A simple strategy to enhance electrical conductivity of nanotube-conjugate polymer composites via iodine-doping

Abstract

Iodine-doping is useful in the development of high-conductivity carbon nanotube (CNT)–polymer composites. We prepared three types of CNT–polymer composites by in situ polymerization and obtained iodine-doped samples by mechanical mixing. A series of studies on electrical conductivity has proven that iodine doping is an effective way to obtain high-conductivity composites with heteroatoms in the polymer matrix. The conductivities of the doped samples increased by 4 orders of magnitude compared to the undoped samples. Based on the Hall-effect, Raman and XPS spectra, we propose that the synergistic effect between CNT and iodine results in superior properties. When the heteroatom is N, the synergistic effect of iodine and CNT helps to form a stronger p–π conjugated system. The N cation radicals (as a type of carrier) increase with the enhanced conjugation, resulting in the improved conductivity. When the heteroatom is S, CNT and iodine form a separate π–π conjugated system and charge-transfer complex with S. The combination of the two interactions induces a boost in the carrier concentration, as well as the conductivity.

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1. Introduction

Conductive polymer composites containing insulating polymers and conductive fillers, such as carbon black (CB),^1–3 carbon nanotubes (CNTs)^4–10 and graphene,^11–14 have found wide industrial applications in the fields of antistatic materials, self-regulating heaters, over-current and over-temperature protection devices, and electromagnetic interference shielding.^15–19 Among the numerous nano-fillers, CNTs are of great interest because of their special structures and properties, and the percolation threshold of the composites could achieve ultralow values.^20–23 However, it is always imperative for the current technical trend to develop advanced CNT composite materials with superior performances, especially for their electrical properties.

Iodine-doping, as a type of classical method to improve the conductivity of polymers,^7,24–27 is rarely used to obtain high-conductivity CNT–polymer composites. The very limited numbers of research efforts reported so far^28–30 have not gone deep into researching the mechanism systematically. To develop a general applicable method of iodine doping, we prepared CNT–polyaniline (CNT–PANI), CNT–polypyrrole (CNT–PPy) and CNT–polythiophene (CNT–PTh), and explored their conductivities, chemical states and molecular structures. Based on the scientific research, we found that only the CNT–polymer composites containing heteroatoms had increased conductivity after iodine doping. Finally, we obtained the mechanism of synergy, which varies when the heteroatom is varied.

2. Experimental

2.1 Materials

CNTs used in this study were supplied by Chengdu Organic Chemicals Co. CNTs are multi-walled tubes with average diameter of 20–30 nm and length of 5–15 μm. Pyrrole, aniline and thiophene monomers were obtained from Sinopharm Chemical Reagent Co. Ammonium persulfate (APS) and hydrochloric acid were also obtained from Sinopharm Chemical Reagent Co. Ethanol and iodine were provided by Shanghai Chemical Reagent Co. and Dongguan United Chemical Co., respectively. All the chemicals were used as received without further treatment. Deionized water was used throughout the experiments.

2.2 Preparation of iodine-doped CNT–polymer composites

The CNT–PPy and CNT–PANI composites were synthesized through in situ polymerization with APS as the oxidizing agent.^31,32 CNT and thiophene were in situ polymerized by FeCl₃ in anhydrous chloroform solutions.³³ These CNT–polymer composite powders were then blended with iodine by grinding in an agate mortar. After being fully blended, these samples were pressed into wafers with thickness of about 2 mm and diameter of about 13 mm at a constant pressure of 20–30 MPa with pressure holding time of 1 min.

2.3 Characterization

The electrical conductivity of the composites was obtained by a standard four-probe technique. The carrier concentration and carrier mobility were obtained by a Hall effect measuring instrument (HM2000). The measurements were obtained at room temperature. Raman spectra were obtained on a Jobin-Yvon LabRAM HR 800 UV spectrometer with a 633 nm He–Ne laser as the excitation source. The surfaces of the composites were analysed by X-ray photoelectron spectroscopy (XPS). The XPS measurements were made on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer).

3. Results and discussion

3.1 Electrical characterization subject to different iodine mass fractions

In this study, we focused on using iodine-doping to obtain high-conductivity composites. Fig. 1 shows how the conductivities of different CNT–polymer composites, CNT–PANI (a), CNT–PPy (b) and CNT–PTh (c), vary with increasing weight fractions of iodine. On one hand, the conductivities of the three types of composites are improved after iodine doping. The conductivity of CNT–PTh increases sharply (up to 4 orders of magnitude) compared with the undoped samples, whereas that of CNT–PANI has a weak growth (about 2 orders of magnitude) and the increase of CNT–PPy is the most feeble (only ∼3.4 times).

Fig. 1 Conductivities of different CNT–polymer composites with increasing weight fractions of iodine. (a) is for CNT–PANI, (b) is for CNT–PPy and (c) is for CNT–PTh.

On the other hand, the influence of the components on the conductivity is noticeable. In Fig. 1(a) and (c), the conductivities of pure PANI and PTh have a little growth after iodine doping. When the mass fraction of CNT is 5%, the conductivities of the two composites show a sharp increase with increasing iodine. As the CNT content goes up to 20%, the doped iodine does little to enhance the conductivities of the PANI- and PTh-matrix composites. The conductivity of the pure PPy has no obvious change after doping, but that of 10% CNT–PPy first increases and then drops. For the 20% CNT–PPy, the conductivity has almost no increase and only shows a dramatic decrease after iodine doping.

Based on the electrical conductivity equation of semiconductors: σ = neμ, where σ is the conductivity, e is the electron charge, n is the carrier concentration and μ is the mobility ratio, we explored n and μ of the three composites (Fig. 2). The carrier concentration of the three composites all increase with the increasing content of the doped iodine, and the mobility ratios show a decreasing trend after the first increase. The upward tendency of the conductivities is attributed to the carrier concentration and the first increasing stage of the mobility ratio. The tendency of the mobility to drop is too weak to influence the conductivity of CNT–PANI and CNT–PTh. However, the changes for CNT–PPy in carrier concentration and mobility are both weak, and in this case, the huge decrease in the mobility ratio results in the decrease in the conductivity.

Fig. 2 Carrier concentrations and mobility ratios of (a) 5% CNT–PANI, (b) 10% CNT–PPy, and (c) 5% CNT–PTh.

3.2 Effect of iodine doping on the chemical bonding states

Fig. 3 shows the Raman spectra of the three composites before and after iodine doping. The curves in Fig. 3(a) contain most of the markers for PANI bands. The band located at around 1584 cm⁻¹ corresponds to carbon–carbon double bond stretching vibrations in benzene and quinine rings, the C=N stretching modes of imine are located at 1473 cm⁻¹ and the C–N stretches of amine sites are seen at 1216 cm⁻¹. The stretching vibration of an intermediate bond C–N⁺ is also observed with characteristic frequency around 1314 cm⁻¹.³⁴ Compared with the undoped samples, the band located at 1314 cm⁻¹ has an obvious enhancement after iodine doping. Other bands are pretty much the same in CNT–PANI.

Fig. 3 Raman spectra of (a) CNT–PANI, (b) CNT–PPy and (c) CNT–PTh before and after iodine doping.

In the spectra of CNT–PPy (Fig. 3(b)), a strong peak at 1573 cm⁻¹ represents the backbone stretching mode of C–C bonds and the weak peaks at 1380 and 1320 cm⁻¹ are attributed to antisymmetric C–N stretching and the D-band of CNTs, respectively. The peaks at 935 and 1085 cm⁻¹ are associated with the bipolaron structure (N⁺) and those at 970 and 1060 cm⁻¹ are associated with the polaron structure (N⁺⁺).³⁵ Evidently, the peaks of the polaron and bipolaron reinforced after iodine-doping. This indicates that iodine in both CNT–PANI and CNT–PPy accelerates the generation of nitrogen cation radicals (N⁺ and N⁺⁺).

Unlike CNT–PANI and CNT–PPy, the Raman spectra of CNT–PTh (Fig. 3(c)) do not show apparent changes after iodine-doping. This means the effect of iodine in CNT–PTh is different from the other two.

3.3 Effect of iodine-doping on the chemical states of the heteroatoms

XPS analysis was performed to investigate the chemical states of the heteroatoms in the CNT–polymer composites (as shown in Fig. 4). The N 1s spectra of CNT–PANI and CNT–PPy were deconvolved into the same set of two components. In Fig. 4(a), the peaks located at 398.7 and 401.1 eV were assigned to the benzenoid amine and nitrogen cationic radicals, respectively. After iodine-doping (Fig. 4(d)), the two peaks then shifted in the direction of higher binding energy to 399.2 and 400.3 eV, and the percentage of nitrogen cationic radicals increased from 19.9% to 30.3%; thus, iodine helps in the production of more nitrogen cationic radicals.

Fig. 4 N 1s XPS spectra of (a) undoped and (d) iodine (5%)-doped 5% CNT–PANI; N 1s XPS spectra of (b) undoped and (e) iodine (5%)-doped 10% CNT–PPy; S 2p XPS spectra of (c) undoped and (f) iodine (5%)-doped 5% CNT–PTh.

Similarly, the two components of CNT–PPy shift slightly from 399.6 and 401.9 eV to 400.0 and 402.4 eV, respectively (Fig. 4(b) and (e)), and the percentage of nitrogen cationic radicals went from 21.0% to 28.5%. We therefore initially concluded that iodine plays an important role in generating more nitrogen cationic radicals when the N heteroatom exists in the CNT–polymer composites.

The S 2p XPS spectra of CNT–PTh are exhibited in Fig. 4(c) and (f). The two components at 163.3 and 164.2 eV were assigned to the neutral S 2p_3/2, and the oxidized S 2p_1/2. Unlike the former two composites, we did not find the positively charged sulfur (S^δ+) at about 167.5 eV after iodine doping.³⁶ The two components shifted slightly in the direction of low binding energy, and were finally located at 162.9 and 163.6 eV. This further demonstrates that the effect of iodine in CNT–PTh is different from that in the other two composites. To further explore the function of iodine, the I 3d XPS spectra were obtained and are shown in Fig. 5. The two peaks of the standard iodine spectra are located at 619.5 and 631.0 eV. When doped into the three composites, the peaks display various degrees of movement. For I₂–CNT–PANI, the two peaks are located at 619.0 (−0.5 eV relative shift) and 630.6 eV (−0.4 eV relative shift). The changes in the spectra of I₂–CNT–PPy are similar to those in I₂–CNT–PANI, and the two peaks shift from 619.5 and 631.0 eV to 618.7 and 630.2 eV, respectively, with a relative shift of −0.8 eV. The peaks then shift further toward higher binding energies and finally centered at 620.7 and 632.0 eV, as shown in Fig. 5(c). This means the electron density around iodine in CNT–PANI and CNT–PPy increased, but decreased in CNT–PTh.

Fig. 5 I 3d spectra of (a) 5% I₂–(5% CNT–PANI); (b) 5% I₂–(10% CNT–PPy) and (c) 5% I₂–(5% CNT–PTh).

3.4 Mechanism

In our former research, we doped CNTs (ESI Fig. S1†), conjugated polymers and CNT–polymer composites with iodine. We found that the conductivity of pure CNTs had a small decrease after iodine doping and that of pure polymers had a slight increase with the exception of PANI. For PANI, the protonation effect led to the increase in the conductivity. Only the conductivity of the iodine doped CNT–polymer composite increased sharply compared to the undoped sample. On this basis, we proposed that the synergistic effect between iodine and CNT led to the increase in the conductivity.

To move a step further, we obtained several CNT–conjugated polymer composites to research the iodine-doping effect. Surprisingly, only the CNT–polymer composites containing heteroatoms in the polymer matrix showed a boost in conductivity after iodine-doping. The other composites, such as CNT–PPA (ESI Fig. S2†), even showed a decrease in conductivity after doping. Based on these facts, we believe there is synergy of the CNT and iodine functions toward the heteroatom in the polymer matrix. In addition, we found the mechanisms of action to be dissimilar when the heteroatom was different.

CNT–PANI and CNT–PPy both contain nitrogen atoms, and the synergistic effect helps to generate more carriers. The process is as follows: after iodine-doping, there is a p–π conjugated system between the doped iodine and CNT–polymer. This system is surely considerably stronger than that in the iodine-doped polymer because of the synergy of CNT and iodine. The enlarged conjugation makes the lone electron pair of N delocalized over a larger range, which promotes the generation of more nitrogen cationic radicals. These increased cationic radicals, as evidenced from Raman and N 1s XPS spectra, provide more carriers in the doped composites, leading to stronger conductive ability (shown in Fig. 1 and 2), and the increased degree of conjugation in the composites improves the mobility ratio at the same time. With increasing iodine-doping concentration, the impact of neutral impurity scattering becomes more significant. It adversely reduces the mobility ratio in this case. As a result, the conductivity of iodine-doped CNT–PPy exhibits a downward trend. However, for CNT–PANI, the influence of the mobility ratio is considerably smaller than carrier concentration, and the conductivity does not decrease.

However, the synergistic effect is totally different when the heteroatom is S. For CNT–PTh, there is a π–π conjugation between CNT and PTh. Only one lone pair electron from S could be involved in the conjugated system because of the directivity of the electron pair. After iodine doping, the other lone pair of electrons of S are shifted to iodine due to the stronger electronegativity of iodine. This means the composites and iodine form a charge-transfer complex,³⁷ besides the π–π conjugated system. As a result, the S 2p peaks move in the direction of higher binding energy and the peaks of I 3d, in the opposite direction. With the synergistic effect of the so-called charge-transfer complex and π–π conjugation system, the concentration of carriers is boosted and the conductivity of the material shoots up. However, for iodine doped PTh, there is only one type of interaction to improve the carrier concentration, which is why the conductivity of iodine doped PTh does not have such a sharp increase. With the increasing iodine content, the excess iodine lowers the mobility ratio, and the conductivity does not show an obvious decrease because the influence of the mobility ratio is really small compared with the carrier concentration.

4. Conclusions

In conclusion, we successfully demonstrated an effective way to obtain highly conductive CNT–polymer composites via an iodine-doping process. Only the composites containing heteroatoms in the polymer matrix have increased conductivity after iodine doping. The conductivity of such doped composites could increase by 4 orders of magnitude compared to the undoped samples. The mechanism is different if the heteroatom differs. When the heteroatom is N, the conductivity increasing process is a nitrogen cationic radical-growth process. Based on the synergistic effect of iodine and CNT, the p–π conjugation is enhanced and more nitrogen cationic radicals are produced. Moreover, in the polymer-matrix composites containing S atoms, there is a charge-transfer complex and conjugated system at the same time. Under the influence of the two interactions, the carrier concentration, as well as the conductivity increases. Having explored the mechanism, we find that iodine doping is a valid means of obtaining a class of high-conductivity polymer composites such as CNT–PANI, CNT–PPy and CNT–PTh. It is a significant step toward developing both advanced materials and the iodine-doping theory.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The conductivity of CNTs and CNT–PPA composite after iodine-doping are provided. See DOI: 10.1039/c5ra15107d

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