ORIGINAL PAPER

Effect of pulse current on the electrodeposition of copperfrom choline chloride-ethylene glycol

Sujie Xing & Caterina Zanella & Flavio Deflorian

Received: 2 August 2013 /Revised: 14 January 2014 /Accepted: 20 January 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract Deep eutectic solvents (DESs) have been consid-ered as alternatives to classic aqueous deposition baths used atroom temperature. This work describes a preliminary studyabout the copper deposition process from a bath based oncholine chloride/ethylene glycol deep eutectic solvents. Thephysical properties, such as viscosity and conductivity of thebath, are compared before and after the addition of CuCl2·2H2O. The process kinetics during copper deposition wasinvestigated. By optimizing the concentration of metallic saltsand operating temperature, deposits with compact surfacesand small grain size were obtained. However, the processhas a very limited current efficiency. Pulse plating was appliedto improve the mass transport and refine grain size. In thiscase, an important improvement was achieved. Current effi-ciency (CE) increased significantly compared to the resultsobtained in direct current. This proves that pulse plating canbe used as an effective method to reduce deposition time andcosts. Surface morphology of deposits was observed by scan-ning electron microscopy (SEM), and compositional analysiswas quantified by energy-dispersive X-ray spectroscopy(EDXS).

Keywords Deep eutectic solvent . Copper .

Electrodeposition . Pulse plating . Current efficiency

Introduction

Room temperature ionic liquids are very interesting compo-nents for the metallic layer electrodeposition of new baths.This is because of several important advantages such as wider

electrochemical potential window in comparison to aqueoussolutions, negligible vapor pressure, and low hydrogen evo-lution [1]. The most widely studied ionic liquid systems arethose formed from the chloroaluminate [2] andalkylimidazolium [3] salts. They have been used to try todeposit various materials including metals such as Ti [4] andwere used successfully in same cases such as Al [5] andW [6]which cannot be deposited from aqueous solutions in usualconditions. Most studies on metal deposition from ionic liq-uids aim to prepare new alloys, semiconductors, and newcoatings in replacement of toxic coatings or baths [7–9].However, up to now, these liquid systems suffer from mainlimitations. They are highly sensitive to moisture, and there-fore often they require to be processed under the protection ofan inert atmosphere. Moreover, very often they are econom-ically expensive.

Deep eutectic solvents (DESs) are a subset of ionic liquids,usually formed by a quaternary ammonium salt and a hydrogenbond donor [10]. Such mixtures are not, strictly speaking, ionicliquids since they, in general, contain an organic molecularcompound and a salt. Unlike the conventional ionic liquids,these deep eutectic solvents are easy to be prepared in a purestate. Despite their tendency to absorb water, they are not waterreactive. Moreover, from an environmental point of view, theirbiodegradability is proven [10]. A wide range of DESs hasbeen reported in literature. These are generally produced usingamides, alcohols, and carboxylic acids as the hydrogen bonddonor [11–13]. In addition, some of these baths based on DESshave recently been started to be applied in the industrial fieldfor different applications. For instance, electropolishing, re-placing aqueous-based processes which often require stronginorganic acids or toxic chemical compounds [14–17]. Abbottet.al have characterized some of the physical properties of thesemixtures such as viscosity and conductivity [18]. More recent-ly, they have expanded the research characterizing alloy layersdeposited from these systems [19].

S. Xing :C. Zanella (*) : F. DeflorianDepartment of Industrial Engineering, University of Trento,Via Mesiano 77, 38123 Trento, Italye-mail: caterina.zanella@ing.unitn.it

J Solid State ElectrochemDOI 10.1007/s10008-014-2400-8

The electrodeposition of copper is an essential step formany industrial and decorative processes, including large-scale applications as a pre-coating [17]. In this case, the copperprovides a protective layer for the substrate to allow furthercoating depositions [20]. Recently, copper has been electro-deposited from a range of ionic liquids baths [21, 22].However, properties of deep eutectic solvents, such as simplepreparation and tolerance to humidity, make them an interest-ing alternative for large-scale production in industrial applica-tions [23]. Unfortunately, most DESs are suffering from lowmass transfer. This results in low deposition rate, whichmakesit unsuitable for commercial manufacture.

Pulse plating has been widely used in electrodepositionfrom aqueous solutions for several decades to improve de-posits while it is still quite new in the field of ionic liquids. Toour knowledge, few papers deal with pulse plating from ionicliquids, especially from deep eutectic solvents.

In this study, the work was focused to optimize the param-eters for depositing copper layers with higher efficiency, lowdeposition time, and better properties, which are mandatory inindustrial productions.

Experimental

Choline chloride (ChCl) [HOC2H4N (CH3)3Cl] (Aldrich98 %) and ethylene glycol (EG) [HOCH2CH2OH] (Aldrich99 %) were used as received. The mixture was formed bystirring the two components together at 1:2 molar ratio ofChCl:EG at 80 °C until a homogeneous, colorless liquid isproduced. The copper salt, CuCl2·2H2O (Aldrich ≥98 %),was also used as received.

The physical properties, such as viscosity and conductivity,were compared considering the neat 2EG/ChCl solvent andthe solution containing up to 1 M CuCl2·2H2O at varioustemperatures. The viscosity was measured using a BrookfieldDV-E viscometer fitted with a thermostatic jacket for temper-ature control. The conductivity was determined by a Crisonconductimeter 525.

To optimize the process, temperature and concentration ofcomponents and cyclic voltammetries (CV) were conductedusing a PAR model 273A potentiostat/galvanostat (Princetonapplied research) connected to a PC for data acquisition andcontrol. The cell setup consists of three electrodes, a platinumplate (area=3 cm2) as working electrode, a platinum wire asquasi-reference, and a second platinum plate as counter elec-trode. The electrodes were polished until mirror-like surfaceswere obtained, then cleaned by ultrasound for 10 min indistilled water and acetone, and then dried with moisture-free air prior to each experiment. Voltammograms were per-formed at 10 mV/s (if not diversely stated) at various temper-atures (30, 60, and 80 °C) at different CuCl2·2H2O concen-trations (0.01, 0.1, 0.5 M).

Potentiostatic deposition was carried out from 2EG/ChClwith various concentrations of CuCl2·2H2O at different tem-peratures. The deposition was carried out under unstirredconditions between two parallel vertical electrodes. The tem-perature was controlled by a thermostatic jacket cell.Potentials were chosen at the values corresponding to reduc-tion peak of Cu(I) to Cu(0) on cyclic voltagramms. Thedeposition time needed to deposit a layer of 1 μm thicknesswas calculated each time from the average current densityaccording to Faraday’s law.

The copper deposition was performed on brass foil(area=10 cm2). The substrate was cleaned by ultrasound inacetone and rinsed with deionized water to remove the organiccontaminant from the surface. Before the plating process, thesubstrate was dried under moisture-free air. The counter elec-trode for the deposition process was a platinum plate to avoidany impurity. The quasi-reference electrode was a Pt wire tokeep the working potential stable. The final deposits weresequentially rinsed with methanol and deionized water.

Galvanostatic deposition was performed under optimizedconditions applying average current density measured by thepotentiostatic process. In order to obtain 3-μm thick layers,deposition time for the different deposition current densitiestested was calculated by Faraday’s law. Current efficiency wasobtained from the ratio between the actual deposited mass andthe theoretical one.

Pulse plating was applied to study its effect on the processand deposited layer using the same experimental setup previ-ously described for direct current deposition. The schematicdiagram of the square-wave shape of pulse current is shown inFig. 1, where ip refers to the peak current density and ton and toffdenote respectively the on-time and off-time of the pulse. Theduty cycle (θ) is given by ton/(ton+toff) and frequency (f) equalsto 1/(ton+toff). The average current density (iav) in pulse platingis defined as iav=peak current (ip)×duty cycle (θ). Duty cyclein this study was fixed at 0.5; all parameters used are summa-rized in Table 1. Different frequencies were applied to inves-tigate their influence on the deposit morphology. Depositiontime was selected according the pulse parameters in order toobtain the same thickness as under direct current condition.

ton

+ toff

toff

I/ m

A c

m-2

t/ s

ip

ton

Fig. 1 Schematic diagram of the pulse plating wave

J Solid State Electrochem

The surface morphology and chemical compositions of Cu-based deposits were characterized by scanning electron mi-croscopy (SEM) (Philips XL 30), equipped with energy-dispersive X-ray spectrometry (EDXS).

Results and discussion

Physical properties

To check how the addition of metal salts influences the phys-ical properties of bulk DES, 1 M CuCl2·2H2O was added into2EG/ChCl, and the variation of physical properties with tem-perature were measured.

Figure 2 demonstrates the addition of 1 M CuCl2·2H2Ohas no evident effects on the viscosity and conductivity of

2EG/ChCl from 30 to 100 °C. Figure 2a shows that theviscosity of the electrolytes decreases with increasing temper-ature. Similar to the viscosity, the conductivity of 2EG/ChClshown in Fig. 2b is insensitive to CuCl2·2H2O addition andincreases linearly with temperature. The conductivity remainsrather stable with the addition of different concentrations ofsolutes (not shown). While in aqueous solutions, conductivityis generally observed to increase dramatically as the metalconcentration of electrolyte is increased [24].

Differently formed aqueous-based systems and viscosityand conductivity in ionic liquids are related to the relativemobility of ionic species [25]. A model called hole theorydemonstrates a fundamental assumption that the probability ofion mobility is the product of the relative population of holeswith suitable size in solvent and the size of the ions that shouldmove [26].

As reported in the literature [27], viscosity of ionic liquidsis also affected by the tendency of forming hydrogen bondsand the strength of van der Waals interactions among compo-nents. The stronger are the forces, the higher the viscosities.The addition of CuCl2·2H2O is supposed to disrupt the hy-drogen bond network in the solvent leading to an increase inaverage void fraction and hence inducing a decrease in vis-cosity. However, according to physical properties of this sol-vent [28], ethylene glycol acts as a rather weak hydrogen bonddonor to choline chloride that indeed leads to a relativelylower viscosity compared with other deep eutectic solvents[21]. Therefore, the ethylene glycol-based solvent, whichalready contains a high portion of voids, is less susceptibleto solute additions [26]. Similar effects have been observed byadding zinc salts into this solvent [28].

Cyclic voltammetry

The electrochemical window of the neat 2EG/ChCl was foundto be narrower than other ionic liquids [29, 30]. However, inthis case, the electrochemical window of neat solvent, from +0.7 to −1.8 V(vs. Pt), can cover well the redox peaks of Cu(II)/Cu(I) and Cu(I)/Cu(0). The cyclic voltammetry curves shownin Fig. 3 exhibit two pairs of well-defined redox peaks: (1)corresponding to the Cu(II)/Cu(I) redox couple and (2) corre-sponding to the Cu(I)/Cu(0) couple.

A voltammetric study was performed in order to study inmore detail the deposition kinetics of Cu in the solvent. Theimpact of the potential scanning rate (ν) on the voltammo-grams of the electrolyte is presented in Fig. 3. The cathodiccurrent density of the process (II) varied linearly withν1/2(inset of Fig. 3), indicating that the deposition process isunder diffusion control. However, according to the potentialpeak values reported in Table 2, the difference |EIIpa−EIIpc|(ΔEIIp) increases with ν and is larger than the values typicallyfound for reversible reactions [31]. The large peak separationbetween anodic peak and cathodic peak for the same process

Table 1 Details of the pulse current parameters

Pulse frequency (Hz) Current density (mA/cm2)

0.005 0.5 50 Peak Average

Duty cycle (%) Pulse times (on-off), s

50 100-100 1-1 0.01-0.01 7 3.5

20 30 40 50 60 70 80 90 100 110

5

10

15

20

25

30

T/ oC

T/ oC

Vis

cosi

ty/ m

Pa

S

a

20 30 40 50 60 70 80 90 100 1105

10

15

20

25

30

35

40

45

Con

duct

ivit

y/ m

S cm

-2

b

Fig. 2 The a viscosity and b conductivity of neat 2EG/ChCl (solid line)and with addition of 1 M CuCl2·2H2O (dashed line) as a function oftemperature

J Solid State Electrochem

indicates a mixed-diffusion and kinetic-controlled electrolytesystem [32]. The large value of ΔEp is usually indicative of ahigh solution resistance, which is often a characteristic of ionicliquid-based electrolyte [33]. This result can also explain theslow kinetics of the reaction and therefore its difficulty toreach the electrochemical equilibrium during fast voltage scanrate. Moreover, in this situation, the linearity does not passthrough the origin, also suggesting a quasi-reversible system[34].

Effect of temperature and concentration

To achieve higher deposition rates during the process, theeffects of applied temperature and concentration of metallicsalts were investigated in 2EG/ChCl. The effect of tempera-ture on the reduction current of copper was investigated bycarrying out cyclic voltammetry in 2EG/ChCl containingCuCl2·2H2O at 30, 60C, and 80 °C, which is shown inFig. 4. The increase of temperature slightly shifted the reduc-tion potential of the Cu(I) peak from −1.088 V (vs. Pt) at 30 °Cto −1.066 V (vs. Pt) at 60 °C and −1.069 V at 80 °C. Thisreduced overvoltage can be the result of the reduced viscosity,increased conductivity, and increased mass transport of metalion species to the electrode surface at higher temperatures. A

linear relationship between reduction current density and ap-plied temperature can be obtained. Figure 5 reports the surface

-2.0 -1.5 -1.0 -0.5 0.0 0.5

-40

-30

-20

-10

0

10

20

30

40

50

60I/

mA

cm

-2

E(vs. Pt)/ V

3 4 5 6 7 8 9 10 11

10

15

20

25

30

35

I / m

A c

m-2

scan rate1/2/ (mV/s)1/2

I

I

II

II

Fig. 3 Cyclic voltammetry of 2EG/ChCl containing 0.1 M CuCl2·2H2Oobtained at scan rates of 10 mV/s (solid line), 50 mV/s (dotted line), and100 mV/s (dashed line) at 80 °C. Inset is the plot of reduction currentdensity of peak (II) versus the root of scan rate

Table 2 Potential peak values and variation between reduction andoxidation process of Cu(I)/Cu(0) under different scan rates analyzed at80 °C

Scan rate (ν), mV/s EIIpa (V) EIIpc (V) |EIIpa−EIIpc| (V)

10 −0.54 −1.07 0.53

50 −0.48 −1.17 0.69

100 −0.32 −1.24 0.92

-2.0 -1.5 -1.0 -0.5 0.0 0.5-20

-10

0

10

20

30

I/ m

A c

m-2

E(vs. Pt)/ V

Fig. 4 Cyclic voltammograms of 0.1 M CuCl2·2H2O in 30 °C (solidline), 60 °C (dotted line), and 80 °C (dashed line)

a

b

c

Fig. 5 SEM images corresponding to Fig. 3 as a 30 °C, b 60 °C, and c80 °C (Potential versus Pt disk quasi-reference electrode)

J Solid State Electrochem

morphologies of the Cu films deposited under potentiostaticcontrol, corresponding to three different temperatures at 0.1MCuCl2·2H2O. A significant difference among samples pre-pared at different temperatures can be clearly observed in theSEM pictures. Due to the increased current densities resultingfrom elevated temperatures, e.g., 30 to 60 °C, the as-depositedsurface of the Cu film becomes very compact without anyevident columnar structure. This is different from the porousappearance observed at lower temperature. To confirm thechemical composition of the deposited film, EDXS analysiswas carried out and the results report the deposit is composedmainly of copper and by a small amount of oxygen due to thepartial oxidation of the metal surface. Comparing the actualdeposited mass with the theoretical value calculated accordingto Faraday’s law, the current efficiencies for the three depositsshown in Fig. 5 were very low, between 30–50 %.

The further characterization was carried out on the effect ofsalt concentration on the deposition, maintaining a constanttemperature at 80 °C and the same deposition potential. Thedeposition current density tends to increase proportionallywith the salt concentration as shown in Fig. 6. SEM imagesof Cu layers deposited from baths containing different con-centrations of CuCl2·2H2O are reported in Fig. 7. A rough andvery porous surface is obtained at 0.01 M; increasing theconcentration to 0.1 M, the film is compact with larger grainsand shows a smoother topography. At 1 M, the grain sizeincreases further. Moreover, the deposits obtained from bathcontaining 1 M CuCl2·2H2O show inhomogeneous colordistribution. The highest concentration of CuCl2·2H2O avail-able to deposit uniform-colored layers is 0.5 M. The currentefficiency of deposition carried out at 80 °C and 0.5 M re-mains rather poor, at around 50 %.

The results presented in this section (“Effect of temperatureand concentration”) indicate that both temperature and con-centration play important roles in surface topography of the

deposits. Higher temperature can decrease the viscosity of theionic liquid and thus increase ion species mobility, whichwould favor the mass transport and deposition process.Meanwhile, a moderate increase in concentration can be ben-eficial for Cu nucleation which results in a smoother surface.

0 200 400 600 800 1000 12000

-1

-2

-3

-4

-5

0.01 M

0.1 M

0.5 M

1 M

t / s

I/ m

A c

m-2

Fig. 6 Resulting current density versus time deposited from bath con-taining indicated concentrations of CuCl2·2H2O at 80 °C

a

b

c

Fig. 7 SEM images of Cu film on brass substrate deposited from 2EG/ChCl at 80 °C containing CuCl2·2H2O at concentrations of a 0.01 M, b0.1 M, and c 1 M (Potential versus Pt disk quasi-reference electrode)

J Solid State Electrochem

In conclusion, 80 °C and 0.5 M has been chosen as optimizedparameters for electrodeposition.

Galvanic deposition: direct current versus pulse current

To simulate the galvanic deposition process carried out inindustrial production, constant current was applied to depositCu film. The current density was fixed at 3.5 mA/cm2, whichwas similar to the value obtained from potentiostatic deposi-tion at 80 °C and 0.5 M concentration. However, the currentefficiency was around 56 %, which can hardly be acceptablein industrial production. This may be explained by the fact thatthe electrodeposition process is predominantly under masscontrol and a relatively larger overpotential is required forthe crystalline growth with metallic ion mobility near theelectrode surface [35]. So pulse plating was taken into accountsince pulse current is known to be an effective tool to improvemass transport. The average deposition current density wasmaintained equal to the direct current plating and therefore theon-time current was double compared with the value underdirect current.

The current efficiencies obtained applying pulse currentwere between 90 and 98 %. As shown in Fig. 8, the currentefficiency showed a drastic increase when the direct currentwas replaced by pulse current. This could be considered asresult of effective supply of Cu ions during off-time andhigher current density at the beginning of on-time deposition,favoring the instantaneous nucleation [36]. By comparisonwithin pulse range, the current efficiency tends to increasewith frequency. This may be due to the fact that more copperions can be efficiently replenished during off-time in a shortercycle time under higher frequency. This result confirms thelack of copper ions near to the surface is the main cause of thelow efficiency under direct current (DC) [37] and the changefrom amass-controlled to a more kinetically controlled systemby applying pulse current. As a result of the improved current

efficiency, the time needed to deposit 1-μm Cu film can bereduced to 12 min under pulsed mode from the 23 min neededunder DC conditions.

As shown in Fig. 9, the Cu layer deposited under pulsecurrent shows a smoother surface and finer grains with a morehomogeneous size distribution, compared with the coatingsdeposited under direct current. Coatings produced with adifferent pulse frequency do not show noticeable differencesin the surface morphology. The present investigation indicatesa pulse current favors the formation of finer and more regulargrains and inhibits the occurrence of side reactions and voidformation in the coatings.

Pulse current can effectively reduce the concentration gra-dient near the surface of the cathode electrode during off-timeand replenish the Cu ions which have been consumed duringon-time. This mechanism is similar to the well-known effectof pulse plating on the reduction of hydrogen evolution inaqueous solutions [35].

DC 0.005 0.5 5050

60

70

80

90

100

CE

/

f/ Hz

Fig. 8 Effect of frequency on current efficiency of films deposited ataverage current density of 3.5 mA/cm2 with duty cycle of 0.5 or directcurrent

a

b

Fig. 9 SEM images of Cu film on brass substrate deposited from 0.5 MCuCl2·2H2O in 2EG/ChCl at 80 °C applying a direct current and b pulsecurrent at a frequency of 0.5 Hz

J Solid State Electrochem

Conclusions

This paper presents a study on the electrodeposition of Culayers in a deep eutectic solvent based on ethylene glycol andcholine chloride.

Physical properties, such as conductivity and viscosity, areinsensitive to the addition ofmetal salt and strongly dependent ontemperature. Electrodeposition parameters were optimized and0.5 M at 80 °C was considered the best parameter to optimizecoating appearance and morphology. The bath was characterizedas a mixed-diffusion and kinetic-controlled electrolyte system.

Pulse current was applied to deposit a copper layer withgreat improvement in current efficiency from 56 % underdirect current, up to 98 % at 50 Hz. The main cause of thevery low current efficiency was proven to be mass transportlimitation. Pulse current can partially avoid this limit leadingalso to finer and smoother layers.

These results confirm that pulse plating can be used in Cudeposition from DESs with promising improvements. Currentefficiency is indeed increased to 98 % by applying pulseplating, changing the deposition mechanism from a masscontrolled to a more kinetically controlled.

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