Silver nucleation on mercaptoacetic acid covered gold electrodes

Electrochimica Acta 52 (2007) 4818–4824

Silver nucleation on mercaptoacetic acid covered gold electrodes Nianjun Yang a,∗,1 , Xiaoxia Wang b , Qijin Wan c,∗ a

Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba 305-8568, Japan b Graduate School of Engineering, University of Fukui, Fukui 910-8507, Japan c Hubei Key Lab of Novel Reactor and Green Chemical Technology, School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, China Received 10 December 2006; received in revised form 12 January 2007; accepted 18 January 2007 Available online 27 January 2007

Abstract Silver nucleation on the mercaptoacetic acid coated gold electrode under potentiodynamic and potentiostatic conditions was investigated using cyclic voltammetry, chronoamperometry, scanning electron microscopy, and mathematical simulations. Silver was easily adsorbed on the modified electrode surface due to the complexation and electrostatic interaction of silver with the carboxyl groups of mercaptoacetic acid. The rate constants for the electrochemical reduction and oxidation of silver on the mercaptoacetic acid coated gold electrode were evaluated. The variation of the site density of silver nucleus with the deposition potential, the dependences of the nucleation rate on the deposition potential and on the driving force (the concentration of silver) were investigated. The deposition potential affected the activity of the nucleation sites but the driving force did not. These variations were explained by the existence of a distribution of site energies (mercaptoacetic acid molecules) on the electrode surface. © 2007 Elsevier Ltd. All rights reserved. Keywords: Electrodeposition; Nucleation; Silver; Mercaptoacetic acid; Self-assembly

1. Introduction Chemically modified electrodes have been utilized widely to electrodeposit metal film and/or particles in that the artificially composed structures on the electrode surface are favorable to been used as sensors, to prepare metal–organic–metal sandwich structures for electronic applications, and as molds/replica for material sciences. Taking silver electrodeposition as an example, the reported chemically modified electrodes have covered the iodine covered platinum electrode [1], polymer films coated electrode [2–11], thiols self-assembled monolayer modified gold electrodes [12–16]. For instance, Ortega and coworkers [2] investigated silver electrodeposition on polymer film coated platinum electrodes and suggested that silver electrodeposition occurred at the interface of the polymer and the electrode. Yoneyama and co-workers [12–14] pointed out that the under potential deposition of silver initiated at defects of

∗

Corresponding authors. Tel.: +81 29 861 5080; fax: +81 29 861 2771. E-mail addresses: [email protected] (N. Yang), [email protected] (Q. Wan). 1 ISE member. 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.01.017

self-assembled monolayers of thiols and then grew laterally with the intervention of silver between thiol molecules and the Au substrate. Esplandiu and co-workers [15,16] examined the effect of thiols on silver electrodeposition by changing terminally functional groups and chain lengths of thiols which were self-assembled on the gold electrode surface and confirmed the possibility of constructing a flat silver film on the top of thiols. Self-assembled monolayer (SAM) modified gold electrode has been proved to be one of the best candidates to reach the practical applications mentioned above because it has its unique advantages (e.g. ease of preparation, highly ordered structure, and well-documented properties [12–17]). For the metal film/particles deposition, the presence of terminally functional groups is promising to make the deposition of metal film/particles occur at the top of self-assembled layer since it is possible to have an interaction of the deposited species with terminally functional groups of thiols which were self-assembled on the gold electrode [15,16]. This interaction will reduce the possibility of diffusion of deposited particles into self-assembled layer [17–21]. The initial nucleation at the top of SAM has also been confirmed by employing various types of thiols and by using different techniques [22–27]. Among these techniques, scanning probe microscopy (e.g. scanning tunnel microscopy)

N. Yang et al. / Electrochimica Acta 52 (2007) 4818–4824

4819

[26,27] has been extensively utilized because scanning probe microscopy provides the direct images in a micro- to nano-meter range about the initial nucleation of particles on the surface of modified electrodes. However, a detailed investigation on the nucleation process of metal particles on short-chained thiols covered gold electrodes is still missing. Herein, we prepared a self-assembled monolayer modified gold electrode with a short-chained thiol (mercaptoacetic acid) and investigated silver nucleation on the mercaptoacetic acid coated gold electrode under potentiodynamic and potentiostatic conditions by use of cyclic voltammetry, chronoamperometry, scanning electron microscopy, and mathematic simulations. 2. Experimental 2.1. Chemicals and apparatus Silver nitrate, mercaptoacetic acid, and l-dithiothretitol (Shanghai 1st Chem. Com., Shanghai, China) were used as received. Other chemicals were of analytical grade without further purification. The stock silver solution (1.1 mM) was prepared with triply distilled water from silver nitrate. The buffer was 0.1 M KNO3 aqueous solution of which pH value was adjusted with 1.0 M nitric acid. The solutions for silver electrodeposition were bubbled with highly purified nitrogen gas for at least 20 min to remove oxygen. Nitrogen atmosphere was kept during measurements. Electrochemical experiments were performed on a CHI 600 electrochemical workstation (CH Instrument, Austin, USA) with a conventional three-electrode system at room temperature (25 ± 2 ◦ C). A platinum wire and an Ag|AgCl electrode in 3 M sodium chloride solution were used as the counter and reference electrode, respectively. The potentials mentioned in the paper were against this reference electrode. A bare gold electrode 2 mm in diameter or the mercaptoacetic acid coated gold electrode was used as the working electrode. The detailed procedure of the preparation and characterization of the mercaptoacetic acid coated gold electrode has been described elsewhere [28,29]. Briefly, after polishing and cleaning, the gold electrode was activated electrochemically in 0.1 M sulfuric acid within the potential domain of −0.2 to +1.5 V at a scan rate of 0.1 V s−1 . Its real surface area was evaluated to be 0.0278 cm2 and surface roughness factor was calculated to be 1.2 ± 0.1. The gold electrode was then immersed into 10 mM mercaptoacetic acid ethanol solution for about 40 min. After rinsed carefully with twice distilled water, the mercaptoacetic acid coated gold electrode was fabricated and then characterized regarding its capacitance, the amount of the adsorbed mercaptoacetic acid, working potential window, and electrochemical reactivity towards redox couples. Ex situ scanning electron microscopic (SEM) images of a gold foil (0.027 cm2 in area, which was self-assembled with the monolayer of mercapatoacetic acid in the same procedure as described above) before and after silver deposition were recorded on a SEM and EDX integration system (Hitachi, Japan) operated at 10 kV. Silver electrodeposition on the mercaptoacetic acid coated gold foil was performed in 1.1 ␮M silver solution

Fig. 1. Cyclic voltammograms of the mercaptoacetic acid coated gold electrode in the presence of 1.1 ␮M silver (solid line) and the absence of silver (dashed line) in the pH 1.81 buffer solution and cyclic voltammogram (dashed-dotted line) of 1.1 ␮M silver solution (pH 1.81) on a bare gold electrode at a scan rate of 0.1 Vs−1 .

(pH 1.81) at −0.17 V for 2 s. SEM images of the mercaptoacetic acid modified gold foil after silver deposition at −0.17 V for 2 s and dissolution at 0.2 V for 2 s in 1.1 ␮M silver solution (pH 1.81) were also recorded. 3. Results and discussion 3.1. Potentiodynamic deposition and dissolution of silver on mercaptoacetic acid coated gold electrodes Fig. 1 shows cyclic voltammograms of the mercaptoacetic acid coated gold electrode in pH 1.81 KNO3 buffer solution in the presence of 1.1 ␮M silver (solid line) and the absence of silver (dashed line). The scanned potential window was from 0.3 to −0.3 V with an initial potential of 0.3 V. The scan rate was 0.1 V s−1 . The voltammogram (solid line) of silver on the mercaptoacetic acid covered gold electrode showed a cathodic wave at −0.17 V, a sharp anodic wave at 0.01 V, and a broad anodic wave around at 0.15 V. The anodic waves at 0.01 and 0.15 V still appeared even in a blank solution after pre-electrolysis of silver onto the mercaptoacetic acid coated gold electrode at −0.1 V for several seconds. The anodic peak currents also enhanced with an increase in the pre-electrolysis time. The voltammogram (dashed line) of the mercaptoacetic acid coated gold electrode in the blank solution without silver only showed characteristic capacitive current, indicating no interference of hydrogen evolution. Therefore, the cathodic peak in the voltammogram (solid line) is due to the electrochemical reduction (electrodeposition) of silver on the mercaptoacetic acid coated gold electrode and two anodic waves are due to the electrochemical oxidation (dissolution) of deposited silver from the modified electrode surface. In contrast, the voltammogram of silver on a bare gold electrode (dashed-dotted line) exhibited a cathodic wave at 0.43 V due to the reduction of silver and a sharp and symmetric anodic peak due to the oxidation of silver around 0.65 V. The difference of the peak potentials for the reduction and oxidation of silver on the bare gold electrode from those on the mercaptoacetic acid coated gold electrode possibly results from the changed surface properties of electrodes (e.g. higher resistance for the mercapa-

4820

N. Yang et al. / Electrochimica Acta 52 (2007) 4818–4824

Fig. 3. Dependence of the peak currents density, jp , for the anodic wave at 0.01 V (squares) and for the cathodic wave (circles) on scan rate, v. The peak current density was obtained from cyclic voltammograms of 1.1 ␮M silver solution (pH 1.81) on the mercaptoacetic acid coated gold electrode at different scan rate.

Fig. 2. Ex situ SEM images of the surface of the mercaptoacetic acid covered gold foil (a) before silver electrodeposition, (b) after silver electrodepostion at −0.17 V for 2 s in 1.1 ␮M silver solution (pH 1.81) and (c) after silver electrodepostion at −0.17 V for 2 s and then dissolution at 0.2 V for 2 s in 1.1 ␮M silver solution (pH 1.81).

toacetic acid coated gold electrode than a bare gold electrode) and/or special interactions (e.g. complexation and electrostatic interaction) of silver with the modified electrode. Silver electrodeposition and dissolution on the mercaptoacetic acid coated gold electrode was further checked by ex-situ scanning electron microscopy. The scanning electron microscopic image in Fig. 2(a) showed clearly a smooth electrode surface before silver electrodeposition. The electrode surface was populated with relatively large particles (see image in Fig. 2(b)) at some fixed locations after silver electrodeposition at −0.17 V for 2 s in 1.1 ␮M silver solution (pH 1.81). More negative electrolysis potential and longer electrolysis time increased the number and sizes of particles on the electrode surface. Cleaning the resulted electrode with water stream or in a supersonic bath did not wash away these particles. However, electrochemical dissolution of the resulted electrode at a positive potential (0.20 V) for 2 s in 1.1 ␮M silver solution (pH

1.81) led to a decrease in the number and sizes of particles on the electrode surface and lastly a comparatively clear electrode surface was re-obtained (see image in Fig. 2(c)). This kind of site-selective electrodeposition and dissolution indicates an interaction of silver with mercaptoacetic acid self-assembled on the gold electrode. In order to understand the interaction (e.g. physical or chemical adsorption) of silver with mercaptoacetic acid self-assembled on the gold electrode surface, cyclic voltammetry of silver on the mercaptoacetic acid coated gold electrode at different scan rate and at different concentration of silver were conducted. Fig. 3 shows the variations of the peak current densities of the cathodic wave at −0.17 V and of anodic one at 0.01 V as a function of scan rate in the range from 0.02 to 0.15 V s−1 , indicating proportionality. The peak current density of these waves also increased proportionally to the concentration of silver. Consequently, the peak current for silver electrodepostion and dissolution is controlled by adsorption. On the other side, chemical titration of silver (0.1 M) with mercaptoacetic acid (0.01 M), acetic acid (0.01 M), and ldithiothretitol (0.01 M) was performed. Notice there that the concentration of silver was 10 times larger than that of mercaptoacetic acid, acetic acid, and l-dithiothretitol because over-amount of silver will make the reactions of silver with mercaptoacetic acid, acetic acid, or l-dithiothretitol finish completely. White precipitate easily obtained when the solution of silver ion was mixed with mercaptoacetic acid or acetic acid. In contrast, no deposition was obtained when the silver solution was mixed with l-dithiothretitol. Thus, the precipitate is silver–carboxyl complex which results from the chemical reaction of silver ions with the carboxyl group of mercaptoacetic acid (acetic acid) and the interaction of the sulfur–silver is negligible. This was further proved by the following quantitative calculation. 0.2014 g white precipitate was collected for the titration of silver with mercaptoacetic acid and 0.1522 g for the titration of silver with acetic acid after filtering, washing, and drying. Taking the molecular weight of CH3 COOAg (167 g mol−1 ) and HSCH2 COOAg (199 g mol−1 ) into account, the amount of sil-

N. Yang et al. / Electrochimica Acta 52 (2007) 4818–4824

Fig. 4. Variation of the peak potentials, Ep , with the natural logarithm of scan rate, lnv. The peak potential was obtained from cyclic voltammograms of 1.1 ␮M silver solution (pH 1.81) on the mercaptoacetic acid coated gold electrode at different scan rate.

ver (0.0010 mol) which reacted with mercaptoacetic acid was almost the same as that (0.0009 mol) with acetic acid. Therefore, the original and main adsorption force probably due to the interaction (complexation and/or electrostatic force) of silver with the terminally functional groups (carboxyl) of mecaptoacetic acid self-assembled on the gold electrode. The complexation and/or electrostatic force make silver adsorb easily and reduce/oxidize at more negative potentials on the modified electrode surface than on a bare gold electrode. For an adsorption-controlled electrode process, the peak potential, Ep , is given by [30] Ep = Eo −

RTk0 RT ln nαF αvnF

4821

Fig. 5. Variation of the cathodic peak current density with pH values of the solutions. The cathodic peak current was obtained from cyclic voltammograms of 1.1 ␮M silver dissolved in buffer solution with different pH values at a scan rate of 0.1 V s−1 on the mercaptoacetic acid coated gold electrode.

ide and/or silver oxide was noticed in the solution. In the pH range of 3.4–5.02, the cathodic peak current density was not so high and not sensitive. Although it seems that main interactions (complexation and electrostatic interaction) of silver with mercaptoacetic acid are favorable in solutions with higher pH values, large and sensitive cathodic peak current density which was still unclear to us was obtained when the pH value of silver solution was within the pH range from 3.4 to 1.0. 3.2. Potentiostatic deposition of silver on mercaptoacetic acid coated gold electrode

(1)

in which Eo is the formal potential of the reaction of AgOOCCH2 S–Au + e → Ag, n the total number of electrons transferred in the reaction of AgOOCCH2 S–Au + e → Ag, α the electron transfer coefficient and k0 is the rate constant of the above electrode reaction. Other symbols have their usual meanings. We plotted out the cathodic peak potential, Ep,c and the anodic one, Ep,a , as a function of the natural logarithm of scan rate, lnv, in Fig. 4 where shows linear relationship. Inputting the slopes and interceptions obtained from Fig. 4 into Eq. (1) resulted in the values of α = 0.42 and k0 = 8.4 × 10−6 s−1 for silver reduction (electrodeposition) on the mercaptoacetic acid coated gold electrode; α = 0.35 and k0 = 3.7 × 10−6 s−1 for silver oxidation (dissolution) from the mercaptoacetic acid coated gold electrode. Since terminally functional groups (carboxyl) of mercaptoacetic acid are in contact with the solutions and play a role in the charge state of monolayer of mecaptoacetic acid on the electrode surface, the amount of the species adsorbed and the charge/electron transfer will be greatly affected by the acidity of the solutions used. Fig. 5 shows the variation of the cathodic peak current density for silver electrodeposition in 1.1 ␮M silver on the mercaptoacetic acid coated gold electrode with the pH values of silver solutions. As expected, changes in pH values of silver solutions produced great effect on the cathodic peak current density. When the pH value was over than 5.02, white precipitate resulted from the formation of silver hydrox-

Chronoamperometry was adopted as a quantitative method for the investigation of silver electrodeposition and simultaneously as the diagnostic technique for the investigation of silver nucleation on the mercaptoacetic acid covered gold electrodes. Fig. 6 shows the chronoamperogram of silver electrodeposition on the mercaptoacetic acid coated gold electrode at −0.27 V where the silver deposition should be controlled by diffusion from 1.1 ␮M silver solution. The chronoamperogram included three transients of current density: a sharp decrease of the cur-

Fig. 6. Chronoamperogram (solid line) of silver electrodepostion at −0.27 V on the mercaptoacetic acid coated gold electrode in 1.1 ␮M silver solution (pH 1.81) and the fitted chronoapmerogram (open circles) with fitting parameters of the site density of nuclei of 3.7 × 104 cm−2 and the nucleation rate of 4.8 s−1 . The potential was held at 0.3 V for 0.5 s and then stepped to −0.27 V.

4822

N. Yang et al. / Electrochimica Acta 52 (2007) 4818–4824

rent (Regime I) due to the formation of the first nuclei on the electrode after charging the double layer, an increase in current and reaching into maximum current (imax ) at time (tmax ) (Regime II) attributed to an increase of the active surface area of the electrode, namely the growth of nuclei on the electrode; and the relative steady current (Regime III) in which the current was proportional to the square root of the pre-electrolysis time, and hence a decrease in current is due to the typical limitation of the reaction process controlled by the semi-infinite linear diffusion of silver ions, as we expected. Under these conditions, the system made a transition from no reaction to the steady reaction which will be controlled by the rate of mass transfer silver ions towards the mercaptoacetic acid coated gold electrode. For a heterogeneous system, an increase in initial current is due to an increase of surface area when a silver nucleation process is involved. As silver nucleation progresses, silver nuclei will begin overlapping. Each nucleus will define its own zone through which silver particles have to diffuse around, representing the masssupplying mechanism for the continuation of growth. Since the diffusion zones of silver ions are much larger than the underlying nuclei, the overlapping zones will eventually cover the entire electrode area. Further reaction (silver deposition) is then strictly controlled by the rate of mass transfer via the control of the diffusion zone (i.e. under steady state conditions). Within the diffusion zone, the growth of already-established silver nuclei continues, or additional silver nucleation is initiated on other sites. However, both are quite possible to be governed by the steady state conditions, as described by the Cottrell equation. The chronoamperogram was then fitted with the Heerman– Tarallo theory [31] to determine kinetic parameters of silver deposition on the mercaptoacetic acid coated gold electrode by varying the values of the site density of nuclei (N0 ) and the nucleation rate of silver on the mercaptoacetic acid coated gold electrode (A). According to the Heerman–Tarallo theory, the current transient for silver deposition on the mercaptoacetic acid coated gold electrode can be expressed as a function of electrolysis time: 1 Φ i = nFDc {1 − exp[αN0 (πDt)1/2 t 1/2 ]Θ} 1/2 Θ (πDt)

(2)

where Φ=1−

e−At (At)



1/2

1 − e−At At

N(t) = N0 [1.0 − exp(−At)]

(At)1/2

eλ dλ 2

(3)

0

(4)

where c is the concentration of silver ion and other parameters have the same meaning as described previously. The circles in Fig. 6 present the best simulation when the value of N0 (3.7 × 104 cm−2 ) and A (4.8 s−1 ) were taken for the simulation. The smaller values of N0 and A for silver deposition on the mercaptoacetic acid than those on glassy carbon electrode (N0 = 5.4 × 105 cm−2 , A = 5.9 s−1 ) and on n-Si(1 1 1) surface

(5)

If A is large and At  1 in a short time then N(t) = N0 . Conversely, if A is small and At  1 in a short time then N(t) = AN0 t, namely the density of nuclei increase with time. The former case corresponds to an instantaneous nucleation and the latter one corresponds to a progressive nucleation. In the instantaneous nucleation mechanism, the nuclei are created in a slow growth rate on a small amount of active sites simultaneously at the beginning of electrolysis. While in the progressive nucleation mechanism new nuclei are continuously generated in a fast growth rate on many active sites during the period of electrolysis. For an instantaneous nucleation followed by diffusion limited growth, the rate law is given by: i(t) =

nFcD1/2 1/2 {1 − exp[N0 D(8π3 cVm ) t]} π1/2 t 1/2

(6)

For a progressive nucleation followed by diffusion limited growth, the rate law is given by: i(t) =

nFcD1/2 2 1/2 {1 − exp[− AN0 D × (8π3 cVm ) t 1/2 ]} (7) π1/2 t 1/2 3

where Vm is the volume of the solution used. Other terms have the same physical meaning as mentioned above. For t → ∞, Eqs. (6) and (7) approach the following common expression: i(t) =

nFcD1/2 π1/2 t 1/2

(8)

on the other hand, for t → 0, Eq. (6) becomes i(t) = nFcN0 Aπ1/2 t 1/2

is related directly with the Dawson’s integral [32] and reflects the ‘retardation’ of the current by slow nucleation and Θ=1−

(N0 = 9.4 × 106 cm−2 and that of A = 148.4 s−1 ) [33] suggest a fast silver nucleation on the mercaptoacetic acid which possibly results from the strong interaction of silver with the carboxyl groups of mercaptoacetic acid on the electrode surface, as observed under potentiodynamic conditions. Theoretically, at a constant potential (under potentiostatic conditions), the nucleus density which varies as a function of time, N(t), can be described as a function of A in the first order [34]:

(9)

and Eq. (7) is reduced to i(t) = nFcD2/3 N0 Aπ1/2 t 1/2

(10)

Unfortunately, it is more difficult to test the fulfillment of Eqs. (9) and (10) because of uncertainty in evaluating the initial decreasing current related to the adsorption and electrochemical formation of the first monolayer. This drawback can be circumvented by using dimensionless plots and by comparing to the growth laws normalized in terms of the maximum current, imax , and the time at which the maximum current is observed, tmax . For instantaneous nucleation, Eq. (6) is converted into i2 i2max

= 1.9542

tmax  tmax 2 1 − exp(−1.2564 ) t t

(11)

N. Yang et al. / Electrochimica Acta 52 (2007) 4818–4824

2 Fig. 7. Comparison of the variation of j2 /jmax with t/tmax (solid line) for silver electrodeposition on the mercaptoacetic acid coated gold electrode with the theoretical curves for progressive (full circles) and instantaneous (open circles) nucleation.

where tmax =

1.2564 , imax = 0.6382nFDc(AN0 )1/2 , and N0 πAD

i2max tmax = 0.1629(nFc)2 D. For progressive nucleation, Eq. (7) is re-written as:   t 2 2 i2 tmax max = 1.2254 1 − exp(−2.3367 i2max t t

(12)

where tmax =

4.6733 , N0 πAD

imax = 0.4615nFD3/4 c(AN0 )1/4 ,

and i2max tmax = 0.2598(nFc)2 D. Fig. 7 shows experimental result (solid line) and mathematical simulations for an instantaneous nucleation (open circles) and for a progressive nucleation (full circles) on the base of Eqs. (11) and (12) by taking the values of imax and tmax from the solid line in Fig. 6. Note there that we used current density rather than the current for the mathematical simulations in Fig. 7. The chronoapmerometric curve obtained at −0.27 V (solid line) agreed well with the open circles, indicating an instantaneous nucleation for silver deposition on the mercaptoacetic acid coated gold electrode. This agrees with the observation shown in Fig. 6 where the current of silver electrodeposition (solid line) on the mercaptoacetic acid coated gold electrode reached the maximum value in a quite short time. Fig. 8 shows the variation of the natural logarithm of the nucleation rate, ln A, and the logarithm of the site density, log N0 , of silver deposition on the mercaptoacetic acid coated gold electrode as a function of the deposition potential, E. Both ln A and log N0 increased linearly with the deposition potential. From the following equation [35]: d lnA Ncrit = d E RT

(13)

4823

Fig. 8. Dependence of the natural logarithm of the site density of nuclei, ln A (full circles) and of the logarithm of the nucleation rate, log N0 (full squares) on the potential, E, for silver electrodeposition on the mercaptoacetic acid coated gold electrode in 1.1 ␮M silver solution (pH 1.81).

the critical amount of silver nucleus can be determined. The slope of ln A versus E was 14.4 and the critical value of silver nucleus was calculated to be composed by one atom within the potential range investigated. The linearity of the logarithm of the site density, log N0 , against the deposition potential for silver deposition on the mercaptoacetic acid coated gold electrode indicates a remarkable influence of deposition potential during the initial nucleation. The chronoamperomatric curves for silver electrodepostion on the mercaptoacetic acid coated gold electrode at −0.27 V in different concentration of silver solutions were also recorded to examine the influence of driving force (silver concentration) for silver nucleation during the initial stage on the mercaptoacetic acid coated gold electrode. As seen in Fig. 9, the site density did not change with an increase in the concentration of silver solution. Since during the initial stage of silver electrodeposition the site density is actually decided by the amount of carboxyl functional groups on the electrode surface, the independence of N0 on the concentration of silver solutions was observed. This dispersion of nucleation rate which has also been reported on other systems [31,36,37] agrees well with the experimental result of the site density dependence of deposition potential.

Fig. 9. Dependence of the logarithm of the nucleation rate, log N0 , on the logarithm of the concentration of silver solutions, c, for silver electrodeposition on the mercaptoacetic acid coated gold electrode at −0.27 V.

4824

N. Yang et al. / Electrochimica Acta 52 (2007) 4818–4824

4. Conclusion Silver nucleation on the mercaptoacetic acid coated gold electrode occurred in the continuous steps: adsorption and formation of silver complexes, and then reduction of silver complexes into silver metal on the electrode surface. Silver nucleation was affected remarkably by the pH values of supporting electrolyte and the deposition potential. In contrast, the driving force (the concentration of silver) did not affect silver nucleation on the modified electrode surface. Silver nucleation on the mercaptoacetic acid coated electrode also exhibited the site-selective electrodeposition and dissolution. This was explained by the strong interaction (complexation and electrostatic force) of silver with the carboxyl groups of mercaptoacetic acid on the electrode surface because the carboxyl groups of mercaptoacetic acid on the gold electrode surface appear site energies and a large fraction of there sites becomes more active as the potential increases. This kind of site-selective electrodeposition and dissolution of particles on modified electrode is promising to have potential applications for the construction of sensors with high selectivity, for the fabrication of nano-structured templates, and for the design of electronic devices. Acknowledgement The financial support from the Science Foundation of Education Department of Hubei Province, China (Project D200515003) is greatly acknowledged. References [1] C.A. Jeffrey, W.M. Storr, D.A. Harrington, J. Electroanal. Chem. 569 (2004) 61. [2] N.H. Lndez, J.M. Ortega, M. Choy, R. Ortiz, J. Electroanal. Chem. 515 (2001) 123. [3] F.Y. Song, K.K. Shiu, J. Electroanal. Chem. 498 (2001) 161. [4] F. Fourcade, T. Tzedakis, J. Electroanal. Chem. 493 (2000) 20. [5] F.D. Eramo, J.J. Silber, A.H. Arevalo, L.E. Sereno, J. Electroanal. Chem. 494 (2000) 68. [6] K.H. Lubert, L. Beyer, J. Casabo, C. Perez-Jimenez, L. Escriche, J. Electroanal. Chem. 475 (1999) 73. [7] N.L. Pickup, J.S. Shapiro, D.K.Y. Wong, Anal. Chim. Acta 364 (1998) 41.

[8] E. Lorenzo, H.D. Abruna, J. Electroanal. Chem. 328 (1992) 111. [9] A.Q. Zhang, C.Q. Cui, J.Y. Lee, J. Electroanal. Chem. 413 (1996) 143. [10] B. Bottger, U. Schindewolf, J.L. Avila, R. Rodriguez-Amaro, J. Electroanal. Chem. 432 (1997) 139. [11] A. Alqudami, S. Annapoorni, P. Sen, R.S. Rawat, Synth. Mater. 157 (2007) 53. [12] D. Oyamatsu, M. Nishizawa, S. Kuwabata, H. Yoneyama, Langmuir 14 (1998) 3298. [13] D. Oyamatsu, M. Nishizawa, H. Yoneyama, J. Electroanal. Chem. 473 (1999) 59. [14] M. Nishizawa, T. Sunagawa, H. Yoneyama, J. Electroanal. Chem. 436 (1997) 213. [15] H. Hagenstrom, M.J. Esplandiu, D.M. Kolb, Langmuir 17 (2001) 839. [16] M.J. Esplandiu, H. Hagenstrom, Solid State Ionics 150 (2002) 39. [17] D. Qu, K. Uosaki, J. Phys. Chem. B. 110 (2006) 17570. [18] B. Boer, M.M. Frank, Y.J. Chabal, W. Jiang, E. Garfunkel, Z. Bao. Langmuir 20 (2004) 1539. [19] T. Ohgi, H.-Y. Sheng, H. Nejoh, Appl. Surf. Sci. 130/132 (1998) 919. [20] T. Ohgi, H.-Y. Sheng, Z.-G. Dong, H. Nejoh, D. Fujita, Appl. Phys. Lett. 79 (2001) 2453. [21] R.P. Andres, T. Bein, M. Dorogi, S. Feng, J.I. Henderson, C.P. Kubiak, W. Mahoney, R.G. Osichin, R. Reifenberger, Science 272 (1996) 1323. [22] X.D. Cui, A. Primak, X. Zarate, J. Tomfohr, O.F. Sankey, A.L. Moore, T.A. Moore, D. Gust, S.M. Lindsay, Science 294 (2001) 571. [23] Y. Xiao, F. Patolsky, E. Katz, J.F. Hainfeld, I. Willner, Science 299 (2003) 1877. [24] G.K. Ramchandram, T.J. Hopson, A.M. Rawlett, L.A. Primak, S.M. Lindsay, Science 300 (2003) 1413. [25] J. Zheng, Y. Zhou, X. Li, Y. Ji, T. Lu, R. Gu, Langmuir 19 (2003) 632. [26] Y.-C. Yang, S.-L. Yau, Y.-L. Lee, J. Am. Chem. Soc. 128 (2006) 3677. [27] M. Petri, A.M. Kolb, U. Memmert, H. Meyer, Electrochim. Acta. 49 (2003) 183. [28] N. Yang, Q. Wan, X. Wang, Eletrochim. Acta. 50 (2005) 2175. [29] N. Yang, X. Wang, Q. Wan, Electrochim. Acta. 51 (2006) 2050. [30] A.J. Bard, L.R. Faulkner, Electrochemical Methods, second ed., Wiley, New York, 2001. [31] L. Heerman, A. Tarallo, J. Electroanal. Chem. 451 (1998) 101. [32] M. Abramowitz, I.A. Stegun, Handbook of Mathematical Functions, Dover, New York, 1965. [33] K. Marquez, G. Staikov, J.W. Schultze, Electrochim. Acta. 48 (2003) 875. [34] M. Fleischmann, H.R. Thirsk, in: P. Delahay (Ed.), Advances in Electrochemistry and Electrochemical Engineering, vol. 3, Wiely, New York, 1963. [35] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase Formation and Growth—An Introduction to Initial Stages of Metal Deposition, VCH, Weinheim, 1996. [36] V. Tsakova, A. Milchev, J. Electroanal. Chem. 235 (1987) 237. [37] J. Mostany, J. Parra, B.R. Scharifker, J. Appl. Electrochem. 16 (1986) 333.