Article pubs.acs.org/Macromolecules
Effect of Hydrophobic Moieties in Water-Soluble Polymers on Physical Exfoliation of Graphene Hyeon Woo Shim,†,‡ Ki-Jin Ahn,†,‡ Kyungun Im,† Seonmyeong Noh,† Min-Sik Kim,† Younghee Lee,† Hojin Choi,† and Hyeonseok Yoon*,†,§ §
Alan G. MacDiarmid Energy Research Institute, School of Polymer Science and Engineering, and †Department of Polymer Engineering, Graduate School, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 500-757, South Korea
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S Supporting Information *
ABSTRACT: Graphene is a fascinating material with unique properties, such as high mechanical strength and excellent electronic and thermal conductivities, as well as many other beneficial properties. Despite much recent effort, the facile synthesis and colloidal stabilization of graphene in aqueous solutions remains central to both academic research and practical applications. Here, we provide an in-depth insight into how the hydrophobic moieties of polymers affect the physical exfoliation of graphite into graphenes in aqueous solution. Four different polymers with graphene-like moieties, such as phenyl- and pyrenylfunctionalized side chains, were synthesized on the basis of the two water-soluble polymers poly(vinyl alcohol) (PVA) and dextran. Simply, sonication of graphite with the polymers in an aqueous solution produced stable graphene dispersions even after centrifugation. The ability of the polymers to exfoliate graphene sheets from the graphite was systematically investigated. Notably, 10 wt % phenyl-PVA led to the production of 46.7% bilayer and 26.7% 3- or 4-layer graphene flakes. An in-depth study into this and similar results was performed using density functional theory and MMFF94 computational tools, which led to better understanding of the interaction between graphene and the polymers in the solution. The polymers used can efficiently cleave graphite into graphene pieces without significant degradation of the sp2 carbon bonding network and then stabilize them in the aqueous solution, in contrast with the so-called “reduced graphene oxide”. This approach is advantageous for the large-scale production of high-quality, few-layer graphene. Moreover, a judicious combination of the parent polymer backbone and the functional side chains might allow the control of the size and number of layers of the graphene flakes made in the aqueous solution. Lastly, graphene gels were directly prepared from the aqueous graphene/polymer solution; further, their potentials for two model applications, as a dye adsorbent and gel electrolyte, were demonstrated.
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INTRODUCTION The extraordinary properties of graphene have been experimentally measured.1 Typically, graphene absorbs only 2.3% of white light and has an electron mobility of 2.0 × 105 cm2 V−1 s−1, maximum thermal conductivity of 5000 W m−1 K−1, and a breaking strength of 42 N m−1.2−4 Owing to these excellent electronic, optical, thermal, and mechanical properties, graphene possesses strong potentials for innovative applications in the fields of electronics, optoelectronics, energy devices, and composites.5−10 With the well-known excellent properties of graphene, extensive efforts have been made in the preparation of graphene flakes. Graphene has a two-dimensional sp2 carbon lattice, which is the basic building block for other carbon © XXXX American Chemical Society
materials such as carbon nanotubes and graphite. As an interesting example of graphene extraction, it was reported that direct scissoring of single-walled carbon nanotubes along their long axes yielded graphene flakes.11 Graphite consists of many graphene sheets, held by van der Waals forces in a regular pattern. Accordingly, most top-down approaches for obtaining graphene flakes have focused on graphite, including its chemical or physical exfoliation and cleavage. Received: June 29, 2015 Revised: August 16, 2015
A
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 1. Candidate polymers for graphene exfoliation in aqueous solution: (a) PVA, (b) phenyl-PVA (Ph-PVA), (c) pyrenyl-PVA (Py-PVA), (d) DT, (e) phenyl-dextran (Ph-DT), and (f) pyrenyl-dextran (Py-DT).
Graphene oxide employed as a precursor is a typical synthesis method for chemically derived graphene.12 Graphene is isolated by the exfoliation of oxidized graphite, followed by its reduction. The so-called reduced graphene oxide is usually synthesized using the Hummers and Offeman method, in which strong oxidizing agents, such as sulfuric acid, nitric acid, and potassium permanganate, are used to produce graphene oxide.13 Subsequent reduction of the oxide is performed with reducing agents such as hydrazine and sodium borohydrate. Unfortunately, chemical treatment like this creates many unavoidable defects in the resulting graphene, which degrades its properties. Mechanical cleavage is another simple approach for obtaining graphene. Graphene layers have been separated from graphite using adhesive tapes, elastomeric stamps, and the tips of atomic force microscopy units.14 Likewise, chemical exfoliation produces graphene flakes with the aid of intercalation compounds. Commonly, alkali metals with ionic radii smaller than the graphite interlayer spacing have been used to isolate graphene. Although these approaches have fewer negative effects on the structure and properties of graphene, they are not suitable for large-scale applications. Extensive effort has also been applied to achieving the bottom-up synthesis of graphene. Representatively, chemical vapor deposition growth on catalytic metal substrates, such as copper or nickel, has an exceptional potential for producing large-area, high-quality graphene. However, the method requires a high reaction temperature, a sacrificial metal catalyst, and multistep substrate-transfer process. The precise control of many synthesis variables is required to produce high-quality graphene, including such varied parameters as the mixing ratio and feeding rate of source gases, temperature, and cooling rate. Each synthesis route to graphene has strengths and weaknesses.
Many research groups are still conducting studies to ameliorate the current synthetic methods.15−18 At earlier stages, most research focused on obtaining monolayer graphene. However, current studies mainly focus on the development of applicationspecific synthetic techniques. The liquid-phase production of graphene has received much attention due to its inexpensive source materials and high yield.19−21 The liquid-phase approach allows the production of stable dispersions of graphene flakes, which enable solvent-assisted processing such as spin-coating or -casting, roll-to-roll printing, and filtration. Many large-scale graphene-based applications, including transparent conductive electrodes, sensors, catalytic supports, supercapacitors, batteries, and nanocomposites, can use the colloidal dispersion of graphene.22−25 Graphene suspension or dispersion has been most successfully achieved in organic solvents such as N-methyl-2pyrrolidone, N,N-dimethylformamide, dichlorobenzene, and trichlorobenzene.19−27 Aromatic compounds such as pyrene derivatives have been used as surfactants to facilitate the liquidphase dispersion of graphene.19 As a notable example, Samori group has been systematically investigated the liquid-phase exfoliation of graphene using supramolecular building blocks in the organic solvents.28,29 Owing to graphene’s inherent hydrophobicity, graphene dispersions in aqueous solutions are quite difficult to achieve. Both covalent and noncovalent functionalization of graphene has been used to facilitate its dispersion in aqueous solutions.30−32 Compared to covalent functionalization, noncovalent functionalization is more favorable in retaining the quality of graphene. Here, we investigate a top-down, physical production of graphene flakes in an aqueous solution, for which four different polymers with aromatic-ring side chains were synthesized based on two existing polymers. B
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Graphene flakes colloidally stabilized by the synthetic polymers were obtained via sonication in an aqueous solution, depending on the chemical structure and concentration of the polymers. In-depth studies into these mechanisms lead to better understanding of the interaction between graphene and the polymers in solution. Furthermore, the use of polymers as both intercalating and stabilizing agents, rather than ions or small molecules, extends the range of graphene applications. The aqueous dispersion of the graphene/polymer was readily made into gels, which demonstrated good performance in model applications such as absorbents and separators.
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polymer−graphene interactions in aqueous solution. It is expected that aromatic ring groups can interact with the basal plane of graphene through π−π interaction. Phenyl- and pyrenyl-containing functional chains were grafted to the parent polymers, enabling the comparison study of a single aromatic ring versus multifused aromatic rings as pendant groups. The degree of grafting was set to a moderate level ([phenyl group]/ [repeat unit] = 1/5; [pyrenyl group]/[repeat unit] = 1/20) to prevent interpendant group association, where the pyrenyl group’s four benzene rings were considered. The functionalization of the polymers with the aromatic rings was confirmed by ultraviolet (UV)-visible spectroscopy (see Supporting Information). The simple design of the polymers facilitated the elucidation of the polymer−graphene interactions in an aqueous solution. To determine the ability of each polymer to exfoliate graphene sheets from graphite, the polymer was added to an aqueous solution with graphite flakes at two different concentrations (5 and 10 wt %), and then the mixture was sonicated for 30 min. Once the polymer interacts with the exposed surface of graphite in solution, it can intercalate into the graphitic layers from the exposed end surfaces and form polymer/graphene layered structures. All polymers produced black solutions after the sonication, although the intensity or opacity of the color was different. The resulting solution was centrifuged to retrieve graphene sheets with colloidal stability. Photos of the final solutions in centrifuge tubes are provided in Figure 2. Only sonication of graphite with the polymers in
EXPERIMENTAL SECTION
Synthesis of Water-Soluble Derivatives. Four different polymers were synthesized (see Supporting Information). Exfoliation of Graphite in Aqueous Solution. All polymers were dissolved in 5 mL of distilled water at concentrations of 5 and 10 wt %. Then, 0.9 mg graphite flakes (Sigma-Aldrich) as a graphene precursor were added to the solution. The mixture solution was sonicated for 30 min at an amplitude of 60%. The resulting black solution was centrifuged at 8000 g for 10 min to separate impurities, including unstable graphene or residual graphite. Theoretical Calculation. First, a density functional theory (DFT) functional, B3LYP, in Gaussian09 was used with the 6-31G basis set to calculate the interaction of graphene with the different polymers. A counterpoise correction for basis-set superposition errors was included in the calculation. All computations were carried out using the Gaussian09 program package. Next, the MMFF94 energy minimization (with a minimum RMS gradient of 0.1) tool, implemented in ChemBio3D Ultra Software 12, was used to predict the chain conformation of the polymers. Synthesis and Applications of Gels. The hydrogel was prepared from 5.0 mL of graphene/polymer solution to which 11 μL of glutaraldehyde and 2.5 mL of 0.08 M H2SO4 were added, as a crosslinker and catalyst, respectively. The solution was poured into a mold and allowed to set overnight. After gelation, the hydrogel was rinsed with excess distilled water until the pH was constant. The hydrogel was freeze-dried under vacuum at −45 °C for 24 h to remove water completely, which resulted in an aerogel. To examine the removal efficiency of methylene blue by the gel, 0.45 g of the gel was added to 10 mL of 1 mM methylene blue solution. The solution was magnetically stirred at room temperature for 2 h. The concentration of methylene blue was calculated from the intensity of the solution’s 665 nm peak (see Supporting Information). For the solid-state supercapacitor application, two carbon nanofiber (CNF) electrodes were prepared at the same dimensions (8 mm × 15 mm). The graphene/polymer aerogel was immersed in a 1 M H2SO4 electrolyte for 1 h and then assembled between the CNF electrode materials. Two stainless steel foils (thickness 0.001 in) were employed as current collectors, and last the supercapacitor cell was wrapped in polypropylene film. PVA-only/H2SO4 gel electrolyte was used as a control. Galvanostatic charge/discharge curves were obtained at a current density of 0.5 A g−1.
Figure 2. Photographs of the graphene solutions after sonication followed by centrifugation. Graphite was sonicated in an aqueous solution containing (a) 5 wt % and (b) 10 wt % of the polymer, and then the resulting solution was further centrifuged to examine the dispersion stability.
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RESULTS AND DISCUSSION Figure 1 illustrates the molecular structures of the six candidate polymers employed to provide in-depth insight into polymergraphene interactions in aqueous solutions. First, two existing water-soluble polymers, poly(vinyl alcohol) (PVA) and dextran (DT), were chosen due to their ability to disperse carbon nanomaterials in aqueous solutions.33−35 Both of these polymers are amphiphilic in nature, with polar groups attached to nonpolar backbones. Structurally, PVA features a linear hydrocarbon chain, while dextran, a polysaccharide, consists of linked heterocyclic rings. Derivatives of the two polymers were synthesized with hydrophobic moieties to generate further
aqueous solutions produced uniformly opaque black dispersions even after centrifugation. All polymers, except for Ph-DT, led to stable graphene dispersion without serious precipitation at a concentration of 10 wt %. This shows that all tested polymers (except Ph-DT) can exfoliate graphite to some extent in an aqueous solution. In particular, the deepest black solutions were obtained in the presence of Ph-PVA, Py-PVA, and Py-DT. However, in the cases of Py-PVA and Py-DT, the additional yellow color arising from the pyrenyl group prevented estimating the degree of suspension of the graphene. Atomic force microscopy (AFM) was employed to characterize the graphene pieces dispersed in the aqueous solution. C
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. Representative AFM images of graphene flakes obtained using 5 wt % synthetic polymer in aqueous solution: (a) PVA, (b) Ph-PVA, (c) Py-PVA, (d) DT, (e) Ph-DT, and (f) Py-DT.
Figure 4. Representative AFM images of graphene flakes obtained using 10 wt % synthetic polymer in aqueous solution: (a) PVA, (b) Ph-PVA, (c) Py-PVA, (d) DT, (e) Ph-DT, and (f) Py-DT.
contained many graphene flakes of different sizes and thicknesses. The layer number and size of the graphene sheets is seen to depend on the chemical structure and concentration
Figures 3 and 4 show representative AFM images of graphene sheets obtained with the different polymers in aqueous solution. As expected, the more opaque black solutions D
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. Size and layer distribution of graphene flakes obtained using 5 wt % synthetic polymer in aqueous solution, where the histogram shows the distribution of graphene layers: (a) PVA, (b) Ph-PVA, (c) Py-PVA, (d) DT, (e) Ph-DT, and (f) Py-DT.
several samples. However, characteristic Raman spectrum of single-layer graphene reported previously (e.g., ∼0.5 G-to-2D peak intensity ratio36) was not obtained at all, which would be due to the adsorbed residual polymers or sonication-induced defects. Thus, the intensities of G peak and 2D peak were not directly used to identify the layer number of the graphene flakes, and the position and shape of the 2D peak were considered to calculate the layer number of graphene sheets.36 The parent polymers, PVA and DT (except for 10 wt % DT), produce thicker and larger flakes compared to the functionalized polymers, indicating that the parent polymers are not well-suited for graphene intercalation. Ph-PVA and Py-PVA facilitate the generation of few-layer graphene flakes, such as bilayer and 3- and 4-layer structures. They yield twice the amount of bilayer graphene flakes at the increased polymer concentration, demonstrating that the functionalized PVAs are effective in both exfoliating graphene flakes and stabilizing them in an aqueous solution. Notably, Ph-PVA is found to have produced 43.4% bilayer and 30.0% 3- and 4-layer graphene flakes at 10 wt % concentration. On the other hand, compared with 10 wt % DT, Ph-DT, and Py-DT yield lower amounts of few-layer graphene at both 5 and 10 wt %, implying that the phenyl- and pyrenyl-functionalization of DT elicit adverse effects in producing graphene flakes.
of the polymers. Specifically, we observe that, at the same concentration, pristine PVA yields better-dispersed graphene flakes with larger sizes than pristine DT. At the increased polymer concentration (10 wt %), graphene flakes appear to become both thinner and larger. Unexpectedly, it seems that the pyrenyl-functionalized parent polymers are less effective than their phenyl-functionalized correlates in producing graphene flakes. The AFM observation provides information on the size and thickness of the obtained graphene flakes. However, the polymers used for exfoliation can be adsorbed onto the graphene flakes, causing difficulties in estimating the thickness of only the graphene flakes. For a clearer interpretation, a statistical analysis of the size and thickness distributions for the generated graphene flakes was conducted by Raman spectroscopy coupled with optical microscopy. Figures 5 and 6 show the distributions of graphene flakes obtained at polymer concentrations of 5 and 10 wt %, respectively. The flakes produced from each polymer solution are categorized as monolayer, bilayer, 3- or 4-layer, 5-layer, and 6- to 10-layer graphene via examination of the Raman 2D peak, and the size of the flakes are calculated from the optical images. It should be noted that Raman spectra of graphene is sensitive to defects, electrostatic/contact doping, strain, and so forth. In fact, the AFM allowed to observe flakes with single-layer thickness in E
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. Size and layer distribution of graphene flakes obtained using 10 wt % synthetic polymer in aqueous solution, where the histogram shows the distribution of graphene layers: (a) PVA, (b) Ph-PVA, (c) Py-PVA, (d) DT, (e) Ph-DT, and (f) Py-DT.
Figure 7. Raman spectra of graphene flakes obtained with 5 wt % Ph-PVA (λexc = 532 nm): (a) full spectra, (b) magnified spectra in the 2D peak region. Representative spectra are presented for graphene flakes with different numbers of layers.
In comparing phenyl- versus pyrenyl-functionalization, it is difficult to determine how the functional groups had such distinct differences in the production of graphene flakes. Namely, no common results in Figures 5 and 6 demonstrate one functional group’s greater facility in producing few-layer graphene flakes compared to the other. To summarize, judging from the above data, it appears that the ability to produce fewlayer graphene flakes increases in the order of Ph-DT ≈ Py-DT < DT ≈ PVA < Py-PVA < Ph-PVA. Furthermore, we can
conclude that a judicious combination of the parent polymer and the functional group allows control of the size and layer number of the graphene flakes made in aqueous solution. The Raman spectroscopy provides further information on the quality of the graphene flakes. Raman spectra of few-layer graphene flakes obtained with 5 wt % Ph-PVA were sorted by stacked-layer number; representative spectra are shown in Figure 7a. A small D peak is observed in the spectrum of the bilayer. However, the D peak decreases gradually in intensity F
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
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the simple graphene/trimer models are in reasonable agreement with the experimental observations. In fact, the aromatic ring groups of the polymers can interact with the graphene’s π electron cloud. The aromatic ring pendant group first penetrates into the stacked graphene sheets and then gradually separates the graphene pieces in the aqueous solution by wrapping them with the parent polymer chains. Accordingly, we expected that the phenyl- and pyrenylfunctionalization would boost the chemical affinity of the polymers to graphene and lower the absolute energies of individual bases. However, DT displayed precisely the opposite behavior when decorated with the aromatic ring pendant groups. The interaction energies of graphene with Ph-DT and Py-DT were calculated to be rather lower than that of graphene with DT, indicating the decorated polymers’ lower chemical affinity. To provide an insight into the chain conformation of the polymers, longer 20-mers were modeled using the MMFF94 computational tool, a force field calculation method, and the minimized energies of the 20-mers were calculated. Figure 8 provides the geometrical preferences of the 20-mers obtained after the calculation. The three-dimensional models of the PVA derivatives almost maintain the chain linearity of PVA. We expect that the linear chain conformation of Ph-PVA and PyPVA is beneficial in the exfoliation of graphite. On the other hand, the DT derivatives reveal spiral-coiled geometries, in contrast with the linear chain conformation of DT. Interestingly, the attachment of a pyrenyl side chain to the 20-mer of DT induces a dramatic change in chain conformation from linear to spiral. Considering the geometries of the DT derivatives, a large magnitude of steric hindrance would occur when the DT derivatives intercalate into the stacked graphene sheets. The steric hindrance may account for the lower yields of few-layer graphene flakes when using Ph-DT and Py-DT. We can consequently summarize that the key factors determining the exfoliation and stabilization of graphene pieces from graphite in an aqueous solution include (i) the nature of the parent polymer, (ii) the structure and chemical properties of the side functional group, and (iii) the functionalization degree of the parent polymer. The exfoliation of graphite using intercalants leaves residual intercalants on the isolated graphene pieces. The removal of polymeric intercalants is more difficult than that of small molecular ones, unfortunately, which may inhibit the usefulness of the synthesized graphene. However, in this work, the resulting product is an aqueous dispersion of graphene flakes stabilized by water-soluble polymers. These dispersions can find direct applications without the removal of the polymers. As a model application, graphene hydrogels and aerogels were prepared from the aqueous dispersion of the graphene/polymer (Figure 9a). Simply, the cross-linking of the dispersed polymers in the solution yielded hydrogels with embedded graphene flakes inside the polymer chain networks. The subsequent freeze-drying of the hydrogel resulted in an aerogel. Importantly, no phase separation occurred between graphene and the polymer, indicating that the interactions between graphene and the polymer were sustained at the molecular level during the cross-linking reaction. The graphene/Py-PVA gel can be molded into shape-persistent, free-standing objects by virtue of its great mechanical strength. For example, Figure 9b shows that the graphene-embedded polymer gel can recover its original shape without any structural fatigue after a compression of more than 50% in height. Figure 9c depicts graphene/Py-
with increasing layers and disappears completely in the spectrum of the 6- to 10-layer flakes. This is possibly due to graphene edges’ contribution to the D peak, which is a measure of non-sp2 covalent functionalization and defects. The 2D peak changes with the number of graphene layers, as shown in Figure 7b. The change in shape of the 2D peak confirms that the graphene layers in the flakes are stacked in an AB arrangement. Such findings are commensurate with those for physically exfoliated graphenes reported in the literature.20,36 Consequently, under our experimental conditions, we believe that the polymers used can efficiently cleave graphite into graphene pieces without significantly degrading the sp2 carbon bonding network and then stabilize them in aqueous solution, in contrast with methods using reduced graphene oxide. The interactions of graphene with the polymers were estimated by the DFT framework.37−39 Noncovalent intermolecular interactions, including π−π stacking and van der Waals forces, can aid in stabilizing the graphene/polymer system. The interaction energies were calculated using the B3LYP functional in conjunction with the 6-31G basis set. A 5 × 5 graphene sheet and central-functionalized trimer structures of the polymers were employed as simple miniature models for the calculation. Owing to the finite sizes, the properties estimated using these models are likely to deviate from those of the real systems to some extent. However, qualitatively, the miniature models are expected to yield results similar to those of the real systems. Interaction energies were calculated using the following equation: E int = Ea(graphene/trimer ensemble) − {Ea(graphene‐only) + Ea(trimer‐only)}
When an Eint value has a negative sign, the interaction of graphene with the polymer is energetically preferable. The absolute energies (Ea) were obtained from the ground electronic state. The geometries of individual isolated base and ensemble models were optimized at the B3LYP/6-31G level of theory; these optimized geometries were used for the subsequent calculation of absolute energies. The absolute energies of individual bases and the calculated interaction energies are listed in Table 1. The values of the interaction energies were found to increase in the order of Py-DT < Ph-DT < DT < PVA < Ph-PVA < Py-PVA. Considering that the functionalization degree of the phenyl group exceeded that of the pyrenyl group in the polymers, the interaction energies of Table 1. Interaction Energies (Eint) of Graphene with the Polymersa,b Eab (Hartree) ensemble
graphene/ trimer
graphene
trimer
Eint (Hartree)
graphene/PVA graphene/Ph-PVA graphene/Py-PVA graphene/DT graphene/Ph-DT graphene/Py-DT
−3396.1599 −3895.5594 −4371.1549 −4769.7286 −5269.1031 −4444.9260
−2740.7039 −2740.7039 −2740.7039 −2740.7039 −2740.7039 −2740.7039
−655.4524 −1154.8505 −1630.4439 −2029.0226 −2528.5346 −1704.5374
−0.0036 −0.0050 −0.0071 −0.0021 0.1354 0.3153
a
B3LYP functional/6-31G basis set was used. bThe Eint will be lowered in the presence of water because of the increased interaction between the polymer and graphene. G
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
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Figure 8. Chain-conformational structures for 20-mers of the polymers obtained after energy minimization: (a) PVA, (b) Ph-PVA, (c) Py-PVA, (d) DT, (e) Ph-DT, and (f) Py-DT.
Figure 9. (a) Photographs describing the sequential preparation of hydrogels and aerogels from an aqueous dispersion of graphene/Py-PVA. Photographs showing (b) good mechanical strength and (c) water-mediated swelling/contraction of the graphene/Py-PVA hydrogel. (d) Scanning electron microscopy image showing visible pores in the gel. The cross-linking density (Nc−1) of the gel was 0.08. (e) Photographs (inset) of vials containing 1 mM methylene blue solution before and after the graphene/polymer gel treatment, and UV−visible absorption spectra of the solutions. (f) Typical galvanostatic charge/discharge curves (inset) of the solid-state supercapacitor cells recorded at a current density of 0.5 A g−1 and their long-term cycling performances.
PVA hydrogels in both swollen and contracted states. The graphene-embedded polymer gel sustains a large change in
volume by water absorption. The gels have visible pores, as shown in Figure 9d. H
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
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Clearly, the graphene-embedded polymer gels can be utilized in diverse application areas, due to their unique chemical, electrical, and structural characteristics.39−44 We demonstrated two different model applications in this work. The ability of the gel to remove methylene blue as a dye pollutant from an aqueous solution was first explored, to take advantage of the gel’s porosity, high surface area, and chemical intermolecular interactions (Figure 9e). A small amount of methylene blue was dissolved in water, and the graphene/polymer gel was then introduced into the dye solution. The blue color (the absorption of methylene blue at 665 nm) of the solution disappeared after 2 h with mild stirring. Figure 9e, inset, exhibits photographs of the methylene blue solution before and after the graphene/polymer gel treatment. The gel showed an effective removal efficiency of approximately 100% for the dye in aqueous solution. Methylene blue, a heterocyclic aromatic chemical compound, is assumed to be spontaneously adsorbed onto the graphene flakes embedded into the polymer gel due to their high chemical affinity. A gel electrolyte was demonstrated as a second model application of the graphene/polymer gel (Figure 9f). The mechanically strong graphene/polymer gel was assembled into a solid-state supercapacitor cell, in which an acidic electrolyte (H2SO4) was contained in the gel. Compared with a PVA-only gel electrolyte (84.2 ± 5.2 F g−1) as a control, the use of the graphene/polymer gel electrolyte (107.5 ± 3.1 F g−1) allowed higher specific capacitances and excellent long-term cycling stability over 10000 cycles. We believe that the graphene embedded into the gel enhances the ionic conductivity of the gel.45 These results clearly support the variety of possible applications for graphene/polymer gels.
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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.macromol.5b01423. Supplementary experimental details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +82-62-530-1779. Tel.: +82-62-530-1778. Author Contributions ‡
These authors (H.W.S. and K.-J.A.) contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (NRF-2015R1A2A2A01007166).
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REFERENCES
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CONCLUSIONS
We explored the ability of several water-soluble polymer derivatives to chemically exfoliate graphene flakes from graphite, using sonication in an aqueous solution, and then stabilize the flakes by direct suspension, forming a graphene/ polymer system. The water-soluble polymers with phenyl- and pyrenyl-functionalized side chains facilitated the formation of stable aqueous dispersions of graphene without degrading the sp2 hybridized structure. The ability to produce few-layer graphene flakes increased in the order of Ph-DT ≈ Py-DT < DT ≈ PVA < Py-PVA < Ph-PVA, which was supported by (i) the calculated interaction energies between the graphene and the polymer and (ii) the predicted energy-minimized geometries of the polymers. Graphene-embedded hydrogels were also demonstrated as a promising application of the aqueous graphene/polymer solution. We assume that an optimal hydrophobic moiety content exists for producing the aqueous dispersion of graphene pieces. With further study and optimization, we expect that our top-down approach will provide a simple and efficient methodology for producing graphene flakes with controlled thickness and size in an aqueous solution. Additionally, graphene-embedded gels may be a fascinating class of materials. The main properties of the gels may be further controlled by the amount of graphene embedded, the variety of polymer matrix, and the cross-linking density of the gel, which may provide new pathways for extended applications of the gel. I
DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.macromol.5b01423 Macromolecules XXXX, XXX, XXX−XXX