Efficient mineralization of toluene by W-doped TiO2 nanofibers under visible light irradiation

Article abstract of DOI:10.1166/jnn.2015.9638

Toxic toluene gas caused enormous harm to human health, and the traditional method to deal with this puzzle is using physical adsorption, which just transfer the toluene from one medium to another. Photocatalysis has great potential to mineralize toluene into CO2 under visible light irradiation, but their applications have been limited by difficulties in preparing efficient photocatalysts with fine crystallite size, considerable visible light response, and large surface area to contact with toluene gas. To address this problem, we have developed a film composed of W-doped TiO2 nanofibers to mineralize toluene under visible light irradiation. The electrospinning preparation route allows incorporation of up to 50 wt% of W in substitutional positions of titanium atom in the anatase network. The W-doped TiO2 nanofibers behave finer crystallite size, stronger visible light absorbance, and larger surface area comparing with pure TiO2 nanofibers. The nanofiber structured morphology on the quartz tube promotes the reaction rates for the gas-phase photo-oxidation of toluene. The concentrations of the produced CO2 keep steady during the photodegradation process, indicating the practicality and operability for the whole experiment. This research is conducive to the development of novel photocatalytic materials to efficiently mineralize toxic gas pollutants including toluene for practical application.

Full text of DOI:10.1166/jnn.2015.9638

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 2944–2951, 2015
Copyright © 2015 American Scientific Publishers
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Printed in the United States of America
Efficient Mineralization of Toluene by W-Doped TiO2
Nanofibers Under Visible Light Irradiation
Li Zhang1, Yaogang Li2, Hongyong Xie3, Hongzhi Wang1ꢀ∗, and Qinghong Zhang2ꢀ∗
1State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
College of Materials Science and Engineering, Donghua University,
Shanghai 201620, China
2Engineering Research Center of Advanced Glasses Manufacturing Technology, MOE,
College of Materials Science and Engineering, Donghua University,
Shanghai 201620, China
3School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic University,
Shanghai 201209, China
Toxic toluene gas caused enormous harm to human health, and the traditional method to deal with
this puzzle is using physical adsorption, which just transfer the toluene from one medium to another.
Photocatalysis has great potential to mineralize toluene into CO2 under visible light irradiation, but
their applications have been limited by difficulties in preparing efficient photocatalysts with fine
crystallite size, considerable visible light response, and large surface area to contact with toluene
Delivered by Publishing Technology to: University of New South Wales
gas. To address this problem, we have developed a film composed of W-doped TiO2 nanofibers to
IP: 206.214.8.126 On: Mon, 11 Apr 2016 04:06:16
mineralize toluene under visible light irradiation. The electrospinning preparation route allows incor-
Copyright: American Scientific Publishers
poration of up to 50 wt% of W in substitutional positions of titanium atom in the anatase network.
The W-doped TiO2 nanofibers behave finer crystallite size, stronger visible light absorbance, and
larger surface area comparing with pure TiO2 nanofibers. The nanofiber structured morphology on
the quartz tube promotes the reaction rates for the gas-phase photo-oxidation of toluene. The con-
centrations of the produced CO2 keep steady during the photodegradation process, indicating the
practicality and operability for the whole experiment. This research is conducive to the development
of novel photocatalytic materials to efficiently mineralize toxic gas pollutants including toluene for
practical application.
Keywords: Photocatalyst, Visible Light Photocatalysis, Nanofibers, Toluene, TiO2.
1. INTRODUCTION
with enhanced photocatalytic efficiency, which completely
mineralizes the organic pollutants.
Toluene is a common organic solvent which is widely
used in both industrial and domestic activities, such as
in preparing aviation gasoline,1 dyes,2 adhesives,3 and
paints.4 Repeatedly breathing air contaminated by toluene
over long periods of time can cause death, permanent
brain damage, or depression.5 Accordingly, there is a
growing interest in developing good air purification meth-
ods for the removal of volatile toluene from indoor air.
Physical adsorption, using charcoal or activated carbon,
has been investigated to remove the toluene, but this
method just transfer the toluene from one medium to
another.6 Consequently, it is crucial to develop environ-
mentally benign routes combining effective adsorption
Photocatalytic oxidation that mostly using titania (TiO2)
based materials as the catalyst has attracted substantial
attention in recent years, as it is cost-effective, high cat-
alytic stability and can be carried out at room temperature
and atmospheric pressure.7ꢀ8 However, its practical appli-
cation has been limited because of its large energy band
gap (3.2 eV) that requires UV light to excite the photo-
catalytic reactions.9 In view of better utilization of solar
light, in nowadays, doping with other elements in TiO2
has drawn a great deal of attentions to extend the spectral
response from the UV range into the visible region. For
instance, Asahi et al. found that N doped TiO2 was visible
light active due to the interaction of N2p and O2p orbitals,
which can lead to the up-shifting of TiO2 valence band.10
∗Authors to whom correspondence should be addressed.
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1533-4880/2015/15/2944/008
doi:10.1166/jnn.2015.9638

Zhang et al.
Efficient Mineralization of Toluene by W-Doped TiO2 Nanofibers Under Visible Light Irradiation
Xie et al. have synthesized C doped TiO2 nanoparticles and
studied their photocatalytic activity on toluene.11 Fuerte
et al. have explored the doping of the anatase structure
using high concentrations of nine different dopants and
showed that W was one of the best options for toluene pho-
todegradation using sunlight-type excitation.12 W doping
anatase structure appears to potentially affect photoactiv-
ity by three different factors, namely the increasing visible
light response, the presence of surface W centers enhanc-
ing oxygen activation, and finally, the creation of new
surface centers with modified interaction with toluene.13
In these studies, though anionic or cationic doping TiO2
systems showed good photocatalytic properties, they are
usually in the form of agglomerated particles, so the gas
molecules had a relative low rate to diffuse into the interior
parts. Thin porous films composed of nanofibers provide a
new way to solve this problem. Compared with nanopar-
ticles, nanofibers have more advantages on photodegra-
dation of volatile toluene, because they take advantage
of special characteristics such as large surface area-to-
volume ratio, 3 dimensional (3-D) open structure and ease
of fabrication.14ꢀ15 However, to the best of our knowledge,
there was no report on efficient mineralization of toluene
using porous films composed of W-doped TiO2 nanofibers
under visible light irradiation.
followed by loading into a 10 mL plastic syringe with a
21 gauge stainless steel needle at the tip. The needle was
electrified by a 13 kV high-voltage DC supply. The solu-
tion was pumped continuously by a syringe pump (Model
22, Harvard Apparatus, USA) with a rate of 2.5 mL/h.
A grounded wire netting was used as electrode to collect
the nanofibers. The as-spun fibers were calcined at 500 ꢀC
for 2 h to burn out the organic compounds and obtain
inorganic W-doped TiO2 fibers. Five samples of W-doped
TiO2 nanofibers with different W weight percentage were
fabricated, and were denoted as TW0, TW10, TW20,
TW30 and TW50, respectively (see Table I). The digits
represent the W/(W +Ti) ratio in weight.
2.2. Sample Characterization
The morphology of the samples were characterized with
a field emission scanning electron microscope (FESEM,
Hitachi S-4800) operated at an accelerating voltage of
20 kV. TEM images were obtained using a JEM-2100
F TEM (JEOL Tokyo Japan) operating at 200 kV
equipped with a LINK probe for energy dispersive X-ray
(EDX) spectroscopy analysis. X-ray diffraction (XRD)
was carried out on a Rigaku-D/max 2550 PC (Japan)
diffractometer with Cu Kꢃ radiation (ꢄ = 1ꢂ5406 Å).
X-ray photoelectron spectroscopy (XPS) measurements
were carried out with an SECA Lab 220i-XL spectrom-
eter by using an unmonochromated Al Kꢃ (1486.6 eV)
In this work, we prepared porous films composed of
W-doped TiO2 nanofibers with different doping concentra-
Delivered by Publishing Technology to: University of New South Wales
tions and studied the influence of W-doping level on the
X-ray source. All the spectra were calibrated to the bind-
IP: 206.214.8.126 On: Mon, 11 Apr 2016 04:06:16
photomineralization of toluene. The nanofibers were pre-
pared by electrospinning method allowing doping of up to
ca. 50 wt% of W in substitutional positions of the anatase
structure. The influences of the crystallite size, the crystal-
lographic phases, the surface area, and the chemical states
on the activity of the photocatalysts were also discussed.
Copyright: American Scientific Publishers
ing energy of the adventitious C 1s peak at 284.6 eV. The
porous properties of samples were investigated using phys-
ical adsorption of N2 at liquid-nitrogen temperature on
an automatic volumetric sorption analyzer Autosorb-1 MP
(Quantachrom SI, USA). UV-vis diffuse reflectance spec-
troscopy (DRS) spectra were recorded on a PE Lambda
950 instrument, using BaSO4 as the reference sample.
2. EXPERIMENTAL DETAILS
2.1. Sample Preparation
2.3. Photoreactivity Measurements
The experimental set-up and photoreactor used for gas
phase photo-oxidation of toluene is designed according the
method previously reported.11 The photoreactor was made
of a quartz tube, a fluorescent lamp and light shield. The
quartz tube had an inner diameter of 80 mm, and an effec-
tive length of 1200 mm. The catalyst was painted onto the
inner wall of the quartz tube from a concentrated suspen-
sion of the TiO2 nanofibers. After a day of ambient drying,
The TiO2 nanofibers were typically prepared by elec-
trospinning
a solution containing 4.0 g tetrabutyl
titanate (Ti(OC4H9ꢁ4, Shanghai Chemical Reagent Com-
pany) mixed with tungsten hexachloride (WCl6, 99.99%,
Aldrich), 4.0 mL acetic acid (glacial, Shanghai Chemical
Reagent Company), 1.4 g poly(vinyl pyrrolidone) (PVP)
(Mw ≈ 1ꢂ3 × 106, Aldrich) and 13 mL ethanol. The solu-
tion was ultrasonicated for 2 h to ensure the homogeneity
Table I. W percentage, BET surface area, crystallite size, mean pore diameter, pore volume and mineralization degree of the W-doped TiO2 nanofibers.
W percentage
(W/(W +Ti), wt%)
Surface area
Crystallite
size (nm)
Pore diameter
(nm)
Pore volume
(cm3 g−1
)
Mineralization
degree (%)
Sample
(m2 g−1
)
TW0
0
46ꢂ0
61ꢂ8
79ꢂ2
84ꢂ5
104ꢂ8
26ꢂ0
13ꢂ9
10ꢂ5
10ꢂ4
10ꢂ2
6ꢂ911
3ꢂ621
3ꢂ578
3ꢂ465
3ꢂ235
0ꢂ153
0ꢂ076
0ꢂ074
0ꢂ059
0ꢂ043
20ꢂ5
45ꢂ8
76ꢂ6
58ꢂ5
37ꢂ4
TW10
TW20
TW30
TW50
10
20
30
50
J. Nanosci. Nanotechnol. 15, 2944–2951, 2015
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Efficient Mineralization of Toluene by W-Doped TiO2 Nanofibers Under Visible Light Irradiation
Zhang et al.
the catalyst formed a uniform coating that exhibited good
adhesion on the quartz support. After three times of the
operation, the thickness of thin film composed of TiO2
nanofibers was ca. 300 nm. A fluorescent lamp (Philips
TL-D 36W/54-765), 1200 mm long, was placed in the
center of the quartz tube to give 12 mW/cm2 fluorescent
irradiation with the spectrum in the visible region. The
residence time of the air stream in the photoreactor was
about 5.7 s with the relative humidity of 80% and initial
toluene concentration of 6–300 mg/m3. The air stream was
sampled every 30 min at both the inlet and outlet using
Tenax tubes at a flow rate of 0.5 L/min for 1–5 min corre-
sponding to the air flow rate via the toluene generator from
0.02 to 0.6 L/min. Reaction rates were evaluated under
steady state conditions, typically achieved after 3–4 h from
the beginning of irradiation. The toluene in the tube was
refilled every 3 h to make the toluene concentration in a
constant value. The toluene concentration of the air stream
was analyzed by gas chromatography/mass spectrometry
(GC/MS, Shimadzu QP2010 plus).
3. RESULTS AND DISCUSSION
SEM images of W-doped TiO2 nanofibers after calcin-
ꢀ
ing at 500 C for 2 h were shown in Figures 1(a)–(e).
All these composite fibers have similar one-dimensional
texture structure and are randomly oriented forming non-
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woven mats. Each sample has a narrow diameter distribu-
IP: 206.214.8.126 On: Mon, 11 Apr 2016 04:06:16
tion. The diameters of nanofibers decreased from 100 nm
Copyright: American Scientific Publishers
to 50 nm with increasing W content in the W-doped
TiO2 from 0 to 20 wt%, while the diameters of TW30
and TW50 are 80 nm and 90 nm. The diameters of
TiO2 and WO3 nanofibers prepared by electrospinning
method were reported to be ca. 200 nm and 100–500 nm,
respectively.16ꢀ17 The results indicate that adding differ-
ent contents of WCl6 into the electrospinning solution
would change the physical and chemical properties of the
spinning solution which has a great impact on the spin-
ning process. In contrast to previous reports about single-
component titanium or tungstate salts, the preparations
of the W-doped TiO2 nanofibers started from the sol–
gel process of the tetrabutyl titanate and tungsten hex-
achloride together. The tetrabutyl titanate was hydrolyzed
for 2 h and then tungsten hexachloride were added into
Figure 1. SEM images of (a) TW0, (b) TW10, (c) TW20, (d) TW30,
and (e) TW50. (f) Digital photo of TW0 and TW50.
from white (TW0) to pale yellow (TW50), indicating the
visible photoactivity of the doped nanofibers. TEM image
of TW20 (Fig. 2(a)) confirms the one-dimensional texture
structure of the nanofibers with the diameter of 50 nm.
The nanofibers are porous, as shown in Figure 2(b) of a
single fiber, indicating a large surface area for adsorption
of toluene. As expected, the specific surface area and pore
volume of TW20 are 79.2 m2 g−1 and 0.074 cm3 g−1. As
shown in HRTEM image (Fig. 2(c)), TiO2 nanofibers with
the fringe spacing of 0.35 nm corresponding to (1 0 1)
facets of anatase are observed.19 The inset of Figure 2(c)
shows the selected area electron diffraction pattern of
TW20, and the diffraction rings indicate the polycrys-
talline nature of the nanofibers.20 A dark-field TEM image
with corresponding EDX elemental mapping of the same
nanofiber region indicates spatial distribution of O (blue),
W (green), and Ti (red), as shown in Figure 2(d).
the system. During the process, Ti
and W W bonds were formed and stabilized by
acetylacetone.18 The increasing of Ti
W bonds in
O
Ti, Ti
O
W
O
O
the solution leaded to a low conductivity fluids, which
was beneficial to the formation of thinner nanofibers.
However, in the spinning solutions of TW30 and TW50,
too much W
ing the concentration of Ti
ters of TW30 and TW50 increased compared with TW20.
Figure 1(f) shows the digital photo of TW0 and TW50.
With increasing W content in the spinning solution, the
obtained W-doped nanofibers gradually deepened in color
O
W bonds were formed thus decreas-
W bonds, so the diame-
Figure 3(a) shows the XRD patterns of TW0, TW10,
TW20, TW30 and TW50 after calcinations at 500 ꢀC. The
diffraction peaks of the W-doped TiO2 nanofibers can be
indexed to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0
4) and (2 1 5) crystal planes of TiO2 (JCPDS File No. 21-
1272). All the peaks of the W-doped TiO2 nanofibers can
O
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Zhang et al.
Efficient Mineralization of Toluene by W-Doped TiO2 Nanofibers Under Visible Light Irradiation
Figure 2. (a) TEM, (b) HR-TEM, (c) HR-TEM with the inset of the
SAED patterns, and (d) EDX images of TW20.
be assigned to the anatase phase and no peaks for WO3
were observed. The crystallites sizes are 15.6, 13.9, 10.5,
10.4 and 10.2 nm for TW0, TW10, TW20, TW30 and
TW50, respectively, calculated from X-ray line broadening
(ꢀ) according to the Scherrer formula:
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d = 0ꢁ89ꢂ/ꢀcosꢃ
where ꢂ is the X-ray wavelength, 1.5406 Å (CuKꢄ), and
(1)
IP: 206.214.8.126 On: Mon, 11 Apr 2016 04:06:16
Copyright: American Scientific Publishers
ꢃ is the diffraction angle. The intensity of diffraction peaks
decreased with increasing W content in the composites,
which also suggested that the finer crystallite size of TiO2
were formed in the presence of W-doping TiO2 nanofibers.
The diffraction peaks of TW20 moved to the right by
0.5 degrees compared with the expected peak positions of
the anatase TiO2 (JCPDS File No. 21-1272), as shown in
Figure 3(b). It can be attributed to doping of W6+ into the
TiO2 crystal lattice, as the effective ionic radii of W6+ and
Ti4+ are 0.058 nm and 0.061 nm, respectively.21
Figure 3. (a) XRD patterns of the W-doped TiO2 nanofibers. (b) XRD
pattern of TW20, with the vertical lines showing the standard peak posi-
tions of anatase TiO2.
anatase, and 3.0 eV for rutile). All W-doped TiO2 sam-
ples exhibited spectral response in the visible region (from
400 to 700 nm), and the absorbance became stronger with
increasing of W content from 0 wt% to 50 wt% because of
narrower band gap induced by the energy state of W6+ 5d
levels lying bellow the conduction band of TiO2.24 In pure
anatase, the absorption is associated with the O2−–Ti4+
charge transfer corresponding to electronic excitation from
the valence band (O 2p character) to the conduction band
(Ti 3d character),25 while in the W-doped TiO2 samples the
presence of donor levels (W6+ 5d levels) made the conduc-
tion band broader, thus decreasing the band gap between
the conduction band and the valence band, as shown in
Scheme 1.
Figure 6 shows the nitrogen adsorption/desorption
isotherm curves of pure TiO2 and W-doped TiO2 nano-
fibers. All the samples show similar type-IV isotherms,
indicating the presence of mesopores in the nanofibers.26
The specific surface area of TW0, TW10, TW20, TW30
and TW50 are 46.0, 61.8, 79.2, 84.5 and 104.8 m2/g,
Figure 4 shows the high-resolution Ti 2p and W 4f XPS
spectra of TW20. As shown in Figure 4(a), there are two
peaks in the Ti 2p region. The peak located at 464.5 eV
corresponds to the Ti 2p1/2 and another one located at
22
458.8 eV is assigned to Ti 2p3/2
.
The splitting between
Ti 2p1/2 and Ti 2p3/2 is 5.7 eV, indicating a normal state of
Ti4+ in TW20. In Figure 4(b), the W 4f7/2 peak is located
at 35.5 eV and the W 4f5/2 peak is found at 37.4 eV. The
splitting of the 4f doublet of W is 1.9 eV, indicating that
the valence state of W doped in TiO2 nanofibers is +6.23
The optical absorption property, which is relevant to
the electronic structure feature, is considered as a key
factor in determining photocatalytic behavior. Figure 5
presents the DRS spectra of the as-prepared samples. Pure
TiO2 nanofibers displayed no obvious absorbance in vis-
ible light region due to its large band gap (3.2 eV for
J. Nanosci. Nanotechnol. 15, 2944–2951, 2015
2947

Efficient Mineralization of Toluene by W-Doped TiO2 Nanofibers Under Visible Light Irradiation
Zhang et al.
Scheme 1. Proposed mechanism of photocatalytic reaction on W-doped
TiO2 photocatalyst.
amount of pores increased even though the pores have
smaller sizes. As shown in Figure 6, the volume adsorption
increased and the hysteresis loop became narrower with
increasing W content in the nanofibers. The results con-
firmed the appearance of large amounts of finer pores in
the nanofibers.27 The mean pore diameters of TW0, TW10,
TW20, TW30 and TW50 are 6.91, 3.62, 3.58, 3.47 and
3.24 nm, respectively, according to the calculated pore size
distribution curves based on BJH theory from the desorp-
tion branch of the isotherm. The porosity of the nanofibers
is beneficial to the adsorption of gas molecules.
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Figure 7 shows steady state reaction rates for the gas-
IP: 206.214.8.126 On: Mon, 11 Apr 2016 04:06:16
phase photo-oxidation of toluene by the W-doped TiO
Copyright: American Scientific Publishers
2
nanofibers with different W contents under artificial vis-
ible light irradiation taking into account the different
BET surface area of the samples. For comparison, the
photocatalytic result of the commercial reference TiO2
Degussa P25 is also showed. The reaction rates of TW0,
TW10, TW20, TW30, TW50 and P25 under visible light
irradiation are calculated to be of 1ꢀ02 × 10−8, 2ꢀ59 ×
10−8, 5ꢀ12 × 10−8, 2ꢀ98 × 10−8, 1ꢀ98 × 10−8, and 1ꢀ25 ×
10−8 mol s−1m−2, respectively, which are much higher
Figure 4. X-ray photoelectron spectroscopy of (a) Ti 2p and (b) W 4f
of TW20.
while the pore volumes of the four samples are 0.153,
0.076, 0.074, 0.059 and 0.043 cm3/g, respectively (see
Table I). The increased specific surface area and decreased
pore volumes of the samples from TW0 to TW50 indi-
cates that with increasing W content in the nanofibers the
Figure 5. UV-vis diffuse reflectance spectroscopy (DRS) of (a) TW0,
Figure 6. Nitrogen adsorption/desorption isotherm curves of TW0,
(b) TW10, (c) TW20, (d) TW30 and (e) TW50.
TW10, TW20, TW30 and TW50.
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Zhang et al.
Efficient Mineralization of Toluene by W-Doped TiO2 Nanofibers Under Visible Light Irradiation
Scheme 2. Schematic diagrams of TiO2 nanoparticles and nanofibers
dispersed on the quartz tube, with the right region showing corresponding
SEM images. The scale bars inserted are 100 nm.
Figure 7. Toluene photo-oxidation under visible light irradiation over
different photocatalysts (reaction rates, mol s−1 m−2ꢀ.
oxygen molecules (O2) in the air stream, producing super-
oxide anions (O2•−).30 The similar energy of Ti 3d and W
5d levels produces widening of electronic bonds (roughly
similar crystal field splitting for both cations), so a nar-
row band gap is obtained and lower photo energy is
needed to excite the photocatalyst, as shown in Scheme 1.
The formed superoxide anions (O2•−) may either attack
the organic molecules directly or generate hydroxyl rad-
icals (•OH) by reacting with hydrion (H+) and pho-
togenerated electrons.31ꢄ32 Afterward, hydroxyl radicals
(•OH), as strong oxidizing agents, degraded the organic
molecules.33ꢄ34 Since the radii of both ions are very sim-
ilar (0.061 nm for Ti4+, 0.058 nm for W6+ꢀ, one does
than that reported by Christoforidisa’ group.28 W doping
into TiO2 network can be described as:
TiO2
2WO3 −−→ 2WT••i +VTꢀꢀiꢀꢀ +6Oo
(2)
Two positively charges existed in the Ti position of anatase
network after Ti4+ was replaced by W6+, along with the
Ti vacancy appears. Large amounts of structural defects,
both in the bulk or surface of the materials, can act as the
electron capture centers, and thus increase the amounts of
holes for effective photo-oxidation of toluene. The influ-
ence of W6+ in the creation of electronic states in the band
gap and the concomitant decreasing of the photoabsorption
band gap energy onset, increase the reaction rates of the
W-doped TiO2 nanofibers under visible light irradiation.
Scheme 1 shows the energy band diagram and possible
mechanism of photocatalytic reaction on W-doped TiO2
nanofibers. The detailed photocatalytic processes are pro-
posed as follows:
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not expected large length variations in the shorter W
O
IP: 206.214.8.126 On: Mon, 11 Apr 2016 04:06:16
bonds. However, when the W content further increased,
Copyright: American Scientific Publishers
more defects appearing in the bulk of the materials caused
the distortion of the anatase TiO2 network, as confirmed
by the XRD patterns. Moreover, the defects may make
a significant contribution to charge recombination, which
could result in the decreasing of the reaction rates.
The nanofiber structured morphology on the quartz tube
may also promote the reaction rates for the gas-phase
photo-oxidation of toluene. As shown in Scheme 2, the
toluene gas can pass through the loose fibers on the film,
but can hardly pass through the closely packed particles.
The increased contact areas between photocatalysts and
toluene could greatly increase the photo-oxidation rates.
Figure 8 shows the inlet and outlet concentrations versus
time for toluene and CO2 using TW20 as the photocata-
lyst. The initial toluene concentration remained stable with
the average value of 108.9 ppm in the whole experiment.
Only small changes were found for the toluene concen-
tration at the inlet and outlet of the quartz tube in the
dark. After irradiation by the fluorescent lamp, the con-
centration of toluene at the outlet drastically dropped to
11.7 ppm due to the degradation of the toluene. However,
the intermediate products of benzaldehyde and benzoic
acid were easily formed during the toluene mineralization
into CO2 under visible light irradiation. So it is necessary
to detect the CO2 concentration to evaluate mineralization
degree. As shown in Figure 8, the average CO2 concen-
tration before and after irradiation was about 10 ppm and
W-doped TiO2 +hꢁꢂ700 nm>ꢃ>380 nmꢀ→eC−B +hV+B
+
•
H2O+hVB → OH+H+
eC−B +O2 →O•2−
2O2•− +2H+ →O2 +H2O2
O2•− +2H+ +2eC−B →H2O2
−
H2O2 +eC−B → OH+OH
•
•
C6H5CH3 + OH→CO2 +H2O
When the W-doped TiO2 photocatalyst was radiated by
visible light with a photon energy higher than or equal
to the band gap between the top of the O2− 2p band and
the bottom of the Ti4+ 3d band, electrons (e−ꢀ in the 2p
level of O2− were excited into the conduction band of
TiO2, leaving same amount of holes (h+) in the 2p level of
O2−. Holes were then captured by surface hydroxyl groups
(OH−) on the photocatalyst surface to yield hydroxyl rad-
icals (•OH),29 while the electrons were trapped by the
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2949

Efficient Mineralization of Toluene by W-Doped TiO2 Nanofibers Under Visible Light Irradiation
Zhang et al.
surface were formed in the W-doped TiO2 nanofibers, in
favor of adsorption and thorough mineralization of toluene.
The sample with W content 20 wt% exhibited a degrada-
tion rate as high as 5ꢅ12×10−8 mol s−1m−2, 5 times higher
than that of bare TiO2 nanofibers. However, when the W
content reached 50 wt% in the W-doped TiO2 nanofibers,
more defects appeared in the bulk of the material and made
a significant contribution to charge recombination, which
resulted in decreasing of the reaction rates.
Acknowledgments: We gratefully acknowledge the
financial support by Natural Science Foundation of China
(Nos. 51072034, 51172042), the Cultivation Fund of the
Key Scientific and Technical Innovation Project (No.
708039), Specialized Research Fund for the Doctoral Pro-
gram of Higher Education (20110075130001), Science
and Technology Commission of Shanghai Municipality
(12nm0503900), the Program for Professor of Special
Appointment (Eastern Scholar) at Shanghai Institutions of
Higher Learning, and the Program of Introducing Talents
of Discipline to Universities (No. 111-2-04).
Figure 8. The inlet and outlet concentrations versus time for toluene
and CO2 using TW20 as the photocatalyst.
531.28 ppm. The mineralization degree (MD) was calcu-
lated to be 76.6% according to the following formula:
C
−C
ꢀCO2ꢁoutletꢂ
ꢀCO2ꢁinletꢂ
MD =
(3)
7 ×ꢃCꢀtoluene, inletꢂ −Cꢀtoluene, outletꢂ
ꢄ
The mineralization degree was higher than that reported
in the literature,35 in which nanostructured rutile TiO2 was
utilized for photocatalytic oxidation of aromatic alcohols.
The mineralization degrees of TW0, TW10, TW30 and
TW50 were calculated to be 20.5%, 45.8%, 58.5% and
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IP: 206.214.8.126 On: Mon, 11 Apr 2016 04:06:16
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Received: 23 September 2013. Accepted: 12 January 2014.
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