Physical Characteristics of Titania Nanofibers Synthesized
by Sol-Gel and Electrospinning Techniques
Soo-Jin Park, Yong C. Kang, Ju Y. Park, Ed A. Evans, Rex D. Ramsier, and George G. Chase
University of Akron, Akron, OH UNITED STATES
Correspondence to:
George G. Chase email: [email protected]
ABSTRACT
Titania nanofibers were successfully synthesized by
sol-gel coating of electrospun polymer nanofibers
followed by calcining to form either the pure anatase
or rutile phases. Characterization of these materials
was carried out using scanning electron microscopy
(SEM), transmission electron microscopy (TEM),
diffuse reflectance Fourier transform infrared
spectroscopy (DRIFTS), X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), and UV-vis
spectroscopy techniques. The average diameter of
these ceramic nanofibers was observed to be around
200 nm for both the rutile and anatase forms. The
valence band structure and optical absorption
thresholds differ, however, indicating that nanofibrous mats of titania can be selectively developed
for different applications in catalysis and
photochemistry.
INTRODUCTION
Nanostructured metal oxide materials have attracted
considerable attention because of their potential
applications in many areas such as electronics,
photonics, sensors and catalysis.1-6 Nanosized
ceramics with large surface area per unit mass can
have enhanced electrical, physical, and chemical
properties with respect to their bulk counterparts.7,8 In
nanofiber form these materials can be incorporated
into fabrics, textiles, and filter media.
Titania, for example, is an important ceramic material
used in practical applications such as microporous
membranes,9,10 photocatalysts,11 catalysts12,13 and
chemical sensors.14 The two most commonly used
titania forms are anatase and rutile. Control of the
phase content is important since the optical and
electrical properties of the anatase and rutile phases
differ. Rutile titania has applications in high quality
paints, cosmetics and ultraviolet absorbents because
of its high refractive index and UV absorption crossJournal of Engineered Fibers and Fabrics
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50
section.15 On the other hand, the anatase phase is
chemically and optically active, thus it is generally
used for catalysis and catalytic supports.16,17
Various approaches for the preparation of nano- and
micro-structures of titania have been reported, such
as sol-gel processes, pyrolysis, electrospinning,
chemical vapor deposition and hydrothermal
methods.18-22 In particular, our research focuses on
electrospinning as one of the simplest and most
versatile methods for the fabrication of polymeric or
ceramic nanofiber mats.23
In this paper, titania nanofibers were fabricated by
dip-coating electrospun polymeric fibers into sol-gel
precursors. The fibers are characterized by scanning
electron microscopy (SEM), transmission electron
microscopy (TEM), diffuse reflectance Fourier
transform infrared spectroscopy (DRIFTS), X-ray
diffraction (XRD), X-ray photoelectron spectroscopy
(XPS), and UV-vis spectroscopy techniques.
Characterization of mechanical strength properties
are a topic of future work.
It was found that titania nanofibers formed different
crystal structures (anatase or rutile) depending on the
temperature during post-coating annealing, as
verified by several different methods.
Most
importantly, we compare the electronic structure and
optical properties of the two phases of titania coated
nanofibers to demonstrate that our synthesis methods
can be used to produce nanofibers with different
physical properties which can therefore be tailored
for specific applications.
EXPERIMENTAL
Titania nanofibers were prepared by sol-gel coating
of a template made from electrospun nylon
nanofibers. Nylon-6 (Aldrich, MW. 4,322) was
dissolved in formic acid (Fisher Scientific) at a 20:80
wt % ratio. This solution was electrospun into
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nanofiber
mats
using
a
laboratory-scale
electrospinning apparatus.24 The polymer mixture
was loaded into a 5 ml plastic syringe with a flexible
silicone tube at one end and a 21 gauge stainless steel
needle at the other end. The needle was connected to
a high voltage supply (Gamma High Voltage
Research Inc. Ormond Beach, FL) operated at 20 kV
DC. The feeding rate for the Nylon-6 solution was
controlled using a syringe pump (World Precision
Instruments, Sarasota, FL; SP1011) at 2.0 μl/min
with the needle positioned about 15 cm above the
grounded collector. The collector was aluminum foil
wrapped around a 12.5 cm diameter cylindrical wire
drum. The drum was rotated on its axis at a rate of
about 1 revolution per minute.
In order to deposit titania on the surface of the
Nylon-6 nanofibers, the fiber mats were submerged
into a sol-gel solution which consisted of 144 ml
distilled water, 20 ml of 5 M nitric acid (Fisher
Scientific), 10 ml of isopropyl alcohol (Fisher
Scientific), and 2 ml of titanium isopropoxide
(Aldrich). In order to make the anatase form of titania,
the submerged nanofiber mat was heated from 60 ºC
to 95 ºC at a rate of 5 ºC/min and held at 95 ºC for
about 90 minutes until the solution turned to a milky
white color. To make the rutile form, the coated
nylon nanofibers were heated from room temperature
to 60 ºC at a rate of 0.5 ºC/min and held at 60 ºC for 3
to 4 hrs until the solution became milky. This heating
induced the growth of titania nanoparticles on the
surfaces of the fibers. The titania-coated nanofibers
were removed from the sol solution and washed
several times with methanol to remove any residual
alkoxides. The nanofibers were then heated to 350
ºC to form anatase titania and 700 ºC to form rutile
titania.
The morphology of the aforementioned titania fibers
was observed by scanning electron microscopy
(SEM; JEOL Ltd., JSM 5310, Tokyo, Japan) and by
transmission electron microscopy (TEM, JEOL Ltd.,
JEM 1200XII, Tokyo, Japan). The vibrational spectra
were acquired using diffuse reflectance Fourier
transform infrared spectroscopy (DRIFTS; Bruker
Optics, IFS 66v/S, Tucson, AZ). The crystalline
phase of the titania fibers was identified by wide
angle X-ray diffraction (WAXD; Rigaku Co., Ltd.,
Tokyo, Japan) in the reflection mode with Cu K
radiation.
The chemical nature of the titania nanofibers was
investigated with X-ray photoelectron spectroscopy
(XPS; VG ESCALAB MK II, West Sussex, UK).
The base pressure in the analysis chamber was less
Journal of Engineered Fibers and Fabrics
Volume 5, Issue 1 - 2010
51
than 1 × 10−9 mbar. The XPS system was equipped
with a twin anode X-ray source, Mg Kα (1253.6 eV)
and Al Kα (1486.6 eV), and a concentric
hemispherical analyzer (CHA). During all
experiments discussed here, spectra were obtained
using the Al Kα X-ray source. The parameters used
for XPS experiments were an anode voltage of 9 kV,
an electron multiplier voltage of 2850 eV, anode
current of 20 mA, filament current of 4.2 A, pass
energy of 50 eV, dwell time of 100 ms, and energy
step size of 0.5 eV in constant analyzer energy (CAE)
mode for survey scans. High resolution scans were
performed at an energy step size of 0.02 eV and a
pass energy of 20 eV with all other parameters the
same as used in survey scans. XPS data were
collected from several different samples and all
exhibited similar trends and reproducibility. In each
binding energy region, spectra were signal averaged
up to 15 times in order to improve the signal to noise
ratios.
The optical absorption edge of the nanofibers was
measured using a UV-Vis spectrophotometer (Varian
Inc., Cary 300, Palo Alto, CA). The optical
absorbance spectra of the titania samples was used to
calculate the band gap energy in the wavelength
range of 200–800 nm.
RESULTS AND DISCUSSIONS
The morphology and diameter of sol-gel coated
titania nanofibers were investigated by SEM and
TEM, as shown in Figures 1 and 2. During the solgel process titania precipitates and coats the polymer
nanofibers. The diameter of the titania coated Nylon6 fibers was in the range of about 250 nm after
heating to 120 ºC (Figure 1A) and in the range of 200
nm after heating to 350 ºC (Figure 1B). Rutile titania
nanofibers exhibited similar diameters after heating
to 700 ºC. The decreasing diameter of fibers at higher
temperature is due to the removal of solvent and
hydrocarbons and the decomposition of the nylon
fibers. The TEM images in Figure 2 show the fiber
surface morphology is irregular and rough due to the
crystal grains of the titania.
In order to identify the chemical nature of the
synthesized nanofibers, DRIFT measurements were
carried out yielding spectra as shown in Figure 3.
The characteristic vibrational bands in sol-gel coated
Nylon-6 nanofibers appeared in the range of 35003300 cm-1 and 1650-1580 cm-1 for N-H stretching and
bending modes, 2960-2850 cm-1 and 1470-1350 cm-1
for C-H stretching, scissoring and bending, 17601670 cm-1 for the C-O stretching bands, and 13401020 cm-1 for the C-N stretching band. In Figure 3
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curve A and Figure 3 curve B, we see that the coated
fibers, after heating to 120 ºC and 275 ºC, still have
significant IR absorption features corresponding to
vibrations of Nylon-6. Figure 3 curve C, shows that
Ti-O vibrations dominate around 900 cm-1, indicating
that pyrolysis of the nylon material has occurred after
calcining these fibers to 350 ºC and the anatase
titania phase has formed. Also, the formation of a
sequestered form of carbon dioxide is evidenced by
the feature around 2340 cm-1 .25 The absorption
bands at 1636 and 3000 cm-1 in this spectrum indicate
the presence of hydroxyl groups.
Rutile titania nanofibers (Figure 3 curve D) exhibited
DRIFT spectra very similar to those from the anatase
form.
1m
1m
(A)
(A)
(B)
FIGURE 1. Scanning electron microscopy (SEM) images of, (A) titania sol-coated nylon fibers calcined at 120 ºC, and (B) calcined at 350 ºC.
The coated nylon fibers shrink in diameter from about 250 nm to 200 nm in diameter after heating to 350 ºC.
(A)
(B)
FIGURE 2. Transmission electron microscopy (TEM) images of titania fibers calcined to 700 ºC. In photograph (A) distinct individual crystals
grains are clearly observed on the surface of the fibers. From the TEM images it is uncertain whether the fibers are hollow. The photo in (B) is at
a lower magnification than (A) but still shows the crystals. The surface morphology of the fibers appears to be rough and uneven due to the
granular structure.
Wide angle X-ray diffraction was used to identify the
crystal structures of the nanofibers, as illustrated in
Figure 4. When the heating rate was fast (5 ºC/min),
anatase titania was formed after calcination at 350 ºC
(Figure 4A). It was found that all the sharp features
observed at 2θ = 25.39, 38.11, 48.47, 54.5, 55.01, 62.5
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52
and 68.5o in the XRD pattern are consistent with
anatase (101), (004), (105), (200), (211), (204) and
(116) Miller indices. The lattice constants determined
for the anatase nanofibers were a = 0.3772 nm and c =
0.9505 nm. On the other hand, when the solution was
slowly heated (0.5 ºC/min) and calcinated at 700 ºC,
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High resolution XPS data from the Ti 2p and valence
band regions comparing rutile and anatase titania are
shown in Figures 5A and 5B, respectively. As seen in
Figure 5A, the doublet peaks of Ti 2p were observed at
459 (Ti 2p3/2) and 465 (Ti 2p1/2) eV in both rutile and
anatase titania. These are the characteristic Ti 2p XPS
features of titania. The similar peak positions for both
forms of nanofibers imply that the core level binding
energy of Ti on the surface does not depend
significantly on the crystal structure. However, the
electronic structure of the valence band region was
affected by structure as shown in Figure 5B. This
structure dependence on the valence band observed in
this work has also been identified by Rossi’s group in
their investigation of a nickel-based alloy using XPS.28
They identified that the binding energies of Ni core
levels, such as Ni 2p3/2 and Ni 2p1/2, were not affected
by different crystal structures, but the valence level
(the Ni 3d region in their case) was altered by changes
in phase.
In Figure 5B, the top two curves represent XPS data
from rutile and anatase titania and the bottom trace
means that anatase titania exhibits a higher density of
states than rutile and that anatase has less covalent
shows the spectral intensity obtained by subtraction of
the XPS intensity of rutile titania from that of anatase.
It is clear that the band structure ca. 3 eV below the
Fermi (EF) level shows positive peak intensity. This
bond character. This is consistent with the rutile phase
having the higher thermodynamic stability.27
The difference in electronic structure near the Fermi
level distinguishes the two different titania forms and
is supported by the UV-vis absorption spectra of
Figure 6. The band gap wavelength was determined by
extrapolation of the base line and the absorption edge
to calculate the optical band gap.29
N-H C-O
C-N C-H
o
o
(B) 275 C anatase
OH
CO2
Ti-O
OH
o
(C) 350 C anatase
CO2
OH
o
(D) 700 C rutile
800
1300
1800
2300
2800
3300
-1
Wavenumber (cm )
FIGURE 3. DRIFT spectra from titania-coated Nylon-6 nanofiber
mats after heating to (A)120 ºC, (B) 275 ºC, (C) 350 ºC, and (D)
700 oC.
R(110)
Intensity (a.u.)
R(211)
20
30
40
50
R(301)
70
R(310)
R(002)
R(200)
R(220)
R(111)
R(101)
60
A(116)
40
50
2 Theta (degrees)
A(204)
A(211)
A(105)
30
A(200)
A(004)
Intensity (a.u.)
A(101)
20
N-H
C-H
(A) 120 C anatase
Intensity (a.u.)
rutile titania resulted (Figure 4B). The (110), (101),
(220), (111), (211), (220), (002), (310) and (301)
features can be indexed to the rutile titania crystal
structure with lattice parameters a = 0.4566 nm and c
= 0.295 nm.26 Depending on the heating rate and
temperature the crystal structures of the nanofibers can
therefore be manipulated to yield distinct properties
from the same starting materials.27
60
70
2 Theta (degrees)
(A)
(B)
FIGURE 4. XRD spectra for (A) anatase titania nanofibers calcined at 350 ºC, and (B) rutile titania fibers calcined at 700 ºC (A = anatase; R =
rutile).
Journal of Engineered Fibers and Fabrics
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(B) valence band
Ti 2p3/2
EF
relative intensity (cps)
relative intensity (cps)
(A) Ti 2p
Ti 2p1/2
R
R
A
A
A-R
x3
468
466
464
462
460
458
10
456
8
6
4
2
0
-2
-4
Binding Energy (eV)
Binding Energy (eV)
(A)
(B)
FIGURE 5. High resolution XPS spectra of the Ti 2p region (A) and valence band region (B) comparing rutile and anatase titania. (A = anatase;
R = rutile; A-R = subtraction of rutile XPS intensity from anatase XPS intensity).
Absorbance (a.u.)
E=hc/λ, where h is Planck’s constant (6.626×10-34 Js),
c is the speed of light (2.998×108 m/s) and λ is the
wavelength of light. These calculated band gap
energies are consistent with data reported in the
literature for other forms of titania materials.30
anatase (A)
350
rutile (R)
400
450
500
Wavelength (nm)
FIGURE 6. UV absorption spectra of anatase (A), and rutile (R)
titania nanofibers.
Based on our data, anatase titania has a band gap of
3.2 eV, which corresponds to approximately 385 nm
and rutile titania has band gap energy of 3.0 eV,
which corresponds to approximately 420 nm. These
band gap energies were calculated using the
relationship of photon energy and frequency (c/λ):
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54
Nanofibers offer surface areas (per unit volume and
mass) that are larger than bulk materials and
approaching those of nanoparticles. One benefit over
the latter, however, is the macroscopic handling
capability of nanofiber mats which is not possible
with nanoparticles. Thus, even though our materials
may not exhibit surface areas and defect densities as
large as nanoparticles having diameters less than 100
nm, we can employ these materials for catalysis and
surface chemical reaction and sensor applications
where nanoparticles are not as cost effective. For
example, nanoparticles may be used for the chemical
regeneration of used motor oils by surface anchoring
of sulfate species thus removing them from the oil however the particles may themselves not be
recovered and are therefore not recyclable.
Nanofiber mats, or filters constructed thereof, may be
able to perform the same surface chemistry as the
nanoparticles, but offer the additional possibility of
being cleaned and reused since they can be handled
macroscopically. Controlling the titania nanofibers'
crystalline phases to exhibit different optical and
electronic properties could provide for the fabrication
of advanced materials with significant industrial uses.
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CONCLUSIONS
Titania nanofibers with diameters of 200 nm were
synthesized by sol-gel coating of electrospun nylon
nanofibers followed by calcination. The XRD results
show that the crystalline phases were either anatase
or rutile forms of titania depending on the heating
method. Photoelectron emission data demonstrate
that while the crystal structures of titania affected the
electronic structure of the valence band region, it did
not affect the core level structure, such as Ti 2p3/2 and
Ti 2p1/2. Optical absorption spectra exhibited band
gap differences as expected for these materials.
[8] Matsui, T., Harada, M., Ichihashi, Y. et al., Effect
of noble metal particle size on the sulfur tolerance of
monometallic Pd and Pt catalysts supported on highsilica USY zeolite, Appl. Catal. A 286 (2005) 249257.
[9] Sekulic, J., Ten, E.J.E., Blank, D.H.A., Synthesis
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ACKNOWLEDGMENTS
We thank the Coalescence Filtration Nanomaterials
Consortium
(Ahlstrom,
Cummins
Filtration,
Donaldson Filtration Solutions, Hollingsworth &
Vose Company, and Parker filtration) for financial
support. We acknowledge E.T. Bender and P. Katta
for measuring the DRIFT spectra and assistance in
sol-gel coating of the nanofibers, respectively.
[11] Onozuka, K., Ding, B., Tsuge, Y. et al.,
Electrospinning
processed nanofibrous TiO2
membranes
for
photovoltaic
applications,
Nanotechnology 17 (2006) 1026-1031.
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Soo-Jin Park
Yong C. Kang
Ju Y. Park
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Rex D. Ramsier
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University of Akron
Chemical Engineering
185 E. Mill Street
Whitby Hall 411A
Akron, OH 44325-3906
UNITED STATES
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