Self-embedment of small rectangular parallelepiped platinum particle array in etch pits on {100} planes of diamond crystallites

Article abstract of DOI:10.1246/bcsj.20100330

Small platinum particles less than about 200 nm in size with the unique form of a rectangular parallelepiped array were self-embedded in etch pits that were formed through partial gasification of the diamond with hydrogen and the platinum particles on the {100} planes of synthetic diamond crystallites at 1173 K.

Full text of DOI:10.1246/bcsj.20100330

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Bull. Chem. Soc. Jpn. Vol. 84, No. 4, 376–378 (2011)
Short Articles
Selected Papers
Self-Embedment of Small
Rectangular Parallelepiped
Platinum Particle Array in
Etch Pits on {100} Planes
of Diamond Crystallites
Tatsuya Ohashi, Wataru Sugimoto, and
Yoshio Takasu*
Figure 1. SEM micrograph of a (100) surface of the
synthetic diamond crystallite heat-treated under a gas
mixture stream of high-purity H2(10%) + N2(90%) at
1173 K for 2 h.
Department of Materials and Chemical Engineering,
Faculty of Textile Science and Technology,
Shinshu University, 3-15-1 Tokida, Ueda,
Nagano 386-8567
particles are self-embedded in the surface layers of the {100}
planes of synthetic pure diamond crystallites.
The diamond crystallites (IMS-25, Tomei Diamond Co.,
Ltd.) used in this study were statically produced under 5 GPa
at 1573 K. The average diameter was 0.3 mm. The morphology
of the synthetic diamond crystallites was a cubo-octahedral
structure with eight {111} and six {100} planes. A highly
Received November 24, 2010
E-mail: ytakasu@shinshu-u.ac.jp
{
100}-oriented diamond coating formed on a Si{100} single
crystal wafer using MPACVD, was also used in this study
provided by Kobe Steel, Ltd.). Figure 1 shows an SEM image
Small platinum particles less than about 200 nm in size
with the unique form of a rectangular parallelepiped array
were self-embedded in etch pits that were formed through
partial gasification of the diamond with hydrogen and the
platinum particles on the {100} planes of synthetic diamond
crystallites at 1173 K.
(
of a (100) surface of the diamond crystallites. Steps were found
on the (100) surface of the diamond; however, no etch pits were
observed.
Platinum loading on the diamond crystallites was conducted
by impregnation. The aqueous suspension of diamond crystal-
lites and [Pt(NH ) (NO ) ] was placed in a beaker, heated on a
Diamond has excellent physical properties, such as high
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2
2 2
hardness and strong chemical stability. To extend the function-
ality of diamond, including boron-doped diamond (BDD),
researchers have focused on surface modifications, e.g., those
involving surface morphology and embedment of small metal
particles. Etching of diamond {111} surfaces with dry oxygen
gas, oxygen/water vapor, and molten potassium nitrate resulted
in the formation of triangular micron-order etch pits with
smooth rounded corners.1 Patterning of a diamond surface
was conducted in a hydrogen atmosphere through a thin solid
hot plate at 353 K while stirring, and then dried. The resulting
powder was placed in a silica boat in a silica tube furnace,
¹1
heated in a stream of H (10%) + N (90%) at 5 K min to
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1173 K, retained at that temperature for 2 h, and then cooled to
room temperature in the hydrogen atmosphere, where high-
purity N2 and H2 (both gases; 99.999% purity, TAIYO
NIPPON SANSO CORPORATION) were used. Heating the
diamond crystallites, on which the platinum complex was
loaded, in the hydrogen atmosphere, decomposed the platinum
complex below about 473 K, forming small platinum particles
on the diamond. At 1173 K, the diamond crystallites underwent
partial gasification with hydrogen and platinum as the catalyst,
forming etch pits and channels. Methane was the only gaseous
product detected by gas chromatography.
­3
4
metal layer, such as Fe, Ni, and Pt. A well-ordered nanoporous
honeycomb was prepared on mirror-polished BDD by oxygen
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­7
plasma etching through a porous alumina mask.
The
honeycomb BDD electrodes with various pore diameters were
modified with Pt nanoparticles, and their size-selective electro-
catalytic properties were examined.7 Applying microwave
plasma-assisted chemical vapor deposition (MPACVD) to a
Pt thin-film predeposited on diamond, spherical platinum
Figures 2a and 2b show SEM images of a typical (100)
surface of the diamond crystallites, loaded with 5 mass %
platinum on average, which was heat-treated in the hydrogen
atmosphere at 1173 K. Many rectangular parallelepiped par-
ticles less than about 200 nm wide were embedded in the etch
pits of the diamond, while the geometries of the portion of the
metal particles embedded inside of the pits were unseen by
SEM. The backscattering electron image shown in Figure 2b
strongly suggests the particles to be platinum. However, the
platinum particles might have contained carbon, because the
equilibrium content of carbon in the phase diagram of Pt­C at
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particles were implanted in the diamond film. Various nano-
meter-sized platinum particles were electrodeposited on BDD
9
electrodes. We previously reported the successful catalytic
formation of channels and holes, i.e., etch pits, in the surface
layers of both crushed pure diamond powder and BDD with a
few metal nanoparticles (Co, Ni, Nb, Mo, W, and Pt) by heat
treatment in a hydrogen atmosphere.1
0­12
Here, we report our discovery of the catalytic formation of
etch pits in which rectangular parallelepiped platinum array
1
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1173 K was about 0.7 atom %. As shown in Figure 3, the
Published on the web March 5, 2011; doi:10.1246/bcsj.20100330

T. Ohashi et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 4 (2011)
377
Figure 3. Raman spectra of the Pt-treated diamond crystal-
lites prepared at 1173 K for 2 h and that of the pristine
diamond crystallites.
Figure 2. SEM micrographs of the Pt-loaded {100} surface
of a diamond crystallite prepared by impregnation under a
gas mixture stream of high-purity H2(10%) + N2(90%) at
1173 K for 2 h. 2a is a secondary electron image, and 2b is
its backscattering electron image.
Raman spectrum of the diamond crystallites, which was etched
by platinum particles at 1173 K in the hydrogen atmosphere,
was almost the same as that of the pristine diamond crystallites.
This means that the surface of the diamond structure of the
former specimens was retained even after the Pt treatment.
The form of the agglomerated platinum particles must have
been affected by one or more factors, namely (1) thermo-
dynamics governing the stable form of the platinum particles in
the hydrogen atmosphere, (2) carbon content in the platinum
particles, (3) amount of platinum in the etch pits, (4) shape of
the etch pits, and (5) arrangement of the surface carbon atoms
of the walls of the etch pits. As typically shown in Figure 2, the
platinum particles could adhere closely to the walls of the etch
pits of the diamond. Although the walls of the etch pits must
consist of {111} planes of the diamond, as shown in a previous
paper for cobalt particles embedded in the etch pits of
Figure 4. SEM micrographs of the Pt-loaded surface of the
highly {100}-oriented diamond coatings heat-treated at
(4a) 1173 or (4b) 1273 K for 2 h under a gas mixture stream
of high-purity H (10%) + N (90%). Platinum was loaded
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by vacuum evaporation.
carbide particles are movable in a solid state and contribute
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to the formation of carbon nanotubes, even at 873 K. To
understand the process of the catalytic etching by platinum
particles, platinum was loaded on a highly {100}-oriented
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diamond coating by vacuum evaporation (6.9 © 10
¹
2
Pt atoms cm ) and heat-treated under the hydrogen atmosphere
at 1173 or 1273 K. The SEM micrographs in Figure 4 strongly
suggest that; an increase in the heating temperature provokes
not only the etching of diamond by Pt particles in each etch pit,
but connecting of the pits.
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diamond, this is difficult to confirm from the SEM images
presented in Figure 2. As the condition of platinum particles
contributing to the etching of the diamond, two probabilities
can be proposed. The first one is that the platinum particles
were in a melted state, because the melting point of a metal
The exposed crystal planes of the parallelepiped platinum
particles appear to be {100} planes. We previously reported
that the particle size appeared to determine the exposed crystal
plane of nanosized platinum particles loaded on a BDD surface
by vacuum evaporation followed by heat treatment in a
hydrogen atmosphere; however, the morphology of the
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nanoparticle generally decreases with smaller particle size.
The small platinum particles in a melted state must have
behaved in various manners; etching of the diamond, coales-
cence with the other platinum particles or staying at the
position where the etching of the diamond had initiated. The
second probability is that the platinum particles were in a solid
state, because, as Yoshida and his co-workers found, iron
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platinum particles was too small to be determined. Figure 5
shows the size distribution of platinum particles evaluated from
the SEM images of Figure 2 and the above assumption for the
particle size, where platinum particles of less than 10 nm were
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excluded due to inadequate resolution of the SEM images.

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Bull. Chem. Soc. Jpn. Vol. 84, No. 4 (2011)
© 2011 The Chemical Society of Japan
parallelepiped platinum array in the surface layers of {100}
planes of synthetic diamond crystallites by exposing the {100}
planes by a catalytic method at 1173 K. The platinum particles
were self-embedded in diamond-crystallite etch pits that were
formed through partial gasification of the diamond with
hydrogen and the platinum particles. This simple method not
only contributes to fundamental research on small platinum
particles but also introduces new functionalities to diamonds,
including the development of agglomeration-resistant dia-
mond-supported platinum particle catalysts and platinum
particle catalysts with a specific crystal plane.
The highly {100}-oriented diamond coating was a gift from
Kobe Steel, Ltd. We gratefully acknowledge their help.
Figure 5. Size distribution of the platinum particles eval-
uated from the SEM images of Figure 2, where platinum
particles of less than 10 nm were excluded due to
inadequate resolution of the SEM images.
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Figure 6. SEM micrographs of the Pt-loaded {111} surface
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secondary electron image, and 6b is its backscattering
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1
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The amount of platinum per specimen, 5 mass % Pt, would
1
6
18
¹2
correspond to nearly 2.9 © 10 Pt atoms cm (dia), if the platinum
had been uniformly coated on the diamond on every plane and
crystallite; however, uniform coating of the platinum complex by
impregnation is essentially very difficult. Assuming that all the
platinum particles loaded on the plane shown in Figure 2 are
spheres and that all the cross sections evaluated from the top
view of the reflection SEM image in Figure 2b correspond to those
On the {111} surfaces of the Pt-loaded crystallites, channels
indicated with a white ellipse in Figure 6) and triangular
(
hollows (indicated with a white square in Figure 6) with
distorted sphere-shaped platinum particles are formed as shown
in Figures 6a and 6b. The base of the channels and hollows
formed in the surface layers of the {111} planes is flat with the
close-packed {111} planes of the diamond.
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7
of the spheres, the loading amount of Pt would be 1 © 10
¹
2
In conclusion, we have prepared small platinum particles
less than about 200 nm with the unique form of a rectangular
Pt atoms cm
dia)
, where the amount of the platinum monolayer
(
would be about 1.3 © 1015 Pt atoms cm
¹2
.