Article pubs.acs.org/Langmuir
Effect of Additives on the Properties of Polyaniline Nanofibers Prepared by High Gravity Chemical Oxidative Polymerization Yibo Zhao,† Moses Arowo,†,‡ Wei Wu,*,† and Jianfeng Chen†,‡ †
State Key Laboratory of Organic-Inorganic Composites and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *
ABSTRACT: Polyaniline (PANI) nanofibers with improved properties were prepared by high gravity chemical oxidative polymerization in a rotating packed bed with the assistance of p-aminodiphenylamine (AD) and p-phenylenediamine (AP). The effects of reactor type, additive dosage, reaction temperature, and high-gravity level on the properties of products were investigated in detail. Three conclusions were made: (1) a small amount of additive can significantly improve some properties of the nanofibers such as uniformity, specific surface area, and specific capacitance; (2) in order to obtain high-quality nanofibers, the high-gravity level should coordinate with the reaction rate; (3) the molecular weight and conductivity of PANI decrease with the increase of additive dosage. The products have larger specific surface areas of up to 73.9 and 68.4 m2/g and consequently improved specific capacitance of up to 527.5 and 552 F/g for the PANI nanofibers prepared with AD and AP, respectively. However, the specific surface area and specific capacitance of pure PANI are only 49.1 m2/g and 333.3 F/g, respectively. This research provides a simple, reliable, and scalable method to produce PANI nanofibers of high performances.
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INTRODUCTION Polyaniline (PANI) has received considerable attention in many applications such as energy-storage devices,1−3 sensors,4,5 electrodes,6−8 and electrochromic devices,9−11 owing to its simple synthesis, low cost of raw materials, high conductivity, and good pseudocapacitive performances.12−16 Among the reported approaches for its preparation, chemical oxidative polymerization is the commonly used method because of its simple and convenient operation. However, since the duration of induction period of PANI usually lasts just for a few seconds or several minutes, the raw materials mixed by this technique (typically use stirring paddle or magnetic stirrer) cannot be well mixed at microscopic scale in such a short time, which finally leads to nonuniform morphology of PANI arising from heterogeneous nucleation. Researchers have shown that the properties of PANI such as conductivity,17 specific capacitance,18,19 catalytic activity,20 and gas sensitivity21 are all related to the morphology of PANI. Based on these studies, it is concluded that the PANI nanofibers with long dimension and large specific surface area usually have high performances. In order to produce PANI nanofibers of uniform morphology under homogeneous nucleation environment, the mixing of raw materials and the nucleation of PANI must be separated, and also, the second growth of PANI should be avoided. In our previous work, PANI nanofibers were successfully prepared by high gravity chemical oxidative polymerization (HGCOP) in a rotating packed bed (RPB), and this approach © XXXX American Chemical Society
was demonstrated to be efficient and especially appropriate for industrial scale-up since RPB is capable of greatly intensifying the mixing of raw materials in a short time.22 In addition, the as-prepared nanofibers have also been proved to have great potential in energy storage application.23 However, the nanofibers produced by this technique still have some morphology issues such as aggregation and nonuniformity caused by the heterogeneous nucleation as a result of the discordance between the high mixing rate of RPB and the low nucleation rate of PANI. It has been reported that a small amount of either paminodiphenylamine (AD) or p-phenylenediamine (AP) can greatly improve the morphology of PANI nanofibers as they significantly speed up the nucleation process and effectively suppress the second growth.24−27 Nevertheless, the greatly accelerated reaction needs much higher mixing rate to separate the mixing process and nucleation stage, especially for largescale production of PANI nanofibers. In this work, PANI nanofibers were prepared in RPB using AD or AP as additive in order to improve their properties. The effects of reactor type, the additive dosage, reaction temperature, and high-gravity level on the properties of the products were discussed in detail. The AD- and AP-assisted PANI nanofibers were compared by measurement of SEM images, Received: December 24, 2014 Revised: April 13, 2015
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Figure 1. SEM images of (A) RPB-PANI and (B) STR-PANI, (C) AD-0.5, (D) AP-0.5, (E) AD-1, (F) AP-1, (G) AD-2, and (H) AP-2.
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FT-IR spectra, UV−vis absorption spectra, XRD patterns, molecular weights, conductivities, specific surface areas, and electrochemical properties. It was demonstrated that RPB is highly suitable for this new route, and a small amount of additives can significantly improve the qualities of PANI nanofibers and their electrochemical properties.
EXPERIMENTAL SECTION
Materials. All the reagents used in this work were of analytical grade. Aniline (Tianjin Fuchen Chemical Reagent Company, China) was distilled under vacuum and refrigerated prior to use. Ammonium peroxydisulfate (APS), hydrochloric acid (HCl), and absolute alcohol were all purchased from Beijing Chemical Reagent Company (China). The additives AD and AP were purchased form Tianjin Guangfu Fine B
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Chemical Research Institute (China) and Aladdin Reagent Co. Ltd. (China), respectively, and their molecular structures are shown in Figure S1. Synthesis of PANI Nanofibers. PANI nanofibers were synthesized by the HGCOP method in RPB under different operational conditions including additive amount, reaction temperature, and RPB high-gravity level. The apparatus used is described in Figure S2, and the PANI samples were all prepared according to the following steps unless stated otherwise. Typically, 1.86 g of aniline and 1.14 g of APS were dissolved in 100 mL of HCl solution (1.0 M). The aniline solution was then mixed with 2 mL of absolute alcohol containing certain amount of AD or AP (use the mass ratio of additive to aniline to represent the dosage of additive). Both solutions were adjusted to 20 °C using a circulator bath and then simultaneously pumped into the RPB (at a high-gravity level of 713.7 m/s2) by two peristaltic pumps. The resultant mixture was collected in a tank and aged in the above bath for half an hour to form a green precipitate which was washed and dried at 60 °C overnight. As a contrast test, PANI nanofibers were also prepared by mixing both aniline and APS solution rapidly for 1 min in a stirred tank reactor (STR, made of a beaker and a stirrer), while the other steps remained the same. Preparation of Electrode. Electrode slurry was obtained by directly mixing the PANI filter cake with deionized water instead of ultrasonic treatment to avoid breaking the nanofibers. Afterward, several drops of the obtained slurry were dropped onto a titanium sheet with an area of 1.0 cm2. Finally, the titanium sheet was dried in an oven at 60 °C for 12 h to obtain the electrode. The loading of electrode was around 1.0 mg/cm2. Characterization. The morphologies of the PANI nanofibers were characterized by scanning electron microscopy (SEM, JSM-6701F), and the molecular structures were analyzed by Fourier transform infrared spectroscopy (FTIR, Nicolet model 8700) and UV−vis absorption spectroscopy (UV−vis, UV-5200PC, aqueous samples). Xray diffractometer (XRD, Bruker D8A) analysis was performed to confirm the crystal structure. The specific surface areas of the PANI nanofibers were obtained by the Brunauer−Emmett−Teller method (BET, Quadrasorb SI) at room temperature. The molecular weights of the dedoped PANI samples were measured by gel permeation chromatography (GPC, Agilent 1200 Series) at room temperature. Dimethylformamide was used both as the solvent and mobile phase (1.0 mL/min), and the system was calibrated with polystyrene standards. The conductivities of the PANI samples were tested on a digital multimeter (SX1934) by the four-probe technique. The samples were compressed into tablets at a pressure of 5.0 MPa on a desktop tablet pressing machine (YP-30T). All electrochemical measurements were performed in a threeelectrode setup. Both cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests were carried out on a CHI660D electrochemical workstation. The electrolyte used was 1.0 M H2SO4 solution. A platinum sheet and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The specific capacitances of the electrodes were calculated from the CV and GCD discharge curves according to eqs 1 and 2:
∫
Cm = ( I dV )/(νmV )
RESULTS AND DISCUSSION Effect of Reactor Type. In order to study the effect of reactor type on the polymerization process, aniline was polymerized at a low concentration of 0.025 M in RPB and STR (see Figure S3). The color of the reaction mixture changed from light blue to dark blue, signifying the end of the induction period and the late stage of polymerization, respectively. Results revealed that RPB can significantly reduce the induction period to 5 min and complete reaction time to 10 min as compared to 16 and 27 min for STR, respectively. In addition, the reaction mixture produced by STR was not as well-distributed as that obtained by RPB, indicating the poor mixing effect of STR. In order to investigate the morphology differences between the nanofibers prepared in RPB and STR, aniline was polymerized at a higher concentration of 0.1 M, and the as-obtained products were labeled as RPB-PANI and STRPANI, respectively. In Figure 1A,B it is evident that RPB-PANI exhibits more uniform morphology with a diameter of 30−50 nm, as compared to the STR-PANI which mostly shows rodlike structure (50−100 nm in diameter). Many studies have shown that the mixing degree of raw materials is very important for production of PANI nanofibers with good qualities because the uniformity of mixing greatly influences the nucleation mode of polymerization and the morphologies of the PANI.28,29 On this basis, RPB is more suitable for preparing PANI nanostructures than STR.23,30 Effect of Additive. To investigate the effect of additives on the polymerization process in RPB, aniline was polymerized with 1.0 wt % AD or AP at a low concentration of 0.025 M (see Figure S4). In comparison with the polymerization without additives (see Figure S3), the additive-containing mixtures turned dark blue in just 5 min, illustrating that additives can greatly promote the reaction. Moreover, the AD-containing mixture turned green at 0 s while this color appeared at 30 s for the AP-containing mixture, suggesting that AD is more effective for accelerating the reaction than AP. It is therefore evident that these two additives have different reaction routes in the polymerization process. AD can directly react with aniline monomers with almost no induction period since it is the predominant product of the induction period, i.e., aniline dimer.31,32 However, AP promotes the reaction just by taking part in the formation of aniline oligomers (or called active centers),33,34 which means that an induction period is still required. Nevertheless, the polymerization process is strongly accelerated by both of these additives due to the shortened induction period. The SEM images of additive-assisted nanofibers prepared in RPB are shown in Figure 1C−H. The samples prepared with 0.5, 1.0, and 2.0 wt % of the additives were labeled as AD-0.5, AD-1, AD-2, AP-0.5, AP-1, and AP-2, accordingly. Noticeably, the nanofibers become more uniform in morphology and smaller in diameter with increasing additive dosage. Interestingly, AP-assisted nanofibers tend to twist into bundles, and this trend gets more and more obvious with the increase of AP dosage. To obtain the morphology parameters of PANI nanofibers, samples were dispersed in water by ultrasound and dropped on copper grids for SEM observation. The calculation method of these parameters was similar to others.35,36 After analyzing 150 PANI nanofibers from the SEM images of each sample, the statistical morphology parameters of the samples are listed in Table 1. It is clear that a gradual increase in additive dosage
(1)
where Cm is the specific capacitance based on the mass of electroactive material in F/g, I is the response current in A, V is the potential in V, ν is the potential scan rate in V/s, and m is the mass of the active material on the electrode in g. Cm = I /(dV /dt ·m)
Article
(2)
where Cm is the specific capacitance based on the mass of electroactive material in F/g, I is the discharge current in A, dV/dt is the slope of the linear section on discharge curve in V/s, and m is the mass of the active material on the electrode in g. C
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change. In contrast, the sample with 2.0 wt % AP shows nanoribbon morphology (100 nm in width and 25 nm in thickness) only at 0 °C, while only nanofibers were observed at other temperatures. Additionally, it is evident that only nanofibers were observed when aniline was polymerized with 1.0 wt % AP at 0 °C (see Figure S5), indicating that adequate AP dosage is essential for the formation of nanoribbons. Therefore, it can be inferred that the morphology difference observed at 0 °C is related to the molecular structures of the respective additives since both AD- and AP-assisted samples were synthesized at the same conditions. Additives can form a lot of growth centers prior to aniline monomers because of their much lower oxidant potential,24,37 and the as-formed growth centers subsequently determine the formation of PANI nanostructures in the chain propagation stage.38 Thus, AD-assisted samples simply form nanofibers at any temperature since AD is aniline dimer. However, APassisted samples favor the formation of bundle-like structure as mentioned earlier, and this structure may self-assemble into nanoribbons under the influence of hydrogen bonds (primary amines of AD molecules) at low temperature (see Figure S6). Effect of High-Gravity Level. According to the previous study, high high-gravity level is favorable for preparing PANI nanofibers of thin diameter.22 However, since the reaction conditions have been changed, this factor should be studied further. As shown in Figure 3, the morphology of pure PANI becomes more and more uniform with increasing high-gravity level, but still not as uniform as the additives-assisted PANI nanofibers. However, the additive-assisted PANI nanofibers are
Table 1. Morphology Parameters of the PANI Nanofibers Prepared with Different Additive Dosages samples
av diam (nm)
std dev of diam (nm)
av length (nm)
av aspect ratio
RPB-PANI AD-0.5 AD-1 AD-2 AP-0.5 AP-1 AP-2
42.1 40.6 33.2 29.5 37.9 37.4 31.2
13.2 8.7 7.1 6.7 8.9 7.2 6.7
580 728.2 597.6 573.3 1201.8 1278.5 1166
13.8 17.9 18 19.4 31.7 34.2 37.4
leads to an obvious decrease in the average diameter (av diam) and the standard deviation of diameter (std dev of diam). Also, additives, especially AP, can increase the nanofibers’ average length (av length) effectively. Furthermore, the average aspect ratio (av aspect ratio, equals to av length/av diam) of the nanofibers increased from 13.8 to 19.4 and 37.4 for AD- and AP-assisted samples, respectively. Therefore, it can be established that AP is more effective for improving the morphology of PANI nanofibers than AD. Effect of Reaction Temperature. To study the effect of reaction temperature, aniline was polymerized with no additive, 2.0 wt % AD and 2.0 wt % AP at different temperatures. As shown in Figure 2, the morphology of pure PANI changes from irregular particles to uniform nanofibers with increasing temperature since higher temperature favors homogeneous nucleation.22 However, the samples with 2.0 wt % AD always exhibit uniform nanofibers irrespective of the temperature
Figure 2. SEM images of PANI nanofibers polymerized with (A) no additives, (B) 2.0 wt % AD, and (C) 2.0 wt % AP in RPB at different temperatures (left: 0 °C; middle: 20 °C; right: 40 °C; aniline concentration: 0.1 M; high-gravity level: 713.7 m/s2). D
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Figure 3. SEM images of PANI nanofibers polymerized with (A) no additive, (B) 2.0 wt % AD, and (C) 2.0 wt % AP in RPB at different high-gravity levels (left: 178.4 m/s2; middle: 713.7 m/s2; right: 2854.7 m/s2; aniline concentration: 0.1 M; reaction temperature: 20 °C).
maximum of 17.0 and 42.2 for AD and AP samples at 713.7 m/ s2, respectively. It is notable that the average aspect ratio of the additive-assisted nanofibers decreases distinctly at 2854.7 m/s2. As discussed above, additives are effective in increasing the reaction rate of PANI and higher high-gravity level can also promote the polymerization process. These two factors result in an extremely fast reaction, making the initially formed nanofibers be precipitated in the RPB. Therefore, the growth environment of the nanofibers is disturbed by the strong agitation of RPB, and finally only short nanofibers are formed. Characterization of PANI Nanofibers. The composition of the as-prepared PANI nanofibers was studied by FTIR and UV−vis spectroscopy. Figure 4A shows the FTIR spectra of pure PANI, AD-2, and AP-2. The samples show similar peak position, and the main peaks located at 3438, 1570, 1486, 1297, 1133, and 802 cm−1 are in agreement with the emeraldine form of PANI.39−41 The characteristic bands at 1570 and 1486 cm−1 are assigned to the CC stretching vibration of quinoid rings and benzene rings, while other peaks located at 3438, 1297, 1133, and 802 cm−1 are attributed to the N−H stretching vibration, C−N stretching mode, NQN stretching mode (with Q representing the quinoid ring), and C−H bonds of benzene rings, respectively.42 By comparison, it was found that these three samples have almost the same peaks, indicating that the additives hardly affect the typical molecular structure of PANI.24 The UV−vis absorption spectra of PANI nanofibers are shown in Figure 4B. The spectra are consistent with other literatures.43,44 It is clear that all the samples have three similar characteristic peaks. The peaks at 350, 440, and 850 nm are ascribed to the π−π* transition of the benzenoid rings,
relatively short when the high-gravity level is too high (2854.7 m/s2), especially for AD-assisted samples. As shown in Table 2, the statistical data of the samples were obtained using the same method as described earlier. It is Table 2. Morphology Parameters of the PANI Nanofibers Prepared at Different High-Gravity Levels av diam (nm)
std dev of diam (nm)
av length (nm)
av aspect ratio
A1 A2 A3 B1
43.4 42.1 36.4 37.8
13.7 13.2 11.8 6.8
478.5 580 553.7 496.2
11.0 13.8 15.2 13.1
B2 B3 C1
29.5 36.7 26.1
6.7 8.4 6.9
573.3 420.6 737.5
17.0 11.5 28.3
C2 C3
31.2 34.8
6.7 7.9
1166 632.9
42.2 18.2
samples no additive
2.0 wt % AD
2.0 wt % AP
evident that higher high-gravity level improves the quality of pure PANI nanofibers in terms of both diameter and average aspect ratio as it ensures sufficient micromixing and consequently homogeneous nucleation of PANI.22 Nevertheless, pure PANI nanofibers have less uniform morphology as evidenced by the large standard deviation of diameter (>11 nm). However, the additive-assisted PANI nanofibers display smaller diameter, more centralized diameter distribution (around 7 nm) and larger average aspect ratio than pure PANI nanofibers. The largest average aspect ratio is up to a E
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Figure 4. (A) FTIR spectra, (B) UV−vis absorption spectra, and (C) XRD patterns of (a) pure PANI, (b) AD-2, and (c) AP-2.
polaron−π*, and π−polaron transitions, respectively.45 Further, the peaks at 442 and 850 nm are related to the doping level and the formation of polaron46 and indicate that the obtained PANI is in the doped state.42 To investigate the crystallinity of the samples, XRD patterns of the samples were recorded from 5 to 40°. In Figure 4C, there are four characteristic peaks located at 2θ = 9.1°, 15.1°, 20.3°, and 25.4°, which correspond to the (001), (011), (020), and (200) crystal planes of PANI, respectively.47−49 These peaks further illustrate that the products are in highly doped emeraldine salt form.22 The peaks at 20.3° and 25.4° are more intense for both AD-2 and AP-2 as compared with those of pure PANI, signifying their better crystallinity. Basic Properties of PANI Nanofibers. The molecular weights of PANI nanofibers with different additive dosages are shown in Table S1. It is evident that 0.5 wt % of either AD or AP significantly increases the molecular weight whereas dosages beyond 0.5 wt % result in decreased molecular weight. The variation in polydispersities of the products also follows a similar trend. The explanation of these phenomena is based on two basic points: (1) additives act as the nucleation centers of PANI; (2) the aniline dosage used in the experiments is constant. When the additive dosage is very small, each nucleation center can have more aniline molecules growing on it, thereby making it difficult to guarantee the uniformity of molecular weight. On the other hand, excessive additive dosage can lead to less aniline molecules growing on each nucleation center and thus narrower the molecular weight distribution. Therefore, low additive dosage is beneficial to increase the molecular weight of PANI while high additive dosage will result in PANI of narrow molecular weight distribution. It is well-known that PANI nanofibers have anisotropic conductivity because they are one-dimensional conductors. Usually, long PANI nanofibers have larger conductivity than short ones because the conductivity along the molecular chain is much larger than that along the perpendicular direction. Therefore, the conductivity of PANI is significantly affected by
its morphology. As shown in Figure 5, the conductivity of both AD- and AP-assisted samples first increases and then decreases
Figure 5. Conductivity variation of PANI nanofibers with different additive dosages.
with an increase in additive dosage, and the highest conductivity for AD- and AP-assisted samples is 334.5 and 389.1 S/m, respectively. The increase in conductivity is ascribed to the dramatic change in PANI morphology and the increased molecular weight of PANI (see Table S1), while the reduction in conductivity is related to the decrease of molecular weight. This trend of decrease in conductivity has also been found by other researchers.50,51 In our previous research, the specific surface area of pure PANI prepared in RPB and STR was found to be 49.1 and 38.9 m2/g, respectively, demonstrating that RPB has superior advantages on the preparation of nanomaterials.23 As for the AD-assisted samples, the specific surface area of AD-0.5, AD-1, and AD-2 is 53.5, 62.6, and 73.9 m2/g, respectively. The increase in specific surface area is due to the reduced diameter of PANI nanofibers (see Table 1). However, the specific surface area of AP-0.5, AP-1, and AP-2 is 68.4, 67.1, and 60.5 m2/g, respectively. This trend of decrease may be attributed to the F
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Figure 6. CV curves of the (A) AD-assisted and (B) AP-assisted samples at the scan rate of 10 mV/s; initial GCD curves of (C) AD-assisted and (D) AP-assisted samples at a current density of 0.5 A/g.
Figure 7. Formation mechanism of PANI nanofibers without or with the addition of additives in RPB.
slow during the first cycle. The discharge process started from around 0.74 to 0 V. From these curves, the specific capacitance of pure PANI, AD-0.5, AD-1, AD-2, AP-0.5, AP-1, and AP-2 was calculated to be 333.3, 454.5, 522.8, 527.5, 471.7, 500.3, and 552 F/g, respectively, which agrees well with that deduced from the CV tests. Usually, the specific capacitance of PANI mainly relates to its specific surface area (increase the effective utilization rate of PANI) and conductivity (speed up the charge transfer). However, together with the result of conductivity, it seems that the specific area has a greater influence than conductivity in explaining the variation trend of specific capacitance of PANI samples, meaning that the conductivity is high enough for the electrochemical process. Mechanism Analysis. According to the above discussions, the mechanism is described in Figure 7. When aniline molecules are polymerized with no additives, the as-formed PANI nanofibers exhibit short length and wide distribution of diameter due to the heterogeneous nucleation resulting from
special bundle structure; i.e., several nanofibers combine into one nanofiber, leading to the reduction of the exposed surface area. Therefore, it could be deduced that the real specific surface area of nanofibers should be larger than the measured value since the diameter of nanofibers actually decreases with the AP dosage. The electrochemical performances of PANI nanofibers were evaluated by CV and GCD in a three-electrode system. In Figure 6A,B the CV curves of all the samples have three pairs of redox peaks, indicating the existence of pseudocapacitance. Besides, the CV curves of both AD- and AP-assisted samples have larger areas than that of pure PANI, illustrating the better specific capacitance. According to these CV curves, the specific capacitance of pure PANI, AD-0.5, AD-1, AD-2, AP-0.5, AP-1, and AP-2 was found to be 274.4, 389.5, 423.8, 466.4, 406.3, 425.2, and 445.9 F/g, respectively. The initial GCD curves of all the samples are presented in Figure 6C,D. It is normal that the charge process is relatively G
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(3) Cong, H. P.; Ren, X. C.; Wang, P.; Yu, S. H. Flexible graphene− polyaniline composite paper for high-performance supercapacitor. Energy Environ. Sci. 2013, 6 (4), 1185−1191. (4) Ding, M. N.; Tang, Y. F.; Gou, P. P.; Reber, M. J.; Star, A. Chemical sensing with polyaniline coated single-walled carbon nanotubes. Adv. Mater. 2011, 23 (4), 536−540. (5) Hibbard, T.; Crowley, K.; Killard, A. J. Direct measurement of ammonia in simulated human breath using an inkjet-printed polyaniline nanoparticle sensor. Anal. Chim. Acta 2013, 779, 56−63. (6) Tai, Q. D.; Chen, B. L.; Guo, F.; Xu, S.; Hu, H.; Sebo, B.; Zhao, X. Z. In situ prepared transparent polyaniline electrode and its application in bifacial dye-sensitized solar cells. ACS Nano 2011, 5 (5), 3795−3799. (7) Wang, G. Q.; Xing, W.; Zhuo, S. P. The production of polyaniline/graphene hybrids for use as a counter electrode in dyesensitized solar cells. Electrochim. Acta 2012, 66, 151−157. (8) Pirkarami, A.; Olya, M. E.; Yousefi Limaee, N. Decolorization of azo dyes by photo electro adsorption process using polyaniline coated electrode. Prog. Org. Coat. 2013, 76 (4), 682−688. (9) Zhang, J.; Tu, J. P.; Zhang, D.; Qiao, Y. Q.; Xia, X. H.; Wang, X. L.; Gu, C. D. Multicolor electrochromic polyaniline−WO3 hybrid thin films: One-pot molecular assembling synthesis. J. Mater. Chem. 2011, 21 (43), 17316−17324. (10) Wei, H. G.; Zhu, J. H.; Wu, S. J.; Wei, S. Y.; Guo, Z. H. Electrochromic polyaniline/graphite oxide nanocomposites with endured electrochemical energy storage. Polymer 2013, 54 (7), 1820−1831. (11) Kelly, F. M.; Meunier, L.; Cochrane, C.; Koncar, V. Polyaniline: Application as solid state electrochromic in a flexible textile display. Displays 2013, 34 (1), 1−7. (12) Ć irić-Marjanović, G. Recent advances in polyaniline research: polymerization mechanisms, structural aspects, properties and applications. Synth. Met. 2013, 177, 1−47. (13) Wang, Y.; Tran, H. D.; Liao, L.; Duan, X. F.; Kaner, R. B. Nanoscale morphology, dimensional control, and electrical properties of oligoanilines. J. Am. Chem. Soc. 2010, 132 (30), 10365−10373. (14) Wei, Z. X.; Zhang, Z. M.; Wan, M. X. Formation mechanism of self-assembled polyaniline micro/nanotubes. Langmuir 2002, 18 (3), 917−921. (15) Feng, X. M.; Mao, C. J.; Yang, G.; Hou, W. H.; Zhu, J. J. Polyaniline/Au composite hollow spheres: synthesis, characterization, and application to the detection of dopamine. Langmuir 2006, 22 (9), 4384−4389. (16) Kim, B.-J.; Oh, S.-G.; Han, M.-G.; Im, S.-S. Preparation of polyaniline nanoparticles in micellar solutions as polymerization medium. Langmuir 2000, 16 (14), 5841−5845. (17) Rahy, A.; Yang, D. J. Synthesis of highly conductive polyaniline nanofibers. Mater. Lett. 2008, 62 (28), 4311−4314. (18) Amarnath, C. A.; Chang, J. H.; Kim, D.; Mane, R. S.; Han, S.-H.; Sohn, D. Electrochemical supercapacitor application of electroless surface polymerization of polyaniline nanostructures. Mater. Chem. Phys. 2009, 113 (1), 14−17. (19) Zhou, H. H.; Chen, H.; Luo, S. L.; Lu, G. W.; Wei, W. Z.; Kuang, Y. F. The effect of the polyaniline morphology on the performance of polyaniline supercapacitors. J. Solid State Electrochem. 2005, 9 (8), 574−580. (20) Duić, L.; Grigić, S. The effect of polyaniline morphology on hydroquinone/quinone redox reaction. Electrochim. Acta 2001, 46 (18), 2795−2803. (21) Li, G. F.; Martinez, C.; Janata, J.; Smith, J. A.; Josowicz, M.; Semancik, S. Effect of morphology on the response of polyanilinebased conductometric gas sensors: Nanofibers vs. thin films. Electrochem. Solid-State Lett. 2004, 7 (10), H44−H47. (22) Lu, X. W.; Wu, W.; Chen, J. F.; Zhang, P. Y.; Zhao, Y. B. Preparation of polyaniline nanofibers by high gravity chemical oxidative polymerization. Ind. Eng. Chem. Res. 2011, 50 (9), 5589− 5595. (23) Zhao, Y. B.; Wei, H. G.; Arowo, M.; Yan, X. R.; Wu, W.; Chen, J. F.; Wang, Y. R.; Guo, Z. H. Electrochemical energy storage by
the discordance between the high mixing rate of RPB and the low nucleation rate of PANI. However, the additives can serve as nucleation centers, accelerate the reaction, and eventually make the process more appropriate for RPB. Consequently, uniform nanofibers with thin diameter are obtained. However, AD and AP have different impacts on the products because of their different molecular structure. AD, as a kind of intermediate product of PANI, can serve as nucleation center and therefore greatly promote the reaction. As for AP, since its para-position is hindered, the polymerization will be initiated in its meta- or ortho-position, promoting the formation of crosslinked structure.24 Besides, the hydrogen bonds between the primary amines of AP molecules may promote the formation of bundle-like PANI nanofibers.
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CONCLUSIONS In summary, aniline was polymerized with additives in the RPB to produce PANI nanofibers with good properties. It was observed that the additives can greatly accelerate the reaction rate and make the process more suitable for polymerization in RPB. The additive-assisted nanofibers exhibited higher average aspect ratio, more uniform diameter, and larger specific surface area than pure PANI nanofibers. The specific capacitance was 527.5 and 552 F/g for AD-2 and AP-2, respectively. The greatly improved specific capacitance is mainly due to the high specific areas of products. This research therefore provides a simple, reliable, and scalable route for synthesizing PANI nanofibers with good morphology and energy storage performance, and we believed that the property of other conducting polymers (e.g., polythiophene and polypyrrole) could also be improved by introducing suitable additives in a RPB.
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ASSOCIATED CONTENT
S Supporting Information *
Molecular structures of the additives, schematic diagram of the reaction apparatus, reaction process of PANI without and with the addition of additives, supplementary SEM images, and molecular weights of products. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/la504996c.
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AUTHOR INFORMATION
Corresponding Author
*E-mail [email protected]; Tel +86-10-64443134; Fax +86-10-64434784 (W.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors greatly acknowledge the financial support from the National Natural Science Foundation of China (Grants 21376025 and 21076021).
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REFERENCES
(1) Wei, H. G.; Yan, X. R.; Wu, S. J.; Luo, Z. P.; Wei, S. Y.; Guo, Z. H. Electropolymerized polyaniline stabilized tungsten oxide nanocomposite films: electrochromic behavior and electrochemical energy storage. J. Phys. Chem. C 2012, 116 (47), 25052−25064. (2) Wei, H. G.; Gu, H. B.; Guo, J.; Wei, S. Y.; Guo, Z. H. Electropolymerized polyaniline nanocomposites from multi-walled carbon nanotubes with tuned surface functionalities for electrochemical energy storage. J. Electrochem. Soc. 2013, 160 (7), G3038− G3045. H
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inverse microemulsion polymerization. J. Nanopart. Res. 2001, 3 (5− 6), 399−409. (44) Du, X. S.; Zhou, C. F.; Mai, Y. W. Facile synthesis of hierarchical polyaniline nanostructures with dendritic nanofibers as scaffolds. J. Phys. Chem. C 2008, 112 (50), 19836−19840. (45) Abdiryim, T.; Zhang, X. G.; Jamal, R. Comparative studies of solid-state synthesized polyaniline doped with inorganic acids. Mater. Chem. Phys. 2005, 90 (2), 367−372. (46) Macdiarmid, A.; Epstein, A. J. The concept of secondary doping as applied to polyaniline. Synth. Met. 1994, 65 (2), 103−116. (47) Qiu, J. D.; Shi, L.; Liang, R. P.; Wang, G. C.; Xia, X. H. Controllable deposition of a platinum nanoparticle ensemble on a polyaniline/graphene hybrid as a novel electrode material for electrochemical sensing. Chem. Eur. J. 2012, 18 (25), 7950−7959. (48) Yan, J.; Wei, T.; Shao, B.; Fan, Z. J.; Qian, W. Z.; Zhang, M. L.; Wei, F. Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance. Carbon 2010, 48 (2), 487−493. (49) Yang, F.; Xu, M. W.; Bao, S. J.; Wei, H.; Chai, H. Self-assembled hierarchical graphene/polyaniline hybrid aerogels for electrochemical capacitive energy storage. Electrochim. Acta 2014, 137, 381−387. (50) Prokeš, J.; Stejskal, J.; Křivka, I.; Tobolkova, E. Anilinephenylenediamine copolymers. Synth. Met. 1999, 102 (1), 1205−1206. (51) Prokeš, J.; Křivka, I.; Stejskal, J. Control of electrical properties of polyaniline. Polym. Int. 1997, 43 (2), 117−125.
polyaniline nanofibers: High gravity assisted oxidative polymerization vs. rapid mixing chemical oxidative polymerization. Phys. Chem. Chem. Phys. 2015, 17 (2), 1498−1502. (24) Zujovic, Z. D.; Wang, Y.; Bowmaker, G. A.; Kaner, R. B. Structure of ultralong polyaniline nanofibers using initiators. Macromolecules 2011, 44 (8), 2735−2742. (25) Tran, H. D.; Wang, Y.; D’arcy, J. M.; Kaner, R. B. Toward an understanding of the formation of conducting polymer nanofibers. ACS Nano 2008, 2 (9), 1841−1848. (26) D’aprano, G.; Leclerc, M.; Zotti, G. Electrochemistry of phenylN-capped aniline oligomers. Evaluation of optical and electrochemical properties of ideal polyaniline. Synth. Met. 1996, 82 (1), 59−61. (27) Wei, Y.; Jang, G. W.; Chan, C. C.; Hsueh, K. F.; Hariharan, R.; Patel, S. A.; Whitecar, C. K. Polymerization of aniline and alkyl ringsubstituted anilines in the presence of aromatic additives. J. Phys. Chem. 1990, 94 (19), 7716−7721. (28) Li, D.; Kaner, R. B. Shape and aggregation control of nanoparticles: not shaken, not stirred. J. Am. Chem. Soc. 2006, 128 (3), 968−975. (29) Li, D.; Kaner, R. B. How nucleation affects the aggregation of nanoparticles. J. Mater. Chem. 2007, 17 (22), 2279−2282. (30) Chen, J. F.; Wang, Y. H.; Guo, F.; Wang, X. M.; Zheng, C. Synthesis of nanoparticles with novel technology: high-gravity reactive precipitation. Ind. Eng. Chem. Res. 2000, 39 (4), 948−954. (31) Shim, Y. B.; Won, M. S.; Park, S. M. Electrochemistry of conductive polymers VIII in situ spectroelectrochemical studies of polyaniline growth mechanisms. J. Electrochem. Soc. 1990, 137 (2), 538−544. (32) Yang, H. J.; Bard, A. J. The application of fast scan cyclic voltammetry. Mechanistic study of the initial stage of electropolymerization of aniline in aqueous solutions. J. Electroanal. Chem. 1992, 339 (1), 423−449. (33) Bober, P.; Stejskal, J.; Trchova, M.; Prokeš, J.; Sapurina, I. Oxidation of aniline with silver nitrate accelerated by p-phenylenediamine: A new route to conducting composites. Macromolecules 2010, 43 (24), 10406−10413. (34) Sapurina, I. Y.; Stejskal, J. Oxidation of aniline with strong and weak oxidants. Russ. J. Gen. Chem. 2012, 82 (2), 256−275. (35) Zhang, X.; Zhu, J.; Haldolaarachchige, N.; Ryu, J.; Young, D. P.; Wei, S.; Guo, Z. Synthetic process engineered polyaniline nanostructures with tunable morphology and physical properties. Polymer 2012, 53 (10), 2109−2120. (36) Park, H.-W.; Kim, T.; Huh, J.; Kang, M.; Lee, J. E.; Yoon, H. Anisotropic growth control of polyaniline nanostructures and their morphology-dependent electrochemical characteristics. ACS Nano 2012, 6 (9), 7624−7633. (37) Gospodinova, N.; Terlemezyan, L. Conducting polymers prepared by oxidative polymerization: polyaniline. Prog. Polym. Sci. 1998, 23 (8), 1443−1484. (38) Li, Y.; Zheng, J. L.; Feng, J.; Jing, X. L. Polyaniline micro-/ nanostructures: morphology control and formation mechanism exploration. Chem. Pap. 2013, 67 (8), 876−890. (39) Li, J.; Xie, H. Q.; Li, Y.; Liu, J.; Li, Z. X. Electrochemical properties of graphene nanosheets/polyaniline nanofibers composites as electrode for supercapacitors. J. Power Sources 2011, 196 (24), 10775−10781. (40) Guan, H.; Fan, L. Z.; Zhang, H. C.; Qu, X. H. Polyaniline nanofibers obtained by interfacial polymerization for high-rate supercapacitors. Electrochim. Acta 2010, 56 (2), 964−968. (41) Li, J. B.; Jia, Q. M.; Zhu, J. W.; Zheng, M. S. Interfacial polymerization of morphologically modified polyaniline: from hollow microspheres to nanowires. Polym. Int. 2008, 57 (2), 337−341. (42) Zhao, Y. B.; Wu, W.; Chen, J. F.; Zou, H. K.; Hu, L. L.; Chu, G. W. Preparation of polyaniline/multiwalled carbon nanotubes nanocomposites by high gravity chemical oxidative polymerization. Ind. Eng. Chem. Res. 2012, 51 (9), 3811−3818. (43) Xia, H. S.; Wang, Q. Synthesis and characterization of conductive polyaniline nanoparticles through ultrasonic assisted I
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Effect of Additives on the Properties of Polyaniline Nanofibers Prepared by High Gravity Chemical Oxidative Polymerization Yibo Zhao,† Moses Arowo,†,‡ Wei Wu,*,† and Jianfeng Chen†,‡ †
State Key Laboratory of Organic-Inorganic Composites and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *
ABSTRACT: Polyaniline (PANI) nanofibers with improved properties were prepared by high gravity chemical oxidative polymerization in a rotating packed bed with the assistance of p-aminodiphenylamine (AD) and p-phenylenediamine (AP). The effects of reactor type, additive dosage, reaction temperature, and high-gravity level on the properties of products were investigated in detail. Three conclusions were made: (1) a small amount of additive can significantly improve some properties of the nanofibers such as uniformity, specific surface area, and specific capacitance; (2) in order to obtain high-quality nanofibers, the high-gravity level should coordinate with the reaction rate; (3) the molecular weight and conductivity of PANI decrease with the increase of additive dosage. The products have larger specific surface areas of up to 73.9 and 68.4 m2/g and consequently improved specific capacitance of up to 527.5 and 552 F/g for the PANI nanofibers prepared with AD and AP, respectively. However, the specific surface area and specific capacitance of pure PANI are only 49.1 m2/g and 333.3 F/g, respectively. This research provides a simple, reliable, and scalable method to produce PANI nanofibers of high performances.
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INTRODUCTION Polyaniline (PANI) has received considerable attention in many applications such as energy-storage devices,1−3 sensors,4,5 electrodes,6−8 and electrochromic devices,9−11 owing to its simple synthesis, low cost of raw materials, high conductivity, and good pseudocapacitive performances.12−16 Among the reported approaches for its preparation, chemical oxidative polymerization is the commonly used method because of its simple and convenient operation. However, since the duration of induction period of PANI usually lasts just for a few seconds or several minutes, the raw materials mixed by this technique (typically use stirring paddle or magnetic stirrer) cannot be well mixed at microscopic scale in such a short time, which finally leads to nonuniform morphology of PANI arising from heterogeneous nucleation. Researchers have shown that the properties of PANI such as conductivity,17 specific capacitance,18,19 catalytic activity,20 and gas sensitivity21 are all related to the morphology of PANI. Based on these studies, it is concluded that the PANI nanofibers with long dimension and large specific surface area usually have high performances. In order to produce PANI nanofibers of uniform morphology under homogeneous nucleation environment, the mixing of raw materials and the nucleation of PANI must be separated, and also, the second growth of PANI should be avoided. In our previous work, PANI nanofibers were successfully prepared by high gravity chemical oxidative polymerization (HGCOP) in a rotating packed bed (RPB), and this approach © XXXX American Chemical Society
was demonstrated to be efficient and especially appropriate for industrial scale-up since RPB is capable of greatly intensifying the mixing of raw materials in a short time.22 In addition, the as-prepared nanofibers have also been proved to have great potential in energy storage application.23 However, the nanofibers produced by this technique still have some morphology issues such as aggregation and nonuniformity caused by the heterogeneous nucleation as a result of the discordance between the high mixing rate of RPB and the low nucleation rate of PANI. It has been reported that a small amount of either paminodiphenylamine (AD) or p-phenylenediamine (AP) can greatly improve the morphology of PANI nanofibers as they significantly speed up the nucleation process and effectively suppress the second growth.24−27 Nevertheless, the greatly accelerated reaction needs much higher mixing rate to separate the mixing process and nucleation stage, especially for largescale production of PANI nanofibers. In this work, PANI nanofibers were prepared in RPB using AD or AP as additive in order to improve their properties. The effects of reactor type, the additive dosage, reaction temperature, and high-gravity level on the properties of the products were discussed in detail. The AD- and AP-assisted PANI nanofibers were compared by measurement of SEM images, Received: December 24, 2014 Revised: April 13, 2015
A
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Figure 1. SEM images of (A) RPB-PANI and (B) STR-PANI, (C) AD-0.5, (D) AP-0.5, (E) AD-1, (F) AP-1, (G) AD-2, and (H) AP-2.
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FT-IR spectra, UV−vis absorption spectra, XRD patterns, molecular weights, conductivities, specific surface areas, and electrochemical properties. It was demonstrated that RPB is highly suitable for this new route, and a small amount of additives can significantly improve the qualities of PANI nanofibers and their electrochemical properties.
EXPERIMENTAL SECTION
Materials. All the reagents used in this work were of analytical grade. Aniline (Tianjin Fuchen Chemical Reagent Company, China) was distilled under vacuum and refrigerated prior to use. Ammonium peroxydisulfate (APS), hydrochloric acid (HCl), and absolute alcohol were all purchased from Beijing Chemical Reagent Company (China). The additives AD and AP were purchased form Tianjin Guangfu Fine B
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Chemical Research Institute (China) and Aladdin Reagent Co. Ltd. (China), respectively, and their molecular structures are shown in Figure S1. Synthesis of PANI Nanofibers. PANI nanofibers were synthesized by the HGCOP method in RPB under different operational conditions including additive amount, reaction temperature, and RPB high-gravity level. The apparatus used is described in Figure S2, and the PANI samples were all prepared according to the following steps unless stated otherwise. Typically, 1.86 g of aniline and 1.14 g of APS were dissolved in 100 mL of HCl solution (1.0 M). The aniline solution was then mixed with 2 mL of absolute alcohol containing certain amount of AD or AP (use the mass ratio of additive to aniline to represent the dosage of additive). Both solutions were adjusted to 20 °C using a circulator bath and then simultaneously pumped into the RPB (at a high-gravity level of 713.7 m/s2) by two peristaltic pumps. The resultant mixture was collected in a tank and aged in the above bath for half an hour to form a green precipitate which was washed and dried at 60 °C overnight. As a contrast test, PANI nanofibers were also prepared by mixing both aniline and APS solution rapidly for 1 min in a stirred tank reactor (STR, made of a beaker and a stirrer), while the other steps remained the same. Preparation of Electrode. Electrode slurry was obtained by directly mixing the PANI filter cake with deionized water instead of ultrasonic treatment to avoid breaking the nanofibers. Afterward, several drops of the obtained slurry were dropped onto a titanium sheet with an area of 1.0 cm2. Finally, the titanium sheet was dried in an oven at 60 °C for 12 h to obtain the electrode. The loading of electrode was around 1.0 mg/cm2. Characterization. The morphologies of the PANI nanofibers were characterized by scanning electron microscopy (SEM, JSM-6701F), and the molecular structures were analyzed by Fourier transform infrared spectroscopy (FTIR, Nicolet model 8700) and UV−vis absorption spectroscopy (UV−vis, UV-5200PC, aqueous samples). Xray diffractometer (XRD, Bruker D8A) analysis was performed to confirm the crystal structure. The specific surface areas of the PANI nanofibers were obtained by the Brunauer−Emmett−Teller method (BET, Quadrasorb SI) at room temperature. The molecular weights of the dedoped PANI samples were measured by gel permeation chromatography (GPC, Agilent 1200 Series) at room temperature. Dimethylformamide was used both as the solvent and mobile phase (1.0 mL/min), and the system was calibrated with polystyrene standards. The conductivities of the PANI samples were tested on a digital multimeter (SX1934) by the four-probe technique. The samples were compressed into tablets at a pressure of 5.0 MPa on a desktop tablet pressing machine (YP-30T). All electrochemical measurements were performed in a threeelectrode setup. Both cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests were carried out on a CHI660D electrochemical workstation. The electrolyte used was 1.0 M H2SO4 solution. A platinum sheet and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The specific capacitances of the electrodes were calculated from the CV and GCD discharge curves according to eqs 1 and 2:
∫
Cm = ( I dV )/(νmV )
RESULTS AND DISCUSSION Effect of Reactor Type. In order to study the effect of reactor type on the polymerization process, aniline was polymerized at a low concentration of 0.025 M in RPB and STR (see Figure S3). The color of the reaction mixture changed from light blue to dark blue, signifying the end of the induction period and the late stage of polymerization, respectively. Results revealed that RPB can significantly reduce the induction period to 5 min and complete reaction time to 10 min as compared to 16 and 27 min for STR, respectively. In addition, the reaction mixture produced by STR was not as well-distributed as that obtained by RPB, indicating the poor mixing effect of STR. In order to investigate the morphology differences between the nanofibers prepared in RPB and STR, aniline was polymerized at a higher concentration of 0.1 M, and the as-obtained products were labeled as RPB-PANI and STRPANI, respectively. In Figure 1A,B it is evident that RPB-PANI exhibits more uniform morphology with a diameter of 30−50 nm, as compared to the STR-PANI which mostly shows rodlike structure (50−100 nm in diameter). Many studies have shown that the mixing degree of raw materials is very important for production of PANI nanofibers with good qualities because the uniformity of mixing greatly influences the nucleation mode of polymerization and the morphologies of the PANI.28,29 On this basis, RPB is more suitable for preparing PANI nanostructures than STR.23,30 Effect of Additive. To investigate the effect of additives on the polymerization process in RPB, aniline was polymerized with 1.0 wt % AD or AP at a low concentration of 0.025 M (see Figure S4). In comparison with the polymerization without additives (see Figure S3), the additive-containing mixtures turned dark blue in just 5 min, illustrating that additives can greatly promote the reaction. Moreover, the AD-containing mixture turned green at 0 s while this color appeared at 30 s for the AP-containing mixture, suggesting that AD is more effective for accelerating the reaction than AP. It is therefore evident that these two additives have different reaction routes in the polymerization process. AD can directly react with aniline monomers with almost no induction period since it is the predominant product of the induction period, i.e., aniline dimer.31,32 However, AP promotes the reaction just by taking part in the formation of aniline oligomers (or called active centers),33,34 which means that an induction period is still required. Nevertheless, the polymerization process is strongly accelerated by both of these additives due to the shortened induction period. The SEM images of additive-assisted nanofibers prepared in RPB are shown in Figure 1C−H. The samples prepared with 0.5, 1.0, and 2.0 wt % of the additives were labeled as AD-0.5, AD-1, AD-2, AP-0.5, AP-1, and AP-2, accordingly. Noticeably, the nanofibers become more uniform in morphology and smaller in diameter with increasing additive dosage. Interestingly, AP-assisted nanofibers tend to twist into bundles, and this trend gets more and more obvious with the increase of AP dosage. To obtain the morphology parameters of PANI nanofibers, samples were dispersed in water by ultrasound and dropped on copper grids for SEM observation. The calculation method of these parameters was similar to others.35,36 After analyzing 150 PANI nanofibers from the SEM images of each sample, the statistical morphology parameters of the samples are listed in Table 1. It is clear that a gradual increase in additive dosage
(1)
where Cm is the specific capacitance based on the mass of electroactive material in F/g, I is the response current in A, V is the potential in V, ν is the potential scan rate in V/s, and m is the mass of the active material on the electrode in g. Cm = I /(dV /dt ·m)
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(2)
where Cm is the specific capacitance based on the mass of electroactive material in F/g, I is the discharge current in A, dV/dt is the slope of the linear section on discharge curve in V/s, and m is the mass of the active material on the electrode in g. C
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change. In contrast, the sample with 2.0 wt % AP shows nanoribbon morphology (100 nm in width and 25 nm in thickness) only at 0 °C, while only nanofibers were observed at other temperatures. Additionally, it is evident that only nanofibers were observed when aniline was polymerized with 1.0 wt % AP at 0 °C (see Figure S5), indicating that adequate AP dosage is essential for the formation of nanoribbons. Therefore, it can be inferred that the morphology difference observed at 0 °C is related to the molecular structures of the respective additives since both AD- and AP-assisted samples were synthesized at the same conditions. Additives can form a lot of growth centers prior to aniline monomers because of their much lower oxidant potential,24,37 and the as-formed growth centers subsequently determine the formation of PANI nanostructures in the chain propagation stage.38 Thus, AD-assisted samples simply form nanofibers at any temperature since AD is aniline dimer. However, APassisted samples favor the formation of bundle-like structure as mentioned earlier, and this structure may self-assemble into nanoribbons under the influence of hydrogen bonds (primary amines of AD molecules) at low temperature (see Figure S6). Effect of High-Gravity Level. According to the previous study, high high-gravity level is favorable for preparing PANI nanofibers of thin diameter.22 However, since the reaction conditions have been changed, this factor should be studied further. As shown in Figure 3, the morphology of pure PANI becomes more and more uniform with increasing high-gravity level, but still not as uniform as the additives-assisted PANI nanofibers. However, the additive-assisted PANI nanofibers are
Table 1. Morphology Parameters of the PANI Nanofibers Prepared with Different Additive Dosages samples
av diam (nm)
std dev of diam (nm)
av length (nm)
av aspect ratio
RPB-PANI AD-0.5 AD-1 AD-2 AP-0.5 AP-1 AP-2
42.1 40.6 33.2 29.5 37.9 37.4 31.2
13.2 8.7 7.1 6.7 8.9 7.2 6.7
580 728.2 597.6 573.3 1201.8 1278.5 1166
13.8 17.9 18 19.4 31.7 34.2 37.4
leads to an obvious decrease in the average diameter (av diam) and the standard deviation of diameter (std dev of diam). Also, additives, especially AP, can increase the nanofibers’ average length (av length) effectively. Furthermore, the average aspect ratio (av aspect ratio, equals to av length/av diam) of the nanofibers increased from 13.8 to 19.4 and 37.4 for AD- and AP-assisted samples, respectively. Therefore, it can be established that AP is more effective for improving the morphology of PANI nanofibers than AD. Effect of Reaction Temperature. To study the effect of reaction temperature, aniline was polymerized with no additive, 2.0 wt % AD and 2.0 wt % AP at different temperatures. As shown in Figure 2, the morphology of pure PANI changes from irregular particles to uniform nanofibers with increasing temperature since higher temperature favors homogeneous nucleation.22 However, the samples with 2.0 wt % AD always exhibit uniform nanofibers irrespective of the temperature
Figure 2. SEM images of PANI nanofibers polymerized with (A) no additives, (B) 2.0 wt % AD, and (C) 2.0 wt % AP in RPB at different temperatures (left: 0 °C; middle: 20 °C; right: 40 °C; aniline concentration: 0.1 M; high-gravity level: 713.7 m/s2). D
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Figure 3. SEM images of PANI nanofibers polymerized with (A) no additive, (B) 2.0 wt % AD, and (C) 2.0 wt % AP in RPB at different high-gravity levels (left: 178.4 m/s2; middle: 713.7 m/s2; right: 2854.7 m/s2; aniline concentration: 0.1 M; reaction temperature: 20 °C).
maximum of 17.0 and 42.2 for AD and AP samples at 713.7 m/ s2, respectively. It is notable that the average aspect ratio of the additive-assisted nanofibers decreases distinctly at 2854.7 m/s2. As discussed above, additives are effective in increasing the reaction rate of PANI and higher high-gravity level can also promote the polymerization process. These two factors result in an extremely fast reaction, making the initially formed nanofibers be precipitated in the RPB. Therefore, the growth environment of the nanofibers is disturbed by the strong agitation of RPB, and finally only short nanofibers are formed. Characterization of PANI Nanofibers. The composition of the as-prepared PANI nanofibers was studied by FTIR and UV−vis spectroscopy. Figure 4A shows the FTIR spectra of pure PANI, AD-2, and AP-2. The samples show similar peak position, and the main peaks located at 3438, 1570, 1486, 1297, 1133, and 802 cm−1 are in agreement with the emeraldine form of PANI.39−41 The characteristic bands at 1570 and 1486 cm−1 are assigned to the CC stretching vibration of quinoid rings and benzene rings, while other peaks located at 3438, 1297, 1133, and 802 cm−1 are attributed to the N−H stretching vibration, C−N stretching mode, NQN stretching mode (with Q representing the quinoid ring), and C−H bonds of benzene rings, respectively.42 By comparison, it was found that these three samples have almost the same peaks, indicating that the additives hardly affect the typical molecular structure of PANI.24 The UV−vis absorption spectra of PANI nanofibers are shown in Figure 4B. The spectra are consistent with other literatures.43,44 It is clear that all the samples have three similar characteristic peaks. The peaks at 350, 440, and 850 nm are ascribed to the π−π* transition of the benzenoid rings,
relatively short when the high-gravity level is too high (2854.7 m/s2), especially for AD-assisted samples. As shown in Table 2, the statistical data of the samples were obtained using the same method as described earlier. It is Table 2. Morphology Parameters of the PANI Nanofibers Prepared at Different High-Gravity Levels av diam (nm)
std dev of diam (nm)
av length (nm)
av aspect ratio
A1 A2 A3 B1
43.4 42.1 36.4 37.8
13.7 13.2 11.8 6.8
478.5 580 553.7 496.2
11.0 13.8 15.2 13.1
B2 B3 C1
29.5 36.7 26.1
6.7 8.4 6.9
573.3 420.6 737.5
17.0 11.5 28.3
C2 C3
31.2 34.8
6.7 7.9
1166 632.9
42.2 18.2
samples no additive
2.0 wt % AD
2.0 wt % AP
evident that higher high-gravity level improves the quality of pure PANI nanofibers in terms of both diameter and average aspect ratio as it ensures sufficient micromixing and consequently homogeneous nucleation of PANI.22 Nevertheless, pure PANI nanofibers have less uniform morphology as evidenced by the large standard deviation of diameter (>11 nm). However, the additive-assisted PANI nanofibers display smaller diameter, more centralized diameter distribution (around 7 nm) and larger average aspect ratio than pure PANI nanofibers. The largest average aspect ratio is up to a E
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Figure 4. (A) FTIR spectra, (B) UV−vis absorption spectra, and (C) XRD patterns of (a) pure PANI, (b) AD-2, and (c) AP-2.
polaron−π*, and π−polaron transitions, respectively.45 Further, the peaks at 442 and 850 nm are related to the doping level and the formation of polaron46 and indicate that the obtained PANI is in the doped state.42 To investigate the crystallinity of the samples, XRD patterns of the samples were recorded from 5 to 40°. In Figure 4C, there are four characteristic peaks located at 2θ = 9.1°, 15.1°, 20.3°, and 25.4°, which correspond to the (001), (011), (020), and (200) crystal planes of PANI, respectively.47−49 These peaks further illustrate that the products are in highly doped emeraldine salt form.22 The peaks at 20.3° and 25.4° are more intense for both AD-2 and AP-2 as compared with those of pure PANI, signifying their better crystallinity. Basic Properties of PANI Nanofibers. The molecular weights of PANI nanofibers with different additive dosages are shown in Table S1. It is evident that 0.5 wt % of either AD or AP significantly increases the molecular weight whereas dosages beyond 0.5 wt % result in decreased molecular weight. The variation in polydispersities of the products also follows a similar trend. The explanation of these phenomena is based on two basic points: (1) additives act as the nucleation centers of PANI; (2) the aniline dosage used in the experiments is constant. When the additive dosage is very small, each nucleation center can have more aniline molecules growing on it, thereby making it difficult to guarantee the uniformity of molecular weight. On the other hand, excessive additive dosage can lead to less aniline molecules growing on each nucleation center and thus narrower the molecular weight distribution. Therefore, low additive dosage is beneficial to increase the molecular weight of PANI while high additive dosage will result in PANI of narrow molecular weight distribution. It is well-known that PANI nanofibers have anisotropic conductivity because they are one-dimensional conductors. Usually, long PANI nanofibers have larger conductivity than short ones because the conductivity along the molecular chain is much larger than that along the perpendicular direction. Therefore, the conductivity of PANI is significantly affected by
its morphology. As shown in Figure 5, the conductivity of both AD- and AP-assisted samples first increases and then decreases
Figure 5. Conductivity variation of PANI nanofibers with different additive dosages.
with an increase in additive dosage, and the highest conductivity for AD- and AP-assisted samples is 334.5 and 389.1 S/m, respectively. The increase in conductivity is ascribed to the dramatic change in PANI morphology and the increased molecular weight of PANI (see Table S1), while the reduction in conductivity is related to the decrease of molecular weight. This trend of decrease in conductivity has also been found by other researchers.50,51 In our previous research, the specific surface area of pure PANI prepared in RPB and STR was found to be 49.1 and 38.9 m2/g, respectively, demonstrating that RPB has superior advantages on the preparation of nanomaterials.23 As for the AD-assisted samples, the specific surface area of AD-0.5, AD-1, and AD-2 is 53.5, 62.6, and 73.9 m2/g, respectively. The increase in specific surface area is due to the reduced diameter of PANI nanofibers (see Table 1). However, the specific surface area of AP-0.5, AP-1, and AP-2 is 68.4, 67.1, and 60.5 m2/g, respectively. This trend of decrease may be attributed to the F
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Figure 6. CV curves of the (A) AD-assisted and (B) AP-assisted samples at the scan rate of 10 mV/s; initial GCD curves of (C) AD-assisted and (D) AP-assisted samples at a current density of 0.5 A/g.
Figure 7. Formation mechanism of PANI nanofibers without or with the addition of additives in RPB.
slow during the first cycle. The discharge process started from around 0.74 to 0 V. From these curves, the specific capacitance of pure PANI, AD-0.5, AD-1, AD-2, AP-0.5, AP-1, and AP-2 was calculated to be 333.3, 454.5, 522.8, 527.5, 471.7, 500.3, and 552 F/g, respectively, which agrees well with that deduced from the CV tests. Usually, the specific capacitance of PANI mainly relates to its specific surface area (increase the effective utilization rate of PANI) and conductivity (speed up the charge transfer). However, together with the result of conductivity, it seems that the specific area has a greater influence than conductivity in explaining the variation trend of specific capacitance of PANI samples, meaning that the conductivity is high enough for the electrochemical process. Mechanism Analysis. According to the above discussions, the mechanism is described in Figure 7. When aniline molecules are polymerized with no additives, the as-formed PANI nanofibers exhibit short length and wide distribution of diameter due to the heterogeneous nucleation resulting from
special bundle structure; i.e., several nanofibers combine into one nanofiber, leading to the reduction of the exposed surface area. Therefore, it could be deduced that the real specific surface area of nanofibers should be larger than the measured value since the diameter of nanofibers actually decreases with the AP dosage. The electrochemical performances of PANI nanofibers were evaluated by CV and GCD in a three-electrode system. In Figure 6A,B the CV curves of all the samples have three pairs of redox peaks, indicating the existence of pseudocapacitance. Besides, the CV curves of both AD- and AP-assisted samples have larger areas than that of pure PANI, illustrating the better specific capacitance. According to these CV curves, the specific capacitance of pure PANI, AD-0.5, AD-1, AD-2, AP-0.5, AP-1, and AP-2 was found to be 274.4, 389.5, 423.8, 466.4, 406.3, 425.2, and 445.9 F/g, respectively. The initial GCD curves of all the samples are presented in Figure 6C,D. It is normal that the charge process is relatively G
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(3) Cong, H. P.; Ren, X. C.; Wang, P.; Yu, S. H. Flexible graphene− polyaniline composite paper for high-performance supercapacitor. Energy Environ. Sci. 2013, 6 (4), 1185−1191. (4) Ding, M. N.; Tang, Y. F.; Gou, P. P.; Reber, M. J.; Star, A. Chemical sensing with polyaniline coated single-walled carbon nanotubes. Adv. Mater. 2011, 23 (4), 536−540. (5) Hibbard, T.; Crowley, K.; Killard, A. J. Direct measurement of ammonia in simulated human breath using an inkjet-printed polyaniline nanoparticle sensor. Anal. Chim. Acta 2013, 779, 56−63. (6) Tai, Q. D.; Chen, B. L.; Guo, F.; Xu, S.; Hu, H.; Sebo, B.; Zhao, X. Z. In situ prepared transparent polyaniline electrode and its application in bifacial dye-sensitized solar cells. ACS Nano 2011, 5 (5), 3795−3799. (7) Wang, G. Q.; Xing, W.; Zhuo, S. P. The production of polyaniline/graphene hybrids for use as a counter electrode in dyesensitized solar cells. Electrochim. Acta 2012, 66, 151−157. (8) Pirkarami, A.; Olya, M. E.; Yousefi Limaee, N. Decolorization of azo dyes by photo electro adsorption process using polyaniline coated electrode. Prog. Org. Coat. 2013, 76 (4), 682−688. (9) Zhang, J.; Tu, J. P.; Zhang, D.; Qiao, Y. Q.; Xia, X. H.; Wang, X. L.; Gu, C. D. Multicolor electrochromic polyaniline−WO3 hybrid thin films: One-pot molecular assembling synthesis. J. Mater. Chem. 2011, 21 (43), 17316−17324. (10) Wei, H. G.; Zhu, J. H.; Wu, S. J.; Wei, S. Y.; Guo, Z. H. Electrochromic polyaniline/graphite oxide nanocomposites with endured electrochemical energy storage. Polymer 2013, 54 (7), 1820−1831. (11) Kelly, F. M.; Meunier, L.; Cochrane, C.; Koncar, V. Polyaniline: Application as solid state electrochromic in a flexible textile display. Displays 2013, 34 (1), 1−7. (12) Ć irić-Marjanović, G. Recent advances in polyaniline research: polymerization mechanisms, structural aspects, properties and applications. Synth. Met. 2013, 177, 1−47. (13) Wang, Y.; Tran, H. D.; Liao, L.; Duan, X. F.; Kaner, R. B. Nanoscale morphology, dimensional control, and electrical properties of oligoanilines. J. Am. Chem. Soc. 2010, 132 (30), 10365−10373. (14) Wei, Z. X.; Zhang, Z. M.; Wan, M. X. Formation mechanism of self-assembled polyaniline micro/nanotubes. Langmuir 2002, 18 (3), 917−921. (15) Feng, X. M.; Mao, C. J.; Yang, G.; Hou, W. H.; Zhu, J. J. Polyaniline/Au composite hollow spheres: synthesis, characterization, and application to the detection of dopamine. Langmuir 2006, 22 (9), 4384−4389. (16) Kim, B.-J.; Oh, S.-G.; Han, M.-G.; Im, S.-S. Preparation of polyaniline nanoparticles in micellar solutions as polymerization medium. Langmuir 2000, 16 (14), 5841−5845. (17) Rahy, A.; Yang, D. J. Synthesis of highly conductive polyaniline nanofibers. Mater. Lett. 2008, 62 (28), 4311−4314. (18) Amarnath, C. A.; Chang, J. H.; Kim, D.; Mane, R. S.; Han, S.-H.; Sohn, D. Electrochemical supercapacitor application of electroless surface polymerization of polyaniline nanostructures. Mater. Chem. Phys. 2009, 113 (1), 14−17. (19) Zhou, H. H.; Chen, H.; Luo, S. L.; Lu, G. W.; Wei, W. Z.; Kuang, Y. F. The effect of the polyaniline morphology on the performance of polyaniline supercapacitors. J. Solid State Electrochem. 2005, 9 (8), 574−580. (20) Duić, L.; Grigić, S. The effect of polyaniline morphology on hydroquinone/quinone redox reaction. Electrochim. Acta 2001, 46 (18), 2795−2803. (21) Li, G. F.; Martinez, C.; Janata, J.; Smith, J. A.; Josowicz, M.; Semancik, S. Effect of morphology on the response of polyanilinebased conductometric gas sensors: Nanofibers vs. thin films. Electrochem. Solid-State Lett. 2004, 7 (10), H44−H47. (22) Lu, X. W.; Wu, W.; Chen, J. F.; Zhang, P. Y.; Zhao, Y. B. Preparation of polyaniline nanofibers by high gravity chemical oxidative polymerization. Ind. Eng. Chem. Res. 2011, 50 (9), 5589− 5595. (23) Zhao, Y. B.; Wei, H. G.; Arowo, M.; Yan, X. R.; Wu, W.; Chen, J. F.; Wang, Y. R.; Guo, Z. H. Electrochemical energy storage by
the discordance between the high mixing rate of RPB and the low nucleation rate of PANI. However, the additives can serve as nucleation centers, accelerate the reaction, and eventually make the process more appropriate for RPB. Consequently, uniform nanofibers with thin diameter are obtained. However, AD and AP have different impacts on the products because of their different molecular structure. AD, as a kind of intermediate product of PANI, can serve as nucleation center and therefore greatly promote the reaction. As for AP, since its para-position is hindered, the polymerization will be initiated in its meta- or ortho-position, promoting the formation of crosslinked structure.24 Besides, the hydrogen bonds between the primary amines of AP molecules may promote the formation of bundle-like PANI nanofibers.
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CONCLUSIONS In summary, aniline was polymerized with additives in the RPB to produce PANI nanofibers with good properties. It was observed that the additives can greatly accelerate the reaction rate and make the process more suitable for polymerization in RPB. The additive-assisted nanofibers exhibited higher average aspect ratio, more uniform diameter, and larger specific surface area than pure PANI nanofibers. The specific capacitance was 527.5 and 552 F/g for AD-2 and AP-2, respectively. The greatly improved specific capacitance is mainly due to the high specific areas of products. This research therefore provides a simple, reliable, and scalable route for synthesizing PANI nanofibers with good morphology and energy storage performance, and we believed that the property of other conducting polymers (e.g., polythiophene and polypyrrole) could also be improved by introducing suitable additives in a RPB.
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ASSOCIATED CONTENT
S Supporting Information *
Molecular structures of the additives, schematic diagram of the reaction apparatus, reaction process of PANI without and with the addition of additives, supplementary SEM images, and molecular weights of products. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/la504996c.
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AUTHOR INFORMATION
Corresponding Author
*E-mail [email protected]; Tel +86-10-64443134; Fax +86-10-64434784 (W.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors greatly acknowledge the financial support from the National Natural Science Foundation of China (Grants 21376025 and 21076021).
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REFERENCES
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