Influence of concentration of zinc ions on electrocrystallization process of zinc

Vol. 15 No. 3Trans. Nonferrous Met. Soc. ChinaJun.2005Article ID: 1003 - 6326(2005)03 - 0642 - 06Influence of concentration of zinc ions onelectrocrystallization process of zincZHANG Zhao(张昭)(Department of Chemistry, Zhejiang University, Hangzhou 310027 , China)Abstract: Cyelie voltammetry, chronoamperometry and scanning electron microscopy were employed to study theinfluence of Zn2+ ion concentration in electrolyte solutions on zinc electroplating process. The results show that, athigh overpotentials, the nucleation of zinc is instantaneous, and nuclear density increases with the overpotentials in-creasing. While at low overpotentials, the zinc may be preferentially electrodeposited on surface inhomogeneitiessuch as emergence points of edge, screw dislocations, atomic disorder, kink sites, or monoatomic steps, and no dis-tinguished nucleation current can be observed. The major dissolution peak in cyclic voltammogram drifts positivelydue to the change of the rate determining step of zinc electroplating processes from diffusion to the electrochemicalreaction with the increase of Zn?+ ion concentration.Key words: zinc electroplating; nucleation; growth; mechanismCLC number: 0646. 6Document code: A1 INTRODUCTION2 EXPERIMENTALThe electrodeposited coatings of zinc andThe experimental electrolyte was preparedits alloys have been widely used as protectivewith AR grade reagents and twice distilled waterfilms of metals, and electrodeposition of alloys isaccording to the basic compositions listed in Tablewidely used in the production of new materials1. A three -electrode system consisting of cycloidalwith specific chemical and physical properties,polycrystalline brass electrode with an area ofsuch as nanocrystalline magnetic thin films and0.5027 cm2 exposed which was used as workingfunctional multilayersl-s]. As far as the electropla-electrode( WE), saturated calomel electrode( SCE)ting process is concerned, the nucleation kineticsas reference electrode and a large bright platinumand the growth mechanism of the first metallicfoil as counter electrode, was employed. Beforenuclei formed on the initial substrate are critical,measurements, the exposed surface of WE werewhich determine the physicochemical properties ofpolished with silicon carbide papers from 3 throughthe electrodeposited materials6,n] and are, there-1 to0.5 μm and velvet, rinsed with the twice dis-fore, crucial points to be understood and con-tilled water, washed in acetone, rinsed with thetrolled.twice ditilled water again and then dried in air.On the other hand, the different electrocrys-The experimental temperature was (25士1) C con-tallization nucleation and growth processes of zinctrolled by thermostat water tank.will result in different electrodeposited layer mor-phologies, such as compact, spongy or dendriticTable 1Basic electrolyte compositions forsurfaces[8] .electroplating zinc electrodeposits (g/L)The aim of this paper is to investigate theZnCl2 NasC,HsO, ●2H2O EDTA-2Na H,BO。 KCIinfluence of Zn2+ ions concentration on the20- 9012440 100nucleation and growth mechanism of zinc， detectthe relationship between the structure of depositlayer and the reaction of rate determining stepsCyclic voltammetry and chronoamperometryduring zinc electroplating process, and provide themeasurements were performed with a commercialbasis reference for further studying on theelectrochemical analyzer/ workstation( Modelnucleation and growth mechanisms of zinc base660A, US). After experiments, scanning electronalloys.microscopy( SEM, HITACHI S-570, Japan) was中国煤化工①Foundation item: Projects(20203015, 5049935) supported by the Nation_na; Project supported byState Key Laboratory for Corrosion and Protection, ChinaYHCNMHGReceived date: 2004 -09 - 23; Accepted date: 2005 -01 -05Correspondence; ZHANG Zhao, Associate professor, PhD; Tel; + 86-571-87952318; Fax: +86-571-87951895;E-mail: ealezzy@zjuem. zju. edu. cnVol. 15 No. 3Influence of concentration of zinc ions on electrocrystallization process of zinc● 643 ●used to observe the morphologies of the electrode-of zinc electrodeposits changes from dentritic to theposited layers.compact with the increase of Zn2 + ion concentra-tion (Figs. 2 - 4). Because the electroplating3 RESULTS AND DISCUSSIONprocess under diffusion control favors the forma-tion of less compact deposits, such as spongy and3.1 Voltammetric studydentritic coatings, the rate determining step mayThe influence of the concentration of Zn2+ ionschange from diffusion process of Zn2 + ions throughin the electroplating solution on zinc depositionprocess was first investigated by cyclic voltamme-try. The sweep potential range was set from opencircuit potential to - 1.5 V and always initiated inthe negative direction. Fig. 1 shows the typical vol-tammograms obtained in electroplating solutionwith different Zn2+ ion concentrations. All voltam-mograms presented here are obtained during thefirst cycle with a scanning rate of 10 mV/s. It canbe seen that there are two distinct current peaks ineach voltammogram. The cathodic peak is associat-ed with Zn2+ ions reduction process( electrodeposi-208pmtion); while the anodic peak with the Zn dissolu-tion process from the cupro-zinc substrate. Mean-Fig.2 SEM morphology of zinc electrodepositwhile, in the anodic branches of the zinc deposi-obtained in electrolyte containingtion, the major dissolution peaks are located at dif-30 g/L Zn2+ ionsferent potentials, which implies that different zincphases are involved.40.01 t≤-0.01 t- 30g/L-0.0260g/L90g/L4一100g/L-0.03 tFig.3 SEM morphology of zinc electrodeposit!-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.660 g/L Zn?+ ionsφ(vs SCE)/VFig. 1 Cyclic voltammograms forbrass electrode in basic electrolyte(pH=4. 0)containing different Zn2+ ion concentrations(Cathodic current is taken as positive one)From Fig. 1, it can be seen that the cathodiccurrent peak intensity increases with the concen-tration of Zn2+ ions, indicating that the zinc depo-sitionis controlled ( limited) by the diffusionprocess of Zn2+ ions as far as the investigated con-中国煤化工19/7AY/83centration range concerned]. However, SEM ob-fYHCNMHGservation of the layers obtained in different Zn2ion concentrations contained electrolyte solutionFig. 4 SEM morphology of zinc electrodeposit(J= -1.5 A/dm2 ) shows that, under an identi-cal electroplating current density, the morphology90g/L Zn2+ ionsTrans. Nonferrous Met. Soc. ChinaJun.2005the bulk electrolyte solution to the cathodic surfaceto electrochemical reaction with the increase of0.10(a)Zn2+ ions concentration. .Potential0.08-43.2 Chronoamperometric study一-1.210VIn order to clarify the discrepancy between the4--1 406 V0.06Fresults of cyclic voltammetry and SEM technique,zinc nucleation process is characterized in more de-tail by chronoamperometric analysis.0.043Generally, nuclei are preferentially formed onsurface inhomogeneities, such as emergence points0.02of edge, screw dislocations, atomic disorder, kink21sites, or monoatomic steps. And there are anumber of literatures describing the electrocrystal-23lization process mathematically{9-16], however, up1t/:to date, the most widely employed theoreticalmodel for electrochemical nucleation is the one de-1.5p(bOverpotential 0.763 8 Vveloped by Hills and his colleaguesl1]. In this10-6 N=3.678 cm-2model[12-15]， the authors describe the kinetics of1- (1Imax)2_Instantaneouselectrolytic phase formation at early stages whenProgressivediffusion of the electroactive species from bulk solution to the interface is the rate determining step,0.5Hand the growth of nuclei is considered to be 3Dwhen taking into account of overlap of diffusionzones. According to this model, the rising portionof the current transient can be described, respec-tively for the instantaneous nucleation and progres-sive nucleation by-0.nD/2c 17.510.0I:=T121%[1- exp(- NrkDt )](1)/maxk= (8tcM)"(2)(c)nD1/2 c1.0-Overpotential 0.9598 V1.= ni?e「1-exp(-No,k"D*)](3)10-7N=1 .625 cm-2/Im )2k'=((4)( 8rcM \/2(4)0.8当。where n is the molar charge of electrodepositing曾0.6species, D the diffusion coefficient, c the bulk con-centration of the zinc species, N is the number of0.4nuclei, N。and NA,∞are the total number of ac-tive sites for instantaneous nucleation and progres-.2sive nucleation respectively, M is the molar mass ,ρ is the density of the deposited material, and kand k' are the numerical constants determined by412620the experimental conditions.t/tmaxFigs. 5- 7 show the relationship of the experi-mental non dimensional variables between (t/tmax )Fig. 5 Potentiostatic I- -t transients(a) andnon-dimensional 1/Imx vs t/tmax plot forand (I /Ix)”，where Imx and tmx are the maxi-mum current transient values. Determination oelectrodeposition of zinc from electrolytethe nucleation process involved is achieved by ana-containing 30 g/L ZnCl2(b, c)lyzing the rising section of the current transientand then comparing the curve to the dimensionlesschen中国煤化Iluently rising por-theoretical curves obtained from Eqns. (1) and (3)tion:ss and a posteriorrespectively.decrMYHC N M H Gaiffusion process.From Figs.5-7, it can be seen that, each ofThe rising section appears to reach its maximum inthe“I- -t”curves consists of an initial spike( withinincreasingly shorter time with more negative over-the first 0. 02 s) due to the charging of the electro-potential steps. The maximum in the current tran-Vol.15 No. 3Influence of concentration of zinc ions on electrocrystallization process of zinc● 645 .0.06 (a)Potential2.s5-(b)Overpotential 0.6885 V10-5 N=3.567 cm-0.05. 1一(l/max)22.0fInstantancous44--1.297V3一Progressive0.04≤0.03” 1.00.02 Io.sH0.01 '0.51.01.52.02.53.03.5 4.0t/:t/max1.(1- (lImx)21.0Overpotential 0.8335VInstantaneous10-6 N=2.292 cm-23 - - Progressive- (/Imx)20.8%。%居0.6|Progressive0.4一20.2Overpotential 0.7275 V.310-5 N=-6.864 cm-2012345678012025t/tmaxFig. 6 Potentiostatic I一t transients(a) and non-dimensional I/Imx vs t/tmx plot forelectrodeposition of zinc fromelectrolyte containing 60 g/L ZnCl2(b, c,. d)sient at high overpotential and short time corre-seen that the nuclear densities(N) increase withsponds to the maximum surface area， i. e. theoverpotential, and the overpotential for nucleationpoint at which hemi-spherical nuclei are on thecurrent obviously decreases with the increase of thepoint of collision. Meanwhile, it can also be seenconcentration of zinc ions, which indicates that thethat, in the case of high overpotentials, the experi-zinc electroplating process in electrolyte solutionmental I- -t curves follow closely the theoretic in-containing less Zn2+ ions is mainly controlled bystantaneous nucleation curve. However， at lowdiffusion processes, which coincides with the aboveoverpotential, no distinguished nucleation currentSEM results.is observed because the practical electrode sub-Because the rate determining step of the zincstrate surfaces usually possess many defects suchelectroplating process is changed from diffusionas plateau edges, kinks, vacancies and emergentprocess to activation reaction with the increase ofscrew dislocations, at which crystal growth can oc-zinc ions concentration， the structure of thecur without nucleation.electrodeposited zinc is changed from spongy toIn order to clarify the influence of the concen-compact(Figs. 2 - 4)，resulting in the positive drifttration of zinc ions on the zinc electroplatingof th中国煤化工。cyclic voltammo-process deeply and clearly, the nuclear densitiesgramons concentration(N) at different concentrations of zinc ions are cal-(Fig.YHCNMHGculated according to the theoretical Eqn. (5)11-141and ilustrated in Figs. 5- 7.4 CONCLUSIONS ;Imx =0.638 2 nDc(kN)1/2(5)With the comparison of Figs. 5 -7, it can be1) Zinc electroplating processes are controlled●646●Trans. Nonferrous Met. Soc. ChinaJun.20050.09Potential.0-(6)Overpotential 0.7145 V10-6N=1.306 cm-20.072- -1.174V0.80.062. !一(1/max)20.05InstantaneousS0.04 |3 - - Progressive0.40.023二1:197V0.2一2-30-35468/st/max1.2()Overpotential 0.7375 V1.0(d)Overpotential 0.905 5 V10-6 N=1.812 cm-210-6N=7.883 cm-2了-0.8 |1- (/ma)}2_- Instantaneous3一Progressive- Progressive0.600一2040t/tmaxFig. 7 Potentiostatic I-t transients(a) and non-dimensional I/Imx vs t/tmx plot forelectrodeposition of zinc from electrolyte containing 90 g/L ZnCl2(b, c, d)by diffusion of Zn2+ ion from bulk solution to ca-[2] Bai A，Hu C C. Iron-cobalt and iron-cobalt nickelnanowires deposited by means of ceyclic voltammetrythodic surface. However, SEM and chronoampero-and pulse reverse electroplating [J]. 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