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: [email protected] (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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