Synthesis and Characterization of Metal Nanoparticle Embedded Conducting Polymer–Polyoxometalate Composites
© to the authors 2007
Received: 20 August 2007
Accepted: 20 November 2007
Published: 13 December 2007
Phosphomolybdate has been employed simultaneously as the oxidizing agent for the monomer polymerization and the reduced polyoxometalate is used as reducing agent for the reduction of metal ions. The composites thus obtained have been characterized and may have many potential applications.
The desire to synthesize nanostructures that combine the mechanical flexibility, optical and electrical properties of conducting polymers with the high electrical conductivity and magnetic properties of metal nanaoparticles has inspired the development of several techniques for the controlled fabrication of metal nanoparticle—conducting polymer composites. The incorporation of metal nanoparticles into the conducting polymer offers enhanced performance for both the host and the guest . They have diverse application potentials in electronics because incorporation of metal clusters is known to increase the conductivity of the polymer . The applications of these composites have also been extended to various fields such as, sensors [3, 4], photovoltaic cells , memory devices , protective coatings against corrosion , and supercapacitors . Of particular interest is the application of these composites in catalysis. The polymer allows the control of the environment around the metal center, thus influencing selectivity of the chemical reactions. Polyaniline (PAni) supported Pd nanoparticles have been used for the oxidative coupling of the 2,6-di-t-butylphenol . In terms of engineering applications, conducting polymer-supported metal nanoparticle catalysts are attractive materials for fuel cell design. For example, direct alcohol and proton exchange membrane fuel cell electrocatalysts based on conducting polymers have been studied [10–12]. Dispersing the metal nanoparticles into a conducting polymer matrix maintains the electrical connectivity of the particles to the underlying electrode [13, 14]. Under optimal conditions, this arrangement may result in enhanced electrocatalytic properties compared to the corresponding reactivity of the bulk metal . Various methods for the preparation of nanoparticle embedded conducting polymer composites have been described, including template method for growing metal nanoparticles and polymers into nanostructures , photochemical preparation , and electrochemical methods involving, incorporation of metal nanoparticles during the electrosynthesis of the polymer  or electrodeposition of metal nanoparticles on preformed polymer electrodes , reduction of metal salts dissolved in a polymer matrix , and incorporation of preformed nanoparticles during polymerization of monomers  or nanoparticles generated during polymerization [22, 23]. Creation of ideal reaction conditions for the simultaneous reactions (polymerization and nanoparticle formation) is a challenge. The synthesis of nanoparticle and polymer using the same reagent in aqueous solution for generating nanoparticles and polymer in the form of a composite is particularly important, as it reduces the number of steps in a complex set of sequential reactions to the formation of a composite.
Polyoxometalates are well-defined metal-oxide polyanions that can undergo stepwise and multi-electron reactions while retaining structural integrity . The introduction of polyoxometalates into conducting polymer network can be conveniently accomplished by taking advantage of the doping process of polymer leading to incorporation of charge-balancing species into the structure . The strong oxidizing potential and acidic character of Keggin type polyoxometalate, Phosphomolybdic acid (H3PMo12O40, PMo12) provides perfect environment for the polymerization of monomers such as aniline, pyrrole, or thiophene to yield corresponding polymer–polyoxometalate composites. Different conducting polymers–polyoxometalate composites have been prepared by both chemical and electrochemical routes and used for photoelectrochemical and energy storage applications [26–29], but as such, there are no reports available on the incorporation of metal nanoparticles on the PAni-PMo12 composites by using a single reagent.
The present investigation focuses on the synthesis of Au or Ag nanoparticles embedded PAni-PMo12composites (Ag-PAni-PMo12and Au-PAni-PMo12) and characterization of the formed composites. The PMo12as reagent for simultaneous oxidation of aniline and reduction of metal salts for the synthesis of nanocomposites has not been reported so far. During the oxidation of aniline, PMo12get reduced to heteropoly blue which then serve as reducing agent for the metal (Ag and Au) ions to form metal nanoparticles. The high-resolution transmission electron microscopic analysis revealed formation of metal embedded polymer nanostructures. The present method can also be extended for the preparation of various metal nanoparticles containing nanocomposites with different conducting polymers such as polypyrrole and poly(3,4-dioxy thiophene). Further, the properties of the inorganic–organic composites can be tailored by simply varying the polymer or polyoxometalate which are desired for electrocatalytic and sensor applications.
Aniline from Aldrich was distilled under vacuum prior to use. Phophomolybdic acid (H3PMo12O40, PMo12) was procured from Aldrich and used further without purification. AgNO3and HAuCl4were obtained from Sisco research laboratories and used as received. Ultrasonic treatment of the composites was performed on TOSHCON sonicator (20 KHz, 100 W), India.
Preparation of Metal Nanoparticles Embedded PAni-PMo12Composite
In a typical experiment, an aqueous solution of PMo12(50 mM, 600 μL) was added to aniline monomer (100 μL) and this led to the reduction of PMo12and oxidative polymerization of aniline. The appearance of an intense blue color due to the formation of polyoxomolybdate blue indicated the electron transfer from aniline to PMo12. To this solution, 10 mM aqueous solution of AgNO3was added and ultrasonicated for 5 min. This was then allowed to stand for 24 h. The as prepared sample (Ag-PAni-PMo12) was filtered out, washed, and dried under vacuum. Similar strategy was adopted for the preparation of Au nanoparticles by using 10 mM HAuCl4to prepare Au-PAni-PMo12composite.
UV–Visible spectra were recorded on Cary 5E UV–Vis-NIR spectrometer. FTIR investigations were performed on Perkin–Elmer 1760 in the region 2,000–400 cm−1 with 32 scans by using KBr pellet mode. Powder X-ray diffraction patterns were recorded using a SHIMADZU XD-D1 diffractometer using a Ni-filtered Cu Kα radiation (λ = 1.5418 Å at a 0.2° scan rate (in 2θ). The morphology of the composites was investigated by a scanning electron microscopy (SEM) (FEI, Model: Quanta 200). The transmission electron micrograph (TEM) analysis was performed on CM12/STEM working at a 100 kV accelerating voltage. High-resolution transmission electron microscopy (HRTEM) was carried out on a JEOL-3010 instrument operating at 300 kV. Textural characteristics of composites were determined from nitrogen adsorption/desorption at 77 K using a Micrometrics ASAP 2020 instrument. The specific surface area, average pore diameters were determined. Prior to the measurements, the samples were degassed at 423 K. The BET specific surface area was calculated by using the standard Brunauer, Emmett, and Teller method on the basis of the adsorption data. The pore size distributions were calculated applying the Barrett–Joyner–Halenda (BJH) method. For conductivity measurements the composites were pressed in a manual hydraulic press at 750 MPa into a pellet of 13-mm diameter and 0.56-mm thickness. The conductivity measurements of Au-PAni-PMo12 and Ag-PAni-PMo12 were measured by the four-point Van der Pauw method . The experimental setup included a Keithley 225 current source and Agilent 34401 voltmeter.
Results and Discussion
In conclusion, a simple method has been introduced to prepare Ag and Au nanoparticle containing organic–inorganic nanocomposites of PAni and PMo12 using the excellent electron transfer capability of polyoxometalates. PMo12has served dual role in the formation of the nanocomposites; it served as oxidizing agent for the polymerization of aniline and reducing agent for the formation of metal nanoparticles. In particular, the synthesized nanocomposites exhibited embedded metal nanoparticles in the polymer matrix. Furthermore, the method can be extended to the synthesis of other conducting polymers and opens up a new route to prepare inorganic–organic nanocomposites with wide variation of properties. It should also stimulate the exploration of applications of these nanocomposites especially in fields such as sensors, catalysis, and composite materials.
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