J. Phys. Chem. C 2009, 113, 8023–8029
8023
Electrical, Optical, and Morphological Properties of P3HT-MWNT Nanocomposites Prepared by in Situ Polymerization Viney Saini,† Zhongrui Li,*,† Shawn Bourdo,‡ Enkeleda Dervishi,† Yang Xu,† Xiaodong Ma,† Vasyl P. Kunets,§ Gregory J. Salamo,§ Tito Viswanathan,‡ Alexandru R. Biris,| Divey Saini,⊥ and Alexandru S. Biris*,† Nanotechnology Center, UniVersity of Arkansas at Little Rock, Little Rock, Arkansas 72204, Department of Chemistry, UniVersity of Arkansas at Little Rock, Little Rock, Arkansas 72204, Physics Department, UniVersity of Arkansas, FayetteVille, Arkansas 72701, National Institute for Research and DeVelopment of Isotopic and Molecular Technologies, P.O. Box 700, R-400293 Cluj-Napoca, Romania, and Duke Human Vaccine Institute, Duke UniVersity, Durham, North Carolina 27710 ReceiVed: October 26, 2008; ReVised Manuscript ReceiVed: March 19, 2009
In situ polymerization of thiophene to poly(3-hexylthiophene) (P3HT) was carried out in the presence of different loadings of multiwall carbon nanotubes (MWNTs) (0.1-10 wt %). It was found that the nanotubes are dispersed uniformly in the polymer matrix and the polymer chains wrap around the nanotube walls. NMR analysis indicated the presence of both CH-π and π-π interaction between P3HT and MWNTs, thereby enhancing the optical, thermal, and electrical properties of the nanocomposite for certain loadings of MWNTs. With an increase in MWNT concentration the head-tail (H-T) regioregularity of the composites was found to decrease. The UV-vis spectra of the composite films show a red shift of the π-π* transition band with increasing MWNT concentration, which was attributed to the uncoiling of the P3HT chain on the MWNT surface. Additionally, the charge carrier transport phenomenon was studied for these nanocomposites. The temperature-dependent conductivity measurement revealed that the addition of MWNTs into P3HT polymer shifted the main conduction mechanism from variable-range hopping to fluctuation-assisted tunneling, and the polymer acted as a barrier in bundle-to-bundle hopping. 1. Introduction Recently, conducting polymer-fullerene composites have attracted much attention for their potential applications due to their unique chemical, mechanical, optical, and conducting properties.1-3 Inherent conducting polymers (ICPs) and C60 have shown promise for applications in photovoltaic devices, owing to efficient electron transfer from the conjugated polymer to C60 under visible light.4 In this respect, ICPs and carbon nanotubes (CNTs) are of great interest concerning the novel electronic interaction between these two elements and the mechanical reinforcement of the polymeric materials. Poly[3(2-hydroxyethyl)-2,5-thienylene] (PHET) produced in situ with MWNTs has shown increased conductivity because of the coating of nanotube walls with polymer in opposition to the composite prepared by sonicating a mixture of PHET and nanotubes.5 An increased short-circuit current density has been found in the dye, N-(1-pyrenyl)maleimide (PM), functionalized single wall carbon nanotubes (SWNTs) blended with poly(3octylthiophene) (P3OT) photovoltaic cell versus SWNT-polymer photovoltaic cell without the dye. It has been theorized that the increased photocurrent may be due to the dye acting as an interface for better transfer of charge carriers between SWNTs * To whom correspondence should be addressed. E-mail:
[email protected] and
[email protected]. † Nanotechnology Center, University of Arkansas at Little Rock. ‡ Department of Chemistry, University of Arkansas at Little Rock. § Physics Department, University of Arkansas. | National Institute for Research and Development of Isotopic and Molecular Technologies. ⊥ Duke Human Vaccine Institute, Duke University.
and P3OT mainly because of ground state interactions between SWNT, dye, and P3OT.6 P3HT is an ICP attracting increasing attention because of its solubility in various solvents and high electrical conductivity when electrons are added or removed from the conjugated π-orbitals via dopping.7,8 The alkyl group plays an important role for P3HT in finding applications in electronics and optoelectronics and is incorporated in the thiophene ring with two different regioregularities: head-to-head (HH) and headto-tail (HT). The HT-regioregularity is preferred over HHregioregularity since it improves electroconductivity, optical nonlinearity, and magnetic properties.9 P3HT also exhibits photoluminescence properties based on the tunability of procedures used during synthesis.10,11 The P3HT-CNT composites exhibit interesting physical, optical, and conductivity properties. But for a fruitful use of these properties, a clear understanding of the behavior of the P3HT-CNT composites prepared from different techniques is necessary. Carbon nanotubes are difficult to process due to their insolubility in commonly used solvents. One way to overcome this problem is to wrap them with polymer chains, by which they can be dissolved in many of the solvents.12 Wrapping the polymer onto the walls of CNTs through noncovalent interactions, like, π-π interaction and or CH-π interaction, can improve the dispersity of CNTs in polymeric materials.13 Moreover, the polymerization of monomer can be carried out in the presence of CNTs functionalized by nonpolar groups to prepare polymer wrapped CNTs.14 Nandi et al. reported successful wrapping of the MWNTs with P3HT polymeric chains by in situ polymerization and some preliminary results of the morphology, structure, thermal properties, optical properties, and conductivity for these composites with varying
10.1021/jp809479a CCC: $40.75 2009 American Chemical Society Published on Web 04/17/2009
8024
J. Phys. Chem. C, Vol. 113, No. 19, 2009
MWNT concentration.15 However, they still lack the underlying interplay, especially the H-T regioregularity, and the conductance mechanism of the composites remains poorly understood. In this work, in situ polymerization of soluble conductive polymer P3HT was performed in the presence of different doping levels of MWNTs (named as PM-x, where x is the percentage of MWNT in the composite) in order to uniformly disperse MWNTs in P3HT for applications in optoelectronic devices. For the application in organic photovoltaic devices, MWNTs can provide better carrier transport than SWNTs, since semiconducting tubes usually dominate SWNT product. MWNT also offers a better mechanical strength. Additionally, MWNT can be produced in large quantity with a much lower cost. The MWNTs used in this work were synthesized by radio frequency chemical vapor deposition (CVD) method by pyrolysis of acetylene over Fe-Co/CaCO3 metal catalyst nanoparticles. We also performed a more systematic investigation of the morphological, optical, thermal, and electrical properties of the in situ polymerized P3HT-MWNT nanocomposites in order to understand the underlying interplay, especially the H-T regioregularity and the conductance mechanism of the composites. 2. Experimental Section 2.1. Synthesis of MWNTs, P3HT, and P3HT-MWNT Composites. The MWNTs were prepared by a catalytic chemical vapor deposition (cCVD) method. The synthesis was carried out by catalytic decomposition of acetylene over a Fe-Co/ CaCO3 catalyst system under radio frequency heating at 720 °C for 30 min. The as-produced CNTs has a mixture of byproducts such as the unreacted catalyst metal nanoparticles, amorphous carbon, and other carbon species. Therefore, the asproduced MWNTs were purified by refluxing in HCl for 24 h. The nanotubes were further refluxed in deionized water for 24 h and dried at 100 °C for the next 24 h. The purity level of the nanotubes was above 93% after one acid wash treatment. P3HT was synthesized by oxidative polymerization of the 3-hexylthiophene (3HT) monomer in the presence of anhydrous FeCl3 at room temperature. The thiophene (3HT) monomer (99+%), anhydrous iron(III) chloride (97%), chloroform, and other organic solvents were purchased from Aldrich and were used as provided without any further purification. Anhydrous FeCl3 (20 mmol) was placed in a 500 mL 3-necked bottle with 100 mL of CHCl3. The suspension was stirred for 15 h under nitrogen flow. 3HT (5 mmol) was added to the suspension via a syringe and the solution was stirred for another 24 h under nitrogen flow. The black mixture was transferred to a methanol-HCl mixture (9:1), which was stirred for 5 min. The black precipitate was then filtered with a vacuum filtration system by using a Teflon membrane of 25 nm pore size. The black solid was washed in a Soxhlet’s extraction unit with methanol for 60 h. The P3HT-MWNT composites were synthesized under the same conditions and procedure as described above except by sonicating MWNTs in CHCl3 and then adding them together with 3HT in the FeCl3 suspension and stirring for next 24 h. The P3HT-MWNT nanocomposites were prepared with 0.1, 1.0, 5.0, and 10 wt % (identified as PM-0.1, PM-1, PM-5, and PM10 respectively) MWNTs by the weight of monomer. 2.2. Characterization of P3HT-MWNT Composite. Multiple techniques were utilized to evaluate the properties of P3HTMWNT composites. Raman scattering study of the P3HT was performed at room temperature by using a Horiba Jobin Yvon LabRam HR800 equipped with a charge-coupled detector, a spectrometer with a grating of 600 lines/mm, and a He-Ne
Saini et al. laser (633 nm) as excitation source. The laser beam intensity measured at the sample was kept at 5 mW. The microscope focused the incident beam to a spot size of
