A Potentiostatic and Atomic Force Microscopy Study of the Nucleation

J. Phys. Chem. C 2007, 111, 17541-17550

17541

A Potentiostatic and Atomic Force Microscopy Study of the Nucleation and Growth Mechanisms of Certain Metallic Cyanometalates M. Orellana, R. Del Rı´o, R. Schrebler, and R. Cordova* Instituto de Quı´mica, Pontificia UniVersidad Cato´ lica de Valparaı´so, AVenida Brasil 2950, Valparaı´so, Chile ReceiVed: March 1, 2007; In Final Form: September 12, 2007

Nucleation and growth mechanisms (NGMs) of nickel(II) hexacyanoferrate, nickel(II) octacyanomolibdate, and copper(II) octacyanomolibdate were performed by means of the potential step technique by oxidizing the respective metal electrode in 0.02 M sulfuric acid containing the potassium salt of either hexacyanoferrate or octacyanomolibdate compounds. The obtained current-time transients (I/t) were analyzed using the theory of electrocrystallization of the metallic phase. As evidenced through these experiments, we can state that for the nickel cyanometalate compounds, electroformation takes place through a three-dimensional progressive NGM, and for the copper(II) octacyanomolibdate compound, electroformation occurs by means of a threedimensional instantaneous NGM. The electrochemical results were verified through the use of atomic force microscopy (AFM). From these results, we can conclude that AFM is a good complementary technique to determine the nature of the NGMs through which electroformation of a solid phase occurs on an electrode surface.

Introduction Transition metal cyanometalates constitute a class of compounds that can be represented by the following general formula: AaMb[Z(CN)c]d•eH2O. In this formula, a, b, c, d, and e represent stoichiometric numbers; A is an alkali metal cation, and M and Z are transition metals bonded respectively to the nitrogen atom and the carbon atom of the cyano group present in the structure of the compound. The value of the stoichiometric number c in the more common compounds can only be six or eight. Accordingly, these compounds can be designated as hexacyanometalates or octacyanometalates. Transition metal hexacyanometalates are compounds that lend themselves to a detailed study of insertion electrochemistry. Most of these compounds present reversible electrochemical behavior when studied under certain conditions, such as ambient temperature, in the presence of aqueous electrolyte solutions, and with scan rates up to some hundreds of millivolts per second. Moreover, hexacyanometalates belong to a class of mixed-valence compounds of which iron(III)-hexacyanoferrate(II) (Prussian blue) (where A ) K and M ) Z ) Fe in the above generic formula) is the classical example. Metal hexacyanoferrates (HCFs) have a face-centered cubic lattice with octahedral coordination of the M and Fe ions by cyanide ligands.1,2 The alkali metal cations, A, are located in the tetrahedral sites3 of these structures, which can contain water molecules and also some anions. The alkali metal cations, A, play an important role, acting as charge compensator agents during the electrochemical changes that affect both the M and Fe ions. The M/Fe ratio in the structure of metal HCF analogues depends on the respective oxidation states of both the M and the Fe. Several representative structures and their corresponding stoichiometric compounds have been formulated previously.4 A thermodynamic study of the electrochemical reactions of solid metal hexacyanometalates has been performed.5 In said * Correspondingauthor.Fax: +56-32-273422.E-mailaddress: [email protected].

study, relationships were established between the formal redox potential of the electrochemical insertion reactions presenting metallic hexacyanometalates and their solubility products and also between this potential and the structure parameters such as the ion potential (charge/ionic radii ratio) of the inserted ion, M, and the lattice constant. Transition metal hexacyanometalates form a class of zeolitic inorganic compounds that have been studied extensively because of their exceptional properties. Many of these compounds exhibit reversible insertion electrochemistry6-9 and possess electrocatalytic activity,10-16 electrochromism,17-23 and properties permitting their use in rechargeable batteries.24-28 Some transitionmetalhexacyanometalatesarehigh-temperatureferromagnets.29-36 Ion-exchange, ion-sensing, and photomagnetic properties (along with their associated applications) have also been mentioned for these compounds.4 Furthermore, other solid metal cyanometalates such as octacyanometalates (octacyanotungstates and octacyanomolibdates (OCMs)) have been studied, though to a lesser extent.37 In spite of the great number of published papers related to the properties and applications of metallic cyanometalates, studies related to the nucleation and growth mechanisms (NGM) of these compounds, deposited as phases on an electrode surface, have not been performed. Research in this area is important because it would provide interesting information about how these phases are formed on an electrode substrate, especially if certain properties of these cyanometalate compounds were found to be applicable to the modification of an electrode surface for a determined purpose. In the present paper, the NGMs of Ni-HCF, Ni-OCM, and Cu-OCM phases are determined by applying the methods of potentiostatic pulses and atomic force microscopy (AFM). The cyanometalate phases were formed by oxidizing the respective metal electrode in an electrolytic solution containing the corresponding cyanometalate potassium salt (HCF or OCM). Thus, the control of the ionic product in the interface electrodesolution (which permits the control of the precipitation of the

10.1021/jp0716942 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/26/2007

17542 J. Phys. Chem. C, Vol. 111, No. 47, 2007

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Figure 1. E/t program employed for the determination of the transients (I/t).

TABLE 1: Experimental Conditions for the NGMs of Nickel and Copper Cyanometalates compound

electrolytic media

E/t program

Ni-HCF

0.1 M K4Fe(CN)6 0.5 M K2SO4, pH 1.0 0.05 M K4Mo(CN)8 0.5 M K2SO4, pH 1.0 0.05 M K4Mo(CN)8 0.5 M K2SO4, pH 1.0

E1) -0.45 V for 60 s E2 values) -0.15, -0.14, -0.13, -0.12, -0.11, -0.10, and -0.09 V for 300 s E1) -0.30 V for 30 s E2 values) -0.08, -0.07, -0.06, -0.05, -0.04, -0.03, and -0.09 V for 60 s E1) -0.40 V for 30 s E2 values) -0.06, -0.05, -0.04, -0.03, -0.02, -0.01, and -0.009 V for 60 s

Ni-OCM Cu-OCM

metal cyanometalate phases) was possible by tuning the applied potential value. Experimental The electrochemical measurements were performed using a conventional electrochemical cell with three compartments. An Ag/AgCl electrode (saturated with KCl, E ) 0.197 V vs NHE) was used as a reference electrode, and platinum wire (0.5 cm2) was used as an auxiliary electrode. Nickel rods (5 mm diameter, 99.995% purity, manufactured by Johnson Matthey Chemicals Limited) and copper rods (2.5 mm diameter, 99.9999% purity, manufactured by Aldrich) were used as working electrodes to form the metallic cyanometalate. Before each measurement, the working electrode surface was mirror polished with alumina (0.02 µm) followed by a copious rinse with Milli-Q water. All the electrochemical experiments were carried out at room temperature (18-20 °C) under an argon atmosphere using a Zahner IM-6e potentiostat-galvanostat. Data acquisition and analysis were performed using the THALES package from Zahner Elektrik GMBH & Company. Ultrapure water with a resistivity of 18.2 MΩ was obtained from a Milli-Q water system (Millipore Corporation, Bedford, MA) and was used throughout for the preparation of solutions. Analytical-grade reagents and non-buffered solutions were also used. A solution of pH close to 1 was attained by adding sulfuric acid; this value was electrometrically controlled. The composition of the different solutions and other experimental parameters are shown in Table 1. Morphology studies of the working electrode surface were conducted ex situ with a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) using tapping-mode AFM. A silicon tip with a resonance frequency of 200-400 kHz was used. The formation of the different metallic cyanometalate films studied was carried out by means of the anodic polarization of the corresponding metal electrode in a proper electrolytic media (see Table 1). The E/t program applied is shown in Figure 1. Initially, the metallic electrode was polarized at a potential

step E1 for a predetermined length of time. As can be seen in Table 1, the potential value of E1 depends on the nature of the metal used for the working electrode. In principle, this potential value guarantees a clean and non-attacked electrode surface. However, for the present paper, this result was only attainable for the copper electrodes. Subsequently, an anodic potential step E2 was applied, which is more positive than the respective open circuit potential. At this potential value, the electroformation of the metallic cyanometalate phase takes place, meaning the corresponding transient (I/t) could be recorded. The potential values, E1 and E2, for the metallic cyanometalates studied were estimated from a previous linear potentiodynamic study, which was carried out using the corresponding metal/electrolytic solution interfaces at a potential sweep of 0.05 V s-1. Given the experimental conditions depicted in Table 1, a linear voltammetric scan was performed in the potential range of -0.45 to 0.6 V for the Ni-HCF system, -0.6 to 0.6 V for the Ni-OCM system, and -0.4 to 0.3 V for the Cu-OCM system. Results and Discussion Nucleation and Growth of the Ni-HCF Phase. Figure 2 shows the potentiodynamic profiles corresponding to a nickel rod electrode, taken in an electrolytic media containing 0.5 M K2SO4 (pH ) 1.0) both in the absence (circles) and in the presence (solid line) of 0.1 M K4Fe(CN)6. In the absence of HCF(II) ions, the potentiodynamic I/E profile (circles) shows a single anodic current contribution that starts at -0.230 V and has a current maximum located at 0.166 V. This current contribution represents the nickel electrooxidation process. In this electrolytic media, at the given initial potential value, the current contribution, in thermodynamic terms, corresponds to the active dissolution of the metal. Nevertheless, when the potential value reaches the point corresponding to the current maximum, the deposition of nickel(II)-oxygen-containing species (oxohydroxo compounds) can take place.38 The film formed on the electrode surface causes

NGMs of Ni(II)-HCF, Ni(II)-OCM, and Cu(II)-OCM

J. Phys. Chem. C, Vol. 111, No. 47, 2007 17543

Figure 2. Potentiodynamic profiles of the nickel electrode, taken in an electrolytic media containing 0.5 M K2SO4, pH ) 1.0 (circles) and 0.1 M K4Fe(CN)6, 0.5 M K2SO4, pH ) 1.0 (solid line). Potentiodynamic profiles were registered at a scan rate of 0.05 V s-1.

Figure 3. Anodic contributions to the potentiodynamic profile I/E during Ni-HCF film formation (circles) in an electrolytic media containing 0.1 M K4Fe(CN)6 and 0.5 M K2SO4 (pH ) 1.0). The potentiodynamic profiles were registered at a scan rate of 0.05 V s-1.

passivity of the nickel electrode, which explains the remarkable decrease in the observed anodic current. In the presence of HCF(II) anions (solid line), the peak in the anodic current contribution starts at -0.30 V, as shown in Figure 2. This potential value is more negative than that observed in the absence of the HCF anions; this is due to the electroformation of a Ni-HCF phase. This Ni-HCF formation occurs via a dissolutionprecipitation process of the Ni(II), and takes place when the ionic product of the Ni(II) and HCF(II) ions that are present in the interface attain a value greater than the solubility product constant for the Ni-HCF compound.5 This reaction can be expressed by the following global equation:

Ni + 2K+ + Fe(CN)64- f K2NiFe(CN)6 + 2e-

(I)

However, in the presence of HCF(II) ions, the I/E profile in Figure 2 represents a complex electrochemical process that cannot be represented by means of a single reaction. In this I/E profile, the presence of several processes that occur at different potential values can be distinguished. In order to show such processes, mathematical deconvolution of the profile was carried out. Three processes were postulated for the deconvolution, and it was performed using a nonlinear curvefitting program, and Gaussian curve shapes were assumed

Figure 4. Transients (I/t) for the electroformation of Ni-HCF in 0.1 M K4Fe(CN)6 and 0.5 M K2SO4 (pH ) 1.0) electrolytic solutions. The applied potentials are labeled in the figure.

according to the procedure previously reported.39 The result of the deconvolution of the profile (see Figure 3) shows two anodic current peaks located at 0.238 V (A1) and 0.123 V (A2) and a current contribution starting close to 0.350 V (A3). The current peak labeled as A1 in Figure 3 starts at -0.3 V and represents the process that is conducive to the formation of the Ni-HCF film. Additionally, current peak A2 starts at -0.06 V and extends to a potential value of 0.312 V, which is attributed to the electrodissolution and to the passivity of the nickel electrode surface caused by nickel(II)-containing oxygen species. It is important to note that this electrochemical process starts at a potential value of -0.06 V, which is 0.170 V higher than the profile obtained in the absence of the HCF(II) anion. The profile of A2 appears to be completely developed in a narrower potential interval. It is assumed that the electroformation of nickel-containing oxygen species takes place through the underlying Ni-HCF film. The current contribution labeled A3 starts at 0.350 V and is attributed to the oxidation of the Ni-HCF(II) film to Ni-HCF(III) according to the following reaction:40 [K2NiII[FeII(CN)6]](S) f [KNiII[FeIII(CN)6]](S) + K+ (aq) + e (II)

Taking into account the deconvoluted potentiodynamic profiles, it is evident that the potential range where the NiHCF phase can be formed, free from the interference of nickeloxygen-containing species, is located between -0.150 V and -0.090 V. Therefore, by applying the E/t program shown in Figure 1, the corresponding transients (I/t) were registered at the potential values shown in Figure 4. The transients (I/t) show the typical shape of a nucleation process. At a very early stage (t < 1 s), double-layer charging occurs and the current increases as a result of the formation and growth of the Ni-HCF nuclei. The instantaneous current rises until it reaches a maximum value and finally tends to decrease slightly except when the applied potentials were -0.150 V and -0.140 V (transients (I/t) 1 and 2). This fact is associated with the coalescence of the growing nuclei, since, according to the Avrami theorem, when the nuclei overlap they cause a decrease in the electrode area.41 Consequently, the current associated with the formation of the Ni-HCF phase also decreases. When the potential value applied is either -0.150 V or -0.140 V, equilibrium between the formation and dissolution processes of the nuclei may occur, which would explain why, under these conditions, a current maximum is not attained.


Orellana et al.

Figure 5. Non-dimensional plot for the NP3D mechanism (dotted line), the NI3D mechanism (circles), and the experimental data (solid line) taken from the Ni-HCF phase on a nickel electrode. The data was obtained from the transients (I/t) at (A) -0.130 V and (B) -0.100 V.

Furthermore, the transients (I/t) show that when the overpotential is increased, a decrease in the induction time ti can be observed. The induction time is the time at which the current of the transient (I/t) starts to increase, and, at this time, the first nuclei of the forming phase appear.41 When the overpotential is increased, there is a greater availability of Ni(II) ions in the electrode/electrolytic solution interface, and therefore the ionic product value that guarantees the precipitation of the Ni-HCF phase is attained in less time. All the registered transients (I/t) show a constant current value greater than zero for t > 150 s. This fact suggests the presence of three-dimensional (3-D) growth, which could be controlled by means of a diffusion or charge-transfer process. To analyze the above transients (I/t), the corresponding non-dimensional parameters I/Im and t/tm were obtained and compared with the respective parameters for the theoretical NGMs. These respective parameters are the instantaneous 3-D (NI3D) (eq III) mechanisms and the progressive 3-D (NP3D) (eq IV) mechanisms.41

( ) ( ( ) (

) )

I 2 1.9542 ) [1 - exp-1.2564(t/tmax)]2 Imax t/tmax I

Imax

2

)

1.2254 [1 - exp-2.3367(t/tmax)2]2 t/tmax

(III) (IV)

In the above equation, Imax is the maximum current value attained in the transient (I/t), and tmax is the time at which this maximum value occurs. I and t are the current and time values for all values I * Imax and t * tmax.

Figure 5 shows two non-dimensional plots for the transients (I/t) obtained at the potential values of -0.130 V (A) and -0.100 V (B). In both graphs it can be observed that the experimental data (solid line) lies close to the theoretical plot corresponding to an NP3D mechanism (dotted line), particularly on the left side of the maximum and near the maximum. Nevertheless, this analysis is approximate because an instantaneous bi-dimensional nucleation process (NI2D) cannot be a priori rejected. This is due to the possible presence of specific adsorption (chemisorption) of HCF anions on the clean nickel substrate, which is caused by the chemical affinity established between the nickel(II) ions of the crystal structure and the HCF ions of the solution. Therefore, when the nickel(II) ions are released from the crystal structure during the potential pulse, it may be the case that they are spontaneously reacting with the anions chemisorbed on the electrode surface, allowing an NI2D process to take place. Nevertheless, adsorption measurement to help confirm this assumption was not considered in the present study. Furthermore, after the maximum is reached, the correspondence of the two graphs is not good, as a significant deviation from the theoretical models can be seen. With the aim of explaining the complexity of the Ni-HCF electroformation process, the next step was to fit the experimental transients (I/t) to a theoretical model that considers the following current transient contributions:39,42,43 (i) an NI2D process (eq V), (ii) an NP3D process with a charge-transfer control (NP3Dct) (eq VI), and (iii) an NP3D process with a diffusion control (NP3Ddif) (eq VI). The respective mathematical expressions for these NGMs are as follows: (i) NI2D

INI2D ) P1t exp-P2t

2

(V)

where P1 ) (2πnFhN°K2)/F, and P2 ) (πM2N°K2)/F2. (ii) NP3Dct

INP3Dct ) P3[1 - exp-P4t ] 3

(VI)

where P3 ) nFK′3, and P4 ) (πM2K32A3N°)/3F2. (iii) NP3Ddif

INP3Ddif )

P5 t

1/2

[1 - exp-P6t ] 2

(VII)

where P5 ) (nFD1/2C∞)/π1/2, and P6 ) A′πKD. The meaning of each term present in the above equations is as follows: INP2D represents the current transient contribution associated with the NP2D process under activated control. INP3Dct represents the current transient contribution associated with the NP3D process under activated control. INP3Ddif represents the current transient contribution associated with the NP3D process under diffusion control. n represents the number of electrons involved in the reaction. F represents the Faraday constant (C mol-1). M represents the molar weight of the electroformed phase (g mol-1). h represents the thickness of the monolayer deposits (cm). A represents the rate constant for nucleation (s-1). N° represents the number of sites where the nucleation can occur (cm-2).



Figure 7. Logarithmic I/t plot corresponding to experimental data obtained at -0.130 V at the initial time of electrode polarization in 0.1 M K4Fe(CN)6 and 0.5 M K2SO4 (pH ) 1.0) electrolytic solutions.

Figure 6. Transients (I/t) obtained with a Ni electrode in 0.1 M K4Fe(CN)6 and 0.5 M K2SO4 (pH ) 1.0) at potentials of (A) -0.130 V and (B) -0.100 V. The solid line represents the experimental curve. Open circles represent the theoretical curve that results from summation of the (1) NI2D, (2) NP3Ddif, and (3) NP3Dct current contributions.

K represents the rate constant for the bi-dimensional growth of the nuclei (mol cm-2 s-1). K3 represents the rate constant for the parallel growth of nuclei in a 3-D growth mechanism (mol cm-2 s-1). K3′ represents the rate constant for the perpendicular growth of nuclei in a 3-D growth mechanism (mol cm-2 s-1). F represents the density of the deposited material (g cm-3). C∞ represents the concentration of the chemical species involved in the formation of the deposited phase (mol cm-3). D represents the diffusion coefficient of the chemical species involved in the formation of the deposited phase (cm2 s-1). Figure 6 shows the transients (I/t) obtained at -0.130 V (A) and -0.100 V (B). These potential values correspond to the negative and positive limits in which the formation of a NiHCF phase should occur without interference from any other process. In general, all the transients (I/t) obtained could be fitted adequately to the experimental data taking into account the existence of the three current transient contributions mentioned above. It is important to note that all current contributions start simultaneously from induction time ti ) 0. The first current contribution (1), which is the smallest, is related to an NI2D mechanism under activated control. This affirmation is based in the fact that, in the logarithmic plot of the current versus time for initial time values (Figure 7), the gradient of the slope obtained was 0.83. According to eq V, this value has a theoretical ideal of 1; however, since the given value is close to 1, it can be assumed that it refers to an NI2D process.39 This current contribution is completely developed in a short period of time (t < 50 s) and corresponds to the

formation of a Ni-HCF compound with the participation of HCF(II) anions directly adsorbed on the clean electrode surface. The second current contribution (2), which is higher than the first one, is present during all the transients (I/t). This current contribution is associated with an NP3D mechanism under diffusion control. In this case, the diffusion character of the current component is associated with the movement of nickel(II) ions, which are electrogenerated during the pulse, through a film, assumed to be irregular, that partially covers the surface of the electrode (vide infra). Therefore, once the nickel(II) ions arrive at the interface of the electrode/electrolytic solution and when an ionic product higher than the product solubility constant of the Ni-HCF compound is attained, the corresponding precipitation takes place. Finally, the largest current contribution, denoted as 3, which is also present in all the transients (I/t), represents an NP3D mechanism under charge control. In this case, it is possible to assume that the corresponding process occurs in the electrodic regions, where the underlying film is thinnest. Given the condition of the film, the diffusion of nickel ions that takes place through it does not appear to be the ratedetermining step of the process. It can be observed that the sum of all the partial current contributions adequately fits the experimental transients (I/t). The analysis of the adjusting parameters, P, presented in the above equations and used to determine the NGMs of Ni-HCF (data not shown), shows that, for those parameters directly related to the number of nuclei (P1, P2, and P4 parameters), greater variation is exhibited by their values at higher potential levels. This fact can be explained considering that, when the potential increases, the quantity of available nickel(II) ions required for the apparition of the Ni-HCF nuclei also increases. Nucleation and Growth of the Ni-OCM Phase. When the HCF ions are exchanged for OCM ions in the electrolytic solution, the voltammetric response of the Ni/electrolytic solution interface (see Figure 8) changes appreciably with respect to the previous results observed in the presence of HCF ions. Remarkable passivity of the nickel electrode can be observed in the presence of OCM ions. This fact is responsible for the corresponding I/E potentiodynamic profile (solid line in Figure 8) appearing shifted toward more positive potential values, whereas lesser current values are involved in the process of Ni-OCM formation. As is shown in both of the linear voltammetric profiles depicted in Figure 8, in the range of -0.08 to -0.03 V, only electroformation of the Ni-OCM compound takes place, as it


Orellana et al.

Figure 8. Potentiodynamic profiles of the nickel electrode taken in 0.5 M K2SO4 (pH ) 1.0) (circles) and 0.05 M K4Mo(CN)8, 0.5 M K2SO4 (pH ) 1.0) (solid line) electrolytic solutions. Potentiodynamic profiles were registered at a scan rate of 0.05 V s-1.

Figure 10. Transients (I/t) obtained with a Ni electrode in 0.05 M K4Mo(CN)8 and 0.5 M K2SO4 (pH ) 1.0) electrolytic solutions at potentials of (A) -0.08 V and (B) -0.03 V. The solid line represents the experimental curve. Open circles represent the theoretical curve that results from the summation of (1) NI2D, (2) NP3Ddif, and (3) NP3Dct current contributions. Figure 9. Transients (I/t) for the electroformation of Ni-OCM in 0.05 MK4Mo(CN)8 and 0.5 M K2SO4 (pH ) 1.0) electrolytic solutions. Potentials applied are labeled in the figure.

can be observed free from the interference of the formation of nickel-oxygen-containing species. For this reason, the above potential range was chosen to be applied to the E/t program from Figure 1. The corresponding transients (I/t) are shown in Figure 9 and were registered at the potential values labeled. For all the transients (I/t) in Figure 9, an analysis similar to the one previously described was carried out in this study. According to this analysis, the following results were achieved: (i) The NGM for the Ni-OCM compound takes place by means of an initial NI2D mechanism followed by NP3D mechanisms. (ii) All the transients (I/t) were fitted to the experimental data by considering the existence of three current contributions: INI2D under activated control, INP3D under activated control, and INP3D under diffusion control. (iii) The relative importance of the different current contributions in the NMG process was similar to that observed in the NGM study of the Ni-HCF phase (i.e., INP3Dct > INP3Ddif > INI2D). Figure 10 shows two representative transients (I/t) obtained at the potential values -0.08 V (A) and -0.03 V (B) fitted to the experimental data using the three current contributions mentioned above. Once again, as was obtained in the Ni-HCF study, a good fit can be observed between the experimental and simulated data.

AFM Study of the Ni Cyanometalate Films. In order to observe the main morphological changes that took place on the nickel electrode surface during the Ni cyanometalate phase formation (Ni-HCF or Ni-OCM), 3-D AFM images of the electrode surface were generated for each of the different steps of the processes. The different AFM images are shown in Figures 11-14. They were obtained using a measurement area of 1 µm2. Figure 11A shows the morphology of the polished nickel surface; a flat surface with clearly defined furrows due to the abrasive effect of the alumina. When the nickel electrode was initially polarized, in the range of -0.80 to -0.30 V covering the immunity and corrosion regions38 in the absence of cyanometalate anions, the nickel surface basically maintained the morphology previously described. However, an increase in the nickel surface roughness was observed (Figure 11B,C) as a result of the corrosion of the metallic surface. In fact, the rootmean-square (rms) values obtained for the corresponding AFM images changed from 2.889 nm (Figure 11A) to 4.465 nm (Figure 11B) and 6.606 nm (Figure 11C). The presence of HCF or OCM anions during the polarization at the potential E1 caused the surface morphology of the nickel electrode to change dramatically (see Figure 12A,B) compared to the image of the nickel electrode surface shown in Figure 11. The AFM images show an irregular and globular surface due to the presence of a compound disposed on the electrode surface. This compound of nickel cyanometalate was formed during the polarization of the electrode at the applied potential value. It was observed that the morphological changes of the nickel electrode surface, in the presence of cyanometalate ions, depended on the potential conditions applied to the electrode.



Figure 11. AFM images of the polished Ni electrode surface (A) and the Ni electrode surface polarized at -0.8 V (B) and -0.3 V (C) in 0.5 M K2SO4/H2SO4 (pH ) 1.0) electrolytic solution.

Figure 13. AFM images of the Ni electrode surface polarized in the presence of potassium cyanometalate salt under the conditions (A) -0.080 V, 0.01 s polarization time, 0.03 M potassium OCM (IV), and (B) -0.140 V, 0.2 s polarization time, 0.1 M potassium HCF(II).

Figure 12. AFM images of the Ni electrode surface, polarized in the presence of cyanometalate anions, in immunity (A) and corrosion (B) potential thermodynamic conditions.

The morphological changes of the nickel electrode caused an increase in the rms values of the images obtained under the different potential conditions. In fact, the rms values of the image of the Ni electrode surface, polarized in the presence of the OCM anion, changed from 1.795 nm (at E1 ) -0.80 V) to 12.889 nm (at E1 ) -0.45 V). Similarly, in the presence of the HCF anion, the rms value for the Ni electrode surface image changed from 3.630 nm (at E1 ) -0.80 V) to 9.640 nm (at E1 ) -0.45 V). The above values are concordant with the fact that the surface film formed at -0.80 V appeared with a less globular appearance than those formed at -0.45 V. The superficial nickel(II) cyanometalate compound, formed at -0.80 V, is the product of a corrosion process involving the reduction of protons present in the electrolytic media and the dissolution of the metallic nickel to Ni(II) ions, which are subsequently precipitated by the cyanometalate ions also present in the solution. When the nickel electrode potential was increased to a level over the E1 value (-0.4 V), electrochemical formation of the Ni(II) cyanometalate compound was observed on the electrode surface. This process occurred simultaneously, on the uncovered regions of the nickel electrode and on the film already formed at potential value E1. The previous fitting procedure applied to the transients (I/t) for the nickel electrode in the presence of cyanometalate anions showed that there are three NGMs during the precipitation of the respective compound on the electrode surface. The first mechanism (NI2D) occurs during the first polarization period and can be associated with the deposition of the Ni(II) cyanometalate film on the uncovered nickel sites. In the AFM images (see Figure 13), this mechanism could be responsible for the appearance of the flatter regions (valleys) that can be

Figure 14. AFM images of the Ni electrode surface, obtained under the following conditions: (A) left: 0.080 V, 0.02 s polarization time, 0.03 M potassium OCM(IV); right: -0.140 V, 0.5 s polarization time, 0.1 M potassium HCF(II). (B) left: -0.080 V, 0.03 s polarization time, 0.03 M potassium OCM(IV); right: -0.140 V, 0.7 s polarization time, 0.1 M potassium HCF(II).

observed in the deposit. It is important to note that, in the case of the Ni-OCM deposit (Figure 13A), flat regions are more evident than in Ni-HCF deposits (Figure 13B). This observation is concurrent with the respective transients (I/t) previously shown. In fact, in the case of the Ni-OCM transient (Figure 10), it can be observed that the NI2D contribution reaches a stabilized state in less time than in the case of Ni-HCF (Figure 6). Additionally, the AFM image of Figure 13 also shows, in both cases, the presence of 3-D growth regions. This latter observation is consistent with the fact that the fitted transients (I/t) (Figures 6 and 10) contain 3-D current contributions. Furthermore, the progressive character of the 3-D current contributions, considered in the fitting procedure, can also be observed in Figure 14A,B. As can be seen in these AFM images, the number of growth nuclei of the electroformed compound increased with the polarization time until the nickel electrode


Figure 15. Potentiodynamic profiles of a Cu electrode taken in an electrolytic media containing (1) 0.5 M K2SO4 with pH ) 1.0, and (2,3) 0.03 M K4Mo(CN)8 and 0.5 M K2SO4 with pH ) 1.0. Potentiodynamic profiles were taken at a scan rate of 0.05 V s-1.

surface appeared to be fully covered by the respective Ni(II) cyanometalate compounds. Nucleation and Growth of Cu-OCM Phase. Taking into account the previously discussed results for Ni(II) cyanometalate along with the fact that copper is a more noble metal than nickel and also considering the similar behavior of HCF and OCM anions as precipitating phase agents with metals, the next phase of the research was the study of the NGMs corresponding to the electroformation of the Cu(II)-OCM compound. A similar procedure to that used for the Ni(II) cyanometalate compounds was performed. Figure 15 shows a linear voltammetric profile of a copper electrode obtained using an electrolytic media containing 0.5 M K2SO4/H2SO4 with pH ) 1.0 in the absence (profile 1) and in the presence (profile 2) of 0.03 M of K4Mo(CN)8 registered at 0.05 V s-1. Profile 1 corresponds to the active electrodissolution of the copper electrode, which appears as an exponential current contribution starting at 0.0 V. In the presence of OCM anions, the I/E profile that starts at a potential close to -0.1 V shows an anodic current peak located at 0.150 V, which is attributed to the electroformation of a Cu(II)-OCM compound. The decrease in the current peak and the current corresponding to the hydrogen evolution reaction in the successive scans (profile 3) suggest that the Cu(II)-OCM compound promotes passivity on the electrode surface. The most remarkable observation is the existence of a wide potential range (0.1 V) free from interference from the active electrodissolution of the metal where the NGM study of the electroformation of the Cu(II)-OCM can be performed. Figure 16 shows the transients (I/t) obtained under the experimental conditions indicated in Table 1. As can be seen in Figure 16, for higher t values, the current values of the transients (I/t) obtained tend to constant current values greater than zero. Moreover, the inset of Figure 16 shows a decrease in the induction times for the apparition of the first nuclei of the Cu(II)-OCM phase, as the potential value increases. This is a consequence of a greater availability of copper ions, which allows the value of the ionic product required for the phase precipitation to be reached at a lesser time when the potential value is increased. According to these facts, it is possible to conclude that the NGM for the Cu(II)-OCM phase takes places by means of a 3-D growth mechanism under diffusion control.40,41 Furthermore, beyond the transients’ maximum, the current starts to decrease until it reaches a constant current value. The

Orellana et al.

Figure 16. Transients (I/t) for the electroformation of Cu-OCM in 0.03 MK4Mo(CN)8 and 0.5 M K2SO4 (pH ) 1.0) electrolytic solutions. Potentials applied are labeled in the figure. The inset displays the induction time/E plot for the transients (I/t).

Figure 17. Non-dimensional graph of the NP3D mechanism (dotted line), the NI3D mechanism (circles), and the experimental data (solid line) for the Cu-OCM phase on a Cu electrode obtained from the transients (I/t) at (A) -0.03 V and (B) -0.01 V.

stationary current obtained is due to the growth of the initially electrodeposited film. To analyze the transients (I/t) of Cu(II)OCM electroformation, a corresponding non-dimensional graph of I/Im versus t/tm for the NI3D (III) and NP3D (IV) mechanisms was constructed using the same procedure mentioned above for the Ni(II) cyanometalate compounds. Figure 17 shows the results obtained at -0.03 V (Figure 17A) and at -0.01 V (Figure 17B). Considering the excellent agreement between the experimental and the theoretical data, it is demonstrated that the NGM for the Cu(II)-OCM electroformation takes place by means of an NI3D mechanism. Later, the experimental transients (I/t) obtained at different potential values were adjusted to fit eq VIII, which represents



Figure 19. AFM images of the Cu electrode surface under the following conditions: polished (A) and polarized in 0.03 M K4Mo(CN)8 and 0.5 M K2SO4 (pH ) 1.0) at -0.060 V for 0.2 (B), 0.5 (C), and 0.7 s (D).

Figure 18. Transients (I/t) obtained with a Cu electrode in 0.03 M K4Mo(CN)8 and 0.5 M K2SO4 (pH ) 1.0) at potentials of (A) -0.03 V and (B) -0.01 V. The solid line represents the experimental curve. Open circles represent the theoretical curve of NI3Ddif current contributions.

the variation of the current intensity with time when the NGM is of type NI3D under diffusion control:

1 INI3Ddif ) P1 1/2[1 - exp-P2t] t

(VIII)

where the parameters P1 and P2 correspond to the following expressions containing the terms previously described:

P1 )

(

)

[

]

nFD1/2C∞ 8πC∞M ; P2 ) (N°πKD)yK ) 1/2 F π

1/2

AFM Study of Cu(II)-OCM Film. The main morphological changes that take place on the copper electrode surface during the electroformation of the Cu-OCM phase are shown in the AFM images in Figure 19. A gradual change in surface morphology can be observed. This change is evidenced by the increases in the rms values for the corresponding AFM images, changing from 4.246 nm (Figure 19A), to 6.092 nm (Figure 19B), to 6.224 nm (Figure 19C), and finally to 8.364 nm (Figure 19D). Image 19A is an image of the polished copper surface where the clearly defined furrows are due to the preliminary abrasive treatment. Image 19B shows the apparition of numerous, small 3-D growth nuclei that completely cover the electrode surface. In the following images (19C and 19D), it can be observed that the initial quantity of the growth nuclei apparently does not increase, and only the growth of existing ones takes place. It is important to mention that the above pictures were obtained after polarizing the

electrode at a potential of -0.060 V for 0.7 s. In order to explain the above results, specific adsorption of the OCM anions on the surface copper atoms has been assumed. Moreover, as in the study of the NGM of the Ni cyanometalate phases, it can be observed that there exists good agreement between the results of the AFM measurements performed on the electrode surface after its polarization and the mathematical approach of the NGM models used to fit the data to the experimental transients (I/t). Conclusions The NGMs of nickel and copper cyanometalate compound electroformation, established by means of the potential step technique, showed an excellent correlation with the AFM images acquired from the electrode surface after the electroformation of the respective cyanometalate phases. The NGM study revealed that the progressive or instantaneous nature of the mechanism was dependent on the initial state of the electrode surface. Therefore, when the electrode was partially covered by a preliminary compound film (as in the case of nickel), a progressive nucleation mechanism took place for the electroformation of the corresponding Ni cyanometalate compound. On the other hand, when the electroformation of the cyanometalate phase took place on a clean surface free from previously deposited film (as in the case of copper), an instantaneous nucleation mechanism took place. In this case, previous specific adsorption of the cyanometalate anions is necessary in order to favor this type of NGM. Acknowledgment. We gratefully acknowledge the financial support from FONDECYT-Chile (Project 1040837) and DGIPUCV (Project 125766/04). M.O. thanks DGI-PUCV for the young researcher contract. References and Notes (1) Ludi, A.; Gudel, H. U.; Dunitz, J. D. In Structure and Bonding;Springer-Verlag: Berlin, 1973; Vol. 14, pp 1-21. (2) Herren, F.; Fisher, P.; Ludi, A.; Ha¨lg, W. Inorg. Chem. 1980, 19, 956-960. (3) West, A.R. Basic Solid State Chemistry; Wiley: Chichester, U.K., 1999; Chapter 1, p 25.

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A Potentiostatic and Atomic Force Microscopy Study of the Nucleation

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