- Email: [email protected]
choline chloride ionic liquid
Electrochimica Acta 147 (2014) 513–519
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical deposition of zinc from zinc oxide in 2:1 urea/choline chloride ionic liquid Haoxing Yang a , Ramana G. Reddy b, * a b
Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 June 2014 Received in revised form 19 September 2014 Accepted 27 September 2014 Available online 30 September 2014
To overcome the major difficulties involved in the electrodeposition of zinc (Zn) from traditional aqueous solvents, it is important to discover novel electrolytes that are eco-friendly and highly efficient. Zinc oxide (ZnO) was dissolved in eutectic mixture of urea/choline chloride (2:1 molar ratio) at different temperatures (343-373K). Electrochemical measurements confirmed that the onset reduction potential for Zn2+ to Zn occur at -1.05V using cyclic voltammetry. A linear relationship was obtained between cathodic peak current and square root of scan rate, thus indicating the reaction was governed by diffusion-controlled mechanism. Electrodeposition of Zn followed three dimensional (3D) instantaneous nucleation and growth mechanism for various reduction potentials. Scanning electron microscopy (SEM) images showed the formation of hexagonal-shaped Zn particles at lower applied potentials, while platelike structures of Zn metal were deposited at higher applied potentials. The X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis confirmed the presence of high-pure Zn metal electrodeposits on Cu cathode. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition zinc urea-choline chloride diffusion coefficient nucleation
1. Introduction In electroplating industry, Zn is commonly used as a coating material on reactive metal surfaces due to its anti-corrosive property [1]. More than 80% of total Zn production in the world is produced from zinc sulfate electrolytic process [2]. Zinc sulphate ore is mined from the earth’s crust and several flotation methods are employed to produce zinc sulfide mineral concentrates. These wet concentrates are roasted in hot air to form metal oxides. Leaching of the metal oxides with sulfuric acid produces zinc sulfate solution and Zn metal is electrodeposited from the zinc sulphate electrolyte using constant current electrodeposition at ambient temperatures [3]. However, zinc sulphate electrolytic process is very sensitive to impurities that are very hard to separate from the electrolyte solution [4]. Therefore, the traditional method requires effective purification methods to electrodeposit Zn from the aqueous solutions. Recently, ionic liquids (ILs) have been widely used as novel solvents for dissolving organic and inorganic substances. Other important applications of ionic liquids are: (a) solvents for organic and catalytic reactions, (b) electrolytes for electrochemical
* Corresponding author. Tel.: +1 205 348 4246. E-mail address: [email protected] (R.G. Reddy). http://dx.doi.org/10.1016/j.electacta.2014.09.137 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
processes and (c) solvents for extraction and separation processes [5]. The primary advantages of ionic liquid electrolytes are: (i) negligible vapor pressure, (ii) non-flammable (iii) non-corrosive, (iv) wide electrochemical window, (v) high thermal stability, (vi) high solubility of metal salts and (vii) high electrical conductivity [6]. Unlike traditional solvents that typically consist of neutral molecules, ILs primarily contain charged species i.e., anions and cations [7]. ILs are classified into three types. First-generation type ILs are AlCl3-based ILs and are well-known for their applications at ambient temperatures. However, the hygroscopic nature of AlCl3based ILs limit their wide-scale applicability as room temperature ILs or RTILs[8]. The second-generation type ILs are air and moisture stable electrolytes containing 1-ethyl-3-methylimidazolium as cation and either tetrafluoroborate or hexafluorophosphate as anion. But formation of HF in the presence of water causes decomposition of the IL and therefore, long term stability is a main concern for second-generation ILs [9]. The third-generation ILs are eutectic solvents with general formula: R1R2R3R4N+X.zY, where R1R2R3R4N+ is a quaternary ammonium cation, X is a halide anion, and z is number of Y molecules (MClx or RCONH2) that forms complex with X [10]. Only few studies have been reported on RCONH2 based thirdgeneration ILs, where RCONH2 molecule acts as complexing agent via hydrogen bonding donor groups. Eutectic mixture of urea (NH2CONH2) and choline chloride (HOC2H4N(CH3)3+Cl) have been
514
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
2.3. Electrochemistry
Fig. 1. Chemical structure of (a) urea and (b) choline chloride.
investigated as a potential electrolyte for extracting metals from corresponding metal oxides (MOs). Chemical structures for urea and ChCl are shown in Fig. 1. The melting points of urea and ChCl are 408 K and 575 K respectively [11]. Studies on the structure and electrochemical stability of urea and ChCl show that (1) the eutectic mixture of urea/ChCl (2:1 molar ratio) has low melting point (290 K) compared to both the molecules and(2) the decrease in melting point for urea/ChCl mixture is related to interaction strength between urea and ChCl [12,13]. Unlike other traditional solvents that requires rigorous refining process for zinc production, Urea/ChCl electrolyte is an ideal choice because of its unique ability to selectively dissolve metal oxides. Table 1 lists the solubility of several MOs in eutectic mixture of Urea/ChCl (2:1 molar ratio) at 333 K using inductively coupled plasma atomic emission spectroscopy (ICP-AES) [14].ZnO, PbO2 and Cu2O showed appreciable solubility in Urea/ChCl, thus showing the possibility for quick electrolytic production of Zn, Pb and Cu from respective oxides. In the current study, we discuss the electrodeposition of Zn metal from ZnO using eutectic mixture of Urea/ChCl (2:1 molar ratio) at different temperatures (343 to 373K). The specific objectives of the research work are (i) to investigate the solubility of ZnO in Urea/ChCl IL, (ii) to calculate the diffusion coefficient and the activation energy for Zn electrodeposition process, (iii) to determine the underlying reduction mechanism and (iv) to characterize the electrodeposits of zinc.
The electrochemical experiments were carried out using EG&G PARC model 273 A potentiostat/galvanostat. The instrument was controlled by a desktop computer using Power Suite software (Princeton Applied Research) [15].The cyclic voltammetry (CV) of ZnO-Urea/ChCl electrolyte solution was carried out using threeelectrode system. Platinum wire (0.004” diameter), silver wire (0.004” diameter) and tungsten wire (0.019” diameter) were used as counter, reference and working electrodes respectively. Chronoamperometry (CA) experiments were conducted in the same electrolyte using three-electrode system that contains a copper foil(0.02” thickness, controlled surface area) as working electrode. All the electrodes were rinsed with acetone and deionized water and dried completely before and after the experiments. 2.4. Characterization of the Zn deposits The Zn deposits on Cu cathode were characterized using X-ray diffraction (XRD) pattern obtained from Phillips MPD XRD instrument that uses monochromatic Cu ka radiation [16]. The XRD pattern of the electrodeposits was compared with standard ICDD card of Zn metal. The morphology and elemental analysis of Zn deposits were carried out using scanning electron microscope (SEM, JEOL 7000)and energy dispersive spectrometer (EDS) equipped with the SEM. The SEM images were obtained by setting the accelerating voltage and working distance to 20kV and 10.0mm respectively. 3. Results and discussion 3.1. Dissolution of ZnO in Urea/ChCl electrolyte
2. Experimental Procedure The dissolution of ZnO in Urea/ChCl(2:1 molar ratio) electrolyte was studied using Fourier transform infrared spectroscopy (FTIR). Fig. 2(b) shows the FTIR spectrum of Urea/ChCl (2:1 molar ratio)
2.1. Preparation of the ionic liquid Urea (NH2CONH2, 99.3%) and choline chloride (HOC2H4N (CH3)3+Cl-, 98%) were purchased from Alfa Aesar, Ward Hill, MA (USA). Both the chemical reagents were dried under vacuum at 373K for 2h before electrolyte preparation. The eutectic mixture was prepared by combining urea and ChCl (2:1 molar ratio) at 363 K for 12 h under argon atmosphere until a clear homogeneous solution was obtained. 2.2. Dissolution of ZnO in 2:1 Urea/ChCl ionic liquid Different amounts of ZnO(Fisher Scientific, ACS grade, 99.0%) were dissolved in eutectic mixture of Urea/ChCl (2:1 molar ratio) at different temperatures (343 to 373K) under constant stirring. Dissolution of ZnO in eutectic mixture of Urea/ChCl (2:1 molar ratio) was confirmed from Fourier Transform Infrared Spectroscopy (FTIR, model Perkin Elmer spectrum 400) analysis.
Table 1 Solubility (using ICP-AES) of metal oxide in Urea/ChCl(2:1 molar ratio) at 333 K. Metal Oxide
Solubility (ppm)
ZnO Cu2O CuO PbO2 Al2O3 MnO2 Fe2O3 Fe3O4
8466 8725 470 9157 <1 493 49 40
Fig. 2. FTIR spectrum of (a) ZnO addition (0.82M) to Urea/ChCl(2:1 molar ratio) and (b) Urea/ChCl (2:1 molar ratio) at 363K.
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
515
Table 2 Standard stretching and bond vibration IR frequencies of different functional groups. Functional Group
Characteristic Absorptions (cm-1)
Intensity
-NH2 -RNHC=ONHR -CH3 -R4 N+ -CCO
3202-3444 1620-1668 1360-1474 970-1360 955
Strong Strong Strong Medium Strong
electrolyte and analysis of bond-stretching frequencies for Urea/ ChCl functional groups indicate a close agreement with the corresponding peaks from literature (in Table 2) [12].Fig. 2(a) shows the FTIR spectrum of ZnO-Urea/ChCl electrolyte and a large absorption peak was observed at 2150cm-1 in comparison to Fig. 2(b), thus indicating complete dissolution of ZnO in Urea/ChCl. The Cl- ions from the electrolyte weakens the Zn-O bond of dissolved ZnO and upon interaction of Zn (II) ion with urea results in the formation of complex anion, [ZnClO urea]-. The additional absorption peak at 2150 cm-1 is due to stretching vibration of Zn (II) - urea bond of [ZnClO urea] - ion [17] and therefore, the peak acts as a diagnostic tool to identify the dissolution of ZnO in Urea/ChCl electrolyte. Dissolution of ZnO in Urea/ChCl (2:1 molar ratio) electrolyte depends on (a) the temperature of mixing and (b) the concentration of ZnO in the electrolyte. Fig. 3 shows the effect of temperature on the dissolution of 0.82 M of ZnO in Urea/ChCl (2:1 molar ratio). The area under the characteristic FTIR absorption peak (2100 to 2300 cm-1) increases with the increase in temperature (from 343 to 373K), thus implying a greater solubility of ZnO in Urea/ChCl at higher temperatures. This is because of the additional heat obtained at higher temperatures, which facilitates the dissolving reaction by providing sufficient energy to break bonds in the solid. Fig. 4 shows the effect of ZnO concentration on dissolution of ZnO in Urea/ChCl (2:1 molar ratio) at 373 K. From the inset of Fig. 4, it is clearly showed that FTIR absorption intensities (2100 to 2300 cm-1) increases linearly with ZnO concentration from 0.41 to 1.23 M, which is according to Beer-Lambert law; while no change in peak intensity was observed from 1.23 to 2.05 M. Therefore, the
The solubility measurement of ZnO in Urea/ChCl eutectic mixture lays a good foundation for the electrodeposition of Zn from ZnO-Urea/ChCl mixture. Cyclic Voltammetry (CV) is used to study the electron-transfer process and electrode reaction mechanism for ZnO-Urea/ChCl system.The solid curve of Fig. 5 shows the CV of Urea/ChCl (2:1 molar ratio)electrolyte. Formation of hydrogen gas takes place at the cathodic limit (-1.2V) [12]. The dotted curve in Fig. 5 shows the CV of ZnO-Urea/ChCl electrolyte.
Fig.3. FTIR absorbing peaks (2100 - 2300 cm-1) of 0.82 M ZnO in Urea/ChCl (2:1 molar ratio) at different temperatures.
Fig. 5. Cyclic Voltammetry of (—) blank electrolyte (Urea/ChCl) and (—) 0.41 M ZnO addition to Urea/ChCl (2:1 molar ratio), at 373 K and a scan rate of 50 mV/s.
Fig. 4. FTIR absorbing peaks (2100 - 2300 cm-1) of different ZnO concentrations in Urea/ChCl(2:1 molar ratio) at 373 K and inset displays IR peak absorbance vs. concentration of ZnO.
solubility limit of ZnO in Urea/ChCl(2:1 molar ratio) electrolyte is 1.23 M at 373 K. 3.2. Cyclic Voltammetry of ZnO-Urea/ChCl electrolyte
516
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
The red-ox reactions of Zn occurs within the electrochemical window of the blank electrolyte. Only one set of red-ox reaction peaks was observed in CV for ZnO-Urea/ChCl electrolyte, thus indicating the electrodeposition of Zn takes place via one step two-electron transfer process. The onset reduction potential at -1.05V is due to the reduction of Zn2+ to Zn and similarly, the oxidation peak at -0.8 V is mainly because of stripping of deposited Zn. The electrodeposits were confirmed as pure Zn metal from XRD and EDS (Fig. 10 and 11) analysis. Fig. 6 shows the CV plot of ZnO-Urea/ChCl electrolyte at different scan rates of (a) 20, (b) 35, (c) 50, (d) 65 and (e) 80 mV/s. Asymmetric reduction/oxidation peaks are observed, however, it is hard to judge reversibility of a metal ion/metal redox couple because a solid electrode is used as the working electrode. Nevertheless, it is observed that the maximum reduction potential (Epc) undergoes a negative shift as the scan rate is increased from 20 to 80 mV/s which is a sign of an irreversible process. Meanwhile, the separation of reduction/ oxidation peaks is much larger than the theoretical value 2.3RT/nF (37 mV when n=2), which is expected for a reversible exchange process. These features suggest that the charge-transfer process is very likely to have occurred in an irreversible manner [18–20] and similar observations are made in other literatures [21,22]. It is also noticed that there is a clear interception between cathodic and anodic peak observed when scan direction was reversed (pointed out in Fig. 6). This phenomenon indicates that there is nucleation and growth mechanism involved when Zn deposited on the cathode. Moreover, the cathodic peak current density (jpc) against square root of scan rate (v1/2) is found to be linear, as showed in the inset of Fig. 6, which suggests that the reduction process is most likely to be controlled by diffusion of electroactive species from the electrolyte to the electrode surface. However, the plot of jpc vs. v1/2 does not pass through the origin as expected for simple linear diffusion process. This additional kinetic current may be due to the nucleation and growth involved in the process of diffusion or could be because of the influence of electrolyte resistance [23–25].
Fig. 6. Cyclic voltammograms of 0.41 M ZnO addition to Urea/ChCl(2:1 molar ratio) at 373 K and scan rates (mV/s): (a) 20, (b) 35, (c) 50, (d) 65 and (e) 80 and inset plot of jpc vs. v1/2.
3.3. Nucleation and Growth of Zn particles Chronoamperometry (CA) experiments were performed in Urea/ChCl electrolyte to study the nucleation and growth mechanism of Zn electrodeposition on Cu substrate. Fig. 7 displays the overlay of current-time transients obtained by stepping the potential from an initial value of -1.0 V, where no reduction took place, to a sufficiently negative value that initiated the nucleation and growth of Zn particles. All current-time transients follow a typical trend of diffusion-controlled nucleation and growth process. Because of the charge associated with the electrochemical double layer, a current fluctuation was observed at the initial time of 0.02 s. The subsequent rising portion of the curve is because of increase in electroactive surface area resulting from the nucleation and growth of Zn particles on the Cu cathode. A hemispherical diffusion zone is generated around the nuclei during the growth process and upon overlapping of all these zones, the hemispherical mass transfer of nuclei is converted to planar mass transfer. Thus, the maximum current density (jm) observed at time (tm) starts to decay and the decaying portion of the current-transient curve is described by Cottrell equation i.e., inverse-relationship between current and time for a diffusion-controlled process [26].In addition, the time (tm) needed to reach maximum current density (jm) shifted to lower value at higher negative applied potential. Scharifker and Hills introduced and developed the most widely used model for describing nucleation and growth mechanism [27]. According to their theory, the 3D nucleation and growth process of metal particles can be either (i) instantaneous nucleation (fast nucleation on small number of active sites) or (ii) progressive nucleation (slow nucleation on large number of active sites). The governing equations of instantaneous and progressive nucleation processes are given below in eq. (1) and eq. (2) respectively [28]: 1 t f1 exp½1:2564ðt=tm Þg2 (1) ðj=jm Þ2 ¼ 1:9542 tm
Fig. 7. Current-time transients from chronoamperometry of 0.41 M ZnO addition in Urea/ChCl(2:1 molar ratio) at 373 K for different potentials (V): (a) -1.0, (b) -1.1, (c) -1.2 and (d) -1.3.
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
517
Fig. 9. X-ray diffraction (XRD) pattern of Zn electrodeposits on Cu cathode. Fig. 8. A comparison between dimensionless experimental current-time transients obtained at potentials (V): (&) –1.1, () -1.2 and (D) -1.3 and theoretical models of (a) instantaneous (solid curve) and progressive (— dotted curve) nucleation under diffusion-controlled growth of Zn particles.
ðj=jm Þ2 ¼ 1:2254
t tm
1
2
f1 exp½2:3367ðt=tm Þ2 g
(2)
To understand whether the given process is an instantaneous or a progressive nucleation, a common method is to compare the experimental current-time transients with the theoretical transients obtained for each mechanism. Fig. 8 shows a comparison between plots obtained from dimensionless experimental current density-time transients with the theoretical models of equations (1) and (2). Instantaneous nucleation process takes place at each applied potential during electrodeposition of Zn. At longer time periods, the predicted current density from the instantaneous model is smaller than the experimental current densities, which are likely from the partial kinetic control of the growth process [29,30]or due to the associated cathodic reaction of ionic liquid (reduction of hydrogen). All the nucleation processes are followed by diffusion-controlled growth process of Zn particles. The diffusion coefficient (D in cm2s-1) is calculated by measuring jm and tm values from Fig. 7 and by substituting them in equation (3) [31]: 2
jm tm ¼ 0:1629ðnFCÞ2 D
Reddy’s research group [32–34] and other groups [35,36] and are listed in Table 3. All values of diffusion coefficients fall within reasonable range. The different values of diffusion coefficient for Zn contained species could be due to different temperatures, and the ionic liquid systems considered for the electrodeopsition. 3.4. Electrodeposition of Zn metal The potentiostatic electrodeposition of Zn was conducted at 373K on Cu substrate for 1h based on the reduction potential of Zn2 + ions from the CV experiments. Under these conditions, the presence of [ZnOClurea]-as the predominant species in the electrolyte was also confirmed by other researchers [37,38]. From the FTIR spectrum of the electrolyte, the stretching of C-C=O bond occurs at 955 cm-1 and therefore, the structure of Ch+ is not destroyed in ZnO-Urea/ChCl system. The remaining Ch+ ions react with [ZnOClurea]- ions and the two electrons needed for reduction of Zn2+ is supplied by the anode, thus leading to the electrodeposition of Zn metal on cathode and leaving [ChClOurea]2- as possible zinc free species in the electrolyte. The possible electrochemical reactions are given below:
(3)
The average value of the diffusion coefficient is 1.89 10-8cm2/s which is compared with the various systems investigated by Table 3 Diffusion coefficient of different species in various ionic liquid systems. Ionic liquid
Predominant Species
T(K)
D (cm2 s-1)
Reference
Urea/ChCl Urea/ChCl Urea/ChCl AlCl3-BMIC AlCl3-EMIC [Bu3MeN]Tf2N AlCl3- MeEtimCl
[ZnOClurea][PbOClurea][CoCl3][Al2Cl7][Al2Cl7]Zn2+ Zn2+
373 363 373 363 363 373 313
1.8910-8 2.4210-7 1.710-6 2.210-7 9.110-7 4.1210-8 6.710-7
This work In press [32] [33] [34] [35] [36] Fig. 10. EDS spectrum of Zn electrodeposits on Cu cathode.
518
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
Fig. 11. SEM micrographs of Zn electrodeposits on Cu substrate at different potentials (V): (a) -1.1, (b) -1.2, (c) –1.25 and (d) -1.3. On the right are corresponding images of 5 X magnification.
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
Dissolution reaction: ZnO + Cl + urea ! [ZnOClurea] -
-
(4)
Cathodic reaction: Ch + [ZnOClurea]-+ 2e-!Zn(s)+ [ChClOurea]2+
(5)
[ChClOurea] !Ch + Cl + 2e +0.5O2 +
substrate was carried out using chronoamperometry at different potentials. XRD and EDS analysis confirmed the presence of highpure Zn metal deposits. SEM images showed a transition in surface morphology of Zn deposits, from small hexagonal crystals to agglomerated platelets, upon application of higher potentials. Acknowledgements
Anodic reaction: 2-
519
-
-
(6)
Overall reaction: [ZnOClurea]- ! Zn (s) + 0.5O2 + Cl- + urea
(7)
3.5. Characterization of Zn electrodeposits Fig. 9 shows the characteristic XRD pattern of Zn electrodeposits in addition to the Zn (ICDD File No.00-004-0831) peaks at diffracting angle (2u): 363 , 389 , 432 , 543 , 700 , 821 , 866 and 899 corresponding to (0 0 2), (1 0 0), (1 0 1), (1 0 2), (1 0 3), (11 2), (2 0 1) and (1 0 4) reflections of standard hexagonal Zn metal. The distinct peak observed at 50.4 is because of Cu substrate (from Cu ICDD File No. 00-004-0836). Fig. 10 shows composition analysis of the electrodeposits using EDS spectrum. All strong peaks of the spectrum corespond to high pure Zn depsoits, while the weak peaks are due to Cu substrate. Fig. 11 shows the SEM morphologies of Zn electrodeposits on the Cu cathode obtained at different potentials. Fig. 11(a) display SEM image of the electrodeposits obtained at -1.1 V. The film consists of non-uniform, less compact and nodular particles of Zn metal. In Fig. 11(b), the electrodeposits obtained at -1.2 V contain coarse grains of typical hexagonal like Zn crystals. At this stage, the film is dense and grain-growth was significant with an average particle size of 5 mm. As the potential reaches to more negative value of -1.25 V (Fig. 11(c)), agglomeration of Zn particles takes place without any further increase in size of the particles. However, the individual particles splits into small plate-like structures at higher applied potential. In Fig. 11(d), the electrodeposits obtained at -1.3 V contain platelet-type structures that are aligned perpendicular to surface of the substrate. As shown in the micrograph at higher magnification, the stacked-up platelets agglomerate inside each grain and undergo further nucleation along particular direction to form plate-like crystal. Liu et al., also observed similar changes in morphology of the electrodeposits due to absorption of solvation layers at the growing Zn surface [39]. 4. Conclusions The solubility of ZnO in eutectic mixture of Urea/ChCl (2:1 molar ratio) was studied using FTIR spectroscopy. The optimum values of experimental temperature and ZnO concentration were 373K and 1.23 M respectively. The reduction of Zn2+ to Zn occurred at -1.05V in cyclic voltammetry curve. The nucleation was modeled as instantaneous process and the growth of Zn particles was governed by diffusion-controlled mechanism. Electrodeposition of Zn on Cu
The authors are pleased to acknowledge the financial support from National Science Foundation Grant No. DMR-1310072 and ACIPCO for this research project. We also thank The University of Alabama for providing the experimental and analytical facilities. References [1] H. Geduld, Zinc Plating, ASM International, Teddington, London, 1988. [2] A.P. Brown, J.H. Meisenheider, N. Yao, Ind. Eng. Chem. Prod. Res. Dev. 22 (1983) 263–272. [3] P. Guillaume, J. Appl. Electrochem. 37 (2007) 1237–1243. [4] D.J. Mackinnon, J.M. Brannen, R.C. Kerby, J. Appl. Electrochem. 9 (1979) 55–70. [5] J.F. Brennecke, E.J. Maginn, ALChE. J. 47 (2001) 2384–2389. [6] R.G. Reddy, J. Phase Equilib. Diff. 27 (2006) 210–211. [7] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [8] C.L. Hussey, Adv. Molten Salt Chem. 5 (1983) 185. [9] A.P. Abbott, Chem. Phys. 8 (2006) 4265–4279. [10] F. Endres, S. Abedinw, Phys. Chem. Chem. Phys. 8 (2006) 2101. [11] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, P. Shikotra, Inorg. Chem. 44 (2005) 6497–6499. [12] D. Yue, Y. Jia, Y. Yao, Y. Sun, Y. Jing, Electrochim. Acta 65 (2012) 30–36. [13] H.G. Morrison, S. Neervannan, Int. J. Pharm. 378 (2009) 136–139. [14] A.P. Abbott, G. Capper, D.L. Davies, K.J. McKenzie, S.U. Obi, J. Chem. Eng. Data 51 (2006) 1280–1282. [15] D. Pradhan, D. Mantha, R.G. Reddy, Electrochim. Acta 54 (2009) 6661–6667. [16] D. Pradhan, R.G. Reddy, Electrochim. Acta 54 (2009) 1874–1880. [17] Y. Zheng, K. Dong, Q. Wang, S. Zhang, Q. Zhang, X. Lu, Sci. China. Chem. 55 (2012) 1587–1597. [18] R.S. Nicholson, Anal. Chem. 37 (1965) 1351–1355. [19] R.S. Nicholson, I. Shain, Anal. Chem. 36 (1964) 706–723. [20] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamental and Applications, John Wiley& Sons, New York, 2000, pp. 137–164. [21] A. Gomes, M.I. da Silva Pereira, Electrochim. Acta 52 (2006) 863–871. [22] P. Chen, I. Sun, Electrochim. Acta 46 (2001) 1169–1177. [23] Y. Nuli, J. Yang, P. Wang, Appl. Surf. Sci. 252 (2006) 8086–8090. [24] T. Jiang, M.J. Chollier, G. Dubé, A. Lasia, G.M. Brisard, Surf. Coat. Tech. 201 (2006) 1–9. [25] Y. Zhao, T.J. VanderNoot, Electrochim. Acta 42 (1997) 1639–1643. [26] A. Radisic, J.G. Long, P.M. Hoffmann, C. Searson, J. Electrochem. Soc. 148 (2001) 41–46. [27] B. Scharifker, G. Hills, Electrochim. Acta 28 (1981) 879–889. [28] B. Scharifker, J. Mostany, J. Electroanal. Chem. 177 (1984) 25–37. [29] P. He, H. Liu, Z. Li, J. Li, J. Electrochem. Soc. 152 (2005) 146–153. [30] S. Langerock, L. Heerman, J. Electrochem. Soc. 151 (2004) 155–160. [31] J. Lee, B. Miller, X. Shi, R. Kalish, K.A. Wheeler, J. Electrochem. Soc. 147 (2000) 3370–3376. [32] M. Li, Z. Wang, R.G. Reddy, Electrochimi. Acta 123 (2014) 325–331. [33] D. Pradhan, R.G. Reddy, Mater. Chem. Phys. 143 (2014) 564–569. [34] V. Kamavaram, D. Mantha, R.G. Reddy, Electrochim. Acta 50 (2005) 3286–3295. [35] P. Chen, C.L. Hussey, Electrochim. Acta 52 (2007) 857–1864. [36] W.R. Pitner, C.L. Hussey, J. Electrochem. Soc. 144 (1997) 3095–3103. [37] T. Tsuda, L. Boyd, S. Kuwabata, C.L. Hussey, ECS Trans. 16 (2009) 529–540. [38] A.P. Abbott, G. Capper, K.J. McKenzie, K.S. Ryder, J. Electroanal. Chem. 599 (2014) 288–294. [39] Z. Liu, S. Abedin, F. Endres, Electrochim. Acta 89 (2013) 635–643.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical deposition of zinc from zinc oxide in 2:1 urea/choline chloride ionic liquid Haoxing Yang a , Ramana G. Reddy b, * a b
Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 June 2014 Received in revised form 19 September 2014 Accepted 27 September 2014 Available online 30 September 2014
To overcome the major difficulties involved in the electrodeposition of zinc (Zn) from traditional aqueous solvents, it is important to discover novel electrolytes that are eco-friendly and highly efficient. Zinc oxide (ZnO) was dissolved in eutectic mixture of urea/choline chloride (2:1 molar ratio) at different temperatures (343-373K). Electrochemical measurements confirmed that the onset reduction potential for Zn2+ to Zn occur at -1.05V using cyclic voltammetry. A linear relationship was obtained between cathodic peak current and square root of scan rate, thus indicating the reaction was governed by diffusion-controlled mechanism. Electrodeposition of Zn followed three dimensional (3D) instantaneous nucleation and growth mechanism for various reduction potentials. Scanning electron microscopy (SEM) images showed the formation of hexagonal-shaped Zn particles at lower applied potentials, while platelike structures of Zn metal were deposited at higher applied potentials. The X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis confirmed the presence of high-pure Zn metal electrodeposits on Cu cathode. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition zinc urea-choline chloride diffusion coefficient nucleation
1. Introduction In electroplating industry, Zn is commonly used as a coating material on reactive metal surfaces due to its anti-corrosive property [1]. More than 80% of total Zn production in the world is produced from zinc sulfate electrolytic process [2]. Zinc sulphate ore is mined from the earth’s crust and several flotation methods are employed to produce zinc sulfide mineral concentrates. These wet concentrates are roasted in hot air to form metal oxides. Leaching of the metal oxides with sulfuric acid produces zinc sulfate solution and Zn metal is electrodeposited from the zinc sulphate electrolyte using constant current electrodeposition at ambient temperatures [3]. However, zinc sulphate electrolytic process is very sensitive to impurities that are very hard to separate from the electrolyte solution [4]. Therefore, the traditional method requires effective purification methods to electrodeposit Zn from the aqueous solutions. Recently, ionic liquids (ILs) have been widely used as novel solvents for dissolving organic and inorganic substances. Other important applications of ionic liquids are: (a) solvents for organic and catalytic reactions, (b) electrolytes for electrochemical
* Corresponding author. Tel.: +1 205 348 4246. E-mail address: [email protected] (R.G. Reddy). http://dx.doi.org/10.1016/j.electacta.2014.09.137 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
processes and (c) solvents for extraction and separation processes [5]. The primary advantages of ionic liquid electrolytes are: (i) negligible vapor pressure, (ii) non-flammable (iii) non-corrosive, (iv) wide electrochemical window, (v) high thermal stability, (vi) high solubility of metal salts and (vii) high electrical conductivity [6]. Unlike traditional solvents that typically consist of neutral molecules, ILs primarily contain charged species i.e., anions and cations [7]. ILs are classified into three types. First-generation type ILs are AlCl3-based ILs and are well-known for their applications at ambient temperatures. However, the hygroscopic nature of AlCl3based ILs limit their wide-scale applicability as room temperature ILs or RTILs[8]. The second-generation type ILs are air and moisture stable electrolytes containing 1-ethyl-3-methylimidazolium as cation and either tetrafluoroborate or hexafluorophosphate as anion. But formation of HF in the presence of water causes decomposition of the IL and therefore, long term stability is a main concern for second-generation ILs [9]. The third-generation ILs are eutectic solvents with general formula: R1R2R3R4N+X.zY, where R1R2R3R4N+ is a quaternary ammonium cation, X is a halide anion, and z is number of Y molecules (MClx or RCONH2) that forms complex with X [10]. Only few studies have been reported on RCONH2 based thirdgeneration ILs, where RCONH2 molecule acts as complexing agent via hydrogen bonding donor groups. Eutectic mixture of urea (NH2CONH2) and choline chloride (HOC2H4N(CH3)3+Cl) have been
514
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
2.3. Electrochemistry
Fig. 1. Chemical structure of (a) urea and (b) choline chloride.
investigated as a potential electrolyte for extracting metals from corresponding metal oxides (MOs). Chemical structures for urea and ChCl are shown in Fig. 1. The melting points of urea and ChCl are 408 K and 575 K respectively [11]. Studies on the structure and electrochemical stability of urea and ChCl show that (1) the eutectic mixture of urea/ChCl (2:1 molar ratio) has low melting point (290 K) compared to both the molecules and(2) the decrease in melting point for urea/ChCl mixture is related to interaction strength between urea and ChCl [12,13]. Unlike other traditional solvents that requires rigorous refining process for zinc production, Urea/ChCl electrolyte is an ideal choice because of its unique ability to selectively dissolve metal oxides. Table 1 lists the solubility of several MOs in eutectic mixture of Urea/ChCl (2:1 molar ratio) at 333 K using inductively coupled plasma atomic emission spectroscopy (ICP-AES) [14].ZnO, PbO2 and Cu2O showed appreciable solubility in Urea/ChCl, thus showing the possibility for quick electrolytic production of Zn, Pb and Cu from respective oxides. In the current study, we discuss the electrodeposition of Zn metal from ZnO using eutectic mixture of Urea/ChCl (2:1 molar ratio) at different temperatures (343 to 373K). The specific objectives of the research work are (i) to investigate the solubility of ZnO in Urea/ChCl IL, (ii) to calculate the diffusion coefficient and the activation energy for Zn electrodeposition process, (iii) to determine the underlying reduction mechanism and (iv) to characterize the electrodeposits of zinc.
The electrochemical experiments were carried out using EG&G PARC model 273 A potentiostat/galvanostat. The instrument was controlled by a desktop computer using Power Suite software (Princeton Applied Research) [15].The cyclic voltammetry (CV) of ZnO-Urea/ChCl electrolyte solution was carried out using threeelectrode system. Platinum wire (0.004” diameter), silver wire (0.004” diameter) and tungsten wire (0.019” diameter) were used as counter, reference and working electrodes respectively. Chronoamperometry (CA) experiments were conducted in the same electrolyte using three-electrode system that contains a copper foil(0.02” thickness, controlled surface area) as working electrode. All the electrodes were rinsed with acetone and deionized water and dried completely before and after the experiments. 2.4. Characterization of the Zn deposits The Zn deposits on Cu cathode were characterized using X-ray diffraction (XRD) pattern obtained from Phillips MPD XRD instrument that uses monochromatic Cu ka radiation [16]. The XRD pattern of the electrodeposits was compared with standard ICDD card of Zn metal. The morphology and elemental analysis of Zn deposits were carried out using scanning electron microscope (SEM, JEOL 7000)and energy dispersive spectrometer (EDS) equipped with the SEM. The SEM images were obtained by setting the accelerating voltage and working distance to 20kV and 10.0mm respectively. 3. Results and discussion 3.1. Dissolution of ZnO in Urea/ChCl electrolyte
2. Experimental Procedure The dissolution of ZnO in Urea/ChCl(2:1 molar ratio) electrolyte was studied using Fourier transform infrared spectroscopy (FTIR). Fig. 2(b) shows the FTIR spectrum of Urea/ChCl (2:1 molar ratio)
2.1. Preparation of the ionic liquid Urea (NH2CONH2, 99.3%) and choline chloride (HOC2H4N (CH3)3+Cl-, 98%) were purchased from Alfa Aesar, Ward Hill, MA (USA). Both the chemical reagents were dried under vacuum at 373K for 2h before electrolyte preparation. The eutectic mixture was prepared by combining urea and ChCl (2:1 molar ratio) at 363 K for 12 h under argon atmosphere until a clear homogeneous solution was obtained. 2.2. Dissolution of ZnO in 2:1 Urea/ChCl ionic liquid Different amounts of ZnO(Fisher Scientific, ACS grade, 99.0%) were dissolved in eutectic mixture of Urea/ChCl (2:1 molar ratio) at different temperatures (343 to 373K) under constant stirring. Dissolution of ZnO in eutectic mixture of Urea/ChCl (2:1 molar ratio) was confirmed from Fourier Transform Infrared Spectroscopy (FTIR, model Perkin Elmer spectrum 400) analysis.
Table 1 Solubility (using ICP-AES) of metal oxide in Urea/ChCl(2:1 molar ratio) at 333 K. Metal Oxide
Solubility (ppm)
ZnO Cu2O CuO PbO2 Al2O3 MnO2 Fe2O3 Fe3O4
8466 8725 470 9157 <1 493 49 40
Fig. 2. FTIR spectrum of (a) ZnO addition (0.82M) to Urea/ChCl(2:1 molar ratio) and (b) Urea/ChCl (2:1 molar ratio) at 363K.
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
515
Table 2 Standard stretching and bond vibration IR frequencies of different functional groups. Functional Group
Characteristic Absorptions (cm-1)
Intensity
-NH2 -RNHC=ONHR -CH3 -R4 N+ -CCO
3202-3444 1620-1668 1360-1474 970-1360 955
Strong Strong Strong Medium Strong
electrolyte and analysis of bond-stretching frequencies for Urea/ ChCl functional groups indicate a close agreement with the corresponding peaks from literature (in Table 2) [12].Fig. 2(a) shows the FTIR spectrum of ZnO-Urea/ChCl electrolyte and a large absorption peak was observed at 2150cm-1 in comparison to Fig. 2(b), thus indicating complete dissolution of ZnO in Urea/ChCl. The Cl- ions from the electrolyte weakens the Zn-O bond of dissolved ZnO and upon interaction of Zn (II) ion with urea results in the formation of complex anion, [ZnClO urea]-. The additional absorption peak at 2150 cm-1 is due to stretching vibration of Zn (II) - urea bond of [ZnClO urea] - ion [17] and therefore, the peak acts as a diagnostic tool to identify the dissolution of ZnO in Urea/ChCl electrolyte. Dissolution of ZnO in Urea/ChCl (2:1 molar ratio) electrolyte depends on (a) the temperature of mixing and (b) the concentration of ZnO in the electrolyte. Fig. 3 shows the effect of temperature on the dissolution of 0.82 M of ZnO in Urea/ChCl (2:1 molar ratio). The area under the characteristic FTIR absorption peak (2100 to 2300 cm-1) increases with the increase in temperature (from 343 to 373K), thus implying a greater solubility of ZnO in Urea/ChCl at higher temperatures. This is because of the additional heat obtained at higher temperatures, which facilitates the dissolving reaction by providing sufficient energy to break bonds in the solid. Fig. 4 shows the effect of ZnO concentration on dissolution of ZnO in Urea/ChCl (2:1 molar ratio) at 373 K. From the inset of Fig. 4, it is clearly showed that FTIR absorption intensities (2100 to 2300 cm-1) increases linearly with ZnO concentration from 0.41 to 1.23 M, which is according to Beer-Lambert law; while no change in peak intensity was observed from 1.23 to 2.05 M. Therefore, the
The solubility measurement of ZnO in Urea/ChCl eutectic mixture lays a good foundation for the electrodeposition of Zn from ZnO-Urea/ChCl mixture. Cyclic Voltammetry (CV) is used to study the electron-transfer process and electrode reaction mechanism for ZnO-Urea/ChCl system.The solid curve of Fig. 5 shows the CV of Urea/ChCl (2:1 molar ratio)electrolyte. Formation of hydrogen gas takes place at the cathodic limit (-1.2V) [12]. The dotted curve in Fig. 5 shows the CV of ZnO-Urea/ChCl electrolyte.
Fig.3. FTIR absorbing peaks (2100 - 2300 cm-1) of 0.82 M ZnO in Urea/ChCl (2:1 molar ratio) at different temperatures.
Fig. 5. Cyclic Voltammetry of (—) blank electrolyte (Urea/ChCl) and (—) 0.41 M ZnO addition to Urea/ChCl (2:1 molar ratio), at 373 K and a scan rate of 50 mV/s.
Fig. 4. FTIR absorbing peaks (2100 - 2300 cm-1) of different ZnO concentrations in Urea/ChCl(2:1 molar ratio) at 373 K and inset displays IR peak absorbance vs. concentration of ZnO.
solubility limit of ZnO in Urea/ChCl(2:1 molar ratio) electrolyte is 1.23 M at 373 K. 3.2. Cyclic Voltammetry of ZnO-Urea/ChCl electrolyte
516
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
The red-ox reactions of Zn occurs within the electrochemical window of the blank electrolyte. Only one set of red-ox reaction peaks was observed in CV for ZnO-Urea/ChCl electrolyte, thus indicating the electrodeposition of Zn takes place via one step two-electron transfer process. The onset reduction potential at -1.05V is due to the reduction of Zn2+ to Zn and similarly, the oxidation peak at -0.8 V is mainly because of stripping of deposited Zn. The electrodeposits were confirmed as pure Zn metal from XRD and EDS (Fig. 10 and 11) analysis. Fig. 6 shows the CV plot of ZnO-Urea/ChCl electrolyte at different scan rates of (a) 20, (b) 35, (c) 50, (d) 65 and (e) 80 mV/s. Asymmetric reduction/oxidation peaks are observed, however, it is hard to judge reversibility of a metal ion/metal redox couple because a solid electrode is used as the working electrode. Nevertheless, it is observed that the maximum reduction potential (Epc) undergoes a negative shift as the scan rate is increased from 20 to 80 mV/s which is a sign of an irreversible process. Meanwhile, the separation of reduction/ oxidation peaks is much larger than the theoretical value 2.3RT/nF (37 mV when n=2), which is expected for a reversible exchange process. These features suggest that the charge-transfer process is very likely to have occurred in an irreversible manner [18–20] and similar observations are made in other literatures [21,22]. It is also noticed that there is a clear interception between cathodic and anodic peak observed when scan direction was reversed (pointed out in Fig. 6). This phenomenon indicates that there is nucleation and growth mechanism involved when Zn deposited on the cathode. Moreover, the cathodic peak current density (jpc) against square root of scan rate (v1/2) is found to be linear, as showed in the inset of Fig. 6, which suggests that the reduction process is most likely to be controlled by diffusion of electroactive species from the electrolyte to the electrode surface. However, the plot of jpc vs. v1/2 does not pass through the origin as expected for simple linear diffusion process. This additional kinetic current may be due to the nucleation and growth involved in the process of diffusion or could be because of the influence of electrolyte resistance [23–25].
Fig. 6. Cyclic voltammograms of 0.41 M ZnO addition to Urea/ChCl(2:1 molar ratio) at 373 K and scan rates (mV/s): (a) 20, (b) 35, (c) 50, (d) 65 and (e) 80 and inset plot of jpc vs. v1/2.
3.3. Nucleation and Growth of Zn particles Chronoamperometry (CA) experiments were performed in Urea/ChCl electrolyte to study the nucleation and growth mechanism of Zn electrodeposition on Cu substrate. Fig. 7 displays the overlay of current-time transients obtained by stepping the potential from an initial value of -1.0 V, where no reduction took place, to a sufficiently negative value that initiated the nucleation and growth of Zn particles. All current-time transients follow a typical trend of diffusion-controlled nucleation and growth process. Because of the charge associated with the electrochemical double layer, a current fluctuation was observed at the initial time of 0.02 s. The subsequent rising portion of the curve is because of increase in electroactive surface area resulting from the nucleation and growth of Zn particles on the Cu cathode. A hemispherical diffusion zone is generated around the nuclei during the growth process and upon overlapping of all these zones, the hemispherical mass transfer of nuclei is converted to planar mass transfer. Thus, the maximum current density (jm) observed at time (tm) starts to decay and the decaying portion of the current-transient curve is described by Cottrell equation i.e., inverse-relationship between current and time for a diffusion-controlled process [26].In addition, the time (tm) needed to reach maximum current density (jm) shifted to lower value at higher negative applied potential. Scharifker and Hills introduced and developed the most widely used model for describing nucleation and growth mechanism [27]. According to their theory, the 3D nucleation and growth process of metal particles can be either (i) instantaneous nucleation (fast nucleation on small number of active sites) or (ii) progressive nucleation (slow nucleation on large number of active sites). The governing equations of instantaneous and progressive nucleation processes are given below in eq. (1) and eq. (2) respectively [28]: 1 t f1 exp½1:2564ðt=tm Þg2 (1) ðj=jm Þ2 ¼ 1:9542 tm
Fig. 7. Current-time transients from chronoamperometry of 0.41 M ZnO addition in Urea/ChCl(2:1 molar ratio) at 373 K for different potentials (V): (a) -1.0, (b) -1.1, (c) -1.2 and (d) -1.3.
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
517
Fig. 9. X-ray diffraction (XRD) pattern of Zn electrodeposits on Cu cathode. Fig. 8. A comparison between dimensionless experimental current-time transients obtained at potentials (V): (&) –1.1, () -1.2 and (D) -1.3 and theoretical models of (a) instantaneous (solid curve) and progressive (— dotted curve) nucleation under diffusion-controlled growth of Zn particles.
ðj=jm Þ2 ¼ 1:2254
t tm
1
2
f1 exp½2:3367ðt=tm Þ2 g
(2)
To understand whether the given process is an instantaneous or a progressive nucleation, a common method is to compare the experimental current-time transients with the theoretical transients obtained for each mechanism. Fig. 8 shows a comparison between plots obtained from dimensionless experimental current density-time transients with the theoretical models of equations (1) and (2). Instantaneous nucleation process takes place at each applied potential during electrodeposition of Zn. At longer time periods, the predicted current density from the instantaneous model is smaller than the experimental current densities, which are likely from the partial kinetic control of the growth process [29,30]or due to the associated cathodic reaction of ionic liquid (reduction of hydrogen). All the nucleation processes are followed by diffusion-controlled growth process of Zn particles. The diffusion coefficient (D in cm2s-1) is calculated by measuring jm and tm values from Fig. 7 and by substituting them in equation (3) [31]: 2
jm tm ¼ 0:1629ðnFCÞ2 D
Reddy’s research group [32–34] and other groups [35,36] and are listed in Table 3. All values of diffusion coefficients fall within reasonable range. The different values of diffusion coefficient for Zn contained species could be due to different temperatures, and the ionic liquid systems considered for the electrodeopsition. 3.4. Electrodeposition of Zn metal The potentiostatic electrodeposition of Zn was conducted at 373K on Cu substrate for 1h based on the reduction potential of Zn2 + ions from the CV experiments. Under these conditions, the presence of [ZnOClurea]-as the predominant species in the electrolyte was also confirmed by other researchers [37,38]. From the FTIR spectrum of the electrolyte, the stretching of C-C=O bond occurs at 955 cm-1 and therefore, the structure of Ch+ is not destroyed in ZnO-Urea/ChCl system. The remaining Ch+ ions react with [ZnOClurea]- ions and the two electrons needed for reduction of Zn2+ is supplied by the anode, thus leading to the electrodeposition of Zn metal on cathode and leaving [ChClOurea]2- as possible zinc free species in the electrolyte. The possible electrochemical reactions are given below:
(3)
The average value of the diffusion coefficient is 1.89 10-8cm2/s which is compared with the various systems investigated by Table 3 Diffusion coefficient of different species in various ionic liquid systems. Ionic liquid
Predominant Species
T(K)
D (cm2 s-1)
Reference
Urea/ChCl Urea/ChCl Urea/ChCl AlCl3-BMIC AlCl3-EMIC [Bu3MeN]Tf2N AlCl3- MeEtimCl
[ZnOClurea][PbOClurea][CoCl3][Al2Cl7][Al2Cl7]Zn2+ Zn2+
373 363 373 363 363 373 313
1.8910-8 2.4210-7 1.710-6 2.210-7 9.110-7 4.1210-8 6.710-7
This work In press [32] [33] [34] [35] [36] Fig. 10. EDS spectrum of Zn electrodeposits on Cu cathode.
518
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
Fig. 11. SEM micrographs of Zn electrodeposits on Cu substrate at different potentials (V): (a) -1.1, (b) -1.2, (c) –1.25 and (d) -1.3. On the right are corresponding images of 5 X magnification.
H. Yang, R.G. Reddy / Electrochimica Acta 147 (2014) 513–519
Dissolution reaction: ZnO + Cl + urea ! [ZnOClurea] -
-
(4)
Cathodic reaction: Ch + [ZnOClurea]-+ 2e-!Zn(s)+ [ChClOurea]2+
(5)
[ChClOurea] !Ch + Cl + 2e +0.5O2 +
substrate was carried out using chronoamperometry at different potentials. XRD and EDS analysis confirmed the presence of highpure Zn metal deposits. SEM images showed a transition in surface morphology of Zn deposits, from small hexagonal crystals to agglomerated platelets, upon application of higher potentials. Acknowledgements
Anodic reaction: 2-
519
-
-
(6)
Overall reaction: [ZnOClurea]- ! Zn (s) + 0.5O2 + Cl- + urea
(7)
3.5. Characterization of Zn electrodeposits Fig. 9 shows the characteristic XRD pattern of Zn electrodeposits in addition to the Zn (ICDD File No.00-004-0831) peaks at diffracting angle (2u): 363 , 389 , 432 , 543 , 700 , 821 , 866 and 899 corresponding to (0 0 2), (1 0 0), (1 0 1), (1 0 2), (1 0 3), (11 2), (2 0 1) and (1 0 4) reflections of standard hexagonal Zn metal. The distinct peak observed at 50.4 is because of Cu substrate (from Cu ICDD File No. 00-004-0836). Fig. 10 shows composition analysis of the electrodeposits using EDS spectrum. All strong peaks of the spectrum corespond to high pure Zn depsoits, while the weak peaks are due to Cu substrate. Fig. 11 shows the SEM morphologies of Zn electrodeposits on the Cu cathode obtained at different potentials. Fig. 11(a) display SEM image of the electrodeposits obtained at -1.1 V. The film consists of non-uniform, less compact and nodular particles of Zn metal. In Fig. 11(b), the electrodeposits obtained at -1.2 V contain coarse grains of typical hexagonal like Zn crystals. At this stage, the film is dense and grain-growth was significant with an average particle size of 5 mm. As the potential reaches to more negative value of -1.25 V (Fig. 11(c)), agglomeration of Zn particles takes place without any further increase in size of the particles. However, the individual particles splits into small plate-like structures at higher applied potential. In Fig. 11(d), the electrodeposits obtained at -1.3 V contain platelet-type structures that are aligned perpendicular to surface of the substrate. As shown in the micrograph at higher magnification, the stacked-up platelets agglomerate inside each grain and undergo further nucleation along particular direction to form plate-like crystal. Liu et al., also observed similar changes in morphology of the electrodeposits due to absorption of solvation layers at the growing Zn surface [39]. 4. Conclusions The solubility of ZnO in eutectic mixture of Urea/ChCl (2:1 molar ratio) was studied using FTIR spectroscopy. The optimum values of experimental temperature and ZnO concentration were 373K and 1.23 M respectively. The reduction of Zn2+ to Zn occurred at -1.05V in cyclic voltammetry curve. The nucleation was modeled as instantaneous process and the growth of Zn particles was governed by diffusion-controlled mechanism. Electrodeposition of Zn on Cu
The authors are pleased to acknowledge the financial support from National Science Foundation Grant No. DMR-1310072 and ACIPCO for this research project. We also thank The University of Alabama for providing the experimental and analytical facilities. References [1] H. Geduld, Zinc Plating, ASM International, Teddington, London, 1988. [2] A.P. Brown, J.H. Meisenheider, N. Yao, Ind. Eng. Chem. Prod. Res. Dev. 22 (1983) 263–272. [3] P. Guillaume, J. Appl. Electrochem. 37 (2007) 1237–1243. [4] D.J. Mackinnon, J.M. Brannen, R.C. Kerby, J. Appl. Electrochem. 9 (1979) 55–70. [5] J.F. Brennecke, E.J. Maginn, ALChE. J. 47 (2001) 2384–2389. [6] R.G. Reddy, J. Phase Equilib. Diff. 27 (2006) 210–211. [7] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [8] C.L. Hussey, Adv. Molten Salt Chem. 5 (1983) 185. [9] A.P. Abbott, Chem. Phys. 8 (2006) 4265–4279. [10] F. Endres, S. Abedinw, Phys. Chem. Chem. Phys. 8 (2006) 2101. [11] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, P. Shikotra, Inorg. Chem. 44 (2005) 6497–6499. [12] D. Yue, Y. Jia, Y. Yao, Y. Sun, Y. Jing, Electrochim. Acta 65 (2012) 30–36. [13] H.G. Morrison, S. Neervannan, Int. J. Pharm. 378 (2009) 136–139. [14] A.P. Abbott, G. Capper, D.L. Davies, K.J. McKenzie, S.U. Obi, J. Chem. Eng. Data 51 (2006) 1280–1282. [15] D. Pradhan, D. Mantha, R.G. Reddy, Electrochim. Acta 54 (2009) 6661–6667. [16] D. Pradhan, R.G. Reddy, Electrochim. Acta 54 (2009) 1874–1880. [17] Y. Zheng, K. Dong, Q. Wang, S. Zhang, Q. Zhang, X. Lu, Sci. China. Chem. 55 (2012) 1587–1597. [18] R.S. Nicholson, Anal. Chem. 37 (1965) 1351–1355. [19] R.S. Nicholson, I. Shain, Anal. Chem. 36 (1964) 706–723. [20] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamental and Applications, John Wiley& Sons, New York, 2000, pp. 137–164. [21] A. Gomes, M.I. da Silva Pereira, Electrochim. Acta 52 (2006) 863–871. [22] P. Chen, I. Sun, Electrochim. Acta 46 (2001) 1169–1177. [23] Y. Nuli, J. Yang, P. Wang, Appl. Surf. Sci. 252 (2006) 8086–8090. [24] T. Jiang, M.J. Chollier, G. Dubé, A. Lasia, G.M. Brisard, Surf. Coat. Tech. 201 (2006) 1–9. [25] Y. Zhao, T.J. VanderNoot, Electrochim. Acta 42 (1997) 1639–1643. [26] A. Radisic, J.G. Long, P.M. Hoffmann, C. Searson, J. Electrochem. Soc. 148 (2001) 41–46. [27] B. Scharifker, G. Hills, Electrochim. Acta 28 (1981) 879–889. [28] B. Scharifker, J. Mostany, J. Electroanal. Chem. 177 (1984) 25–37. [29] P. He, H. Liu, Z. Li, J. Li, J. Electrochem. Soc. 152 (2005) 146–153. [30] S. Langerock, L. Heerman, J. Electrochem. Soc. 151 (2004) 155–160. [31] J. Lee, B. Miller, X. Shi, R. Kalish, K.A. Wheeler, J. Electrochem. Soc. 147 (2000) 3370–3376. [32] M. Li, Z. Wang, R.G. Reddy, Electrochimi. Acta 123 (2014) 325–331. [33] D. Pradhan, R.G. Reddy, Mater. Chem. Phys. 143 (2014) 564–569. [34] V. Kamavaram, D. Mantha, R.G. Reddy, Electrochim. Acta 50 (2005) 3286–3295. [35] P. Chen, C.L. Hussey, Electrochim. Acta 52 (2007) 857–1864. [36] W.R. Pitner, C.L. Hussey, J. Electrochem. Soc. 144 (1997) 3095–3103. [37] T. Tsuda, L. Boyd, S. Kuwabata, C.L. Hussey, ECS Trans. 16 (2009) 529–540. [38] A.P. Abbott, G. Capper, K.J. McKenzie, K.S. Ryder, J. Electroanal. Chem. 599 (2014) 288–294. [39] Z. Liu, S. Abedin, F. Endres, Electrochim. Acta 89 (2013) 635–643.