Conjugated

Article pubs.acs.org/JPCC

Interfacial Interactions of Single-Walled Carbon Nanotube/ Conjugated Block Copolymer Hybrids for Flexible Transparent Conductive Films Ho Seok Park,† Bong Gill Choi,‡ Won Hi Hong,‡ and Sung-Yeon Jang*,§ †

Department of Chemical Engineering, College of Engineering, Kyung Hee University, Yongin-si 446-701, Republic of Korea Department of Chem. & Biomolecular Eng. (BK 21), KAIST, 335 Gwahagno, Yuseong-gu, Daejeon 305-701, Republic of Korea § Department of Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Nanohybrids consisting of single-walled carbon nanotubes (SWCNTs) and a conductive block copolymer, perchlorate-doped poly(3,4-ethylenedioxythiophene)-block-poly(ethyleneoxide) (P-PEDOTb-PEO), were successfully prepared. Individual exfoliation of SWCNTs and high solution processability were simultaneously achieved in the supramolecular assembly. The assembly at the molecular level was driven by the interfacial interaction between SWCNT walls and the PEDOT block, as characterized by various spectroscopic analyses (UV−Vis, FT-IR, PL, and Raman). The exfoliation of SWCNTs and the solubility of the nanohybrids, which are facilitated by the soluble PEO block, were confirmed by Raman spectroscopy and a range of other microscopy techniques (AFM, TEM, and SEM). Flexible transparent conductive films of the nanohybrids were fabricated using a vacuum-assisted filtration method. The films displayed high electrical conductivity with good mechanical integrity due to the strong interaction between the SWCNT and the conductive polymer. The strategy described here opens up promising possibilities for the fabrication of CNT/conjugated polymer hybrids as well as for their use in flexible electronics.

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INTRODUCTION Recently, single-walled carbon nanotubes (SWCNTs) have received great attention as a class of next-generation nanomaterials due to their excellent electrical, optical, chemical, and mechanical properties.1 Because of their poor solubility and strong aggregation characteristics, enhancing the processability of SWCNTs while preserving their unique properties has been a main challenge in facilitating their practical applications.2 Hybridization of CNTs with polymers at the molecular level is a popular strategy in terms of obtaining the emerging properties through the synergistic effects of the two components while overcoming the intrinsic limitations of the individual materials.3 Furthermore, the polymers can often afford improved processability of the resulting hybrids. From a materials engineering perspective, the characteristics of the interface between CNTs and polymers should be rationally designed to resolve the poor solubility of CNTs via high compatibility with polymers, whereas the desired properties of the CNTs are wellpreserved in the modified hybrids.4 Supramolecular concepts, in particular, the wrapping CNTs using functional macromolecules such as block copolymers,5 dendrimers,6 polyelectrolytes,7 biomaterials, 8 and liquid crystals,9 paved the way to obtain processable CNT solutions, resulting in well-controlled architectures of CNT/polymer nanohybrids (NHBs) across a range of scales. Although the © 2012 American Chemical Society

hybridization of CNTs with insulating polymers has been widely reported,10 it is not the best combination for electronic applications because the insulating polymers have a detrimental effect on the electrical properties of CNTs by acting as an interfacial resistor. Conjugated polymers (CPs) such as polypyrroles,11 polyanilines,12 poly(3-alkylthiophene),13 poly(phenylene vinylenes),14 and poly(arylene ethynylene),15 have shown efficient decoration of SWCNTs, thereby preserving their electronic properties. The strong van der Waals (vdW) interactions between conjugated π-bonds in both CPs and CNTs enhance the miscibility between the respective components;11−15 however, the intrinsically rod-structured CPs are not highly soluble in most of the common (aqueous and organic) solvents. From this standpoint, the rod−coil block copolymers (RCBCs), which consist of a conjugated rod-block and a soluble nonconjugated coil block, can be regarded as efficient materials to functionalize CNTs for applications in electronic or electrochemical devices.16 The π-conjugated structure in rod blocks is intimate with the CNT sidewalls, and the nonconducting coil blocks tune the solution processability.17 Huo and Zhai et al. reported the dispersion Received: October 12, 2011 Revised: March 21, 2012 Published: March 21, 2012 7962

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PEO (0.05 mg) were vigorously stirred overnight in a solution of DMF (5 mL), and oxygen was removed from the mixture by degassing with nitrogen. The resultant solution was sonicated (110 W) for 60 min, held for 120 min at 20 °C, and then centrifuged at 10 000 rpm for 15 min to separate aggregated particles. SWCNT/P-PEDOT-b-PEO hybrid solution was isolated from the supernatant. Fabrication of SWCNT/P-PEDOT-b-PEO Hybrid Films. SWCNT/P-PEDOT-b-PEO hybrid solutions were first suspended in 300 mL of dimethylformamide, and the solution was filtered through an anodic aluminum oxide (AAO) membrane with a pore size of 0.02 μm (Anodisc 47, Whatman) under mild vacuum. When the SWCNT/P-PEDOT-b-PEO hybrid-coated AAO membrane was floated directly on 3 M aqueous NaOH, the AAO membrane gradually dissolved, and the resulting SWCNT/P-PEDOT-b-PEO hybrid film was transferred onto a PET substrate by gradually aspirating the aqueous NaOH solution. The SWCNT/P-PEDOT-b-PEO hybrid-coated PET film was washed several times with DI water and dried under vacuum at room temperature. Characterization. Transmission electron microscopy (TEM) images were collected with a JEM-3010 high-resolution transmission electron microscope (HR TEM, 300 kV). SEM images were obtained by a field-emission scanning electron microscope (FE SEM, FEI Sirion model) equipped with an inhouse Schottky emitter in high stability. Atomic force microscopy (AFM) images were recorded in the noncontact mode using a Nanoman Digital Instruments 3100 AFM (VEECO) with an etched silicon aluminum-coated tip. Ultraviolet−visible (UV−Vis) spectra were recorded with a CE instrument after the samples were passed to contact the detector. Photoluminescence (PL) spectra were recorded using a SPECTRAmax M2 spectrofluorometer and the SoftMax pro software (Molecular Devices). In PL measurements, the concentration of all solutions was fixed into 1 μg/mL to compensate for reduced absorption in the solvent. Fourier transform infrared (FT-IR) spectra were collected on a JASCO FT-IR 470 plus as attenuated total reflection. Each spectrum, which was recorded as the average of 12 scans, was collected from 4000 to 650 cm−1. The Raman spectra were recorded from 3500 to 100 cm−1 on a Bruker FT Raman spectrophotometer RFS 100/S using a 785 and 1064 nm dual channel laser at a resolution of 1 cm−1. Sheet resistance and film thickness of the TCFs were measured by using the standard four-point probe technique (MCP-T 610, MITSU-BISHI Chemical Analytech) and a surface profiler (TENCOR P-10), respectively. All electrochemical data were obtained at room temperature within the error range of ±5%.

of multiwalled carbon nanotubes (MWCNTs) in some organic solvents (chloroform, tetrahydrofuran, and toluene) using a conjugated block copolymer, P3HT-b-polystyrene (PS).16 They observed a more than two times higher CNT dispersing capability of P3HT-b-PS than that of P3HT homopolymer, with enhanced solution stability.16 However, neither the dispersion of SWCNTs nor their individualized exfoliation was demonstrated. Furthermore, an electronic application such as a flexible transparent conductive film (TCF) on a macroscopic scale prepared through facile assembly of the dispersed CNTs using conjugated RCBCs has not been achieved. In supramolecular concept, the hybridization on the molecular level is often facilitated by the interfacial interaction of the components, and the electronic interaction can occur if the components are electronically active. The electronic interaction between CNTs and other materials such as gas (nitrogen and oxygen),18 alkali metals,19 and inorganic or organic substances20 has often resulted in the adjustment of the Fermi level and the promotion of charge transfer of CNTs. The charge transfer on the nanoscale is a fundamental phenomenon that is applicable for nanoelectronics, optoelectronics, solar cells, photodetectors, lasers, displays, sensors, and so on.21 Of particular interest is the charge transfer between the CNT and the CP (as a donor or an acceptor), which can extend the utility of both by exhibiting unique electronic properties.22 The experimental demonstration of the electronic interaction between a SWCNT and a CP shown in this communication provides a deeper understanding of the interfacial interactions in the hybrid systems as a supramolecular concept while focusing on the electronic properties. Herein, a simple and useful fabrication method for SWCNT/ conjugated RCBCs-based NHBs was demonstrated, and the flexible TCFs using the NHBs were fabricated. The charge transfer behavior in the NHBs was characterized to understand further their interfacial interaction. Perchlorate-doped poly(3,4ethylenedioxythiophene)-b-poly(ethylene oxide) (P-PEDOT-bPEO) block copolymer was used as a conjugated RCBC. (See Figure S1 of the Supporting Information.) The P-PEDOT block acted as a conductor glue that can interact with SWCNT while deriving the desired optoelectronic properties, and the PEO block acted as a dispersing (solubilizing) component. The PEDOT possesses advantages over other CPs in terms of high electronic conductivity with good optical transparency in the visible light region.23 In contrast, the PEO improves the solubility of SWCNTs in a range of solvents, which enables it to fabricate homogeneous, robust, 3D, interpenetrating network films. PEO has been used as flexible blocks to solubilize insoluble rigid-rod-shaped polymers such as poly(phenylene vinylenes) and polyfluorene.24 The flexible TCFs using the SWCNT/P-PEDOT-b-PEO NHBs were prepared by a vacuum-assisted filtration (VF) method. Furthermore, the charge transfer between P-PEDOT blocks and the SWCNTs and the optical and electrical properties of the processable NHBs were investigated to demonstrate their potential for optoelectronic applications.

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RESULTS AND DISCUSSION The spatially uniform distribution of SWCNTs in polymer solutions, triggered by the enhanced solubility of P-PEDOT-bPEO-coated SWCNTs in organic solvents and the favorable interactions at the molecular level in these NHBs, is the main property desired for the fabrication of advanced SWCNT/ polymer NHBs. The SWCNT/P-PEDOT-b-PEO hybrids, which were prepared by supramolecular assembly of the two components, followed by brief sonication, exhibited good solubility in a range of polar organic solvents such as methanol, dimethyl sulfoxide, dimethylformamide, and acetonitrile. (See Figure S2 of the Supporting Information.) Furthermore, the mixtures of water/polar organic solvents could also disperse the NHBs at a range of compositions. The enhanced solubility of

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EXPERIMENTAL SECTION Synthesis of SWCNT/P-PEDOT-b-PEO Hybrids. Purified SWCNTs (ASP-100F, purity >90 wt %, manufactured by reflux in 3.0 M HNO3, oxidation in air, and annealing)25 and PPEDOT-b-PEO were purchased from Iljin Nanotech and Sigma Aldrich, respectively. SWCNTs (0.1 mg) and P-PEDOT-b7963

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The FT-IR spectra in Figure 1b show that the characteristic bands of P-PEDOT-b-PEO were red-shifted except for the band at 1195 cm−1 because the electronic interactions withdrew the charge density of SWCNT acting as an electron donor and softened the stretch vibration frequency of the conjugated blocks. The strong interaction of SWCNT with P-PEDOT-bPEO was also confirmed by the PL quenching of the two emission bands of the P-PEDOT-b-PEO at 430 and 460 nm, indicating that the intermolecular relaxation of the emitted energy occurred through the charge transfer from P-PEDOT blocks to SWCNTs. (See Figure S3 of the Supporting Information.) Therefore, the supramolecular assembly of PPEDOT-b-PEO with SWCNT was strongly influenced by the electronic interactions, as proven by spectroscopic analysis. The influence of P-PEDOT-b-PEO on the isolation and electronic structures of SWCNTs was also investigated by the Raman spectra, as shown in Figure 2. The Raman spectra of

SWCNTs was attributed to the existence of soluble PEO blocks. Despite the good solubility of PEO in water, SWCNT/ P-PEDOT-b-PEO hybrids could be only marginally dispersed in water due to the relatively low fraction of PEO blocks. UV−Vis, FT-IR, and PL spectroscopic methods were used to gain insight into the origin of the interactions between SWCNTs and P-PEDOT-b-PEO. As shown in the UV−Vis spectroscopy results in Figure 1a, a band of P-PEDOT-b-PEO

Figure 1. (a) UV−vis spectra of SWCNT, P-PEDOT-b-PEO, and SWCNT/P-PEDOT-b-PEO hybrid and (b) FT-IR spectra of SWCNT and SWCNT/P-PEDOT-b-PEO hybrid.

at ∼500 nm was assigned to π−π* transition of the PEDOT blocks, whereas the absorption spectrum of SWCNTs was ascribed to its intrinsic electronic structure. The UV−Vis spectrum of SWCNT/P-PEDOT-b-PEO revealed the superposition of characteristic bands of pristine SWCNT and PPEDOT-b-PEO, indicating the absence of any ground-state charge interaction between them. This finding was analogous to previous results concerning the interaction between CPs and CNTs;14 however, the characteristic band of P-PEDOT-b-PEO was broadened upon hybridization in conjunction with the appearance of a shoulder band at ∼400 cm−1, due to the electronic interactions between SWCNTs and the conjugated P-PEDOT blocks. The flexible PEO blocks allow the PPEDOT-b-PEO to coat layers of nanotubes through the enhanced freedom of segmental motion, providing close proximity for the interfacial interactions between conjugated segments of SWCNTs and P-PEDOT blocks.21 In contrast, the conjugated P-PEDOT blocks favorably interact with SWCNTs that can accommodate the electronic modification of SWCNTs.

Figure 2. Raman spectra of SWCNT and SWCNT/P-PEDOT-b-PEO hybrids in the range of (a) 100−300 and (b) 1200−1800 cm−1.

SWCNTs with an excitation wavelength at 532 nm exhibited the characteristic bands from radial breathing mode (RBM), Dmode, and G-mode.26 The RBM frequency was inversely correlated with the tube diameter, following the relation; ωRBM (cm−1) = 204/d + 27.27 The several RBM bands of SWCNTs in the range of 180−250 cm−1 showed a broad diameter range of 0.9 to 1.3 nm. The noticeable weakness of the RBM bands at >200 cm−1 in SWCNT/P-PEDOT-b-PEO relative to the 7964

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PEDOT-b-PEO hybrids indicates that the charge transfer in the NHBs was facilitated by the electronic interactions between SWCNTs and P-PEDOT blocks. An intriguing aspect of the interactions in the NHBs was observed in their structures on the diverse scales as shown in Figure 3. The SWCNT/P-PEDOT-b-PEO hybrid was diluted in a solution, coated on a substrate, and isolated for the observation of microscopic structure. Compared with pristine SWCNTs, NHBs showed more debundled or exfoliated CNTs attributable to the assembly of RCBCs, as proven by AFM and TEM images. As shown in a HR TEM image (Figure 3a), the dispersed moieties were located close to individually isolated tubes. The diameters of SWCNT/P-PEDOT-b-PEO hybrids were measured from their cross sections on the AFM images (Figure 3b) and averaged from measurements of 30 samples. The diameter of an individual NHB was ∼1.0 nm, which was in reasonable agreement with the values calculated by RBM frequency and from TEM images, indicating the exfoliation or individualization of the SWCNT by P-PEDOT-b-PEO. In contrast, the diameters of SWCNT bundles were 2−10 nm. (See Figure S4 of the Supporting Information.) Although the exact coating thickness of P-PEDOT-b-PEO was hard to measure, it is thought to be below 1.0 nm in a typical composition. The identity of P-PEDOT-b-PEO-wrapped SWCNTs was also confirmed by the differences in the electron densities of the two materials. (See Figure S5 of the Supporting Information.) The heavy coating of polymer chains in some samples is indicative of a good wetting onto SWCNTs efficiently dispersed in organic solvents, whereas the CNTs alone are insoluble. The macroscopic structures of SWCNT/PPEDOT-b-PEO hybrid-based thin films were observed by SEM images. Because the NHBs were densely packed and some walls covered by polymers tended to weld one another in the process of VF process, the width of NHBs looked larger than those in TEM and AFM images. Owing to the efficient coating and exfoliation of SWCNTs by P-PEDOT-b-PEO in close proximity, SWCNT/P-PEDOT-b-PEO hybrids created 3D interpenetrated networks, which are beneficial for percolated electron transport.35 Moreover, SWCNT/P-PEDOT-b-PEO hybrids with the high amount of polymer exhibited dense and compact morphology. (See Figure S5 of the Supporting Information.) From a macroscopic point of view, the homogeneous films consisting of 3D hybrid networks could be readily fabricated by VF and spin-coating methods. Mechanical robustness or integrity of SWCNT hybrid films was derived from their densely networked structure, consisting of the individual exfoliated SWCNT/P-PEDOT-b-PEO hybrids. Therefore, the supramolecular self-assembly of PPEDOT-b-PEO and SWCNT leads to the fabrication of homogeneous films and 3D networking structures for electronic or optoelectronic applications. The superiority of SWCNT/P-PEDOT-b-PEO NHBs, which have the 3D interpenetrating structures produced by molecular level interactions, was demonstrated by measuring optoelectronic properties. The transparency and sheet resistance of SWCNT/P-PEDOT-b-PEO hybrid films are shown in Figure 4. The inset image reveals the homogeneity and flexibility of a large-scale NHB film prepared by the VF method. (See Figure S6 of the Supporting Information.)36 The thickness of the NHB films, which determines the sheet resistance and transmittance, can be varied over a wide range by controlling the concentration of the solution (Figure 4b). In this research, the thickness was in the range of 20−120 nm. The sheet

bundled SWCNTs indicates that P-PEDOT-b-PEO effectively coated, isolated, and dispersed the small-diameter nanotubes because of the weak intertube interactions of small SWCNTs, as previously reported.28 The band broadening and shift in the low-frequency range of 150−200 cm−1 was attributed to the debundling of SWCNTs by the RCBC into another component and the vdW interactions between isolated small SWCNTs.28,29 The exfoliation of SWCNT bundles into individualized tubes can be confirmed by the fact that the 1.1 nm diameter of isolated SWCNT calculated by the RBM band was consistent with the results of TEM (Figure 3a). Given that the D-band at

Figure 3. (a) TEM, (b) AFM, and (c,d) SEM images of SWCNT/PPEDOT-b-PEO hybrids.

∼1300 cm−1 is attributable to disordered graphene layers and the degree of conjugation disruption and the G-band at ∼1600 cm−1 is attributable to the tangential C−C stretching vibrations, the intensity ratio of the D-band to the G-band (ID/IG) is regarded as an indicator of sidewall covalent derivation or defect introduction.30 The negligible changes in the ID/IG ratio of SWCNTs indicate that the noncovalent functionalization by P-PEDOT-b-PEO prevents the dramatic destruction of intrinsic electronic structures of SWCNTs. Furthermore, the broad band of the NHBs at ∼1434 cm−1 was assigned to the characteristic band of P-PEDOT-b-PEO. Along with the observation that the UV−Vis and FT-IR spectra of P-PEDOT-b-PEO were changed after the hybridization of SWCNTs, it is hypothesized that the charge transfers may be involved in the supramolecular assembly of hybrids. Because the tangential vibrational modes for SWCNTs are sensitive to the charge transfer,31 the G band of SWCNTs was compared with that of SWCNT/P-PEDOT-bPEO NHBs for verification of the electronic interactions. The G-band of the Raman spectra of SWCNTs at 1578 cm−1 was blue-shifted by 8 cm−1 for SWCNT/P-PEDOT-b-PEO hybrids at 1586 cm−1. The blue shift of the G-band to the higher frequency was attributed to either p-doping (such as bromine as an electron-acceptor) or oxidation (similar to that by HNO3 or H2SO4).32 The carbon bonds are stiffened due to reduction of the delocalized electron density on the SWCNTs resulting from the hole injection from P-PEDOT-b-PEO to SWCNT.33 A similar shift of Raman spectra by means of an extra charge injection into CNTs was reported by Yang and McGuire et al.34 Consequently, the spectroscopic analysis of SWCNT/P7965

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Article

ASSOCIATED CONTENT

S Supporting Information *

Chemical structure of poly(3,4-ethylenedioxythiophene)-blockpoly(ethylene glycol) solution, SWCNT/P-PEDOT-b-PEO hybrid solution in water, MeOH, DMF, DMSO, and ACN, PL emission spectra of SWCNT, P-PEDOT-b-PEO, and SWCNT/P-PEDOT-b-PEO hybrid and changes in the PL emission spectra of SWCNT/P-PEDOT-b-PEO hybrid with the addition of SWCNT, AFM image of SWCNTs, TEM and SEM images of SWCNT/P-PEDOT-b-PEO hybrid film (weight ratio of SWCNT to P-PEDOT-b-PEO of 1 to 2), and photo image of SWCNT/P-PEDOT-b-PEO hybrid film. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology (2011-0008698), the Korea Research Council of Fundamental Science & Technology (KRCF), and KIST for ‘National Agenda Project (NAP)’ program.

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Figure 4. Transmittance and sheet resistance of SWCNT/P-PEDOTb-PEO hybrid films. Inset is a photo image of SWCNT/P-PEDOT-bPEO hybrid film on a plastic substrate.

resistance of NHB films dramatically increased at the transmittance of ca. >80% at 550 nm. The conductivity of CNT-based films is strongly dependent on the connectivity of the constitutive CNT networks as a consequence of the highly resistive tunneling/Schottky barriers at the intertube junctions37 and the injection of extra charge by electron and hole doping.33 From this reason, the charge transfer at the interface of the two components of NHBs and the well-interconnected networking structures of the resultant films enable SWCNT/ conjugated RCBC hybrids to be used in advanced electrical conductors for optoelectronic applications.

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REFERENCES

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CONCLUSIONS

In summary, we have demonstrated SWCNT/P-PEDOT-bPEO NHBs prepared by supramolecular assembly. The charge transfer in the hybrids by means of the electronic interaction was analyzed by various spectroscopic methods (UV−vis, PL, FT-IR, and Raman), whereas their nanostructures were investigated by TEM, SEM, and AFM. The interfacial engineering of the NHBs by the interactions led to the formation of 3D interpenetrated networks used for percolated electron transport and to provide the mechanical robustness and integrity for the flexible TCFs. The strategy delineated herein opens up promising possibilities for applications in flexible electronics, optoelectronics, biological sensors, and optical devices. 7966

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