Spectral Characteristics of Polyaniline Nanostructures Synthesized by

Aug 20, 2008 - potential scan rates, in the presence of ferrocenesulfonic acid. The potential scan rate controlled the formation and growth of polyani...
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J. Phys. Chem. B 2008, 112, 11558–11563

Spectral Characteristics of Polyaniline Nanostructures Synthesized by Using Cyclic Voltammetry at Different Scan Rates Shaolin Mu* and Yifei Yang Department of Chemistry, Yangzhou UniVersity, Yangzhou 225002, Jiangsu ProVince, China ReceiVed: June 12, 2008; ReVised Manuscript ReceiVed: July 12, 2008

The polyaniline nanofibers with different sizes were synthesized by using cyclic voltammetry at different potential scan rates, in the presence of ferrocenesulfonic acid. The potential scan rate controlled the formation and growth of polyaniline nuclei, which plays a key role in controlling nanofiber sizes. The average diameters of nanofibers decreased from about 130 nm to about 80 nm as the potential scan rate increased from 6 to 60 mV s-1. We first observed an ordered change in the following spectra with the nanofiber sizes of polyaniline. The spectra of the X-ray diffraction indicated that the partially crystalline form existed in the polyaniline nanofibers and that the crystallinity of polyaniline increased with decreasing diameter of polyaniline nanofibers. The ESR spectra revealed the fact that the decrease in the intensity of the ESR signal was accompanied by the increase in the value of the peak-to-peak line width ∆Hpp as the diameter of polyaniline nanofibers decreased. The 1H NMR spectra showed that a peak in a triplet caused by the ( NH free radical was split into two peaks with different intensities and that their relative intensity also changed with the diameter of the polyaniline nanofibers. 1. Introduction Among conducting polymers, polyaniline is a promising material because of its high conductivity, good redox reversibility, swift change of color with potential, and stability in aqueous solutions and air. Research on polyaniline is also closely related to the rapid development in nanofibers, nanowires, nanotubes,1-9 and molecular electronics.10 Polyaniline nanostructures have aroused considerable interest in their unique properties11-13 and potential applications.14-16 The nanostructured polyaniline can be prepared by using chemical, electrochemical, and physical methods, which have been widely reviewed and discussed by Huang and Kaner.17 Among these methods, the chemical oxidative polymerization of aniline has received growing interest in recent years because this method can readily and rapidly prepare bulk quantities of nanostructured polyaniline, 5,18 compared to the electrochemical method. Conventional chemical synthesis of nanostructured polyaniline was carried out by polymerizing the monomer with the aid of either insoluble templates,19-21 soluble templates,22-24 or biological templates.25 In addition, both interfacial polymerization26 and rapidly mixed reactions27 have been used for making pure nanofibers by suppressing the secondary growth of polyaniline in the absence of any template. The rapidly mixed reaction is of great significance for the chemical preparation of nanofibers and provides a clearly synthetic mechanism of polyaniline nanofibers. As mentioned above, most of the polyaniline nanostructures were prepared by using the chemical oxidative polymerization of aniline. However, the electrochemical oxidative polymerization of aniline for the preparation of the polyaniline nanostructures has also received considerable attention since the electrochemical method has relatively easy controllability of experimental conditions compared to the chemical oxidative polymerization of aniline, and the electrochemical preparation of polyaniline nanostructures provided a convenient way to study * Corresponding author. E-mail: [email protected].

the electrochemical properties of nanostructures.28 The pure polyaniline nanofibers can be obtained without the need for any template, simply by controlling the electrochemical polymerization kinetics.29-31 Although, many papers regarding polyaniline nanostructures have been reported as mentioned above, it still remains as a great challenge to prepare polyaniline nanofibers with controllable sizes and especially to approach the influence of the nanofiber sizes on the spectral properties of polyaniline. In this article, we reported the synthesis of polyaniline nanofibers by cyclic voltammetry in the presence of ferrocenesulfonic acid, but in the absence of any solid template. Polyaniline nanofiber sizes obtained in this manner are controllable via controlling potential scan rates, and particularly, we first observed that the nanofiber sizes have a significant effect on the spectra of X-ray diffraction, 1H NMR, and ESR of polyaniline. 2. Experimental Section The chemicals used were of analytical reagent grade. Aniline was distilled under reduced pressure before use. Doubly distilled water was used to prepare solutions. The electrolytic cell for the synthesis of polyaniline consisted of two platinum foils and a saturated calomel reference electrode (SCE). The electrochemical experiments were performed on a CHI 407 electrochemical workstation and a PAR Model 173 potentiostatgalvanostat with a Model 179 digital coulometer. A solution containing 0.2 M aniline, 0.2 M ferrocenesulfonic acid, and 0.3 M H2SO4 was used for the electrochemical polymerization of aniline, which was carried out by using cyclic voltammetry between -0.10 and 0.92 V under different scan rates. Polyaniline films used for the measurement of the SEM images were obtained by using cyclic voltammetry for four cycles under the scan rates of 6, 12, and 30 mV s-1, and six cycles under the scan rate of 60 mV s-1. Before measurements of SEM images, polyaniline films were first washed with 0.05

10.1021/jp8051517 CCC: $40.75  2008 American Chemical Society Published on Web 08/20/2008

Polyaniline Nanostructures

Figure 1. Film growth of polyaniline during the electrolysis of a solution containing 0.2 M aniline, 0.2 M ferrocenesulfonic acid, and 0.3 M H2SO4. Curves: (3) third cycle; (4) forth cycle; (5) fifth cycle; (6) sixth cycle, at a scan rate of 60 mV s-1.

M H2SO4 solution to remove unreacted aniline and ferrocenesulfonic acid, and then were immersed in 0.2 M H2SO4 solution to make repeated potential cycling between -0.20 and 0.80 V for seven cycles to remove further ferrocenesulfonic acid adsorbed on the polyaniline films. Finally, they were rinsed with doubly distilled water. The SEM images of polyaniline films deposited on platinum foils were measured on an XL-30 ESEM instrument. A model Tecnai-12 transmission electron microscope was used to determine the TEM images of polyaniline samples. To obtain a well-dispersed system, the samples dissolved in ethanol were treated with ultrasonic agitation for 10 min. Polyaniline samples used for measurements of X-ray diffraction, ESR, 1H NMR, and conductivity were also prepared by using repeated potential cycling at different scan rates. By ending the electrolysis, the potential was kept at 0.75 V. This indicates that polyanilines were in the oxidation state. Polyaniline was peeled off the platinum electrode and then thoroughly washed with 0.05 M H2SO4 solution until the filtrate was colorless. Finally, the product was dried. The X-ray diffraction data of nanostructured polyanilines were recorded by using a Bruker D8 super speed X-ray instrument (Cu KR radiation, λ ) 1.542 Å). The proton NMR spectra of nanostructured polyanilines were conducted on a 600 MHz Bruker spectrometer at 303.1 K in dimethyl sulfoxide-d6 (DMSO-d6) in a 5-mm diameter NMR tube. The ESR measurements were carried out using a Bruker A300 spectrometer operating in X-band (9.862 GHz). The microwave power for the measurements of nanostructured polyanilines in the solid state was 20 mW, and the modulation amplitude was set at 1.0 G. The Bruker Company provides a g-factor marker with S3/2, and its g-value is 1.9800 ( 0.0006. To obtain the g-value of polyaniline, both the sample and the g-factor marker were put into a testing tube to simultaneously measure their ESR signals. The conductivity of polyaniline was measured on a pressed pellet using a four-probe technique. The thickness of the pellet sample was determined by using a QLR digital caliper. 3. Results and Discussion 3.1. Synthesis of Polyaniline Nanofibers with Different Sizes. Figure 1 shows film growth of polyaniline during the electrolysis of a solution containing 0.2 M aniline, 0.2 M ferrocenesulfonic acid, and 0.3 M H2SO4 at a scan rate of 60 mVs-1. As can be seen in Figure 1, the anodic and cathodic currents increase quickly with an increasing number of cycles. As a result, the polyaniline film grows on the platinum electrode with time. The first pair of redox peaks in Figure 1 is attributed

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Figure 2. SEM images of polyaniline films obtained at the different scan rates: (a) 6; (b) 12; (c) 30; (d) 60 mV s-1. the scale bar is 1 µm.

to the redox of ferrocenesulfonic acid itself;32 its anodic and cathodic peak currents increase with an increasing number of cycles, which is caused by polyaniline that catalyzes the redox reaction of ferrocenesulfonic acid. The oxidation peak at the highest potential is caused by the oxidation of aniline. It was found that the size of poly(aniline-co-o-aminophenol) synthesized in the sulfuric acid medium is much larger than that of the copolymer synthesized under exactly the same conditions, but in the presence of ferrocenesulfonic acid.28 This indicates that ferrocenesulfonic acid in the electrolytic solution plays a role in making fiber sizes of the copolymer smaller compared to that of the copolymer synthesized in the absence of ferrocenesulfonic acid. To synthesize polyaniline nanostructures, polyaniline was synthesized in the presence of ferrocenesulfonic acid. This is because the molecule of ferrocenesulfonic acid has a large size with positive charges. On the forward scan, the oxidation of aniline results in the formation of polyaniline; meanwhile, ferrocenesulfonic acid with large size in the polyaniline film is repelled into the solution due to electrostatic interaction, which would play a role in decreasing the growth rate of polyaniline fibers. Therefore, the regular doping and dedoping processes are favorable for the formation of smaller nanofibers. However, the polyaniline nanofiber sizes were mainly controlled by the potential scan rate during the polymerization process, which will be discussed in detail in the following section. As discussed above, ferrocenesulfonic acid can be oxidized and reduced during the polymerization process of aniline, indicating that a portion of charges passed in electrolysis was consumed with the redox of ferrocenesulfonic acid. To know the electrolytic efficiency, the electrochemical polymerization of aniline in the presence and absence of ferrocenesulfonic acid was carried out, respectively, under a constant potential of 0.85 V; the same charge was passed for the electrolysis of both solutions. After electrolysis, both polyaniline films were thoroughly washed by using 0.05 M H2SO4 solution and then were immersed in 0.2 M H2SO4 solution to make cyclic voltammetry in the potential range of -0.20 to 0.80 V at a scan rate of 60 mV s-1. The electrolytic efficiency of aniline in the presence of ferrocenesulfonic acid was calculated from the area of both cyclic voltammograms (omitted here) to be 69.3%, compared to that of the electrolysis of aniline in the absence of ferrocenesulfonic acid. 3.2. Morphology of Polyaniline Nanofibers. Figure 2 shows SEM images of polyaniline films obtained at the scan rates of

11560 J. Phys. Chem. B, Vol. 112, No. 37, 2008 6 (a), 12 (b), 30 (c), and 60 mV s-1 (d). The SEM images in Figure 2 reveal a fact that all films are constructed of interwoven nanofibers with different diameters and lengths. The nanofibers in Figure 2a have an average diameter of about 130 nm with lengths varying from 300 nm to 2.6 µm, and its structure is the loosest among plots in Figure 2. As the scan rate increases, the diameter and the length of the nanofibers become smaller and shorter, respectively, and the nanostructures become more and more compact. At the scan rate of 60 mV s-1, the nanofibers in Figure 2d have an average diameter of about 80 nm with lengths varying from 270 nm 1.1 µm. It is clear that the sizes of the polyaniline nanofibers can be controlled by the potential scan rate. Now, there are two questions here: (1) Why are polyaniline nanofibers obtained at the scan rate of 6 mV s-1 the largest in diameter and the longest in length? (2) Why does the morphology of polyaniline films strongly depend on the scan rate? In general, aniline in the acidic solution can be oxidized beginning at 0.70 V (vsSCE) to form polyaniline. For the synthesis of polyaniline using cyclic voltammetry, a slower scan rate is favorable to slow the formation of polyaniline nuclei. This is because, at the slower scan rate, the retention time becomes longer in the aniline oxidation region of 0.70 to 0.92 V in our experimental conditions shown in Figure 1, which leads to the rapid growth of polyaniline nuclei. As a result, the rate of film growth increases due to the longer time allowing more film growth. Therefore, only few polyaniline nanofibers with large diameters were found in Figure 2a. This process is similar to that of crystallization of a salt in the aqueous solution. That is, there are two steps involved in the electrochemical polymerization of aniline. They are first, the formation of the polyaniline nuclei and second, their growth. As the electrolysis proceeds slowly at the scan rate of 6 mV s-1, polyaniline continuously formed on the polyaniline nuclei since polyaniline can catalyze the oxidation of aniline.33,34 In this case, the polymerization of aniline still took place preferably on the polyaniline nuclei rather than at the naked platinum surface. This is why polyaniline nanofibers in Figure 2a are the largest in diameter and the longest in length, and a certain amount of the naked platinum surface area still remaind in Figure 2a after ending the polymerization of aniline. The scan rate increased from 6 to 12, 30, and 60 mV s-1, which is favorable for the quicker formation of the polyaniline nuclei, but slower growth. As a result, the amount of polyaniline nuclei deposited on the platinum surface increased with increasing scan rate. Therefore, the images in Figure 2a-d show that the surface structure of polyaniline become more and more compact and that the nanofiber sizes are getting smaller and smaller in diameter and shorter and shorter in length as the scan rate increases. Figure 2 shows that the diameters of polyaniline nanofibers decrease with increasing potential scan rate, which is in good agreement with the result of rapidly mixed reaction that was successfully used for the chemical preparation of polyaniline nanofibers.27 Figure 3 shows TEM images of polyanilines synthesized at different scan rates. The average diameter of the nanofibers is about 120 nm in Figure 3a and is about 90 nm in Figure 3d. The trend of the change in the diameter of nanofibers is similar to that shown in Figure 2. The SEM and TEM images in Figures 2 and 3 were measured on polyaniline samples that were purified with washing and using cyclic voltammetry in 0.2 M H2SO4 solution as described in the Experimental Section. Sulfuric acid is a very strong acid, and it is a much better dopant than ferrocenesulfonic acid. Thus, the second cyclic voltammetry during the purification of the

Mu and Yang

Figure 3. TEM images of polyaniline films obtained at the different scan rates: (a) 6; (b) 12; (c) 30; (d) 60 mV s-1. The scale bar is 0.2 µm.

Figure 4. SEM images of polyaniline samples purified without using the second cyclic voltammetry scanning, synthesized at different scan rates: (a) 6; (b) 12; (c) 30; (d) 60 mV s-1. The scale bar is 1 µm.

samples partially leads to the replacement of ferrocenesulfonic acid in polyaniline with sulfuric acid, which would affect the morphology of polyaniline samples. To differentiate and to identify whether the fibers are produced during the electrochemical polymerization or during purification using the second cyclic voltammetry scanning in the sulfuric acid solution, polyaniline samples purified without using the second cyclic voltammetry scanning were directly used to measure their images, after the samples were thoroughly washed with 0.05 H2SO4 solution and dried. Figure 4 shows the SEM images of polyaniline nanofibers purified without using the second cyclic voltammetry scanning, in which the average diameter of the nanofibers also decreases with increasing scan rate. Therefore, the result in Figure 4 is similar to that shown in Figure 2, indicating that the nanofibers are produced during the electrochemical polymerization of aniline, i.e., the sizes of polyaniline nanofibers produced are dependent on the scan rate during the electrochemical polymerization of aniline by using cyclic voltammetry.

Polyaniline Nanostructures

Figure 5. X-ray diffraction spectra of nanostructured polyanilines prepared at different potential scan rates: (1) 6, (2) 12, and (3) 60 mV s-1.

3.3. Diffraction Spectra of Polyaniline Nanofibers. X-ray diffraction was used to further probe the structure of the polyaniline nanofibers. Curves 1-3 in Figure 5 are the diffraction spectra of nanostructured polyanilines prepared at the potential scan rates of 6, 12, and 60 mV s-1, respectively. Two broad peaks at 2θ ) 19.8° and 25.1° appear on each curve in Figure 5, which arise from momentum transfer perpendicular to the chain direction.35 The broad X-ray structure suggests an amorphous polymer. It is noteworthy that there are another two sharp peaks at 2θ ) 39.7° and 46.2° on each curve in Figure 5, and the diffraction intensity of both sharp peaks increases with the decrease in the diameter of nanofibers. This result reveals a fact that the partially crystalline form exists in the polyaniline nanofibers and that the crystallinity of polyaniline increases with decreasing diameter of nanofibers. The latter phenomenon is first observed for polyaniline. In fact, the X-ray diffraction spectrum of polyaniline doped with 1,5-naphthalene disulfonic acid showed a sharp peak at 2θ ) 37.5°, which was obtained in the study of the influence of different naphthalene sulfonic acids on the structure of polyaniline nanotubes.36 In general, there is a peak at 2θ ) 10° in the X-ray diffraction spectrum of polyaniline, which arises from scattering with momentum transfer along the orientation direction,35 but depending on the kind of the counteranions35,36 and doping level.37,38 It was found that there is no peak at 2θ ) 10° in the X-ray diffraction spectra of polyanilines doped with R-naphthalene sulfonic acid and β- naphthalene sulfonic acid36 and that the peak intensity at 2θ ) 10° decreases with decreasing doping level.37,38 However, there is no peak at 2θ ) 10° in Figure 5, which would be caused by ferrocenesulfonic acid in polyaniline. 3.4. ESR Spectra of Polyaniline Nanofibers. The unpaired spin density of polyaniline or aniline copolymer strongly depends on the protonation level and the redox state. Therefore, the ESR technique was used here to approach the influence of polyaniline nanofiber sizes on the ESR spectra of polyaniline. Curves 1-3 in Figure 6 are the ESR spectra of nanostructured polyanilines prepared at the potential scan rates of 6, 12, and 60 mV s-1, respectively. The weight of each sample was 10.0 mg. As can be seen in Figure 6, the ESR signal intensity decreases as the diameter of the polyaniline nanofibers decreases, and the values of the peak-to-peak line width ∆Hpp of nanostructured polyanilines are 18.2 G (curve 1), 24.3 G (curve 2), and 29.3 G (curve 3). It is evident that the value of ∆Hpp of the nanostructured polyaniline increases with decreasing diameter of polyaniline nanofibers. Such ordered changes in both the ESR signal intensity and the ∆Hpp value with nanofiber sizes are also first observed for polyaniline. In general, the increase in the

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Figure 6. ESR spectra of nanostructured polyanilines prepared at different potential scan rates: (1) 6, (2) 12, and (3) 60 mV s-1.

intensity of the ESR signal of polyaniline or aniline copolymer is accompanied with the decrease in the ∆Hpp value,39 indicating the increase in the unpaired spin density. Therefore, the ESR result shown in Figure 6 indicates that the unpaired spin density of polyaniline was affected by its nanofiber sizes. On the basis of the known g-value of the g-factor marker, the g-values of polyaniline samples were determined to be 2.0034, i.e,. the g-value was unchanged with the diameter of the polyaniline nanofibers. This result demonstrates that the electronic structure of polyaniline is not affected by its nanofiber diameter. It must be pointed out here that the ∆Hpp value in Figure 6 is much larger than that of conventionally prepared polyaniline. To gain a better understanding of this question, the above samples were immersed in 0.05 M H2SO4 solution for 4 h, and then dried before the measurement of the ESR signal. The ∆Hpp values of samples 1-3 are 13.9, 15.6, and 16.7 G, respectively. In comparison with data in Figure 6, the ∆Hpp value of the same sample after protonation decreases; but this change is limited. This indicates that the broadening of the line width of the ESR signal in Figure 6 is partially caused by deprotonation of polyaniline because the resulting polyaniline samples used here were rinsed by using doubly distilled water. The above results demonstrate that the ∆Hpp value and the unpaired spin density of polyaniline were affected by the nanofiber sizes of polyaniline, indicating that the nanostructure sizes can influence the magnetic properties of polyaniline because free radicals are contained in polyaniline. This influence substantially arises from the change in the doping and the protonation levels of polyaniline. These changes are very small but are very closely related to radical density in polyaniline. The ESR technique is an extremely sensitive technique for the detection of radicals. Therefore, the changes in the ∆Hpp value and the unpaired spin density caused by the doping and protonation levels of polyaniline nanofibers can be readily detected by the ESR determination. In view of the existence of ferrocenesulfonic acid in polyaniline, the ESR determination of ferrocenesulfonic acid was carried out in 0.33 M ferrocenesulfonic acid solution. The ESR spectrum of ferrocenesulfonic acid (omitted here) shows that there are two ESR signals arising from Fe(III) in ferrocenesulfonic acid. The center of the spectrum line of the first ESR signal of ferrocenesulfonic acid lies at 1650 G, which is out of the range of the applied magnetic field in Figure 6. The center of the spectrum line of the second ESR signal of ferrocenesulfonic acid lies at 3682 G, which is also out of the range of the applied magnetic field in Figure 6. Even though the second ESR signal of ferrocenesulfonic acid has a large Hpp value, no evidence for the influence of ferrocenesulfonic acid in polyaniline on the ESR spectra of polyanilines was detected in Figure

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Mu and Yang reported by Menon et al.42 and Bein et al.43 The above result also shows that the conductivity of polyaniline synthesized in the presence of ferrocenesulfonic acid is lower than that of the conventional polyaniline. A possible reason for this is due to the ferrocenesulfonic acid incorporated in polyaniline.

Figure 7. 1H NMR spectra of nanostructured polyanilines prepared at different potential scan rates:, (1) 6, (2) 12, and (3) 60 mV s-1.

6 because the amount of ferrocenesulfonic acid in polyaniline is low. Thus, the change in the ESR signal in Figure 6 is caused by nanofiber sizes of polyaniline itself. 3.5. Proton NMR of Polyaniline Nanofibers. It was found that the 1H NMR spectrum of aniline copolymer is affected by its protonation level and redox states,40 which is caused by the radicals in the copolymer. As discussed previously, the ESR spectra of polyaniline nanofibers synthesized at different scan rates are related to the nanofiber sizes. Therefore, the NMR technique was used here to study polyaniline nanofibers. Plots A, B, and C in Figure 7 show the 1H NMR spectra of nanostructured polyanilines prepared at the potential scan rates of 6, 12, and 60 mV s-1, respectively. The 1H NMR spectrum of plot A is explained as follows. The peak at 5.89 ppm is caused by the NH resonance in polyaniline. The peak at 6.32 ppm is attributed to the proton resonance of cyclopentadiene rings of ferrocenesulfonic acid, which was doped into polyaniline during the polymerization process. The 1H NMR spectra of protonated and deprotonated poly(aniline-co-o-aminophenol)s indicated that a sharp 1/1/1 triplet around 7.0 ppm is caused by the free radical ( NH proton resonance.40 Therefore, three lines between 6.99 and 7.17 ppm having almost equal intensity in plot A are indicative of the free radical ( NH proton resonance due to the presence of 14N with unit spin, which splits the proton attached to it into a 1/1/1 triplet. The splitting peaks at 7.39-7.55 ppm are assigned to the aromatic proton resonance of polyaniline since the chemical shift for the aromatic protons of the aniline monomer is in the region of 6.9-7.4 ppm.41 Although, there are several subtle differences among the 1H NMR spectra in Figure 7, the most interesting one is the triplet caused by the free radical ( NH proton resonance. When we carefully observe the spectra, we can find that each main peak in the triplet is split into two peaks with different intensities due to a proton attached to nitrogen atom. Their relative difference in the intensity becomes smaller and smaller as the diameter of polyaniline fibers decreases, which is consistent with the change in the intensity of the ESR signal with the diameter of polyaniline nanofibers. The change in the relative intensity of the two splitting peaks with the nanofiber size would be attributed to the alternation of a magnetic interaction between the magnetic moment of unpaired electron and the magnetic nucleus due to the change of the unpaired spin density. However, the situation for this is not as simple as we assumed here, and a more complex situation must be considered. 3.6. Conductivity. The conductivities of polyaniline nanofibers synthesized at the scan rates of 6, 12, and 60 mV s-1 are 2.4 × 10-3, 5.1 × 10-3, and 5.3 × 10-3 S cm-1, respectively. It is obvious that the conductivity increases with decreasing nanofiber diameters of polyaniline, but this change is small because the differences between nanofiber diameters of polyanilines are not large. The reason for the largest conductivity of polyaniline prepared at 60 mV s-1 is probably attributed to its morphology because it is more compact than that prepared at 6 mV s-1. The tendency of the change in the conductivity with diameters is in agreement with that of conducting polymer wires

4. Conclusions In summary, the polyaniline nanofibers with different sizes were synthesized by using cyclic voltammetry, in the presence of ferrocenesulfonic acid. The potential scan rate is closely related to the formation and growth of polyaniline nuclei, which plays a key role in controlling nanofiber sizes. This is very similar to the crystallization of a salt in the aqueous solution. The presence of ferrocenesulfonic acid with large size in the electrolytic solution is favorable for the formation of the smaller diameters of polyaniline nanofibers. The spectra of the X-ray diffraction indicate that the partially crystalline form exists in the polyaniline nanofibers and that particularly the crystallinity of polyaniline increases with decreasing diameter of polyaniline nanofibers. The ESR spectra reveal a fact that the decrease in the intensity of the ESR signal is accompanied by the increase of ∆Hpp value as the diameter of polyaniline nanofibers decreases. The 1H NMR spectra show that a peak at a triplet caused by the ( NH free radical is split into two peaks with different intensities and that their relative intensity also changes with the diameter of the polyaniline nanofibers. The spectral characteristics of the polyaniline nanofibers presented here are first observed. It is to be expected that the spectral characteristics of the polyaniline nanofibers will be conducive to further promoting the study of polyaniline nanostructure in the future. Acknowledgment. We thank the Laboratory Center of Yangzhou University for technical assistance. References and Notes (1) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2385–2400. (2) Choi, S. J.; Park, S. M. AdV. Mater. 2000, 12, 1547–1549. (3) Staii, C.; Johnson, A. T., Jr.; Pinto, N. J. Nano Lett. 2004, 4, 859– 862. (4) Liu, H.; Kameoka, J.; Czaplewski, D. A.; Craighead, H. G. Nano Lett. 2004, 4, 671–675. (5) Huang, J. X.; Virji, S. B.; Weiller, H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314–315. (6) Zhang, X.; Manohar, S. K. Chem. Commun. 2004, 20, 2360–2361. (7) Chiou, N. R.; Epstein, A. J. AdV. Mater. 2005, 17, 1679–1683. (8) Ding, H. J.; Wan, M. X.; Wei, Y. AdV. Mater. 2007, 19, 465–469. (9) Wang, Y. Y.; Jing, X. L. J. Phys. Chem. B 2008, 1157–1162. (10) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581–2590. (11) Wang, H. J.; Ji, L. W.; Li, D. F.; Wang, J. Y, J. Phys. Chem. B 2008, 112, 2671–2677. (12) Anilkumar, P.; Jayakannan, M. J. Phys. Chem. C 2007, 111, 3591– 3600. (13) Li, J.; Zhu, L. H.; Luo, W.; Liu, Y.; Tang, H. Q. J. Phys. Chem. C 2007, 111, 8383–8388. (14) Virji, S.; Kaner, R. B.; Weiller, B. H. J. Phys. Chem. B 2006, 110, 22266–22270. (15) Virji, S.; Huang, J.; Kaner, R. B.; Weiller, B. H. Nano Lett. 2004, 4, 491–496. (16) Tseng, R. J.; Huang, J.; Quyang, J.; Kaner, R. B.; Yang, Y. Nano Lett 2005, 5, 1077–1080. (17) Huang, J. X.; Kaner, R. B. Chem. Commun. 2006, 4, 367–376. (18) Zhang, X. Y.; Goux, W. J.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126, 4502–4503. (19) Wu, C. G.; Bein, T. Science 1994, 264, 1757–1759. (20) Martin, C. R. Chem. Mater. 1996, 8, 1739–1746. (21) Wang, C. W.; Wang, Z.; Li, M. K.; Li, H. L. Chem. Phys. Lett. 2001, 341, 431–434. (22) Michaelson, J. C.; McEvoy, A. J. J. Chem. Soc., Chem. Commun. 1994, 1, 79–80.

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