Identification of different silver nucleation processes on vitreous carbon surfaces from an ammonia electrolytic bath

Journal of Electroanalytical Chemistry 443 Ž1998. 81–93

Identification of different silver nucleation processes on vitreous carbon surfaces from an ammonia electrolytic bath Margarita Miranda-Hernandez, Manuel Palomar-Pardave, ´ ´ Nikola Batina, Ignacio Gonzalez ´

)

UniÕersidad Autonoma Metropolitana-Iztapalapa, Departamento de Quımica, Apdo. Postal 55-534, 09340 Mexico, D.F., Mexico ´ ´ ´ ´ Received 17 March 1997; received in revised form 7 July 1997

Abstract We performed an electrochemical study of silver electrodeposition from an electrolytic bath containing 1 M NH 4OH and 1 M KNO 3 ŽpH s 11., over a AgŽI. concentration range of 10y4 to 0.3 M, and identified silver nucleation processes on the vitreous carbon surface. Depending upon the silver concentration in the deposition bath, growth occurred either two-dimensionally Ž2D., controlled by adatom incorporation, or three-dimensionally Ž3D., controlled by diffusion or lattice incorporation. Qualitative and quantitative characterization of the observed nucleation processes were based on the results of cyclic voltammetry and chronoamperometry analysis. For quantitative characterization, different theoretical models related to electrocrystallization processes were used. Atomic force microscopy ŽAFM. was employed to probe the surface morphology of the silver deposit. AFM images revealed that silver deposits formed from ammonia baths with different concentrations of AgŽI. ion possess different morphological characteristics. Indeed, many of the surface characteristics clearly corroborated the mechanism of silver deposition proposed by electrochemical analysis. q 1998 Elsevier Science S.A. Keywords: Electrodeposition; Atomic force microscopy; 2D nucleation; 3D nucleation; Amine silver complex

1. Introduction The use of electrodeposition in the preparation of new materials with specific characteristics Žthin layers, dispersed materials, nanostructures. requires an in depth comprehension of the different phases of the electrodeposition process. Nucleation kinetics and the growth of the first metallic nuclei formed on the initial substrate are critical steps determining the physicochemical properties of the electrodeposited materials w1x and are, therefore, crucial points of understanding and control. Electrochemical characterization of the nucleation process generally is performed by collecting different kinds of current transients obtained when potential pulses are imposed on the metal
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monly related to only one kind of nucleation process w4–6x. Complex deposition systems, consisting of two or more nucleation processes with sophisticated transitions between the different types have rarely been examined. Recent studies have reported using scanning tunnelling microscopy ŽSTM. in combination with current transients analysis, to detect 2D nuclei formation Župd electrodeposition. w7x and 3D nucleation controlled by incorporation of atoms to the nucleus Žoverpotential deposition. w8,9x of copper adatoms on a gold electrode. Utilizing this same powerful combination, 2D to 3D transformation during electrocrystallization of silver onto a graphite electrode surface was recognized w10,11x. These studies were all performed on single crystal electrode surfaces w7–11x, due to easy recognition and understanding of major characteristics of different electrocrystallization processes. Although the findings could have a significant impact on future nanotechnologies, it will be necessary to define experimental conditions and ultimately to control the electrocrystallization processes on the polycrystalline substrates for other applications. However, due to the heterogeneous nature and undefined physicochemical characteristics of these surfaces, this task is difficult to achieve. Definition of the experimental conditions, in which different types of

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M. Miranda-Hernandez et al.r Journal of Electroanalytical Chemistry 443 (1998) 81–93 ´

electrocrystallization processes can be obtained in the same chemical system on the polycrystalline substrate, is indeed, a worthwhile challenge. It is important as well to define the relationship between the type of nucleation predicted by electrochemical techniques, and surface observation by different imaging and microscopic techniques. In the past, this relationship has been reported for scanning electron microscopy ŽSEM. w12–15x and metallurgical microscopy w16x. Because of the relatively low resolution of these techniques, instantaneous or progressive nucleation is distinguishable only for nuclei formed after a relatively long period of time, i.e., during the later stages of the electrocrystallization process w12–16x. The early stages of nucleation or less energetic nucleation processes on the polycrystalline surface Že.g., 2D growth. can be identified by the newly developed, high resolution STM and atomic force microscopy ŽAFM. techniques. However, the nature of the polycrystalline substrate can make such STM and AFM analyses very difficult w17,18x. We present an electrochemical study of silver electrodeposition from an electrolytic bath containing 1.6 M NH 4 OH and 1 M KNO 3 ŽpH s 11., for an AgŽI. concentration range between 10y4 and 0.3 M. Under these chemical conditions, it was possible to identify different silver nucleation processes over vitreous carbon ŽVC. surfaces as functions of AgŽI. concentration: 2D growth, controlled by adatom incorporation, 3D growth controlled either by diffusion or lattice incorporation of adatoms. Different nucleation processes identified by electrochemical techniques, cyclic voltammetry and chronoamperometry, were related to the content of AgŽI. ions in the deposition baths. Each type of silver electrocrystallization process was defined quantitatively, using different theoretical methods. AFM analysis of the deposit surface morphology completely supported the electrochemical data, clearly showing that the surface characteristics of silver deposits depended upon the AgŽI. concentration in the deposition baths.

2. Experimental The electrochemical techniques, cyclic voltammetry and chronoamperometry, as well as AFM, were employed to the study silver electrocrystallization mechanisms on VC substrates. All electrochemical experiments were carried out in a conventional three electrode cell system with a VC disc, surface area 0.071 cm2 , ŽRadiometer, Tacussel. as a working electrode. The surfaces were polished with 0.3 m m alumina powder and treated in a pure water in an ultrasonic bath for 10 min. Prior to use, the electrode surface was pretreated Žactivated. electrochemically by exposing the electrode surface to 15 voltammetric cycles in the potential range between y0.7 to 0.7 V vs. SCE in a 10y2 M K 4 wFeŽCN. 6 x q 1 M KCl solution, at a scan rate of 50 mV sy1 . A saturated calomel electrode ŽSCE. served as a

reference electrode. All potentials in our study are quoted vs. SCE. The counter electrode was a graphite rod with a much larger surface area than the working electrode. Potentials were controlled with a PAR 273 ŽUSA. potentiostat coupled to a computer with a commercial M-270 ŽPAR. software package. Silver deposition was carried out from electrolyte solutions containing: AgNO 3 , 1.6 M NH 3 and 1 M KNO 3 ŽpH s 11.. The silver concentration was varied from 10y4 to 0.3 M. All solutions were prepared using reagent grade chemicals and ultra pure Milli-Q water. Prior to electrochemical experiments, all solutions were carefully deaerated by clean nitrogen gas. Scan rates in cyclic voltammetry from 50 to 250 mV sy1 , were applied. Electrochemical data concerning kinetics of the silver electrocrystallization were evaluated quantitatively using a homemade software package, developed for the PC computer system. AFM ŽNanoscope III, Digital Instruments, USA. was used for visualization of the silver deposition process on the VC electrode. AFM was performed in the contact mode, under a laboratory air atmosphere, using standard geometry silicon nitride probes ŽDigital Instruments.. The scan head with a maximum scan range of 17.5 m m, was used. All images were collected with a relatively slow scan rate of 1–2 Hz. In order to evaluate the progress and the mechanism of silver deposition, the electrode surface was visualized before and after the deposition process, regularly. Other than altering the silver concentration Žfrom 10y4 to 0.3 M., other deposition parameters such as deposition overpotential and the total time of the deposition process Ž1 s., were kept constant. All deposits were, therefore, studied at similar stages of growth, to permit comparison of morphological characteristics of the silver deposit as a function of the AgŽI. content in the deposition bath. For each sample, AFM images were recorded at four different places. Imaging regularly started by visualization of a small area, 1 m m = 1 m m, and proceeded by scanning over 15 m m = 15 m m. During imaging, no destruction of the sample was noticed. All images are presented in so called ‘height mode’ where the higher parts appears brighter.

3. Results and discussion 3.1. Voltammetric study Cyclic voltammetry was first employed to define the potential region of the deposition process at each of the AgŽI. concentrations studied. The voltammetric studies were consistently performed in the potential range y0.7 to 0.7 V. The sweep potential was initiated at 0.7 V and proceeded in the negative direction. Fig. 1 shows typical voltammograms obtained in solution with different concentrations of AgŽI. ion. All voltammograms presented were

M. Miranda-Hernandez et al.r Journal of Electroanalytical Chemistry 443 (1998) 81–93 ´

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Fig. 2. Linear dependence of peak Ž j . current density vs. log c Žconcentration of the AgŽI. electroactive species in the deposition bath.. The values were obtained from voltammograms at 50 mV sy1 .

obtained during the first cycle with a scan rate of 50 mV sy1 . In each voltammogram, two distinct voltammetric peaks ŽI and II. were resolved clearly. Peak ŽI. is associated with the AgŽI. reduction process Ždeposition. whereas Peak ŽII. represents the Ag dissolution process from the VC substrate. The current magnitude and potentials of voltammetric peaks changed significantly as the AgŽI. concentration was varied in the deposition bath. As the AgŽI. concentration increased, potentials of peak ŽI. and peak ŽII. moved toward more positive values. With concentration increases, both peaks also changed in shape and in the relationship between scans in the two directions. At concentrations of 10y3 M, 10y2 M and 10y1 M, some overcrossings on the cathodic branches were noticed. Such behavior usually indicates the existence of a nucleation process on the electrode surface w19x. At higher concentrations, the voltammetric peaks broadened. In order to describe the above mentioned changes in the silver deposition process quantitatively, we performed a detailed analysis of the voltammetric peak shape, position and magnitude according to the change in AgŽI. concentration. We studied the behavior of peak current ŽI., for example, for each concentration as a function of the potential scan rate Ž Õ 1r2 . Ž50–250 mV sy1 .. A linear relationship between peak current vs. Õ 1r2 was found. This indicates that during the cyclic voltammetric scan, silver deposition was controlled Žlimited. by the diffusion process w20x. The linear relationship determined between log Ip for a fixed scan potential and log AgŽI. concentration ŽFig. 2. supports this conclusion. Fig. 3 shows the relationship between deposition peak potential ŽI. vs. log c. An increase in the peak potential seems to be almost uniform with an increase of concentra-

Fig. 1. Cyclic voltammograms of the vitreous carbon electrode in AgŽI.q 1.6 M NH 4 OHq1 M KNO 3 , ŽpH s11. deposition bath, with different AgŽI. concentrations: Ža. 10y4 M, Žb. 10y3 M, Žc. 10y2 M, Žd. 10y1 M and Že. 3=10y1 M. Each scan started from 0.7 V toward y0.7 V. Scan rate: 50 mV sy1 . Two distinct peaks show reduction ŽI. and oxidation ŽII. of the silver species.

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M. Miranda-Hernandez et al.r Journal of Electroanalytical Chemistry 443 (1998) 81–93 ´

Fig. 3. A silver reduction peak ŽI. potential Ž EpI . dependence with log c Žconcentration of AgŽI. in the deposition bath.. Žl. experimental value of EpI obtained from cyclic voltamogram; Ž – – – . theoretical plot calculated from equilibrium conditions and Nernst equation and Žv . experimentally estimated values from crossing potentials Ž Ec ..

tion Žl. in particular for the three middle concentrations. However, for each concentration the peak potential Ž EpI . was found to be more negative than the value expected from Nernst calculations Ž — — .. The estimation of equi0 librium potentials for the AgŽNH 3 .q couple in an 2 rAg ammonium bath is presented in Appendix A. Comparing the calculated to experimental Ž EpI ., we observed that the AgŽI. deposition process requires an electrocrystallization overpotential, which also, interestingly, depends on the AgŽI. concentration in the deposition bath. Note that the calculated Ž — — . and experimental data Žl. for peak ŽI. possess different slopes. In particular, the difference is more significant at very low Ž10y4 M. and very high Ž0.3 M. concentrations of AgŽI.. This could indicate different kinetics or a different silver deposition mechanism. For high AgŽI. concentrations IR u Žohmic drop. could have influenced the potential values. However, this influence is assumed to be negligible for further discussion. Encouraged by these findings, we evaluated and analyzed the equilibrium potential Ž E . Žwithout overpotential influence. for the silver deposition process from an ammo-

nium bath with different contents of AgŽI.. This was achieved by evaluation of the so called crossing potential Ž Ec ., from our experimental data. As previously demonstrated, at such equilibrium conditions the Ec value can be safely related to the thermodynamic equilibrium potential Ž E . w21–24x. Ec was estimated from a separate set of cyclic voltammetric scans, in which the negative scan was purposely reversed at different electrode potentials Ž El .. Note that El was always more positive than the deposition peak potential Ž EpI . w22x. Fig. 4 shows a typical set of cyclic voltammograms used for Ec evaluation for 0.1 M AgŽI. concentration where the reversal potential was in the range from y0.080 to y0.200 V. The current of the reversed scan was always larger than that of the forward scan. This behavior is typical for a mechanism involving metal nuclei formation on a substrate of a different nature w21–25x. The same type of studies were performed with other concentrations as well. For each concentration, Ec was found to be constant, i.e., independent of El, confirming Ec as an equilibrium value w22x. Thus, determined Ec values Žv . plotted in Fig. 3, show perfect agreement with Ž E . values calculated from the Nernst relation. The results presented herein demonstrate how estimation of Ec can be a very elegant method for evaluation of the equilibrium potential for metal deposition on a substrate of a different nature. From differences between equilibrium potentials Žfrom both theoretical calculation and Ec estimation. and experimental values for the electrode deposition potentials Žpeak I., we learned that the silver electrocrystallization process under our experimental conditions was strongly affected by non-thermodynamic, kinetic contributions. We also recognized the possibility that deposition from baths with different contents of AgŽI. proceeds by different mechanisms. Since the evaluation of the electrocrystallization behavior by cyclic voltammetry analysis is limited, we characterized the nucleation processes in more detail using chronoamperometric analysis.

Fig. 4. A set of cyclic voltammograms obtained for the vitreous carbon electrode in 10y1 M AgŽI. q 1.6 M NH 4 OH q 1 M KNO 3 , ŽpH s 11. bath, at a scan rate of 50 mV sy1 . In order to estimate the crossing potential Ž Ec ., the negative going scan was reversed at different potentials Ž El ., as indicated in the figure.

M. Miranda-Hernandez et al.r Journal of Electroanalytical Chemistry 443 (1998) 81–93 ´

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Fig. 5. Family of current density transients for the deposition of AgŽI. on vitreous carbon from 10y4 M AgŽI. in aqueous 1.6 M NH 4 OH q 1 M KNO 3 . The potential imposed at the electrode was different: Ža. y0.100 V, Žb. y0.150 V, Žc. y0.200 V, Žd. y0.250 V vs. SCE.

3.2. Chronoamperometric study Chronoamperometric studies were performed in the same AgŽI. concentration range as the voltammetric study, 10y4 to 0.3 M. Different shapes of potentiostatic current transient curves were observed for different concentrations, indicating that the silver electrocrystallization process proceeded by different mechanisms. Chronoamperometric analyses for three different concentrations, 10y4 M, 10y2 M and 10y1 M, are presented. The theoretical formalisms often utilized for the description of electrolytic phase formation onto a foreign substrate usually consider two extreme cases of nucleation: instantaneous and progressive. In this paper, in order to identify graphically the mechanism involved during the deposition of silver onto the VC electrode, we used theoretical non-dimensional curves of these extreme cases of nucleation for 2D or 3D growth. It helps us to define the

dominant mechanism of the nucleation and growth for each of the studied concentrations. 3.3. Deposition bath with 10 y 4 M Ag(I) Fig. 5 shows a family of current transients obtained at different silver deposition potentials in a solution with 10y4 M AgŽI.. All the current transients possess similar shapes regardless of the applied potential. This suggests that they can be analyzed by the theoretical model proposed by Bewick et al. w25x. This model describes the kinetics of electrolytic phase formation at the early stages of 2D growth where the rate determining step of the electrocrystallization process is the incorporation of adatoms at the expanding periphery of the centers. It also takes into account the overlap of nuclei. The model considers two kinds of nucleation processes, instantaneous and

Fig. 6. Non-dimensional plot jrjm vs. trtm of the experimental data for the deposition of Ag on vitreous carbon from 10y4 M AgŽI. in aqueous 1.6 M NH 4 OH q 1 M KNO 3 compared with theoretical curves for instantaneous nucleation obtained by Eq. Ž3. Ž – – – . and progressive nucleation obtained by .. Currents transients were recorded at the following potentials: Žv . y0.100 V, ŽI. y0.150 V, Ž'. y0.250 V vs. SCE. Eq. Ž4. Ž

M. Miranda-Hernandez et al.r Journal of Electroanalytical Chemistry 443 (1998) 81–93 ´

86

progressive, which can be described by Eqs. Ž1. and Ž2., respectively w25x. jŽinstantaneous. s

2p nFMhN0 k g2 t

r

ž

exp y

p N0 M 2 k g2 t 2 r2

/

Table 1 Kinetic parameters obtained from the experimental transients in Fig. 5 by means of the BFT theory for a 2D progressive nucleation, controlled by the lattice incorporation of adatoms w25x y ErmV vs. SCE

10 2 t m rsy1

10 4 Im r A cmy2

N0 Ak g2 rmol 2 cmy6 sy3

0.100 0.150 0.200 0.250

6.53 5.89 5.68 5.47

2.29 2.66 3.07 3.54

21.69 29.55 32.95 36.90

Ž 1. jŽ progressive. s

p nFMhAN0 k g2 t 2 r

ž

exp y

p AN0 M 2 k g2 t 3 3r 2

/ Ž 2.

N0 is the number density of active sites, k g is the growth rate constant of a nucleus, A corresponds to the nucleation rate constant, M is the molar mass, r is density of the deposited material, nF is the molar charge transferred during electrodeposition, and h is height of the monolayer. These two equations can be reduced to more convenient forms ŽEqs. Ž3. and Ž4.. after normalization of Eqs. Ž1. and Ž2. by means of the current transient maximum values Ž jm , t m .. Therefore, Eqs. Ž3. and Ž4. become non-dimensional equations and can be used as a criterion for distinguishing between the above mentioned types of nucleation w26x. j

t s

jm

tm

j

t s

jm

t m2

ž ž

exp y

t 2 y t m2

exp y2

2 t m2

3.4. Deposition bath with 10 y 2 M Ag(I)

/

instantaneous

Ž t 3 y tm3 . 3t m3

instantaneous. Besides determining the type of nucleation, using Eq. Ž2. we evaluated several important quantitative parameters concerning the silver electrocrystallization process, such as the product of AN0 and k. The estimated values are presented in Table 1. Our values agreed with those previously reported for similar nucleation processes w4x.

/

progressive

Ž 3. Ž 4.

Fig. 6 shows our experimental data presented in a non-dimensional plot compared with theoretical curves calculated using Eqs. Ž3. and Ž4.. The nucleation of silver on the VC electrode from an ammonium bath with 10y4 M AgŽI. mainly follows the response predicted for progressive nucleation. The fit is essentially perfect between the experimental data and the theoretical curve for progressive nucleation, in particular for lower values of the silver deposition potential. An increase in the potential, shifts the nucleation slightly in the region between progressive and

Fig. 7 shows a family of current transients obtained at different potentials for a silver concentration of 10y2 M AgŽI.. The shape of these experimental current transients differs significantly from those obtained during deposition from 10y4 M AgŽI.. Our transients are similar to the those described by the theoretical models proposed by Hills and Scharifker w27x, Gunawardena et al. w28x and by Scharifker and Mostany w29x. Their models describe kinetics of electrolytic phase formation at the early stages when diffusion of the electroactive species from bulk to the interface is the slowest step of the whole process. The growth of nuclei is considered to be 3D taking into account overlap of diffusion zones. According to the model, which has been used extensively for many different systems w6,8,16,30–32x, the

Fig. 7. Potentiostatic transients at different potentials, for the deposition of Ag on vitreous carbon from 10y2 M AgŽI. in aqueous 1.6 M NH 4 OH q 1 M KNO 3 . The potentials were at: Ža. y0.100 V, Žb. y0.150 V, Žc. y0.170 V, Žd. y0.200 V, Že. y0.250 V vs. SCE.

M. Miranda-Hernandez et al.r Journal of Electroanalytical Chemistry 443 (1998) 81–93 ´

instantaneous and progressive type nucleation are described by Eqs. Ž5. and Ž6. w27,28x. jŽinstantaneous. s ks

ž

ks

3

ž

Ž 5.

/

r

4

1 y exp Ž yN0 p kDt .

p 1r2 t 1r2 1r2

8p cM

jŽ progressive. s X

nFD1r2 c

nFD1r2 c

p 1r2 t 1r2

8p cM

r

ž

1 y exp y

AN0 p kX Dt 2 2

1r2

/

Ž 6.

/

where D is the diffusion coefficient, c is the bulk concentration of the silver species. Other parameters were described above for Eqs. Ž1. and Ž2. in Section 3.3. For 3D nucleation with crystal growth controlled by localized hemispherical diffusion, the following expressions ŽEqs. Ž7. and Ž8.. can be applied w27,28x. Normalized variables jrjm and trtm are derived using jm and t m , the current and time at the current density maximum. j

2

ž /

s 1.9542

jm

t

3.5. Deposition bath with 10 y 1 M Ag(I)

ž / tm

½

ž / jm

2

s 1.2254

½

Fig. 8 shows a non-dimensional plot of the experimental current transients at different potentials for silver electrodeposition onto VC from 10y2 M solution in comparison with theoretical curves from Eqs. Ž7. and Ž8.. Silver nucleation follows closely the response predicted for 3Dprogressive nucleation controlled by diffusion of AgŽI. ions. After establishing the type of nucleation process we calculated other parameters such as A and No , usually useful for quantitative description of the deposition process. All parameters calculated by employing previously described methodology w32x are presented in Table 2. Our values agreed with those previously reported for similar nucleation process w16,32,33x Ži.e., D s 1.0 = 10y5 cm2 sy1 w33x. The theoretical formalism used above to describe the ‘early stages’ of either 2D or 3D, considers the ‘early stages’ to involve not only the formation and growth of the initial metallic nuclei on the substrate surface Ž trtm - 1., but also the physical coalescence of the growing nuclei andror the overlap of the diffusion zones around them Ž trtm s 1. and the growth of the initial metal layer on the substrate Ž trtm ) 1..

y1

= 1 y exp y1.2564 j

87

t

2

t

ž /5

Ž 7.

tm

y1

ž / tm

= 1 y exp y2.3367

t

ž / tm

2

2

5

Ž 8.

The current transients for 10y1 M and 3 = 10y1 M AgŽI. deposition baths were completely different from those obtained at lower concentrations. Again, this indicates that the mechanism of silver deposition at high concentrations is different from that at lower concentrations, Fig. 9 shows a family of current transients recorded in 10y1 M AgŽI. solution at different potentials. For all potentials, the rise of the deposition current as a function of the deposition time was observed. In the final stage, the current reached a steady state condition. The current tran-

. Fig. 8. A comparison of the theoretical non-dimensional plots Ž jrjm . 2 vs. trtm for instantaneous Eq. Ž7. Ž – – – . and progressive Eq. Ž8. Ž nucleation with experimental data. The experimental transients were obtained at different potentials: Ž`. y 0.100 V, Ž'. y0.150 V and Že. y0.200 V vs. SCE.

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Table 2 Kinetic parameters obtained from the experimental transients in Fig. 7 by means a 3D nucleation controlled by the diffusion of electroactive ions to the electrode–electrolyte interface w27x y ErmV vs. SCE

t m rsy1

10 3 Im rArcmy2

Arsy1

10y7 N0rcmy2

10 5 Drcm2 sy1

0.150 0.170 0.190 0.200 0.250 0.300 0.320 0.350

0.12 0.08 0.05 0.05 0.034 0.034 0.034 0.034

6.2 7.5 8.79 9.76 11.42 12.58 13.20 13.53

0.59 0.76 1.5 2.87 2.35 4.18 4.18 4.63

16.15 25.75 25.63 21.07 45.31 23.55. 21.52 18.50

1.92 1.90 1.71 1.85 1.80 2.10 2.41 2.53

Fig. 9. Family of current transients for deposition of Ag on vitreous carbon from 10y1 M AgŽI. in aqueous 1.6 M NH 4 OH q 1 M KNO 3 The potentials were Ža. y0.030 V, Žb. y0.050 V, Žc. y0.150 V, Žd. y0.200 V, Že. y0.250 V vs SCE.

Fig. 10. j vs. t 2 Ž^. and j vs. t 3 Žv . dependence for the rising part of the experimental current transient recorded at y0.250 V Žtransient Že. in Fig. 9..

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89

sients follow the behavior described and predicted by the theoretical model proposed by Armstrong et al. w34x. The model is based on 3D growth of the nuclei with a specific geometry. Each nucleus is treated as a well-defined circular cone which grows on a foreign substrate in a direction parallel to the surface with a rate constant Ž k 1 .. In the direction perpendicular to the substrate surface, nuclei grow with a different rate constant defined as Ž k 2 .. The growth proceeds as the nuclei physically overlap. Eqs. Ž9. and Ž10. could be employed to describe instantaneous and progressive nucleation, respectively, in this model w34x. jŽinstantaneous. s nFk 2 1 y exp jŽ progressive. s nFk 2 1 y exp

ž

ž

yp M 2 k 12 N0 t 2

r2

yp M 2 k 12 AN0 t 3 3r 2

/ /

Ž 9. Ž 10 .

All parameters mentioned in these equations were described previously. Evaluation of the type of nucleation using this model was slightly different than in the previous cases. Namely, the model does not offer a non-dimensional form of Eqs. Ž9. and Ž10. due to the difficulty of defining the exact value of tmax . To overcome this limitation we analyzed only the initial parts of the current transients, usually during a very short time, less than 1 s. At this early stage we supposed that overlap between nuclei does not take place, and therefore, the whole process is less complex than that described by Eqs. Ž9. and Ž10.. We compared our experimentally obtained current transients with calculated sets for instantaneous and progressive nucleation and plotted all data showing current density vs. t 2 and t 3 ŽFig. 10.. However, only the initial part of each current transient was available for analysis by this method. In Fig. 10, the nucleation process in our experiments at high concentration levels of AgŽI. could be defined as instantaneous Ž I is linear with t 2 .. Furthermore, the nucleation was followed by 3D growth, limited by adatom incorporation into the substrate lattice. At high and very low concentrations Ž10y1 M and 10y4 M. the growth was limited by the same process Žadatom incorporation into the substrate lattice.. Exclusively for deposition from 10y2 M AgŽI. solution, the nuclei growth was controlled by diffusion of the electroactive species in solution. This contradicts our cyclic voltammetric data, which supported diffusion control over the whole concentration range. The difTable 3 Kinetic parameters obtained from the experimental transients in Fig. 9 by means of a 3D instantaneous nucleation controlled by the lattice incorporation of adatoms w34x 2

10

7

y ErmV vs. SCE

10 Im r A cmy2

10 N0 k 2 k 12 rmol 3 cmy8 sy3

10 k 2 rmoly1 cmy2 sy1

0.150 0.70

1.2 1.0

1.50 0.85

4.98 4.39

Fig. 11. Surface morphology of the mechanically Ža and b. and electrochemically polished Žc. vitreous carbon electrode substrate, revealed by AFM, with typical polishing scratches. Image size: 15 m m=15 m m Ža. and 2 m m=2 m m Žb and c., with z-range: 0–60 nm.

ferent kinds of electrode perturbation of cyclic voltammetry and chronoamperometry, must be considered to account for this discrepancy. After establishing that silver nucleation from 10y1 M AgŽI. solution was instantaneous with 3D growth Žoverlap control., k 12 N0 k 2 parameters at different overpotential were calculated. These values are presented in Table 3. The value of the k 2 constant Žperpendic-

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M. Miranda-Hernandez et al.r Journal of Electroanalytical Chemistry 443 (1998) 81–93 ´

Fig. 12. AFM images show different electrode surface morphology for Ag deposits formed from ammonium bath containing: Ža. 10y3 M, Žb. 10y2 M and Žc. 0.3 M of AgŽI..

M. Miranda-Hernandez et al.r Journal of Electroanalytical Chemistry 443 (1998) 81–93 ´

ular growth. were found to be similar to literature results w5x. In general, the results obtained here demonstrate that silver deposition depends upon the active species concentration in the deposition bath, and proceeds by at least three different mechanisms. 3.6. Atomic force microscopy analysis The main motive for using the AFM in this study was to reveal surface morphological characteristics of silver deposits and mechanisms of deposition onto the VC electrode from solutions of different AgŽI. concentration. In order to distinguish morphological differences between the bare and the silver coated electrode surface, we first focused on characterization of the bare electrode substrate. A set of AFM images Žtopviews. in Fig. 11a,b, shows typical surface morphology of our VC electrode substrate during a preliminary step of preparation. Prior to using the electrode for deposition purposes, its surface was mechanically polished with alumina and activated by electrochemical methods. Following mechanical treatment the electrode surface was rough, with typical polishing lines Žscratches. running all over the electrode surface. They appeared to be about 15 nm deep. We later found that silver deposits tended to form along these scratches, indicating that the scratches acted as active sites for the deposition process. More details were visible in smaller images Ži.e., 2 m m = 2 m m. as presented in Fig. 11b. This image reveals that the VC electrode substrate consisted of nodule-like features, with average diameter of 50–70 nm. All observations are in agreement with previously published findings concerning STM analysis for the same or similar materials w35–38x. However our surface appeared to be smoother, most likely due to the sample preparation. The real substrate in our study was an electropolished VC electrode surface ŽFig. 11c.. The surface morphology of the electropolished electrode differed significantly from a mechanically polished surface w38–40x. After the electrochemical pretreatment we found that the electrode was drastically enriched in a number of surface features. Some kind of surface partitioning or surface roughness was introduced during the electrochemical treatment w41x. However, even on the electrochemically pretreated surface, the polishing scratches were still present. Also images revealed some kind of large protrusions, like large clouds, all over the electrode surface. Although, we were unable to identify the origin of the protrusions, we suppose they consist of a FeCN-complex deposited on the electrode’s top surface layer w42,43x. Thus the VC electrode surface seems to be, at least partially, covered by protective film. Indeed, images obtained in the early stages of the silver deposition process Žat lower silver concentrations. show the silver deposit between these big protrusions. However, in the later stage, a smoother surface, with silver clusters aligned along the polishing scratches, were observed com-

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monly. Fig. 12 presents three AFM images of the silver deposits formed from ammonium baths with different AgŽI. concentrations, Ža. 10y3 M, Žb. 10y2 M and Žc. 3 = 10y1 M. Deposits were prepared utilizing the chronocoulometric technique, during 1 s, with the electrode potential kept constant at y0.15 V Ža. and y0.1 V Žb and c.. As shown in the cyclic voltammograms presented in Fig. 1 and taking into account the shift of the deposition peak ŽI. by concentration, all samples have been prepared at the same overpotential. However, as revealed by AFM, they differ significantly in surface morphology characteristics. Small silver clusters along the sharp polishing scratches are dominant surface features on deposits prepared from the 10y3 M AgŽI. solution. Since the silver clusters possess a diameter similar to the nodule-like features of the bare VC substrate, it is quite difficult to distinguish the substrate from the silver deposit. Note that here we are using the real electrode material without a well-defined surface. It is, therefore, difficult to draw any firm conclusion about the nucleation type and the silver deposit growth. In fact, the image showing 2D growth is masked by the electrode surface roughness. Conversely, the silver deposit grown from 10y2 M AgŽI. solution, possesses its own morphology Žimage presented in Fig. 12b.. Numerous 3D silver clusters can be recognized on the uniformly covered and flat electrode surface. Most of them have developed along the polishing lines. Once again this emphasizes the influence of surface imperfections Žsuch as step edges or polishing scratches. on the course of the electrocrystallization process. The silver clusters are of similar size, on average three to four times larger than substrate features. All AFM images of silver deposits are of the same size Ž4 m m = 4 m m. and presented in the same 3D surface plot mode. Silver clusters which obviously grow on top of other clusters Žbirth and growth mechanism. can be recognized in the image presented in Fig. 12c. Such deposits were formed from 0.1 M and 0.3 M AgŽI. solutions. Therefore, the deposit appears to be rough. Surface corrugation Ž z-scale in the image. exceeds 500 nm. The size and orientation of the silver clusters, seem to follow random distribution with no evidence for strict alignment along the substrate defects. All clusters possess clear 3D characteristics. The early stages of such new cluster phase formation over the smooth deposit adlayer is also apparent in Fig. 12b. A few isolated silver 3D clusters were formed on the top of two neighboring rows. The silver deposits formed from an ammonia bath, could have different morphological characteristics, depending on the concentration of AgŽI. ions in the deposition bath. Indeed many of the deposit characteristics indicate the mechanism of silver deposition proposed by electrochemical analysis. Therefore, in combination with the above data concerning evaluation of the kinetics and mechanistic parameter, the AFM data strongly support the overall picture that the mechanism of silver electrocrystallization depends on the AgŽI. concentration in the deposition

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bath. To evaluate the mechanism of the metal deposition by the AFM technique alone, an in situ AFM with the possibility of direct monitoring of all phases of the deposition process, should be used. 4. Conclusions Silver electrodeposition from an electrolytic bath containing 1.6 M NH 4 OH and 1 M KNO 3 ŽpH s 11., over a AgŽI. concentration range between 10y4 and 0.3 M, was studied. Depending on AgŽI. concentration, several different silver nucleation processes over the VC surface were observed: 2D growth, controlled by adatoms incorporation; 3D growth controlled either by diffusion or lattice incorporation of adatoms. Qualitative and quantitative determinations of nucleation processes were performed based on results of cyclic voltammetry and chronoamperometry measurements. Each type of silver electrocrystallization process was defined quantitatively using the available theoretical methods. Our study clearly points out the way to evaluate the kinetics and mechanistic parameters from current transient data, with rather complex and unusual shapes. Here, we dealt with a system where the nucleation mechanism was dependent on the active species concentration in the deposition bath. Certainly different, even more complex systems, can be treated by the same approach. Initially, from the current transient shape, we attempted to determine which theoretical formalism was the most appropriate for evaluation of kinetics and mechanistic parameters from our experimental set of data. We then performed an ex situ visualization of the silver deposit on the electrode surface by AFM analysis to verify the deposit surface morphology according to our electrochemical findings. AFM observations, complemented the electrochemical data, clearly showing that silver deposits formed from baths with different concentrations of AgŽI. possess different surface morphological characteristics.

in which AgŽI. concentration varied from 10y4 to 0.3 M. However, the AgŽI. species was the same in the deposition bath at all concentrations. We demonstrated previously that the dominant species in this kind of silver ammonium bath w32x. is AgŽNH 3 .q 2 In general, the silver deposition from silver can be described by the following reactions: Agqq eym Ag 0

Ž 11 .

0 q E s EAg < Ag 0 q 0.06 log

q

w Ag x

0 q1 where: E s EAg rAg 0 s 0.56 V vs. SCE. The reaction of silver complexion in the ammonia bath is: q

Agqq 2NH 3 m Ag Ž NH 3 . 2

Ž 13 .

q

Kf s

Ag Ž NH 3 . 2

w Agq xw NH 3 x 2

s 10 7.5

q

Ag Ž NH 3 . 2 q eym Ag 0 q 2NH 3 0 0 y 0.12 log E s EAg ŽNH 3 .q 2 < Ag

Appendix A. Estimation of the equilibrium potential using the Nernst equation As described in the experimental part of our paper, silver deposition was carried out from an ammonium bath,

Ž 15 . q

w NH 3 x q 0.06 log Ag Ž NH 3 . 2 Ž 16 .

< 0 E is the equilibrium potential of the AgŽNH 3 .q 2 Ag sys0 tem and EAg ŽNH 3 .q2 < Ag 0 is the normal potential, which was calculated in our experimental conditions using Eqs. Ž12. and Ž14. and substituting in Eq. Ž16.. 0 0 s 0.56 y 0.06 log K s 0.135 V vs. SCE EAg ŽNH 3 .q f 2 < Ag

Ž 17 . From Eqs. Ž17. and Ž16., the expression of the equilibrium potential for silver deposition from the ammonium bath, at concentration of NH 3 s 1.6 M, was: q

We thank Dr. J.G. Ibanez ˜ from Depto. de Ingenierıa ´ y Ciencias Quımicas, Universidad Iberoamericana, Mexico, ´ who allowed us to use the AFM facilities. Financial assistance was received from CONACYT ŽProject 0913E-P; Catedra Patrimonial de Excelencia Nivel II, for N.B.., ´ ŽProjects: L0081-E9608 and 1779P-A9507.. M. M-H. and M. P-P. also acknowledge CONACYT for scholarship support.

Ž 14 .

where Ž K f . is the complexation constant w44x, Therefore, the silver deposition from the ammonium bath can be described as:

E s 0.11 q 0.06 Ag Ž NH 3 . 2 Acknowledgements

Ž 12 .

Ž 18 .

Eq. Ž18. is plotted in Fig. 3.

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