influence of ph on the electrolytic deposition of ni–co films
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Thin Solid Films 516 (2
Influence of pH on the electrolytic deposition of Ni–Co films
Renáta Oriňáková a,⁎, Andrej Oriňák a, Guido Vering b, Ivan Talian a, Roger M. Smith c,Heinrich F. Arlinghaus b
a Institute of Chemistry, Faculty of Science, P.J.Šafárik University, Moyzesova 11, SK-04154 Košice, Slovak Republicb Physical Institute, Wilhelm Westphalen University, Wilhelm-Klemm-Str. 10, D-48149 Muenster, Germanyc Department of Chemistry, Loughborough University, Loughborough, Leics, LE11 3TU, United Kingdom
Received 4 December 2006; received in revised form 12 November 2007; accepted 4 December 2007
Available online 15 December 2007Abstract
Ni–Co alloys obtained by electrolytic deposition on a paraffin impregnated graphite electrode have been characterised by cyclic voltammetry,scanning electron microscopy with energy dispersive X-ray microanalysis, and time of flight secondary ion mass spectrometry (ToF–SIMS), withthe objective of qualitatively assessing and comparing their composition and the distribution of chemical species. The effect of pH on thecomposition and morphology of the deposit and on the proportion of the hydrogenated forms of Ni and Co has been investigated. It has beendetermined that the predominant species, which give rise to the ToF–SIMS positive ion spectra of the deposits, are the pure metals, their hydrides,hydroxides and oxides as a hydrogen. In negative secondary ion spectra, the most favourable species were found to be: O, OH, as well as hydrides,hydroxides and hydrated ions of the two metals. The proportion of individual species changed depending on the electrolyte pH. The content of Coin the deposit corresponds to the content of Co in the electrolyte and increased moderately with increasing pH value, corresponding to the regulardeposition behaviour of Ni–Co film rather than an anomalous deposition.© 2007 Elsevier B.V. All rights reserved.
Keywords: Electrolytic deposition; Ni–Co; Hydrogen evolution reaction; ToF–SIMS; Anomalous co-deposition; Thin films
1. Introduction
Electrochemical methods of producing coatings of metalliclayers are attractive due to a high degree of control which can beobtained by varying the experimental conditions.
Nickel, cobalt and their alloys are important engineeringmaterials in many applications because of their uniquephysicochemical properties [1]. According to Ref. [2,3], theelectrochemical deposition of Ni–Co alloys can show anom-alous behaviour, in which there is a preferential deposition ofthe less noble rather than the more noble metal. This is observedas a much higher content of the less noble metal (Co) in thedeposit than in the deposition bath [3,4]. Similar results wereobtained for a typical Watts bath [5], chloride and acetateelectrolytes [6,7] as well as sulphate and sulphamate baths [8,9].This phenomenon has been reviewed extensively [10,11] for the
⁎ Corresponding author. Tel.: +421 55 2342328; fax: +421 55 6222124.E-mail address: [email protected] (R. Oriňáková).
0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.12.081
iron group binary alloys and models to predict this behaviourhave been developed. One proposal is the competitionadsorption of the metal hydroxides, monohydroxides, or metalions of the less noble metal at the cathode, caused by a localincrease in the pH. The hydroxide may suppress deposition ofthe more noble metal. However, recent studies [11,12] showedthat anomalous co-deposition occurred when the pH at theelectrode surface was lower than that required for the formationof the metal hydroxide. The formation of the Ni–Co alloy wasalso studied by Fan and Piron [12] in chloride and citrate baths.They reported that anomalous co-deposition only occurred atcurrent densities lower than 100 mA cm−2 in the chloride bath.In the citrate bath, the formation of complex ions inhibited anypreferential deposition of Co. In early work, Correia andMachado [13] investigated the electrodeposition of thin layersof Ni–Co alloys from dilute chloride baths. Chemical analysisof the coatings revealed a regular deposition behaviour and theysuggested that the small amounts in the very thin deposits wasnot sufficient to promote anomalous behaviour. In recent work,
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Bai and Hu [1] used cyclic voltammetry to analyse theelectrodeposition of Ni–Co alloys from chloride plating baths.Deposits, with a composition approximately equivalent to theircorresponding plating solutions, were formed when thepotential range was between 0 and −1.2 V at pH 2. Thesuppression of the anomalous co-deposition was attributed tothe anodic dissolution of the freshly deposited metal atomsresulting in the simultaneous dissolution of the adsorbedmonohydroxide species between −0.3 and 0 V.
Alloy deposition is strongly dependent on the experimentalparameters, such as the bath composition, pH, depositionpotential or current, temperature, stirring, time, etc. Changes inthe conditions may result in electrodeposits with a differentcomposition, morphology or phase structure. Hence theanomalous co-deposition can be either enhanced or suppressedby changing deposition conditions.
Several mechanisms have been proposed for the electro-deposition of Ni–Co alloys. It is generally acknowledged thatthe deposition process occurs in several steps and utilizes thepresence of an adsorbed metal compounds and/or intermediatespecies. The generally accepted mechanism for this electroplat-ing behaviour, based on the formation and adsorption of themetal hydroxyl ions on the deposits, can be expressed asfollows:
2H2O þ 2e−↔H2 þ 2OH− ð1Þ
M2þ þ OH−↔MOHþ ð2Þ
MOHþ→MðOHÞþads ð3Þ
MðOHÞþads þ 2e−↔M þ OH− ð4Þ
where M represents Co or Ni atoms. The newly formed OH− inEq. (4) favours the further formation of MOH+ and enhances theadsorption of MOH+. The adsorption ability of CoOH+ isconsidered to be higher than that of NiOH+ [1].
Electroplating of metallic coatings proceeds simultaneouslywith the hydrogen evolution reaction (HER), which depends onpH. Hydrogen evolution can produce a reduction of currentefficiency and a local increase in pH that can cause theprecipitation of metal hydroxides on the cathode. A number ofhypotheses have been proposed concerning the state ofhydrogen in metal coatings [14]. The hydrogen may beembedded into the growing film and may give rise to markedmechanical effects, such as hydrogen embrittlement [15].
There are two generally accepted mechanisms for HER duringthe deposition of the iron group metals and their alloys fromaqueous solution [16]: i) discharge (Volmer reaction) (5) fol-lowed by Tafel recombination (6a) or ii) discharge (5) followedby electrochemical desorption (Heyrovsky reaction) (6b):
M þ Hþ þ e−→M−Hads ð5Þ2M−Hads→2Mþ H2 ð6aÞM−Hads þ Hþ þ e−→M þ H2: ð6bÞ
The rate-determining step is determined by the strength ofthe hydrogen bond with the surface [17]. Reaction (5) is an
adsorption step in which a chemical bond M−Hads is formed. Amajor portion of the adsorbed hydrogen reacts to give hydrogenmolecules followed by a desorption stage according either toreaction (6a), which implies that the Hads atoms are mobile onthe metal surface, or to reaction (6b) in which a second proton isinvolved. A small proportion of the adsorbed hydrogen isadsorbed into the metallic lattice, M(Hads) as follows [16]:
M−Hads↔MðHadsÞ: ð7Þ
The interaction of hydrogen with the transition metal surfaceis a topic of considerable experimental, theoretical, andindustrial interest. The purpose of this contribution is to studythe influence of pH on the electrolytic deposition of the Ni–Cofilm, the surface morphology and chemical composition ofbinary coating as well as on the nature and distribution of thehydrogen forms presented in the deposits.
2. Experimental details
Ni–Co alloy electrodepositions were carried out by potentialscanning from 0 V to −1.5 V (vs. Ag/AgCl/3 M KCl) in adeaerated electrolyte containing 0.7 M NiSO4+0.125 MCoSO4+0.26 M NaCl adjusted to the required pH (2, 3 or 4).The polarization rate was 10 mV/s for all measurements. Thecontent of Ni and Co in the deposit was determined by atomicabsorption spectrometry after dissolution in diluted (1:1) nitricacid.
The electrochemical experiments were carried out in aconventional three-electrode cell employing a potentiostatEcaStat, Model 110 V (Istran, Slovak Republic) computercontrolled using the ISTRAN software. The working electrodewas a paraffin impregnated graphite electrode (PIGE) with anarea of 2.83×10−5 m2. The auxiliary electrode was a Pt foilwith an area of 87.9×10−5 m2, and the potential was referencedto the Ag/AgCl/3 M KCl electrode. To obtain reproducibleresults, the surface of the PIGE was mechanically polished withfine sand and filter paper before each measurement. The mainadvantage of this electrode was the ability to provide areproducible surface in contrast to the poor reproducibility ofsome metallic electrode surfaces and hence the voltammetricmeasurements were based on the relatively constant back-ground current corresponding to almost constant electrodesurface area. The platinum electrode was cleaned in nitric acid(1:1) and rinsed with distilled water before use. The rate ofstirring was 200 rpm.
Time of flight secondary ion mass spectrometry (ToF–SIMS) experiments were performed with a ToF–SIMS IVinstrument built at the University of Münster which used a11 keV Ar+ primary ion-gun. The primary ion beam wasrastered on 30×30 μm2 or 100×100 μm2 areas with a current of0.6 pA and a primary ion dose density of 1.33×1013 ions/cm2.
Scanning electron microscopy in combination with energydispersive X-ray microanalysis (SEM/EDX) (Tesla BS 340 withEDX LINK ISIS microanalyser operated at 20 kV with acollection time of 120 s) was employed for microstructuralevaluation of the deposits using a copper standard. The
Fig. 1. Polarization curves for binary Ni–Co films electrodeposition on PIGE(with surface area of 2.83×10−5 m2) for different values of pH.
Table 1Atomic ratio of Ni and Co in the plating bath and Ni–Co films on PIGE fordifferent value of bath pH
Ni:Co in electrolyte Ni:Co in deposited layer
pH 2 5.6:1 6:1pH 3 5.6:1 4.5:1pH 4 5.6:1 4:1
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operating voltage for the SEM was maintained at 20 kVthroughout the analysis.
3. Results and discussion
3.1. Voltammetric analysis
The polarization curves for a binary alloy deposited on thePIGE at different pH values are presented in Fig. 1. Changes inthe slopes of the cathodic part of the curves reflect increase inthe deposition rate with increasing pH, this is accompanied byan increase in the anodic peak area as well in the thickness of thefilms. The single oxidation peak at 430 mV, observed for pH 2,indicates that the dissolution of both metals in the binary Ni–Cofilm either progresses simultaneously or at very similarpotentials. Two separated oxidation peaks observed at higherpH values correspond to the consecutive dissolution of Ni andCo from the deposited binary layer at 400 mV and 840 mV forpH 3, and at 480 mV and 930 mV for pH 4, respectively. Adetailed study of the electrochemical behavior of the individualmetals (Ni and Co) confirmed this hypothesis, as correspondinganodic peaks were found in the polarization curves of the singlemetals. Increase in the anodic peak at the more positive potentialwas also correlated with an increasing content of Co in binaryfilm (Table 1) yielding further confirmation.
Although molar ratios of the Ni:Co ions in the bulk solutionwere the same for each sample, the deposited film contained aslightly higher content of Co with increasing the electrolyte pHvalue. However, this enrichment did not correspond toanomalous co-deposition it can be attributed to the sameeffects: (i) the less acidic bath favors the formation of theM(OH)+
species and (ii) the adsorption of Co(OH)+ is considered to bestronger than that of NiOH+ [1]. Both these effects could resultin the preferential deposition of cobalt, and are enhanced by adecrease in the proton concentration. Moreover, Gómez et al.[6] explained the anomalous co-deposition by following steps:(i) nickel is deposited at the initial stage and then (ii) Co2+ isadsorbed onto the freshly deposited Ni and begins to bedeposited. This cobalt deposition was supposed to inhibitsubsequent Ni deposition, although it was not considered to
block it completely. On the basis of above results anddiscussion, the regular deposition behaviour observed in thepresent work can be associated with the insufficient thicknessof the deposited layer and it can be expected that at longerdeposition time the anomalous co-deposition would beobserved.
3.2. SEM/EDX analysis
The surface morphology of the deposited films wasinvestigated by SEM and EDX analysis. A grain shapedmorphology is related to the presence of Co while ahomogeneous morphology is associated with Ni. In general,the deposits were cracked and not adhesive (Fig. 2) and themorphology became slightly more granular with increasing pHvalue, which can be attributed to an increased content of cobalt.Evidence of the simultaneous hydrogen reduction at pH 2 and 3(Fig. 2a, b) was apparent from the dark circles created byhydrogen bubbles growing on the electrode surface. Thedestructive influence of hydrogen was less noticeable at pH 4(Fig. 2c), but cracking of the deposit occurred to a higherdegree. EDX analysis revealed the homogeneous distribution ofboth metals in the binary films.
3.3. ToF–SIMS analysis
The SEM/EDX analysis has been supplemented withinformation obtained from ToF–SIMS analysis. These elementmaps clearly showed a uniform distribution of both metals in thebinary coating on the PIGE (Figs. 3 and 4). The mass spectralanalysis revealed that the ionic residues containing hydrogenpresent were almost the same but their intensity changed withpH. The highest intensities in positive secondary ion spectrumwere detected for: Ni, Co, H, H2, NiH, CoH, NiH2, CoH2, NiOH,CoOH, NiH2O, CoH2O. Peaks with lower intensities wereobserved for: CoNiH, CoNiOH, CoNiH2, Ni(OH)2, Co(OH)2 aswell many different oxides. At lower pH, atomic hydrogen andhydrides dominated in the deposits (Fig. 3) and thought tooriginate from simultaneous HER, indicating that hydrogenevolution has a significant influence on binary alloy deposition.Hydrides and hydroxides predominated at higher pH. Theintensities of the ions for pure Ni as well nickel species werealways higher than those of pure Co or cobalt species.
In the negative spectra the intensities of most cobalt specieswere slightly higher than the intensities of the nickel species.The most intensive signals in the negative secondary ionspectrum were observed for: O, OH, CoH, NiH, CoOH, NiOH,NiH2, CoH2, Ni, Co, NiH2O, CoH2O. Less intensive signals
Fig. 2. SEM micrograph of Ni–Co films deposited on PIGE a) pH 2, b) pH 3, c) pH 4.
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were recorded for: CoNiOH, Ni(OH)2, Co(OH)2, CoNi(OH)2,CoNiH2O and also many different oxides. Again the intensitiesof these species varied with pH in the same manner as in thepositive spectrum. However, significant differences wereobserved in the intensities of the most intensive species; Oand OH. At pH 2, signal for O was the stronger (Fig. 4), while atpH 4 that of OH dominated. At pH 3 the intensities of these twospecies were comparable. The higher content of OH species athigher pH may be responsible for the enhancement of thedeposition rate detected by CVand thus it can indirectly confirm
Fig. 3. ToF–SIMS image of the distribution of some positive hyd
the importance of MOH+ species in Ni–Co electrodepositionmechanism.
From the ToF–SIMS analysis of the Ni–Co depositsobtained on the PIGE at different pH, it is evident that thehydrogen resulting from the cathodic reaction is chemisorbed orchemically bonded in the electrodeposited layer. The generalconclusion is that the highest intensity among the hydrogencontaining species was detected for OH ions at all pH values.Furthermore, at lower pH the hydrogen present in the depositedlayer is primarily in the pure form (H, H2), less as the metallic
rogen containing species in Ni–Co deposit on PIGE at pH 2.
Fig. 4. ToF–SIMS image of the distribution of some negative hydrogen containing species in Ni–Co deposit on PIGE at pH 2.
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hydrides (MH, MH2) and least as the metallic monohydroxidespecies (MOH). In addition the signal for atomic oxygen wasvery intense at lower pH values and was higher than that of OHspecies, probably due to the absence of boric acid in theelectrolyte. This high content of oxygen may eventuallypromote the formation of the OH species. At higher pH, inaddition to the OH ion, very high intensities were also registeredfor metallic hydrides (MH, MH2) as well mono- anddihydroxides (MOH, M(OH)2).
From the intensities of the hydrogen and hydrogenatedspecies in both positive and negative spectra, it can be deducedthat the amount of hydrogen incorporated in the deposit washighest at pH 2. This corresponds to the highest concentration ofhydrogen protons in the electrolyte solution. In spite ofcontaining the highest level of evolved hydrogen, the qualityof the binary Ni–Co film that was deposited at pH 2 seems to beno worse than at the other pH values (Fig. 2). For moreinformation on the quality of the deposits the exact analysis isrequired.
4. Conclusions
Ni–Co films were deposited by linear sweep voltammetry bypotential scanning from 0 V to −1.5 Von PIGE at pH 2, 3 and 4.Rate of polarization was 10 mV/s. Obtained deposits werestudied by cyclic voltammetry, SEM/EDX and ToF–SIMS.
It was found that increasing pH value increased the rate ofbinary alloy deposition by linear sweep voltammetry onto aPIGE electrode, the content of cobalt in the binary deposit andthe intensity of a second anodic peak. The uncovered sphericalspots corresponding to hydrogen bubbles attached to theelectrode surface were observed at pH 2. From this point ofview the quality of the deposit was affected less by the co-evolution of the hydrogen at higher pH but the deposit was morecracked and granular. In general, the distribution of both metalsin the deposited films was uniform, and cobalt content in thedeposit was nearly proportional to that in the plating bath,indicating a regular deposition of Ni–Co alloy. Anomalousdeposition was not observed under the conditions examined.The composition of the binary Ni–Co deposits was determinedin detail by ToF–SIMS analysis depending on pH and principalhydrogen species in the deposited layer were identified. At allthe pH values examined the highest intensity of the hydrogenspecies was found for the OH ion. In addition to the pure metals(Ni, Co), the next most intensive species at lower pH wereatomic hydrogen and the hydrides of both metals, while athigher pH, apart from OH ion, the highest intensity wasobserved for atomic oxygen and the hydroxides of both metals.
ToF–SIMS analysis yielded sensitive information about thechemical composition and distribution of hydrogen compoundsin deposit. Application of ToF–SIMS in more detailed study canprovide deeper insight into the deposition mechanism.
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Acknowledgements
This work was financially supported by the Slovak GrantAgency VEGA; Grant No. 1/2118/05. The authors wish tothank ION-TOF GmbH, Germany for temporary licence ofToF–SIMS software providing.
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