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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

    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 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

    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

    www.elsevier.com/locate/matresbu

    Materials Research Bulletin 42 (2007) 14851491

    * 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 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 [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 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 [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.

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