Reactive & Functional Polymers 68 (2008) 1435–1440

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Reactive & Functional Polymers

journal homepage: www.elsevier .com/locate / react

A novel strategy for the synthesis of polyaniline nanostructures withcontrolled morphology

Junsheng Wang a, Jixiao Wang b,*, Zhen Yang b, Zhi Wang b, Fengbao Zhang a, Shichang Wang b

a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR Chinab State Key Laboratory of Chemical Engineering, Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University,Tianjin 300072, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 February 2008Received in revised form 21 June 2008Accepted 1 July 2008Available online 8 July 2008

Keywords:PolyanilineNanostructuresNanosheetsNanofibersNanoparticles

1381-5148/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.reactfunctpolym.2008.07.002

* Corresponding author. Tel.: +86 22 27404533; faE-mail address: jxwang@tju.edu.cn (J. Wang).

Polyaniline (PANI) nanostructures with sheets-, fiber- and spherical-like morphologieswere synthesized from p-toluene sulfonic acid (p-TSA) aqueous solution. The results dem-onstrate that the morphology of PANI nanostructures was significantly influenced by themolar ratio of aniline to p-TSA. Other experimental parameters, such as polymerizationtemperature and the concentration of ammonium peroxydisulfate (APS), also have aninfluence on the morphology of PANI nanostructures. A rational mechanism based on theself-assembly of micelles is proposed for the formation of PANI nanostructures. Scanningelectron microscopy (SEM), Fourier transform infrared (FTIR), UV–visible spectroscopy(UV–vis) and X-ray diffraction (XRD) were applied to characterize the products.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Nanomaterials with different morphologies are attrac-tive owing to their unique optical, electronic, magneticand chemical properties [1,2]. Control over the morphol-ogy and organization of nanomaterials provides an impor-tant strategy in tuning their physical and chemicalproperties [3–5]. Numerous imaginative methods havebeen developed for the synthesis of nanomaterials withdifferent morphologies [6–10]. Although traditional meth-ods such as altering the shape of templates [11,12], adjust-ing the concentration of the surfactant [13], addingfunctional polymers [14], varying the feeding manner ofthe reagent [15], and using interfacial reactions [16] haveshown significant effects on the morphology control andorganization of nanomaterials, facile and effective transi-tion of the morphology remains an important challenge.

Polyaniline (PANI) is unique among the family of con-ductive polymers due to its good electrical, electrochemi-

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x: +86 22 27404496.

cal, and optical properties, ease of preparation, andexcellent environmental stability [17,18]. Nanoscale PANIhas gained a wide range of interest for promising applica-tions in nanoactuators [19], sensors [20], microcapacitors[21], and microelectronic devices [22]. Various strategiessuch as solution-phase chemical, bi-phase interfacial andelectrochemical polymerization techniques have been re-ported for the fabrication of PANI nanostructures suchas nanofiber, nanotubes, nanobelts and nanoparticles[23–30]. Among these methods developed for synthesisof PANI nanomaterials, the solution-phase chemicalmethod has been considered as the most promising routein terms of low cost and large-scale production potential.Generally, the solution-phase chemical approach may beclassified into the ‘template’ and ‘template-free’ method,which uses a structure-directing agent or self-assembledmicelles to direct the growth of the nanostructured PANI.With the advance in the solution-phase ‘template-free’synthesis method of PANI nanostructures, there is grow-ing interest in the morphological control of the nano-structured PANI. For example, PANI nanostructures,including spheres, fibers and nanotubes, were obtained

1436 J. Wang et al. / Reactive & Functional Polymers 68 (2008) 1435–1440

under the direction of the unique amphiphilic dopant (4-[4-hydroxy-2((Z)-pentadec-8-enyl) phenylazo]-benzene-sulfonic acid) [31]. PANI nanostructures with tubular,spherical, polyhedral, dendritic and belt-like morpholo-gies were also synthesized by tuning the preparation con-ditions in a two-phase medium [16]. Recently, wereported the synthesis of PANI nanostructures includingnanosheets, nanofibers, and nanoparticles at organic/aqueous interfaces by controlling the diffusion rate andthe polymerization induction time of aniline [32]. In thiscomplicated process, an organic compound is requiredto form the organic phase, which is unfriendly to the hu-man body and environment.

In the present work, we developed a novel strategy tosynthesize PANI nanostructures including nanosheets,nanofibers and nanoparticles by simply changing the molarratio of aniline to p-TSA in the aqueous solution in the ab-sence of any functional additives. The results provide insightinto the specific function of the molar ratio of aniline to p-TSA in the morphological control of PANI nanostructures.The formation mechanism of PANI nanostructures was dis-cussed and the products were characterized to understandthe chemical and electronic structure of synthesized PANI.

2. Experimental

2.1. Materials

The aniline (reagent grade) was distilled until colorlessunder reduced pressure prior to use. Other chemicals werereagent grade and used as-received without further treat-ment. All of the aqueous solutions were prepared withdouble distilled water.

2.2. Synthesis of PANI nanostructures

The synthesis of PANI nanostructures was simplyachieved by changing the concentration of p-toluene sul-fonic acid (p-TSA) at ambient temperature. A general poly-merization process is as follows: 0.138 mL aniline wasdissolved in 5.0 mL aqueous solution containing p-TSA.To this solution, 5.0 mL aqueous solution containing0.114 g ammonium peroxydisulfate (APS) was added rap-idly. The mixture was left undisturbed at room tempera-ture for the growth of PANI. After 24 h, the entireaqueous phase was filled homogeneously with dark greenPANI. The aqueous solution was suction filtered and theproducts were washed by methanol and water to removethe by-products. The dark green PANI was dried in airand stored in desiccators. In an attempt to investigate thefactors affecting the morphology of PANI nanostructures,a series of experiments were carried out by varying theamount of aniline and p-TSA to change the molar ratio ofaniline to p-TSA.

2.3. Characterization

The morphologies of the PANI nanostructures wereinvestigated by field emission scanning electron micros-copy (SEM, JSM-6700F). The structure of the PANI

nanostructures was characterized by Fourier transforminfrared (FTIR) and UV–visible spectroscopy techniques.FTIR spectra in the range of 4000–400 cm�1 were mea-sured on a Nicolet MANGA-IR 560 FTIR spectrophotometerusing KBr pressed disks. UV–vis spectra were measured inaqueous solution. XRD of the polymer samples was re-corded with X’ Pert Pro X-ray diffractometer in the rangeof 2h = 5–40� with Cu Ka emission.

3. Results and discussion

The typical scanning electron microscopy (SEM) imagesof the PANI nanostructures are shown in Fig. 1. The SEMimages show that PANI with morphologies of nanosheets,nanofibers and nanoparticles were obtained by tuning themolar ratio of aniline to p-TSA (represented by [An]/[p-TSA]). Fig. 1A shows that sheet-like PANI nanostructureswere obtained when [An]/[p-TSA] ratio is 3:1. When the[An]/[p-TSA] ratio was changed to 1:1, the produced PANIassumed the form of nanoparticles, while PANI nanofiberswere formed when the [An]/[p-TSA] ratio was changed to1:2 (Fig. 1C).

Anilinium cation is an amphiphilic structure that con-sists of protonated hydrophilic –NHþ3 and hydrophobic –C6H5 functional groups. Just like a surfactant, aniliniumcations have the ability to form micelles in aqueous solu-tion. It has been reported that such micelles formed by ani-linium cations might be the templates to form differentPANI nanostructures in the absence of a surfactant[33,34]. In our experiment, when anilinium cations wereadded into aqueous solution, nanoscale micelles were alsoformed, which was confirmed by dynamic light scattering(DLS) measurements (Fig. 2). Fig. 3 shows the typical mor-phology of the resulting PANI nanostructures at an earlystage of the polymerization. According to the morphologyof the resulting PANI and the DLS results, the formed mi-celles might be a layer configuration in shape. Whenoxidant was added to the reaction system, the polymeriza-tion took place at nanoscale micelles and was followed byPANI growth. Comparing Fig. 1A with Fig. 3, it is evidentthat the morphology of PANI has no obvious change withpolymerization time, but that the size of the nanostruc-tures increases with an increase in the polymerizationtime. These results indicate that a two-dimensional growthprocedure occurred during the polymerization process.

The shapes of the micelles, such as spheres, cylinders,and layers, can be controlled by adjusting the surfactantconcentration and the interaction between surfactantsand counter-ions [31,35]. In the reaction system, aniliniumcations perform as the surfactant and result from the reac-tion of aniline with p-TSA. With the change in the [An]/[p-TSA] ratio, the concentration of anilinium cations changes,and thus induces a change in the shape of the micelle. Asthe molar ratios of [An]/[p-TSA] decrease, the formationof cylinder-shaped and sphere-shaped micelles will yieldpolyaniline nanofibers and nanoparticles, respectively.The experimental results clearly indicate that the [An]/[p-TSA] ratio plays a predominant role in controlling PANImorphologies such as nanosheets, nanofibers andnanoparticles.

Fig. 1. SEM images of PANI nanostructures (experimental conditions: [An]/[p-TSA] = A, 3:1; B, 1:1; C, 1:2 and [APS] = 0.1 M; D; 3:1, [APS] = 0.2 M, 20 �C).

Fig. 2. DLS measurements of aniline and p-TSA complexes, [An]/[p-TSA] = 3:1. Fig. 3. SEM images of PANI nanostructures prepared after 5 min of

polymerization (experimental conditions: [An]/[p-TSA] = 3:1,[APS] = 0.1 M, 20 �C).

J. Wang et al. / Reactive & Functional Polymers 68 (2008) 1435–1440 1437

The influence of the molar ratio of aniline to p-TSA onthe morphologies of the PANI nanostructures was alsoinvestigated. Fig. 4 presents SEM images of PANI nano-structures synthesized with different [An]/[p-TSA] ratios.The experimental results clearly show that PANI nano-sheets can be obtained when the [An]/[p-TSA] ratio in-creases from 3:1 to 5:1 (Fig. 4B). However, the mixture of

PANI nanosheets coexisting with microspheres were ob-tained by further increasing the [An]/[p-TSA] ratio to 6:1as demonstrated in Fig. 4A. This result may be due to theincrease in the amount of aniline monomers. On the basisof the resulting PANI morphology, it is inferred that the

Fig. 4. SEM images of PANI nanostructures (experimental conditions: [APS] = 0.1 M, [An]/[p-TSA] = A, 6:1; B, 5:1; C, 1:3; D, 3:1 with stirring, 20 �C).

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excess aniline monomers may assemble into sphere mi-celles in aqueous solution and subsequent oxidation andgrowth of these sphere micelles produces microspherePANI. When the [An]/[p-TSA] ratio is 1:3, PANI nanofiberswere also formed (Fig. 4C). There is no obvious differencein morphology between that seen in Figs. 1C and 4C. TheSEM results indicate that the shape-controllable PANInanostructures, varying from nanosheets to nanofibers tonanoparticles, can be easily formed by controlling the mo-lar ratio of aniline to p-TSA.

The temperature and concentration of APS are the otherexperimental parameters affecting the morphology of PANInanostructures. From the SEM images of Fig. 5, we canclearly see that sheet-like PANI structures were obtainedat 5 �C and 30 �C, respectively, and that the thickness ofthe PANI sheet increases at a temperature of 30 �C (Fig.5B). This behavior might be ascribed to the more activeaniline monomers at higher temperatures. More anilinemonomers would diffuse into the layer-shaped micellesand thus a thicker sheet-like PANI is fabricated. With theincrease in APS concentration, PANI nanofibers rather thannanosheets were formed as displayed in Fig. 1D. One pos-sible explanation is that the high APS concentrationchanges the surface energy of the solution and further in-duces the micelles aggregate form change. Stirring wasalso found to play an important role in the formation of

PANI nanostructures. When the reaction was carried outunder stirring, a mixture of sheet-, fiber- and sphere-likePANI were obtained (Fig. 4D). Stirring prevents the aggre-gation of monomers into stable micelles and results in var-ious PANI nanostructures.

The chemical structure of the PANI nanostructures wascharacterized by UV–vis and FTIR spectroscopy. UV–visspectra of the PANI nanostructures doped with p-TSA dis-persed in water are shown in Fig. 6. The peak at 351 nmseen for the PANI nanofibers is attributed to the p–p* tran-sition in benzenoid units of the polymer chains [36]. ForPANI nanosheets, a peak corresponding to a blue shift ap-peared at 311 nm, which might indicate that the conjuga-tion length of the PANI nanosheets decreased comparedwith that of PANI nanofibers. This may be due to the or-dered special arrangement effect of sheet-like PANI poly-mer chains, which causes a decrease in the planarconjugation length of aromatic rings. Both PANI nanostruc-tures show an absorbance peak at 440 nm correspondingto the partial protonation of the PANI chains [36]. Thisindicates that both PANI nanostructures are in the dopedstate. The relative intensity of the peak indicates that thePANI nanosheets have a higher protonation level than PANInanofibers. The peaks at 700 nm for PANI nanofibers and800 nm for PANI nanosheets could be assigned to the po-laron transition of the polymer chains [36].

Fig. 5. SEM images of PANI nanostructures (experimental conditions: [An]/[p-TSA] = 3:1, [APS] = 0.1 M. A, 5 �C; B, 30 �C).

Fig. 6. UV–vis spectra of polyaniline nanostructures dispersed in water.

Fig. 7. FTIR spectra of polyaniline nanostructures.

Fig. 8. XRD patterns of polyaniline nanostructures.

J. Wang et al. / Reactive & Functional Polymers 68 (2008) 1435–1440 1439

The Fourier transform infrared spectra of the PANInanostructures are shown in Fig. 7. The FTIR spectra ofboth the nanostructures main peaks are in good agreementwith the previously reported results [37]. The peaks at

1578 cm�1 and 1494 cm�1 are ascribed to C@C stretchingvibration of quinoid and benzenoid rings, respectively,and the peak at 1304 cm�1 to the C–N stretching vibration.The peaks at 1146 cm�1 and 821 cm�1 can be assigned tothe C–H in-plane bending vibration and the out-of-planevibration in the 1,4-disubstituted benzene ring, respec-tively. The absorptions at 3050 cm�1 are due to the N–Hstretching vibration. The peaks at 1050 and 697 cm�1 arerelative to the S@O and S–O stretching vibration of the sul-phonate groups attached to the aromatic rings, which alsoindicates the prepared PANI nanostructures were in thedoping state.

X-ray diffraction (XRD) was used to further probe thestructure of the PANI nanostructures as shown in Fig. 8.The PANI nanofibers presented a broad signal centered at2h = 22� and 30� which can be ascribed to the periodicityparallel and perpendicular to the polymer chains of PANI,respectively [38]. This broad X-ray structure suggested anamorphous character of the polymer, similar to that of bulkPANI prepared by conventional methods. Fig. 8 also pre-sents the X-ray scattering patterns of the PANI nanosheetsthat showed three peaks centered at 2h = 7.4�, 22�, and 30�,respectively. The newly appeared sharp peak centered at

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2h = 7.4� corresponds to the formation of highly crystallinematerials. It is assigned to the periodicity distance betweenthe dopant and N atom on adjacent main chains and indi-cates the ordering of the dopant molecules in the tunnelsbetween the PANI chains [38]. The half widths of the peaksat 2h = 22� and 30� decrease when the PANI morphologychanged from nanofibers to nanosheets. The result alsosuggests that the sheet-like PANI has a better orientationof the periodicity parallel and perpendicular to the poly-mer chains than that of fiber-like PANI. The crystallinityof PANI nanomaterials could be changed not only by thestructure of the dopants as demonstrated in the literature[39], but also by controlling the morphology of PANInanomaterials.

4. Conclusions

In summary, the morphology-controllable PANI nano-structures varying from nanosheets to nanofibers to nano-particles were simply prepared by a solution-phasechemical method. It was observed that the [An]/[p-TSA] ra-tio, the concentration of APS, and the reaction temperature,have significant influence on the morphology of PANInanostructures. The micelles formed by anilinium cationswere regarded as ‘soft template’ in the formation of theself-assembled PANI nanostructures. The result demon-strates that PANI with controllable nanostructures can beobtained by tuning the molar ratio of aniline to p-TSA.Moreover, the crystallinity of PANI nanomaterials couldalso be changed by controlling the morphology of PANInanomaterials. The route reported here might be used forthe controlled synthesis of other polymer nanostructures.

Acknowledgements

This work was supported by the Program of IntroducingTalents of Discipline to Universities, No. B06006, the Na-tional Natural Science Foundation of China (No.20676095), and the Program for New Century ExcellentTalents in University.

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