Multifunctional Hybrid Materials Based on Carbon Nanotube

Nov 20, 2013 - Multifunctional Hybrid Materials Based on Carbon Nanotube Chemically Bonded to Reduced Graphene Oxide ... *E-mail [email protected]; ...
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Multifunctional Hybrid Materials Based on Carbon Nanotube Chemically Bonded to Reduced Graphene Oxide Moumita Kotal and Anil K. Bhowmick* Department of Chemistry, Indian Institute of Technology, Patna 800013, India S Supporting Information *

ABSTRACT: A novel chemical approach is explored to design three-dimensional porous network of reduced graphene oxide (GO)/multiwalled carbon nanotube (MWCNT) hybrid from reduced GO connected to MWCNT by sp2 carbons. The process involved simultaneous functionalization, reduction, and stitching of GO by p-phenylenediamine and subsequent diazotization followed by C−C coupling with MWCNT. For comparison, a physical mixture was also prepared. The resulting hybrids were characterized by infrared, Raman, UV−vis, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and scanning tunneling microscopy. The chemical hybrid was shown to exhibit good electrical conductivity (210.48 S m−1), promising specific capacitance (277 F g−1) even at high current density (10 A g−1), remarkable energy density (21.32 W h kg−1) especially at high power density (3.13 kW kg−1), outstanding cyclability (89%) even after 2000 cycles, and good dye adsorption capacity (245 mg g−1) for crystal violet owing to its extended conjugated network, larger surface area, and porous structure. Therefore, the hybrid is attractive for supercapacitors, dye removal, and other electronic applications.

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graphene),17,18 in situ growth of hybrids on Cu foil coated with an Fe catalyst layer,19 and microwave plasma chemical vapor deposition.20 Although these methods have been established to fabricate hybrids with outstanding properties, most of these hybrids require multiple synthesis steps and represent a dense CNT layer. These limitations can only be defeated by establishing a covalent C−C bonding between these two materials owing to the availability of full potential held by CNT−graphene hybrids. Although different CVD techniques formed covalent C−C bonding between them, they have some serious drawbacksone was the requirement of long processing times and high growth temperatures (typically 700−1000 °C) resulting in a high power consumption, and the other was the unstable catalyst nanoparticles on graphene or metal substrates for CNT growth during CVD. Therefore, the formation of covalent C−C bonding between these two graphene derivatives is still a great challenge in nanoscience and nanotechnology, which in turn may facilitate them to be more intimate resulting in increased surface area, decreased interfacial resistance, enhanced electron and ion transport, and thereby leading to improved performance of the hybrid nanostructures. Also, one of the ideas is to break the layers of graphene by CNT to utilize full potential of the graphene structure.

INTRODUCTION Owing to superior electrical and thermal conductivity, high specific surface area, sufficient porosity, high electrochemical and thermal stability, graphene sheets, and their tubular variants, carbon nanotubes (CNTs) have gained importance in a wide variety of applications like supercapacitors, batteries, field effect transistors, polymer composites, solar cells, actuators, and chemical- and biosensors.1−3 Mostly, the geometric structure of graphene is fundamental to its unique electronic properties including zero band gap, nonchirality, quantum Hall effect, high Fermi velocity, long spin relaxation lengths, and charge transport on a scattering free platform.4,5 Being mutually complementary in both structure and few properties, graphene and CNTs have their own demerits. CNTs are easy to entangle and aggregate easily through their lengthy geometric shape and large aspect ratio, while graphene tends to restack owing to their strong π−π interactions and large van der Waals force.6,7 As a result, the surface areas of CNTs and graphene are largely reduced and thereby diminishing the bulk conductivity, energy storage, and dye adsorption capacity of these carbon allotropes.8,9 One of the promising solutions may be their hybridization, which would presumably generate a new technological frontier for materials research, to give more exciting performance than individual counterparts.10,11 Many attempts have been explored to fabricate graphene/CNT hybrids including postorganization methods through layer-by-layer self-assembly,11−13 electrodeposition,14 liquid phase reaction15,16 and chemical vapor deposition (CVD) of graphene (or CNT) layers on CNTs (or © 2013 American Chemical Society

Received: September 30, 2013 Revised: November 17, 2013 Published: November 20, 2013 25865

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Scheme 1. Schematic Diagram for the Formation of Chemically Bonded rGO−MWCNT Hybrid

and MWCNT without using any catalyst and surfactant. As a result, a large scale π−π conjugated network is developed which may assist in increasing surface area, electrical conductivity, and electron and ion transport. The aim of this study is to explore the effect of chemical bonding between rGO and MWCNT on the capacitive, electrical, and dye adsorption behavior of chemically bonded rGO−MWCNT hybrid over that of physical mixture of rGO and MWCNT and their individual components. Scheme 1 illustrates the formation of chemically bonded rGO−MWCNT hybrid (GO-PPD-MWCNT hybrid). Briefly, GO and PPD were used as the precursors for the GO− PPD. The nucleophilic ring-opening reaction of epoxy group of GO by amine group of PPD could not only reduce and functionalize GO but also stitching it. The addition of NH3 solutions rendered alkaline environment useful for the nucleophilic substitution reaction. This structure facilitates the formation of 3D hybrid network by diazotization of GO−PPD and sequential C−C coupling with MWCNTs via radical generation. For comparison, physical mixtures were also prepared by simply mixing equal weight ratio using mortar− pestle.

Importantly, relative to MWCNTs, the synthesis of SWCNTs requires much tougher conditions, and the rapid connection of SWCNTs on graphene substrate is extremely difficult, leading to poor interconnection.21 On the contrary, graphene oxide (GO) is an economical precursor.6,22 However, owing to its insulating behavior and lack of porosity, the supercapacitive performance as well as dye adsorption efficiency of GO/CNT mixture was inferior to those of the reduced GO (rGO)/CNT mixture.23,24 Most of the reducing agents used so far are highly toxic, corrosive, harmful to the environment, and often damage the integral structure of GO due to bursting of H2 bubbles during reduction.25 Conversely, chemical functionalization followed by reduction of GO is shown to exhibit inferior conductivity compared to pristine graphene.26 Therefore, suitable reduction process is essential to be developed by exploring simultaneous functionalization and reduction so as to improve the electrochemical properties as well as the dye adsorption capacity of rGO. Recently, reduced GO was prepared using ethylenediamine and p-phenylenediamine (PPD), rendering high electrical conductivity.27,28 Focusing the effect of large-scale π−π conjugated structure of rGO, PPD is expected to be more efficient in reducing and functionalizing GO. Therefore, such molecules are ideally suited for coupling with MWCNT by direct covalent C−C bonding to satisfy our goal. A novel chemical approach is established in this article to design three-dimensional (3D) MWCNT decorated rGO hybrid by the formation of covalent bonding between rGO

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EXPERIMENTAL SECTION Materials. Expanded graphite powder and MWCNTs were procured from Asbury Graphite Mills Inc. and HELIX Materials Solutions Inc., respectively. MWCNTs were purified by thermal treatment at 300 °C for 1 h in air to remove the amorphous carbon. Sodium nitrate (NaNO3), potassium permanganate 25866

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(KMnO4, 98%), 30% H2O2, p-phenylenediamine (PPD), and sodium nitrite (NaNO2) were purchased from Sigma-Aldrich. Concentrated H2SO4 (98%) and ethanol (99%) were procured from Merck. All other reagents were of analytical reagent grade and used without further purification. Preparation of Simultaneously Reduced and Functionalized Graphene Oxide (GO) with PPD. Graphite oxide was synthesized from natural graphite powder by the modified Hummers method.13 Briefly, graphite powder (2.5 g) was mixed with NaNO3 (1.25 g) and H2SO4 (50 mL) under stirring in an ice bath for 30 min. Under vigorous stirring, KMnO4 (7.5 g) was slowly added into the suspension maintaining temperature below 20 °C. Then, the ice bath was removed and stirred at 35 °C for 6 h followed by dilution of the mixture and stirring at 98 °C for 2 h. In order to reduce the residual KMnO4, 30% H2O2 and deionized water were added. After that, the mixture was centrifuged at 15 000 rpm for 30 min, and the residue was washed out using deionized water. The asprepared powder (80 mg) was redispersed in deionized water and exfoliated under sonication for 1 h to get graphene oxide (GO). For the synthesis of reduced and functionalized GO with PPD, PPD (800 mg) and NH3 solution (480 μL) were added consequently. The mixture was refluxed at 95 °C under stirring for 6 h and filtered with a 0.22 μm poly(tetrafluoroethylene) (PTFE) filter paper. The obtained filtrate cake was rinsed with ethanol by using ultrasonication and then filtered. The rinsing− filtration procedures were repeated until the complete removal of physically adsorbed PPD. Finally, the obtained product (GO−PPD) was dried in a vacuum oven at 60 °C for 24 h. Synthesis of Reduced-Functionalized Graphene Oxide/MWCNT Hybrid. Briefly, diazotization reaction of amine-functionalized reduced GO was carried out so as to chemically grafted onto MWCNT via C−C coupling as shown in Scheme 1. In a typical experiment, 40% HCl was dropwise added to the mixture of GO−PPD (0.07 g) and NaNO2 (1.26 g) under stirring at 0−5 °C until the completion of reaction. Before using MWCNT for reaction, presonicated MWCNTs (0.07 g) were made slightly acidic (pH 6) by adding dilute HCl and NH3 to remove catalytic impurities. It was further introduced into the reaction mixture under stirring, and the resulting slurry was heated to 75 °C under a nitrogen atmosphere to generate the radicals which were subsequently scavenged by MWCNT during the reaction to form reduced GO−MWCNT hybrid. The resulting product was filtered using PTFE filter paper, initially washed with dilute HCl to convert any unreacted amine to water-soluble salt, and washed with water to remove any salt and undesired product. The obtained 1:1 chemically bonded reduced GO/MWCNT hybrid (Chem GO−PPD−MWCNT) was vacuum-dried for further characterization. In order to investigate the effect of chemically bonded hybrid, a 1:1 physical mixture was also prepared simply by mixing 0.05 g of MWCNT and 0.05 g of GO−PPD in a mortar−pestle. Materials Characterization. Fourier transform infrared (FITR) spectra were recorded on PerkinElmer spectrum 400 system. Raman spectra were collected on a STR500 series (Seki Technotron) with 514 nm laser source. UV−vis absorption spectra were analyzed on Shimadzu UV-2550 spectrometer. Xray diffraction (XRD) were recorded using a Rigaku TT RAX 3 diffractometer with Cu Kα radiations (35 kV, 20 mA, λ = 0.154 nm). X-ray photoelectron spectroscopic (XPS) measurements were carried out on a VG Scientific ESCA Lab II spectrometer

using electrostatic lens mode with pass energy of 160 eV. Nitrogen adsorption/desorption isotherms were performed with an accelerated surface area and porosimetry system (ASAP 2020) to measure the surface area of the samples using the Brunauer−Emmett−Teller (BET) method at 77 K. Field emission scanning electron microscopic (FESEM) images were acquired on a FESEM S4800 Hitachi while tappingmode atomic force microscopic (AFM) images taken on Agilent 5500 AFM. The chemical bonding of MWCNT on GO−PPD sheets was studied using a transmission electron microscope (JEOLJEM2100) at an acceleration voltage of 200 kV. Scanning tunneling microspic images and spectra were recorded on the Nanorev series. The bulk conductance as well as resistivity of GO−PPD and chemical hybrids was measured with the INDOSAW four-point probe resistivity meter by preparing circular pellet of 10 mm diameter. Therefore, the resistivity was measured by using the equation ρ=

F=

V × 2πS × F I × f (W / S )

(1)

ln 2 ⎧ ( D )2 + 3 ⎫ ln 2 + ln⎨ DS 2 ⎬ ⎩(S ) − 3 ⎭ ⎪

⎪

⎪

⎪

(2)

where ρ is the resistivity, V is the voltage in mV, I is the current in mA, W is the thickness of the pellet, D is the diameter of the pellet (10 mm), S is the distance between the two voltage-point probes (2.4 mm), f(W/S) is the finite thickness of the respective sample, and F is the lateral size of the sample. ρ is multiplied by F as the current-probe point is at the edges of the pellet in either case.29 Electrochemical Measurements. The electrochemical measurements were performed with a CH760D electrochemical workstation (CH Instruments) using a standard three-electrode cell setup. The hybrids (both chemically grafted and physical mixture) along with MWCNT and reducedfuctionalized GO were individually loaded on a glassy carbon electrode (GCE) acting as the working electrode. For this purpose, separately MWCNT, reduced-functionalized GO, and their hybrids were dispersed in N-methylpyrrolidinone (NMP) with 200 μL of Nafion solution by ultrasonicating for 6 min to get homogeneous slurry, of which about 40 μL of the mixtures was transferred on GCE followed by drying at 70 °C. The amount of chemical hybrid, physical mixture, MWCNT, and reduced-functionalized GO loadings were calculated to be 1.52, 1.49, 1.51, and 1.50 mg, respectively. The platinum wire and Ag/AgCl with saturated KCl solution were used as the counter electrode and the reference electrode, respectively, while 1 M aqueous H2SO4 solution used as the electrolyte. The applied potential range for cyclic voltammetry and galvanostatic charge−discharge was carried out from −0.05 to 0.7 V at varying scan rate (10−100 mV s−1) and current density (0.3− 10.0 A g−1). Electrochemical impedance spectroscopy (EIS) was recorded under the sweep frequency range of 105−0.05 Hz and ac voltage amplitude of 5 mV against open circuit potential. The specific capacitance (Cs) was calculated from the slope of discharge curve following the equation Cs = 25867

I × Δt m × ΔV

(3)

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Figure 1. (a) FTIR spectra, (b) Raman spectra, and (c) XRD patterns of GO, GO−PPD, MWCNT, chemically bonded GO−PPD−MWCNT hybrid, and physical mixture.

where I is the discharge current, Δt is the discharge time, ΔV is the discharge voltage difference excluding the IR drop, and m is the mass of the active material deposited on GCE. The imaginary capacitance (C″) was estimated from EIS using the equation

C″ =

Z′ ω |Z|2

absorbance of each dye. The amount of dye adsorbed by each samples at time t was calculated following the equation qt =

■

where ω is the angular frequency and Z′ and |Z| are the real part and magnitude of impedance, respectively. ω is given by ω = 2πf. The energy density (E) and power density (P) in the Ragone plots were calculated according to the equations 1 CsV 2 2

(5)

P=

E Δt

(6)

(7)

where C0 and Ct (mg/L) are the initial and time t concentration of the dye, respectively, while V is the volume of dye solution (L) and M is the mass of the samples (g).

(4)

E=

(C0 − Ct ) × V M

RESULTS AND DISCUSSION The reduction of oxygen-containing groups as well as the functionalization of GO by PPD and the existence of covalent bonding of GO−PPD with MWCNT in the hybrid were confirmed by various spectroscopic measurements, including Fourier transform infrared (FTIR), Raman, and X-ray diffraction spectroscopy (Figure 1). Figure 1a presents FTIR spectra of GO, GO−PPD, MWCNT, chemically bonded GO− PPD−MWCNT hybrid, and physical mixture. As expected, the absorption bands of GO appear at 3430, 1717, 1628, 1224, and 1047 cm−1 corresponding to −OH, CO in carboxyl group, CC in aromatic ring, C−OH, and C−O stretching vibrations, respectively. However, after reaction of GO with PPD, the intensities of the bands related to the oxygen functionalities are found to decrease, implying the reduction of GO by PPD. Moreover, the appearance of new peaks at 3460, 3340, 1538, and 780 cm−1 associated with −NH2 double stretching, N−H bending, and N−H wagging vibrations, respectively, suggests functionalization of GO with PPD. Interestingly, another new peak at 1175 cm−1 corresponding to C−N stretching vibration confirms the formation of C−NH−C bands due to the nucleophilic substitution of epoxide carbon in GO with −NH2 in PPD (shown in Scheme 1). These observations confirm not only the reduction of GO but also the formation of amine

Dye Adsorption Measurements. Generally, carbonaceous materials are effective in adsorbing dye from aqueous solutions. Considering this advantage, two basic dyes (rhodamine 6G and crystal violet) were investigated for their adsorption behavior with the as-prepared chemically bonded reduced GO− MWCNT hybrid, physical mixture, and their individual components for comparison. In a typical experiment, 10 mg of each sample was inserted into 100 mL of 10 mg/L aqueous dye solution under mixing in a vortex mixer at ambient temperature. At a series of intervals, the mixture was centrifuged, and the dye concentration of the supernatant was determined through a UV−vis spectrometer and calculated from the standard spectrophotometric method at the maximum 25868

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Figure 2. XPS spectra of GO, GO−PPD, MWCNT, chemically bonded GO−PPD−MWCNT hybrid, and physical mixture: (a) wide scan survey spectra, (b) C 1s spectrum of GO−PPD, (c) O 1s spectrum of GO−PPD, (d) N 1s spectrum of GO−PPD, (e) C 1s spectrum of chem GO−PPD− MWCNT, (f) O 1s spectrum of chem GO−PPD−MWCNT, and (g) N 1s spectrum of chem GO−PPD−MWCNT hybrid.

PPD includes defects at the open edges of GO−PPD sheets. However, MWCNT is made of sp2 carbons that should contribute to the G band signal while open-end carbon atoms and defects on the cylindrical carbon walls may contribute to the D band when integrating with GO−PPD sheets through C−C coupling, resulting in an increased ID/IG compared to MWCNT.31 The observed blue-shift of the band positions compared to MWCNT indicates stronger bonding between GO−PPD and MWCNT. Interestingly, the prominent 2D band in the hybrid compared to GO−PPD suggests the presence of more sp2 carbon domains after chemical bonding of GO−PPD with MWCNT.17 Furthermore, the additional peaks at 1506 cm−1 is attributed to C−C stretching of benzene ring, confirming the chemical bonding of GO−PPD with MWCNT. Our results are also in accordance with the results observed by others.20,32 On the contrary, in the case of the physical mixture, D, G, and 2D bands remain nearly at the same locations with that of MWCNT with marginally higher ID/IG (0.66) compared to MWCNT (0.63). However, the absence of the peak corresponding to C−C stretching indicates negligible effect of GO−PPD in their physical mixture. UV−vis spectra of the samples (Figure S2) also support the stronger interaction of GO−PPD with MWCNT in the hybrid as discussed in the Supporting Information. Further evidence for the lattice expansion of GO by PPD intercalation and the chemical bonding of GO−PPD with MWCNT is obtained from their XRD patterns (Figure 1c). It is mentioned that pristine graphite reveals an intense crystalline peak at 26.5° corresponding to the (002) plane with a d-spacing of 0.336 nm.27 However, the peak for GO was observed at 11.5° with d spacing of 0.77 nm due to the introduction of oxygen functionalities on carbon sheets.27 After functionalization of GO with PPD, it exhibits a peak at lower diffraction angle of 7.35° (d001 = 1.20 nm) due to the stitching of individual GO layers by PPD via nucleophilic substituion reaction. MWCNT exhibits hexagonal graphitic peak at 26.2° (d002 = 0.34 nm), while the chemically bonded

functionalized GO. The amount of PPD covalently functionalized with entire GO known as percentage of PPD grafting on GO was calculated from the calibration curve of the absorbance of −NH2 stretching frequency against mmol amount of PPD. The calibration curve (Figure S1) as well the procedure is shown in the Supporting Information. From the calibration curve of PPD, 29.2% of grafting of PPD with GO was observed. Pristine MWCNT shows a weak absorption band at 1070 cm−1 and a strong absorption band at 3430 cm−1 due to C−O stretching and O−H stretching vibrations, respectively. However, after chemical bonding of MWCNT with GO− PPD, the peak related to N−H stretching of aromatic amine at 3340 cm−1 disappears, confirming the covalent functionalization of GO−PPD with MWCNT via diazotization, as shown in Scheme 1. On the contrary, in the physical mixture, the peak of N−H stretching is still present, indicating the absence of chemical bonding. As shown in Figure 1b, the Raman spectrum of MWCNT exhibits G, D, and 2D bands centered at 1344, 1571, and 2685 cm−1, respectively. In contrast, GO shows prominent D and G bands at 1350 and 1591 cm−1. The appearance of G and D peaks in higher regions with respect to those of graphite (D: 1343 cm−1 and G: 1567 cm−1) indicates the destruction of the sp2 character and the formation of defects in the graphene sheets due to extensive oxidation.30 After reaction with PPD, D and G bands of GO are shifted to 1340 and 1570 cm−1, and the additional peaks at 1490 and 1269 cm−1 due to C−C stretching of benzene ring and C−N stretching suggest successful functionalization of GO with PPD. Interestingly, the intensity ratio of D to G band (ID/IG) of GO (0.97) decreases for GO−PPD (0.92), inferring deoxygenation of GO by PPD and thereby acting as reducing agent, in line with earlier observations.27,28 After hybridization through chemical bonding of GO−PPD with MWCNT, D, G, and 2D bands appear at 1355, 1582, and 2694 cm−1 with lower ID/ IG (0.85) compared to that of GO−PPD (0.92) and higher compared to MWCNT (0.63). The disordered carbon in GO− 25869

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Figure 3. FESEM images of (a) GO−PPD (inset shows wrinkled GO sheets), (b) chem GO−PPD−MWCNT hybrid (inset shows individual dispersion of MWCNT on GO sheets), and (c) TEM image of chem GO−PPD−MWCNT hybrid showing the inherent bonding of GO−PPD with MWCNT and STM imaging of (d) GO-PPD, (e) chem GO−PPD−MWCNT hybrid (set point: 210 pA) on HOPG, and (f) representative STS I− V spectra.

confirming the covalent functionalization of PPD with GO. As PPD is bifunctionalized, therefore, half of the N/C atomic ratio (3.8453/84.26) participates in grafting with GO. Therefore, the grafting level was calculated based on the atomic percentage value for nitrogen and carbon atoms, and the percentage of grafting was calculated to be 26%, supporting the value obtained from FTIR analysis. The core level C 1s spectrum of MWCNT (Figure S3c) presents three peaks at 284.6, 285.9, and 286.8 eV corresponding to C−C, defects on MWCNT, and −C−O, respectively. In contrast, in chemical hybrid (Figure 2e−g), O/C decreased to 0.038 whereas N/C decreased to 0.028−0.03 due to the chemical bonding of MWCNTs with GO−PPD. Interestingly, the peak at 285.7 eV arises in the hybrid due to the presence of the C−N bond, which is absent in MWCNT, confirming the chemical bonding of GO−PPD with MWCNT. Additionally, in the N 1s spectrum, the vanishing of 402.0 eV peak corresponding to diazonium salt and appearance of peak at 400.2 eV due to C−N and 406.0 eV for unreacted absorbed NO2 on the hybrid surface further indicated that diazotized GO reacted with MWCNT via C−C coupling reaction.33 On the contrary, in the physical mixture (Figure S3e−g), O/C and N/C slightly decreased to 0.064 and 0.056, respectively. Interestingly, the peak at 401.4 due to terminal −NH2 confirms the absence of chemical bonding. A salient difference of morphology between GO and GO− PPD sheets is observed. The surface of GO sheets is almost smooth (Figure S4a), whereas GO−PPD exhibits a threedimensional network structure composed of hierarchical pores: macropores with 1−2 μm in diameter formed by the side walls of wrinkled GO sheets and mesopores in few nanometers diameter formed by GO sheets (shown by the arrow in Figure 3a). Such porous structure may be due to the presence of phenyl groups of PPD which act as spacers through covalent linking between the GO sheets. The appearance of wrinkles shown in the inset of Figure 3a and TEM image of GO−PPD (Figure S5a) further confirms face-to face stacking of GO

hybrid and the physical mixture show intense peak at 26.1° and 26.2° with slightly broadening nature. These findings are further supported by their crystallite size which is calculated from the Scherrer formula to be 3.4, 3.8, and 4 nm for chemically bonded hybrid, physical mixture hybrid, and MWCNT, respectively. The sharp peak is due to the presence of MWCNT whereas the broadening is attributed to the loose stacking of GO−PPD sheets by MWCNT. The aim of XPS analysis was to evaluate the elemental compositions and functionalities of the hybrids, thereby providing a better understanding of their surface chemistries that are directly correlated to electrical properties. Figure 2a represents the XPS survey spectra for GO, GO−PPD, MWCNT, chemically bonded GO−PPD−MWCNT hybrid, and their physical mixture. Compared with GO, GO−PPD, chemical hybrid, and the physical mixture showed a strong suppression for the oxygen-containing groups followed by appearance of the N 1s peak. These findings clearly indicate the efficient reduction of oxygen-containing functional groups of GO by PPD and simultaneous functionalization. Further chemical bonding of MWCNT with GO−PPD is also proven from the spectra. The C 1s spectrum of GO (Figure S3a) shows three pronounced deconvoluted peaks at 284.6, 286.0, and 287.3 eV corresponding to C−C in unoxidized graphite carbon skeleton, C−OH in hydroxyl group, and C−O−C in epoxide group, respectively. The O 1s spectrum of GO (Figure S3b) shows a deconvoluted peak at 532.3 eV corresponding to −C O in GO, thus confirming successful carboxylation of graphite. More particularly, in GO−PPD (Figure 2b−d), the atomic ratio of oxygen and carbon (O/C) decreased from 0.444 for GO to 0.095, and the appearance of a new characteristic peak at 285.6 eV corresponding to C−N with the atomic ratio of nitrogen and carbon (N/C) as 0.091 suggests simultaneous reduction and functionalization of GO by PPD. Furthermore, the deconvoluted N 1s of GO−PPD exhibits two peaks at 399.2 and 401.0 eV attributed to C−N−C and −NH2, respectively, 25870

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supports the finding obtained from STS. The resistivity of the samples has been further calculated using eq 1, and thereby the electrical conductivity of GO−PPD and chemical hybrid is evaluated to be 59.53 and 210.48 S m−1, respectively. The porous attributes of the chemical hybrid along with their individual components were further confirmed by nitrogen sorption analysis as shown in Figure 4. GO−PPD, MWCNT,

sheets. Such stacking of GO sheets in GO−PPD is further confirmed by height profile AFM analysis showing its higher thickness (16.0 nm) compared to that of GO (1.0 nm) (Figure S6a,b). However, after chemical bonding of GO−PPD with MWCNT (Figure 3b and inset), the porous structure is found with individual dispersion of MWCNT on GO−PPD sheets. Moreover, TEM images of chemical hybrid (Figure S5b) show that MWCNTs are attached simultaneously at multiple GO− PPD sheets supported with the observation obtained from the AFM analysis (Figure S6c). Additionally, TEM image of chemical hybrid shows that MWCNT is chemically bonded to the host GO−PPD sheets (Figure 3c), indicating the inherent bonding of GO−PPD with MWCNT. Interestingly, the height profile AFM image showed that the thickness of the hybrid decreases (5.2 nm) compared to that of GO−PPD, suggesting the decrease in number of graphene layers (Figure S6c). This is because of the fact that grafted MWCNT prevents the restacking of graphene sheets and acts as spacers to separate sheets. On the contrary, in physical mixture (Figure S4c), few MWCNTs are dispersed on GO sheets due to the weaker interaction between them, signifying the benefit of covalent interaction of GO−PPD with MWCNT over noncovalent interaction. STM was used to study the lattice structure of the hybrid and GO−PPD as well as to investigate the effect of hybridization on conductance by measuring I−V characteristics of the samples through STS. GO−PPD (Figure 3d) represents the formation of 3-fold symmetry patterns of six carbon atoms showing different visibility within the honeycomb atomic lattice of the graphite layer. This may be due to the stacking of the graphene planes by rigid phenyl groups of PPD via covalent linkage. On the contrary, in the case of MWCNT (Figure S7a), the hexagon centers appear elongated along the tube due to the geometrical distortion arising from the locally changing tip− sample arrangement. However, the chemically bonded hybrid (Figure 3e) exhibits the distorted hexagons which are shown to overlap with 3-fold symmetry patterns of six carbon atoms in a honeycomb atomic lattice inferring the presence of C−C bonding connection between GO−PPD and MWCNT. In contrast, no such overlapped structure is detected in physical mixture (Figure S7b); the well-separated elongated hexagons and the stacking of GO−PPD sheets are clearly visible. Representative I−V curves obtained by STS were averaged for each sample and are shown in Figure 3f. It infers that the electrical conductance in the chemical hybrid is higher compared to GO−PPD due to the formation of extended conjugated network through chemical bonding of GO−PPD with MWCNT. These observed conductance correlated nicely with the calculated band gap (Figure S8) which revealed a systematic lowering of the gap between the occupied and unoccupied states in the series GO−PPD (0.2611 eV), physical mixture (0.1131 eV), chemically bonded hybrid (0.0975 eV), and MWCNT (0.0731 eV). STS measures a local spot conductance of the samples, and hence, the bulk conductance as well as resistivity of GO−PPD and chemical hybrids has been measured with the four-point probe resistivity meter by preparing circular pellet of 10 mm diameter. The corresponding I−V plots for GO−PPD and the chemical hybrid are shown in Figure S9. The linear I−V relationship was observed for both GO−PPD and the chemical hybrid although the conductance was found to be higher for the hybrid (0.237 80 S) compared to that of GO−PPD (0.043 06 S) due to the increase of number of mobile electrons through extended conjugation formed by chemical bonding of MWCNT with GO−PPD,34 which also

Figure 4. Nitrogen adsorption−desorption isotherms of MWCNT, GO−PPD, physical mixture, and chemical hybrid.

physical mixture, and the chemical hybrid showed a typical IUPAC type I−V curve characteristic indicating several mesopores in the samples, supporting earlier observations from FESEM. N2 adsorption amount of chemical hybrid was higher than those of their individual counterparts due to the coupling reaction of GO−PPD with MWCNT. The surface area of the chemical hybrid is 210 m2 g−1, which is higher than that of GO−PPD (160 m2 g−1) and significantly higher than that of MWCNT (79 m2 g−1) and physical mixture (100 m2 g−1) obtained by fitting the isotherm to the BET model. The increased surface area of the hybrid is ascribed to the synergistic effect of GO−PPD and MWCNT through chemical bonding. The insertion of phenyl groups of PPD and MWCNT can bridge the adjacent reduced GO sheets by covalent linking, preventing restacking and thereby leading to increase of the surface area. Such a high surface area of the chemical hybrid is very useful for better supercapacitive and adsorption performance as high surface area is suitable for more surface contact with electrolyte and dye, respectively. The 3D porous network as discussed above was designed for suprcapacitor applications. The measured voltammetry curves (Figure 5a,b) show rectangular-like shape, attractive for capacitor applications. For MWCNT, no peaks were found showing the typical electrical double-layer capacitor behavior. However, broad peak characteristics were observed for all other samples except MWCNT, corresponding to redox reactions of surface functionality of PPD and oxygen groups in the samples which provide pseudocapacitance behavior. Interestingly, the area of the CV curve is higher in GO−PPD compared to MWCNT and even higher than that of the physical mixture. The redox peaks of PPD give the pseudocapacitive effect in GO−PPD, responsible for higher specific capacitance compared to MWCNT and their physical mixture. Conversely, the lack of interconnected network carbon structures in physical mixture may inhibit the diffusion of fast ions, resulting in lower 25871

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Figure 5. Electrochemical performance of MWCNT, GO−PPD, physical mixture, and chem GO−PPD−MWCNT hybrid in 1.0 M H2SO4: (a) CV curves at 10 mV/s scan rate, (b) CV curves of chem GO−PPD−MWCNT hybrid at 10, 20, 50, 100, and 200 mV/s scan rate, (c) galvanostatic charge/discharge curves at 0.3 A g−1 current density, (d) specific capacitance as a function of current densities, (e) Nyquist plots, and (f) normalized C″ vs frequency plots.

specific capacitance. On the contrary, chemically bonded hybrid is shown to exhibit highest capacitance due to the amine functionalization of GO which facilitates chemical bonding with MWCNT via diazotization. Importantly, the shape of the CV curve of the chemically bonded hybrid is maintained even at high scan rate 0.2 V/s, suggesting good capacitive behavior. As a result, the obtained hybrid having interconnected network carbon structures was useful for fast ion diffusion, good electrical conductivity, and high capacitance performance even at high scan rate 0.2 V/s.11,35,36 These findings suggest the potential for the use of this new hybrid in high-performance supercapacitors. The galvanostatic charge/discharge (GCD) curves of the hybrids along with their individual component (Figure 5c) are also found to be in line with the findings from CV. The specific capacitance of the hybrids of both chemically

bonded and physical mixtures along with GO−PPD and MWCNT was calculated following eq 3 to be 390, 90, 188, and 30 F g−1, respectively, at 0.3 A g−1. Compared with the hybrid materials obtained by CVD,18 arc discharge,37 and hydrothermal route,38 the chemically bonded hybrid shows better specific capacitance. The specific capacitance of the samples at different current densities (Figure 5d) represents that ∼71% of specific capacitance of the chemically bonded hybrid is retained even at 10 A g−1. Such higher rate capability of chemically bonded hybrid than GO−PPD (which retains 46% at 10 A g−1) further implies the advantage of covalent interaction of MWCNT with GO−PPD which improves the diffusion of electrolyte ions into the interior of the hybrid electrode. This was confirmed by electrochemical impedance analysis of the samples. As shown in the Nyquist plots (Figure 5e), the 25872

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Figure 6. (a) Ragone plots of MWCNT, GO−PPD, physical mixture, and chem GO−PPD−MWCNT hybrid and (b) cycling behavior of GOPPD, physical mixture, and chem GO−PPD−MWCNT hybrid.

Figure 7. (a) Adsorption capacity of crystal violet and (b) pseudo-second-order kinetic curves of crystal violet dye by MWCNT, GO−PPD, physical mixture, and chemically bonded GO−PPD−MWCNT hybrid.

−68° with shallower slope due to pseudocapacitance effects from the PPD groups as well as the oxygen groups in GO.42,43 Using eqs 5 and 6, energy and power density were calculated, and accordingly a Ragone plot was made (Figure 6a). The maximum energy density of 30.47 W h kg−1 is achieved at power density of 115.06 W kg−1 for the chemically bonded hybrid, while the physical mixture, GO−PPD, and MWCNT exhibit 7.03, 14.84, and 2.34 W h kg−1 of energy density, respectively. More importantly, when the power density increases to 3.13 kW kg−1, the energy density of the chemically bonded hybrid is still as high as 21.32 W h kg−1, implying that the hybrid can provide high energy density and high power density concomitantly due to the significant synergistic effect of GO−PPD and MWCNT through chemical bonding. Figure 6b represents that the chemically bonded hybrid retained 89% of its initial specific capacitance even after 2000 cycles at 0.3 A g−1. Comparatively, the specific capacitance of physical mixture and GO−PPD is shown to retain 63% and 65% of capacitance at the same condition. The results demonstrate that the chemical anchoring effect through introduction of MWCNT in GO− PPD sheets via diazotization as well synergistic effect of MWCNT and GO−PPD promotes electrochemical stability of the hybrids over physical mixture. Figure 7 represents the adsorption curves of crystal violet dye with chemically bonded hybrid, physical mixture, MWCNT, and GO−PPD individually. Figure S11a displays the representative adsorption curves for rhodamine 6G dye. It is noted that the removal of dyes has attained almost equilibrium in 10 h in all cases while it takes 18 h for MWCNT. More importantly, the chemically bonded hybrid exhibits maximum

projected length of the Warburg-type line on the real axis, which characterizes the ion diffusion process from solution into the electrode structure, is shorter in chemically bonded hybrid compared to that of GO−PPD and physical mixture, confirming the fast ion diffusion from electrolyte to the hybrid and lower internal resistance of the hybrid electrode. The larger Warburg-type line is due to the poor interconnection of MWCNT with GO−PPD through mixing by mortar−pestle. Knee frequencies of 5615 Hz for MWCNT, 3174 Hz for GO− PPD, 3906 Hz for chemical hybrid, and 2686 Hz for the physical mixture were observed (Figure 5e), representing the maximum frequency at which capacitive behavior is dominant. Such ultrahigh frequency of the chemical hybrid indicates superior frequency response compared with the earlier literatures.39−41 Rapid frequency response of our fabricated supercapacitor was further confirmed by short relaxation time constant (τ0). τ0 accounts for the point of maximum energy dissipation and the transition point from capacitive to resistive behavior of the electrochemical capacitor. τ0 is estimated from the reciprocal of the frequency (f 0) at the peak in the plot of normalized imaginary capacitance (C″) against frequency (Figure 5f). The C″ which corresponds to the dielectric loss of the electrolyte occurring during movement of the molecules of electrolyte was estimated using eq 4. τ0 is calculated to be 0.0671 s for the chemical hybrid, closer to that of MWCNT (0.0178 s), indicating rapid frequency response as well as excellent power response ability of our fabricated supercapacitor. The Bode (phase−frequency) plot of the samples is further shown in Figure S10. It is noted that for frequencies up to 1.82 Hz the phase angle for chemical hybrid is about 25873

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adsorption capacity of 245 and 219 mg g−1 for crystal violet and rhodamine 6G, which are larger than those of crystal violet dye with magnetic-modified MWCNT (228 mg g−1),44 rhodamine 6G dye with cane sugar reduced graphene,45 and crystal violet lactone dye with bifunctionalized graphene.46 The adsorption capacity of the hybrid is highest in crystal violet as the order of basicity is as follows: crystal violet > rhodamine 6G. Besides, steric effects contributed by phenyl rings for rhodamine 6G also play an important role in adsorption capacity. MWCNT possesses minimum adsorption capacity in either case as it contains multiple atomic layers requiring more energy for diffusion of dye, leading to slower adsorption rate. In addition, lack of oxygen-containing groups in pristine MWCNT causes poor affinity toward basic dyes, while all other samples have higher O content (as evident from XPS analysis) and are favorable for the basic dye adsorption by electrostatic interaction. However, the chemically bonded hybrid exhibit highest adsorption due to the strong π−π interaction between the extended delocalized π electron of GO−PPD with MWCNT surface through chemical bonding and the free electrons of dye molecules (aromatic rings, −NC−CC− bonds). Therefore, the adsorption behavior of dyes with the hybrid is the result of the synergistic effect of π−π and electrostatic interactions. The adsorption process of dyes with the chemically bonded hybrid, physical mixture, and their individual components fitted well with pseudo-second-order kinetic model (Figure 7b and Figure S11b).

and adsorption capacity of rhodamine 6G and their pseudosecond-order kinetic curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

*E-mail [email protected]; Tel (+91-6122) 277380; Fax (+916122) 277384. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge to IIT Patna, India, for the financial support. REFERENCES

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CONCLUSIONS In summary, three-dimensional hierarchical porous network of reduced GO/MWCNT hybrid comprised of reduced, functionalized GO directly chemically bonded to MWCNT was successfully developed. The hybrid was synthesized using PPD as simultaneous functionalizing and reducing agent for GO followed by diazotization of the terminal amine of the functionalized stitched GO and subsequently C−C coupling with MWCNTs. Unlike the use of any catalyst or surfactant, the guest MWCNT was directly bonded to host GO−PPD by sp2 carbons forming interconnected carbon structures of welldefined pores. Owing to the high conductance of GO−PPD, the fully accessible surface, extended conjugated network with well-defined pores, the obtained hybrid exhibits good electrical conductivity, attractive specific capacitance even at high current density, remarkable energy density especially at high power density, outstanding cyclability even after 2000 cycles, excellent dye adsorption capacity for the application in supercapacitors, removal of dyes, and other electronic applications. Therefore, this research provides a new route involving a simple, costeffective, and environment friendly strategy to access a hybrid without using metal electrocatalyst with a high activity under mild conditions.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

Calibration curve of PPD, UV−vis spectra of the samples, XPS spectra of C 1s and O 1s spectrum of GO, C 1s and O 1s spectrum of MWCNT, C 1s, O 1s, and N 1s spectrum of physical mixture, FESEM images of GO and MWCNT, TEM images of GO−PPD and chemical hybrid, AFM images of GO, GO−PPD, and chemical hybrid and their corresponding height profiles, STM images of MWCNT and physical mixture, d[ln I]/d[ln V] against V curves, I−V curves of GO−PPD and chemical hybrid, Bode (phase−frequency) plot of the samples, 25874

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