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Nucleation and growth of cobalt onto different substrates
Journal of Electroanalytical Chemistry 545 (2003) 39 /45 www.elsevier.com/locate/jelechem
Nucleation and growth of cobalt onto different substrates Part II. The upd-opd transition onto a gold electrode L.H. Mendoza-Huizar a,b,*, J. Robles b, M. Palomar-Pardave´ c a
Centro de Investigaciones Quı´micas, Universidad Auto´noma del Estado de Hidalgo, Unidad Universitaria, Km 4.5 Carretera Pachuca-Tulancingo, Pachuca-Hidalgo, Mexico b Facultad de Quı´mica, Universidad de Guanajuato, Noria Alta S/n, Guanajuato, GTO. 36050, Mexico c Departamento de Materiales, Universidad Auto´noma Metropolitana-Azcapotzalco, Mexico D.F. C.P., 02200, Mexico Received 16 October 2002; accepted 29 January 2003 Dedicated to the memory of Professor Rotislaw Kaischew
Abstract A study of the electrochemical deposition of cobalt was carried out using electrochemical techniques. The formation kinetics and growth of cobalt nuclei onto a polycrystalline gold electrode were studied employing an aqueous 10 2 M CoCl2 1 M NH4Cl solution (pH 9.5). We obtain the potentiostatic j /t plots when the potential step jumps from a potential value in the underpotential deposition zone to a final potential in the opd region. It was found that these current density transients can be described through a kinetic mechanism that involves three different contributions: (a) a Langmuir type adsorption process, (b) 2D diffusion-controlled instantaneous nucleation and (c) 3D nucleation limited by a mass transfer reaction. In order to describe the contribution due to 3D growth we test two different approaches by Scharifker and Mostany (J. Electroanal. Chem. 177 (1984) 13) and Heerman and Tarallo (J. Electroanal. Chem. 470 (1997) 70), the values of the experimental parameters A (nucleation rate constant), N0 (number of active nucleation sites) and D (diffusivity of the depositing ions) obtained in the two cases are quite close, however if the influence of the adsorption and 2D nucleation and growth processes is not considered, A and N0 are overestimated and underestimated, respectively. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Cobalt; Gold; opd; upd; Nucleation; Kinetics
1. Introduction Considerable efforts have been expended on the study of cobalt materials obtained by electrochemical techniques [1 /6]. It has been reported that cobalt can be deposited onto a gold electrode in both underpotential [2] and overpotential conditions [3,4]. In Part I [2] of this series of papers we have analyzed the underpotential deposition (upd) of cobalt onto a polycrystalline gold electrode and we have found that the cobalt upd process may be considered as an initial stage of the cobalt opd
* Corresponding author. Tel.: /52-17-17-2000x6785/6786/6787; fax: /52-17-17-2000x6502. E-mail address: [email protected] (L.H. MendozaHuizar).
process on polycrystalline gold. However, in spite of interest in these mechanisms, there is little available information on the kinetics of the electrodeposition process when the upd and opd processes are coupled [7 / 10]. It is evident that the understanding of the kinetic deposition to gain better control of the deposition processes is important. For the case of cobalt deposition onto a polycrystalline gold electrode, the influence of the upd on the opd process, to our knowledge, has not been analyzed. Therefore, in order to gain deeper insight into this system, in the present paper, which is the second in a set of five papers, we report on the electrochemical deposition of cobalt on polycrystalline gold, using cyclic voltammetry and the potential step technique in the upd /opd transition during cobalt electrochemical deposition on a gold electrode. In later papers we shall analyze the influence of the crystallinity of the gold
0022-0728/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0728(03)00087-1
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L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
substrate (part III), the coordination sphere of the depositing cobalt(II) ion (part IV) and the nature of the substrate on the cobalt deposition process (part V).
2. Experimental Cobalt deposits on a polycrystalline gold electrode were carried out from an aqueous solution containing 102 M of CoCl2, and 1 M NH4Cl, at pH 9.5 (adjusted with NaOH) at 25 8C. Under these conditions, the predominant chemical species of the Co(II) ion is the [Co(NH3)5(H2O)]2 complex [6]. All solutions were prepared using analytical grade reagents with ultra pure water (Millipore-Q system) and were deoxygenated by bubbling N2 for 15 min before each experiment. The working electrode was a polycrystalline gold, rotating disc electrode tip provided by Radiometer Tacussel, of 0.0707 cm2 area. The exposed surface was polished to a mirror finish with different grades of alumina down to 0.05 mm and cleaned ultrasonically before experiments. A graphite bar with an exposed area greater than the working electrode was used as a counter electrode. A saturated calomel electrode (SCE) was used as the reference electrode, with all measured potentials referred to this scale. The electrochemical experiments were carried out with a PARC 273 potentiostat /galvanostat connected to a personal computer running ECHEM software to allow control of experiments and data acquisition. To verify the electrochemical behavior of the electrode in the electrodeposition bath, cyclic voltammetry was performed in the 0.4 to /1.3 V potential range at different scan rates. The kinetic mechanism of cobalt overpotential and underpotential deposition on polycrystalline gold was studied under potentiostatic conditions by means of the analysis of the experimental potentiostatic current density transients obtained with the double potential step technique. The perturbation of the potential electrode started always at 0.400 V (SCE). The first potential step was imposed at different potentials detailed in this work. The second step always ended at 0.400 V.
Fig. 1. Cyclic voltammograms obtained in the Au/102 M CoCl2/1 M NH4Cl (pH 9.5) system at different potential scan rates of (1) 100, (2) 80, (3) 60, (4) 40 and (5) 20 mV s 1. In all cases the potential scan was started at 0.4 V toward the negative direction. Cathodic current density peaks (A /C) and anodic peaks (D /F) are also indicated in the figure.
current density peaks D, E and F appeared at potentials of /0.700, /0.430, and 0.330 V, respectively. Peak D (which appears as a shoulder) could be due to hydrogen oxidation [2]. Peaks E and F may be related to oxidation of different cobalt phases. Peak F has been related to the dissolution of cobalt previously deposited in the upd region [2]. In order to determine the type of control limiting the upd process, the maximum current density (jp) value associated with both peaks A and B was plotted as a function of n1/2. A linear relationship was found for both cases [2] indicating a diffusion-controlled process [12,13]. The plot corresponding to peak C is shown in Fig. 2. Note in this plot a linear relationship for this process, which may indicate a diffusion-controlled opd process according to the equation of Berzins and Delahay [12,13].
3. Results and discussion 3.1. Cyclic voltammetry study Fig. 1 shows a set of cyclic voltammograms obtained in the Au/10 2 M CoCl2/1 M NH4Cl (pH 9.5) system at different potential scan rates. During the direct scan it is possible to note the formation of three peaks, A, B and C at the potentials 0.01, /0.34 and /1.02 V, respectively. Peaks A and B have been related to upd processes, while peak C corresponds to opd processes [2 /4]. During the inverse potential scan three anodic
Fig. 2. jp vs. scan potential rate (n1/2) for Peak C (Fig. 1). The straight line corresponds to the linear fit of the experimental data (k).
L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
3.2. Potentiostatic study
3.2.1. Kinetic analysis of the 3D nucleation and growth process Formation of new phases generally occur through nucleation and growth mechanisms and corresponding current transients can provide valuable information about the kinetics of electrodeposition. Fig. 3 shows a set of current density transients recorded at different potentials in the opd region by a double pulse potential technique. These transients were obtained by applying an initial potential (E0) of 0.400 V on the surface of the gold electrode. At this potential value, the cobalt deposition had still not begun, see Fig. 1. After the application of this initial potential, a step of negative potential (Ec) was varied on the surface of the electrode for 40 s. All the current density transients were obtained when the potential step jumped from a potential value in the upd zone to a final potential in opd region. The dissolution process (not shown) involved the application of a second step potential beginning in the final potential value of the first pulse and ending at 0.400 V. From Fig. 3, it may be observed that at shorter times there is a falling current transient that, in this case, can be associated with a process where adsorption and diffusion-controlled 2D instantaneous nucleation of cobalt occurs simultaneously at the gold electrode surface [2]. Note that after this falling current, in each case, the j /t plot passes through a maximum and then approaches the limiting diffusion current to a planar electrode. This behavior has been related to multiple 3D nucleation and growth processes controlled by a mass transfer reaction [6,8]. We plotted the current density, which falls after the maximum, vs. 1/t1/2. We obtained a linear relationship according to the Cottrell equation [14]. This fact, together with the votammetric evidence, also indicates
Fig. 3. A set of experimental current density transients recorded in the Au/102 M CoCl2/1 M NH4Cl (pH 9.5) system for different potential step values (V) indicated in the figure. In all the cases the initial potential was 0.400 V
41
an opd process, controlled by diffusion. However, it is important to mention that, even though the latter fact (the current after the maximum exhibits Cottrell behavior) has been widely used as sufficient evidence to consider the process as mass transfer controlled, recently, Cao and West [15] have shown, through direct numerical simulation, that a slow charge transfer has qualitatively the same effects as a slow nucleation rate constant. Thus, the transient for instantaneous nucleation with initial growth controlled by charge transfer can look as if the nucleation is progressive controlled by diffusion (see Fig. 2, Ref. [15]). Unfortunately there is no way to recognise this possible kinetic limitation from any feature of the experimental current density transient. Notwithstanding, if we consider the overpotential range studied in this work and that the system under investigation, [Co(NH3)5(H2O)]2/Co, is kinetically fast, at least compared with the [Co(H2O)6]2/Co system, see Palomar-Pardave´ et al. [6,16], we decided to use the theoretical model proposed by Scharifker et al. [17,18], developed for a nucleation process with diffusion-controlled growth, in order to describe the current transients shown in Fig. 3, after the initial falling current. According to Scharifker et al. [17,18] the current density, j3D-dc, associated with 3D nucleation with diffusion-controlled growth can be described through Eq. (1): j3D-dc (t)zFDc
1 (pDt)1=2
f1exp[aN0 (pDt)1=2 t1=2 U]g
(1)
with U 1
(1 exp(At)) At
(2)
where, z is the number of exchanged electrons, F is the Faraday constant, D is the diffusion coefficient, N0 is the numeric density of active sites, a 2[2Vm Dc]1=2 ; and Vm is the molar volume of the deposit. To determine the type of nucleation and growth mechanism, progressive or instantaneous, we compared experimental transients, with their dimensionless curves. These curves were plotted substituting the coordinates of the experimental local maximum (tm, jm), in 2 j 2 1:9542 t 1exp 1:2564 (3) (t=tm ) tm jm2 for instantaneous nucleation and in 2 2 j 2 1:2254 t 1exp 2:3367 (t=tm ) tm jm2 for progressive nucleation.
(4)
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L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
Fig. 5 shows a comparison of an experimental current density transient with a theoretically generated curve, obtained by non-linear fitting of experimental data to Eq. (1). It is clear that the model expressed by this equation predicts successfully the behavior of the experimental current transient corresponding to the 3D nucleation and growth process. Heerman and Tarallo [19] have proposed a correction to the model suggested by Scharifker et al. proposing that Eq. (1) could be improved if written as: Fig. 4. Comparison of an experimental transient (k) normalized through the coordinates of its respective local maximum (tm, jm) , obtained during the electrodeposition of cobalt at /0.930 V with the theoretical non-dimensional curves corresponding to 3D instantaneous nucleation (Eq. (3)) (---) and 3D progressive nucleation ( */) (Eq. (4)).
In Fig. 4 we compare the theoretical dimensionless transient (Eqs. (3) and (4)) with an experimental dimensionless current density transient. Note that at t/ tm B/1 the experimental curve closely follows the response predicted for a 3D instantaneous nucleation meanwhile for t/tm /1 it closely follows the response predicted for a 3D progressive nucleation. As a transition from instantaneous to progressive nucleation would seem impossible, this may be indicative of the presence of other contributions to the overall current during the early stages of the cobalt deposition process (t/tm B/1) apart from the 3D nucleation contribution. Even so, it is clear that most of the experimental data fall within the range of validity of the theory proposed by Scharifker et al. [17,18] These arguments suggest carrying out a nonlinear fitting of the experimental transient using Eq. (1).
j3D-dc (t)zFDc
1 F (pDt)1=2 U
f1exp[aN0 (pDt)1=2 t1=2 U]g
(5)
where F 1
exp(At) (At)1=2
g
(At)1=2
exp(l2 )dl
(6)
0
We compared these models and obtained quite similar fittings to those shown in Fig. 5, see inset in Fig. 5. The physical parameters obtained from the adjustments, Eqs. (1) and (5), are summarized in Table 1. Comparing the results obtained for both models, see Table 1, it is possible to note that the values of the parameters are quite similar. Arbib et al. [11] had arrived at the same result when they analyzed the experimental current density transients recorded during Rh electrodeposition onto gold substrates. This means that within the potential interval investigated, both theories could be applied with sufficient accuracy; however it is important to note that Eq. (1) is easier to solve than Eq. (5).
3.2.2. Kinetic analysis of the upd /opd transition during the nucleation and growth process So far we have shown that both Eqs. (1) and (5) can be used successfully to describe most of the experimental transients recorded during Co electrodeposition onto a gold substrate, however neither could explain the falling current transient observed at short times. In a previous work by this group [2] it has been determined that this current can be attributed to Co upd onto a gold electrode and this involves the simultaneous presence of an adsorption and a 2D nucleation and growth process. Thus, we propose to separate the total current density transient, jtotal(t), in Fig. 5. Comparison between an experimental current density transient ( */) recorded during cobalt electrodeposition onto a polycrystalline gold electrode when a potential value of /0.930 V was applied (Fig. 3) with a theoretical transient (k) generated by non-linear fitting of Eq. (1) to the experimental data. Physical constants obtained from best-fit parameters are shown in Table 1. The inset depicts the same comparison when Eq. (5) was used to generate the theoretical transient.
jtotal (t)j2Di-dc (t)jad (t)j3D-dc (t)
(7)
where j2Di dc is the density current associated with 2D nucleation and growth [2,20,21] and is given by: j2Di-dc (t)k1 exp(k2 t) where
(8)
L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
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Table 1 Potential dependence for the physical constants involved during cobalt opd on a polycrystalline gold electrode Scharifker et al.
Heerman and Tarallo
/E vs. SCE/V
A /s 1
105D /cm s 1
10 8No /cm 2
A /s 1
105D /cm s 1
10 8N0/cm 2
0.890 0.895 0.900 0.910 0.920 0.930
0.132 0.187 0.290 0.569 2.378 7.417
3.32 2.58 2.21 2.20 2.00 2.01
0.53 0.79 1.96 2.37 3.10 4.65
0.094 0.251 0.549 0.565 3.081 6.563
3.44 2.48 2.17 2.18 1.98 2.00
0.38 0.75 1.54 1.53 3.18 4.29
The values were obtained from best-fit parameters found through the fitting process of the experimental j /t plots using Eq. (1), Scharifker et al. and Eq. (5), Heerman and Tarallo.
k1 qmon pS2 D
(9)
and k2 pS 2 DN0
(10)
In these equations qmon is the charge density associated with the monolayer formation, S is a constant controlled by the potential. Note that k1/k2 /Qnucl and Qnucl is the charge density due to the 2D nucleation process. jad(t) is the contribution associated with the adsorption process [2,20 /22] and is given by jad (t)k3 exp(k4 t)
(11)
Note that k3/k4 /Qads and Qads is the charge density due to the adsorption process. j3D dc is given by Eq. (1) [17,18] or Eq. (5) [19]. Fig. 6 shows a comparison of an experimental current density transient with a theoretically generated curve, obtained by non-linear fitting of the experimental data
to Eq. (7). It is clear that the model expressed in Eq. (7) predicts the behavior of the whole experimental current density transient. In Table 2 the physical parameters obtained from this adjustment are summarized. Note that we carried out adjustments for both the Scharifker et al. [17,18] and Heerman and Tarallo [19] models for j3D dc. Note, again that, independently of which model was used to describe j3D dc in Eq. (7), the physical parameters obtained are similar (Table 2). If we compare Tables 1 and 2, several important observations can be made. If the currents due to the adsorption and 2D nucleation and growth processes are considered, then A is lower and N0 is two orders of magnitude greater than in the case where these processes are not considered. The diffusion coefficient values are close to 2 /105 cm2 s1, the experimentally reported value [1]. Fig. 7 shows a plot of ln A vs. /E. Note that a linear relationship is found using the models represented in Eqs. (5) and (7). In the framework of the atomistic theory of electrolytic nucleation [23,24], it is possible to estimate the critical size of the nuclei [24] using: nc
Fig. 6. Comparison between an experimental current density transient ( */), recorded during cobalt electrodeposition onto a polycrystalline gold electrode when a potential value of /0.930 V was applied (Fig. 3) with a theoretical transient (k) generated by a non-linear fit of Eq. (7) to the experimental data. In this figure are depicted the different individual contributions that form Eq. (7). Physical constants obtained from best-fit parameters are shown in Table 2.
kT dlnA a ze0 dE
(12)
where k is the Boltzmann constant, T is the absolute temperature and e0 is the elementary electric charge. Substituting in Eq. (12) the value of a / 0.8 [2], and the slope d(ln A )/d(E ) :/100 (Eqs. (1) and (5)) or 111 (Eq. (7)) of the experimental potential dependence of A (Tables 1 and 2) within the potential interval studied, one obtains nc /1. According to the atomistic theory [23], a complex for which the probability for attachment of one atom is less than 1/2 is defined as critical. The attachment of a new atom, however, converts the complex into a stable one, for which the probability for attachment of the next atom is already higher than 1/ 2. Therefore, in this case, the critical nucleus is composed by one atom adsorbed on an active site and the stable nucleus should be formed by two atoms.
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Table 2 Potential dependence for the physical constants involved during cobalt opd on a polycrystalline gold electrode Scharifker et al.
Heerman and Tarallo
/E vs. SCE/V
A /s 1
105D /cm s 1
10 10No /cm 2
A /s 1
105D /cm2 s 1
10 10N0/cm 2
0.890 0.895 0.900 0.910 0.920 0.930
0.011 0.138 0.214 0.378 1.463 2.056
2.31 2.18 2.18 1.78 1.99 2.01
0.41 1.42 2.70 4.04 3.96 5.59
0.031 0.130 0.236 0.465 1.843 2.363
1.63 2.01 1.95 2.02 1.97 1.97
0.59 1.42 5.97 7.93 9.60 9.60
The values were obtained from best-fit parameters found through the fitting process of the experimental j /t plots using Eq. (7) and Eq. (1), Scharifker et al. or Eq. (5), Heerman and Tarallo.
Acknowledgements L.H.M.H. is grateful for a partial graduate student fellowship from CONCYTEG and CONACYT. This work was done in partial fulfilment of L.H.M.H. Ph.D. requirements. We gratefully acknowledge financial support from CONACYT through projects number 25059E and 32158-E and to DCBI-UAMA through project 2260220. We are indebted to Dr Mario Romero-Romo for his interest and fruitful discussions.
Fig. 7. ln A vs. /E , the applied potential, relationship. The A values were obtained from: (k) Eq. (5) and (I) Eq. (7) (using Eq. (5) in order to describe j3D dc).
These facts suggest that the processes occurring before the 3D nucleation and growth process modify the electronic density on the surface active sites [25], and the need to consider them in the analysis of the deposit processes becomes obvious.
4. Conclusions An electrochemical study of cobalt electrodeposition onto a polycrystalline gold electrode from an aqueous 102 M CoCl2, 1 M NH4Cl (pH 9.5) solution was carried out through cyclic voltammetry and potential step techniques. Analysis of the experimental data indicates that formation of cobalt deposits involve the simultaneous occurrence of an adsorption process, and a 2D and 3D nucleation and diffusion-controlled growth processes. We compared the models proposed by Scharifker et al and Heerman and Tarallo and we obtained similar conclusions from both models. However, if the influence of the adsorption and 2D nucleation and growth processes is not considered, the nucleation rate and the number of active nucleation sites are overestimated and underestimated, respectively.
References [1] S. Floate, M. Hyde, R.G. Compton, J. Electroanal. Chem. 523 (2002) 49. [2] L.H. Mendoza-Huizar, J. Robles, M. Palomar-Pardave´, J. Electroanal. Chem. 521 (2002) 95. [3] L. Cagnon, T. Devolder, R. Cortes, A. Morrone, J.E. Schmidt, C. Chappert, P. Allongue, Phys. Rev. B 63 (2001) 104419. [4] L. Cagnon, A. Gundel, T. Devolder, A. Morrone, C. Chappert, J.E. Schmidt, P. Allongue, Appl. Surf. Sci. 164 (2000) 22. [5] E. Herrero, J. Li, H. Abrun˜a, Electrochim. Acta 44 (1999) 2385. [6] M. Palomar-Pardave´, I. Gonza´lez, A.B. Soto, E.M. Arce, J. Electroanal. Chem. 443 (1998) 125. [7] A.I. Danilov, E.B. Molodkina, Y.M. Polukarov, V. Climent, J.M. Feliu, Electrochim. Acta 46 (2001) 3137. [8] M. Palomar-Pardave´, N. Batina, I. Gonzalez, J. Phys. Chem. B 104 (2000) 3545. [9] G. Staikov, E. Budevski, W. Obretenov, W.J. Lorenz, J. Electroanal. Chem. 349 (1993) 355. [10] D. Cecilia Alonzo, B.R. Scharifker, J. Electroanal. Chem. 274 (1989) 167. [11] M. Arbib, B. Zhang, V. Lazarov, D. Stoychev, A. Milchev, C. Buess-Herman, J. Electroanal. Chem. 510 (2001) 67. [12] T. Berzins, P. Delahay, J. Am. Chem. Soc. 75 (1953) 555. [13] P. Delahay, New Instrumental Methods in Electrochemistry, Interscience, New York, 1954, p. 122. [14] A.J. Bard, L.R. Faulkner, Electrochemical Methods. Fundamental and Applications (Ch. 5), Wiley, New York, 2001. [15] Y. Cao, A.C. West, J. Electrochem. Soc. 149 (2002) C223. [16] L.H. Mendoza-Huizar, M. Palomar-Pardave´, J. Robles, Electrochim. Acta 46 (2001) 2749. [17] B.R. Scharifker, J. Mostany, J. Electroanal. Chem. 177 (1984) 13.
L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45 [18] B.R. Scharifker, J. Mostany, M. Palomar-Pardave´, I. Gonza´lez, J. Electrochem. Soc. 146 (1999) 1005. [19] L. Heerman, A. Tarallo, J. Electroanal. Chem. 470 (1999) 70. [20] M. Palomar-Pardave´, M. Miranda-Hernandez, N. Batina, I. Gonza´lez, Recent Res. Dev. Electrochem. 1 (1998) 15. [21] M. Palomar-Pardave´, M. Miranda-Herna´ndez, I. Gonza´lez, N. Batina, Surf. Sci. 398 (1998) 80.
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[22] M.H. Ho¨lzle, U. Retter, D.M. Kolb, J. Electroanal. Chem. 371 (1994) 101. [23] A. Milchev, S. Stoyanov, R. Kaischev, Thin Solid Films 22 (1974) 267. [24] A. Milchev, Contemp. Phys. 32 (1991) 321. [25] L.H. Mendoza Huizar, J. Robles, J. Silva Domı´nguez, in: B.K. Rao, P. Jena (Eds.), Clusters and Nanostructure Interfaces, World Scientific, Singapore, 2000, p. 589.
Nucleation and growth of cobalt onto different substrates Part II. The upd-opd transition onto a gold electrode L.H. Mendoza-Huizar a,b,*, J. Robles b, M. Palomar-Pardave´ c a
Centro de Investigaciones Quı´micas, Universidad Auto´noma del Estado de Hidalgo, Unidad Universitaria, Km 4.5 Carretera Pachuca-Tulancingo, Pachuca-Hidalgo, Mexico b Facultad de Quı´mica, Universidad de Guanajuato, Noria Alta S/n, Guanajuato, GTO. 36050, Mexico c Departamento de Materiales, Universidad Auto´noma Metropolitana-Azcapotzalco, Mexico D.F. C.P., 02200, Mexico Received 16 October 2002; accepted 29 January 2003 Dedicated to the memory of Professor Rotislaw Kaischew
Abstract A study of the electrochemical deposition of cobalt was carried out using electrochemical techniques. The formation kinetics and growth of cobalt nuclei onto a polycrystalline gold electrode were studied employing an aqueous 10 2 M CoCl2 1 M NH4Cl solution (pH 9.5). We obtain the potentiostatic j /t plots when the potential step jumps from a potential value in the underpotential deposition zone to a final potential in the opd region. It was found that these current density transients can be described through a kinetic mechanism that involves three different contributions: (a) a Langmuir type adsorption process, (b) 2D diffusion-controlled instantaneous nucleation and (c) 3D nucleation limited by a mass transfer reaction. In order to describe the contribution due to 3D growth we test two different approaches by Scharifker and Mostany (J. Electroanal. Chem. 177 (1984) 13) and Heerman and Tarallo (J. Electroanal. Chem. 470 (1997) 70), the values of the experimental parameters A (nucleation rate constant), N0 (number of active nucleation sites) and D (diffusivity of the depositing ions) obtained in the two cases are quite close, however if the influence of the adsorption and 2D nucleation and growth processes is not considered, A and N0 are overestimated and underestimated, respectively. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Cobalt; Gold; opd; upd; Nucleation; Kinetics
1. Introduction Considerable efforts have been expended on the study of cobalt materials obtained by electrochemical techniques [1 /6]. It has been reported that cobalt can be deposited onto a gold electrode in both underpotential [2] and overpotential conditions [3,4]. In Part I [2] of this series of papers we have analyzed the underpotential deposition (upd) of cobalt onto a polycrystalline gold electrode and we have found that the cobalt upd process may be considered as an initial stage of the cobalt opd
* Corresponding author. Tel.: /52-17-17-2000x6785/6786/6787; fax: /52-17-17-2000x6502. E-mail address: [email protected] (L.H. MendozaHuizar).
process on polycrystalline gold. However, in spite of interest in these mechanisms, there is little available information on the kinetics of the electrodeposition process when the upd and opd processes are coupled [7 / 10]. It is evident that the understanding of the kinetic deposition to gain better control of the deposition processes is important. For the case of cobalt deposition onto a polycrystalline gold electrode, the influence of the upd on the opd process, to our knowledge, has not been analyzed. Therefore, in order to gain deeper insight into this system, in the present paper, which is the second in a set of five papers, we report on the electrochemical deposition of cobalt on polycrystalline gold, using cyclic voltammetry and the potential step technique in the upd /opd transition during cobalt electrochemical deposition on a gold electrode. In later papers we shall analyze the influence of the crystallinity of the gold
0022-0728/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0728(03)00087-1
40
L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
substrate (part III), the coordination sphere of the depositing cobalt(II) ion (part IV) and the nature of the substrate on the cobalt deposition process (part V).
2. Experimental Cobalt deposits on a polycrystalline gold electrode were carried out from an aqueous solution containing 102 M of CoCl2, and 1 M NH4Cl, at pH 9.5 (adjusted with NaOH) at 25 8C. Under these conditions, the predominant chemical species of the Co(II) ion is the [Co(NH3)5(H2O)]2 complex [6]. All solutions were prepared using analytical grade reagents with ultra pure water (Millipore-Q system) and were deoxygenated by bubbling N2 for 15 min before each experiment. The working electrode was a polycrystalline gold, rotating disc electrode tip provided by Radiometer Tacussel, of 0.0707 cm2 area. The exposed surface was polished to a mirror finish with different grades of alumina down to 0.05 mm and cleaned ultrasonically before experiments. A graphite bar with an exposed area greater than the working electrode was used as a counter electrode. A saturated calomel electrode (SCE) was used as the reference electrode, with all measured potentials referred to this scale. The electrochemical experiments were carried out with a PARC 273 potentiostat /galvanostat connected to a personal computer running ECHEM software to allow control of experiments and data acquisition. To verify the electrochemical behavior of the electrode in the electrodeposition bath, cyclic voltammetry was performed in the 0.4 to /1.3 V potential range at different scan rates. The kinetic mechanism of cobalt overpotential and underpotential deposition on polycrystalline gold was studied under potentiostatic conditions by means of the analysis of the experimental potentiostatic current density transients obtained with the double potential step technique. The perturbation of the potential electrode started always at 0.400 V (SCE). The first potential step was imposed at different potentials detailed in this work. The second step always ended at 0.400 V.
Fig. 1. Cyclic voltammograms obtained in the Au/102 M CoCl2/1 M NH4Cl (pH 9.5) system at different potential scan rates of (1) 100, (2) 80, (3) 60, (4) 40 and (5) 20 mV s 1. In all cases the potential scan was started at 0.4 V toward the negative direction. Cathodic current density peaks (A /C) and anodic peaks (D /F) are also indicated in the figure.
current density peaks D, E and F appeared at potentials of /0.700, /0.430, and 0.330 V, respectively. Peak D (which appears as a shoulder) could be due to hydrogen oxidation [2]. Peaks E and F may be related to oxidation of different cobalt phases. Peak F has been related to the dissolution of cobalt previously deposited in the upd region [2]. In order to determine the type of control limiting the upd process, the maximum current density (jp) value associated with both peaks A and B was plotted as a function of n1/2. A linear relationship was found for both cases [2] indicating a diffusion-controlled process [12,13]. The plot corresponding to peak C is shown in Fig. 2. Note in this plot a linear relationship for this process, which may indicate a diffusion-controlled opd process according to the equation of Berzins and Delahay [12,13].
3. Results and discussion 3.1. Cyclic voltammetry study Fig. 1 shows a set of cyclic voltammograms obtained in the Au/10 2 M CoCl2/1 M NH4Cl (pH 9.5) system at different potential scan rates. During the direct scan it is possible to note the formation of three peaks, A, B and C at the potentials 0.01, /0.34 and /1.02 V, respectively. Peaks A and B have been related to upd processes, while peak C corresponds to opd processes [2 /4]. During the inverse potential scan three anodic
Fig. 2. jp vs. scan potential rate (n1/2) for Peak C (Fig. 1). The straight line corresponds to the linear fit of the experimental data (k).
L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
3.2. Potentiostatic study
3.2.1. Kinetic analysis of the 3D nucleation and growth process Formation of new phases generally occur through nucleation and growth mechanisms and corresponding current transients can provide valuable information about the kinetics of electrodeposition. Fig. 3 shows a set of current density transients recorded at different potentials in the opd region by a double pulse potential technique. These transients were obtained by applying an initial potential (E0) of 0.400 V on the surface of the gold electrode. At this potential value, the cobalt deposition had still not begun, see Fig. 1. After the application of this initial potential, a step of negative potential (Ec) was varied on the surface of the electrode for 40 s. All the current density transients were obtained when the potential step jumped from a potential value in the upd zone to a final potential in opd region. The dissolution process (not shown) involved the application of a second step potential beginning in the final potential value of the first pulse and ending at 0.400 V. From Fig. 3, it may be observed that at shorter times there is a falling current transient that, in this case, can be associated with a process where adsorption and diffusion-controlled 2D instantaneous nucleation of cobalt occurs simultaneously at the gold electrode surface [2]. Note that after this falling current, in each case, the j /t plot passes through a maximum and then approaches the limiting diffusion current to a planar electrode. This behavior has been related to multiple 3D nucleation and growth processes controlled by a mass transfer reaction [6,8]. We plotted the current density, which falls after the maximum, vs. 1/t1/2. We obtained a linear relationship according to the Cottrell equation [14]. This fact, together with the votammetric evidence, also indicates
Fig. 3. A set of experimental current density transients recorded in the Au/102 M CoCl2/1 M NH4Cl (pH 9.5) system for different potential step values (V) indicated in the figure. In all the cases the initial potential was 0.400 V
41
an opd process, controlled by diffusion. However, it is important to mention that, even though the latter fact (the current after the maximum exhibits Cottrell behavior) has been widely used as sufficient evidence to consider the process as mass transfer controlled, recently, Cao and West [15] have shown, through direct numerical simulation, that a slow charge transfer has qualitatively the same effects as a slow nucleation rate constant. Thus, the transient for instantaneous nucleation with initial growth controlled by charge transfer can look as if the nucleation is progressive controlled by diffusion (see Fig. 2, Ref. [15]). Unfortunately there is no way to recognise this possible kinetic limitation from any feature of the experimental current density transient. Notwithstanding, if we consider the overpotential range studied in this work and that the system under investigation, [Co(NH3)5(H2O)]2/Co, is kinetically fast, at least compared with the [Co(H2O)6]2/Co system, see Palomar-Pardave´ et al. [6,16], we decided to use the theoretical model proposed by Scharifker et al. [17,18], developed for a nucleation process with diffusion-controlled growth, in order to describe the current transients shown in Fig. 3, after the initial falling current. According to Scharifker et al. [17,18] the current density, j3D-dc, associated with 3D nucleation with diffusion-controlled growth can be described through Eq. (1): j3D-dc (t)zFDc
1 (pDt)1=2
f1exp[aN0 (pDt)1=2 t1=2 U]g
(1)
with U 1
(1 exp(At)) At
(2)
where, z is the number of exchanged electrons, F is the Faraday constant, D is the diffusion coefficient, N0 is the numeric density of active sites, a 2[2Vm Dc]1=2 ; and Vm is the molar volume of the deposit. To determine the type of nucleation and growth mechanism, progressive or instantaneous, we compared experimental transients, with their dimensionless curves. These curves were plotted substituting the coordinates of the experimental local maximum (tm, jm), in 2 j 2 1:9542 t 1exp 1:2564 (3) (t=tm ) tm jm2 for instantaneous nucleation and in 2 2 j 2 1:2254 t 1exp 2:3367 (t=tm ) tm jm2 for progressive nucleation.
(4)
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L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
Fig. 5 shows a comparison of an experimental current density transient with a theoretically generated curve, obtained by non-linear fitting of experimental data to Eq. (1). It is clear that the model expressed by this equation predicts successfully the behavior of the experimental current transient corresponding to the 3D nucleation and growth process. Heerman and Tarallo [19] have proposed a correction to the model suggested by Scharifker et al. proposing that Eq. (1) could be improved if written as: Fig. 4. Comparison of an experimental transient (k) normalized through the coordinates of its respective local maximum (tm, jm) , obtained during the electrodeposition of cobalt at /0.930 V with the theoretical non-dimensional curves corresponding to 3D instantaneous nucleation (Eq. (3)) (---) and 3D progressive nucleation ( */) (Eq. (4)).
In Fig. 4 we compare the theoretical dimensionless transient (Eqs. (3) and (4)) with an experimental dimensionless current density transient. Note that at t/ tm B/1 the experimental curve closely follows the response predicted for a 3D instantaneous nucleation meanwhile for t/tm /1 it closely follows the response predicted for a 3D progressive nucleation. As a transition from instantaneous to progressive nucleation would seem impossible, this may be indicative of the presence of other contributions to the overall current during the early stages of the cobalt deposition process (t/tm B/1) apart from the 3D nucleation contribution. Even so, it is clear that most of the experimental data fall within the range of validity of the theory proposed by Scharifker et al. [17,18] These arguments suggest carrying out a nonlinear fitting of the experimental transient using Eq. (1).
j3D-dc (t)zFDc
1 F (pDt)1=2 U
f1exp[aN0 (pDt)1=2 t1=2 U]g
(5)
where F 1
exp(At) (At)1=2
g
(At)1=2
exp(l2 )dl
(6)
0
We compared these models and obtained quite similar fittings to those shown in Fig. 5, see inset in Fig. 5. The physical parameters obtained from the adjustments, Eqs. (1) and (5), are summarized in Table 1. Comparing the results obtained for both models, see Table 1, it is possible to note that the values of the parameters are quite similar. Arbib et al. [11] had arrived at the same result when they analyzed the experimental current density transients recorded during Rh electrodeposition onto gold substrates. This means that within the potential interval investigated, both theories could be applied with sufficient accuracy; however it is important to note that Eq. (1) is easier to solve than Eq. (5).
3.2.2. Kinetic analysis of the upd /opd transition during the nucleation and growth process So far we have shown that both Eqs. (1) and (5) can be used successfully to describe most of the experimental transients recorded during Co electrodeposition onto a gold substrate, however neither could explain the falling current transient observed at short times. In a previous work by this group [2] it has been determined that this current can be attributed to Co upd onto a gold electrode and this involves the simultaneous presence of an adsorption and a 2D nucleation and growth process. Thus, we propose to separate the total current density transient, jtotal(t), in Fig. 5. Comparison between an experimental current density transient ( */) recorded during cobalt electrodeposition onto a polycrystalline gold electrode when a potential value of /0.930 V was applied (Fig. 3) with a theoretical transient (k) generated by non-linear fitting of Eq. (1) to the experimental data. Physical constants obtained from best-fit parameters are shown in Table 1. The inset depicts the same comparison when Eq. (5) was used to generate the theoretical transient.
jtotal (t)j2Di-dc (t)jad (t)j3D-dc (t)
(7)
where j2Di dc is the density current associated with 2D nucleation and growth [2,20,21] and is given by: j2Di-dc (t)k1 exp(k2 t) where
(8)
L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
43
Table 1 Potential dependence for the physical constants involved during cobalt opd on a polycrystalline gold electrode Scharifker et al.
Heerman and Tarallo
/E vs. SCE/V
A /s 1
105D /cm s 1
10 8No /cm 2
A /s 1
105D /cm s 1
10 8N0/cm 2
0.890 0.895 0.900 0.910 0.920 0.930
0.132 0.187 0.290 0.569 2.378 7.417
3.32 2.58 2.21 2.20 2.00 2.01
0.53 0.79 1.96 2.37 3.10 4.65
0.094 0.251 0.549 0.565 3.081 6.563
3.44 2.48 2.17 2.18 1.98 2.00
0.38 0.75 1.54 1.53 3.18 4.29
The values were obtained from best-fit parameters found through the fitting process of the experimental j /t plots using Eq. (1), Scharifker et al. and Eq. (5), Heerman and Tarallo.
k1 qmon pS2 D
(9)
and k2 pS 2 DN0
(10)
In these equations qmon is the charge density associated with the monolayer formation, S is a constant controlled by the potential. Note that k1/k2 /Qnucl and Qnucl is the charge density due to the 2D nucleation process. jad(t) is the contribution associated with the adsorption process [2,20 /22] and is given by jad (t)k3 exp(k4 t)
(11)
Note that k3/k4 /Qads and Qads is the charge density due to the adsorption process. j3D dc is given by Eq. (1) [17,18] or Eq. (5) [19]. Fig. 6 shows a comparison of an experimental current density transient with a theoretically generated curve, obtained by non-linear fitting of the experimental data
to Eq. (7). It is clear that the model expressed in Eq. (7) predicts the behavior of the whole experimental current density transient. In Table 2 the physical parameters obtained from this adjustment are summarized. Note that we carried out adjustments for both the Scharifker et al. [17,18] and Heerman and Tarallo [19] models for j3D dc. Note, again that, independently of which model was used to describe j3D dc in Eq. (7), the physical parameters obtained are similar (Table 2). If we compare Tables 1 and 2, several important observations can be made. If the currents due to the adsorption and 2D nucleation and growth processes are considered, then A is lower and N0 is two orders of magnitude greater than in the case where these processes are not considered. The diffusion coefficient values are close to 2 /105 cm2 s1, the experimentally reported value [1]. Fig. 7 shows a plot of ln A vs. /E. Note that a linear relationship is found using the models represented in Eqs. (5) and (7). In the framework of the atomistic theory of electrolytic nucleation [23,24], it is possible to estimate the critical size of the nuclei [24] using: nc
Fig. 6. Comparison between an experimental current density transient ( */), recorded during cobalt electrodeposition onto a polycrystalline gold electrode when a potential value of /0.930 V was applied (Fig. 3) with a theoretical transient (k) generated by a non-linear fit of Eq. (7) to the experimental data. In this figure are depicted the different individual contributions that form Eq. (7). Physical constants obtained from best-fit parameters are shown in Table 2.
kT dlnA a ze0 dE
(12)
where k is the Boltzmann constant, T is the absolute temperature and e0 is the elementary electric charge. Substituting in Eq. (12) the value of a / 0.8 [2], and the slope d(ln A )/d(E ) :/100 (Eqs. (1) and (5)) or 111 (Eq. (7)) of the experimental potential dependence of A (Tables 1 and 2) within the potential interval studied, one obtains nc /1. According to the atomistic theory [23], a complex for which the probability for attachment of one atom is less than 1/2 is defined as critical. The attachment of a new atom, however, converts the complex into a stable one, for which the probability for attachment of the next atom is already higher than 1/ 2. Therefore, in this case, the critical nucleus is composed by one atom adsorbed on an active site and the stable nucleus should be formed by two atoms.
44
L.H. Mendoza-Huizar et al. / Journal of Electroanalytical Chemistry 545 (2003) 39 /45
Table 2 Potential dependence for the physical constants involved during cobalt opd on a polycrystalline gold electrode Scharifker et al.
Heerman and Tarallo
/E vs. SCE/V
A /s 1
105D /cm s 1
10 10No /cm 2
A /s 1
105D /cm2 s 1
10 10N0/cm 2
0.890 0.895 0.900 0.910 0.920 0.930
0.011 0.138 0.214 0.378 1.463 2.056
2.31 2.18 2.18 1.78 1.99 2.01
0.41 1.42 2.70 4.04 3.96 5.59
0.031 0.130 0.236 0.465 1.843 2.363
1.63 2.01 1.95 2.02 1.97 1.97
0.59 1.42 5.97 7.93 9.60 9.60
The values were obtained from best-fit parameters found through the fitting process of the experimental j /t plots using Eq. (7) and Eq. (1), Scharifker et al. or Eq. (5), Heerman and Tarallo.
Acknowledgements L.H.M.H. is grateful for a partial graduate student fellowship from CONCYTEG and CONACYT. This work was done in partial fulfilment of L.H.M.H. Ph.D. requirements. We gratefully acknowledge financial support from CONACYT through projects number 25059E and 32158-E and to DCBI-UAMA through project 2260220. We are indebted to Dr Mario Romero-Romo for his interest and fruitful discussions.
Fig. 7. ln A vs. /E , the applied potential, relationship. The A values were obtained from: (k) Eq. (5) and (I) Eq. (7) (using Eq. (5) in order to describe j3D dc).
These facts suggest that the processes occurring before the 3D nucleation and growth process modify the electronic density on the surface active sites [25], and the need to consider them in the analysis of the deposit processes becomes obvious.
4. Conclusions An electrochemical study of cobalt electrodeposition onto a polycrystalline gold electrode from an aqueous 102 M CoCl2, 1 M NH4Cl (pH 9.5) solution was carried out through cyclic voltammetry and potential step techniques. Analysis of the experimental data indicates that formation of cobalt deposits involve the simultaneous occurrence of an adsorption process, and a 2D and 3D nucleation and diffusion-controlled growth processes. We compared the models proposed by Scharifker et al and Heerman and Tarallo and we obtained similar conclusions from both models. However, if the influence of the adsorption and 2D nucleation and growth processes is not considered, the nucleation rate and the number of active nucleation sites are overestimated and underestimated, respectively.
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