Preparation Of Nano-Fibrous Polyaniline Via Electrochemical Method

Nowadays polyaniline (PANI) is considered as one of the promising conducting polymers owing to its favorable processibility and relative stability and hence its great variety of applications in many fields, such as electrochemical catalysis, electrochromic displays, sensors, corrosion protections, etc. PANI can be prepared by chemical oxidation which produces it in powder form, or by electrochemical oxidation which produces a PANI film adherent to certain electrode surface. Many researches indicated that the morphology and properties of the PANI produced electrochemically are influenced by the nature of the electrolytes or by the electropolymerization methods. The main results obtained in this paper are shown as follows: 1. Polyaniline films were deposited on various metallic electrodes by pulse galvanostatic method (PGM). The chronopotentiograms of aniline polymerization in 0.3 mol L–1 aniline + 1 mol L–1 HNO3 aqueous solution and the anodic polarization curves of various metals in monomer–free HNO3 aqueous solution indicated that PANI films can rapidly grow on inert metals platinum (Pt) and ruthenium (Ru) which are stable in HNO3. Although stainless steel (SS), aluminum (Al), titanium (Ti) and zirconium (Zr) all remain passive state in HNO3, PANI films can only form on SS and Al after the electropolymerization potentials reach a relatively high value for a period of time. Ti and Zr cannot be employed for aniline electrosynthesis by PGM, due to the fact that their potentials cannot maintain steady state within the electropolymerization process. Active metals such as Pb, Cu and Ag, which can undergo dissolution before the electropolymerization potential of the monomer is reached, are not suitable for aniline electrodeposition. The scanning electron microscope (SEM) images demonstrated that the PANI films prepared by PGM on Pt, Ru, SS and Al all exhibit a similar fibrous morphology with a diameter of 80 – 100 nm and many pores and gaps exist among the fibers. Nevertheless, the results of cyclic voltammograms (CV) and electrochemical impedance spectroscopies (EIS) showed that polyaniline films on various substrates presented different electrochemical reactivity in aniline–free 1 mol L–1 HNO3 aqueous solution. 2. The growth of nano–fibrous polyaniline film deposited on stainless steel by pulse galvanostatic method in 0.3 mol L–1 aniline + 1 mol L–1 HNO3 aqueous solution at 1 mA cm–2 was studied. The electrochemical behavior during the polymerization and the morphologies of the PANI films with various deposition charge were characterized by chronopotentiogram and scanning electron microscope, respectively. Moreover, the rate of aniline electropolymerization was determined by the cyclic voltammetry technique. The results showed that the polymerization of aniline on SS involved two separate stages. Firstly, polyaniline grew on bare SS electrode and represented granular structure. In this stage, the potential of aniline polymerization was relatively positive (at ca. 1.1 V). After about 30 s, the surface of the SS electrode was entirely covered by a thin PANI film, where the polymer grew further. Finally, the polymerization potential maintained ca. 0.75 V and nano–fibrous PANI film formed. 3. Polyaniline was deposited on Ti electrode from aqueous solution containing 0.3 mol L?1 aniline and 1 mol L?1 HNO3 by pulse potentiostatic method (PPM). Scanning electron microscope observation indicates that the polyaniline films prepared on Ti substrate exhibited nano-fibrous structure and therefore possessed high porosity and large specific surface. The growth of the PANI film was studied via chronoamperogram and SEM. Two different stages of growth were observed, the first corresponding to the formation of a compact granular layer, the second corresponding to the growth of a nano-fibrous structure. The influences of the synthesis parameters, such as anodic pulse duration ( t a), cathodic pulse duration ( t c), lower limit potential ( Ec ) and upper limit potential ( Ea ), on the morphology and electrochemical properties of the PANI films have been investigated. The electroactivity of the PANI film strongly depends on the morphology, which is determined by preparation conditions. At the following conditions, t a = 0.8 s, t c = 0.1 s, E c = 0 V and Ea = 1.0 V, the high quality nano-fibrous PANI film with the best electrochemical properties can be obtained. 4. The growth processes of PANI on Pt, Zr and Al electrodes were similar to that of PANI on SS electrode. However, the catalytic capacity of different substrates for aniline polymerization in initial stage is different and in the following descending order: Pt > Ti and Zr > Al. The kinetics rule of the nucleation and growth of the PANI was studied based on the analysis of chronoamperograms during aniline polyrization. The results showed that the PANI growth follows 2-D progressive mechanism at initial stage, producing granular morphology, and then transfers into 3-D progressive mechanism after the whole electrode surface was covered by a thin compact granular PANI film, producing fibrous morphology.