UDK 621.793	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 48(2)221(2014)
ELECTRODEPOSITION AND CHARACTERIZATION OF Cu-Zn ALLOY FILMS OBTAINED FROM A SULFATE
BATH
ELEKTRONANOS IN KARAKTERIZACIJA PLASTI ZLITIN Cu-Zn, NASTALIH IZ SULFATNE KOPELI
Abdelouahab Redjechta1, Kzmel Loucif1, Loubna Mentar2, Mohamed Redha Khelladi2,
Abdelkrim Beniaiche3
1Laboratoire des Materiaux Non Metalliques, Institut d'Optique et Mecanique de Precision, Universite Ferhat Abbas-Setif 1, 19000 Setif, Algeria 2Laboratoire de Chimie, Ingenierie Moleculaire et Nanostructures, Ferhat Abbas-Setif 1, 19000 Setif, Algeria 3Laboratoire des Systemes Photoniques et Optiques Non Lineaires, Institut d'Optique et Mecanique de Precision, Universite Ferhat
Abbas-Setif 1, 19000 Setif, Algeria redjechtaabdelouahab@yahoo.fr
Prejem rokopisa - received: 2013-02-26; sprejem za objavo - accepted for publication: 2013-04-16
In this work, we report the influence of the deposition potential on the electrodeposition process, current efficiency, surface morphology and microstructure of Cu-Zn alloys deposited on a Ru substrate from a sulfate solution with an addition of EDTA. The study was carried out by means of cyclic voltammetry (CV), chronoamperometry, atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques analyzing the electrochemical behavior, surface morphology and structural characterization, respectively. The experimental results show that the electrochemical behavior of Cu-Zn electrodeposits varied with the deposition potential. The AFM measurement showed that the Cu-Zn thin films obtained at all the potentials are homogenous in appearance being of a small crystallite size, and a variation in the film roughness with deposition potentials is established. An analysis of X-ray diffraction patterns indicates that the electrodeposited Cu-Zn alloys exhibit ß- and y-phases. Keywords: copper-zinc, electrodeposition, cyclic voltammetry, morphology, X-ray diffraction
V tem delu poročamo o vplivu potenciala nanosa pri postopku elektronanašanja na učinkovitost toka, morfologijo površine in mikrostrukturo zlitine Cu-Zn, nanesene na podlago iz Ru iz sulfatne raztopine z dodatkom EDTA. Za analizo elektrokemijskega vedenja, morfologije površine in značilnosti strukture so bile uporabljene ciklična voltametrija (CV), kronoamperometrija, mikroskopija na atomsko silo (AFM) in rentgenska difrakcija (XRD). Rezultati raziskav kažejo, da se elektrokemijsko vedenje elektronanosov Cu-Zn spreminja s spreminjanjem potenciala pri nanašanju. AFM-meritve so pokazale, da so tanke plasti Cu-Zn, dobljene pri vseh potencialih, na videz homogene z majhnimi kristalnimi zrni, spreminjanje potenciala pa vpliva na hrapavost površine. XRD-analize pokažejo, da zlitina Cu-Zn po elektronanosu vsebuje ß- in y-faze.
Ključne besede: baker-cink, elektronanos, ciklična voltametrija, morfologija, rentgenska difrakcija
1 INTRODUCTION	maintainable equipment.1 The control of the solution
composition and deposition parameters determines the
The production of the coatings made of zinc and its	properties of a deposit. In electrodeposition, the mecha-
alloys has recently been of interest since alloy coatings	nism growth, the morphology and the micro-structural
provide a better corrosion protection than pure-zinc	properties of a film depend on electrodeposition condi-
coatings. In addition, alloy coatings are very interesting	tions such as the electrolyte composition, the electrolyte
due to their high strength, good plasticity and excellent	pH and the deposition potential.2
mechanical properties. There are several methods for	Numerous studies of the electrodeposition of Cu-Zn
obtaming these alloys: physical^apor deposition (PVD),	alloys from aqueous baths have been carried out.3,4 It has chemical vapor deposition (CVD), sputtering and mole-
been reported that different electrochemical deposition para-
cular beam epitaxy (MBE) techniques are just a few of
them. These methods have several advantages and are	meters such as deposition potential or current density,
used for specific applications. However, due to certain	temperature, pH, substrate-surface preparation and bath
limitations, such as high capital and high-energy costs,	composition affect considerably the properties of depo-
an alternative method is required. Recently, the electro-	sits.5-9 It is known that high-quality micrometer-thick
chemical deposition (electrodeposition) has been used as	films (smooth and bright deposits) can be prepared at a
an alternative technique for producing these structures on	reasonably high deposition rate using baths with a high
different surfaces. Electrochemical processes offer many	metallic-ion concentration and small amounts of additi-
advantages, including a room-temperature operation,	ves.10,11 In the same way, due to a large difference bet-
low-energy requirements, fast deposition rates, a fairly	ween the standard electrode potentials of Cu and Zn (=1.1
uniform deposition over complex three-dimensional	V), these ions should be complex in electrodeposition
objects, low costs and a simple scale-up with an easily	solutions to facilitate their codeposition. Therefore, in
this work, an organic additive of C10H14Na2O8, 2H2O, called EDTA, was added to the sulfate bath.
The objective of the present work was to study the electrodeposition process and properties of the Cu-Zn alloys from a sulfate electrolyte with EDTA. The morphology and structure of the deposits were examined.
2 EXPERIMENTAL WORK
Table 1: Bath composition and conditions for the Cu-Zn electrodepo-sition
Tabela 1: Sestava kopeli in razmere pri elektronana{anju Cu-Zn
Bath	ZnSO4, 7H2O/M	CuSO4, 5H2O/M	Na2SO4 /M	H3BO3 /M	EDTA /M
Zn	0.06		1	0.5	0.35
Cu		0.14	1	0.5	
Cu-Zn	0.06	0.14	1	0.5	0.35
A deposition of Cu-Zn alloys was carried out in a bath of 0.14 M CuSO4 for Cu, 0.06 M ZnSO4 for Zn (Aldrich) with the Na2SO4 support electrolyte, 0.5 M H3BO3 (in order to control the pH of the solution and improve the quality of the deposit) and 0.35 M C10H14Na2O8, 2H2O (EDTA) at pH « 4.2 (Table 1). All the measurements were made at room temperature. Ethylenediaminetetra acetic acid, widely abbreviated as EDTA, was chosen as the complexing agent for a deposition of theses alloys. Plating baths were prepared from the chemicals of analytical grade and bidistilled water, and the pH was adjusted with dilute sulfuric acid when needed. Before the electrodeposition, each solution was stirred with a nitrogen gas flow. The conventional electrochemical measurements were taken using a glass cell consisting of a three-electrode assembly that was connected to a VoltaLab 40 (PGZ301 and Volta Master 4) controlled by a personal computer. A platinum sheet was used as the counter electrode (anode) and the cathode (the Ru substrate) potentials were referred with respect to the saturated calomel electrode (SCE). The working electrode was an thick Ru layer approximately 200 nm deposited by sputtering onto each silicon wafer at a low temperature (150 °C) to get a better adherence. Before the electrodeposition, the substrates were first cleaned ultrasonically in acetone and ethanol and then also with distilled and deionized water. The Cu-Zn thin-film deposition onto the Ru surface (an area of 0.5 cm2) was studied by means of cyclic voltammetry (CV) and chronoamperometry (CA) techniques. The CV for all the solutions was initially carried out between -1.2 V and 0.2 V (SCE) at a scan rate of 20 mV s1. The surface morphologies of the deposits were examined with atomic force microscopy (AFM). The roughness (the root-mean-square height deviation) of the samples was obtained directly from the AFM software (PicoScan 5.3 from Molecular Imaging). The crystalline structures of the deposits were identified with X-ray diffraction using a Philips diffractometer with a 26 range 10-100° and Cu K« radiation (X = 0.15406 nm).
3 RESULTS AND DISCUSSIONS 3.1 Electrochemical study
Cyclic voltammetry was performed to understand the electrochemical behavior of the Cu(II) and Zn(II) species on the Ru electrode. Figure 1 shows the cyclic voltam-mograms of the Ru electrode recorded in different ion
solutions of Cu, Zn and Cu-Zn. In effect, Figure 1a shows a cyclic voltammogram of a solution containing 0.14 M CuSO4 with a cathodic scan limit of -0.8 V vs. SCE. Two sharp peaks are observed at -0.141 V and 0.072 V, corresponding to a reduction and a dissolution of Cu, respectively. In the Cu electrodeposition, the charge transfer step is fast and the growth rate is controlled with the rate of the Cu-ion mass transfer to the
10
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E 0
-5 20
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-60
40
20
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a) - t /	1
i . 1 . I .	1 . 1
-b)	j/
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-C)	A
1 . 1 . 1 .	1 . 1
-1,5
-1,0
-0,5
0,0
0,5
1,0
E (V VS. SCE)
Figure 1: Cyclic voltammograms obtained for: a) 0.14 M CuSO4, b) 0.06 M ZnSO4 + 0.35 M EDTA and c) 0.14 M CuSO4 + 0.06 M ZnSO4 + 0.35 M EDTA with the cathodic scan limit of -1.20 V vs. SCE, at the scan rate of 20 mV s-1; supporting electrolyte is 1 M Na2SO4 + 0.5 M H3BO3 (pH 4.2)
Slika 1: Ciklični voltamogrami, dobljeni z: a) 0,14 M CuSO4, b) 0,06 M ZnSO4 + 0,35 M EDTA in c) 0,14 M CuSO4 + 0,06 M ZnSO4 + 0,35 M EDTA s katodno omejitvijo -1,20 V proti SCE, pri hitrosti skeniranja 20 mV s-1; osnovni elektrolit je 1 M Na2SO4 + 0,5 M H3BO3 (pH 4,2)
growing centers. The consistency of the cyclic-voltam-metry behavior upon the potential cycling indicates that the anodic stripping process completely removes Cu from the electrode surface. The data in this figure indicates the absence of an underpotential deposition peak, with the Cu reduction occurring at the significant overpotential to the Nernstian value. This is due to a weak deposit/substrate interaction, as the early stages of an electrodeposition of Cu on Ru surfaces correspond to the Volmer-Weber growth mechanism.12
Figure 1b shows a cyclic voltammogram obtained in 0.06 M ZnSO4. During a direct scan, it is possible to note that the increase in the current begins at -0.7 V; this is due to the electrodeposition of Zn and hydrogen evolution. In the reverse potential scan, the absence of the peak corresponding to the dissolution of the previously deposited Zn is observed. For the Cu and Zn solution (Figure 1c), the voltammogram obtained shows the presence of cathodic and anodic peaks related to the deposition and dissolution of the metals. In the cathodic scan, it can be observed that the increases in the current were detected at -0.152 V and -0.75 V, being characteristic of the potential deposition processes of Cu and Zn onto Ru surfaces, respectively. After this limit, the hydrogen evolution is predominant. During the inverse of the potential scan, it is possible to observe, in all the curves, the presence of crossovers which are typical of the formation of a new phase involving a nucleation pro-cess.13
To elucidate the role of an applied potential, the current efficiency (CE) during the codeposition process was determined. The deposition CE was calculated from the ratio of the cathodic electric charge, which passed during the electrodeposition of the Cu-Zn alloy, to that of the anodic one required for the total alloy dissolution. The Cu-Zn alloy thin films were obtained in the poten-tiostatic mode at different deposition potentials. The dependence of CE on different deposition potentials is
shown in Figure 2. The efficiencies of the deposits decrease considerably with the potential and then reach their minima at more negative potentials. This decrease in CE is due to the process of hydrogen evolution. It is clear from these results that there is an appreciable decrease in the value of CE as the deposition potentials become more negative than ~ -1.0 V. This is due to the hydrogen evolution reaction (HER) that becomes more significant than the Zn and Cu electrodeposition. This, in turn, increases the pH level at the cathode, causing the metal hydroxide to be included in the deposit.11 These observations indicate that the control of the deposition potential is very important for realizing a high CE in the Cu-Zn deposition process. Similar results for Cu-Zn deposits are observed by de Almeida et al.14
The current was recorded as a function of time to study the deposition mechanisms of Cu-Zn alloys during their growth. The electrochemical deposition was performed using the standard chronoamperometry technique to study the nucleation and growth mechanism of Cu-Zn on ruthenium. Deposition potentials were chosen according to the reduction peaks appearing on the cyclic voltammograms. Deposition is conducted at the constant potentials in the potentiostatic mode, during which current transients are recorded. Figure 3 shows the current transients obtained at four different potentials: -1.0 V, -1.1 V, -1.2 V and -1.3 V vs. SCE. In this figure, at the beginning of the applied potentials, a high cathodic current is seen for a short time. After that, the current rapidly decreases due to a depletion of the metal-ion concentrations close to the electrode surface, subsequently reaching a stable value. The current-time transients have a normal dependence on the overpotentials, whereas the current density increases with an increase in the overpotential. This is specific to the nucleation and growth process and for longer times, merging into a common curve caused by the diffusion-controlled pro-
Figure 2: Effect of the deposition potential on the current efficiency of the Cu-Zn electrodeposition process
Slika 2: Vpliv potenciala nanosa na učinkovitost elektronanašanja Cu-Zn
Figure 3: Evolution of current densities versus deposition time during the deposition of Cu-Zn on the Ru surface at different deposition potentials
Slika 3: Razvoj gostote toka v primerjavi s časom nanosa Cu-Zn na površino Ru pri različnih potencialih nanašanja
cess and described with the Cottrell equation.15 According to the i-t curves, each transient has one well-defined, recognizable current maximum seen as a clear first peak followed by a sharp fall and subsequent growth. The i-t transients have a normal dependence on the overpotentials, whereas the current density increases with an increase in the overpotential. The peak corresponds to the nucleation of the metallic sites on the surface and it is followed by a reduction in the current exhibiting a three-dimensional (3D) growth. An increase in the peak current at higher overpotentials means that the number of sites (nucleation rate) increases due to a higher nucle-ation rate.15
3.2 Morphological and Structural Analysis
The morphology of the electrodeposited surface was imaged ex situ after the Cu-Zn electrodeposition using AFM measurements. Figure 4 shows 2 pm x 2 pm AFM
images of the deposited Cu-Zn alloy films obtained at different deposition potentials. The figures reveal that, with all the applied potentials, the images have a granular surface. However, the dimensions of the visible features in the images are different. It is known that a film electrodeposited on a polycrystalline substrate is also polycrystalline. During electrolysis, Cu-Zn crystallites randomly grow on the polycrystalline substrate and may form conglomerates. The composition and crystallite sizes strongly depend on the applied potential. If the current density is very small, there will be insufficient crystalline growth centers and the deposited layer will be rough-grained. If the current density is high, the deposited layer will be porous and soft.
The surface topography is traditionally analyzed with surface-roughness measurements such as the root-mean-square (RMS) roughness, the average roughness and the peak-to-valley roughness.16 In brief, the surface roughness R^ (denoted also as RMS) and the mean roughness Ra were calculated using the standard software (Table 2). The surface roughness increases with the film thickness for the films deposited using both potentials, and the variation in the surface morphology with the applied potential is consistent with the general theory.17
Table 2: Dependence of the surface roughness and crystallite size of the electrodeposited Cu-Zn thin films on deposition potentials Tabela 2: Odvisnost hrapavosti površine in velikosti kristalnih zrn Cu-Zn tanke plasti po elektronanašanju od potenciala pri nanosu
E/(V vs. SCE)	Rq/nm	Ra/nm	D/nm
-1.10	42.72	33.12	48.20
-1.20	53.04	65.60	46.50
-1.30	108.62	82.08	46.10
Figure 4: AFM images of the Cu-Zn thin films prepared at different cathodic potentials: a) -1.1 V, b) -1.2 V and c) -1.3 V vs. SCE Slika 4: AFM-posnetki tankih plasti Cu-Zn, pripravljenih pri različnih katodnih potencialih: a) -1,1 V, b) -1,2 V in c) -1,3 V v odvisnosti od SCE
The effect of the current density on this surface morphology can be explained since a high current density results in higher rates of the crystal nucleation (a higher mobility of atoms), giving rise to finer crystal structures and, hence, a smoother surface.17 However, a new theory18,19 has emerged, proposing that the concentration of metallic ions does change the bath homogeneously, but rather preferentially increases near the substrate (cathode). This relative concentration in the discharging zone is of a little significance at a low current density, at which the surface of a deposited film is rough. At a high current density, the convexity of the film increases, being associated with a relative concentration of ions in the discharging zone. The surface of a film deposited under these conditions is very smooth. The convex part of the film attracts more ions by acting as a focus of discharge, further increasing the convexity of the film. This may explain the mechanism of deposition.20
Figure 5 shows the XRD patterns of the Cu-Zn films deposited on the Ru substrates in the sulfate/EDTA bath under different deposition potentials of -1.1 V, -1.2 V and -1.3 V vs. SCE in the 26 scan range of 25-60°. All the XRD patterns show many peaks corresponding to two distinct phases, ß and y, respectively. It is clear that
Figure 5: X-ray diffraction patterns of the electrodeposited Cu-Zn alloy films obtained at different deposition potentials: a) -1.1 V, b) -1.2 V and c) -1.3 V vs. SCE
Slika 5: XRD-posnetki tankih plasti Cu-Zn, dobljeni pri različnih potencialih nanašanja: a) -1,1 V, b) -1,2 V in c) -1,3 V v primerjavi s SCE
the XRD patterns from the Cu-Zn electrodeposits are different from those of pure Zn and Cu, indicating that crystalline alloys are indeed formed in these Cu-Zn electrodeposits. Furthermore, the y phase diffraction lines increase in intensity as the deposition potential becomes more negative; in other words, the y phase increases as the Cu content in the electrodeposited Cu-Zn alloy decreases. At a higher deposition potential, a decrease in the Cu concentration is explained with the fact that, at these potentials, a reduction in Cu is masstransport limited. A further increase in the deposition overpotential would only increase the amount of Zn being deposited.21-23 From these results, the y phase was more dominant than the ß phase in the Cu-Zn alloy thin films.
The average crystallite size of the particles is calculated from the full width at half maximum (FWHM) of the respective peaks using the Scherrer relation:24
0.91
D=lü9se	(1)
where D is the crystallite size, X is the wavelength of X-ray radiation (X = 0.15406 nm), ß is the FWHM of the peak and d is the diffraction angle.
Also, Table 2 shows the average crystallite size obtained from XRD for [001] planes, for the ß phase of the alloys electrodeposited at three different applied potentials. The average crystallite size decreases with the increasing Zn concentration in the films and with the increasing applied potentials. This observation shows
that, at a more negative potential, the deposition rate is high and, hence, the atoms are incorporated in the film with little surface migration, thus limiting the grain size.
4	CONCLUSIONS
Smooth, compact and bright binary Cu-Zn alloys were deposited on a Ru substrate from a sulfate electrolyte with an EDTA additive. The electrodeposition behavior of the sulfate electrolyte was studied using cyclic voltammetry. A possibility of depositing pure copper and zinc with a trace of copper was revealed during a cathodic scan of the substrate potential. The effects of deposition potentials on the microstructures of Cu-Zn were investigated by means of AFM and XRD techniques. The AFM images showed Cu-Zn clusters of an equivalent size randomly distributed in the surface defects acting as active sites. An X-ray diffraction measurement reveals that the Cu-Zn alloy exhibits two phases, the ß and y phases. According to these results, the y phase was more dominant than the ß phase in the Cu-Zn alloy thin films.
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