Spectroscopic Characterization of Carbon Nanotube-Polypyrrole

Jul 15, 2014 - ... nanotubes (MWNTs) with in situ chemically polymerized polypyrrole (PPy) films (approximately 8 nm) by resonance Raman spectroscopy...
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Spectroscopic Characterization of Carbon Nanotube-Polypyrrole Composites Fabiana Inoue,† Rômulo A. Ando,† Celly M. S. Izumi,‡ and Paola Corio*,† †

Instituto de Química, Universidade de São Paulo, São Paulo SP, Brazil Departamento de Química, Universidade Federal de Juiz de Fora, Juiz de Fora, MG Brazil

‡

ABSTRACT: The purpose of this study is to investigate the chemical interaction between carboxylfunctionalized single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) with in situ chemically polymerized polypyrrole (PPy) films (approximately 8 nm) by resonance Raman spectroscopy. The composites (SWNT/PPy and MWNT/PPy) were synthesized using nitric acid functionalized nanotubes as the starting material. The resulting composites were characterized by transmission electronic microscopy (TEM) and by resonance Raman spectroscopy (λexc. = 514.5, 632.8, and 785 nm). The TEM images of the nanocomposites revealed that PPy wrapped around the carbon nanotubes. The resonance Raman data showed evidence of charge transfer between composite moieties and a stronger interaction between the carbon material and the polymer in the SWNT/PPy compared to MWNT/PPy. Specifically, an intense charge-transfer process occurred between PPy and the SWNTs, significantly perturbing the electronic structures of both moieties. Analysis of the SWNTs in the SWNT/PPy spectra showed a softening of the C−C bond in both metallic and semiconducting SWNTs and an increase in the Breit−Wigner−Fano (BWF) contribution to the G-band shape of the metallic SWNTs, which resulted from an increase in the electronic density of the SWNT π band. Accordingly, the resonant Raman spectra of PPy in SWNT/PPy showed an increase in the bipolaron/polaron ratio compared to standard PPy, indicating an increase in the polymer’s doping level. Thus, the resonance Raman data were consistent with an increase in the electronic density of metallic and semiconducting nanotubes and a decrease in the electronic density of the polymer, indicating a PPy → SWNT charge transfer in the composite. Our data suggested the formation of a true composite material in the SWNTs with enhanced conductive properties because the interaction with the SWNTs resulted in the stability of the most doped and conductive form of PPy.

1. INTRODUCTION The formation of hybrid materials is an interesting approach to enhance the properties of materials because composite structures may exhibit characteristics that differ from those of the individual component compounds.1 The remarkable physical and chemical properties of carbon nanotubes (CNTs) have stimulated the investigation of CNTs-based nanostructures, such as CNTs/polymer hybrid materials.2 Hybrid composites formed between single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) and insulating or conducting polymers attracted considerable attention in part because the synergic combination between both constituents offers new electronic and mechanical properties that can be tailored for a wide range of applications.3 The combination of CNTs with conducting polymers in particular provides synergistic results by enhancing the thermal and mechanical stability, electronic properties, and processability of the material.4 CNT/conducting polymer nanocomposites are well-established in various applications, such as in solar cells and sensors.5,6 The combination of CNT properties with functional polymers, such as polypyrrole (PPy), polyaniline, and polythiophene, is particularly interesting.7 The primary interaction mechanism during the preparation of conjugated polymers/CNT composites consists of a charge transfer between the two constituents, regardless of the final © 2014 American Chemical Society

form (i.e., whether the polymer functionalizes the CNTs or if the polymers are doped with CNTs).3 Previous studies addressing the properties of composites between CNTs and conducting polymers showed that the interaction with CNTs significantly changes the conductivity of the polymer.8,9 One important approach to the synthesis of carbon nanotube-based nanostructures is polymer wrapping, which results in the dispersion of CNTs throughout the host material. The effective utilization of CNTs in composite applications depends on the ability to disperse them throughout the polymeric matrix; thus, understanding the interfacial interaction between the carbon material and the polymer would benefit in the preparation of such composite materials. Raman spectroscopy is a well-established technique used to investigate the electronic and vibrational properties of CNTs; it can be used to probe modifications of the CNT electronic and vibrational properties caused by interactions between different chemical species.10,11 Raman spectroscopy is an important technique to study both the electronic structure and vibrational spectra of conducting polymers. Therefore, Raman spectroscopy can be a powerful technique in the study of chemical Received: June 4, 2014 Revised: July 11, 2014 Published: July 15, 2014 18240

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interactions between CNT/polymer composite moieties because it is well-established in the investigation of the electronic and vibrational properties of both moieties. The detailed analysis of resonance Raman spectra may provide insights into the nanotube−polymer interaction. Recently, individual MWCNTs have been coated with an unprecedentedly uniform PPy layer that is less than 15 nm thick.12 Typically, composites where CNTs are embedded in bulk PPy13 are coated with thick (50 to 80 nm) and nonuniform layers of PPy14,15 or covered by PPy nanoparticles (approximately 50 nm),16 thus the possibility of achieving an ultrathin layer of polypyrrole is very favorable for the investigation of electronic interaction between the polymer and CNTs. In this study, this protocol was used for MWNTs and adapted for coating SWNT bundles. In this study, SWNTs, MWNTs, and polypyrrole (PPy) hybrid nanocomposites were synthesized by the in situ chemical polymerization of PPy films, where nitric acid functionalized SWNTs and MWNTs were used as starting material. The composites (hereafter called SWNT/PPy and SWNT/PPy) were characterized by transmission electron microscopy (TEM) and electronic and resonance Raman spectroscopy (λexc. = 514.5, 632.8, and 785 nm). Our goal was to study the modification of the nanotube and polymer properties during their interactions using resonance Raman spectroscopy, which was highly sensitive to the electronic properties of both nanotubes and conducting polymers. Herein, the electronic interaction between CNTs and the polymer is discussed.

suspended in water and drop wised to copper TEM grids. The grids were analyzed using a FEI/PHILIPS CM120 microscope. 2.3. Synthesis of the CNT-Polypyrrole Composites. The procedure was based on the work of Pumera et al.,17 who developed a coating procedure for MWNTs; slight modifications were made to the procedure when coating the SWNTs. Oxidation of SWNTs18 and MWNTs.19 The SWCNTs and MWNTs were functionalized in a 125 mL reaction flask equipped with a reflux condenser, magnetic stirrer, and thermometer in an oil bath. Approximately 100 mg MWNTs was refluxed in 15 mL of concentrated nitric acid (65%) at 120 °C for 48 h, and approximately 100 mg SWNTs was refluxed in 15 mL of 3 mol L−1 nitric acid at 120 °C for 24 h. The acid/ CNTs mixtures were washed with distilled water and centrifuged until the aqueous solution reached a neutral pH. The carboxylic acid-functionalized CNTs (f-SWNT and fMWNT) were filtered through a 0.45 μm Millipore membrane and dried at 60 °C for 24 h. Coating of f-MWNTs and f-SWNTs. Initially, pyrrole was distilled under reduced pressure. The coating step involved the dispersion of 2 mg of f-MWCNTs or f-SWNTs in 4 mL distilled water followed by 1 h of mixing by ultrasonication. A 100 μL pyrrole solution (451 mmol L−1) in 2-propanol and 410 μL 10 mmol L−1 (NH4)2S2O8 in water were added to the CNT suspension. The mixtures were stirred for 24 h, filtered through a 0.2 μm membrane, washed with distilled water, and left to dry in air. Synthesis of Polypyrrole. For comparison, polypyrrole was synthesized identically to the CNT/PPy composites in the absence of CNTs.

2. EXPERIMENTAL SECTION 2.1. Materials. The MWNTs were kindly donated by the ́ Laboratório de Nanomateriais, Departamento de Fisica, Universidade Federal de Minas Gerais (UFMG). The MWNTs were synthesized by chemical vapor deposition (CVD) using ferrocene as a catalyst and ethylene gas as a carbon source. The process temperature ranged from 700 to 800 °C. The SWCNTs produced by the arc discharge method were purchased from Carbon Solutions Inc. (2−10 nm in diameter and >95% purity). Pyrrole and other chemicals were purchased from SigmaAldrich or Merck. All chemicals were analytical reagent grade and used as received. 2.2. Equipment. Ultraviolet−visible/near-infrared (UV− vis/NIR) spectra were obtained with a Shimadzu UVPC-3101 scanning spectrophotometer. The Raman spectra were acquired on a Renishaw Raman InVia equipped with a CCD detector and coupled to a Leica microscope that allowed a rapid accumulation of Raman spectra with a spatial resolution of approximately 1 μm (micro-Raman technique). The laser beam was focused on the sample with a 50× lens. The samples were irradiated at 632.8 nm with a He−Ne laser or 782 nm from a diode laser. Raman spectra were also acquired on a Renishaw Raman System 3000 equipped with a CCD detector and coupled to an Olympus microscope (BTH2) with a 50× lens and a spatial resolution of approximately 1−2 μm (microRaman technique). The samples were irradiated at 514.5 nm using an Ar laser. The laser power was always kept below 0.7 mW when analyzing the samples. The experiments were performed at ambient conditions using backscattering geometry. The morphology of the samples was characterized by transmission electron microscopy. CNTs or composites were

3. RESULTS AND DISCUSSION 3.1. Electronic and Morphological Characterization. The TEM images of the f-MWCNTs and MWCNTs/PPy are shown in Figure 1. The f-MWCNTs (Figure 1A) present a

Figure 1. TEM images of (A) f-MWCNTs and (B) MWCNT/PPy.

tubelike structure characteristic of the CTNs bundles. The TEM image of MWCNT/PPy (Figure 1B) clearly shows that the surface of MWCTs presents a polymer coating confirming that a PPy layer is formed on CNTs surface. The coating is continuous and relatively uniform over the CNTs with thickness varying from 7 to 15 nm. PPy exists in two oxidation states, one that is doped (oxidized), which has good electrical conductivity, and one that is undoped (reduced), which has very low electrical conductivity. The polaron and bipolaron states are associated with the intermediary energy levels that arise within the electronic band gap region of the polymer due to oxidation.20 Because the sub-band gap transitions become allowed due to the polaron−bipolaron formation, these new electronic levels provide intermediate channels to promote electrons from the valence band (VB) to the conduction band (CB), decreasing 18241

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spectra were dominated by a broad, single band located between 700 and 1300 nm that could be related to the conductive form of PPy (dication). The sharper band centered at approximately 450 nm was assigned to the neutral polymer.22 No well-defined absorption bands could be observed for the CNTs and CNTs/PPy composites. The fact that the composites did not show absorption band characteristics of the polymer may be due to the fact that an ultrathin layer of PPy might have been wrapped around the nanotubes, and therefore, UV−vis spectroscopy did not achieve enough sensitivity to reveal the presence of the polymer. 3.2. Resonant Raman Characterization. Resonance Raman spectroscopy is sensitive to the electronic structure and vibrational properties of both CNTs and conducting polymers. To characterize the interaction between CNTs and PPy, Figures 3 and 4 show the resonance Raman spectra of pristine CNTs, carboxyl-functionalized CNTs and CNT/PPy composites obtained from different laser excitation energies. The resonance Raman spectra of both SWNT/PPy and MWNT/PPy composites were strongly dependent on the laser energy (Figures 3 and 4). At 514.5 nm, the spectra of the SWNT/PPy and MWNT/PPy composites were dominated by the vibrational mode characteristics of the nanotubes (the strong band in the frequency range of 1500−1650 cm−1, associated with the ν(C−C) stretching modes (tangential G band), the disordered disorder-induced D-band feature at approximately 1350 cm−1, and the highly dispersive secondorder harmonic of the D-band (G′ band) at approximately 2683 cm−1 for SWNT/PPy and 2706 cm−1 for MWNT/PPy; no significant contribution of the PPy Raman bands were observed. In addition, the presence of PPy in the composites could not be concluded from the Raman data at this wavelength. As the laser wavelength increased, a preresonance with the polymer electronic band occurred. At 632.8 nm, the

the energy required for the electronic transition and leading to a red shift in the UV−Vis spectrum.21 Figure 2 shows the

Figure 2. Electronic absorption spectra of PPy, f-SWNT, f-MWNT, SWNT/PPy, and MWNT/PPy obtained by diffuse reflectance. The wavelengths used for acquiring the Raman spectra are indicated.

visible and near-infrared (NIR) region of the electronic absorption spectra of the PPy and CNT/PPy composites obtained by diffuse reflectance. The PPy electronic absorption

Figure 3. Raman spectra of pristine SWNTs, p-SWNT, nitric acid functionalized SWNT, f-SWNT, and SWNT/PPy. The spectra were obtained at λexc. = 514.5, 632.8, and 785 nm. 18242

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Figure 4. Raman spectra of pristine MWNT, p-MWNT, nitric acid functionalized MWNT, f-MWNT, and MWNT/PPy. The spectra were obtained at λexc. = 514.5, 632.8, and 785 nm.

Table 1. Average Peak Positions of G-Band and G′-Band at Distinct Excitations Wavelengths λexc = 514.5 nm −1

G band (cm ) f-SWNT SWNT/PPy Δ (cm−1) f-MWNT MWNT/PPy Δ (cm−1)

1603 1595 8 1582 1583 −1

± 1.9 ± 0.5 ± 0.5 ± 0.6

λexc = 632.8 nm −1

G′ band (cm ) 2686 2683 3 2706 2706 0

± 2.3 ± 0.7 ± 0.4 ± 0.4

−1

G band (cm ) 1603 1595 8 1583 1591 8

± 0.9 ± 0.7 ± 0.5 ± 0.9

λexc = 785 nm −1

G′ band (cm ) 2645 2639 6 2666 2665 1

± 0.6 ± 0.5 ± 0.4 ± 0.5

−1

G band (cm ) 1606 1598 8 1584 1586 −2

± 1.3 ± 0.5 ± 0.4 ± 0.5

G′ band (cm−1) 2588 2605 17 2622 2620 2

± 0.6 ± 1.9 ± 0.5 ± 0.6

treatment. In fact, changes of nanotube diameter distribution have been reported after oxidative purification processes.24 The observed shift of RBM peak to higher wavenumbers is associated with a decrease of SWNT diameter. Thus, the changes in the low frequency Raman spectrum in the f-SWNT sample as compared to pristine SWNT can be explained by the preferential destruction of SWNTs with larger diameters during the oxidation process. This behavior can be expected because SWNTs with larger diameters usually contain more defects, being more prone to oxidative damage.24 3.3. CNT Spectra: Frequency Shifts. Our discussion initially focused on the changes to the nanotube spectra that occurred when interacting with the polymer. Exposure of the nanotubes to species with reducing or oxidizing properties can modify the occupation of the electronic states of the nanotubes, as has previously been observed by electronic optical absorption measurements on thin films of CNTs.25 Resonance Raman data are useful in monitoring the perturbation of the sidewall π-electron density in carbon nanotubes because of sidewall oxidation or reduction reactions, and the frequency of CNT vibrational bands are sensitive to the density of electrons in the CNT.26 The charge transfer process induced a softening or hardening of the C−C bond, which affected the frequency of the CNT vibrational modes. Displacement to higher wave-

spectrum showed the vibrational features of nanotubes; however, some characteristic bands of the polymer were also observed with a weak relative intensity especially for SWNT/ PPy, at approximately 939 and 1092 cm−1 as the exciting radiation energy approached the PPy electronic transition. At 785 nm, the vibrational mode characteristics of both the nanotubes and PPy were clearly observed in the Raman spectra of the SWNT/PPy and MWNT/PPy composites (at 939 and 936 cm−1, respectively). It is important to point out that for the 785 nm excitation, the Raman spectrum of the pristine SWNT is different from the acid-oxidized and polymer-coated samples in the low-frequency region of 150−200 cm−1. The Raman signal in this region is characteristically associated with the A1g radial breathing (RBM) mode of SWNTs. The RBM is a unique feature in the Raman spectrum of SWNTs and involves a collective vibrational movement of the carbon atoms toward and away from the central axis of a SWNT.23 The RBM frequency shows a strong dependence on the SWNT diameter and shifts to lower wave numbers as the diameter of the nanotube increases. As shown in Figure 3, while the pristine SWNT spectrum shows a RBM band at 153 cm−1, the f-SWNT and SWNT/PPy spectra show a RBM band at 172 cm−1. This suggests a structure modification of the SWNTs during nitric acid 18243

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frequency of the G′ band. However, the displacement of the G′-band was significantly dependent on the energy of the exciting wavelength due to its dispersive behavior, which was different from the behavior of the G band. However, the displacement of the G′-band was significantly dependent on the energy of the exciting wavelength, which was different from the behavior of the G band. The obtained results showed that this displacement increased as the energy of the exciting wavelength decreased. In fact, a strong dependence of the doping-induced shift second order G′ band on the excitation energy has been previously reported by Rafailov and Thomsen.31 These authors investigated electrochemically doped CNTs and observed different frequency shifts per unit applied potential in the second order G′ band. Measurements of the doping-induced G′ shift (in inverse centimeters per voltage) at several excitation wavelengths revealed increasing values for longer exciting wavelengths. This is in agreement with the results in Table 1, where the obtained results showed that the displacement of the G′ band upon interaction with the polymer increases as the energy of the exciting wavelength decreases. In accordance with Rafailov and Thomsen,29 this behavior can be explained with consideration of the framework of a double-resonant model for Raman scattering, where the G′ band is considered to originate from phonons in the vicinity of the graphite K-point and appear in the Raman spectra due to a defect-induced double-resonant process. Doping shifts the phonon branch, altering the doubleresonance condition. Different frequency slopes of the G′-band at different excitation energies thus imply that the dopinginduced shift varies with wave vector. Therefore, the strong dependence of the G′- band shift with doping on excitation energy gives evidence about the doping-induced shift of whole regions of phonon dispersion branches. 3.4. Analysis of the Tangential G-Band of SWNTs. In addition to frequency shifts, changes in line shapes were also evident in the SWNT bands. The line shapes of the tangential G-band mode observed at λexc. = 632.8 and 785 nm (favorable for the resonance and preresonance with an E11 transition of metallic tubes) were analyzed in terms of the Breit−Wigner− Fano (BWF) contributions for the lower frequency G-band component (Figure 5). The BWF line shape in metallic SWNTs has been assigned to an electron−phonon coupling mechanism.32 Electron−phonon coupling is a key physical parameter in nanotubes, playing a key role in defining the phonon dispersions and the Raman spectra of nanotubes. This mechanism occurs through a resonance between phonons and electron−hole pairs and explains the difference in the Raman spectra of metallic and semiconducting nanotubes.28 It is strongly dependent on the electronic density near the Fermi level, being the major source of broadening for the Raman G and G peaks in graphite and metallic nanotubes.28 Only the metallic nanotubes exhibit the BWF line shape, and its existence is frequently used to distinguish between the metallic and semiconducting SWNTs in any given sample.33,34 There was a significant increase in the relative intensity of the BWF component at approximately 1550 cm−1, when the f-SWNTs G-band shape was compared to the SWNT/PPy. While the BWF feature of the f-SWNT sample contributed ca. 9% to the total line integrated intensity, its contribution was significantly larger for the SWNT/PPy (approximately 15%) for data obtained at 632.8 nm. Therefore, the changes in the G-band line shape indicated a transition to a more metallic line shape when the SWNTs interacted with PPy.

numbers was related to the nanotubes interaction with electron acceptors, while displacement to lower wavenumbers occurred when nanotubes interacted with electron donors.27 Table 1 shows the mean position of the G and G′-bands in fCNTs and CNT/PPy. To account for possible inhomogeneity from the sample, peak positions are shown as the average results from ten spectra taken at different spots on the sample. Data from the three wavelengths showed that the primary vibrational bands of the SWNTs shifted to lower wavenumbers for coated SWNTs, suggesting electronic interactions between both moieties in the composites. The results in Table 1 do not show a significant displacement during the interaction between the MWNTs and the polymer. This suggested a stronger interaction between the SWNTs and the polymer compared to the MWNTs. Table 1 shows that the G-band peak frequency downshifted 8 cm−1 for the PPy coated-SWNTs at all investigated laser wavelengths. The displacement of the tangential G-band mode to a lower frequency observed in the case of semiconducting nanotubes (for λexc. = 514.5 nm, for example) and in the case of metallic nanotubes (for λexc. = 632.8 nm, for example) indicated the addition of electrons to the SWNT π band. Thus, spectral changes indicated a charge transfer from PPy to metallic and semiconducting SWNTs in the composite. The dependence of the G-band peak frequency to the electronic density of the carbon nanotubes valence bands was accounted for by the changes in C−C bonds lengths. As a consequence of a charge transfer between the two constituents, the electronic density of the CNTs increased. This increased electronic density populated antibonding states. These charge transfer processes caused an increase of the C−C bonds lengths, and the decreased force constant caused a frequency downshift of the ν(C−C) stretching modes. In this study, the displacement of the G-band frequency was used to estimate the magnitude of the charge transfer in the SWNT/PPy composite. The tangential mode frequency of CNTs is significantly upshifted (downshifted) by exposure to acceptor (donor) dopants due to the transfer of electrons from (to) the carbon π (π*) states in the tubes to (from) the dopant species.28 It has been proposed that the displacement of the tangential G band is proportional to the degree of charge transfer. Consequently, a quantitative estimation about the charge transfer can be made by analyzing changes in the Raman spectrum of the pristine tubes due to chemical doping. The degree of charge transfer may be defined by the quantity f, which is the transferred charge per host C atom.29 Average values of Δω/Δf were obtained for acceptor and donor SWNTs. In this work, we use results from Raman scattering studies of the continuous electrochemical charging of SWNTs from which Δω/Δf = +320 cm−1 is obtained.30 Under the assumption that this value can be roughly extrapolated to polymer interaction with CNTs, we conclude that the magnitude of the charge transfer between the nanotubes and the polymer in the SWNT/PPy composite can be estimated as 0.025 e per C atom. The second order G′ band, located in the 2550−2700 cm−1 region, was also sensitive to the carrier density of the nanotube π band. Table 1 shows that the position of this overtone G′ band also shifts toward lower wavenumbers for the polymer coated-SWCNTs. These shifts observed in this second-order Raman feature were consistent with the increase in the SWNT electron carrier density. The softening of the C−C bond by the PPy → SWNT charge transfer effect was also observed in the 18244

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Figure 5. Lineshape analysis of the tangential G-band feature for fSWNT and SWNT/PPy composites (λexc. = 632.8 and 785 nm). A Lorentzian line shape was used to fit the G feature for both metallic and semiconducting tubes; a BWF line was fit to the lower frequency G modes from metallic tubes, while a Lorentzian line shape was fit to the G mode for the semiconducting tubes.

Figure 6. Raman spectra of PPy obtained at λexc. = 514.5, 632.8, and 785 nm.

This behavior was in accordance with the expected outcome of electrons flowing from the polymer to the nanotubes in the composite and results from the increased density of electrons in the nanotubes. The conduction electrons at the Fermi level play a crucial role in the appearance of the BWF line shape for metallic SWNTs. Therefore, the intensity of the BWF feature was strongly dependent on the electronic density at the Fermi level. This significant increase in the BWF line shape contribution was another indication that electrons were added to the conducting π band of the metallic nanotubes because of the interaction with the polymer. These data were also consistent with previous results obtained in an electrochemically controlled environment.35 Previously, applying positive electrochemical potentials have shown a decrease in the BWF contribution as a result of a downshift of the Fermi level as the SWNT electron carrier density decreased. The spectra of SWNT/PPy were similar to those observed when SWNTs were reduced in an electrochemical environment. 3.5. Polypyrrole Spectra. In addition, it is important to discuss changes to the PPy’s Raman spectra that occurred when interacting with the nanotubes. Raman spectroscopy was used to correlate the molecular structure of polypyrrole with its conducting properties, which were strongly dependent on the oxidation state in the polymer. The vibrational Raman spectra provided information on the charge distribution of the polymer and could probe its “state of conjugation”.36 Figure 6 shows the resonance Raman spectra of PPy prepared under polymerization conditions identical to those of the CNT/PPy composites. The peaks at 936 cm−1 (ring deformation) and 1086 cm−1 (CH in-plane bending) were associated with the bipolaron structure, and those at 969 cm−1 (ring deformation) and 1050 cm−1 (CH in-plane bending) were assigned to the polaron structure. The peak at 1595 cm−1, related to the CC stretching mode, was considered an

overlap of the two oxidized structures.37 The relative intensity of the ring deformation and CH in-plane bending bands were characteristics of polaron and bipolaron structures that correlated with the doping level; this method was valuable for estimating the doping level of PPy samples from their Raman spectra.31 Table 2 shows the band intensity ratios associated with bipolaron/polaron structures for standard PPy at different laser Table 2. Average Values for Band Intensity Ratios Associated with the Bipolaron/Polaron Structures in PPy for Standard PPy, SWNT/PPy, and MWNT/PPy [(I939 cm−1 + I1078 cm−1)/(I969 cm−1 + I1049 cm−1)] λexc (nm)

PPy

SWNT/PPy

MWNT/PPy

514.5 632.8 785

0.7 1.4 2.2

− − 4.1 ± 0.3

− − 1.5 ± 0.2

exciting wavelengths and for SWNT/PPy and MWNT/PPy at λexc. = 785 nm. The peaks assigned to the bipolaron structure of the standard PPY were resonantly enhanced as the exciting wavelength increased and approached the PPy electronic transition at the near-infrared region as the oxidized polymer absorbed in this region. Calculating the changes in the ratio between the intensity of the band sensitivity to the oxidation state of the polymer at a given wavelength provided a measurement of the changes in proportion to both types of charge carriers (polaron and bipolaron structures) in the polymer. Because of the low contribution of the PPy bands to the spectra of CNTs/PPy observed at 514.5 and 632.8 nm, the intensity ratio was not 18245

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determined. The PPy bands of the Raman spectra of CNTs/ PPy acquired at 785 nm are clearly seen, and it was possible to calculate the [(I939 cm−1 + I1086 cm−1)/(I984 cm−1 + I1052 cm−1)] ratio and compare it to the standard PPy. The PPy in the composite exhibited different characteristics compared to the standard ones for PPy. The bipolaron/polaron ratio increased significantly compared to the standard PPy for the SWNT/ PPy composite from 2.2 in standard PPy to 4.1 in SWNT/PPy. This observation was attributed to changes in the polaron− bipolaron ratio in PPy because of its electronic interaction with the SWNTs, suggesting that electrons were removed from the polaronic polymeric chain and gave rise to the formation of bipolarons. This formation of bipolarons resulted in the stabilization of the most doped and conductive form of PPy. Thus, the resonance Raman spectra of PPy also indicated that the polymer was oxidized by the nanotubes. Therefore, this analysis indicated that the polymer was oxidized by the nanotubes to produce an increase in the polymer’s doping level, which was consistent with the prior observation of a charge transfer PPy → SWNT. A different behavior however was observed for MWNT/PPy. A decrease in the oxidation state of the polymer occurred during the interaction with the nanotubes. However, the difference between standard PPy and MWNT/PPy was not as pronounced. This result was consistent with the smaller perturbation that occurred to the MWNT characteristic spectra during the interaction with the polymer compared to the SWNT. 3.6. ID/IG Analysis of CNTs. The relative intensity of the two characteristic graphitic peaks at approximately 1350 and 1590 cm−1, corresponding to D- and G-bands, respectively (ID/ IG), could be used to reveal the degree of disorder in the carbon network. The average values of ID/IG for p-CNT, f-CNT, and CNT/PPy calculated from the D and G band integrated areas are shown in Table 3. These values were calculated from the band areas from a data set of 10 spectra obtained in distinct and random spots in each sample.

In such conditions, the observed ID/IG ratio arises primarily from the unmodified inner tubes resembling the pristine CNT Raman spectrum. The average value of ID/IG for the MWNT/ PPy (ID/IG = 0.34 ± 0.03) was even smaller compared to the one obtained for p-MWNT (ID/IG = 0.42 ± 0.04). This result suggested that the internal tubes in the composite were chemically oxidized and had a quantity of defects that was even smaller than that in most external tubes in the pristine material. In the previous study,35 such effects were discussed for MWNTs; in the present study, SWNTs were also investigated. Because of the van der Waals interactions, the tubes of the SWNT/PPy were dispersed in bundles. Thus, the ID/IG ratio observed may have originated from more internal tubes in the bundles. However, the ID/IG ratio for the SWNT/PPy (ID/IG = 0.25 ± 0.03) was in between the values observed for p-SWNT (ID/IG = 0.058 ± 0.03) and f-SWNT (ID/IG = 0.43 ± 0.06). This result suggested that the nitric acid treatment modified the surface of the most external tubes in the bundles and the inner tubes, introducing functional groups or creating structural defects that provided symmetry breaks in the inner tubes of the bundles. Therefore, the value of ID/IG observed for SWNT/PPy originated from inner tubes in the bundles, which were also oxidized by the nitric acid treatment. The changes in the ID/IG ratio followed the same trend for SWNTs and MWNTs, but a more pronounced effect was observed for the SWNTs compared to the MWNTs. Thus, after coating with PPy, the observed Raman signal arose from inner tubes in both cases. The functionalized outer tubes did not contribute significantly to the observed Raman spectra. The difference arose from the effect of the nitric acid treatment on the different types of nanotubes. The oxidation of the SWNTs occurred in the outer and inner tubes in bundles because nitric acid penetrated into the interstitial channels of the SWNT lattice, while oxidation occurred almost exclusively in the outer tubes for the MWNTs. While the inner tubes of the SWNTs in bundles were oxidized, the inner tubes of the MWNTs were not. Figure 7 represents the proposed model.

Table 3. Average Values of the D/G Ratio of p-SWNT, fSWNT, f-SWNT/PPy and p-MWNT, f-MWNT, f-MWNT/ PPy

4. CONCLUSIONS In this study, we reported the spectroscopic characterization of nanostructures composed of PPy and CNTs obtained by chemically synthesizing PPy on carboxyl functionalized single and multi-walled carbon nanotubes. Raman spectroscopy is a powerful technique used to study the chemical interaction between the composite moieties because it is well-established in the investigation of electronic and vibrational properties of both carbon nanotubes and polymeric materials. The Raman data provided evidence of different behaviors between SWNTs and MWNTs and their chemical interaction with PPy. While Raman spectroscopy did not provide conclusive evidence of the specific interaction between the PPy and MWNT, clear evidence of an electronic interaction between PPy and CNTs could be obtained for composites made with SWNTs; this was possibly because the MWNT Raman spectrum represented an average of several concentric tubes, and the polymer only coated the most external tubes. Changes to the resonant Raman spectra of the SWNTs and PPy indicated a charge transfer from PPy to metallic and semiconducting SWNTs in the composite. The composite prepared by the use of functionalized SWNTs showed a downshift in the G and G′ Raman bands of the nanotubes when interacting with the polymeric material. However, the MWNT/PPy composite did not show a significant displacement of the characteristic features of the

sample

average value of ID/ IG

sample

average value of ID/ IG

p-SWNT f-SWNT f-SWNT/PPy

0.058 ± 0.03 0.43 ± 0.06 0.25 ± 0.03

p-WWNT f-MWNT f-MWNT/PPy

0.42 ± 0.04 0.50 ± 0.06 0.34 ± 0.03

The f-CNTs exhibited higher ID/IG ratios compared to the corresponding ratio of the pristine CNTs. The value of the ID/ IG ratio increased from 0.058 ± 0.03 to 0.43 ± 0.06 and from 0.42 ± 0.04 to 0.50 ± 0.06 after functionalization of the SWNTs and MWNTs, respectively. This change was attributed to the oxidation process that occurred during the chemical functionalization of the CNTs.38 However, this increase was followed by a decrease of the ID/IG ratio upon in situ polymerization. This behavior was analogous to the one observed between the interaction of the MWNTs and nanostructured TiO2.39 On the basis of the model previously proposed for explaining such a behavior, the presence of the PPy thin film induced a modification of the electronic structure of the most external nanotubes, decreasing their contribution to the observed Raman spectra in the composites. 18246

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Figure 7. Schematic representation of the nitric acid treatment for SWNTs and MWNTs and its effect on the observed ID/IG value after polymerization. Polymers/Carbon Nanotubes Composites. Curr. Org. Chem. 2001, 15, 1160−1196. (3) Lefrant, S.; Baibarac, M.; Baltog, I. Raman and FTIR Spectroscopy as Valuable Tools for the Characterization of Polymer and Carbon Nanotube based Composites. J. Mater. Chem. 2009, 19, 5690−5704. (4) Baibarac, M.; Baltog, I.; Scocioreanu, M.; Ballesteros, B.; Mevellec, J. Y.; Lefrant, S. One-dimensional Composites based on Single Walled Carbon Nanotubes and Poly(o-phenylenediamine). Synth. Met. 2011, 161, 2344−2354. (5) Zhang, X. H.; Wang, S. M.; Xu, Z. X.; Wu, J.; Xin, L. Poly(ophenylenediamine)/MWNTs Composite Film as a Hole Conductor in Solid-state Dye-sensitized Solar Cells. J. Photochem. Photobiol., A 2008, 198, 288−292. (6) Zhang, X. H.; Wang, S. M.; Jia, L.; Xu, Z. X.; Zeng, Y. Electrochemical Properties of Colchicine on the PoPD/SWNTs Composite-modified Glassy Carbon Electrode. Sens. Actuators, B 2008, 134, 477−482. (7) Panhuis, M. Carbon Nanotubes: Enhancing the Polymer Building Blocks for Intelligent Materials. J. Mater. Chem. 2006, 16, 3598−3605. (8) Cochet, M.; Maser, W. K.; Benito, A. M.; Callejas, M. A.; Martinez, M. T.; Benoit, J.; Schreiber, J.; Chauvet, O. Synthesis of a New Polyaniline/nanotube Composite: “In-situ” Polymerisation and Charge Transfer through Site-selective Interaction. Chem. Commun. 2001, 16, 1450−1451. (9) Ramamurthy, P. C.; Harrel, W. R.; Gregory, R. V.; Sadanadan, B.; Rao, A. M. Electronic Properties of Polyaniline/Carbon Nanotube Composites. Synth. Met. 2003, 137, 1497−1498. (10) Corio, P.; Santos, A. P.; Santos, P. S.; Temperini, M. L. A.; Brar, V. W.; Pimenta, M. A.; Dresselhaus, M. S. Characterization of Single Wall Carbon Nanotubes filled with Silver and with Chromium Compounds. Chem. Phys. Lett. 2004, 383, 475−480. (11) Jorio, A.; Pimenta, M. A.; Souza Filho, A. G.; Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Characterizing Carbon Nanotube Samples with Resonance Raman Scattering. New J. Phys. 2003, 5, 139.1−139.17. (12) Pumera, M.; Smid, B.; Peng, X.; Golberg, D.; Tang, J.; Ichinose, I. Spontaneous Coating of Carbon Nanotubes with an Ultrathin Polypyrrole Layer. Chem.Eur. J. 2007, 13, 7644−7649. (13) Wang, J.; Dai, J.; Yarlagadda, T. Carbon Nanotube− Conducting-Polymer Composite Nanowires. Langmuir 2005, 21, 9− 12.

nanotubes compared to the pristine material. However, both composites (SWNT/PPy and MWNT/PPy) showed a decrease in the ID/IG ratio in the presence of the polymeric material. These results suggested that a modification of the electronic structure of the nanotubes, which was significantly more pronounced for the SWNTs compared to the MWNTs, was caused by the interaction with the polymeric material. The characteristic chemical properties of PPy also changed significantly because of its interaction with the SWNTs. In particular, the most oxidized state of PPy was stabilized when interacting with the SWNTs.

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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 The authors acknowledge FAPESP, CNPq, CAPES, and Rede Nacional de Pesquisa em Nanotubos de Carbono for financial support. We are grateful to Dr. Michele Lemos de Souza for help in improving the manuscript and to Laboratório de ́ Nanomateriais do Departamento de Fisica, Universidade Federal de Minas Gerais (UFMG) for donation of the carbon nanotubes. We also sincerely acknowledge Dr. Sandeep Kumar Vashist and A. G. Venkatesh for the permission to use the MWNT figure from J. Nanomed. Nanotechol. 2012, 3(8), 1−2.

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