Enhanced optical and electrical properties of PEDOT: PSS films by the addition of MWCNT-sorbitol

Synthetic Metals 159 (2009) 1701–1704

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Enhanced optical and electrical properties of PEDOT: PSS films by the addition of MWCNT-sorbitol Y. Chen, K.S. Kang ∗ , K.J. Han, K.H. Yoo, Jaehwan Kim Center for EAPap Actuator, Department of Mechanical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Ku, Incheon 402-751, South Korea

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Article history: Received 16 April 2008 Accepted 8 May 2009 Available online 24 July 2009 Keywords: Carbon nanotubes Conducting electrode Sorbitol Functionalized carbon nanotube

a b s t r a c t A thin layer of carbon nanotubes (CNTs) presents a strong candidate for application as a transparent conducting electrode and a high frequency Schottky diode. Multiwalled carbon nanotubes (MWCNTs) were modified using nitric acid to form –OH and –COOH groups on the MWCNT surface. Functionalized MWCNTs (FMWCNTs) were further modified using sorbitol molecules. N, N -carbonyldiimidazole (CDI) was utilized as an activating agent for carboxylic acids in a homogeneous one-pot reaction of FMWCNTs in N,N-dimethylacetamide (DMAc). The activated FMWCNTs were mixed with sorbitol and heated up to 60 ◦ C with stirring. Due to the mild conditions and efficiency of the reaction, a large amount of sorbitol molecules were covalently attached with increasing reaction time. The FMWCNTs with sorbitol (FMWCNTSORs) were mixed with poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate) (PEDOT:PSS). The FMWCNTSORs were homogeneously dispersed into PEDOT:PSS solution without any precipitation. The FMWCNTSORs/PEDOT:PSS film showed stronger FTIR absorption peaks in the case of samples reacted for longer time. The UV–vis transmittance and the conductivity of the FMWCNTSORs/PEDOT:PSS film was increased as the reaction time increased. Although the field emission scanning electron microscope (FESEM) surface image of the 2 h reacted FMWCNTSORs/PEDOT:PSS film showed large number of small aggregated particles, only a small number of aggregated particles was found for the sample reacted for 6 h. These results indicate that the appropriate amount of sorbitol molecules on the MWCNT can increase the conductivity and transmittance of the PEDOT:PSS film. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Optically transparent conducting electrodes have been the focus of considerable research due to their application in flat pannel displays and organic light emitting diodes (OLEDs). Flexible displays are expetced to play a major role in the future display industry. Conducting polymers are considered the best candidates for flexible transparent conducting electrodes. Among these polymers, poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate) (PEDOT:PSS) is particularly attractive due to its high conductivity, structural stability, optical transparency, and processibility. Despite its versatile advantages, the conductivity is still a limitting factor for practical application in optoelectronic devices. However, PEDOT:PSS having significantly enhanced conductivity has been developed by the addition of sorbitol [1], glycerol [2], ethylene glycol [3], dimethyl sulfoxide [4], meso-erythritol [4], and poly(ethylene glycol) [5], respectively, to the PEDOT:PSS solution prior to film casting. HCl treatment [6] and rinsing in water [7] also increased the conductivity.

∗ Corresponding author. Tel.: +82 32 874 7325; fax: +82 32 832 7325. E-mail address: [email protected] (K.S. Kang). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.05.009

Carbon nanotubes (CNTs) have attracted much attention due to their excellent strength and high electrical and thermal conductivity. They also allow versatile versitile surface modification. However, difficulties in making a stable dispersion in a liquid system due to CNT interactions and entanglements has resulted in their poor processibility. Effective reinforcement or electrical conductivity in polymer composites can be achieved by establishing a proper dispersion and good interfacial bonding between the CNTs and the polymer materix. A proper dispersion can be achieved via surface modification of the CNTs. The chemical oxidation of CNTs with HNO3 , O3 , OsO4 , RuO4 , or KMnO4 provides oxygen containing groups, such as carboxylate groups, ether groups, and –OH groups, on the ends and surfaces of the nanotubes. Following the succesful incorporation of oxygen containing groups on CNTs, other studies reported further modification performed using silanization [8], (3-aminopropyl)-triethoxysilane and biomolecules [9], and 1,2-bis-(10,12-tricosadiynoyl)-sn-glycero-3phosphoethanolamine phospholipid [10]. Research on CNT-polymer mixtures to complement indium tin oxide (ITO) could provide a new class of transparent conducting materials for certain applications such as OLEDs and solar cells. One of the most important advantages of CNT-polymer composites is superior flexibility with respect to ITO. Another important

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Fig. 1. Synthetic process of the FMWCNT and sorbitol. The –COOH groups were activated by the CDI molecules and reacted with –OH groups of the sorbitol molecules.

consideration is that CNTs are a naturally abuntant material compared with indium. In a previous study, transparent, conductive, and flexible CNT films were fabricated, and a smooth surface was achieved through PEDOT:PSS passivation [11]. A double layer conductive transparent electrode using CNTs and PEDOT:PSS was also fabricated on a transparent polyethylene terephthalate (PET) film [12,13]. Another work investigated CNTs incorporated into a PEDOT:PSS mixture using polyethyleneimine and gum arabic, which acts as a surfactant for better dispersion [14,15]. In this investigation, we report two-step modification processes of the MWCNT including surface functionalization with HNO3 and covalent bond formation with sorbitol using CDI as an activating agent. FTIR spectra of the Multiwalled carbon nanotubes (MWCNTs)ORs, UV–vis transmittance spectra, electrical resistivity, and surface FESEM images of PEDOT:PSS films mixed with Functionalized MWCNTs (FMWCNTSORs) were also reported in this paper.

2. Experimental PEDOT-PSS, DMAc, CDI, sorbitol, and MWNTs were purchased from Sigma-Aldrich and used without further purification. Concentrated nitric acid (60%) was obtained from DC Chemical Co., Ltd. For the sample preparation, 500 mg of MWCNTs was dispersed in 100 ml of concentrated nitric acid using an ultrasonic device (Fisher Scientific, FS30H). The dispersion was refluxed at 90 ◦ C for 24 h. The resulting solution was filtered using a nanofilter having a pore size of 200 nm. The remaining solid was repeatedly washed

using deionized water until the pH reached 7. The FMWCNTs were dried in an oven at 80 ◦ C for 24 h. FMWNTs (25 mg) were dispersed in 50 mL of DMAc. CDI (95 mg) was added to the FMWCNTs solution. The mixture was stirred for 24 h at 60 ◦ C to activate the carboxylic acids attached to the FMWCNT surface. A sufficient amount of sorbitol (212 mg, 2:1 molar ratio with respect to CDI) was dissolved in DMAc solvent and added to the reaction mixture. The mixture was stirred for various times at 60 ◦ C. The reactant was filtered and washed with acetone and DI water repeatedly to completely remove residual CDI and sorbitol. The resulting product was dried in an oven for 24 h at 80 ◦ C. Finally, the FMWCNTSORs were obtained. FMWCNTSORs were dissolved in DMAc solvent. A fixed amount of FMWCNTSORs was added to 3 g of PEDOT:PSS. The solutions were spin-coated onto a glass substrate using a spin-coater (Laurell, EDC2-100) and dried in an oven for 24 h at 80 ◦ C. UV–vis spectra and conductivity were measured using an HP 8452A UV–vis spectrometer and a CMP-200 probe station equipped with a Precision Premier II ferroelectric system (Radiant Technology INC), respectively. The solutions were spin-coated onto a silicon wafer to investigate the FESEM surface morphology. FTIR spectra for the sorbitol and FMWCNTSORs were recorded using a Bio-RAD FTS-3000 IR spectrometer. Surface images of the films were obtained using a HITACHI S-4000 FESEM.

3. Results and discussion The MWCNTs were functionalized using HNO3 to make –OH and –COOH groups on the surface of the MWCNTs. The –COOH

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Fig. 3. UV–vis transmittance spectra. As the reaction time is increased, the transmittance is increased.

Fig. 2. FTIR spectra of (a) pure sorbitol molecule and (b) FMWCNTSORs. The spectra from the top to bottom are 2, 4, and 6 h reacted sample. As the reaction time is increased, the characteristic sorbitol absorption peaks increased.

groups were utilized for the next covalent bonding process. To increase the hydrophilic nature of the MWCNT, sorbitol molecules were selected to form covalent bonds to the FMWCNT. The International Union of Pure and Applied Chemistry (IUPAC) designates sorbitol as hexane-1,2,3,4,5,6-hexanol. It has 6 –OH groups in one molecule. When sorbitol is attached on CNTs, the CNTs can be well dissolved in a polar solvent due to the large number of polar –OH groups. A schematic representation of the synthetic route is shown in Fig. 1. The structure of CDI is shown in I in Fig. 1. The carboxylic acids attack the center carbon, which has relatively low electronic density, remove one imidazole, and form the intermediate structure designated as II. Due to structure II’s instability, it transforms to structure III. This structure then releases one CO2 molecule and forms structure IV. Finally, molecule IV reacts with sorbitol molecules, yielding FMWCNTSORs. The FTIR spectrum of pristine sorbitol is shown in Fig. 2a. Strong peaks of –OH bond stretching vibration –C–O stretching vibrations are observed at 3400 and 1100 cm−1 , respectively. The peak of C–H stretching vibration appears at 2900 cm−1 . Several broad peaks of C–H bending vibrations appear between 1250 and 1450 cm−1 . FTIR spectra of the 2, 4, and 6 h reacted samples are shown in Fig. 2b. No specific peaks appeared for the 2 h reacted sample, and absorption peaks of 1100, 1300, 1700, and 2900 cm−1 were observed for the 4 h reacted sample. Major FTIR absorption peaks of sorbitol were observed for the 6 h reacted sample. This implies that sorbitol molecules were attached to the FMWCNTs.

The synthesized FMWCNTSORs were dispersed to the PEDOT:PSS. FMWCNTSORs were expected to disperse well to the PEDOT:PSS because the solvent of PEDOT:PSS is water. Generally, when CNTs are added to a polymer, the transmittance of the polymer film is reduced. This reduction of transmittance may be due to the absorption by CNTs or scattering by nonuniformly distributed or aggregated CNTs. Since the diameter of CNTs is much smaller than the visible light wavelength, the scattering loss could be negligible when CNTs are uniformly distributed in the polymer matrix. Fig. 3 shows the transmittance of the PEDOT:PSS-FMWCNTSORs films with various reaction times. As the reaction time increased, the transmittance increased. The visible transmittance of the films was about 70, 75, 78, and 79% at 500 nm for the 2, 4,and 6 h reacted samples, and the sample without FMWCNTSORs, respectively. The enhanced transmittance of the sample might be due to the homogeneous and uniform distribution of FMWCNTSORs in the PEDOT:PSS matrix. Fig. 4 shows the resistance of the FMWCNTSORs/PEDOT:PSS samples with reaction time. The resistance values for the pristine PEDOT:PSS, 2, 4, and 6 h-FMWCNTSORs/PEDOT:PSS films were 830000, 780, 430, and 260  cm−1 , respectively. As the reaction

Fig. 4. Resistance of the pristine PEDOT:PSS, 2, 4, and 6 h reacted FMWCNTSORs/PEDOT:PSS film. The resistance reduced drastically by the introduction of FMWCNTSORs to the PEDOT:PSS. As the reaction time is increased, the resistance is reduced.

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Fig. 5. Surface FESEM images of the FMWCNTSORs/PEDOT:PSS films.

time increased, the resistance was reduced. This may be due to both the doping effect of the sobitol and the uniform distribution of the FMWCNTSORs in the PEDOT:PSS matrix. Surface FESEM images are shown in Fig. 5. Numerous aggregated FMWCNTSORs particles were distributed across the surface for the 2 h reacted FMWCNTSORs/PEDOT:PSS film. The number of particles was reduced significantly for the 4 h reacted FMWCNTSORs/PEDOT:PSS film. The size and number of particles were further reduced for the 6 h reacted FMWCNTSORs/PEDOT:PSS film. These results and the FTIR results imply that longer reaction time increases the number of sorbitol molecules on the surface of the FMWCNTs and facilitates uniform distribution of the FMWCNTSORs in the PEDOT:PSS matrix. As a result, the UV–vis transmittance and conductivity of the PEDOT:PSS films were enhanced. 4. Conclusions To improve conductivity and transmittance of the PEDOT:PSS films after mixing MWCNT, two-step modification processes of MWCNTs were performed to uniformly disperse MWCNTs in PEDOT:PSS. MWCNTs were functionalized using HNO3 to form –OH and –COOH groups onto the MWCNT surface. For the second modification process, the reaction between the carboxyl groups from FMWCNTs and hydroxyl groups from sorbitol molecule formed covalent bonds between FMWCNTs and sorbitol molecules. The more sorbitol molecules on the FMWCNT showed the better uniform dispersion of FMWCNTSORs in a PEDOT:PSS solution without any precipitation. Aggregated FMWCNTSORs par-

ticles in PEDOT:PSS matrix were dramatically reduced when the reaction time was 6 h, which means that FMWCNTSORs were well-dispersed in the PEDOT:PSS matrix. As a result, UV–vis transmittance and electrical conductivity were enhanced for the PEDOT:PSS films having the more sorbitol molecules on the FMWCNTs. This work was performed under support from the Creative Research Initiatives (EAPap Actuator) of KOSEF/MOST, South Korea. References [1] F. Zhang, M. Johansson, M.R. Andersson, J.C. Hummelen, O. Inganas, Adv. Mater. 14 (9) (2002) 662. [2] D. Bagchi, R. Menon, Chem. Phys. Lett. 425 (2006) 114. [3] B. Yoo, A. Dodabalapur, D.C. Lee, T. Hanrath, B.A. Korgel, Appl. Phys. Lett. 90 (2007) 072106. [4] J. Ouyand, C.W. Chu, F.C. Chen, Q. Xu, Y. Yang, Adv. Funct. Mater. 15 (2005) 203. [5] T. Wang, Y. Qi, J. Xu, X. Hu, P. Chen, Appl. Surf. Sci. 250 (2005) 188. [6] T.P. Nguyen, S.A. de Vos, Appl. Surf. Sci. 221 (2004) 330. [7] D.M. DeLongchamp, B.D. Vogt, C.M. Brooks, K. Kano, J. Obrzut, C.A. Richter, O.A. Kirilov, E.K. Lin, Langmiur 21 (2005) 11480. [8] P.C. Ma, J.K. Kim, B.Z. Tang, Carbon 44 (2006) 3232. [9] L. Yu, C.M. Li, Q. Zhou, Y. Gan, Q.L. Bao, Nanotech 18 (2007) 115614. [10] P.H. Merek, M.W. Urban, Biomacromolecules 6 (2005) 2455. [11] D. Zhang, K. Ryu, X. Liu, E. Polikarpov, J. Ly, M.E. Tompson, C. Zhou, Nano Lett. 6 (2006) 1880. [12] A.J. Miller, R.A. Hatton, S.R. Silva, Appl. Phys. Lett. 89 (2006) 133117. [13] L. Hu, G. Gruner, D. Li, R.B. Kaner, J. Cech, J. Appl. Phys. 101 (2007) 016102. [14] R. Bandyopadhyaya, E. Nativ-Roth, O. Regev, R. Yerushalmi-Rozen, Nano Lett. 2 (2002) 25. [15] C.C. Oey, A.B. Djurisic, C.Y. Kwong, C.H. Cheung, W.K. Chan, P.C. Chui, Mater. Res. Soc. Symp. Proc. 871E (2005), I9.16.1.