Electrodeposition Study of Ni Coatings on Copper from Choline Chloride-Based Deep Eutectic Solvents

Nickel was electrodeposited from choline chloride (ChCl)-based deep eutectic solvents (DESs) containing ethylene glycol (EG) and urea (U) on Cu substrate. The influence of the temperature and of DESs nature was evaluated. The diffusion coefficients of nickel varied from 2.2 × 10 to 3.7 × 10 cm s for ChCl:2U and from 1.3 × 10 to 5.8 × 10 cm s for ChCl:2EG with increasing temperature. The nucleation process of Ni on Cu at 70 °C was determined as instantaneous in ChCl:2EG and progressive in ChCl:2U using Scharifker and Hills model. The SEM characterization showed that with increasing temperature, more compact coatings were obtained. Linear polarization and electrochemical impedance spectroscopy were used to evaluate the corrosion performance of the obtained deposits in 3.5% NaCl. The coatings were more protective as higher the temperature of deposition, with polarization resistance (Rp) values from 3.53 to 19.11 kΩ cm.


Introduction
Nickel (Ni) is known to enhance corrosion resistance of metallic materials, 1 which makes it largely applicable in industry in order to improve the corrosion performance of various alloys including stainless steels and high Ni alloys manufacturing, as well as its application as electrodeposited coatings onto different metallic substrates. 2,3In the case of the industrial Ni electroplating process, the most traditional plating solution is based on the use of the aqueous Watt's bath. 1,2,4,5However, electrodeposition of metals and alloys from aqueous solution is strongly influenced by hydrogen evolution reaction.For instance, the parallel production of hydrogen decreases the electrodeposition efficiency, causing deleterious effects to the corrosion resistance of the electrodeposited coatings, such as the hydrogen embrittlement.[8][9][10][11] So, in order to avoid the disadvantage related above with the electrodeposition in aqueous medium, the interest to obtain and to comprehend the metals and alloys electrodeposition from non-aqueous solutions is rising.
][33][34][35] However, there are some lacks in understanding nickel electrochemical behavior and nucleation/growth kinetics in DESs.In this context, a systematic study of voltammetric profile of nickel onto copper and a comparative study of the nucleation and growth of Ni onto Cu in different DESs has not been reported.So, in the present work two different systems ChCl:2EG and ChCl:2U containing NiCl 2 •6H 2 O were used in order to evaluate the influence of nature of DESs and temperature in the nucleation and

Experimental
Choline chloride (Sigma-Aldrich ® , > 98%), urea (Sigma-Aldrich ® , 99.0-100.5%)and ethylene glycol (Sigma-Aldrich ® , 99.8%) were all used as received.DESs solutions were formed by the mixture of the two components in a molar ratio of 1ChCl:2U or 1ChCl:2EG at 80 °C until obtaining a homogeneous, colorless and transparent liquid.The solutions containing nickel ions were prepared by addition of 0.2 mol L −1 NiCl 2 •6H 2 O (P.A. 97%, VETEC ® ) to the DESs until a homogeneous green liquid was obtained.The water content of neat DESs and electroplating solutions was determined by Karl Fischer coulometric titration using a Titrino Coulometer model 756 from Metrohm ® and it was less than 1% for all tested samples.
Electrochemical experiments were carried out using an Autolab ® PGSTAT 128N potentiostat/galvanostat, controlled with NOVA ® software.A heated water-jacketed cell of 30 mL was used for DESs experiments with a conventional three-electrode setup.For aqueous medium, a three-electrode conventional electrochemical cell with a volume of 80 mL was used.
In electrochemical experiments, the working electrode was a copper disc with 0.03 cm 2 of geometric area.A Pt gauze, with an exposed area greater than that of the working electrode, was used as counter electrode and an Ag wire was used as the quasi-reference electrode in DES and an Ag|AgCl|Cl − (saturated KCl) was used as reference electrode for the measurements carried out in aqueous medium.Experiments of cyclic voltammetry were carried out in the absence and in the presence of 0.2 mol L −1 NiCl 2 •6H 2 O in potentials more negative than −0.40 V to avoid oxidation of Cu substrate in anodic scan. 36For neat DESs, the potential range was from −0.40 to −1.50 V and the sweep rate was 10 mV s −1 , varying the temperature from 25 to 70 °C.To evaluate the influence of temperature on nucleation loop of Ni onto Cu substrate, scans were recorded at 25, 45 and 70 °C with a sweep rate of 10 mV s −1 , starting at −0.40 V and reversing the scan as soon as the nickel reduction wave has formed.Current transients were obtained by chronoamperometry at 70 °C for nucleation and growth studies.Chronoamperometry experiments at large overpotentials were carried out to calculate diffusion coefficients of nickel in both DESs by Cottrell's equation at 25, 45 and 70 °C.
Ni electrodeposits were obtained under galvanostatic control using Cu discs with 0.2 cm 2 of geometric exposed area.A current density of 1 mA cm −2 was applied for 3600 s in order to obtain Ni coatings at 25, 45 and 70 °C in both DESs.Prior to each experiment, Cu electrodes were abraded with SiC paper up to 1200, degreased with 10% NaOH aqueous solution, activated with 10% HCl solution, rinsed with Milli-Q ® grade water and dried with heated air.
Polarization curves and impedance spectra were obtained in aqueous 3.5% NaCl (VETEC ® ) after 3600 s of open circuit potential stabilization using Cu discs with the Ni galvanostatic electrodeposits as working electrodes at 22 ± 1 °C.Linear polarization experiments were carried out at 1 mV s −1 and electrochemical impedance spectroscopy (EIS) data were acquired in the frequency range from 10 kHz to 10 mHz with an AC sinusoidal wave of 10 mV of amplitude.The equivalent electric circuit (EEC) analysis was carried out in order to fit the EIS data.The choice of best fit was based on the lowest number of circuit's components, χ 2 error equal to or minor than 0.001 and up to 5% of errors associated with circuit's components.
The surface morphology of the Ni electrodeposited coatings was analyzed by FEI-Quanta 450 FEG microscope for characterization of Ni deposits.Firstly, this figure shows that adsorption peaks are absent in the spectra obtained for both neat DESs.On the other hand, the spectrum obtained with NiCl 2 •6H 2 O dissolved in ChCl:2EG presents three bands centered at 412, 690 and 750 nm, while the achieved in ChCl:2U presented two bands, centered at 360 and 580 nm.Abott et al. 18 reported bands centered in 425 and 655-715 nm for NiCl 2 •6H 2 O in ChCl:2EG which corroborates with our results.Gu and Tu, 37 studying the thermochromic behavior of chloro-nickel(II) species in DESs, obtained UV-Vis spectra with a band at around 420 nm and at least three bands in the range of 600-800 nm for 0.05 mol L −1 NiCl 2 •6H 2 O in ChCl:2EG in the temperature range from 30 to 70 °C.In ChCl:2U, these authors observed one band at around 400 nm and two bands in the range of 600-850 nm for 0.1 mol L −1 NiCl 2 •6H 2 O with temperature varying from 29 to 72 °C. 37Zhang et al. 38 also observed bands in the ranges of 350-500 nm and 600-800 nm for the UV-Vis absorption spectrum of NiCl 2 in aqueous medium.2][43][44] These bands have been explained by the ligand field theory and have been assigned as characteristic of Ni II d-d transitions, 38,43,[45][46][47][48] since Ni II forms d 8 complexes.Tanabe and Sugano 49 diagrams for d 8 electronic configuration in octahedral ligand field can be used to predict the allowed transitions for the complexes in study.According to these, there are three allowed transitions, 3 A 2g (F) → 3 T 2g (F), 3 A 2g (F) → 3 T 1g (F) and 3 A 2g (F) → 3 T 1g (P), which lead to a prediction of up to three bands in the UV-Vis spectrum for Ni II complexes. 45,49The bands observed in 412 nm in ChCl:2EG and in 360 nm in ChCl:2U can be attributed to the 3 A 2g (F) → 3 T 1g (P) transition. 43,45,49ands in the region of 550-800 nm can be attributed to 3 A 2g (F) → 3 T 1g (F) transition. 38,43,49Moreover, in this range, a split in the band could occur, due to a spin-orbit coupling that mixes the 3 T 1g (F) and 1 E g states. 38,43This split was observed in ChCl:2EG but not in ChCl:2U spectrum.Lastly, the difference in the wavelengths of the obtained bands in each solvent indicated that the transitions in ChCl:2U involve more energy than that occurring in ChCl:2EG, which was confirmed by the green tones of these solutions, being a pale one to the ChCl:2EG and a more intense green in ChCl:2U.Griffiths and Scarrow 50 reported the formation of complexes of the type Ni(EG) x 2+ , NiCl(EG) y + and NiCl 4 2-using ethylene glycol as solvent and adding chloride with different salts.According to them, x is a number between 3 and 6 and y is between 3 and 5, since EG can act as a bidentate ligand. 50harma et al. 51

UV-Vis spectroscopy analysis
by adding an aqueous solution of urea to a solution containing Ni 2+ ions in water.So, the obtained bands in Figure 1 can be related to the formation of Ni II complexes with ethylene glycol and urea, with or not Cl − in the structure, for each corresponding DES medium.For both neat DESs, a pronounced increase in the cathodic current occurs at potentials more negative than −1.0 V, with the potential increasing with decreasing temperature.Furthermore, a well-defined cathodic peak was displayed in all obtained cyclic voltammograms obtained in ChCl:2U solvent.An analogous behavior was found  by Yue et al. 29 studying the electrochemical behavior of ChCl:2U.These authors demonstrated that these processes was associated with choline cation decomposition forming triethylamine (at −1.2 V, 100 °C) and hydrogen (at −1.4 V, 100 °C). 29However, the mentioned process occurring before the abrupt increase in current is very discrete to be related to choline cation decomposition, because the concentration of this chemical is high.So, the hypothesis of this process be related to water content in the ChCl:2U was tested by the addition of known amounts of water to this DES at 70 °C.These results are shown in Figure 3.

Electrochemical characterization of the electrolytes
From Figure 3, it can be seen that increasing the amount of added water to the ChCl:2U leads to an increase in currents of the mentioned process, confirming that it is related to water.Nevertheless, the changing in currents is relatively low when compared to the amount of water added, indicating little influence in currents with increasing water amounts.In fact, Shah and Mjalli 52 concluded that amounts of water minor than 5% did not influence ChCl:2U physical properties, like melting point and viscosity.So, once the water content of all solutions used in the present work was less than 1%, as determined by Karl-Fischer method, no significant changes in the physical properties of the DESs can be assured.Additionally, the similarity of the morphology of the obtained deposits with data published in literature for Ni from DESs or ionic liquids shows that no influence of this water content was detected in the electrodeposits. 20,53cleation and growth studies Figure 4 shows the voltammograms obtained for 0.2 mol L −1 NiCl 2 •6H 2 O in both DESs at 10 mV s −1 with increasing temperature at a restricted potential range.It could be seen the so called nucleation loop 54 for voltammograms recorded in both DES.It indicates that nickel electrodeposition onto copper needs a certain overpotential for the initial nucleation and growth process. 55n ChCl:2U, the nucleation overpotential 56 reduced from 320 to 210 mV with increasing temperature from 25 to 70 °C.In ChCl:2EG, these values varied from 380 to 360 mV in the same temperature range.As the nucleation overpotential decreases with increasing temperature, it can be inferred that rising the temperature favors nickel electrodeposition.Also, comparing the nucleation overpotential observed for each DES, those referring to ChCl:2U were always minor than in ChCl:2EG, indicating that nucleation and growth initiation was more favorable to occur in ChCl:2U than in ChCl:2EG.
It is important to note that in the presence of NiCl 2 •6H 2 O, for both media, discharge of solvent occurs after nickel reduction, having no interference on this process.Besides, increasing temperature leads to a shift in onset potential towards less negative values and no additional process was observed with changing temperature.This means that with increasing thermal  energy, less potential energy is required for nickel reduction to occur, which is in accordance with theoretical expectations.Also, for the same nickel concentration, electrode area and temperature, current values obtained in ChCl:2EG were always larger than that of ChCl:2U.It can be attributed to the difference of viscosity between these DESs.8][59] So, species and charge transports are much more difficult in ChCl:2U than in ChCl:2EG at studied temperature range, leading the current observed in ChCl:2U to be lower than in ChCl:2EG.
The Cottrell (equation 1) plots 56 obtained from chronoamperometry at 2.0 V and 70 °C in both DES are shown in Figure 5. (1) In equation 1, I is the current in A, z is the number of electrons involved in the process, F is the Faraday constant (C mol −1 ), A is the area of the electrode (cm 2 ), D is the diffusion coefficient (cm 2 s −1 ), c is the bulk concentration (mol cm −3 ) and t is the time (s).
In Figure 5, it can be noted that the obtained correlation coefficients for the |I| vs. t 1/2 plots are in good agreement with the linearity predicted by Cottrell equation.So, they were used to calculate the diffusion coefficients of nickel in both DESs, as shown in Table 1.
From Table 1, a rise in diffusion coefficients with increasing temperature for both DESs can be seen, which is in agreement with the decrease of viscosity of the solvent.At the same temperature, the diffusion coefficients obtained in ChCl:2EG are always higher than that for ChCl:2U, as expected by early explanation on viscosity.
The nucleation and growth processes of nickel onto copper substrate were studied in detail by chronoamperometry at 70 °C.The plots of current-time transients are presented in Figures 6a and 6b for ChCl:2U and ChCl:2EG, respectively.The shape of the transients was in accordance with the expected for the j vs. t relation when new metallic phase nucleation occurs. 60As can be seen in Figure 6, the current density increased during a defined period till a maximum and, then, decreased due to overlapping of diffusion zones around the nuclei formed. 2,60Equations 2 and 3 are the basis of analyzing experimental current-time transients obtained at a certain applied potential. (2) (3) In equations 2 and 3, F, D and t have the same meaning as in equation 1. M is the molar mass of the deposit (g mol −1 ), ρ is the specific weight of the deposit (g cm −3 ).For the progressive nucleation (equation 2), AN ∞ is the rate of nucleation each cm 2 per s.For the instantaneous nucleation (equation 3), N is the number of nuclei formed per cm 2 .
According to current-time transients of Figure 6, the growth current density was dependent on the magnitude of the applied potential, since the former increases with increasing the latter.It implies that the AN ∞ and N were potential dependent, too.So, the major the potential, the major are the AN ∞ and N and the shorter the time for overlapping to occur, as observed in Figure 6. 60or current-time transients obtained in ChCl:2U, the current density j followed a linear correlation with t 3/2 and with t 1/2 for those obtained in ChCl:2EG, which means  that nickel nucleation is progressive in ChCl:2U and instantaneous in ChCl:2EG. 60,61Besides, the AN ∞ and N can be easily calculated from the slope of the j vs. t 3/2 or j vs. t 1/2 plots for known D and concentration to a certain potential.Thus, the AN ∞ , and N calculated 60,61 for the current-time transients presented in Figure 6 are shown in Table 2, as well as the applied potential, −E step , the maximum current density j max and maximum time t max obtained.
From this, it can be noted that there is an increase in the rate of nuclei formation, for ChCl:2U, and in the number of nuclei, for ChCl:2EG, when potentials became more negative. 60igure 7 displays the normalized coordinates of experimental current-time transients with the corresponding theoretical curves for progressive (equation 4) and instantaneous (equation 5) nucleation for ChCl:2U (Figure 7a) and ChCl:2EG (Figure 7b) according to Scharifker-Hills model.61 (4) (5)   In equations 4 and 5, j max is the maximum value of the current transient in A cm −2 ; and t max is the time corresponding to j max .
As it can be seen, the experimental curves fit well with the theoretical profile for progressive nucleation in ChCl:2U and for instantaneous nucleation theoretical curve in ChCl:2EG.
It can be observed that coatings obtained from ChCl:2U exhibited a similar morphology characteristic for thin films of Ni electrodeposited at 25, 45 and 70 °C. 62The Ni coatings  were dense and adherent at all temperature studied, which were consistent with previous works. 12,62For Ni coatings deposited at 25 °C, no nodule was observed in SEM micrograph, suggesting the formation of a very thin film.At 45 °C, the nodular morphology was revealed and, finally, for Ni deposits obtained at 70 °C, the nodular morphology was maintained, but the size of the nodules was noticeably smaller than that observed at 45 °C.These changes in morphology indicated that the increasing temperature favored the Ni nucleation onto Cu and that the coating was more homogeneously distributed when obtained at 70 °C.
On the other hand, the Ni coatings obtained from ChCl:2EG at 25, 45 and 70 °C can be seen in SEM micrographs presented in Figures 8d-8f, respectively.It is obviously seen that the surface morphology of Ni deposits changed significantly as the nature of DES at 25 and 45 °C, but not at 70 °C.For ChCl:2EG, a heterogeneous film with different morphologies was obtained at 25 °C.At 45 °C, the film presented many cracks like dry lake morphology.The crack formation in Ni films can be induced by the internal stress due to the rapid nucleation during the coating formation. 63As evidenced in Figure 6, the current densities obtained in transients for nickel in ChCl:2EG are always major than in ChCl:2U and the t max in Table 2 is always minor in ChCl:2EG than in ChCl:2U, which implies that the nucleation is faster in the former than in the latter and result in the internal stress accumulation on Ni deposits obtained from ChCl:2EG and consequently promote the cracks formation.These results are in accordance with the instantaneous nucleation observed for Ni electrodeposition in ChCl:2EG and not observed for ChCl:2U in the present study.Although, at 70 °C, no cracks were observed and the morphology was very similar with the deposit obtained in ChCl:2U.solution, R 1 is the charge transfer resistance, Q 1 is the constant phase element related to the capacitance of the double layer, R 2 is the resistance of the corrosion products and Q 2 is the constant phase element associated to the capacitance of the corrosion products film.
It can be noted that the capacitive loops increased with the increase of the temperature of the electrodeposition, indicating that the coatings conferred corrosion protection to the substrate.The resistance values were listed in Table 4. Values of n close to 0.5 indicated that the processes presented diffusion characteristics at low frequencies. 72

Conclusions
Temperature had a significant influence in DESs viscosity and in diffusion coefficients of nickel and in the morphology of the obtained Ni electrodeposits.Increasing values of the temperature, Ni deposition was favored.The nature of the DESs also influenced in the characteristic of the deposits.In this case, ChCl:2EG led to a cracked coating at low temperatures and a homogeneous nodular coating at 70 °C.In ChCl:2U, no cracks were observed.All deposits obtained in ChCl:2U and the deposit obtained from ChCl:2EG at 70 °C presented corrosion protective properties.Polarization curves and EIS spectra revealed that the R p values increased with increasing of the DESs temperature during electrodeposition.It was in accordance with physical characterization, which showed more effective coatings at higher temperatures.

Figure 1
Figure 1 displays the UV-Vis spectra obtained for both neat DESs and for 5 × 10 −3 mol L −1 NiCl 2 •6H 2 O in ChCl:2U and in ChCl:2EG.Firstly, this figure shows that adsorption peaks are absent in the spectra obtained for both neat DESs.On the other hand, the spectrum obtained with NiCl 2 •6H 2 O dissolved in ChCl:2EG presents three bands centered at 412, 690 and 750 nm, while the achieved in ChCl:2U presented two bands, centered at 360 and 580 nm.Abott et al.18 reported bands centered in 425 and 655-715 nm for NiCl 2 •6H 2 O in ChCl:2EG which corroborates with our results.Gu and Tu,37 studying the thermochromic behavior of chloro-nickel(II) species in DESs, obtained UV-Vis spectra with a band at around 420 nm and at least three bands in the range of 600-800 nm for 0.05 mol L −1 NiCl 2 •6H 2 O in ChCl:2EG in the temperature

Table 1 .
Diffusion coefficients (D) calculated from Cottrell's equation for Nickel at Cu substrate in ChCl:2U and ChCl:2EG at different temperatures

Figure 11 .
Figure 11.Electrical equivalent circuit for the fitted values on Figure 10.

Table 2 .
Analysis of the current-time transients considering the nucleation processes of nickel at copper substrate at 70 °C in ChCl:2U and ChCl:2EG : applied potential; t max : maximum time; j max : maximum value of the current transient; AN ∞ : rate of nucleation each cm 2 per s; N: number of nuclei formed per cm 2 . step

Table 3 .
The polarization resistance (R p ), corrosion current (j corr ) and corrosion potential (E corr ) values obtained from polarization curves acquired at room temperature (22 ± 1 °C) for the Ni electrodeposits in 3.5% NaCl

Table 4 .
Fitted values calculated from equivalent circuits of impedance measurements carried out at room temperature (22 ± 1 °C) for the Ni coatings in 3.5% NaCl : resistance of the solution; Q 1 : constant phase element related to the capacitance of the double layer; R 1 : charge transfer resistance; Q 2 : constant phase element related to the capacitance of the corrosion products film; R 2 : resistance of the corrosion products. s