Synthesis, characterization and formation process of transition metal oxide nanotubes using carbon nanofibers as templates

Article abstract of DOI:10.1016/j.jssc.2009.04.005

Mono and binary transition metal oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air. The transition metal oxide nanotubes were composed of

Full text of DOI:10.1016/j.jssc.2009.04.005

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Journal of Solid State Chemistry 182 (2009) 1587–1592
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Journal of Solid State Chemistry
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Synthesis, characterization and formation process of transition metal oxide
nanotubes using carbon nanofibers as templates
Ã
ÃÃ
Hitoshi Ogihara a,b, ,1, Sadakane Masahiro a, Yoshinobu Nodasaka c, Wataru Ueda a,
a Catalysis Research Center, Hokkaido University, N21-W10 Kita-ku, Sapporo 001-002, Japan
b Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo 102-8472, Japan
c Oral Functional Science, Graduate School of Dental Medicine, Hokkaido University, N13-W7 Kita-ku, Sapporo 060-8586, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Mono and binary transition metal oxide nanotubes could be synthesized by the immersion of carbon
nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air.
The transition metal oxide nanotubes were composed of nano-crystallites of metal oxides. The
functional groups on the carbon nanofiber templates were essential for the coating of these templates:
they acted as adsorption sites for the metal nitrates, ensuring a uniform metal oxide coating. During the
removal of the carbon nanofiber templates by calcination in air, the metal oxide coatings promoted the
combustion reaction between the carbon nanofibers and oxygen.
Received 6 January 2009
Received in revised form
31 March 2009
Accepted 3 April 2009
Available online 10 April 2009
Keywords:
Transition metal oxides nanotubes
Template
& 2009 Published by Elsevier Inc.
Carbon nanofibres
1. Introduction
Thus, in order to deposit the metal oxides on the template
surfaces selectively, one needs to exploit the interactions between
the templates and metal oxides, such as electrostatic or hydrogen
bonding interactions. For example, negatively charged silica
species, formed through the hydrolysis of tetraethyl orthosilicate
(TEOS), could be adsorbed on positively charged templates,
leading to the formation of SiO2 nanotubes [11–13]. Another
selective deposition process was demonstrated by van Bommel
and Shinkai [14]. The catalysts that promote the hydrolysis and
polymerization reaction were deposited on the template surfaces
to enable selective coating of the templates because the formation
of metal oxide occurred only near the template surface.
The sol–gel method is certainly an effective method for coating
templates, but it has a number of shortcomings. The most serious
is that the constituent elements of metal oxide nanotubes are
limited. Metal alcoxides are the most common starting reagents
for the sol–gel method, but the production of transition metal
alcoxides involves complex, costly processes. Thus, it is difficult to
fabricate transition metal oxide nanotubes using the sol–gel
method. This has created a demand for the development of a
fabrication process of metal oxides nanotubes from readily
available precursors.
Since the discovery of carbon nanotubes, nanotubular materi-
als have drawn attention owing to their attractive properties [1,2].
Various nanotubular materials, such as metal, metal oxide, nitride,
and sulfide, have been synthesized and characterized. Nanotub-
ular materials, in particular metal oxide nanotubes, have been
synthesized using the so-called ‘‘template method’’ [3–10]. The
synthesis of metal oxide nanotubes through the template method
involves a two-step process: the templates are first coated with
the metal oxide and are then removed to yield metal oxides with
nanotubular hollow interiors. The metal oxides should form on
the templates during coating. This type of selective coating is
needed because if metal oxides were to form in the absence of
templates, the nanotube yield would drop.
The sol–gel method is widely used to produce metal oxides,
and has often been employed to coat templates in the template
method. The sol–gel method involves a hydrolysis reaction and a
polymerization reaction of metal precursors in liquid phase. But,
given the sol–gel reaction mechanism, metal oxides presumably
form throughout the solution, not only on the template surface.
Recently, we have reported the synthesis of nanoscale
materials with various metal oxides, such as SiO2, ZrO2, Al2O3,
SiO2–Al2O3, SiO2–TiO2, LaMnO3, NiFe2O4 and CoFe2O4, by using
carbon nanofibers as templates [15–17]. Although there have been
the oxide nanotube synthesis processes using carbon nanotubes/
carbon nanofibers as removal templates [8–10,18–22], comparing
to the previous methods, advantages of our fabrication process are
that oxide nanotubes with various shapes and containing a variety
Ã
Corresponding author at: Catalysis Research Center, Hokkaido University,
N21-W10 Kita-ku, Sapporo 001-002, Japan. Fax: +8111706 9163.
ÃÃ
Corresponding author. Fax: +8111706 9163.
E-mail addresses: ogihara@cms.titech.ac.jp (H. Ogihara),
ueda@cat.hokudai.ac.jp (W. Ueda).
1
Present address: Department of Chemistry and Materials Science, Tokyo
Institute of Technology, 2-12-1 S1-44, Ookayama, Meguro-ku, Tokyo 152-8552,
Japan.
0022-4596/$ - see front matter & 2009 Published by Elsevier Inc.
doi:10.1016/j.jssc.2009.04.005

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of elements can be obtained. One of the important features of our
synthesis process is that not only metal alcoxides but also
inorganic metal salts (e.g., metal nitrates) can be used as raw
materials, which lead to the formation of oxide nanotubes
containing a variety of elements. Most metal salts are cheaper,
more available, and more stable than metal alcoxides. In our
process, templates are coated by immersion into solutions
containing the metal precursor, followed by drying of the solution.
Metal oxide layers were deposited on carbon nanofiber templates
through the simple processes, although the coating mechanism
(e.g., the reason why only carbon nanofiber is selectively coated
with oxide layers) remains unclear. Thus, in the present study, the
synthesis and characterization of metal oxides, and the coating
mechanism of carbon nanofiber templates will be discussed on
the basis of TEM, SEM, X-ray diffraction (XRD), thermogravimetry
(TG) and N2 adsorption measurements.
100 nm
100 nm
Fig. 1. TEM images of (a) VGCFs and (b) VGCFs(gr).
Fig. 1 shows TEM images of VGCFs and VGCFs(gr). Both VGCFs and
VGCFs(gr) are straight and have a diameter of 100–150 nm. XRD
patterns of VGCFs and VGCFs(gr) are shown in Fig. 2. Both XRD
patterns exhibit a diffraction line at 20–301, which is attributed to
graphite (002). The diffraction line of VGCFs(gr) is much sharper
than that of VGCFs, indicating that VGCFs(gr) have highly
graphitized structures. It is well known that the graphitization
degree of carbonaceous materials increases upon heat treatment
in an inert atmosphere. Thus, it is reasonable to assume that the
heat treatment increased the graphitization degree of the carbon
nanofibers. Their specific surface areas are almost the same
(18 m2/g for VGCFs and 23 m2/g for VGCFs(gr)). These results
suggest that the shape and the surface area of carbon nanofibers
do not change upon heat treatment, while their graphitization
degree increases dramatically.
2. Experimental
Two types of vapor grown carbon fibers (VGCFs, Showa Denko
Co.) are used as templates. One is treated at 3273 K under an inert
gas atmosphere, while the other is not subjected to this heat
treatment. Carbon fibers obtained with and without heat treatment
will hereafter be referred to as VGCFs(gr) and VGCF, respectively.
Before being used as templates, the carbon fibers were suspended in
a concentrated H2SO4/HNO3 mixture (3:1 v/v) and ultrasonicated for
2 h. After filtration, the carbon fibers were washed with ion-
exchanged water three times, and dried at 393 K. Through the acid
treatment, functional groups such as hydroxyl groups and carboxyl
groups are formed onto the surface of the carbon fibers [23].
A set of VGCF or VGCF(gr) templates was placed in a suction
filtering unit (Buchner funnel with filter paper), and a metal nitrate
solution (Fe(NO3)3 ꢀ 9H2O, Co(NO3)2 ꢀ 6H2O, Ni(NO3)2 ꢀ 6H2O,
La(NO3)3 ꢀ 6H2O, Fe(NO3)3 ꢀ 9H2O or Mn(NO3)2 ꢀ 6H2O), diluted with
ethanol (total metal nitrate concentration: 0.3 M), was dropped
onto the templates. Immediately, the nitrate solution infiltrated
into the surface-connected pore of the fibrous structure. The
excess solution was removed by filtration. The sample obtained
was dried in air at 573 K. During the drying process, the ethanol in
the pores of the fibrous structure evaporated. The metal nitrates
that were left behind adsorbed onto the template surfaces and
transformed into metal oxides through thermal decomposition.
This process was repeated 10 times. Finally, the templates were
removed by calcination in air at 773–923 K for 5 h (the standard
calcination temperature: 773 K).
3.2. Mono metal oxide nanotubes
Iron, cobalt, and nickel oxide nanotubes were prepared by
using VGCFs as templates. The crystallite structure of the
nanotubes was identified by XRD pattern measurements (Fig. 3).
For the iron oxides, the XRD patterns corresponded to
no other characteristic diffraction lines, such as
Fe3O4, were observed. For the cobalt and nickel oxides, diffraction
lines corresponded to Co3O4 and NiO were confirmed. In these
XRD patterns, no diffraction lines attributable to graphite were
observed, indicating that the VGCF templates were removed
completely. Average crystallite sizes estimated on the basis of
Scherrer’s equation were 34.0 nm for Fe2O3, 17.9 nm for Co3O4 and
11.7 nm for NiO. By applying the template method to VGCFs,
nanoscale crystallites of metal oxides were formed.
Fig. 4 shows TEM images of the Fe2O3, Co3O4 and NiO prepared.
Clearly, they all have a nanotubular structure, but the nanotube
walls of NiO are not well formed. Fe2O3 nanotubes seem to be
made up of flat particles ca. 20 nm in thickness. Meanwhile, the
walls of the Co3O4 nanotubes consist of granular particles
10–30 nm in size. In the case of NiO, small particles ca. 10 nm in
diameter have aggregated into a nanotube-like structure. These
TEM images show that the type of constituent elements
influences the nanotube shape. The particle sizes of Co3O4 and
NiO observed in TEM images are almost the same as the average
crystallite sizes estimated from XRD patterns as described above.
On the other hand, in the case of Fe2O3 nanotubes, since the Fe2O3
particles are not spherical but plate-like, a comparison between
particle size (estimated from TEM images) and average crystallite
size (estimated from XRD patterns) is difficult. The crystallite size
estimated on the basis of Scherrer’s equation means the thickness
of layers that are perpendicular to a certain crystal face. Therefore,
for a spherical crystallite, the particle size estimated from the TEM
image often corresponds to the crystallite size estimated by XRD.
In contrast, for a crystallite with anisotropic shape, the particle
size does not always correspond to the crystallite size.
a
-Fe2O3 and
g-Fe2O3 and
Powder X-ray diffraction patterns were recorded with
a
diffractometer (Rigaku, RINT Ultima+) using CuK radiation.
a
TEM images were obtained using a H-800 (Hitachi) operated at
200 kV. SEM images were obtained with a JSM-6300 or JSM-6500F
(JEOL) using an accelerating voltage of 5 kV. Thermogravimetry
analysis was performed with a TG-8120 (Rigaku) thermogravi-
metric analyzer. Samples were placed in an Al2O3 holder and the
temperature was increased at 10 K/min under flowing air.
Nitrogen adsorption measurements were performed on a Auto-
sorb 3 (YUASA IONICS) sorption analyzer. Prior to the sorption
measurements, samples were degassed under vacuum at 573 K for
3 h. Surface areas were calculated by Brunauer–Emmet–Teller
(BET) method. Elemental analysis for C, H, and N atoms were
carried out with a CHN corder MT-6 (YANACO).
3. Results and discussion
3.1. Carbon nanofiber templates
The properties of the carbon nanofiber templates were
characterized by TEM, XRD, and surface area measurements.
An increase in temperature induces sintering of Fe2O3 oxides
above a certain temperature. Thus, the thermal stability of the

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200 nm
50 nm
50 nm
200 nm
200 nm
Fig. 2. XRD patterns of VGCFs and VGCFs(gr).
50 nm
Fig. 4. TEM images of (a, b) Fe2O3, (c, d) Co3O4, and (e, f) NiO nanotubes.
100 nm
100 nm
100 nm
)
Fig. 5. TEM images of Fe2O3 nanotubes prepared at different calcination
temperatures: (a) 823, (b) 873, and (c) 923 K.
ferrites, and silica-based mixed oxides [16]. To examine the
structure of binary metal oxide nanotubes, TEM and SEM
measurements were carried out. TEM images indicate that the
NiFe2O4 nanotube walls consist of crystallites ca. 20–50 nm in
diameter, while clear crystallites could not be observed in LaMnO3
nanotubes (Fig. 6(a) and (b)). A low magnification SEM image
clearly showed the high yield of nanotubes (Fig. 6(c)). In addition,
many ends/openings of the rod-shaped materials in the SEM
image (Fig. 6(d)) suggest that the resulting oxide materials have
certainly nanotube structures.
As shown in the previous investigation, the crystallite sizes
were estimated using XRD patterns on the basis of Scherrer’s
equation [16]. In the case of NiFe2O4 nanotubes, the crystallite size
estimated by a XRD pattern (29.4 nm) is equivalent to the
crystallite size observed by TEM measurements (Fig. 6(b)). In
contrast, clear crystallites could not be observed in the TEM image
of LaMnO3 although the crystallite size could be estimated to be
22.1 nm by using an XRD pattern. In addition, only diffraction lines
corresponding to NiFe2O4 or LaMnO3 appeared in the XRD
patterns, i.e., mono-oxides, such as NiO, Fe2O3, La2O3 and MnO2,
were not formed. We note that a binary oxide, rather than an
amorphous structure or mono-oxide, formed upon calcination at
Fig. 3. XRD patterns of Fe2O3, Co3O4, and NiO nanotubes.
metal oxide nanotubes was examined. As described above, VGCF
templates were removed at 773 K. Their thermal stabilities were
tested by removal at different temperatures (823, 873, and 923 K).
As shown in Fig. 5, the nanotubular structure was retained at 823
and 873 K, but the average particle size of Fe2O3 formed at 873 K
was larger. At 923 K, the nanotubes were completely broken by
the sintering, and a fibrous structure was formed. The particle size
of nano-fibrous Fe2O3 is 50–100 nm, which is much larger than
the particles observed in Fe2O3 nanotubes formed at 773 K. Thus,
the sintering process transformed the nanotubular structures into
fibrous structures.
3.3. Binary metal oxide nanotubes
Carbon nanofiber templates can also be used to prepare binary
metal oxide nanotubes, such as perovskite-type compounds,

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500 nm
100 nm
20 nm
50 nm
100 nm
100 nm
Fig. 7. TEM images of VGCFs coated with (a, b) Fe2O3, (c) Co3O4 and (d) NiO heated
at 573 K.
5 µm
1 µm
Fig. 6. TEM images of (a) LaMnO3 and (b) NiFe2O4 nanotubes.
773 K, which is a relative low temperature for producing mixed
oxides. Usually, to mix metal elements of oxides at the atomic
level requires high-temperature steps. To obtain mixed metal
oxides at low temperature, precursors must be mixed thoroughly
at the atomic level before calcination. For this purpose, sol–gel
process is a suitable method because the gentle conditions during
the formation of the oxide/hydroxide networks allow the forma-
tion of atomically dispersed species if the hydrolysis rates of the
precursors are matched. In our case, the two precursors adsorbed
on the template surface were well-dispersed for reasons that
remain unclear, so that mixed metal oxides nanotubes were
formed upon calcination at relative low temperatures. In addition,
carbon nanofiber templates were almost entirely removed,
because elemental analysis for C atoms in LaMnO3 nanotubes
indicated that the amount of carbon were less than 0.99 wt%. As
described later, carbon nanofiber templates are combusted at
around 600 K, and the coating oxide layers play a role as catalysts
for the combustion of carbon nanofiber templates. Therefore, the
amount of residual carbon nanofibers would depend on the
calcination temperature and type of metal oxide layers.
500 nm
100 nm
Fig. 8. TEM images of VGCFs(gr) coated with Fe2O3.
In addition, Fe2O3 particles were observed only on the VGCF
surfaces, and nowhere else, indicating that selective deposition of
metal oxides on the template surfaces had been achieved. This
explains the high yield of metal oxide nanotubes. In the case of
Co3O4 and NiO as well, VGCF surfaces were coated uniformly and
completely (Figs. 7(c) and (d)). Interestingly, when VGCFs(gr)
were used as templates instead of VGCFs, the coating behavior of
the metal oxides changed. As can be seen in Fig. 8, not every
VGCF(gr) was coated with metal oxides on its entire surface. As
mentioned above, VGCFs(gr) have a higher graphitization degree
than VGCFs (Fig. 2). Thus, the deposition of metal oxides may be
influenced by the graphitization degree of the carbon nanofiber
templates.
3.4. Coating process
However, it is not easy to understand the relationship between
the deposition of metal oxide and the graphitization degree of
carbon nanofibers because the graphitization degree correspond
to bulk structure, while the deposition of metal oxide layers occur
on only surfaces. Hereafter, we will focus on the surface structure
of carbons with different graphitization degrees. An important
factor which determines surface properties of carbons is the
functional group. There are a vast number of investigations on the
functional groups present on carbons. For example, with ultra-
sonication in a concentrated H2SO4/HNO3 mixture, functional
groups (e.g., hydroxyl groups, carboxyl groups, carbonyl groups,
and sulfate groups) were introduced onto carbon nanotubes,
which were confirmed with FT-IR measurements [23]. In the
present study, such functional groups must be also introduced
In order to form metal oxide nanotubes, VGCF templates must
be coated with metal oxide and then be removed without
collapsing the oxide layers. The formation of a variety of metal
oxide nanotubes reflecting the shapes of the VGCF templates
indicate that VGCF templates could be coated with metal oxides
by infiltration them with a metal nitrate solution, followed by
heat treatment at 573 K. It should be stressed that coating of the
templates was achieved by such simple processes. To examine
the coating process, TEM measurements of samples before the
removal of VGCF templates were carried out. Figs. 7(a) and (b)
show TEM images of VGCFs coated with Fe2O3. Comparing to the
TEM image of ‘‘bare’’ VGCF templates (Fig. 1(a)), it is clear that
VGCF surfaces were uniformly and completely coated with Fe2O3.

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metal precursor
1591
OH
OH
OH
OH
OH
OH OH
OH
OH OH
OH
COOH
COOH
carbon nanofiber
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
Fig. 9. Schematic representation for the coating processes of carbon nanofibers. (a) Adsorption of metal precursors to functional groups on carbon nanofiber templates, (b)
transformation of metal precursors into oxide layers with heat treatment and regeneration of surface functional groups, (c) adsorption of metal precursors to functional
groups on metal oxide layers, (d) transformation of metal precursors into oxide layers with heat treatment and regeneration of surface functional groups.
onto the surface of carbon nanofiber templates because the same
acid treatment was carried out.
0
-10
-20
-30
-40
-50
-60
Carbonaceous materials with large and/or regular stacks of
graphene sheets show
a high graphitization degree, while
carbonaceous materials with small and/or disordered stacks of
graphene sheets (turbostractic structure) exhibit an amorphous
structure. It is considered that amorphous carbons have more
edges and defects in their structures. Since the reaction to
introduce the functional groups involves a chemical reaction
between the carbon surface and acid solution, functional groups
preferentially form on reactive sites (i.e., edges and defects of
graphene sheets). Thus, a larger concentration of functional
groups could be introduced on VGCFs than VGCFs(gr). We believe
that these functional groups act as adsorption sites for metal
nitrates. When VGCFs (without acid treatment) were used as
templates, they were only partially coated with a metal oxide
layer. This strongly suggests that the functional groups introduced
by the acid treatment are essential for the uniform coating of
VGCFs. This adsorption mechanism would be the general process
because several researchers also proposed similar mechanism
(i.e., functional groups on carbonaceous materials acted as the
adsorption sites for metal cations) [10,24,25].
The coating mechanism we propose for the carbon nanofiber
templates is the following. The ethanol solution of metal nitrates
penetrates the carbon nanofiber templates, filling the pores of the
fibrous structure. During the drying process, metal nitrates adsorb
on the functional groups of the carbon nanofibers. These metal
nitrates are transformed into metal oxides by thermal decom-
position at 573 K, so that the carbon nanofiber templates become
coated with metal oxides.
400
600
Temperature/ k
800
1000
Fig. 10. TG calcination curves of VGCFs coated with Fe2O3 (solid line), and VGCFs
diluted with Al2O3 powders (dotted line).
nanofiber surface because functional groups act as adsorption
sites. After the second cycle, the hydroxyl groups reproduced on
the metal oxide surface act as adsorption sites for the metal
nitrates. Fig. 9 shows the schematic representations for coating
processes of carbon nanofibers.
3.5. Template removal process
By removing the VGCF templates from VGCFs coated with
metal oxides, metal oxide nanotubes are formed. VGCFs are
removed by heat treatment in air via the following reaction:
However, after the second coating cycle, there must be no
functional groups left on the templates because the carbon
nanofiber surfaces are covered with thin metal oxide layers.
Nevertheless, metal oxides continue to deposit on the templates
after the second cycle.
A surface sol–gel process for producing ultrathin films has
been reported, where film growth is achieved by repeated
saturation adsorption of alcoxides and subsequent regeneration
of a uniform hydroxyl surface [26]. Alcoxides adsorb on the
surface hydroxyl groups, and are hydrolyzed. The oxide surfaces
formed have hydroxyl groups, so that repeated chemisorption can
occur. In this surface sol–gel process, the hydroxyl groups that are
regenerated on the metal oxide surface play an important role for
the synthesis of the metal oxide film. We believe that a similar
coating mechanism is at play in our case. During the drying
process in the first cycle, metal nitrates accumulate on the carbon
C þ O2 ! CO2
This VGCF template removal process was examined by thermo-
gravimetry analysis. Fig. 10 shows TG calcination curves of VGCFs
coated with Fe2O3 and VGCFs diluted with Al2O3 powders. In both
samples, a weight decrease was observed starting from around
600 K. This corresponds to the decomposition of VGCF templates
in air. Thus, VGCFs decomposed at the same temperature,
regardless of the coating with Fe2O3. Above 600 K, VGCFs coated
with Fe2O3 were combusted more rapidly than VGCFs diluted
with Al2O3 powder. This is presumably due to the catalytic
behavior of Fe2O3 for combusting carbons. In the case of VGCFs
coated with Fe2O3, VGCFs were inside the Fe2O3 nanotubes. Since
the nanotube walls are composed of small metal oxide crystallites,
the walls have pores through which gases can diffuse. Thus,
during the heat treatment in air, oxygen reaches VGCFs through

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the pores in the Fe2O3 nanotube walls and Fe2O3 would catalyze
the combustion between the VGCFs and oxygen.
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