Synthesis and Photocatalytic Performance of TiO2 Nanofibers
Consisted of a Mixture of Anatase/Rutile Crystalline
Ming-Chung Wu* and Pei-Huan Lee
Department of Chemical and Materials Engineering, College of Engineering, Chang
Gung University
Address corresponding to [email protected]
ABSTRACT
Photocatalytic processes can be used to address several aspects of modern
renewable energy production and management of environmental pollution.
Titanium dioxide (TiO2) is probably the most promising photocatalyst being
environmentally friendly, with low cost, good photocatalytic activity and excellent
photostability as demonstrated in electrochemical photolysis of water to produce
hydrogen, and in abatement of volatile organic compounds from air. In this study,
TiO2 nanofibers were synthesized by hydrothermal method from TiO 2 powders in
alkaline solutions and then forming TiO2 nanofibers by simple thermal annealing in
air. In order to prepare the catalytically active TiO2 nanofibers from the sodium
hydrogen titanate nanofibers, a quick screening was carried out to find the
appropriate calcination temperature. The samples were heated in air at 250, 400,
550, 600, 700, 850 and 1000 °C for 4 hours then analyzed by X-ray diffraction and
Raman spectrometer. The desired phase of anatase and rutile mixture is found to be
calcinated at 850 oC for 4 hours. This photodegradation process of the TiO2 catalyzed
organic dyes follows the Langmuir-Hinshelwood model. The kinetics is of first-order
and can be described as ln(C0/C) = kt, where C is the concentration of the dye at time
1
t; Co denotes the initial concentration and k is the apparent reaction rate constant.
The reaction rate constants of the calcined TiO2 nanofibres can be calculated from
the decolouration rate of the brilliant green added to be ~1.96x10-3 s-1, higher than
that of the commercial Degussa P25 TiO2 nanoparticle, 1.75x10-3 s-1. As a result, we
may assume that the obtained TiO2 nanofibers consisted of a mixture of
anatase/rutile crystalline can be a reasonable alternative of the traditional Degussa
P25 for photocatalytic applications.
Keywords : TiO2, photocatalyst, nanofibers, anatase
INTRODUCTION
In recent years, energy and environmental issues are important topics around
the world.[1,2] Photocatalytic processes can be used to address several aspects of
modern renewable energy production, such as efficient solar cell electrodes,
photocatalytic, hydrogen production by photocatalytic water splitting and
management of environmental pollution. Up to now, titanium dioxide (TiO2) have
been the subject of intensive research as the most promising photocatalyst because
of good photocatalytic activity and excellent photostability at the same time being
environmentally friendly and low cost.[3-7] The alkaline hydrothermal synthesis has
opened new possibilities for large scale and simple production of various types of
titanate nanostructures such as nanoparticles, nanofibers and nanotubes. [8-12] These
titanates can be used as the starting materials for the synthesis of nanostructured
highly photoactive TiO2 -based materials by a simple thermal annealing procedure.
TiO2 is a wide-band-gap semiconducting material with several natural crystalline
phases, such as anatase, rutile, brookite, etc. According to previous studies, the
anatase based photocatalysts offer the most viable alternative for degradation of
organic contaminants in both water and air.[13-16] However, in this study, we
observed TiO2 nanofibers consisted of a mixture phase of anatase/rutile crystalline
2
showing better photocatalytic activity than pure phase of anatase crystalline. TiO2
nanofibers were synthesized by hydrothermal method from TiO2 powders in alkaline
solutions and of the forming TiO2 nanofibers by simple thermal annealing in air. In
order to prepare the various catalytically active TiO2 nanofibers from the sodium
hydrogen titanate nanofibers, a quick screening was carried out to find the
appropriate calcination temperature. The obtained TiO2 nanofibers consisted of a
mixture of anatase/rutile crystalline might be a reasonable alternative of the
traditional Degussa P25 for photocatalytic applications.
EXPERIMENTAL DETAILS
For the preparation of sodium hydrogen titanate nanofibers, we suspend 2.5 g
TiO2 anatase powder in 10.0 M NaOH aqueous solution of 62.5 mL, followed by a
treatment in a teflon-lined autoclave at 150 oC for 24 hours, applying revolving
around its short axis. The product, i.e., sodium titanate nanofibers was then washed
in 0.10 M HCl to exchange sodium ions to protons. The neutralized product was
washed with deionized water and finally filtered and dried in air at 70 oC.
In order to prepare the different phase of TiO2 nanofibers from sodium hydrogen
titanate nanofibers, a quick screening for finding the optimum calcination
temperature was carried out (calcination in air at 250, 400, 550, 600, 700, 850 and
1000 oC for 4 hours). The heat-treated nanofibers were tested by with UV-B light
induced photodegradation of brilliant green in aqueous solutions. The various
samples were suspended in the dye solutions and after 75 min UV-B irradiation
(Sankyo Denki, G8T5E, 8 W × 6 sets). The decoloration of the dye in the dispersion
was evaluated by UV-vis absorption spectrometer.
The comparison between photocatalytic activity of TiO2 anatase/rutile
nanofibers and commercial TiO2-based photocatalyst (Degussa P25) was performed
by monitoring the decoloration of brilliant green. Degussa P25 is a common standard
for TiO2-based photocatalyst materials comparison. It contains anatase and rutile
nanoparticles in a ratio of about 3:1 having size of 85 nm and 25 nm, respectively. In
a typical experiment, 20.0 mg of catalyst was sonicated for 5 min in 150 mL brilliant
green aqueous solution (20 mg/L). The suspension was irradiated with UV-B light
(Sankyo Denki, G8T5E, 8 W X 6 sets) under vigorous stirring at ambient conditions.
3
After a centrifuging process (for 15 min at 3500 rpm), the UV-Vis spectrum of the
remained brilliant green and its derivatives in the supernatant was recorded in the
spectral range from 400 nm to 900 nm (Jasco Analytical Instruments, V-650 UV-Vis
Spectrophotometer). The brilliant green concentration was calculated from the
absorbance at λ= 624 nm extrapolated to a previously plotted calibration curve.
The microstructure and morphology of TiO2 nanofibers were studied by scanning
electron microscopy (SEC SNE-4500M MiniSEM). The crystalline of TiO2 nanofibers
were studied by XRD operating at 100 keV and 300 mA (Rigaku TTRAX 3). In order to
obtain the Raman scattering spectra, the sodium hydrogen titanate nanofibers
calcined at different temperatures were positioned on a high-resolution piezoelectric
stage of the scanning microscopy (WITec, Alpha300 S) and excited by a He-Ne laser
of 632.8 nm. The laser beam was focused with a 100 X objective lens (Nikon plane
objective, NA ˜0.9), and the diameter of the laser beam focus was about 1 µm.
RESULTS AND DISCUSSION
The microstructure of the sodium hydrogen titanate nanofibers synthesized by
the alkaline hydrothermal method is shown in Figure 1. The nanofibers are having
length of up to a few micrometers and diameter of 50-300 nm.
Fig. 1 SEM image of sodium hydrogen titanate nanofibers.
4
X-ray diffraction patterns of the thermally treated sodium hydrogen titanate
nanofibers (Fig. 2(a)) show a phase transformation occurred. The results show that
TiO2 nanofibers calcined at 850 oC for 4 hours start generating rutile-phase as shown
by the appearance of (110) reflection at ~27.2o. Although TiO2 nanofibers with
rutile-phase might lead to relative lower photocatalytic activity, the crystallinity of
anatase-phase at 850 oC is still better than others. We also calcined samples at 850
o
C for different time durations. As shown by Fig. 2(b). The results present that with
increasing time the rutile phase grows up and anatase phase decreases. So we can
assume TiO2 NFs calcined at 850 oC for 4 hours shows the highest photocatalytic
activity.
The anatase and rutile phases of TiO2 can be sensitively identified by Raman
spectroscopy. The anatase phase shows major Raman bands at 144, 197, 399, 515,
519 and 639 cm-1. These bands can be attributed to the six Raman-active modes of
anatase phase with the symmetries of Eg , Eg , B1g , A1g , B1g , and Eg , respectively.
The typical Raman bands due to rutile phase appear at 143 (superimposed with the
144 cm-1 band due to anatase phase), 235, 447, and 612 cm-1 , which can be ascribed
to the B1g , two-phonon scattering, Eg , and A1g modes of rutile phase, respectively[17].
Obviously, the results of Raman scattering show that at 400 oC present an unknown
mix-phase. At 550 oC the phases transform to anatase-phase and up to 850 oC the
rutile phase start generating. This result is similar to the results of XRD. As shown by
Fig 3(a). And at 1000 oC, it shows rutile phase major Raman band at 447 and 612
cm-1 obviously. Fig 3(b) shows that with calcination time increasing, the rutile phase
major band at 447 cm-1 starts growing up.
5
Fig 2. (a) X-ray diffraction patterns of sodium hydrogen titanate nanofibers calcined
in air at 250, 400, 550, 600, 700, 850 and 1000 oC for 4 hours (heating rate of 5
o
C/min). (b) Sodium hydrogen titanate nanofibers calcined at 850 oC for 4 hours, 8
hours and 12hours (heating rate of 5 oC/min).
6
Fig 3 (a) Raman spectra of TiO2 calcined at different temperature, room temperature,
400, 550, 600, 700, 850 and 1000 oC, for 4 hours. (b) Raman spectra of TiO2 calcined
at 850 oC for 4, 8 and 12 hours.
7
The photocatalytic activities of the synthesized TiO2 NFs were tested with UV-B
light-induced photodegradation of brilliant green in aqueous solution. UV-Vis spectra
of brilliant green as function of UV light irradiation time were recorded (Fig 4(c)); and
from the absorbance measured at λ= 624 nm, the corresponding dye concentration
could be calculated using a calibration curve measured previously. In the
photocatalytic screening experiments (visual observation of brilliant green
decoloration after 75 min UV irradiation), the low temperature annealed (400 oC)
and high temperature annealed (1000 oC) sodium hydrogen titanate fibers showed
poor activity. The overall order of photocatalytic activity results are 850 oC > 700 oC >
600 oC > 400 oC ~1000 oC as shown by Fig 4(a),indicating that the nanofibers treated
at 850 oC consisting of anatase phase with a little rutile phase showing the best
photocatalytic activity. This mixture crystalline phase of TiO2 nanostructure is very
promising for photocatalytic applications. TiO2-catalyzed photodegradation of
different dyes essentially follow Langmuir-Hinshelwood kinetics,[18] which can be
simplified to an apparent first-order kinetics at lower initial dye concentrations,
mathematically described as ln(C0/C) = kt; where C is the concentration of the dye at
time t, C 0 is the initial concentration and k is the apparent reaction rate constant.
Plotting the logarithm of the reciprocal of the measured dye concentrations as a
function of time, we have obtained linear slopes for each catalyst studied and they
are in good agreement with the Langmuir–Hinshelwood model. TiO2 NFs at 850 oC
for 4h show better activity than commercial TiO2 (Degussa P25). As shown by Fig. 4(b)
The brilliant green decoloration over both the TiO2 nanofibers calcined at 850 °C for
4 hours and the commercial Degussa P25 gives the fastest decoloration
phenomenon with calculated rate constants of ∼1.96
10-3 s−1 and 1.75
10-3 s−1,
respectively. Consequently, we may assume that the obtained TiO2 nanofibers
consisted of a mixture of anatase/rutile crystalline can be a reasonable alternative of
the traditional Degussa P25 for photocatalytic applications.
8
Fig 4. (a) Photocatalytic activities of various materials over the photodegradation of
brilliant green under UV-B irradiation. (b) Linearized kinetic plots for the degradation
of brilliant green using different types of TiO2 catalyst materials. (c) UV-Vis
absorbance spectra of brilliant green (initial concentration of 20.0 mg/L) as a
function of illumination time with the TiO2 NFs at 850 oC catalyst (20.0 mg dispersed
in 150 mL of solution).
9
CONCLUSION
Sodium hydrogen titanate nanofibers calcined at 850 oC for 4 hours leads to the
formation of TiO2 nanofibers consisted of a mixture of anatase/rutile crystalline. The
brilliant green decoloration over both the TiO2 nanofibers calcined at 850 °C for 4
hours and the commercial Degussa P25 shows better reaction rates compared with
other TiO2 nanostructure samples and have calculated rate constants of ∼1.96
10-3 s−1 and 1.75
10-3 s−1, respectively. In this study, TiO2 nanofibers consisted of a
mixture of anatase/rutile crystalline can be a reasonable alternative of the
traditional Degussa P25 for photocatalytic applications.
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