Synthesis of polyoxometalates-functionalized carbon nanotubes composites and relevant electrochemical properties study

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<ul><li><p>Synthesis of polyoxometalates-functionalized carbon nanotubes</p><p>composites and relevant electrochemical properties study</p><p>Yanli Song, Enbo Wang *, Zhenhui Kang, Yang Lan, Chungui Tian</p><p>Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun Jilin 130024, PR China</p><p>Received 24 May 2006; received in revised form 24 October 2006; accepted 1 November 2006</p><p>Available online 18 December 2006</p><p>Abstract</p><p>Carbon nanotubes (CNTs)-based polyoxometalates (POMs)-functionalized nanocomposites were synthesized by simply</p><p>functionalizing CNTs with Keggin and Dawson-type POMs. The positively charged polyelectrolyte poly (diallyldimethylammo-</p><p>nium chloride) (PDDA) was introduced to assemble negatively charged POMs and CNTs. The composition, structure and</p><p>morphology were investigated by UVvisible (UVvis), Fourier transform infrared spectroscopy (FTIR) and transmission electron</p><p>microscopy (TEM). Cyclic voltammetry (CV) was employed to investigate the electrochemical properties of the resulting</p><p>nanocomposites. The cyclic voltammograms indicate that the electrochemical properties of POMs are fully maintained.</p><p>Functionalizing CNTs with POMs not only retains the unique properties of nanotubes, but also endows CNTs with the reversible</p><p>redox activity of POMs.</p><p># 2006 Elsevier Ltd. All rights reserved.</p><p>Keywords: A. Composites; A. Nanostructures; B. Electrochemical properties</p><p>1. Introduction</p><p>During the past decades nanometer-scale materials have attracted considerable interest due to their fundamental</p><p>significance for physical properties and potential applications owing to their unique particle sizes and surface effects</p><p>[1]. Up to now, a variety of techniques have been applied to fabricate nanostructures of a broad class of materials,</p><p>ranging from semiconductors, metal oxides to metal nanoparticles with different morphologies [2]. However, some of</p><p>the proposed applications of these nanomaterials remain a far-off dream; others are closed to technical realization.</p><p>Recent developments of reliable strategies for functionalizing and processing the nanomaterials provide an additional</p><p>impetus towards extending the scope of their applications [3]. More and more efforts have been paid to functionalize</p><p>nanomaterials since the functional properties of the obtained nanoscaled composites are greatly improved compared</p><p>with the original materials [4].</p><p>Since the discovery of carbon nanotubes (CNTs) [5], they have triggered intensive research for their unique</p><p>properties including high surface area, specific electrical conductivity, exceptional physicochemical stability and</p><p>significant mechanical strength. In fact, as the best and most available one-dimensional (1D) nanomaterials, carbon</p><p>nanotubes show wide applications in material science, sensor technology, catalysis and biomedical fields [6].</p><p>However, electrochemical inertness and low chemical reactivity of raw nanotubes lead to the basal limitations of their</p><p>www.elsevier.com/locate/matresbu</p><p>Materials Research Bulletin 42 (2007) 14851491</p><p>* Corresponding author. Tel.: +86 431 5098787; fax: +86 431 5098787.</p><p>E-mail address: wangenbo@public.cc.jl.cn (E. Wang).</p><p>0025-5408/$ see front matter # 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.materresbull.2006.11.001</p></li><li><p>applications [7]. Enhancing the activity and extending the applications of carbon nanotubes show strong dependence</p><p>on the development of methods for functionalizing and processing these nanotubes. Recently, several methods have</p><p>been reported for the attachment of nanoparticles, biomacromolecule and other functional materials with various</p><p>natures onto CNTs [8]. The obtained functionalized nanocomposites preserve the unique properties of the nanotubes,</p><p>simultaneously endowing the materials with novel functions that cannot otherwise be acquired by raw nanotubes [3].</p><p>Polyoxometalates (POMs), a class of molecularly defined inorganic metal oxide clusters, have won particular</p><p>attention for their various applications in many fields of science, such as medicine, biology, catalysis, and materials</p><p>owing to their chemical, structural, and electronic versatility [9]. Accordingly, the development of POMs-containing</p><p>functional nanomaterials and nanodevices is steadily increasing [10], and the functional properties have also been</p><p>investigated, which provide a spark to the combination of nanoscience and polyoxometalates chemistry.</p><p>On the basis of the excellent redox properties and the high protonic conductivity of polyoxometalates, both Keggin-</p><p>andDawson-typeheteropolyanionshavebeenextensivelyapplied ashighly selectiveand long-timestable redox catalysts</p><p>[11]. Therefore, functionalizing CNTs with POMs will make CNTs more attractive in catalysis and electrochemistry</p><p>fields by comparison with pristine nanotubes. The reported POMs-functionalized nanocomposites exhibit voltammetric</p><p>response in the potential window commonly used, which indicates that the electrochemical properties of POMs may be</p><p>fully maintained when they are introduced to functionalize CNTs [12]. Hence, CNTs-based nanocomposites bearing</p><p>POMs with redox activity are of potential importance to electrocatalysis and charge storage in redox capacitors. In the</p><p>present work, three types of POMs were chosen to prepare CNTs-based POMs-functionalized nanocomposites on the</p><p>basis of electrostatic interactions. The positively charged polyelectrolyte PDDAwas used to assemble negatively charged</p><p>POMs and CNTs, and the obtained nanocomposites were expressed with CNTs-PDDA/POMs.</p><p>2. Experimental</p><p>2.1. Chemicals</p><p>CNTs with diameters of 1520 nm were purchased from Tsinghua-Nafine Nano-power Commercialization</p><p>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</p><p>(H2SO4, 98%), nitric acid (HNO3, 70%), and sodium bromide (NaBr, AR), were all purchased from commercial</p><p>market and used without further purification.</p><p>2.2. Instruments</p><p>An AA10200A ultrasonic cleaner was used to oxidative cutting CNTs and coating CNTs with polyelectrolyte. UV</p><p>vis absorption spectra were recorded on a 756 PC UVvis spectrophotometer. FTIR patterns were measured in the</p><p>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</p><p>connected to a digital-586 personal computer was used for the control of the electrochemical measurements and for</p><p>data collection. A conventional three-electrode cell, consisting a carbon paste electrode (CPE) as the working</p><p>electrode, a saturated calomel electrode (SCE) served as reference electrode and a platinum foil was applied as the</p><p>counter electrode. All potentials were measured and reported versus the SCE.</p><p>2.3. Fabrication of CNs-PDDA/POMs nanocomposites</p><p>The received CNTs were sonicated with 37% hydrochloric acid (HCl) for 2 h to remove the catalysts (support and</p><p>metal particles). The precipitate was kept overnight and then diluted with deionized water. The obtained mixture was</p><p>chemically oxidized by ultrasonification in a mixture of sulfuric acid and nitric acid (3:1) for 8 h, and then washed with</p><p>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</p><p>concentration of 0.1 mg/mL, and then the CNTs bearing carboxylic groups were coated with PDDA through</p><p>Y. Song et al. /Materials Research Bulletin 42 (2007) 148514911486</p></li><li><p>dispersing CNTs in PDDA solutions for 3 h with sonification. The obtained PDDA-wrapped CNTs formed stable,</p><p>uniform aqueous solution and did not precipitate for at least 12 h [14]. As demonstrated in the literature, POMs can</p><p>combine with PDDA via electrostatic action forming small domains [15]. Electrolyte NaBr was added to separate</p><p>excess polyelectrolyte from PDDA-CNTs, the mixture was centrifugated three times and the supernatant solution was</p><p>decanted. POMs (SiW12, PMo12, P2Mo18) was subsequently deposited by redispersing PDDA coated CNTs</p><p>nanohybrid materials in 0.1 M POMs aqueous solution with sonication for 1 h. The last procedure was carried out by</p><p>three repeated centrifugation/wash cycles. Then, the black precipitate was dried in vacuum.</p><p>3. Results and discussion</p><p>Scheme 1 illustrates the preparation procedure of the nanocomposites. The oxidatively treated CNTs were</p><p>negatively charged owing to the anionic carboxylic acid groups generated at both the defect sites along the side walls</p><p>and the open ends of the tubes [16], and the carbon nanotubes were shortened at the same time. The driving force for</p><p>the formation of CNTs-PDDA/SiW12 is electrostatic attraction between oppositely charged species. The reaction</p><p>mechanism is similar to the mechanism of LbL technique first introduced by Decher [17] and can be described as</p><p>follows: negatively charged CNTs interact with cationic polyelectrolyte PDDA and form carboxylates, then anionic</p><p>POMs absorb on CNTs by combining with PDDA. Fig. 1(a) displays typical TEM images of raw CNTs, while Fig. 1(b)</p><p>and (c) show lower and higher magnification images of CNTs-PDDA/SiW12 nanocomposites. As shown in Fig. 1, the</p><p>surface of raw CNTs was smooth but when POMs deposited on CNTs small domains formed, which is owing to the</p><p>aggregation of POMs in the presence of PDDA [15].</p><p>The as-prepared products were characterized by UVvis spectroscopy. Fig. 2 shows the UVvis spectra of the raw</p><p>CNTs (a), CNTs-PDDA (b) and CNTs-PDDA/POMs (c), respectively. The broad peak around 240 nm can be assigned</p><p>Y. Song et al. /Materials Research Bulletin 42 (2007) 14851491 1487</p><p>Scheme 1. Schematic illustration of the procedures for the fabrication of PDDA-CNTs/POMs nanocomposites. (Step a) Ultrasonic oxidation of</p><p>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</p><p>nanohybrid materials. (Step c) Adsorption of POMs to functionalize PDDA-wrapping CNTs.</p><p>Fig. 1. TEM images of CNTs (a) and CNTs-PDDA/SiW12 nanocomposites (b) lower and (c) higher magnification.</p></li><li><p>Y. Song et al. /Materials Research Bulletin 42 (2007) 148514911488</p><p>Fig. 2. UVvis spectra of raw CNTs (a); CNTs-PDDA (b); CNTs-PDDA/SiW12 (c-1), CNTs-PDDA/PMo12 (c-2), and CNTs-PDDA/P2Mo18 (c-3).</p></li><li><p>to the absorption of CNTs suspended in ethanol [18]. However, the absorption wave shifted to 267 nm when CNT-</p><p>PDDA formed, which is consistent with literature [19]. As shown in Fig. 2(c)-1, the remarkable peaks at 200, 260 nm</p><p>attributed to the oxygen ! tungsten charge transfer (CT) transition of SiW12. Furthermore, in Fig. 2(c)-2, theOd ! Mo and Ob (Oc) ! Mo charge transfer (CT) transition of PMo12 are observed at 215 and 325 nm, while twopeaks at 212 and 320 nm in Fig. 2(c)-3 are the characteristic absorption of P2Mo18. The combined results demonstrate</p><p>that the expectant products have been synthesized.</p><p>Fourier transform infrared spectroscopy is an effective characterization method to reveal the composition of the</p><p>products. The IR spectrum of CNTs was shown in Fig. 3(a)-1, the band at 1725 cm1 is attributed to the C O stretchmode of carboxylic acid groups, which demonstrates the formation of carboxyl on CNTs. In Fig. 3(a)-2, six peaks were</p><p>observed in the IR spectrum of CNTs-PDDA, the ns(OH), nas(CH2), ns(CH2), d(CH2) and n(CN) appeared at 3435,</p><p>2922, 2855, 1458 and 138 4 cm1, respectively. Furthermore, the peak at 1636 cm1 can be assigned to carboxylatesaccording to the nas(RCOOR</p><p>0) at 16501545 cm1 [20]. The results above combined further verified the formation ofCNTs-PDDA nanohybrid materials. In Fig. 3(b)-1, the four characteristic peaks at 974, 921, 884 and 790 cm1 areattributed to n(W Od), n(SiOa), n(WObW), n(WOcW), respectively. As shown in Fig. 3(b)-2, the bands at</p><p>1061, 958, 878, 792 cm1are the characteristic peaks of PMo12, while six characteristic bands of P2Mo18 wereobserved at 1077, 1003, 940, 905, 834, 777 cm1 in Fig. 3(b)-3. All of the results attest the adsorption of POMsclusters on the PDDA-wrapped CNTs and the basic structure of POMs are still preserved in the nanocomposites [21].</p><p>The electrochemical behavior of CNTs-PDDA/POMs nanocomposites was investigated by fabricating carbon paste</p><p>electrode (CPE). The CPE was fabricated according to the literature [22]. Fig. 4 shows the cyclic voltammograms of</p><p>Y. Song et al. /Materials Research Bulletin 42 (2007) 14851491 1489</p><p>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).</p></li><li><p>CNTs (a) and CNTs-PDDA/POMs (b) nanocomposites. It can be seen from Fig. 4(a) that in the potential range 400to +400 mV (versus SCE), there is no redox peak at the working electrode. While at the SiW12-functionalized carbon</p><p>nanotubes working electrode, three reversible redox peaks appear and the formal potentials E1/2 = (Epa + Epc)/2 for the</p><p>three pairs of peaks are0.16,0.44,0.77 V in 1 M CH3COOH1 M CH3COONa buffer at scan rate = 10 mV/s, asshown in Fig. 4(b)-1. Furthermore, Fig. 4(b)-2 shows three waves of CNTs-PDDA/PMo12 at +0.27, +0.12, 0.11 Vwhen scanned in 0.05 M H2SO4 at 5 mV/s. When P2Mo18-functionalized nanocomposites were scanned in 0.5 M</p><p>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</p><p>the wave formal potential values of three POMs are similar to the literature [21]. The above experimental results attest</p><p>to the successful fabrication of CNTs-PDDA/POMs nanocomposites. Considering that POMs have excellent</p><p>electrocatalysis properties and meanwhile CNTs can probably be used as active and stable catalysts for certain</p><p>reactions [23], the obtained nanocomposites may have potential applications in electrocatalytic fields.</p><p>4. Conclusion</p><p>In summary, we have showed a facile strategy through noncovalent functionalization of CNTs with Keggin and</p><p>Dawson type POMs. The experimental results presented in this paper demonstrated that the basic structure and</p><p>electrochemical activity of POMs are maintained in the nanocomposites. Furthermore, the electrochemical properties</p><p>Y. Song et al. /Materials Research Bulletin 42 (2007) 148514911490</p><p>Fig. 4. Cyclic voltammograms of CNTs (a); CNTs-PDDA/SiW12 (b-1), CNTs-PDDA/PMo12 (b-2), and CNTs-PDDA/P2Mo18 (b-3).</p></li><li><p>of CNTs are enhanced when they are functionalized. The electroactivity of the nanocomposites are envisaged to make</p><p>them very useful for basic electrochemical studies.</p><p>Acknowledgment</p><p>Financial support for this work was provided by the National Natural Science Foundation of China (Grant</p><p>20371011).</p><p>References</p><p>[1] L.N. Lewis, Chem. Rev. 93 (1993) 2693.</p><p>[2] B.L. Cushin...</p></li></ul>

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