Synthesis of polyoxometalates-functionalized carbon nanotubes

composites and relevant electrochemical properties study

Yanli Song, Enbo Wang *, Zhenhui Kang, Yang Lan, Chungui Tian

Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun Jilin 130024, PR China

Received 24 May 2006; received in revised form 24 October 2006; accepted 1 November 2006

Available online 18 December 2006

Abstract

Carbon nanotubes (CNTs)-based polyoxometalates (POMs)-functionalized nanocomposites were synthesized by simply

functionalizing CNTs with Keggin and Dawson-type POMs. The positively charged polyelectrolyte poly (diallyldimethylammo-

nium chloride) (PDDA) was introduced to assemble negatively charged POMs and CNTs. The composition, structure and

morphology were investigated by UV–visible (UV–vis), Fourier transform infrared spectroscopy (FTIR) and transmission electron

microscopy (TEM). Cyclic voltammetry (CV) was employed to investigate the electrochemical properties of the resulting

nanocomposites. The cyclic voltammograms indicate that the electrochemical properties of POMs are fully maintained.

Functionalizing CNTs with POMs not only retains the unique properties of nanotubes, but also endows CNTs with the reversible

redox activity of POMs.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Composites; A. Nanostructures; B. Electrochemical properties

1. Introduction

During the past decades nanometer-scale materials have attracted considerable interest due to their fundamental

significance for physical properties and potential applications owing to their unique particle sizes and surface effects

[1]. Up to now, a variety of techniques have been applied to fabricate nanostructures of a broad class of materials,

ranging from semiconductors, metal oxides to metal nanoparticles with different morphologies [2]. However, some of

the proposed applications of these nanomaterials remain a far-off dream; others are closed to technical realization.

Recent developments of reliable strategies for functionalizing and processing the nanomaterials provide an additional

impetus towards extending the scope of their applications [3]. More and more efforts have been paid to functionalize

nanomaterials since the functional properties of the obtained nanoscaled composites are greatly improved compared

with the original materials [4].

Since the discovery of carbon nanotubes (CNTs) [5], they have triggered intensive research for their unique

properties including high surface area, specific electrical conductivity, exceptional physicochemical stability and

significant mechanical strength. In fact, as the best and most available one-dimensional (1D) nanomaterials, carbon

nanotubes show wide applications in material science, sensor technology, catalysis and biomedical fields [6].

However, electrochemical inertness and low chemical reactivity of raw nanotubes lead to the basal limitations of their

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Materials Research Bulletin 42 (2007) 1485–1491

* Corresponding author. Tel.: +86 431 5098787; fax: +86 431 5098787.

E-mail address: wangenbo@public.cc.jl.cn (E. Wang).

0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2006.11.001

applications [7]. Enhancing the activity and extending the applications of carbon nanotubes show strong dependence

on the development of methods for functionalizing and processing these nanotubes. Recently, several methods have

been reported for the attachment of nanoparticles, biomacromolecule and other functional materials with various

natures onto CNTs [8]. The obtained functionalized nanocomposites preserve the unique properties of the nanotubes,

simultaneously endowing the materials with novel functions that cannot otherwise be acquired by raw nanotubes [3].

Polyoxometalates (POMs), a class of molecularly defined inorganic metal oxide clusters, have won particular

attention for their various applications in many fields of science, such as medicine, biology, catalysis, and materials

owing to their chemical, structural, and electronic versatility [9]. Accordingly, the development of POMs-containing

functional nanomaterials and nanodevices is steadily increasing [10], and the functional properties have also been

investigated, which provide a spark to the combination of nanoscience and polyoxometalates chemistry.

On the basis of the excellent redox properties and the high protonic conductivity of polyoxometalates, both Keggin-

andDawson-typeheteropolyanionshavebeenextensivelyapplied ashighly selectiveand long-timestable redox catalysts

[11]. Therefore, functionalizing CNTs with POMs will make CNTs more attractive in catalysis and electrochemistry

fields by comparison with pristine nanotubes. The reported POMs-functionalized nanocomposites exhibit voltammetric

response in the potential window commonly used, which indicates that the electrochemical properties of POMs may be

fully maintained when they are introduced to functionalize CNTs [12]. Hence, CNTs-based nanocomposites bearing

POMs with redox activity are of potential importance to electrocatalysis and charge storage in redox capacitors. In the

present work, three types of POMs were chosen to prepare CNTs-based POMs-functionalized nanocomposites on the

basis of electrostatic interactions. The positively charged polyelectrolyte PDDAwas used to assemble negatively charged

POMs and CNTs, and the obtained nanocomposites were expressed with CNTs-PDDA/POMs.

2. Experimental

2.1. Chemicals

CNTs with diameters of 15–20 nm were purchased from Tsinghua-Nafine Nano-power Commercialization

Engineering Center. Poly (diallyldimethylammonium chloride) (PDDA, 20% in water, MW � 100,000–2,000,000)

was purchased from Aldrich and used as received. Polyoxometalates (POMs) with the composition H3PMo12O40�14H2O (abbreviated PMo12), K4SiW12O40�14H2O (abbreviated SiW12), (NH4)6P2Mo18O62�14H2O (abbreviated

P2Mo18) were synthesized according to the literature procedure [13]. Hydrochloric acid (HCl, 37%), sulfuric acid

(H2SO4, 98%), nitric acid (HNO3, 70%), and sodium bromide (NaBr, AR), were all purchased from commercial

market and used without further purification.

2.2. Instruments

An AA10200A ultrasonic cleaner was used to oxidative cutting CNTs and coating CNTs with polyelectrolyte. UV–

vis absorption spectra were recorded on a 756 PC UV–vis spectrophotometer. FTIR patterns were measured in the

range 400–4000 cm�1 on an Alpha Centauri FTIR spectrophotometer. TEM images were obtained using a JEM-2010

transmission electron microscope at an acceleration voltage of 200 kV. A CHI 660-electrochemical workstation

connected to a digital-586 personal computer was used for the control of the electrochemical measurements and for

data collection. A conventional three-electrode cell, consisting a carbon paste electrode (CPE) as the working

electrode, a saturated calomel electrode (SCE) served as reference electrode and a platinum foil was applied as the

counter electrode. All potentials were measured and reported versus the SCE.

2.3. Fabrication of CNs-PDDA/POMs nanocomposites

The received CNTs were sonicated with 37% hydrochloric acid (HCl) for 2 h to remove the catalysts (support and

metal particles). The precipitate was kept overnight and then diluted with deionized water. The obtained mixture was

chemically oxidized by ultrasonification in a mixture of sulfuric acid and nitric acid (3:1) for 8 h, and then washed with

deionized water and separated by centrifuging/washing till the pH � 7. After being dried in vacuum at 60 8C, the

oxidative CNTs were dispersed in deionized water. In this work, PDDA was dissolved in deionized water at a

concentration of 0.1 mg/mL, and then the CNTs bearing carboxylic groups were coated with PDDA through

Y. Song et al. / Materials Research Bulletin 42 (2007) 1485–14911486

dispersing CNTs in PDDA solutions for 3 h with sonification. The obtained PDDA-wrapped CNTs formed stable,

uniform aqueous solution and did not precipitate for at least 12 h [14]. As demonstrated in the literature, POMs can

combine with PDDA via electrostatic action forming small domains [15]. Electrolyte NaBr was added to separate

excess polyelectrolyte from PDDA-CNTs, the mixture was centrifugated three times and the supernatant solution was

decanted. POMs (SiW12, PMo12, P2Mo18) was subsequently deposited by redispersing PDDA coated CNTs

nanohybrid materials in 0.1 M POMs aqueous solution with sonication for 1 h. The last procedure was carried out by

three repeated centrifugation/wash cycles. Then, the black precipitate was dried in vacuum.

3. Results and discussion

Scheme 1 illustrates the preparation procedure of the nanocomposites. The oxidatively treated CNTs were

negatively charged owing to the anionic carboxylic acid groups generated at both the defect sites along the side walls

and the open ends of the tubes [16], and the carbon nanotubes were shortened at the same time. The driving force for

the formation of CNTs-PDDA/SiW12 is electrostatic attraction between oppositely charged species. The reaction

mechanism is similar to the mechanism of LbL technique first introduced by Decher [17] and can be described as

follows: negatively charged CNTs interact with cationic polyelectrolyte PDDA and form carboxylates, then anionic

POMs absorb on CNTs by combining with PDDA. Fig. 1(a) displays typical TEM images of raw CNTs, while Fig. 1(b)

and (c) show lower and higher magnification images of CNTs-PDDA/SiW12 nanocomposites. As shown in Fig. 1, the

surface of raw CNTs was smooth but when POMs deposited on CNTs small domains formed, which is owing to the

aggregation of POMs in the presence of PDDA [15].

The as-prepared products were characterized by UV–vis spectroscopy. Fig. 2 shows the UV–vis spectra of the raw

CNTs (a), CNTs-PDDA (b) and CNTs-PDDA/POMs (c), respectively. The broad peak around 240 nm can be assigned

Y. Song et al. / Materials Research Bulletin 42 (2007) 1485–1491 1487

Scheme 1. Schematic illustration of the procedures for the fabrication of PDDA-CNTs/POMs nanocomposites. (Step a) Ultrasonic oxidation of

CNTs in a mixture of sulfuric acid and nitric acid (3:1). (Step b) Deposition of PDDA on negatively charged CNTs with formation of CNTs-PDDA

nanohybrid materials. (Step c) Adsorption of POMs to functionalize PDDA-wrapping CNTs.

Fig. 1. TEM images of CNTs (a) and CNTs-PDDA/SiW12 nanocomposites (b) lower and (c) higher magnification.

Y. Song et al. / Materials Research Bulletin 42 (2007) 1485–14911488

Fig. 2. UV–vis spectra of raw CNTs (a); CNTs-PDDA (b); CNTs-PDDA/SiW12 (c-1), CNTs-PDDA/PMo12 (c-2), and CNTs-PDDA/P2Mo18 (c-3).

to the absorption of CNTs suspended in ethanol [18]. However, the absorption wave shifted to 267 nm when CNT-

PDDA formed, which is consistent with literature [19]. As shown in Fig. 2(c)-1, the remarkable peaks at 200, 260 nm

attributed to the oxygen! tungsten charge transfer (CT) transition of SiW12. Furthermore, in Fig. 2(c)-2, the

Od!Mo and Ob (Oc)!Mo charge transfer (CT) transition of PMo12 are observed at 215 and 325 nm, while two

peaks at 212 and 320 nm in Fig. 2(c)-3 are the characteristic absorption of P2Mo18. The combined results demonstrate

that the expectant products have been synthesized.

Fourier transform infrared spectroscopy is an effective characterization method to reveal the composition of the

products. The IR spectrum of CNTs was shown in Fig. 3(a)-1, the band at 1725 cm�1 is attributed to the C O stretch

mode of carboxylic acid groups, which demonstrates the formation of carboxyl on CNTs. In Fig. 3(a)-2, six peaks were

observed in the IR spectrum of CNTs-PDDA, the ns(OH), nas(CH2), ns(CH2), d(CH2) and n(C–N) appeared at 3435,

2922, 2855, 1458 and 138 4 cm�1, respectively. Furthermore, the peak at 1636 cm�1 can be assigned to carboxylates

according to the nas(RCOOR0) at 1650–1545 cm�1 [20]. The results above combined further verified the formation of

CNTs-PDDA nanohybrid materials. In Fig. 3(b)-1, the four characteristic peaks at 974, 921, 884 and 790 cm�1 are

attributed to n(W Od), n(Si–Oa), n(W–Ob–W), n(W–Oc–W), respectively. As shown in Fig. 3(b)-2, the bands at

1061, 958, 878, 792 cm�1are the characteristic peaks of PMo12, while six characteristic bands of P2Mo18 were

observed at 1077, 1003, 940, 905, 834, 777 cm�1 in Fig. 3(b)-3. All of the results attest the adsorption of POMs

clusters on the PDDA-wrapped CNTs and the basic structure of POMs are still preserved in the nanocomposites [21].

The electrochemical behavior of CNTs-PDDA/POMs nanocomposites was investigated by fabricating carbon paste

electrode (CPE). The CPE was fabricated according to the literature [22]. Fig. 4 shows the cyclic voltammograms of

Y. Song et al. / Materials Research Bulletin 42 (2007) 1485–1491 1489

Fig. 3. IR spectra of the CNTs (a-1) and CNTs-PDDA (a-2); CNTs-PDDA/SiW12 (b-1), CNTs-PDDA/PMo12 (b-2), and CNTs-PDDA/P2Mo18 (b-3).

CNTs (a) and CNTs-PDDA/POMs (b) nanocomposites. It can be seen from Fig. 4(a) that in the potential range �400

to +400 mV (versus SCE), there is no redox peak at the working electrode. While at the SiW12-functionalized carbon

nanotubes working electrode, three reversible redox peaks appear and the formal potentials E1/2 = (Epa + Epc)/2 for the

three pairs of peaks are�0.16,�0.44,�0.77 V in 1 M CH3COOH–1 M CH3COONa buffer at scan rate = 10 mV/s, as

shown in Fig. 4(b)-1. Furthermore, Fig. 4(b)-2 shows three waves of CNTs-PDDA/PMo12 at +0.27, +0.12, �0.11 V

when scanned in 0.05 M H2SO4 at 5 mV/s. When P2Mo18-functionalized nanocomposites were scanned in 0.5 M

H2SO4 solution at scan rate of 5 mV/s, three redox waves were observed at +0.42, +0.33, +0.16 V in Fig. 4(b)-3. All of

the wave formal potential values of three POMs are similar to the literature [21]. The above experimental results attest

to the successful fabrication of CNTs-PDDA/POMs nanocomposites. Considering that POMs have excellent

electrocatalysis properties and meanwhile CNTs can probably be used as active and stable catalysts for certain

reactions [23], the obtained nanocomposites may have potential applications in electrocatalytic fields.

4. Conclusion

In summary, we have showed a facile strategy through noncovalent functionalization of CNTs with Keggin and

Dawson type POMs. The experimental results presented in this paper demonstrated that the basic structure and

electrochemical activity of POMs are maintained in the nanocomposites. Furthermore, the electrochemical properties

Y. Song et al. / Materials Research Bulletin 42 (2007) 1485–14911490

Fig. 4. Cyclic voltammograms of CNTs (a); CNTs-PDDA/SiW12 (b-1), CNTs-PDDA/PMo12 (b-2), and CNTs-PDDA/P2Mo18 (b-3).

of CNTs are enhanced when they are functionalized. The electroactivity of the nanocomposites are envisaged to make

them very useful for basic electrochemical studies.

Acknowledgment

Financial support for this work was provided by the National Natural Science Foundation of China (Grant

20371011).

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