Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

RSC Advances
View Article Online

PAPER

Cite this: RSC Adv., 2023, 13, 13809

View Journal | View Issue

Solvothermal synthesis of hair-like carbon
nanotubes onto sub-micron-sized spherical metaloxide catalyst cores†
Kazuya Kobiro, *ab Hinako Kimura,a Saki Hirose,a Makoto Kinjoa
and Hiroshi Furuta *ac
Long- and uniform-length, and high-density hair-like carbon nanotubes (CNTs) were produced by CNT
growth on sub-micron-sized spherical catalyst supports. The nanosized catalysts (FeOx, CoOx, and NiOx)
that were supported in/on the sub-micron-sized spherical metal oxides (TiO2, ZrO2, SnO2, and CeO2)
were prepared via one-step solvothermal and/or two-step impregnation methods. The nanosizedcatalysts supported in/on the spherical metal oxide supports were converted into CNT conjugates with
the CNT-hair morphology via a chemical thermal vapor deposition technique using ethyne gas asa carbon source; the CNTs grew on the central spherical metal oxide core under the base growth
process conditions. Among the many types of candidate spherical catalyst materials, the combination of
FeOx as a catalyst for CNT growth and ZrO2 as a support led to the best growth of CNT-hair under thereaction conditions, which included a temperature of 730 °C, pressure of 65 Pa, a 10 sccm ethyne gas
ﬂow, and a reaction time of 10 s. The CNTs consisted of ﬁve-to-eight-layered multi-wall structures withlengths of approximately 3 mm. The CNT-hair that was obtained using the solvothermally embedded
catalyst showed higher crystallinity and was dense, thick, and straight, while the corresponding CNT-hairobtained using the impregnated catalyst was slightly sparse, thin, and curly. A unique layer structure
constructed using large quantities of uniform CNT-hair, including multiple CNT yarns similar to fuzzyballs or cotton candies, was assembled. The CNT-hair conjugate, speciﬁcally constructed in a layer

Received 5th February 2023
Accepted 28th April 2023

structure with core FeOx/ZrO2 catalysts and tangled CNT yarns, had an appearance similar to frog eggs.
Therefore, we successfully prepared suitable catalysts for CNT-hair production and fabricated a layer

DOI: 10.1039/d3ra00770g

structure consisting of large numbers of CNT-hair conjugates. The unique structures are expected asa new metamaterial with intriguing physical properties, including isotropic absorption of polarized lightrsc.li/rsc-advancesand electromagnetic waves.

Introduction
Carbon nanotubes (CNTs) have attracted researchers' interest
because of their unique electrical, thermal, and mechanicalproperties since they were rst reported by Iijima.1 A wide
variety of CNT-based applications have been reported that use
their specic properties in transistors, chemical sensors,thermal absorbers, composites with carbon nanotubes, loudspeakers, and wires for space-elevators. Both quality control andaGraduate School of Engineering, Kochi University of Technology, 185 Miyanokuchi,
Tosayamada, Kochi 782-8502, Japan. E-mail: kobiro.kazuya@kochi-tech.ac.jp;
furuta.hiroshi@kochi-tech.ac.jpbCenter for Structural Nanochemistry, Research Institute, Kochi University ofTechnology, 185 Miyanokuchi, Tosayamada, Kochi 782-8502, Japanc
Center for Nanotechnology, Research Institute, Kochi University of Technology, 185
Miyanokuchi, Tosayamada, Kochi 782-8502, Japan† Electronic supplementary information
https://doi.org/10.1039/d3ra00770g(ESI)

available.

See

DOI:

© 2023 The Author(s). Published by the Royal Society of Chemistrymass production of these CNTs are desired for further development of future applications based on CNTs.
Commercially available CNTs such as HiPCo and CoMoCAT
are produced via gas-phase synthesis using catalytic thermal
chemical vapor deposition (CVD). In these processes, residual
metal catalysts remain in the bodies of the nanotubes, which
can cause their electrical, thermal, and mechanical properties
to degrade. A relatively new method to accomplish high-purity
fabrication of CNTs was investigated using catalytic thermal
CVD on substrates, by which CNTs are grown with high-areadensity and uniform lengths on catalysts on the substrates.
High-quality CNTs with low amounts of residual catalytic
metals in the CNT bodies are expected to be produced by thecatalytic thermal CVD process on the substrates.
Increasing the mass of CNT production in a single batch
during high-quality thermal CVD growth on at substrates isstill challenging. Noda et al. reported higher mass production of
CNTs on 0.1 mm-sized Al2O3 beads using a thermal CVD
process that combined a uidized-bed method with catalytic

RSC Adv., 2023, 13, 13809–13818 | 13809

View Article Online

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

RSC Advances
thermal CVD by increasing the surface area of the substrates.2,3
Verpillere et al. reported the smallest diameter for the sphericalsubstrate to increase the surface area of the substrates,4 but theuniformity of the diameter of the spherical substrate was notinvestigated.
Vertically aligned high-areal density CNT lms were grownon substrates to a uniform length to produce so-called CNT
forests, which opened up a wide variety of new applications for
CNTs in the electrical, mechanical, and optical elds, including
eld emitters, electrode structures for secondary batteries, andmetamaterials. We have previously reported uniform and highdensity growth of CNT forests using thin lm metal catalysts
followed by thermal CVD using carbon gas sources. The heightof the CNTs was controlled from an optical wavelength scale of<1 mm to a mechanical scale of 250 mm in length via pulse gas
source control using an automated gas bulb.5,6 Optical metamaterial features including resonance absorption eﬀects wereobserved successfully when using these high-density CNT lms.
In addition, many spherical core catalysts for CNT growth
applications have been reported to date. For example, He et al.
reported diameter- and length-dependent self-organization of
CNTs on micro-spherical alumina particles with sizes of 3–10
mm.7 Park et al. prepared sea urchin-like CNT conjugates using
a combination of spray pyrolysis and thermal CVD in the gas
phase.8 Piao et al. prepared sea urchin-shaped CNTs using
hollow carbon spheres that contained iron oxide on theirsurfaces.9 Kim et al. reported a semi-continuous uidized-bed
process that yielded CNT using approximately 500 mm-sizedalumina beads as cores.10 Jo et al. investigated continuousow synthesis of carbon-coated silicon/iron silicide particles.11
In addition, Boi et al. developed radial ferromagnetically lledCNT structures.12 Chen et al. reported sea urchin-like CNT/
polyhedra structures starting from a zeolitic imidazolate
framework (ZIF).13 Boron nitrate was also used as a core material to yield CNT conjugates.14 However, it is still diﬃcult toaccomplish homogeneous growth of dense CNTs witha uniform length from sub-micron-sized spherical core catalysts, because (i) core spherical catalysts larger than 1 mm in sizeare commercially available, but their sub-micron-sized counterparts are not easy to obtain; (ii) the availability of practicalpreparation techniques that can yield a variety of porous metaloxide cores with almost perfect spherical morphologies andsub-micron sizes is also limited; and (iii) the uniformity of thecatalyst lms on the catalyst supports is a specic feature of themagnetron sputtering method for planar substrates.
As an alternative, we have studied a unique and simple
synthesis method to obtain sub-micron-sized spherical porous
assemblies consisting of metal oxide nanoparticles using onepot and single-step solvothermal reactions.15 For example,
MgO,16 TiO2,17 ZrO2,18,19 Nb2O5,20 SnO2,21 and CeO2 (ref. 22)
assemblies were prepared successfully using this process. The
assemblies obtained have almost perfect spherical morphologies with huge specic surface areas. We called these structures meso-/micro-porously architected roundly integrated
metal oxide (MARIMO) particles.15 Composite MARIMOs,
including several metal oxides, were synthesized easily using
the solvothermal method, including Al2O3/TiO2,23 SiO2/TiO2,2413810 | RSC Adv., 2023, 13, 13809–13818

Paper
Nb2O5/TiO2,25 SiO2/ZrO2,26 ZrO2/CeO2,27 POx/SnO2,21 and CoOx/
MnOy particles.28 Industrial scale synthesis of some of thesematerials is also possible.29 The MARIMOs obtained havecharacteristic nano-concave–convex surface structures andlarge numbers of pores that can capture noble metal nanoparticles eﬀectively, leading them to support nanometal catalysts. Nanometal catalysts supported on MARIMOs such as Au/
MARIMO TiO2 for use in CO oxidation30 and Ni/MARIMO ZrO2
for dry reforming of methane19,26 were easy to prepare andexhibited high performance and durability when used in thesereactions. Therefore, we speculated that our sub-micron-sized
spherical catalysts would yield well-dened sea urchin-like
CNT conjugates with uniform growth of CNTs if we usedthem as catalysts for CNT growth.
In this paper, preparation of spherical core catalysts suitablefor CNT growth using the solvothermal method, detailed reaction conditions for production of CNT-MARIMO conjugates,
and characterization of the conjugates obtained are discussed.

Experimental details
Materials
85% zirconium(IV) butoxide (Zr(OnBu)4) in 1-butanol, iron(III)nitrate nonahydrate (Fe(NO3)3$9H2O), cobalt(II) nitrate hexahydrate (Co(NO3)2$6H2O), nickel(II) nitrate hexahydrate(Ni(NO3)2$6H2O), zirconium dioxide (ZrO2), cerium(III) nitratehexahydrate (Ce(NO3)3$6H2O), benzoic acid, diethylene glycol,
acetonitrile, and methanol were purchased from FUJIFILM
Wako Pure Chemical Corporation. Acetylacetone was obtained
from Tokyo Chemical Industry Co., Ltd. Ethanol was purchasedfrom Kishida Chemical Co., Ltd. UEP-100 ZrO2 nanoparticles
were obtained from Daiichi Kigenso Kagaku Kogyo Co., Ltd. All
reagents were used as-received. TiO2,15,17 ZrO2,19 SnO2,21 andCeO2 (ref. 22) MARIMOs were prepared according to the previously reported procedure.
Synthesis of catalysts for CNT formation
Solvothermal method. Nanosized FeOx catalysts embedded
into porous Zr-based MARIMO support were solvothermallysynthesized. First, Fe(NO3)3‧9H2O (13.7 mg, 0.0339 mmol) was
dissolved in 0.70 mL of ethanol. 85% Zr(OnBu)4 in 1-butanol(58.5 mL, 0.136 mmol) and acetylacetone (0.99 mL, 9.6 mmol)
was then dissolved in 1.7 mL of ethanol, to which the Fe(NO3)3
solution in ethanol was added. The precursor solution obtained
was stirred and then transferred to an SUS316 stainless steelpressure vessel with an inner volume of 10 mL; the vessel wasthen heated up to 250 °C and this temperature was maintainedfor 1 h. The reactor was then placed in an ice-water bath toquench the reaction. Aer it was cooled, the reaction mixture
was washed several times with methanol using centrifugationand decantation. The powdery product obtained was then driedin a vacuum to yield 20 mg of s20-FeOx/ZrO2-M porous spheres;
here, “s” and “M” denote “prepared by the solvothermal
method” and “MARIMO support,” respectively, and the number
following “s” represents the atomic percentage of the catalyst
metal element (Fe) from the total amount of the catalyst© 2023 The Author(s). Published by the Royal Society of ChemistryView Article Online

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

Paper

RSC Advanceselement (Fe) and the support metal element (Zr) (Table 1).
Similar procedures using diﬀerent amounts of Fe(NO3)3‧9H2O
aﬀorded fabrication of s5-FeOx/ZrO2-M, s10-FeOx/ZrO2-M, s30FeOx/ZrO2-M, and s50-FeOx/ZrO2-M porous spheres. Similarly,
s20-CoOx/ZrO2-M and s20-NiOx/ZrO2-M spheres were preparedusing Co(NO3)2‧6H2O and Ni(NO3)2$6H2O, respectively. In
addition, s20-NiOx/ZrO2-M, s30-NiOx/ZrO2-M, s50-NiOx/ZrO2-M,
and s70-NiOx/ZrO2-M spheres were all prepared using Ni(NO3)2‧
6H2O and Zr(OnBu)4 in 1-butanol as metal sources and diethylene glycol as an additive in acetonitrile. A similar solvothermal procedure using Fe(NO3)3‧9H2O and Ce(NO3)3‧6H2O
as the metal sources and diethylene glycol and benzoic acid asadditives in methanol yielded s50-FeOx/CeO2-M spheres.
Impregnation method. The as-prepared ZrO2 MARIMO
support19 was calcined in air at 350 °C for 1 h. Aer cooling,
200 mg of the support was dispersed in 2.5 mL of reverse
osmosis water. The resulting dispersion was then sonicated forseveral minutes. A solution of Fe(NO3)3‧9H2O (164 mg, 0.406mmol) in 2.5 mL of reverse osmosis water was subsequently
added to the dispersion. The resulting dispersion was mixedusing a planetary centrifugal mixer (AR-100, Thinky) to produce
a dry powder. This powder was then dried further at 110 °C for
2 h in an oven and subsequently calcined at 300 °C in air for 2 h
to yield i20-FeOx/ZrO2-M spheres, where “i” and “M” represent“prepared by the impregnation method” and “MARIMO
support,” respectively; additionally, the number represents theatomic percentage of the catalyst metal element (Fe) from thetotal amount of the catalyst element (Fe) and the support metalelement (Zr) (Table 2). A similar procedure with diﬀerent
quantities of Fe(NO3)3‧9H2O produced i5-FeOx/ZrO2-M, i10FeOx/ZrO2-M, i30-FeOx/ZrO2-M, and i50-FeOx/ZrO2-M porous
spheres. In a similar manner, a nanosized NiOx catalyst
impregnated in ZrO2 MARIMO (i8-NiOx/ZrO2-M) was preparedusing Ni(NO3)2‧6H2O. Nanosized FeOx catalysts impregnated incommercially available ZrO2 nanoparticles, denoted by i20FeOx/ZrO2-W and i20-FeOx/ZrO2-U, were also prepared by theimpregnation method, where “W” and “U” represent “Wako
ZrO2” and “UEP-100 ZrO2,” respectively. Nanosized FeOx was

Table 1

Table 2

Catalysts prepared by the impregnation method

Sample namea

Catalyst metaloxide

i8-NiOx/ZrO2-M
i5-FeOx/ZrO2-M
i10-FeOx/ZrO2-M
i20-FeOx/ZrO2-M
i30-FeOx/ZrO2-M
i50-FeOx/ZrO2-M
i20-FeOx/ZrO2-W
i20-FeOx/ZrO2-U
i20-FeOx/TiO2-M
i20-FeOx/SnO2-M
i20-FeOx/CeO2-M

NiOx
FeOx
FeOx
FeOx
FeOx
FeOx
FeOx
FeOx
FeOx
FeOx
FeOx

Atomic ratio of
catalyst metal/
support metal inprecursor solution
7.6/92.4
5/95
10/90
20/80
30/80
50/80
20/80
20/80
20/80
20/80
20/80

a

i, the number, M, W, and U represent prepared by the impregnation
method, the atomic percentage of the catalyst metal element from thetotal amount of catalyst and support metal elements, MARIMO ZrO2,
Wako ZrO2 and UEP-100 ZrO2, respectively.

also impregnated in MARIMO TiO2, MARIMO SnO2, and MARIMO CeO2 to yield i20-FeOx/TiO2-M, i20-FeOx/SnO2-M, and i20FeOx/CeO2-M spheres, respectively.
Characterization. Transmission electron microscopy (TEM)and scanning electron microscopy (SEM) images of the catalysts
were acquired using a JEOL JEM-2100F microscope and a Hitachi SU8020 microscope, respectively. Energy-dispersive X-ray
spectroscopy (EDX) measurements were performed using an
Oxford INCA X-Max 80 EDX spectrometer attached to the TEM
and a HORIBA X-Max80 spectrometer attached to the SEM. X-ray
diﬀraction (XRD) patterns were recorded using both Rigaku
SmartLab and Rigaku SmartLab SE diﬀractometers with nickelltered Cu-Ka radiation (X-ray wavelength: 1.5418 Å). Nitrogen
adsorption/desorption isotherms were obtained using theMicrotracBEL BELSORP-mini X and BELSORP-mini II instruments at 77 K. Specic surface areas were calculated via theBrunauer–Emmett–Teller (BET) method. Pore size distribution

Catalysts prepared by the solvothermal method

Sample namea

Atomic ratio of catalyst
Catalyst metal metal/support metal in precursor
solutionoxides5-FeOx/ZrO2-M FeOx
s10-FeOx/ZrO2-M FeOx
s20-FeOx/ZrO2-M FeOx
s30-FeOx/ZrO2-M FeOx
s50-FeOx/ZrO2-M FeOx
s20-CoOx/ZrO2-M CoOx
s20-NiOx/ZrO2-M NiOx
s30-NiOx/ZrO2-M NiOx
s50-NiOx/ZrO2-M NiOx
s70-NiOx/ZrO2-M NiOx
s50-FeOx/CeO2-M FeOx

5/95
10/90
20/80
30/70
50/50
20/80
20/80
30/70
50/50
70/30
50/50

Metal sources

Additive

Solvent

Fe(NO3)3 + Zr(OnBu)4 Acetylacetone
EthanolFe(NO3)3 + Zr(OnBu)4 Acetylacetone
EthanolFe(NO3)3 + Zr(OnBu)4 Acetylacetone
EthanolFe(NO3)3 + Zr(OnBu)4 Acetylacetone
EthanolFe(NO3)3 + Zr(OnBu)4 Acetylacetone
Ethanol
Co(NO3)2 + Zr(OnBu)4 Acetylacetone
Ethanol
Ni(NO3)2 + Zr(OnBu)4 Acetylacetone
Ethanol
Ni(NO3)2 + Zr(OnBu)4 Diethylene glycol
Acetonitrile
Ni(NO3)2 + Zr(OnBu)4 Diethylene glycol
Acetonitrile
Ni(NO3)2 + Zr(OnBu)4 Diethylene glycol
Acetonitrile
Fe(NO3)3 + Ce(NO3)3 Diethylene glycol + benzoic acid Methanolas, the number, and M represent prepared by the solvothermal method, the atomic percentage of the catalyst metal element from the total amountof catalyst and support metal elements and MARIMO ZrO2, respectively.

© 2023 The Author(s). Published by the Royal Society of Chemistry

RSC Adv., 2023, 13, 13809–13818 | 13811

View Article Online

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

RSC Advances

Schematic illustration of the thermal chemical vapor deposition
apparatus.

Fig. 1was estimated by the Barrett–Joyner–Halenda (BJH) method.
Raman spectra were observed to evaluate the crystallinity of the
CNTs on a Horiba HR-800 Raman spectrometer under excitation at a laser wavelength of 532 nm.
Thermal chemical vapor deposition. The CVD instrument
was set up as described in previous reports.5,6 A schematic
illustration of the setup is shown in Fig. 1. An aliquot amount of
the MARIMO catalyst was dispersed in either methanol or
acetone. The dispersion was then drop-cast on a thermal CVD
SiO2 (th-SiO2) substrate. The thickness of the oxide layer of thSiO2 substates, formed by thermal CVD on Si substrates, was
100 nm. Aer drying, the th-SiO2 substrate with the catalyst wastransferred into the quartz tube of the CVD instrument. The
catalysts on the th-SiO2 substrate in the quartz tube were driedat 120 °C under vacuum conditions until the pressure reachedless than 5.0 × 10−4 Pa. The reactor temperature was raised to730 °C. Ethyne (C2H2) gas was then introduced into the tube
with a 10 sccm gas ow under pressure of 65 Pa for 10 s, and thereactor was then cooled to room temperature.

Results and discussion
Synthesis of the core MARIMO catalysts
Nanosized metal oxide catalysts supported in the MARIMO submicron-sized spheres were synthesized by two diﬀerent
methods: the one-pot solvothermal method and the two-step
impregnation method. Fig. 2 illustrates the two synthesis

Paperapproaches used to yield FeOx catalysts supported in/on MARIMO ZrO2 as an example.
First, nanosized metal oxide catalysts (FeOx, CoOx, and NiOx)
that was embedded in the support metal oxides were preparedusing our original one-pot and single-step solvothermal
method.19 The synthesis procedure was very simple. For
example, a precursor solution that included Zr(OnBu)4,Fe(NO3)3‧9H2O, and acetylacetone in ethanol was heated up to
250 °C in an SUS316 pressure vessel. The powdery product obtained was washed with methanol and dried in a vacuum,
yielding sFeOx/ZrO2-M with diﬀerent FeOx contents (see Table1). In the sNiOx/ZrO2-M case except for s20-NiOx/ZrO2-M,
a solution that included Zr(OnBu)4, Ni(NO3)2‧6H2O, and diethylene glycol in acetonitrile was used as the precursor solution.
The combination of Co(NO3)2 and Zr(OnBu)4 with acetylacetone
in ethanol and the combination of Fe(NO3)3 and Ce(NO3)3 with
diethylene glycol and benzoic acid in methanol producedsCoOx/ZrO2-M and sFeOx/CeO2-M spheres, respectively, using
procedures that were slightly modied relative to the previously
reported method.19,22
Nanosized metal oxide catalysts (FeOx, CoOx, and NiOx) thatwere impregnated in the support metal oxides (TiO2, ZrO2,
SnO2, and CeO2) were prepared by the conventional impregnation method, which is quite a versatile technique in the catalyst
preparation eld for fabrication of supported nanometal catalysts.31 TiO2,15 ZrO2,19 SnO2,21 and CeO2 (ref. 22) MARIMOs wereprepared using the previously reported solvothermal methods;
these oxides are ideal materials for dispersion of catalyst metal(oxide) particles on their surfaces because they have unique
nano-concave-convex surfaces, high specic surface areas (150–
220 m2 g−1; see Fig. S1 and Table S1†), and large numbers ofpores for good dispersion of the nanometal oxide catalysts used
for CNT growth. FeOx was loaded on these MARIMOs by theimpregnation method using Fe(NO3)3‧9H2O to yield i5-FeOx/
ZrO2-M, i10-FeOx/ZrO2-M, i20-FeOx/ZrO2-M, i30-FeOx/ZrO2-M,
i50-FeOx/ZrO2-M, i20-FeOx/TiO2-M, i20-FeOx/SnO2-M, and i20FeOx/CeO2-M spheres (Table 2). Similarly, i20-FeOx/ZrO2-W and
i20-FeOx/ZrO2-U spheres were also prepared as reference catalysts by the impregnation method, where “W” and “U” represent“Wako ZrO2” and “UEP-100 ZrO2,” respectively. Similarly, NiOx
was loaded on ZrO2-M using Ni(NO3)2$6H2O to yield i8-NiOx/
ZrO2-M.

Physical properties of the core MARIMO catalysts

Fig. 2 One-pot solvothermal synthesis of FeOx catalyst embedded in
ZrO2 MARIMO (top) and step-wise preparation of FeOx catalyst
impregnated in MARIMO ZrO2 (bottom).

13812 | RSC Adv., 2023, 13, 13809–13818

Specic surface areas of impregnated catalysts. We havestudied the solvothermal synthesis of spherical porous aggregates of metal oxide(s) called MARIMO particles. The main
characteristic of the process is the simple one-pot, single-stepreaction that yields embedded nanosized catalysts in porous
metal oxide supports with spherical morphologies, highporosity, and nano-convex–concave surface structures. We
applied this technique to produce FeOx, CoOx, and NiOx catalysts embedded in MARIMO ZrO2 support.
Specic surface areas of the solvothermally embedded catalysts were estimated to compare with those of native MARIMO
supports. In the cases of sFeOx/ZrO2-M, s20-CoOx/ZrO2-M, and© 2023 The Author(s). Published by the Royal Society of ChemistryView Article Online

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

Paper
s20-NiOx/ZrO2-M, their specic surface areas are so high (182–269 m2 g−1) (see Fig. S1 and Table S1†). However, further
incorporation of NiOx into ZrO2-M support by the solvothermal
method resulted in drastic decrease of the specic surface areas(6–36 m2 g−1), which can be ascribed to blockage of the pores ofthe MARIMO supports with the much amount of the introduced
nanosized catalyst metal oxides as BJH plots indicate (seeFig. S1B†). Similarly, s50-FeOx/CeO2-M also exhibited quite low
specic surface area (8 m2 g−1).
Microscope observations and XRD spectra. SEM images of
the catalysts obtained from the procedures above are shown in
Fig. 3 and 4. Scanning TEM (STEM)-EDX and SEM-EDX imagesof the catalysts obtained for elemental analysis are shown in
Fig. S2 and S3.† Atomic ratios obtained from the elemental
analysis on STEM/EDX or SEM/EDX are listed in Tables S2 andS3.† The XRD spectra are shown in Fig. S4 and S5.†
As shown in Fig. 3A–C, ZrO2 catalysts that included 5 to 20
atom% FeOx with striking spherical morphologies were obtained; their XRD spectra showed quite broad peaks that wereascribed to cubic ZrO2 (see Fig. S4A–C†). Judging from the peak
positions (2q), FeOx, CoOx, or NiOx did not yield solid solutions
with MARIMO ZrO2 support (see Fig. S4†). Further introductionof larger amounts of FeOx caused swellings on the MARIMO
surfaces (Fig. 3D and E), for which the corresponding XRD
spectra showed clear peaks that were ascribed to cubic Fe3O4(see Fig. S3D and E†). Therefore, introduction of too much FeOxcaused irregular shapes to occur and led to the formation oflarge-sized Fe3O4 particles. In contrast, 20 atom% CoOx catalyst
embedded in ZrO2 MARIMO produced perfectly sphericalFig. 3 SEM images of catalysts prepared using the solvothermal
method. (A) s5-FeOx/ZrO2-M, (B) s10-FeOx/ZrO2-M, (C) s20-FeOx/
ZrO2-M, (D) s30-FeOx/ZrO2-M, (E) s50-FeOx/ZrO2-M, (F) s20-CoOx/
ZrO2-M, (G) s20-NiOx/ZrO2-M, (H) s30-NiOx/ZrO2-M, (I) s50-NiOx/
ZrO2-M, (J) s70-NiOx/ZrO2-M and (K) s50-FeOx/CeO2-M.

© 2023 The Author(s). Published by the Royal Society of Chemistry

RSC Advances

SEM images of catalysts prepared using the impregnation
method. (A) i8-NiOx/ZrO2-M, (B) i5-FeOx/ZrO2-M, (C) i10-FeOx/ZrO2M, (D) i20-FeOx/ZrO2-M, (E) i30-FeOx/ZrO2-M, (F) i50-FeOx/ZrO2-M,(G) i20-FeOx/ZrO2-W, (H) i20-FeOx/ZrO2-U, (I) i20-FeOx/TiO2-M, (J)i20-FeOx/SnO2-M and (K) i20-FeOx/CeO2-M.
Fig. 4morphologies (Fig. 3F). In addition, 20–70 atom% NiOx catalysts embedded in ZrO2 MARIMOs also exhibited perfectly
spherical morphologies (Fig. 3G–J). 50 atom% FeOx catalyst
embedded in CeO2 MARIMO also showed a sphericalmorphology, including both large- and small-sized spheres(Fig. 3K).
As an alternative, the impregnation method is a simple
preparation technique that can be used to obtain supported
catalysts.31 In addition, this method is a versatile technique thatis applicable to almost all types of support. We applied thistechnique to obtain NiOx and FeOx catalysts that were supported on several types of MARIMO support. XRD spectra
indicated that the ZrO2 MARIMO supports for the impregnatedcatalysts showed a cubic crystal phase (see Fig. S5B–F†). Interestingly, i8-NiOx/ZrO2-M consisted of cubic and monoclinic
crystal phases (see Fig. S5A†). The inherently sphericalmorphologies of the MARIMO supports were clearly preservedin the cases of 5 to 20 atom% impregnation of NiOx and FeOx in
the ZrO2 MARIMO (Fig. 4A–D). However, further loading oflarger quantities of FeOx into the ZrO2 MARIMO (30 and 50
atom%) resulted in catalysts with rugged structures (Fig. 4E and
F). Therefore, loading of more than 30 atom% of FeOx on the
ZrO2 MARIMO support by the impregnation method was notsuitable for our purpose of preparing spherical catalysts for CNT
growth. Furthermore, commercially available ZrO2 particles,
comprising the Wako ZrO2 and UEP-100 ZrO2 particles, without
spherical morphologies were also converted into FeOx/ZrO2
catalysts via the impregnation method to act as reference
samples (Fig. 4G and H, respectively), with the ZrO2 supports
showing a monoclinic crystal phase in this case (see Fig. S5G

RSC Adv., 2023, 13, 13809–13818 | 13813

View Article Online

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

RSC Advances

Paper

and H†). In the cases of the spherical TiO2, SnO2, and CeO2
MARIMO supports, globular morphologies were observed forthe impregnated catalysts aer FeOx loading (Fig. 4I–K,
respectively). Therefore, we prepared catalyst candidate materials to obtain the MARIMO-CNT conjugates successfully.
Thermal CVD of the prepared catalysts
Embedded catalysts obtained by one-pot solvothermal
synthesis. As a rst test, thermal CVD using ethyne gas asa carbon source was performed under conditions of a temperature of 730 °C, pressure of 65 Pa, 10 sccm ethyne gas ow, anda reaction time of 10 s using 20 atom% FeOx, CoOx, and NiOx
catalysts that were solvothermally embedded in MARIMO ZrO2
and CeO2 supports. The performances of these catalysts were
evaluated via SEM observations (Fig. 5). As a result, the FeOx
catalyst embedded in MARIMO ZrO2 showed the best results,
producing CNT-hair (Fig. 5A) rather than CNT sea urchin
structures, as compared with the other CoOx (Fig. 5B) and NiOx
(Fig. 5C) catalysts embedded in MARIMO ZrO2. In addition, thecombination of FeOx/CeO2 resulted in poor growth of CNTs withlow density (Fig. 5D). Therefore, the CNT-hair growth process isstrongly dependent on the combination of the nanosized metaloxide catalyst and the support metal oxide. In our case, thecombination of the nanosized FeOx catalyst with ZrO2 as the
support (FeOx/ZrO2) produced the best results for formation of
the CNT-hair.
Next, the amount of loaded Fe was varied to optimize the
CNT-hair formation process (Fig. 6). The amount and the lengthof CNT-hair increased for Fe loading of up to 20 atom%(Fig. 6A–C), but the spherical catalyst cores themselves can beseen through the CNT-hair when the amount of Fe exceeded 30
atom% (Fig. 6D and E). Therefore, we decided to use 20 atom%of Fe catalyst in the later experiments.
The reaction temperature was varied (670, 730, and 760 °C)
while using s20-FeOx/ZrO2-M as the catalyst (Fig. 7). The CNThair seemed to grow well at all temperatures tested. To determine the quality of the CNTs, the IG/ID intensity ratios in the
Raman spectra were estimated (see Fig. S6†), whose ratios of the
G and D bands (IG/ID) in the Raman spectra were used todetermine the crystallinity of the CNT-hair obtained. G band, in-

SEM images of CNT-hair obtained using diﬀerent catalysts,
including (A) s20-FeOx/ZrO2-M, (B) s20-CoOx/ZrO2-M, (C) s20-NiOx/
ZrO2-M and (D) s50-FeOx/CeO2-M at 730 °C under pressure of 65 Pawith a 10 sccm ethyne gas ﬂow for 10 s.
Fig. 513814 | RSC Adv., 2023, 13, 13809–13818

SEM images of CNT-hair obtained using diﬀerent quantities ofcatalysts, including (A) s5-FeOx/ZrO2-M, (B) s10-FeOx/ZrO2-M, (C)s20-FeOx/ZrO2-M (the same as Fig. 5A), (D) s30-FeOx/ZrO2-M and (E)s50-FeOx/ZrO2-M at 730 °C under pressure of 65 Pa with a 10 sccm
ethyne gas ﬂow for 10 s.
Fig. 6

Fig. 7 SEM images of CNT-hair obtained using s20-FeOx/ZrO2-M
catalysts at temperatures of (A) 670 °C, (B) 730 °C (the same as Fig. 5A)and (C) 760 °C under pressure of 65 Pa with a 10 sccm ethyne gas ﬂow
for 10 s.

plane bond stretching mode of the C–C bonds in the hexagonallattice,32 represents the higher concentration of sp2 carbon
atoms and higher-ordered graphitic structures. While the Dband, hexagonal sp2 breathing mode, indicates the presence of
defects in sp2 network.33 The CNT-hair obtained at 730 °C (1.88)
was found to show the best crystallinity than those obtained at
670 °C (0.71) and 760 °C (0.82).
With regard to the reaction time, small perturbances wereobserved with the short reaction time of 0.5 s (Fig. 8A), while
CNT-hair of suﬃcient length was produced aer 10 s (Fig. 8D),
this indicating that a suﬃcient quantity of the ethyne gas is
indispensable for formation of the CNT-hair conjugates.

Fig. 8 SEM images of products obtained using s20-FeOx/ZrO2-M
catalysts with diﬀerent reaction times at 730 °C under pressure of 65
Pa with a 10 sccm ethyne gas ﬂow. (A) 0.5 s, (B) 1 s, (C) 3 s and (D) 10 s(the same as Fig. 5A).

© 2023 The Author(s). Published by the Royal Society of ChemistryView Article Online

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

Paper
In summary for this section, in which nanosized metal oxide
catalysts embedded in metal oxide supports were used, thereaction conditions when using the s20-FeOx/ZrO2-M catalysts
at 730 °C under pressure of 65 Pa with 10 sccm ethyne gas ow
for 10 s were the best conditions for CNT-hair formation.
Impregnated catalysts obtained by two-step impregnation
method. Catalysts obtained by the impregnation method werealso subjected to thermal CVD. With regard to the catalyst
supports, nanosized FeOx catalysts impregnated in TiO2 andSnO2 MARIMOs did not yield CNTs (Fig. 9A and C), while thoseimpregnated in ZrO2 and CeO2 MARIMOs yielded CNT-haireﬀectively (Fig. 9B and D). Of the two supports, ZrO2 seemed
to be a better support than CeO2 based on the amount of CNThair obtained. As a matter of course, the reference nanosized
FeOx catalysts impregnated into the commercially available
ZrO2 nanoparticles without spherical morphologies did not
produce CNT-hair (Fig. 9E and F).
The quantities of the loaded FeOx were critical to eﬀective
production of the CNT-hair. Smaller amounts of Fe atoms (5 and 10
atom%) in the MARIMO ZrO2 support were not eﬀective in yielding
CNT-hair (Fig. 10A and B). However, larger quantities of Fe atoms(20 atom%) on the MARIMO ZrO2 support led to eﬀective CNT-hairgrowth (Fig. 10C). The reaction temperature was also important to
the formation of the CNT-hair. Almost zero CNT-hair formation wasobserved at the lower temperature of 670 °C (Fig. 11A). However,use of the higher temperatures of 730 °C and 760 °C resulted in theformation of satisfactory CNT-hair (Fig. 11B and C).
As a conclusion for thermal CVD of the prepared catalysts
Section, the combination of nanosized FeOx as the CNT

RSC AdvancesFig. 11 SEM images of products obtained using i20-FeOx/ZrO2-M at
diﬀerent temperatures. (A) 670 °C, (B) 730 °C (the same as Fig. 9B) and(C) 760 °C under pressure of 65 Pa with a 10 sccm ethyne gas ﬂow for
10 s.

catalyst and ZrO2 as the support was shown to be the best for
production of CNT-hair under our reaction conditions, irrespective of use of a solvothermally embedded or impregnated
FeOx/ZrO2 catalyst. In light of all the results obtained, weconcluded that the best catalyst for CNT-hair formation was
the 20 atom% FeOx catalyst supported in the MARIMO ZrO2
porous spheres. From this point, the reaction conditions
were xed as follows: use of the 20 atom% FeOx/ZrO2 catalyst
at 730 °C under pressure of 65 Pa with a 10 sccm ethyne gas
ow for 10 s.

Fig. 9 SEM images of products obtained using FeOx catalysts supported on diﬀerent metal oxides. (A) i20-FeOx/TiO2-M, (B) i20-FeOx/
ZrO2-M, (C) i20-FeOx/SnO2-M, (D) i20-FeOx/CeO2-M, (E) i20-FeOx/
ZrO2-W and (F) i20-FeOx/ZrO2-U at 730 °C under pressure of 65 Pawith a 10 sccm ethyne gas ﬂow for 10 s.

SEM images of products obtained using catalysts with diﬀerent
quantities of FeOx. (A) i5-FeOx/ZrO2-M, (B) i10-FeOx/ZrO2-M and (C)i20-FeOx/ZrO2-M (the same as Fig. 9B) at 730 °C under pressure of 65
Pa with a 10 sccm ethyne gas ﬂow for 10 s.
Fig. 10© 2023 The Author(s). Published by the Royal Society of Chemistry

SEM images of CNT-hairs at diﬀerent magniﬁcations obtainedusing (A) s20-FeOx/ZrO2-M and (B) i20-FeOx/ZrO2-M catalysts at 730 °C under pressure of 65 Pa with a 10 sccm ethyne gas ﬂow for 10 s.
Fig. 12RSC Adv., 2023, 13, 13809–13818 | 13815

View Article Online

RSC Advances

Paper

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

Detailed observation of the CNT-hair

SEM images of CNT-hairs at diﬀerent magniﬁcations obtainedusing (A) s20-FeOx/ZrO2-M and (B) i20-FeOx/ZrO2-M catalysts at 730 °C under pressure of 65 Pa with a 10 sccm ethyne gas ﬂow for 10 s.
Fig. 13

Fig. 12 shows SEM images of the products obtained using thesolvothermally embedded s20-FeOx/ZrO2-M catalyst (Fig. 12A)
and the impregnated i20-FeOx/ZrO2-M catalyst (Fig. 12B). In theembedded s20-FeOx/ZrO2-M case, a waved sheet-like product ona th-SiO2 substrate was observed under low magnication(Fig. 12A1). Expansion of the ripped area shows that the sheet
consists of large numbers of particles (Fig. 12A2). The 5 mmscale image clearly indicates that these particles are aggregates of multiple yarns, similar to fuzzy balls or cotton candies,with diameters of less than 3 mm (Fig. 12A3). Further expansion
clearly shows that the aggregates include dark spherical coresand long gray threads (ca. 3 nm in length) (Fig. 12A4), which canbe ascribed to the FeOx/ZrO2 MARIMO core and the CNTs,
respectively. In the same manner, the impregnated i20-FeOx/
ZrO2-M catalyst produced similar CNT-hair (Fig. 12B), but thedensity of the CNTs on the impregnated i20-FeOx/ZrO2-M
catalyst appeared to be slightly lower than that obtained on theembedded s20-FeOx/ZrO2-M catalyst (Fig. 12A3, A4, B3, B4 and
S7, S8†).
Further expansions clearly show that the CNTs grew upwardfrom the surface of the core FeOx/ZrO2 MARIMOs (Fig. 13A3, A4,
B3 and B4). Another unique feature of the CNT-hair can be seenin Fig. 13A1, A2, B1 and B2. The CNT-hairs were entangled witheach other to show a structure similar to a nonwoven fabric,
which constructs CNT grids with junctions formed by the ZrO2
MARIMOs. Furthermore, the combination of the black core
FeOx/ZrO2 MARIMOs with the CNTs appears similar to frog
eggs. A single long CNT is so and exible. However, when
multiple CNTs come together, they support each other, creatinga new CNT forest morphology.5,6 Similar to the CNT forest, theCNTs in the CNT-hair are so and exible. However, when large
numbers of CNT-hairs come together, they become tangled witheach other, yielding a new morphology like the observed CNT-

Fig. 14 TEM images of CNT-hairs at diﬀerent magniﬁcations obtained using (A) s20-FeOx/ZrO2-M and (B) i20-FeOx/ZrO2-M catalysts at 730 °C
under pressure of 65 Pa with a 10 sccm ethyne gas ﬂow for 10 s.

13816 | RSC Adv., 2023, 13, 13809–13818© 2023 The Author(s). Published by the Royal Society of ChemistryView Article Online

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

Paper

RSC AdvancesFig. 15 Diameter distributions of CNT-hairs obtained using (A) s20-FeOx/ZrO2-M catalyst and (B) i20-FeOx/ZrO2-M catalyst at 730 °C underpressure of 65 Pa with a 10 sccm ethyne gas ﬂow for 10 s.

Fig. 16 Raman spectra of CNT-hairs obtained using (A) s20-FeOx/ZrO2-M and (B) i20-FeOx/ZrO2-M catalysts at 730 °C under pressure of 65 Pawith a 10 sccm ethyne gas ﬂow for 10 s. The Si peaks at 520 cm−1 were observed in both samples on the th-SiO2 substrates on which MARIMO
catalysts were drop-cast.

frog eggs structure. Based on these observations, the layeredmaterials obtained by constructed with large amounts of CNThair can show intriguing physical properties, including
isotropic absorption of polarized electromagnetic waves.
TEM images clearly show that the CNT-hairs grew from thesurface of the core MARIMO catalysts regardless of whether thecatalyst was embedded or impregnated (Fig. 14). In addition,
the CNTs obtained were ascertained to be multi-walled structures with 5–8 layers. To determine the diameter distribution of
the CNT-hairs, the diameters of 100 tubes in the TEM images
were estimated (Fig. 15). As a result, the CNT diameter distribution obtained when using the embedded s20-FeOx/ZrO2-M
catalyst showed two maxima of 12 and 24 nm, with an averagevalue of 14.8 nm and a 5.4 nm dispersion, while the corresponding results obtained using the impregnated i20-FeOx/
ZrO2-M catalyst showed a smaller average value (13.8 nm) witha lower dispersion (2.3 nm). Therefore, the CNT-hair obtainedusing the embedded s20-FeOx/ZrO2-M catalyst was dense, thick,and straight, while that obtained using the impregnated i20FeOx/ZrO2-M catalyst was a little sparse, thin, and curly (seeFig. 12–15).
To determine the crystallinity of the CNT-hair obtained, theintensity ratios of the G and D bands IG/ID in the Raman spectra
were estimated (Fig. 16). Consequently, the CNT-hair obtainedusing s20-FeOx/ZrO2-M (1.88) was found to show better crystallinity than that obtained using i20-FeOx/ZrO2-M (1.12). The G′(or 2D) band at around 2700 cm−1 originates from the secondorder Raman scattering, arising from a two-phonon, intervalley,second-order Raman scattering process.34 The sharp and higher© 2023 The Author(s). Published by the Royal Society of Chemistry

intensity of the G′ band represents the lower concentration of
defects and disorders and lower interaction between graphene
layers, indicating a high-quality lower number of layers, especially for single-walled carbon nanotubes. The intensity ratio of
the G′ and G band in CNTs grown at 730 °C on s20-FeOx/ZrO2-M(IG′/IG = 0.56) was higher than that on i20-FeOx/ZrO2-M catalysts(IG′/IG = 0.45). Sharp G′ band and higher IG′/IG of CNTs on s20FeOx/ZrO2-M catalysts indicate higher quality of carbon nanotube formation, compared with i20-FeOx/ZrO2-M catalysts. As
shown in Fig. 14A4 and B4, crystal structure order wasconrmed higher for CNTs on s20-FeOx/ZrO2-M catalysts(Fig. 14A4) compared with i20-FeOx/ZrO2-M catalysts(Fig. 14B4).

Conclusions
Catalysts that were suitable for CNT-hair preparation werestudied extensively. FeOx, CoOx, and NiOx were selected as the
CNT catalysts, and TiO2, ZrO2, SnO2, and CeO2 were selected as
the support materials. Supported catalysts with diﬀerent
combinations of these catalysts and supports were prepared by
two diﬀerent methods, comprising the one-step solvothermal
method and the two-step impregnation method, and yielded
impregnated catalysts and embedded catalysts, respectively.
Among the catalyst candidates, the combination of FeOx as thecatalyst for CNT growth and ZrO2 as the support resulted in thebest growth of CNTs under the required reaction conditions ofa temperature of 730 °C under pressure of 65 Pa with a 10 sccm
ethyne gas ow for a reaction time of 10 s. These CNTs consisted

RSC Adv., 2023, 13, 13809–13818 | 13817

View Article Online

Open Access Article. Published on 05 May 2023. Downloaded on 10/16/2024 12:41:16 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

RSC Advancesof 5–8 layered multi-wall structures and the length of these
CNTs reached approximately 3 mm. The CNT-hair that was obtained using the solvothermally embedded catalyst showed
higher crystallinity and was dense, thick, and straight, whilethat obtained using the impregnated catalyst was a little sparse,
thin, and curly. A unique structure consisting of multiple CNT
yarns, similar to fuzzy balls or cotton candies, was produced.
Multiple CNT-hair conjugates came together to yield a layer
structure, in which the core FeOx/ZrO2 catalysts and CNT yarns
appeared to be structured like frog eggs. Based on theseobservations, the layer materials obtained from construction
with large amounts of CNT-hair are expected to exhibit
intriguing physical properties, including isotropic absorption ofpolarized light and electromagnetic waves. Further studies ofthe physical properties of these materials are now in progress.

Author contributions
Kazuya Kobiro: funding acquisition, project administration,
resources, supervision, writing – review & editing. Hinako
Kimura: data curation, formal analysis, investigation, methodology, visualization. Saki Hirose: data curation, formal analysis,
investigation, methodology, visualization. Makoto Kinjo: data
curation, formal analysis, investigation. Hiroshi Furuta:
conceptualization, methodology, project administration,
writing – review & editing.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
This work was partially supported by the JSPS KAKENHI (grant
numbers 20K05093 and 22K04819). We thank David MacDonald, MSc, from Edanz (https://jp.edanz.com/ac) for editinga dra of this manuscript.

References
1 S. Iijima, Nature, 1991, 354, 56–57.
2 D. Y. Kim, H. Sugime, K. Hasegawa, T. Osawa and S. Noda,
Carbon, 2012, 50, 1538–1545.
3 M. Li, S. Hachiya, Z. Chen, T. Osawa, H. Sugime and S. Noda,
Carbon, 2021, 182, 23–31.
4 J. De La Verpilliere, S. Jessl, K. Saeed, C. Ducati, M. De Volder
and A. Boies, Nanoscale, 2018, 10, 7780–7791.
5 A. Pander, K. Takano, A. Hatta, M. Nakajima and H. Furuta,
Opt. Express, 2020, 28, 607–625.
6 H. Miyaji, A. Pander, K. Takano, H. Kohno, A. Hatta,
M. Nakajima and H. Furuta, Diamond Relat. Mater., 2018,83, 196–203.
7 D. He, M. Bozlar, M. Genestoux and J. Bai, Carbon, 2010, 48,
1159–1170.

13818 | RSC Adv., 2023, 13, 13809–13818

Paper
8 S.-Y. Park, W.-D. Kim, D.-G. Kim, J.-K. Kim, Y.-S. Jeong,J. H. Kim, J. K. Lee, S. H. Kim and J.-W. Kang, Sol. Energy
Mater. Sol. Cells, 2010, 94, 750–754.
9 Y. Piao, K. An, J. Kim, T. Yu and T. Hyeon, J. Mater. Chem.,
2006, 16, 2984–2989.
10 D. Y. Kim, H. Sugime, K. Hasegawa, T. Osawa and S. Noda,
Carbon, 2011, 49, 1972–1979.
11 C. Jo, A. S. Groombridge, J. De La Verpilliere, J. T. Lee, Y. Son,
H.-L. Liang, A. M. Boies and M. De Volder, ACS Nano, 2020,
14, 698–707.
12 F. S. Boi, G. Mountjoy and M. Baxendale, Carbon, 2013, 64,
516–526.
13 T. Chen, B. Cheng, G. Zhu, R. Chen, Y. Hu, L. Ma, H. Lv,
Y. Wang, J. Liang, Z. Tie, Z. Jin and J. Liu, Nano Lett., 2017,17, 437–444.
14 A. B. D. Nandiyanto, Y. Kaihatsu, F. Iskandar andK. Okuyama, Mater. Lett., 2009, 63, 1847–1850.
15 P. Wang and K. Kobiro, Pure Appl. Chem., 2014, 86, 785–800.
16 E. K. C. Pradeep, M. Ohtani, T. Kawaharamura andK. Kobiro, Chem. Lett., 2017, 46, 940–943.
17 P. Wang and K. Kobiro, Chem. Lett., 2012, 41, 264–266.
18 P. Wang, H. Takigawa, K. Ueno and K. Kobiro, J. Supercrit.
Fluids, 2013, 78, 124–131.
19 K. Kan, E. Yamamoto, M. Ohtani and K. Kobiro, Eur. J. Inorg.
Chem., 2020, 4435–4441.
20 Y. Kumabe, H. Taga, K. Kan, M. Ohtani and K. Kobiro, RSC
Adv., 2020, 10, 14630–14636.
21 A. Taniguchi, R. Miyata, M. Ohtani and K. Kobiro, RSC Adv.,
2022, 12, 22902–22910.
22 A. Taniguchi, Y. Kumabe, K. Kan, M. Ohtani and K. Kobiro,
RSC Adv., 2021, 11, 5609–5617.
23 E. K. C. Pradeep, M. Ohtani and K. Kobiro, Eur. J. Inorg.
Chem., 2015, 5621–5627.
24 H. T. T. Nguyen, T. Habu, M. Ohtani and K. Kobiro, Eur. J.
Inorg. Chem., 2017, 3017–3023.
25 Y. Tanaka, H. Usui, Y. Domi, M. Ohtani, K. Kobiro and
H. Sakaguchi, ACS Appl. Energy Mater., 2019, 2, 636–643.
26 M. Meilieana, T. Nakayashiki, E. Yamamoto, H. Hayashi,
M. Ohtani and K. Kobiro, Nanoscale Res. Lett., 2022, 17, 47.
27 E. K. C. Pradeep, T. Habu, H. Tooriyama, M. Ohtani andK. Kobiro, J. Supercrit. Fluids, 2015, 97, 217–223.
28 M. Ohtani, T. Muraoka, Y. Okimoto and K. Kobiro, Inorg.
Chem., 2017, 56, 11546–11551.
29 https://www.ujiden-net.co.jp/marimo/, accessed Feb 05,
2023.
30 F. Duriyasart, A. Irizawa, K. Hayashi, M. Ohtani andK. Kobiro, ChemCatChem, 2018, 10, 3392–3396.
31 X. Song, F. Shao, Z. Zhao, X. Li, Z. Wei and J. Wang, ACS
Catal., 2022, 12, 14846–14855.
32 A. Jorio and R. Saito, J. Appl. Phys., 2021, 129, 021102.
33 A. C. Ferrari and J. Robertson, Phys. Rev. B: Condens. Matter
Mater. Phys., 2000, 61, 14095–14107.
34 M. S. Dresselhaus, G. Dresselhaus, R. Saito and A. Jorio,
Phys. Rep., 2005, 409, 47–99.

© 2023 The Author(s). Published by the Royal Society of Chemistry