synthesis of polyoxometalates-functionalized carbon nanotubes composites and relevant...
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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
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 UVvisible (UVvis), 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
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
. 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 . 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 . 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 .
Since the discovery of carbon nanotubes (CNTs) , 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 .
However, electrochemical inertness and low chemical reactivity of raw nanotubes lead to the basal limitations of their
Materials Research Bulletin 42 (2007) 14851491
* Corresponding author. Tel.: +86 431 5098787; fax: +86 431 5098787.
E-mail address: email@example.com (E. Wang).
0025-5408/$ see front matter # 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.materresbull.2006.11.001
applications . 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 . 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 .
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 . Accordingly, the development of POMs-containing
functional nanomaterials and nanodevices is steadily increasing , 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
. 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 . 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.
CNTs with diameters of 1520 nm were purchased from Tsinghua-Nafine Nano-power Commercialization
Engineering Center. Poly (diallyldimethylammonium chloride) (PDDA, 20% in water, MW 100,0002,000,000)was purchased from Aldrich and used as received. Polyoxometalates (POMs) with the composition H3PMo12O4014H2O (abbreviated PMo12), K4SiW12O4014H2O (abbreviated SiW12), (NH4)6P2Mo18O6214H2O (abbreviatedP2Mo18) were synthesized according to the literature procedure . 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.
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 UVvis spectrophotometer. FTIR patterns were measured in the
range 4004000 cm1 on an Alpha Centauri FTIR spectrophotometer. TEM images were obtained using a JEM-2010transmission 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, theoxidative 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) 148514911486
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 . As demonstrated in the literature, POMs can
combine with PDDA via electrostatic action forming small domains . 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 , 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  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.