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Synthetic Metals 159 (2009) 1583–1588

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

reparation of polyaniline microplates via a novel template-free method

unsheng Wang, Jixiao Wang ∗, Zhongde Dai, Zhi Wang, Fengbao Zhangtate Key Laboratory of Chemical Engineering, Chemical Engineering Research Center, School of Chemical Engineering and Technology,ianjin University, Weijin Rd. 92, Tianjin 300072, PR China

r t i c l e i n f o

rticle history:eceived 8 December 2008eceived in revised form 8 April 2009ccepted 20 April 2009

a b s t r a c t

Two-dimensional plate-like PANI are successfully synthesized by a self-assembly process using benzoylperoxide as oxidant. The micelles formed by aniline/p-TSA salt serves as ‘soft template’ to form PANImicro/nanostructures and proper [An]/[acid] ratios are required for the formation of plate-like PANI. Itis found that at high [An]/[p-TSA] ratios (5:1 to 2:1) plate-like PANI are obtained, while at low [An]/[p-

vailable online 23 May 2009

eywords:onducting polymersolyanilineenzoyl peroxide

TSA] ratios ranging from 1:1 to 1:3 PANI nanoparticles are obtained. The influence of the other syntheticparameters, such as temperature, the concentration of benzoyl peroxide and stirring, on the morphologiesof the PANI micro/nanostructures has also been investigated. Fourier transform infrared (FTIR), UV–visspectroscopy (UV–vis), X-ray photoelectron spectroscopy (XPS), as well as X-ray diffraction (XRD) andcyclic voltammeter method (CV) are applied to characterize the products.

lateicrostructure

. Introduction

Among the family of conductive polymers, polyaniline (PANI)as attracted much interest due to its low cost, facile syntheticrocess, good environmental stability, and unique redox tunability1,2]. Recently, PANI micro/nanostructures have been extensivelytudied due to its strong influence on the associated propertiesnd applications. Considerable efforts have been made towardshe synthesis of low dimensional PANI micro/nanostructures suchs nanoparticles, nanorods, nanofibers, nanotubes, nanobelts, andicrospheres [3–10]. Besides the template methods [11–13], many

ew synthetic approaches including rapid mixing reaction [14],nterfacial polymerization [15], dilute polymerization [16], seed-ng polymerization [17], oligomer-assisted polymerization [18],ubstrate surfaces polymerization [19], and soft-template guidedoutes such as surfactants [20,21] and specific dopants [22,23], haveeen employed to prepare PANI micro/nanostructures. Among var-

ous morphologies of PANI, two-dimensional PANI nanostructuresre important for studying and understanding electron transport inwo-dimensional systems [24]. Therefore, the fabrication of two-imensional micro/nanostructured PANI has become one of theost important domains and a lot of recent researches have focused

n this issue.Two-dimensional PANI nanostructures have been prepared by

sing layered minerals such as montmorillonite, vanadium oxidend hectorite as hard templates [25–27]. However, it is difficult to

∗ Corresponding author. Tel.: +86 22 27404533; fax: +86 22 27404496.E-mail address: jxwang@tju.edu.cn (J. Wang).

379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2009.04.020

© 2009 Elsevier B.V. All rights reserved.

obtain pure PANI nanoplates by removing these minerals layers.Leaf-like PANI has been successfully prepared in the presence ofpoly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)as structural directing surfactants [28]. Very recently, PANI nan-odisks have been synthesized by chemical oxidation of anilinewith or without any acids [24,29–31] when proper molar ratio ofaniline/oxidant or aniline/dopants is used. We also reported thatPANI nanoplates were formed by interfacial polymerization wheninduction time and aniline diffusion rate are controlled properly[32]. Of these reports, ammonium persulfate (APS), as a famil-iar oxidative reagent, was used to initiate the oxidation of anilinefor preparing two-dimensional PANI micro/nanostructures. How-ever, ammonium persulfate is unstable on storage, i.e. the acidityincreases due to moisture absorption and undergoes decomposi-tion [33]. Moreover, ammonium persulfate being a strong oxidizingagent and aniline polymerization being exothermic, controlling thereaction temperature is rather difficult. The removal of inorganicbyproduct (ammonium sulfate) from the polymer is also difficult[34]. Benzoyl peroxide, a well-known mild initiator in polymer-ization reactions, is soluble in most of organic solvents with goodstability. It has been used as a novel oxidant by Sathyanarayana[35,36] in the inverse emulsion polymerization process of ani-line. In the reports using benzoyl peroxide as oxidant, only PANInanoparticles were prepared. The reports on the fabrication of two-dimensional PANI micro/nanostructures using benzoyl peroxide as

oxidant, to the best of our knowledge, were very sparse yet.

In the present work, we demonstrate a novel one-step template-free method for fabricating exclusive PANI plates by a self-assembleprocess using benzoyl peroxide as oxidant for the first time. Thesynthesis of two-dimensional PANI micro/nanostructures is carried

1 Metals 159 (2009) 1583–1588

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ut without the aid of any surfactants or templates. The growthechanism of plate-like PANI is proposed. The chemical structure

f PANI plates is also investigated.

. Experimental

.1. Materials

The aniline (analytical grade) was distilled until colorless undereduced pressure prior to use. Other chemicals were analyticalrade and used as-received without further treatment.

.2. Synthesis of PANI plates

An optimized procedure for the synthesis of exclusive PANIlates is described as follows: aniline was dissolved in 5.0 mLthanol solution containing 0.1 M p-toluene sulphonic acid (p-TSA).o this solution, another 5.0 mL ethanol containing 0.121 g benzoyleroxide was added rapidly under stirring condition. The mixtureas left undisturbed at room temperature for the growth of PANI.

he precipitated PANI was filtered by a filter membrane with poreize of 0.22 �m, washed with ethanol to remove the byproducts. Theample was dried under vacuum condition at room temperature forharacterization.

.3. Characterization

The morphologies of the resulting products were investigated byEM (Tecnai G2 F20) and SEM (JEOL, JSM6700F), respectively. TheEM samples were prepared by suspending an appropriate amountf product in ethanol under supersonic condition and then cast ontoopper TEM grids. The grids were placed on filter paper to facilitateapid drying. Samples for SEM experiments were made by placingome products on conducting stages and observed with gold coat-ngs. The structure of the PANI nanostructures was characterized byourier transform infrared (FTIR), UV–vis spectroscopy (UV–vis),nd X-ray photoelectron spectroscopy (XPS). FTIR spectra in theange of 4000–400 cm−1 were measured on a Nicolet MANGA-IR60 spectrophotometer using KBr pressed disks. WXRD of the poly-er samples was recorded with an X’ Pert Pro X-ray diffractometerith Cu K� emission. The spectra were recorded in the range of

� = 5–40◦. The electrochemical behavior of PANI plats is charac-erized by cyclic voltammeter method (CV) using PAR273A (USA).he conductivity of the compressed pellets (pressed at 8 MPa formin) at room temperature was measured by a standard four-probeethod using digital DC resistance measurer (XC2853).

. Results and discussion

Typical scanning electron microscopy (SEM) image of PANI syn-hesized in ethanol is shown in Fig. 1. It is shown that exclusiveANI plates are formed. The SEM image indicates that the typicalateral dimensions of the plates are about 5–10 �m as shown inig. 1A. A TEM image of PANI plates is represented in Fig. 1B. The 2Dicrostructure is seen clearly and lateral dimensions of the plates

re consistent with the SEM image of Fig. 1A.The chemical structure of plate-shaped PANI was characterized

y Fourier-transform infrared (FTIR), UV–vis spectroscopy (UV–vis)nd X-ray photoelectron spectroscopy (XPS). The FTIR spectra ofANI plates show typical characteristic bands of PANI reported pre-

iously (Fig. 2). The bands at 1588 and 1504 cm−1 are ascribed to

C stretching vibration of quinoid and benzenoid rings, respec-ively, and the band at 1302 cm−1 to the C–N stretching vibration.he band at 851 cm−1 can be assigned to the 1,4-substituted phenyling stretching. The bands at 1170 and 1062 cm−1 are relative to the

Fig. 1. Images of PANI plates synthesized in ethanol. Polymerization conditions:[An]/[p-TSA] = 1:3, [oxidant] = 0.1 M (A: SEM; B: TEM).

asymmetric and symmetric O S O stretching vibrations, respec-tively, and the band at 696 cm−1 to the S–O stretching vibration ofthe sulphonate groups attached to the aromatic rings, which indi-cates the prepared PANI are in doping state. In addition to the mainabsorption bands of PANI base, bands at 1445 and 1208 cm−1 arealso present in the spectra of the products which correspond to thepresence of ortho-coupled aniline units and phenazine-like units[37]. Fig. 2 also represents the UV–vis spectrum of PANI plates dis-persed in deionized water. The peak at 310 nm is assigned to the�–�* electron transition of benzenoid ring segments along thePANI chains. The strong absorption peak at 510 nm which shouldbe assigned to �–�* quinonoid transition exhibits a blue shift

compared with conventional PANI. This shift further attests to thepresence of phenazine segment in the polymer chains [38]. X-rayphotoelectron spectroscopy (XPS) was used to further analyze thestructure and the doping level of PANI plates by way of three

J. Wang et al. / Synthetic Metals 159 (2009) 1583–1588 1585

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odisks reported, which indicates that the molecular structure ofPANI plates is similar to the PANI nanodisks that are composedof emeraldine base, emeraldine salt, and phenazine-like seg-ment [24]. The room-temperature conductivity of plate-like PANI

Table 1

ig. 2. FTIR and UV–vis spectra as well as N1s XPS core level spectra of PANI plates.

ifferent nitrogen environments with specific N1s binding ener-ies. Fig. 2 shows the N1s XPS core level spectra for as-synthesizedANI plates. The two main peaks with binding energies at 398nd 399 eV are assigned to quinoid imine (–N ) and benzenoidmine (–NH–), respectively, and the peak with binding energy at02 eV was assigned to positively charged nitrogen (–N+–) [39].he area of imine peak was found to be larger than the imine peak62% vs. 32% of the N content) indicating benzenoid over quinoid

nits. The doping level estimated from the peak area of –N+– withespect to the total nitrogen was 0.06 for the as-synthesized PANIlates.

Fig. 3 shows the XRD patterns of the resultant PANI plates.everal main peaks centered at 2� = 22.0◦, 25.3◦ together with an

Fig. 3. XRD pattern and the cyclic voltammetric curve of PANI plates (scanning rate:50 mV s−1).

intensive sharp peak centered at 2� = 7.8◦ are observed, and twounfamiliar peaks centered at 2� = 24.0◦, 24.5◦ are also appeared(Table 1). Similar to the PANI prepared by conventional methods,the peaks centered at 2� = 22.0◦ and 25.3◦ are ascribed to the peri-odicity parallel and perpendicular to the polymer chains of PANI,respectively. The newly appeared sharp peak centered at 2� = 7.8◦

corresponds to the periodicity distance between the dopant and Natom on adjacent main chains, which is only observed for highlyordered PANI samples [40–42]. The XRD result suggests that PANIplates possess increased solid state ordering compared to that ofconventional PANI. The cyclic voltammetric curve of PANI platesperformed in 0.5 M p-TSA aqueous solution with a scan rate of50 mV s−1 is also shown in Fig. 3. Two cathodic peaks at 0.02and 0.85 V is assigned as the reduction of PANI from pernigrani-line to emeraldine and to a further reduction from emeraldine toleucoemeraldine. The result is consistent with that of PANI nan-

Yield, conductivity and WXRD peaks of PANI plates.

Yield (%) Conductivity (S cm−1) WXRD peaks at 2�

54.3 3.2 × 10−4 7.8◦ , 22.0◦ , 24.0◦

24.5◦ , 25.3◦

1586 J. Wang et al. / Synthetic Metals 159 (2009) 1583–1588

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ig. 4. DLS measurements of aniline and p-TSA complexes in ethanol ([An]/[p-SA] = 3:1).

Fig. 5. Typical SEM images of the PANI obtained from ethanol with different [An]/[p-TSA] ratios: A, 5:1; B, 2:1; C, 1:1; D, 1:3.

Fig. 6. SEM images of PANI nanostructures synthesized in water. Polymerization conditions: [An]/[p-TSA] = 1:3, [oxidant] = 0.1 M.

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ellet, measured by a four-probe method, is about 3.2 × 10−4 S cm−1

Table 1).Anilinium cation is an amphiphilic structure that consists of pro-

onated hydrophilic –NH3+ group and hydrophobic –C6H5 group.ust like a surfactant, anilinium cations have the ability to form

icelles in aqueous solution. According to our previous reports [29],he micelles formed by aniline/p-TSA salt serves as ‘soft template’o form PANI nanosheets at high [An]/[p-TSA] ratios. When anilinend p-TSA at the same molar ratio are added into ethanol solution,imilar micelles are also formed which is confirmed by dynamicight scattering (DLS) measurement (Fig. 4). When BPO was addednto the ethanol solution, the polymerization took place and the

icelles serve as an initial growth template for PANI plates.The growth mechanism revealed above indicates that proper

An]/[acid] ratio is required for the growth of plate-like PANI. Inrder to further confirm the growth mechanism, the influence ofAn]/[p-TSA] ratios on the morphology of PANI have been inves-igated (Fig. 5). The experimental results clearly show that PANIlates are obtained exclusively under the condition of high [An]/[p-SA] ratio ranging from 5:1 to 2:1, while PANI nanoparticles are

btained under the condition of low [An]/[p-TSA] ratio ranging from:1 to 1:3, respectively. The shapes of the micelles, such as spheres,ylinders, and layers, can be controlled by adjusting the surfactantoncentration. Anilinium cation results from the reaction of aniline

Fig. 7. Images of PANI plates synthesized in ethanol. Polymerization conditions: [

s 159 (2009) 1583–1588 1587

with p-TSA, and the concentration of anilinium cations dependson the concentration of aniline and 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 which resultsin the change of obtained PANI morphologies.

The stability of the micelles plays a major role in fabricatingexclusive plate-shaped polymeric materials. When the solvent waschanged from ethanol to water, although PANI nanoplates wereobtained, nanostructures with other morphologies also appeared inthe final products as shown in Fig. 6. This might be attributed to theless stability of micelles in aqueous solution than that in ethanol.As we know, ethanol has an amphiphilic molecule structure anddifferent properties such as polarity, surface tension comparedwith H2O. The molecule interaction between anilinium cations andamphiphilic ethanol makes the micelles more stable and exclusivePANI plates can be obtained.

The influence of the other synthetic parameters, such as tem-perature, the concentration of benzoyl peroxide and stirring, onthe morphologies of the PANI nanostructures has also been inves-tigated. From the SEM images of Fig. 7A and B, it is evident that

PANI plates are obtained in the temperature of 5 and 50 ◦C. Withthe increasing of benzoyl peroxide concentration, the PANI plateswere also formed as displayed in Fig. 7C. The PANI plates were alsoobtained in other temperature (15 and 30 ◦C) and in higher benzoyl

An]/[p-TSA] = 1:3. (A: 5 ◦C; B: 50 ◦C; C: [oxidant] = 0.15 M; D: under stirring).

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eroxide concentration (0.2 M). The result indicates that temper-ture and the concentration of benzoyl peroxide have no obviousnfluence on the morphologies of the PANI nanostructures underhe range of our experimental conditions. This behavior suggestshat the reaction could carry out conveniently at high tempera-ure. Stirring was found to have effect on the formation of PANIanostructures. When the reaction was carried out under stirring,lthough the plate-shaped PANI was obtained (Fig. 7D), it was muchmaller in size and irregular in shape. Shearing stress produced bytirring may break the plates and thus causes the formation of smallnd irregular PANI plates.

. Conclusion

In conclusion, PANI plates were successfully synthesizedhrough a chemical polymerization method in ethanol solutionsing benzoyl peroxide as oxidant. The [An]/[p-TSA] ratio has a sig-ificant influence on the morphology of PANI. The experimentalesults indicate that temperature and the concentration of benzoyleroxide have no obvious influence on the morphologies of the PANIicro/nanostructures under the range of our experimental condi-

ions. XRD result indicates that the synthesized PANI plates haveore ordered polymer chains compared with conventional PANI.

he chemical polymerization of PANI plates in ethanol solution cane easily carried out and reproduced. This simple synthetic strat-gy provides a promising way for large scale synthesis of exclusiveimed polymeric micro/nanomaterials.

cknowledgments

This work was supported by the National Natural Science Foun-ation of China (Nos. 20676095, 20836006), the Program for Newentury Excellent Talents in University and Advanced Materialstd., Co. (Tianjin).

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