Notes
Cobalt Electroplating in Choline Chloride-ethylene Glycol: A Comparative Study
2021 年 89 巻 6 号 p. 602-605
詳細
2021 年 89 巻 6 号 p. 602-605
In this work, cobalt electroplating from homogeneous oxidation of cobalt powder via iodine as a chemical oxidizing agent and from CoCl2·6H2O in choline chloride-ethylene glycol electrolytic bath was carried out at 90 °C. As relatively high temperature and corrosion resistive coating material, cobalt electroplating was performed. A number of electrochemical, spectroscopic and microscopic techniques, such as, cyclic voltammetry (CV), chronocoulometry, UV-visible spectroscopy and scanning electron microscopy (SEM) were used in fabrication and characterizations of the electroplated cobalt. The progressive nucleation mechanism is followed in case of cobalt electroplating using cobalt powder in the presence of iodine. The mirror-like surface (i.e., smooth surface) surface has been obtained when cobalt powder utilized with the aid of iodine as shown in the SEM images. The effectiveness of the existence of iodine and cobalt powder was evidenced from chronocoulometry in which huge number of charge was released during the electrochemical course.
Electroplating of metals and alloys are of great importance in the fundamental and industrial values.1,2 Throughout literature, electroplating of metals and alloys in aqueous solutions is well-defined.3,4 Over last two decades, a new class of electrolytic baths has appeared as media of different electrochemistry, such as wide potential window, thermal stability and relatively high current efficiency.5–7 The electrolyte baths that fulfill these properties are known as deep eutectic solvents (DESs).8,9
It is well-known that electrodeposition or electroplating is the process of finishing metal ions on suitable substrates. This process has invaluable meaning at both fundamental and industrial levels.8 The process of reduction of metal ions can be carried out both chemically and electrochemically. The later one has wide applications which is principally facile and straightforward.9,10 In electrochemistry, there are reverse of electroplating processes which is so called anodization, particularly, electropolishing.11–13
It is well-known that cobalt coating protects substrate from impact of high temperature and also corrosion in aggressive environments. In this regard, a promising methodology has been carried out to achieve an efficient electroplating, for instance, addition of additives.14,15 The complexity of the process of electroplating in DESs, such as speciation, relatively high temperature and addition of additives turn the process of electroplating to be complicated. Abbott et al. investigated speciation of a number of metal ions in different DESs via using extended X-ray absorption fine structure (EXAFS) and UV-visible spectroscopy.16,17
This work is addressed at electroplating of cobalt from chemical dissolution of cobalt powder using dissolved iodine as chemical oxidizing agent in CoCl2·6H2O in choline chloride-ethylene glycol electrolytic bath. It is also study that the mechanism of electroplating of cobalt is clarified using two different electrolytic baths; dissolved cobalt salt and cobalt powder in a deep eutectic solvent. The importance of using cobalt powder in the electroplating, releasing cobalt ion into bulk electrolyte homogeneously followed by electrodeposing.
Choline chloride; HOC2H4N(CH3)3Cl (ChCl) (99 %), ethylene glycol (EG) (purity ≥ 99 %), cobalt salt; CoCl2·6H2O (Purity ≥ 98 %), hydantoin (HD) (Purity ≥ 99.5 %), ammonium persulphate, (NH4)2S2O8 (Purity ≥ 98 %) and iodine (Purity ≥ 99.99 %), all were received from Sigma Aldrich and sulphuric acid, H2SO4, was purchased Fisher.
2.2 Electrolytic bath preparationsThe cobalt concentration (CoCl2·6H2O) in all electrolytic baths was 0.40 mol L−1; otherwise, it will be specified. In another electrolytic bath, cobalt powder was added into the mixture of choline chloride and ethylene glycol (1 : 2 mol ratio of ChCl : ethylene glycol) in the presence of 0.2 mol L−1 of iodine.
2.3 UV-visible acquisitionsAll UV-visible spectra were acquired from a Shimadzu model UV-1601 spectrophotometer using a cuvette cell (a path length of 1 cm).
2.4 Electrochemical measurementsIn a three-electrode cell using Autolab PGSTAR12 potentiostat controlled via the GPES2 software, all electrochemical measurements were conducted. Both working and counter were platinum disk (1 mm2 Pt) and Pt flag, respectively and an Ag wire as pseudo-reference electrode was used. To make sure that the working electrode was clean, a 0.05 µm γ-alumina paste was used as a mechanical mean prior to electrochemical measurements.
Copper sheet (50 mm × 42 mm × 1 mm) was used and degreased using anopol for five minutes prior to bulk analysis. In an attempt to remove a very thin oxide layer over and activate Cu sheet, a mixture of 0.87 mol L−1 (NH4)2S2O8 and 0.20 mol L−1 H2SO4 solution was utilized. The copper substrate was inserted into the electrolytic bath. Copper sheet and iridium oxide-coated Ti meshes were used as cathode and anode poles, respectively. In all electrochemical experiments 2.8 mA cm−2 was applied for one hour at 90 °C. Ultimately, the coated copper sheet with cobalt film was cleaned by rinsing into water and dried using acetone.
2.5 Surface characterizationsIn the surface characterizations, scanning electron microscope (SEM) images was recorded using a Philips XL-30 Field Emission Gun equipped to a Bruker AXS XFlash 4010 EDS detector at 25 kV as operating voltage. The working distance (5 mm) and an accelerating voltage (20 kV) were verified by secondary electron images. Performing acquisitions of SEM was carried out through exposing sample cross sections after covering in Conductomount (Met-Prep) on a Struers Labopress-3 by heating to 180 °C for three minutes at 25 kN.
The UV-Vis spectra of Co (II) in the bath electrolyte containing choline chloride-ethylene glycol and in the presence of iodine are shown in Fig. 1 (left). The Co (II) ion is originated from oxidation of cobalt powder by iodine as shown in the following equations:
| \begin{equation} \text{3I$_{2}$} + \text{2e$^{-}$} \to \text{2I$_{3}^{-}$} \end{equation} | (1) |
| \begin{equation} \text{2I$^{-}$} + \text{2e$^{-}$} \to \text{3I$^{-}$} \end{equation} | (2) |
UV-visible spectra of CoCl2·6H2O and cobalt powder in the presence of iodine (left) and photos of solution (right). Both in choline chloride-ethylene glycol.
It has already been confirmed that the speciation of Co (II) ion is supposed to be in the form of tetrahedral [CoCl4]2− is formed in the such environment that enriched in chloride ion. In Fig. 1 (right), it is observed that in the existence of iodine, the Co (II) ion solution is blue and slightly darker, indicating the an increasing in concentration of cobalt as a consequence of providing more cobalt ion into the solution by effective chemical oxidation of the cobalt powder.17,18 Furthermore, no sign of different speciation in the presence of iodine from the spectra.
3.2 Electrochemical studyThe electrochemical responses of Co (II) ion in the presence and absence of iodine in choline chloride-ethylene glycol mixture at 90 °C is exhibited in Fig. 2. It is seen that the onset of reduction potential shifts to more positive potential in the case of iodine, suggesting efficient cobalt powder dissolution chemically and re-deposition electrochemically. In other words, the kinetic of cobalt electroplating is accelerated as a result of providing cobalt ion homogeneously in the electrolytic bath. It is seen that existence of iodine causes change in both onset and maximum peak position on anodic sweep.
Cyclic voltammograms of CoCl2·6H2O and cobalt powder in the presence of iodine from choline chloride-ethylene glycol at 20 mV s−1 at 90 °C.
Figure 3 (left) shows the effect of scan rate on the peak height of cobalt powder in iodine-choline chloride-ethylene glycol electrolytic bath at 90 °C. It is observed that it obeys Randles-Sevcik equation where the Faradaic current is directly proportional concentration and square root of scan rate.19 It is likely to occur when temperature is elevated as a result of diffusion process accelerated and increasing of free Co (II) ion as exhibited in Fig. 3 (right).
Cyclic voltammograms of cobalt powder in the presence of iodine from choline chloride-ethylene glycol at 90 °C at various scan rates (left) and the same electrolytic at 20 mV s−1 bath at different temperature (right).
To examine the mass transport, chronocoulometry is applied as shown in Fig. 4. An overpotential (relatively high charge transfer) is applied in which the mass transport at its minimum level using Cottrell equation in two forms, as shown in Eqs. (3) and (4):
| \begin{equation} I = \frac{nFAC_{\text{o}}\sqrt{D}}{\sqrt{\pi\tau}} \end{equation} | (3) |
| \begin{equation} Q = \frac{2nFAC_{\text{o}}D^{1/2}t^{1/2}}{\pi^{1/2}} \end{equation} | (4) |
Chronocoulometry of CoCl2·6H2O in the ethylene glycol-ChCl bath (black) and the same solution containing iodine (red). The held potential was −0.8 V for 10 s, followed by a step potential to −0.4 V for ca. 40 min. using a 1 mm Pt disc working electrode with a platinum flag counter-electrode and a silver wire as the reference electrode and at 90 °C.
It is obviously seen that the mass transport of Co (II) ion in choline chloride-ethylene glycol electrolytic bath at 90 °C is substantially higher than that in the case of using metal salt, CoCl2·6H2O.
To recognize the mechanism of electroplating of Co (II) ion from cobalt powder in iodine-choline chloride-ethylene glycol electrolytic bath at 90 °C, Scharifker and Hills models were implemented. According to this model, there are two different nucleation processes; instantaneous and progressive.20,21
From the fitting of the plots, the nucleation process of Co (II) ion electroplating from the cobalt powder-based electrolytic bath is confirmed and quite close to progressive mechanism at 0.6 and 0.7 V as shown in Fig. 5.
The experimental i-t transient plots of cobalt powder-choline chloride-ethylene glycol electrolytic bath: at 0.6 V (left) and 0.7 V (right). All experiments on a Pt disc (1 mm dia.) at 25 °C versus a silver wire reference electrode and at 90 °C.
The SEM of cobalt film from the electrolytic baths are shown in Fig. 6. It is clearly seen that cobalt film obtained from choline chloride-ethylene glycol based on CoCl2·6H2O enriches in many defects as shown in Fig. 6 (left). On the other hand, cobalt film yielded from choline chloride-ethylene glycol based on cobalt powder is relatively very smooth as shown in Fig. 6 (right), indicating the low roughness surface that will be useful as corrosive resistant surface.22,23
SEM images of electroplated cobalt films from CoCl2·6H2O (left) and cobalt powder-iodine (right) at 90 °C.
It seems from this study that cobalt electroplating from homogeneous electrolytic bath using iodine as chemically oxidizing agent in choline chloride-ethylene glycol at 90 °C is of significant importance in terms of achieving smooth surface of cobalt.
It is concluded that homogeneous releasing of cobalt ion from its powder as a consequence of oxidation gradually by iodide (I−) ion and then electroplating of the produced cobalt (II) ion yields dense, uniform, adherent and smooth films.
From the fundamental perspective, it is important to confirm that the mechanism of nucleation of Co (II) ion is progressive when cobalt powder and iodine is used in the electrolytic bath.
I would like to acknowledge the University of Sulaimani for supporting.
Jamil A. Juma: Conceptualization (Equal), Writing – original draft (Equal), Writing – review & editing (Equal)
Wrya O. Karim: Conceptualization (Equal), Data curation (Equal), Formal analysis (Equal), Writing – original draft (Equal), Writing – review & editing (Equal)
Shujahadeen A. Aziz: Writing – original draft (Supporting)
Khalid M. Omer: Writing – original draft (Supporting)
W. O. Karim: ECSJ Active Member
