The Anodic Behaviour of Bulk Copper in Ethaline and 1-Butyl-3-Methylimidazolium Chloride

applied sciences

Article The Anodic Behaviour of Bulk Copper in Ethaline and 1-Butyl-3-Methylimidazolium Chloride

Wrya O. Karim 1,2, Shujahadeen B. Aziz 3,4,* , Mohamed. A. Brza 1,5, Ranjdar M. Abdullah 1 and Mohd. F. Z. Kadir 6

1 Department of Chemistry, College of Science, University of Sulaimani, Qlyasan Street, Sulaimani 46001, Kurdistan Regional Government-Iraq; [email protected] (W.O.K.); [email protected] (M.A.B.); [email protected] (R.M.A.) 2 Department of Chemistry, University of Leicester, Leicester LE1 7RH, UK 3 Prof. Hameeds Advanced Polymeric Materials Research Lab., Department of Physics, College of Science, University of Sulaimani, Qlyasan Street, Sulaimani 46001, Kurdistan Regional Government-Iraq 4 Komar Research Center (KRC), Komar University of Science and Technology, Sulaimani 46001, Kurdistan Regional Government-Iraq 5 Faculty of Engineering, International Islamic University of Malaysia, Kuala Lumpur, Gombak 53100, Malaysia 6 Centre for Foundation Studies in Science, University of Malaya, Kuala Lumpur 50603, Malaysia; [email protected] * Correspondence: [email protected] or [email protected]

Received: 23 August 2019; Accepted: 11 October 2019; Published: 17 October 2019

Abstract: The anodic dissolution of bulk metallic copper was conducted in ionic liquids (ILs)—a deep eutectic solvent (DES) ((CH3)3NC2H4OH) comprised of a 1:2 molar ratio mixture of choline chloride Cl (ChCl), and ethylene glycol (EG)—and imidazolium-based ILs, such as C4mimCl, using electrochemical techniques, such as cyclic voltammetry, anodic linear sweep voltammetry, and chronopotentiometry.To investigate the electrochemical dissolution mechanism, electrochemical impedance spectroscopy (EIS) was used. In addition to spectroscopic techniques, for instance, UV-visible spectroscopy, microscopic techniques, such as atomic force microscopy (AFM), were used. The signiﬁcant industrial importance of metallic copper has motivated several research groups to deal with such an invaluable metal. It was conﬁrmed that the speciation of dissolved copper from 2 the bulk phase at the interface region is [CuCl3]− and [CuCl4] − in such chloride-rich media, and the EG determine the structure of the interfacial region in the electrochemical dissolution process. A super-saturated solution was produced at the electrode/solution interface and CuCl2 was deposited on the metal surface.

Keywords: copper electrodissolution; (choline chloride-based IL) ethaline eutectic solvent; butyl-3-methylimidazolium chloride ionic liquid; linear sweep voltammetry; electrochemical impedance spectroscopy; electropolishing

1. Introduction The electrodeposition of copper has been investigated for a variety of applications, and the anodic dissolution of metallic copper has been thoroughly studied in ionic liquid (IL) and deep eutectic solvent (DES) media [1–10]. Previously, the anodic dissolution of metallic copper has been thoroughly examined in aqueous media [11–13]. This invaluable metal has wide applications, including electronics, decorating, pre-coating, and electro-catalysis [14–19].

Appl. Sci. 2019, 9, 4401; doi:10.3390/app9204401 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 4401 2 of 15

Gu et al. studied electrodeposition and the corrosion behaviour of copper in a choline chloride-ethylene glycol DES. In the electrodeposition process, a rough surface was obtained with the addition of ethylene diamine; the corrosion decreased and a relatively uniform surface was gained [5]. Many research groups have tried to deal with the physical properties of ethaline (choline chloride–ethylene glycol). it was found that the mass transport could be increased with an increasing temperature; when the temperature was increased to 50 ◦C, the viscosity decreased by 65% [7]. Concerning the physical properties of the electrolyte, such as conductivity and viscosity, it was found that it is insensitive to metal-salt addition but strongly temperature dependent [9]. The speciation of 2 2 copper is well-defined in choline-based DESs. It was determined to be [CuCl3] − and [CuCl4] − for Cu(I) and Cu(II), respectively [20]. The dissolution of bulk copper was studied previously in these media, in the present work attempt was directed to deal with copper in more detail [21]. Deep eutectic solvents (DESs) are attracting significant research interest as electrochemical media, especially for electrodeposition, as they share many of the useful properties of ILs, but are typically greener and cheaper [22]. The influence of water and the changing potential on the interfacial region between a platinum electrode and a DES were studied. It was found that the nanostructural feature of interface decreased in the presence of small amount of water and was also strongly potential dependent [23]. The aim of this work is to investigate the dissolution mechanism of copper in a DES and its comparable IL and compare it to the aqueous system. This is important for counter-electrode processes during electrodeposition, for electropolishing, and also to understand whether metallic copper corrodes in the DESs.

2. Experimental The DES was prepared by mixing choline chloride (ChCl) (Aldrich, 99%) and EG (Aldrich, >99%) in a stoichiometric molar ratio of 1:2 (ChCl:EG). Then, it was heated to 60◦C with continuous stirring until a clear liquid was produced. The IL one was purchased from (Aldrich, 99%). The 1-Butyl-3-methylimidazolium chloride, (C4mim)(Cl), was dried under vacuum before use, but had a water content of ca. 0.1 wt.% (thermogravimetric analysis, Mettler Toledo TGA/DSC1 STARe system) which enabled it to be liquid at 70◦C.The copper wire was purchased from Alfa Aesar (99.9% purity). Regarding electrochemical measurements, both cyclic voltammetry and linear sweep voltammetry were conducted by means of both stationary and rotating disk electrodes. The galvanostatic and the AC impedance were performed using an Autolab PGSTAT 12: controlling by GPES software and then ﬁtting with an FRA impedance module. The impedance spectra acquisition was in the frequency range of 1–65,000 Hz with small amplitude of 10 mV of the AC signal. All electrochemical measurements were carried out in a three electrode cell system involving a 1 mm diameter metal disc Cu working electrode, sealed in glass; a platinum ﬂag (1 cm2 area) as a counter electrode; and Ag/AgCl (0.1 M in 1:2ChCl:EG) as the reference electrode. All measurements are performed at 20◦C and 70◦C at a 5 mV s 1 scan rate except for the determination of time considered necessary for copper electrode to · − reach passivation. The sweep rate was changed from 5 up to 50.5 mV s 1. · − The UV spectra were recorded by means of a Shimadzu model Uv-1601 spectrophotometer with a cell path length of 10 mm. The morphological examinations were conducted using atomic force microscopy (AFM). The acquisition of images was by means of a Digital Instruments Nanoscope IV Dimension 300 (Veeco) atomic force microscope with a 100 mm scanning head contact mode. The controlling software was Nanoscope version 6.13 during image acquisition in air. Appl. Sci. 2019, 9, 4401 3 of 15

3. Results and Discussion

3.1. Anodic Dissolution Mechanism

Cyclic and Linear Sweep Voltammetry Figure1a exhibits the cyclic voltammetric response a of metallic, copper disc electrode in choline chloride-based IL at 20 ◦C. Within the anodic potential range, two oxidation processes can be clearly seen. The anodic current begins to increase at 0.3 V, peaking at ca. 0 V. The current sharply drops Appl. Sci. 2019, 9, x FOR PEER REVIEW − 3 of 14 in a manner which is a characteristic of a quasi-passivation process. This might be due primarily to presencepresence of of EG, EG, and and partly, partly, the the chloride chloride ion. ion. The The second second anodic anodic current current rises rises to a to peak a peak at +0.3 at + V0.3 and V and fallsfalls down down to to a a steady steady state state cu currentrrent of of approximately approximately 23 23 mA·cm mA cm−2. The2. The second second anodic anodic peak peak beyond beyond · − +0.25+0.25 V V could could be be linked linked to to extra extra oxidation oxidation of ofcopper copper from from Cu(I) Cu(I) to Cu(II). to Cu(II). The The other other ting ting peaks peaks are are artefactsartefacts that that might might be be linked linked to to complexity complexity of of di dissolutionssolution process process of of bulk bulk copper copper metal metal within within such such a aviscous viscous liquid.liquid.

50 40 a b 40 30 ) ) -2

-2 30 20 20 ( mA cm ( mA ( mA cm ( mA j 10 j 10

0 0

-10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 E (V) vs. Ag/AgCl E (V) vs. Ag/AgCl

FigureFigure 1. 1. CyclicCyclic voltammetric voltammetric responses responses of ofa Cu a Cu disc disc (solid (solid line) line) and and 0.1 0.1M CuCl M CuCl2·2H22HO (dashedO (dashed line) line) 2· 2 inin choline choline chloride-based chloride-based ionic ionic liquid liquid (IL) (IL) on ona platinum a platinum electrode electrode at 20 at °C 20 (◦aC(), anda), and a Cu a Cudisc disc (solid (solid line)line) and and 0.2 0.2 M M CuCl CuCl2·2H2H2O Oin inimidazolium-based imidazolium-based IL (dashed IL (dashed line) line) at 70 at °C 70 (bC(), allb), at all a atsweep a sweep rate rateof of 2· 2 ◦ 55 mV·s mV −s1. 1. · − OnOn the the cathodic cathodic scan, scan, the the current current was was approximately approximately constant constant until until ca. ca.0.2 V, 0.2 when V, when a noisy a noisy phenomenonphenomenon occurred occurred at at the the same same potential potential that that pass passivationivation film film formation formation occurred: occurred: on the on reversed the reversed sweep.sweep. Indeed, Indeed, the the process process is notnot anan artefactartefact but but is is very very reproducible. reproducible. The The main main cathodic cathodic process process begins beginsat ca. 0.0at V,ca. peaking 0.0 V, atpeaking0.4 V, at with −0.4 a shoulderV, with ata shoulder0.2 V. The at noise−0.2 isV. likely The owednoise tois redissolutionlikely owed to of the − − redissolutionCu(II) species of formed the Cu(II) on species the electrode’s formed surface.on the electrode’s surface. InIn the the interpretation interpretation of of the the whole whole processes processes that that occurs, occurs, it is it helpful is helpful to compare to compare the thecyclic cyclic voltammogramsvoltammograms of of the the metallic metallic copper copper electrode electrode in choline in choline chloride-based chloride-based DES with DES that with for that at Pt for at electrodePt electrode in a in solution a solution of 0.1 of 0.1M MCuCl CuCl2·2H22HO inO the in theelectrolyte. electrolyte. Abbott Abbott et al. et al.documented documented the the 2· 2 electrodepositionelectrodeposition of of copper copper using using CuCl CuCl2·2H2H2O inO the in the electrolyte, electrolyte, observing observing that that two two distinct distinct processes processes 2· 2 taketake place, place, with Cu(II)Cu(II) undergoingundergoing aa one-electron one-electron reduction reduction to to Cu(I)at Cu(I)at+0.43 +0.43 V,followed V, followed by aby one-electron a one- electronreduction reduction to metallic to metallic copper atcopper0.45 at V [−240.45]. The V [24]. significance The significance of this finding of this is finding that both is processesthat both are − processesreversible. are It reversible. is worth-mentioning, It is worth-mentioning, however, that however, direct comparisonthat direct comparison between the between redox the potentials redox in potentialsFigure1 and in Figure the previously 1 and the previously reported study reported are study not helpful are not due helpful to the due use to the of considerablyuse of considerably di fferent differentreference reference electrodes electrodes (a silver (a wire silver quasi-reference wire quasi-reference electrode electrode in the latterin the case). latter case). Figure 1a reveals an over-laid cyclic voltammetry CV (dashed line) of 0.1 M CuCl2·2H2O in Figure1a reveals an over-laid cyclic voltammetry CV (dashed line) of 0.1 M CuCl 2 2H2O in choline + choline chloride-based DES versus Ag/Ag+ at 20 °C. It can obviously be seen that the onset· potential chloride-based DES versus Ag/Ag at 20 ◦C. It can obviously be seen that the onset potential on the on the anodic scan are similar to large extent, indicating that the metallic copper first dissolves as anodic scan are similar to large extent, indicating that the metallic copper first dissolves as Cu(I) in Cu(I) in the complex form. The quasi-passivation which is observed for the bulk copper electrode the complex form. The quasi-passivation which is observed for the bulk copper electrode dissolution dissolution occurs at the same potential as the Cu(I)/(II) oxidation in solution. One can conclude that occurs at the same potential as the Cu(I)/(II) oxidation in solution. One can conclude that the second the second process in metallic copper dissolution occurs owing the change in oxidation state of the metal. When Cu(I) salts solubilise in choline chloride-based DES, the speciation was found to be in the [CuCl2]− form while the Cu(II) salts tend to produce [CuCl4]2−[20,24]. Both of these complexes are known to be soluble largely and so the cause of the quasi-passivation is not immediately clear. From Figure 1a,b, one can see the differences which are caused due to a specific cation effect or because of the lower chloride concentration in choline chloride-based DES compared to imidazolium- based IL. To confirm this further, 1-butyl-3-methylimidazolium chloride (C4mimCl) was diluted by adding EG. The addition of EG into imidazolium-based IL one results in a shift in the oxidation onset potential of metallic copper by approximately 300 mV, as presented in Figure 2. Similarly, a quasi- Appl. Sci. 2019, 9, 4401 4 of 15 process in metallic copper dissolution occurs owing the change in oxidation state of the metal. When Cu(I) salts solubilise in choline chloride-based DES, the speciation was found to be in the [CuCl2]− 2 form while the Cu(II) salts tend to produce [CuCl4] − [20,24]. Both of these complexes are known to be soluble largely and so the cause of the quasi-passivation is not immediately clear. From Figure1a,b, one can see the di fferences which are caused due to a specific cation effect or because of the lower chloride concentration in choline chloride-based DES compared to imidazolium-based IL. To confirm this further, 1-butyl-3-methylimidazolium chloride (C4mimCl) was diluted by adding EG. The addition of EG into imidazolium-based IL one results in a shift in Appl.the oxidationSci. 2019, 9, x onset FOR PEER potential REVIEW of metallic copper by approximately 300 mV, as presented in Figure4 of 142. Similarly, a quasi-passivation profile is also observed showing that passivation is probably associated passivation profile is also observed showing that passivation is probably associated to the chloride to the chloride concentration. concentration.

50 )

-2 40

30 A cm

( m 20 j

-10 -0.5 0.0 0.5 1.0 1.5 E (V) vs. Ag/AgCl Figure 2. Linear sweep voltammetry (LSVs) of copper discs in choline chloride-based IL Figure 2. Linear sweep voltammetry (LSVs) of copper discs in choline chloride-based IL (ChCl:EG (ChCl:EG 1:2, solid line), and the other three shifted, LSV-overlapped responses of copper discs 1:2, solid line), and the other three shifted, LSV-overlapped responses of copper discs in 1-butyl-3- in 1-butyl-3-methylimidazolium chloride (C4mimCl):EG with ratios of 1:2, 1:3, and 1:4, represented by methylimidazolium chloride (C4mimCl):EG with ratios of 1:2, 1:3, and 1:4, represented by dashes, dashes, dots, and solid lines, respectively, at 5 mV s 1 and 20 C. dots, and solid lines, respectively, at 5 mV·s−1and 20· −°C. ◦ As the concentration of chloride declines due to addition of EG, shifting in the onset potential As the concentration of chloride declines due to addition of EG, shifting in the onset potential occurs. It is noteworthy that the chloride ion caused a higher current response. occurs. It is noteworthy that the chloride ion caused a higher current response. The eﬀect of anion concentration on copper dissolution in choline chloride-based IL was The effect of anion concentration on copper dissolution in choline chloride-based IL was investigated as shown in Figure3. In the study the concentration of chloride in electrolyte (ChCl:EG investigated as shown in Figure 3. In the study the concentration of chloride in electrolyte (ChCl:EG 1:2) was manipulated by diluting the electrolyte with EG. As the molar ratio altered from 1:2 to 1:3 and 1:2) was manipulated by diluting the electrolyte with EG. As the molar ratio altered from 1:2 to 1:3 1:4, the passivation potential was changed to less positive potential values. This was predictable, as in and 1:4, the passivation potential was changed to less positive potential values. This was predictable, such cases, there is less chloride available to the electrode’s surface, so it is more diﬃcult to produce as in such2 cases, there is less chloride available to the electrode’s surface, so it is more difficult to CuCl4 − and more likely forms CuCl2. produce CuCl42− and more likely forms CuCl2.

40 )

-2 30

20 ( mA cm mA ( j

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 E (V) vs. Ag/AgCl

Figure 3. The influence of electrolyte composition on the equilibrium passivation potential of copper disc electrodes in ethaline ChCl:EG, 1:2 (solid line);ChCl:EG, 1:3 (dashed line); and ChCl:EG, 1:4 (dotted line).

3.2. UV-Visible Spectroscopy It is of utmost significance to know about the speciation of dissolved copper in choline chloride/EG system. After metallic bulk copper’s dissolution, electrochemically, into Ethaline, a coloured solution of dissolved copper was generated. Of great importance to the dissolved bulk metal is the determination of the identity of the species that arose from copper dissolution into a chloride Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 14 passivation profile is also observed showing that passivation is probably associated to the chloride concentration.

50 )

-2 40

30 A cm

( m 20 j

-10 -0.5 0.0 0.5 1.0 1.5 E (V) vs. Ag/AgCl

Figure 2. Linear sweep voltammetry (LSVs) of copper discs in choline chloride-based IL (ChCl:EG 1:2, solid line), and the other three shifted, LSV-overlapped responses of copper discs in 1-butyl-3- methylimidazolium chloride (C4mimCl):EG with ratios of 1:2, 1:3, and 1:4, represented by dashes, dots, and solid lines, respectively, at 5 mV·s−1and 20 °C.

As the concentration of chloride declines due to addition of EG, shifting in the onset potential occurs. It is noteworthy that the chloride ion caused a higher current response. The effect of anion concentration on copper dissolution in choline chloride-based IL was investigated as shown in Figure 3. In the study the concentration of chloride in electrolyte (ChCl:EG 1:2) was manipulated by diluting the electrolyte with EG. As the molar ratio altered from 1:2 to 1:3 and 1:4, the passivation potential was changed to less positive potential values. This was predictable, as in such cases, there is less chloride available to the electrode’s surface, so it is more difficult to Appl. Sci. 2019, 9, 4401 5 of 15 produce CuCl42− and more likely forms CuCl2.

40 )

-2 30

20 ( mA cm mA ( j

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 E (V) vs. Ag/AgCl Figure 3. The influence of electrolyte composition on the equilibrium passivation potential of copper Figure 3. The influence of electrolyte composition on the equilibrium passivation potential of copper disc electrodes in ethaline ChCl:EG, 1:2 (solid line); ChCl:EG, 1:3 (dashed line); and ChCl:EG, 1:4 disc electrodes in ethaline ChCl:EG, 1:2 (solid line);ChCl:EG, 1:3 (dashed line); and ChCl:EG, 1:4 (dotted line). (dotted line). 3.2. UV-Visible Spectroscopy 3.2. UV-Visible Spectroscopy It is of utmost significance to know about the speciation of dissolved copper in choline chloride/EG system.It is After of utmost metallic significance bulk copper’s to dissolution, know about electrochemically, the speciation into of Ethaline,dissolved a colouredcopper in solution choline of chloride/EGdissolved copper system. was After generated. metallic Of greatbulk importancecopper’s dissolution, to the dissolved electrochemically, bulk metal is the into determination Ethaline, a colouredof the identity solution of the of dissolved species that copper arose was from generated. copper dissolution Of great importance into a chloride to the rich dissolved electrolyte, bulk as metal that iscan the be determination regarded as a of key the factor identity in dealing of the specie with thes that kinetics arose of from the electrochemicalcopper dissolution processes. into a chloride Herein, a facile, non-destructive technique was used to conduct this task: the UV-Vis spectroscopic technique. In the visible wavelength range, to gain information about any species, the solution must be coloured. To fulfil this condition, the chromophores/electron transition within the d-d orbitals have to exist [22]. From Figure4, one can see similar UV-Vis spectra of 0.1 mM CuCl 2H O in ethaline and the 2· 2 solution of dissolved copper obtained electrochemically by holding a copper disc electrode in ethaline at a potential of 1.2 V for 1 h, from which dissolved copper ion in the electrolyte, i.e.,ethaline, was obtained at 20 ◦C. However, there are two observations to make: first, there is a large peak at around 220 nm present in the copper dissolved electrochemically, which is entirely missing in the control solution; next, the peak at 281 nm in the control solution underwent blue-shifting in the solution of the copper product dissolved electrochemically. These can be associated to the impurities which might be present in the used copper wire (99.9%). The spectra obtained in both cases were identical, indicating that the speciation of stripped copper from the bulk copper metal into ethaline is exactly the same as that for the solution species 2 obtained by dissolving CuCl2.2H2O in solution. The formation of [CuCl4] − was emphasised from the spectra where three distinct peaks were seen at 233, 281, and 407 nm and this was confirmed using EXAFS [20,22]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 14 rich electrolyte, as that can be regarded as a key factor in dealing with the kinetics of the electrochemical processes. Herein, a facile, non-destructive technique was used to conduct this task: the UV-Vis spectroscopic technique. In the visible wavelength range, to gain information about any species, the solution must be coloured. To fulfil this condition, the chromophores/electron transition within the d-d orbitals have to exist [22]. From Figure 4, one can see similar UV-Vis spectra of 0.1 mM CuCl2·2H2O in ethaline and the solution of dissolved copper obtained electrochemically by holding a copper disc electrode in ethaline at a potential of 1.2 V for 1 h, from which dissolved copper ion in the electrolyte, i.e.,ethaline, was obtained at 20 °C. However, there are two observations to make: first, there is a large peak at around 220 nm present in the copper dissolved electrochemically, which is entirely missing in the control solution; next, the peak at 281 nm in the control solution underwent blue-shifting in the solution of the copper product dissolved electrochemically. These can be associated to the impurities which might be present in the used copper wire (99.9%). The spectra obtained in both cases were identical, indicating that the speciation of stripped copper from the bulk copper metal into ethaline is exactly the same as that for the solution species obtained by dissolving CuCl2.2H2O in solution. The formation of [CuCl4]2− was emphasised from the spectra where three distinct peaks were seen at 233, 281, and 407 nm and this was confirmed using EXAFSAppl. Sci. [20,22].2019, 9, 4401 6 of 15

1.2

0.8 Abs.

0.4

0.0

200 250 300 350 400 450 500 λ Wavelength, / nm Figure 4. UV-Vis spectra of a dissolved metallic copper disc (solid line) at 50 mA cm 2 for 1 h at ambient Figure 4. UV-Vis spectra of a dissolved metallic copper disc (solid line) at ·50 mA·cm− −2 for 1 h at temperature and 0.1 mM (CuCl2) 2H2O (dash line) in 1:2EG:ChCl electrolyte (choline chloride-based ambient temperature and 0.1 mM· (CuCl2)·2H2O (dash line) in 1:2EG:ChCl electrolyte (choline IL) on a platinum electrode. chloride-based IL) on a platinum electrode. 3.3. The Effect of Temperature 3.3. The Effect of Temperature Figure1a exhibits the corresponding experiments in the IL, C 4mimCl; in other words, Figure 1a exhibits the corresponding experiments in the IL, C4mimCl; in other words, Cu Cu electrochemical dissolution and the voltammetry of 0.1 M CuCl2 2H2O on a Pt electrode. electrochemical dissolution and the voltammetry of 0.1 M CuCl2·2H2O· on a Pt electrode. This This experiment had to be performed at 70 ◦C owing to the high melting point of C4mimCl. Clearly, experimentit is seen that had a comparableto be performed current at 70 density °C owing was to observed the high formelting copper point dissolution of C4mimCl. to that Clearly, of choline it is seenchloride-based that a comparable IL. Moreover, current the samedensity onset was potential observed for electrochemicalfor copper dissolution copper dissolutionto that of wascholine also chloride-basedobtained. However, IL. Moreover, the most noticeablethe same onset difference pote betweenntial for theelectrochemical electrochemical copper dissolution dissolution responses was alsoin choline obtained. chloride-based However, ILthe and most imidazolium-based noticeable difference IL is that between there is the no quasi-passivationelectrochemical dissolution response in responsesthe latter andin choline there is chloride-based an inflection point IL and occurring imidazolium the identical-based IL potential is that there as the is Cu(I) no quasi-passivation/(II) process. It is responsealso worth in notingthe latter that and the there voltammogram is an inflection for CuClpoint occurring2H O in C themimCl identical is quite potential comparable as the Cu(I)/(II) to that in 2· 2 4 process.ethaline, It showing is also twoworth one-electron noting that reductions the voltammogram and oxidation for processes. CuCl2·2H2O in C4mimCl is quite comparableBasically, to thethat ligands in ethaline, for copper’s showing dissolution two one-electron electrochemically, reductions should and oxidation be the same processes. in both liquids Basically, the ligands for copper’s dissolution electrochemically, should be the same in both (Cl−). However, there is a discrepancy in the electrochemical behaviour in both electrolytes, which liquids (Cl−). However, there is a discrepancy in the electrochemical behaviour in both electrolytes, can, therefore, only be due to kinetic factors (diffusion of copper from the electrode and Cl− to the − whichelectrode) can, ortherefore, thermodynamic only be du factorse to kinetic owing factors to the solubility(diffusion ofof thecopper chlorometallate from the electrode complexes and Cl in the to both electrolytes. From the literature survey, one can see a huge number of studies on copper dissolution and deposition in a various electrolyte solutions. The kinetics of the electrodeposition of a copper salt in choline chloride-based IL was studied, and it was determined that the Cu(I)/II process is quasi-reversible with a rate constant of 9.5 10 4 cm s 1 [25]. For copper to be deposited from its salt in the electrolyte, ± − · − the amount of copper deposited on the electrode’s surface must be quite small so it can all be dissolved in the anodic sweep (returning the current to approximately zero), and a diffusion-limited current is seen for the oxidation of Cu(I) to Cu(II). When copper dissolution is conducted electrochemically at a bulk copper electrode, the copper is effectively at infinite concentration. The solution close to the electrode’s surface, i.e., at the interface region, could become saturated. It is proposed that that is what occurred in Figure1a at the potential about +0.2 V. Saturating the solution at the interfacial region with the copper complex will result in a decrease in the oxidative current as the electrode becomes blocked with dissolution product, and as a consequence, an asymmetric peak is gained. The number of moles involved in the phase transitions at first oxidation peaks have been computed and are presented in Table1. One can conclude that copper’s conversion from metallic copper to Cu(I) in Ethaline is about 90 times higher than stripping of electrodeposited copper one in choline chloride-based and imidazolium based electrolytes, using platinum as a substrate at 20 ◦C and 70 ◦C, Appl. Sci. 2019, 9, 4401 7 of 15 respectively. The number of moles copper stripped electrochemically in the choline chloride-based IL at 20 ◦C is slightly lower than the moles calculated in the imidazolium based IL at 70 ◦C. This might be owing to higher viscosity of imidazolium-based IL even at 70 ◦C (142 cP) to choline chloride-based IL (36 cP) at 20 ◦C[26].

Table 1. The number of moles of copper stripped in the ﬁrst oxidation process, calculated from the overall charge under the peak of the LSV using the GPES software.

Electrolyte Cu/nmole Choline chloride IL 90.6 CuCl 2H O Choline chloride IL at 20 C 1.20 2· 2 ◦ CuCl 2H OC mimCl at 70 C 1.76 2· 2 4 ◦

The quasi-passivation process in the electrochemical dissolution of bulk copper in choline based IL at 20 ◦C could be as a result of the low solubility of CuCl2. So as to verify this, the experiment was repeated in the choline-based IL at 70 ◦C to create a comparison with the imidazolium-based IL experiment and the result are shown in Figure5. It was anticipated that as the temperature raised, so would the anodic current (approximately 10 fold) because of the lowering of the solution viscosity. It ought to, however, be noticed that the sharp decline in current still occurred at 0.6 V instead of 0.2 V. This is presumably as a result of the super-saturation at elevated temperature; i.e., higher concentration relative to saturated one, and the diﬀusion of chloride was also higher. One thing that has to be well-known is the oxidation onset potential that shifted to be more negative; i.e., there was less anodic onset potential as the temperature is raised, demonstrating the kinetic accelerating oxidation reaction of the bulk copper. Ultimately, the overall electrochemical behaviour did not alter at that elevated temperature; in the other words, the passivation was still eﬀective. The time for the bulk copper electrode to passivate in the choline chloride-based IL as a function of scan rate is presented in Figure6. At faster sweep rates, a higher concentration of electrochemically dissolved copper is put more rapidly into the solution and the interfacial region between the electrode’s surface and the electrolyte saturates more quickly. It appears as if the potential of passivation increases, Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 14 but this is just an artefact of the system not being at equilibrium state.

300

250

) 200 -2

150

100 ( mA cm (

j 50

-50 -1.0 -0.5 0.0 0.5 1.0 1.5 E (V) vs. Ag/AgCl

Figure 5. Cyclic voltammograms of bulk Cu disc in choline chloride-based IL at 20 ◦C (black line) and 70FigureC (red 5. Cyclic line) atvoltammograms 5 mV s 1. of bulk Cu disc in choline chloride-based IL at 20 °C (black line) and ◦ · − 70 °C (red line) at 5 mV·s−1.

0.55 140 0.50 120 0.45

100 0.40

80 0.35

60 0.30 Passivation time / s / time Passivation 0.25 40 Passivation potential / V

0.20 20

0.00 0.01 0.02 0.03 0.04 0.05 Scan rate (V/s)

Figure 6. The influence of sweep rate on the equilibrium passivation potential; square-dotted and circle-dotted lines correspond to passivation time and passivation potential, respectively.

3.4. Influence of Mass Transport

When the potential of bulk copper electrode was held at +0.18 V versus Ag/Ag+ for 10 min in choline chloride-based IL, the surface initially darkened and a green film slowly formed on the electrode’s surface, as presented in the previous work [21]. At this potential, the most likely salt is CuCl, which is only sparingly soluble in the electrolyte. The light green colour indicates that the further oxidation of Cu(II) occurs; however, given the applied electrode potential, the oxidation would also be caused by the existence of dissolved oxygen. After the green film was washed off, the electrode metal was darkened quite considerably owing to surface roughening. To study the morphology of the surface, it is important to determine the role of mass transport; the experiment was repeated using a rotating disc electrode, as presented in Figure 7. The morphology of the surface before and after anodic polarisation can be seen in later section for a bulk copper electrode with and without stirring. The manipulation of mass transportation was tested using a rotating disc electrode, as presented in Figure 7. It is important that in the absence of stirring, passivation occurs, even at stirring rates of up to 500 rpm. Only at rotation speeds above 1000 rpm, the passivation response was lost. As the rotation speed increased, so did the current, due to the anions provided to the surface by which more reactions occurred. The current did not, however, reach a steady state value as would be predicted for a solution based species, so it must be limited by the diffusion of oxidised copper from the Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 14

300

250

) 200 -2

150

100 ( mA cm (

j 50

-50 -1.0 -0.5 0.0 0.5 1.0 1.5 E (V) vs. Ag/AgCl

Figure 5. Cyclic voltammograms of bulk Cu disc in choline chloride-based IL at 20 °C (black line) and Appl. Sci. 2019, 9, 4401 8 of 15 70 °C (red line) at 5 mV·s−1.

0.55 140 0.50 120 0.45

100 0.40

80 0.35

60 0.30 Passivation time / s / time Passivation 0.25 40 Passivation potential / V

0.20 20

0.00 0.01 0.02 0.03 0.04 0.05 Scan rate (V/s) Figure 6. The influence of sweep rate on the equilibrium passivation potential; square-dotted and Figure 6. The influence of sweep rate on the equilibrium passivation potential; square-dotted and circle-dotted lines correspond to passivation time and passivation potential, respectively. circle-dotted lines correspond to passivation time and passivation potential, respectively. 3.4. Influence of Mass Transport 3.4. Influence of Mass Transport When the potential of bulk copper electrode was held at +0.18 V versus Ag/Ag+ for 10 min in cholineWhen the chloride-based potential of IL,bulk the copper surface electrode initially was darkened held at and +0.18 a green V versus film Ag/Ag slowly+ formedfor 10 min on thein cholineelectrode’s chloride-based surface, as presentedIL, the surface in the initially previous da workrkened [21 and]. At a this green potential, film slowly the most formed likely on salt the is electrode’sCuCl, which surface, is only as sparingly presented soluble in the in previous the electrolyte. work The[21]. light At this green potential, colour indicates the most that likely the furthersalt is CuCl,oxidation which of Cu(II)is only occurs; sparingly however, soluble given in the appliedelectrolyte. electrode The light potential, green the colour oxidation indicates would that also the be furthercaused oxidation by the existence of Cu(II) of dissolved occurs; however, oxygen. given the applied electrode potential, the oxidation wouldAfter also thebe caused green film by the was existence washed oofff ,dissolved the electrode oxygen. metal was darkened quite considerably owing to surfaceAfter the roughening. green film To was study washed the morphology off, the electr ofode the metal surface, was it darkened is important quite to determineconsiderably the owing role of tomass surface transport; roughening. the experiment To study wasthe morphology repeated using of th a rotatinge surface, disc it is electrode, important as to presented determine in the Figure role7 . ofThe mass morphology transport; ofthe the experiment surface before was repeated and after using anodic a rotating polarisation disc electrode, can be seen as presented in later section in Figure for a 7.bulk The coppermorphology electrode of the with surface and without before and stirring. after anodic polarisation can be seen in later section for a bulkThe copper manipulation electrode ofwith mass and transportation without stirring. was tested using a rotating disc electrode, as presented in FigureThe manipulation7. It is important of mass that transportation in the absence was of stirring, tested using passivation a rotating occurs, disc evenelectrode, at stirring as presented rates of inup Figure to 500 7. rpm. It is important Only at rotation that in speedsthe absence above of1000 stirring, rpm, passivation the passivation occurs, response even at wasstirring lost. rates As theof uprotation to 500 speed rpm. increased,Only at rotation so did thespeeds current, above due 1000 tothe rpm, anions the passivation provided to response the surface was by lost. which As more the rotationreactions speed occurred. increased, The current so did the did current, not, however, due to reach the anions a steady provided state value to the as surface would by be predictedwhich more for reactionsa solution occurred. based species, The current so it must did benot, limited however, by the reach diff usiona steady of oxidised state value copper as would from thebe electrode’spredicted forsurface a solution rather based than a species, diffusion so of it chloride must be from limited the bulk by the electrolyte diffusion region of oxidised to the electrode’s copper from surface. the Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 14 electrode’s surface rather than a diffusion of chloride from the bulk electrolyte region to the Appl. Sci. 2019, 9, 4401 9 of 15 electrode’s surface.

no stirring 40 1000 rpm 3000 rpm 5000 rpm

30 ) -2 20 ( mA cm( mA j 10

-0.5 0.0 0.5 1.0 1.5 E (V) vs. Ag/AgCl 1 Figure 7. Anodic LSVs of Cu disc in ethaline versus Ag/AgCl, at a sweep rate of 5 mV s−1 and at Figure 7. Anodic LSVs of Cu disc in ethaline versus Ag/AgCl, at a sweep rate of 5 mV·s· − and at variousvarious rotation rotation speeds. speeds.

The formation of films on the electrode’s surface is known to influence the surface morphology The formation of films on the electrode’s surface is known to influence the surface morphology of the dissolved copper at the surface. The process of electropolishing to small extent, i.e., surface of the dissolved copper at the surface. The process of electropolishing to small extent, i.e., surface levelling, is thought to occur because of film formation which restricts metal ions diffusing away from levelling, is thought to occur because of film formation which restricts metal ions diffusing away from the electrode’s surface as a consequence of compact film formation, i.e., resistive film. the electrode’s surface as a consequence of compact film formation, i.e., resistive film. 3.5. Electrochemical Impedance Spectroscopy (EIS) 3.5. Electrochemical Impedance Spectroscopy (EIS) EIS technique is a novel approach to examine the electrical properties of bulk and interfacial EIS technique is a novel approach to examine the electrical properties of bulk and interfacial regions of various materials [27–29]. Figure8 shows the EIS spectra of metallic copper in choline regions of various materials [27–29]. Figure 8 shows the EIS spectra of metallic copper in choline chloride-based IL at various direct current (DC) potentials. At 0.2 V, there was a single semi-circle for chloride-based IL at various direct current (DC) potentials. At− −0.2 V, there was a single semi-circle an electron transfer process which got smaller at 0 V indicating an increase in electron transfer rate for an electron transfer process which got smaller at 0 V indicating an increase in electron transfer constant. At +0.2 V, a second semi-circle was enlarged at +0.4 V, and above that voltage, dominated, rate constant. At +0.2 V, a second semi-circle was enlarged at +0.4 V, and above that voltage, suggesting that an insulating layer formed on the electrode’s surface. At +0.2 V, there were two dominated, suggesting that an insulating layer formed on the electrode’s surface. At +0.2 V, there semi-circle responses which could represent the first and second oxidations of copper. were two semi-circle responses which could represent the first and second oxidations of copper. The data acquired for impedance at different polarisation potentials in Figure8b were fitted to an electrical equivalent circuit (EEC) involving a two Randle’s circuits in series with Warburg impedance. EEC is a straight method to represent the behaviour of the medium with circuit elements, thus understanding the electrical properties of the system under study [30,31]. From the data analysis, when applying a potential of +0.8 V, the film capacitance was found to be 4.9 10 6 F cm 2, assuming × − · − a dielectric constant of 8.0; the thickness of the film was estimated to be 1.4 µm [32]. This is about an order of magnitude more than expected from the films formed in aqueous media. The film thickness was, however, remarkably thicker than that found for stainless steel electropolishing in the same liquid under the comparable conditions, and less thick compared to cobalt in the same electrolyte. As a consequence, the cobalt surface underwent a well-mirror-like electropolishing. The thickness was estimated to be only 16 nm for that layer [32]. It should, however, be noted that speciation is different where for stainless steel, the iron complex formed is glycolate. Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 14 Appl. Sci. 2019, 9, 4401 10 of 15

50 a

40 )

-2 30

20 ( mA cm j 10

-10 -0.4 0.0 0.4 0.8 1.2 E (V) vs. Ag/AgCl

Figure 8. 8. (a)( aComparative) Comparative anodic anodic linear linear sweep sweep voltammogr voltammogramsams of a bulk of copper a bulk in copper choline inchloride- choline

chloride-basedbased IL at 20 °C IL (solid at 20 line)◦C and (solid imidazo line)lium-based and imidazolium-based IL at 70 °C (dashed IL at line) 70 ◦ C(b). (dashed Electrochemical line) (b). Electrochemicalimpedance spectra impedance of copper spectra in choline of copper chloride-based in choline chloride-basedIL at 20 °C at various IL at 20 ◦potentialsC at various with potentials an AC withamplitude an AC of amplitude10 mV in the of frequency 10 mV in range the frequency of 1–65,000 range Hz (c), of as 1–65,000 in (b) but Hz using (c), imidazolium-based as in (b) but using

imidazolium-basedIL at 70 °C. IL at 70 ◦C.

The experimentdata acquired was for repeated impedance in cholineat different chloride-based polarisation IL potentials at 70 ◦C in (not Figure shown) 8b were and thefitted same to mechanisman electrical ofequivalent dissolution circuit was gained.(EEC) involving A comparison a two withRandle’s copper circuits dissolution in series at 20with◦C showedWarburg a capacitanceimpedance. EEC of 2.17 is a straight10 5 F cm method2 at 70 to representC and + 0.8the Vbehaviour which corresponds of the medium to a with layer circuit of about elements, 0.3 µm × − · − ◦ thickthus understanding [32]. It would seemthe electrical logical to properties say that theof the diff systemusion layer under in study the duplex [30,31].From salt film the model data shouldanalysis, be thinnerwhen applying at a higher a potential temperature of +0.8 as V, the the salt film would capacitance be more was soluble. found to be 4.9 × 10−6 F·cm−2, assuming a dielectricThe structure constant of of the8.0; doublethe thickness layer of at the the film electrode’s was estimated surface to in be molecular 1.4 µm [32]. solvent This is systems about an is completelyorder of magnitude different more from than that expected in ILs, from in such the afilms way formed that in in the aqueous former, media. the electrodeThe film thickness charge is compensatedwas, however, by remarkably both adsorbed thicker counter than ions that and found the diforffuse stainless layer, whilesteel electropolishing in the latter, the structurein the same may involveliquid under a monolayer the comparable of counter conditions, ions as compensation, and less thick followedcompared by to a cobalt multilayer in the involving same electrolyte. cations andAs anionsa consequence, adjacent the to each cobalt other surface [33,34 underwent]. a well-mirror-like electropolishing. The thickness was estimatedIn Figure to be8c only the impedance16 nm for that of a layer copper [32]. electrode It should, in imidazolium-basedhowever, be noted that IL was speciation measured is different at 70 ◦C aswhere a function for stainless of DC steel, potential. the iron A single complex semicircle formed was is glycolate. observed at 0.2 V corresponding to the process − of electronThe experiment transfer that was was repeated likely the in oxidation choline chloride-based of copper. In this IL experiment,at 70 °C (not the shown) polarisation and the potential same wasmechanism altered overof dissolution the potential was windowgained. A from comparis negativeon towith positive. copper Asdissolution the DC potential at 20 °C was showed shifted a fromcapacitance negative of to2.17 0.0 ×10 V,− the5F·cm semicircle−2at 70 °C lessened and + 0.8 its V widthwhich as corresponds a result of ato faster a layer electron of about transfer 0.3 µm process thick that[32]. wasIt would predicted seem given logical the to increase say that in the overall diffusion potential. layer From in the+ 0.4duplex to + 1.2salt V, film a vertical model straight should linebe wasthinner observed, at a higher signifying temperature the characteristic as the salt would of a series be more RC circuitsoluble. which was caused by an insulating layerThe on thestructure electrode’s of the surface. double Thelayer response at the electr of theode’s series surface RC circuit in molecular did not altersolvent with systems potential, is indicatingcompletely that different once the from film that forms, in ILs, it is notin such permeable a way and that insulates in the former, the electrode’s the electrode surface. charge It would is therefore,compensated be expectedby both adsorbed that this wouldcounter make ions and the electrodethe diffuse unelectropolished. layer, while in the This latter, reveals the structure how the Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 14

may involve a monolayer of counter ions as compensation, followed by a multilayer involving cations and anions adjacent to each other [33,34]. In Figure 8c the impedance of a copper electrode in imidazolium-based IL was measured at 70 °C as a function of DC potential. A single semicircle was observed at −0.2 V corresponding to the process of electron transfer that was likely the oxidation of copper. In this experiment, the polarisation potential was altered over the potential window from negative to positive. As the DC potential was shifted from negative to 0.0 V, the semicircle lessened its width as a result of a faster electron transfer process that was predicted given the increase in overall potential. From +0.4 to +1.2 V, a vertical straight line was observed, signifying the characteristic of a series RC circuit which was caused by an insulating layer on the electrode’s surface. The response of the series RC circuit did not alter with potential, indicating that once the film forms, it is not permeable and insulates the Appl. Sci. 2019, 9, 4401 11 of 15 electrode’s surface. It would therefore, be expected that this would make the electrode unelectropolished. This reveals how the speciation of dissolved copper can affect the behaviour of speciationthe electrode. of dissolved From examining copper canthe linear aﬀect thesweep behaviour voltammetry of the electrode.for the imidazolium-based From examining IL the system, linear sweepit can be voltammetry seen that a fordiagonal the imidazolium-based line is gained, indicating IL system, a resistor. it can be This seen can that be a diagonalattributed line to isa layer gained, of indicatingcopper chloride a resistor. formation This can on be the attributed copper metal. to a layer of copper chloride formation on the copper metal.

3.6. Electropolishing The electropolishingelectropolishing can can basically basically be describedbe describe asd a controlledas a controlled electrochemical electrochemical dissolution dissolution process ofprocess a surface of a surface in an attemptin an attempt to construct to construct it less it le roughlyss roughly at theat the macroscale macroscale (levelling) (levelling)> >11 µ µmm andand microscale (brightening)(brightening) <<11 µmµm [[35].35]. The The basic basic principles of this process are filmfilm formationformation and aa mass transport limitedlimited currentcurrent plateauplateau fromfrom thethe polarisationpolarisation response.response. SoSo as to achieve the conditions forfor macrosmoothing,macrosmoothing, thethe ohmicohmic controlcontrol oror thethe massmass transporttransport controlcontrol hashas toto bebe conducted,conducted, whilewhile forfor microsmoothing, thethe massmass transporttransport mechanismmechanism isis susufficientfficient [[35].35]. InIn the present work, work, metallic metallic copper copper was was electropol electropolishedished in in choline choline chloride-based chloride-based IL IL at at20 20 °C◦ atC ata potential a potential of of+1.2+1.2 V, V,as asexhibited exhibited in inFigure Figure 9. 9This. This is isthe the first first time time that that a DES a DES has has been been shown shown to tobe beuseful useful electropolishing electropolishing electrolyte electrolyte for a for single a single metal. metal. Figure Figure 9exhibits9 exhibits a metallic a metallic copper copper surface surface which whichis brighter is brighter but there but thereare obvious are obvious signs signs of pitting of pitting on onthe the surface. surface. It Itcould could actually actually be be questioned whetherwhether thisthis isis trulytruly electropolishedelectropolished oror justjust brightenedbrightened alone.alone.

Figure 9.9. AA photograph photograph of of a samplea sample of metallicof metallic copper copper sheet sheet after electrochemicalafter electrochemical dissolution dissolution in choline in

chloride-basedcholine chloride-based IL after beingIL after held being at + held0.18 at V +0.18 for 10 V min for at10 20 min◦C, at unpolished 20 °C, unpolished (u) and (u) polished and polished (p). (p). 3.7. Atomic Force Microscopy (AFM) 3.7. AtomicThe AFM Force images Microscopy for a bulk(AFM) copper sheet before and after electrochemical dissolution in choline chloride-basedThe AFM images IL are presentedfor a bulk incopper Figure sheet 10. before It can beand seen after that electrochemical metallic copper dissolution undergoes in choline pitting underchloride-based several conditions IL are presented and electropolishing in Figure 10. under It can the be others, seen that noticing metallic that copper on the native undergoes (unpolished) pitting surface,under several machining conditions marks and and scratches electropolishing can clearly beunder seen. the Over others, the anodic noticing sweep, that it canon bethe seen native that the(unpolished) average surface surface, roughness machining value, marks Ra, and of scratche the electrodes can priorclearly to be anodic seen. polarisationOver the anodic was sweep, 0.75 µm, it whilecan be theseen value that the after average polarisation surface was roughness 3.75 µm. value, When Ra, the of the electrode electrode was prior rotated to anodic at 3000 polarisation rpm, the 0.63was µ0.75m was µm, gained. while the If thevalue liquid after was polarisation stirred (by wa rotatings 3.75 µm. the electrodeWhen the at electrode 3000 rpm) was then rotated no film at 3000 was producedrpm, the 0.63 at the µm electrode’s was gained. surface If the andliquid the was solution stirre becamed (by rotating green. the electrode at 3000 rpm) then no film Galvanostaticwas produced etching at the electrode’s for short times surface resulted and the in pitting solution taking became place, green. leading to an uneven surface, whereasGalvanostatic increasing etching the etch for time short led totimes a visibly resulted brighter in pitting surface taking with place, less microscopic leading to roughness.an uneven Thesurface, machining whereas marks increasing were removed the etch by time the electroled to polishinga visibly process.brighter Similarly,surface with the sameless microscopic pattern was observedroughness. for The the machining electropolishing marks were of stainless removed steel by inthe choline electro chloride-based polishing process. IL [ 36Similarly,]. In commercial the same electropolishingpattern was observed electrolytes, for the it electropolishing is well-known that of levellingstainless onlysteel reallyin choline takes chloride-based place once the electrolyteIL [36]. In iscommercial saturated withelectropolishing metal ions. Electropolishing electrolytes, it is the well-known metallic copper that levelling samples only with 0.81really M takes CuCl place2 added once to the solution caused two different morphologies on the electrode’s surface. It is worth noting that at low etch times, pitting was more observable, whereas longer timescales gave more even surface finishes. It is worth mentioning the hydrogen evolution occurs at the cathode during the anodic dissolution of metallic copper in choline chloride-based IL at 20◦C. On the one hand, a negligible gas evolution at low current density (<20 mA cm 2) is observed in the form of bubble. On the other hand, at higher current · − density the evolution is remarkable in the addition of water at any amount. This can be related to either the electrolysis of a trace of water or EG [36]. The hydrogen gas evolution resulted in a lowering of the current efficiency of the electrochemical dissolution process, which is undesirable. DESs are generally insensitive to moisture compared to ILs, but the impact of water on the electrochemical process is still effective [37]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 14 the electrolyte is saturated with metal ions. Electropolishing the metallic copper samples with 0.81 M CuCl2 added to the solution caused two different morphologies on the electrode’s surface. It is worth noting that at low etch times, pitting was more observable, whereas longer timescales gave more even surface finishes. It is worth mentioning the hydrogen evolution occurs at the cathode during the anodic dissolution of metallic copper in choline chloride-based IL at 20°C. On the one hand, a negligible gas evolution at low current density (<20 mA·cm−2) is observed in the form of bubble. On the other hand, at higher current density the evolution is remarkable in the addition of water at any amount. This can be related to either the electrolysis of a trace of water or EG [36]. The hydrogen gas Appl. Sci. 2019, 9, 4401 12 of 15 evolution resulted in a lowering of the current efficiency of the electrochemical dissolution process, which is undesirable. DESs are generally insensitive to moisture compared to ILs, but the impact of waterOn on thethe oneelectrochemical hand, there isprocess an alteration is still effective in morphology [37]. of metallic copper surface from dark to surfaceOn brightness,the one hand, as shownthere is in an Figure alteration 10. On in themorp otherhology side, of roughness metallic copper is revealed surface in Figurefrom dark 10; the to surfacecopper brightness, sheet after dissolutionas shown in made Figure the 10. subjects On the rougher,other side, but roughness the fewer is crevices revealed are in considered Figure 10; the the copperkey feature sheet of after the copperdissolution sheet. made So as the to makesubjects a decision rougher, on but whether the fewer electropolishing crevices are occurredconsidered or not,the keyherein feature at least, of the the copper brightness sheet. accomplished So as to make had a deci tosion be su onffi cient.whether Moreover, electropol thisishing is desirable occurred from or not, the hereinelectropolishing at least, the perspective brightness that accomplished can be linked had to to the be kind sufficient. of electrolyte Moreover, because this the is desirable electropolishing from the of electropolishingcopper robustly dependsperspective on that the naturecan be oflinked the electrolyte. to the kind of electrolyte because the electropolishing of copper robustly depends on the nature of the electrolyte.

(a) Ra = 44 nm

(b) Ra=148 nm (c) Ra=336 nm

(d) Ra=163 nm (e) Ra=188 nm

FigureFigure 10. Resonant mode (ca.(ca. 300300 kHz)kHz) AFMAFM images images (recorded (recorded in in air air at at a scana scan rate rate of 0.5of 0.5 Hz, Hz, with with 256 256lines) lines) of the of sample:the sample: (a) native (a) native copper, copper, after dissolution after dissolution in pure in choline pure chloride-basedcholine chloride-based IL at 50 mA IL atcm 502 · − mA·cmfor (b) 20−2for min (b or) 20 (c )min 1 h, or and (c under) 1 h, theand same under conditions the same but conditions with the additionbut with of the 0.81 addition M CuCl of2H 0.81O M to 2· 2 CuClthe ethaline2·2H2O to for the (d) ethaline 10 min orfor ( e(d) 1) 10 h. min (Arithmetic or (e) 1 h. average (Arithmetic height, average Ra: quantiﬁes height, Ra: vertical quantifies deviation vertical of a deviationsurface from of a normal surface vector.) from normal vector.)

ItIt is is also also impressive impressive to to notice notice that that the the natu naturere of of interfacial region region governs governs the the nature nature of of electrochemical-polishingelectrochemical-polishing which which is is different diﬀerent to to large large extent from that of aqueous counterparts. It It is is well-reported that electrochemical polishing occurs at the interface interface region region between between the the electrode electrode and and thethe electrolyte electrolyte where where dissolved ions diffuse diﬀuse from thethe electrode’s surfacesurface intointo bulkbulk electrolyteelectrolyte [[38,39].38,39].

4. Conclusions In the present work, the study of mechanism of metallic copper dissolution in two chloride- containing electrolytes, choline chloride-based deep eutectic solvent, ethaline, and imidazolium-based IL, C4mimCl, has revealed that CuClads and CuCl2ads formed in the first oxidation region which was a compact film, and that was followed by a second oxidation resulting in the complexation of oxidised 2 copper in the form CuCl3− and CuCl4 − producing a porous film which then diffused away from the Appl. Sci. 2019, 9, 4401 13 of 15 metallic copper surface. In other words, the release of dissolved copper from the bulk solid phase into both electrolytes in the form Cu(II)rather than Cu(I) occurred. The Cu(I)/Cu(II) process can be identified by means of an asymmetric peak at the anodic regime as a result of passivation of the copper surface primarily with saturated corrosion products. This is evidenced by comparing the voltammogram of CuCl 2H O and metallic copper disc in both 2· 2 electrolytes. This can be linked with the chemistry of what actually happens at the interface, considering the composition properties (composition and structure). It was also seen that EG was responsible for saturation at the interface region, resulting in withdrawing chloride ions into the interfacial region. The quasi-passivation of the copper in this electrolyte depends upon the EG. The postulated mechanism involves the electrochemical formation of CuClads and CuCl2−, followed by oxidation to CuCl2 leading to super-saturation of the interfacial region with [CuCl3]− and 2 [CuCl4] − restricted by the availability of Cl−. It is probable that the Cl− ions interact with oxidised metallic copper, creating insulating CuCl and CuCl2 which may not cover the entire surface, and some parts keep free of coverage in non-stoichiometric proportions which are vulnerable to solvation with Cl− ions, and as a result, diffusion away from the surface occurs. Additionally, the kinetics of metallic copper dissolution in both electrolytes of interest was studied to some extent. The temperature influence and use of RDE have demonstrated that electrochemical dissolution raised as the temperature elevated and the mass transport effect changed towards faster electrochemical metallic copper dissolution. Finally, the electropolishing of copper in deep eutectic fashion was accomplished to some extent, which cannot be acquired by means of aqueous chloride electrolyte. The electropolishing of copper in this electrolyte (ChCl:EG 1:2) involves levelling (macrosmoothing) and to some extent brightening with a diminution in the surface roughness. The use of dissolved copper as a counter electrode in a range of experiments to produce copper ions slowly on the working electrode on a large scale level is another advantage of studying the anodic behaviour of copper metal.

Author Contributions: Conceptualisation, W.O.K.; formal analysis, W.O.K.; investigation, W.O.K.; methodology, W.O.K. project administration, S.B.A.; validation, M.A.B., R.M.A., and M.F.Z.K.; writing—original draft, W.O.K.; writing—review and editing, S.B.A., M.A.B., R.M.A., and M.F.Z.K. Funding: This research was funded by University of Leicester, Leicester, UK and Ministry of Higher Education and Scientific Research-Kurdish National Research Council (KNRC), Kurdistan Regional Government/Iraq. The financial support from the University of Sulaimani and Komar Research Center (KRC), Komar University of Science and Technology is greatly appreciated. Acknowledgments: The authors gratefully acknowledge the financial support for this study from Ministry of Higher Education and Scientific Research, Kurdistan Regional Government/Iraq and the University of Leicester, Leicester, UK. The authors are grateful to scientist Andrew P.Abbott for fruitful discussion regarding the dissolution of bulk copper. Conflicts of Interest: The authors declare no conflict of interest.

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