Electrochemical nucleation and growth of Cu onto Au nanoparticles supported on a Si (111) wafer electrode

Journal of Electroanalytical Chemistry 791 (2017) 1–7

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Electrochemical nucleation and growth of Cu onto Au nanoparticles supported on a Si (111) wafer electrode M. Romero-Romo a, J. Aldana-González a, L.E. Botello a, M.G. Montes de Oca a, M.T. Ramírez-Silva b, S. Corona-Avendaño a, M. Palomar-Pardavé a,⁎ a b

Universidad Autónoma Metropolitana-Azcapotzalco, Departamento de Materiales, Área Ingeniería de Materiales, Av. San Pablo #180, Col. Reynosa-Tamaulipas, CDMX C.P. 02200, Mexico Universidad Autónoma Metropolitana-Iztapalapa, Departamento de Química, Área de Química Analítica Av. San Rafael Atlixco #186, Col. Vicentina, CDMX C.P. 09340, Mexico

a r t i c l e

i n f o

Article history: Received 12 January 2017 Received in revised form 28 February 2017 Accepted 2 March 2017 Available online 4 March 2017 Keywords: Gold nanoparticles Si (111) wafer electrode Cu upd-opd Nucleation Kinetics

a b s t r a c t This work showed that chemically-synthesized gold nanoparticles, AuNPs, supported onto a Si (111) wafer electrode, can be selectively modified with a copper adlayer through underpotential deposition (upd) conditions, using both: potentiodynamic or potentiostatic electrochemical means. From analysis of experimental potentiostatic current density transients, it is shown that Cu upd onto the AuNPs occurs by a mechanism involving the simultaneous presence of a Langmuir-type adsorption-desorption and an instantaneous two-dimensional, 2D, nucleation process. The influence of the applied potential on the Cu upd kinetics and on the extent of Cu atoms coverage over the AuNPs was also reported. Furthermore, it is shown that the Cu overpotential deposition, opd, onto these AuNPs, starting from a potential in the upd region where the AuNPs surface is free from Cu atoms, occurs through a 2D-3D mechanism, where the 3D nucleation is mass-transfer controlled. Notwithstanding, when Cu opd started at the equilibrium potential the mechanism solely involved 3D nucleation. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The metallization of semiconductor surfaces for the formation of Schottky barriers, ohmic contacts and diffusion barriers in electronic devices can be performed, in vacuum, by evaporation and sputtering [1], electrodeposition [2–8], or through charged particle beams [9], Si-C covalent bonds [10] and self-assembled method [11]. Recently, we have shown that chemically synthesized gold nanoparticles, AuNPs, can be electrostatically adsorbed onto an ITO substrate, previously modified with Poly-L-Lysine, for its metallization [12–14] and that the resulting ITO/AuNPs electrode can be used for dopamine selective quantification [15]. Furthermore, we have also shown that these AuNPs surfaces can be selectively covered with a copper full 2D monolayer via underpotential deposition, Cu upd [16]. Some researchers have taken advantage of this sort of adlayers to control the synthesis of platinum catalysts on AuNPs towards methanol oxidation [17]. Since the amount of Cu upd coverage, on the AuNPs, is susceptible to be modulated by controlling the electrode potential [18–32], in this work the modification of AuNPs,

⁎ Corresponding author. E-mail address: [email protected] (M. Palomar-Pardavé).

http://dx.doi.org/10.1016/j.jelechem.2017.03.003 1572-6657/© 2017 Elsevier B.V. All rights reserved.

supported onto a Si (111) wafer electrode with 2D adlayers formed through Cu upd is considered, with specific attention to the formation mechanism and kinetics, using both potentiodynamic and potentiostatic electrochemical techniques. Moreover, the Cu 2D-3D transition when varying the applied potential from the upd zone to the overpotential deposition (opd) region, is also shown. 2. Experimental 2.1. Chemicals and solutions All solutions were prepared with analytical grade reagents and deionized water (18.2 MΩcm) from Millipore Milli-Q. HAuCl4·3H2O, 99.9%, C2H5Na3O7·2H2O, 99.5%, CuSO4, 99.999%, H2SO4, 99.999%, Poly-L-Lysine (Mw 30.000–70.000), NaCl, 99.5% and Si(111) wafer were purchased from Sigma-Aldrich. CuSO4 and H2SO4, ultrapure grade, were from Merck. 2.2. Synthesis of AuNPs AuNPs were synthesized following Turkevich and Frens method [33, 34], slowly adding C2H5Na3O7 to HAuCl4 under reflux and strong agitation, during 1 h. The reaction is ended when the solution turns red.

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2.5. Electrochemical setup Cyclic voltammetry and chronoamperometry served to study Cu electrodeposition at Si (111)-AuNPs substrates from a 5.0 mM CuSO4 and 0.5 M H2SO4 solution. Electrochemical experiments were carried out in a system using a Si (111)-AuNPs working electrode with a 0.071 cm2 geometric area, a Pt wire as counter electrode and a Cu wire as pseudo-reference electrode, to which all potentials are herein quoted. Before the experiments, solutions deaeration was done for 30 min using pure N2. Potentials were controlled by a PAR 273 (USA) potentiostat-galvanostat coupled to a PC running the M270 electrochemical research software (EG&G PAR) for experimental control and data acquisition. Before initiating the Cu upd experiments, the working electrode was constantly maintained under the potential control (400 mV), from initial contact with solution. 3. Results and discussion Fig. 1. UV–Vis spectrum of AuNPs.

3.1. AuNPs characterization 2.3. AuNPs characterization 2.3.1. UV–Vis absorption spectroscopy AuNPs UV–Vis spectra were obtained with a Perkin Elmer lambda 35 UV–Vis spectrometer with a 10 mm quartz cell, using Milli-Q water as blank.

2.4. Si (111)-AuNPs electrodes preparation The Si (111) wafer electrode was ultrasound-cleansed in acetone, ethanol and deionized water, 15 min in each, and dried under pure N2. The clean surfaces were modified by dipping in 1 mg cm−3 Poly-LLysine, PLL, solution for 10 min, followed by deionized Milli-Q water rinsing and drying under pure N2. For electrochemical experiments, five layers were stacked on the modified PLL-Si (111) wafer electrode, having the AuNPs electrostatically adsorbed. This allowed control of the AuNPs number, dependent on the Si (111) wafer electrode immersion time in the AuNPs solution [12–14]. The PLL cationic nature makes it an intermediary between the negatively charged substrate's structure, and the AuNPs enclosed in citrates' chains [14]. The electroactive surface area of the Si (111)-AuNPs electrode was estimated as (0.285 ± 0.043) cm2 following the methodology reported by Aldana-González et al. [16].

2.4.1. SEM characterization The Si (111)-AuNPs was characterized through a SEM SUPRA PV Zeiss instrument, using 10 kV accelerating voltage and 20 KX or 100 KX magnifications.

From analysis of the chemically synthesized AuNPs by means of XRD [15] and UV–Visible spectroscopy, see Fig. 1, it was found they are polycrystalline, with a predominant Au(111) plane, see Fig. 4 in [15], and display a 5–20 nm average diameter (a plasmon resonance peak appeared around 520 nm), see Fig. 1, respectively.

3.2. Si (111)-AuNPs electrode characterization 3.2.1. SEM analysis The AuNPs were supported onto a Si (111) wafer electrode, see Section 2.4. Fig. 2 shows SEM images of the AuNPs electrostatically adsorbed on a Si (111) wafer surface. It is possible to note that the AuNPs are dispersed over the whole Si(111) wafer surface, see Fig. 2a, and in spite that some of them agglomerated, their nanometric nature was preserved, see Fig. 2b. 3.2.2. Cyclic voltammetry analysis Fig. 3 shows typical CVs recorded using the bare Si (111) wafer electrode, showing practically null faradaic activity in the potential range considered (see the red CV), or that modified with AuNPs, Si (111)AuNPs (blue CV), immersed in a sulfuric acid aqueous solution. The latter shows the faradaic current associated with the oxidation of the AuNPs to form gold oxides (O1) and its corresponding reduction to metallic gold (R1). It is important to note that in the potential range: 0 ≤ E ≤ 400 mV there is not any faradaic activity and solely the capacity current associated to the electrochemical double layer can barely be observed, see the inset in Fig. 3.

Fig. 2. SEM images of the AuNPs (one layer was stacked) electrostatically adsorbed on a Si (111) wafer electrode surface: a) 20,000× and b) 100,000×.

M. Romero-Romo et al. / Journal of Electroanalytical Chemistry 791 (2017) 1–7

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Fig. 3. CVs recorded in the systems: Si (111) / 0.5 M H2SO4 (red line) and Si (111)-AuNPs / 0.5 M H2SO4 (blue line), in both cases the potential scan started at − 420 mV in the positive direction at 5.0 mVs−1. The inset shows a close up of the region (0 ≤ E ≤ 400 mV) of the CVs.

3.3. Cu upd onto AuNPs 3.3.1. Potentiodynamic study Fig. 4 depicts an experimental CV recorded during Cu upd onto the AuNPs supported on the Si (111) wafer electrode. Two well resolved voltammetric peaks were formed. The cathodic one is associated with the formation of a copper monolayer covering the whole electroactive surfaces of the AuNPs, and the anodic one its electrochemical dissolution to form Cu(II) ions in solution. It is important to mention that Cu electrodeposition onto Si(111) surface has been reported; however, it occurs at potential values more negative than − 1.0 V vs. Cu(II) /

Fig. 5. Comparison of the experimental i-θ plots, corresponding to both: the anodic (Fig. 5a) and cathodic (Fig. 5b) branches of the CV shown in Fig. 4, where the θ values where obtained by integration of the respective voltammetric branch, with that obtained by non-linear fitting of Eq. (2). The best fit parameters obtained were: for the anodic branch P1 = (27 ± 0.1) μA and P2 = − 0.8 ± 0.1 and for the cathodic branch, P1 = (27 ± 0.2) μA and P2 = −2.0 ± 0.3.

Cu(0) [2–6]. It is important to point out that the CV recorded during Cu upd onto Au(111) single crystals extended surfaces [19,25,32], at the same experimental conditions, showed two very well defined

Fig. 4. Cyclic voltammogram recorded in system: Si (111)-AuNPs / 5.0 mM CuSO4, 0.5 M H2SO4. The potential scan started at 375 mV in the negative direction at 5.0 mVs−1.

Fig. 6. Comparison of the experimental (circles) i-t plots recorded in the system: Si (111)AuNPs / 5.0 mM CuSO4, 0.5 M H2SO4 stepping the potential from 400 mV to the different potential values indicated in the Figure in mV, and the theoretical ones (solid lines) obtained by non-linear fit of Eq. (3) to the experimental data. The inset shows the individual contributions of the theoretical overall current, for the i-t plot recorded at 60 mV due to an adsorption process (iad) and an instantaneous 2D nucleation process, limited by ad-atoms incorporation (i2Di-LI).

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Table 1 Current maxima coordinates and best fitting parameters obtained through non-linear fitting of Eq. (3) to the experimental transients shown in Fig. 6. E /mV

tm a /s

100 80 60 40 20 0

6.05 5.25 4.47 3.16 2.63 2.10

a b

± ± ± ± ± ±

0.02 0.01 0.04 0.02 0.01 0.02

ima /μA

k1a /μA

k2a /s−1

2.20 ± 0.04 5.53 ± 0.08 9.21 ± 0.06 19.92 ± 0.08 24.82 ± 0.02 29.58 ± 0.01

8.65 + 0.31 9.50 ± 0.39 10.83 ± 0.68 11.20 ± 0.44 11.85 ± 0.56 12.92 ± 0.52

1.02 0.91 0.90 0.71 0.32 0.30

0.04 0.04 0.07 0.04 0.07 0.05

103 k4a /s−2

qb /μC

im tm/q

0.60 ± 0.01 1.44 ± 0.02 2.85 ± 0.05 6.20 ± 0.02 9.11 ± 0.27 12.78 ± 0.30

11.6 ± 0.2 13.9 ± 0.2 19.2 ± 0.3 40 ± 1 55 ± 1 72.6 ± 1.1

21.5 54.4 68.2 81.9 93.7 103.5

0.62 0.53 0.60 0.77 0.70 0.60

The uncertainty values were evaluated on the basis of 3 repetitions for each applied potential. Obtained by integration of the cathodic branch of the CV reported in Fig. 4.

cathodic peaks which have been associated [32] with structural changes on the Cu monolayer formation. The first one (occurring at more positive potential values) to the transition from Cu randomly adsorbed to ð pffiffiffi pffiffiffi 3  3 ÞR30 ° , honeycomb-like superstructure, and the second one to the transition from the latter to that corresponding with the isomorphic (1 × 1) full Cu monolayer formation. Therefore, a plausible explanation in the case of Cu upd onto AuNPs is that it occurs through just one structural transition namely: from Cu randomly adsorbed to a (1 × 1) adlayer structure. These features clearly indicate that the kinetics and mechanism of the Cu upd onto Au are very sensitive to the structure of the gold surfaces. The complexity of the Cu upd on Au flat surfaces has been addressed by Rikvold and coworkers [26,27] and by Oviedo et al. [31], by means of different simulations namely: numerical studies of lattice-gas [26,27] and quantum mechanical methods models [31]. 3.3.1.1. Voltammogram simulation. Considering the following: a) the shape of the experimental CV recorded during Cu upd onto AuNPs supported onto the Si (111) wafer electrode, b) that the electroactive species is adsorbed and c) that this process involves the formation of a copper monolayer onto the surfaces of the AuNPs, it was decided to analyze the CV shown in Fig. 4 in terms of surface coverage derived from transient measurements as described in Eq. (1) [35]. When lateral interactions exist between the oxidized, Ox, and the reduced, Re, form of the atoms that will give rise to the two-dimensional, 2D, film, the shape of the i-E curve depends upon the energies of the interactions of Ox with Ox, Re with Re and Ox with Re. If a Frumkin-type isotherm is assumed, the expression of the current as a function of the surface coverage, θ, is given by Eq. (1). This methodology is similar to that described by Barradas and Porter [36], for the analysis of two-dimensional nucleation-growth processes in terms of surface coverage, derived from transient measurements, and also described by Noel and Vasu [37].

i¼

± ± ± ± ± ±

k3a /μAs−1

z2 F 2 SvΓ 0 RT



 θRe ð1−θRe Þ 1−2εgθT θRe ð1−θRe Þ

ð1Þ

where θRe is the fractional area of Re, S is the electrode surface area, z is the number of electrons transferred during the heterogeneous reaction, v is the potential scan rate, R, T and F are the universal gas constant, absolute temperature and Faraday constant, respectively, Γο⁎ is the total surface coverage, ε is the number of water molecules displaced from the surfaces by absorption of Ox or Re, θT = θRe + θOx (with θOx being the fractional area of Ox) and g = aOx + aRe − 2aOxRe (where aOxRe, aOx and aRe are the Ox-Re, Ox-Ox and Re-Re interaction parameters (dimensionless)). Eq. (2) is a parameterized form of Eq. (1) with P1 ¼ P2 = 2εgθT  i ¼ P1

 θRe ð1−θRe Þ 1−P 2 θRe ð1−θRe Þ

z2 F 2 Sv0 RT

and

ð2Þ

Fig. 5 shows a comparison of the experimental i-θ plots, corresponding to both: the anodic (Fig. 5a) and cathodic (Fig. 5b) branches of the CV shown in Fig. 4, where the θ values were obtained by integration of the respective voltammetric branch, with that obtained by non-linear fitting of Eq. (2). It is possible to note that both cases can be adequately described by the model of monolayers formation and dissolution in terms of surface coverage, derived from transient measurements [35–37] and furthermore, since the interaction parameter (P2) is different to 0 in both cases, it means that lateral interactions between the copper ad-atoms that form the monolayer are not negligible (as compared with the interaction of these ad-atoms with the gold substrate); this is congruent with the formation of a Cu monolayer throughout a nucleation mechanisms [36]. 3.3.2. Potentiostatic study 3.3.2.1. upd. Hölzle et al. [25], studied the mechanisms and kinetics of Cu upd onto Au(111) and Palomar-Pardavé et al., in both Au(111) [19–21] and polycrystalline gold [21] extended surfaces by means of potentiostatic current transients, i(t). In these works, it was shown that regardless of the crystalline nature of the Au surface, the

Fig. 7. Potential variation of a) the current time's maxima (tm) of the experimental i-t transients reported in Fig. 5 and b) parameter k4 obtained by non-linear fit of Eq. (3) to the experimental potentiostatic current transients. The lines and the equations correspond to the linear fitting of the data obtained (points).

M. Romero-Romo et al. / Journal of Electroanalytical Chemistry 791 (2017) 1–7

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mechanism that described the experimental potentiostatic current transients, see Eq. (3), involves the simultaneous presence of the double layer charge process, a Langmuir-type adsorption-desorption equilibrium, iad (t), Eq. (4) and at least an instantaneous 2D nucleation process limited by the lattice incorporation, i2Di-LI (t), of Cu ad-atoms to the 2D nuclei Eq. (5). iðt Þ ¼ iad ðt Þ þ i2Di−LI ðt Þ

ð3Þ

with: iad ðt Þ ¼

k1 expð−k2 t Þ

ð4Þ

1 where t is the potential perturbation time, k1 ¼ SE Rs , k2 ¼ Rs C

and   i2Di−LI ðt Þ ¼ k3 t exp −k4 t 2 with k3 ¼

2πzSFMhN0 K 2g ρ

and k4 ¼

ð5Þ πM2 N0 K 2g ρ2

where E is the applied potential, Rs is the solution's resistance, C the double layer capacitance, Kg is the nuclei growth rate constant (mol cm−2s− 1), M and ρ are the molecular mass and the deposit density, respectively; h is the layer formed height and N0 is the overall number density of active sites available for the nucleation process on the substrate surface.

Fig. 8. Potential variation of the charge density (left y-axis) and the corresponding AuNPs surface coverage with Cu atoms (right y-axis). The individual contributions to the total Cu upd process (circles) due to 2D nucleation (squares) and the adsorption process (triangles) were obtained from Eqs. (8) and (9) using the parameters reported in Table 1.

fitting of the Eq. (3) gives a good support to the model used to describe the mechanisms and kinetics of Cu upd on AuNPs. Moreover, for instantaneous 2D nucleation [38–39] the product im tm must be equal to 0.606

3.3.2.2. Influence of the applied potential on Cu upd onto AuNPs. Fig. 6 compares the experimental potentiostatic current density transients obtained during Cu upd on the AuNPs surfaces, supported onto Si (111) wafer electrode, for different applied potentials and those resulting from Eq. (3) fitting to the respective experimental data. The best fitting parameters are shown in Table 1. Note that Eq. (3) describes adequately the experimental evidence in all cases. Moreover, it is plain from the inset in Fig. 6 that at very short times the adsorption process is the dominant contribution. However, after a few seconds the 2D instantaneous nucleation becomes the more important contribution to the total current. This transition can be related to the structural changes observed during Cu upd on Au(111) extended surfaces, namely: random adsorption of pffiffiffi pffiffiffi Cu atoms to ð 3  3ÞR30 ° or honeycomb structure, and eventually, if the applied potential corresponds to the equilibrium potential, to a (1 × 1) isomorphic structure corresponding to the formation of a full monolayer [18]. As can be noted from Fig. 6 all these current transients possess wellresolved current maxima with tm, im coordinates, see Table 1. At this current maximum, see inset in Fig. 6, the contribution associated to the double layer charging is negligible, thus at this point the current is practically due to the 2D nucleation process and then the temporal derivative of Eq. (5) must be equal to zero. Thus, differentiation of Eq. (5) with respect to time yields:  tm ¼

1 2k4

1=2 ¼

ρ ð2πN0 Þ1=2 MK 2g

ð6Þ

Taking the logarithm of Eq. (6) and then its partial differentiation with respect to E yields:     ∂ logt m 1 ∂ logk4 ≅− 2 ∂E ∂E

ð7Þ

Fig. 7 shows the potential dependence of the logarithm of current time's maxima (tm), Fig. 7a, and the logarithm of parameter k4, Fig. 7b. In both cases, the observed behaviors were linear. The slopes were 0.004 (for log (tm) vs. E) and −0.008 (for log (k4) vs. E). This agreement between direct experimental evidence (tm) and that (k4) obtained by

Fig. 9. a) Comparison of the experimental (circles) j-t plot recorded in the system: Si (111)-AuNPs / 5.0 mM CuSO4, 0.5 M H2SO4 stepping the potential from 400 to −60 mV (upd-opd), and the theoretical one (solid line) obtained by non-linear fit of Eq. (10) to the experimental data. The individual contributions of the theoretical overall current density due to adsorption (iad), instantaneous (i2Di-LI) and progressive (i2Dp-LI) 2D nucleation, limited by ad-atoms incorporation and a 3D nucleation process mass transfer-controlled (i3D-dc) are also shown. b) Comparison of experimental (points) j-t plots recorded in the system: Si (111)-AuNPs / 5.0 mM CuSO4, 0.5 M H2SO4 stepping the potential from 400 to −60 mV (upd-opd) (triangles) and from 0 to −60 mV (points) with the theoretical j-t traces generated by Eq. (10) and (17), respectively. For both cases the best fit parameters obtained are reported in Table 2.

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Table 2 Adsorption and nucleation parameters obtained through non-linear fitting of Eqs. (10) or (17) to the experimental transients shown in Fig. 9. Potential perturbationa

400 to −60 mV 0 to −60 mV a

j2Di−LI(t)

j2Dp −LI(t)

k1 /mA cm− 2

jad(t) k2 /s− 1

N0K2g /mol2 cm− 6 s− 1

AN0K2g /mol2 cm− 6 s− 3

105 D /cm2 s− 1

A /s− 1

j3D−DC(t) 10−6 N0 /cm− 2

0.81 ± 0.02 1.56 ± 0.05

6.6 ± 0.3 4.0 ± 0.3

0.002 ± 0.001 –

0.00005 ± 0.00002 –

1.3 ± 0.2 1.3 ± 0.1

0.00004 ± 0.00001 0.27 ± 0.01

1582 ± 8 4.45 ± 0.01

The uncertainty values were evaluated on the basis of 3 repetitions for each applied potential.

qM, where qM is the monolayer charge. As can be noted from Table 1, the value of this product, calculated from the experimental transients, and divided by the charge of the monolayer, obtained by integration of the cathodic branch of the CV reported in Fig. 4, adequately agrees with the theoretical value of 0.606, for any of the applied potentials considered. From analysis of Eqs. (4) and (5) it is possible to estimate the charge densities due to adsorption, qad and 2D nucleation, q2D, processes respectively as: qad ¼ k1 =k2

with

P1 ¼

2FD1=2 c0 π1=2

ð13Þ

P 2 ¼ N 0 πkD

ð14Þ

P3 ¼ A

ð15Þ

k ¼ ð8πc0 =ρÞ1=2

ð16Þ

ð8Þ

q2D ¼ k3 =2 k4

ð9Þ

3.3.2.3. upd-opd transition. Fig. 9a shows a comparison of an experimental potentiostatic current density transient, j(t), recorded during the copper electrodeposition onto the Si(111)/AuNPs electrode, starting the potential (Ei) at 0.4 V, where the AuNPs surfaces are copper-free, and then stepped the potential (Ef) to a value within the opd zone (Ef b Eeq), with the theoretical one generated by non-linear fit of Eq. (10) proposed by Palomar-Pardavé et al. [19,40], in order to describe the potentiostatic current transients involving 2D-2D and 2D-3D nucleation transitions, where the three-dimensional nucleation contribution, j3D-dc(t), see Eq. (12), is a mass transfer-controlled process described by Scharifker and Mostany [41].

where A is the nucleation rate, and c0 and D are the concentration and diffusion coefficient of the metal ion in the solution's bulk, respectively. From Fig. 9a it becomes plain that when Cu deposition onto the AuNPs is carried out from an Ei value, where the AuNPs are free from Cu atoms, Ei N Eeq, to a final potential value within the opd zone, Ef b Eeq, where the 3D nuclei are thermodynamically favored, the copper nucleation occurs. At the early stages, t b 2 s, the potentiostatic current density transient is mainly formed by instantaneous 2D nucleation however, for longer times, t N 2, the diffusion-controlled 3D nucleation, is the main contribution to the total current density. Notwithstanding, the participation of a 2D progressive nucleation was observed. The origin and nature of this second monolayer is unclear and further research on this matter would be required. Nevertheless, if the potentiostatic Cu opd is started at the Eeq (where the Cu full monolayer is already formed, see Fig. 8) the experimental current density transient, see Fig. 9b, is formed by a single current density maximum that can be adequately described by Eq. (17) which solely involves a single 3D nucleation process. Furthermore, from Fig. 9b and Table 2, it is possible to note that when the 2D–3D nucleation transition is occurring, the 3D nucleation becomes slower (its maximum occurs a longer times with a lower nucleation rate values); however, the 3D nuclei always are formed on top of a 2D layer, this crystal growth model is known as the StranskiKrastanov mechanism [24]. The influences of the applied potential on both experimental cases namely 2D-3D and·3D nucleation can be found as supporting material of this work.

jðt Þ ¼ jad ðt Þ þ j2Di−LI ðt Þ þ j2Dp−LI ðt Þ þ j3D−DC ðt Þ

jðt Þ ¼ jad ðt Þ þ j3D−DC ðt Þ

Fig. 8 shows the potential dependencies of the total charge density, q, (q = qad + q2D) along with those corresponding to the individual contributions due to adsorption (Cu and co-adsorption of bisulfate and/or sulfate anions) and Cu nucleation. It is possible to note that the surface coverage can be modulated by controlling the electrode potential and that even when the 2D nucleation contribution always predominates, as the applied potential approaches the equilibrium one, the adsorption contribution becomes more notorious. Furthermore, from the data in Table 1, k1 increases linearly with the potential excursion, as expected, and the rate of adsorption k2 decreases significantly as the potential is varied from 100 to 0 mV. This is probably related to the drop in adsorption capacity as the copper monolayer is completed.

ð10Þ

ð17Þ

where the progressive 2D nucleation is given by Eq. (11) 4. Conclusions

  j2Dp−LI ðt Þ ¼ k5 t 2 exp −k6 t 3

with k5

¼

πzFMhAN0 K 2g and k6 ρ

ð11Þ

¼

πM2 AN0 K 2g 3ρ2

And the mass transfer-controlled 3D nucleation contribution is represented by Eq. (12) 

   1− expð−P 3 t Þ jðt Þ ¼ P 1 t −1=2 1− exp −P 2 t− P3

ð12Þ

It has been demonstrated that the electrochemical formation of a Cu adlayer, under upd conditions, onto the AuNPs, supported onto a Si (111) wafer electrode, followed a mechanism involving the simultaneous presence of an adsorption process and an instantaneous 2D nucleation processes. From the theoretical model herein described, both contributions can be deconvoluted, where the individual contribution to the Cu 2D layer, as a function of the applied potential, was also reported. Moreover, it was also shown that Cu opd on the AuNPs always involves the formation of 3D nuclei onto a 2D Cu monolayer.

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Acknowledgements The authors like to thank CONACyT for projects 258487 and 237327 and SEP-PRODEP for RedNIQAE. MTRS thank CONACyT for the cathedra 2159. JAG and LEB thanks CONACyT for the support given to undertake a postdoctoral stay at UAMA and the scholarship granted to pursue postgraduate studies at UAMA, respectively. JAG, MGMY, MTRS, SCA, MRR and MPP wish to thank the SNI for the distinction of their membership and the stipend received. MGMY is indebted to Bristol University and its Electrochemistry group for her Ph.D. formation, CONACyT for paying the corresponding tuition fees and L'oreál-UNESCO-CONACyT-AMC for the grant Woman in Science 2016. We also like to express our gratitude to the anonymous reviewers of this paper for their criticisms and suggestions that contributed to improve our work.

[17]

[18]

[19]

[20] [21]

[22] [23]

Appendix A. Supplementary data [24]

Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2017.03.003.

[25] [26]

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