Electrodeposition of gold at glassy carbon electrodes in room-temperature ionic liquids

Article abstract of DOI:10.1149/1.2724706

The cyclic voltammetric behavior of [Au Cl4] - on glassy carbon (GC) and gold electrodes in room-temperature ionic liquids, i.e., 1-ethyl-3- methylimidazolium tetrafluoroborate (EMIBF4) and 1- n -butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) has been examined. A series of two-electron (2e-) and one-electron (1e-) reductions of the [Au Cl4] - - [Au Cl2] - -Au redox system could be observed at GC electrode. For example, the cathodic and anodic peaks corresponding to the [Au Cl4] - [Au Cl2] - redox couple were observed at ca. 0.2 and 1.2 V vs a Ag wire quasi-reference electrode, respectively, in EMIBF4, while those observed at -0.5 and 0.5 V were found to correspond to the [Au Cl2] - Au redox couple. The disproportionation reaction of the 2e-reduction product of [Au Cl4] -, i.e., [Au Cl2] - to [Au Cl4] - and Au metal, was also found to occur significantly. A single reduction peak corresponding to the three-electron (3e-) reduction of [Au Cl4] - to Au metal was observed at Au electrode. The electrodeposition of Au nanoparticles was carried out on GC electrode in these ionic liquids containing [Au Cl4] - by applying potential-step electrolysis in a different potential range, i.e., the potential was stepped from 0.4 V to 0 and -1.0 V, at which the reduction of [Au Cl4] - to [Au Cl2] - and Au, respectively, takes place. The results obtained demonstrate that the electrodeposition of gold may occur via a disproportionation reaction of [Au Cl4] - to [Au Cl2] - and Au as well as via a series of the reductions of [Au Cl4] - to [Au Cl2] - and further, [Au Cl2] - to Au. The size and morphology of the prepared Au nanoparticles as well as the relative ratio of the Au(111), Au(110), and Au(100) crystalline orientation domains constituting the polycrystalline Au nanoparticles electrodeposited were found to largely depend on the stepped potential (i.e., 0 and -1.0 V). Interestingly, the Au nanoparticles prepared by a potential-step electrolysis from 0.4 to 0 V are enriched in the Au(110) single-crystalline domain.

Full text of DOI:10.1149/1.2724706

D322
Journal of The Electrochemical Society, 154 ͑6͒ D322-D327 ͑2007͒
0013-4651/2007/154͑6͒/D322/6/$20.00 © The Electrochemical Society
Electrodeposition of Gold at Glassy Carbon Electrodes
in Room-Temperature Ionic Liquids
Taku Oyama, Takeyoshi Okajima, and Takeo Ohsaka
,z
*
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, Midori-ku, Yokohama 226-8502, Japan
−
The cyclic voltammetric behavior of ͓AuCl4͔ on glassy carbon ͑GC͒ and gold electrodes in room-temperature ionic liquids, i.e.,
1-ethyl-3-methylimidazolium tetrafluoroborate ͑EMIBF4͒ and 1-n-butyl-3-methylimidazolium tetrafluoroborate ͑BMIBF4͒ has
been examined. A series of two-electron ͑2e-͒ and one-electron ͑1e-͒ reductions of the ͓AuCl4͔−–͓AuCl2͔−–Au redox system could
−
be observed at GC electrode. For example, the cathodic and anodic peaks corresponding to the ͓AuCl4͔−/͓AuCl2͔ redox couple
were observed at ca. 0.2 and 1.2 V vs a Ag wire quasi-reference electrode, respectively, in EMIBF4, while those observed at −0.5
and 0.5 V were found to correspond to the ͓AuCl2͔−/Au redox couple. The disproportionation reaction of the 2e-reduction product
of ͓AuCl4͔−, i.e., ͓AuCl2͔− to ͓AuCl4͔− and Au metal, was also found to occur significantly. A single reduction peak corresponding
−
to the three-electron ͑3e-͒ reduction of ͓AuCl4͔ to Au metal was observed at Au electrode. The electrodeposition of Au nano-
−
particles was carried out on GC electrode in these ionic liquids containing ͓AuCl4͔ by applying potential-step electrolysis in a
−
different potential range, i.e., the potential was stepped from 0.4 V to 0 and −1.0 V, at which the reduction of ͓AuCl4͔ to
−
͓AuCl2͔ and Au, respectively, takes place. The results obtained demonstrate that the electrodeposition of gold may occur via a
−
−
−
−
disproportionation reaction of ͓AuCl4͔ to ͓AuCl2͔ and Au as well as via a series of the reductions of ͓AuCl4͔ to ͓AuCl2͔ and
−
further, ͓AuCl2͔ to Au. The size and morphology of the prepared Au nanoparticles as well as the relative ratio of the Au͑111͒,
Au͑110͒, and Au͑100͒ crystalline orientation domains constituting the polycrystalline Au nanoparticles electrodeposited were
found to largely depend on the stepped potential ͑i.e., 0 and −1.0 V͒. Interestingly, the Au nanoparticles prepared by a potential-
step electrolysis from 0.4 to 0 V are enriched in the Au͑110͒ single-crystalline domain.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2724706͔ All rights reserved.
Manuscript submitted December 4, 2006; revised manuscript received February 5, 2007. Available electronically April 18, 2007.
Gold nanoparticles have recently received considerable interest
used as RTILs. Based on the results obtained, it is demonstrated that
the electrodeposition of gold may occur via a disproportionation
as they have size-dependent unique chemical, electrical, and optical
properties1-7 and are very promising for practical applications in
diverse fields, like electronic nanodevices, molecular catalysts, mul-
tifunctional reagents, and biosensors.8-23 Several techniques have
been employed aiming at the fabrication of gold nanoparticles, e.g.,
−
−
reaction of ͓AuCl2͔ to ͓AuCl4͔ and Au, as well as via a series of
reductions of ͓AuCl4͔ to ͓AuCl2͔ and further ͓AuCl2͔ to Au,62
together with the characterization of the prepared nanoparticles of
gold by scanning electron microscopy ͑SEM͒ and electrochemical
techniques.
−
−
−
the chemical reaction of ͓AuCl4͔ complex,24-30 metal vapor
−
synthesis,31,32 sol-gel technique,33,34 and electrochemical
deposition.35-46 Of these, electrodeposition is of great use because of
the facial control of the characteristics of gold nanoparticles35,38-46
͑e.g., morphology, shape, size, and thickness͒, which may result in
various functions ͑e.g., electrocatalysis͒ of gold nanoparticle-
electrodeposited electrodes by adjusting the electrolysis conditions
͑current density, electrolysis potential, electrolysis time, etc.͒, bath
chemistry ͑solvent, pH, additives͒, electrode substrate, and tempera-
ture.
Experimental
Reagents.— RTILs ͓1-ethyl-3-methylimidazolium tetrafluorobo-
rate ͑EMIBF4͒ and 1-n-butyl-3-methylimidazolium tetrafluoroborate
͑BMIBF4͔͒ were purchased from Stella Chemifa Co. ͑Japan͒. So-
dium tetrachloroaurate͑III͒ ͑Na͓AuCl4͔, 95%, Kanto Chemical Co.,
Ltd.͒, 4-mercaptobenzoic acid ͑4-MBA, Kanto͒, potassium chloride
͑Kanto͒, potassium hydroxide ͑Kanto͒, and sulfuric acid ͑Wako Pure
Chemicals Industries͒ were of analytical grade and were used as
received. All aqueous solutions were prepared with Milli-Q water
purified by a Millipore Milli-Q system ͑Millipore Japan͒.
In this study, the electrodeposition of gold nanoparticles on
glassy carbon ͑GC͒ electrode has been performed in room-
temperature ionic liquids ͑RTILs͒ containing ͓AuCl4͔−. Various
kinds of RTILs have recently been employed as new possible media
for electrodeposition of metals, for example, Co,47 Al,48,49 Mg,50
Ta,51 Ag,52,53 Co alloys,54-57 Pd–Au alloys,58 and Li–Na alloys,59
because RTILs have many advantages such as a wide electrochemi-
cal potential window, acceptable ionic conductivity, high thermal
stability, and negligible vapor pressure.60,61 The electrodeposition of
gold from RTILs has been originally reported by Xu and Hussey in
1992,62 who have examined the electrochemistry of gold͑III͒ and
gold͑I͒ at GC electrode in the basic aluminum chloride-1-ethyl-3-
methyl imidazolium chloride ͑AlCl3-EMICl͒ ionic liquid. They
Electrodeposition of gold.— All electrochemical experiments
were performed with a three-electrode electrochemical cell using
ALS/Chi 630A electrochemical analyzer at room temperature
͑25 1°C͒. Glassy carbon ͑GC͒ or gold working electrode, plati-
num auxiliary electrode, and potassium chloride-saturated silver/
silver chloride ͑Ag͉AgCl͉KCl ͒ reference or Ag wire quasi-
͑sat͒
reference electrodes were used. The formal potential ͑E0 ͒ of the
Ј
ferrocene ͑Fc͒/ferricinium ion ͑Fc+͒ couple was used as an internal
standard of electrode potential. The E0 value of the Fc/Fc+ redox
Ј
−
couple in EMIBF4, which is 0.40 V vs Ag͉AgCl͉KCl͑sat͒,63 was
0.20 V vs Ag wire irrespective of the kind of electrode substrate.
Prior to each measurement, nitrogen ͑N2͒ or oxygen ͑O2͒ gas was
bubbled directly into the cell solution for at least 15 min to obtain a
N2- or O2-saturated solution, and during every measurement N2 or
O2 gas was flushed over the cell solution.
found that the voltammetric reduction of ͓AuCl4͔ to Au metal oc-
curs in two steps, with the intermediate formation of a gold complex
−
−
͓͑AuCl2͔ ͒ and the electrodeposition of gold from the ͓AuCl2͔ ex-
hibiting three-dimensional progressive nucleation with hemispheri-
cal diffusion-controlled growth of the nuclei. In the present study,
1-ethyl-3-methylimidazolium tetrafluoroborate ͑EMIBF4͒ and 1-
n-butyl-3-methylimidazolium tetrafluoroborate ͑BMIBF4͒, in which
sodium tetrachloroaurate͑III͒ ͑Na͓AuCl4͔͒ is soluble, have been
Before electrodeposition of gold, GC electrodes ͑diameter, ␾
= 3 and 6 mm͒ were polished on polishing microcloth ͑Marumoto
Struers Kogyo͒ with alumina powder ͓particle diameter, 1.0 and
0.06 ␮m ͑Marumoto͔͒/water slurries. After polishing, the GC elec-
trodes were rinsed, sonicated for 10 min, and stored in Milli-Q wa-
ter. Gold deposition onto the GC electrodes was accomplished in a
*
Electrochemical Society Active Member.
z E-mail: ohsaka@echem.titech.ac.jp
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Journal of The Electrochemical Society, 154 ͑6͒ D322-D327 ͑2007͒
D323
N2-saturated EMIBF4 containing 10 mM ͓AuCl4͔−. Note that
−
͓AuCl4͔ is stable ͑ligand-substitution inert͒ in EMIBF4 within the
examined period of 1 week based on the UV-visible absorption
spectra in the absence and the presence of Cl− ions ͑as EMICl͒. Two
reduction waves were obtained at ca. 0.2 and −0.5 V vs Ag wire in
the potential scan from 1.0 to −1.0 V, and two oxidation waves ap-
peared at ca. 0.5 and 1.2 V in the reverse potential scan. The current
for the first reduction wave was about twice that for the second
wave. The similar voltammetric behaviors were also obtained in
BMIBF4. These results are essentially the same as those obtained by
Xu and Hussey for the basic aluminum chloride-1-methyl-3-
ethylimidazolium chloride ͑AlCl3-EMICl͒ molten salt.62 Therefore,
the first and second cathodic waves are considered to correspond to
−
−
the reduction of ͓AuCl4͔ to ͓AuCl2͔ and further reduction of
͓AuCl2͔− to Au metal, respectively ͑i.e., Eq. 1 and 2͒, and the anodic
wave at ca. 0.5 V is ascribable to the oxidation of the deposited Au
−
metal to ͓AuCl2͔ ͑i.e., the backward reaction of Eq. 2͒
−
−
͓AuCl4͔ + 2e− → ͓AuCl2͔ + 2Cl−
͓1͔
͓2͔
͓AuCl2͔ + e− → Au + 2Cl−
−
Another anodic wave observed at ca. 1.2 V, which has not been
reported by Xu and Hussey,62 is considered to correspond to the
−
−
oxidation of ͓AuCl2͔ to ͓AuCl4͔ ͑vide infra͒. Thus, the cathodic
and anodic peaks observed at ca. 0.2 and 1.2, respectively, corre-
spond to the ͓AuCl4͔−/͓AuCl2͔− redox couple, while those observed
at −0.5 and 0.5 V correspond to the ͓AuCl2͔−/Au redox couple.
Figure 1. ͑A͒ CVs obtained at GC electrode ͑␾ = 3 mm͒ in N2-saturated
EMIBF4 containing 10 mM ͓AuCl4͔−. Potential scan range: ͑1, solid line͒
1.5 to −1.0 V vs Ag wire; ͑2, dashed line͒ 1.5–0 V. Potential scan rate
10 mV s−1. ͑B͒ Linear sweep voltammograms obtained in EMIBF4 at
10 mV s−1 in the ͑1͒ absence and ͑2,3͒ the presence of 10 mM ͓AuCl4͔−.
Voltammogram 2 is the same as that ͑enlarged one͒ shown by the dashed line
in Fig. 1A. Voltammogram 3 was measured in the anodic scan after the
electrode potential was held at 0 V for 200 s.
That is,
a series of 2e- and 1e-electrode reactions of the
͓AuCl4͔−–͓AuCl2͔−–Au redox system could be observed in the
RTILs. These CV behaviors are largely different from those ob-
tained in aqueous media. For example, in 0.5 M H2SO4 in which the
−
reduction waves of adsorbed ͓AuCl4͔− and solution bound ͓AuCl4͔
to Au metal were observed at a GC electrode and the oxidation wave
corresponding to the dissolution of deposited Au was not observed
in the potential range −0.4 to 1.5 V vs Ag͉AgCl͉KCl ͑sat͒.35
The anodic peak could be also observed at the same potential ͑ca.
0.5 V͒ as the first anodic peak potential in Fig. 1A ͑1͒ when the
potential scan was reversed at 0 V after observing the first reduction
peak ͓see Fig. 1B ͑2͒, i.e., the enlarged CV of Fig. 1A ͑2͔͒. This
anodic peak current was found to increase when the electrode po-
tential was held at 0 V for 200 s and then was scanned in the posi-
tive direction of potential ͓Fig. 1B ͑3͔͒. In the AlCl3-EMICl, how-
ever, the corresponding oxidation wave was not observed and no Au
deposit was obtained when the GC electrode was held at a potential
ca. 0.1 V more negative than the first reduction peak potential for
several minutes.62
bath of EMIBF4 or BMIBF4 containing 10 mM Na͓AuCl4͔ under
N2 gas atmosphere by applying a potential step from 0.4 to 0 V vs
Ag wire or from 0.4 to −1.0 V for 1000 s. The electrodeposition
procedures, based on a potential step from 0.4 to 0 V vs Ag wire
and from 0.4 to −1.0 V vs Ag wire, are hereafter called procedures I
and II, respectively. The electrodeposited Au on the GC electrodes
was electrochemically activated by repeatedly scanning the potential
between −0.2 and +1.5 V vs Ag͉AgCl͉KCl
at potential scan rate
͑sat͒
of 10 V s−1 for 10 min in 0.5 M H2SO4 aqueous solution under N2
gas atmosphere.
Characterization of electrodeposited gold.— Size and morphol-
ogy of the Au nanoparticles electrodeposited onto the GC electrode
were observed using SEM ͑Keyence Co., Japan͒ at an acceleration
voltage of 20 kV and a working distance of 7.5–7.6 mm.
The fact that in the present case the anodic peak was observed at
ca. 0.5 V when the potential scan was reversed after the first reduc-
tion wave was traversed strongly demonstrates the deposition of Au
metal on the GC electrode surface, even though the electrode poten-
Self-assembled monolayers ͑SAMs͒ of 4-MBA were formed on
the Au-electrodeposited GC electrode by immersion into
a
−
tial was polarized in such a way that ͓AuCl4͔ is reduced to form
N2-saturated 10 mM 4-MBA methanol solution for 15 min. After
immersing, the electrode was rinsed carefully in pure methanol to
remove the nonchemisorbed 4-MBA. The ratio of Au single-
crystalline domains ͓i.e., Au͑111͒, Au͑110͒, and Au͑100͔͒ on the
surface of Au particles was estimated by measuring the reductive
desorption patterns of a SAM of 4-MBA formed on the Au-
electrodeposited GC electrodes in N2-saturated 0.5 M KOH
solution.41,44,45,64,65,67 The cyclic voltammetric measurements for re-
ductive desorption were performed by scanning the potential be-
−
͓AuCl2͔ ͑not Au metal͒. This remarkable observation can be ex-
plained as follows. The formal potential ͑E0 ͒ of Eq. 2 is more
Ј
positive than that of Eq. 162,66 and thus, in general, the reduction of
−
͓AuCl4͔ in both aqueous and nonaqueous media is a three-electron
process leading directly to Au metal without a clear indication of an
intermediate ͓AuCl2͔−. However, the reaction of Eq. 2 at GC elec-
trode in the RTILs is very slow kinetically, i.e., the nucleation re-
quires a large overpotential. As a result, the two-step reduction pro-
cesses corresponding to Eq. 1 and 2 were observed. Under such a
tween −0.3 and −1.3 V vs Ag͉AgCl͉KCl
at 50 mV s−1
.
͑sat͒
−
Oxygen reduction reaction at the Au-electrodeposited GC elec-
trodes was examined using cyclic voltammetry in O2-saturated
0.5 M KOH solution.45
condition, the disproportionation reaction of ͓AuCl2͔ ͑Eq. 3͒ can be
expected to take place and consequently, once the Au metal is
formed on
−
−
3͓AuCl2͔ ꢀ ͓AuCl4͔ + 2Au + 2Cl−
͓3͔
Results and Discussion
Ϫ
−
Redox behavior of ͓AuCl4͔ in RTILs.
on the GC electrode surface, the subsequent reduction of ͓AuCl4͔
.— Figure 1 shows the
typical cyclic voltammograms ͑CVs͒ obtained at GC electrode in
can occur on these Au nuclei with little or no nucleation
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D324
Journal of The Electrochemical Society, 154 ͑6͒ D322-D327 ͑2007͒
Figure 3. SEM images of the Au particles electrodeposited on GC elec-
trodes. Au particles were electrodeposited in N2-saturated EMIBF4 contain-
Figure 2. CVs obtained at Au electrode ͑␾ = 1.6 mm͒ in N2-saturated
EMIBF4 containing 10 mM ͓AuCl4͔−. Potential scan rate 10 mV s−1. Poten-
tial scan range ͑solid line͒ 1.5 to −1.0 V and ͑dashed line͒ 1.5–0 V vs Ag
wire.
−
ing 10 mM ͓AuCl4͔ by applying a potential-step electrolysis ͑a,a ͒ from
Ј
0.4 to 0 V vs Ag wire ͑procedure I͒ and ͑b,b ͒ from 0.4 to −1.0 V ͑proce-
Ј
dure II͒ for 1000 s.
The observed differences in the particle density as well as the
size and morphology of the Au particles prepared by procedures I
and II result from the differences in the electrodeposition of
͓AuCl4͔− by both procedures. In procedure II, the nucleation of gold
overpotential.62 This can be clearly confirmed from the CVs ob-
served on Au electrode in EMIBF4 ͑Fig. 2͒. One main reduction
wave was observed at ca. 0.2 V and two oxidation waves at ca. 0.5
and 1.2 V. The observed cathodic wave corresponds to the 3e-
−
−
may result from a series of reductions of ͓AuCl4͔ to ͓AuCl2͔ and
further ͓AuCl2͔− to Au. After the electrochemical nucleation of gold,
the growth of the nucleus to particles or crystalline and the conse-
quent coalescence of mature particles occurs; the deposition is based
on the nucleation-coalescence growth mechanism.68,69 In procedure
I, the ͑initial͒ nucleation of gold can be considered to result from the
disproportionation of the 2e-reduction product of ͓AuCl4͔−, i.e.,
−
reduction of ͓AuCl4͔ to Au metal. Based on the above-mentioned
discussion on the CVs in Fig. 1, the first and second anodic waves
−
are ascribable to the oxidations of Au metal to ͓AuCl2͔ and
−
͓AuCl2͔ to ͓AuCl4͔−, respectively.
Xu and Hussey62 reported that no Au deposition occurred on the
GC electrode in the AlCl3-EMICl melt, unlike in the RTILs used
here, when the electrode potential was reversed after the first reduc-
tion wave was traversed, possibly due to the different solution com-
positions in both cases. The AlCl3-EMICl melt previously contained
an excess ͑1.0 M͒ of Cl− ions that are not covalently bound to
aluminum and thus, the equilibrium of Eq. 3 is largely declined to
the left side ͑i.e., unfavorable for the Au metal formation͒, while the
present RTILs contained no Cl− initially and consequently, the equi-
librium is declined to the right side, resulting in the appreciable
formation of Au metal.
−
−
͓AuCl2͔ to ͓AuCl4͔ and Au metal. A part of the formed nuclei
seem to predominately grow to Au particles of larger size without
the coalescence between particles. In this case, once the Au nuclei
are formed on the GC electrode surface, the subsequent deposition
−
of Au ͑i.e., the overall 3e-reduction of ͓AuCl2͔ to Au metal͒ may
mainly take place on the Au surface, because little or no nucleation
overpotential of Au is required on a Au electrode, compared with the
Au deposition on the GC electrode that requires a large overpoten-
tial to drive the nucleation and subsequent growth of a Au deposit
͑as mentioned above͒.
SEM analysis of electrodeposited Au particles.— Figure
3
Electrochemical characterization of deposited Au particles:
Characteristic current potential curves.— Figure 4 shows the char-
acteristic current-potential curves obtained at the Au particle-
deposited GC electrodes prepared by procedures I and II. Both
curves are essentially the same, as expected for a clean Au electrode
in H2SO4 solution, and it is clear that the amount of Au electrode-
posited by procedure I is much smaller than that by procedure II,
being in agreement with the SEM images shown in Fig. 3. The real
surface areas ͑S cm2͒ of Au particles were estimated from the
charge consumed during the reduction of the Au surface oxide
shows typical SEM images of the Au particles electrodeposited on
GC electrodes by applying a potential-step electrolysis in the differ-
ent potential ranges ͓i.e., procedure I ͑a,a ͒ and procedure II
Ј
͑b,b ͔͒. From the comparison of Fig. 3a and b, at first glance, we
Ј
can see that the particle density of the Au particles electrodeposited
by procedure II is much higher than that of the Au particles prepared
by procedure I using the same time of electrolysis, i.e., 1000 s. In
addition, the size and morphology of the Au particles prepared by
both procedures are also different ͑see Fig. 3a and b ͒. When elec-
Ј
Ј
trodeposited by procedure I, crystals composed of pentagonal crys-
talline planes with a size of ca. 500 nm were obtained and much
smaller Au particles ͑typically 10–20 nm͒ were also prepared
around 500 nm particles. The Au particles prepared by procedure II
possess an aggregate morphology composed of nanoparticles with a
particle size ranging from ca. 50 to 250 nm. The shapes of the Au
particles prepared from RTILs are different from those of the Au
particles electrodeposited from aqueous media, for example, by ap-
plying the same potential-step electrolysis as in this study in 0.5 M
monolayer observed at around 0.93 V vs Ag͉AgCl͉KCl
using a
͑sat͒
reported value of ͑390 10͒ ␮C cm−2 70 and the obtained values of
,
S are summarized in Table I. The surfaces of the Au particles pre-
pared by procedure II in EMIBF4 and BMIBF4 using the same elec-
trolysis times are about 9.4 and 5.3 times larger, respectively, than
those of the Au particles prepared by procedure I, which may reflect
the difference in the electrodeposition processes in both cases, espe-
cially in the initial nucleation processes as mentioned above. In
addition, it is obvious that the surface areas of the Au particles
prepared by procedure I in EMIBF4 and BMIBF4 are almost the
−
H2SO4 solution containing 1.0 mM ͓AuCl4͔ in which Au particles
of spherical shape were prepared.67
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Journal of The Electrochemical Society, 154 ͑6͒ D322-D327 ͑2007͒
D325
electrodeposition of Au by procedure II is significantly affected by
the viscosity of RTILs, i.e., ͑26 − 27͒ cP ͑1 cP = 10−3 Pa s͒ of
EMIBF4 and 180 cP of BMIBF4 at 25°C.71
Reductive desorption of 4-mercaptobenzoic acid (4-MBA) self-
assembled monolayer (SAM).— Based on the comparison of CV
patterns for the reductive desorption of the SAMs of thiols ͑e.g.,
4-MBA and L-cysteine͒ formed on polycrystalline and three low-
index ͓i.e., ͑111͒, ͑100͒, and ͑110͔͒ single-crystalline Au electrodes,
we have recently proposed that three reductive desorption waves
observed at polycrystalline Au electrode are strongly correlated with
those from each domain of the low-index faces ͓i.e., Au͑111͒,
Au͑100͒ and Au͑110͔͒ on the surface,64 and that the relative ratio of
the low-index faces over polycrystalline Au surfaces can be esti-
mated as the relative ratio of three peak current intensities in CVs
obtained for the reductive desorption of thiol SAMs. This idea has
been successfully applied to estimate the relative ratios of the
Au͑111͒, Au͑110͒, and Au͑100͒ crystalline orientation domains con-
stituting the surface of the polycrystalline Au nanoparticles elec-
trodeposited from aqueous solutions under various electrolysis
conditions.41,44,45,67
Figure 5 shows typical CVs obtained for the reductive desorption
of 4-MBA from the surfaces of the Au particles which were elec-
trodeposited on GC electrodes by procedures I and II in N2-saturated
0.5 M KOH solution at a scan rate of 50 mV s−1. In both cases,
three reductive peaks ͑denoted by ء͒ were obtained at around −0.6,
−0.8, and −1.0 V vs Ag͉AgCl͉KCl , which correspond to the de-
͑sat͒
sorption of 4-MB A from the single-crystalline orientation domains
of Au͑111͒, Au͑100͒, and Au͑110͒, respectively, over the surfaces of
Au particles. The amounts of 4-MBA adsorbed on each single-
crystalline face of Au͑111͒, Au͑100͒, and Au͑110͒ on the Au particle
surfaces were estimated from the corresponding peak areas of CVs,
and the relative ratios of three low-index domains on the deposited
Au particles were calculated. The obtained results are summarized
in Table I, together with the total surface coverage ͑⌫͒ of 4-MBA.
The ⌫ values are ͑2–9͒ ϫ 10−10 mol cm−2, which correspond to a
saturated adsorption of 4-MBA onto the Au particle surfaces in a
monolayer fashion.72,73 From Table I, very interestingly, it is obvi-
ous that the Au particles, prepared by procedure I, are enriched in
the Au͑110͒ single-crystalline domain, i.e., the percentages of
Au͑110͒ domain of the Au particles prepared in EMIBF4 and
BMIBF4 are 92 and 99%, respectively. The relative ratios of the
Au͑111͒, Au͑110͒, and Au͑100͒ of the Au particles prepared by pro-
cedure II are similar to those of the Au particles electrodeposited
from aqueous solutions.41,44,45,67 In the case of the Au particles pre-
pared by a potential-step electrolysis ͑for 1000 s͒ in 0.5 M H2SO4
solution containing 1.0 mM ͓AuCl4͔−, the relative ratio of three
low-index facets has been evaluated as Au͑111͒:Au͑110͒
+ Au͑100͒ = 25͑%͒:75͑%͒ based on the reductive desorption of
SAMs of L-cysteine.67
Figure 4. CVs obtained at Au-particle-electrodeposited GC electrodes ͑␾
= 6 mm͒ in N2-saturated 0.5 M H2SO4 solution. Potential scan rate
100 mV s−1. The electrodes ͑a͒ and ͑b͒ were prepared by procedures I and II,
respectively, under the same conditions as in Fig. 3 and then electrochemi-
cally pretreated by repeatedly scanning the potential between −0.2 and 1.5 V
vs Ag͉AgCl͉KCl ͑sat͒ at 10 V s−1 for 10 min in the same sulfuric acid solu-
tion before CV measurements.
same, but those of the Au particles prepared by procedure II are
different in EMIBF4 and BMIBF4. This fact may suggest that the
Table I. Relative ratios of the Au(111), Au(110), and Au(100) single-crystalline orientation domains constituting the surface of the polycrystalline
Au nanoparticles electrodeposited on GC electrodes.
c
c
b
c
Sa
⌫
Au͑111͒
͑%͒
Au͑100͒
͑%͒
Au͑110͒
͑%͒
Procedure
I
Solvents
͑cm2͒
͑mol cm−2
͒
EMIBF4
BMIBF4
EMIBF4
BMIBF4
0.17
0.18
1.59
0.95
9.0 ϫ 10−10
3.0 ϫ 10−10
1.9 ϫ 10−10
2.2 ϫ 10−10
3.9
0.9
27
4.6
0.4
5.1
6.1
92
99
68
66
II
28
a The real surface of Au particles.
b The total surface coverage of 4-MBA.
c The relative ratios of the amount of charge consumed during the reductive desorption of the SAMs of 4-MBA from the Au͑111͒, Au͑100͒, and Au͑110͒
surface domains.
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D326
Journal of The Electrochemical Society, 154 ͑6͒ D322-D327 ͑2007͒
Figure 6. CVs obtained for O2 reduction at the Au-particle-electrodeposited
GC electrodes ͑␾ = 6 mm͒ in O2-saturated 0.5 M KOH solution. Potential
scan rate 50 mV s−1. The Au-particle-electrodeposited GC electrodes were
prepared by procedures I and II under the same conditions as in Fig. 3.
Voltammograms a and b were obtained at the Au-particle-electrodeposited
GC electrodes prepared by procedures I and II, respectively.
Au͑111͒ and/or Au͑110͒, because the ORR is quasi-reversible at
Au͑111͒ and Au͑110͒ single-crystalline electrodes but is completely
irreversible at Au͑100͒ single-crystalline electrode.45,74,75 In addi-
tion, by considering this together with the above-mentioned reduc-
tive desorption measurements ͑Fig. 5͒, it can be safely said that the
ORR observed at the Au-particle-electrodeposited electrode pre-
pared by procedure I takes place at the Au͑110͒ faces, while the
ORR at the Au particle electrode prepared by procedure II occurs at
the Au͑111͒ and Au͑110͒ faces, i.e., the main reduction peak at
−0.16 V corresponds to the ORR at the Au͑111͒ faces of the Au
nanoparticles and a small reduction peak observed as a shoulder at
around −0.28 V is ascribed to the ORR at the Au͑110͒ faces.
Figure 5. CVs obtained for reductive desorption of the SAMs of 4-MBA
adsorbed on Au-particle-electrodeposited GC electrodes ͑␾ = 6 mm͒ in
N2-saturated 0.5 M KOH solution. Potential scan rate 50 mV s−1. Electrodes
͑a͒ and ͑b͒ were prepared by procedures I and II, respectively, and the same
conditions as in Fig. 3.
Conclusions
−
We have examined the CV behavior of ͓AuCl4͔ at GC and Au
electrodes in EMIBF4 and EMIBF4 ionic liquids. The results show
that a series of 2e- and 1e-reductions of the ͓AuCl4͔−–͓AuCl2͔−–Au
redox system occurs at the GC electrode, while a single reduction of
͓AuCl4͔ to Au metal takes place at the Au electrode. In the former
case, a significant disproportionation reaction of ͓AuCl2͔ to
−
−
Oxygen reduction reaction.— The oxygen reduction reaction
͑ORR͒ at gold electrodes in alkaline solutions is well known to be
very dependent on the crystallographic orientation of the gold
surface.45,74,75 Thus, CVs for the ORR at the Au-particle-
electrodeposited GC electrodes prepared in the RTILs were mea-
sured in O2-saturated 0.5 M KOH solution to examine the surface
character of the Au particles
Figure 6 shows the typical CVs obtained for the ORR at the
Au-particle-electrodeposited GC electrode prepared by procedures I
and II in EMIBF4. The CV ͑voltammogram a͒ obtained at the Au-
particle-deposited electrode prepared by procedure I gave a couple
of cathodic and anodic peaks at −0.26 and −0.03 V with a ratio
͑iap/icp͒ of anodic to cathodic peak current of ca. 0.44 and a anodic to
cathodic peak separation ͑⌬Ep͒ of 230 mV. That at the electrode
prepared by procedure II ͑voltammogram b͒ gave one main couple
of cathodic and anodic peaks at −0.16 and −0.08 V with iap/ipc = ca.
0.63 and ⌬Ep = 80 mV. Thus, in both cases, the ORR is quasi-
reversible. Similar results were also obtained at the Au-particle-
electrodeposited electrodes prepared by procedures I and II in
BMIBF4. These results suggest that the surfaces of the electrodepos-
ited Au particles are enriched in the single-crystalline faces of
−
͓AuCl4͔ and Au was found to occur. The electrodeposition of Au
nanoparticles was found to occur via a disproportionation of
−
−
͓AuCl4͔ to ͓AuCl2͔ and Au metal as well as via a series of reduc-
−
−
−
tions of ͓AuCl4͔ to ͓AuCl2͔ and further, ͓AuCl2͔ to Au ͑i.e., 3e-
−
reduction of ͓AuCl4͔ ͒. In addition, the size and morphology of the
Au particles and the relative ratio of the Au͑111͒, Au͑110͒, and
Au͑100͒ crystalline orientation domains constituting their surface
were found to largely depend on the potential-step electrolysis po-
tential used in the electrodeposition, i.e., on whether this potential
−
−
corresponds to the reduction of ͓AuCl4͔ to ͓AuCl2͔ or Au.
Acknowledgments
This research was financially supported by Grant in Aids for
Scientific Research ͑no. 14050038͒ to T. Ohsaka, from the Ministry
of Education, Science, Sports, Culture and Technology, Japan, and
also by New Energy and Industrial Technology Development Orga-
nization ͑NEDO͒, Japan. The authors thank H. Masumitsu ͑Keyence
Co., Japan͒ for his support in the SEM measurements. Helpful dis-
cussions with Professor F. Kitamura and Dr. M. S. El-Deab at Tokyo
Institute of Technology and Professor N. Oyama at Tokyo Univer-
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Journal of The Electrochemical Society, 154 ͑6͒ D322-D327 ͑2007͒
D327
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Tokyo Institute of Technology assisted in meeting the publication costs of
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