Copper Alloys: Preparation, Properties and Applications : Preparation, Properties and Applications [1 ed.] 9781620815441, 9781612095042

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Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Copper Alloys: Preparation, Properties and Applications : Preparation, Properties and Applications, Nova Science Publishers, Incorporated, 2011.

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Copper Alloys: Preparation, Properties and Applications : Preparation, Properties and Applications, Nova Science Publishers, Incorporated, 2011.

MATERIALS SCIENCE AND TECHNOLOGIES

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COPPER ALLOYS: PREPARATION, PROPERTIES AND APPLICATIONS

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MATERIALS SCIENCE AND TECHNOLOGIES

COPPER ALLOYS: PREPARATION, PROPERTIES AND APPLICATIONS

MICHAEL NABOKA AND

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JENNIFER GIORDANO EDITORS

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Library of Congress Cataloging-in-Publication Data Copper alloys : preparation, properties, and applications / editors, Michael Naboka and Jennifer Giordano. p. cm. Includes bibliographical references and index. ISBN:  (eBook)

1. Copper alloys--Metallurgy. I. Naboka, Michael. II. Giordano, Jennifer. TN693.C9C67 2011 669'.3--dc22

2010051521

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CONTENTS Preface Chapter 1

Chapter 2

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Chapter 3

Chapter 4

vii Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode I. Danaee, M. Jafarian, F. Forouzandeh, F. Gobal and M. G. Mahjani Electrodeposition of Copper Based Alloys and Multilayer for Giant Magneto Resistance Applications N. Rajasekaran and S. Mohan Development and Characterization of Nickel-Copper-Based Substrates for 2nd Generation High-Critical Temperature Superconducting Tapes Angelo Vannozzi and Giuseppe Celentano Magnetoresistance Effects in Cu and Co, Co/Cu Multilayers and Granular Cu-Co Alloys and in Rapidly Quenched Gd-Doped Cu–Co Alloys Jacek Jaworski and Eric Fleury

1

45

67

89

Chapter 5

Mechanical Properties of Copper Under Dynamic Load G. G. Savenkov

107

Chapter 6

Copper Alloy Microstructures for Application in Microreactors Juan Manuel Zamaro and Eduardo Miró

127

Chapter 7

Composite Electrodeposits With C60 Fullerene V. N. Tseluikin

141

Chapter 8

Influence of Alloying with Ni, Al, Zn, Sn and Ag on the Anneal Hardening Effect in Sintered Copper Base Alloys Svetlana Nestorovic

151

High Strength Copper-Based Conductor Materials J. Freudenberger

159

Chapter 9

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vi Chapter 10

Chapter 11

Contents New Materials for Technological Applications from the TiCrCu Ternary System A. Provino, P. Manfrinetti, D. Mazzone, C. Bernini, C. Boffito, A. Corazza and A. Coda

213

Chemical Mechanical Polishing of Copper Alloys Yong X. Gan, Ioan D. Marinescu and Xuesong Han

247

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Index

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259

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PREFACE This new books presents current research in the study of the preparation, properties and applications of copper alloys. Topics discussed include the electrodeposition of copper based alloys for giant magneto resistance applications; the mechanical properties of copper under dynamic load; copper alloy micro-structures for application in microreactors and high strength copper-based conductor materials. Chapter 1- Nickel-copper alloy modified glassy carbon electrodes (GC/NiCu) prepared by galvanostatic deposition were examined for their redox process and electrocatalytic activities towards the oxidation of methanol and glucose in alkaline solutions. The methods of cyclic voltammetery (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. The cyclic voltammogram of NiCu alloy demonstrates the formation of β/β crystallographic forms of the nickel oxyhydroxide under prolonged repetitive potential cycling in alkaline solution. In CV studies, in the presence of methanol NiCu alloy modified electrode shows a significantly higher response in oxidation reaction. The peak current of the oxidation of nickel hydroxide increase is followed by a decrease in the corresponding cathodic current in presence of methanol and glucose. The anodic peak currents show linear dependency with the square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the CA regime the reaction followed a Cottrellian behavior and the diffusion coefficient of methanol and glucose was found to be 2 × 10-6 and 1 × 10-5 cm2 s-1 in agreement with the values obtained from CV measurements. The impedance behavior show different patterns, capacitive, and inductive loops and negative resistances, at different applied anodic potential. The influence of the electrode potential on the impedance pattern is studied and a quantitative explanation for the impedance behavior of oxidation reaction is put forward by a proposed mathematical model. The conditions required for the reversing of impedance pattern are delineated with the use of the impedance model. The previously proposed electrooxidation mechanism on GC/NiCu electrode was found to reproduce the experimental impedance plots. Chapter 2 - Copper based alloys and multilayer which exhibits the giant magneto resistance effect, have been the subject of numerous studies. They have great potential for technological applications, such as magneto resistive sensors and magnetic recording devices. These types of materials are produced by electrodeposition. GMR effect is more usually seen in multilayer and alloys structure, when two magnetic layers are closely separated by a thin non-magnetic spacer layer. The first magnetic layer allows electrons in only one spin state to

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viii

Michael Naboka and Jennifer Giordano

pass through it easily. If the second magnetic layer is aligned than that spin channel can easily pass through the structure, and the resistance is low. If the second magnetic layer is misaligned then the neither spin channel can get through the structure easily and the resistance is high. The minimum requirement for the GMR effect is the existence of interface between the magnetic and non-magnetic materials. This chapter deals with the review of literature available on electrodeposition of copper based alloys and a multilayer for giant magneto resistance application. Electrodeposition of some copper based alloys and multilayer will be discussed in detail. The effect of thickness of magnetic, non-magnetic layers and bi-layers are presented. Chapter 3 - The employment of cuprate superconductors such as YBa2Cu3O7-δ (YBCO) for power application requires the realization of quasi-single crystal wires in order to avoid depression of supercurrent caused by weaks links originated by high-angle grain boundaries. One solution is the epitaxial deposition of μm-thick films on flexible, biaxially textured metal tapes known as coated conductors. The proper crystallographic orientation {001}, i.e. cube orientation, can be induced in fcc metals with a medium-high stacking fault energy by cold-rolling to a high deformation degree followed by high-temperature recrystallization treatment. Ni and Ni-alloys were thoroughly studied during the past 15 years. In particular Ni-W alloys are presently successfully employed as textured substrates. More recently, copper and nickel-copper alloys tapes were investigated as alternative substrates for YBCO coated conductors, in order to overcome the possible limitations due to the ferromagnetic losses and at the same time reduce the cost of the raw materials. In this chapter, the authors report about the recent developments of ternary nickel-copper-based substrates, since the introduction of a third element lead to a stable recrystallization texture with respect to the binary Ni 50 at% Cu (Ni-Cu) alloy, which has been taken as parent alloy. In particular, two systems have been studied. The first is the Ni-Cu-Wx alloy, with x = 0.5-3 at%, which shows a Curie temperature below 22 K. For x > 2 at% the alloy is inhomogeneous and a cube texture is formed for x < 1 at%. The other is the Ni-Cu-Co3 alloy, which shows a Curie temperature as high as 155 K and develops a sharp cube texture. For this alloy also Ni:Cu atomic ratios 2:3 and 3:2 were studied. Cu-poor alloy has a sharp cube texture development and a Curie temperature as high as 275 K, besides Cu-rich alloy has a depressed Curie temperature to 60 K but develops a less sharp cube texture. An application of YBCO film deposition to test the suitability of Ni-Cu-based substrates for the development of YBCO coated conductor is reported. Chapter 4 - The chapter is a compendium of the current knowledge on the magnetoresistance effect (MR) in pure copper metal and alloys. Several variants of MR effect are described such as the ordinary MR (OMR) connected with the Lorenz force and Kohler‘s rule for pure copper and also copper doped a little by cobalt, the anisotropic MR (AMR), domain wall MR (DWMR) observed in pure cobalt and cobalt doped by copper atoms, and the giant MR (GMR) measured for Cu – Co multilayer systems with and without pin-holes, as well as granular alloys with either coarse or fine structures. The present chapter also reports new results of a study on the effect of the structure on the MR properties of Gd-doped Co–Cu granular alloys prepared by two different processing techniques. The dependence of the MR properties of Cu metal and alloys with the structure, composition and temperature is presented with an emphasis on the processing method. A new type of transistor taking advantage of the GMR effect of one of its elements is also described.

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Preface

ix

Chapter 5 – Measurements and valuations have shown: at the submicro, micro and millisecond range of dynamic loading breaking stress is significantly exceeds the various degree of purity polycrystal copper static limit of strength. In this work dynamic tensile strength values are comparing with possible limiting ones and one can notice that the dynamic tensile strength values don‘t surpass 7 percent of the limited ones that were estimated on a minimum on the volume compressibility curve. The shock adiabatic of copper was used as the volume compressibility curve and the limiting value of stress equal  th  23 GPa. and was found from  th   0 c0 / 4b where b

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– the factor of linear equation D  c0  bu of the shock adiabatic [30]. Chapter 6 - Microreactors are catalytic systems that allow reaching high mass and heat transfer rates. They exhibit advantages to be applied in highly exo and endothermic reactions that are hard to manipulate with powder or pellet catalysts. The microreactor core is made up of shaped microstructured supports having a thin catalytic film deposited onto the wall surface of their small channels. The heat transfer behavior and subsequent performance of these systems could be further improved if highly conductive materials are used as supports. Copper alloys are low-cost materials, with high thermal conductivity and high thermal diffusivity, easy to handle and commercially available within a broad range of geometrical configurations. All these characteristics make them interesting materials to be used in microreactors. In this work, the authors discuss the preparation of copper alloy microstructures covered with thin zeolite films as catalytic material, in various supporting geometries as plates, microchannels and microgrids. Zeolite synthesis variables are studied and the synthetic procedures are adapted to each support geometry, which allow obtaining thin, homogeneous and adherent zeolite coatings. The obtained zeolite/copper alloy microstructures have a great potential to be applied as catalytic units in microreactors for highly exo-endothermic gas phase reactions. Chapter 7 - A method for obtaining composite electrodeposits (CED) with C60 fullerene is proposed. The process of deposition of CED nickel–fullerene C60 under potentiostatic and galvanostatic conditions is studied. The structure and composition of deposits has been determined by means of secondary-ion mass spectrometry. It is shown that fullerene species are hydrogenated during the deposition. The corrosion and electrochemical behavior of CED nickel – fullerene C60 in 0.5 M H2SO4 is studied with a help of potentiodynamic method. Sliding friction coefficients of fullerene-containing nickel coatings are measured. Chapter 8 - This chapter reports results of investigation carried out on sintered copper alloys: Cu-8at%Ni, Cu- 8at %Zn, Cu-8at%Al, Cu-8at%Ag, Cu-4at%Ni-4at%Sn, Cu-4at%Ni4at%Zn and Cu-4at%Au. For comparison, investigation carried out on the sintered copper. The sintered alloys, as well as pure copper, were subjected to the same thermomechanical treatment. Thermomechanical treatment included cold rolling of 30,50 and 70% in reduction. Isochronal and isothermal annealing up to the recrystallization temperature were performed, followed with hardness and electrical conductivity measuring. This investigation shows that hardness and electrical conductivity of cold deformed copper-alloys increase after annealing in the temperature range of 160 - 400°C, due to anneal hardening effect. Electrical conductivity decreases with increasing the amount of alloying elements. During the annealing the electrical conductivity slowly increases for both set of the samples copper and alloys due to recovery and recrystallization.

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Michael Naboka and Jennifer Giordano

Chapter 9 - Materials with apparently conflictive combination of materials properties, such as high conductivity and strength are facing growing interest. Their availability is claimed in a wide scope of application. On the one hand there are microelectromechanical units in which the energy absorption, which is caused by mechanical loading, has to be raised beyond the present limits. Thus, the efficiency can be increased, while the power durability is warranted even at high energy. On the other (macroscopic) side highly strengthened conductors with a good formability are required as e.g. for the windings in non-destructive high field magnets which are operated in a pulsed mode. The mechanical and physical properties of the conductors is strongly related to their microstructure. Thus, the control of the microstructure is a crucial item when adjusting an optimised combination of materials properties. Due to its high thermal and electrical conductivity Copper is the ideal candidate to form alloys and materials that serve as conductor. Alloying bears the potential to apply several hardening mechanisms to enhance the strength. A special interest is paid to the increase of the strength due to the formation of precipitates. In contrast to dispersion strengthened materials they face a higher strengthening potential. However, they are less thermal stable and their potential can be used at low temperatures, only. Nevertheless, if the temperature of the conductor can be kept at or below room temperature these conductors are highly recommended to be used. The possibility to adjust the microstructure by thermal and mechanical treatments builds the basis to develop highly strengthened conductor materials, which materials properties have to be fitted to the case of applications within a broad range of application. This article reviews the properties of age hardenable, highly strengthened Copper based conductor materials that are developed for applications at room temperature and below. Chapter 10 - The phase equilibria in the TiCrCu system have been investigated by means of metallographic analysis, local electronprobe microanalysis and Xray powder diffraction and the isothermal crosssection at 800C was drawn. About the terminal solubilities, it has been found that βTi dissolves both Cr and Cu up to a large composition ranges (up to Ti80Cr14Cu6), while the addition of Ti makes possible either to Cr to be dissolved in the fcc lattice of Cu (up to Ti05Cr05Cu88100) or Cu to be dissolved in Cr (up to Ti04Cr95100Cu02.5). The known binary phases form large solubility fields upon addition of the third element; their composition ranges have been determined. A new ternary compound is formed in this system: Ti3(Cr,Cu)5. The authors have found it crystallizes in the tetragonal Ti3Pd5 prototype (tP8, P4/mmm) [from a = 3.134(1) Å and c = 11.222(4) Å to a = 3.120(1) Å and c = 11.248(1) Å]; no other isotypic binary or ternary compounds have never been reported in literature up to now. In this phase the Cr content, rising up to a maximum value of about 12.5 at. %, brings to a stoichiometry of 3:1:4 (that is the exact composition Ti3CrCu4); furthermore, Xray data suggest a ternary ordered variant structure for this compound. The section is characterized by the existence of large binary fields. Lattice parameters for nearly all the phases, either the pseudobinaries or the ternary, and their trends with the Cr(Cu) atomic content, were also determined. Some of the physical properties were moreover measured on two alloys, selected as being more representatives: Ti2.5Cr0.5Cu97 (fcc Cu solid solution) and Ti37.5Cr12.5Cu50 (Ti3CrCu4 ternary phase). Chapter 11 - This chapter provides a brief overview on the chemical mechanical polishing (CMP) of copper alloys. The first part presents the commonly used copper CMP with hydrogen peroxide as the based slurry. The mechanisms by which removal and

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xi

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planarization occur during the chemical mechanical polishing of copper are discussed. Both the dissolution of copper and the formation of a surface layer on the CMP specimens will be examined in view of the fundamentals of chemistry. The second part deals with the CMP of copper using various complex agents such as amino acid and/or oxidizers including fenicyanide ion and nitrate ion plus corrosion inhibitors. Finally, recent advances in slurryfree CMP of copper using electric rheological fluids will be discussed.

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In: Copper Alloys: Preparation, Properties and Applications ISBN 978-1-61209-504-2 Editor: Michael Naboka and Jennifer Giordano © 2011 Nova Science Publishers, Inc.

Chapter 1

ELECTROCATALYTIC OXIDATION OF METHANOL AND GLUCOSE ON NICU ALLOY ELECTRODE I. Danaee1, M. Jafarian2, F. Forouzandeh2, F. Gobal3 and M. G. Mahjani2 1

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Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran 2 Department of chemistry, K. N. Toosi University of Technology, Tehran, Iran 3 Department of chemistry, Sharif University of Technology, Tehran, Iran

ABSTRACT Nickel-copper alloy modified glassy carbon electrodes (GC/NiCu) prepared by galvanostatic deposition were examined for their redox process and electrocatalytic activities towards the oxidation of methanol and glucose in alkaline solutions. The methods of cyclic voltammetery (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. The cyclic voltammogram of NiCu alloy demonstrates the formation of β/β crystallographic forms of the nickel oxyhydroxide under prolonged repetitive potential cycling in alkaline solution. In CV studies, in the presence of methanol NiCu alloy modified electrode shows a significantly higher response in oxidation reaction. The peak current of the oxidation of nickel hydroxide increase is followed by a decrease in the corresponding cathodic current in presence of methanol and glucose. The anodic peak currents show linear dependency with the square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the CA regime the reaction followed a Cottrellian behavior and the diffusion coefficient of methanol and glucose was found to be 2 × 10 -6 and 1 × 10-5 cm2 s-1 in agreement with the values obtained from CV measurements. The impedance behavior show different patterns, capacitive, and inductive loops and negative resistances, at different applied anodic potential. The influence of the electrode potential on the impedance pattern is studied and a quantitative explanation for the impedance behavior of oxidation reaction is put forward by a proposed mathematical model. The conditions

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2

I. Danaee, M. Jafarian, F. Forouzandeh et al. required for the reversing of impedance pattern are delineated with the use of the impedance model. The previously proposed electrooxidation mechanism on GC/NiCu electrode was found to reproduce the experimental impedance plots.

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1. INTRODUCTION Electrocatalytic processes involving the oxidation of organic compounds are of great interest in many areas, ranging from medical applications to wastewater treatment and from the construction of biological fuel cells to analytical applications in the food industry [1,2]. Fuel cell has recently received a good deal of attention for both mobile and stationary applications [3,4]. Methanol and glucose as fuel has numerous advantages such as simple operation and ease of fuel storage and distribution. However, compared to the hydrogen based fuel cells, direct methanol fuel cell (DMFC) still remains to be further developed. One of the problems still unsolved is the slow kinetics of methanol oxidation on the fuel cell‘s anode [2]. Considerable efforts have been directed towards the study of methanol electro-oxidation at high pH. The use of alkaline solutions in a fuel cell has many advantages such as increasing its efficiency [1,5], a wider selection of possible electrode materials, a higher efficiency of both anodic and cathodic processes, almost no sensitivity to the surface structure [6] and negligible poisoning effects in alkaline solutions were observed [7,8]. In the electrochemical oxidation of organic compounds, the electrode material is clearly an important factor where a highly efficient electrocatalyst is needed. As described previously [9-14], a considerable increase in power density and fuel utilization was obtained by optimizing different components of fuel cells. Pure Cu electrodes have demonstrated activity for the anodic oxidation of carbohydrates in alkaline media. However, the corresponding response mechanism remains somewhat controversial. It has been suggested that a Cu(II)/Cu(III) redox couple at the surface of the anodized Cu electrodes works through an electron-transfer mediated mechanism for carbohydrate oxidation [15,16]. Pt-binary electrodes were commonly used as a catalyst for the electrochemical oxidation of methanol. As catalysis is a surface effect, the catalyst needs to have the highest possible surface area. Therefore, carbon-supported electrodes are generally used as catalyst [17], such as Pt–Ru and Pt–Ru–P/carbon nano-composites [18], Pt/Ni and Pt/Ru/Ni alloy nanoparticles [19]. It is well established that nickel can be used as a catalyst due to its surface oxidation properties. Many electrodes involving nickel have been used as catalysts in fuel cells. Ni has commonly been used as an electrocatalyst for both anodic and cathodic reactions in organic synthesis and water electrolysis [20-23]. One of the very important uses of nickel as a catalyst is for the oxidation of alcohols. Several studies of the electro-oxidation of alcohols on Ni have been reported [24,25]. Taraszewska and Roslonek [26] found that glassy carbon/Ni(OH)2 modified electrode acts as an effective catalyst for the oxidation of methanol. Most oxidizable organic compounds were found to oxidize at the same potential and this potential coincided exactly with that at which the surface of the nickel anode becomes oxidized [25-29]. Van Effen and Evans [30] found that the oxidation of ethanol in KOH solution involved the formation of a higher valent nickel oxide, which acts as a chemical oxidizing agent. This fact was confirmed by both cyclic voltammetry and impedance spectroscopy [31].

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

3

The study of alloy electrodes is motivated primarily from the anticipation of a synergistic electrocatalytic benefit from the combined properties of the component metals of alloys. Furthermore, the use of preanodized alloy electrodes offered the advantage of ease of preparation and long-term stability in comparison with thermally prepared and electrolytically deposited mixed-oxide film electrodes. Kuwana and coworkers investigated the alloy electrodes in their studies of carbohydrate reaction at Ni-based alloys containing high percentages of Cu and Cr [32,33]. One promising characteristic of such alloy electrodes is the resistance to corrosion in the alkaline media. However, semi-conductive oxide films, e.g. Cu2O and NiO, if formed might cause a slowly increasing IR drop at the electrode surface as the thickness of these oxide films increases with time [34,35]. The addition of small amounts of alloying elements to Cu has been demonstrated to provide increased electrocatalytic activity in comparison to the pure Cu electrode. For example, the presence of 5% Mn to Cu, denoted as Mn5Cu95, shows a much improved electrochemical activity for the oxidation of glucose in alkaline media in comparison to that of the pure Cu electrode [36]. The fact that pure Ni and Cu metals have the same facecentered cubic structure with similar lattice parameters (a = 3.523 for Ni and 3.616 for Cu) makes it possible to have a wide range of composition for Ni-Cu alloys. Numerous papers have described chemical and physical properties of Ni-Cu alloys. An excellent review was presented by Khulbe et al. on the behavior of Ni-Cu alloys in a variety of catalytic mechanisms including hydrogenation reactions, ortho-para hydrogen conversion and H2/D2 exchange reaction [35]. Electrochemical impedance spectroscopy (EIS) is a good tool to analyze the kinetics of electrode reactions. The advantage of EIS over DC techniques is that this steady-state technique is capable of probing relaxation phenomena over a wide frequency range. The measured impedance can be presented in the form of imaginary vs. real parts at various measurement frequencies, Nyquist plots, which appear as a multitude of semicircles and lines [37-40]. Equivalent electrical circuits capable of generating the same impedance plots in response to a potential stimuli are used for the interpretations that associate kinetics and transport properties with the circuit elements. Often, discrepancies and ambiguities hamper the analysis [38]. Many equivalent circuits can show the same impedance characteristics and supplementary data and chemical intuition help to select the most relevant one. Also, it may be difficult to find electrochemical equivalent to some electrical circuit elements and vice versa [41-42]. Inductive loops are often difficult to account for and are related to desorptive generation of sites for the charge transfer processes of electroactive constituents [41-44]. There has been a recent resurgence of interest in the studies of the negative impedance behavior in electrochemical systems like electrocatalysis, electrodeposition and electrodisolution [45,46]. Although the mechanistic origin of the negative impedance is quite different for various systems (desorption from a catalyst, adsorption of an inhibitor, double layer repulsion), they are all described by essentially the same mathematical model whose properties are clearly insensitive to the precise chemical origin of the negative dIF(E)/dE with E and IF are the applied potential and Faradaic current. The purpose of the present work is to study the electrochemical oxidation of methanol and glucose on a nickel-copper alloy modified glassy carbon electrode and the analysis of reaction mechanism of the electrooxidation of methanol and glucose in NaOH solution by impedance spectroscopy dominated by negative time constants. The analysis of the theoretical impedance function provides important information on the kinetic parameters.

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2. MATERIALS AND METHODS Sodium hydroxide, nickel sulfate, sodium citrate, methanol and glucose used in this work were from Merck (Darmstadt, Germany), products of analytical grade and were used without further purification. Doubly distilled water was used throughout. Electrochemical studies were carried out in a conventional three electrode cell powered by an electrochemical system comprising of an EGandG model 273 potentiostat/galvanostat and Solartron model 1255 frequency response analyzer. The system is run by a PC through M270 and M389 commercial software via a GPIB interface. The frequency range of 100 kHz to 15 mHz and the modulation amplitude of 5 mV were employed for impedance measurements. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of home written least squares software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance [47,48]. A dual Ag/AgCl-saturated KCl electrode, a Pt wire and a glassy carbon (GC) disk electrode were used as the reference, counter and working electrodes, respectively. All studies were carried out at 298 ± 2 K. The GC disk electrode supplied by EGandG was further polished with 0.05mm alumina powder on a polishing microcloth and rinsed thoroughly with doubly distilled water prior to modification. Films of nickel were formed on the GC surface by galvanostatic deposition from a solution composed of 0.7 M NiSO4.6H2O + 0.26 M C6H5Na3O7.2H2O for Ni deposition at the current density of 10 mA cm-2 and for 300 s and addition of 0.05 M CuSO4.5H2O for NiCu alloy deposition. The working electrode was placed in the middle of the electrolyte and the electrolyte was stirred with a magnetic stirrer during electrodeposition. The surface morphology of the deposit was evaluated by metallographic microscope Neophot 32. The chemical composition of the deposit was evaluated by scanning electron microscopy (SEM, Philips XL30) equipped with energy dispersive X-ray (EDX) facilities. Structural investigation was conducted by the X-ray diffraction (XRD) method using a Philips (Xpert) diffractometer and Cu Ka radiation. AFM imaging was performed with a thermo microscope autoprobe CP-Research from Veeco Society.

3. RESULTS AND DISCUSSION 3.1. Surface Analysis of Electrodeposited NiCu Alloy The metallographs presented in figure 1a and b display the morphology of the electrodeposited Ni and NiCu alloy. Smooth surfaces are observed for the electrodeposited Ni and NiCu alloy. Figure 1c and d presents the scanning electron micrographs showing the cross-section and the average thickness of the deposited films varying between 5 and 7 μm for Ni and NiCu alloy. The films are fairly compact with virtually no pores and cavities. The compactness and crystalline nature of the films are also seen in AFM images of Ni and NiCu deposited layer, figure 1e and f. The result of chemical composition analysis obtained by EDX revealed that the composite contains 79% Ni and 21% Cu. The compositions are given in weight percents, figure 1g.

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

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Figure 1. (Continued). Copper Alloys: Preparation, Properties and Applications : Preparation, Properties and Applications, Nova Science Publishers, Incorporated, 2011.

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

7

Figure 1. Metallograph of electrode surface after electrodeposition of (a) Ni and (b) NiCu alloy from a solution composed of 0.7 M NiSO4 + 0.26 M C6H5Na3O7.2H2O for Ni deposition at the current density of 10 mA cm-2 and for 300 s and addition of 0.05 M CuSO 4 for NiCu alloy deposition. Cross section and the thickness of (c) Ni and (d) NiCu deposited layer by SEM. AFM images of electrode surface after electrodeposition of (e) Ni and (f) NiCu alloy. (g) EDX results of the chemical composition of NiCu alloy at the surface. (h) X-ray diffractogram (XRD) of electrodeposited NiCu alloy, in the range 10°k6 and v5>v6. In this case    i

   0 

 i ,   E

   0 and thus  

 I F  i    i  E

   0. 

The capacitive arc at the intermediate frequencies in the Nyquist plot flips to the second quadrant with the real component of the impedance becoming negative. This means that passivation of the electrode surface has occurred [79]. The EIS data can also be simulated using the equivalent circuit of figure 12. Melnick et al. [80] indicated that the passivation of the Pt electrode during methanol electrooxidation is probably due to the reversible formation of oxide species. Meanwhile, with reaction (9) being the rate determining step the oxidation of intermediates with Ni3+ is much slower than the generation of intermediates by reaction (8) and the passivation at higher potentials can be explained by the formation of a large amount of intermediates on the surface of the catalyst. Therefore, adsorption of methanol is inhibited due to an increase of the coverage of intermediates and the electro-oxidation rate shows almost no significant increase. As can be seen from the simulation of impedance plots with equivalent circuit (Table 2), it is observed that with the other elements remaining positive, the only parameter that will cause the reversal of the impedance pattern to the second quadrant is R2, the value of which is determined by the electrode potential. At potentials lower than 0.55 V R2 decreases with the increase of applied anodic potential but at higher potential R2 jumps to very negative values. Further increase of potential will lead to a decrease in the absolute value of R2. By comparing the potential dependence of the simulated impedance pattern, it is found that, if R2 is positive, the impedance will show a pattern similar to figure 14a,b. If R2 is negative, the reversing impedance pattern, as in figure 14c, can be observed. The other element in the equivalent circuit remains positive and decreases with increasing applied potential. So from the analyses of EIS, reaction (9) as the rate-determining step at high potential range can well explain the experimental results. The negative resistance in impedance plots for GC/Ni is not observed due to the lower catalytic activity of GC/Ni in electro-oxidation of methanol caused by the fact that Eq. (8) was slower. Assuming Fleischmann‘s mechanism for the electrooxidation of methanol no negative resistance was obtained in the derivation of the impedance equation. Therefore Fleischmann‘s mechanism is not complete for the electro-oxidation of methanol on Ni and Ni alloy surfaces. The theoretical impedance diagrams obtained according to our proposed electro-oxidation mechanism are in agreement with the negative resistance observed in the experimental impedance plots; therefore the proposed mechanism is a complete mechanism for the electrooxidation of methanol on GC/Ni and GC/NiCu electrodes.

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

29

3.4. Electro-Oxidation of Glucose

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Figure 18 shows cyclic voltammograms of GC/Ni and GC/NiCu electrode in 1 M NaOH solution in the presence of 5 mM glucose at a potential sweep rate of 10 mV s-1. As can be seen in 5 mM glucose, GC/NiCu electrode generates higher current density for the electrooxidation in NaOH solution due to higher surface concentration of β-NiOOH form. Figure 19 shows cyclic voltammograms of GC/NiCu electrode in the absence (1) and presence (2) of various concentrations of glucose at a potential sweep rate of 10 mV s-1. At GC/NiCu electrode, oxidation of glucose appeared as a typical electrocatalytic response. The anodic charge increased with respect to that observed for the modified surface in the absence of glucose, and it was followed by decreasing the cathodic charge upon increasing the concentration of glucose in solution. In the presence of 7 mM glucose with the potential sweep rate of 10 mV s-1, the charge associated with the anodic peak was 99% of that of the corresponding cathodic peak, while in the absence of glucose, this ratio was 58%. The anodic current in the positive sweep was proportional to the bulk concentration of glucose, and any increase in the concentration of glucose caused an almost proportional linear enhancement of the anodic current (figure 19b). Moreover, in the presence of glucose, the onset potential of the Ni(II) moiety oxidation shifted to positive value and enhanced upon increasing the concentration of glucose. In fact, this indicated a strong interaction of glucose with the surface already covered by low valance nickel species. Therefore, catalytic electrooxidation of glucose on GC/NiCu seems to be certain. The electrocatalytic oxidation of glucose occurs not only in the anodic but also continues in the initial stage of the cathodic half cycle. Glucose molecules adsorbed on the surface are oxidized at higher potentials parallel to the oxidation of Ni(II) to Ni(III) species. The anodic current passes through a maximum as the potential is anodically swept.

Figure 18. Cyclic voltammograms of the (1) GC/Ni and (2) GC/NiCu electrode in 1M NaOH solution in the presence of 5 mM of glucose in the solution. Potential sweep rate was 10 mV s -1.

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Figure 19. (a) Cyclic voltammograms of the GC/NiCu electrode in 1M NaOH solution in the presence of (1) 0 mM; (2) 1 mM; (3) 3 mM; (4) 5 mM; (5) 7 mM (6) 9 mM of glucose. Potential sweep rate was 10 mV s-1. (b) Dependency of the anodic peak current on the concentration of glucose in solution.

In the reverse half cycle, the oxidation continues, and its corresponding current goes through a maximum due to the regeneration of active sites for the adsorption of glucose as a result of removal of adsorbed intermediates and products. Surely, the rate of glucose oxidation as signified by the anodic current in the cathodic half cycle drops as the unfavorable cathodic potentials are approached. In addition, continuous cycling of GC/NiCu in the presence of glucose showed that glucose reacted with the surface, and no poisoning effect on the surface was observed. Glucose is oxidized on the modified surface via the following reaction v2 Ni3+ +glucose   Ni2+ +intermediate

(26)

v3 Ni3+ +intermediate   Ni2+ +products

(27)

v4 Ni3+ -glucose   Ni3+ -intermediate+e

(28)

v5 Ni3+ -intermediate   Ni3+ -products+e

(29)

Observation of a new oxidation peak for glucose oxidation at a potential much more positive than that of the oxidation of Ni(OH)2 potential is according to Eqs. (28) and (29). Gluconolactone [81] as well as formats and oxalates [82] have been reported as the oxidation products.

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

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Figure 20. Repeated cyclic voltammograms of 3 mM glucose oxidation on GC/NiCu electrode at 10 mV s-1, cycle number: 1, 10, 20, 50, 100, 150.

Figure 20 shows the cyclic voltammograms of GC/NiCu electrode in 1 M NaOH in the presence of 3 mM glucose at a scan rate of 10 mV s-1 for 150 cycles. It is observed that the current density of the electro-oxidation of glucose is almost constant in 150 cycles, signifying the stability of electrocatalyst. Cyclic voltammograms of GC/NiCu in the presence of 5 mM glucose at various potential sweep rates and the proportionality of anodic peak currents to the square root of sweep rates in a range of 2 to 2000 mV s-1 are illustrated in figure 21a,b, respectively. The cathodic peak was not observed in low scan rates, but appeared upon increasing the sweep rate. This reveals that the oxidation of glucose on Ni may belong to a slow process. In higher scan rates, a new oxidation peak was observed for glucose oxidation at a potential much more positive than that of the oxidation of Ni(OH)2 potential. Meanwhile, the anodic peak currents that are linearly proportional to the square root of scan rate (figure 21b) suggest that the overall oxidation of glucose at this electrode is controlled by the diffusion of glucose to the surface redox sites. Moreover, a plot of the so-called current function, I/v1/2, with respect to the scan rate (figure 21c), exhibited a typical shape of an electrochemical-chemical (EC′) catalytic process [68]. The value of electron transfer coefficient for the reaction which is totally irreversiblediffusion-controlled can be obtained as 0.8 through equation (4). Figure 22a shows double step choronoamperograms for the GC/NiCu in the absence (1) and presence (2–6) of glucose over a concentration range of 1 to 9 mM with applied potential steps of 560 and 320 mV, respectively. Plotting of net current with respect to the inverse of the square roots of time, after removing the background current, presents a linear dependency (figure 22b). The dominance of a diffusion-controlled process is evident. Using the slope of this line in Cottrell equation the diffusion coefficient of glucose has been obtained to be 1× 10-5 cm2 s-1, which is in good agreement with value reported in the literature [83]. The current is also negligible when potential is stepped down to 320 mV, indicating the irreversibility of glucose oxidation process. From the slope of the Icat/IL vs. t1/2 plot, presented in figure 22c,

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32

I. Danaee, M. Jafarian, F. Forouzandeh et al.

the mean value of k for the concentration range of 1 to 9 mM of glucose was obtained as 5.8×105 cm3 mol-1 s-1.

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Figure 21. (a) Typical cyclic voltammograms of the GC/NiCu in1 M NaOH in the presence of 5 mM glucose at various potential sweep rates of 2, 10, 50, 75, 100, 200, 350, 500, 750, 1000, 1250 and 1500 mV s-1. (b) Dependence of anodic peak current during the forward sweep on the square roots of sweep rate. (c) The anodic current function (I/ν1/2) vs. potential sweep rate v. (d) Dependence of the peak potential on lnν for the oxidation of glucose at GC/NiCu electrode.

Figure 22. (a) Double steps chronoamperograms of GC/NiCu electrode in 1 M NaOH solution with different concentrations of glucose of: (1) 0 mM, (2) 1 mM, (3) 3 mM, (4) 5 mM, (5) 7 mM and (6) 9 mM. Potential steps were 560 mV and 320 mV, respectively. (b) Dependency of transient current on t1/2 . (c) Dependence of Icatal/IL on t1/2 derived from the data of chronoamperograms of 1 and 3 in panel (a). (d) Plot of sampled transient current at fixed time of 10 s vs. glucose concentration.

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

33

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Figure 23. Nyquist diagrams of GC/NiCu electrode in different concentration of glucose in 1 M NaOH: (1) 7, (2) 9, and (3) 11 mM. DC potential is 0.55 V vs. Ag/AgCl. Inset: high frequency part of the impedance diagram.

Figure 22d shows the plot of sampled current at fixed time (10 s) with respect to the concentration of glucose. A good linearity has been witnessed, and a limit of detection of 0.8 μM has been obtained from three times the standard deviation of the blank per the slope of calibration curve [84]. Figure 23 shows the Nyquist diagrams of GC/NiCu electrode recorded at the oxidation peak potential for glucose concentrations in the range of 7-11 mM. The Nyquist diagrams consist of three slightly depressed overlapping semicircles in high, medium and low frequency. The depressed semicircle in high frequency region can be related to the combination of charge transfer resistance and the double layer capacitance. It is observed that the charge transfer characteristic appear in the first quadrant, two loops in medium and low frequencies are located in the second, third, and forth quadrants. Figure 24 shows Nyquist plots of the impedance of glucose electrooxidation at different potentials in 9 mM glucose. At 0.48 V vs. Ag/AgCl, two large depressed capacitive semicircle are observed, figure 24a, revealing a slow reaction rate of glucose oxidation. The semicircle in the high frequency side is due to charge transfer and the one at the low frequency end is due to the adsorption of the intermediates. Bode phase plots are shown in figure 25a where two well resolved peaks are observed pointing to two depressed and overlapping semicircles in Nyquist plot. The equivalent circuit compatible with the Nyquist diagram is depicted in figure 12. To corroborate equivalent circuit the experimental data are fitted to equivalent circuit and the magnitudes of the circuit elements are obtained. Table 4 illustrates the equivalent circuit parameters derived from the impedance spectra of glucose oxidation. In the potential range of 0.5-0.52 V, a pseudoinductive behavior was observed, figure 24b, where the large semicircles at high and medium frequencies are terminated to a small arc in the forth quadrant at low frequency and with all the diameters decreasing sharply with

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I. Danaee, M. Jafarian, F. Forouzandeh et al.

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increasing potential. This inductive behavior is due to the relaxation phenomenon characteristics of the generation of further active sites upon the desorption of the intermediates [75,76] and further adsorption of electroactive constituents, glucose, on active sites.[76] Negative phase angle also is observed in the corresponding bode phase shift, figure 25b. As potential arrives at 0.53 V vs. Ag/AgCl, a change in the shape of impedance plots happens where the two loops in the medium and low frequencies reversing to the second, third and forth quadrants presumably due to the passivation of electrode surface [75,77] as shown in figure 24c. The phase sift, figure 25c, of experimental impedance data shows an abrupt jump between the positive and negative values of phase angle, indicating the change of the rate determining step of elecrooxidation of glucose on GC/NiCu in potential range 0.470.55 V.

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

35

Figure 24. Experimental Nyquist diagrams as a function of applied potential for glucose electrooxidation on GC/NiCu electrode in 9 mM glucose: (a) at potentials (1) 0.47, (2) 0.48, and (3) 0.49 V vs. Ag/AgCl; (b) at potentials (1) 0.5, (2) 0.51, and (3) 0.52 V vs. Ag/AgCl; (c) at potentials (1) 0.53, (2) 0.54, and (3) 0.55 V vs. Ag/AgCl.

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Table.4 Equivalent circuit parameters of electrooxidation of 9 mM glucose on GC/NiCu electrode in NaOH solution obtained from figure 24 and 25 E (V) 0.42 0.45 0.47 0.49 0.52 0.55 0.59 0.62 0.64

Rs (Ω) 12 12.5 12.1 12.2 12 12 12.2 11.9 12.1

R1 (Ω) 120 85 62 49 42 35 18 12 8

Q1×104 (F) 4 5 5.2 6.9 8 8.2 9 10 11.1

R2×10-2 (Ω) 12.1 8 6.1 4.9 3.5 2.7 -2.5 -2 -1.6

Q2×104 (F) 5.1 5.95 7 8.9 9.5 10.8 10.9 11 12.9

R3×10-2 (Ω) 8 6.2 4.8 1.7 1.2 0.9

L×10-3 (H) 6.1 7.1 4.3 0.7 0.5 0.3

n1

n2

0.88 0.85 0.84 0.9 0.85 0.86 0.87 0.84 0.88

0.92 0.93 0.93 0.93 0.9 0.94 0.96 0.95 0.92

The equivalent circuit compatible with the Nyquist diagram recorded in potential range of 0.5-0.55 V is depicted in figure 16. Table 4 illustrates the equivalent circuit parameters for the impedance spectra of glucose oxidation in different applied potential. As previously mentioned the first depressed semicircle in the entire potential range is due to charge transfer resistance and the inductive behavior appearing in higher potentials is due to the regeneration of active sites for the adsorption of glucose. The EIS results indicate that the glucose electrooxidation on GC/NiCu catalyst at various potentials shows different impedance behaviors. On the basis of reaction mechanism and equation (25) and the kinetic parameters are determined by fitting the model equation to the experimental Nyquist diagram (Table 5). The

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I. Danaee, M. Jafarian, F. Forouzandeh et al.

corresponding coverage‘s of Ni3+ and the intermediates as a function of potential can be obtained from the model calculation and are presented in figure 26. With the increase of potential, Ni3+ and the intermediates coverage‘s rise. Also Ni3+ coverage approach unity in higher applied anodic potential. So the impedance behaviors of glucose electrooxidation in different potential ranges can be categorized as: (i) At low potential region (0.48 V), assuming reaction (28) is rate-determining step and

 I F   i

then: 

 F  v4 v5    0 ,     0 and the impedance diagram should show capacitive qi  E E  

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behavior with two overlapped capacitive semicircles appearing in the Nyquist plot and signifying a reaction with one adsorbed intermediate [76].

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

37

Figure 25. Experimental phase shift plots as a function of applied potential for glucose electrooxidation on GC/NiCu electrode in 9 mM glucose: (a) at potentials (1) 0.47, (2) 0.48, and (3) 0.49 V vs. Ag/AgCl; (b) at potentials (1) 0.5, (2) 0.51, and (3) 0.52 V vs. Ag/AgCl; (c) at potentials (1) 0.53, (2) 0.54, and (3) 0.55 V vs. Ag/AgCl.

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Table 5. Values of rate constant calculated from equation 25 and figure 24 for electrooxidation of 9 mM glucose on GC/NiCu electrode in NaOH solution k1o×1012 (mol cm-2 s-1) 4.9

k-1o×105 (mol cm-2 s-1) 2

k2×108 (mol cm-2 s-1) 4.1

k3×107 (mol cm-2 s-1) 2.5

k4o×1010 (mol cm-2 s-1) 3.5

k5o×106 (mol cm-2 s-1) 1.6

Figure 26. Calculated surface coverage of Ni3+ (1) and intermediates (2) for 9 mM glucose electrooxidation on GC/NiCu electrode.

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I. Danaee, M. Jafarian, F. Forouzandeh et al.

(ii) When glucose is electrooxidized at intermediate potential range (0.51 V), increasing the potential, enhances the rate of reaction (28), but not enough to exceed that of reaction (29). In this case the rate-determining step of glucose electrooxidation is in a transition region

 I 

  

 I 

F   0 ,  i   0 . According to impedance parameters  F  and and also    i    i   E 

 i   E 

  , inductive behavior in glucose electrooxidation reveals that the coverage‘s of the  

intermediates decreases with increasing potential and lead to an increase of Faradaic current. Apparently with increasing potential large amounts of Ni3+ are formed on the electrode surface and react with intermediates to decrease their coverage and also higher active site is available for reaction (28)and(29). (iii) At high potential range (0.54 V), reaction (29) can be assumed as rate-determining

 I F   i

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step. In this case 

   0 

 i    0 and thus   E 

, 

 I F  i    i  E

   0 . The capacitive arc 

at the intermediate frequencies in the Nyquist plot will flips to the second quadrant with the real component of the impedance becoming negative. This means that passivation of electrode surface has occurred [79]. Meanwhile, with reaction (29) being the rate-determining step the oxidation of intermediates with Ni3+ is much slower than the generation of intermediates by reaction (28) and the passivation at higher potentials can be explained by the formation of a large amount of intermediates on the surface of the catalyst. Adsorption of glucose is inhibited due to an increase of the coverage of intermediates and the electrooxidation rate shows almost no significant increase. Therefore the proposed mechanism is a complete mechanism for the electrooxidation of glucose on GC/NiCu electrode and is in agreement with negative resistance observed in the experimental impedance plots.

CONCLUSION The nickel oxide film was formed electrochemically on electrodeposited nickel-copper alloy in a regime of cyclic voltammetry on a glassy carbon electrode and tested for electrooxidation of methanol and glucose in alkaline media. The addition of copper to the electrodeposited nickel is a very effective method of suppressing the formation of γ-NiOOH species during prolonged cycling processes in alkaline medium. Also from repeated cycling observed that the overpotential for O2 evolution increases for NiCu alloy modified electrode. The modified electrodes showed electrocatalytic activity for the oxidation of methanol at around 650 mV vs. Ag/AgCl, while the glassy carbon electrode presents no activity. More specifically, the response for electro-oxidation at the NiCu alloy modified electrode is significantly larger than the response obtained for pure electrodeposited Ni. Double steps choronoamperograms for electrocatalytic oxidation show irreversible process and the dominance of a diffusion controlled process is evident. The diffusion coefficient of methanol

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Electrocatalytic Oxidation of Methanol and Glucose on NiCu Alloy Electrode

39

and glucose has been obtained to be 2 × 10-6 and 1 × 10-5 cm2 s-1 in agreement with cyclic voltammetry. Electrochemical impedance studies of oxidation reaction on GC/NiCu electrode demonstrate the potentialities of this method as a tool for investigating the mechanism of electrooxidation. Different impedance patterns are observed for Ni and NiCu alloy electrode. Electrooxidation of glucose and methanol on GC/NiCu show negative resistance in impedance plots. The impedance data recorded at different potentials are analyzed and show evidence for two processes occurring at the interface: one is associated with methanol electrooxidation leading to the intermediates formation on the surface and the other is assigned to the oxidation of intermediates. A theoretical impedance model based on kinetics is proposed which captures and explains all of the features of potential dependence of experimental impedance. The impedance behaviors in different potential regions reveal that the mechanism and rate-determining step in electrooxidation vary with potential. At low potential region, first oxidation step is rate-determining, while at higher potentials, the oxidation and removal of adsorbed intermediates became rate-determining step. Meanwhile, at intermediate potential, the rate-determining step in electrooxidation is in transition region. The theoretical impedance diagrams obtained according to our proposed electrooxidation mechanism are in agreement with the experimental impedance plots.

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Parsons, R; Vander Noot, T; The oxidation of small organic molecules: a survey of recent fuel cell related research. J. Electroanal. Chem., 1988, 257, 9-45. Jarvis, TD; Stuve, EM. In: Lipkowski J, Ross P, editors. Electrocatalysis. Wiley-VCH; 1998. p. 109. Scott, K; Taama, WM; Argyropoulos, P; Engineering aspects of the direct methanol fuel cell system. J. Power Sources, 1999, 79, 43-59. Kordesch, K; Simander, G. Fuel cells and their application. Weinheim: VCH Verlagsgesellschaft; 1996. p. 151. Nishimura, K; Machida, K; Enyo, M. Electrooxidation of formate and formaldehyde on electrodes of alloys between Pd and Group IB metals in alkaline media: part II. The possibility of complete oxidation of formaldehyde in weak alkali. J. Electroanal. Chem., 1988, 251, 117-125. Morallon, E; Cases, FJ; Vazquez, JL; Aldaz, A. Irreversible adsorption of methanol on Pt(110) in carbonate solution. Electrochim. Acta, 1992, 37, 1883-1886. Biswas, PC; Nodasaka, Y; Enyo, M. Electrocatalytic activities of graphite-supported platinum electrodes for methanol electrooxidation. J. Appl. Electrochem., 1996, 26, 3035. Jafarian, M; Forouzandeh, F; Danaee, I; Gobal, F; Mahjani, MG. Electrocatalytic oxidation of glucose on Ni and NiCu alloy modified glassy carbon electrode. J. Solid State Electrochem., 2009, 13, 1171-1179.

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I. Danaee, M. Jafarian, F. Forouzandeh et al. Ren, X; Zelenay, P; Thomas, S; Davey, J; Gottesfeld, S. Recent advances in direct methanol fuel cells at Los Alamos National Laboratory. J. Power Sources, 2000, 86, 111-116. Schultz, T; Zhou, S; Sundmacher, K. Current status of and recent developments in the direct methanol fuel cell. Chem. Eng. Technol., 2001, 24, 1223-1233. Carrette, L; Friedrich, KA; Stimming, U. Fuel cells – fundamentals and applications. Fuel Cells, 2001, 1, 5-39. Arico, AS; Crety, P; Baglio, V; Modica, E; Antonucci, V. Influence of flow field design on the performance of a direct methanol fuel cell. J. Power Sources, 2000, 91, 202-209. Dohle, H; Divisek, J; Jung, R. Process engineering of the direct methanol fuel cell. J. Power Sources, 2000, 86, 469-477. Baldauf, M; Preidel, W. Experimental results on the direct electrochemical oxidation of methanol in PEM fuel cells. J. Appl. Electrochem., 2001, 31, 781-786. Zhao, J; Wang, F; Yu, J; Hu, H. Electro-oxidation of glucose at self-assembled monolayers incorporated by copper particles. Talanta, 2006, 70, 449-454. Miller, B. Split-Ring Disk Study of the Anodic Processes at a Copper Electrode in Alkaline Solution. J. Electrochem. Soc., 1969, 116, 1675-1680. Antolini, E. Formation of carbon-supported PtM alloys for low temperature fuel cells: a review. Mater. Chem. Phys., 2003, 78, 563-573. King, WD; Corn, JD; Murphy, OJ; Boxall, DL; Kenik, EA; Kwiatkowski, KC et al. Pt– Ru and Pt–Ru–P/carbon nanocomposites: synthesis, characterization, and unexpected performance as direct methanol fuel cell (DMFC) anode catalysts. J. Phys. Chem., 2003, 107, 5467-5474. Park, KW; Choi, JH; Kwon, BK; Lee, SA; Sung, YE; Ha, HY et al. Chemical and electronic effects of Ni in Pt/Ni and Pt/Ru/Ni alloy nanoparticles in methanol electrooxidation. J. Phys. Chem., 2002, 106, 1869-1877. Wen, TC; Lin, SM; Tsai, JM. Sulphur content and the hydrogen evolving activity of NiSx deposits using statistical experimental strategies. J. Appl. Electrochem., 1994, 24, 233-238. Fan, C; Piron, DL; Sleb, A; Paradis, P. Study of electrodeposited nickel–molybdenum, nickel–tungsten, cobalt–molybdenum, and cobalt–tungsten as hydrogen electrodes in alkaline water electrolysis. J. Electrochem. Soc., 1994, 141, 382-387. Raj, IA; Vasu, KI. Transition metal-based hydrogen electrodes in alkaline solution – electrocatalysis on nickel based binary alloy coatings. J. Appl. Electrochem., 1990, 20, 32-38. Casadei, MA; Pletcher, D. The influence of conditions on the electrocatalytic hydrogenation of organic molecules. Electrochim. Acta, 1988, 33, 117-120. Berchmans, S; Gomathi, H; Prabhakara Rao G. Electrooxidation of alcohols and sugars catalysed on a nickel oxide modified glassy carbon electrode. J. Electroanal. Chem., 1995, 394, 267-270. Fleischmann, M; Korinek, K; Pletcher, D. The oxidation of organic compounds at a nickel anode in alkaline solution. J. Electroanal. Chem., 1971, 31, 39-49. Taraszewska, J; Roslonek, G. Electrocatalytic oxidation of methanol on a glassy carbon electrode modified by nickel hydroxide formed by ex situ chemical precipitation. J. Electroanal. Chem., 1994, 364, 209-213.

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[27] Allen, JR; Florido, A; Young, SD; Daunert, S; Bachas, LG. Nitrite selective electrode based on an electropolymerized cobalt phthalocyanine. Electroanalysis, 1995, 7, 710713. [28] Bettelheim, A; White, BA; Raybuck, SA; Murray, RW. Electrochemical polymerization of amino-, pyrrole-, and hydroxy-substituted tetraphenyl porphyrins. Inorg. Chem., 1987, 26, 1009-1017. [29] Macor, KA; Spiro, TG. Porphyrin electrode films prepared by electrooxidation of metalloprotoporphyrins. J. Am. Chem. Soc., 1983, 105, 5601-5607. [30] Van Effen, RM; Evans, DH. A study of aldehyde oxidation at glassy carbon, mercury, copper, silver, gold and nickel anodes. J. Electroanal. Chem., 1979, 103, 383-397. [31] Motheo, AJ; Machado, SAS; Rabelo, FJB; Santos Jr, JR. Electrochemical study of ethanol oxidation on nickel in alkaline media. J. Braz. Chem. Soc., 1994, 5, 161-165. [32] Marioli, JM; Luo, P; Kuwana, T. Nickel–chromium alloy electrode as a carbohydrate detector for liquid chromatography. Anal. Chim. Acta, 1993, 282, 571-580. [33] Marioli, JM; Kuwana, T. Electrochemical detection of carbohydrates at nickel–copper and nickel–chromium–iron alloy electrodes. Electroanalysis, 1993, 5, 11–15. [34] Trasatti, S; editor. Electrodes of conductive metallic oxides, part B. New York: Elsevier; 1980. [35] Khulbe, KC; Mann, RS; Manoogian, A. Behavior of nickel–copper alloy in hydrogenation, ortho hydrogen–para hydrogen conversion and H2–D2 exchange reaction. Chem. Rev., 1980, 80, 417-428. [36] Yeo, IH; Johnson, DC. Anodic response of glucose at copper based alloy electrodes. J. Electroanal. Chem., 2000, 484, 157-163. [37] Boukamp, BA. A nonlinear least squares fit procedure for analysis of immittance data of electrochemical systems. Solid State Ion., 1986, 20, 31-44. [38] Harrington, DA; Conway, BE. AC impedance of faradaic reactions involving electrosorbed intermediates I. Kinetic theory. Electrochim. Acta, 1987, 32, 1703 -1712. [39] Danaee, I; Jafarian, M; Forouzandeh, F; Gobal, F; Mahjani, MG. Electrochemical impedance studies of methanol oxidation on GC/Ni and GC/NiCu electrode. Int. J. Hydrogen Energy, 2009, 34, 859-869. [40] Danaee, I; Jafarian, M; Forouzandeh, F; Gobal, F; Mahjani, MG. Impedance spectroscopy analysis of glucose electro-oxidation on Ni-modified glassy carbon electrode. Electrochim. Acta, 2008, 53, 6602-6609. [41] Gimenez-Romero, D; Garcia-Jareno, JJ; Vicente, F. Analysis of an impedance function of zinc anodic dissolution. J. Electroanal. Chem., 2004, 572, 235-247. [42] Muller, JT; Urban, PM; Holderich, WF. Impedance studies on direct methanol fuel cell anodes. J. Power Sources, 1999, 84, 157-160. [43] Jafarian, M; Gobal, F; Danaee, I; Biabani, R; Mahjani, MG. Electrochemical studies of the pitting corrosion of tin in citric acid solution containing Cl-. Electrochim. Acta, 2008, 53, 4528-4536. [44] Hassel, BAV; Boukamp, BA; Burggraaf, AJ. Electrode polarization at the Au, O2(g)/yttria stabilized zirconia interface. Part I: Theoretical considerations of reaction model. Solid State Ion., 1991, 48, 139-154. [45] Orlik, M; Jurczakowski, R. The kinetic parameters of the electrocatalytic reduction of the coordinated azide anions found from comparison of dc and ac measurements for the tristable Ni(II)-N3- system. J. Electroanal. Chem., 2008, 614, 139-148.

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[46] Gregori, J; Garc´ia-Jaren˜o, J J; Keddam, M; Vicente, F. A kinetic interpretation of a negative time constant in impedance equivalent circuits for the dissolution/passive transition. Electrochim. Acta, 2007, 52, 7903-7909. [47] Jafarian, M; Gobal, F; Danaee, I; Mahjani, MG. Impedance spectroscopy study of aluminum electrocrystallization from basic molten salt (AlCl3–NaCl–KCl). Electrochim. Acta, 2007, 52, 5437-5443. [48] Danaee, I; Jafarian, M; Forouzandeh, F; Gobal, F; Mahjani, MG. Kinetic Interpretation of a Negative Time Constant Impedance of Glucose Electrooxidation. J. Phys. Chem. B, 2008, 112, 15933-15940. [49] Alper, M; Kockar, H; Safak, M; Celalettin Baykul, M. Comparison of Ni–Cu alloy films electrodeposited at low and high pH levels. J. Alloys Compd., 2008, 453, 15-19. [50] El-Shafei, AA. Electrocatalytic oxidation of methanol at a nickel hydroxide/glassy carbon modified electrode in alkaline medium. J. Electroanal. Chem., 1999, 471, 89-95. [51] Briggs, GWD; Snodin, PR. Ageing and the diffusion process at the nickel hydroxide electrode. Electrochim. Acta, 1982, 27, 565-572. [52] Hahn, F; Beden, B; Croissant, MG; Lamy, C. In situ UV visible reflectance spectroscopic investigation of the nickel electrode alkaline solution interface. Electrochim. Acta, 1986, 31, 335-342. [53] Desilvestro, J; Corrigan, DA; Weaver, MJ. Characterization of redox states of nickel hydroxide filmelectrodes by in situ surface Raman spectroscopy. J. Electrochem. Soc., 1988, 135, 885-892. [54] Barnard, R; Randell, CF. Studies concerning charged nickel hydroxide electrodes. VII. Influence of alkali concentration on anodic peak positions. J. Appl. Electrochem., 1983, 13, 89-95. [55] Conway, BE; Liu, TC. Experimental evaluation of adsorption behaviour of intermediates in anodic oxygen evolution at oxidized nickel surfaces. J. Chem. Soc. Faraday Trans., 1987, 83, 1063-1080. [56] Druska, P; Strehblow, HH; Golledge, S. A surface analytical examination of passive layers on Cu/Ni alloys: part I. Alkaline solution. Corros. Sci., 1996, 38, 835-851. [57] Luo, P; Prabhu, SV; Baldwin, RP. Constant potential amperometric detection at a copper-based electrode: electrode formation and operation. Anal. Chem., 1990, 62, 752755. [58] Bode, H; Dehmelt, K; Witte, J. Zur kenntnis der nickelhydroxidelektrode-I. U¨ ber das nickel (II)- hydroxidhydrat. Electrochim. Acta, 1966, 11, 1079-1087. [59] Schrebler Guzman, RS; Vilche, JR; Arvia, AJ. Rate processes related to the hydrated nickel hydroxide electrode in alkaline solutions. J. Electrochem. Soc., 1978, 125, 15781587. [60] Chen, J; Bradhurst, DH; Dou, SX; Liu, HK. Nickel hydroxide as an active material for the positive electrode in rechargeable alkaline batteries. J. Electrochem. Soc., 1999, 146, 3606-3612. [61] Singh, DJ. Characteristics and effects of γ-NiOOH on cell performance and a method to quantify it in nickel electrodes. J. Electrochem. Soc., 1998, 145, 116-120. [62] Oliva, P; Leonardi, J; Laurent, JF. Review of the structure and the electrochemistry of nickel hydroxides and oxyhydroxides. J. Power Sources, 1982, 8, 229-255.

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[63] Oshitani, M; Watada, M; Lida, T. Hydrogen and metal hydride batteries. In: Bennett, PD; Sakai, T; editors. PV 94-27. The electrochemical society proceedings series. Pennington, NJ; 1995. p. 303. [64] Luo, PF; Kuwana, T; Paul, DK; Sherwood, PMA. Electrochemical and XPS study of the nickel–titanium electrode surface. Anal. Chem., 1996, 68, 3330-3337. [65] Danaee, I; Jafarian, M; Forouzandeh, F; Gobal, F; Mahjani, MG. Electrocatalytic oxidation of methanol on Ni and NiCu alloy modified glassy carbon electrode. Int. J. Hydrogen Energy, 2008, 33, 4367-4376. [66] Laviron E. General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems. J. Electroanal. Chem., 1979, 101, 19-36. [67] Danaee, I; Jafarian, M; Mirzapoor, A; Gobal, F; Mahjani, MG. Electrooxidation of methanol on NiMn alloy modified graphite electrode. Electrochim. Acta, 2010, 55, 2093-2100. [68] Nicholson, RS; Shain, I. Theory of stationary electrode polarography. Single scan and cyclic methods applied to reversible, irreversible, and kinetic systems. Anal. Chem., 1964, 36, 706-723. [69] Fleischmann, M; Korinek, K; Pletcher, D. The kinetics and mechanism of the oxidation of amines and alcohols at oxidecovered nickel, silver, copper, and cobalt electrodes. J. Chem. Soc. Perkin Trans. 2, 1972, 10, 1396-1402. [70] Robertson, PM. On the oxidation of alcohols and amines at nickel oxide electrodes: mechanistic aspects. J. Electroanal. Chem., 1980, 111, 97-104. [71] Vertes, G; Horanyi, G. Some problems of the kinetics of the oxidation of organic compounds at oxide-covered nickel electrodes. J. Electroanal. Chem., 1974, 52, 47-53. [72] Bard, AJ; Faulkner, LR. In: Bard AJ, editor. Electrochemical methods, fundamentals and applications. New York: Wiley; 2001. p. 209 [chapter 5]. [73] Pariente, F; Lorenzo, E; Tobalina, F; Abruna, HD. Aldehyde biosensor based on the determination of NADH enzymically generated by aldehyde dehydrogenase. Anal. Chem., 1995, 67, 3936-3944. [74] Maritan, A; Toigo, F. On skewed ARC plots of impedance of electrodes with an irreversible electrode process. Electrochim. Acta, 1990, 35, 141-145. [75] Amstrong, RD; Henderson, M. Impedance plane display of a reaction with an adsorbed intermediate. J. Electroanal. Chem., 1972, 39, 81-90. [76] Seland, F; Tunold, R; Harrington, DA. Impedance study of methanol oxidation on platinum electrodes. Electrochim. Acta, 2006, 51, 3827-3840. [77] Amstrong, RD. Electrode impedance for the active-passive transition. J. Electroanal. Chem., 1972, 34, 387-390. [78] Cao, CN. On the impedance plane displays for irreversible electrode reactions based on the stability conditions of the steady-statedI. One state variable besides electrode potential. Electrochim. Acta, 1990, 35, 831-836. [79] Bagotzky, VS; Vassilyew, YB. Mechanism of electro-oxidation of methanol on the platinum electrode. Electrochim. Acta, 1967, 12, 1323-1343. [80] Melnick, RE; Palmore, GTR. Time-dependent impedance of the electro-oxidation of methanol on polished polycrystalline platinum. J. Phys. Chem. B, 2001, 105, 94499457.

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[81] Zhao, C; Shao, C; Li, M; Jiao, K. Flow-injection analysis of glucose without enzyme based on electrocatalytic oxidation of glucose at a nickel electrode. Talanta, 2007, 71, 1769-1773. [82] Mho, S; Johnson, DC. Electrocatalytic response of carbohydrates at copper-alloy electrodes. J. Electroanal. Chem., 2001, 500, 524-532. [83] Torto, N; Ruzgas, T; Gorton, L. Electrochemical oxidation of mono- and disaccharides at fresh as well as oxidized copper electrodes in alkaline media. J. Electroanal. Chem., 1999, 464, 252-258. [84] Goyal, RN; Gupta, VK; Bachheti, N. Fullerene-C60-modified electrode as a sensitive voltammetric sensor for detection of nandrolone—An anabolic steroid used in doping. Anal. Chim. Acta, 2007, 597, 82-89.

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In: Copper Alloys: Preparation, Properties and Applications ISBN 978-1-61209-504-2 Editor: Michael Naboka and Jennifer Giordano © 2011 Nova Science Publishers, Inc.

Chapter 2

ELECTRODEPOSITION OF COPPER BASED ALLOYS AND MULTILAYER FOR GIANT MAGNETO RESISTANCE APPLICATIONS N. Rajasekaran and S. Mohan Central Electrochemical Research Institute, Karaikudi-630 006, India

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ABSTRACT Copper based alloys and multilayer which exhibits the giant magneto resistance effect, have been the subject of numerous studies. They have great potential for technological applications, such as magneto resistive sensors and magnetic recording devices. These types of materials are produced by electrodeposition. GMR effect is more usually seen in multilayer and alloys structure, when two magnetic layers are closely separated by a thin non-magnetic spacer layer. The first magnetic layer allows electrons in only one spin state to pass through it easily. If the second magnetic layer is aligned than that spin channel can easily pass through the structure, and the resistance is low. If the second magnetic layer is misaligned then the neither spin channel can get through the structure easily and the resistance is high. The minimum requirement for the GMR effect is the existence of interface between the magnetic and non-magnetic materials. This chapter deals with the review of literature available on electrodeposition of copper based alloys and a multilayer for giant magneto resistance application. Electrodeposition of some copper based alloys and multilayer will be discussed in detail. The effect of thickness of magnetic, non-magnetic layers and bi-layers are presented.

INTRODUCTION In the past few years magnetic multilayers have been the subject of tremendous scientific effort. This effort is mainly based on the very interesting magnetic and electrical phenomena 

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that can be observed in these artificially layered structures, such as the oscillatory exchange coupling between magnetic layers across nonmagnetic spacer layers and the so called giant magneto resistance (GMR) effect. The GMR has attracted a great deal of attention for fundamental interest and technological applications. GMR discovered in certain Fe/Cr structures [1], and these materials have been made mostly by sputtering and molecular beam epitaxy (MBE), which require high or ultra high vacuum. For this to be observed a fair control over the deposition parameters is needed, since rough interfaces between subsequent layers can easily destroy the effect. That is why most of the samples which exhibit large coupling strengths and high MR values have been prepared by sputtering, molecular beam epitaxy (MBE), or other vacuum-based techniques [2]. However, recently electrodeposition has attracted many researchers worldwide for its successful utility as a tool in producing thin films for GMR applications. In 1993 Alper et al. [3] and Hua et al. [4] have reported giant magnetoresistance in Co-Ni/Cu multilayers grown by electrodeposition in a single electrolyte. The concept of multilayer growth by electrodeposition is not a new one [5]. The advantage of electrodeposition is over vacuum-based techniques mainly lies in the simplicity of the experimental setup, less expensive apparatus, large area of epitaxial growth, well-controlled film orientation, minimum inter-diffusion, flat individual layers and the multilayers with the individual layer thicknesses below 10Å. However, growing high quality multilayers that exhibit appreciable giant MR values still remains a challenge. Electrodeposition offers the promising possibility of growing wire-like multilayer structures with very large height-to-width aspect ratios as recently demonstrated by Blonde1 et al. [6] and Piraux et al. [7], which enables the study of the giant MR effect with the current perpendicular to the multilayer planes (the so-called CPP MR). Until recently, this could only be realized either by superconducting contacts [8], which limits the measurements to low temperatures, or by advanced micro structuring [9], which results in structures having rather poor aspect ratios compared to the electrodeposited wires. As mentioned above, high qua lity electrodeposited samples which show giant MR values comparable with those of sputtered or MBE-grown multilayers are scarce, mainly due to a lack of thickness control and growth homogeneity, making the very thin spacer-layer thickness regime of antiferromagnetic (AF) coupling (5-3 nm) not yet accessible. For the CPP MR this is less a problem than for the case where the current is flowing in the planes of the layers (the CIP geometry) because of the different length scales involved for the MR effect [10]. Reviews on GMR have been published by Fert and Bruno [13] Levy [14] and Dieny [15] covering the field upto 1994. Other reviews by Gijs and Bauer [16] Ansermet [17] Bass and Pratt [18] Fert and Piraux [19] and Gijs [20] are devoted specifically to the CPP GMR. Coehoorn [21] and by Barthelemy et al. [22] have also reported a review of GMR. The first one highlights the theoretical and experimental results, which are of particular interest for applications of spin valves in read heads. The second one discusses the nature of GMR by accenting the importance of CPP geometry and gives a full list of experimental papers. All the above reviews discusses the materials prepared by the physical techniques as well as electrochemical techniques. In addition, all the reviews are covering the research work up to 2000 only. The present review is devoted to the electrodeposition of giant magneto resistance materials. We emphasize the experimental data on GMR in magnetic multilayer, alloys and

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discuss the dependence of GMR on magnetic layer thickness, non-magnetic layer thickness and bi-layer numbers.

EXPERIMENTAL SURVEY In this section we mainly overview the experimental results of GMR materials prepared by electrodeposition. GMR was first discovered in 1988 by the group of Albert Fert on Fe/Cr magnetic multilayer [23] and the group of Peter Grunberg on Fe/Cr/Fe trilayer [24]. In both cases the samples were grown using MBE and had [001] orientation of the layers. The GMR has been observed in many multilayered structures of the form B tB /n* (F tF Å/ NM tNM Å)/C tC, in which B and C designate a buffer and a capping layer, respectively, F is a magnetic layer and NM is a non-ferromagnetic layer. tB, tF, tNM and tC refers to the thickness of the corresponding layers and n* is number of bi-layers. The amplitude of the GMR depends considerable on the pair of F and NM materials and on the thickness of the various layers. The GMR values are calculated from the following formula R H - R0 GMR (%) = ------------- X 100 R0

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where, RH – resistance in the presence of magnetic field (H) R0 – resistance in the absence of magnetic field.

EFFECT OF NUMBER OF BILAYERS Giant magneto resistance of the multilayer increases strongly with increase in the number of bi-layers [25-30]. The increase in GMR with increasing number of interfaces (increasing overall thickness) might be due to the decreasing shunting effect of the metallic substrate [3031]. The addition of more bi-layers decreases the total resistance but increases the importance of spin-dependent scattering in the film over spin- independent scattering in the substrate and thus, increases the magneto resistance [28]. Cyrille et al. [32] explained the increasing the magnitude of GMR with bi-layers number and this is due to the overall increasing of coherent interface roughness. The magnitude of this increase in GMR with bi-layer or interface number is greater than that which might be assigned to a shunting effect and so eliminates the possibility. The GMR becomes almost double when the number of layers doubled [28]. Whatever the reason is it is clear that GMR is dependent on the number of interfaces. Therefore, it is important to mention the number of bi-layers when GMR effects are reported. For example, Bird and Schlesinger [33] reported 55% GMR in electrodeposited Co/ Cu multilayers when the number of bilayers are 6000. In the light of the present data, a 55% GMR for a multilayer consisting of 6000 bi-layers is consistent with GMR effects reported by other groups as their values indicate less than 1% GMR per 100 bi-layers. On the other hand,

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Weihnacht et al. [34] have reported 10% GMR in electrodeposited Co (Cu)/Cu multilayers, which consisted of 300 bi-layers. This means they have achieved 3.3% GMR per100 bilayers.

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EFFECT OF MAGNETIC LAYER THICKNESS ON THE GMR The effect of magnetic layer thickness at a fixed non-magnetic layer thickness in electrochemically deposited GMR [28, 35-42] multilayer was found to show variations in GMR characteristics. The results have been understood in terms of a gradual morphological transition of the magnetic layer from a granular to a continuous layer pattern as the magnetic layer thickness increases. Dulal et al. [28] reported the multilayer with the very thin (2nm) magnetic layer exhibit lower GMR than that of 4nm. The GMR decreases with further increase in magnetic layer thickness, when the layer thickness 2nm the layer may not be continuous [43], this may lead to the lower GMR value. The Chowdhury et al [35]reported the value of GMR increases with tmag, attains a sharp maximum at tmag = 0.5 nm, and then decreases to have a sharp minimum at 0.6 nm, followed by a broad maximum between 1 and 3 nm and decreases thereafter. This variation in GMR with tmag is unexpected, while the magnetic coupling between layers, which depends on non-magnetic layer thickness tnon-mag is normally a cause of a similar behavior. The initial change is due to an increase of the amount of ferromagnetic spins in the ferromagnetic layers caused by the ferromagnetic exchange interactions associated with the increase of ferromagnetic layer thickness [36, 29, 44]. The average dimension of magnetic particles and magnetic field efficiency are also increasing as a consequence of increasing tmag. Further observation at higher thicknesses shows a decrease in GMR due to the decrease in the anti-ferromagnetic interactions existing between the two magnetic layers near the nonmagnetic layer [44]. An increase of magnetic layer thickness contributes to a different saturation state and its resistance, thus, enhancing GMR [36]. E.Y.Tsymbal et al. Observed the decrease in GMR at large magnetic layer thickness and this is due to the increasing shunting of the current in the inner part of the ferromagnetic layers. The decrease in GMR at low thickness is due to the scattering at the outer boundaries (substrate, buffer layer or capping layer). This scattering significantly affects GMR when the thickness of the ferromagnetic layer becomes smaller than the longer of the two mean-free paths associated with the up- and down-spin electrons [45] When The magnetic layer thickness as high as 10nm and more, the measured MR values are decreased (comparable to alloy system) [41, 46-47]. This fact indicates that at such high magnetic layer thicknesses, the GMR contribution to the total MR from the interfaces [47] is strongly reduced and the AMR contribution from the interior of the magnetic sub-layer dominates clearly. The increasing dominance of the AMR contribution at high magnetic layer thicknesses clearly indicates that the GMR effect in multilayer is mostly connected with spin dependent scattering at the FM/NM interfaces and not in the bulk of the FM layer.

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EFFECT OF NON-MAGNETIC LAYER THICKNESS ON THE GMR The dependence of MR on the spacer layer thickness [48-49], tCu, for multilayers, the MR initially increases with thickness and attains its maximum value for certain tCu and decreases for higher thicknesses [35,39,50]. The drop of GMR beyond dCu =1.5 nm is due to the absence of the sharp low-field GMR contribution in this thickness range [51]. The decrease in GMR in thin Cu layers may be due to pinholes in these layers [52]. Rafaja et al.[53] observed an increase in the GMR magnitude with increasing thickness of Cu layers in the ED [Co(4.0 nm)/Cu(2.4 nm)]125, [Co(3.3 nm)/Cu(4.0 nm)]91 and [Co(3.6 nm)/Cu(9.3 nm)]55 multilayers. Still, the increase in the GMR amplitude was much larger between [Co(4.0 nm)/Cu(2.4 nm)]125 and [Co(3.3 nm)/Cu(4.0 nm)]91 than between [Co(3.3 nm)/Cu(4.0 nm)]91 and [Co(3.6 nm)/ Cu(9.3 nm)]55. Concurrently, the relative importance of the AMR contribution (i.e. the difference LMR – TMR) with respect to the GMR term was reduced considerably for larger Cu layer thicknesses. The GMR exhibits oscillations in its magnitude as the thickness of Cu layer was varied [38,27, 54-57, 34, 58-63, 3, 64-65]. These behaviors observed due to structural imperfections [38, 54-56]. Pandya et al. [38] reported, that can be attributed to the higher surface roughness induced in the multilayer by the initial surface roughness of substrate. The magnitude of GMR varies from study to study (probably due to differences in actual layer thicknesses, preferred texture, substrate material, and other details of the electrodeposition process) the general trend is that (i) a clear GMR effect develops above a certain Cu-layer thickness of about 1 nm only, (ii) the GMR magnitude increases monotonically with dCu, and (iii) a saturation or maximum occurs for Cu-layer thicknesses around and above 4 nm. Q.X. Liu et al. [69] not able to obtain an oscillation of the GMR with increasing Cu layer thickness. This reason is besides the weak or vanishing AF coupling, , under the current deposition conditions a complete separation of the magnetic layers by the Cu spacer materials is achieved beyond 2 nm average Cu layer thicknesses only. The GMR values are decreased with increasing the thickness of non-magnetic layer [43]. This is due to the reduction of the dipole-dipole interaction between the adjacent magnetic layers.

CO–CU/CU MULTILAYER FILMS About half of the papers reported on a GMR study of ED multilayer films were dealing with Co–Cu systems [26-27, 30, 33-34, 37-39, 48, 53, 55-56, 58, 61, 66-69, 70-123, 188]. The driving force for this extended research effort was the fact that, among physical deposited (PD) materials, the Co/Cu system exhibited the largest GMR being as high as 50% at room temperature [54,60,124]. Unfortunately, the GMR magnitude of electrodeposited (ED) Co– Cu/Cu multilayer films has hardly reached only 20% (apart from some reports [33,111,114] the results of which could not yet be reproduced by other laboratories). Due to the differences in the electrochemical behavior of Co and Cu as well as the immiscibility of the two elements, the multilayer growth process appears to be especially unfavorable for achieving appropriate GMR characteristics in this system by electrodeposition.

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Bird and Schlesinger [33] electrodeposited Co–Cu/Cu multilayer films from a sulfamate/sulfate bath. The Co layer thickness was kept constant at 3.2 nm and the Cu layer thickness was varied from 0.5 to 8 nm with bilayer repeat numbers between 800 and 6000. The total multilayer thickness was at least several micrometers and in many cases it may have exceeded even 20 µm. The saturation GMR values were displayed for ten different Cu layer thicknesses up to dCu 4.3 nm and this is due to the expected oscillatory (RKKY-type) behavior as a function of the Cu layer thickness. A maximum room-temperature GMR of 55%, equal to that reported for sputtered Co/Cu multilayers [54, 60, 124], was obtained for dCu around 0.75 nm, with the second and third GMR maximum being around 2 nm and 3.5 nm, respectively. Unfortunately, because of lack of sufficient details about the preparation conditions and the magnetoresistance measurements, especially the shape of the MR(H) curves, one cannot properly assess the validity of these results which could not be reproduced in subsequent studies. L. Piraux et al. reported multilayered Co/Cu nanowire with approximately 7nm Co rich and 3nm Cu rich layer with MR 19 % and at room temperature [165]. When the cobalt layer thickness varied in the range of 2-22nm high GMR upto 12-14 % was attained. GMR was found to increase with increase in tco in the range 5-20nm. The saturation field values were also found to increase with the increase in Co layer thickness. Comparison of GMR measurement of the Co-Cu and Ni-Cu multilayer shows the replacement of Ni by Co on multilayer significantly enhance the GMR effect [33]. The Co-Cu system has 8 times greater GMR value than Cu-Ni system. Weihnaclt et al [34] reported that the following conditions were to be most favorable for a pronounced GMR effect on Co-Cu multilayer.

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i. Large negative current density for Co deposition eg.106mA/cm2 ii. Low negative potential for Cu deposition eg. -0.25V iii. Relatively thin Cu layer eg. A nominal thickness of about 2nm The GMR results of the some Co-Cu systems are presented in table 1.

Ni-Cu/Cu Multilayer Films Kubota et al. [60] reported the GMR of sputtered Ni/Cu multilayers is nearly by an order of magnitude smaller than for corresponding Co/Cu multilayers. This may be explained by the fact that much less efforts were devoted to the study of GMR in ED of Ni–Cu/Cu multilayers [33, 40, 46, 47, 65, 125-135] in comparison with the available literature on ED Co–Cu/Cu multilayers. In Ni-Cu system at room temperature in magnetic field up to 7 KOe, AMR was formed for alloy deposited by DC plating, where as both GMR and AMR was formed for Ni-Cu/Cu multilayer [46]. The GMR value of Ni/Cu multilayer system is 2.5% with current flowing in the plane of the multilayer and the applied magnetic filed was 0 to 1.5 T at 50K [65]. The GMR behavior of Ni(Cu)/Cu was obtained at tcu above 2nm for the constant current/constant potential sources, where galvanostatic and potential control was used for the deposition of magnetic and non-magnetic layer.

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The electrolyte pH [46], orientation of the deposits [52, 127, 143,] and crystalline sizes [97] have influence on the GMR properties [135]. The two factors may significantly influence the GMR of electrodeposited multilayer [41]. i.

The position and orientation of the investigated sample section on the cathode surface during deposition. ii. The deterioration of the particular citrate/sulphate electrolyte used. The GMR results of some Ni-Cu/Cu system presented in table 2.

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Co–Ni–Cu/Cu Multilayer Films An intensive study of GMR has been carried out on ED Co–Ni–Cu/Cu multilayers [2, 3, 52, 63, 50, 28, 136-151]. This wide interest was motivated mainly by the fact that whereas the addition of Ni to the magnetic layers of Co/Cu multilayers prepared by PD methods was found to reduce the GMR [60], the deleterious exchange reaction and magnetic metal dissolution is less effective in the case of using Ni as magnetic metal when preparing GMR multilayers by electrodeposition as compared to the case of Co. The first observation of GMR in ED multilayers which was made in the Co–Ni–Cu/Cu system [133] was achieved by finding a successful compromise between reduced GMR and better controlled electrochemistry during multilayer preparation. Similarly to the ED Co–Cu multilayers, various bath combinations were elaborated to prepare ED Co–Ni–Cu/Cu multilayers for GMR studies and the review of results will be presented mainly along this line. The Co-Ni-Cu/Cu multilayer had a significantly higher GMR than Co-Cu/Cu multilayer deposited with galvanostatic control. The composition of Ni on Co-Ni-Cu alloy layer have beneficial effect on the GMR , Co-Ni(3.5)-Cu/Cu system has 11 % GMR [150]. The GMR value of Co-Ni-Cu/Cu multilayer increases with the Cu layer thickness in the 1-2.3 nm range [151]. The same observation was observed for Co-Cu/Cu multilayer system [67, 69]. Co-Ni-Cu/Cu multilayers are electrodeposited on n-GaAs substrates with two different crystallographic orientations. The GMR in multilayer grown on n-GaAs (111) is suppressed for tcu