A new gold(III)-aminoethyl imidazolium aurate salt as precursor for nanosized Au electrocatalysts

Article abstract of DOI:10.1016/j.electacta.2010.10.009

In this work the novel, robust, air- and moisture-stable gold(III)-aminoethyl imidazolium aurate salt [Cl₃AuNH ₂(CH₂)₂ImMe][AuCl₄] (1) has been employed as precursor for the electrosynthesis of gold nanoparticles (AuNPs) onto ITO electrodes in 0.5 mol dm^-3 H₂SO₄ aqueous solution without the use of additional additives and/or stabilizers. The electrode termed ILNH₂-Au_NPs200 prepared at a fixed electrodeposition time (t_d) of 200 s was characterized with AFM, SEM, XRD diffraction as well as cyclic voltammetry (CVs) and compared with an electrode in which the AuNPs have been electrodeposited from KAuCl₄ in the presence of KI as additive at the same t_d (Au _NPs200). The effect of the t_d (i.e. 50 s or 500 s; electrodes ILNH₂-Au_NPs50 and ILNH₂-Au _NPs500, respectively) was also investigated together with the electrocatalytic activity towards methanol oxidation in alkaline medium of all the prepared electrodes. The comparison with Au_NPs obtained with KI as additive shows that the presence of the amino-functionalized imidazolium moiety although not relevant in relation to the particle size, favours the metal electrodeposition process and plays an important role in the enhancement of the following parameters: (i) surface coverage (S.C.), (ii) electroactive particle coverage Φ_p %, (iii) electroactive surface area σ_red(exp)/σ_red(theor) and (iv) catalytic efficiency.

Full text of DOI:10.1016/j.electacta.2010.10.009

Electrochimica Acta 56 (2010) 676–686
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Electrochimica Acta
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A new gold(III)-aminoethyl imidazolium aurate salt as precursor for nanosized
Au electrocatalysts
Barbara Ballarina,∗, Maria Cristina Cassania,∗, Massimo Gazzanob, Gavino Solinasa
a Dipartimento di Chimica Fisica e Inorganica, Viale Risorgimento 4, INSTM UdR Bologna, 40136 Bologna, Italy
b Istituto per la Sintesi Organica e la Fotoreattività, CNR, Via Selmi 2, 40126 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work the novel, robust, air- and moisture-stable gold(III)-aminoethyl imidazolium aurate salt
[Cl3AuNH2(CH2)2ImMe][AuCl4] (1) has been employed as precursor for the electrosynthesis of gold
nanoparticles (AuNPs) onto ITO electrodes in 0.5 mol dm−3 H2SO4 aqueous solution without the use of
additional additives and/or stabilizers. The electrode termed ILNH2-AuNPs200 prepared at a fixed electrode-
position time (td) of 200 s was characterized with AFM, SEM, XRD diffraction as well as cyclic voltammetry
(CVs) and compared with an electrode in which the AuNPs have been electrodeposited from KAuCl4 in
the presence of KI as additive at the same td (AuNPs200). The effect of the td (i.e. 50 s or 500 s; electrodes
ILNH2-AuNPs50 and ILNH2-AuNPs500, respectively) was also investigated together with the electrocatalytic
activity towards methanol oxidation in alkaline medium of all the prepared electrodes. The comparison
with AuNPs obtained with KI as additive shows that the presence of the amino-functionalized imidazolium
moiety although not relevant in relation to the particle size, favours the metal electrodeposition process
and plays an important role in the enhancement of the following parameters: (i) surface coverage (S.C.),
(ii) electroactive particle coverage ˚p %, (iii) electroactive surface area ꢀred(exp)/ꢀred(theor) and (iv) catalytic
efficiency.
Received 27 August 2010
Received in revised form
27 September 2010
Accepted 2 October 2010
Available online 28 October 2010
Keywords:
Amino-functionalized imidazolium salts
Gold nanoparticles
Electrodeposition
Cyclic voltammetry
Methanol electro-oxidation
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
and the relative ratio of the Au(1 1 1), Au(1 1 0), and Au(1 0 0) crys-
talline orientation domains constituting their surface were largely
The deposition of metal nanoparticles onto conducting sub-
strates has drawn considerable attention due to its great
importance in fundamental studies as well as in the fabrication
of practical devices such as heterogeneous catalytic systems, elec-
tronic sensors and biosensors [1,2]. To accomplish this purpose,
electrodeposition has proved to be a very attractive technique
because of the simple instrumentation and the capability to readily
control the size and morphology of the as-deposited nanostruc-
tures through adjusting the electrochemical parameters such as
potential and current density and/or composition of the electrode-
position bath. With regard to the latter, the nature of the solvent
and the presence of additives play an important role [3–7] and in
this field room temperature ionic liquids (RTILs) have proved to
be electrochemically advantageous over other kinds of solvents in
terms of their excellent properties [8–13].
dependent on the potential used. With respect to the use of gold
containing imidazolium salts for the electrodeposition of AuNPs,
to the best of our knowledge the only example was reported
by Douce et al. and consisted in the electrolysis at high poten-
tial (+1.78 to +2.27 V) and high temperature (111 ◦C and 117 ◦C)
of a melted dicyanoaurate(I) derivative of a mesomorphic 3-(4-
dodecyloxybenzyl)-1-methyl-1H-imidazolium compound [18]. In
this contest, since it has been recently reported that functionalized
ILs with a terminal amine group on the pendant side chain (IL-
NH2) plays an active role in stabilizing the AuNPs [19–21], with
the aim of preparing a well-defined AuNPs precursor we have
recently synthesized the robust gold(III)-aminoethyl imidazolium
aurate salt [Cl3AuNH2(CH2)2ImMe][AuCl4] (1) shown in Scheme 1
[22,23].
Moreover, considering the high cost of carrying out an elec-
trodeposition reaction in neat ionic liquid and the negative effect
connected with the high viscosity of these solvents [12], the
observation that the cyclic voltammetric behaviour of 1 in ace-
tonitrile [22], closely resembled that reported for NaAuCl4 in neat
[bmim][NTf2] [16], prompted us to devise a novel strategy based
on the use of 1, in which the imidazolium and Au(III) moieties
are already in intimate contact, as precursor for the electrosyn-
thesis of AuNPs onto indium tin oxide (ITO) electrodes [24] in an
Concerning the electrodeposition of gold nanoparticles (AuNPs),
various kinds of RTILs have been employed [14–17] and it has
been demonstrated that the size and morphology of the AuNPs
∗
Corresponding author.
E-mail addresses: ballarin@ms.fci.unibo.it (B. Ballarin),
cassani@ms.fci.unibo.it (M.C. Cassani).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.10.009

B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
677
Scheme 1. The gold(III)-aminoethyl imidazolium aurate salt 1: syntheses and crystal structure.
acid aqueous media without the use of additional additives and/or
stabilizers.
configuration with ITO as working electrode, a SCE as reference
electrode and a platinum wire as counter electrode. All the electro-
chemical measurements were conducted at room temperature in
nitrogen saturated solutions. The morphological characterization
of AuNPs was investigated using a scanning electron microscopy
(SEM, EVO 50-RENISHOW) operating at 15 kV equipped with
an energy-dispersive spectrometer (EDS). ITO-electrode surface
coverage (S.C.) was calculated from SEM images at higher mag-
nification (70,000×) by using the surface analysis programme
ImageJ. AFM measurements were performed using a Scanning
Probe Microscope Vista 100, Burleigh Instruments in air and at
room temperature, in a non-contact mode with standard silicon
nitride tips with a nominal tip radius of 10 nm and resonant fre-
quencies of about 190 kHz. Typically, the surface was scanned at
0.5 Hz (25% amplitude decrease) with 256 lines per image resolu-
tion (all the image sizes equal 10 ␮m × 10 ␮m in this article). The
image processing and roughness analysis was carried out using a
VISTA ImageStudio software. X-ray diffraction (XRD) analysis were
carried out on a X’PertPro PANalytical diffracto◦meter using Cu K␣
The AuNPs were electrodeposited at a fixed potential of −0.155
vs SCE/V and a deposition time (td) of 200 s (hereafter termed
ILNH2-AuNPs200), characterized by means of AFM, SEM, XRD as well
as cyclic voltammetry (CVs) and compared with AuNPs obtained
from KAuCl4 in the presence of KI, as additive, at the same poten-
tial and td (AuNPs200) [7]. Next, the effect of the deposition time
was examined and AuNPs were electrosynthesized from 1 at 50 s
and 500 s (ILNH2-AuNPs50 and ILNH2-AuNPs500), furthermore the
electrocatalytic activity of all the prepared electrodes towards
methanol oxidation in alkaline medium was studied.
2. Experimental
2.1. Chemicals and materials
The reagents H2SO4, KI (99,7%), KOH, l-cysteine were of ana-
lytical grade (Aldrich) and used without further purification.
ITO glasses were purchased from Optical Filters Ltd., England
(surface resistivity = 12 ꢁ cm2). The gold compounds KAuCl4 and
[Cl3AuNH2(CH2)2ImMe][AuCl4] (1) were synthesized according to
previously reported methods [22,25]; solutions were prepared
from ultrapure water purified with the Milli-Q plus system (Mil-
lipore Co., resistivity over 18 Mꢁ cm). All glassware used in the
following procedures were cleaned in a bath of freshly prepared
3:1 HCl/HNO3 and rinsed thoroughly with H2O prior to use.
˚
radiation (ꢂ = 1.5418 A) in the 2ꢃ range 36–80 , with performing
steps of 0.066◦ (2ꢃ), counting 500 s/step in the 2ꢃ angular region
between 34◦ and 48◦ and counting 800 s/step elsewhere. The elec-
trodes were placed on a flat sample holder and directly analysed.
The cell parameters were calculated as the best values that fit the
distances of the reflections 1 1 1, 2 0 0 and 2 2 0. The crystal sizes
perpendicular to (1 1 1) planes were calculated by using the Scher-
rer method with the empirical constant K = 0.94 [26].
2.2. Instruments and measurements
2.3. AuNPs electrochemical synthesis
Electrochemical measurements were performed using a poten-
tiostat (Autolab PGSTAT20) in a conventional three-electrode cell
The ITO glass slides (exposed geometry area 1.28 cm2) were son-
icated for 15 min in each of the following solvents: soapy water,

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B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
water, acetone, and ethanol. Next, the electrodes were rinsed with
distilled water and dried with a stream of nitrogen. AuNPs were
electrodeposited onto the ITO surface from a 0.5 mol dm−3 H2SO4
solution containing 1.0 × 10−3 mol dm−3 of 1 at a fixed poten-
tial of −0.155 vs SCE/V for three different deposition time td:
50 s (ILNH2-AuNPs50), 200 s (ILNH2-AuNPs200), and 500 s (ILNH2-
AuNPs500). For comparison, AuNPs were electrodeposited from a
0.5 mol dm−3 H2SO4 solution containing 1.0 × 10−3 mol dm−3 of
KAuCl4 and 1.0 × 10−4 mol dm−3 of KI at a fixed potential −0.155 V
for td of 200 s (AuNPs200). The tests with cysteine self-assembled
monolayer (SAM) were carried out by immersing the modified ITO
electrodes into a 1.0 × 10−3 mol dm−3 aqueous l-cysteine solution
for 20 min and then rinsing with water before running the electro-
chemical measurements [3–5].
3. Results and discussion
3.1. AFM, SEM and XRD characterizations
The electrodeposited AuNPs modified ITO electrodes have been
examined using AFM, SEM and XRD diffraction. The AFM images
reported in Fig. 1 show a homogeneous distribution of nano-
structured particles on ITO surface.
Fig. 1. From left to right: AFM 2D topographic images, magnified three-dimensional (3D) views and cross-sectional analysis graphs of (A) AuNPs200, (B) ILNH2-AuNPs200, (C)
ILNH2-AuNPs50, and (D) ILNH2-AuNPs500
.

B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
679
Fig. 2. SEM photographs (20,000×) and size distribution of AuNPs electrodeposited onto ITO electrodes: (A) AuNPs200, (B) ILNH2-AuNPs200, (C) ILNH2-AuNPs50, and (D) ILNH2-
AuNPs500; inset: magnification 70,000×.

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B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
Table 1
Values of roughness parametes for samples of Fig. 1a.
b
b
b
b
Morphological parameter (nm)
AuNPs200
ILNH2-AuNPs200
ILNH2-AuNPs50
ILNH2-AuNPs500
ITO barec
Rms roughness, Sq
Average roughness, Sa
Skewness, Ssk
26.97
22.03
2.12
19.56
15.20
1.02
18.63
14.32
1.20
21.88
22.49
2.14
64.69
55.24
0.59
Kurtosis, Sku
13.03
5.34
4.75
10.47
2.18
a
Scan area is 10 ␮m×10 ␮m.
b
The mean value is 0.01 nm.
c
The mean value is 14.65.
Table 2
SEM surface parameters.
Inspection of surface morphology of the electrodeposited gold
nanoparticles by AFM provides a statistical analysis of the sur-
face roughness features. The following parameters were chosen
as key descriptors of the electrode surface morphology: root-
mean-square (rms) roughness of z data (Sq), average roughness
(Sa), skewness (Ssk) and kurtosis (Sku) [27,28]. Sq and Sa are the
most widely used amplitude roughness parameters that respec-
tively describe the standard deviation of the height values and
the average deviation of the measured z-values from the mean
plane. Surface skewness Ssk describes the asymmetry of the height
distribution: positive values of Ssk refer to a surface in which
the local summits dominate over the valleys. Surface kurtosis
Sku, a measure of the sharpness of the surface height distribu-
tion, approaches 3.0 for a Gaussian height distributions; smaller
values indicate a broader height distributions and vice versa for
values greater than 3.0. The selected roughness parameters for
the AuNPs modified electrodes and for bare ITO are reported in
Table 1.
Sample
Particle density/
S.C. (%)
AuNPs geom.
surf./cm2
␮m2
AuNPs200
20.7
13.6
14.0
13.2
11
21
19
28
0.095
0.157
0.256
0.207
ILNH2-AuNPs200
ILNH2-AuNPs50
ILNH2-AuNPs500
In addition, assuming an hemispherical shape of the AuNPs on
the ITO substrate and combining the average diameters derived
from SEM images with the particle density, we have estimated the
nanoparticle geometrical surface area (AuNPs geom. surf.) for the
different electrodes and the values are also reported in Table 2.
In order to examine the crystalline structure of the elec-
trodeposited nanoparticles on ITO, XRD measurements have been
performed and the XRD patterns of the as-prepared samples are
reported in Fig. 3.
The analysis of the data reveal the following features: (i) Sq and
Sa of the bare ITO are significantly higher than the AuNPs mod-
ified ones; (ii) for the ILNH2-AuNPs electrodes an increase of Sq
and Sa with the deposition time td from 50 to 500 s is observed;
(iii) the AuNPs200 electrode presents amplitude roughness parame-
ters higher than the corresponding ILNH2-AuNPs200 but analogous
to ILNH2-AuNPs500. With the exclusion of bare ITO, the Ssk and Sku
values approximately follow the same trend with the latter always
greater than 3.0.
Several peaks appear in each profile giving evidence of a well
formed crystalline material. Some peaks are due to the ITO sub-
SEM microscopy has been used to reinforce the information
obtained by AFM and to rule out artefacts due to the SPM tip
convolution effects. SEM photographs and size distribution his-
tograms relative to AuNPs200, ILNH2-AuNPs200, ILNH2-AuNPs50 and
ILNH2-AuNPs500 are reported in Fig. 2A–D.
In all electrodes examined although the nanoparticle morphol-
ogy results very similar presenting the typical rose-like shape,
the size distribution, determined by measuring the sizes of indi-
vidual particles in the SEM images (more than 100 particles),
depends on the composition of the electrodeposition bath. Con-
sistent with previous reports, we found that the average particle
size for the ILNH2-AuNPs electrodes is always larger than that of
AuNPs200 i.e. in the presence of I− ions, but significantly smaller
than AuNPs obtained in the absence of any additive (ca. 250 nm)
[4,5]. Within the ILNH2-AuNPs electrodes, a nanoparticles size
increase from td = 50 s to td = 200 s is observed whereas when the
deposition time is raised to 500 s (ILNH2-AuNPs500, Fig. 2D) no sig-
nificant enlargement occurs. Concurrently with a lower particle
size an higher particle density (number of particles/␮m2) equal
to 20.7 is observed for AuNPs200, whereas the values 14.0, 13.6,
13.2 were respectively found for ILNH2-AuNPs50, ILNH2-AuNPs200
and ILNH2-AuNPs500. The surface coverage (S.C.), calculated from
the SEM images, is 21% for ILNH2-AuNPs200 and 11% for AuNPs200
whereas a variation in td gives a S.C. of 19 and 28% for ILNH2-
AuNPs50, and ILNH2-AuNPs500 respectively (see Table 2). The larger
amount of deposited Au always found for ILNH2-AuNPs electrodes
with respect to AuNPs200 can be related to the presence of two Au(III)
metal centers in compound 1.
Fig. 3. X-ray diffraction patterns of (A) AuNPs200, (B) ILNH2-AuNPs200, (C) ILNH2-
AuNPs50, and (D) ILNH2-AuNPs500. The Miller indexes of the Au phase are reported; the
reflections due to ITO are marked by ♦; the intensities of the two pattern sections
at angles greater than 62◦ are multiplied by 2.

B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
681
Table 3
Net peak area in counts/1000.
Crystal
Size
peak 111
(100)a
peak 200
(50)a
peak 220
(30)a
sample
(nm)b
AuNPs200
2401 (100) (227) (187)
8172 (100) (161) (176)
3116 (100) (156) (125)
12488 (100) (111) (143)
532 (22) (50) (41)
2507 (31) (50) (54)
1004 (32) (50) (40)
5604 (45) (50) (64)
394 (16) (36) (30)
1401 (17) (27) (30)
747 (24) (37) (30)
2643 (21) (23) (30)
30 1
31 1
26 1
32 1
ILNH2-AuNPs200
ILNH2-AuNPs50
ILNH2-AuNPs500
aRelative intensities as reported in reference 29. bEstimated from the width of 111 reflection. The red
values are the relative intensities respect to the 111 peak taken as 100% of intensity. The blue ones are
the relative intensities scaled matching to 50% the relative intensity of the 200 peak; the green ones are
the relative intensities scaled matching to 30% the relative intensity of the 220 peak. The values have
been normalized for the different counting times used in the data collection of the different peaks.
strate, while those appearing at 2ꢃ = 38.2, 44.4, 64.6, 77.6 were
assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) reflections of the face-
centered cubic gold structure. The cell parameter of the Au phase
found to be 0.4075 ( 0.0001) nm, well compares with that one typ-
ical for Au metal reported in the literature (a = 0.4078 nm) [29].
While the intensity of the substrate peaks are roughly constant,
the intensity of the Au reflections in the electrodes obtained from
the electrodeposition of 1 increase as a function of td indicating a
higher amount of scattering material and confirming the forma-
tion of Au crystals all along the process (see Fig. 3B–D). In contrast,
the intensity of the Au reflections of AuNPs200 is comparable with
that of ILNH2-AuNPs50 suggesting that the amino-functionalized
imidazolium moiety present in 1 affects the nanoparticle growth
favouring the metal electrodeposition. At the moment the origin
of this behaviour is not clear, and even if we do not have direct
evidence at present, we speculate that there may be a surface com-
plexation of the IL with gold atoms on the particle surface. The
reasons for this peculiar behaviour are currently under investiga-
tion.
In Table 3 the crystal size (similar for all samples) as well as
the integrated intensities of the main XRD reflections are reported
(peaks 1 1 1 are related with Au(1 1 1) facets, peak 2 0 0 with
Au(1 0 0) facets and peak 2 2 0 with Au(1 1 0) facets). As can be
appreciated by the relative values (red in parentheses), only for
the ILNH2-AuNPs500 sample, the intensities of the reflections are
roughly close to those reported in the literature for isotropic crys-
tallites (values marked with asterisks in the top line) [29]. If we
rescale the relative intensity matching to 50% the intensity of 2 0 0
peak (values in blue in Table 3) it is more evident that the most part
of reflection intensities are comparable with the references data,
but the peak 1 1 1 is over expressed particularly for the samples
AuNPs200, ILNH2-AuNPs200 and ILNH2-AuNPs50
.
The variation of the reflection intensities is very often corre-
lated with preferential orientation usually caused by a non isotropic
morphology of the material, nevertheless the morphology of the
samples herein investigated is roughly spherical and as a con-
sequence the phenomenon should be ascribed to an increase of
particles with (1 1 1) facets parallel to the ITO slide, with respect
to the statistical occurrence. Indeed the samples have been ana-
lyzed in reflection mode putting the electrode parallel to the
sample holder: in this geometry the planes parallel to the film
surface are those that give the maximum contribution to the
signal.
Fig. 4. Typical CVs for (A) AuNPs200 and (B) ILNH2-AuNPs50
, ILNH2-AuNPs200,
and ILNH2-AuNPs500 electrodes. Potential sweep rate: 50 mV s−1; electrode area:
1.28 cm2.

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B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
Fig. 5. CVs relatives to the first (black path) and second (red path) scan for the reductive desorption of cysteine SAMs on AuNPs200, ILNH2-AuNPs200, ILNH2-AuNPs50 and
ILNH2-AuNPs500 and in N2-saturated 0.5 mol dm−3 KOH solution.
3.2. Electrochemical characterization
in Table 4:
ꢀ
ꢁ
ꢀred(exp) (␮C)/ꢀred(theor) (␮C/cm2)
The electrodeposited AuNPs were characterized by means of
cyclic voltammetry in a 0.5 mol dm−3 KOH solution in a potential
range between −0.20 and +0.55 vs SCE/V and with a scan rate of
50 mV s−1. The voltammetric paths (Fig. 4) relative to the AuNPs
samples showed a broad oxidation wave in the potential range
+0.32 to +0.40 vs SCE/V and a reduction peak at ca. +0.00 vs SCE/V,
respectively corresponding to the formation of surface Au oxides
and their reduction [30–33].
The increase of both the capacitive and faradic current observed
in the voltammograms relatives to ILNH2-AuNPs50, ILNH2-AuNPs200
and ILNH2-AuNPs500 samples, moving from 50 to 500 s was
attributable to the enhancement of the S.C. (as estimated from SEM
images).
From the integration of the cathodic peaks corresponding to
the reduction of the Au oxides relative to the stable cyclic voltam-
mograms obtained at 5 mV s−1 in 0.5 mol dm−3 KOH, the charge
density ꢀred(exp) has been calculated for all the electrodes. This data
allows the consequent determination of two important parame-
ters: (i) the electroactive AuNPs surface area given by the ratio
ꢀred(exp)/ꢀred(theor) [34], assuming the theoretical charge density for
the reduction of the Au oxide (ꢀred(theor)) is 723 ␮C/cm2 [35–37] and
(ii) the particle coverage (˚p %) defined as the ratio between the
electroactive surface area and the geometric area of the ITO slides in
contact with the electrolyte solution (Eq. (1)). The data are reported
˚p (%) =
× 100
(1)
ITO geometric (cm2)
The values of the electroactive surface area are in good agreement
with the XRD data that showed an increase of electrodeposited
AuNPs with td whereas the discrepancies found when the data
are compared with the geometrical surface area determined by
SEM (reported again in Table 4 for sake of clarity) can be ascribed
to the fact that the hypothesis of a hemispherical shape is not
appropriate.
In order to further assure the conclusions drawn from the XRD
analysis l-cysteine SAMs were first grafted onto the AuNPs and the
succeeding desorption was measured in N2-saturated 0.5 mol dm−3
KOH solutions by CVs [4,5,38]; the voltammograms relatives to the
Table 4
Electrochemical parameters.
Samplea
Electroactive surface area
AuNPs geom.
surf. (cm2)
˚ %
p
(cm2) ꢀred(exp)/ꢀred(theor)
AuNPs200
0.115
0.179
0.141
0.486
0.095
0.157
0.256
0.207
9
ILNH2-AuNPs200
ILNH2-AuNPs50
ILNH2-AuNPs500
14
11
38
ITO glass exposed geometry area equal to 1.28 cm2.
a

B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
683
Table 5
Comparison of the relative ratios of the amount of charge consumed during the reductive desorption of l-cysteine.
a
a
b
Sample
td (s)
QAu(1 1 1) (%)
Q[Au(1 0 0)+Au(1 1 0)] (%)
QTotal (␮C)
Qꢀc (␮C/cm2)
AuNPs200
200
200
50
57
87
46
48
43
13
54
52
2.4
5.3
6.3
6.8
21.1
29.8
44.6
14.0
ILNH2-AuNPs200
ILNH2-AuNPs50
ILNH2-AuNPs500
500
a
QAu(1 1 1) and Q[Au(1 0 0)+Au(1 1 0)] represent the relative ratio of the amount of charge consumed during the reductive desorption of the cysteine SAMs from the (1 1 1) and
(1 0 0) + (1 1 0) facets respectively.
b
The total amount of charge consumed during the reductive desorption of the cysteine SAMs.
QTotal/electroactive surface area.
c
Table 6
Methanol electro-oxidation activity for AuNPs catalysts in KOH 0.5 mol dm−3
.
Sample
Methanol oxidation peak (V)a
Onset potential (V)a
Catalytic efficiencyb
Calibration feetsc
Ref.
AuNPs200
0.423
0.436
0.428
0.458
0.434
0.447
0.544
0.459
0.499
0.273
0.272
0.278
0.260
0.234
0.244
n.d.
3.2
6.3
6.4
5.0
5.3
5.5
n.d.
n.d.
n.d.
Not linear
This work
This work
This work
This work
[41]
[41]
[40]
[30]
[46]
ILNH2-AuNPs200
ILNH2-AuNPs50
ILNH2-AuNPs500
AuNPs-MPTMS/ITO
AuNPs-APTES/ITO
AuNPs-NDT/GC
AuNPs-NDT/EQCN
AuNPs/C
y = 161.91x − 73.19R2 = 0.998
y = 137.54x − 32.21R2 = 0.999
Not linear
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
NDT, 1,9-nonanedithiol; MPTMS, (3-mercaptopropyl)trimethoxysilane; APTES, 3-aminopropyltriethoxysilane; GC, glassy carbon; EQCN, electrochemical quartz-crystal
nanobalance; C, carbon black.
a
Potentials are given versus SHE.
b
Calculated as (icat − iblank)/iblank
.
c
y = Ip* (␮A); x = [MeOH] (mol dm−3).
2.5x10-4
2.0x10-4
1.5x10-4
1.0x10-4
-4
1.5x10
no MeOH
2.7 M
ILNH2-AuNPs200
-4
Au
no MeOH
2.7 M
NPs200
1.0x10
-5
5.0x10
5.0x10-5
0.0
0.0
-5.0x10-5
-5
-5.0x10
-0.20
0.00
0.20
0.40
0.60
-0.20
0.00
0.20
E vs SCE / V
0.40
0.60
E vs SCE / V
2.5x10-4
2.0x10-4
1.5x10-4
1.0x10-4
-4
1.5x10
no MeOH
2.7 M
ILNH2-AuNPs500
no MeOH
2.7M
ILNH-Au
2
NPs50
-4
1.0x10
5.0x10-5
0.0
-5
5.0x10
0.0
-5.0x10-5
-1.0x10-4
-5
-5.0x10
-0.20
0.00
0.20
0.40
0.60
-0.20
0.00
0.20
E vs SCE / V
0.40
0.60
E vs SCE / V
Fig. 6. CVs of AuNPs200, ILNH2-AuNPs200, ILNH2-AuNPs50, ILNH2-AuNPs500 and electrodes in 0.5 mol dm−3 KOH solution before and after 2.7 mol dm−3 methanol addition. Stable
voltammograms have been obtained after 5 cycles respectively; scan rate 50 mV s−1
.

684
B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
700.0
600.0
500.0
400.0
300.0
200.0
100.0
0.0
ILNH2-AuNPs500
ILNH2-AuNPs200
ILNH2-AuNPs50
AuNPs200
0.0
1.0
2.0
3.0
4.0
5.0
[MeOH] / mol dm-3
-4
-4
1.5x10
2.0x10
no MeOH
ILNH2-AuNPs200
1.0 M
2.7 M
4.2 M
AuNPs200
-4
no MeOH
1.0 M
1.5x10
-4
1.0x10
2.7 M
-4
4.2 M
1.0x10
-5
5.0x10
-5
5.0x10
0.0
0.0
-5
-5.0x10
-5
-5.0x10
-0.20
0.00
0.20
0.40
0.60
-0.20
0.00
0.20
0.40
0.60
E vs SCE / V
E vs SCE / V
-4
-4
2.0x10
2.0x10
no MeOH
1.0 M
ILNH2-AuNPs500
-4
ILNH2-AuNPs50
1.5x10
2.7 M
-4
1.5x10
no MeOH
1.0 M
4.2 M
-4
1.0x10
2.7 M
-4
1.0x10
4.2 M
-5
5.0x10
-5
5.0x10
0.0
0.0
-5
-5.0x10
-5
-4
-5.0x10
-1.0x10
-0.20
0.00
0.20
0.40
0.60
-0.20
0.00
0.20
0.40
0.60
E vs SCE / V
E vs SCE / V
Fig. 7. CVs of AuNPs200, ILNH2-AuNPs200, ILNH2-AuNPs50, ILNH2-AuNPs500 and electrodes in 0.5 mol dm−3 KOH solution without and with 1.0, 2.7, 4.2 mol dm−3 methanol. Stable
voltammograms have been obtained after 5 cycles respectively; scan rate 50 mV s−1. Inset: dependence of the Ip* upon MeOH concentration.
first (black path) and second (red path) scan for the reductive of the
cysteine SAMs are reported in Fig. 5.
still discernible [3,5,38,39]. In Table 5 the total quantity of the
faradaic charge (QTotal) consumed during the reductive desorp-
tion (estimated from CVs) is reported together with Qꢀ calculated
as the ratio between QTotal and the electroactive surface area (see
Table 4).
The values of QAu(1 1 1) and QAu(1 0 0)+Au(1 1 0) respectively defined
as the quantity of the faradaic charge consumed during the reduc-
tive desorption from Au(1 1 1) and [Au(1 0 0) + Au(1 1 0)] facets, are
also reported separately in Table 5; in perfect agreement with the
XRD data, the preferential orientation with (1 1 1) facets prevail in
the case of ILNH2-AuNPs200 and AuNPs200 samples.
The voltammograms show the existence of multiple reductive
desorption peaks at around −0.78 and −0.95 vs SCE/V having dif-
ferent current intensity. The first reductive peak, observed at −0.78
vs SCE/V in the first scan, disappears completely in the second
one and is due to the reductive desorption of cysteine from (1 1 1)
facets of the Au crystals whereas the second enlarged peak at ca.
−0.95 vs SCE/V, corresponding to the reductive desorption of cys-
teine from the (1 0 0) and (1 1 0) facets decreased in the second
scan but in two cases (ILNH2-AuNPs50 and ILNH2-AuNPs500) it is

B. Ballarin et al. / Electrochimica Acta 56 (2010) 676–686
685
3.3. Electrocatalytic oxidation of methanol in alkaline media
reported in Table 5 relative to the l-cysteine desorption and can be
explained with a lower amount of catalytic sites for surface unit.
The electro-oxidation of methanol is a commonly used method
to evaluate the catalytic activity of gold-coated electroactive sub-
strates. Because the adsorption of oxygen species (such as OH−
anion) on the Au surface has a promoting role in the electro-
oxidation of methanol or other alcohols, Au exhibits much higher
electrocatalytic activity in alkaline media than in acidic ones
[33,40]. Recently it has been reported that methanol can be oxi-
dized at low potentials in alkaline solutions at very rough golden
electrodes with high surface area, as poisoning does not occur dur-
ing the electro-oxidation process [33,40,41]. Furthermore detailed
studies [42,43] have demonstrated that methanol oxidation on Au
electrode proceedsindependently in twopotential regions with dif-
ferent forms of mechanisms: in the first region up to +0.40 vs SCE/V
methanol is mainly oxidized to formate as shown in Eq. (2) whereas
at higher potentials, from ca. +0.40 vs SCE/V onward, oxidation to
carbonates occurs as in Eq. (3) [41]:
4. Conclusions
This study investigated the use of the gold(III)-aminoethyl
imidazolium aurate salt [Cl3AuNH2(CH2)2ImMe][AuCl4] (1) as pre-
cursor for the electrodeposition of AuNPs on ITO electrodes without
further additives at td of 50, 200 and 500 s. A combination of AFM,
SEM, XRD and cyclic voltammetry was employed in order to have
a thorough view over the size, morphology and electrochemical
properties of such nanoparticles. Furthermore the electrocatalytic
behaviour towards methanol oxidation was studied. The compar-
ison of AuNPs200 obtained with KI as additive with ILNH2-AuNPs200
shows that although the amino-functionalized imidazolium moi-
ety is not acting as a strong capping agent therefore leading to the
formation of larger nanoparticles, it favours the metal electrode-
position process and plays an important role in the enhancement
of the following parameters: (i) surface coverage (S.C.), (ii) elec-
troactive particle coverage (˚p %), (iii) electroactive surface area
(ꢀred(exp)/ꢀred(theor)) and (iv) catalytic efficiency. Moreover the
comparison of the ILNH2-AuNPs electrodes obtained with increas-
ing td shows that to a rise in the amount of electrodeposited AuNPs
corresponds an increase of the electrocatalytical performances only
up to 200 s whereas at 500 s a drastic worsening is observed.
CH3OH + 5OH− → HCOO− + 4H2O + 4e−
(2)
(3)
2−
CH3OH + 8OH− → CO3 + 6H2O + 6e−
The catalytic activity of our electrodes was investigated and
Fig. 6 reports the CVs obtained for AuNPs200, ILNH2-AuNPs200
,
ILNH2-AuNPs50 and ILNH2-AuNPs500 before and after 2.7 mol dm−3
methanol addition. The analysis of the CVs after methanol addi-
tion in the lower potential region shows a large anodic wave at
about +0.20 vs SCE/V for all samples, on the contrary in the high
potential region (>+0.40 vs SCE/V) a sharp increase of the anodic
current occurs significantly only for ILNH2-AuNPs200, ILNH2-AuNPs50
electrodes [30,32,44–46].
Acknowledgements
The authors wish to thank the University of Bologna and the
Ministero dell’ Università e della Ricerca (MUR) (project: “New
strategies for the control of reactions: interactions of molecular
fragments with metallic sites in unconventional species”, PRIN
2007) for financial support and Dr. Sandra Stipa for the SEM images.
The voltammetric paths are very stable against repetitive cycling
in the potential windows used, indicating that AuNPs are free
from poisoning effects. In Table 6 the electrocatalytic parame-
ters for the AuNPs are listed and in order to compare our data
with those reported in the literature the potentials are given ver-
sus SHE reference electrode [30,32,45,44,46]. The data in Table 6
show that the catalytic efficiency, calculated as (icat − iblank)/iblank
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