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Electrochemical Organization of Gold Nanoclusters in Three Dimensions as Thin Films from an Aminosilicate-Stabilized Gold Sol and Their Characterization S. Bharathi,*,† M. Nogami,† and O. Lev‡ Department of Material Science and Engineering, Gokiso-Cho, Showa-Ku, Nagoya 466-8555, Japan, and Fredy and Nadine Herrmann School of Applied Science, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received August 7, 2000. In Final Form: February 19, 2001 Electrochemical organization of a thin film of gold nanoparticles from an aminosilicate-stabilized gold sol on an indium tin oxide coated glass is demonstrated. Films are semitransparent to reflectivity depending on the thickness and have three-dimensional conductivity. Characterization of the films using UV-visible spectroscopy, IR spectroscopy, atomic force microscopy, and cyclic voltammetry has shown that they are made of a network of gold nanoparticles that are interconnected by aminosilicate moieties. This methodology is successfully extended to codeposited glucose oxidase enzyme, based on which a glucose biosensor is demonstrated.
Introduction Functionalized metal nanoparticles exhibit desirable optical and catalytic properties that make them ideal building blocks for two- and three-dimensional molecular/ nanoarchitectures. The practical use of such metal nanoclusters is coupled to our ability to organize them efficiently in three, two, and one dimension. Rapid advances are being made in the construction of organized superstructures from metal colloids.1-4 Schiffrrin and co-workers5,6 and Wheeten et al.7 have reported an organized threedimensional (3D) network of gold nanoparticles using thiol ligands that are used to cover the particle surface and also to achieve equidistance between the particles in network. However, most of the studies reported are concerned with the organization of metal nanoclusters in two dimensions. This is often achieved by two different strategies as described below. (a) The first strategy is self-organization of ligand stabilized clusters onto a smooth surface, from a solution, without any chemical interaction between cluster and surface. An example is the well-ordered two-dimensional (2D) network of gold clusters demonstrated by Whetten et al.7 and Andres and co-workers8 using an alkanethiol * Correspondingauthor.Fax: 0081-52-735-5285.E-mail: bharathi@ mse.nitech.ac.jp. † Nagoya Institute of Technology. ‡ The Hebrew University of Jerusalem. (1) Freeman, R. G.; Graber, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (2) Graber, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (3) Graber, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353; Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726. (4) Schmid, G.; Peschel, St.; Sawitowski, Th. Z. Anorg. Allg. Chem. 1997, 623, 719. (5) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (6) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (7) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vermar, I.; Wang, Z.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landmann, U. Adv. Matter. 1996, 8, 428.
spacer molecule. The interparticle distance, which is defined by the length of the thiol spacers, has a pronounced effect on the electronic properties of the resulting 2D arrays.8 (b) The other strategy is based on the (chemical) interaction between the surface and the cluster. For example, many metallic clusters have a high affinity for -NH2, -CN, and -SH functional groups. Au and Ag nanoparticles were anchored on these functional groups as terminal groups on organosilane polymers on silica, alumina, or SnO2 substrates.8-11 Natan et al.12 have demonstrated the use of 2D arrays of gold and silver nanoparticles as substrates for surfaceenhanced Raman spectroscopy. The reported enhancement factor of 4 orders makes this monolayer electrode a very attractive substrate for both basic and applied uses. The same authors have studied the electrochemistry of cyctochrome c on this monolayer electrode,13 which revealed that the kinetics depended on the size of the gold nanoparticles in the monolayer. Willner and co-workers14 have used a similar procedure to anchor gold nanoparticle on an ITO substrate and further derivatized them with self-assembled monolayers. Many other variants of this methodology have been used to prepare 2 D arrays of metal nanoparticles.15-18 Despite the electronic and electrochemical significance of conductive 3D nanometallic structures, very little effort (8) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science, 1996, 273, 1690. (9) Spatz, J. P.; Roescher, A.; Moller, M. Adv. Mater. 1996, 8, 337. (10) Spatz, J. P.; Mobner, S.; Moller, M. Chem. Eur. J. 1997, 3, 1552. (11) Mirkin, C. D.; Letsinger, R. L.; Mucic, R. C.; Storkoff, J. J. Nature 1996, 382, 607. (12) Graber, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (13) Brown, K. R.; Fox, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1154. (14) Dhoron, A.; Katz E.; Willner, I. Langmuir 1995, 11, 1313. (15) Chumanov, G.; Skolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466. (16) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (17) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. J. Electroanal. Chem. 1996, 409, 137. (18) Colvin, V. L.; Schlamp, M. C.; Alivistatos, A. P. Nature 1994, 370, 354.
10.1021/la001136d CCC: $20.00 © 2001 American Chemical Society Published on Web 04/06/2001
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was devoted to employ this fast growing field of stabilized nanoparticle sols for the construction of 3D structures. Reports by Murray19 and Natan20 on conductive 3D nanostructures are notable exceptions. Murray and coworkers have deposited alkanethiol-stabilized gold clusters on interdigitated array electrodes and examined their electrochemistry.19a Nonlinear I-V curves were reported for these systems that depended on the length of the alkane chain. The conductivities of the arrays calculated from the I-V curves revealed an order of 2 decrease in the magnitude for every four carbons in the alkane chain. The same authors19b have also described the formation of controlled and reversible gold nanoparticle aggregates and films using Cu2+-carboxylate chemistry. Natan and coworkers20 have deposited gold colloids layer by layer using bifunctional molecular bridges (e.g., alkanedithiol) between the stabilized nanocrystalline layers. The conductivity of the gold films depended on the chain length of the spacer molecule. Natan20 claimed that this step-by-step process is time-consuming and the possibility of contaminating gold colloid due to improper rinsing between stages. In addition, the derivatization time varies with the size of the gold nanoparticle. Electrochemical characterization of the gold films prepared by the stepwise assembly of gold nanoparticles have shown that the film still has considerable resistivity that needed IR compensation for achieving approximate electrochemical reversibility.21a Very recently, Natan et al.20b have demonstrated the preparation of Ag, Au, and Ag-Au metal films by stepwise assembly of corresponding metal nanoparticles from solution. Schiffrin and co-workers21b have reported on the self-assembled multilayer thin films prepared by successive self-assembly of 6 nm gold nanoparticles and organic dithiol molecules, and the films exhibited nonmetallic optical and electronic properties. Willner et al.22a have recently demonstrated superstructures based on gold nanoparticles, on the similar lines of Natan et al., using bipyridinium-bridging molecules. Chen22b has recently reported on the self-assembly of monolayer-protected gold nanoparticles onto a gold electrode surface. The resulting monolayer exhibited discrete electron-transfer features that were ascribed to the quantized capacitance charging of the particle double layer. Zhong et al.22c have described a gold thin film assembly formed by an exchange reaction between alkanethiolates on the nanocrystal shells and dithiols in solution. Herein we report an electrochemical method to organize a 3D gold-aminosilicate film, consisting of a network of gold nanoparticles that are interconnected by aminosilicate moieties, from an aminosilicate-stabilized gold sol. This study was influenced by the research of Murray23 (19) (a) Therill, R. H.; Postlethwaite, T. A.; Chen, C. H.; Poon, C. D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnoson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (b) Templeton, A. C.; Zamborini, F. P.; Wuelfing, P. W.; Murray, R. W. Langmuir 2000, 16, 6682. (20) (a) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (b) Musick, M. D.; Keating, C. D.; Lyon, A.; Botsko, S. L.; Pena, D. J.; Holliway, W. D.; M.McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Chem. Mater. 2000, 12, 2869. (21) (a) Musick, M. D.; Pena, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Langmuir 1999, 15, 844. (b) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (22) (a) Lahav, M.; Gabriel, T.; Shipway, A. N.; Willner, I. J. Am. Chem. Soc. 1999, 121, 258. Lahav, M.; Gabai, R.; Shipway, A. N.; Willner, I. Chem. Commun. 1999, 1937. (b) Chen, S. J. Phys. Chem. 2000, 104, 663. (c) Zhong, C. J.; Zheng, W. X.; Leibowitz, F. L. Electrochem. Commun. 1999, 1, 72. (23) Lenhard J. R.; Murray, R. W. J. Am. Chem. Soc. 1978, 100, 7870.
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and others (see ref 24 for a review) that deposited an aminosilicate monolayer on ITO and on noble metal electrodes by anodic formation of oxide layers (e.g., PtO or AuO) and subsequent bonding of aminoalkylalkoxysilane moieties on these surfaces. We have demonstrated25,26a that it is possible to stabilize gold sols in aqueous solutions by aminosilicate shells in which the amine groups are oriented toward the gold surface. These two advances paved the road for multilayer electrochemical deposition of aminosilicate-modified gold nanoparticles. A preliminary report on these results has been published by us.27 Experimental Section Chemicals. N-[3-(Trimethoxysilyl)propylethylenediamine] (EDAS), HAuCl4, and glucose oxidase (GOx) were obtained from Sigma Chemicals Co., USA. All other chemicals used in this work were of analytical grade. Double distilled water was used through out the studies. Indium tin oxide (ITO) coated glass slides with 10 Ω cm resistance were obtained from M/S Delta Technologies (USA). Gold Sol Preparation. Gold sols used in this work were prepared using our earlier reported procedure.25,26 For example, a typical sol was prepared by dissolving the required amount of EDAS in 0.1 M KH2PO4 followed by sonication for 10 min. pH of the solution was adjusted using concentrated HCl to the required value followed by the addition of HAuCl427 (a molar ratio of 1:100 (Au/EDAS) was used throughout this study). The solution was sonicated for another 10 min, after which a few drops of a freshly prepared NaBH4 were added with vigorous stirring resulting in the formation of the gold sol. Absorption spectra of the gold sol showed the characteristic surface plasmon absorption peak at ca. 520 nm. The size of the gold nanoparticles was determined by transmission electron microscopy (TEM) and was found to be 4-6 nm with narrow size distribution.25,26 Pretreatment of ITO Slides. ITO slides were cleaned in methanol, rinsed in water, dipped in 0.1 M NaOH solution for 2 min, and rinsed with copious amount of distilled water. Electrodes were stored under dry conditions. Electrochemical Measurements. The electrochemical cell used in this study was a single-compartment cell, consisting of an ITO slide working electrode of 0.25 cm2 area, a Pt wire counter electrode, and an Ag/AgCl reference electrode. All the potentials are referred to this electrode unless specified otherwise. A computer-controlled EG&G potentiostat (model 263A) was used to record the cyclic voltammograms and for the electrodeposition. After electrodeposition, films were washed with copious amount of water and subsequently dried using a stream of air. UV-Visible Spectroscopy. The absorption spectra of the nanocrystalline gold films were recorded using either a Cary E1 or a Jasco Ubest 50 UV-vis spectrophotometer. All the spectra were recorded in air. IR Spectroscopy. IR spectra of the typical films were taken by reflection of the incident beam at an angle of incidence of 80° in a Brukker Fourier transform spectrophotometer with DTGS detector. Typically, 800 scans were averaged to yield a spectrum. All the spectra were corrected for freshly cleaned ITO background. Atomic Force Microscopy (AFM). The surface morphology and the growth pattern of the nanocrystalline films were examined using an atomic force microscope (Topometerix, USA). “Tapping mode” measurements were used to avoid the damaging of the films. Multiple images at different areas of each film were taken. (24) Lev, O.; Wu, Z.; Bharathi, S.; Modestov, A.; Glezer, V.; Gun, J.; Rabinovich, L.; Sampath, S. Chem. Mater. 1997, 9, 2354. (25) Bharathi, S.; Lev, O. Chem. Commun. 1997, 2303. (26) (a) Bharathi, S.; Fishelson, N.; Lev, O. Langmuir 1999, 15, 1929. (b) Zhmud B. V.; Sonnefeld, J. J. Non-Cryst. Solids 1996, 195, 16. Bellamy, L. J. The Infrared Spectra of Complex molecules; 2nd ed.; Methuen: London, 1960. (c) Boerio, F. J.; Armogan, L.; Cheng, S. Y. J. Colloid Interface Sci. 1980, 73, 416. Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (27) Bharathi, S.; Joseph, J.; Lev, O. Electrochem Solid State Lett. 1999, 2, 284. (28) EDAS-phosphate salt precipitate at high concentration. If a higher concentration of phosphate is required, it is recommended to add the buffer after gold reduction.
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Figure 1. Absorption spectra of the gold films on an ITO electrode, deposited for 30 min by cycling the electrode potential between -0.4 and 1.0 V at 0.1 V/s from (a) 0.1 mM, (b) 1 mM, (c) 6 mM, and (d) 10 mM of gold sol. pH ) 4.5, EDAS:Au molar ratio 100:1.
Results and Discussion Films have been electrodeposited from gold sols, with concentrations ranging from 0.1 to 10 mM, by cycling the electrode potential between -0.4 and 1.0 V at a scan rate of 0.1 V/s. Influence of the pH (of the sol), anodic potential limit (APL), and the deposition time has been investigated to understand the film growth and its kinetics. The strong surface plasmon resonance of the gold nanoparticle and the characteristic electrochemical behavior of gold are monitored to follow the film growth by UV-vis spectroscopy and cyclic voltammetry. AFM is used to observe the surface morphology of the films. Figure 1 shows the absorption spectra of the films deposited, for 30 min, in gold sols containing 0.1-10 mM of gold, at a pH of 4.5 (the deposition potential window is -0.4 to 1.0 V at 0.1 V/s). It can be seen from the figure that all the spectra show the characteristic surface plasmon resonance (SPR) peak at ≈530 nm. Further, for the identical deposition times, the intensity at the SPR maxima and the extent of its red shifting increase with increasing gold sol concentration. The SPR maxima of the deposited film shift from ≈530 to ≈650 nm, as the gold sol concentration is increased from 0.1 to 10 mM. To study the influence APL during deposition, the deposition potential was varied from 0.4 to 1.0 V. Figure 2 illustrates the plot of the relative intensities at the SPR maxima of the films, deposited for 30 min, as a function of APL at different gold sol concentrations. It is obvious from the figure that the relative intensity at the SPR maxima increases with increasing APL. However, the increase (in intensity at SPR maxima) in this potential range has two linear ranges intersecting at 0.75 V. The larger slope of the linear range between 0.75 and 1.0 V as compared to the range between 0.4 and 0.75 V suggests a faster growth rate at higher APL. Besides, the magnitude of the red shifting of the SPR peak increases with increasing APL and gold sol concentration. Figure 3 depicts a series of absorption spectra of the films deposited over different times, ranging from 15 min to 24 h, from 0.1 and 10 mM of gold sols, at a pH of 4.5 (deposition potential window is -0.4 to 1.0 V, at a scan rate of 0.1 V). As can be seen from Figure 3, with increasing deposition time the SPR maxima red shift by 60 and 100 nm for films deposited from 0.1 and 1 mM (not shown in figure) of gold sol. Whereas, for the films deposited from
Bharathi et al.
Figure 2. Relative intensity at SPR maxima as a function of anodic potential limit during deposition for films deposited for 30 min from solution containing increasing concentration of Au sol. All other conditions are the same as in Figure 1.
Figure 3. Absorption spectra of the films obtained by cycling the electrode potential between -0.4 and 1.00 V at a scan rate of 0.1 V/s from Au sol concentrations (A) 0.1 and (B) 10 mM for (a) 15, (b) 30, (c) 180, (d) 270, (e) 540, and (f) 1000 min. pH ) 4.5 and EDAS:Au is 100:1.
6 (not shown in Figure) and 10 mM gold sol, in addition to the more pronounced red shifting of SPR maxima (by 150 to 200 nm), a broad and continuous absorption band characteristic of bulk gold film was noticed at longer deposition times (more than 30 min). Interestingly, the films obtained at deposition times more than 30 min in
Organization of Gold Nanoparticles
10 mM gold sol appear like a continuous metallic film with a characteristic reflection and yellow color of the gold. Ideally, we would like to relate the UV spectra of the films to the particle size and shape. However, the SPR maxima depend not only on the particle size and shape but also on other variables such as refractive index and interparticle spacings.29,30 Nevertheless, qualitative information about the particle size and the environment can be inferred from the optical spectra of the films. It is well-known that the SPR maxima blue shifts with decreasing particle size and red shifts with increasing particle size. In addition to the increasing particle size, red shifting could also occur when the interparticle spacing is smaller than the wavelength of light. The results presented above for films deposited from different concentrations can be grouped into two broad categories based on the extent of red shifting. SPR maxima of the films deposited from 0.1 and 1 mM of gold sol red shifts by 60-100 nm. This probably indicates that the gold nanoparticle in the film has more or less the same particle size, which is comparable to the size of the nanoparticle (4-6 nm) in the sol used for the electrodeposition. By comparison, the films deposited from higher gold concentrations (viz., 6 and 10 mM) have shown a larger red shifting of about 150-200 nm, and also the spectra were very broad and continuous. This may be either due to the aggregation of nanoparticles in the film or due to decreased interparticle distance as more and more particles are packed into the film bringing the particles in close proximity.1,2,29,30 Moreover, the observed red shifting of the plasmon band to 600-700 nm occurs gradually over time. This observation is somewhat different from other studies1 that demonstrated gradual evolution of a second plasmon band at ca. 600-700 nm in addition to the first one (at 500-550 nm). This suggests structural differences between the films prepared by other procedures and the electrodeposited films, probably implying a more homogeneous distribution of the gold nanoparticles in our film. Electrochemical Characterization. Films are further characterized by cyclic votammetry (CV) using two electrochemical probe reactions that are sensitive to the surface area of the gold: (a) by following the gold oxide formation31 and its reduction in 0.5 M H2SO4 and (b) by the underpotential deposition32 (UPD) of copper from a solution containing 1 mM copper sulfate and 0.5 M H2SO4. These two electrochemical reactions are capable of shedding light on the electrochemically active surface area of the film, which is both exposed to solution and connected to the substrate. A typical cyclic voltammetric response of the film, deposited for 900 min from 1 mM gold sol (pH 4.5), in 0.5 M H2SO4 at a scan rate of 0.003 V/s, is presented in Figure 4A. As expected, the CV shows the peaks corresponding to the gold oxide formation and its subsequent reduction. UPD behavior of the copper on the same electrode (Figure 4B) has revealed the characteristic peaks corresponding to the underpotential deposition and dissolution. The observed underpotential shift of ≈150 mV, the difference between EoCu2+/0 and the UPD potential, is in close agreement with the underpotential shift reported for the bulk gold electrode. The Coulombic charge under the oxide reduction peak is calculated by integrating (29) Bohren, C. F.; Huffman, D. F. Absorption and scattering of light by small particles; Wiley: New York, 1983. (30) Mulvaney, P.; Underwood, S. Langmuir 1994, 10, 3427. LizMarzan, L. M.; Mulvaney, P. Recent Res. Dev. Phys. Chem. 1998, 2, 1. (31) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1971, 32, 29. (32) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11.
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Figure 4. Typical voltammetric response of the thin film of a gold electrode, deposited for 900 min from 1 mM gold sol (deposition potential window -0.4 to 1.0 V, scan rate 0.1 V/s), at a scan rate of 3 mV/s in (A) 0.5 M H2SO4 and (B) in 1 mM CuSO4 and 0.5 M H2SO4.
the area under the peak. These data for the films deposited under different experimental conditions are presented and discussed in the following paragraph. Previous studies31,32 have shown that the charge associated with the reduction of a monolayer of gold oxide (and for the deposition/dissolution of a monolayer of copper) is ≈430 µC/cm2. Hence, the observed charges under these peaks provide an estimate of the electrochemically active surface area of the gold. Figure 5 gives the charge under the Au oxide reduction peak, plotted as a function of APL, for films deposited for 30 min from 0.1 to 10 mM of gold sol. The charges under the oxide reduction peak increase with increasing APL in general but has two different linear ranges similar to Figure 2. Moreover, the amount of gold in the films deposited from 0.1 and 1 mM of gold sol is found to be a submono-/monolayer ( 10) At pH 1-3, EDAS is positively charged or neutral. At pH 3-10 it exists in zwitterionic form and is mono- or dinegatively charged. At pH above 10 it is trinegatively charged. Electrophoretic mobility of EDAS and the EDASstabilized gold sol at various pH values have revealed that the EDAS-stabilized gold sol has more negative electrophoretic mobility as compared to EDAS at any given pH. In fact, the EDAS-Au particles were always (down to pH 2) negatively charged while at pH 2-3 EDAS is positively charged. The negative charge of EDAS-gold sol even at acidic pH values suggests that NH2 the group is locked onto the gold nanoparticle surface and prevented from protonation. On the basis of this result and our previous results,26 the presence of a layer of an aminosilicate shell on the gold nanoparticle is well established. Because of this, the particles are negatively charged down to pH 2. This also explains the preferential deposition of the EDAS-gold moiety over EDAS alone, as the EDASgold moiety is attracted more toward the electrode surface than the pure aminosilane moieties. The proposed second mechanism is essentially a combination of two processes, viz., silanization and the electrochemical oxidation of the gold nanoparticle. Initially a monolayer of gold is formed by a silanization process, (37) Pludermann, E. P. Silane Coupling Agents, 2nd Ed.; Plenum Press: New York, 1991.
Organization of Gold Nanoparticles
subsequent electrochemical oxidation of this monolayer results in the creation of active sites for further condensation with the silanol shells of the gold nanoparticles that are attracted to the surface by the electrostatic process, leading to the multilayer deposition, as shown in Scheme 1. Existence of this pathway is confirmed by an experiment in which CV of the gold film electrode was recorded in a solution containing identical aminosilane concentration and pH. Under this condition, Au oxide formation starts around 0.75 V as noticed by the raising anodic current, even though the oxide peak is observed at 1.2 V.38 This explains why the films deposited at potentials up to 0.7 V always had lower coverage as compared to the films deposited at 1.0 V (see Figure 6). Further, for the films deposited at 0.7 V from 0.1 and 1 mM of gold sol, the coverage stabilizes at a monolayer level (400-500 µC/ cm2) while for films deposited from 6 and 10 mM it is around 2000 µC/cm2. These results suggest the coexistence of both the mechanisms at potentials >0.75 V, as evidenced from the multilayer deposition noticed at all the concentration range studied. The deposition mechanism at potentials
