Platinum layer formation on a self-assembled monolayer by electrochemical deposition

Article abstract of DOI:10.1246/cl.2006.258

A platinum metal-self-assembled monolayer (SAM)-gold substrate sandwich structure without a short circuit between the two metals was successfully constructed by electrochemical deposition of platinum on top of a 1,4-benzenedimethanethiol (BDMT) SAM-covered Au(III) substrate. Deposition of Pt was carried out by electrochemical reduction of Pt ions, which were adsorbed on free thiol end groups of the BDMT SAM, in a Pt ion-free sulfuric acid solution. Electrochemical measurements and X-ray photoelectron spectroscopy (XPS) showed the reduction of the adsorbed Pt ions to metallic platinum on top of the SAM. After a cathodic potential scan of the Pt-SAM-Au electrode to -1.25 V in 0.1 M KOH solution, a typical cyclic voltammogram of the Au(III) electrode was obtained, showing that the BDMT SAM was reductively desorbed with a Pt layer and that no platinum was deposited directly on the Au(III) surface. Copyright

Full text of DOI:10.1246/cl.2006.258

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58
Chemistry Letters Vol.35, No.3 (2006)
Platinum Layer Formation on a Self-assembled Monolayer by Electrochemical Deposition
ꢀ
Deyu Qu and Kohei Uosaki
Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science,
Hokkaido University, Sapporo 060-0810
(Received November 25, 2005; CL-051461; E-mail: uosaki@pcl.sci.hokudai.ac.jp)
A platinum metal-self-assembled monolayer (SAM)–gold
In this study, we propose another approach for metal-layer
formation on top of an SAM with strong interaction between
the SAM and the formed metal layer as shown in Figure 1.
At first, a Au(111) electrode was modified with a self-
assembled monolayer of BDMT in a 1 mM ethanolic BDMT
solution.
substrate sandwich structure without a short circuit between
the two metals was successfully constructed by electrochemical
deposition of platinum on top of a 1,4-benzenedimethanethiol
(
BDMT) SAM-covered Au(111) substrate. Deposition of Pt
was carried out by electrochemical reduction of Pt ions, which
were adsorbed on free thiol end groups of the BDMT SAM, in
a Pt ion-free sulfuric acid solution. Electrochemical measure-
ments and X-ray photoelectron spectroscopy (XPS) showed
the reduction of the adsorbed Pt ions to metallic platinum on
top of the SAM. After a cathodic potential scan of the Pt–
SAM–Au electrode to ꢁ1:25 V in 0.1 M KOH solution, a typical
cyclic voltammogram of the Au(111) electrode was obtained,
showing that the BDMT SAM was reductively desorbed with
a Pt layer and that no platinum was deposited directly on the
Au(111) surface.
After a full monolayer of BDMT was formed on the
Au(111) surface with a free thiol end group face up, the SAM-
covered electrode was then immersed in an aqueous solution
2ꢁ
containing 5 mM of PtCl4 ions for 20 min. The Pt ions in the
solution was expected to bound to free SH end groups on the
1
7–19
SAM surface.
The presence of Pt 4f7=2 and Pt 4f5=2 peaks
at 72.9 and 75.6 eV, respectively, and Cl2p peak at 200.1 eV
in XP spectra with the relative amount of Pt and Cl of 3.8 con-
firmed that the platinum ions were adsorbed on the SAM in the
2ꢁ
form of platinum tetrachloride complex (PtCl4 ). The binding
energy of Pt 4f7=2 negatively shifted from 73.4 eV of Pt 4f7=2
2
1
of K2PtCl4. This suggests that Pt was coordinated with the S
end group and that the oxidation state of Pt was changed from
Metal-layer formation on an organic monolayer has been an
important research subject in recent years because of its connec-
tion with an attractive field of molecular electronics in which an
organic monolayer will be used as electronic components. Self-
assembled monolayers (SAMs) of thiols are good candidates to
be used as molecular layers for this purpose because of their ease
of preparation, highly ordered structure and well-documented
characteristics. A metal–SAM–metal sandwich structure without
a short circuit between the two metals is used to determine the
electrical properties of the molecules and is also an essential
component of molecule devices. However, most attempts to
electrochemically deposit a metal on top of an SAM have
failed.1 The reasons of the failures are the existence of defect
sites in the SAM and presence of metal ions in solution. Even
if some metal ions are immobilized on top of an !-functional
thiolate SAM, excess amounts of free metal ions in the bulk so-
lution can still penetrate the organic film through these imperfec-
tions and metal deposition occurs underneath the SAM not on
top of it. Recently, Kolb and co-workers proposed a new ap-
proach to form a metal layer on an organic surface by electro-
1
7,19
+2 to +1.
After adsorption of Pt ions, the S2p peak seemed
to be broadened with decreased intensity of the peak at 164 eV
and a slight shift to lower binding energy of the shoulder around
162.5 eV. This suggests that thiol was converted to thiolate after
the Pt ion adsorption, indicating that the free thiol groups on the
surface were coordinated with platinum complex ions.
Finally, the electrode was transferred to a platinum ion-free
0.05 M H2SO4 solution and the adsorbed Pt ions on the BDMT
SAM-covered Au(111) electrode were reduced electrochemical-
ly by holding an electrode potential at +0.4 V, which is more
2
ꢁ
negative than the redox potential of PtCl4 /Pt (ca. +0.55
20
–13
V), to form a Pt layer on top of the SAM.
A cyclic voltammogram (CV) of the resulting electrode re-
corded in a 0.05 M H2SO4 solution was in good agreement with a
typical CV of a platinum electrode with waves of adsorption and
desorption of hydrogen. This proves that a platinum layer was
formed. The charge associated with the desorption of hydrogen
ꢁ
2
was found to be 61 mC cm , which is equal to about 28% of the
desorption of one full monolayer of hydrogen on a Pt surface.
This is reasonable as the amount of thiolate on Au(111) surface,
i.e., Pt adsorption site, is only 1/3 of the surface Au atoms of the
Au(111) substrate.
0
chemical reduction of Pd(II) ion pre-adsorbed on a 4,4 -dithiodi-
1
4–16
pyridine-modified Au(111) surface.
In their system, howev-
er, the formed metal layer was only weakly adsorbed on the
SAM.
XP spectra of a Pt–BDMT–Au(111) electrode showed peaks
of Pt 4f7=2 and Pt 4f5=2 at 71.7 and 75.2 eV, respectively, which
are shifted from 72.9 and 75.6 eV obtained before the reduction,
respectively, confirming that a metallic platinum layer was
formed. The Cl2p peak totally disappeared after electrochemical
reduction of Pt ions. This result also shows that the Pt complex
ions were reduced to metallic platinum. The C1s signal and the
position of the S2p peak were found to be the same before and
after the electrochemical reduction.
Figure 1. Schematic illustration of metal-layer formation on
top of a SAM.
Angle-resolved XPS was applied to a Pt–BDMT–Au(111)
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.3 (2006)
259
following scans.
Furthermore, the XP spectra obtained after the negative scan
to ꢁ1:25 V in a 0.1 M KOH solution showed the disappearance
of Pt4f and S2p peaks. These results clearly show that platinum
was removed with BDMT by the cathodic scan in alkaline solu-
tion, indicating that a Pt was not deposited on the Au(111) sur-
face but only on top of the SAM.
In summary, a novel method to construct a Pt/SAM/Au
sandwich structure without a short circuit between two metals
was proposed. XPS and electrochemical measurements con-
firmed that metallic platinum only deposited on top of the
SAM and that no metallic platinum existed on the Au(111) sur-
face. The metal deposition on Au underneath the SAM through
defect site was avoided because electrochemical deposition was
carried out in a platinum-ion free solution. In this way, no plat-
inum ions penetrated through imperfections in the course of
electrochemical reduction of Pt ions and only Pt ions pre-adsorb-
ed on the BDMT SAM were reduced.
Figure 2. Dependencies of relative intensities of (A) IPt=Ic and
(
B) Ipt=IAu on a take-off angle. IPt, IAu, and IC represent the
intensities of XP spectra of Pt 4f7=2, Au 4f7=2, and C1s peaks,
respectively.
electrode to determine the position of Pt. The relationship
between the relative intensities of Pt4f over Au4f and C1s and
the take-off angles was shown in Figure 2. The figure clearly
shows that the relative intensities of Pt4f to those of Au4f
and C1s increased as the take-off angle decreased. This proves
that Pt was indeed deposited not underneath but on top of the
BDMT SAM.
The results of electrochemical measurements also provide
evidence that the platinum layer formed only on top of the
SAM. A solid line in Figure 3 shows a current response when
the potential of the Pt–BDMT SAM-modified Au(111) electrode
was scanned negatively from 0 to ꢁ1:25 V. The cathodic current
increased significantly when the potential became more negative
than ꢁ1:0 V and a shoulder was observed at ꢁ1:18 V. The large
cathodic current is due to hydrogen-evolution reaction (HER)
and the shoulder is due to desorption of the Pt–BDMT SAM.
The reductive desorption took place at more negative potential
than the reductive desorption potential of the BDMT SAM
D. Qu acknowledges the Japan Society for the Promotion of
Science (JSPS) for a Postdoctoral Fellowship for foreign re-
searchers. Professor K. Shimazu is acknowledged for use of
the XPS. This work was partially supported by a Grant-in-Aid
for Scientific Research in the Priority Area of ‘‘Molecular Nano
Dynamics’’ (No. 16072202) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
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Published on the web (Advance View) January 28, 2006; DOI 10.1246/cl.2006.258