Electrical and magnetic properties of polyaniline/Fe₃O₄ nanostructures

Abstract

We report on electrical and magnetic properties of polyaniline (PANI) nanotubes (∼150 nm in diameter) and PANI/Fe₃O₄ nanowires (∼140 nm in diameter) containing Fe₃O₄ nanoparticles with a typical size of 12 nm. These systems were prepared by a template-free method. The conductivity of the nanostructures is 10⁻¹–10⁻² S/cm; and the temperature dependent resistivity follows a ln ρ∼T^−1/2 law. The composites (6 and 20 wt% of Fe₃O₄) show a large negative magnetoresistance compared with that of pure PANI nanotubes and a considerably lower saturated magnetization (M_s=3.45 emu/g at 300 K and 4.21 emu/g at 4 K) compared with the values measured from bulk magnetite (M_s=84 emu/g) and pure Fe₃O₄ nanoparticles (M_s=65 emu/g). AC magnetic susceptibility was also measured. It is found that the peak position of the AC susceptibility of the nanocomposites shifts to a higher temperature (>245 K) compared with that of pure Fe₃O₄ nanoparticles (190–200 K). These results suggest that interactions between the polymer matrix and nanoparticles take place in these nanocomposites.

Introduction

Recently, conducting polymers such as polyaniline (PANI), polypyrrole have attracted considerable attention especially from technical interest due to their novel properties (e.g., good environmental stability, high conductivity, easy processing, light weight, low cost and so on) and potential applications in electromagnetic radiation shielding [1], [2], microwave absorbent [3] et al. A series of methods such as template synthesis [4], [5], electrospinning [6] have been reported for synthesizing micro-/nanotubes of conducting polymers. Wan et al. have proposed a template-free method to prepare tubular PANI and polypyrrole, and reported that the resulting PANI nanotubes can obviously increase electromagnetic loss at microwave frequency [3], [7], [8], [9].

Since nanocomposites often exhibit improved chemical and physical properties over their single-component counterparts, in order to realize the full potential of technological applications of inorganic nanomaterials and conducting polymers, a large number of articles have been published reporting on the polymeric nanocomposites, i.e., PANI or polypyrrole composites containing nanoparticles such as TiO₂ [10], [11], ZrO₂ [12], γ-Fe₂O₃ [13], [14], [15], [16], Fe₃O₄ [17], [18], [19], [20], [21], SnO₂ [22], etc. Multi-component conducting polymer systems with nanoparticles of metal oxides can be tailored to exhibit, besides novel electrical, magnetic and optical properties, also good film forming and processing properties. For example, Dey et al. [11] reported that TiO₂/PANI nanocomposites show a large dielectric constant. Embedding Fe₃O₄ particles into PANI matrix could improve the composite thermal stability [21]. Polymeric nanocomposites have potential applications in electrochromic device [23], nonlinear optical systems [24], light emitting diodes [25], photodiodes and photovoltaic solar cells [26], gas and vapor sensors [27], and magnetic storage materials [28]. Several good review articles have been written on the synthesis, characterization and applications of the polymeric nanocomposites (e.g. see Refs. [29], [30]).

In recent years, electrical properties such as electrical conductivity [13], [14], [15], [19], [20] and magnetic properties such as magnetization [13], [15], [16], [17], [18], [19], [20], [30] of iron oxide–polymer composites have been reported extensively. For instance, Gangopadhyay et al. [14] have reported temperature-dependent conductivity and thermoelectric power for Fe₂O₃/polypyrrole composites. Superparamagnetism was also observed at room temperature in polymer nanocomposites containing Fe₃O₄ or γ-Fe₂O₃ nanoparticles [13], [19], [30]. However, magnetoresistance (MR) and AC susceptibility of the composites especially in the form of nanowires have not been intensively reported yet.

In this article, β-naphthalene sulfonic acid (NSA)-doped PANI nanotubes and PANI/Fe₃O₄ nanorods were prepared by the template-free method. The electrical conductivity, magnetization and AC magnetic susceptibility as a function of temperature and magnetic field have been studied. The results indicate there could be physical interactions between the polymer matrix and embedded Fe₃O₄ nanoparticles.