The electrochemical nucleation of copper on disc-shaped ultramicroelectrode in industrial electrolyte

Electrochimica Acta 54 (2008) 801–807

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

The electrochemical nucleation of copper on disc-shaped ultramicroelectrode in industrial electrolyte O. Gladysz a , P. Los b,∗ a b

Department of Inorganic Chemistry, Wroclaw Medical University, Szewska 38, 50-139 Wroclaw, Poland Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 5 February 2008 Received in revised form 29 May 2008 Accepted 23 June 2008 Available online 2 July 2008 Keywords: Copper nucleation Ultramicroelectrodes Chronoamperometry Industrial electro-refining electrolytes Levelling agents

a b s t r a c t The understanding of the very early stage of copper crystal formation kinetics and mechanism is very important for both fundamental and applied aspects. Particularly the quality of the industrial electrolytic copper deposit can be improved and better controlled. In this paper, copper nucleation mechanism is investigated in real refinery electrolytes. The main aim is to study the influence of potential, temperature and copper concentration on potentiostatic reduction of copper ions. The model of electrocrystallization is considered on the basis of the equations describing the nucleation controlled by diffusion towards a disc-shaped ultramicroelectrode. In the present paper the rates of copper nuclei formation are estimated under different applied potential, temperature, copper and levelling agent concentrations. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction The electrocrystallization of copper is a very important step during copper electro-refining process. It is believed that the quality of electro-deposited copper depends on the rate of electrocrystallization and especially of the nucleation. The detailed knowledge about the copper nucleation mechanism is very valuable for technological process. There are data in the literature concerning the nucleation mechanism during copper electro-deposition. The results are different depending on the methods of measurements, type of electrodes, electrolyte composition, electrode potential and amount of levelling agent’s concentration [1–8]. Most of those studies were carried out using synthetic electrolytes whose basic ingredients compositions and concentrations are similar to those used in copper refineries. The electro-deposition of copper from refinery electrolyte has been investigated [9] but no copper nucleation mechanism has been established. The main aim of this work is to establish copper nucleation mechanism and kinetics from industrial refinery electrolytes using an ultramicroelectrode (UME). Miniaturization of an electrode to the micrometer scale results in its new properties and improves electrochemical measurements [10–13]. Particularly, UME is the best tool in the studies of concentrated electrolytes as copper industrial electrolytes are due to low current values and ohmic drop values. In particular, diffusion form ∗ Corresponding author. Fax: +48 225682061. E-mail addresses: [email protected], [email protected] (P. Los). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.06.055

of mass transport changes from linear to spherical in the case of ultramicroelectrode [14]. Due to this fact the conventional treatment of electrocrystallization models (Scharifker and Hills [15]) becomes inappropriate when applied to an ultramicroelectrode. The proper equations, describing the nucleation process controlled by spherical diffusion, were delivered by Correira and Machado [16]. This model was successfully applied in the studies of electrocrystallization on disc-shaped ultramicroelectrodes [17–18]. 2. Experimental In this paper chronoamperometry was used as a method to study kinetics and mechanism of electrocrystallization [19]. Gold disc ultramicroelectrode of diameter 25 ␮m was employed as the working electrode. The auxiliary electrode was a copper plate of high purity (grade A, 99.99% Cu) with a surface of about 0.3 cm2 . The auxiliary electrode acts as the reference electrode as well. Due to high currents, routinely used microelectrodes (of millimetre linear size) cannot be applied at such high current densities because of high ohmic resistance drop. High current densities result from the fact that chronoamperometric experiments were carried out at the potential where copper reduction is controlled by the diffusion, i.e., at the plateau of the voltammetric curve that is registered in industrial electrolyte at an ultramicroelectrode. All chronoamperometric measurements were carried out in twoelectrode system. It can be assumed that copper ion activity was constant in the studied electrolytes. Additionally the aux electrode

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(0.3 cm2 copper plate) had a large surface in comparison with the disc-shaped ultramicroelectrode (4.9 × 10−6 cm2 ). It can be claimed that such the electrode did not undergo the polarization. The results of these studies were to be applied in copper refinery technology [20–21]. Therefore, simplicity and convenience of the electrochemical cell was very important factor. The AUTOLAB GSTST30 system was used to carry out chronoamperometric measurements and General Purpose Electrochemical System version 4.5 was the program used for data acquisition. Potentiostatic current–time transients for electro-deposition of copper were registered at −0.4 V (vs. copper reference electrode) for 0.084–0.1 s. The cell temperature was maintained at 20–40 ◦ C by thermostated water. The ultramicroelectrode was mechanically polished with wet alumina powder. Copper sulphate solution was kindly supplied by KGHM S.A. copper refinery in Legnica (Poland) and Norddeutsche Affinerie Hamburg (Germany). The major components of the electrolytes are: 19.9–58.1 g/dm3 copper, 170–200 g/dm3 H2 SO4 , 0.02–0.05 g/dm3 Cl− and the electrolyte contains numerous impurities (e.g., 25 g/dm3 Ni, 20 g/dm3 As, 2 g/dm3 Fe, 0.7 g/dm3 Sb, 0.6 g/dm3 Bi) and also 1–10 parts per million of levelling and grain-refining agents (e.g., animal glue and thiourea). Some electrolytes containing 40.99 g/dm3 (Legnica), 44.3 g/dm3 (Hamburg) were taken from electro-refining cells, where copper electro-deposition on cathodes took place. Small fraction of the electrolyte is treated in electrolyte purification circuit. The impurities and copper are removed in some steps including electrowinning of copper and the remainder was recirculated to refining. These electrolytes that contained lower copper concentration came from NA Hamburg and were used to study the influence of the temperature and copper concentration. Different copper concentrations from 19.9 to 58.1 g/dm3 were obtained by dissolving appropriate quantities of CuSO4 ·5H2 O. In order to study the influence of animal glue on copper nucleation it is required to know glue concentration. Because in most copper refinery grain-refining agents are dosed experimentally so that their precise concentrations were not known. Therefore, before chronoamperometric measurements leveling agents (previously added in the refinery) were removed by heating for 4 h. A reflux condenser was used to protect the analyzed solutions from copper concentration changes. The biopolymeric structure of animal glue is degraded by acid catalyzed hydrolysis to its amino acid constituents [22]. According to the literature data only glue with molar mass higher than 10,000 units are active during electrolysis and degradation products do not act as leveling agents [23]. Glue degradation to less active species (molar mass below 3700 units) under typical conditions in copper refinery tank is completed after 2 h. When the influence of levelling agents’ concentrations was studied the fresh portions of them were added. Thiourea produced by Hengyang Hong Xiang Chemical Co. Ltd. (China) and animal glue manufactured by Smits Vuren B.V (Netherlands) was used. The same reagents are added to tank house electrolyte during copper electro-refining process. Scanning electron micrographs were recorded on JEOL JSM 5800 LV instrument. Copper deposits were obtained using the electrolyte containing 44.3 g/dm3 copper ions.

Fig. 1. Reproductibility of chronoamperometric measurements in copper refinery electrolyte, using gold ultramicroelectrode (diameter 25 ␮m) without polishing.

maximum of current is achieved in very short time and little amount of copper is electro-reduced. As the result the high reproducibility of chronoamperometric measurements can be observed in Fig. 1. It is worth noticing that although the ultramicroelectrode has not been polished between measurements standard deviation did not exceed 1%. 3.2. Potential dependence of copper deposition Potential dependence of chronoamperometric transient is presented in Fig. 2. In agreement with literature data [1] copper nucleation is preceded by double-layer charging process followed by the induction period. When copper nuclei are created the current increases. Parameters tmax and Imax describe the coordinates of current maximum during chronoamperometric measurement. As can be expected the higher absolute value of potential the shorter tmax is reached. On the contrary, the higher absolute value of potential the higher Imax is reached. The relation between logarithm of current density ln jmax and potential is linear as can be observed in Fig. 3. 3.3. Temperature and concentration dependence of copper deposition Fig. 4 presents current–time curves registered at 20–40 ◦ C temperatures. It can be observed that Imax increases and tmax decreases with the increase of temperature.

3. Results and discussion 3.1. Reproducibility During measurements the potential was fixed near the plateau of voltamperometric curve. Therefore, copper deposition was controlled by copper ion diffusion. At an ultramicroelectrode the

Fig. 2. Chronoamperometric curves in industrial electrolyte at different potentials; Cu 41.0 g/dm3 , 60 ◦ C.

O. Gladysz, P. Los / Electrochimica Acta 54 (2008) 801–807

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Fig. 3. The plot of the relation ln jmax vs. potential.

Fig. 6. The plot of the relation ln jmax vs. temperature.

This dependence has been applied to the method of copper concentration measurements in real refinery electrolyte and described in patent application [21]. In electrolyte with higher copper ion concentration shorter times Imax are registered. There is linear relation between logarithm of maximal current density ln jmax and temperature. The proper equations are presented in Fig. 6. 3.4. Effect of leveling agents

Fig. 5 presents current–time curves obtained in electrolytes with different copper concentrations ranging from 19.9 to 58.1 g/dm3 at 40 ◦ C. It can be observed that current values at the end of chronoamperometric transient are higher when copper concentration increases. This result is predicted by the ultramicroelectrode theory [14] and in case of disk shaped ultramicroelectrode the steady-state current iss is given by the Eq. (1):

In order to produce smooth and pure copper deposit leveling agents (animal glue) and grain-refining agents (thiourea) are added to the electrolyte [25]. It is thought that glue is adsorbed on new forming copper grains and Cu–Cl–thiourea complexes form nucleation sites for copper crystals [24]. The detailed mechanisms have not been explained yet. It can be observed in Fig. 7 that the nucleation is slowest when animal glue concentration amounts to 1 mg/dm3 . Imax is reached in relatively longer tmax . In temperature 60 ◦ C animal glue in this concentration, acts as inhibitor of copper nucleation. In Fig. 8 synergic effect of thiourea and animal glue can be seen. When their concentrations are equal 1 mg/dm3 then the longer tmax is recorded and the slowest nucleation takes place.

iss = 4nFDCr

3.5. Nucleation model

Fig. 4. Chronoamperometric curves in industrial electrolyte at different temperatures; Cu 58.1 g/dm3 .

(1)

where n is the number of electrons transferred during the reaction, F is Faraday’s constant, D is the diffusion coefficient, C is the concentration and r is the radius of disc.

Fig. 5. Chronoamperometric curves in industrial electrolyte at different copper concentrations, 40 ◦ C.

The experimental results are analyzed according to two models of electrocrystallization on disc-shaped ultramicroelectrodes [16]: instantaneous and progressive. During instantaneous nucleation all

Fig. 7. Chronoamperometric curves in industrial electrolytes with different animal glue concentrations (60 ◦ C, 43.3 g/dm3 Cu, without thiourea).

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Fig. 8. Chronoamperometric curves in industrial electrolytes with different thiourea concentrations (60 ◦ C, 43.3 g/dm3 Cu, 1 mg/dm3 animal glue).

nuclei are formed at the beginning of potentiostatic transient. In the second model progressive formation of nucleus is assumed. Progressive and instantaneous nucleation model were described by proper current–time equations in terms of dimensionless time variable : =

4Dt r2

(2)

where t is time of experiment. Depending on the time scale of the experiment chronoamperogram obtained at UME may differ from this obtained at an electrode of conventional size because mass transport rates to and from UME are increased because of non-planar diffusion. When diffusion to ultramicroelectrode’s surface is analyzed, the smallest linear size of UME has to be taken into consideration. In case of a disk ultramicroelectrode radius r is a kinetic parameter. At large value of  the diffusion is nonlinear (spherical or hemispherical) and dimensions of diffusion layer exceed the dimensions of an electrode. In order to support current, ions diffuse from large volume. On the contrary at small value of  the diffusion is linear because the electrode dimension is larger than the diffusion layer. The relationship for both types of nucleation, that take into account the mass transport by spherical diffusion and large value of dimensionless time parameter , can be expressed by Eqs. (3) and (4), respectively [16]. For progressive nucleation: I = (nFD1/2 c ∞ r 2 1/2 t −1/2 + nFc ∞ rD)[1 − exp(−0.5AN∞ k Dt 2 )] (3) For instantaneous nucleation: I = (nFD1/2 c ∞ r 2 1/2 t −1/2 + nFc ∞ rD−3/2 )[1 − exp(−NkDt)] (4) where AN∞ is the rate of nuclei formation, N is the number of nuclei, c∞ is the bulk concentration and r is the radius of disc. k and k are the numerical constants for progressive and instantaneous nucleation, respectively, and they are expressed by Eqs. (5) and (6) 4 k = 3 k=

 8c∞ M 1/2 

 8c∞ M 1/2 

(5)

(6)

where M is molar mass,  is the copper density. Eqs. (3) and (4) are very useful in the analysis of the experimental results. By fitting chronoamperometric curves to experimental transients parameters D and AN∞ or N are available. Furthermore the good agreement between calculated and experimental

Fig. 9. Experimental and simulated chronoamperometric curves in industrial electrolyte, Cu 44.3 g dm3 , 1 mg/dm3 animal glue, 20 ◦ C.

curves can be the diagnostic criterion for determination of nucleation model. Summing up the analysis enable to make distinction between instantaneous and progressive nucleation. Our experimental conditions such as time of experiment and disc radius enabled us to calculate parameters D and AN∞ using the equations concerning the small value of . The trial–error fitting procedure method was used in the paper. Computer simulations were made using Eqs. (3) and (4). When the Eq. (3), concerning progressive nucleation, was used then simulated curves were the best fitted to experimental curves. In Fig. 9 the precision of fitting procedure for progressive nucleation is presented. The diagnostic criterion for accuracy between experimental and fitted curves were the mean square errors (MSE) which have been calculated using the equation: MSE () =

1 (j − )2 n

(7)

j−1

where  j is experimental current and  is calculated current. In all cases MSE in reference to the diffusion current squared did not exceed 4%. From the fitting procedure parameters AN∞ and D were calculated. The detailed analysis of diffusion coefficients in industrial electrolytes was discussed in previous paper [26]. The calculated rates of nuclei formation AN∞ at different potential are presented in Table 1. The relation between logarithm of AN∞ and potential is linear as can be observed in Fig. 10. Nucleation rates calculated for electrolytes containing varied copper concentration are presented graphically in Fig. 11. There are graphical presentations of leveling agents’ influence on nucleation rates in Figs. 12 and 13. The parameters AN∞ for the electrolytes containing both glue and thiourea are collected in Table 2. The nucleation rates AN∞ are significantly lower in case when both thiourea and glue are present in electrolyte. At 50 and 60 ◦ C the optimal glue concentration is 1 mg/dm3 and this is in agreement with the concentration used during techTable 1 The calculated values of AN∞ at different potential E (mV)

t (◦ C)

CCu2+ (g/dm3 )

AN∞ × 109 (cm−2 s−1 )

−500 −475 −450 −425 −400 −375 −350 −325 −300

60

40.99

1103.45 509.41 254.39 130.85 52.13 25.05 8.26 2.90 2.31

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Fig. 10. The plot of the relation ln AN∞ vs. potential.

805

Fig. 13. Nucleation rates AN∞ as a function of thiourea concentration (industrial electrolyte, without glue, 43.3 g/dm3 Cu). Table 2 The calculated values of AN∞ at different animal glue concentration and constant thiourea concentration E (mV)

t (◦ C)

CCu (g/dm3 )

Cthiourea (mg/dm3 )

Cglue (mg/dm3 )

AN∞ × 109 (cm−2 s−1 )

−400

60

44.3

1 1 1 1 1

0.0 0.5 1.0 1.5 2.0

17.5 26.0 6.8 8.8 155.0

3.6. SEM micrographs

Fig. 11. Nucleation rates AN∞ as a function of copper concentration (industrial electrolytes without thiourea and animal glue).

nological process in copper refinery [25]. This is quite surprising since cathodic current densities used in our study are much higher than in the case of industrial electro-refining process. When glue concentration is about 3 mg/dm3 at 50 and 60 ◦ C the nucleation rate AN∞ is even higher than in case of absence of levelling agents. At lower temperatures ranging from 20 to 40 ◦ C the catalytic effect of animal glue is not observed. So, too high concentration of the animal glue in the industrial process (temperatures between 50 and 60 ◦ C) can have catalytic effect rather than inhibiting (when it is added in lower amounts) on the copper electrocrystallization process. Our results show the importance of the proper control of active animal glue concentration in the industrial process.

Fig. 12. Nucleation rates AN∞ as a function of animal glue concentration (industrial electrolyte, without thiourea, 43.3 g/dm3 Cu).

When a nucleation process takes place there is possibility to form crystals of three morphologies (one-dimensional, twodimensional and three-dimensional). Scanning electron micrographs, for copper deposition after 0.1 s, is shown in Fig. 14. It can be observed that the nuclei are growing with hemispherical shapes characteristic for 3D morphology. Furthermore, different sizes of nuclei suggest that they were formed progressively. Fig. 15 shows SEM picture of a single copper crystal after 0.01 s of electrolysis. Depending on the temperature it is the average time of electrolysis when maximum of a current–time curve is reached. Fig. 16 presents

Fig. 14. SEM micrograph for copper deposition for 0.1 s on gold UME (diameter 25 ␮m), magnificence 2500× [29].

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Fig. 15. SEM micrograph for copper deposition for 0.01 s on gold UME (diameter 25 ␮m) magnificence 20,000× [29].

Fig. 16. SEM micrograph for copper deposition for 10 s on gold UME (diameter 25 ␮m), magnificence 20,000×.

tures varying between 50 and 60 ◦ C. Furthermore, when levelling agents are overdosed, the copper nucleation undergoes even faster in comparison to electrolyte without any organic additives. Our study confirms that active animal glue and thiourea concentrations should be controlled on-line during copper electro-refining process. It should be noticed that current densities at the ultramicroelectrodes used in the present study, are up to hundred times higher than optimal current densities that are currently used in copper refineries [25]. The usage of higher current densities in the industrial process might improve the yield of copper production. The quality (structure and purity) of copper, deposited under condition of such a high current density, demands further investigation. Most of the electroanalytical studies were carried out historically in diluted solutions probably due to the analytical application of electrochemical methods and leading role of polarographic methods in the past. Taking into account that there is no general theory of concentrated electrolytes (at such high ionic force all kind of semi-empirical corrections are used to, e.g., Debye-Huckel theory to describe thermodynamic properties of the concentrated solutions) it is very important to collect as much as possible experimental data to enable further development of the theory and models of concentrated electrolytes. It is especially important since all kinetic equations (e.g., mass transport, i.e., Fick’s law) in concentrated electrolytes are function of activity of electroactive species and not their concentrations as it is in diluted solutions. So, the kinetics of electrochemical processes should be studied experimentally in concentrated electrolytes to develop the theory (as it is in the case of, e.g., molten salts). In general the laws governing the kinetics of electrochemical processes are rarely linear and consequently, incremental approach (e.g., extrapolation) cannot be used to get kinetics data of most applied electrochemical processes, e.g., copper electro-deposition. So, the experimental conclusions and kinetics parameters presented in this paper cannot be deducted from the previous studies obtained in laboratory prepared sulfate electrolytes. Our measurements give unique opportunity to understand better electrochemical processes in concentrated aqueous electrolytes. Finally we believe that our paper is a contribution to development of methodology of concentrated electrolytes studies, too.

all surface of the ultramicroelectrode with polycrystalline copper deposit obtained after about 10 s. Characteristic strong edge effect can be seen. It is due to the not uniform current distribution over the disk-shaped ultramicroelectrode surface [27–28].

Acknowledgements

4. Conclusions

References

In the present study we showed that copper electrocrystallization from industrial electrolytes occurs according to the progressive model on disc-shaped ultramicroelectrodes. Spherical diffusion towards a gold ultramicroelectrode controls three-dimensional growth of copper. In the studied range of copper concentrations no change in nucleation model is observed. No change was found in case of different potential and temperature as well. Although the nucleation model does not change, the rate of nuclei formation is increasing with higher absolute value of potential. Thus uneven distribution of potential on the cathodic surface can cause a rough copper deposition. Temperature and copper concentration influence the rate of nucleation as well. The higher the temperature and copper concentration are, the faster nuclei are formed. It does not seem that the presence of levelling agents can change the nucleation mechanism but they influence the nucleation rate. 1 mg/dm3 glue and 1 mg/dm3 thiourea is the optimal concentration to inhibit copper nucleation process from industrial electrolytes at tempera-

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This study has been carried out under R&D projects with KGHM Polska Miedz´ S.A. and Norddeutsche Affinerie Hamburg.

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