Electrochemical nucleation and growth of poly-N-Methylpyrrole on copper M.A. Raso a, ⁎, M.J. González-Tejera a , I. Carrillo b , E. Sanchez de la Blanca a , M.V. García a , M.I. Redondo a a Dpt. Química Física I. Fac. CC.Químicas. Univ. Complutense. 28040, Madrid, Spain b Dpt. Química Industrial y Polímeros, E.U.I.T. Industrial, Univ. Politécnica de Madrid, 28012, Madrid, Spain abstract article info Article history: Received 3 February 2010 Received in revised form 16 November 2010 Accepted 17 November 2010 Available online 27 November 2010 Keywords: Poly-N-Methylpyrrole Electropolymerization Nucleation electrocrystallization theory Conducting polymers The potentiostatic electropolymerization of poly-N-Methylpyrrole on Cu at 1.5 V, 2.0 V and 2.5 V vs saturated calomel electrode potentials has been carried out to establish the successive steps of polymer coating generation. The chronoamperograms obtained allow the different steps of the nucleation and growth mechanism to be analyzed by fitting the experimental data to the mathematical equations established in the electrocrystallization theory. In all cases a mechanism consisting of a two-dimensional growth at short time followed by a three-dimensional diffusion controlled growth and at longer times a subsequent two- dimensional growth mechanism, with progressive nucleation, is suggested. Scanning electron microscopy micrographs support this mechanism. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The generation of a new phase in a polarisable solid/liquid interface can be controlled from the beginning by electrochemical methods. These methods allow the formation rate of critical nuclei to be determined either by a charge transfer or by a diffusion mechanism and the different type of nucleation (instantaneous or progressive) [1]. The process is quite complicated and some approaches have been described in the literature in the absence or not of overlapped phenomena [2]. On the other hand, conducting polymer coatings have been recently used as corrosion inhibitors on different industrial substrates, such as iron [3–6], steel [5–11], mild steel [12–14], stainless steel [15– 20], copper and copper alloys [21–24]. To obtain the polymer coatings, different electrochemical techniques are used. Galvanostatic tech- nique is one of the most often used. In the potential–time curves two stages are usually registered. The first one corresponding to the induction time, related with the possible dissolution of the substrate and the second one attributed to the polymeric phase formation [4,6,8,9,11,14]. However in some systems no measurable induction time is found, which means that no substrate dissolution takes place before the beginning of the electropolymerization process [15]. Potentiodynamic methods yield knowledge about the different oxidation states of the metal substrate [24] and about the passivation of the electrode but no information about the nucleation mechanism is provided. Potentiostatic technique implies that a constant potential is applied and the nuclei as discrete centres start to grow when they exceed a critical size on the adsorbed polymeric layer producing a maximum current in the current density–time (j–t) transients. This technique allows studying all the nucleation and growth processes of the conducting polymers in the different regions of the j–t transient [25–32]. In this study the potentiostatic method is applied to analyze the nucleation and growth mechanism (NG) process and to establish the electrogeneration mechanism of poly-N-Methylpyrrole onto Cu in an oxalic acid electrolyte medium (Cu/PNMPy/Ox). It has been tested by Fourier Transform Infrared spectroscopy that insoluble Cu(I) and Cu (II) oxalates were formed before the NMPY polymerization on Cu electrode in oxalic acid medium [33]. This layer which adheres to the electrode is conducting enough to allow the PNMPy deposition. Up to date no data devoted to the nucleation mechanism of this polymer coating on this metal have been reported. 2. Experimental details Electropolymerization of N-Methylpyrrole (NMPy) was carried out in an unstirred single compartment, three-electrode cell at room temperature. The working electrodes were 0.39 cm 2 copper rods embedded in an epoxy resin inside a Teflon™ holder, the auxiliary electrode was a platinum wire and the reference electrode was saturated KCl calomel (SCE). Working electrodes were polished to a smooth surface using 800 and 1200 grit size emery paper rinsed with acetone in an ultrasonic bath, cleaned with distilled water and dried in air. The auxiliary electrode was washed with hot concentrated sulphuric acid, rinsed copiously with distilled water and dried in air. The monomer N-Methylpyrrole (Aldrich Chemical 99%) was distilled under vacuum before use. A monomer concentration of 0.1 mol L -1 in an aqueous electrolytic medium 0.2 mol L -1 oxalic acid (ox 2-hidrate PRS, Quimivita) (Ox), pH = 1.5, was used. Thin Solid Films 519 (2011) 2387–2392 ⁎ Corresponding author. Tel.: +34 91 3944280; fax: +34 91 3944135. E-mail address: marg@quim.ucm.es (M.A. Raso). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.11.020 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf