Article pubs.acs.org/cm
Influence of Stacking Morphology and Edge Nitrogen Doping on the Dielectric Performance of Graphene−Polymer Nanocomposites Mahmoud N. Almadhoun,‡ M. N. Hedhili,† Ihab N. Odeh,‡ Prince Xavier,‡ Unnat S. Bhansali,† and H. N. Alshareef*,† †
Materials Science & Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia Corporate Research and Innovation Center, Saudi Basic Industries Corporation (SABIC), Thuwal 23955-6900, Saudi Arabia
‡
S Supporting Information *
ABSTRACT: We demonstrate that functional groups obtained by varying the preparation route of reduced graphene oxide (rGO) highly influence filler morphology and the overall dielectric performance of rGO-relaxor ferroelectric polymer nanocomposite. Specifically, we show that nitrogen-doping by hydrazine along the edges of reduced graphene oxide embedded in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) results in a dielectric permittivity above 10 000 while maintaining a dielectric loss below 2. This is one of the best-reported dielectric constant/dielectric loss performance values. In contrast, rGO produced by the hydrothermal reduction route shows a much lower enhancement, reaching a maximum dielectric permittivity of 900. Furthermore, functional derivatives present in rGO are found to strongly affect the quality of dispersion and the resultant percolation threshold at low loading levels. However, high leakage currents and lowered breakdown voltages offset the advantages of increased capacitance in these ultrahigh-k systems, resulting in no significant improvement in stored energy density.
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INTRODUCTION Nanocomposites using insulating, semiconducting, and conducting fillers dispersed in a polymer matrix are actively being researched to enhance the dielectric properties of polymeric materials in applications such as high charge storage capacitors and electromechanical devices.1 Polymers with high dielectric breakdown strength generally suffer from very low dielectric permittivity (εr), which limits the charge storage capacity of the material. One strategy of enhancing the dielectric permittivity is to disperse insulating fillers, such as high-k ceramics, at high loadings (30−50 vol %) into the polymer matrix. However, problems such as porosity, voids, and agglomeration at high loadings degrade the breakdown performance and mechanical properties of these composite systems.2−4 On the other extreme, using conductive fillers, a substantial enhancement in the dielectric permittivity can be obtained using much lower filler loadings (1−5 vol %). The drawback in these conductivefiller percolative systems is the huge rise in the dielectric loss due to the insulator−conductor transition near the percolation threshold.5 Focusing on percolative systems, graphene-based composites are of huge interest due to their remarkable enhancements in dielectric permittivity at significantly low filler loadings, an attractive property for nanodielectrics used in high charge− storage applications.6−9 There are many reports that discuss the electrical and dielectric properties of nanocomposites using graphene dispersed in various polymer matrices, but a clear understanding of the underlying mechanisms is inadequately explained.10−15 Unlike well-established models that can predict © 2014 American Chemical Society
dielectric enhancements in polymer/ceramic composites, percolative composites display large discrepancies in their dielectric performance, possibly due to the combined structural, morphological, and interfacial effects present in these systems. The power laws in the standard percolation theory are often used to predict the incremental increase in the conductivity and dielectric permittivity, but more experiments are needed to fully understand the origin of these variations reported in numerous publications.16 In this article, we investigate the dielectric performance of two graphene-based polymer composites. We implement two different methods to convert graphite oxide (GO) to reducedgraphene oxide (rGO): hydrothermal (HT-rGO) and hydrazine (HZ-rGO). For consistency, the polymer matrix, poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE), and graphite oxide (GO) are used from the same batch. Without further functionalization, we will show how the structure of rGO formed by both reduction techniques strongly influences the quality of dispersion and the magnitude of dielectric enhancement. Finally, we will briefly discuss how exceptionally high capacitances in such systems may not necessarily lead to higher energy density due to negative impact on the breakdown voltage of the nanocomposites. Received: February 7, 2014 Revised: April 18, 2014 Published: April 21, 2014 2856
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EXPERIMENTAL METHODS
Preparation of Reduced Graphene Oxides. GO was prepared using graphite purchased from Sigma-Aldrich (powder, 10 000 was achieved using 1.7 vol % HZ-rGO, while a dielectric constant of only ∼900 was reached using a 7.7 vol % HT-rGO loading. Further increase in filler loading in either case leads to a drop in the dielectric permittivity due to increasing leakage currents associated with switching from non-Ohmic to Ohmic
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RESULTS AND DISCUSSION Figure 1a shows powder X-ray diffraction (XRD) patterns of GO, HZ-rGO, and HT-rGO. The sharp peak of GO at around 2857
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Figure 3. Bar chart comparing the maximum dielectric permittivity and corresponding dielectric loss reported in the literature using P(VDF-TrFE-CFE) polymer as a matrix in percolative composites.14,26−28 Statistical data for the HT-rGO and HZ-rGO samples are shown in Table S2 in the Supporting Information.
remained relatively low compared to previous studies above their percolation threshold.13,14,23−25 The effective a.c. conductivity as a function of rGO loading is shown in Figure 2c. A steep insulator-to-conductor transition at very low filler loadings is clearly observed in the HZ-rGO system reaching a maximum conductivity of ∼0.03 Sm−1. The increase in a.c. conductivity with filler loading is attributed to the gradual formation of an interconnecting graphene nanosheet network. More conductive rGO sheets in the HZ-rGO composites reflect better restoration of sp2 carbon upon reduction.30 The stability of rGO in solvents has been shown to contribute to making better performing composites.31 In our study, graphene suspensions that are stable for long periods were achieved following 1 h sonication of both HT-rGO and HZ-rGO in DMF. Solvent stability, however, does not explain the different rGO morphology upon casting and annealing the films. Cross-section SEM of 2 wt % HT-rGO and HZ-rGO in the polymer is shown in Figure 4a,b, respectively. Typically described as microcapacitors, such lamellar morphologies are common to graphene-based percolative composites. In Figure 4a, wrinkled HT-rGO sheets are randomly oriented in the polymer medium as opposed to flat and layered HZ-rGO nanosheets seen in Figure 4b. We surmise that intercalation of HZ-rGO sheets separated by the ultrathin polymer layers contribute to the significantly larger dielectric enhancements observed in Figure 2a. Thus, depending on the reduction technique of GO, the resultant functional groups and surface chemistries can highly influence polymer/filler interaction, which lead to the different morphologies of HT-rGO and HZrGO in the polymer. Figure 5 shows FTIR characterization of the polymer composites at different loadings of rGO. All bands between 400 and 1500 cm−1 can be attributed to P(VDF-TrFE-CFE), in line with literature reports.32−34 However, when HT-rGO is added to the polymer, two additional bands at 1582 and 1745 cm−1 appear, Figure 5a, which correspond to the stretching vibrations of aromatic CC and carboxyl CO, respectively.35,36 The presence of the carboxyl peak reflects partial restoration of the sp2 carbon network following hydrothermal reduction. Similarly, as shown in Figure 5b, bands at 1576 and 1745 cm−1 appear for the composite containing HZ-rGO.
Figure 2. Dependence of (a) the permittivity, (b) dielectric loss, and (c) conductivity of the P(VDF-TrFE-CFE)/(HZ-rGO) and P(VDFTrFE CFE)/(HT-rGO) on the loading of rGO in vol %, measured at room temperature and 1 kHz. Statistical data corresponding to these plots are summarized in Table S2 in the Supporting Information.
conduction.5 Similarly, Figure 2b shows that the dielectric loss increases with loading, also due to the formation of conductive paths within the composites. With the exception of an abrupt increase in loss at 2.7 vol % HZ-rGO, both systems displayed losses ranging between 0.8 and 2. Although these dielectric loss values are high, our values are much lower than has been reported for similar composites above their percolation threshold.13,14,23−25 Until HZ-rGO filler loading exceeded 1.7 vol %, the devices showed normal dielectric response as can, for example, be seen in the linear a.c. conductivity dependence on frequency on a log−log plot (Figure S1, Supporting Information). Beyond 1.7 vol % loading, the HZ-rGO devices showed a large drop in permittivity and deviated significantly from the normal dielectric behavior. To the best of our knowledge, the HZ-rGO system we report displayed the highest dielectric permittivity value reported for percolative composites using P(VDF-TrFE-CFE) as the polymer matrix, as summarized in the chart in Figure 3.14,26−29 It is also interesting to point that, at HZ-rGO filler loadings above the percolation threshold and up to the maximum dielectric permittivity (0.7− 1.7 vol %: red-shaded area in Figure 2), the dielectric loss 2858
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Figure 4. Cross-section SEM of (a) P(VDF-TrFE-CFE)/HT-rGO and (b) P(VDF-TrFE-CFE)/HZ-rGO nanocomposites.
These vibrations clearly originate from the rGO sheets present in the polymer composite. After normalizing all spectra relative to the polymer CF2 stretching vibration at 1194 cm−1, the C C band intensities increase linearly with rGO loading as shown in Figure 5c. Notably, the band at 1576 cm−1 from HZ-rGO, relative to the 1582 cm−1 from HT-rGO, increases more rapidly with rGO loading. We attribute this to a combination of aromatic CC skeletal and CN stretching vibrations found in HZ-rGO.37 From the FTIR spectra, we can qualitatively confirm that the HZ-rGO composite contains more aromatic sp2 carbon leading to the higher conductivity depicted in the dielectric characterization. The presence of some remaining oxygen groups in both rGOs is known to contribute to dispersion stability in solvents but cannot explain the distinctly high dispersion efficiency unique to the HZ-rGO composite after annealing.31 A recent paper describing the reaction mechanism of hydrazine-treated rGO shows the formation N−N moieties (pyrazole/pyrazoline) at the graphene edges, better described as a reduction/ substitution reaction.38 Thus, we suspect that these moieties along the HZ-rGO edges play an important role in rGO/ polymer interactions and the resultant dielectric performance. Thus, high-resolution XPS spectra of HZ-rGO before/after dispersion in polymer were investigated. In Figure 6a, pyrazolelike graphene edges (Figure 6a inset) reflect the two wellresolved nitrogen peaks at BE = 399.0 eV for 2-fold coordinated N1 with a lone pair of electrons and BE = 400.4 eV for 3-fold coordinated N2 with a hydrogen atom attached along the sp2 plane. The peak at 402.0 eV is attributed to graphitic/quaternary.38,39
Figure 5. FTIR spectra of (a) P(VDF-TrFE CFE)/HT-rGO and (b) P(VDF-TrFE CFE)/HZ-rGO under different rGO loadings. (c) Comparing rGO peak intensities as a function of loading for both composite systems.
Upon dispersion of HZ-rGO in the polymer, only one symmetrical peak around 400.0 eV is observed, as shown in Figure 6b. Previous XPS studies on similar aromatic systems have shown that the removal of the CC double bond in the pyrazole ring confines the electron lone-pair in the adjacent N2 atom, shifting its core-level energy to nearly coincide with that of the N1 atom, which leads to the unresolved peak separation. This signifies the formation of pyrazoline-like structure at the edges of graphene (Figure 6b inset) after mixing with the polymer.39 The N1 and N2 atoms in the pyrazoline-like structure have two relatively free electron lone-pairs permitting possible electrostatic interaction with the polymer. This interaction permits effective intercalation of high-k P(VDF2859
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Figure 6. XPS spectra of HZ-rGO (a) before and (b) after addition to P(VDF-TrFE-CFE). The inset is the corresponding structure of the 5membered ring on the edge of HZ-rGO sheets.
of pristine graphene, whereas the hydrogens at the edges yield strong H-bonding with the fluorine atoms. However, as shown in Figure 7b, the ESP distribution changes upon the addition of a pyrazole ring to graphene. The pyridinic-like sp2 nitrogen (N2) shifts electron charges (blue region) at the graphene/ polymer interface, leading to more enhanced electrostatic interactions, particularly near the N−N moieties. The entrapment of these electron-rich regions in graphene via aromatic N−N doping may also be responsible to our observed variation in filler morphology along the polymer film and the resultant interfacial polarization. The huge dielectric values at low frequencies due to the Maxwell−Wagner−Sillars polarization do not necessarily reflect large charge storage at higher fields. Common to percolative composites, the increase in conductivity and the resultant leakage currents are detrimental to the dielectric breakdown.14 However, and unfortunately, most publications focus on the increase in the permittivity of the system, with little discussion on the effects on breakdown voltage of the percolated composite. As shown in Figure 8 (inset), the incremental
TrFE-CFE) polymer between the HZ-rGO sheets, forming ultrathin microcapacitors embedded within the bulk film that give rise to the very high capacitance. At the microscopic level, intermolecular interaction is among several factors that would contribute to the overall performance of graphene-based nanocomposites. Electrostatic potential (ESP) distribution using DFT was used to visualize how the presence of pyrazole along the graphene edges may influence interfacial charge distribution upon polymer interaction. As a conceptual model, we assumed one monolayer graphene with seven aromatic rings in close proximity with the fluorine atoms of an all-trans conformed terpolymer chain. In Figure 7a, we observe uniform ESP distribution on the surface of graphene, with more negative charges (blue region) confined to the fluorine atoms along the polymer chain. Polymer/graphene interactions are primarily due to the π-electric field in the plane
Figure 8. Dielectric breakdown strength of the P(VDF-TrFE-CFE) composite as a function of HZ-rGO loading (vol %).
increase in leakage currents with rGO loading directly affects the breakdown performance of the composites. The dielectric breakdown strength of the bare polymer (∼110 MV/m) in Figure 8 drops below 30 MV/m when adding HZ-rGO as low as 0.2 wt %, a degradation of almost a factor of 10×. Thin polymer layers sandwiched between rGO sheets ultimately lowers the effective thickness of the microcapacitors, thus forming large electric fields distributed across the bulk film that lead to high tunneling currents between the rGO sheets.40,41 Therefore, more work is needed to address this aspect of these promising composites. Surface-modified and shape-controlled fillers that can suppress charge transfer in tunneling currents by the effect of coulomb blockade have shown more promising results toward the fabrication of low dielectric loss and high energy density capacitors.24,42−44 Other sample variables such as film thickness, voids, and roughness may also need to be investigated as possible variables to improve the breakdown performance of such devices.45−47
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CONCLUSIONS The dielectric performance of two polymer/graphene composite systems, fabricated under similar conditions, was compared as a function of the graphite oxide reduction method. The
Figure 7. Electrostatic potential distribution using DFT describing the influence of interfacial charge distribution (a) without and (b) with an edge-pyrazole group on the edge of graphene. 2860
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reduction routes resulted in significant variation in the quality of dispersion, magnitude of dielectric enhancement, and the percolation threshold. Dispersing hydrazine-reduced graphene oxide in the polymer displayed a more superior performance. The hydrazine based composite displayed a peak dielectric of ∼10 000 while simultaneously maintaining a dielectric loss below 2, an excellent performance that can be attributed to a combination of nitrogen doping and microcapacitor morphology. Although nitrogen moieties may have drastically improved the dielectric permittivity, the formation of microcapacitors degrades the dielectric breakdown strength in these types of nanocomposites.
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ASSOCIATED CONTENT
* Supporting Information S
AC conductivity vs frequency; slope of the curves shown in Figure S1 for different levels of rGO loading; summary of the measurements on HZ-rGO and HT-rGO samples with different rGO loading levels. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(H.N.A.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS H.N.A. acknowledges the financial support from the Saudi Basic Industries Corporation (SABIC) Grant No. 2000000015. REFERENCES
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