A Template-Free Method toward Urchin-Like Polyaniline Microspheres

Communication

604

A Template-Free Method toward Urchin-LikePolyaniline Microspheresa

Junsheng Wang, Jixiao Wang,* Zhi Wang, Fengbao Zhang

Urchin-like PANI microspheres with an average diameter of 5–10 mm have been successfullyprepared. Their surfaces consist of highly oriented nanofibers of �30 nm diameter and 1 mmlength. The solvent composition plays an important role in the formation process of urchin-likePANI microspheres. The structure of the products has beencharacterized by FT-IR, UV-vis, and XRD. To investigate theself-assembly of urchin-like PANI microspheres, the effectof polymerization time on the morphology of the productshas been studied. The morphological evolution processindicates that the urchin-like microspheres originatefrom the self-assembly of nanoplates, which then growinto urchin-like microstructures with nanofibers on thesurface.

Introduction

Among conducting polymers, polyaniline (PANI) has

attracted considerable attention because of its low cost,

ease of synthesis, good optical and electrical properties, as

well as excellent environmental stability.[1,2] The design

and synthesis of PANI nanostructures have received great

attention in nanoscience and nanotechnology because of

their unique properties and potential applications.[3–6]

Various approaches, such as template methods,[7,8]

template-free methods,[9,10] and electrochemical meth-

ods,[11] have been widely employed for the fabrication

of PANI nanostructures. Among these methods, the

J. Wang, J. Wang, Z. Wang, F. ZhangState Key Laboratory of Chemical Engineering, ChemicalEngineering Research Center, School of Chemical Engineering andTechnology, Tianjin University, Tianjin 300072, ChinaE-mail: [email protected]

a: Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.

Macromol. Rapid Commun. 2009, 30, 604–608

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

template-free method has been considered as the most

promising route in terms of low cost and large-scale

production. It is found that the morphology of PANI

micro/nanostructures fabricated by template-free meth-

ods are strongly affected by the nature and concentration

of the dopant, and the concentrations of oxidant,

monomer, as well as the molar ratios of the dopant

and oxidant to monomer.[12–17] By changing these

polymerization parameters, various PANI micro/nano-

structures from one dimensional (1D) to three dimen-

sional (3D), such as nanotubes, nanofibers, nanosheets,

and hollow spheres, have been successfully synthesized

with template-free methods.[16,18–21] It is believed that

the multidimensional assembly of nano-units has a

marvellous ability to control the physical and chemical

properties of materials because of their novel architec-

tures.[21,22] Three-dimensional microstructures assembled

from 1D nanostructures have been proposed to provide

high functionality and performance for applications in

technology as recently reviewed by Wan.[17] Therefore,

developing a facile route to fabricate 3D microstructures

assembled from 1D nanostructures into desired struc-

tures is a significant challenge in the design of advanced

nanodevices.

DOI: 10.1002/marc.200800726

A Template-Free Method toward Urchin-Like Polyaniline Microspheres

Self-assembly driven by various molecular interactions

such as hydrogen bonding, p-p stacking, and van der

Waals interactions is an effective strategy for the

formation of nanostructures.[23] These molecular interac-

tions are affected by the composition of the solution such

as the dopants and solvent, etc. Thus, varying the dopants

and solvents might provide an effective approach to

fabricate 3D microstructures assembled from 1D nano-

structures. Three-dimensional rambutan-like hollow

microstructures assembled from nanofibers of PANI were

successfully prepared using perfluorosebacic acid (PFSEA)

and perfluorooctane sulfonic acid (PFOSA) as multi-

functional dopants.[24,25] However, to the best of our

knowledge, the report on the fabrication of PANI 3D

microstructures assembled from nanofibers of PANI by

varying the solvent composition under common dopants

[such as p-toluene sulfonic acid (p-TSA) and HCl], has not

been reported yet. Herein, we report a novel approach to

the self-assembly of urchin-like PANI microspheres using a

mixture of ethanol and H2O as solvent. The growth process

of the urchin-like PANI microspheres is discussed, and the

molecular structure of the synthesized PANI is character-

ized by Fourier-transform infrared (FT-IR) and UV-vis

spectroscopy and X-ray diffraction (XRD).

Experimental Part

Aniline monomer (analytical grade) was distilled until colorless

under reduced pressure prior to use. Other chemicals were of

analytical grade and used as received without further treatment.

In a typical synthesis, 5.0 mmol of aniline was dissolved in

4.0 mL of ethanol solution that contained 1.0 mmol of p-TSA.

An aqueous solution of ammonium peroxydisulfate (APS, 6 mL,

1.0 mmol) was added to the above solution as the oxidant. The

reaction was allowed to proceed without agitation for 24 h at

room temperature. Finally, the products were washed with

deionized water until the filtrate became colorless and dried in a

vacuum at 60 8C for 24 h. Other experiments were carried out by

Figure 1. SEM and TEM images of PANI microstructures. (The volume ratio of ethanolto H2O¼ 4:6, [An]¼0.5 M, [p-TSA]¼0.1 M, and [APS]¼0.1 M.)

varying the volume of ethanol and H2O.

The morphologies of the resulting products

were characterized by scanning electron

microscopy (SEM, JEOL, JSM6700F) and trans-

mission electron microscopy (TEM, Tecnai G2

F20), respectively. Samples for SEM experi-

ments were prepared by placing the product

on conducting stages and were observed with

gold coatings. The TEM samples were pre-

pared by suspending an appropriate amount

of product in ethanol by sonication and

casting onto copper TEM grids. The grids were

placed on filter paper to facilitate rapid drying.

The structure of the PANI nanostructures was

characterized by FT-IR, UV-vis, as well as XRD

techniques. FT-IR spectra in the range of 4 000

to 400 cm�1 were measured on a Nicolet



MANGA-IR 560 spectrophotometer using KBr pressed disks. The

XRD of the polymer samples was recorded with an X’ Pert Pro X-

ray diffractometer with Cu Ka emission. The spectra were recorded

in the range of 2u¼ 5 to 408 with Cu Ka emission. The conductivity

of the compressed pellets (pressed at 8 MPa for 5 min) at room

temperature was measured by a standard four-probe method

using digital DC resistance measurer (XC2853).

Results and Discussion

The morphology of the as-synthesized PANI was char-

acterized by SEM and TEM techniques. As shown in

Figure 1, the resulting PANI is spherical in shape with a

diameter of 5–10 mm. From the SEM and TEM images, it

can be found that the surface of the obtained microspheres

consist of highly oriented nanofibers of about 30 nm in

diameter and 1 mm in length, which is similar to the

structure of a sea urchin.

Figure 2 shows the morphologies of PANI prepared in

the solution that contained 0.5 M aniline, 0.1 M p-TSA, and

different proportions of ethanol. When the volume ratio of

ethanol to H2O is 1:9, PANI nanoplates can be obtained

exclusively, and the same result is also observed at a 2:8

volume ratio of ethanol to H2O. Figure 2b and c show that

when the volume ratio of ethanol to H2O increases from

3:7 to 6:4, the urchin-like PANI microspheres are obtained.

However, with a further increase in ethanol content, PANI

nanoparticles become the dominating units in morphol-

ogy as demonstrated in Figure 2d. The experimental

results clearly show that the solvent composition has an

obvious effect on the morphology of PANI, and choosing a

proper solvent is an effective strategy for the self-assembly

of nanostructures as mentioned above.

The structure of the PANI microspheres was character-

ized by FT-IR, UV-vis, and XRD techniques. A typical FT-IR

spectrum of the urchin-like PANI is shown in Figure 3. All

the characteristic peaks of PANI appear in the spectrum of

www.mrc-journal.de 605

J. Wang, J. Wang, Z. Wang, F. Zhang

Figure 2. Effect of different proportions of ethanol on the morphology of PANI. Thevolume ratios of ethanol to H2O¼ a) 1:9, b) 3:7, c) 5:5, d) 9:1. ([An]¼0.5 M, [p-TSA]¼0.1 M,and [APS]¼0.1 M.)

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the urchin-like microstructures.[26] The peaks at 1 599 and

1 528 cm�1 are ascribed to the C––C stretching vibration of

the quinonoid and benzenoid rings, respectively, and the

peak at 1 306 cm�1 to the C–N stretching vibration. The

peaks at 858 and 3 200 cm�1 are assigned to the out-of-

plane vibration in the 1,4-disubstituted aromatic rings and

the N–H stretching vibration, respectively. The peaks at

1 179 and 1 074 cm�1 are assigned to the asymmetric and

symmetric O––S––O stretching vibrations, respectively, and

the peak at 700 cm�1 to the S–O stretching vibration of the

sulfonate groups attached to the aromatic rings. A FT-IR

spectrum of the product after 2 h is also presented in

Figure 3 and shows all the characteristic peaks of PANI.

Comparing the FT-IR spectra of the product at 24 h with

that at 2 h, the relative intensity of the peaks at 1 599 and

1 528 cm�1 becomes stronger. This suggests that the

content of the quinonoid and benzenoid units increases in

Figure 3. FT-IR spectra and pH values of the solution at different polymerization time,and XRD pattern of PANI microstructures after 24 h. The volume ratio of ethanolto H2O¼ 4:6, [An]¼0.5 M, [p-TSA]¼0.1 M, and [APS]¼0.1 M.



the formed PANI at 24 h. In addition to

the above main absorption peaks of

PANI, peaks at 1 445 and 1 414 cm�1

are also observed in the FT-IR spectra of

the products at different polymerization

time. The appearance of these peaks

indicates the presence of ortho-linked

aniline constitutional units and phena-

zine-like units, which are commonly

generated by the branching and cross-

linking reactions among polymer

chains.[27–29] The UV-vis spectra of

urchin-like PANI microspheres is mea-

sured with samples dispersed in water

(Supporting Information, Figure S1). The

peak at 286 nm is attributed to the p-p�

benzenoid transition, while the peaks at

435 and 839 nm are related to the partial

protonation and polaron transition of

the PANI chains.[16] The results indicate

that the urchin-like PANI microspheres

are in the conductive emeraldine salt

form. The room-temperature conductiv-

ity of the urchin-like PANI pellet, mea-

sured by a four-probe method, is about

6.5� 10�2 S � cm�1. XRD was used to further characterize

the structure of the urchin-like PANI microstructures, as

shown in Figure 3. Three peaks centered at 2u¼ 7.4, 22, and

288 are observed for the urchin-like microstructures.

Similar to the PANI prepared by conventional methods,

the peaks centered at 2u¼ 22 and 288 can be ascribed to the

periodicity parallel and perpendicular to the polymer

chains of PANI, respectively, and the newly appeared sharp

peak centered at 2u¼ 7.48 corresponds to the periodicity

distance between the dopant and the N atom on adjacent

main chains, which indicates the ordering of the dopant

molecules in tunnels between the PANI chains.[30] The

results suggest that the urchin-like PANI has a better

crystallinity than that of conventional PANI.

The products at different reaction stages were obtained

to examine the morphology evolution process of the

urchin-like microstructures, as shown in Figure 4. It is

found that the morphology of PANI is

obviously affected by the polymerization

time, and the formation process of

urchin-like microspheres is divided into

three stages. At the early stage, the

as-synthesized product with a platelet

morphology forms in solution

(Figure 4a), while the spherical morphol-

ogy begins to appear in the product after

15 min (Figure 4b). When the polymer-

ization time is further extended to 2 h,

the morphology of the product comple-

DOI: 10.1002/marc.200800726

A Template-Free Method toward Urchin-Like Polyaniline Microspheres

Figure 4. SEM images of PANImicrostructures synthesizedwith different polymerizationtimes: a) 5 min, b) 15 min, c) 2 h, and d) 4 h. The volume ratio of ethanol to H2O¼ 4:6,[An]¼0.5 M, [p-TSA]¼0.1 M, and [APS]¼0.1 M.

tely evolves into a spherical architecture, which is

assembled by the platelet oxidation products as seen

from the inset (Figure 4c). As the polymerization time

increases to 4 h, nanofibers with a high orientation are

formed on the surface of these spheres to form urchin-like

microstructures (Figure 4d).

In order to further elucidate the self-assembly process of

the micro/nanostructures, the effect of polymerization

time on pH values of the reaction system was measured, as

shown in Figure 3. It is shown that the pH values of the

solution decrease with the polymerization time because of

the released protons.[31–33] The polymerization starts in

weak acidic conditions (pH> 5.0) at a [An]/[p-TSA] ratio of

5:1, thus, plate-like products are formed under this

condition at the early stage.[19,21] These formed platelets

gradually assemble into spherical architectures at a

volume ratio of 4:6 of ethanol to water (Figure 4) as the

reaction time increases. However, the formed platelets will

grow into PANI nanoplates rather than microspheres

when pure water is employed as the solvent.[19] In other

words, the self-assembly of these nanoplates into micro-

spheres takes place only when the solvent changes from

pure water to the mixture of water and ethanol with

certain volume ratios. As we know, ethanol has an

amphiphilic molecular structure and different properties,

such as polarity and surface tension, compared

with H2O. When the solvent changes from pure water

to the mixture of water and ethanol with certain volume



ratios, the molecular interactions (such

as hydrogen bonding and van der Waals

forces) between the products and sol-

vents, and the surface energy of the

products, may also change in the reac-

tion solution. The plate-like products

obtained at the early stage may assem-

ble to counterbalance the changed forces.

Therefore, it is concluded that the self-

assembly of these nanoplates is a result

of the interplay of molecular interactions

between the products and solvents that

result from the effect of ethanol

and H2O in the reaction system. Since

the pKa value of aniline is 4.6 at 25 8C,[28]

neutral aniline molecules are the dom-

inating monomer species when the self-

assembly process starts. Neutral aniline

molecules can be oxidized by the cou-

pling of aniline in the ortho- or para-

position.[27–29] From the analysis of the

FT-IR spectra (Figure 3 and Figure S2), it

can be concluded that branching and

cross-linking reactions also occur in this

self-assembly process. From Figure 3, it is

seen that the pH value of the reaction

system reduces to below 4.6 (the pKa of aniline) after 1 h of

polymerization and anilinium cations begin to be the

dominating monomer form in solution. The growth of

PANI nanofibers is intrinsic to the polymerization of

anilinium cations, which act as a ‘soft-template’ for the

formation of nanofibers.[11,34] This may result in the

formation of nanofibers on the surface of the PANI

microstructures. When a common inorganic acid HCl with

a low concentration is used as dopant, a similar process

occurs and microstructures are also obtained as shown in

the Supporting Information (Figure S3). The results

indicate that the 3D microstructures can be easily

fabricated with common organic/inorganic acids using

this facile template-free method. It is expected that the

urchin-like PANI microspheres with a large specific area

might find potential applications in microelectronic

devices, sensors, energy storage, drug release/delivery

vehicles, and separation systems, etc.

Conclusion

In summary, urchin-like polyaniline microspheres with an

average diameter of 5–10 mm have been successfully

fabricated by a template-free method. The surface of the

obtained microspheres consists of highly oriented nano-

fibers of about 30 nm in diameter and 1 mm in length. The

volume ratios of ethanol to H2O play an important role in

www.mrc-journal.de 607

J. Wang, J. Wang, Z. Wang, F. Zhang

608

the formation of the urchin-like PANI microspheres. It is

proposed that the self-assembly of nanoplates is driven by

the molecular interactions between the products and

solvents and branching/cross-linking reactions also occur

in the growth process of the polymer chains. After the self-

assembly of nanoplates into microspheres, nanofibers are

fabricated on the surface of these microspheres, and

urchin-like microspheres are formed eventually. The

results indicate that this strategy is facile, effective, and

controllable for the self-assembly of conducting polymer

micro/nanostructures.

Acknowledgements: This work was supported by the Program ofIntroducing Talents of Discipline to Universities, No. B06006, andthe Program for New Century Excellent Talents in University.

Received: November 21, 2008; Revised: January 4, 2009; Accepted:January 7, 2009; DOI: 10.1002/marc.200800726

Keywords: conducting polymers; microspheres; microstructure;polyaniline; self-assembly

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DOI: 10.1002/marc.200800726

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A Template-Free Method toward Urchin-Like Polyaniline Microspheres