new insights in nano-electrodeposition: an electrochemical ... · methods such as sputtering or...

New Insights in Nano-electrodeposition: An Electrochemical AggregativeGrowth Mechanism

Jon Ustarroz*, Annick Hubin and Herman TerrynDepartment Materials and Chemistry, Research Group Electrochemical and Surface Engineering (SURF), Vrije UniversiteitBrussel (VUB), Brussels, Belgium

Abstract

Supported nanostructures represent the cornerstone for numerous applications in different fields such aselectrocatalysis (fuel cells) or electroanalysis (sensors). In contrast to other methods, electrochemicaldeposition allows the growth of the nanostructures directly on the final support, improving the electronpathway within the substrate, nanostructure, and electrolyte. However, despite the increasing number ofpublications in the field, the early stages of electrochemical nanocrystal formation are still underdiscussion.

In this chapter, we first provide a survey on the traditional approaches to study the early stages ofelectrochemical nucleation and growth, together with the classical theories used to understand them. Next,we describe our most recent findings which have led to reformulate the Volmer-Weber island growthmechanism into an electrochemical aggregative growth mechanism which mimics the atomistic processesof the early stages of thin-film growth by considering nanoclusters of few nm as building blocks instead ofsingle atoms. We prove that the early stages of nanoelectrodeposition are strongly affected by nanoclusterself-limiting growth, surface diffusion, aggregation, and coalescence.

Keywords

Supported nanoparticles; Electrodeposition; Nucleation and growth; Aggregative growth

Introduction

Supported nanostructures represent the cornerstone for numerous applications in different fields such aselectrocatalysis (fuel cells) [1–3] or electroanalysis (sensors) [4–6] among others [7]. In both cases,supported metal nanoparticles (NPs) are the active materials whose development can boost both technol-ogies to levels almost unimaginable a few years ago.

Metal nanoparticles can be synthesized by multiple methods either in solution or in the gas phase, asreviewed many times [3, 8, 9], colloidal synthesis and other solution-based methods being the mostcommon approach [10–13]. Nonetheless, when generated particles are required to be attached to asurface, previously mentionedmethods do not always provide the best solution. Physical vapor depositionmethods such as sputtering or electron beam deposition require expensive high-vacuum facilities,

*Email: [email protected]



Handbook of NanoelectrochemistryDOI 10.1007/978-3-319-15207-3_10-1# Springer International Publishing Switzerland 2015

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whereas colloids may lose some of their properties due to the organic ligands used during the synthesisprocedure or because of unwanted aggregation during deposition on a given support [14–16].

Alternatively, electrochemical deposition allows the growth of the nanostructures in one step, directlyonto the final support, without needing further sample preparation, thus improving the electron pathwaywithin the substrate, nanostructure, and environment. Consequently, the technique has been proveneffective to obtain highly electroactive nanostructures with potential for fuel cell [17–20] or (bio)sensing[5, 4] applications. Moreover, the technique is surfactant free, highly selective, cost effective, and allowsthe nature of the nanoclusters to be easily tuned by changing electrolyte composition and depositionparameters [21, 22].

However, compared to other synthesis methods, the reliability of the synthesis procedures, size andshape tunability, or size dispersion control is far from being at the same level of development. The mainreason for this is a lack of knowledge of the fundamental aspects of nanoscale electrodeposition. Despitethe increasing number of publications in the field, the early stages of nanocrystal formation mechanisms inelectrochemical processes are still under discussion.

Early Stages of Electrochemical Nucleation and Growth: Classical Approach

Nucleation and growth phenomena have been thoroughly studied since more than a century for colloidalsyntheses [23], thin-film growth [24], and electrochemical deposition processes [25] among others,resulting in a classic nucleation and growth theory which predicts that nanocrystals grow irreversiblyby atomic addition until the reaction is halted.

Contrarily to other nanoparticle synthesis or thin-film deposition methods, electrochemical depositionprocesses can be followed in situ by recording the current (or potential) transients after applying differentpotential (or current) pulses. In the case of potentiostatic single-pulse electrodeposition, the evaluation ofthe current-time transients, or chronoamperograms, provides invaluable time-resolved information aboutnucleation and growth processes and is hence performed and reviewed on countless occasions [26–30].

If we consider a homogeneous surface such as amorphous carbon, when a given overpotential, �, isapplied, nuclei are supposed to form at random locations over the surface according to Eq. 1:

N tð Þ ¼ N0 1� exp �Atð Þ½ � (1)

N0 is the saturation number density (maximum number of nuclei within the surface), A is the nucleationrate constant, and AN0 is the nucleation rate. In principle, all formed nuclei have r � rCrit, so they willgrow irreversibly until the potential pulse is stopped or the concentration of active species in theirsurroundings decreases below a certain level. By considering that the only growth mechanism is that ofdirect reduction of ions onto the surface of three-dimensional hemispherical nuclei (direct attachment),plenty of theoretical work has been carried out to relate the chronoamperometric response I(t) to thenucleation rate and N(t) (Eq. 1) [26, 31–33, 28].

In short, the growth of each nucleus affects both the concentration of active species and theoverpotential distribution in the cluster vicinity, creating zones of reduced concentration and overpotentialand thus reduced nucleation rate. Then, if multiple clusters are considered, their local zones of reducednucleation rate spread and gradually overlap. This is a very complex problem which is frequently solvedby approximating the areas of reduced nucleation rate by overlapping planar diffusion zones in whichnucleation is fully arrested. Then, if it is considered that these growth centers, which are randomlydistributed on the electrode surface, grow under spherical diffusion control, the total electrochemicalcurrent can be expressed by


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I tð Þ ¼ Sj tð Þ ¼ SbzFcD

pt

� �1=2

� 1� exp � 1

2AN 0pg

8pcMr

� �1=2

Dt2 !" # (2)

if the nucleation is progressive, and

I tð Þ ¼ Sj tð Þ ¼ SzFcD

pt

� �1=2

� 1� exp �N0p 8pcMr

� �1=2Dt

� �� (3)

if the nucleation is instantaneous. It is important to note here that Eq. 1 accounts for progressivenucleation, but instantaneous nucleation can be thought as the limit A ! 1 and then N = N0, so N0

nuclei are instantaneously formed at t = 0. S is the electrode area, j(t) is the current density, F is theFaraday constant, z is the number of transferred electrons, c is the electrolyte concentration, D is thediffusion coefficient, r is the density,M is the molar mass, b equals 1 according to references [26, 31] and4/3 according to references [32, 33], and g equals 4/3 according to references [26, 34] and 1 according tothe other references. In the next sections, we will consider b = 1 and g = 4/3. Equations 2 and 3 lead toI(t) profiles which display a peaked shape, characteristic of 3D nucleation and diffusion-limited growthprocesses.

Other widely cited reformulations of the model have been developed by Scharifker and Mostany(SM) [31], Mirkin and Milov (MM) [32], and Heerman and Tarallo (HT) [33]. Additional theoreticalapproaches considered to study electrochemical nucleation and growth phenomena can be found else-where [35–38, 30]. It is not the purpose of this chapter to elaborate more on details of these theoreticalmodels, but the reader can find more information in the bibliographic references provided in Table 1.

Table 1 Selected bibliographic references that describe theoretically the early stages of electro-chemical nucleation andgrowth

Ref. Year Type of publication Comments

1 [26] 1983 Journal paper Scharifker-Hills Model (SH)

2 [31] 1984 Journal paper Scharifker-Mostany Model (SM)

3 [32] 1990 Journal paper Mirkin-Nilov Model (MN)

4 [35] 1998 Journal paper Non-stationary nucleation approach

5 [36] 1998 Journal paper Non-stationary nucleation approach

6 [33] 2000 Journal paper Herman-Tarallo Model (HT)

7 [37] 2005 Journal paper Model including proton co-reduction

8 [38] 2008 Journal paper Multi-Step electrochemical reactions

9 [27] 1999 Review paper –




13 [25] 1997 Book Electrochemical nucleation and growth

14 [21] 2006 Book Electrodeposition

15 [22] 2007 Book Nano-Electrodeposition


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The important feature to emphasize here is that independently of the slight modifications of themathematical description of the processes, all the models describing three-dimensional electrochemicalnucleation and growth are based on several assumptions that have no direct proof. One of them is the factthat all nuclei are pinned to a specific site on the surface, and another is that they only grow by directattachment. This concept will be thoroughly discussed throughout this chapter. Further description ofelectrochemical nucleation and growth phenomena can be found in several books and review papers (seeTable 1).

Qualitative agreement of the current transients with the referred models has been reported on countlessoccasions. However, the models allow a quantitative analysis which has been carried out on much lessoccasions. The most typical approach is to first fit the chronoamperograms obtained at differentoverpotentials. This way, if we take Eq. 2 as an example, the diffusion coefficient D and the nucleationrate AN0 can be obtained by a standard least-squares algorithm. On the other hand, morphologicalcharacterization of the substrates is carried out to estimate the number of particles at different depositiontimes in order to get N(t). The nucleation rate constant A and saturation number density N0 can be inferredfrom Eq. 1 in order to compare the values with these obtained with the model fit and hence check thevalidity of the models for different experimental systems.

One important aspect which is emphasized here is related to the experimental determination of N(t),N being the number of formed nuclei after a given deposition time. Obviously, an accurate determinationof N will depend on the dimensions of the nuclei and on the size resolution of the characterizationtechnique employed. Inherent to any electrochemical deposition system, the attachment of thenanoparticles to a surface has restricted traditionally the range of applicable techniques to surfaceanalytical techniques such as Field Emission Scanning Electron Microscopy (FESEM) [19, 39, 40] orAtomic ForceMicroscopy (AFM)/Scanning TunnelingMicroscopy (STM) [41–44]. In practice, however,the resolution of a FESEM is limited to particles with diameters larger than 5–6 nm, and AFM/STM canonly be carried out accurately on very flat substrates, its lateral resolution being much affected by thegeometry of the tip. Also, in situ Transmission Electron Microscopy (TEM) imaging has been developedand used to study the electrodeposition of copper nuclei on gold thin films [45–49]. Unfortunately, the factthat the electrons had to be transmitted through the electrolyte made their TEM spatial resolution rise up toabout 5 nm, making it again impossible to detect smaller nuclei.

Several groups have reported quantitative analyses in which experimental observations agree [40, 41,50, 51] or are in conflict with electrochemical nucleation and growth models [42, 44, 46, 52]. Furtherdetails and more bibliographic references are provided in Table 2. Anyhow, it is clear that even in the bestof the cases, the presence or absence of nanoparticles of d � 5 nm cannot be ascertained. Critical clustersare known to be much smaller than 5 nm. Hence, it is doubtful to which extent experimental observationsof particle number densities can be directly related to N(t). Therefore, it can be asserted that the earlystages of electrochemical nucleation and growth are far from being fully understood.

New Insights into Nanoelectrodeposition: An Electrochemical AggregativeGrowth Mechanism

To clear the uncertainties derived from a lack of resolution, a novel approach has been introduced by ourresearch group. Such an approach consists in using carbon-coated TEM grids (CCTGs) as electrochem-ical electrodes. In this way, an accurate determination of the nanoparticle size distributions and structuralcharacterization at the atomic scale can be combined with electrochemical measurements. More details onthe use of CCTGs as electrochemical electrodes can be found elsewhere [57–59]. The next two sectionsdescribe the studies, performed by means of this approach, of silver [58] and platinum [60]


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Tab

le2

Publisheddataaboutthe

comparisonof

electrochemicalnucleatio

nratesobtained

from

I(t)modelfitsandexperimentalobservatio

nsof

N(t).GCglassy

carbon,

HOPGhighly

oriented

pyroliticgraphite,S

WNTs

single-w

alledcarbon

nanotubes,GRgraphene,O

Mopticalmicroscopy

Ref.

Mat.

Substr.

Model

FitparametersAN0(p/cm

2s),N

0(p/cm

2)

Charact.

Exp.p

aram

etersAN0(p/cm

2s),N

0(p/cm

2)

Exp/Theor

1[19

]Pd

SWNTs

SH

N<

<<

Exp.

AFM,FESEM

N=

1.5�

108

>>

>1

2[39

]Cu

GC

SH

N=

2.6�

105

SEM

N=

9.7�

107

4�

102

3[39

]Cu

GC

[53]

N=

1.4�

1012

SEM

N=

9.7�

107

10�4

4[39

]Cu

GC

SH

AN0=

9�

104

SEM

AN0=

1.5�

105

1

5[40

]Ag

GC

[40]

N=

1�

9�

109

SEM

N=

1�

7�

109

1

6[41

]Ag

HOPG

SH

N=

4�

108�

2�

1010

NC-A

FM

N=

3�

109�

1.2�

1010

1�

10

7[42

]Pt

HOPG

SH

N=

1�

2�

106

STM

N=

5�

109�

1�

1010

103

8[43

]Cu

Si

SH

AN0=

4�

106

AFM

AN0=

3�

107

10

9[44

]Au

Si

SH

N=

106�

5�

107

AFM

N=

1010

5�

102�

104

10[44]

Au

Si

SH

N=

106�

108

AFM

N=

109

10�

103

11[45]

Au

GaA

sSH

N=

106�

107

SEM,T

EM

N=

109�

1012

103�

105

12[46]

Cu

Au

SH

N=

1�

104�

1�

106

TEM

N=

1�

108�

1�

1010

104

13[50]

Ag

GC

SM

N=

2�

105�

7�

107

OM

N=

1�

106�

2�

107

1

14[51]

Ag

H-Si

SH

N=

2�

1010

NC-A

FM

N=

1�

109

10�1

15[52]

Cu

TiN

SH

N=

104�

107

AFM

N=

108�

1011

104

16[54]

Au

GC

SH

N=

3�

106

FESEM

N=

2�

5�

108

102

17[55]

Hg

GC

SM

N=

105�

106

OM

N=

5�

105�

107

5�

10

18[56]

Pd

GR

SM

N=

6�

107

OM

N=

3�

109

50�

100


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electrodeposition. The authors would like to acknowledge Prof. Sara Bals, Dr. Xiaoxing Ke and ThomasAltantzis for the excellent TEM analysis carried out at the Electron Microscopy for Materials Science(EMAT) research group of the University of Antwerp. The authors would also like to acknowledgeDr. Joshua Hammons for his active involvement in the research described in this chapter.

Silver ElectrodepositionPotentiostatic single-pulse depositions were performed by pulsing the electrode potential to E = �0.4 Vvs Ag/AgCl, in a solution of 0.1 M KNO3 + 1 � 10�3 M AgNO3, where the process should be diffusioncontrolled and no other reactions such as hydrogen evolution are present. The early stages of Agelectrochemical nucleation and growth were then analyzed according to the classical electrochemicalmodels described in the section “Early Stages of Electrochemical Nucleation and Growth: ClassicalApproach.” More details on the experiments explained hereafter can be found elsewhere [57–59].

Comparison Between Theoretical Models and Experimental DataOn the one hand, the fact that the potentiostatic current transients satisfy the SH model for progressivenucleation implies that the dimensional chronoamperogram can be fitted to Eq. 2 so that the nucleationrate AN0 is obtained. Good agreement is obtained with the best-fitting parameter being AN0 = 4.21� 1011 � 5.69 � 109 particles/cm2s. Figure 1a shows the experimental chronoamperogram and thetheoretical model for the best-fitting parameters and different values of AN0.

On the other hand, it is possible to determine experimentally the evolution with time of the particledensity, N(t). Figure 1b shows N(t), after analyzing FESEM pictures such as this of Fig. 2a. This meansthat it is possible to fit these data to an exponential curve to obtain A and N0, as shown in Fig. 1b

We clearly identify an exponential behavior which agrees with Eq. 1, A = 92 � 43 s�1 and N0 = 5.28� 109 � 8.85 � 108 particles/cm2 being the best-fitting parameters. The product of these two valuesgives AN0 = 4.86 � 1011 � 2.27 � 1011 particles/cm2s, which is in good agreement with the valueobtained from the fitting of the chronoamperometric curve to Eq. 2. This means that the evolution withtime of the particle distribution agrees with the classical progressive nucleation model.

Particle Analysis: Bimodal Size Distribution and High-Resolution TEM studiesIn this case, High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) has also been used to analyze particle distributions after potentiostatic electrodeposition (seeFig. 2b). This way, no doubt should arise from the limits of resolution because this TEM mode can yieldsub-Angstrom resolution.

In contrast with FESEM images (Fig. 2a), two populations are clearly distinguished corresponding to abimodal size distribution in which “large” particles have d � 6 nm whereas the diameter of the “small”particles is always smaller than 2.5 nm. Figure 2 shows the evolution of particle density (c) and diameter(d) as a function of deposition time. The number of “large” particles grows exponentially with time until itbecomes saturated after 30 ms. Accordingly, their average diameter and size dispersion also growwith time.

On the other hand, the number of randomly dispersed “small” particles also grows with time during thefirst 10 ms but then decreases during the following 20–30 ms. For longer times, the number of those kindsof particles is negligible. Surprisingly, the size of these particles remains constant with d = 1.70 � 0.55nm, independently of the deposition time.The presence of such a bimodal size distribution in the early stages of the process in which such “large”

growing particles coexist with randomly dispersed nongrowing “small” clusters cannot be predicted bythe classical models. It is logical that the nucleation of the “small” clusters is not reflected in thechronoamperograms because their contribution to the total current compared to the larger particles is


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very small, though the understanding of why these nuclei are present and which parameters may affecttheir size and number density is certainly of great importance for nanoscale electrodeposition.

In addition, probe-corrected HAADF-STEM was also used to investigate the atomic-scale structure ofthe particles. Contrarily to what would be expected from a growth mechanism based on direct attachment,most of the medium-sized particles (6 � d � 15 nm) obtained after short deposition times (tdep � 20 ms)presented polycrystalline structures in which the crystalline domains had dimensions ranging from 1 to3 nm (Fig. 2f). It must be pointed out that the dimension of these structural domains coincides with the sizeof the dispersed nongrowing shown in Fig. 2b. Only a small fraction of medium-sized nanoparticles(Fig. 2e) was found to be monocrystalline. This suggests that a mechanism such as aggregation–coa-lescence of small clusters governs the early electrochemical growth of the nanoparticles.

Fig. 1 (a) Experimental chronoamperogram of the potentiostatic electrodeposition of silver at E = �0.4 V vs Ag/AgCl anddimensional plots of the SH model for progressive nucleation (Eq. 2) with different values of the nucleation rate constant AN0.(b) Evolution of “large” nanoparticle number density with deposition time obtained from the analysis of HAADF-STEMpictures and the best exponential fit of 1


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In contrast, larger particles obtained at longer deposition times (tdep � 20 ms) showed in most of thecases monocrystalline structures with defects such as stacking faults and twinnings, typical fromcoalescence events [61, 62]. This may be explained by the tendency of polycrystalline nanoparticles toreduce its internal energy by recrystallizing into a monocrystalline structure [63] and indicates that furthernanoparticle growth is again governed by direct attachment because otherwise larger nanoparticles shouldpresent polycrystalline domains at least in their outer surface.

Ag Electrodeposition on Carbon: An Electrochemical Aggregative Growth MechanismThe data here presented indicates that classical electrochemical nucleation and growth theories do notfully account for some nanoscale phenomena occurring during the early stages of silver electrodeposition.Hence, we have suggested that an Electrochemical Aggregative Growth Mechanism might be the clue tounderstand the observed phenomena on the early stages of electrochemical nanoparticle formation and

1012

6560555045403530d,

nm

2520151050

1 10 100 1000

1011

1010

N, p

artic

les/

cm2

109

108

1 10 100

Small particles (d<2.5 nm)Large particles (d>6 nm) Small particles (d<2.5 nm)

a

e f

b

c d

Large particles (d>6 nm)Randomly disperse

‘small’ nuclei notdetected anymore

t, mst, ms

1000

Fig. 2 Typical FESEM (a) and HAADF-STEM (b) pictures after potentiostatic electrodeposition of silver at E = �0.4 V vsAg/AgCl during 10 ms. Evolution of “large” and “small” nanoparticle number density (c) and size (d) with deposition timeobtained from the analysis of STEM pictures. Probe-corrected HAADF-STEM pictures of a monocrystalline (e) andpolycrystalline (f) Ag nanoparticle


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growth. Figure 3 represents schematically the stages of the proposed mechanism. Further informationabout the discussion presented in this section can be found elsewhere [58, 59].

In the first instants after the application of a negative potential, very small clusters of r � rc are formedrandomly distributed through the carbon substrate (particles such as 1 in Fig. 3a). Then, as the nucleationis progressive, more small clusters will be formed until the nucleation exclusion zones have overlapped

Fig. 3 Top: Electrochemical Aggregative Growth mechanism. Red dots represent the nanoparticles and blue circles theprojection of their corresponding nucleation exclusion zones. Black stripes within a particle represent defects, whereas theabsence of stripes represents a defect-free monocrystalline structure. Bottom: Representation of the classical electrochemicalgrowth by direct attachment and the Electrochemical Aggregative Growth mechanism


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and covered the entire surface. Although those small clusters do not dissolve because r � rc, they areunstable and can easily move through the surface because of their small size [64] and the weak van derWaals (VDW) forces between carbon and silver atoms.

These small clusters aggregate with each other or with other aggregates. This aggregation mechanismtakes place due to a thermodynamic driving force altering the system toward its lowest energy configu-ration, as small nuclei may move through the surface because of the same reasons that adatoms movetoward edges or kink sites [64].

Then, until the conditions of planar diffusion are reached (i.e., when nucleation exclusion zones haveoverlapped and covered the entire surface as in Fig. 3c), three phenomena happen in parallel: nucleation ofnew small clusters, surface movement of small clusters which aggregate with each other or with otheraggregates, and direct attachment (i.e., incorporation of Ag+ ions onto aggregates and small clusters).A small fraction of monocrystalline structures (particles such as 4 in Fig. 3b) is also present. Therefore, wesuggest that both aggregation and direct attachment are responsible for early nanoparticle growth,aggregation being predominant over the classical growth mechanism.

Afterward, when nucleation exclusion zones have completely covered the surface (Fig. 3c), no morenucleation is supposed to happen on the carbon surface so the concentration of the randomly dispersedsmall clusters decreases due to their aggregation or incorporation in the other aggregates.

Finally, when all the small clusters have been consumed (Fig. 3d), the number of aggregates reachessaturation. At this stage, previous polycrystalline and aggregated particles undergo recrystallization intomonocrystalline structures so most of the larger particles are monocrystalline with a high amount ofdefects (particles such as 7 and 8 in Fig. 3c and d). In later stages, as no more small clusters are present onthe carbon surface and nucleation exclusion zones cover all the carbon surface, further nanoparticlegrowth should be due to direct attachment.

The most significant feature introduced by the proposed mechanism is that in the early stages ofelectrochemical nucleation and growth, surface movement and aggregation of clusters of d � 1 � 2 nmcannot be considered negligible and determine in great extent the size and structure of electrodepositednanostructures. This is a striking result not only because these mechanisms had never been considered inthe widely accepted electrochemical models for nucleation and growth but also because as a result,nanoparticles in the early stages of their electrochemical growth are polycrystalline instead of monocrys-talline, which is the most commonly accepted hypothesis.

Platinum ElectrodepositionTo get new insights into the electrodeposition of another metal, platinum deposition was studied with asimilar strategy to the one described in the section “Silver Electrodeposition.” Hence, potentiostaticsingle-pulse depositions were performed by pulsing the electrode potential to various potentials in asolution of 1 � 10�3 M H2PtCl6 + 0.1 M KCl. The early stages of Pt electrochemical nucleation andgrowth were again analyzed according to the classical electrochemical models described in the section“Early Stages of Electrochemical Nucleation and Growth: Classical Approach.” More details on theexperiments explained hereafter can be found elsewhere [60, 59].

Comparison Between Theoretical Models and Experimental DataContrarily to the case of Ag electrodeposition, the current-time transients obtained for the early stages ofplatinum electrodeposition, shown in Fig. 4a, do not always display the typical peaked-shape character-istic of the 3D island growth mechanism [26, 25].

However, for some potentials, such as E = �0.5 V, the peaked shape of the current transients allows ananalysis according to the theoretical models described in the section “Early Stages of ElectrochemicalNucleation and Growth: Classical Approach.” In this case, a good fit is found for an instantaneous


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nucleation model, leading to a saturation particle density, N � 4 � 105 particles/cm2 [65]. It must benoted that platinum electrodeposition occurs together with different hydrogen reduction reactions cata-lyzed by the actual platinum surface. Therefore, the obtained current-time transients represent a convo-lution of both phenomena. If we take into account the effect of H adsorption in the model fit [37, 56], agood fit is in this case found for a progressive nucleation model. In this case, the nucleation rate is found tobe approximately AN0 = 7.8 � 104 particles/cm2.

On the other hand, it is possible to determine experimentally the particle density, N, for differentpotentials and times, as shown in Fig. 4b. For E = �0.5 V and t = 20s, N = 3 � 1010 particles/cm2,being approximately five orders of magnitude larger than the value obtained by fitting the chronoam-perograms to the classical models. Although this reflects an important contradiction, a literature analysisreveals that for Pt electrodeposition, the difference in number density between experimental and calcu-lated values has also been reported to rise up to several orders of magnitude [42]. Similar phenomena alsooccur with other metals such as Cu [46] or Pd [56].

Fig. 4 (a) Chronoamperommetric current transients obtained for the electrodeposition platinum onto CCTGs from a solutionof 1 � 10�3 M H2PtCl6 + 0.1 M KCl. (b) Evolution of particle number density with applied potential for different depositiontimes


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Particle Analysis: Bimodal Size Distribution and High-Resolution TEM StudiesTo clear the doubts arising from a lack of resolution, HAADF-STEM was also used to analyze particledistributions. HAADF-STEM images at low magnification have shown that for high overpotentials(E � �0.4 V), a population of nanoclusters of d � 2 � 4 nm coexists with larger nanostructuresregardless of deposition time. However, only large dendritic structures are observed at low overpotentials(E � �0.2 V) [60].

Although the morphology of the deposits and the current transients are very different from the onesobtained for Ag electrodeposition (Section “Silver Electrodeposition”), bimodal size distributions for theearly stages of Pt electrodeposition at certain potentials suggest that similar growth by nanoclusteraggregation may be taking place.

To get more hints on their formation mechanism, electron tomography and HAADF-STEM at highermagnification were used to observe representative nanostructures obtained at the two potential regimesand different deposition times. Figure 5a–c are representative for the low overpotential regime, andFig. 5d–f are representative for the high overpotential regime.

At low overpotentials, nanodendritic morphologies are depicted in all cases, consisting of manyrandomly oriented “branches” which build up a quasicircular shape (Fig. 5a, b). Different depositiontimes do not influence the dendritic morphology. In all cases, many spherical protuberances of d � 2 nmcan be seen on the outer edges of the nanostructure, most of the times linked to the rest of the body by anarrower neck. Even if lattice fringes extend coherently through large domains within the nanostructure,domains with different crystallographic orientation coexist within the same “particle” as shown byFig. 5c.

When deposited at high overpotentials, platinum nanoparticles also display an irregular porous shapebut are more compact (Fig. 5d). After short deposition times, Fig. 5e shows that many spherical pro-tuberances are again connected to the body of the structure by necks as narrow as few atomic layers. These

Fig. 5 Three-dimensional electron tomography reconstructions (top views: electrode surface below the particle) of platinumdendritic nanostructures obtained by electrodeposition at potential of (a) �0.1 V for 20 s and (d) �0.6 V for 200 s. Highmagnification HAADF-STEM images of platinum nanostructures obtained after electrodeposition at a potential of (b)�0.1 Vfor 200 s, (e) �0.6 V for 3 s and (f) �0.6 V for 200 s. (c) Region showing several domains with different crystallographicorientation, together with the corresponding FFT shown as inset


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protuberances have an equivalent diameter of 2–3 nm, just as in the case of low overpotentials. Figure 5e, fshow that in contrast with the case of low overpotentials, an increase in deposition time leads to morecompact structures and smoother edges. In these cases, lattice fringes are coherent within larger domains.However, structural defects such as twin planes or the open pores indicated by arrows in Fig. 5f pointagain toward nanocluster coalescence events [61, 62].

Pt Electrodeposition on Carbon: An Electrochemical Aggregative GrowthMechanism. Influence ofRecrystallization and CoalescenceTraditionally, the growth of dendritic nanostructures had been associated to diffusion-limited growth[66]. However, pure epitaxial growth can be ruled out in the cases that spherical protuberances are linkedthrough narrow necks (Fig. 5a or e) [67, 62, 61, 68, 69], that domains with different crystallographicorientation coexist (Fig. 5c) [70, 71, 62, 61, 72], or that structural defects such as twin planes or stackingfaults are abundant [61, 62], as all these features indicate cluster coalescence events. At highoverpotentials, the coexistence of large nanostructures with isolated nongrowing small nanoclusterspoints again toward nanocluster aggregation-mediated growth [71, 58] (see Section “Silver Electrodepo-sition”). Accordingly, dendrite branch thickness, spherical protuberances, and isolated nanoclusters havethe same dimensions (d � 2 nm), regardless of applied potential and deposition time.

This is an important concept to take into account as metal electrodeposition had traditionally beenconsidered to proceed by nucleation and direct attachment, due to the assumption that growing nucleiwould be pinned to the surface and motionless. Although some groups have found irregular Ptnanostructures and suggested that cluster aggregation could take part [73, 42, 74], cluster aggregationmechanisms and their influence in electrochemical growth have not been discussed in detail.Alternatively, the suggested growth mechanism is schematized in (Fig. 6).

At low overpotentials, even at later stages of the electrochemical deposition process, largenanostructures keep growing by the addition of clusters of the same size. This in turn indicates that aself-limiting growth mechanism stops the epitaxial growth of primary clusters and dendritic aggregates.

At high overpotentials, though, the situation is slightly different. After long deposition times, theelectrodeposited nanostructures are more compact, less porous, and their lattice fringes extend over verylarge domains, indicating that they have undergone a higher degree of recrystallization and epitaxialgrowth by atomic incorporation. Firstly, this indicates that recrystallization kinetics are overpotentialdependent and are accelerated at high overpotentials. Secondly, this indicates that a certain degree of

Fig. 6 Schematic representation of the electrochemical aggregative growth mechanism of platinum porous dendriticnanostructures


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nanocluster coalescence and recrystallization is needed to overcome the self-limiting growth mechanismand allow further growth by atomic incorporation.

A Generalized Electrochemical Aggregative Growth MechanismSections “Silver Electrodeposition” and “Platinum Electrodeposition” describe the discovery of newinsights into Ag and Pt electrodeposition on carbon substrates, by using carbon-coated TEM grids aselectrodes, and different TEM characterization modes. Although many differences exist between thegrowth mechanisms inferred for both materials, it has been shown in both cases that nanocluster self-limiting growth, surface diffusion, aggregation, and coalescence need to be taken account to understandthe early stages of electrochemical growth. In this section, we analyze both systems together and elaborateon a Generalized Electrochemical Aggregative Growth Mechanism that can be used to describe theprocess of metal electrodeposition on low-energy substrates. More details can be found elsewhere [65,59].

Evaluation of Different Growth PathwaysFigure 7 shows a summary of the different growth pathways of Ag and Pt electrodeposition on carbon.

When the main growth mechanism is atomic addition, monocrystalline defect-free nanoparticles areexpected to be the most abundant species. However, in the case of silver electrodeposition, only a verysmall portion of particles are found to present such a structure. Contrarily, most of the silver nanoparticlesare found to grow through a mechanism such as the one depicted by growth pathway “a” (see section“Ag Electrodeposition on Carbon: An Electrochemical Aggregative Growth Mechanism”). Platinum,though, forms irregular porous dendritic nanostructures rather than hemispherical nanoparticles, asindicated by the growth pathways “b” and “c” (see section “Platinum Electrodeposition”).

A first important conclusion that can be derived from this analysis is that such an ElectrochemicalAggregative Growth Mechanism is common for two metals from different rows and columns of theperiodic table such as Ag and Pt. Therefore, it can be suggested as a general metal electrodepositionmechanism onto low-energy substrates such as carbon. Secondly, recrystallization and coalescencekinetics are dependent on material, overpotential, and adsorbed species and dictate the morphology of

Fig. 7 Schematic diagram showing the growth pathways inferred by HAADF-STEM observation during the electrodepositionof silver (a) and platinum (b, c) onto CCTGs. (a) Growth by nanocluster aggregation and full coalescence resulting intomonocrystalline nanoparticles with defects. (b) Growth by nanocluster aggregation and partial coalescence resulting in porousnanostructures. (c) Growth by nanocluster aggregation and small extent of coalescence resulting in ultra-porous dendriticnanostructures


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the final nanostructures to a large extent. Thirdly, direct attachment must not be excluded from the growthprocess, but its contribution is only noticeable after the aggregates have undergone a high degree ofrecrystallization.

To fully understand the described mechanism and its different growth pathways, an evaluation ofelectrochemical chronoamperograms and resulting nanoparticle size distributions is carried out in the nextsections. Some concepts such as self-limiting growth, cluster surface diffusion, coalescence, and recrys-tallization will be used. Although they are common in other new phase formation fields, they are barelyused in electrochemical systems. Therefore, the reader is encouraged to look up the discussion provided ina recent publication [65].

Evaluation of Nanoparticle Size DistributionsFigure 8 shows characteristic FESEM pictures presenting an overview of the distribution of silver (a–c)and platinum (d–f) nanoparticles electrodeposited at high overpotentials and different deposition times.Figure 8 shows also the corresponding evolution of average diameter (g) and particle number density(h) of both “large” and “small” particle populations for silver and platinum electrodeposition. Silvernanoparticles are formed at much shorter deposition times than platinum due to differences in reactionkinetics, and so the deposition time is represented in a logarithmic scale for the sake of comparability.

Figure 8g shows that small particles do not grow with deposition time in any of the cases. However, thesize of such isolated small clusters is material dependent. This implies that a self-limiting growthmechanism prevents the growth of primary nanoclusters and stabilizes particles of different sizesdepending on the material. On the other hand, we have shown that Pt aggregates which undergo a verysmall amount of recrystallization still show a self-limiting growth mechanism, whereas fullyrecrystallized aggregates do not (see section “Platinum Electrodeposition”) [60]. One possible explana-tion, linked to closed-shell magic sizes, would be that the nanoclusters have a metastable atomicconfiguration, which hinders epitaxial growth, but become unstable after coalescence. However, in allthe cases, primary nanoclusters have relative large size dispersion (33 %), indicating that adsorption-driven stabilization is more plausible than closed-shell magic sizes. Therefore, another possible explana-tion would be that stabilization is related to specific adsorption onto the exposed facets of the primarynanoclusters, which change after coalescence. On the other hand, Fig. 8g also shows that “large” Ag andPt nanoparticles only start to grow after a certain induction time (tInd � 30 ms for Ag; tInd � 10 s for Pt);before this time, their size remains more or less constant (d � 15 � 5 nm for Ag; d � 8 � 2 for Pt). Suchan induction time corresponds to the aggregation of primary nanoclusters onto larger entities which maygrow at later stages [58]. Such a phenomenon, which can be considered an aggregative nucleationprocess, has been recently reported in the field of colloidal synthesis [71, 75, 76].

In the case of Ag electrodeposition, the induction time corresponds to the period in which the number ofaggregates increases by the assembly of isolated primary nanoclusters. Figure 8h shows that after theinduction period (tInd � 30 ms), no more aggregates are created because the surface has been depleted ofprimary nanoclusters. In the case of platinum, large aggregates start to grow after tInd � 10 s, but smallprimary clusters are continuously being formed on the carbon surface, and consequently, more and morelarge aggregates are also created continuously.

In fact, if one looks at the evolution of the aggregate diameter (i) and number density (j) with the surfacecoverage, shown in Fig. 8, it becomes clear that in the case of silver, the number of aggregates reachessaturation at small surface coverage of about 1 %, whereas for platinum, the large-particle number densitykeeps increasing even at large surface coverage of about 30–40 %. Correspondingly, for silver, the large-particle size starts to increase at low surface coverage, whereas platinum aggregates remain small evenafter large surface coverage has been reached. Under the assumption of growth by direct attachment,growth under diffusion control generates diffusion zones around growing islands of about 10 times their


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Fig. 8 (continued)


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diameter [77]. This would imply that when diffusion fields cover the whole surface, surface coverageshould be about 1 %. This is exactly the case for silver electrodeposition, indicating that silver aggregatesgrow by direct attachment under diffusion control. This behavior is confirmed by their monocrystallinestructure after long deposition times, shown in the growth pathway “a” in Fig. 7. Such generation ofdiffusion zones around the growing aggregates implies that the concentration of ions close to the surfacegets gradually smaller, which in turn implies that after a certain moment, no more primary clusters andhence no more aggregates can be generated. On the other hand, Fig. 8j shows that the number of platinumaggregates keeps increasing at high surface coverage, meaning that new primary nanoclusters arecontinuously generated over the carbon surface. This implies that no (or very small) diffusion fields arecreated around the aggregates, which in turn indicates that their growth by atomic incorporation is notlimited by diffusion of active species toward the surface.

As shown in Fig. 7, the main difference between platinum and silver growth pathways is the fact thatsilver aggregates recrystallize into monocrystalline nanoparticles, whereas platinum forms porous den-dritic structures which only seem to partially recrystallize at large overpotentials. Therefore, silveraggregates behave as traditional islands and grow by atomic incorporation once they have undergonetotal recrystallization. This is why the evolution of large-particle or aggregate distributions follows thetrends established by conventional electrochemical nucleation and growth theories and correlates with

Fig. 8 Representative FESEM pictures after the electrodeposition of silver onto CCTGs at a potential of�0.4 V for (a) 10, (b)100 and (c) 500 ms. Representative FESEM pictures after the electrode-position of platinum onto CCTGs at a potential of�0.6 V for (d) 3, (e) 20 and (f) 200 s. Insets: high magnification HAADF-STEM figure of a monocrystalline silver nanoparticle(c) and 3D reconstruction of a porous platinum nanostructure (f). Time evolution of silver and platinum nanoparticle (g)average diameter and (h) number density after electrodeposition onto CCTGs at potentials of �0.4 and �0.6 V respectively.Evolution of “large” particle (i) average diameter and (j) number density with surface coverage for silver and platinumelectrodeposition onto CCTGs at potentials of �0.4 and �0.6 V respectively


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classical chronoamperometric models with good agreement (see section “Silver Electrodeposition”). Onthe other hand, partially recrystallized platinum dendritic nanostructures behave halfway between grow-ing islands and small “stabilized” nanoclusters which cannot grow due to a self-limiting growth mech-anism. Therefore, even under electrochemical diffusion control, the evolution of particle morphology andsize distribution does not follow the assumptions of the Volmer-Weber 3D island growth mechanism.

Evaluation of Chronoamperometric DataThe potentiostatic current transients obtained for silver (a) and platinum (b) electrodeposition have beenshown in previous sections (see Figs. 1a and 4a). The analysis of the silver current transients performed inthe section “Silver Electrodeposition” leads to a calculated nucleation rate, AN0, in agreement with thenumber of “large” aggregates derived from HAADF-STEM analysis. Silver aggregates, which have fullyrecrystallized into monocrystalline islands, grow by direct attachment. Hence, the assumptions from theclassical models are fulfilled, and good agreement between experimental and theoretical data is achieved.

On the other hand, it has been shown that most of the current-time transients obtained for the earlystages of platinum electrodeposition do not display the typical peaked-shape characteristic of the 3Disland growth mechanism [26, 25]. At E = �0.1 V, I(t) has a constant value, characteristic of a chargetransfer–limited electrochemical reaction. However, kinetically limited island growth should lead to anincreasing current density due to an increasing active surface area, until physical island overlap is reached.This is not the case, indicating that the deposited islands do not act as active surface area for direct atomicincorporation. Under these conditions, platinum aggregates are ultraporous nanostructures which havenot undergone almost any recrystallization and present no signs of growth by direct attachment (Fig. 7,growth pathway “c”). At E = �0.2 V, I(t) shows long-term decaying characteristics that cannot be due todouble-layer charging because they extend over � 15 s. The current decay in this case is due to the factthat platinum reduction takes place in diffusion-limited or mixed-control regime. Still, no signs of currentincrease indicate that primary nanoclusters are being formed under planar diffusion limitations and thatlarge aggregates are not contributing to an increase in active surface area for direct attachment. This isagain correlated to the ultraporous nanodendritic morphology obtained at these potentials (Fig. 7, growthpathway “c”).

At E = �0.4 V, I(t) starts by a long-term decay, followed by a slight increase in current, indicating thata certain degree of uncoupled diffusion toward growing active centers occurs. At E = �0.5 V andE = �0.6 V, current starts increasing at earlier times indicating an earlier onset of island growth bydirect attachment. This is again linked with the fact that the obtained nanostructures have undergone acertain degree of recrystallization and are thus smoother and less porous than those obtained at smalleroverpotentials (Fig. 7, growth pathway “b”).

We have shown in the section “Comparison Between Theoretical Models and Experimental Data” thatthe fitting of Pt chronoamperograms to classical models leads to a saturation particle density approxi-mately five orders of magnitude smaller than the value observed experimentally. The fact that weexperimentally observe much more particles and much smaller than predicted by the models pointsagain toward a self-limiting growth mechanism, which prevents primary nanoclusters to grow by atomicincorporation.

Revision of the Nucleation and Growth Mechanism: A Generalized Electrochemical AggregativeGrowth MechanismIn conclusion, the data presented throughout this chapter indicate that metal electrodeposition ontolow-energy substrates, such as carbon, proceeds by an Electro-chemical Aggregative Growth Mechanisminstead of by a classical Volmer-Weber mechanism in which only direct attachment is considered. Theproposed mechanism starts with the nucleation of primary nanoclusters that grow until a certain self-


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limiting growth mechanism stabilizes them at a given size. Then, electrochemical potential-driven surfacediffusion of these primary nanoclusters leads to aggregation, which can be interpreted as aggregative-nucleation events. Finally, the degree to which aggregates undergo partial or full coalescence dictates towhich extent further growth by direct attachment (or classical island growth) occurs. Figure 9 (top)schematically represents the stages of the proposed mechanism. Its implications on the interpretation ofthe potentiostatic current transients are shown in Fig. 9 (bottom).

In the first moments after the application of a negative potential, very small primary nanoclusters areformed, randomly distributed over the substrate as shown in Fig. 9.1. These clusters are single crystallinenanoparticles that grow by direct attachment until they reach a metastable size. These size-stabilizedprimary nanoclusters can diffuse over the carbon surface due to their small size and weak VDW forcesbetween them and the carbon support. Due to particle-particle attractive VDW forces, primarynanoclusters stick together when they hit each other, resulting in aggregative-nucleation events, whichgive birth to aggregated particles from nanocluster building blocks (Fig. 9.2 and second column of Fig. 7).Silver aggregates (Figs. 9.2a and 7a) are much more compact than platinum ones (Figs. 9.2b, c and 7b, c)because they undergo more and faster recrystallization. Until this stage, new primary nanoclusters andconsequently new aggregates are continuously formed on the substrate in both cases, because even underthe assumptions of island growth by direct attachment, the diffusion zones of potentially growing nucleiwould not have yet covered the whole carbon surface.

Fig. 9 TOP: Schematic diagram showing the different stages of the Generalized Electrochemical Aggregative GrowthMechanism and respective potentiostatic current transients. Dots represent the non-growing nanoclusters and blue circlesaround the aggregates represent the projection of their corresponding nucleation exclusion zones. BOTTOM: (2.a) Fullrecrystallization. (1) Induction time: Formation of non-growing primary nanoclusters, (2) Formation of primary nanoclusters +uncoupled “island” growth of completely recrystallized aggregates, (3) Coupled “island” growth of completely recrystallizedaggregates. (2.c) Almost no recrystallization. Continuous formation and aggregation of non-growing primary nanoclusters


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An important point here is related to the interpretation of the electrochemical current-time transients.Traditionally, the first decaying part is related to double-layer charging, whereas island nucleation andgrowth is correlated to hemispherical diffusion to an increasing active surface, leading to an increasingcurrent. We have shown, though, that longer current decays should be of faradic origin, and hence webelieve that such I(t) feature reflects the formation of primary nanoclusters that do not grow beyond agiven size. This period may be considered an induction time because large aggregates do not growsignificantly yet. Such a phenomenon is normally correlated to a prenucleation stage where metal ions arebeing discharged before nuclei have been formed. However, we believe that such induction timecorresponds to a preaggregation or precoalescence stage where primary nanoclusters have alreadynucleated but cannot grow over a given size unless they undergo aggregation and recrystallization.Subsequent current rise and maximum are due to the growth of recrystallized aggregates. In fact, thedegree to which such aggregates undergo coalescence and recrystallization dictates the subsequentgrowth pathways.

A first possibility is that the aggregates fully coalesce and recrystallize fast into monocrystallinenanoparticles (Figs. 9.2a and 7a), as it happens for silver electrodeposition. Then, the self-limiting growthmechanism vanishes, and the particles may grow by direct attachment. This is probably the most commoncase, and it implies that classical island growth concepts apply. Thus, diffusion zones are generatedaround growing particles until they cover the entire surface and particle number density reaches saturationat small surface coverage of 1 %, while small primary nanoclusters are consumed. Therefore, a classicalinterpretation of the potentiostatic current-time transients can be carried out taking into account that thederived nucleation rate corresponds to an “aggregative-nucleation + recrystallization” rate rather than toprimary nanocluster formation (Fig. 9.2a).

A second possibility is that aggregates undergo partial recrystallization as it happens for platinumelectrodeposition at high overpotentials (Figs. 9.2b and 7b). In this case, classical growth concepts do notapply, as partially recrystallized aggregates behave halfway between traditional islands and nongrowingclusters. This way, small diffusion zones may be generated around growing aggregates, slowing down thenucleation rate of both primary nanoclusters and aggregates. However, the extent to which the particlesgrow by direct attachment is smaller than in the first case, and hence, diffusion zones do not cover theentire surface until longer deposition times. In this case, particle number density keeps increasing even athigh surface coverage. The fact that many aggregates do not grow by atomic incorporation may favor thenucleation of new primary nanoclusters, as discharged atoms may be repelled by nongrowing particles,thus increasing their concentration on the carbon surface. In this case, the shape of the current transientsdepends on the degree of coalescence and recrystallization and growth by direct attachment.

A third possibility is that recrystallization happens to a very small extent leading to a lower degree ofdirect attachment (Figs. 9.2c and 7c). In these cases, highly porous dendritic nanostructures are obtained,ever decaying current transients are measured (Fig. 9.2c), and aggregates behave as nongrowing primarynanoclusters, which do not contribute to an increase in reactive surface area.

In conclusion, particle number density, size, and morphology depend on the balance between primarynanocluster nucleation, cluster surface diffusion, cluster aggregation, and coalescence kinetics. So do theobtained potentiostatic current transients and their interpretation.

Conclusions

Although the synthesis of supported nanostructures by electrochemical deposition offers many advan-tages compared to other synthesis methods, a lack of knowledge of the fundamental aspects of the earlystages of electrochemical nucleation and growth had been identified. This book chapter summarizes the


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research which has been carried out in this field by using CCTGs as electrochemical electrodes, which hasallowed to combine electrochemical characterization with structural characterization at the atomic scale.

By the combination of aberration-corrected HAADF-STEM, FESEM, and electrochemical character-ization of different electrochemical deposition systems, a reformulation of the Volmer-Weber islandgrowth mechanism for the early stages of metal electrodeposition on low-energy substrates has beenprovided. A Generalized Electrochemical Aggregative Growth Mechanism has been elaborated, whichmimics the atomistic processes of the early stages of thin-film growth by considering nanoclusters of a fewnm as building blocks instead of single atoms. This way, the influence of new processes, such asnanocluster self-limiting growth, surface diffusion, aggregation, and coalescence, on the growth mech-anism, morphology of the resulting nanostructures, and interpretation of potentiostatic current transientshave been discussed.

This mechanism, apart from being an important scientific breakthrough from the fundamental point ofview, is crucial to gain better control of electrochemical deposition processes in order to obtain supportednanostructures with desired morphology and enhanced properties. Besides, it provides exciting possibil-ities for electrochemical nanostructuring with nanoclusters as building blocks.

Acknowledgments

The authors acknowledge the support from the FondsWetenschappelijk Onderzoek in Vlaanderen (FWO,contract no. FWOAL527), the Flemish Hercules 3 program for large infrastructure and the SociétéFrançaise de Bienfaisance et d’Enseignement (S.F.B.E.) de San Sebastian-Donostia (Spain).

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new insights in nano-electrodeposition: an electrochemical ... · methods such as sputtering or...

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