Article pubs.acs.org/Langmuir
Water-Processable Laponite/Polyaniline/Graphene Oxide Nanocomposites for Energy Applications Isaac A. Ramphal and Michael E. Hagerman* Department of Chemistry, Union College, Schenectady, New York 12308, United States
ABSTRACT: Graphene−polyaniline (GP) nanocomposites have demonstrated remarkable ability as supercapacitive materials and are typically synthesized via chemical reduction of graphene oxide/polyaniline (GOP) precursors. We report the formation of novel nanomaterials combining GOP nanocomposites with Laponite nanodisks. Host−guest interactions within GOP systems were studied with and without Laponite nanoparticle templating agents. Incorporating Laponite clay into the composite synthesis enhances aqueous dispersibility as well as facilitates the casting of homogeneous films. Structural and morphological characterization confirmed porous heterointerfaces and control of polymer and nanoclay loading. These results may enable the development of flexible supercapacitive and solar nanocomposites with improved device utility, water dispersibility, and film processability. We demonstrate that these films can be easily cast and that the composites maintain their electrical transport properties.
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INTRODUCTION Two of the most significant challenges facing society today are the rising costs of nonrenewable resources and the environmental impacts associated with their acquisition and usage.1 Carbon-based materials in general have been widely studied for supercapacitor applications.2−4 Graphene especially has been the subject of extensive study upon demonstration of its excellent mechanical and electrical properties.5 Graphene’s high thermal and electrical conductivity, exceptional surface area, outstanding mechanical strength, large capacitance values, and good cycling stability are several of the reasons it is an attractive material system.6−8 Greener synthetic strategies should employ materials with high water solubility as often as possible to avoid organic solvent waste. As such, a key challenge of working with graphene is its poor aqueous solubility. Typically, graphene is oxidized to afford the insulating graphene oxide (GO), which is generally accepted to have alcohol and epoxide substituents decorating the basal plane, and carbonyl moieties along the sheet edges,9 which is much more amenable to aqueous processing and can be readily reduced back to graphene if desired.10−12 Electroactive polymers such as polyaniline, polythiophene, and polypyrrole have likewise been studied for their electrical capabilities.13,14 Polyaniline (PANI) has found widespread use as a conductive polymer due to its solution processability for varied applications including sensors, actuators, memory devices, and energy storage devices.15 PANI is an attractive © XXXX American Chemical Society
material in these systems due to its environmental stability, low cost, and large surface area.16,17 PANI’s conductivity has been shown to vary considerably with changes in its oxidation state and morphology.14,18 Depending on the morphosynthetic conditions used, PANI has been isolated as fibers, wires, microwebs, hemispheres, nests, rods, and 2D nanosheets along with many other morphologies.14,19 Graphene intercalation compounds of conductive polymers such as PANI have been studied insofar as the composites provide a lightweight nanomaterial with improved electrical performance, especially as supercapacitor electrodes.13,16,20−22 Previous research has demonstrated GP nanocomposites with capacitance values as high as 1145 F/g due to a combination of electrical double-layer capacitance of graphene and pseudocapacitance of PANI.23 Supercapacitive composite materials have also been made using PANI and unreduced GO.24 Consequently, the ability to tune the extent of oxidation of graphene coupled with the variable oxidation states available to PANI provides a versatile composite material system. Despite high versatility of this hybrid system, several obstacles must be overcome in order to realize inexpensive, water-processable device implementation. These include failure upon cycling, thermal degradation, and delamination.25 Many Received: July 31, 2014 Revised: January 7, 2015
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Figure 1. FTIR absorbance (left) and Raman scattering (right) vibrational spectra of powders of (a) GO, (b) GOP80, (c) GOP50, (d) GOP10, (e) Laponite, (f) GOPL80, (g) GOPL50, (h) GOPL10, and (i) PANI. Laponite has no Raman-active vibrational modes in the spectral region shown and is thus omitted. such that there was a 1:4 mole ratio of APS to aniline. These solutions were left to stir overnight. As aniline polymerizes, the solution undergoes a color change from brown to green, with higher aniline concentrations producing darker green solutions. The products were then washed in succession with H2O, absolute ethanol, and hexanes. Each rinse was added to the reactant solution, centrifuged for 30 min (2500 rpm, 5 °C), and then decanted. The products were heated overnight at 65 °C under vacuum to fully dry the composites. The final composites were obtained as dark green powders. Preparation of GOPL Composites. The GOP synthesis described above was repeated with the exception that Laponite and GO were simultaneously added to the aniline solution. Laponite was added such that it was 25 wt % of the combined GO and aniline mass. The final composites were obtained as dark green powders. These composite materials are labeled GOPL10, GOPL50, and GOPL80, respectively. Preparation of GOPL Films. The GOPL composites were dispersed in an aqueous solution with a concentration of 8 mg/mL. These solutions were vigorously agitated, pipetted onto a glass slide, and allowed to air-dry slowly in a humid environment for 24 h to yield the GOPL films. Characterization. Vibrational Spectroscopy. ATR-FTIR absorbance spectra were obtained using a Nicolet Avatar 330 FTIR spectrophotometer with Smart Orbit ATR attachment. Spectra were collected using 128 scans at 2 cm−1 resolution. Raman spectra were acquired with a Bruker Senterra Raman microscope (excitation laser λ = 786 nm, 1 mW) with 3 cm−1 resolution, and fluorescence corrections were applied. XRD Analysis. Powder X-ray diffraction spectra were obtained using a Phillips PW-1840 X-ray diffractometer with a Co Kα (λ = 1.7902 Å) source operating at 45 kV and 35 mA. TGA Analysis. Thermal stability was measured using a PerkinElmer TGA-7 thermogravimetric analyzer. Samples were characterized from 100 to 500 °C with a heating rate of 2 °C/min in an N2 atmosphere.
researchers have explored nanoparticle templating to control heterointerfaces and improve functionality.26,27 The synthetic clay Laponite offers new classes of electroactive and energy storage materials, as robust, transparent films can be coated onto a variety of substrates through facile self-assembly from the aqueous phase. Laponite nanoparticles provide versatile inorganic scaffolds with nanoarchitectures which can be selectively tuned to direct interactions across heterointerfaces, including polyaniline nanoassemblies.28−30 While previous studies have demonstrated kinetic control within graphene/ Laponite colloidal dispersions and Laponite/GO films for oxygen-barrier applications,31,32 we report herein the first detailed study of Laponite/polymer/graphene oxide films. We are currently employing Laponite nanoparticles to improve the aqueous self-assembly of GOP nanocomposites (GOPL), afford thin-film processability, and increase viability for device implementation.
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EXPERIMENTAL SECTION
Materials. Graphite powder (crystalline, −300 mesh, 99%) was purchased from Alfa Aesar. Sodium Laponite RD (Na0.7[(Li0.3Mg5.5Si8)O20(OH)4]) was obtained from Southern Clay Products in the USA and donated by Laporte Industries in the UK. All other reagents and solvents were purchased from Sigma-Aldrich. Graphene oxide was prepared according to a procedure from the published literature.33 Preparation of GOP Composites. GOP nanocomposites were synthesized using a modified literature procedure.16 Aniline and GO were added to 1 M HCl in weight ratios of 90:10, 50:50, and 20:80 and sonicated for 1 h. These composite materials are labeled GOP10, GOP50, and GOP80, respectively. To each of these, a solution of ammonium persulfate in 1 M HCl was added with vigorous stirring B
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Langmuir Morphology Studies. Nitrogen adsorption−desorption isotherms were measured using a Micromeritics Tristar 3000 automated gas adsorption analyzer at 77.4 K. Samples were purged under nitrogen flow for 2 h at 45 °C followed by 20 h at 60 °C in a Micromeritics Smartprep programmable degas system prior to analysis. Specific surface areas were calculated from the adsorption isotherms using the Brunauer−Emmett−Teller (BET) method (relative pressure range P/ P0 = 0.01−0.3) according to IUPAC recommendations.34 Pore size distributions were estimated from the adsorption isotherm using the Barrett−Joyner−Halenda (BJH) method. SEM images were collected using a Zeiss EVO W filament system scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDX) was carried out in the SEM using a Bruker Quantax 200 EDX system. Tapping mode AFM images were collected using an Asylum Research MFP-3D atomic force microscope with commercially available rotated silicon probes with a tip radius of curvature of 8 nm and a spring constant of 20−80 N/m. Optical micrographs were acquired using an Olympus BX51 optical microscope. Electrochemical Measurements. Cyclic voltammetry (CV) data were collected using a PAR VersaSTAT 3 AMETEK model potentiostat with a standard three-electrode setup consisting of screen-printed carbon black working and counter electrodes and Ag/ AgCl reference electrode (CH Instruments, TE100). Voltammograms were collected in the range of −0.2 to 0.8 V at a scan rate of 100 mV/s in 2 M H2SO4 electrolyte. Samples were mixed with 5 wt % aqueous Nafion solution at a ratio of 1 μL per mg of material and pressed onto the working electrode. Local charge transport properties were studied in contact mode using a Veeco Dimension V scanning probe microscope and commercially available platinum-coated silicon probes with a tip radius of curvature of 30 nm. For stable current−voltage characterization (CVC) responses, applied voltage biases ranged from 250 mV up to 2.5 V.
provides further evidence of interactions between the polymeric polaronic lattice and the negatively charged GO sheet. The GOPL system afforded upon incorporation of Laponite displays a similar shift to lower frequency with increasing PANI loading as seen in the GOP system. This has been attributed above to interactions between PANI and the negatively charged GO sheet, which are now also possible between PANI and the negatively charged silica surfaces of Laponite nanoparticles. This is supported by spectral broadening of the 1280 and 1225 cm−1 C−N and C−N•+ stretching modes in the GOPL system. Further evidence of host−guest interactions can be seen in shifts in the Si−O vibrational modes of Laponite: the edge and surface silicate bands shift to 790 and 950 cm−1, respectively, leading to three distinct silicate stretching modes including the 1050 cm−1 internal vibration (pink shaded region) and providing clear evidence that Laponite is interacting with the composite matrix. Raman spectroscopy was used to probe the local environment at the graphene oxide and PANI interface (Figure 1, right panel). The Raman spectrum of GO contains the typical Gband of graphite at 1595 cm−1 corresponding to first-order scattering of the E2g vibrational mode.40 The more prominent D-band beak at 1310 cm−1 is due to disruption of the carbon sp2 lattice of pristine graphitic materials and is indicative of extensive oxidation of the graphitic surface. For pure PANI, Raman bands are observed at 1162, 1250, 1332, 1367, 1500, and 1595 cm−1 corresponding to in-plane C−H bending, C− N•+ stretching, C−H deformation, symmetric C−N stretching, CN quinoid, and C−C benzenoid breathing modes, respectively. These bands are consistent with the ES form of PANI.17,21 The persistence of these peaks in the composite spectra indicates PANI remains in the acid-doped ES form upon polymerization in the presence of GO. The shift in the C−H bending mode to 1170 cm−1 in GOP50 provides evidence of interactions between PANI and GO, while the return to 1165 cm−1 in GOP10 shows dominance of PANI−PANI interactions at the highest PANI loading. In addition, the PANI peaks at 1332 and 1595 cm−1 cause broadening of the overlapping Gband of GO in the intermediate loading composite. In the highest PANI loading, GOP10 shows a shift of this Raman band to 1628 cm−1, while GOPL10 shows further broadening and stiffening of the G-band of GO (blue shaded region). These shifts are consistent with hole-doping of GO by interactions between carboxylic acid groups of GO and the polaronic PANI backbone and to π−π interactions between PANI and GO.21,41 Powder XRD. Structure of the composites was investigated using powder XRD. XRD patterns of PANI, GO, Lap, GOP, and GOPL composites are shown in Figure 2. GO exhibits a major diffraction peak centered at 2θ = 11.3° which corresponds to an interlayer d-spacing of 9.1 Å. This peak has been assigned to the (001) plane and may vary depending upon materials processing and the amount of interlamellar water. 42 Laponite has two broad peaks occurring at approximately 2θ = 7.7° and 2θ = 22.8°, corresponding to the (001) and (003) planes of the tetrahedral−octahedral− tetrahedral layer with d-spacings of 13.3 and 4.5 Å, respectively.43 The PANI pattern has two major, broad peaks centered at 2θ = 23.5° and 2θ = 29.3°, corresponding to dspacings of 4.4 and 3.5 Å and attributed to the (020) and (200) planes, respectively.44,45 The overall shape of this pattern is
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RESULTS AND DISCUSSION GOP and GOPL Composite Powders. Vibrational Spectroscopy. Molecular structures of the composites were investigated using ATR-FTIR (Figure 1, left panel). GO exhibited bands at 1060, 1220, 1610, and 1730 cm−1, which are attributed to stretching frequencies of C−O−C epoxide, C−O hydroxide, quinoidal CC, and carboxyl CO, respectively.33 PANI showed characteristic bands located at 810, 835, 1110, 1225, and 1280 cm−1 associated with C−H in-plane and out-ofplane bends in benzene rings, CN quinoid stretching, C− N•+ in the emeraldine salt (ES) form of PANI, and C−N stretching modes, respectively.24 The characteristic peaks of the benzenoid and quinoid ring breathing modes are present at 1450 and 1550 cm−1, respectively. The slightly less than unitary quinoid/benzenoid peak intensity ratio is indicative of the ES form of PANI.35 Laponite RD has characteristic vibrational modes at 850 and 1000−1100 cm−1, corresponding to edge and a combination of surface and internal Si−O stretches, respectively.36,37 In the GOP system the benzenoid breathing mode at 1450 cm−1 shifts to lower frequency with increased PANI loading, indicative of interactions with the GO sheets. These interactions may include intermolecular electrostatic, hydrogen bonding, and π−π forces and have been reported previously by Sarker and Hong.38 By contrast, the C−N stretching frequency at 1280 cm−1 increases with increasing GO content (green shaded region), which may be evidence of a PANI-GO chargetransfer complex similar to the findings of Vallés et al.39 A better understanding of this reversible charge transfer is needed to control the switching between neutral and charged states to optimize supercapacitive behavior. In addition, the shift to lower frequency in the C−N•+ with increasing GO content C
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subsequently smaller change in interlayer spacing may also indicate that Laponite is influencing the PANI−GO heterointerface. Further evidence of Laponite’s influence on PANI− GO heterointerfaces is apparent in thermal studies of the composites. Thermal Stability. Thermal stability of the composites was studied using thermogravimetric analysis (TGA), with results for GOP80, GOPL80, PANI, GO, and Lap shown in Figure 3.
Figure 3. TGA of GO (), PANI (◊), Laponite (- - -), GOP80 (△), and GOPL80 (○).
GO shows a significant mass loss of ∼40% between 175 and 250 °C owing to the decomposition of labile oxygen-containing functional groups, with subsequent mass loss attributed to pyrolysis of more stable oxygen moieties.46,47 The PANI sample exhibits a steady mass loss of approximately 20% over the temperature range studied. Both of the graphene oxide/ polyaniline composites displayed larger thermal stability than pure GO with lower mass losses of around 30% for GOP80 and 25% for GOPL80. These results are consistent with those reported in Zhang et al., who have previously demonstrated improvement in thermal stability and decreased mass loss for GOP nanocomposites.16 We have further improved this stability via addition of Laponite nanoparticles to the composite, evidenced by a 65 °C increase in the midpoint of thermal decomposition temperature. This effect was observed for all composite powder systems presented here. This improved stability is a direct consequence of Laponite and GOP interactions and may mitigate the effects of local heating which prove deleterious to capacitive performance. Morphological Characterization. Nitrogen adsorption− desorption isotherms were used to determine the BET surface area (SBET), peak pore diameter (ϕp), and peak pore volume (Vp) for graphene oxide, polyaniline, Laponite, and their composite powders (Table 1). The SBET values of 210 and 70 m2/g for GO and PANI, respectively, are both significantly higher than those typically reported.16,20 These exceptionally large surface areas are attributed to a high degree of chemical exfoliation during GO synthesis and to the very fibrous microweb polyaniline morphology determined by XRD studies. The surface area and porosity data of Laponite matches previously reported values.48 In general, the GOPL composites displayed an appreciable increase in surface area compared to their non-Laponite counterparts. Addition of Laponite resulted in a nearly 3-fold increase in SBET for the lowest polyaniline loading (from 50 to 130 m2/g), an 11-fold increase for the intermediate loading
Figure 2. Powder X-ray diffraction patterns of (a) GO, (b) GOP80, (c) GOP50, (d) GOP10, (e) Laponite, (f) GOPL80, (g) GOPL50, (h) GOPL10, and (i) PANI. Blue shaded region shows the fwhm of the native GO peak.
consistent with previously reported powder XRD patterns for the microweb structure of PANI.19 The powder XRD data for the GOP and GOPL composite systems confirm a varied loading of PANI intercalated between GO sheets. The pattern for GOP80 shows a major diffraction peak at 2θ = 12.9° corresponding to a d-spacing of 8.0 Å. We attribute the decrease in d-spacing relative to pure GO to changes in the amount of interlamellar water and/or disruption of the ordered hydrogen-bonded water network formed with GO hydroxyl and epoxide moieties. As PANI loading increases to 50% in the GOP50 composite, this angle decreases to 12.1° (d = 8.5 Å). This expansion in interlayer GO sheet distance is likely a result of the surface adsorption and intercalation of PANI. Broadening of this peak with increased PANI loading is evidence of exfoliation of the GO sheets, and its disappearance in the GOP10 composite spectrum indicates the complete exfoliation of GO under highest PANI loading. Addition of Laponite as a dispersing and templating agent preserves the two-dimensional nanoarchitecture of the GOP composite crucial for supercapacitive behavior, as no new peaks appear in the diffraction patterns. Moreover, a similar shift to lower angle upon increased PANI loading is seen in the GOPL composites. The GOPL80 pattern has a major peak at 2θ = 12.9° (d = 8.0 Å) as in GOP80. This peak shifts to 12.6° (d = 8.2 Å) in GOPL50, with complete exfoliation in GOPL10. Laponite nanodisks may compete with GO for PANI heteronucleation and polymerization sites, leading to a smaller amount of PANI insertion between GO sheets. The D
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The morphology of GO, GO with Laponite and PANI, and their composites were further characterized using SEM (Figure 5). Figure 5a shows nano- and microsheets of GO ranging in size from 100 nm to 150 μm, with a terraced morphology indicative of lamellar stacking. The GO/Lap image (Figure 5b) shows Laponite nanodisks (100 nm diameter) stippling the GO sheet surface. These nanoparticles appear white and are abundant in the upper portion of the image. The PANI image (Figure 5c) reveals a microweb of fibrous PANI consistent with the X-ray and N2 physisorption analyses and typical for the morphosynthetic conditions used.14 PANI fibers range from 20 to 100 nm in diameter and extend up to 500 nm in length. Figure 5d−f show SEM images of GOP80, GOP50, and GOP10 composites, respectively. GOP80 and GOP50 show preservation of lamellar stacking of GO important for supercapacitive response, with no visible exfoliation arising from polyaniline growth. As PANI content increases, surface polymer growth is increasingly evident. While GOP80 shows a GO crystallite surface with little evidence of polymeric coverage, in GOP50 the templating influence of GO leads to irregular spherical agglomerates of PANI residing on the surface of the GO microsheets.50 As PANI loading is increased to 90 wt % for GOP10 (Figure 5f), the polymer forms a fibrous web as seen for pure PANI growth, with complete coverage of the GO surface and clear evidence of exfoliation of GO sheets in the bottom of the image. SEM images of the GOPL composites are shown in Figure 5g−i and demonstrate that incorporation of Laponite is not deleterious to the lamellar stacking of GO. In contrast to GOP80, nucleation of PANI spheroids is apparent on the surface of GOPL80 in the bottom left of the image, consistent with X-ray evidence supporting less intercalation due to the templating effects of Laponite nanoparticles and competition for nucleation sites during polymerization. In addition, the dispersing capabilities of Laponite lead to a more even distribution of polymer growth on the surface of GO terraces in GOPL50, rather than the phase-separated polymer aggregates in GOP50. The intimate mixing of GO and PANI is maintained in GOPL10. Rather than the fibrous overgrowth of PANI in GOP10, the GOPL10 composite system shows clear evidence of a more wedded, sponge-like composite composed of wellmixed graphitic and polymeric morphologies in which the lateral stacking of GO sheets and fibrous polyaniline is maintained. In addition, the extent of GO exfoliation is dramatically decreased in the GOPL10 composite in Figure 5i, and the polyaniline fibers show a decrease in thickness and length. This is concomitant with growth of a polymer network containing higher pore volume in the microporous regime as indicated by the N2 physisorption data in Figure 4. Water-Processable GOPL Composite Films. Morphology and Spatial Distribution of Materials. Energy-dispersive X-ray spectroscopy and elemental mapping were used to verify the presence of all three materials in the composite films (Figure 6). The EDX spectrum shows characteristic Kα X-ray emission lines of carbon, nitrogen, oxygen, sodium, magnesium, and silicon. The presence of Na, Mg, and Si peaks provides direct confirmation of the presence of Laponite in the composite after material washing and processing. The digital and SEM images inset in the EDX spectrum show the macroscopic appearance and morphology of the film on the micron scale. EDX elemental maps show the spatial distribution
Table 1. Surface Area and Porosity of GO, PANI, Lap, and Their Powder Composites sample
SBET (m2/g)
ϕp (nm)
Vp (cm3/g)
GO PANI Lap GOP80 GOP50 GOP10 GOPL80 GOPL50 GOPL10
210 70 370 50 10 30 130 110 150
11 20 3.7 8.4 49 35 2.8 3.0 6.2
0.13 0.35 0.24 0.04 0.05 0.26 0.09 0.08 0.30
composite (from 10 to 110 m2/g), and a 5-fold increase for the highest polyaniline loading (from 30 to 150 m2/g). This effect, coupled with a concomitant decrease in ϕP and increase in VP, indicates that the addition of Laponite nanoparticles leads to formation of a more fibrous polymer morphology. This is supported by the N2 physisorption isotherms and BJH poresize distributions for GOP10 and GOPL10 (Figure 4), where
Figure 4. Nitrogen adsorption−desorption isotherms for GOP10 (●) and GOPL10 (⧫). Open points represent desorption isotherms. Inset: corresponding BJH pore size distributions determined from the adsorption isotherm.
the large increase in adsorbed volume at high P/P0 (0.9−1.0) indicates the presence of mesopores and macropores.49 This increase is significantly larger in GOP10 than in GOPL10. In addition, the pore-size distributions (inset) show a sharp increase in pore volume associated with the microporous regime in GOPL10, in agreement with a more fibrous, higher surface-area polyaniline morphology. Zhang et al. have attributed low current densities and specific capacitance values in their GO/PANI composites to low surface areas.16 The formation of an electrical double-layer and faradaic redox processes both occur at or across interfaces. The inclusion of Laponite nanoparticles offers a simple synthetic means of significantly increasing the surface area of GOP composites, leading to greater potential for implementation in charge storage applications. E
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Figure 5. SEM images of (a) GO, (b) GO/Lap, (c) PANI, (d) GOP80, (e) GOP50, (f) GOP10, (g) GOPL80, (h) GOPL50, and (i) GOPL10.
of carbon, nitrogen, and silicon. The uniform distribution of all three of these elements provides strong evidence of a fully integrated composite material, with minimal aggregation and phase separation of any component. Structure of Composite Films. Powder XRD patterns of GOPL films are presented in Figure 7 showing the characteristic diffraction peak of the GO (001) plane. In GOPL80 this peak is centered at 2θ = 13.9°, corresponding to an interlayer dspacing of 7.4 Å. This angle decreases to 13.5° in the GOPL50 film, indicating a larger d-spacing of 7.6 Å between GO sheets. In the GOPL10 composite film, this peak occurs at 2θ = 12.7°, with a corresponding d-spacing of 8.1 Å. This peak was notably absent in the GOPL composite powders, and this was attributed to complete exfoliation of the GO sheets.
The presence of this peak here indicates that exploitation of the dispersive and templating properties of Laponite nanoparticles leads to a more ordered nanocomposite film. This action likely improves interfacial contact within the composite film, with implications for improved performance in device applications. In addition, the composite film d-spacing values are less than their powder counterparts by 0.6 Å for GOPL80 and GOPL50. This further supports the theory that Laponite is affecting interactions at the GO−PANI interface, in this case resulting in an overall improvement in wedding of the materials as shown in Figure 6c. It is well-known that electroactive polymers display inhomogeneous changes in volume upon charge cycling, which may result in premature degradation of the composite. 51,52 The tightening effect of Laponite F
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Figure 6. EDX spectrum and elemental maps of GOPL10 film.
(Figure 8). The curve for GO shows the quinone/hydroquinone redox couple typical for oxygenated carbon materials.16 The sharp increase in current from 0.6 to 0.8 V is due to reaction of the electrolyte. The PANI voltammogram contains two peaks evident at approximately 0.3 V (present as a shoulder) and 0.5 V in the oxidizing sweep. These correspond respectively to the redox transitions from the leucoemeraldine semiconducting state to the conducting emeraldine state and from the emeraldine state to the fully oxidized pernigraniline state.53 The CV curves for all composites are rectangular in shape, indicating increased efficiency of charge transport within the composite materials compared to pure GO or polyaniline. In general, increased polyaniline loading in the composites leads to higher specific current measured. The shifts observed in both reduction and oxidation potentials within the composite materials have been previously attributed to strong interactions between PANI and GO.39 There was no significant decrease in specific current observed over 10 cycles for any of the composites. It is clear from the voltammetry data that the inclusion of Laponite nanoparticles does not prohibit the use of polyaniline/graphene oxide composite materials for electrochemical and energy applications involving redox cycling and charge transport. Conductive AFM. Local structure and electronic properties were investigated through conductive AFM (cAFM) experiments (Figure 9). Tapping mode height profiles of GOPL10 are presented in Figures 9a and 9b, showing surface morphology of this composite. In Figure 9a, agglomerates of nanofibrous PANI on GO surfaces are evident with diameters on the order of 100 nm. In Figure 9b, Laponite nanodisks with diameters ranging from 25 to 40 nm are dispersed across the composite surface. There is clear evidence of both stacked and unstacked Laponite nanoparticles, given the range in height from 1 to 20 nm. The local electrical transport properties of the GOPL10 film were studied by employing cAFM to conduct CVC experiments, shown in Figure 9c. Application of an electrical potential
Figure 7. Powder X-ray diffraction patterns of (a) GOPL80, (b) GOPL50, and (c) GOPL10 films.
demonstrated here is expected to ameliorate this effect by locking the composite materials more firmly in place, thereby increasing the functional lifetime of devices. Electrochemical Properties of GOPL Composites. Cyclic Voltammetry. Bulk electrochemical performance of GO, PANI, and GOPL composites were studied using CV G
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Figure 8. Cyclic voltammograms of (a) GO, (b) PANI, (c) GOPL80, (d) GOPL50, and (e) GOPL10 in 2 M H2SO4 at a scan rate of 100 mV/s.
Figure 9. AFM tapping mode images of GOPL10 showing (a) PANI polymer growth and (b) Laponite nanodisks. (c) Local electron transport properties of GOPL10. Contact mode (d) topography and (e) corresponding cAFM current map for GOPL80 with a 2.5 V applied bias. H
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conductive response to an applied tip bias. Our work herein supports key efforts by Shen and co-workers, who have shown improvements in asymmetric supercapacitor design by tuning the interfacial polymerization of polyaniline on graphene nanosheets.58 A better understanding of the specific interactions (electrostatic, π−π stacking, hydrogen bonding)21,59,60 that mediate this self-assembly will inform new strategies to direct the stacking of graphene sheets to realize supercapacitive activity. Importantly, the incorporation of Laponite nanoparticles did not result in the loss of electrochemical and redox properties of the GOP system. CV and cAFM studies showed that at both the bulk and local levels there were CVC responses indicating stable redox cycling necessary for electrochemical and energy applications dependent on charge transfer processes. This water-processable system may support future roll-to-roll production methods of film preparation common in industrial device fabrication.
between the AFM tip and the composite material surface produces a current on the order of tens of nA entering the tip. There is a nonlinear current response with an applied bias of 0−1 V, typical of semiconductor materials, followed by a linear current response at applied biases larger than 1 V due to formation of the conductive polyaniline state. This CVC response shows that even under experimental conditions widely used to produce polyaniline in the conductive emeraldine salt form, local domains exist where the polymer is present in varied oxidation states. This has been previously reported for polyaniline films, where higher conductivity has been associated with shorter synthesis times.54 Figure 9 shows contact mode topography (d) and current (e) images obtained simultaneously on a 1 μm by 1 μm area of the GOPL80 film. This topographical map shows PANI growth on the surface of GO as a ridge up to 300 nm in step-height running vertically through the center of the image. The current map was acquired with an applied bias of 2.5 V and shows localized regions of high conductivity along the polyaniline nanoridge shown in the topographical map as well as in globular domains of varying sizes across the full map area. It is clear that even with the lowest polyaniline loading, conductive polymeric growth is observed to be well distributed at nanometer length scales. This type of distribution is necessary to enable energy applications involving charge separation processes. Indeed, the electric properties of localized polyaniline nanostructures have been shown previously to have a significant dependence on the aggregation, morphology, and connectivity of polymer chains.55−57 The cAFM studies reported here extend these insights to an important new class of water-processable carbon-based nanocomposites which show great promise for energy applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] (M.E.H.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Science Foundation (CMMI-1342577, DMR-1229142, and EEC-0939322). We thank Rebecca Cortez, Palmyra Catravas, Jared Mondschein, Yixin Hu, James McGarrah, and Mary Carroll for materials and characterization support.
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CONCLUSIONS Xu et al. have constructed hierarchical GOP nanocomposites and controlled nanowire arrays by vertically aligning polyaniline on GO nanosheets.24 Control of dimensionality in these and related systems has been shown to have a strong influence on capacitive response. To this end, we have successfully used Laponite to cast stable and homogeneous films of GOPL nanocomposites, increasing their viability for implementation into devices through a facile and easily scalable waterprocessable synthesis. Infrared and Raman spectroscopy indicated successful formation of composite materials evidenced by shifts in typical absorption bands and strong interactions between GOP and Laponite domains. These methods provided strong evidence for the presence of the conductive emeraldine state of polyaniline and of polymer acid doping from carboxylic acid moieties of GO. XRD analyses demonstrate that at low PANI loadings lamellar nanostructures that support capacitive response are preserved while the loss of the graphitic diffraction peak at high PANI loading indicated complete exfoliation of GO domains. The incorporation of Laponite nanoparticles reestablishes the lamellar nanoarchitecture of the composite even under the highest polymer loading. Gas physisorption studies show that Laponite inclusion results in the formation of more fibrous polyaniline, leading to appreciably higher surface area composite materials. This leads to a greater interfacial area and is a key requirement for the improvement of chargetransfer processes between materials. SEM and AFM showed nanostructures and interfacial domains between component materials and the local
REFERENCES
(1) Lewis, N.; Nocera, D. Powering the planet: chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103 (43), 15729−15735. (2) Murali, S.; Dreyer, D.; Valle-Vigón, P.; Stoller, M.; Zhu, Y.; Morales, C.; Fuertes, A.; Bielawski, C.; Ruoff, R. Mesoporous carbon capsules as electrode materials in electrochemical double layer capacitors. Phys. Chem. Chem. Phys. 2011, 13, 2652−2655. (3) Hahm, M.; Reddy, A.; Cole, D.; Rivera, M.; Vento, J.; Nam, J.; Jung, H.; Kim, Y.; Narayanan, N.; Hashim, D.; Galande, C.; Jung, Y.; Bundy, M.; Karna, S.; Ajayan, P.; Vajtai, R. Carbon nanotube-nanocup hybrid structures for high power supercapacitor applications. Nano Lett. 2012, 12 (11), 5616. (4) Tan, Y.; Xu, C.; Chen, G.; Liu, Z.; Ma, M.; Xie, Q.; Zheng, N.; Yao, S. Synthesis of ultrathin nitrogen-doped graphitic carbon nanocages as advanced electrode materials for supercapacitors. ACS Appl. Mater. Interfaces 2013, 5 (6), 2241−2248. (5) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 3, 183−191. (6) Lee, C.; Wei, X.; Kysar, J.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321 (5887), 385−388. (7) Xu, Z.; Li, Z.; Holt, C.; Tan, X.; Wang, H.; Amirkhiz, B.; Stephenson, T.; Mitlin, D. Electrochemical supercapacitor electrodes from sponge-like graphene nanoarchitectures with ultrahigh power density. J. Phys. Chem. Lett. 2012, 3, 2928−2933. (8) Ni, G.; Bae, S.; Tan, C.; Kahya, O.; Wu, J.; Hong, B.; Yao, K.; Ö zyilmaz, B. Graphene-ferroelectric hybrid structure for flexible transparent electrodes. ACS Nano 2012, 6 (5), 3935−3942. (9) Dreyer, D.; Todd, A.; Bielawski, C. Harnessing the chemistry of graphene oxide. Chem. Soc. Rev. 2014, 43, 5288−5301. (10) Kuila, T.; Mishra, A.; Khanra, P.; Kim, N.; Lee, J. Recent advances in the efficient reduction of graphene oxide and its
I
DOI: 10.1021/la5046783 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir application as energy storage electrode materials. Nanoscale 2013, 5, 52−71. (11) Fan, Z.; Wang, K.; Wei, T.; Yan, J.; Song, L.; Shao, B. An environmentally friendly and efficient route for the reduction of graphene oxide by aluminum powder. Carbon 2010, 48, 1670−1692. (12) Dreyer, D.; Murali, S.; Zhu, Y.; Ruoff, R.; Bielawski, C. Reduction of graphite oxide using alcohols. J. Mater. Chem. 2011, 21, 3443−3447. (13) Bissessur, R.; Liu, P. K. Y.; White, W.; Scully, S. F. Encapsulation of polyanilines into graphite oxide. Langmuir 2006, 22 (4), 1729− 1734. (14) Tran, H. D.; D’Arcy, J. M.; Wang, Y.; Beltramo, P. J.; Strong, V. A.; Kaner, R. B. The oxidation of aniline to produce “polyaniline”: a process yielding many different nanoscale structures. J. Mater. Chem. 2011, 21 (11), 3534−3550. (15) Li, D.; Huang, J.; Kaner, R. Polyaniline nanofibers: a unique polymer nanostructure for versatile applications. Acc. Chem. Res. 2009, 42 (1), 135−145. (16) Zhang, K.; Zhang, L.; Zhao, X.; Wu, J. Graphene/polyaniline nanofiber composites as supercapacitor electrodes. Chem. Mater. 2010, 22 (4), 1392−1401. (17) Boyer, M. I.; Quillard, S.; Louarn, G.; Froyer, G.; Lefrant, S. Vibrational study of the FeCl3-doped dimer of polyaniline; a good model compound of emeraldine salt. J. Phys. Chem. B 2000, 104 (38), 8952−8961. (18) Peace, B.; Topka, M.; Skorenko, K.; Kowalski, A.; Williams, U.; Hagerman, M.; Cortez, R. Morphosynthesis and morphology characterization of aniline and aniline/Laponite films. Mater. Lett. 2011, 65 (21-22), 3208−3211. (19) Ma, H.; Li, Y.; Yang, S.; Cao, F.; Gong, J.; Deng, Y. Effects of solvent and doping acid on the morphology of polyaniline prepared with the ice-templating method. J. Phys. Chem. C 2010, 114 (20), 9264−9269. (20) Li, Z.; Zhang, H.; Liu, Q.; Sun, L.; Stanciu, L.; Xie, J. Fabrication of high-surface-area graphene/polyaniline nanocomposites and their application in supercapacitors. ACS Appl. Mater. Interfaces 2013, 5 (7), 2685−2691. (21) Wang, H.; Hao, Q.; Yang, X.; Lu, L.; Wang, X. Effect of graphene oxide on the properties of its composite with polyaniline. ACS Appl. Mater. Interfaces 2010, 2 (3), 821−828. (22) Potts, J.; Dreyer, D.; Bielawski, C.; Ruoff, R. Graphene-based polymer nanocomposites. Polymer 2011, 52 (1), 5−25. (23) Yu, P.; Li, Y.; Zhao, X.; Wu, L.; Zhang, Q. Graphene-wrapped polyaniline nanowire arrays on nitrogen-doped carbon fabric as novel flexible hybrid electrode materials for high-performance supercapacitor. Langmuir 2014, 30, 5306−5313. (24) Xu, J.; Wang, K.; Zu, S.; Han, B.; Wei, Z. Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage. ACS Nano 2010, 4 (9), 5019−5026. (25) Kumar, N.; Baek, J. Electrochemical supercapacitors from conducting polyaniline-graphene platforms. Chem. Commun. 2014, 50, 6298−6308. (26) Akcora, P.; Liu, H.; Kumar, S.; Moll, J.; Li, Y.; Benicewicz, B.; Schadler, L.; Acehan, D.; Panagiotopoulos, A.; Pryamitsyn, V.; Ganesan, V.; Ilavsky, J.; Thiyagarajan, P.; Colby, R.; Douglas, J. Anisotropic self-assembly of spherical polymer-grafted nanoparticles. Nat. Mater. 2009, 8, 354−359. (27) Dag, O.; Samarskaya, O.; Coombs, N.; Ozin, G. The synthesis of mesostructured silica films and monoliths functionalised by noble metal nanoparticles. J. Mater. Chem. 2003, 13, 328−334. (28) Kehlbeck, J.; Hagerman, M.; Cohen, B.; Eliseo, J.; Fox, M.; Hoek, W.; Karlin, D.; Leibner, E.; Nagle, E.; Nolan, M.; Schaefer, I.; Toney, A.; Topka, M.; Uluski, R.; Wood, C. Directed self-assembly in Laponite/CdSe/polyaniline Nanocomposites. Langmuir 2008, 24, 9727−9738. (29) Peace, B.; Topka, M.; Williams, U.; Skorenko, K.; Kowalski, A.; Hagerman, M.; Cortez, R. Atomic force microscopy examination of polymeric nanocomposite layers. Mater. Lett. 2011, 65, 3208−3211.
(30) Mondschein, J.; Kowalski, A.; Kehlbeck, J.; Hagerman, M.; Cortez, R. Comparitive AFM studies of water processable polyaniline films: influence of reaction time on nanomorphology and conductivity. Mater. Lett. 2014, 131, 262−265. (31) Alhassan, S.; Qutubuddin, S.; Schiraldi, D. Graphene arrested in Laponite-water colloidal glass. Langmuir 2012, 28, 4009−4015. (32) Yoo, J.; Lee, S.; Lee, C.; Hwang, S.; Kim, C.; Fujigaya, T.; Nakashima, N.; Shim, J. Graphene oxide and Laponite composite films with high oxygen-barrier properties. Nanoscale 2014, 6, 10824−10830. (33) Marcano, D.; Kosynkin, D.; Berlin, J.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L.; Lu, W.; Tour, J. Improved synthesis of graphene oxide. ACS Nano 2010, 4 (8), 4806−4814. (34) Sing, K.; Everett, D.; Haul, R.; Moscou, L.; Pierotti, R.; Rouquérol, J.; Siemieniewska, T. Reporting physisorption data for gas/ solid systems. Pure Appl. Chem. 1985, 57 (4), 603−619. (35) Sariciftci, N. S.; Kuzmany, H.; Neugebauer, H.; Neckel, A. Structural and electronic transitions in polyaniline: a Fourier transform infrared spectroscopic study. J. Chem. Phys. 1990, 92 (7), 4530−4539. (36) Pálková, H.; Madejová, J.; Zimowska, M.; Serwicka, E. Laponitederived porous clay heterostructures: II. FTIR study of the structure evolution. Microporous Mesoporous Mater. 2010, 127 (3), 237−244. (37) Breen, C.; Zahoor, P. Characterization and catalytic activity of acid-treated, size-fractionated smectites. J. Phys. Chem. B 1997, 101, 5324−5331. (38) Sarker, A. K.; Hong, J. Layer-by-layer self-assembled multilayer films composed of graphene/polyaniline bilayers: high-energy electrode materials for supercapacitors. Langmuir 2012, 28 (34), 12637−12646. (39) Vallés, C.; Jiménez, P.; Muñoz, E.; Benito, A. B.; Maser, W. K. Simultaneous reduction of graphene oxide and polyaniline: dopingassisted formation of a solid-state charge-transfer complex. J. Phys. Chem. C 2011, 115 (21), 10468−10474. (40) Tuinstra, F.; Koenig, J. Raman spectrum of graphite. J. Chem. Phys. 1970, 53 (3), 1126−1130. (41) Voggu, R.; Das, B.; Rout, C.; Rao, C. Effects of charge transfer interaction of graphene with electron donor and acceptor molecules examined using Raman spectroscopy and cognate techniques. J. Phys.: Condens. Matter 2008, 20 (47), 1−5. (42) Liu, Z.; Wang, Z.; Yang, X.; Ooi, K. Intercalation of organic ammonium ions into layered graphite oxide. Langmuir 2002, 18 (12), 4926−4932. (43) Hagerman, M. E.; Salamone, S. J.; Herbst, R. W.; Payeur, A. L. Tris(2,2′-bipyridine)ruthenium(II) cations as photoprobes of clay tactoid architecture within Hectorite and Laponite films. Chem. Mater. 2003, 15 (2), 443−450. (44) Jiménez, P.; Castell, P.; Sainz, R.; Ansón, A.; Martínez, M. T.; Benito, A. M.; Maser, W. K. Carbon nanotube effect on polyaniline morphology in water dispersable composites. J. Phys. Chem. B 2010, 114 (4), 1579−1585. (45) Pouget, J. P.; Josefowicz, M. E.; Epstein, A. J.; Tang, X.; MacDiarmid, A. G. X-ray structure of polyaniline. Macromolecules 1991, 24 (3), 779−789. (46) McAllister, M. J.; Li, J.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem. Mater. 2007, 19 (18), 4396−4404. (47) Shen, J.; Hu, Y.; Shi, M.; Lu, X.; Qin, C.; Li, C.; Ye, M. Fast and facile preparation of graphene oxide and reduced graphene oxide nanoplatelets. Chem. Mater. 2009, 21 (15), 3514−3520. (48) Herrera, N.; Letoffe, J.; Putaux, J.; David, L.; Bourgeat-Lami, E. Aqueous dispersions of silane-functionalized Laponite clay platelets. A first step toward the elaboration of water-based polymer/clay nanocomposites. Langmuir 2004, 20 (5), 1564−1571. (49) Bippus, L.; Jaber, M.; Lebeau, B. Laponite and hybridsurfactant/Laponite particles processed as spheres by spray-drying. New J. Chem. 2009, 33, 1116−1126. (50) Xu, G.; Wang, N.; Wei, J.; Lv, L.; Zhang, J.; Chen, Z.; Xu, Q. Preparation of graphene oxide/polyaniline nanocomposite with J
DOI: 10.1021/la5046783 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir assistance of supercritical carbon dioxide for supercapacitor electrodes. Ind. Eng. Chem. Res. 2012, 51, 14390−14398. (51) Kötz, R.; Carlen, M. Principles and applications of electrochemical capacitors. Electrochim. Acta 2000, 45, 2483−2498. (52) Gurunathan, K.; Murugan, A. V.; Marimuthu, R.; Mulik, U. P.; Amalnerkar, D. P. Electrochemically synthesised conducting polymeric materials for applications towards technology in electronics, optoelectronics and energy storage devices. Mater. Chem. Phys. 1999, 61 (3), 173−191. (53) Kumar, N.; Choi, H.; Shin, Y.; Chang, D.; Dai, L.; Baek, J. Polyaniline-grafted reduced graphene oxide for efficient electrochemical supercapacitors. ACS Nano 2012, 6 (2), 1715−1723. (54) Mondschein, J.; Kowalski, A.; Kehlbeck, J.; Hagerman, M.; Cortez, R. Comparative AFM studies of water processable polyaniline films: influence of reaction time on nanomorphology and conductivity. Mater. Lett. 2014, 131, 262−265. (55) Wu, C.; Chang, S. Nanoscale measurements of conducting domains and current-voltage characteristics of chemically deposited polyaniline films. J. Phys. Chem. B 2005, 109, 825−832. (56) Khalid, M.; Tumelero, M.; Zoldan, V.; Pla Cid, C.; Franceschini, D.; Timm, R.; Kubota, L.; Moshkalev, S.; Pasa, A. Polyaniline nanofibers-graphene oxide nanoplatelets composite thin film electrodes for electrochemical capacitors. RSC Adv. 2014, 4, 34168−34178. (57) Chang, S.; Wu, C. Polymerization media on the nanoscale conductivity and current-voltage characteristics of chemically synthesized polyaniline films. J. Phys. Chem. B 2005, 109, 18275−18282. (58) Shen, J.; Yang, C.; Li, X.; Wang, G. High-performance asymmetric supercapacitor based on nanoarchitectured polyaniline/ graphene/carbon nanotube and activated graphene electrodes. ACS Appl. Mater. Interfaces 2013, 5 (17), 8467−8476. (59) Tran, H.; Wang, Y.; D’Arcy, J.; Kaner, R. Toward an understanding of the formation of conducting polymer nanofibers. ACS Nano 2008, 2 (9), 1841−1848. (60) Compton, O.; Cranford, S.; Putz, K.; An, Z.; Brinson, L.; Buehler, M.; Nguyen, S. Tuning the mechanical properties of graphene oxide paper and its associated polymer nanocomposites by controlling cooperative intersheet hydrogen bonding. ACS Nano 2012, 6 (3), 2008−2019.
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Water-Processable Laponite/Polyaniline/Graphene Oxide Nanocomposites for Energy Applications Isaac A. Ramphal and Michael E. Hagerman* Department of Chemistry, Union College, Schenectady, New York 12308, United States
ABSTRACT: Graphene−polyaniline (GP) nanocomposites have demonstrated remarkable ability as supercapacitive materials and are typically synthesized via chemical reduction of graphene oxide/polyaniline (GOP) precursors. We report the formation of novel nanomaterials combining GOP nanocomposites with Laponite nanodisks. Host−guest interactions within GOP systems were studied with and without Laponite nanoparticle templating agents. Incorporating Laponite clay into the composite synthesis enhances aqueous dispersibility as well as facilitates the casting of homogeneous films. Structural and morphological characterization confirmed porous heterointerfaces and control of polymer and nanoclay loading. These results may enable the development of flexible supercapacitive and solar nanocomposites with improved device utility, water dispersibility, and film processability. We demonstrate that these films can be easily cast and that the composites maintain their electrical transport properties.
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INTRODUCTION Two of the most significant challenges facing society today are the rising costs of nonrenewable resources and the environmental impacts associated with their acquisition and usage.1 Carbon-based materials in general have been widely studied for supercapacitor applications.2−4 Graphene especially has been the subject of extensive study upon demonstration of its excellent mechanical and electrical properties.5 Graphene’s high thermal and electrical conductivity, exceptional surface area, outstanding mechanical strength, large capacitance values, and good cycling stability are several of the reasons it is an attractive material system.6−8 Greener synthetic strategies should employ materials with high water solubility as often as possible to avoid organic solvent waste. As such, a key challenge of working with graphene is its poor aqueous solubility. Typically, graphene is oxidized to afford the insulating graphene oxide (GO), which is generally accepted to have alcohol and epoxide substituents decorating the basal plane, and carbonyl moieties along the sheet edges,9 which is much more amenable to aqueous processing and can be readily reduced back to graphene if desired.10−12 Electroactive polymers such as polyaniline, polythiophene, and polypyrrole have likewise been studied for their electrical capabilities.13,14 Polyaniline (PANI) has found widespread use as a conductive polymer due to its solution processability for varied applications including sensors, actuators, memory devices, and energy storage devices.15 PANI is an attractive © XXXX American Chemical Society
material in these systems due to its environmental stability, low cost, and large surface area.16,17 PANI’s conductivity has been shown to vary considerably with changes in its oxidation state and morphology.14,18 Depending on the morphosynthetic conditions used, PANI has been isolated as fibers, wires, microwebs, hemispheres, nests, rods, and 2D nanosheets along with many other morphologies.14,19 Graphene intercalation compounds of conductive polymers such as PANI have been studied insofar as the composites provide a lightweight nanomaterial with improved electrical performance, especially as supercapacitor electrodes.13,16,20−22 Previous research has demonstrated GP nanocomposites with capacitance values as high as 1145 F/g due to a combination of electrical double-layer capacitance of graphene and pseudocapacitance of PANI.23 Supercapacitive composite materials have also been made using PANI and unreduced GO.24 Consequently, the ability to tune the extent of oxidation of graphene coupled with the variable oxidation states available to PANI provides a versatile composite material system. Despite high versatility of this hybrid system, several obstacles must be overcome in order to realize inexpensive, water-processable device implementation. These include failure upon cycling, thermal degradation, and delamination.25 Many Received: July 31, 2014 Revised: January 7, 2015
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Figure 1. FTIR absorbance (left) and Raman scattering (right) vibrational spectra of powders of (a) GO, (b) GOP80, (c) GOP50, (d) GOP10, (e) Laponite, (f) GOPL80, (g) GOPL50, (h) GOPL10, and (i) PANI. Laponite has no Raman-active vibrational modes in the spectral region shown and is thus omitted. such that there was a 1:4 mole ratio of APS to aniline. These solutions were left to stir overnight. As aniline polymerizes, the solution undergoes a color change from brown to green, with higher aniline concentrations producing darker green solutions. The products were then washed in succession with H2O, absolute ethanol, and hexanes. Each rinse was added to the reactant solution, centrifuged for 30 min (2500 rpm, 5 °C), and then decanted. The products were heated overnight at 65 °C under vacuum to fully dry the composites. The final composites were obtained as dark green powders. Preparation of GOPL Composites. The GOP synthesis described above was repeated with the exception that Laponite and GO were simultaneously added to the aniline solution. Laponite was added such that it was 25 wt % of the combined GO and aniline mass. The final composites were obtained as dark green powders. These composite materials are labeled GOPL10, GOPL50, and GOPL80, respectively. Preparation of GOPL Films. The GOPL composites were dispersed in an aqueous solution with a concentration of 8 mg/mL. These solutions were vigorously agitated, pipetted onto a glass slide, and allowed to air-dry slowly in a humid environment for 24 h to yield the GOPL films. Characterization. Vibrational Spectroscopy. ATR-FTIR absorbance spectra were obtained using a Nicolet Avatar 330 FTIR spectrophotometer with Smart Orbit ATR attachment. Spectra were collected using 128 scans at 2 cm−1 resolution. Raman spectra were acquired with a Bruker Senterra Raman microscope (excitation laser λ = 786 nm, 1 mW) with 3 cm−1 resolution, and fluorescence corrections were applied. XRD Analysis. Powder X-ray diffraction spectra were obtained using a Phillips PW-1840 X-ray diffractometer with a Co Kα (λ = 1.7902 Å) source operating at 45 kV and 35 mA. TGA Analysis. Thermal stability was measured using a PerkinElmer TGA-7 thermogravimetric analyzer. Samples were characterized from 100 to 500 °C with a heating rate of 2 °C/min in an N2 atmosphere.
researchers have explored nanoparticle templating to control heterointerfaces and improve functionality.26,27 The synthetic clay Laponite offers new classes of electroactive and energy storage materials, as robust, transparent films can be coated onto a variety of substrates through facile self-assembly from the aqueous phase. Laponite nanoparticles provide versatile inorganic scaffolds with nanoarchitectures which can be selectively tuned to direct interactions across heterointerfaces, including polyaniline nanoassemblies.28−30 While previous studies have demonstrated kinetic control within graphene/ Laponite colloidal dispersions and Laponite/GO films for oxygen-barrier applications,31,32 we report herein the first detailed study of Laponite/polymer/graphene oxide films. We are currently employing Laponite nanoparticles to improve the aqueous self-assembly of GOP nanocomposites (GOPL), afford thin-film processability, and increase viability for device implementation.
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EXPERIMENTAL SECTION
Materials. Graphite powder (crystalline, −300 mesh, 99%) was purchased from Alfa Aesar. Sodium Laponite RD (Na0.7[(Li0.3Mg5.5Si8)O20(OH)4]) was obtained from Southern Clay Products in the USA and donated by Laporte Industries in the UK. All other reagents and solvents were purchased from Sigma-Aldrich. Graphene oxide was prepared according to a procedure from the published literature.33 Preparation of GOP Composites. GOP nanocomposites were synthesized using a modified literature procedure.16 Aniline and GO were added to 1 M HCl in weight ratios of 90:10, 50:50, and 20:80 and sonicated for 1 h. These composite materials are labeled GOP10, GOP50, and GOP80, respectively. To each of these, a solution of ammonium persulfate in 1 M HCl was added with vigorous stirring B
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Langmuir Morphology Studies. Nitrogen adsorption−desorption isotherms were measured using a Micromeritics Tristar 3000 automated gas adsorption analyzer at 77.4 K. Samples were purged under nitrogen flow for 2 h at 45 °C followed by 20 h at 60 °C in a Micromeritics Smartprep programmable degas system prior to analysis. Specific surface areas were calculated from the adsorption isotherms using the Brunauer−Emmett−Teller (BET) method (relative pressure range P/ P0 = 0.01−0.3) according to IUPAC recommendations.34 Pore size distributions were estimated from the adsorption isotherm using the Barrett−Joyner−Halenda (BJH) method. SEM images were collected using a Zeiss EVO W filament system scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDX) was carried out in the SEM using a Bruker Quantax 200 EDX system. Tapping mode AFM images were collected using an Asylum Research MFP-3D atomic force microscope with commercially available rotated silicon probes with a tip radius of curvature of 8 nm and a spring constant of 20−80 N/m. Optical micrographs were acquired using an Olympus BX51 optical microscope. Electrochemical Measurements. Cyclic voltammetry (CV) data were collected using a PAR VersaSTAT 3 AMETEK model potentiostat with a standard three-electrode setup consisting of screen-printed carbon black working and counter electrodes and Ag/ AgCl reference electrode (CH Instruments, TE100). Voltammograms were collected in the range of −0.2 to 0.8 V at a scan rate of 100 mV/s in 2 M H2SO4 electrolyte. Samples were mixed with 5 wt % aqueous Nafion solution at a ratio of 1 μL per mg of material and pressed onto the working electrode. Local charge transport properties were studied in contact mode using a Veeco Dimension V scanning probe microscope and commercially available platinum-coated silicon probes with a tip radius of curvature of 30 nm. For stable current−voltage characterization (CVC) responses, applied voltage biases ranged from 250 mV up to 2.5 V.
provides further evidence of interactions between the polymeric polaronic lattice and the negatively charged GO sheet. The GOPL system afforded upon incorporation of Laponite displays a similar shift to lower frequency with increasing PANI loading as seen in the GOP system. This has been attributed above to interactions between PANI and the negatively charged GO sheet, which are now also possible between PANI and the negatively charged silica surfaces of Laponite nanoparticles. This is supported by spectral broadening of the 1280 and 1225 cm−1 C−N and C−N•+ stretching modes in the GOPL system. Further evidence of host−guest interactions can be seen in shifts in the Si−O vibrational modes of Laponite: the edge and surface silicate bands shift to 790 and 950 cm−1, respectively, leading to three distinct silicate stretching modes including the 1050 cm−1 internal vibration (pink shaded region) and providing clear evidence that Laponite is interacting with the composite matrix. Raman spectroscopy was used to probe the local environment at the graphene oxide and PANI interface (Figure 1, right panel). The Raman spectrum of GO contains the typical Gband of graphite at 1595 cm−1 corresponding to first-order scattering of the E2g vibrational mode.40 The more prominent D-band beak at 1310 cm−1 is due to disruption of the carbon sp2 lattice of pristine graphitic materials and is indicative of extensive oxidation of the graphitic surface. For pure PANI, Raman bands are observed at 1162, 1250, 1332, 1367, 1500, and 1595 cm−1 corresponding to in-plane C−H bending, C− N•+ stretching, C−H deformation, symmetric C−N stretching, CN quinoid, and C−C benzenoid breathing modes, respectively. These bands are consistent with the ES form of PANI.17,21 The persistence of these peaks in the composite spectra indicates PANI remains in the acid-doped ES form upon polymerization in the presence of GO. The shift in the C−H bending mode to 1170 cm−1 in GOP50 provides evidence of interactions between PANI and GO, while the return to 1165 cm−1 in GOP10 shows dominance of PANI−PANI interactions at the highest PANI loading. In addition, the PANI peaks at 1332 and 1595 cm−1 cause broadening of the overlapping Gband of GO in the intermediate loading composite. In the highest PANI loading, GOP10 shows a shift of this Raman band to 1628 cm−1, while GOPL10 shows further broadening and stiffening of the G-band of GO (blue shaded region). These shifts are consistent with hole-doping of GO by interactions between carboxylic acid groups of GO and the polaronic PANI backbone and to π−π interactions between PANI and GO.21,41 Powder XRD. Structure of the composites was investigated using powder XRD. XRD patterns of PANI, GO, Lap, GOP, and GOPL composites are shown in Figure 2. GO exhibits a major diffraction peak centered at 2θ = 11.3° which corresponds to an interlayer d-spacing of 9.1 Å. This peak has been assigned to the (001) plane and may vary depending upon materials processing and the amount of interlamellar water. 42 Laponite has two broad peaks occurring at approximately 2θ = 7.7° and 2θ = 22.8°, corresponding to the (001) and (003) planes of the tetrahedral−octahedral− tetrahedral layer with d-spacings of 13.3 and 4.5 Å, respectively.43 The PANI pattern has two major, broad peaks centered at 2θ = 23.5° and 2θ = 29.3°, corresponding to dspacings of 4.4 and 3.5 Å and attributed to the (020) and (200) planes, respectively.44,45 The overall shape of this pattern is
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RESULTS AND DISCUSSION GOP and GOPL Composite Powders. Vibrational Spectroscopy. Molecular structures of the composites were investigated using ATR-FTIR (Figure 1, left panel). GO exhibited bands at 1060, 1220, 1610, and 1730 cm−1, which are attributed to stretching frequencies of C−O−C epoxide, C−O hydroxide, quinoidal CC, and carboxyl CO, respectively.33 PANI showed characteristic bands located at 810, 835, 1110, 1225, and 1280 cm−1 associated with C−H in-plane and out-ofplane bends in benzene rings, CN quinoid stretching, C− N•+ in the emeraldine salt (ES) form of PANI, and C−N stretching modes, respectively.24 The characteristic peaks of the benzenoid and quinoid ring breathing modes are present at 1450 and 1550 cm−1, respectively. The slightly less than unitary quinoid/benzenoid peak intensity ratio is indicative of the ES form of PANI.35 Laponite RD has characteristic vibrational modes at 850 and 1000−1100 cm−1, corresponding to edge and a combination of surface and internal Si−O stretches, respectively.36,37 In the GOP system the benzenoid breathing mode at 1450 cm−1 shifts to lower frequency with increased PANI loading, indicative of interactions with the GO sheets. These interactions may include intermolecular electrostatic, hydrogen bonding, and π−π forces and have been reported previously by Sarker and Hong.38 By contrast, the C−N stretching frequency at 1280 cm−1 increases with increasing GO content (green shaded region), which may be evidence of a PANI-GO chargetransfer complex similar to the findings of Vallés et al.39 A better understanding of this reversible charge transfer is needed to control the switching between neutral and charged states to optimize supercapacitive behavior. In addition, the shift to lower frequency in the C−N•+ with increasing GO content C
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subsequently smaller change in interlayer spacing may also indicate that Laponite is influencing the PANI−GO heterointerface. Further evidence of Laponite’s influence on PANI− GO heterointerfaces is apparent in thermal studies of the composites. Thermal Stability. Thermal stability of the composites was studied using thermogravimetric analysis (TGA), with results for GOP80, GOPL80, PANI, GO, and Lap shown in Figure 3.
Figure 3. TGA of GO (), PANI (◊), Laponite (- - -), GOP80 (△), and GOPL80 (○).
GO shows a significant mass loss of ∼40% between 175 and 250 °C owing to the decomposition of labile oxygen-containing functional groups, with subsequent mass loss attributed to pyrolysis of more stable oxygen moieties.46,47 The PANI sample exhibits a steady mass loss of approximately 20% over the temperature range studied. Both of the graphene oxide/ polyaniline composites displayed larger thermal stability than pure GO with lower mass losses of around 30% for GOP80 and 25% for GOPL80. These results are consistent with those reported in Zhang et al., who have previously demonstrated improvement in thermal stability and decreased mass loss for GOP nanocomposites.16 We have further improved this stability via addition of Laponite nanoparticles to the composite, evidenced by a 65 °C increase in the midpoint of thermal decomposition temperature. This effect was observed for all composite powder systems presented here. This improved stability is a direct consequence of Laponite and GOP interactions and may mitigate the effects of local heating which prove deleterious to capacitive performance. Morphological Characterization. Nitrogen adsorption− desorption isotherms were used to determine the BET surface area (SBET), peak pore diameter (ϕp), and peak pore volume (Vp) for graphene oxide, polyaniline, Laponite, and their composite powders (Table 1). The SBET values of 210 and 70 m2/g for GO and PANI, respectively, are both significantly higher than those typically reported.16,20 These exceptionally large surface areas are attributed to a high degree of chemical exfoliation during GO synthesis and to the very fibrous microweb polyaniline morphology determined by XRD studies. The surface area and porosity data of Laponite matches previously reported values.48 In general, the GOPL composites displayed an appreciable increase in surface area compared to their non-Laponite counterparts. Addition of Laponite resulted in a nearly 3-fold increase in SBET for the lowest polyaniline loading (from 50 to 130 m2/g), an 11-fold increase for the intermediate loading
Figure 2. Powder X-ray diffraction patterns of (a) GO, (b) GOP80, (c) GOP50, (d) GOP10, (e) Laponite, (f) GOPL80, (g) GOPL50, (h) GOPL10, and (i) PANI. Blue shaded region shows the fwhm of the native GO peak.
consistent with previously reported powder XRD patterns for the microweb structure of PANI.19 The powder XRD data for the GOP and GOPL composite systems confirm a varied loading of PANI intercalated between GO sheets. The pattern for GOP80 shows a major diffraction peak at 2θ = 12.9° corresponding to a d-spacing of 8.0 Å. We attribute the decrease in d-spacing relative to pure GO to changes in the amount of interlamellar water and/or disruption of the ordered hydrogen-bonded water network formed with GO hydroxyl and epoxide moieties. As PANI loading increases to 50% in the GOP50 composite, this angle decreases to 12.1° (d = 8.5 Å). This expansion in interlayer GO sheet distance is likely a result of the surface adsorption and intercalation of PANI. Broadening of this peak with increased PANI loading is evidence of exfoliation of the GO sheets, and its disappearance in the GOP10 composite spectrum indicates the complete exfoliation of GO under highest PANI loading. Addition of Laponite as a dispersing and templating agent preserves the two-dimensional nanoarchitecture of the GOP composite crucial for supercapacitive behavior, as no new peaks appear in the diffraction patterns. Moreover, a similar shift to lower angle upon increased PANI loading is seen in the GOPL composites. The GOPL80 pattern has a major peak at 2θ = 12.9° (d = 8.0 Å) as in GOP80. This peak shifts to 12.6° (d = 8.2 Å) in GOPL50, with complete exfoliation in GOPL10. Laponite nanodisks may compete with GO for PANI heteronucleation and polymerization sites, leading to a smaller amount of PANI insertion between GO sheets. The D
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The morphology of GO, GO with Laponite and PANI, and their composites were further characterized using SEM (Figure 5). Figure 5a shows nano- and microsheets of GO ranging in size from 100 nm to 150 μm, with a terraced morphology indicative of lamellar stacking. The GO/Lap image (Figure 5b) shows Laponite nanodisks (100 nm diameter) stippling the GO sheet surface. These nanoparticles appear white and are abundant in the upper portion of the image. The PANI image (Figure 5c) reveals a microweb of fibrous PANI consistent with the X-ray and N2 physisorption analyses and typical for the morphosynthetic conditions used.14 PANI fibers range from 20 to 100 nm in diameter and extend up to 500 nm in length. Figure 5d−f show SEM images of GOP80, GOP50, and GOP10 composites, respectively. GOP80 and GOP50 show preservation of lamellar stacking of GO important for supercapacitive response, with no visible exfoliation arising from polyaniline growth. As PANI content increases, surface polymer growth is increasingly evident. While GOP80 shows a GO crystallite surface with little evidence of polymeric coverage, in GOP50 the templating influence of GO leads to irregular spherical agglomerates of PANI residing on the surface of the GO microsheets.50 As PANI loading is increased to 90 wt % for GOP10 (Figure 5f), the polymer forms a fibrous web as seen for pure PANI growth, with complete coverage of the GO surface and clear evidence of exfoliation of GO sheets in the bottom of the image. SEM images of the GOPL composites are shown in Figure 5g−i and demonstrate that incorporation of Laponite is not deleterious to the lamellar stacking of GO. In contrast to GOP80, nucleation of PANI spheroids is apparent on the surface of GOPL80 in the bottom left of the image, consistent with X-ray evidence supporting less intercalation due to the templating effects of Laponite nanoparticles and competition for nucleation sites during polymerization. In addition, the dispersing capabilities of Laponite lead to a more even distribution of polymer growth on the surface of GO terraces in GOPL50, rather than the phase-separated polymer aggregates in GOP50. The intimate mixing of GO and PANI is maintained in GOPL10. Rather than the fibrous overgrowth of PANI in GOP10, the GOPL10 composite system shows clear evidence of a more wedded, sponge-like composite composed of wellmixed graphitic and polymeric morphologies in which the lateral stacking of GO sheets and fibrous polyaniline is maintained. In addition, the extent of GO exfoliation is dramatically decreased in the GOPL10 composite in Figure 5i, and the polyaniline fibers show a decrease in thickness and length. This is concomitant with growth of a polymer network containing higher pore volume in the microporous regime as indicated by the N2 physisorption data in Figure 4. Water-Processable GOPL Composite Films. Morphology and Spatial Distribution of Materials. Energy-dispersive X-ray spectroscopy and elemental mapping were used to verify the presence of all three materials in the composite films (Figure 6). The EDX spectrum shows characteristic Kα X-ray emission lines of carbon, nitrogen, oxygen, sodium, magnesium, and silicon. The presence of Na, Mg, and Si peaks provides direct confirmation of the presence of Laponite in the composite after material washing and processing. The digital and SEM images inset in the EDX spectrum show the macroscopic appearance and morphology of the film on the micron scale. EDX elemental maps show the spatial distribution
Table 1. Surface Area and Porosity of GO, PANI, Lap, and Their Powder Composites sample
SBET (m2/g)
ϕp (nm)
Vp (cm3/g)
GO PANI Lap GOP80 GOP50 GOP10 GOPL80 GOPL50 GOPL10
210 70 370 50 10 30 130 110 150
11 20 3.7 8.4 49 35 2.8 3.0 6.2
0.13 0.35 0.24 0.04 0.05 0.26 0.09 0.08 0.30
composite (from 10 to 110 m2/g), and a 5-fold increase for the highest polyaniline loading (from 30 to 150 m2/g). This effect, coupled with a concomitant decrease in ϕP and increase in VP, indicates that the addition of Laponite nanoparticles leads to formation of a more fibrous polymer morphology. This is supported by the N2 physisorption isotherms and BJH poresize distributions for GOP10 and GOPL10 (Figure 4), where
Figure 4. Nitrogen adsorption−desorption isotherms for GOP10 (●) and GOPL10 (⧫). Open points represent desorption isotherms. Inset: corresponding BJH pore size distributions determined from the adsorption isotherm.
the large increase in adsorbed volume at high P/P0 (0.9−1.0) indicates the presence of mesopores and macropores.49 This increase is significantly larger in GOP10 than in GOPL10. In addition, the pore-size distributions (inset) show a sharp increase in pore volume associated with the microporous regime in GOPL10, in agreement with a more fibrous, higher surface-area polyaniline morphology. Zhang et al. have attributed low current densities and specific capacitance values in their GO/PANI composites to low surface areas.16 The formation of an electrical double-layer and faradaic redox processes both occur at or across interfaces. The inclusion of Laponite nanoparticles offers a simple synthetic means of significantly increasing the surface area of GOP composites, leading to greater potential for implementation in charge storage applications. E
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Figure 5. SEM images of (a) GO, (b) GO/Lap, (c) PANI, (d) GOP80, (e) GOP50, (f) GOP10, (g) GOPL80, (h) GOPL50, and (i) GOPL10.
of carbon, nitrogen, and silicon. The uniform distribution of all three of these elements provides strong evidence of a fully integrated composite material, with minimal aggregation and phase separation of any component. Structure of Composite Films. Powder XRD patterns of GOPL films are presented in Figure 7 showing the characteristic diffraction peak of the GO (001) plane. In GOPL80 this peak is centered at 2θ = 13.9°, corresponding to an interlayer dspacing of 7.4 Å. This angle decreases to 13.5° in the GOPL50 film, indicating a larger d-spacing of 7.6 Å between GO sheets. In the GOPL10 composite film, this peak occurs at 2θ = 12.7°, with a corresponding d-spacing of 8.1 Å. This peak was notably absent in the GOPL composite powders, and this was attributed to complete exfoliation of the GO sheets.
The presence of this peak here indicates that exploitation of the dispersive and templating properties of Laponite nanoparticles leads to a more ordered nanocomposite film. This action likely improves interfacial contact within the composite film, with implications for improved performance in device applications. In addition, the composite film d-spacing values are less than their powder counterparts by 0.6 Å for GOPL80 and GOPL50. This further supports the theory that Laponite is affecting interactions at the GO−PANI interface, in this case resulting in an overall improvement in wedding of the materials as shown in Figure 6c. It is well-known that electroactive polymers display inhomogeneous changes in volume upon charge cycling, which may result in premature degradation of the composite. 51,52 The tightening effect of Laponite F
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Figure 6. EDX spectrum and elemental maps of GOPL10 film.
(Figure 8). The curve for GO shows the quinone/hydroquinone redox couple typical for oxygenated carbon materials.16 The sharp increase in current from 0.6 to 0.8 V is due to reaction of the electrolyte. The PANI voltammogram contains two peaks evident at approximately 0.3 V (present as a shoulder) and 0.5 V in the oxidizing sweep. These correspond respectively to the redox transitions from the leucoemeraldine semiconducting state to the conducting emeraldine state and from the emeraldine state to the fully oxidized pernigraniline state.53 The CV curves for all composites are rectangular in shape, indicating increased efficiency of charge transport within the composite materials compared to pure GO or polyaniline. In general, increased polyaniline loading in the composites leads to higher specific current measured. The shifts observed in both reduction and oxidation potentials within the composite materials have been previously attributed to strong interactions between PANI and GO.39 There was no significant decrease in specific current observed over 10 cycles for any of the composites. It is clear from the voltammetry data that the inclusion of Laponite nanoparticles does not prohibit the use of polyaniline/graphene oxide composite materials for electrochemical and energy applications involving redox cycling and charge transport. Conductive AFM. Local structure and electronic properties were investigated through conductive AFM (cAFM) experiments (Figure 9). Tapping mode height profiles of GOPL10 are presented in Figures 9a and 9b, showing surface morphology of this composite. In Figure 9a, agglomerates of nanofibrous PANI on GO surfaces are evident with diameters on the order of 100 nm. In Figure 9b, Laponite nanodisks with diameters ranging from 25 to 40 nm are dispersed across the composite surface. There is clear evidence of both stacked and unstacked Laponite nanoparticles, given the range in height from 1 to 20 nm. The local electrical transport properties of the GOPL10 film were studied by employing cAFM to conduct CVC experiments, shown in Figure 9c. Application of an electrical potential
Figure 7. Powder X-ray diffraction patterns of (a) GOPL80, (b) GOPL50, and (c) GOPL10 films.
demonstrated here is expected to ameliorate this effect by locking the composite materials more firmly in place, thereby increasing the functional lifetime of devices. Electrochemical Properties of GOPL Composites. Cyclic Voltammetry. Bulk electrochemical performance of GO, PANI, and GOPL composites were studied using CV G
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Figure 8. Cyclic voltammograms of (a) GO, (b) PANI, (c) GOPL80, (d) GOPL50, and (e) GOPL10 in 2 M H2SO4 at a scan rate of 100 mV/s.
Figure 9. AFM tapping mode images of GOPL10 showing (a) PANI polymer growth and (b) Laponite nanodisks. (c) Local electron transport properties of GOPL10. Contact mode (d) topography and (e) corresponding cAFM current map for GOPL80 with a 2.5 V applied bias. H
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conductive response to an applied tip bias. Our work herein supports key efforts by Shen and co-workers, who have shown improvements in asymmetric supercapacitor design by tuning the interfacial polymerization of polyaniline on graphene nanosheets.58 A better understanding of the specific interactions (electrostatic, π−π stacking, hydrogen bonding)21,59,60 that mediate this self-assembly will inform new strategies to direct the stacking of graphene sheets to realize supercapacitive activity. Importantly, the incorporation of Laponite nanoparticles did not result in the loss of electrochemical and redox properties of the GOP system. CV and cAFM studies showed that at both the bulk and local levels there were CVC responses indicating stable redox cycling necessary for electrochemical and energy applications dependent on charge transfer processes. This water-processable system may support future roll-to-roll production methods of film preparation common in industrial device fabrication.
between the AFM tip and the composite material surface produces a current on the order of tens of nA entering the tip. There is a nonlinear current response with an applied bias of 0−1 V, typical of semiconductor materials, followed by a linear current response at applied biases larger than 1 V due to formation of the conductive polyaniline state. This CVC response shows that even under experimental conditions widely used to produce polyaniline in the conductive emeraldine salt form, local domains exist where the polymer is present in varied oxidation states. This has been previously reported for polyaniline films, where higher conductivity has been associated with shorter synthesis times.54 Figure 9 shows contact mode topography (d) and current (e) images obtained simultaneously on a 1 μm by 1 μm area of the GOPL80 film. This topographical map shows PANI growth on the surface of GO as a ridge up to 300 nm in step-height running vertically through the center of the image. The current map was acquired with an applied bias of 2.5 V and shows localized regions of high conductivity along the polyaniline nanoridge shown in the topographical map as well as in globular domains of varying sizes across the full map area. It is clear that even with the lowest polyaniline loading, conductive polymeric growth is observed to be well distributed at nanometer length scales. This type of distribution is necessary to enable energy applications involving charge separation processes. Indeed, the electric properties of localized polyaniline nanostructures have been shown previously to have a significant dependence on the aggregation, morphology, and connectivity of polymer chains.55−57 The cAFM studies reported here extend these insights to an important new class of water-processable carbon-based nanocomposites which show great promise for energy applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] (M.E.H.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Science Foundation (CMMI-1342577, DMR-1229142, and EEC-0939322). We thank Rebecca Cortez, Palmyra Catravas, Jared Mondschein, Yixin Hu, James McGarrah, and Mary Carroll for materials and characterization support.
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CONCLUSIONS Xu et al. have constructed hierarchical GOP nanocomposites and controlled nanowire arrays by vertically aligning polyaniline on GO nanosheets.24 Control of dimensionality in these and related systems has been shown to have a strong influence on capacitive response. To this end, we have successfully used Laponite to cast stable and homogeneous films of GOPL nanocomposites, increasing their viability for implementation into devices through a facile and easily scalable waterprocessable synthesis. Infrared and Raman spectroscopy indicated successful formation of composite materials evidenced by shifts in typical absorption bands and strong interactions between GOP and Laponite domains. These methods provided strong evidence for the presence of the conductive emeraldine state of polyaniline and of polymer acid doping from carboxylic acid moieties of GO. XRD analyses demonstrate that at low PANI loadings lamellar nanostructures that support capacitive response are preserved while the loss of the graphitic diffraction peak at high PANI loading indicated complete exfoliation of GO domains. The incorporation of Laponite nanoparticles reestablishes the lamellar nanoarchitecture of the composite even under the highest polymer loading. Gas physisorption studies show that Laponite inclusion results in the formation of more fibrous polyaniline, leading to appreciably higher surface area composite materials. This leads to a greater interfacial area and is a key requirement for the improvement of chargetransfer processes between materials. SEM and AFM showed nanostructures and interfacial domains between component materials and the local
REFERENCES
(1) Lewis, N.; Nocera, D. Powering the planet: chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103 (43), 15729−15735. (2) Murali, S.; Dreyer, D.; Valle-Vigón, P.; Stoller, M.; Zhu, Y.; Morales, C.; Fuertes, A.; Bielawski, C.; Ruoff, R. Mesoporous carbon capsules as electrode materials in electrochemical double layer capacitors. Phys. Chem. Chem. Phys. 2011, 13, 2652−2655. (3) Hahm, M.; Reddy, A.; Cole, D.; Rivera, M.; Vento, J.; Nam, J.; Jung, H.; Kim, Y.; Narayanan, N.; Hashim, D.; Galande, C.; Jung, Y.; Bundy, M.; Karna, S.; Ajayan, P.; Vajtai, R. Carbon nanotube-nanocup hybrid structures for high power supercapacitor applications. Nano Lett. 2012, 12 (11), 5616. (4) Tan, Y.; Xu, C.; Chen, G.; Liu, Z.; Ma, M.; Xie, Q.; Zheng, N.; Yao, S. Synthesis of ultrathin nitrogen-doped graphitic carbon nanocages as advanced electrode materials for supercapacitors. ACS Appl. Mater. Interfaces 2013, 5 (6), 2241−2248. (5) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 3, 183−191. (6) Lee, C.; Wei, X.; Kysar, J.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321 (5887), 385−388. (7) Xu, Z.; Li, Z.; Holt, C.; Tan, X.; Wang, H.; Amirkhiz, B.; Stephenson, T.; Mitlin, D. Electrochemical supercapacitor electrodes from sponge-like graphene nanoarchitectures with ultrahigh power density. J. Phys. Chem. Lett. 2012, 3, 2928−2933. (8) Ni, G.; Bae, S.; Tan, C.; Kahya, O.; Wu, J.; Hong, B.; Yao, K.; Ö zyilmaz, B. Graphene-ferroelectric hybrid structure for flexible transparent electrodes. ACS Nano 2012, 6 (5), 3935−3942. (9) Dreyer, D.; Todd, A.; Bielawski, C. Harnessing the chemistry of graphene oxide. Chem. Soc. Rev. 2014, 43, 5288−5301. (10) Kuila, T.; Mishra, A.; Khanra, P.; Kim, N.; Lee, J. Recent advances in the efficient reduction of graphene oxide and its
I
DOI: 10.1021/la5046783 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir application as energy storage electrode materials. Nanoscale 2013, 5, 52−71. (11) Fan, Z.; Wang, K.; Wei, T.; Yan, J.; Song, L.; Shao, B. An environmentally friendly and efficient route for the reduction of graphene oxide by aluminum powder. Carbon 2010, 48, 1670−1692. (12) Dreyer, D.; Murali, S.; Zhu, Y.; Ruoff, R.; Bielawski, C. Reduction of graphite oxide using alcohols. J. Mater. Chem. 2011, 21, 3443−3447. (13) Bissessur, R.; Liu, P. K. Y.; White, W.; Scully, S. F. Encapsulation of polyanilines into graphite oxide. Langmuir 2006, 22 (4), 1729− 1734. (14) Tran, H. D.; D’Arcy, J. M.; Wang, Y.; Beltramo, P. J.; Strong, V. A.; Kaner, R. B. The oxidation of aniline to produce “polyaniline”: a process yielding many different nanoscale structures. J. Mater. Chem. 2011, 21 (11), 3534−3550. (15) Li, D.; Huang, J.; Kaner, R. Polyaniline nanofibers: a unique polymer nanostructure for versatile applications. Acc. Chem. Res. 2009, 42 (1), 135−145. (16) Zhang, K.; Zhang, L.; Zhao, X.; Wu, J. Graphene/polyaniline nanofiber composites as supercapacitor electrodes. Chem. Mater. 2010, 22 (4), 1392−1401. (17) Boyer, M. I.; Quillard, S.; Louarn, G.; Froyer, G.; Lefrant, S. Vibrational study of the FeCl3-doped dimer of polyaniline; a good model compound of emeraldine salt. J. Phys. Chem. B 2000, 104 (38), 8952−8961. (18) Peace, B.; Topka, M.; Skorenko, K.; Kowalski, A.; Williams, U.; Hagerman, M.; Cortez, R. Morphosynthesis and morphology characterization of aniline and aniline/Laponite films. Mater. Lett. 2011, 65 (21-22), 3208−3211. (19) Ma, H.; Li, Y.; Yang, S.; Cao, F.; Gong, J.; Deng, Y. Effects of solvent and doping acid on the morphology of polyaniline prepared with the ice-templating method. J. Phys. Chem. C 2010, 114 (20), 9264−9269. (20) Li, Z.; Zhang, H.; Liu, Q.; Sun, L.; Stanciu, L.; Xie, J. Fabrication of high-surface-area graphene/polyaniline nanocomposites and their application in supercapacitors. ACS Appl. Mater. Interfaces 2013, 5 (7), 2685−2691. (21) Wang, H.; Hao, Q.; Yang, X.; Lu, L.; Wang, X. Effect of graphene oxide on the properties of its composite with polyaniline. ACS Appl. Mater. Interfaces 2010, 2 (3), 821−828. (22) Potts, J.; Dreyer, D.; Bielawski, C.; Ruoff, R. Graphene-based polymer nanocomposites. Polymer 2011, 52 (1), 5−25. (23) Yu, P.; Li, Y.; Zhao, X.; Wu, L.; Zhang, Q. Graphene-wrapped polyaniline nanowire arrays on nitrogen-doped carbon fabric as novel flexible hybrid electrode materials for high-performance supercapacitor. Langmuir 2014, 30, 5306−5313. (24) Xu, J.; Wang, K.; Zu, S.; Han, B.; Wei, Z. Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage. ACS Nano 2010, 4 (9), 5019−5026. (25) Kumar, N.; Baek, J. Electrochemical supercapacitors from conducting polyaniline-graphene platforms. Chem. Commun. 2014, 50, 6298−6308. (26) Akcora, P.; Liu, H.; Kumar, S.; Moll, J.; Li, Y.; Benicewicz, B.; Schadler, L.; Acehan, D.; Panagiotopoulos, A.; Pryamitsyn, V.; Ganesan, V.; Ilavsky, J.; Thiyagarajan, P.; Colby, R.; Douglas, J. Anisotropic self-assembly of spherical polymer-grafted nanoparticles. Nat. Mater. 2009, 8, 354−359. (27) Dag, O.; Samarskaya, O.; Coombs, N.; Ozin, G. The synthesis of mesostructured silica films and monoliths functionalised by noble metal nanoparticles. J. Mater. Chem. 2003, 13, 328−334. (28) Kehlbeck, J.; Hagerman, M.; Cohen, B.; Eliseo, J.; Fox, M.; Hoek, W.; Karlin, D.; Leibner, E.; Nagle, E.; Nolan, M.; Schaefer, I.; Toney, A.; Topka, M.; Uluski, R.; Wood, C. Directed self-assembly in Laponite/CdSe/polyaniline Nanocomposites. Langmuir 2008, 24, 9727−9738. (29) Peace, B.; Topka, M.; Williams, U.; Skorenko, K.; Kowalski, A.; Hagerman, M.; Cortez, R. Atomic force microscopy examination of polymeric nanocomposite layers. Mater. Lett. 2011, 65, 3208−3211.
(30) Mondschein, J.; Kowalski, A.; Kehlbeck, J.; Hagerman, M.; Cortez, R. Comparitive AFM studies of water processable polyaniline films: influence of reaction time on nanomorphology and conductivity. Mater. Lett. 2014, 131, 262−265. (31) Alhassan, S.; Qutubuddin, S.; Schiraldi, D. Graphene arrested in Laponite-water colloidal glass. Langmuir 2012, 28, 4009−4015. (32) Yoo, J.; Lee, S.; Lee, C.; Hwang, S.; Kim, C.; Fujigaya, T.; Nakashima, N.; Shim, J. Graphene oxide and Laponite composite films with high oxygen-barrier properties. Nanoscale 2014, 6, 10824−10830. (33) Marcano, D.; Kosynkin, D.; Berlin, J.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L.; Lu, W.; Tour, J. Improved synthesis of graphene oxide. ACS Nano 2010, 4 (8), 4806−4814. (34) Sing, K.; Everett, D.; Haul, R.; Moscou, L.; Pierotti, R.; Rouquérol, J.; Siemieniewska, T. Reporting physisorption data for gas/ solid systems. Pure Appl. Chem. 1985, 57 (4), 603−619. (35) Sariciftci, N. S.; Kuzmany, H.; Neugebauer, H.; Neckel, A. Structural and electronic transitions in polyaniline: a Fourier transform infrared spectroscopic study. J. Chem. Phys. 1990, 92 (7), 4530−4539. (36) Pálková, H.; Madejová, J.; Zimowska, M.; Serwicka, E. Laponitederived porous clay heterostructures: II. FTIR study of the structure evolution. Microporous Mesoporous Mater. 2010, 127 (3), 237−244. (37) Breen, C.; Zahoor, P. Characterization and catalytic activity of acid-treated, size-fractionated smectites. J. Phys. Chem. B 1997, 101, 5324−5331. (38) Sarker, A. K.; Hong, J. Layer-by-layer self-assembled multilayer films composed of graphene/polyaniline bilayers: high-energy electrode materials for supercapacitors. Langmuir 2012, 28 (34), 12637−12646. (39) Vallés, C.; Jiménez, P.; Muñoz, E.; Benito, A. B.; Maser, W. K. Simultaneous reduction of graphene oxide and polyaniline: dopingassisted formation of a solid-state charge-transfer complex. J. Phys. Chem. C 2011, 115 (21), 10468−10474. (40) Tuinstra, F.; Koenig, J. Raman spectrum of graphite. J. Chem. Phys. 1970, 53 (3), 1126−1130. (41) Voggu, R.; Das, B.; Rout, C.; Rao, C. Effects of charge transfer interaction of graphene with electron donor and acceptor molecules examined using Raman spectroscopy and cognate techniques. J. Phys.: Condens. Matter 2008, 20 (47), 1−5. (42) Liu, Z.; Wang, Z.; Yang, X.; Ooi, K. Intercalation of organic ammonium ions into layered graphite oxide. Langmuir 2002, 18 (12), 4926−4932. (43) Hagerman, M. E.; Salamone, S. J.; Herbst, R. W.; Payeur, A. L. Tris(2,2′-bipyridine)ruthenium(II) cations as photoprobes of clay tactoid architecture within Hectorite and Laponite films. Chem. Mater. 2003, 15 (2), 443−450. (44) Jiménez, P.; Castell, P.; Sainz, R.; Ansón, A.; Martínez, M. T.; Benito, A. M.; Maser, W. K. Carbon nanotube effect on polyaniline morphology in water dispersable composites. J. Phys. Chem. B 2010, 114 (4), 1579−1585. (45) Pouget, J. P.; Josefowicz, M. E.; Epstein, A. J.; Tang, X.; MacDiarmid, A. G. X-ray structure of polyaniline. Macromolecules 1991, 24 (3), 779−789. (46) McAllister, M. J.; Li, J.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem. Mater. 2007, 19 (18), 4396−4404. (47) Shen, J.; Hu, Y.; Shi, M.; Lu, X.; Qin, C.; Li, C.; Ye, M. Fast and facile preparation of graphene oxide and reduced graphene oxide nanoplatelets. Chem. Mater. 2009, 21 (15), 3514−3520. (48) Herrera, N.; Letoffe, J.; Putaux, J.; David, L.; Bourgeat-Lami, E. Aqueous dispersions of silane-functionalized Laponite clay platelets. A first step toward the elaboration of water-based polymer/clay nanocomposites. Langmuir 2004, 20 (5), 1564−1571. (49) Bippus, L.; Jaber, M.; Lebeau, B. Laponite and hybridsurfactant/Laponite particles processed as spheres by spray-drying. New J. Chem. 2009, 33, 1116−1126. (50) Xu, G.; Wang, N.; Wei, J.; Lv, L.; Zhang, J.; Chen, Z.; Xu, Q. Preparation of graphene oxide/polyaniline nanocomposite with J
DOI: 10.1021/la5046783 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir assistance of supercritical carbon dioxide for supercapacitor electrodes. Ind. Eng. Chem. Res. 2012, 51, 14390−14398. (51) Kötz, R.; Carlen, M. Principles and applications of electrochemical capacitors. Electrochim. Acta 2000, 45, 2483−2498. (52) Gurunathan, K.; Murugan, A. V.; Marimuthu, R.; Mulik, U. P.; Amalnerkar, D. P. Electrochemically synthesised conducting polymeric materials for applications towards technology in electronics, optoelectronics and energy storage devices. Mater. Chem. Phys. 1999, 61 (3), 173−191. (53) Kumar, N.; Choi, H.; Shin, Y.; Chang, D.; Dai, L.; Baek, J. Polyaniline-grafted reduced graphene oxide for efficient electrochemical supercapacitors. ACS Nano 2012, 6 (2), 1715−1723. (54) Mondschein, J.; Kowalski, A.; Kehlbeck, J.; Hagerman, M.; Cortez, R. Comparative AFM studies of water processable polyaniline films: influence of reaction time on nanomorphology and conductivity. Mater. Lett. 2014, 131, 262−265. (55) Wu, C.; Chang, S. Nanoscale measurements of conducting domains and current-voltage characteristics of chemically deposited polyaniline films. J. Phys. Chem. B 2005, 109, 825−832. (56) Khalid, M.; Tumelero, M.; Zoldan, V.; Pla Cid, C.; Franceschini, D.; Timm, R.; Kubota, L.; Moshkalev, S.; Pasa, A. Polyaniline nanofibers-graphene oxide nanoplatelets composite thin film electrodes for electrochemical capacitors. RSC Adv. 2014, 4, 34168−34178. (57) Chang, S.; Wu, C. Polymerization media on the nanoscale conductivity and current-voltage characteristics of chemically synthesized polyaniline films. J. Phys. Chem. B 2005, 109, 18275−18282. (58) Shen, J.; Yang, C.; Li, X.; Wang, G. High-performance asymmetric supercapacitor based on nanoarchitectured polyaniline/ graphene/carbon nanotube and activated graphene electrodes. ACS Appl. Mater. Interfaces 2013, 5 (17), 8467−8476. (59) Tran, H.; Wang, Y.; D’Arcy, J.; Kaner, R. Toward an understanding of the formation of conducting polymer nanofibers. ACS Nano 2008, 2 (9), 1841−1848. (60) Compton, O.; Cranford, S.; Putz, K.; An, Z.; Brinson, L.; Buehler, M.; Nguyen, S. Tuning the mechanical properties of graphene oxide paper and its associated polymer nanocomposites by controlling cooperative intersheet hydrogen bonding. ACS Nano 2012, 6 (3), 2008−2019.
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DOI: 10.1021/la5046783 Langmuir XXXX, XXX, XXX−XXX
