Low temperature synthesis of carbon fibres and metal-filling carbon nanoparticles with laser irradiation into near-critical benzene

Article abstract of DOI:10.1039/c4ra15709e

We irradiate near-critical benzene set at 290°C, in which either an alloy rod composed of mainly iron, chromium, and nickel is placed or copper complex molecules; bis(t-butylacetoacetato)copper(ii): Cu(tbaoac)₂, are dissolved, with the second (532 nm wavelength), third (355 nm), and fourth (266 nm) harmonics generated from a neodymium doped yttrium/aluminium/garnet (Nd:YAG) laser and show that carbon structures such as fibres, coils, and metal-filling carbon nanoparticles are efficiently produced. The operational temperature is 290°C, which is much lower than that in the conventional synthetic methods of nano materials, and the laser power density can be as low as 3.9 mW mm^-2.

Full text of DOI:10.1039/c4ra15709e

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Low temperature synthesis of carbon fibres and
metal-filling carbon nanoparticles with laser
irradiation into near-critical benzene†
Cite this: RSC Adv., 2015, 5, 12671
Takahiro Fukuda,a Yasuhiro Hayasaki,b Takashi Hasumura,a Yoshihiro Katsube,b
Received 3rd December 2014
Accepted 15th January 2015
ab
Raymond L. D. Whitbycd and Toru Maekawa
*
DOI: 10.1039/c4ra15709e
www.rsc.org/advances
We irradiate near-critical benzene set at 290 ꢀC, in which either an drug delivery.7–9 Carbon materials such as fullerenes, carbon
alloy rod composed of mainly iron, chromium, and nickel is placed or nanotubes (CNTs), graphene, bres, and coils are commonly
copper complex molecules; bis(t-butylacetoacetato)copper(II): produced by arc discharge,4 laser ablation,10,11 and chemical
Cu(tbaoac)2, are dissolved, with the second (532 nm wavelength), third vapour deposition (CVD).12
(355 nm), and fourth (266 nm) harmonics generated from
a
It is well known that the gas–liquid coexistence curves
neodymium doped yttrium/aluminium/garnet (Nd:YAG) laser and terminate at the critical points, where clusters formed by the
show that carbon structures such as fibres, coils, and metal-filling uids' molecules percolate the uids' systems.13 As a result,
carbon nanoparticles are efficiently produced. The operational incident light, scattered by the clusters, cannot penetrate the
temperature is 290 ꢀC, which is much lower than that in the uids' systems when the uids' conditions are set at or near
conventional synthetic methods of nano materials, and the laser their critical points; known as critical opalescence.13 Near- and
power density can be as low as 3.9 mW mmÀ2
.
super-critical uids are extremely interesting and important
from a fundamental scientic point of view thanks to their
unique physical properties, universal features in the static and
dynamic structures, and nonequilibrium transport character-
istics.13–18 Super-critical uids are also important from technical
and engineering points of view. They are oen used in chem-
ical, electronic, and environmental sciences and engineering;
e.g., reactions are encouraged,19,20 chemicals are extracted,21,22
and semiconductors are cleaned and puried.23,24 Nano-
materials can also be efficiently synthesised using super-critical
uids; e.g., nano/micro particles have been successfully syn-
thesised via the super-critical anti-solvent (SAS) and rapid
expansion of super-critical solutions (RESS) processes, where
the differences in the solubilities of solutes in super-critical
uids and several advantageous characteristics of super-
critical uids such as gas-like high diffusivities and low
viscosities and liquid-like high mass densities are utilised for
rapid syntheses of nano/micro particles.25,26 Focusing on critical
opalescence, it is supposed that incident light denitely hits
uids' molecules in uids' systems set at or near the critical
points and that ultraviolet (UV) photons may induce decom-
position of uids' molecules captured in large clusters without
raising the molecular temperature. It has actually been
demonstrated that carbon dioxide molecules are decomposed
and carbon nano/micro particles are formed by irradiating a UV
laser into carbon dioxide under its near-critical conditions at
31.0 ꢀC, although UV photons are not absorbed by carbon
dioxide.27 It is therefore supposed that the dissociation of uids'
A variety of structures are formed by carbon; e.g., diamond,
bres, fullerenes, nanotubes, and graphene.1–5 Several carbon
nano/micro materials have already been used in various elds
including mechanical, aeronautical, electronic, and biological
engineering and technology thanks to their unique mechanical,
electronic, and chemical properties. Carbon bres, in partic-
ular, are known as an important industrial material and have
been actively used in aircra, vehicles, and sports gear due to
their lightness, exibility, and strength, whereas carbon coils,
one type of carbon bre, are expected to be utilised as electro-
magnetic absorbers, micro-aerials, and intelligent tactile
sensors.6 Metal-lling carbon nanostructures are also of great
importance particularly in the eld of bio-medicine; e.g., metal-
lling carbon nanostructures may well be used as bio-imaging
agents, nano media for hyperthermia, and nano vehicles for
aBio-Nano Electronics Research Centre, Toyo University, Kawagoe 350-8585, Japan.
E-mail: maekawa@toyo.jp; Tel: +81 492391375
bGraduate School of Interdisciplinary New Science, Toyo University, Kawagoe 350-
8585, Japan
cSchool of Pharmacy and Biomolecular Sciences, University of Brighton, Brighton BN2
4GJ, UK
dSchool of Engineering, Nazarbayev University, Astana 010000, Republic of
Kazakhstan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra15709e
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molecules and formation of carbon nanostructures in uids
under their near-critical conditions subjected to irradiation of
UV photons may be universal phenomena.27–30
Here, we irradiate near-critical benzene set at 290 ꢀC, in
which either an alloy rod composed of mainly iron, chromium,
and nickel is placed or copper complex molecules; bis(t-buty-
lacetoacetato)copper(II): Cu(tbaoac)2, are dissolved, with the
second, third and fourth harmonics, the wavelength of which
are, respectively, 532, 355 and 266 nm, generated from a
neodymium doped yttrium/aluminium/garnet (Nd:YAG) laser.
We show that benzene molecules are dissociated and various
carbon structures such as bres, coils, and metal-lling carbon
nanoparticles are created. Particles produced by laser ablation
of the alloy rod act as a catalyst for the formation of bres and
metal-lling carbon nanoparticles, whereas those created via
pyrolysis and photolysis of Cu(tbaoac)2 act as a catalyst for the
formation of carbon coils. The effect of the wavelength of the
incident laser beam on the production of the above carbon
nanostructures is investigated and claried. The operational
temperature of the present method is 290 ꢀC, which is much
lower than that of the conventional methods; e.g., laser ablation
and CVD. Furthermore, the laser power density can be as low as
3.9 mW mmÀ2. The experimental details are explained in the
second section and the experimental results are shown and
discussed in the third section. We summarise the results
obtained in the present study in the nal section.
Fig. 1 Schematic diagram of the experimental system. Benzene is
confined in a cylindrical stainless steel container. Two pieces of
synthetic quartz are mounted at the top of the container for the
introduction of a laser beam. The temperature is controlled by a heater
installed around the container and a PID controller. The second (532
nm), third (355 nm) and fourth (266 nm) harmonics are irradiated into
benzene under its near-critical conditions at 290 ꢀC. Either a cylin-
drical alloy rod composed of mainly iron, chromium and nickel is
placed on the surface of the bottom wall of the container as a catalyst
or bis(t-butylacetoacetato)copper(II); Cu(tbaoac)2, is dissolved in
benzene.
critical temperature, Tc, critical pressure, Pc,ꢀand critical mass
density, rc, of benzene are, respectively, 289 C, 4.92 MPa, and
300 kg mÀ3 31 The mass density of benzene being constant, the
.
pressure is thermodynamically determined once the tempera-
ture is set at a certain value. It is known that the state of sub-
critical benzene is described by the Antoine equation,32–36
whereas the state of super-critical benzene can be well
described by the van der Waals equation.36,37 Note that
controlling the temperature is much easier and more precise
than regulating the pressure. We irradiated a laser beam of 532,
355, or 266 nm wavꢀelength into benzene under its near-critical
conditions at 290 C as mentioned. The diameter of the laser
beam was 10 mm and the power density was set at 3.9 mW
mmÀ2. The duration of each laser pulse and the frequency of
the pulse generation were 4.3 ns and 10 Hz. 50 000 pulses of a
laser beam were irradiated into benzene, in which either a
cylindrical alloy rod composed of mainly iron, chromium and
nickel (ISO-TR15510L no. 26) was placed as a catalyst or copper
complex molecules; that is, bis(t-butylacetoacetato)copper(II):
Cu(tbaoac)2, (99%, Strem Chemical Inc.) were dissolved. The
diameter and height of the catalytic alloy rod were 10 mm. The
Experimental details
First, we irradiated liquid benzene (99.5%, Wako Pure Chemical
Industries, Ltd.) conned in a glass container through a quartz
window mounted at the top of the container with the second
(532 nm), third (355 nm) and fourth (266 nm) harmonics
generated from an Nd:YAG laser (Brilliant Quantel Co. Ltd.) at
25 ꢀC and 1 atm to check any changes in colour and absorption
spectra of liquid benzene. We measured the absorption spectra
of liquid benzene at 25 ꢀC and 1 atm before and aer laser
irradiation by ultraviolet-visible spectroscopy (DH2000-DUV,
USB2000, Ocean Optics Inc.). We then carried out experi-
ments of laser irradiation into benzene under near-critical
conditions at 290 ꢀC. The schematic diagram of the experi-
mental system is shown in Fig. 1. Benzene was conned in a
cylindrical stainless steel container. The inner and outer
diameters, and inner and outer heights of the container were,
respectively, 13 and 60 mm, and 19 and 66 mm. Two pieces of
synthetic quartz were mounted at the top of the container for
the introduction of a laser beam. The diameter and thickness of
the upper synthetic quartz were 20 and 5 mm, whereas those of
the lower quartz 20 and 10 mm. The mass density of benzene
was set at 428 kg mÀ3. A platinum resistance thermometer
(Pt100, Chino Co. Ltd.) was embedded in the container wall.
The temperature of benzene was regulated by a heater installed
around the container and a PID controller (LT470, Chino Co.
Ltd.). The accuracy of the temperature control was within Æ0.1
mass concentration of Cu(tbaoac)2 was set at 3.47 mg mlÀ1
.
Aer each experiment, the temperature was decreased gradually
down to room temperature. We observed the structures of the
materials produced in the container by scanning electron
microscopes (SEMs) (JSM-7400F, JEOL; SU8030, Hitachi Ltd.)
and transmission electron microscopes (TEMs) (JEM2200FS,
JEOL; JEM2100, JEOL). The elementary components of the
structures were analysed by energy-dispersive X-ray spectros-
copy (EDX) (JED2300T, JEOL). The size of the bres and parti-
cles was measured targeting at least 100 samples from SEM
images.
Results and discussion
ꢀC. The uid conditions were changed from a sub-critical First, we irradiated liquid benzene with laser at 25 ꢀC and 1 atm
liquid–gas two-phase region at 25 ꢀC to a near-critical one at to check any changes in colour and absorption spectra of liquid
290 ꢀC by controlling the uid temperature. Note that the benzene as mentioned. The colour of liquid benzene changed
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signicantly from transparent to yellow aer irradiation of
photons of 266 nm wavelength into liquid benzene, whereas
there was no noticeable change in colour aer irradiation of
photons of 532 nm (see Fig. 2). In the case of the irradiation of
photons of 355 nm wavelength, the colour of the liquid slightly
changed (Fig. 2). The absorption spectra of liquid benzene at 25
ꢀC and 1 atm aer the laser irradiation is shown in Fig. 2, where
the raw absorption spectra aer the irradiation are subtracted
by those before the laser irradiation. It is supposed that
aromatic hydrocarbons such as naphthalene, biphenyl, and
anthracene, and amorphous carbon were produced by photo-
dissociation of benzene induced by a laser of 266 nm wave-
length, noting that it is well known that photons of 240–280 nm
wavelengths are absorbed by liquid benzene,38,39 whereas the
absorption spectrum hardly changed aer irradiation of
photons of 532 nm wavelength into benzene.
We then irradiated Nd:YAG laser into benzene, in which an
alloy rod was placed, under its near-critical conditions at 290 ꢀC.
We observed the structures of materials produced in near-
critical benzene by SEMs and TEMs and conrmed that amor-
phous and graphitic carbon structures, carbon bres, and iron/
chromium-lling carbon nanoparticles were created aer irra-
diation of laser beams of 532, 355, and 266 nm wavelengths into
near-critical benzene. Fig. 3 shows SEM images of carbon bres
and iron/chromium-lling carbon nanoparticles created in
near-critical benzene. Carbon bres and iron/chromium-lling
carbon nanoparticles were evenly formed on the surface of the
catalytic alloy rod aer irradiation of photons of 532 nm wave-
length into near-critical benzene, while carbon bres were not
uniformly distributed on the surface of the alloy rod and the
quantity of iron/chromium-lling carbon nanoparticles was
Fig. 3 SEM images of carbon structures created on the surface of a
catalytic alloy rod in near-critical benzene after irradiation of a laser
beam of different wavelengths. (a) Carbon fibres created by irradiation
of photons of 532 nm wavelength. (b) Metal-filling carbon nano-
particles created by irradiation of photons of 532 nm. (c) Carbon fibres
created by irradiation of photons of 355 nm. (d) Carbon fibres created
by irradiation of photons of 266 nm.
very small in the case of irradiation of photons of 355 and 266
nm (see also Fig. S1 in the ESI† for SEM images of low magni-
cations of the bres). According to the TEM image and EDX
mappings shown in Fig. 4(a)–(c), a nickel particle was captured
at the tip of each carbon bre, which indicates that the particle
acted as a catalyst. We also observed that nanoparticles
composed of iron and chromium were encapsulated in carbon
shells (Fig. 4(d)–(g)). The structures of the carbon shells were
mostly amorphous (see Fig. S2 in the ESI† for TEM images of
iron/chromium carbon nanoparticles). The diameter of iron/
chromium-lling carbon nanoparticles produced by irradia-
tion of photons of 532 nm varied from 10 to 150 nm. The
diameter of carbon bres was changed depending on the
wavelength of the incident laser beam (see Fig. S3 in the ESI† for
the diameter distributions of carbon bres). The diameter of
carbon bres created by irradiation of photons of 532, 355, and
266 nm wavelengths, respectively, ranged from 70 nm to 3.5 mm,
70 to 850 nm, and 20 to 400 nm. Note that the diameter
distribution of bres created by irradiation of photons of 532
nm wavelength was particularly broad since a number of cata-
lytic nanoparticles were produced by irradiation of photons of
532 nm and those nanoparticles coagulated to form larger
particles during the formation and growth process of the bres.
The maximum length of carbon bres was approximately 0.1
mm irrespective of the wavelength of the incident laser. The
diameter of the bres decreased as the wavelength of incident
photons decreased since the diameter of metal nanoparticles
created by laser ablation, which acted as a catalyst, decreased
with a decrease in the wavelength of photons.40 The diameter of
nanoparticles created by irradiation of photons of 532, 355, and
266 nm wavelengths in the air was, respectively, 180 Æ 120, 140
Æ 100, and 100 Æ 70 nm (see Fig. S4 in the ESI† for SEM images
of nanoparticles created on the surface of the alloy rods by laser
ablation).40 Thin and short carbon bres were created by
Fig. 2 Absorption spectra and photographs of benzene after irradia-
tion of the second (532 nm), third (355 nm) and fourth (266 nm)
harmonics into liquid benzene at 25 ꢀC and 1 atm. The raw absorption
spectra of benzene after irradiation of the laser were subtracted by
those before irradiation. It is supposed that benzene was dissociated
and some hydrocarbons of low molecular weights and amorphous
carbon were created by irradiation of photons of 266 nm wavelength,
whereas there was no significant change in the absorption spectra
after irradiation of photons of 532 nm.
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Fig. 4 TEM images and EDX mappings of carbon fibres and metal-filling carbon nanoparticles created in near-critical benzene by irradiation of
photons of 532 nm wavelength. (a) TEM image of carbon fibres. (b) EDX mapping of carbon corresponding to TEM image (a). (c) EDX mapping of
nickel corresponding to TEM image (a). (d) TEM image of iron/chromium-filling carbon nanoparticles. (e) EDX mapping of carbon corresponding
to TEM image (d). (f) EDX mapping of iron corresponding to TEM image (d). (g) EDX mapping of chromium corresponding to TEM image (d).
irradiating photons of 266 nm wavelength, whereas thick and wavelengths into benzene). In terms of the creation of carbon
long ones were created by laser irradiation of 532 nm, noting bres and iron/chromium-lling carbon nanoparticles,
that the maximum length of the bres was more or less the however, irradiation of photons of a short wavelength; i.e., 266
same irrespective of the wavelength of incident laser beams as nm, was not very efficient, in which case amorphous carbon was
mentioned. It is worth stressing that benzene molecules were mostly created due to high absorption of photons by benzene
dissociated and carbon bres and iron/chromium-lling carbon and the less number of catalytic nanoparticles since photons of
nanoparticles were efficiently created by irradiating photons of 266 nm could not reach the alloy rod placed at the bottom of the
532 nm wavelength into benzene under near-critical conditions container, whereas a number of metal nanoparticles were
at 290 ꢀC, although liquid benzene was not dissociated at 25 ꢀC formed by ablation in the case of irradiation of photons of 532
and 1 atm aer irradiation of photons of 532 nm as mentioned nm, thanks to which carbon bres and iron/chromium-lling
(see also Fig. 2). It is supposed as explained in ref. 28 that at carbon nanoparticles were efficiently formed in near-critical
least two- or three-photon absorption is required for the benzene at 290 ꢀC. We suppose that liquid particles were
dissociation of a hydrogen atom from a benzene molecule since initially produced by laser ablation of the alloy rod, considering
the photon energies of 532, 355, and 266 nm wavelengths are, the energy of a single photon of 532 nm wavelength is 2.33 eV.
respectively, 2.33, 3.50, and 4.66 eV, knowing that the dissoci- Liquid particles were nally transformed to solid ones during
ation energy is 4.90 eV.41 The dissociation energy of a second the cooling process of the particles. The phase transformation
hydrogen atom is lower than that of the rst one once the rst may be explained by the ternary phase diagram under high
hydrogen atom has been dissociated, which is lower than the pressure, but in the present study, at least nickel and iron/
energy of a single photon of 355 and 266 nm. Therefore, six- chromium particles were formed; the former initiated the
membered rings of carbon atoms may be quite easily growth of carbon bres, whereas the latter encouraged the
produced by irradiation of the third (355 nm) and fourth (266 formation of metal-lling carbon nanoparticles. Nevertheless, if
nm) harmonics, whereas in the case of the second (532 nm) catalytic nanoparticles are produced and placed on a substrate
harmonic photons, two-photon absorption is still required to in advance, carbon bres and iron/chromium-lling carbon
dissociate a second hydrogen atom from a benzene molecule.41 nanoparticles were efficiently created even by irradiation of
We suppose that the possibility of multi-photon absorption photons of 266 nm wavelength as in the case of irradiation of
increases as the state of benzene approaches its critical point photons of 532 nm wavelength (see Fig. S6 in the ESI† for the
due to the formation of large exible molecular clusters.27 The formation of carbon bres created in benzene by irradiation of
weight of carbon structures formed by laser irradiation photons of 266 nm wavelength).
increased with a decrease in the wavelength of incident photons
Carbon bres were also created by irradiating a laser beam of
(see Fig. S5 in the ESI† for the weight of carbon structures 532, 355, and 266 nm wavelengths into benzene, in which
created aer irradiation of photons of 532, 355, and 266 nm Cu(tbaoac)2 was dissolved, under near-critical conditions at
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ꢀ
290 C. Most of the carbon bres were coiled (see Fig. 5). It is
In this study, we developed a low temperature synthetic
known that polygonal metal nanoparticles initiate the formation methodology utilising near-critical benzene. Carbon nano-
of coiled carbon bres due to the difference in the diffusion structures such as bres and metal-lling carbon nanoparticles
speed of carbon atoms in the particles depending on the polyg- were formed under near-critical conditions. The laser power
onal facet.42 The diameter of the coils created by the second (532 density was as low as 3.9 mW mmÀ2 and the effect of the
nm), third (355 nm), and fourth (266 nm) harmonics was, wavelength of incident laser beams on the creation of carbon
respectively, 300 Æ 80, 185 Æ 90, and 170 Æ 55 nm, while the nanostructures was claried. Note that carbon bres were
helical diameter was 970 Æ 610, 550 Æ 420, and 365 Æ 190 nm. previously synthesised by the hyperbaric-pressure laser chem-
According to the EDX mappings of the coils, which are shown in ical vapour deposition method using benzene, but the pressure
Fig. 5(d)–(f), a copper particle was encapsulated at the tip of each was set at up to only 0.25 MPa.44 Carbon nanoparticles were
coil. The size of a copper particle formed aer irradiation of a produced by decomposing liquid benzene by a femto-second
laser beam of 532, 355, and 266 nm wavelengths was 220 Æ 50, laser beam, but the laser power density was as high as 1 TW
110 Æ 100, and 100 Æ 60 nm. Thermogravimetric analysis (TGA) mmÀ2
.
45 We demonstrated that there are two ways of producing
of Cu(tbaoac)2 carried out in nitrogen at a ow rate of 100 ml carbon bres; that is, irradiation of a laser beam into near-
minÀ1 showed that the melting and pyrolytic decomposition critical benzene, in which either (a) a solid alloy rod is placed
ꢀ
temperatures of Cu(tbaoac)2 were, respectively, 110 and 190 C or (b) complex molecules are dissolved. In the former case, the
(ref. 43) and we conrmed that copper particles and lms were surface of the rod was ablated by laser and nanoparticles, which
formed on the inner surface of the quartz window and the acted as a catalyst, were formed, whereas in the latter case,
container at 290 ꢀC (see Fig. S7 and S8 in the ESI† for the TGA of Cu(tbaoac)2 was decomposed via both pyrolysis and photolysis
Cu(tbaoac)2 and an SEM image of nanoparticles created by and catalytic nanoparticles were formed. In the former case,
pyrolytic decomposition of Cu(tbaoac)2).43 We therefore suppose irradiation of photons of 532 nm wavelength into benzene
ꢀ
that polygonal copper nanoparticles were formed by both pyro- under near-critical conditions at 290 C worked efficiently for
lytic and photo decomposition of Cu(tbaoac)2 in benzene and the the formation of carbon bres and metal-lling carbon nano-
growth of carbon coils was initiated on the catalytic polygonal particles, although the energy of a single photon of 532 nm is
copper nanoparticles, which had been deposited on the inner lower than that of 355 and 266 nm and photons of 532 nm are
surface of the quartz window. Carbon coils were efficiently not absorbed by benzene under standard conditions, whereas
created by irradiation of photons of 355 and 266 nm wavelengths in the latter case, bres were efficiently produced by irradiation
thanks to the effective dissociation of benzene near the quartz of photons of 355 and 266 nm since the catalytic nanoparticles
window and catalytic nanoparticles deposited on the surface of were deposited on the inner surface of a quartz window and the
quartz window. Note that photons of short wavelengths cannot dissociation of benzene was encouraged by photons of those
penetrate benzene under near-critical conditions as mentioned, short wavelengths near the window. Interestingly, when a laser
but can dissociate benzene molecules near the quartz window. beam of 266 nm wavelength was irradiated into near-critical
The diameter of the bres decreased with a decrease in the benzene, in which metallocenes were dissolved, neither bres
wavelength of a laser beam since the size of the catalytic particles nor coils were formed, but only metal-lling carbon nano-
decreased as the wavelength of photons decreases.40
particles were created,30 whereas in the present case, bres and
Fig. 5 SEM and TEM images of carbon fibres created after irradiation of a laser beam of 532, 355 and 266 nm wavelengths into near-critical
benzene. Cu(tbaoac)2 was dissolved in benzene. (a) SEM image of carbon fibres created by irradiation of photons of 532 nm wavelength. (b) SEM
image of carbon fibres created by irradiation of photons of 355 nm. (c) SEM image of carbon fibres created by irradiation of photons of 266 nm.
(d) TEM image of carbon fibres created by irradiation of 355 nm. (e) EDX mapping of carbon corresponding to TEM image (d). (f) EDX mapping of
copper corresponding to TEM image (d).
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coils were successfully produced. We suppose that aligned
carbon bres can be grown by irradiating photons of 532 nm
wavelength into near-critical benzene, in which pre-arranged
catalytic nanoparticles are placed on a substrate, or by irradi-
ating photons of 335 or 266 nm into benzene through a quartz
window, on the inner surface of which pre-arranged catalytic
nanoparticles are deposited.
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carbon bres and coils, and metal-lling carbon nanoparticles
were created via the interactions among the incident photons,
catalytic nanoparticles and benzene molecules captured in large
clusters formed under near-critical conditions. The size of
carbon bres and coils was dependant on the wavelength of the
incident photons. Carbon bres and coils, and metal-lling
carbon nanoparticles were efficiently created by irradiating
photons of 532 nm wavelength into benzene, in which an alloy
rod was placed, under near-critical conditions, although
photons of 532 nm are not absorbed by liquid benzene under
standard conditions and the energy of a single photon of 532
nm is lower than that of 355 and 266 nm. When Cu(tbaoac)2 was
dissolved in benzene under near-critical conditions, irradiation
of 355 and 266 nm wavelengths efficiently created carbon coils.
The operational temperature of the present synthetic method is
much lower than that of the conventional synthetic methods of
nano materials and what is more, the laser power density can be
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Acknowledgements
Part of the present study has been supported by a Grant for the
Programme for the Strategic Research Foundation at Private
Universities S1101017 organised by the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan, since
April 2011 and a Grant-in-Aid for Scientic Research; 24656148,
organised by the Japanese Society for the Promotion of Science
(JSPS), since April 2012. We would like to thank Mr Ryota
Hamasu and Mr Hikaru Ouchi for their experimental support.
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