Nitrogen-doped graphitic carbon synthesized by laser annealing of sumanenemonoone imine as a bowl-shaped π-conjugated molecule

Article abstract of DOI:10.1002/asia.201300500

Time for tuning: Nitrogen-doped graphitic carbon was synthesized from a bowl-shaped π-conjugated compound bearing an imine moiety; this bowl-shaped compound underwent carbonization through laser annealing while almost retaining the nitrogen/carbon ratio.

Full text of DOI:10.1002/asia.201300500

COMMUNICATION
DOI: 10.1002/asia.201300500
Nitrogen-doped Graphitic Carbon Synthesized by Laser Annealing of
Sumanenemonoone Imine as a Bowl-shaped p-Conjugated Molecule
Yuhi Inada,[a] Toru Amaya,[a] Yasutomo Shimizu,[a] Akinori Saeki,[a] Takeo Otsuka,[b]
Ryotaro Tsuji,[b] Shu Seki,[a] and Toshikazu Hirao*[a, c]
Nitrogen-doped graphitic carbons (NGCs, Figure 1a) have
attracted much interest owing to their electrical and catalyt-
ic properties, thus leading to significant contributions
toward their potential applications.[1–14] Recently, many re-
searchers have revealed one of the critical factors influenc-
ing their effectivities to be the bonding pattern of nitrogen
atoms in the graphitic lattice.[4,15–18] However, the composi-
tion ratio of nitrogen-to-carbon (N/C) is also a crucial pa-
rameter as the amount of doped nitrogen atoms determines
the properties. Synthetic methods that allow rational control
make the design of materials more efficient, thereby provid-
ing facile modulation and enhancement of the performance.
The N/C ratio of the resulting NGC is controlled in most of
the previously-reported syntheses, including direct synthesis
such as chemical vapor deposition (CVD)-based,[6,19,20] seg-
regation,[21] and arc-discharge[1,22,23] methods, and postsyn-
thesis such as thermal,[24–27] plasma,[8,10,28,29] hydrazine reduc-
tion,[30,31] or laser[32] treatment of graphene, graphene oxide
(GO), or graphite with various nitrogen sources. However,
it is necessary to adjust multiple parameters, determined by
experiments, to control the N/C ratio. Therefore, we adopt-
ed an approach to transform nitrogen-containing carbon-
based compounds with a fixed N/C ratio (as a starting mate-
rial) into NGC, with the N/C ratio unchanged.
Forming the graphitic skeleton of NGCs is often per-
formed by pyrolysis using a furnace[12,13,33–44] or laser treat-
ment.[45–48] Although carbonization generally progresses with
increasing temperature, most nitrogen atoms are diminished
in the process when heated up above 600–8008C. Therefore,
to increase the degree of carbonization and to suppress loss
of nitrogen atoms, a system favorable to carbonization has
to be designed.[50]
Bowl-shaped
compounds
(hereinafter
called
by
“p bowls”),[51] which belong to nonplanar p-conjugated com-
pounds, have also attracted significant interest.[52–55] p Bowls
are represented as partial structures of fullerene, including
sumanene (1, C21H12, Figure 1b),[56] corannulene (C20H10),[57]
and so on. There have been limited studies on p bowls as
compared to those on fullerenes and carbon nanotubes.
Therefore, we have been actively researching sumanene
chemistry to show the potential characteristics of
p bowls.[49,52,58] One of the features of p bowls is the strain
attributed to the nonplanar p-conjugated shape (Figure 1c).
Figure 1. (a) Our approach to synthesize nitrogen-doped graphitic carbon
(gray: carbon, dark gray: nitrogen) from a nitrogen-containing strained
compound. (b) Sumanene (1, C21H12) and sumanenemonoone imine (2,
C28H17N). (c) Molecular structure of 1 based on X-ray crystallographic
analysis: p-Conjugated plane is strained to be nonplanar.[49]
[a] Y. Inada, Dr. T. Amaya, Y. Shimizu, Dr. A. Saeki, Prof. S. Seki,
Prof. T. Hirao
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
Yamada-oka, Suita, Osaka 565-0871ACTHNUTRGNE(UNG Japan)
Fax : (+81)6-6879-7415
À
Because of the strain, we expected a C C bond of 1 to be
cleaved with ease as compared to those of planar ones, and
this might support the carbonization.
E-mail: hirao@chem.eng.osaka-u.ac.jp
Moreover, as mentioned above, laser is known as one of
the heating sources that can locally promote carbonization
owing to its photothermal effect, thus allowing a target to
be rapidly heated with positional selectivity. Therefore, this
method is considered to be attractive for potential applica-
tions such as in electronic devices. As for the other nonpla-
nar p-conjugated compounds, a laser-induced conductivity
increase of fullerene C60 has been reported.[59–62] If monolay-
[b] Dr. T. Otsuka, Dr. R. Tsuji
KANEKA Fundamental Technology Research Alliance Laboratories
Graduate School of Engineering, Osaka University
Yamada-oka, Suita, Osaka 565-0871
ACHTUNGTRNE(NUNG Japan)
[c] Prof. T. Hirao
JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300500.
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Toshikazu Hirao et al.
er or a few-layer NGC can be synthesized by using this
unique technique in the future, it might be possible to fabri-
cate not only conductors but also semiconductors on a thin
film.[6,21,22,26]
Herein, we report the synthesis of NGC through laser an-
nealing of strained nitrogen-containing sumanenemonoone
imine 2. The NGC has the same carbonization degree as
that formed by pyrolysis at 800–10008C, while almost retain-
ing the N/C ratio (Figure 1b).
wise from 36 to 46 mJcmÀ2 (per pulse) by 5 mJcmÀ2 (per
pulse). The film was exposed to the laser beam through an
iris diaphragm and quartz cover glass (Figure 2b). The elec-
trical resistance of the film was measured by the direct-cur-
rent (DC) method with each annealing for a few minutes
under ambient conditions (Figure 2c). Consequently, with
increasing exposure time, the film color changed from
yellow to black. Each electrical resistance value measured
was converted into conductivity, as plotted in Figure 3.
First, sumanenemonoone imine 2 was synthesized through
the dehydrative condensation reaction of monooxosuma-
nene[63] and p-toluidine (see the Supporting Information).
The change in electrical conductivity of the sample film was
probed as one of the facile methods to confirm graphitiza-
tion. Figure 2 illustrates the experimental procedure. A film
Figure 3. Laser annealing time dependence of the film conductivity of
laser-annealed sumanenemonoone imine 2. White, gray, and dark gray
tracks are the irradiated laser pulse energies of 36, 41, and 46 mJcmÀ2
(per pulse), respectively.
While no change in conductivity was observed for the first
4 min (less than 4ꢁ10À3 ScmÀ1),[64] afterwards, the conductiv-
ity abruptly increased during laser annealing with a fluence
of 36 mJcmÀ2 (per pulse). After continuing to increase to
a fluence of 41 mJcmÀ2 (per pulse), the conductivity reached
a maximum value of approximately 4 ScmÀ1, and then
began to decrease with a fluence of 46 mJcmÀ2 (per pulse).
The control experiment using the fluorene derivative, N-
(9H-fluoren-9-ylidene)-4-methylaniline (see the Supporting
Information) did not show any changes in conductivity. Fur-
thermore, changes in color and form were not observed,
thereby implying that the annealing conditions to start car-
bonization were absent. One possible cause is that more
energy is required to cleave the bonds in fluorenone imine,
owing to the lack of strain as compared to that of sumane-
nemonoone imine 2. The electrical conductivity change was
also studied on the molecular level by using the time-re-
solved microwave conductivity (TRMC) method, in which
use of microwave as a probe instead of electrodes allows the
intrinsic intramolecular conductivity to be evaluated by min-
imizing the effect of impurities, defects, and interfaces as
compared to conventional methods such as time-of-flight
(TOF) and field-effect transistor.[65–69] The film of compound
2 was prepared on a quartz substrate by the drop-casting
method. The above-mentioned Nd:YAG laser (355 nm, 5–
8 ns, 10 Hz) was used again not only as an annealing source
for carbonization but also as an excitation source for the
TRMC measurement. The pulse energy was fixed at
Figure 2. Illustrations of (a) film preparation (drop-casting of the di-
chloromethane solution of 2, followed by covering with a glass cover),
(b) experimental setup for laser annealing, and (c) electrical resistance
measurement of an annealed sample. Resistance value was measured by
a circuit tester after removing the cover.
of compound 2 was formed by the drop-casting method
(Figure 2a) on a glass substrate, on which a pair of indium
tin oxide (ITO) electrodes with a gap (width: 200 mm,
height: 150 nm) were placed. The prepared sample was cov-
ered with a quartz glass, and then fixed using clips. We used
a Nd:yttrium aluminum garnet (Nd:YAG) laser as an an-
nealing source. The wavelength, pulse duration, and pulse
repetition rate were selected at 355 nm, 5–8 ns, and 10 Hz,
respectively, to avoid strong elevation of the temperature.
To avoid photochemical ablation of the starting material, an
excitation wavelength of 355 nm was used, at which imine 2
has a relatively small absorbance coefficient (see the Sup-
porting Information). The pulse energy was increased step-
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Toshikazu Hirao et al.
5 mJcmÀ2 (per pulse) for the TRMC measurement and
36 mJcmÀ2 (per pulse) for annealing, respectively. To moni-
tor the extent of carbonization in the sample, TRMC meas-
urements were performed with each annealing for several
seconds to estimate values of fSm (f: photocarrier genera-
tion yield, Sm: sum of mobilities for positive and negative
charge carriers); this indicates transient conductivity. Each
measurement provides a transient, which has a maximum
fSm, as shown in the inset in Figure 4. These maximum
grazing-incidence X-ray diffractometry (GI-XRD), and X-
ray photoemission spectroscopy (XPS) were conducted. The
sample film was prepared by spin-coating method on a glass
substrate by using the same ITO electrodes as those used in
Figure 2a. The film thickness was measured to be around
100 nm.[70] Laser beam was passed through an iris diaphragm
and glass cover at room temperature and ambient atmos-
phere. Annealing the sample with a Nd:YAG laser (355 nm,
5–8 ns, 12 Hz, 44 mJcmÀ2 (per pulse)) for three minutes
changed the film color from yellow to dark brown. Howev-
er, a large weight loss is unlikely to happen because the film
was encapsulated between the substrate and cover. A
Raman spectrum of the film in the gap between the ITO
electrodes was measured after laser annealing.[71] The typical
spectrum with two signature bands is shown in Figure 5. The
Figure 4. Laser annealing time dependence of fSm for the film of laser-
annealed sumanenemonoone imine 2. The inset shows the conductivity
transient measured after laser annealing at 36 mJcmÀ2 (per pulse) for
437 seconds in total, where the maximum value of fSm is 4.5ꢁ
10À3 cm2 VÀ1 sÀ1
. The transients were measured under excitation at
355 nm and 5 mJcmÀ2 (per pulse).
Figure 5. Raman spectrum of the laser-annealed sumanenemonoone
imine 2. G and D are the graphitic and disordered bands, respectively.
The excitation wavelength is 532 nm.
values were plotted against time in Figure 4, and thus repre-
sent the laser annealing time dependence on the transient
conductivity. After 7 seconds, fSm decreased a little and
then steeply increased to settle at 4.5ꢁ10À3 cm2 VÀ1 sÀ1.
Comparing the results obtained from the DC method
(Figure 3) with those from the TRMC method (Figure 4),
the induction time of the conductivity increase was observed
for the conductivity increasing in the former method. These
differences might be explained by considering a critical dif-
ference in the detection principle of the conductivity among
these measurement methods. Because the TRMC method
allows to immediately detect the enhanced conductivity de-
rived from even isolated graphitic crystallites, fSm continu-
ously increases, as shown in Figure 4. In contrast, the DC
method depends on electrodes, therefore it takes longer to
form and multiply a conductive path between the electrodes,
and this results in a higher threshold and gradual increase in
the conductivity, as shown in Figure 3. Because of these rea-
sons, we can probably also explain the difference in the an-
nealing time scale between the TRMC and DC methods,
where it took 3 and 25 min to reach the maximum conduc-
tivity, respectively. Thus, it is suggested that the laser anneal-
ing process transformed sumanene derivative 2 into the
graphitic compounds.
D band at around 1335 cmÀ1 and G band around 1590 cmÀ1
could be assigned to the disordered carbon that arose from
structural defects and sp3-bonded carbon, and sp2-bonded
graphitic carbon, respectively.[72] The spectrum seems to be
similar to that of the corresponding material formed by
heating at around 700–10008C.[73,74] Although many re-
searchers have estimated the graphitic crystallite size from
the intensity ratio between the D and G band, however, for
this approach the two bands are not separated well enough.
For this reason, the crystallinity of the graphitic film was ex-
amined through GI-XRD measurements. The GI angle was
fixed at 0.088 to gain as much information from the thin film
on the substrate as possible. Meanwhile, the detector was
swept from 10 to 708 (2q-scan) in the out-of-plane configura-
tion (see the Supporting Information). Figure 6 shows the
narrow-range GI-XRD spectrum of the sample. The very
broad band around 248 and very weak band around 428
were observed and assigned to the (002) and (10) planes of
graphitic crystallites present in the thin film, respectively.
The detection of both (002) and (10) planes indicates that
there are at least two configurations of graphitic crystallites
in the film (see the Supporting Information). Moreover, the
low-angle shift of the (002) diffraction band suggests the ex-
To characterize the graphitic compounds from 2, three
measurement techniques such as Raman microspectroscopy,
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Toshikazu Hirao et al.
Figure 6. GI-XRD spectrum of the laser-annealed sumanenemonoone
imine 2. The inset illustration shows the (002) and (10) planes of a graph-
ite crystallite. Here, the enlarged spectrum is displayed in the range of
15–458.
pansion of the interplanar spacing, thus inferring the forma-
tion of turbostratic structures, which represent basal planes
that have slipped out of the crystal structure alignment.[75]
This result is in good agreement with the results of the
Raman microspectroscopy. Also, the sharp diffraction line
observed around 308 came of the (222) plane of ITO elec-
trodes, with which the graphitic thin film was underlaid. To
obtain information about the N/C ratio in the graphitic thin-
film surface, binding energies of C1s and N1s in the same
sample were also determined by XPS. The obtained spectra
are shown in Figures 7a and 7b. The peaks observed around
284.6 and 399.8 eV correspond to C1s of graphitic carbon
with sp2 bonds and N1s of doped nitrogen, respectively. The
graphitic product is mainly composed of carbon, and the N/
C ratio is estimated to be 0.0373. Comparing this value to
that before laser annealing (0.0359[76]), a 3.9% increase in
the N/C ratio was observed, thereby implying that number
of nitrogen atoms was almost retained and incorporated
into the graphitic lattice. A slight increase in the N/C ratio
might be caused by a decrease in the number of carbon
atoms owing to ablation.[77] Although the N/C ratio of the
resulting product (0.0373) is relatively small compared to
previously reported ratios,[5] we expect it to be enough to
effect the NGC properties.[78] To examine the component
ratio of the bonding pattern on nitrogen atoms, the N1s
spectrum can be deconvoluted as shown in Figure 7b, thus
yielding the three peaks at 398.9, 400.0, and 401.7 eV attrib-
uted to “pyridine-like”, “pyrrole-like”, and “quaternary-like
(graphite-like)” nitrogen atoms, respectively.[44] The mea-
surement of their peak areas revealed the ratio to be
26.9:64.5:8.7. It is described that the bonding pattern en-
hancing catalytic activity toward oxygen reduction reaction
(ORR) is the “quaternary-like (graphite-like)” and/or “pyri-
dine-like” types.[16,18,79] Therefore, our NGC includes ap-
proximately 36% of valuable components in total. These
characterization results suggest that laser annealing induced
the structures shown in Figure 8, in which nitrogen atoms
Figure 7. (a) C1s and (b) N1s XPS spectra of the laser-annealed sumane-
nemonoone imine 2. The spectra were measured in vacuo at room tem-
perature using monochromatic focused AlKa radiation (1486.6 eV) as the
X-ray source. The binding energies were referenced to the Ag 3d5/2 line
at 368.2 eV. The deconvolution of the N1s spectrum gave three compo-
nents: (b), (d), and (g) are attributed to pyridine-like, pyrrole-
like, and quaternary-like (graphite-like) nitrogens, respectively. The
*
(c) and ( ) represent the convoluted spectrum of the three compo-
nents and observed data, respectively.
Figure 8. A proposed structure of the nitrogen-doped graphitic product
(gray: C, light gray: H, dark gray: N). Note that the figure does not re-
flect the crystallite size, N/C ratio, and turbostratic structure in the prod-
uct.
are doped into the graphitic lattice with various bonding
patterns.
In conclusion, we have synthesized NGC by laser anneal-
ing of strained nitrogen-containing sumanenemonoone
imine 2. The NGC has the same carbonization degree as
that formed by pyrolysis at 800–10008C, while almost retain-
ing the N/C ratio. It is difficult to synthesize such an NGC
by conventional pyrolysis because of severe loss of nitrogen
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Acknowledgements
We thank Prof. Nobuhito Imanaka, Dr. Toshiyuki Masui, and Dr.
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for the XPS measurements. This work was partially supported by Grants-
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Keywords: bowl-shaped compounds
chemistry · nitrogen-doped graphitic carbon · p-conjugated
molecules
·
carbon
·
laser
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[70] Measured with a surface profiler (Veeco, Dektak150).
[71] Raman measurement was also carried out outside the gap to give
a similar spectrum.
Received: April 11, 2013
Revised: May 13, 2013
Published online: July 10, 2013
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