Decorating Polypyrrole Nanotubes with Au Nanoparticles by an In Situ Reduction Process

Communication

936

Decorating Polypyrrole Nanotubes with AuNanoparticles by an In Situ Reduction Process

Jingjing Xu, Jianchen Hu, Baogang Quan, Zhixiang Wei*

Au nanoparticle-decorated polypyrrole nanotubes (defined as PPy/Au nanocomposites) areprepared by an in situ reduction process. Polypyrrole (PPy) nanotubes are prepared by a self-degraded template method, and Au nanoparticles are deposited in situ by the reduction ofHAuCl4. The size and uniformity of the Au nanoparticles that decorate the PPy nanotubes canbe controlled by adjusting the experimental con-ditions, such as the stabilizers used and the reactiontemperature. The morphologies and optical proper-ties of the nanocomposites have been characterizedby scanning electron microscopy, transmission elec-tron microscopy, UV-vis, and FT-IR spectroscopy.Conductivity measurements show that the conduc-tivities of the nanocomposites decrease with adecrease of temperature, and the conductivity–temperature relationship obeys the quasi-onedimensional variable range hopping model.

Introduction

Combining the electrical, optical, and/or magnetic proper-

ties of functional nanomaterials, multi-functional nano-

materials have shown potential applications in biosensing

and therapy,[1–7] energy conversion and storage,[8–11] and

electromagnetic radiation shieldings.[12] Organic/inor-

ganic hybrid nanomaterials possess the merits of their

organic and inorganic components, and may also exhibit

new properties that one component doesn’t have. There-

fore, it is an efficient way to create novel functional

nanomaterials by careful selection and composition of

J. Xu, J. Hu, B. Quan, Z. WeiNational Center for Nanoscience and Technology, Beijing 100190;P. R. ChinaE-mail: [email protected]. XuGraduate School of the Chinese Academy of Sciences, Beijing,100039, P. R. China

Macromol. Rapid Commun. 2009, 30, 936–940

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

functional components. Au nanoparticles are a typical

inorganic nanomaterial with excellent chemical and

physical properties. For example, dispersed Au nanopar-

ticles exhibited ferromagnetic behavior,[13–15] electrocata-

lytic activity,[16,17] and plasma resonance[18] at room

temperature. On the other hand, one-dimensional nano-

structures of conducting polymers (1D-CPs), which include

nanotubes and nanofibers, have attracted intensive

interest because of their unique properties and potential

applications in nanoelectronic devices and highly sensitive

chemical or biological sensors.[19,20] 1D-CP/Au nanocom-

posites combine the optical properties of Au nanoparticles

with the anisotropic electrical properties of 1D-CPs. In

addition, new properties are observed because of the

electron transfer in the nanocomposites. For example,

Yang et al.[21] prepared a relatively uniform distribution of

Au nanoparticles in polyaniline (PANI/Au) nanofibers by

reducing chloroauric acid. They found that the PANI/Au

nanocomposite exhibited bistability of the non-volatile

memory when the Au nanoparticles were smaller than

DOI: 10.1002/marc.200800764

Decorating Polypyrrole Nanotubes with Au Nanoparticles by . . .

20 nm. Among different types of CPs, polypyrrole (PPy) has

shown high performance including environmental stabi-

lity, electronic conductivity, ion exchange capacity, and

biocompatibility. Up to now, there are only a few reports

on Au nanoparticle/1D-PPy nanocomposites.[22,23]

Herein, we report a facile method to prepare Au

nanoparticle-decorated PPy nanotubes (PPy/Au nanocom-

posites) by an in situ reduction process. PPy nanotubes are

prepared by a self-degraded template method, and Au

nanoparticles are deposited in situ by the reduction of

HAuCl4. The size and uniformity of the Au nanoparticles

that decorate the PPy can be controlled by adjusting the

experimental conditions, such as the stabilizers used and

the reaction temperature. The electrical properties of the

PPy/Au nanocomposites are measured by a Physical

Property Measurement System. The results show that ln

s (T) retained a linear relationship with T�1/2 for PPy and

PPy/Au composite nanotubes, which suggests the con-

ductivity obeyed the one-dimensional variable range

hopping (1D-VRH) model.

Experimental Part

Materials

All chemicals were purchased from Beijing Chemicals Company,

including pyrrole, ferric chloride hexahydrate (FeCl3 �6H2O),

methyl orange (MO), chloroauric acid trihydrate (HAuCl4 �3H2O),

polyoxyethylenesorbitan monooleate (Tween-80), cetyltrimethy-

lammonium bromide (CTAB), and sodium dodecyl sulfonate (SDS).

Pyrrole monomer was distilled under reduced pressure and other

reagents were of analytical grade and used as received without

further treatment.

Preparation of PPy Nanotubes

The preparation of PPy nanotubes was carried out by a self-degraded

template method as reported in the literature.[24] In a typical

procedure, 0.243 g (1.5 mmol) of FeCl3 was dissolved in 30 mL

of 5�10�3M MO (sodium 4-[40-(dimethylamino)phenyldiazo]

phenylsulfonate) ((CH3)2NC6H4-N¼NC6H4SO3Na) deionized water

solution (0.15 mmol). A flocculent precipitate appeared immedi-

ately. Pyrrole monomer (105 mL, 1.5 mmol) was then added and

the mixture was stirred at room temperature for 24 h. The formed

PPy precipitate was washed with deionized water, methanol, and

ether several times, repectively, until the filtrate was colorless

and neutral, and the PPy was finally dried under a vacuum at

60 8C for 24 h.

Figure 1. SEM images of PPy (a) and PPy/Au nanocomposites (b: nosurfactant, c: CTAB, d: SDS, and e: Tween-80 were used as astabilizer respectively). f) TEM image of the PPy/Au nanocompo-site using Tween-80 as a stabilizer. PPy¼ 16.7 mg � L�1,HAuCl4¼ 34.5 mg � L�1. The concentration of the surfactants is0.042 wt.-%, the reactions were all carried out at 0 8C for 30 min.

Preparation of PPy/Au Nanocomposites

In a typical procedure, 2 mg of PPy nanotubes were added into

120 mL of deionized water with surfactant (e.g., 50 mg of Tween-

80) and ultrasonicated for several minutes until the PPy

nanotubes were well dispersed. A HAuCl4 aqueous solution



(0.414 mL, 10 mg �mL�1) was then added and the mixture was

magnetically stirred for half an hour at a defined temperature (e.g.,

0 8C). The mixture was centrifuged and the precipitate was

dispersed in deionized water and sonicated for one hour, and then

centrifuged and washed with deionized water several times. At

last, the final product was dried under vacuum at 60 8C for 24 h.

Characterization

Scanning electron microscopy (SEM) was performed using a

Hitachi S-4800 instrument, and a Tecnai G220 S-TWIN operating at

200 kV was used for transmission electron microscopy (TEM)

measurement. Optical absorption spectra were acquired with a

Lambda 650/850/950 UV-vis spectrophotometer (Perkin–Elmer).

FT-IR spectra were recorded on a Spectrum One FT-IR spectrometer

(Perkin–Elmer). The electrical properties were determined by a

Physics Property Measurement System (PPMS, Quantum Design).

Results and Discussion

Conducting polymers can be oxidized to an over-oxidized

state by HAuCl4, and at the same time, the AuCl�4 ions are

reduced to Au nanoparticles, which are deposited on the

polymer in situ.[21,22] In order to prepare PPy/Au nano-

composites, PPy nanotubes (Figure 1a) were prepared with

an average diameter of about 100 nm and a length of

www.mrc-journal.de 937

J. Xu, J. Hu, B. Quan, Z. Wei

Figure 2. SEM images of PPy/Au composites (a, b, c, and d, thereaction temperature is 0, 25, 50, and 100 8C, respectively) and theplot of influence of temperature on the size of the Au nanopar-ticles (e). PPy¼ 16.7 mg � L�1, HAuCl4¼ 34.5 mg � L�1. The concen-tration of the surfactants is 0.042%, reacting for 30 min.

938

several micrometers by a self-degraded template

method.[24] PPy nanotubes were dispersed in the water

by sonication. After adding chloroauric acid, Au nanopar-

ticles were deposited on the nanotubes by reduction, and

appear as bright dots in Figure 1b. Although some small

nanoparticles with an average diameter of 13 nm are

dispersed uniformly on the nanotubes, large nanoparticles

with an average diameter of 80 nm are also present

because of the overgrowth of the nanoparticles. Large

nanoparticles originate from the overgrowth of the Au

nanoparticles since they are not well protected in the case

without surfactant.

In order to obtain monodispersed Au nanoparticles on

PPy nanotubes, water-soluble surfactants, such as the

cationic surfactant CTAB, the anionic surfactant SDS, and

the non-ionic surfactant Tween-80, were selected as

stabilizers, respectively. CTAB is a typical stabilizer for

the synthesis of Au nanoparticles.[25,26] But when CTAB is

used as a stabilizer to synthesize the PPy/Au hybrid

nanotubes, almost no Au nanoparticles are deposited on

the PPy nanotubes (Figure 1c) because of the repulsive

electrostatic interactions of CTAB and PPy, which are both

considered cationic electrolytes. When SDS is used as a

stabilizer, large nanoparticles with diameters of �50 nm

are formed on the surface of the PPy nanotubes (Figure 1d),

which indicates that the protecting function of SDS is not

quite as good in this case. Using the non-ionic surfactant

Tween-80, Au nanoparticles with an average diameter of

13 nm are deposited on the PPy nanotubes (Figure 1e). TEM

images also confirm that the Au nanoparticles are

uniformly distributed on the PPy nanotubes. Therefore,

the size and uniformity of the Au nanoparticles are highly

dependent on the surfactants: using Tween-80 as the

stabilizer, a PPy/Au nanocomposite with monodispersed

Au nanoparticles is obtained successfully.

As is commonly known, the properties of Au nanopar-

ticles, such as plasmonic absorption, are highly dependent

on their shapes and sizes.[27] Therefore, adjusting the size

of the nanoparticles was carried out by changing the

reaction temperature. Since both the stability of the

surfactants and the reducing speed of HAuCl4 are highly

dependent on the temperature, the influence of tempera-

ture on the size of the Au nanoparticles should quite

obvious (see Figure 2). Accordingly, PPy/Au nanocompo-

sites at four temperatures were prepared, namely 0, 25, 50,

and 100 8C (Figure 2a–d). As expected, the size of the Au

nanoparticles increased from 13 to 49 nm on average, and

the size distribution became broader with an increase of

temperature (Figure 2e). For the synthesis of monometallic

Au nanoparticles, it is common that at higher tempera-

tures the formation of metallic nuclei is more favorable in

a homogeneous system and, therefore, more seeds for

nucleation result in a decrease of the particle sizes and an

increment of the particle numbers.[28,29] But for PPy/Au



nanocomposites, Au nanoparticles are formed in a

heterogeneous system. PPy nanotubes act as the support-

ing template and reducing agent simultaneously. With an

increase in temperature, the formation of nuclei is limited

by the diffusion of AuCl�4 , and the increase of the reactivity

of PPy produces larger Au nanoparticles on its surface.

Therefore, the size of the nanoparticles could be facilely

adjusted by changing the temperature using the same

surfactant.

In order to study the influence of nanoparticle size on

the properties of the nanocomposites, the structures of the

PPy nanotubes and PPy/Au nanocomposites were inves-

tigated by UV-vis and FT-IR spectroscopy. The samples for

UV-vis measurement were dispersed in aqueous solution.

The samples for FT-IR were prepared by grinding the

nanocomposites with KBr powder, and then pressing the

mixture into pellets. In Figure 3a, the absorption peak at

465 nm for the PPy nanotubes is assigned as the p–p�

transition of PPy.[24] In comparison with the PPy nano-

tubes, no new peak appears, but a blue shift from 465 to

420 nm for the PPy/Au nanotubes is observed when the

reactions are carried out at 0 and 25 8C, which indicates

that: 1) the amount of Au nanoparticles on the surface of

the PPy nanotubes is so small that plasmon resonance

absorption is very weak and is overlayed by the absorption

of PPy, and 2) the conjugation degree of PPy decreases as a

DOI: 10.1002/marc.200800764

Decorating Polypyrrole Nanotubes with Au Nanoparticles by . . .

Figure 4. a) ln R versus T plots and b) ln s versus T�1/2 plots of PPyand PPy/Au nanocomposites.

Figure 3. a) UV-vis and b) FT-IR spectra of PPy and PPy/Aunanocomposites obtained at the reaction temperature of 0, 25,50, and 100 8C, respectively. Other reaction conditions: PPy¼16.7 mg � L�1, HAuCl4¼ 34.5 mg � L�1, the concentration of surfac-tants is 0.042%, reacting for 30 min.

result of being partially over-oxidized by HAuCl4.[30]

Furthermore, with a temperature increase, the blue-shift

becomes more apparent. When the temperature is 50 8C, a

new peak at 535 nm appears, which is attributed to the

surface plasmon resonance absorption of the Au nano-

particles.[31,32] When the temperature is 100 8C, a red-shift

occurs from 535 to 550 nm, and the peak intensity

increases, which suggests that the Au nanoparticles

become larger and the plasmon resonance absorption

becomes stronger, which coincides with the SEM images

(Figure 2a–d) very well.

In the Figure 3b, the five spectra of the PPy nanotubes

and PPy/Au nanocomposites are quite similar to each

other, which indicates the main polymer chains of the PPy-

Au composite nanotubes are similar to that of the PPy

nanotubes. For example, the C¼C and C�N stretching

vibration peaks at 1 547 and 1 470 cm�1, the C�H in-plane

vibration at 1 313 and 1 174 cm�1, the C�H in-plane

bending at 1 038 cm�1, and the ring deformation at



900 cm�1 are observed.[33–35] However, there is a new

weak band at 1 724 cm�1, which is attributed to the

carbonyl group, and indicates that PPy is somewhat over-

oxidized by HAuCl4 during the in situ precipitation

process.[35] The over-oxidation of PPy shortens the

conjugation length, which is consistent with the blue-

shift phenomena in the UV-vis spectra.

The temperature-dependent resistances of the blocks of

PPy nanotubes and PPy/Au composite nanotubes were

measured by a four-terminal technique in the temperature

range of 2–300 K as shown in Figure 4a. The conductivities

of the PPy nanotubes and PPy/Au composite nanotubes

decrease with a decrease in temperature, and show a semi-

conductor behavior. The conductivities of the PPy/Au

nanocomposites are much lower than that of the PPy

nanotubes because of the over-oxidation of PPy. Metallic

Au nanoparticles are evenly distributed on the conducting

PPy nanotubes, but two conducting materials are sepa-

rated by an insulating layer of over-oxidized PPy.[21] The

lower conductivity of the PPy/Au nanocomposites also

indicates that the Au nanoparticles did not form a

www.mrc-journal.de 939

J. Xu, J. Hu, B. Quan, Z. Wei

940

conducting network in the pellets because of their low

population density on the PPy nanotubes.

On the other hand, a linear plot of ln s to T�1/2 is

observed for all samples, as shown in Figure 4b, which is in

agreement with the 1D-VRH model,[36] and it can be

expressed as:

sðTÞ ¼ s0 exp½�ðT0=TÞ1=2�

where s0 is a constant, T0 is the hopping barrier, and T is

the temperature in Kelvin. From Figure 4b, we can

calculate that the hopping barriers of the PPy nanotubes,

and PPy/Au (13 nm) and PPy/Au (49 nm) nanocomposities

are 625 K, 17 161 K, and 23 716 K, respectively.

Conclusion

In summary, we have prepared Au nanoparticle-decorated

PPy nanotubes (PPy/Au nanocomposites) by an in situ

precipitation process. Au nanoparticles with controllable

sizes can be realized by adjusting the surfactant used and

the reaction temperature. PPy is partially over-oxidized by

HAuCl4 during the in situ precipitation process, which is

confirmed by UV-vis and FT-IR spectra. Conductivity

measurements show that ln s (T) is linear plotted with

T�1/2 for PPy or PPy/Au composite nanotubes, which

suggests their conductivities obey the 1D VRH model.

Acknowledgements: The authors gratefully acknowledge theNational Natural Science Foundation of China (Grants20604008), the National Basic Research Program of China(2006cb932100, 2009CB930400), and the Chinese Academy ofScience for financial support.

Received: December 8, 2008; Revised: February 16, 2009;Accepted: February 17, 2009; DOI: 10.1002/marc.200800764

Keywords: conducting polymers; nanocomposites; nanoparti-cles; nanotubes; polypyrroles

[1] M. Zayats, A. B. Kharitonov, S. P. Pogorelova, O. Lioubashevski,E. Katz, I. Willner, J. Am. Chem. Soc. 2003, 125, 16006.

[2] I. Willner, B. Willner, Pure Appl. Chem. 2002, 74, 1773.[3] J. M. Abad, S. F. L. Mertens, P. Marcos, V. M. Fernandez, D. J.

Schiffrin, J. Am. Chem. Soc. 2005, 127, 5689.[4] S. Kramer, H. Xie, J. Gaff, J. R. Williamson, A. G. Tkachenko, N.

Nouri, D. A. Feldheim, D. L. Feldheim, J. Am. Chem. Soc. 2004,126, 5388.

[5] J. L. Gardea-Torresdey, J. G. Parsons, E. Gomez, J. Peralta-Videa,H. E. Troiani, P. Santiago, M. Jose Yacaman, Nano Lett. 2002, 2,397.



[6] D. Wang, J. He, N. Rosenzweig, Z. Rosenzweig, Nano Lett. 2004,4, 409.

[7] D. C. Hone, P. I. Walker, R. Evans-Gowing, S. FitzGerald, A.Beeby, I. Chambrier, M. J. Cook, D. A. Russell, Langmuir 2002,18, 2985.

[8] W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 2002, 295,2425.

[9] D. W. Hatchett, R. Wijeratne, J. M. Kinyanjui, J. Electroanal.Chem. 2006, 593, 203.

[10] G. Girishkumar, K. Vinodgopal, P. V. Kamat, J. Phys. Chem. B2004, 108, 19960.

[11] K. Park, S. B. Schougaard, J. B. Goodenough, Adv. Mater. 2007,19, 848.

[12] Y. Long, Z. Chen, J. Duvail, Z. Zhang, M. Wan, Physica B 2005,370, 121.

[13] Y. Yamamoto, T. Miura, M. Suzuki, N. Kawamura, H. Miya-gawa, T. Nakamura, K. Kobayashi, T. Teranishi, H. Hori, Phys.Rev. Lett. 2004, 93, 116801.

[14] P. Crespo, R. Litran, T. C. Rojas, M. Multigner, J. M. de la Fuente,J. C. Sanchez-Lopez, M. A. Garcıa, A. Hernando, S. Penades, A.Fernandez, Phys. Rev. Lett. 2004, 93, 087204.

[15] J. d. l. Venta, A. Pucci, E. F. Pinel, M. A. Garcıa, C. d. J. Fernandez,P. Crespo, P. Mazzoldi, G. Ruggeri, A. Hernando, Adv. Mater.2007, 19, 875.

[16] B. K. Jena, C. R. Raj, Langmuir 2007, 23, 4064.[17] B. K. Jena, C. R. Raj, Chem. Eur. J. 2006, 12, 2702.[18] J. Yang, J. Wu, Y. Wu, J. Wang, C. Chen, Chem. Phys. Lett. 2005,

416, 215.[19] J. Huang, S. Virji, B. H. Weiller, R. B. Kaner, Chem. Eur. J. 2004,

10, 1314.[20] S. Virji, J. Huang, R. B. Kaner, B. H. Weiller, Nano Lett. 2004, 4,

491.[21] R. J. Tseng, J. X. Huang, J. Y. Ouyang, R. B. Kaner, Y. Yang, Nano

Lett. 2005, 5, 1077.[22] P. Xu, X. Han, C. Wang, B. Zhang, X. Wang, H. Wang, Macro-

mol. Rapid Commun. 2008, 29, 1392.[23] R. M. Hernandez, L. Richter, S. Semancik, S. Stranick, T. E.

Mallouk, Chem. Mater. 2004, 16, 3431.[24] X. Yang, Z. Zhu, T. Dai, Y. Lu, Macromol. Rapid Commun. 2005,

26, 1736.[25] F. X. Chen, G. Q. Xu, T. S. Andy, Mater. Lett. 2003, 57, 3282.[26] C. Cao, S. Park, S. J. Sim, J. Colloid Interface Sci. 2008, 322, 152.[27] Y. Yang, S. Matsubara, M. Nogami, J. Shi, Mater. Sci. Eng. B

2007, 140, 172.[28] D. I. Garcia-Gutierrez, C. E. Gutierrez-Wing, L. Giovanetti, J. M.

Ramallo-Lpez, F. G. Requejo, M. Jose-Yacaman, J. Phys. Chem. B2005, 109, 3813.

[29] P. Y. Silvert, K. Tekaia-Elhsissen, Solid State Ionics 1995, 82, 53.[30] Y. Shen, M. Wan, J. Polym. Sci., Part A: Polym. Chem. 1997, 35,

3689.[31] X. Ji, X. Song, J. Li, Y. Bai, W. Yang, X. Peng, J. Am. Chem. Soc.

2007, 129, 13939.[32] H. J. Shin, I. W. Hwang, Y. N. Hwang, D. Kim, S. H. Han, J. S. Lee,

G. Cho, J. Phys. Chem. B 2003, 107, 4699.[33] A. J. F. Romero, J. J. L. Cascales, T. F. Otero, J. Phys. Chem. B 2005,

109, 21078.[34] X. Li, M. Wan, Y. Wei, J. Shen, Z. Chen, J. Phys. Chem. B 2006,

110, 14623.[35] G. W. Lu, Ch. Li, G. Q. Shi, Polymer 2006, 47, 1778.[36] N. F. Mott, E. A. Davis, ‘‘Electronic Processes in Non-Crystalline

Materials’’, 2nd edition, Clarendon Press, Oxford 1979,p. 34.

DOI: 10.1002/marc.200800764

Documents

Decorating Polypyrrole Nanotubes with Au Nanoparticles by an In Situ Reduction Process