Construction of highly oriented self-assembled monolayer of alkyldithiol with ferrocene on gold (111) using underpotentially deposited lead submonolayer as a template

Article abstract of DOI:10.1246/cl.2007.1216

Highly oriented self-assembled monolayer (SAM) of alkyldithiol with a ferrocene group was constructed on Au(111) surface using underpotentially deposited (UPD) Pb submonolayer as a template. Orientation of this SAM was confirmed by comparing amounts of ferrocene moiety and Au-S bonding. Copyright

Full text of DOI:10.1246/cl.2007.1216

1216
Chemistry Letters Vol.36, No.10 (2007)
Construction of Highly Oriented Self-assembled Monolayer of Alkyldithiol
with Ferrocene on Gold (111) Using Underpotentially Deposited
Lead Submonolayer as a Template
Toshihiro Kondo,ꢀ Saori Sato, and Wakana Maeda
Graduate School of Humanities and Sciences, Ochanomizu University, Ohtsuka, Bunkyo-ku, Tokyo 112-8610
(Received July 4, 2007; CL-070711; E-mail: kondo.toshihiro@ocha.ac.jp)
Highly oriented self-assembled monolayer (SAM) of alkyl-
submonolayer on Au(111), SAM of both rod and bridge types
should form. The bridge-type SAM connects both to Au and
Pb atoms because the alkylthiol and alkyldithiol SAMs were
generally closed-packed and the spacing of 0.87 nm is too small
that both terminated S atoms of the dithiol molecule adsorbed
only on Pb atoms or only on Au atoms. After Pb is oxidatively
desorbed, only a rod-type dithiol SAM should remain. When this
substrate is dipped in the solution containing the same dithiol
molecule, it is expected to form the highly oriented dithiol
SAM with a rod-type. Here, we report the formation of the
rod-type SAM of the dithiol molecule with a ferrocene group
on Au(111) using UPD Pb submonolayer as a template. It was
confirmed that this SAM is very highly well-ordered by compar-
ison of amounts of ferrocene moiety and Au–S bonding.
dithiol with a ferrocene group was constructed on Au(111) sur-
face using underpotentially deposited (UPD) Pb submonolayer
as a template. Orientation of this SAM was confirmed by com-
paring amounts of ferrocene moiety and Au–S bonding.
Self-assembly (SA) technique is one of the most promising
methods to construct highly oriented molecular layers with a
molecular dimension, and self-assembled monolayer (SAM) of
alkylthiol on gold has been extensively studied in various fields
such as molecular devices.1–3 SA multilayer should be more
useful to construct molecular devices. Although many studies
on highly oriented SAMs were reported, ordered SA multilayers
have hardly been studied because, during multilayer formation,
yield of surface reactions for the bond between the molecular
layers is relatively low as compared with that for the Au–S bond-
ing between the SAM and gold. For example, Duevel and Corn
reported4 that alkylcarboxylic acid layer was constructed on the
amine-terminated SAM on gold through amide bonding but
adsorbed amount of the second layer of alkylcarboxylic acid
molecule was low, and, therefore, the orientation of alkylcarbox-
ylic acid layer should be low. Kim et al. and Kohli et al. constrct-
ed covalently coupled disulfide multilayer on gold using (ꢀ,!)-
dithiol derivatives with relatively longer alkyl chain.5 However,
a dithiol molecule with a longer alkyl chain often adsorbs on
gold surface not only through one terminated thiol, which is so
called as a rod type, but also through both terminated thiol
groups, which is so called as a bridge type. If the rod-type
SAM is constructed with a dithiol molecule, we can get a
well-ordered SA multilayer using a disulfide bonding.
Scheme 1.
Figure 1. Fc(C6SH)2 molecule.
On the other hand, Shimazu et al. constructed the highly ori-
ented mixed SAMs on Au(111) using underpotentially deposited
(UPD) Pb submonolayer as a template.6 As a first, Pb submono-
layer (coverage, ꢁ ¼ 0:6) was UPD on Au(111), followed by the
formation of a SAM of an alkylthiol. In this stage, the pinstripe
structure was observed by STM. Spacing of each row was ca.
0.87 nm, and then this pinstripe was considered as a row of Pb.
UPD Pb was then oxidatively desorbed from the gold surface
to create a vacant site, where another thiol molecule was adsorb-
ed to finally form the highly oriented mixed SAMs. In this
final stage, the pinstripe structure was also observed, and spacing
of each row was ca. 0.87 nm, which was matched well with the
previous one. If a dithiol SAM is constructed on this UPD Pb
Bis(6-mercaptohexyl)ferrocene (Fc(C6SH)2, shown in
Figure 1) was synthesized by the previously reported proce-
dures.7 Formation procedures of Fc(C6SH)2 SAM is shown in
Scheme 1 and described in the followings. After commercially
available Au(111) disk (SPL, surface area: 0.875 cm2)8 was an-
nealed by a gas flame and quenched under air, Pb was UPD with
ꢁ of 0.6 by scanning the potential from þ0:65 V (vs. Ag/AgCl)
to ꢁ0:15 V in the electrolyte solution containing 1 mM Pb2þ
(step 1 in Scheme 1). Amount of UPD Pb was confirmed by
the electrochemical crystal microbalance (EQCM) measure-
ment. During keeping the potential of ꢁ0:15 V, ethanol solution
of 10 mM Fc(C6SH)2 was added to the electrolyte solution to
construct the Fc(C6SH)2 SAM until total concentration of
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.10 (2007)
1217
300
200
100
0
0
-5
500 mV s-1
200 mV s-1
100 mV s-1
50 mV s-1
-10
-15
-20
-100
-200
-300
-1.4 -1.2
-1
-0.8 -0.6 -0.4 -0.2
0
0.2
Potential / V vs. Ag/AgCl
Figure 3. LSV of Fc(C6SH)2-SAM-modified Au(111) elec-
trode measured in 0.1 M KOH with a scan rate of 20 mV sꢁ1
0
0.2
0.4
0.6
0.8
.
Potential / V vs. Ag/AgCl
a single Au–S bond, namely, is constructed as a rod-type. When
the Fc(C6SH)2 SAM was prepared just by the dipping of
Au(111) substrate into the Fc(C6SH)2 solution, the amount of
Au–S bonding was 1.7 times larger than that of the adsorbed
Fc(C6SH)2 molecule. Thus, it is concluded that the present
Fc(C6SH)2 SAM is highly oriented on the Au(111) surface.
Figure 2. CVs of Fc(C6SH)2-SAM-modified Au(111) electrode
measured in 0.1 M HClO4 with various scan rates.
Fc(C6SH)2 became 1 mM. After that, the substrate was directly
immersed in the ethanol solution of 1 mM Fc(C6SH)2 for 1 h
to form complete Fc(C6SH)2 SAM on UPD Pb/Au(111)
substrate (step 2). UPD Pb was then oxidatively desorbed in
the electrolyte solution without Pb2þ by scanning the potential
from ꢁ0:15 V to þ0:65 V (step 3). Finally, the substrate was
immersed again in ethanol solution of 1 mM Fc(C6SH)2 (step 4).
Figure 2 shows cyclic voltammograms (CVs) of the
Fc(C6SH)2-SAM-modified Au(111) electrode, which was pre-
pared by the above procedures. Relatively sharp wave due to
the redox of ferrocene moiety was observed at þ0:27 V. Linear
relationship between peak current and sweep rate showed that
this peak is due to the redox of the attached ferrocene moiety
on electrode surface. Amount of Fc moiety, i.e., surface concen-
tration of Fc(C6SH)2 molecule, was estimated from the integrat-
ed anodic charge of the redox peak to be ca. 7:1 ꢂ 10ꢁ10
mol cmꢁ2 (4:3 ꢂ 1014 molecules cmꢁ2). Redox potential and
surface concentration observed in the present study were match-
ed well with those previously observed at the ferrocenylalkyl
thiol SAM on gold.7,9 Fwhm (full width at half maximum) of
the peak and the peak separation were 0.07 and 0.04 V, respec-
tively. Although the former value was quite similar to, the latter
was slightly larger than those at the previous one,9 indicating that
the ferrocene moiety should be placed inside the alkyl chain;
namely, this SAM is highly oriented as a rod type.
This work was partially supported by Grants-in-Aid for
Scientific Research (KAKENHI) in Priority Area of ‘‘Molecular
Nano Dynamics’’ (No. 17034017) from Ministry of Education,
Culture, Sports, Science and Technology.
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Figure 3 shows a linear sweep voltammogram (LSV) of
Fc(C6SH)2-SAM-modified Au(111) electrode, which was pre-
pared by the above procedures. A relatively sharp cathodic peak
was observed at ꢁ1:05 V, which is due to electrochemically re-
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Amount of Au–S bonding estimated from reductive charge of
this cathodic peak was ca. 8:1 ꢂ 10ꢁ10 mol cmꢁ2 (4:9 ꢂ 1014
molecules cmꢁ2). When we consider the charge required to
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6
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a) K. Uosaki, Y. Sato, H. Kita, Langmuir 1991, 7, 1510. b)
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the observed value) is calculated to be 7:3 ꢂ 10ꢁ10 mol cmꢁ2
,
which is matched well with that of the amount of adsorbed
Fc(C6SH)2 molecule. This result indicated that the Fc(C6SH)2
SAM prepared in the present study should be connected with
10 T. Kawaguchi, H. Yasuda, K. Shimazu, M. D. Porter, Lang-
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Published on the web (Advance View) September 8, 2007; doi:10.1246/cl.2007.1216