Elsevier

Synthetic Metals

Volume 159, Issues 7–8, April 2009, Pages 561-567
Synthetic Metals

Nanocomposites of polyaniline, its derivatives and platinum prepared using aqueous Pt sol

https://doi.org/10.1016/j.synthmet.2008.11.019Get rights and content

Abstract

Polyaniline (PANI), poly(o-methoxyaniline) (POMA) and poly(o-toluidine) (POT) have been used as matrices for incorporation of Pt nanoparticles from the previously prepared aqueous Pt sol. Polymer–Pt nanocomposites thus obtained have been characterized by X-ray diffraction, transmission electron microscopy and IR spectroscopy. It has been revealed that metal nanoparticles (predominantly of sizes in the range between 2 and 4 nm) have been present in all the composites. It can be postulated that the polymers prevent Pt nanoparticles from agglomeration, i.e. act as their stabilizers. As has been found, charge transfer interactions between polymer chains and Pt nanoparticles are responsible for this stabilization. Among the polymers studied, POT shows the weakest stabilizing effect.

Catalytic isopropyl alcohol conversion has shown that PANI–Pt nanocomposite obtained using aqueous Pt sol exhibits strong redox properties. Its overall catalytic activity is ca. three times higher than that of PANI–Pt composite containing larger Pt particles. This is connected with better dispersion of Pt particles in the composite prepared using aqueous Pt sol.

Introduction

Nanostructured materials exhibit very interesting properties, different from those shown by bulk systems which are characterized by significantly smaller surface-to-volume ratios. Physico-chemical properties of nanometric objects are influenced not only by their size but also by their size distribution. It has been reported, for example, that luminescent spectra of nanocrystalline A(II)B(VI) type semiconductors can be tuned in a wide range by changing the mean size of the nanocrystals. Full width at half maximum of the spectral line is, in turn, a function of the size distribution and it reaches ca. 20 nm for the fractions with this parameter not exceeding 4% [1].

Preparation of nanocrystalline materials is not simple. The process should be conducted in the conditions which limit the growth of the crystals upon their formation and prevent them from subsequent agglomeration. Since the surface of nanocrystals is usually very reactive, it is often necessary to apply stabilizing agents. They coat the particles and thus do not allow their coalescence. In the case of transition metal nanoparticles this stabilization is frequently achieved by using polymers. Poly(N-vinyl-2-pyrrolidone) (PVP) has been widely applied as stabilizer for nanoparticles of e.g. Ag [2], [3], Pt [4], [5], [6], [7], Rh [7], [8], Au [9], [10], Co [11], and Ni [12], [13]. Stabilizing effect of PVP can be attributed to the interactions between its electron donor atoms (nitrogen and oxygen) and surface metal atoms of the nanoparticles. The involvement of PVP carbonyl groups in such interactions has been well documented [7], [13].

Conjugated polymers, whose molecules contain spatially extended π electron system, are extremely promising stabilizers for metal nanoparticles. Therefore intensive research work on the preparation of nanometric metals dispersed in these polymer matrices has been carried out in recent years. Polythiophene–Au [14], Pd [15], polypyrrole–Au [16], Ag [17], polyaniline–Pd [18], [19], and Au [20], [21] nanocomposites have been obtained and their optical [16], sensing [19], [21] and catalytic [15], [18] properties investigated.

Among noble metals, platinum holds a special position since it is a very efficient redox catalyst. Polyaniline (PANI) is, on the other hand, an environmentally stable, easy to prepare conjugated polymer. Hence, a large number of studies presented in the literature concern PANI–Pt composites. In most cases they have been obtained electrochemically, with only a few exceptions describing their chemical preparation [22], [23], [24], [25].

In our previous paper [26] we have shown that Pt nanoparticles embedded in PANI can be prepared chemically using two methods. The first one involves reduction of Pt4+ ions conducted in the presence of the polymer. Another one takes the advantage of basic properties of PANI which make it possible to incorporate Pt-containing anions into the polymer via protonation. Their reduction results in the formation of Pt nanoparticles dispersed in PANI. We have observed, however, that in both methods a strong tendency of the metallic particles towards agglomeration exists. Therefore in the present studies we have chosen a different procedure to obtain Pt nanoparticles dispersed in PANI matrix. First, we have prepared aqueous Pt sol containing small (mainly in the range of 1–3 nm) metallic particles which have been subsequently incorporated into PANI. As it is demonstrated, such procedure allows to obtain PANI–Pt nanocomposites in which metallic nanoparticles whose sizes correspond to those found in the sol are present. Catalytic properties of the obtained PANI–Pt nanocomposites are illustrated by investigations of isopropyl alcohol conversion.

Moreover, in the present work we show that similar procedure can be applied to prepare PANI derivatives, namely poly(o-toluidine) (POT) and poly(o-methoxyaniline) (POMA)–Pt nanocomposites. It should be noted that, in comparison with the parent polymer, significantly less attention has been paid so far to the studies of PANI derivatives–Pt composites. POT–Pt [27] and POMA–Pt [28] composites have been prepared electrochemically and their electrocatalytic properties studied. Due to improved processability of POT and POMA with respect to PANI, application of these polymers as matrices for metallic Pt particles may be advantageous, particularly in the cases when a definite shape of the composite material is needed. Chemical method, on the other hand, allows to prepare large amounts of the material.

It can be postulated that in the composites obtained in the present work, PANI, POMA and POT play the role of the stabilizing agents for metallic Pt nanoparticles.

Section snippets

Experimental

PANI, POMA and POT have been synthesized by oxidation of aniline, o-methoxyaniline and o-toluidine, respectively with ammonium peroxydisulphate carried out according to the similar protocol. Thus, the reactions were performed in 2 mol/dm3 HCl aqueous solution, at 0 °C for 4 h. Molar ratio of the oxidant to aniline or its derivatives in all the systems was equal to 0.25. PANI, POMA and POT hydrochlorides thus obtained were then deprotonated in 0.3 mol/dm3 aqueous ammonia solution for 48 h. The final

Pt sol

For the preparation of Pt sol in the present study the “citrate method” has been chosen since it allows to obtain in a simple way aqueous dispersions of metallic Pt nanoparticles which are stable for a long time [29]. In this procedure, sodium citrate plays a double role. It is a reducing agent for Pt4+ ions initially present in the solution and a stabilizer for metallic Pt particles formed in the reduction process. After the reaction, excess of sodium citrate and other ionic species are

Conclusions

Based on the investigations that have been carried out in the present work the following conclusions can be drawn:

  • 1.

    PANI, POMA and POT–Pt composites containing metal nanoparticles can be prepared using aqueous Pt sol.

  • 2.

    The polymers play the role of stabilizers for Pt nanoparticles in the composites. This stabilization is achieved by charge transfer interactions between the polymer backbones and metal nanoparticles resulting in withdrawal of electrons from the polymer chains.

  • 3.

    Polymer–Pt

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