A Facile Synthesis of Polypyrrole/Carbon Nanotube Composites with Ultrathin, Uniform and Thickness-Tunable Polypyrrole Shells
© Zhang et al; licensee Springer. 2011
Received: 5 October 2010
Accepted: 17 June 2011
Published: 17 June 2011
An improved approach to assemble ultrathin and thickness-tunable polypyrrole (PPy) films onto multiwall carbon nanotubes (MWCNTs) has been investigated. A facile procedure is demonstrated for controlling the morphology and thickness of PPy film by adding ethanol in the reaction system and a possible mechanism of the coating formation process is proposed. The coated PPy films can be easily tuned by adding ethanol and adjusting a mass ratio of pyrrole to MWCNTs. Moreover, the thickness of PPy significantly influences the electronic conductivity and capacitive behavior of the PPy/MWCNT composites. The method may provide a facile strategy for tailoring the polymer coating on carbon nanotubes (CNTs) for carbon-based device applications.
Over the last two decades, carbon nanotubes (CNTs) have been widely used as fillers in desirable combinations with functional polymers because of their high electrical conductivity, chemical stability, low mass density, and large surface area [1–3]. Composite materials of CNTs and polymers have attracted great interest because they may possess novel combinations with superior characteristics than either of the individual components [4–9]. Among them, it has been already confirmed that the composites consisted of electronically conducting polymers (ECPs) and CNTs possess the superior electrical properties than either of the individual components , which are potential materials for the development of organic electronic devices, such as organic photovoltaic cells,  biologic sensors  and flexible light-emitting diodes. To the best of our knowledge, the interfacial structure between nanotube and polymer including the morphology and thickness of polymer is critical to tailor their structures and properties in many potential applications.
So far, a variety of methods such as chemical oxidation process, electrochemical or chemical polymerization through surfactants and template synthesis [14–19] have been investigated for producing composites from the combination of CNTs with conducting polymers. Unfortunately, CNTs have often been coated with thick and nonuniform layers, which range from 50 to 80 nm [14–18], and encapsulated aggregation of CNTs within the bulk polymer matrix due to the poor solubility of CNTs and partial exfoliation of nanotube bundles. Moreover, successful results of the PPy/CNT composites with tunable thickness of the polymer shell have rarely been obtained [21, 22]. The major problem exists in the processibility of CNTs in solution and the controll of interfacial bonding in polymer/CNTs composites. Due to the hydrophobic nature and strong van der Waals interactions between CNTs, as-produced CNTs pack into crystalline ropes and tangle networks which are found to act as an obstacle to most applications, especially diminishing the special mechanical and electrical properties of the individual tubes . Furthermore, inherently weak nanotube-polymer interactions result in the poor interfacial adherent , which will lead to the agglomeration of conjugated polymers. The polymer chains incline to form deposits of irregular nanoparticles or sediments with a diameter of about 50 nm[19, 20, 23]. Consequently, one way to overcome these limitations is to control the polymerization rate of the pyrrole monomers and improve the processibility of CNTs in solution.
Herein, we report a facile approach to assemble ultrathin and uniform PPy films onto multiwall carbon nanotubes [MWCNTs] to form a one-dimensional hybrid nanostructure by an improving in situ chemical oxidation polymerization. The addition of ethanol in the aqueous reaction system is a key point for tuning the morphology and thickness of PPy shell by controlling the polymerization rate , which overcomes the significant challenge in enhancing the interfacial bonding between polymer and carbon nanotubes. The PPy/MWCNT composites possess the core (individual MWCNT)/shell (PPy film) structure and no agglomerations or irregular nanoparticles of polymer are found on the surface of the composites. Furthermore, the synthesis process does not need any surfactant assistance and the thickness of the polymer shell can be precisely controlled by adding ethanol and changing the mass ratio of PPy/MWCNT. Moreover, the influences of the thickness of coating-polymer on the electrical properties of the PPy/MWCNT composites have been explained systemically. The results can provide the basis for tuning the polymer thickness to improve the properties of carbon-based device.
Results and Discussion
Normally, there are significant challenges in tuning the thickness of the polymer shell, since it is intractable in processing chemically the synthesized polymer onto the surface of the carbon nanotubes. Fortunately, an ultrathin and strongly adherent polypyrrole shell grown on the surface of carbon nanotubes are readily obtained directly by our improved method. The morphology and the thickness of polypyrrole shells were kept nicely in our reproducible tests, permitting tuning the thickness of polymer shell by changing the mass ratio of Pyrrole monomers to MWCNTs. Therefore, the PPy/MWCNT composites with tunable thickness of polymer shell were easily fabricated.
On the other hand, the specific capacitance of electrochemical supercapacitor depends strongly on not only the rates of ionic mass transport but also the series resistance (R) [34, 35, 44]. For further understanding the relationship between the thickness of polymer shell and electrochemical properties, the resistance of the PPy/CNT composites are investigated by the electrochemical impedance spectroscopy [EIS], which is another powerful tool for mechanistic analysis of interfacial processes and for evaluation of double-layer capacitance, rate constants, etc . The EIS can be observed as a single and distorted semicircle in the high-frequency region and a near-vertical line in the low-frequency region for both the Nyquist plots. The semicircle portion corresponds to the electron transfer-limited process, whereas the linear part is characteristic of the lower frequencies range and represents the diffusion-limited electron-transfer process [46, 47]. It can show all of resistances of supercapacitors, which are the electrolyte resistance (Rs) and the sum of the electrode itself and the contact resistance between the electrode and the current collector (Rf). The electrolyte resistance and the contact resistance are identical under the same test condition. Therefore, an increase of Rf indicates an increase of the PPy/CNT electrode resistance which is represented by the diameter of the semicircle on the Z' axis in impedance plots (Z*plots)  Based upon this, as shown in Figure 7B, it is clear that the diameters of semicircle of PPy/MWCNTs composites increase with the increase of PPy thickness, indicating a clear dependence of charge-transfer resistances on the polymer thickness. Therefore, the relationship between PPy thickness and electrical properties of the PPy/CNT composites should include: 1) Thin PPy shell is facile to enter into/eject cations and anions. As the PPy thickness increases, the ionic mass transport becomes slow to reach all the available interfaces between PPy and CNT due to the more compact polymer [34, 35]. 2) Compared with Figures 7A and 7B, for PPy/CNT composites with a thinner PPy shell (< around 30 nm), the diffusion-limited electron-transfer process may dominate the electrical properties of the composite because the electrode itself resistance plays a major role in the specific capacitance. Like in a metallic system, the diffusion of the charge carriers is determined by the band structure around the Fermi energy and hence, much information about the electronic band structure of polymer/CNT composite can be obtained , However, when the thickness of polymer is thicker than 40 nm, other factors such as the rates of ionic mass transport and compactness of the sample may become more important. Thus, the electron transfer-limited process dominates the electrical properties of the composites because the electron transfer resistance of the polymer/CNTs composites increases with the polymer thickness . Thus, controlling the thickness of the polymer coating on the CNTs plays an important role in functionalizing the CNTs. Furthermore, this approach could provide a more efficient way for further researches in the carbon nanotube based composites.
In summary, an ultrathin and uniform polypyrrole (PPy) film has been successfully coated on MWCNTs through an improved in situ chemical oxidation polymerization. The thickness of the polymer can be precisely controlled by adding ethanol in the reaction system and adjusting the mass ratio of PPy/MWCNT. The possible mechanism is that ethanol has a pivotal effect on controlling the degree of self-polymerization of pyrrole monomers and the morphology of polymer film on MWCNTs surface by restraining the polymerization reaction rate. The thickness of PPy film has a great effect on the electrical properties of polymer/CNT composites. The facile synthesis method may provide a very promising candidate avenue in controlling the morphology of polymers coating on carbon nanotubes, especially in fabricating the desirable performance of electronic devices.
The milled MWCNTs were carefully separated through 200 mesh screen and then functionalized in 2.6 M nitric acid (HNO3) at 80°C for 14 h to get abundant carboxyl groups at the defect sites and the end of the nanotubes . Subsequently, the carboxylic acid-functionalized MWCNTs were thoroughly washed with distilled water and centrifuged several times until the aqueous solution reached a neutral pH and left to dry in air. Whereafter, they were dispersed in the mixed solution with 1 M HClO4 solution and ethanol (Vethanol/Vacid solution = 1:5) followed by 10 min of ultrasonication. Pyrrole monomers were added to the solution and the mixture was vigorously stirred for 30 minutes. The equal molar of ammonium persulfate (APS) dissolved in acid solution was slowly added to initiate the polymerization at 0~5°C. This mixture was stirred by magnetic stirring for 8 h. At the end of the reaction, a litte acetone was added to terminate the reaction. Following the typical preparation, the PPy/MWCNT composites can be prepared with various thickness of PPy shell by changing the mass ratio of pyrrole monomer/MWCNT.
High-resolution microscopy measurements were performed using a JEM-2010HR transmission electron microscope (TEM) with operating voltage of 120 kV. Raman spectra were recorded at room temperature utilizing back scattering mode on a Renishaw inVia system. The 514.5 nm line of an Ar+ laser was used as the excitation resource. A thermogravimetric analysis [TGA] was carried out in a NetzschTG-209 system. The samples were scanned from 0 to 900°C at a heating rate of 10°C/min in the presence of nitrogen. Morphology and microstructure of the as-obtained composites were performed using X-ray diffraction (XRD, Rigaku D/MAX 2200 VPC, Rigaku company, Japan). The cyclic voltammetry was conducted by an electrochemical station (CH Instruments 660 C, Shanghai Chenhua, China) using conventional three-electrode conFigureuration with a platinum sheet as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode . The electrolyte containing 1 M KCl dissolved in aqueous solution was deoxygenated under a flow of N2 for 30 min. The specific capacitance obtained from the CV curve could be calculated according to the equation C = I/sm, where 'I' was the average current, 's' was the potential sweep rate, and 'm' was the mass of each electrode. The composite electrodes were prepared by dispersing the PPy/MWCNT composites or carbon nanotubes, pure PPy samples and PTFE (5%), followed by adding a small amount of ethanol and NMP to yield a homogenous paste. The paste was spread onto the nickel foam collectors (1 × 1 cm2) and then pressed under 10 MPa. These electrodes were dried in vacuum at 60°C for 24 h. Electrochemical impedance spectroscopy (EIS) measurements (excitation signal: 5 mV; frequency range: 100 kHz down to 10 mHz) were carried out using an IME6X electrochemical workstation.
Financial support from the program of National Natural Science Foundation of China (Grant no. 50673104) and Natural Science Foundation of Guangdong province (Grant no. 7003702) are gratefully acknowledged.
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