Fabrication of Pt nano-dot-patterned electrode using atomic force microscope-based indentation method

Article abstract of DOI:10.1016/j.electacta.2011.12.096

We developed a new technique for fabricating an electrocatalytic surface of defined morphology in which the particle size and interparticle distance are controlled independently on an electroconductive substrate. Firstly, an electroconductive glassy carbon substrate was overcoated by a uniform insulating polymer layer as a mask. Secondly, the polymer coated substrate was indented in nano size using an atomic force microscope (AFM) cantilever to expose the carbon surface. Thirdly, Pt particles were electrodeposited only on the exposed area of the glassy carbon. To expose a small area of the glassy carbon, a 10 ± 1-nm thick insulating polymer overcoat layer was prepared, and a cantilever indentation was performed at an optimized loading force of 3.6 μN. The smallest exposed area obtained was 20 nm × 40 nm. Thereafter, Pt electrodeposition was conducted at an electrode potential of 0.12 V vs RHE so that the Pt particles could be located only at the exposed deposition sites. Thus, Pt deposition of 30-60 nm in diameter was successfully achieved at an arbitrary proportion of the interparticle distance.

Full text of DOI:10.1016/j.electacta.2011.12.096

Electrochimica Acta 63 (2012) 251–255
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Fabrication of Pt nano-dot-patterned electrode using atomic force
microscope-based indentation method
Minoru Umeda∗, Akira Kishi, Sayoko Shironita
Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, 1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We developed a new technique for fabricating an electrocatalytic surface of defined morphology in which
the particle size and interparticle distance are controlled independently on an electroconductive sub-
strate. Firstly, an electroconductive glassy carbon substrate was overcoated by a uniform insulating
polymer layer as a mask. Secondly, the polymer coated substrate was indented in nano size using an
atomic force microscope (AFM) cantilever to expose the carbon surface. Thirdly, Pt particles were elec-
trodeposited only on the exposed area of the glassy carbon. To expose a small area of the glassy carbon, a
10 1-nm thick insulating polymer overcoat layer was prepared, and a cantilever indentation was per-
formed at an optimized loading force of 3.6 ␮N. The smallest exposed area obtained was 20 nm × 40 nm.
Thereafter, Pt electrodeposition was conducted at an electrode potential of 0.12 V vs RHE so that the
Pt particles could be located only at the exposed deposition sites. Thus, Pt deposition of 30–60 nm in
diameter was successfully achieved at an arbitrary proportion of the interparticle distance.
© 2011 Elsevier Ltd. All rights reserved.
Received 9 August 2011
Received in revised form
22 December 2011
Accepted 22 December 2011
Available online 30 December 2011
Keywords:
Nano-dot pattern fabrication
Pt electrodeposition
Glassy carbon electrode
Nano indentation
Polymer overcoat layer
1. Introduction
and mask fabrication [22–39]. Maskless fabrication uses a dip pen
[11–13], microcontact printing [14,15], metal deposition onto a
Electrocatalysts comprised of nano-sized and zero-dimension
particles on an electroconductive support are widely utilized in
practical applications in fuel cells [1], electrolyzers [2], and elec-
trochemical sensors [3–5], due to their high effective surface area.
Almost all of the electrocatalysts have a two-component structure
in which nano-sized catalytic particles are loaded on an electro-
conductive support. To functionalize the electrocatalytic activity,
the particle size and interparticle distance are believed to play an
important role in the electrocatalysts [6–8]. A smaller particle size
yields a larger surface area, which will provide a larger number of
reaction sites. However, there is no research report in the field of
electrocatalysts in which particle size and interparticle distance are
independently controlled. With regard to the field of electrocata-
lysts, it is worthwhile to clarify the relationship between particle
size and interparticle distance.
sample using a charged surface potential [16–20], and an indent
method [21]. For mask fabrication, part of the mask prepared
on a substrate is removed in certain areas by an electron beam
[22–24], ion-beam [25–27], chemical etching [28–30], an atomic
force microscope (AFM) cantilever [31–34], mold [35–39], etc.,
where the metallic areas are selectively deposited on the substrate
in places where the mask was removed.
However, thus far, metallic nano fabrication studies have
focused on electrode-pattern formation for semiconductor devices
[13,15,17,19,20,22–25,28,30–36,39]. Therefore, the metallic nano
fabrication has used a technique that deposits a single-dimension
line of metal on a semiconductor Si substrate, but there are no
research reports on the deposition of zero-dimensional metal dots
on an electroconductive substrate.
To fabricate the electrocatalytic surface of defined morphol-
ogy, our attention was focused on a mask fabrication method that
deposits metallic nano particles on the substrate. Previously, we
developed a nano peel-off fabrication technique for Pt electrode-
position, in which an insulating polymer overcoat layer was first
prepared on an electroconductive glassy carbon substrate, and
then the polymer overcoat layer was peeled off by a scratch tech-
nique using an AFM cantilever. Consequently, Pt deposition of
100–150 nm in diameter was successfully achieved by adjusting
the interparticle distance [40].
To realize a well-characterized electrocatalyst model in which
the particle size and interparticle distance are controlled inde-
pendently, a metallic nano fabrication on an electroconductive
substrate seems to be an effective approach [9,10]. There are two
metal nano fabrication techniques: Maskless fabrication [11–21]
∗
Corresponding author. Tel.: +81 258 47 9323; fax: +81 258 47 9323.
E-mail address: mumeda@vos.nagaokaut.ac.jp (M. Umeda).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.12.096

252
M. Umeda et al. / Electrochimica Acta 63 (2012) 251–255
rinsed using Milli-Q water and then dried. The roughness of the pol-
ished surface was measured using an AFM (Shimadzu, SPM-9500
J3), and the arithmetic average roughness (Ra) was around 2.2 nm.
Next, the polished glassy carbon was immersed in a perfluori-
nated resin solution (Asahi Glass Co., Ltd, Cytop, CTX-109, 1–2 wt.%),
and it was removed at a constant speed of 180 ␮m s−1 using a
motorized micromanipulator (Narishige Co., Ltd., MM-80). After
this dip-coating procedure, the glassy carbon was dried at 80 ◦C
in an oven chamber for 1 h to form a 10–22-nm thick polymer
overcoat layer.
Fig. 1. Schematic illustration of the Pt nano-dot patterning process: (a) a thin poly-
mer insulating layer is overcoated on glassy carbon; (b) nano indentation of the
overcoat layer using a cantilever of AFM; and (c) Pt electrodeposition.
2.2. Nanoindentation of the polymer overcoat
In the present work, the mask fabrication method will be fur-
ther improved, because the previously prepared 100–150 nm Pt is
much larger than that used for the electrocatalysts. Because the
Pt deposition size is proportional to the exposed electrode area,
we will utilize the mask fabrication method by employing a nano-
indentation technique. Firstly, a thin insulating polymer overcoat
layer was prepared on an electroconductive glassy carbon sub-
strate. Secondly, a small part of the polymer overcoat layer is
removed by an indentation with the AFM cantilever. In this process,
the exposed electrode area and interexposed distance of the poly-
mer overcoat layer are controlled by changing the indentation force
of the cantilever and the interindentation distance. Pt particles are
then electrodeposited only on the exposed area of the glassy carbon.
Using this procedure, the Pt electrocatalyst of defined morphology
which has much smaller Pt particles is fabricated successfully.
Nano indentation of the polymer overcoat layer was conducted
using an AFM equipped with a Si cantilever (Nano World, PPP-
NCHR) to expose the glassy carbon surface. The curvature radius
of the used Si-cantilever tip was smaller than 10 nm so that the
aspect ratio of the prepared indentation pinhole could be high. The
loading force of the cantilever was 0.9–4.3 ␮N. After the indenta-
tion, the sample was immersed for 5 min in Milli-Q water to remove
the polymer debris by ultrasonic cleaning.
2.3. Pt electrodeposition on the nano-indented substrate
The nano-indentation sample was used as a working electrode
for Pt electrodeposition. Ag/Ag2SO4 [41], and Pt wire was used
for both the reference and counter electrodes. Using a potentio-
stat (ALS, Model 802B), Pt electrodeposition was carried out in a
solution mixture of 5 mmol dm−3 H2Pt(OH)6 + 3.0 mol dm−3 H2SO4
for 3 s at a stationary electrode potential of −0.65 to −0.45 V vs
Ag/Ag2SO4 (0.1–0.3 V vs RHE)at 20 ◦C. Here, H2Pt(OH)6 was used
for a stable Pt deposition. The sample was then thoroughly rinsed
in Milli-Q water and dried.
2. Experimental
2.1. Preparation of glassy carbon with polymer overcoat
A glassy carbon plate was purchased from Tokai Carbon Co., Ltd.
(GC-20SS). The glassy carbon plate was cut into 1 cm2 pieces. One
side of the glassy carbon plate was polished using a buff (Refinetec
Co., Ltd., No. 55-308) with an alumina suspension (Union Carbide,
0.05 ␮m in diameter). Thereafter, the polished glassy carbon was
The sample surface was characterized by AFM and a scanning
electronmicroscope (SEM) (JEOL, JSM-6060A) coupled with energy
dispersive X-ray spectroscopy (EDS) (JEOL, JED-2300).
Fig. 2. (a) AFM topography of a glassy carbon substrate with a polymer overcoat layer of 21 1 nm thick after the cantilever indentation at 0.9 ␮N loading force. (b) A
magnification of the indented spot. (c) Cross-sectional profile of the dashed line of (a). (d) Illustration of the cantilever tip.

M. Umeda et al. / Electrochimica Acta 63 (2012) 251–255
253
According to the concept shown in Fig. 1, nano indentations
were performed on a 21 1-nm thick insulating layer formed on
the glassy carbon at a cantilever loading force of 0.9 ␮N. Fig. 2(a)
shows the surface topography of the resulting sample measured
by an AFM, in which nine indented pinholes prepared at an
interindentation distance of 500 nm are seen. Fig. 2(b) demon-
strates a magnification of one pinhole. The triangular dark spot
reflects the shape of the indented cantilever as illustrated in the
figure. Fig. 2(c) represents a cross-sectional view of the dotted line
of Fig. 2(a). The cross-sectional profile points out that the depth
of the indented pinhole is less than the thickness of the polymer
layer. Therefore, a part of polymer layer still remains and prevents
the following electrodeposition.
Next, the influence of the loading force to the indentation shape
was investigated. As is demonstrated in Fig. 2(b), the indentation
shape can be defined by the X- and Y-widths. Fig. 3 shows the rela-
tionships between the loading force and the indentation widths and
also between the loading force and the indentation depth. It can
be seen in the figure that the X-width and the indentation depth
increase according to an increase in the loading force. The inden-
tation depth becomes equal to the polymer thickness at 2.7 ␮N
loading force and exceeds that at a loading force of larger than
3.6 ␮N. However, the Y-width increases up to the 2.7 ␮N loading
force and then decreases. This strongly suggests that the tip of the
cantilever was damaged. It is therefore considered that a loading
force of 2.7–3.6 ␮N is appropriate to create the indentation for the
Pt electrodeposition.
Fig. 3. Cantilever loading force dependence of the indentation depth and indenta-
tion widths for the 21 1-nm polymer-coated glassy carbon. The X- and Y-widths
are defined in Fig. 2(b).
3. Results and discussion
3.1. Relationship between indentation force and indentation
widths
3.2. Pt electrodeposition at the mask-indentation electrode
Previously, we employed the mask-scratch technique which
was developed to peel-off the polymer overcoat layer by scan-
ning the AFM cantilever in the X and Y directions. The cantilever
operating area of 80 nm × 80 nm and the following electrodeposi-
tion resulted in Pt particles of 100–150 nm in diameter [40]. To
diminish the peel-off area, the mask-scratch technique has been
improved with a mask-indentation technique in the present study.
This is a new technique in which an AFM cantilever indentation
on the polymer-coated electrode creates a small pinhole for the Pt
electrodeposition.
First, Pt electrodeposition was performed for the 2.7 ␮N-
indentation sample. However, no Pt deposition occurred. This result
can be explained by a residual portion of the insulating poly-
mer at the bottom of the pinhole. Accordingly, an exposure of the
glassy carbon to an electrolytic solution will be needed for the Pt
deposition.
Next, by employing the 3.6-␮N indentation sample, another
Pt electrodeposition was attempted. Fig. 4(a) shows the surface
profile of the sample before Pt deposition. Nine indentation spots
Fig. 4. (a) AFM image of a glassy carbon substrate coated with a 21 1 nm polymer layer using the cantilever indentation at 3.6 ␮N loading force. (b) Cross-sectional profile
of the dashed line of (a). (c) SEM image after the Pt electrodeposition on the same sample. (d) EDS analysis of the top-right particle of (c).

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M. Umeda et al. / Electrochimica Acta 63 (2012) 251–255
than those of Fig. 3. This is because the wedge-shaped cantilever
(see Fig. 2(d)) makes smaller indentation widths on the thinner
polymer. In Fig. 5, the indentation depth is equal to the polymer
thickness at 2.7 ␮N loading force and becomes larger than that at
a loading force of >3.6 ␮N. Thus, 3.6 ␮N-indentation samples were
employed in the following experiments.
Fig. 6(a) and (b) respectively show an AFM image of the 3.6-␮N
indentation sample and an SEM image of the same sample after
the Pt electrodeposition was conducted at an electrode potential of
0.1 V vs RHE. In Fig. 6(a), nine indentation spots of 20 nm × 40 nm
widths, which are smaller than those seen in Fig. 4, are obtained
with an interdistance of 500 nm.
From Fig. 6(b), Pt electrodeposition is known to occur not only
at the indentation spots but also on other sites outside the inden-
tation. The thinner 10 1-nm polymer seems to make it difficult to
retain the insulation.
3.4. Optimization of electrode potential for Pt electrodeposition
Fig. 5. Cantilever loading force dependence of the indentation depth and indenta-
tion widths for the 10 1-nm polymer-coated glassy carbon. The X- and Y-widths
are defined in Fig. 2(b).
In order to ensure that Pt deposition occurs only at the inden-
tation spots on the 10 1 nm polymer layer, an optimization
of electrode potential for the electrodeposition will be needed.
By changing the electrode potential between 0.1 and 0.3 V vs
RHE, the Pt deposition was carried out for the same substrate as
that shown in Fig. 6(a). First, the electrode potential was grad-
ually changed from 0.30 to 0.10 V vs RHE. Pt depositon was not
observed at 0.30, 0.20, and 0.15 V vs RHE; however, particu-
late deposition occurred at indentation spots and other sites at
0.10 V vs RHE. This means that the data reproduce the results of
Fig. 6(b).
For the second substrate, the electrode potential was gradually
shifted from 0.15 to 0.12 V vs RHE. As a result, Pt deposition did not
occur at 0.15, 0.14, and 0.13 V vs RHE. After that, the Pt deposition
occurred only at the indentation spots at 0.12 V vs RHE.
The third one was used for Pt electrodeposition at 0.12 V vs RHE,
again, to ensure the results of the second substrate. Then, the Pt
deposition certainly occurred. Fig. 7(a) and (b) respectively show
an AFM image before the deposition and an SEM image after the
deposition controlled at 0.12 V vs RHE. It is confirmed from the EDS
analysis that the Pt deposition occurs only at the indentation spots.
The deposited particle size is effectively reduced to 30–60 nm in
diameter.
of 70 nm × 50 nm width and a 500-nm interdistance are seen.
A cross-sectional profile of the dotted line demonstrated in Fig. 4(b)
reveals that the depths of the indentations are larger than the poly-
mer thickness. This ensures the exposure of the glassy carbon.
Fig. 4(c) shows an SEM image after Pt electrodeposition at 0.1 V
vs RHE on the sample in Fig. 4(a). In the figure, nine single particles
of 60–140 nm in diameter can be recognized. The result of SEM-EDS
analysis of the top-right particle is shown in Fig. 4(d). This ensures
that the Pt deposition certainly occurred only at the indentation
spots.
However, when we compare Fig. 4(a) and (c), the Pt particle
size is known to be larger than that of the indentation spots. This is
because that the Pt deposition progresses in the X- and Y-directions
on the surface of polymer after the indented pinhole was filled by
the deposited Pt.
3.3. Downsizing of deposited Pt using thinner-mask indentation
An effective way to downsize the Pt deposition is to use a thinner
polymer layer coated on the glassy carbon, because the indenta-
tion width can be decreased. A further thinner polymer layer of
10 1 nm was then prepared for utilization in the indentation, and
the results are shown in Fig. 5. With an increase in the loading force,
the X- and Y-indentation widths as well as the indentation depth
increase. It is noted here that all sizes shown in Fig. 5 are smaller
Consequently, the developed mask indentation technique for
the thin 10 1 nm polymer layer enables creating 20 nm × 40 nm
width spots at an arbitrary distance. After electrodeposition at
0.12 V vs RHE, small 30–60 nm Pt particles were successfully
deposited only at the indentation spots. The thus prepared Pt
electrocatalyst of defined morphology can provide high effective
Fig. 6. (a) AFM image of a glassy carbon substrate coated with a 10 1-nm polymer layer using the cantilever indentation at 3.6 ␮N loading force. (b) SEM image of the same
sample after the Pt electrodeposition at 0.10 V vs RHE. Dashed circles indicate Pt deposition occurred outside the indentation area.

M. Umeda et al. / Electrochimica Acta 63 (2012) 251–255
255
Fig. 7. A 10 1-nm polymer-coated glassy carbon substrate indented by a cantilever at 3.6 ␮N loading force. (a) AFM image and (b) SEM image respectively show the same
sample before and after the Pt electrodeposition at 0.12 V vs RHE.
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(1) An electroconductive glassy carbon substrate was overcoated
by a uniform insulating polymer layer of 10 1 nm thick as a
mask.
(2) A part of the polymer layer was removed by an indentation
with the AFM cantilever. An optimized loading force of 3.6 ␮N
formed the smallest exposed area of 20 nm × 40 nm.
(3) Pt electrodeposition was conducted at an electrode potential of
0.12 V vs RHE so that the Pt particle could be deposited only at
the exposed deposition sites. Thus, Pt deposition of 30–60 nm
in diameter was successfully formed.
Consequently, the Pt electrocatalyst of defined morphology in
which the particle size and interparticle distance are independently
controlled is fabricated.
Acknowledgement
This work was supported by a Grant-in-Aid for Scientific
Research (B, 21360358) from the Japan Society for the Promotion
of Science (JSPS), Japan.
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