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Synthetic Metals 159 (2009) 1061–1066

Contents lists available at ScienceDirect

Synthetic Metals

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

emplating fabrication of polypyrrole nanorods/nanofibers

in Wei, Yun Lu ∗

epartment of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,anjing University, 22# Hankou Road, Nanjing 210093, PR China

r t i c l e i n f o

rticle history:eceived 26 July 2008eceived in revised form 7 January 2009ccepted 20 January 2009vailable online 23 February 2009

a b s t r a c t

Nanostructured polypyrrole have been fabricated in high yield by chemical oxidation polymerizationusing one-dimensional rod-like aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4)as a template. This aggregate has the advantages of both soft and hard template such as stable, goodversatility (for hard template), cheap, and easy removal (for soft template). Also, the molar ratio of TPPS4

and pyrrole monomer could be as low as 1:1000, showing a high efficiency of the template. Polypyr-

eywords:emplateolypyrroleanorodanofiber

role nanorods with variational size have been achieved successfully by using this versatile template andcooperating with the oxidants with different redox potentials. Moreover, in the presence of TPPS4 tem-plate and polyvinylpyrrolidone (PVP), Polypyrrole (PPy) nanofibers were obtained even though using aweak oxidant. The formation of PPy nanorods/nanofibers was discussed and their properties includingconductivity and thermalstability were evaluated.

orphyrinPPS4

. Introduction

Polypyrrole (PPy) has attracted growing attention as a conduct-ng polymer for its high conductivity, good environmental stability,nd hitherto a large variety of applications with the merits ofhe convenience of the preparation method and cheapness of the

onomer in recent years [1,2]. The morphology of PPy, especiallyhe PPy with different nanostructures is of great significance in theesearch of this conducting polymer due to their special proper-ies and promising applications in nanodevices [3]. Several effectiveynthesis methods [4] have been used for the construction andorphology control of PPy nanostructures. For example, “hard tem-

lates” such as porous polycarbonate films [5], fibrillar V2O5 [6], andorous alumina [7] or “soft templates” such as reverse microemul-ion [8,9], micelles [10] and so on have been reported for thereparation of nanostructured PPy.

However, the “hard template-assisted” approach has its ownemerits: the “hard template” usually has to be removed after theynthesis of nanostructured PPy [11]. The environment-unfriendlyough removal procedure (using strong acid/basic or organicolvent or rising temperature) not only increases the cost for

arge-scale manufacture, but also has the possibility to destroyhe nanostructured PPy. The “soft template-assembly” approach,hough can be free of the removal procedure, may still has the dis-dvantages of instability, low efficiency and lack of versatility for

∗ Corresponding author. Tel.: +86 25 83686423; fax: +86 25 83317761.E-mail address: yunlu@nju.edu.cn (Y. Lu).

379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2009.01.031

© 2009 Elsevier B.V. All rights reserved.

a fixed monomer. As a result, until now, a facile, high-efficient andversatile approach for synthesis of nanostructured PPy is still inneed.

It is well known that 5,10,15,20-tetrakis (4-sulfonatophenyl)porphyrin (TPPS4), a water-soluble dye of well-defined chemicalstructure, can form J-aggregates under appropriate conditions, suchas in acidic aqueous solutions [12,13]. The hierarchical structure anddynamics of the TPPS4 aggregation has been well studied [14]. Theresult of atomic force microscopy (AFM) has proved that in acidicaqueous solutions with different pH values or different TPPS4 con-centrations, the width and thickness of the aggregates remained40 and 4.5 nm, respectively, while the length ranged from 200 to1000 nm [15]. Molecular structurally, such stable aggregates withrelatively fixed size in the acidic aqueous solutions are especiallypropitious to be as a hard template for the preparation of the nanos-tructured PPy because the sulfonic groups appended in TPPS4 areof great advantage to act as a dopant. Experimentally, the acidicaqueous system in which the template existed is exactly a suitablecondition for the chemical polymerization of pyrrole. In addition,the aggregates of TPPS4 could be dissolved in neutral aqueous solu-tion and removed easily by routine water-washing. Hereby, in thisarticle, we employed the J-aggregates of TPPS4 to fabricate var-ious nanostructured PPy. The molar ratio of TPPS4 and pyrrolemonomer could be as low as 1:1000, which was very economi-

cal and highly effective. Unlike common hard template system, theachieved PPy with different nanostructures here was free from apost-removal procedure. Also, different from the previous reports[16–19], the action of traces of TPPS4 for constructing and adjust-ing of conductive polymer nanostructure and a synergistic effect of

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062 M. Wei, Y. Lu / Synthetic

VP with a slow but competent oxidant (AgNO3) on formation ofPy nanofibers were discussed in detail.

. Experiment

.1. Materials

Pyrrole monomer was purchased from Aldrich and distillednder reduced pressure before use. Silver nitrate (AgNO3, AR) and

erric chloride (FeCl3, CP) were purchased from Sinopharm Chem-cal Reagent Co. Ltd. Ammonium peroxydisulfate (APS, AR) wasought from Shanghai Lingfeng Chemical Reagent Co. Ltd. All thesexidants and other reagents were used as received without fur-her treatment. TPPS4 was obtained from Aldrich and formed its-aggregate solution by dissolving itself in acidic aqueous mediumHCl was added to reach pH 1). The structure of TPPS4 at pH 1 washown in Fig. 1.

.2. Fabrication of PPy nanorods

Under stirring, 4 ml TPPS4 stock solution (0.0248 g TPPS4 dis-olved in 100 ml deionized water (0.0002 M)) and 1 mmol pyrroleere dissolved in 10 ml deionized water (pH 1, adjusted by HCl).n aqueous solution of APS or FeCl3 (10 ml, 1 mmol oxidant) wasdded dropwise into above solution. The reaction was allowed toroceed for 12 h under room temperature with vigorous stir. Theesulting product was filtered and washed with deionized waternd ethanol repeatedly. The final product was dried by infraredehydration.

.3. Fabrication of PPy nanofibers

Under stirring, 4 ml TPPS4 stock solution and 1 mmol pyrroleere dissolved in 10 ml deionized water (pH 1, adjusted by HNO3).n aqueous solution of AgNO3 or AgNO3/PVP (10 ml, 1 mmol AgNO3,.05 g PVP) was added dropwise into above solution. The reaction

as allowed to proceed for 24 h under room temperature with

igorous stir. The resulting product was filtered and washed witheionized water and ethanol repeatedly. The final product was driedy infrared dehydration.

Fig. 1. Structure of TPPS4 at pH 1.

s 159 (2009) 1061–1066

3. Characterizations

The morphologies of the PPy were observed using a scanningelectron microscope (SEM, JEOL SEM-5610, with energy dispersivespectroscopy (EDS)) and a transmission electron microscope (TEM,JEOL JEM-200CX). Powder X-ray diffraction (XRD) patterns weretaken on a Philip-X’Pert X-ray diffractometer with a Cu K� X-raysource. Fourier-transform infrared (FTIR) spectroscopy measure-ments were performed on a Bruker Fourier-transform spectrometermodel VECTOR22 using KBr pressed discs. Conductivity was mea-sured using a four-probe method on a WR-2B digital multimeterat room temperature using compressed pellets of powders. Foreach value reported, at least three measurements were averaged.UV–vis absorption spectra of TPPS4 in acidic aqueous solution wererecorded on a UV-240 spectrometer (Shimadzu, Japan). A SDT 2960thermogravimetric analyzer was used to investigate the thermalstability of the sample with nitrogen as pure gas at a flow rate of50 ml/min and a heating rate of 10 ◦C/min.

4. Results and discussion

The UV–vis absorption spectra of TPPS4 in acid and neutral aque-ous solutions and TEM image of TPPS4 J-aggregates were shownin Fig. 2. TPPS4 molecules do not aggregate in neutral aqueoussolutions due to the repulsion from the sulfonic groups. In acidicmedia (pH < 4.8), the two nitrogen atoms on the central ring ofTPPS4 are protonated, that is to say, TPPS4 exists in the biproto-nated form (H4

2+TPPS44−). In this case, the static attraction between

the protonated nitrogen atoms and sulfonic groups could cause theaggregation of TPPS4. It could be seen from Fig. 2, that both theappearance of an absorption band peaking at around 490 nm andincreasing absorbance around 709 nm indicated the formation ofJ-aggregates [20,21]. The origin of red shift of soret band from 413to 430 nm can be explained by the formation of an excitonic statethrough the electronic coupling of tightly packed porphyrin units[22]. The inset of Fig. 2 showed TEM image of aggregates of TPPS4which displayed a rod-like form with a unique diameter of 20 nmand a length ranging from 200 to 1000 nm. We found that the con-

centration of TPPS4 aqueous solutions and the storage time of theacidic solution could influence the length but not the diameter ofthe aggregates, which is in favor of the fabrication of nanostructuredPPy with uniform diameter.

Fig. 2. Absorption spectra of TPPS4 J-aggregates in acidic aqueous solution (pH 1,black line) and unprotonated TPPS4 in neutral solution (pH 7, dashed line). The con-centration of TPPS4 was 5 × 10−7 M. Inset: TEM image of TPPS4 aggregates in acidicaqueous solution (pH 1).

M. Wei, Y. Lu / Synthetic Metals 159 (2009) 1061–1066 1063

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and curly nanofiber structure was achieved (Fig. 4c and d). Thelength of these nanofibers ranged from 200 to 3000 nm, which wasmuch longer than that of the templates. What was the reason forthe formation of PPy nanofiber morphology?

Fig. 3. TEM and SEM images of PPy with nanorod

By using the rod-like aggregates of TPPS4 as templates, we dobtain the PPy samples as expected. As shown in Fig. 3, PPy obtainedy oxidation polymerization with APS and FeCl3 as oxidants, respec-ively, all had nanorod structures similar to the template. The twoamples had a good agreement with the TPPS4 templates in lengthanging from 200 to 1000 nm but unconformity in diameter. Fornstance, their average diameter showed ca. 200 nm for APS anda. 75 nm for FeCl3. Such results may be caused by the differencen standard redox potentials of the oxidants we used, Eox = 2.05 Vor APS and Eox = 0.77 V for FeCl3, because the increasing oxidantedox potential is propitious to the increase of PPy nanorod diam-ter [23]. This suggested that the diameter of the PPy nanorods cane controlled by the oxidants with different redox potentials.

It is worthy of emphasis that, in this work, the amount of theemplate was trace in the polymerization system with a molar ratiof TPPS4 and pyrrole monomer of 1:1000, implying the high effi-iency of the template. We interpreted this to be related to thepecific structure of the template. A “cylinder” model hypothesizedased on spectroscopic data and mathematical modeling had beeneported that TPPS4 single molecule in acid aqueous solution firstormed linear one-dimensional aggregates like a thread because ofhe strong intermolecular interaction between the sulfonic groupnd protonated nitrogens in the center of the molecule. The thread,hose length was limited by the number of the molecule and widthas just as that of one TPPS4 molecule of 2 nm [19], resulted in a

ing-shaped structure of about 20 nm in diameter. The rings thentacked together to organize to a nanotube (shown in Scheme 1).pparently in the case of the fixed amount of TPPS4, the forma-

ion of hollow structure was helpful to provide more templatesompared with the solid one. This may be the cause of the highfficiency of the template in inducing pyrrole monomer to formanorod structure. The sulfonic groups on the aggregate nanotubeurface including the inner surface may play a crucial role in attract-ng the pyrrole monomers and inducing their polymerization on theemplate surface. The abundant sulfonic groups of template (four

roups in one TPPS4 molecule) favored the high yield of nanostruc-ured PPy (almost 100%) and the easy removing by water rinse afterhe polymerization because of its excellent hydrophilicity. Becausef the very thin wall of the template, the PPy resulted finally showednanorod structure after washing.

ure oxidized by APS (a and b) and FeCl3 (c and d).

Silver ion as an oxidant, which has a standard redox potentialsimilar to Fe3+, fails to produce PPy within 12 h [24]. A possiblereason for this is that the reaction, although thermodynamicallyfavourable, is kinetically very slow. In our experiment, if TPPS4 wasomitted, few PPy could be obtained even after 48 h. However, inthe presence of TPPS4, a lot of PPy nanorods were formed within24 h. The acceleration effect of the TPPS4 may come from the con-gregation of the silver ion on the surface of the template. Thiscongregation, caused by the static force between the silver ion andsulfonic groups, favors the in situ polymerization of pyrrole and theformation of rod-like structure. Further study indicated that whensilver nitrate was applied as an oxidant, only short and gatherednanorods were obtained (Fig. 4a and b). The diameter of the rodswas about 65 nm, very close to that of the nanorods oxidized byFe3+, testifying the controllability of the diameter of PPy nanorodsby selecting oxidants with different standard redox potentials. Tomake the pyrrole dispersed evenly in its polymerization, we appliedpolyvinylpyrrolidone (PVP) as a dispersant [25]. Surprisingly, long

Scheme 1. Schematic diagram for the formation of TPPS4 nanotubes and PPynanorods.

1064 M. Wei, Y. Lu / Synthetic Metals 159 (2009) 1061–1066

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Fig. 4. TEM and SEM images of PPy oxidized by AgNO3 without (a a

In general, the aggregates of TPPS4 alone in acidic aqueous solu-ion can extend with the storage time, even to a visible netliketructure by crossing of numerous curvaceous fibers [19]. Thus,“simultaneous growth” (the polymerization of pyrrole and the

xtension of the aggregates) may happen due to the slow oxida-ion procedure of silver ion and the dispersion effect of the PVP.ased on the observations described above, a possible mecha-ism for the “simultaneous growth” (shown in Scheme 2) coulde proposed as follows: in the presence of PVP, both the silver ionsnd pyrrole monomer were dispersed adequately around the tem-late [26]. In the initial stage of the polymerization, because of theeak oxidation ability of silver ions, PPy formed mainly on the

emplate surface instead of the end in a relatively slow rate. Andhen this “uncapping end” of template was capable of extendingn its length direction at the same time of the slow polymeriza-ion. By comparison, the aggregates of TPPS4 could not be extended

hen pyrrole was oxidized by APS or FeCl3 due to the rapid oxida-

ion polymerization of the pyrrole. In this case, PPy wrapped thehole template surface quickly, no space available and no timeas afforded for the templates to extend. To sum up, there were

wo key factors in the preparation of PPy nanofibers with TPPS4: a

Scheme 2. Schematic diagram for “simultaneous

and with (c and d) PVP. The reduced Ag could be confirmed by EDS.

slow but competent oxidant and good dispersion of pyrrole in itspolymerization.

Fig. 5 (left) showed the FT-IR spectra of the PPy nanorods (oxi-dized by APS and FeCl3) and PPy granules, which were almost same.The characteristic C C stretching, C N stretching, C–N stretchingand C–H stretching of PPy were located at 1560, 1480, 1290 and1050 cm−1. The strong peaks near 1190 and 920 cm−1 implied thedoping state of PPy [27]. It indicated that the TPPS4 aggregates,although played a “hard template” role, could be washed away eas-ily by water and ethanol. Fig. 5 (right) showed the FT-IR spectraof the PPy oxidized by AgNO3 (with and without PVP) and purePVP. For both PPy samples, a strong absorption band assignable toNO3

− could be observed at 1383 cm−1, indicating that the PPy wasdoped by NO3

− [25]. The band observed at 1652 cm−1 for the sam-ple with PVP was assigned to C O stretching of PVP, showing thatPVP remained in the nanofiber of PPy.

The XRD patterns of the PPy nanorods and PPy granules werepresented in the insert of Fig. 6. The XRD pattern of the PPy nanorodswas almost the same as the one of PPy granules, showing onlya reflection centered at 2� = 25◦, which was the characteristic ofthe doped PPy structured features [25]. By comparison, one could

growth”: the formation of PPy nanofibers.

M. Wei, Y. Lu / Synthetic Metals 159 (2009) 1061–1066 1065

Fig. 5. FTIR spectra of PPy nanorods oxidized by (a) FeCl3 and (b) APS; (c) PPy granules; PPy oxidized by AgNO3 with (d) and without (e) PVP and (f) PVP.

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ig. 6. The XRD pattern of PPy nanorods oxidized by (a) APS and (b) by FeCl3; (c)Py granules; PPy oxidized by AgNO3 with (d) and without (e) PVP.

learly find the existence of Ag in the samples oxidized by AgNO3Fig. 6). Apart from the weak peak at 2� = 25◦, another five diffrac-ion peaks above 30◦ corresponded to (1 1 1), (2 0 0), (2 2 0), (3 1 1)nd (2 2 2) reflection of Ag, which were in agreement with thoseeported for Ag particles [28].

The thermogravimetric results indicated that the decompositionemperature of the PPy nanorods was similar to that of granulesFig. 7), and the weight loss below 100 ◦C was due to the releasef water. The more weight loss of samples ‘b and c’ indicated thathey had higher specific surface area than the granule. Although the

Fig. 8. TEM images of carbonized (a) PPy nanorods oxidized by

Fig. 7. The thermal analysis diagrams of PPy oxidized by AgNO3 with PVP (a); PPynanorods oxidized by (b) FeCl3 and (c) APS and (d) PPy granules.

PPy rods started to decompose at 250 ◦C similar to the PPy granule,the decompose rate was much slower and the residual is more,showing a better stability. This could be interpreted to the improvedspatial order of the nanorods. The more residual of sample ‘a’ maybe attributed to contribution of reduced Ag particles.

As can be seen in Fig. 8, even after the temperature rose to 700 ◦C,

the carbonized PPy nanorods or nanofibers could still keep theirmorphologies. It also provided a new approach for the fabrication ofcarbon nanostructure by using conduction polymer as its precursor[29].

APS and (b) PPy nanofibers oxidized by AgNO3 with PVP.

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[[[24] M.C. Henry, C.C. Hsueh, B.P. Timko, J. Electrochem. Soc. 148 (2001) 155.[25] S.X. Xing, G.K. Zhao, Mater. Lett. 61 (2007 2040).[26] YangF X.M., Y. Lu, Mater. Lett. 59 (2005) 2484.

066 M. Wei, Y. Lu / Synthetic

The conductivity of PPy nanorods oxidized by APS was 2.7 S/cm,hich was almost a doubled number of 1.4 S/cm of PPy granules.

he conductivity of PPy oxidized by AgNO3 with and without PVPas found to be 0.6 and 8.6 S/cm, respectively. Obviously, the tiny

ilver deoxidized, as a conductor, was beneficial for improving theonductivity of PPy. And the low conductivity of the other sampleay be caused by the existence of the insulated PVP in PPy, which

ad been indicated by the IR result.

. Conclusion

In summary, we have demonstrated a simple and high effec-ive strategy for construction of the PPy nanorod with variationalize by using aggregates of TPPS4 as template and cooperatingith the oxidants with different redox potentials. This TPPS4 tem-late showed considerable merits from both soft and hard templateuch as stable, good versatility, cheap and easy removal in the fab-ication of nanostructured PPy. In particular, the molar ratio ofPPS4 and pyrrole monomer could be as low as 1:1000, showinghigh efficiency of the template. Interestingly, in the presence of

PPS4 template and polyvinylpyrrolidone, PPy nanofibers could bebtained even though using a weak oxidant, which was consideredo be related with a “simultaneous growth” of the polymerizationf pyrrole and the extension of the TPPS4 aggregates. It was foundhat PPy nanorods/nanofibers had the main chain structure iden-ical to granular PPy, and their conductivity and thermalstabilityere superior to that of granular PPy.

cknowledgments

This work was supported by the National Natural Science Foun-ation of China (No. 20574034) and the Testing Foundation ofanjing University.

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s 159 (2009) 1061–1066

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