dispersant on exfoliated graphene, drop-cast films and the washed graphene powder were characterized using Raman and XPS. A characteristic Raman spectrum of graphene contains three main bands at ∼1350, ∼1570 and ∼2690 cm−1 named D, G, and 2D, respectively.52 The D band may be attributed to the first-order phonons and indicates disorder, the G band is related to the sp2 carbon stretching mode, and the 2D band may be attributed to the second-order phonons and is related to the number of layers. Some minor bands may also appear,
while samples prepared with PySASS contained flake-like structures (Figure 3). The formation of these structures was not affected by the presence of graphene or graphite, as they were also formed in solutions containing identical concentrations of the dispersant in the absence of graphite and without sonication. These aggregates demonstrated very similar
Figure 3. Graphene-free PySASS: cryo-TEM images of (a) large flakelike and small (inset) aggregates formed by PySASS (bars = 200 nm) and flakes formed after solvent evaporation in SEM (b, bar = 10 μm). C
DOI: 10.1021/acs.langmuir.6b03379 Langmuir XXXX, XXX, XXX−XXX
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Langmuir for instance, the D′ band (∼1610 cm−1) corresponding to defects in the sp2 plane. Before dispersant removal, it is difficult to detect all bands due to the fluorescence from the dispersant molecules. No characteristic signal was detected in samples containing PCA (Figure 5, red solid line). In PBA and PySASS
Figure 6. XPS spectra of expanded graphite (a) and graphene dispersions prepared with PySASS (b), PBA (c), and PCA (d) before (solid line) and after (dashed line) removal of the dispersant. The atomic percent of carbon and oxygen before and after dispersant removal is indicated.
available in the Supporting Information). However, compared to graphene, they have a higher atomic percent of carbonbound oxygen and they may show the presence of sulfur (Figure 1b). Thus, their C 1s peak has components at higher binding energies.59,60 After removing the dispersant, the obtained spectrum and atomic composition were almost identical to those of the source graphite/graphene (Figure 6b−d, dashed line). To summarize, pyrene derivatives were used to form stable graphene dispersions. Since these dispersants were used at concentrations above their critical aggregation concentration, we found that in addition to their ability to stabilize graphene, these dispersants formed self-assembled structures. In solution, the dispersants formed two types of structure: derivatives with a carboxylic moiety assembled into ribbon-like structures, and the derivative with a sulfonic moiety assembled into a flake-like morphology. When the solvent was removed by evaporation, these structures retained their distinct original shapes. The selfassembled dispersant structure was not affected by the presence of graphene, or vice versa.12,13 The difficulty inherent in imaging graphene produced in the type of sonication process described here lies in the resemblance of dispersants’ selfassembled structures to those of graphene, both in solution and in the dry state. In addition, the self-assembled dispersant structures have a high aromatic carbon content, and it is thus difficult to differentiate them from graphene or graphite by XPS. The dispersant molecules covering the graphene were detectable by Raman spectroscopy due to their fluorescence; however, in the case of PCA their presence made it impossible to obtain the spectrum of graphene. Finally, despite their selfassembly, the pyrene derivatives efficiently disperse graphene with a low dispersant-to-graphite ratio and a high exfoliation yield.
Figure 5. Raman spectra of graphene solutions prepared with PBA, PCA, and PySASS before (solid line) and after (dashed line) removal of the dispersant.
dispersions, the spectrum of graphene was detectable (Figure 5a, solid line) and showed the above-mentioned characteristic graphene bands. This difference may possibly be due to a better coverage of graphene by PCA after solvent removal. An additional peak is observed for PBA at 1230 cm−1, which was observed previously53 in graphene−PBA films. Washing the dispersant away did not considerably change the relative intensities of the characteristic peaks in PySASS and PBA (Figure 5, dashed lines). Washing did, however, enable the detection of the signal from graphene prepared with PCA. Raman spectroscopy also provides information on the type of graphene produced. For instance, the intensity of the D′ band is characteristic of in-plane defects. Compared to expanded graphite, the increase in the intensity of the D′ band in the dispersed graphene (Figure 5) was a result of a higher defect density.54 The defect density increased during sonication due to the reduction in size (from ∼10 μm, Figure 1a, to hundreds of nanometers, Figure 4) and to the creation of vacancies and sp3hybridized carbon. A comparison of the 2D band of graphenein terms of location and symmetric line shape55,56with the original graphite 2D band indicated that graphene flakes comprising a few layers were formed. In XPS, which detects atom binding energies characteristic of each element and charging state,57 graphite and graphene can be characterized by their prominent C 1s peaks. Deconvoluting this peak into its components in expanded graphite (Figure 6a) revealed a main peak (284.8 eV), corresponding to graphitic sp2, and two additional peaks with much lower intensities (285.1 and 286.8 eV), which may be attributed to either structural defects or to various energy loss mechanisms.57,58 The XPS spectra of our solution cast films presented an overlap of the dispersant and graphene signals (Figure 6b−d, solid line). The overlap between these signals showed a C 1s spectrum and atomic composition (indicated in Figure 6b−d) similar to graphene oxide and not to graphene. The pyrenebased dispersants have a high content of sp2 carbon and produce a graphitic-like peak in XPS (deconvoluted spectra are
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CONCLUSIONS While water-soluble pyrene derivatives are efficient dispersants for graphene,39,41,42 they have the inherent drawback of interfering with the characterization of graphene by electron microscopy (Figures 2−4), Raman spectroscopy (Figure 5), or XPS (Figure 6). The pyrene derivatives self-assembled into the types of structure previously reported for sonicated graphD
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Langmuir ite,12,13 namely, PBA and PCA self-assembled into ribbon-like structures, while PySASS assembled into a flake-like morphology. It therefore appears that removing the dispersant by filtration and washing is a possible remedy for the misinterpretation. The resultant dispersant-free material will reveal the true graphene structure (microscopy) and quality (Raman and XPS).
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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.langmuir.6b03379. Deconvoluted C 1s XPS spectra of expanded graphite and graphene after dispersant washing (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(M.V.) E-mail
[email protected]. Funding
The work at TAMU was supported by the U.S. National Science Foundation (NSF) under CAREER Award CMMI1253085. The work at BGU was supported by the Grand Technion Energy Program (GTEP), the Adelis Foundation, and the Joint Research Centre (project A9). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Dr. Einat Nativ-Roth is thanked for her excellent technical help in cryo-TEM imaging. We also thank Dr. Ayelet Vilan and Dr. Michael Shtein for critical reading and discussions. The input of Prof. Jaime Grunlan of TAMU and Dr. Sriya Das of Texas Tech, now at Intel, is gratefully acknowledged.
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ABBREVIATIONS Cryo-TEM, cryogenic transmission electron microscopy; PCA, 1-pyrenecarboxylic acid; PBA, 1-pyrenebutyric acid; PySASS, 1pyrenesulfonic acid sodium salt; DI, deionized water; SEM, scanning electron microscopy; XPS, X-ray photoelectron spectroscopy.
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REFERENCES
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DOI: 10.1021/acs.langmuir.6b03379 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.6b03379 Langmuir XXXX, XXX, XXX−XXX
