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Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183
Titanium oxide nanotubes, nanofibers and nanowires
Zhong-Yong Yuan, Bao-Lian Su
∗
Laboratory of Inorganic Materials Chemistry, I.S.I.S., The University of Namur (FUNDP), 61 rue de Bruxelles,
B-5000 Namur, Belgium
Available online 25 May 2004
Abstract
A simple one-step hydrothermal reaction among TiO
2
powders and alkaline solution has been developed to synthesize low-dimensional
titanate nanostructures. The morphologies of the obtained nanomaterials depend on the process parameters: the structure of raw material,
the nature and concentration of alkaline solution, reaction temperature and time, which suggests that the nanostructure synthesis could be
controllable. Trititanate nanotubes with the diameters of about 10 nm were synthesized via the hydrothermal reaction of TiO
2
crystals of either
anatase or rutile phase and NaOH solution in the temperature range of 100–160
◦
C. Nanofibers with an interlinked structure were formed
when amorphous TiO
2
or commercial TiOSO
4
was treated with NaOH at 100–160
◦
C. Pentatitanate nanoribbons with high aspect ratio were
obtained by autoclaving of either crystalline or amorphous TiO
2
in NaOH solution at the temperature above 180
◦
C. Octatitanate nanowires
with the diameters of 5–10nm were prepared from TiO
2
particles treated with KOH solution. These nanostructures were analyzed by a range
of methods including powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), selected area electron
diffraction (SAED), energy-dispersive X-ray spectroscopy (EDX) and infrared spectroscopy (IR).
© 2004 Elsevier B.V. All rights reserved.
Keywords: Titania; Titanate; Nanotube; Nanofiber; Nanowire
1. Introduction
Low-dimensional nanostructured materials have attracted
considerable attention recently due to their unique physical
properties and their potential applications in nanoelectronics
and optoelectronic nanodevices. Dimensionality is a crucial
factor in determining the properties of nanomaterials [1] and,
thus, the control of size and shape is of great interest. In con-
trast to size control, shape control of particulates is a more
difficult and challenging topic. The tubes, flakes or fibers
with the size range in the nanometer region are expected
to possess novel properties [2–4]. Many efforts are being
devoted to develop new methods (for example, so called
“bottom-up” and “top-down” methods) [5] for synthesizing
one-dimensional nanomaterials such as nanowires and nan-
otubes. However, soft-chemical routes are specially focused
with much interest on the preparation of such nanostructures
of metastable oxide materials, though general methods are
not yet available.
∗
Corresponding author. Tel.: +32-81-724531; fax: +32-81-725414.
E-mail address: bao-lian.su@fundp.ac.be (B L. Su).
Titanium oxide is a n-type semiconductor and a typical
photocatalyst, attracting much attention from both funda-
mental and practical viewpoints. It has been used in many
industrial areas including environmental purification, so-
lar cell, gas sensors, pigments and cosmetics [6,7].To
explore novel approaches for the nanostructured titanias
of various nature with the control of the particle size in
nanometer-scale and the morphology is quite interesting,
since the performance of titania in its various applica-
tions depends on its crystalline phase state, dimensions
and morphology [8].TiO
2
tubes [9–11] and fibers [12,13]
have been prepared using a sol–gel template processing
making use of porous alumina, polymer fibers, or super-
molecular compound as a template, and their diameters
were normally larger than 50 nm. The walls of the titania
nanotubules, prepared by deposition in porous alumina
membrane, consisted of anatase nanoparticles (10–20 nm
size) and contained mesopores arising from the spaces be-
tween the anatase particles [14]. Smaller TiO
2
tubes with
a diameter of about 8nm were also reported [15], however,
their structure and formation mechanism are in question
[16]. It is found that the titanium oxide nanotubes pro-
duced via the reaction of TiO
2
crystals and NaOH aqueous
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2004.04.030
174 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183
solution had the structure of neither anatase nor rutile phase
[16]. Sasaki et al. [4,17,18] reported a colloidal suspension
of TiO
2
sheets by the swelling and subsequent exfolia-
tion of a layered titanate in aqueous tetrabutylammonium
(TBA) hydroxide. After heating above 400
◦
C, anatase thin
flakes were formed [4]. However, a considerable drawback
to this synthetic approach lies in the process required for
exfoliation of the titania sheets, which requires a relatively
long time, a low solids content, and a high TBA concen-
tration. These factors make the approach seem unfavorable
for large-scale reactions and the production of low-cost
materials [19]. In this paper, we report the preparation of
titanium oxide nanotubes, nanofibers and nanosheets by a
hydrothermal soft-chemical process. It is found that these
nanoparticles can be shape-controlled by the hydrother-
mal temperature and the crystallization of TiO
2
precur-
sors.
2. Experimental
The hydrolysis of an ethanol solution of titanium triiso-
propoxide [Ti(O
i
C
3
H
7
)
4
] gave, after aging, the precipita-
tion of amorphous TiO
2
gel. After calcination at 600
◦
C
for 2 h, the obtained amorphous TiO
2
gel transformed to
crystalline powders of pure anatase phase. The hydrolyzed
amorphous gel, crystalline anatase, and commercial TiO
2
powders (P-25; a mixture of crystalline rutile and anatase
phases with surface area of 52 m
2
/g) were used as the
TiO
2
precursors for the wet chemical nanoparticle prepa-
ration. 0.1–0.3g of the precursor was mixed with 20ml
of NaOH or KOH aqueous solution with the concentra-
tion of 4–20 mol/L, followed by hydrothermal treatment
at 100–250
◦
C in a Teflon-lined autoclave for one or two
days. The treated powders were washed throughly with
water and 0.1 mol/L HCl aqueous solution until the pH
value of the washing solution lower than 7, and subse-
quently filtered and dried at 60
◦
C. The structural and
chemical natures of the obtained materials were studied us-
ing powder X-ray diffraction (XRD; Philips PW1820 with
CuK␣ radiation), infrared spectroscopy (IR; Perkin-Elmer
Spectrum-2000), N
2
adsorption analysis (Micromeritics
Tristar 3000 system), scanning electron microscopy (SEM;
Philips XL-20), transmission electron microscopy (TEM;
Jeol JEM-2010 at 200kV, Philips CM-200 at 200kV and
Philips TECNAI-10 at 100 kV) and energy-dispersive X-ray
spectroscopy (EDX).
3. Results and discussion
3.1. Nanotubes
Titanium oxide nanotubes can be formed in the range
of reaction temperature of 100–180
◦
C when either crys-
talline anatase or rutile or commercial P-25 was used as
the raw materials. The yield of nanotubes increased with
the hydrothermal temperature when the temperature was
in the range of 100–150
◦
C, resulting in the higher yield
of nanotubes of 80–90%, based on the TEM observations.
Little nanotubes formed when the temperature was lower
than 100
◦
C or higher than 180
◦
C. This is in agreement
with the BET surface areas of the products (Fig. 1). The
surface areas of the products are also influenced by the
particle size of the raw materials. The commercial P-25
powder is known to be very fine with a particle size of
25–30 nm, while the laboratory-made anatase particles
are relatively large (more than 50 nm). Correspondingly,
the surface areas of the produced nanotubes from P-25
is higher than that from the lab-made anatase (Fig. 1),
though the effect of the particle size of the raw materi-
als to the yield of the produced nanotubes is quite small
in comparison to the hydrothermal temperature. Addition-
ally, the concentration of NaOH solution is also a critical
parameter during the formation of nanotubes. Little nan-
otubes were observed when the NaOH concertration is
lower than 5 mol/L or as strong as 20 mol/L. High yield of
nanotubes can be obtained when the NaOH concentration
was 10–15 mol/L, and their surface areas can be as large as
350 m
2
/g.
Fig. 2a shows an overall view of titania nanotubes, re-
vealing large quantity of tubular materials with narrow size
distribution. The diameters of the tubular materials are al-
most uniform around 8–10 nm, and the length range from
several tens to several hundreds of nanometers. High reso-
lution transmission electron microscopy (HRTEM) images
(Fig. 2b) clearly show that the tubular structures are well
crystalline tubes with multiple shells of 1–5 layers, and a
shell spacing is about 0.78 nm. The tubes are hollow and
Fig. 1. BET surface areas of the products obtained from anatase and P-25
with 10 mol/L NaOH at different temperatures for 24h, and the products
from P-25 with different content of NaOH at 110
◦
C for 20 h.
Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 175
Fig. 2. HRTEM images of titania nanotubes: (a) overall view; (b) detailed view on the tube structure.
open ended with an inner diameter of about 5–6 nm. There
are many defects presented in the shell layers of the tubes.
The tubes have been shown to be of circular cross-section,
by rotating the sample holder along the tube axis during
the observation. Besides nanotubes, other phases includ-
ing rolled-up sheets and flat sheets can be found in the
samples.
Normally, the known nanotubes of carbon [20] and in-
organic dichalcogenide MX
2
(M = W, Mo; X = S, Se)
[21–23] have uniform structure with the layers evenly dis-
tributed on either side. Such uniform nanotube structure can
also be seen in titania nanotubes (Fig. 3a). However, in
some titania nanotubes, the shell layers are unequally dis-
tributed on either side of the tubes, for example two lay-
ers on one side and three or four layers on another side
(Fig. 3b). Considering the existence of rolled-up sheets,
the tubes can be described as scroll-type rather than con-
centric structure. This is similar to the tubular structure
of vanadium oxide nanotubes synthesized hydrothermally
by Krumeich et al. [24]. Fig. 4 shows a rolling-up nan-
otube together with a rolled-up sheet with two layers on
one side, which provide strong evidence for the dispar-
ity of the layers of the tube walls. The rolling-up nan-
otube is bended in the middle, having an unequal diameter.
Since the numbers of the layers on both sides of the nan-
otubes could differ by one or two layers (Fig. 3b) and the
thin sheets with two or more layers have often been ob-
served, the nanotubes are reasonably believed to be formed
by rolling of a one- or two-layered sheet of titanium ox-
ide.
176 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183
Fig. 3. HRTEM images of single nanotubes with the different shell
numbers.
The EDX analysis of an area containing a large amount
of tubular particles reveal the existence of Na, Ti and O
elements. However, when only the freestanding individual
nanotubes were selected for EDX study, sodium was not
detected. Infrared spectroscopy of the dried sample indicated
that OH bonds constantly exist in the nanotubes (Fig. 5),
implying that the nanotubes might be the H-form of titanates
rather than titania.
Fig. 6 shows the XRD patterns of the products obtained
from anatase with 10 mol/L NaOH solution at different
temperatures. The diffraction peaks of unreacted anatase
Fig. 4. A typical HRTEM image showing one rolling-up nanotube.
Fig. 5. Infrared spectra of the products obtained from with: (a) anatase and
NaOH (10mol/L) at 150
◦
C; (b) amorphous TiO
2
and NaOH (10 mol/L)
at 180
◦
C; (c) anatase and NaOH (10 mol/L) at 220
◦
C; and (d) anatase
and KOH (10mol/L) at 200
◦
C. The samples were degassed at 150
◦
C
overnight before IR measurements.
can be seen in the product obtained at 100
◦
C, and only
a small yield of nanotubes (about 30%) was observed by
TEM. The products obtained in the temperature range of
130–180
◦
C show some broadened diffraction peaks with
the intensities of progressively temperature-depended in-
crease. The broadening of diffraction peaks is due to the
nanometer size of the tubes and the bending of some atom
planes of the tubes. The structure of the nanotubes can be
indexed as trititanate H
2
Ti
3
O
7
[25]. These nanotubes can
be stable at the calcination temperature lower than 400
◦
C.
However, after calcination at 540
◦
C, the nanotubes have
sintered into nanorods with a perfect circular cross-section
perpendicular to its axis (Fig. 7). No changes of the di-
ameters of the nanorods were observed by tilting the
angles.
3.2. Nanofibers
When amorphous TiO
2
powders were used as the raw
materials, no nanotubes were found after the similar
Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 177
Fig. 6. XRD patterns of the products obtained from anatase with 10mol/L
NaOH at different temperatures for 24 h.
hydrothermal chemical treatment in the NaOH solution. The
products obtained with NaOH concentration of 5–15 mol/L
at the temperature range of 100–160
◦
C are in the mor-
phology of non-tubular needle-shaped fibers. Fig. 8a shows
a low-magnification TEM image of the obtained titanium
oxide nanofibers. A large number of nanofibers with the
thickness of 5–30nm and the length of a few tens to sev-
eral hundreds of micrometres are interlaced to form an
intertexture-like hierarchical structure. Most of nanofibers
have a curl-shaped morphology, and the surface of the
nanofibers is uneven. Fig. 8b shows a HRTEM image of
a nanofiber, indicating that nanofibers are composed of
layered structure. A fringe spacing of about 0.7 nm can
be seen in the lattice resolved image, which is similar to
the shell spacing of the nanotubes. Many defects and dis-
locations were observed in the stucture of the nanofibers
(Fig. 8b).
The electron diffraction patterns of these nanofibers pre-
sented diffuse rings of amorphous phases. However, the
phase of nanofibers is not truly amorphous. XRD patterns
shown in Fig. 9 support the HRTEM results of the layered
structure. The patterns are similar to that from the nanotubes
shown in Fig. 6, though we did not found hollow nanotubes
in our specimens synthesized from amorphous TiO
2
in-
stead of crystalline titania. EDX of the samples revealed
that they consist of titanium and oxygen. The structure of
these nanofiber materials should be similar to that expected
for a layered titanate of nominal H
2
Ti
n
O
2n+1
, probably an
analogue of titanate H
2
Ti
3
O
7
.
Nanoscale fibrous structures can also be obtained with
good yields when commercial TiOSO
4
powders were used
as the starting materials. XRD and HRTEM results suggest
that nanofibers started from TiOSO
4
have the similar inter-
linked structure and composition as those from amorphous
TiO
2
(Figs. 8 and 9). N
2
adsorption–desorption isotherms of
the synthesized nanofibers revealed the presence of meso-
pores (Fig. 10), probably arising from the interparticle space,
meshy holes of intertextural nanofibers. BET surface areas
of these fibrous materials are quiet large, values of 267, 354
and 375m
2
/g being obtained when amorphous TiO
2
pow-
ders reacted with NaOH (10 mol/L) solution at 100, 130 and
150
◦
C, respectively. Values of 325 and 371 m
2
/g obtained
when TiOSO
4
powder was used as raw material treated
with NaOH solution at 100 and 150
◦
C, respectively. It is
expected that such interlinked titanium oxide nanofiberous
materials will attract interest on catalysis. The morphol-
ogy of nanofibers can remain unchanged after calcination
at 540
◦
C, though surface layers became amorphous after
high-temperature calcination.
3.3. Nanoribbons
When the hydrothermal temperature was in the range of
180–250
◦
C with the NaOH concentration of 5–15mol/L,
the products are mainly nanoribbons with very high yields
(almost 100%) whatever the raw materials are crystalline or
amorphous TiO
2
powders. SEM observations reveal that the
products consist of a large quantity of wirelike nanostruc-
tures with typical lengths in the range of several hundreds
to several tens micrometers. Fig. 11a is a low-magnification
TEM image of TiO
2
powders treated with 10 mol/L NaOH
solution at 200
◦
C, showing the ribbon-like structure. The
width of the ribbons varies from 30 to 500 nm, and the ge-
ometry of the ribbons is uniform. The nanoribbon structure
has a high aspect ratio with a quite small thickness. Fig. 11b
shows an individual nanoribbon with a rolled end. From the
rolled region one can clearly see that the ribbon is very thin
(≤5 nm).
Fig. 12 shows the XRD patterns of the synthesized and
calcined nanoribbons. The patterns of the as-synthesized
nanoribbons can be readily indexed to a monoclinic phase
of H
2
Ti
5
O
11
·H
2
O. The infrared spectra of the nanorib-
bons shown in Fig. 5 clearly reflect two kinds of protonic
species. Three absorption bands centered at 3400, 1630 and
950 cm
−1
can be assigned to O–H stretching mode for in-
terlayer water, oxonium ions and hydroxyl groups, H–O–H
bending for water and oxonium ions, and O–H bending
for hydroxyls, respectively [26]. After calcination at the
temperature range of 400–600
◦
C, the materials are com-
pletely dehydrated and re-crystallized into the metastable
form of titanium dioxide, TiO
2
(B) [26,27] (Fig. 12). In
the temperature range 700–900
◦
C, the metastable TiO
2
was transformed into anatase, which was then changed
into rutile. Fig. 11c is a TEM image of a nanoribbon
after calcination at 540
◦
C for 2 h, showing many holes
on the surface of the nanoribbons, which may be due to
dehydration.
Fig. 13a is a typical HRTEM image of a nanoribbon
with well-defined structure, growing along the [0 0 2] di-
rection. Two sets of lattice fringes can be observed in the
178 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183
Fig. 7. HRTEM images of nanotubes after calcined at 540
◦
C, recorded on the specimen holder tilted at different angles: (a) y-axis, −15
◦
; (b) y-axis,
+12
◦
; (c) x-axis, −15
◦
; (d) x-axis, +12
◦
.
lattice-resolved image (Fig. 13b). The fringes parallel to the
ribbon axis (or [002] plane) correspond to an interplan-
nar distance of about 0.65nm. Another set of fringes with
smaller spacing of 0.36 nm skewed in the direction of the
ribbon axis at an angle can correspond to [1 1 0] plane of the
H
2
Ti
5
O
11
·H
2
O crystal structure. The nanocrystals viewed
along the [1 0 0] axis, oriented parallel to the electron
beam. The corresponding selected area electron diffrac-
tion (SAED) pattern (Fig. 13c) can be indexed to be the
[2 0 0] crystal zone of pentatitanate, which could match the
main diffraction peaks at 0.82 nm of d-spacing in the XRD
patterns. HRTEM images also displayed that some of the
nanoribbons consist of two side layers of amorphous phases,
especially at the end parts of ribbons (Fig. 13d). The thick-
ness ratio of the side and core layers is less than 1/15. The
nanoribbons can also bend along the ribbon axis during the
growth.
3.4. Nanowires
When KOH solution was used to react with TiO
2
pow-
ders instead of NaOH solution, the products were nei-
ther tubular nor ribbonlike, but wirelike. Fig. 14a shows
a typical low-magnification TEM image of the obtained
nanowires. The nanowires are almost uniform with a di-
ameter of 5–10nm and a length in the range of several
micrometers to several tens micrometers. Such nanowires
can be synthesized with good yields in the reaction tem-
perature range of 130–240
◦
C. The concentration of KOH
solution in the range of 4–25 mol/L did not affect the
Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 179
Fig. 8. (a) Low-magnification TEM image of titanium oxide nanofibers, showing a well-interlinked structure; (b) HRTEM image of a nanofiber, revealing
the layered structure.
structure of the products, as revealed by XRD and TEM
results.
The powder XRD patterns of the nanowires are shown
in Fig. 15, which can be indexed as a single phase of
K
2
Ti
8
O
17
with monoclinic structure. No other polymorphs
of titania are observed. The broadening of the diffraction
peaks indicates small size of nanocrystals. The crystallite
size calculated by the Sherrer formula is about 10 nm. EDX
and chemical analysis reveal the existence of Ti, K and O in
the sample with a molar ratio of K:Ti close to 1:4.Fig. 14b
is the corresponding SAED pattern from Fig. 14a, which is
consistent with the result of XRD. Fig. 14c shows HRTEM
image of single nanowires. One set of distinctive lattice
fringes parallel to the nanowire axis is observed. The
fringe spacing is about 0.78nm, which corresponds well
Fig. 9. XRD patterns: (a) amorphous TiO
2
precursor; (b–d) products from
amorphous TiO
2
gel treated with 10 mol/L NaOH solution at 150, 130
and 100
◦
C, respectively; (e) calcined sample of (d) at 540
◦
C; (f and g)
products from TiOSO
4
treated with 10 mol/L NaOH at 100 and 150
◦
C,
respectively.
with the reported interplanar distance for [200] plane of
K
2
Ti
8
O
17
. The growth direction of these titanate nanowires
is along the [0 1 0] crystal direction. Some nanowires
aligned well to present a form of coaxial nanocables or
nanoropes (Fig. 14d). Several individual nanowires coiled
together.
Conventionally, K
2
Ti
8
O
17
can be prepared by ion-
exchange reaction of K
2
Ti
4
O
9
followed by a thermal
treatment of KHTi
4
O
9
·nH
2
O [27–29]. In the present
work, K
2
Ti
8
O
17
nanowires were synthesized by a sim-
ple hydrothermal reaction of TiO
2
particles and KOH
solution, and the growths of fine nanowires are oriented
along [010] direction. N
2
adsorption analysis revealed
that these nanowires have a high specific surface area of
250–320 m
2
/g, which should be expected to attract much
interest in the application of catalysis. Moreover, since
K
2
Ti
8
O
17
has high ion conductivity because of easy hole
generation compared to K
2
Ti
6
O
13
[28], it is expected to be
Fig. 10. N
2
adsorption–desorption isotherms of nanofibers obtained: (a)
from amorphous TiO
2
gel at 130
◦
C and (b) from TiOSO
4
at 100
◦
C.
180 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183
Fig. 11. (a) Low-magnification TEM images of titania nanoribbons; (b)
one nanoribbon with a rolled end revealing the small thickness; (c) TEM
image of nanoribbons after calcination at 540
◦
C.
useful as a novel functional material. Calcination at a con-
ventional Muffle above 600
◦
C, K
2
Ti
8
O
17
nanowires tend
to decompose to K
2
Ti
6
O
13
and TiO
2
(anatase and rutile)
(Fig. 15).
3.5. Formation mechanism
It has been found that the morphology of the obtained
products depends on the process parameters: the structure
of raw material, the nature and concentration of alkaline
solution, reaction temperature and time. The hydrother-
mal reaction of crystalline TiO
2
and concentrated NaOH
solution at the temperature of 100–160
◦
C results in the
production of nanotubes, whilst amorphous TiO
2
treated at
the same conditions results in the production of nanofibers.
Either crystalline or amorphous TiO
2
can be used as raw
material to produce nanoribbons when the synthesis was
performed with NaOH solution at the temperature above
Fig. 12. XRD patterns of nanoribbons: (a) as-synthesized from amorphous
TiO
2
gel with 10 mol/L NaOH at 180
◦
C; (b) after calcination of (a) at
540
◦
C; (c) as-synthesized from anatase powders with 10mol/L NaOH at
220
◦
C; (d–g) after calcination of (c) at 400, 500, 600, 700
◦
C, respectively.
A: anatase; B: TiO
2
(B).
180
◦
C. Whereas nanowires were obtained in the KOH,
and nanoparticles were observed in the LiOH treated
samples.
The crystalline structure of the crystalline TiO
2
poly-
morphs (such as anatase and rutile) is described with rep-
resentative Ti–O
6
octahedra which share vertices edges to
build up the three-dimensional framework of the oxide. It
can be proposed that some of Ti–O–Ti bonds of the raw
materials are broken when reacted with alkaline solution,
and layered titanates composed of octahedral TiO
6
units
with the complication of alkali metal ions are formed in
the form of thin small sheets. Under autoclaving, the ti-
tanate sheets were exfoliated into nanosheets with one or
two layers, and then the nanosheets rolled into nanotubes
with a slow growth rate possibly due to the high concen-
tration of NaOH. Simultaneously, nanosheets with several
trititanate layers were formed three-dimensionally, which
may be difficult to roll up completely, so the edges of these
nanosheets were often bent (Figs. 2 and 4). The Na
+
ions
attached in nanotubes and nanosheets could be exchanged
and removed after washing with water and diluted acid so-
lution.
Titanium in the amorphous TiO
2
gel is octahedrally
coordinated by both oxygen atoms, which linked with ti-
tanium atoms of adjacent octahedra, and –O–H groups,
possessing a different structure from crystalline titania
Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 181
Fig. 13. (a) HRTEM image of a single nanoribbon with a well-defined structure; (b) a local enlarged image of (a); (c) SAED pattern; (d) HRTEM image
of a nanoribbon with amorphous sides.
polymorphs. When reacted with NaOH, some of Ti–O–H
(and Ti–O–Ti) bonds are broken by the cauterization
of caustic soda solution to generate new coordina-
tions. Then the octahedra interact between each other
to give long-rang order, partly transform into layered
structure of titanate, and self-assemble into thin titanate
nanosheets. With the process of hydrothermal reac-
tion, more and more nanosheets could be formed with
anisotropic growth. Some of individual sheets may merge
to form nanofibers and further interlink into a hierar-
chical intertextural structure. Most of nanofibers can
interlink chemically rather than physically. Similar phenom-
ena were taken place when TiOSO
4
was used as the raw
material, since two of oxygen in the octahedral TiO
6
units
were coordinated with SO
4
2−
in place of another titanium
atom of adjacent octahedra in crystalline titania. It is be-
lieved that local order of the structure of the raw materials
may be important to orientedly formnanostructure of shaped
oxides.
When the hydrothermal temperature was higher than
180
◦
C, the forms of the products should be controlled
mainly by thermodynamics. With the increase of the
temperature (>160
◦
C), the nanoribbons grew accom-
panied with the decrease of titanate nanotubes. And
the crystalline structure transformed from trititanate to
a fairly stable phase of pentatitanate H
2
Ti
5
O
11
·H
2
O.
Since the nanoribbons curved along the ribbon axis
were observed, it is reasonable to believe that the mor-
phology is temperature-depended. The form of flat
nanoribbons is stable during elevated-temperature autoclav-
ing.
K
2
Ti
8
O
17
nanowires were formed in the autoclaving of
TiO
2
powders and KOH solution. The TEM observation of
the growth process of the nanowires reveals that nanowires
182 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183
Fig. 14. (a) Low-magnification TEM images of titanate nanowires; (b) corresponding SAED pattern from the region of (a); (c) HRTEM image of nanowires
showing lattice fringes of 0.78nm parallel to the nanowire axis; (d) HRTEM images of a bundle of nanowires and its corresponding SAED pattern (insert).
grew out directly from the titania particles. When raw
material of anatase reacted with KOH solution, some of
Ti–O–Ti bonds of titania crystals are broken, and layered
octatitanates are formed on the titania surface along the
(0 1 0) lattice plans of TiO
2
. Their (2 0 0) plans may parallel
to the (101) lattice plans of TiO
2
. Further, hydrothermal
reaction cause the nanowires grow out along the [0 1 0]
direction.
[...]...Z.-Y Yuan, B.-L Su / Colloids and Surfaces A: Physicochem Eng Aspects 241 (2004) 173–183 183 Acknowledgements This work was supported by the Belgian Federal Government PAI-IUAP-5/01 project and the European Program of InterReg III (Programme France-Wallonie-Flandre, FW-2.1.5) References Fig 15 Powder XRD pattern of the as-synthesized titanate nanowires and after calcination at different temperatures... at the temperature above 180 ◦ C Octatitanate nanowires with the diameters of 5–10 nm were prepared from TiO2 particles treated with KOH solution These synthesized nanostructured materials may be significant on the understanding dimensionally confined transport phenomena and developing new catalyst materials Ion-exchange and intercalation of other metal ions and organic molecules may lead to functionalization... K2 Ti8 O17 ; H: K2 Ti6 O13 ; A: anatase; R: rutile 4 Conclusions Low-dimensional nanostructured titanium oxide materials have been prepared controllably Trititanate nanotubes with the diameters of about 10 nm were synthesized via the hydrothermal reaction of TiO2 crystals of either anatase or rutile phase and NaOH solution in the temperature range of 100–160 ◦ C Nanofibers with an interlinked structure... functionalization of titanate nanostructures for future applications [1] J Hu, T.W Odom, C.M Lieber, Acc Chem Res 32 (1999) 435 [2] R Saito, G Dresselhaus, M.S Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998 [3] Characterization of Nanophase Materials, Z.L Wang (Ed.), Wiley-VCH, Weinheim, 2000 [4] T Sasaki, S Nakano, S Yamauchi, M Watanabe, Chem Mater 9 (1997)... F Bieri, B Schnyder, R Nesper, J Am Chem Soc 121 (1999) 8324 [25] Q Chen, G.H Du, S Zhang, L.M Peng, Acta Cryst B 58 (2002) 587 [26] T Sasaki, Y Komatsu, Y Fujiki, Chem Mater 4 (1992) 894 [27] R Marchand, L Brohan, M Tournoux, Mater Res Bull 15 (1980) 1129 [28] C.T Lee, M.H Um, H Kumazawa, J Am Ceram Soc 83 (2000) 1098 [29] K Sasaki, Y Fujiki, J Solid State Chem 83 (1989) 45 . Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183
Titanium oxide nanotubes, nanofibers and nanowires
Zhong-Yong Yuan,. CM-200 at 200kV and
Philips TECNAI-10 at 100 kV) and energy-dispersive X-ray
spectroscopy (EDX).
3. Results and discussion
3.1. Nanotubes
Titanium oxide nanotubes
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