Oxidation of copper in the presence of the Organic Metal polyaniline
J. Posdorfer , B. Wessling
Ormecon Chemie GmbH & Co. KG, Ferdinand-Harten-Str. 7, D-22949 Ammersbek, Germany
Recieved 15 July 2000
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Abstract
By immersing copper in a water dispersion of the Organic Metal (OM) polyaniline (PAni) a thin layer is adsorbed at the surface forming a Cu(I)-PAni complex. XPS analysis showed only Cu(I) signals. Upon aging, the PAni treated Cu surface shows an exponentially decreasing Cu2O layer, and an increase of CuO with Ö t. An acid etched Cu surface shows a linear increase of both Cu2O and CuO. The PAni treatment passivates Cu. The role of polyaniline in the passivation process of copper is discussed and kinetic data are presented.
Keywords: Polyaniline, copper oxides, chronopotentiometric reduction, SEM images
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1. Introduction
During storage, unprotected copper surfaces are oxidized, leading to the formation of Cu2O and later to a mixture of Cu2O and CuO. By coating copper with organic agents (imidazole, benzotriazole or PAni) the surface can be protected [1-3]. When treated with a dispersion of PAni, copper is covered with a thin film and the corrosion rate of the metal is reduced [4-6]. Upon aging at high temperatures, various colors are developing. We studied the mechanism of PAni-Cu interaction. Results are presented on the formation of Cu(I)-complexes, Cu(I) and Cu(II)-oxides with polyaniline and without any surface protection.
2. Experimental
The copper foil (99.999% Goodfellow) used was polished using 6 µm, 3 µm and 1 µm diamond paste. It was rinsed with ethanol and cut into 1.5 x 1.5 cm pieces. Copper was etched for 1 min in 1 molar sulfuric acid, rinsed with deionized water and a) air dried or b) pretreated by immersion in a water dispersion of OM (0.01% and 3% PAni, Ormecon) for 1 min, rinsed with water and air dried. They were annealed in a convection oven (Nabertherm) at 155°C for different hours.
Electrochemical experiments were performed using a potentiostat/galvanostat (EG&G, model 263A) computer controlled by an IEEE-488 GPIB interface board. For chronopotentiometric reduction of copper oxide films a current of -100 µA/cm2 was applied. The electrochemical reduction was performed in a borate buffer at a pH of 8.5 in a 50 ml glass cell with three 14.5/23 standard tapers. The copper foil served as working electrode, a platinum wire as counter electrode and a Ag/AgCl (3 mol/l KCl) as reference electrode, all mounted with taper joints.
Surface morphology and appearance were determined by scanning electron microscopy using a Philips XL30 SEM.
3. Results and discussion
Aging of copper pretreated with the OM dispersion in comparison with sulfuric acid etched plates causes the color to change with annealing time from various violet and bluish tones into silver and bright golden. This is due to interference of light between the upper and lower thin oxide layers. Varying oxide layer thickness causes different colors. The copper oxide thickness was measured by sequential electrochemical reduction [7-9]. The electric current applied is directly proportional to the oxide mass, provided the reduction current and the depleted area remain constant.
A relationship between color and oxide layer thickness was found. The change of oxide layer thickness for Cu2O and CuO is shown in Fig. 1.
Fig. 1a and 1b. Formation of Cu oxide layers depending on aging time.
The growth of copper oxides on surfaces etched with acids is proportional to time as expected for oxidation of metals in the gas phase. The PAni pretreated surface behaves different. The formation of CuO is proportional to Ö t while Cu2O decreases exponentially to a minimum of 0.6 nm. From the slope of a CuO thickness vs. Ö t plot, the rate constant for the formation of CuO was calculated.
Rate constants at different temperatures were determined for both sample sets, summarized in Table 1. The slope of an Arrhenius plot of rate constant vs. 1/T gives an activation energy of 35 kJ/mol for CuO formation. Without PAni an activation energy of 50 kJ/mol was calculated.
The Ö T dependence occurs because the formation of CuO is a diffusion limited reaction: copper is oxidized by PAni, the copper ions are transported through the film and form a Cu(I)-PAni complex; by diffusion Cu(I) ions are
Table 1
Rate constants for CuO formation with and without OM (PAni).
With PAni |
Without PAni |
|||
T [°C] |
k [nm s-1/2] |
k [nm s-1] |
||
100 |
0.044 |
0.00012 |
||
125 |
0.199 |
0.00095 |
||
155 |
0.363 |
0.00175 |
||
175 |
0.428 |
0.00223 |
||
200 |
0.537 |
0.00507 |
transported to the film-gas interface undergoing partial oxidation to Cu(II) easily because of the lower activation energy.
Fig.2. SEM image of golden copper/PAni surface after 2 h at 155 °C.
Analysis of the surface morphology and appearance by using SEM shows that an uniform copper oxide layer is formed on the PAni treated sample (Fig 2). The untreated copper is very rough (Fig. 3).
Fig. 3. SEM image of copper oxidized by annealing at 155 °C for 2 h.
4. Conclusion
During immersion of Cu in a dispersion of PAni a Cu(I)-PAni complex forms on the surface which determines the oxidation of the copper and the passivation of the surface. A Cu2O layer with almost constant thickness is formed, the growth of the CuO layer is proportional to Ö t of aging.
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