Transparent, Conductive, and Flexible Multiwalled Carbon Nanotube

Feb 9, 2012 - Jesus Garoz-Ruiz , David Ibañez , Edna C. Romero , Virginia Ruiz , Aranzazu Heras , Alvaro Colina. RSC Advances 2016 6 (37), 31431-3143...
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Transparent, Conductive, and Flexible Multiwalled Carbon Nanotube/Graphene Hybrid Electrodes with Two Three-Dimensional Microstructures Luowen Peng, Yiyu Feng, Peng Lv, Da Lei, Yongtao Shen, Yu Li, and Wei Feng* School of Materials Science and Engineering, Tianjin University, and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, P.R. China S Supporting Information *

ABSTRACT: Transparent, conductive, and flexible multiwalled carbon nanotube (MWCNT)/graphene hybrids with two three-dimensional microstructuresan interconnected network and a double-layer structurewere prepared. The conductivity and performance of MWCNT/graphene films can be controlled by different microstructures. A photoswitch using a layered heterostructure of a CdTe quantum dot on an interconnected MWCNT/graphene (IN-MWCNT/graphene) electrode shows an enhanced reversible photocurrent with a higher on/off ratio than that of double-layer structures (DL-MWCNT/graphene). Electrochemical capacitors using a IN-MWCNT/graphene network also exhibit an outstanding rate capability and good cycling stability due to a large surface area and high porosity. Results indicate that the IN-MWCNT/ graphene hybrid with porous structures and strong π-interaction is an excellent conductive network for multifunctional flexible devices. The performance of MWCNT/graphene hybrid films can be further optimized by the improved interface and microstructures.

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INTRODUCTION Advanced transparent, conductive, and flexible electrodes, which are lightweight, low cost, printable, mechanically strong, and compatible with substrates, are essentially important to establish the new generation of novel flexible optoelectronics ranging from conducting plastics to organic nanocomposites.1 The traditional indium tin oxide (ITO)-coated flexible electrodes are severely limited due to the deficiency of element indium, the instability at an acid environment, the susceptibility of the ion diffusion, and the poor transparency in the nearinfrared region. Therefore, much effort is spent to identify and evaluate new materials for high-quality conductive flexible electrodes rivaling ITO. Among intense attempts, carbon nanostructures, as well as nanospheres, nanotubes, and nanosheets, are addressed to be ideal candidates based on the control of electronic and optical properties, structures, processability, and compatibility.2,3 Initial progress indicates that the interconnected network of carbon nanotube (CNT) with low resistance and good transparency shows the potential application in organic lightemitting diodes (OLED), solar cells, field emission transistors, and sensors based on high functionality and various processing technologies.4,5 However, the high performance relying on the high carrier mobility and current-carrying capability of a robust individual CNT is affected by an electronic and structural complex of sidewalls, chiralities, semiconducting or metallic natures, the bandgap, and work function.6−8 Blackburn et al. © 2012 American Chemical Society

found that the conductivity and transparency of films with the precise ratios of semiconducting and metallic single-walled CNT depended on an interplay between the localized and the delocalized carriers determined by electronic properties, tubejunction, and redox dopants.8 Moreover, the low yield and labor-intensive separation or selective growth of highly purified CNTs results in the product being prohibitively expensive for widescale use or applications.2,4 The intrinsic problems of CNT strive to develop a new contendera two-dimensional (2D) graphene with a single sheet of sp2-bonded carbon atoms. The geometric structure of graphene is fundamental to its unique electronic properties including a zero bandgap, nonchirality, room-temperature Hall effect, high Fermi velocity, and charge transport on a scatteringfree platform.9,10 The extensive works on a graphene anode for OLED and solar cells have been presented.11,12 The most inspiring result found by Wu et al. was that,13 with the resistance 2 orders of magnitude higher than ITO, the graphene-based OLED exhibited the competitive external quantum efficiency and power efficiency matching ITO. Furthermore, the performance can be further improved by many orders of magnitude with the enhanced charge injection using low sheet resistance (1−10 Ω/□) and 1 nm thick single-layer graphene. Although many methods were Received: September 22, 2011 Revised: December 23, 2011 Published: February 9, 2012 4970

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applied to prepare high-quality electrodes,14,15 the solutionprocessing approach is apparently advantageous to the largescale manufacture of the conductive graphene or nanocomposite electrodes due to the broad usability for physical and chemical functionalization, processability for the film formation, no sophisticated transfer, and good compatibility.16 However, a bottleneck of solution-processed graphene oxide films is the high resistance within the industrially relevant transparency range due to the structural defects with innumerable multiple grain boundaries. Several methods such as chemical reductant, photocatalytic reduction, and heat treatment are used to restore the conjugated large-scale graphitic structures;17,18 nevertheless, few attempts achieve the original excellent performances. Moreover, the reduction results in large multilayered aggregates with poor dispersion, brittle and inflexible film, and incompatibility.19,20 Thus, a graphene nanocomposite with the enhanced conductivity is considered to be a favorable structure to form a high-quality conductive electrode. On the basis of the mutual complementarity in structures and properties,12 a three-dimensional (3D) CNT/graphene building block is regarded to be an ideal nanostructured hybrid for conductive flexible electrodes due to electronic interaction, high conductivity, transparency, and flexibility.20−26 Paulson et al. found the contact or junction resistance can be increased or decreased by more than an order of magnitude in a controlled fashion.20 Results implied a new method of controlling the interface resistance through the lattice alignment and surface properties based on the momentum conservation. Recently, Alam et al. demonstrated ultratransparent, highly conductive graphene electrodes percolation-doped by a subpercolating network of CNTs. On the basis of three different models, they showed that the sheet resistance of the CNT/graphene hybrid can be significantly improved without a loss of the transparency due to unique nanostructures.27 Furthermore, several researchers fabricated a sandwich nanostructure with CNT pillars grown on graphene for high-performance devices.23,24 Yang et al. prepared chemically reduced CNT/graphene nanocomposite flexible films by a low-temperature solution-processing method.21 Preliminary application indicated that the performance of the devices using CNT/graphene electrodes was not limited in the optimization of layer structures. Chen et al. found that the capacitor bahavior of a single-wall CNT (SWCNT)/graphene hybrid was superior to that of both SWCNT and graphene due to the porous structure and a large surface area.25,26 In spite of intriguing initial results, to our knowledge, an excellent solution-processed CNT/graphene flexible electrode for energy conversion or storage devices is still an incomplete project because of the difficulty in control of the structures and the interaction. In this paper, conductive, transparent, and flexible multiwalled CNT (MWCNT)/graphene hybrids with two micostructures were prepared. The 3D nanostructured and multifunctional MWCNT/graphene electrodes combined superior conductivities, good electrical stability over 3000 bending cycles, the mechanical flexibility, high surface properties, porous structures, and good compatibility. The performances of MWCNT/graphene electrodes for photoswitches and supercapacitors were studied using a CdTe quantum dot (QD) with high optical absorption28 and a charge-storage conducting polymer (CP), poly(3,4-ethylenedioxythiophene) (PEDOT),29 respectively. The charge transfer across the interface of 3D interconnected electrodes with high porosity resulted in the increased photoresponse and high capacitance with good cycling stability.

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EXPERIMENTAL SECTION

Uniform MWCNT/Graphene Oxide Solution. Singlelayer graphene oxides with a ∼4 μm aperture were prepared through the acid oxidation of flake graphite.12,30 A highly purified MWCNT synthesized by chemical vapor deposition (CVD) with high length−diameter ratios was prepared by the gas oxidation, H2O2 treatment, and reflux in hydrochloric acid (HCl).31 MWCNT (1 mg) was dispersed in 0.5 wt % sodium dodecyl sulfate by the ultrasonication to form a homogeneous solution (20 mL), and graphene oxide was well water-dispersed due to a wide range of functional groups such as hydroxyl, epoxide, carboxyl, and carbonyl groups. Uniform aqueous MWCNT/graphene oxide hybrids at the different weight ratios of 1:1, 1:2, and 1:5 were prepared by blending MWCNT and graphene oxide solution by ultrasonication. The MWCNT/ graphene oxide hybrid showed the good stability of standing over three months without any insoluble bundles or precipitates (see Figure S1 in Supporting Information). MWCNT/Graphene Hybrid with Two Different Microstructures. MWCNT/graphene flexible electrodes were processed by four steps:32 (i) vacuum-filtering an aqueous solution onto a filtration membrane to form uniform films; (ii) transferring onto a polyethylene terephthalate (PET) sheet by dissolving the filtration membrane in acetone; (iii) chemically reducing graphene oxide to graphene by HI and repeatedly washing by ethanol; (IV) HNO3 treatment for the p-type doping and the removal of the surfactant. Scheme 1 shows an Scheme 1. Schematic Routes of (Left) IN-MWCNT/ Graphene and (Right) DL-MWCNT/Graphene Flexible Electrodes

interconnected network (IN-MWCNT/graphene) and a double-layer structure (DL-MWCNT/graphene) of films. MWCNT films with different transparencies were prepared by the same method. Photoswitch. CdTe QDs at 4−6 nm were synthesized according to the literature.33 A flexible photoswitch using a layered heterostructure of CdTe/MWCNT/graphene was fabricated by the electrostatic layer-by-layer adsorption. The IN-MWCNT/ graphene (Rs = 240 Ω/□, T = 80%) flexible electrode was placed in a cationic poly(diallyldimethylammonium chloride) (PDDA, 2 mg mL−1) solution for 20 min. After being washed by DI water and dried in argon, the film was immersed in an anionic CdTe solution for 1 h to form the single-layer device by the electrostatic adsorption. The photoswitch with 12 layers of CdTe on the MWCNT/graphene was obtained by repeating this self-assembly 4971

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Figure 1. (a) Photograph of plain and flexed MWCNT/graphene hybrid electrode on a PET sheet with “Tianjin University” printed underneath to illustrate the transparency. (b) I−V charateristics of the MWCNT-graphene flexible electrode after bending 100, 500, 1000, 2000, and 3000 times, indicating good electrical stability.

graphene, platinum foil, and SCE electrode as the working, counter, and reference electrode, respectively. All the electrochemical measurements were performed at room temperature on a CHI660D electrochemical workstation (Shanghai CH Instrument Company, China).

procedure. Photoswitches using a DL-MWCNT/graphene (Rs = 180 Ω/□, T = 80%) and a MWCNT film (Rs = 95 Ω/□, T = 80%) were also fabricated by the same method. Electrochemical Capacitor. All electrochemical experiments were carried out in a three-electrode system with INMWCNT/graphene (Rs = 90 Ω/□, T = 60%) or MWCNT (Rs = 78 Ω/□, T = 73%) electrodes or Tantalum (Ta) foils, platinum foil, and SCE electrode as the working, counter, and reference electrodes, respectively. The aqueous solution of 0.01 M EDOT and 0.005 M sodium dodecyl sulfate (SDS) with pH of 1 adjusted by p-toluenesulfonic acid was prepared. PEDOT was deposited on different electrodes by dynamic polymerization in the potential range from −0.5 to 1.2 V at a scanning rate of 50 mV/s. After the polymerization, the resultant sample was washed repeatedly by deionized water and cleaned in 1 M KCl aqueous solution by cyclic voltammetry (CV) to remove the remaining SDS and EDOT. Structures, Properties, and Transparency versus Sheet Resistance Measurements. Microstructures of several individual MWCNTs, single-layer graphene oxide sheets, and MWCNT/graphene hybrids were observed by scanning electronic microscopy (SEM, PhilipsXL-30), atomic force microscopy (AFM, Veeco Multimode III), and transmission electron microscopy (TEM, Phlilps Tecnai G2 F20). X-ray photoelectron spectroscopy (XPS) analyses were preformed on a PHI 1600 model surface analysis system with a 450 W Mg Kα X-ray (1250 eV) source at a base pressure in the 10−8 to 10−9 Torr range. Transparency and sheet resistance were characterized using UV−vis spectroscopy (Hitachi 330 UV−vis spectrophotometer) and the four-point probe method (RTS-2, Guangzhou 4-probe Technology), respectively. I−V characteristics of flexible films was measured by a Keithley model 2635 SourceMeter. Photoelectrochemical Measurements. Photoelectrochemical measurements were carried out in a three-electrode system at room temperature: a CdTe/MWCNT/graphene film as the working electrode and platinum foil and Ag/AgCl electrode as the counter and reference electrode. The electrolyte was 0.1 M KCl solution. Photoresponse was recorded by a CHI 660D electrochemical analyzer after the illumination of UV light (a 500 W mercury lamp) with ON and OFF for 60 s. Electrochemical Measurements. The electrochemical properties were systematically investigated by CV and a galvanostatic charge/discharge technique using a three-electrode system in 1 M KCl aqueous solution with PEDOT/MWCNT/

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RESULTS AND DISCUSSION

Figure 1a shows the photograph of plain and flexed MWCNT/ graphene flexible films (∼4 cm aperture) on the PET sheet with “Tianjin University” printed underneath to illustrate the transparency. The versatile transfer technology can be applied to prepare the arbitrarily massive scale and free-standing uniform MWCNT/graphene hybrid flexible films with good compatibility, limiting only the size of the filtration membranes.32 The thickness and transparency can be tuned by the CNT/graphene ratios and the volumes of the solution. Furthermore, different from the hairline fracture of ITO upon bending,21 as shown in Figure 1b, this MWCNT/graphene hybrid flexible film maintains an excellent stability of the conductivity (over 95%) after the repeated flexure over 120° more than 3000 times, which is much better than flexible films reported recently.34 Good performance of the MWCNT/ graphene hybrid flexible films indicates a wide availability in flexible optoelectronics. The AFM image of single-layer graphene oxide sheets with the aperture of ∼4 μm and the thickness of 1 nm was given in Figure 2. Purified MWCNTs are a mixture of nanotubes with double, triple, quadruple, or quintuple walls (see Figure S2 in Supporting Information) and display the appropriate combination of structural perfection and remarkable electronic properties. The microstructures of IN-MWCNT/graphene and DL-MWCNT/graphene films were observed by TEM (Figure 3). It can be seen that nanotubes and graphene sheets are well-dispersed in six films without large bundles or aggregates. IN-MWCNT/graphene (Figure 3a and b) shows a 3D percolated network of interconnected nanotubes with crumpled graphene sheets filling the intertube void space. The nanostructure can be tuned by weight ratios of MWCNT to graphene (CNT/graphene). A large amount of MWCNT scattering or connected on graphene was obtained with a high CNT/graphene ratio of 1:1. Results show that graphene sheets and MWCNTs are individually separated at nanoscale and form the interconnected network. The strong stacking between nanotubes and nanosheets was observed in high-resolution images (arrowed in Figure 3e and f) due to π-electronic 4972

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Figure 2. (a) AFM and (b) TEM image of single-layer graphene oxide sheets with the thickness of 1 nm.

interaction.25 This interaction can faciliate the charge transfer and transport across the interface. DL-MWCNT/graphene films were also prepared. Figure 3c and d show that the intertwined MWCNT are well stacked horizontally on 2D graphene layers. The 2D graphene sheets serve as a large area of strong “patchs” at the bottom to support the percolated nanotube network. Importantly, a 180°-folded crumpled silk veil (arrowed in Figure 3a and c) is a indicative of a mechanically strong and flexible film. TEM images show that the nanostructural integrity and continuity of MWCNT/ graphene hybrid are essentially attributed to the good intrinsic affinity of graphitic nature between nanotubes and graphene.35,36 The nanostructures affect charge transport and optical absorption, resulting in the difference of the transparency and conductivity of films.2,36 Figure 4 shows the sheet resistance versus transparency of (a) MWCNT, (top) IN-MWCNT/graphene, and (bottom) DL-MWCNT/graphene flexible films with CNT/graphene ratios of (b) 1:1, (c) 1:2, and (d) 1:5. Curves reveal the dependence of the decreased sheet resistance on the sacrifice of the transparency. It can be seen that the increase in CNT/ graphene ratios leads to the successive increase in conductivity, indicating that the electrical performance can be modulated by CNT/graphene ratios. Note that the sheet resistance of INMWCNT/graphene flexible electrodes (240 Ω/□, at 80%; 110 Ω/□, at 70%) with the CNT/graphene ratio of 1:1 is nearly 1 order of magnitude lower than reduced and doped graphene.37,38 IN-MWCNT/graphene films show a steady increase in conductivity versus transparency with the increasing CNT/graphene ratios, depending on the electronic interaction and charge transport through the 3D interconnected structures.35 The performances of flexible electrodes relying on different structures were illustrated. In contrast, interestingly, the DL-MWCNT/graphene films with the CNT/ graphene ratio of 1:1 (180 Ω/□, at 80%) exhibit the almost identical conductance to that of 1:2 (200 Ω/□, at 80%) under similar transparency, but the conductivity decreases dramatically with the CNT/graphene ratio of 1:5 due to a low amount of CNTs on graphene sheets. Results indicate that, different from the IN-MWCNT/graphene films, the conductivities of

DL-MWCNT/graphene films are dominated by the electronic properties of nanotubes on the top layer. Importantly, compared to previously reported solutionprocessed hybrid films with similar structures, 21,39 our MWCNT/graphene hybrid films with the CNT/graphene ratio of 1:1 show an enhanced combination of the conductivity and transparency due to the strong bonding within 1D/2D extended π-conjugated networks (Figure 3), in which an individual nanotube bridges the gaps among graphene sheets.21 Graphene serves as a large scale of patches by the coverage of surface areas including the intertube void space, while CNT acts as wires connecting the large pads together. The strong interaction was confirmed by the high conductivity of DLMWCNT/graphene films comparable to that of films fabricated by CVD.35 Graphene oxides were chemically reduced by HI acid to improve poor conductivity and restore structural defects. The chemical structures of graphene oxide and graphene (reduced graphene oxide) were investigated by high-resolution C1s peaks of XPS (see Figure S3 in Supporting Information). The original graphene oxide consists of three main components, CC/C−C (sp2/sp3 hybridized carbon atoms in aromatic rings, ∼284.6 eV), C−O/C−O−C (∼286.5 eV), and CO (∼288.3 eV), and a minor component of the C(O)−(OH) (∼290.3 eV) species.40 Compared with graphene oxide, the spectrum of graphene exhibits a remarkably strong single peak of CC/C−C and a weak tail in the higher binding energy region. The surface C/O atomic ratio of graphene is 8, which is nearly 2-fold higher than that of graphene oxide. This feature indicates that the majority of oxygen-containing groupshydroxyl and epoxy groupsis effectively removed by the strong reduction of HI acid, and a large amount of sp3-hybridized carbon rings is restored to the sp2-hybridized structures.40 The effect of HNO3 treatment on a MWCNT/graphene hybrid with different structures was shown in Figure S4 (Supporting Information). It can be found that HNO3 treatment results in the combination of the decreased resistance and the increased transparency. The improvement of conductivity is attributed to the increased delocalized carrier density by p-type doping,15,41,42 while the enhanced transparency is contributed 4973

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Figure 3. TEM images of (a, b) IN-MWCNT/graphene and (c, d) DL-MWCNT/graphene with the CNT/graphene ratios of (a, c) 1:5 and (b, d) 1:1 and (e, f) high-resolution images.

nanomaterials. Our work illustrates the marked difference of the improvement controlled by precise CNT/graphene ratios due to HNO3 treatment (see Figure S5 in Supporting Information). MWCNT/graphene hybrid electrodes

by the removal of metal catalysts, the amorphous carbon, and surfactant impurities.43 The remarkable increase in conductivity of both CNT and graphene films was reported previously,15,43 but few studies were focused on different effects of two carbon 4974

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Figure 5c shows photoconductive ON/OFF switching characteristics of CdTe stacked on (1) a IN-MWCNT/ graphene (Rs = 240 Ω/□, T = 80%), (2) a DL-MWCNT/ graphene (Rs = 180 Ω/□, T = 80%), and (3) a MWCNT (Rs = 95 Ω/□, T = 80%) electrode. The switch was illuminated by UV light at 365 nm. Each cycle of the photocurrent consists of three transient regime a sharp increase and constant state under the illumination and a sharp decay when the light is off. Photoswitches exhibit the reversible dramatic increase in photocurrent due to the photoinduced charge or energy transfer from the excited state of CdTe to MWCNT and graphene by the close proximity of two components.43,45,48−51 The photoresponse indicates that the heterostructure benefits from an efficient dissociation process of excitons generated from solar energy. The interaction was found by Holleitner that the photoconductance of CNTs can be adjusted and sensitized by the photoabsorption of nanocrystals on the surface.52 Figure 5d illustrates the energy-level diagram of a heterostructure of a CdTe/MWCNT/graphene hybrid. According to previous studies,47,53 a large delocalized π−π conjugated electron structure favors electron transfer to carbon nanohybrids. Specifically, the excited electron of CdTe with the conduction band of 2.3 eV54,55 transfers to graphene (4.5 eV)56 and MWCNT (4.8 eV) sequentially determined by the difference of Fermi level. The electron moved on interconnected nanotubes and nanosheets prior to the recombination with photogenerated holes in a radiative trap and formed the enhanced photocurrent. An interesting feature that is also found is that, with relatively high resistance, the IN-MWCNT/graphene network shows a much higher photocurrent with the ON/OFF ratio of 10 than that using DL-MWCNT/graphene and the MWCNT electrode. MWCNT shows the instable and low photoresponses due to weak interaction. A slow decay of ON/OFF ratio during 50 reversible cycles indicates the good stability of switchable properties. The photocurrent and reversibility of photoswitching using IN-MWCNT/graphene films significantly outperform nanocomposites of nanocrystallines/molecules/CNTs/graphene.51,57−60 The 3D percolated network of interconnected nanotubes with a crumpled nanosheet filling the intertube void space serves as an excellent nanostructured platform for the transport and collection of photoinduced charges. This performance is supported by the results that the conductivity of INMWCNT/graphene networks depends on the interplay between graphene and MWCNT, while the DL-MWCNT/graphene films are dominated by the electronic properties of nanotubes on the top layer. The IN-MWCNT/graphene electrodes were also utilized for PEDOT-based flexible supercapacitors. In spite of large potential window and chemical stability, the energy and power density of CP-based supercapacitors were lowered by low conductance, ohmic polarization, limited ion diffusion, and weight loss.61,62 Among many nanocomposites,61−64 a CNT/graphene hybrid was found to be a transformable and viable electrode for energy storage devices due to a core−shell structure, unique quantum coupling, and high porosity.25,65 Figure 6 shows (left) CV and (right) galvanostatic charge/discharge curves of PEDOT electrodeposited on (a) Ta foils, (b) MWCNT, and (c) IN-MWCNT/ graphene electrodes at a scanning rate of 50 mV s−1 and at a current density of 2 A/g, respectively. CV curves show the remarkable increase in both the current density and the electrochemical area of PEDOT/IN-MWCNT/graphene, compared with MWCNT and Ta. The excellent capacitive behavior of

Figure 4. Sheet resistance versus transparency of (a) MWCNT, (top) IN-MWCNT/graphene, and (bottom) DL-MWCNT/graphene flexible electrodes with the CNT/graphene ratios of (b) 1:1, (c) 1:2, and (d) 1:5 comparing refs 21, 22, 35, and 39.

(T = 80%) show a steady decrease in the improvement of the resistance (RsHNO3/Rs untreated) from 46% to 82% with the increasing CNT/graphene ratios, while the increase in the transparency (TsHNO3/Ts untreated) from 160% to 101% is also shown. This feature indicates that HNO 3 treatment of MWCNT shows a much larger effect on the conductivity improvement than graphene because of the strong p-type doping of nanotubes, leading to lower Fermi level to the valence band.8,42 The performance of MWCNT/graphene flexible electrodes for photoswitches controlled by different microstructures was investigated based on a layered heterostructure of CdTe (donor)−MWCNT/graphene hybrid (acceptor).43 CdTe is widely used to form a large area, high-quality thin film of polycrystalline for energy conversion devices due to the high optical absorption coefficient (105 cm−1), narrow bandgap of 1.5 eV matching the preferred range of solar spectrum, the exceptional photostability, and high quantum yield.29,44 Many studies reported the quenching of excitonic luminescence emission of a QDs/ nanocarbon heterostructure with the chemical linker or polymer spacers due to fast photoinduced charge or energy transfer.43,45−47 The schematic route of a photoswitch based on a heterostructure of CdTe/MWCNT/graphene by the electrostatic layer-by-layer adsorption is given in Figure 5a. The absorption and photoluminescence spectra of CdTe are shown in Figure S6 (Supporting Information). TEM images (Figure 5b) show that CdTe QDs were densely packed on the networks of a MWCNT/graphene hybrid. CdTe with the lattice fringes and the diameter of 4−6 nm filled into the voids among 3D networks by electrostatic adsorption.48 The strong interaction favors the charge or energy transfer at the interface.43,45,49 4975

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Figure 5. (a) Schematic routes of a CdTe/MWCNT/graphene photoswitch. (b) TEM images of a heterostructure of the CdTe/MWCNT/graphene hybrid. (c) Photoconductive ONN/OFF switching charactersitics of (1) CdTe/MWCNT, (2) CdTe/DL-MWCNT/graphene, and (3) CdTe/ IN-MWCNT/graphene. (d) Energy-level diagram of a CdTe/MWCNT/graphene hybrid.

Figure 6. (Left) CV curves of PEDOT electro-deposited on (a) Ta foils, (b) MWCNT, and (c) MWCNT/graphene electrodes at a scanning rate of 50 mV s−1. (Right) Galvanostatic charge/discharge curves of PEDOT electro-deposited on (a) Ta foils, (b) MWCNT, and (c) MWCNT/graphene electrodes at current density of 2 A/g with the specific capacitance at 2, 4, 6, 8, and 10 A/g in the inset.

an excellent cycling ability are essentially critical to the fabrication of high-energy storage devices.72 The outstanding performance of MWCNT/graphene hybrid films results from a unique interconnected nanostructure with highly porous flakes and a large surface area,24,26 which shows a synergic effect and improves the penetration of electrolyte ions, the ion transport within a short distance, the insertion/ desertion of cations/protons, and electronic transport on a conducting network. Therefore, a fast doping/dedoping process during the redox reactions results in the high rate cability and good cycling stability with fast charge/discharge rate. Different from other nanocomposites relying on the complicated functionalization and interaction,25,64,65,71 3D MWCNT/ graphene electrodes with different nanostructures can be used to load a varitey of CPs and transition metal oxides for

MWCNT/graphene hybrid electrodes was also confirmed by galvanostatic charge/discharge curves. The linear and symmetrical curves are indicative of the excellent electrochemical stability and charge/discharge properties. IN-MWCNT/graphene with a large surface area and high porosity shows the maximum specific electrochemical capacitance of 150 F/g, which is much higher than MWCNT (100 F/g) and Ta (93 F/g). Importantly, the MWCNT/graphene exhibits a high rate capability of 141 F/g at high current density (10 A/g) with 96% retention. The capacitance and good cycling stability are superior to nanostructured PEDOT66−70 and PEDOT/CNT or graphene nanocomposites.62−64,71 CV curves at the scaning rate from 10 to 200 mV s−1 and galvanostatic charge/discharge curves of PEDOT/MWCNT/ graphene at current densities from 2 to 10 A g−1 were given in Figure S7 (Supporting Information). The high rate capability and 4976

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outstanding flexible supercapacitors. Results indicate the INMWCNT/graphene is an excellent conductive nanostructured and highly porous electrode for multifunctional devices.

4. CONCLUSIONS We presented transparent and conductive MWCNT/graphene hybrids with the interconnected network and double-layer structure by the design of the microstructure to control the interaction. MWCNT/graphene flexible films showed a good stability of electrical performance after bending 3000 times. INMWCNT/graphene films with the tube−sheet interface and high porosity show high conductivity and strong bonding attributed to π-electronic interaction with conjugated structures. The performance can be can be tuned by micostructures and CNT/graphene ratios. A layered heterostructure of CdTe/ IN-MWCNT/graphene exhibited the enhanced photocurrent and a high ON/OFF ratio of 10, which outperforms DL-MWCNT/graphene and MWCNT due to unique nanostructures. The great potential of nanostructured MWCNT/ graphene electrodes as a supercapacitor was confirmed by a high rate capability and good cycling stability due to a large surface area and high porosity. The MWCNT/graphene hybrid with unique structure and strong π-interaction can be developed for many energy transport, conversion, or storage integrated devices based on nano-/micromaterials.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of individual MWCNT, XPS C1s peak of graphene oxide and graphene, the sheet resistance versus transparency of flexible electrodes treated by HNO3, absorption and photoluminscence spectra of CdTe aqueous solution, pictures of well-dispersed MWCNT/graphene oxide solution over three months, CV and galvanostatic charge/discharge curves of PEDOT deposited on Ta foils, and MWCNT and MWCNT/graphene electrodes at different scanning rates and current densities, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-22-87402059. E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Grant No. 2010CB934700), the National Natural Science Foundation of China (Grant No. 51073115, 51003072, 51173127, and 51011140072), and Research Fund for the Doctoral Program of Higher Education of China (No. 20110032110067) and the Natural Science Foundation of Tianjin City (No. 10JCZDJC22400).

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