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Materials Chemistry and Physics 105 (2007) 380–384

Fabrication and characterization of self-doped poly(aniline-co-anthranilic acid) nanorods in bundles

Dongxue Han a, Jixia Song a, Xuefeng Ding a,b, Xiaoyu Xu a,b, Li Niu a,∗a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, and Graduate School of

the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, PR Chinab Department of Chemistry, Jilin University, Changchun 130021, PR China

Received 13 May 2006; received in revised form 17 April 2007; accepted 1 May 2007

Abstract

Poly(aniline-co-anthranilic acid) (PANANA) nanorods in bundles was prepared successfully in an alcohol/aqueous media without assistance ofany other kinds of acids. Anthranilic acid played all roles of monomer, acid-media provider, and dopant in the reaction system, and ammoniumpersulfate (APS) served as the oxidant. The morphologies of PANANA nanorods in bundles were investigated by scanning electron microscopy(SEM). Influences of the monomer molar ratio on the resulting morphology were investigated. Moreover the formation mechanism of the nanos-tructured copolymer was proposed. FT-IR, UV–vis and X-ray diffraction (XRD) measurements were used to confirm the molecular and electricalstructure of the self-doped PANANA. The intrinsic properties, such as conductivity, electrochemical redox activity and room-temperature solubilityof the resulting copolymer were explored.© 2007 Elsevier B.V. All rights reserved.

Keywords: Anthranilic acid; Copolymer; Nanorods in bundles

1. Introduction

Conducting polymers have attracted considerable attentionsdue to their increasing applications in numerous fields, suchas batteries, chemical sensors, electrochromic devices, lightemitting diodes, etc. [1–4]. Polyaniline (PANI) is a kind of elec-trically conducting polymers with many features that could beexploited in various applications [5,6]. Furthermore, it is uniquein the family of the conducting polymers due to its simplepreparation and reversible doping/de-doping properties based onacid/base reactions [7,8]. But the main disadvantages of polyani-line are its poor solubility and poor processability both in meltand solution due to its stiffness of the backbone [9] which lim-ited its further extensive applications in many areas. A majorbreakthrough in this field was the copolymerization of anilineand ramificate aniline [10,11]. The self-doped polyaniline hasseveral advantages, including better solubility as well as redoxactivity and conductivity over a wider pH range. A great dealof comonomers involved –SO3H, –COOH groups, etc., havebeen used in the preparation of self-doped polyaniline in recent

∗ Corresponding author. Tel.: +86 431 8526 2425; fax: +86 431 8526 2800.E-mail address: lniu@ciac.jl.cn (L. Niu).

years [12–15]. For example, to introduce a –SO3H group intothe polyaniline chain instead of the conventional inorganic acidsaffects the properties of the polymer without substantially sac-rificing its conductivity, while the solubility of polyaniline inaqueous solution as well as most common organic solvents aregreatly improved by the presence of the –SO3H groups [12].

Anthranilic acid (2-amino benzoic acid) is an importantmonomer and dopant for synthesizing carboxylic acid-dopedpolyaniline. In previous reports [12,13], it was shown that anilineand anthranilic acid monomers were almost totally copolymer-ized in an acid aqueous media generally due to the solubility ofanthranilic acid in neutral aqueous solution. When anthranilicacid is dissolved into pure water, the solubility is so poor andthe polymerization cannot be achieved under the acidity of thesystem. In this work, a neutral alcohol/aqueous media was usedto dissolve aniline and anthranilic acid monomers instead ofinorganic acid. Actually, the anthranilic acid in alcohol/aqueousmedia can offer enough acidity to the copolymerization (e.g.2.5 mmol anthranilic acid dissolved in 10 ml ethanol and 50 mldeionized water, pH 3.58) and no other kinds of acid were furtherrequired. Here, anthranilic acid plays both the roles of acid-media provider and the monomer/dopant in the reaction system.When copolymerization was carried out with proper condi-tions, the morphologies of the copolymer were confirmed by

0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2007.05.002

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SEM measurements where nanorods in bundles can be observedin large scale. For each nanobundle (∼15 �m in length), sev-eral nanorods (500–800 nm in diameter) interlaced themselvesacross their middles. Influences of monomer molar ratio on themorphology of the products were investigated. Furthermore, themolecular structure, crystallinity, redox activity and solubilityof the products were also explored.

2. Experimental

2.1. Materials

The aniline monomer was twice distilled under reduced pressure beforeuse. Anthranilic acid, ammonium persulfate (APS), absolute ethanol, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran(THF) as well as CHCl3 of analytical grade were purchased from Beijing Chem-ical Company and used without any purification. All aqueous solutions wereprepared using ultrapure water from a UPW system.

2.2. Preparation of PANANA nanorods in bundles

A certain amount of anthranilic acid and 0.2325 g aniline monomer weredissolved into 10 ml of alcohol and 40 ml deionized water at room temperature.Twenty millilitres of APS [(NH4)2S2O8] (0.1 M) was added into this mixedsolution. The mixture was ultra-sonicated for 1 min at room temperature. Thenthe mixture was placed into a 4 ◦C ambient for 24 h. The resulting precipitate wascollected by centrifugal separation and was washed several times with ethanoland distilled water. Finally the product was dried under vacuum at 60 ◦C for 24 h.

2.3. Preparation of EB-PANI

Preparation of HCl-doped polyaniline (HCl-PANI) was done as earlyreported [5]. A certain amount of HCl-PANI was placed into 5% ammoniumhydroxide with a constant stirring for 24 h to de-dope HCl from the polymer.Then the obtained emeraldine base (EB-PANI) was filtered, washed, dried asthe same way as PANANA.

2.4. Characterizations

The morphology of the resulting products was measured by a scanningelectron microscope (SEM, XL 30 ESEM FEG, on Au substrate). FT-IR mea-surements were performed using a BRUKER Vertex 70 instrument (in a KBrpellet). UV–vis spectra (Cary 500 Scan UV/Vis–NIR Spectrophotometer, Var-ian, USA) of the samples dissolved in DMF medium were obtained (DMF asreference). The crystal structures of PANANA and EB-PANI samples were stud-ied on X-ray diffraction instrument (XRD, D/max-IIB). Cyclic voltammogramsof PANANA was recorded by using an Ag/AgCl electrode as the referenceelectrode. The cyclic voltammograms were obtained at room temperature on aPANANA–DMSO modified glassy-carbon electrode. After the evaporation ofDMSO, the glassy-carbon electrode was put into a 0.5 M H2SO4 aqueous solu-tion and run the potential scanning between −0.4 and 1.0 V at different scanningrate. Room-temperature solubility of the product in different organic solventswas examined as following: a certain quality of dry sample was placed into 2 mlof organic solvents with ultrasonication for 10 min, then centrifugally separatedand washed with deionized water for several times before drying under vacuumat 60 ◦C for 24 h, finally weighed and calculated.

3. Results and discussion

3.1. Morphology of PANANA and their formationmechanism

SEM images show that morphology of the copolymerproducts changed with different molar ratio of the mixed

Fig. 1. SEM images of PANANA synthesized with different aniline/anthranilicacid molar ratios: (a) 2:1, (b) 1:1 and (c) 1:2.

monomers. Fig. 1 presents the SEM images of the PANANAsynthesized with different aniline/anthranilic acid molar ratio(represented by [An]/[AA]). It is found that samples preparedwith [An]/[AA] = 2:1 (Fig. 1a) and 1:2 (Fig. 1c) both depict adisordered and amorphous morphology. As is shown in Fig. 1a,only a few nanobundles can be found, while in Fig. 1c evennot a single bundle can be clearly observed. When [An]/[AA] is1:1, abundant nanorods in bundles can be obviously observed,as shown in Fig. 1b. It indicates that the structure and morphol-ogy of the copolymer significantly depend on the dosage of themonomers.

Typical SEM images obtained from [An]/[AA] = 1:1 demon-strate the nanostructure of the resulting product, as shown inFig. 2. A low magnification image (Fig. 2a) shows a large scaleof nanorods in bundles, and these nanobundles were not onlyuniform in size but also separated from each other. It was foundin a single nanobundle image (Fig. 2b) that the nanorods are

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Fig. 2. SEM images of PANANA ([An]/[AA] = 1:1): (a) at low magnification, (b) of a single nanobundle and (c) at high magnification.

Fig. 3. Schematic diagram of the formation mechanism for bundles-structured nanorod of PANANA.

in the diameter of 500–800 nm and interlaced themselves acrosstheir middles to form a nanobundle with a length of ca. 15 �m. Itcan also be observed that surface of the nanorods are not smoothas well. High magnification SEM image (Fig. 2c) further revealsthat, a single rod in the bundles consists of a series of orientatednanoparticles–nanorods with a diameter of ca. 50 nm.

The possible formation mechanism for the prepared bundle-structured PANANA nanorods is depicted in Fig. 3. Sinceanthranilic acid dissolved in the alcohol/aqueous solution canprovide enough acidity (pH 3.58) for the copolymerization, inthis system, aniline and anthranilic acid monomers were firstoxidized by APS into copolymer nanoparticles. Then thesenanoparticles might assemble themselves into a rod-like struc-ture via a self-assembly process. At the same time, free anilineand anthranilic acid molecules adsorbed upon the nanorodcan be oxidized by APS continuously and evolved into amulti-strips nanorod. With the proceeding of polymerization,the subsequently formed nanorods deviated from the previousone into other directions probably due to special resistancebetween each other. However, more detailed formation pro-cess of the copolymerization is much dependent on furtherinvestigations.

SEM images obtained at room temperature revealed thatlarge scale of bundle-structured PANANA nanorods can alsobe obtained in a relatively wide temperature range. The resultindicates that reaction temperature can hardly affects on thecopolymerization. However, when the reaction system wasultrasonically vibrated or disturbed instead of static state, nobundle-structured nanorods can be obtained, which is revealedby an optical observation (400 times magnification).

3.2. Structure characterization

The FT-IR, UV–vis and X-ray diffraction measurements wereused to characterize the structure of the self-doped PANANA.

Fig. 4 shows the spectra of PANANA (a) and EB-PANI (b).In curve a, the absorption bands at 1504 and 1571 cm−1 arederived from typical quinoid and benzenoid moieties, respec-tively [16]. It is consistent with those characteristic peaks ofEB-PANI (curve b). And the peaks at 1685 and 1448 cm−1

are corresponding to the C O and C–O stretching vibrationsof the carboxyl group [17]. It confirmed that –COOH and/or

Fig. 4. FT-IR spectra of: (a) PANANA and (b) EB-PANI.

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Fig. 5. UV–vis spectra of PANANA in DMF: (a) fresh solution and (b) stalesolution set for 2 weeks.

–COO− groups are involved in the resulting polymer chain, andthe copolymer was successfully self-doped by anthranilic acid.

UV–vis measurements of PANANA nanorods in bundleswere carried out in DMF solution. Fig. 5 shows the correspond-ing UV–vis spectrum. In either curve a (fresh solution) or curveb (the same solution which was set for 2 weeks without dis-turbance), there are two main absorption peaks at ca. 385 and550 nm. The peak observed at 550 nm corresponds to exciton-like transition in quinoid diimino units [18]. And the other peaksappear at 385 nm is attributed to �–�* transition in benzenoidunits of the polymer chain. Compared with the same character-istic absorption peak of EB-PANI [19], it shifts ca. 60 nm to thelong wave direction, which indicates that the conjugate conjunc-tion of PANANA decreased compared with EB-PANI. It maybe caused by the following reasons: (a) –COOH groups dopedinto the backbone of the polymer chain are strongly attractingelectron radicals which intensively decreased the electron clouddensity of aromatic rings; (b) due to the special resistance effectof –COOH groups, the planar conjugate conjunction of aromaticrings decreased. Therefore, the together cooperation caused thereducing of the conjugate conjunction around the whole poly-mer chain. In addition, curve b is similar to curve a except fora little weakness in intensity of each characteristic peak whichsuggests the stability of the resulting copolymer.

X-ray diffraction patterns were taken to compare the crystal-lizability of PANANA and EB-PANI. In Fig. 6b, a broad peakaround 2θ = 19.90◦ (0 0 1) can be distinctly observed [20] whichwas characteristic peak of amorphous emeraldine base form ofPANI [21]. While PANANA (Fig. 6a) shows a peak not onlyat 20.46◦, but also at 13.52◦, 15.42◦, 17.72◦, 18.38◦, 23.40◦,25.52◦, 27.04◦, 27.93◦, etc. Among these peaks, the peak at25.52◦ (1 1 0) [20] may be ascribed to periodicity perpendicularto the polymer chain. And the other peaks may be caused bythe groups doped into the polymer chains in various directions.From the XRD results, it suggests that PANANA has a bettercrystallizability than EB-PANI.

3.3. Instinctive properties of PANANA

The electrochemical behavior of PANANA film on glassy-carbon electrode was tested through cyclic voltammetric

Fig. 6. XRD patterns of: (a) PANANA and (b) EB-PANI.

measurements in an acidic electrolyte. Fig. 7a depicts the oxi-dation/reduction process of PANANA film on glassy-carbonelectrode. Unlike polyaniline and many of its substitutedforms, PANANA film does not show well-defined redoxpeaks in the potential range from −0.4 to 1.0 V. Only twopairs of relatively weak current peaks at 0.233 V/0.003 V and0.577 V/0.450 V can be observed which were correspondingto leucomeraldine/emeraldine and emeraldine/pernigranilinereversible transition, respectively. Different scanning rates wereused to examine the dynamic property of PANANA during theredox process (Fig. 7b). As is shown in Fig. 7c, the peak currentis directly proportional to the scan rates, suggesting an electriccharge-transfer controlled process.

Finally, room-temperature solubilities of the copolymerPANANA in several organic solvents such as DMSO, THF, DMFas well as CHCl3 were examined, and the results were shown inTable 1. In order to compare the solubility of PANANA and the

Fig. 7. Electrochemical properties of PANANA tested on a glassy carbon elec-trode in 0.5 M H2SO4: (a) cyclic voltammogram at 100 mV s−1; (b) at differentscan rates from 10 to 1000 mV s−1 between −0.4 and 1.0 V; (c) the plot of peakcurrent vs. scan rates.

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Table 1The solubility of PANANA in several solvents (mg ml−1)

Solvent DMF DMSO THF CHCl3

PANANA 4.40 11.75 5.70 2.80HCl-PANI 2.12 2.96 2.45 2.22

conventional polyaniline doped with inorganic acid, for exam-ple, HCl, some relative data were collected and listed in the table[19]. It can be obviously seen from this table that PANANA hasa relatively higher solubility in all the above solvents, especiallyin DMSO, it reaches 11.75 mg ml−1. It reveals that the solu-bility of the resulting product in most organic solvents can beextensively improved by introducing –COOH groups into thebackbone of polyaniline chain.

4. Conclusions

In conclusion, bundle-structured nanorod of poly(aniline-co-anthranilic acid) has been successfully fabricated via anaqueous/ethanol medium without participant of any other acids.Anthranilic acid plays both the roles of acid-media providerand the monomer/dopant in the reaction. It can be consid-ered as a real-content fabrication of self-doped copolymerof aniline and anthranilic acid. Optimal reaction conditionswere investigated, discussed and the possible formation mech-anisms were also proposed. The molar ratio of monomers andthe reaction mode (stirring/static state) affect significantly onthe morphology of the copolymer. FT-IR, UV–vis and XRDmeasurements show that the main chain structure and elec-trical structure of the synthesized copolymer are identical tothose previous investigations, and also confirmed the exis-tence of –COOH groups. Finally, some instinctive propertiesinvolved oxidation/reduction behavior and room-temperaturesolubility of PANANA in several organic solvents wereexamined.

Acknowledgements

The authors are most grateful to the NSFC, China (No.20475053), and to Department of Science and Technology ofJilin Province (No. 20050102) for their financial supports.

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