Electrocrystallization of Au nanoparticles on glassy carbon from HClO4 solution containing [AuCl4]-

Article abstract of DOI:10.1016/j.electacta.2008.08.013

The mechanism and kinetics of electrocrystallization of Au nanoparticles on glassy carbon (GC) were investigated in the system GC/1 mM KAuCl₄ + 0.1 M HClO₄. Experimental results show that the gold electrodeposition follows the so-called Volmer-Weber growth mechanism involving formation and growth of 3D Au nanoparticles on an unmodified GC substrate. The analysis of current transients shows that at relatively positive electrode potentials (E ≥ 0.84 V) the deposition kinetics corresponds to the theoretical model for progressive nucleation and diffusion-controlled 3D growth of Au nanoparticles. The potential dependence of the nucleation rate extracted from the current transients is in agreement with the atomistic theory of nucleation. At sufficiently negative electrode potentials (E ≤ 0.64 V) the nucleation frequency becomes very high and the nucleation occurs instantaneously. Based on this behaviour is applied a potentiostatic double-pulse routine, which allows controlled electrodeposition of Au nanoparticles with a relatively narrow size distribution.

Full text of DOI:10.1016/j.electacta.2008.08.013

Electrochimica Acta 54 (2008) 168–172
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrocrystallization of Au nanoparticles on glassy carbon
from HClO4 solution containing [AuCl4]−
L. Komsiyska, G. Staikov∗,1
Institute of Bio- and Nanosystems (IBN 3), and CNI-Centre of Nanoelectronic Systems for Information Technology,
Research Centre Jülich, D-52425 Jülich, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2008
Received in revised form 6 August 2008
Accepted 8 August 2008
Available online 19 August 2008
The mechanism and kinetics of electrocrystallization of Au nanoparticles on glassy carbon (GC) were
investigated in the system GC/1 mM KAuCl4 + 0.1 M HClO4. Experimental results show that the gold elec-
trodeposition follows the so-called Volmer–Weber growth mechanism involving formation and growth
of 3D Au nanoparticles on an unmodified GC substrate. The analysis of current transients shows that at
relatively positive electrode potentials (E ≥ 0.84 V) the deposition kinetics corresponds to the theoretical
model for progressive nucleation and diffusion-controlled 3D growth of Au nanoparticles. The poten-
tial dependence of the nucleation rate extracted from the current transients is in agreement with the
atomistic theory of nucleation. At sufficiently negative electrode potentials (E ≤ 0.64 V) the nucleation fre-
quency becomes very high and the nucleation occurs instantaneously. Based on this behaviour is applied
a potentiostatic double-pulse routine, which allows controlled electrodeposition of Au nanoparticles with
a relatively narrow size distribution.
Keywords:
Electrodeposition
Gold
Growth
Nanoparticles
Nucleation
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
electrodeposition of metals on glassy carbon (GC) [9–20], highly
oriented pyrolytic graphite (HOPG) [4,21–26] and H-terminated
Gold nanoparticles exhibit interesting size-dependent elec-
tronic, optical and chemical properties and are very attractive for
various technological applications in nanoelectronics, sensorics,
optics and catalysis [1–3]. A powerful technique for modification
of electrodes with metal nanoparticles is the electrodeposition
since the driving force and the rate of the deposition process
can be easily controlled by the electrode potential and the cur-
rent density [4–8]. The key to electrodeposition of nanoparticles
with controlled shape and size is the knowledge of the kinetics
and mechanism of initial electrocrystallization stages in the par-
ticular electrochemical system. Electrodeposition of isolated metal
nanoparticles can be achieved successfully in systems with weak
metal–substrate interaction. No underpotential deposition (UPD)
takes place in such systems and the electrocrystallization pro-
cess starts in the overpotential deposition (OPD) range with the
formation and growth of 3D metal nanoparticles on an unmod-
ified foreign substrate (Volmer–Weber growth mechanism) [6].
This growth mechanism operates on electrode substrates with
low surface energy and has been observed most frequently by
n-Si [27–38]. Detailed studies of nucleation and growth kinet-
ics by gold electrodeposition have been performed on GC and
n-Si substrates in the systems GC/[Au(NH3)2]+ + 1 M NH4Cl [10],
GC/[AuCl4]− + 6 M LiCl [11] and n-Si/[Au(CN)2]− + 0.2 M KCN(pH 14)
[33–35]. Several groups have used as electrolytes for electrode-
position of Au nanoparticles [AuCl4]−-containing acidic HClO4 or
H2SO4 aqueous solutions as well as room-temperature ionic liquids
and investigated the influence of deposition conditions on the size,
structure, morphology and electrochemical properties of electrode-
posited nanoparticles [14–20,39–41]. However, in these systems
detailed studies of involved nucleation and growth phenomena are
absent, which are important for the optimization of polarization
routines for controlled electrodeposition of Au nanoparticles.
In this paper, we report a study of the kinetics and mecha-
nism of electrocrystallization of Au nanoparticles in the system
GC/1 mM[AuCl4]− + 0.1 M HClO4 by means of cyclic voltammetry,
chronoamperometry and scanning electron microscopy (SEM). It
is shown that the electrodeposition of Au nanoparticles follows
the theoretical model for nucleation and 3D growth controlled by
hemispherical diffusion. The potential dependences of the nucle-
ation rate and the size of critical nuclei were determined from
the analysis of initial parts of current transients at relatively posi-
tive electrode potentials. A double-pulse polarization routine was
applied to deposit Au nanoparticles with a narrow size distribution.
∗
Corresponding author. Tel.: +49 2461 612064; fax: +49 2461 613907.
E-mail address: g.staikov@fz-juelich.de (G. Staikov).
1
ISE member.
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.08.013

L. Komsiyska, G. Staikov / Electrochimica Acta 54 (2008) 168–172
169
2. Experimental
Electrochemical measurements were carried out at room tem-
perature (T = 294 K) in a conventional three-electrode cell using
an Autolab Potentionstat/Galvanostat PGSTAT30 (Eco Chemie, The
Netherlands). The electrodeposition of Au nanoparticles was per-
formed from a solution consisting of 1 mM KAuCl4 and 0.1 M HClO4.
The electrolyte solution was prepared from suprapur chemicals and
Millipore® water and was deoxygenated prior to each measurement
with pure nitrogen. A saturated mercury sulfate electrode (SMSE)
was used as reference electrode. All electrode potentials E given in
this report are referred to the potential of the standard hydrogen
electrode (SHE).
The GC substrates (0.6 cm × 0.6 cm) were polished with dia-
mond paste (up to 0.3 ␮m) and subsequently cleaned ultrasonically
for 10 min in ethanol and 10 min in Millipore® water. The mor-
phology and the size of electrodeposited Au nanoparticles were
characterized by SEM using a Leo 1550 SEM equipment with elec-
tron beam energy of 20 keV.
Fig. 2. Current transients for electrocrystallization of gold at different electrode
potentials in the system GC/1 mM[AuCl4]− + 0.1 M HClO4.
[AuCl4]−-containing electrolytes with low concentration of free
Cl− ions [43,44]. According to this mechanism the overall depo-
sition reaction (1) involves two charge transfer steps coupled with
a preceding chemical reaction:
3. Results and discussion
The cyclic voltammogram shown in Fig. 1 illustrates the electro-
chemical behaviour of the system GC/1 mM[AuCl4]− + 0.1 M HClO4.
In this system the active gold species in the electrolyte solution are
the [AuCl4]− complex ions and the overall deposition reaction can
be expressed by
[AuCl4]− ⇔ [AuCl2]+ + 2Cl−
[AuCl2]+ + 2e− ⇒ [AuCl2]−
[AuCl2]− + e− ⇒ Au + 2Cl−
(2)
(3)
(4)
[AuCl4]− + 3e− ⇒ Au + 4Cl−
(1)
The cyclic voltammogram in Fig. 1 shows that the anodic dis-
solution of the gold deposit is strongly inhibited. This inhibition
can be assigned to the low chloride concentration in the electrolyte
solution and/or to the formation of a passivating film of “adsorbed
oxygen” or oxide on the gold surface, which starts in the same
potential range and can impede significantly the gold dissolution
reaction [45,46].
The absence of cathodic current at potentials more positive
than 1 V in the initial cathodic scan of the cyclic voltammogram in
Fig. 1 shows that an Au UPD does not occur in this system and the
deposition process follows the Volmer–Weber growth mechanism.
The sharp increase of the cathodic current at potentials below 1 V
indicates the onset of gold electrodeposition. The constant current
observed during the reversed anodic scan shows that the deposi-
tion process becomes diffusion limited. The higher cathodic current
observed during the reversed scan for E > 0.97 V indicates that the
deposition of gold on gold is easier than the nucleation of gold on
GC. A crossover of reverse current and zero current line is observed
at a potential E ≈ 1.13 V, which is more positive than the stan-
dard equilibrium potential EA0u(III)/Au(0) = 1.00 V corresponding to
The nucleation and growth phenomena in the initial stages of
gold deposition were investigated by potentiostatic transient mea-
surements. Fig. 2 shows current transients obtained at different
electrode potentials in the range 0.64 V ≤ E ≤ 0.94 V. The transients
exhibit an initial current decay in the range 0.001–1 s, which can-
not be related to the double-layer charging. Such falling currents in
the initial parts of transients have been often observed in various
electrodeposition systems and were related to different processes
such as adsorption [47,48], multistep charge transfer reactions [49]
and redox-processes influencing the state of the substrate sur-
face [50]. After the initial current decay, the transients in Fig. 2
show the characteristic current increase due to the nucleation and
growth of gold nanoparticles. The current maximum observed at
high cathodic potentials results from the overlap of growing par-
ticles and/or their diffusion zones [5–9]. According to the model
including diffusion-controlled growth of hemispherical particles
proposed by Scharifker and Hills [9] the deposition transients for
progressive and instantaneous nucleation can be described by the
equations:
the reaction (1) [42]. This behaviour could be related to a reaction
mechanism proposed previously for gold electrodeposition from
ꢂ
ꢃ
ꢄꢅ
2
ꢀ
ꢀ
ꢁ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
2
2
i
tmax
t
t
=1.2254
1− exp −2.3367
(5)
(6)
imax
tmax
prog
2
ꢂ
ꢃ
ꢄꢅ
2
ꢀ
ꢁ
ꢀ
ꢁ
2
i
tmax
t
=1.9542
1− exp −1.2564
imax
t
tmax
inst
Experimental current transients for 0.84 V and 0.64 V are repre-
sented in Fig. 3 in a non-dimensional (i/imax
2
)
vs. t/tmax plot and
Fig. 1. Cyclic voltammogram recorded in the system GC/1 mM[AuCl4]− + 0.1 M
HClO4. The potential scan is initiated at E = 1.32 V in the cathodic direction. Scan
rate: |dE/dt| = 1 mV/s.
are compared with the theoretical curves given by Eqs. (5) and
(6). It is clearly seen, that the experimental current transient at

170
L. Komsiyska, G. Staikov / Electrochimica Acta 54 (2008) 168–172
2
Fig. 3. Non-dimensional (i/imax
) vs. (t/tmax) plots of current transients for electro-
crystallization of gold at E = 0.84 V and E = 0.64 V compared to the theoretical curves
corresponding to the models for progressive nucleation (solid line) and instanta-
neous nucleation (dashed line).
Fig. 5. i2/3 vs. t plots of the initial rising parts of the transients shown in Fig. 2.
Fig. 4 presents a current transient measured at a relatively low
electrode potential E = 0.54 V. The inset of the figure shows that the
current transient follows the Cottrell relation [51]:
0.84 V corresponds to the model involving progressive nucleation.
The transient at 0.64 V fits relatively well with the theoretical curve
for instantaneous nucleation. In this case the deviation of experi-
ꢀ ꢁ
1/2
D
i = zFc
t−1/2
(8)
mental points from the theoretical curve at short times (t < tmax
)
ꢀ
can be connected with the contribution of the initial falling cur-
rent discussed above to the total current. Assuming that at t = tmax
this contribution is negligible the number density N0 of growing
nanoparticles generated instantaneously on the active nucleation
sites can be estimated from the coordinates of the maximum imax
and tmax, which are coupled according to the theoretical model by
[9]:
where D is the diffusion coefficient of gold ions in the elec-
trolyte. From the slope of the linear i − t−1/2 dependence in Fig. 4
and Eq. (8) was estimated a value for the diffusion coefficient of
D = 1.1 × 10−5 cm2 s−1, which is close to the values reported previ-
ously [13,25].
Information about the nucleation kinetics was obtained by ana-
lyzing the initial rising parts of current transients obtained in the
potential range 0.84 V ≤ E ≤ 0.94 V, where the nucleation is progres-
sive. The transient analysis was performed at relatively short times,
i.e. before the overlapping of the diffusions zones of growing gold
nanoparticles. Fig. 5 shows the initial rising parts of current tran-
sients from Fig. 2 as i2/3 vs. t plots. The observed linear dependences
are in agreement with the model for progressive nucleation and
diffusion-controlled growth and can be expressed by [9]
ꢆ
1/2
c3
2
inst
(imaxtmax
)
= 0.065(zF)2
(7)
8ꢀVmN02
where z = 3 is the number of electrons involved in the overall
deposition reaction, F is the Faraday constant, c = 10−6 mol cm−3
is the concentration of gold ions in the bulk electrolyte and
Vm = 10.19 cm3 mol−1 is the molar volume of gold. A value of
N0 = 3 × 106 cm−2 was estimated with Eq. (7) from the coordinates
of the maximum of experimental current transient at 0.64 V.
i2/3 = −1.53Dc(ꢀJzF)2/3Vm1/3(t − t0)
(9)
where t0 represents the induction period. The nucleation rate J was
extracted from the slopes of the straight lines in Fig. 5 using Eq.
(9) and the value for the diffusion coefficient D = 1.1 × 10−5 cm2 s−1
estimated above. The obtained data for the nucleation rate at dif-
ferent electrode potentials were analyzed on the basis of atomistic
theory of electrochemical nucleation [5–7,11]. According to this the-
ory the nucleation rate J is given by
ꢇ
ꢈ
(1 − ˛ + ncrit)zeE
J(E) = A(ncrit) exp
−
(10)
kT
where ncrit is the number of atoms in critical nucleus for a given
potential range, (1 − ˛) is the cathodic charge transfer coefficient
and e is the elementary charge. Eq. (10) predicts a change of the
slope of the ln J vs. E plot of experimental data if clusters with dif-
ferent size ncrit are critical nuclei in neighbouring potential ranges.
Fig. 6 represents the experimental data for the nucleation rate
at different electrode potentials obtained from the linear i2/3 − t
dependences in Fig. 5. The ln J vs. E plot of the experimental data
shows two distinct linear regions in good agreement with the
predictions of the atomistic nucleation theory. From the slopes
of the straight lines in Fig. 6 using Eq. (10) with ˛ = 0.5 were
Fig. 4. Current transient for electrocrystallization of gold at E = 0.54 V. The inset
shows the i vs. t−1/2 plot of the transient.

L. Komsiyska, G. Staikov / Electrochimica Acta 54 (2008) 168–172
171
rent transients. Such disagreement between the theoretical model
and experimental results has been reported also for other elec-
trodeposition systems [34,47,48,52–57]. It should be noted that
an agreement between particle number densities obtained from
analysis of current transients and from microscope images has
been reported for systems involving a single charge transfer step
[58,59], whereas a disagreement is observed particularly in sys-
tems involving multiple charge transfer steps [47,48,53–56] or
single charge transfer step coupled with a preceding chemical
reaction step [34,52]. However, a clarification of the exact reason
for the observed disagreement is still missing and requires fur-
ther experiments and improved theoretical modelling of involved
processes taking into account the above discussion. Therefore,
at present reliable information for the real number density of
electrodeposited nanoparticles can be obtained by microscopic
imaging.
The instantaneous nucleation observed in the potential range
E ≤ 0.64 V offers the possibility for electrodeposition of Au nanopar-
ticles with uniform size. However, at relatively long deposition
times the particles deposited under conditions of instantaneous
nucleation are characterized by a relatively broad size distribution.
As demonstrated by Penner [4], the broadening of size distribu-
tion results from the so-called “interparticle diffusion coupling”
(IDC), which leads to a dispersion in the growth rates of indi-
vidual particles. This IDC effect can be minimized applying a
double-pulse polarization technique [4,5]. In this technique the
first short pulse with duration ꢁ1 to a sufficiently low potential
E1 generates small Au nanoclusters under conditions of instan-
taneous nucleation. This pulse is followed by a second pulse to
a potential E2, which is more positive than the critical poten-
tial for nucleation. Under these conditions the probability for
formation of new nuclei becomes practically zero and the Au
nanoclusters generated during the first nucleation pulse grow
slowly under kinetic or mixed control. The particle diameter can
be varied easily by the duration ꢁ2 of the second growth pulse.
Fig. 6. Potential dependence of the nucleation rate obtained from the linear i2/3 − t
plots in Fig. 5.
estimated values of ncrit = 1 and ncrit = 0 for the potential ranges
0.94 V > E > 0.91 V and 0.91 V > E > 0.84 V, respectively. These values
indicate that a single Au adatom is critical nucleus in the poten-
tial range 0.94 V ≥ E ≥ 0.91 V, whereas at potentials more negative
than 0.91 V each nucleation site plays the role of critical nucleus
and the single Au atom adsorbed on a nucleation site represents
a supercritical cluster, which can grow spontaneously. The results
are in agreement with previous studies on gold electrodeposition in
the system GC/5 mM HAuCl4 + 6 M LiCl, where values of ncrit below
1 were reported for relatively low electrode potentials (E < 0.84 V)
[13].
Information on the shape, size and number density of electrode-
posited gold nanoparticles was obtained by SEM observations. Fig. 7
shows a typical SEM image of Au nanoparticles deposited under
conditions of instantaneous nucleation at E = 0.64 V and t = 100 ms.
The nanoparticles have an approximately hemispherical shape in
accordance with the theoretical model applied above for the anal-
ysis of current transients. However, the analysis of SEM images
shows N0-values ranging between 2 × 108 cm−2 and 5 × 108 cm−2
,
which are about two orders of magnitude higher than the value
(N0 = 3 × 106 cm−2) estimated from the coordinates of the current
maximum of the transient at E = 0.64 V (cf. Fig. 2) using Eq. (7).
This result indicates that the theoretical model does not describe
realistically the interaction between the growing particles in the
advanced deposition stages giving rise to the maximum of cur-
Fig. 8. SEM image of Au nanoparticles electrodeposited by the double-pulse tech-
nique (E1 = 0.64 V, ꢁ1 = 1 ms; E2 = 1.02 V, ꢁ2 = 120 s). The corresponding particle size
histogram is shown at the bottom of the image.
Fig. 7. SEM image of Au nanoparticles electrodeposited at E = 0.64 V for t = 100 ms.

172
L. Komsiyska, G. Staikov / Electrochimica Acta 54 (2008) 168–172
Basically, the double-pulse technique separates the nucleation and
growth processes in time and has been developed originally for
experimental studies of electrocrystallization [5–7]. In the last
decade this technique has been successfully applied for elec-
trodeposition of nanoparticles [4,24,60–62], nanowires [4,63] and
compact metal films [32,35]. Fig. 8 represents a SEM image of Au
nanoparticles deposited by the double-pulse method in the sys-
tem investigated in this work. The corresponding particle diameter
histogram shows that this technique can be successfully applied
for deposition of Au nanoparticles with a relatively narrow size
distribution.
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The results of this study show that electrocrystallization
of gold in the system GC/1 mM[AuCl4]− + 0.1 M HClO4 fol-
lows the Volmer–Weber growth mode. In the potential range
0.84 V ≤ E ≤ 0.94 V the electrocrystallization kinetics corresponds
to the theoretical model for progressive nucleation and diffusion-
controlled growth of 3D nanoparticles. The potential dependence
of nucleation rate obtained from the initial parts of current tran-
sients is analyzed on the basis of the atomistic theory of nucleation
and the size of critical nuclei in this potential range is determined.
The results indicate that at more negative electrode potentials
(E ≤ 0.64 V) the process of electrocrystallization occurs under con-
ditions of instantaneous nucleation. It is demonstrated that gold
nanoparticles with a uniform size can be deposited applying
the double-pulse technique involving an instantaneous nucleation
pulse followed by a slow growth pulse.
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Acknowledgements
We gratefully acknowledge the financial support of this work by
the Research Centre Jülich and the help of H.P. Bochem in the SEM
imaging.
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