Inorganic Chemistry
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layered ruthenate. A weight loss up to 500 °C (see Figure S1 in
the Supporting Information, SI) revealed that its metallization
began to occur at ∼115 °C, which seems to be comparable to that
7c
of rutile-type RuO nanoparticles, above 100 °C.
2
Next, the reduction of a thin film composed of restacked
nanosheets, which was obtained by drying a droplet of the
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exfoliated nanosheet suspension on the silicon substrate, was
conducted. XRD analysis of an as-deposited film exhibited a
series of strong diffraction peaks typical of lamellar ordering with
d = 1.65 nm (not shown), indicating reconstruction of a layered
structure composed of RuO slabs interleaved with hydrated
2
tetrabutylammonium ions. After treatment with 5% H + 95% N2
2
gas at 200 °C, a metallic film exhibiting only one diffraction peak
assignable to 002 of the ruthenium metal (Figure 1b) was
formed. This indicates that the ruthenium film has a strong c-axis
orientation, despite the use of an amorphous substrate. SEM
images show that the lamellar morphology is preserved with a
thickness of several hundred nanometers.
Figure 3. AFM images of an as-grown film of RuO nanosheets (a) and
its heated product (b).
2
nanosheets were adsorbed on the substrate as a monolayer state,
although some gaps between the nanosheets and overlapping
were inevitable under the present fabrication recipe. Interest-
ingly, the nanosheet morphology remained nearly unchanged
after metallization. The results strongly suggest that a topotactic
The above results suggest that a topotactic reduction to
ruthenium metal occurred; in other words, the atomic arrange-
ment in the RuO nanosheet slabs is preserved in the reduced
2
metallic ruthenium. What transition would occur in the extreme
case of a single nanosheet as an ultimately thin reactant? A self-
conversion from RuO to ruthenium metal nanosheets occurs.
2
The average thickness of ∼0.6 nm acquired from the AFM
images for the ruthenium metal nanosheets is much smaller than
those of conventional oxide-type nanosheets (see Figure S2 in
the SI). This thickness can only be rationalized by a release of O
atoms from the RuO2 nanosheets. Furthermore, coverage
analysis based on the height histogram of Figure 3a,b presents
only a faint decrement in coverage (from ∼90% to ∼85%). The
slight shrinkage coincides with the change in the in-plane
periodic structure from 0.29 to 0.27 nm, about 7% shrinkage.
This excellent agreement in the structural feature is concrete
evidence that topotactic metallization took place.
assembled monolayer film of which the RuO nanosheets lay flat
2
to SiO glass substrate was used for this case. The in-plane XRD
2
pattern of the as-deposited monolayer film exhibited at least two
diffraction peaks assignable to 10 and 11 of a 2D hexagonal cell
having a = 0.2929(6) nm (see Figure 2a). After reduction at 200
A previous study with Ti0.91O nanosheets revealed that the
2
structural transformation is affected by the stacking number of
the nanosheet layers because thermal diffusion is a decisive factor
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in the ultrathin reactant. Hence, investigating the transition
behavior of the multilayered RuO nanosheets should be of great
2
help in understanding topotactic metallization of the RuO2
nanosheet monolayer. In-plane XRD analysis revealed that
reduction leads to polycrystalline-like ruthenium metals with no
preferential orientation (see Figure S3 in the SI), similar to the
case for reduction of layered H RuO ·0.5H O. Consequently,
Figure 2. In-plane XRD patterns for (a) an as-grown monolayer film and
that heated at (b) 200, (c) 300, (d) 400, and (e) 500 °C.
0
.2
2
2
topotactic metallization is believed to be peculiar to nanoscopic
°
C, two peaks that were shift to a larger 1/d region were
systems such as the monolayer RuO nanosheet.
I−V measurement of these samples using a four-probe system
shows unusual behavior of sheet resistance. The as-deposited
2
observed. Judging from the metallization behavior of the bulk
systems, the two XRD peaks can be indexed as 100 and 110 of
hcp-Ru. The lack of indexes associated with 00l in the in-plane
direction can be explained by the formation of ruthenium metal
having a preferential orientation with the c axis perpendicular to
the substrate, consistent with the result observed in the restacked
film of the nanosheets. Upon an increase in the reduction
temperature, a gradual surge of a small peak at 4.7 nm− is
evident. This peak is assignable to 101 of ruthenium metal, which
is the strongest reflection in polycrystalline ruthenium. Heat
treatment above 300 °C induces a rearrangement of the Ru
atoms via thermal diffusion, consequently leading to a collapse of
the c-axis orientation.
monolayer film of the RuO nanosheets exhibited a sheet
2
6
resistance of 4 × 10 Ω/square. The sheet resistance of the as-
deposited RuO nanosheet film is inversely proportional to the
2
deposition number of the nanosheet layer (see Figure S4 in the
SI), which is similar to the case of our previous work on the self-
1
10
assembled film of RuO2.1 nanosheets. Surprisingly, reducing
the monolayer RuO nanosheet yielded a black film, which
2
showed poor electronic conduction. The sheet resistance of the
nanosheet films has a strong dependence on the in-plane
network of the nanosheets, i.e., the amount of overlapping.
Because the present self-assembled films had adequate coverage
of the adsorbed nanosheets, the shrinkage of the sheet size after
metallization may have decreased the amount of overlapped
portion of the nanosheets, which, in turn, may have affected
measurement of the sheet resistance. On the other hand, the
multilayered ruthenium metal films exhibited sheet resistance
10
The topographical change before and after metallization was
examined by tapping-mode atomic force microscopy (AFM; see
Figure 3). As-deposited nanosheets are visualized as lamellar
objects with a thickness of about 1.2 nm and a lateral size ranging
from submicrometer to micrometer. The majority of the RuO2
2
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dx.doi.org/10.1021/ic302720d | Inorg. Chem. 2013, 52, 2280−2282
Fukuda, Katsutoshi
