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Author's personal copy
Composites Science and Technology 70 (2010) 1469–1475
Contents lists available at ScienceDirect
Composites Science and Technology
journal homepage: www.elsevier.com/locate/compscitech
Visible-light photocatalytic activity of semiconductor composites supported
by electrospun fiber
Tieshi He a,c, Zhengfa Zhou a, Weibing Xu a,*, Yan Cao b, Zhifeng Shi a, Wei-Ping Pan b,**
a
School of Chemical Engineering, Hefei University of Technology, Hefei 230009, China
Institute for Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green, 42101, USA
c
Liaoning Key Laboratory of Applied Chemistry, Bohai University, Jinzhou 121000, China
b
a r t i c l e
i n f o
Article history:
Received 22 January 2010
Accepted 2 May 2010
Available online 19 May 2010
Keywords:
A. Polymer–matrix composites (PMCs)
B. Synergism
D. Scanning/transmission electron
microscopy (STEM)
D. Thermo-gravimetric analysis (TGA)
E. Electro-spinning
a b s t r a c t
The preparation and photocatalysis of TiO2–ZnS/fluoropolymer fiber composites were investigated. The
fluoropolymer nanofiber mats with carboxyl groups were prepared by electrospinning, and then titanium and zinc ions were introduced onto the fiber surfaces by the coordinating of carboxyl of fluoropolymer in solution. The TiO2–ZnS composites with diameters 15 nm to 1 lm were immobilized on
the surface of fluoropolymer electrospun fiber using hydrothermal synthesis. The Fourier transform
infrared spectroscopy and X-ray photoelectron spectroscopy analysis reveal that some chemical interaction exists between TiO2–ZnS composites and fluoropolymer fibers, so the semiconductor composites
were immobilized tightly on the surface of fluoropolymer fibers. The ultraviolet–visible absorption spectra show the TiO2–ZnS/fluoropolymer fiber composites have low band gap and good visible-light
response ability. The degradation rate of methylene blue in TiO2–ZnS/fluoropolymer fiber composites
system was considerably higher than that of TiO2 or TiO2–ZnS nanoparticles system under visible-light
irradiation, because the TiO2–ZnS/fluoropolymer fiber composites possess good visible-light response
ability, high specific surface areas, and adsorption–migration–photodegradation process. The photocatalytic activity of TiO2–ZnS/fluoropolymer fiber composites changes indistinctively after 10 repeating
photocatalysis tests.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Growing concerns over the threat of chemical warfare agents
and exposure to toxic industrial chemicals have drawn much
attention to the challenge of developing new harmless treatment
methods for the toxic organic materials [1]. Photocatalytic degradation of harmful organic pollutants in the air and the water using
semiconductor particles, such as titanium dioxide (TiO2), is one of
the most widely studied methods [2]. The semiconductor particles
are able to convert abundant solar energy into effective chemical
energy, and mineralized the organic pollutants completely [3].
However, the photocatalytic degradation of toxic organic pollutants using semiconductor is still challenged, in terms of the low
photocatalytic efficiency under natural sunlight, easy agglomerating and losing in the using.
Immobilization of semiconductor particles on the carrier is
one of the best effective methods to prevent the agglomerating
and losing of semiconductor particles in using [4]. The semicon-
* Corresponding author. Tel./fax: +86 551 2901455.
** Corresponding author. Tel.: +1 270 7452221.
E-mail addresses: xwb105105@sina.com (W. Xu), wei-ping.pan@wku.edu (W.-P.
Pan).
0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compscitech.2010.05.001
ductor particles directly depositing onto polymer electrospun fibers are also used to prepare photocatalytic materials [5].
However, polymers usually have troubles in compounding with
inorganic powders, and easy are degraded in the photocatalytic
process [6]. Fluoropolymers like poly(vinylidene difluoride)
(PVDF), which has excellent weather, radiation, chemical and
thermal resistance due to stable –C–F bond in the main chain
[7]. The fluoropolymers electrospun fiber mats with micro-sized
porous structure [8] are able to offer high specific area and good
enrichment ability for organic compounds, are suitable as photocatalyst carrier [9]. The visible-light photocatalytic activity of solitary TiO2 is able to improved greatly by doping it with other
elements, and the synthesis of nanocrystalline TiO2 capped ZnS
under hydrothermal conditions is a convent way [10].
In this paper we demonstrate a novel method to prepare visible-light photocatalytic activity TiO2–ZnS particles loaded by fluoropolymer electrospun fiber with carboxyl groups under
hydrothermal condition. The photocatalytic activity and stability
were investigated through degradation of methylene blue using
TiO2–ZnS/fluoropolymer fiber composites as photocatalyst under
visible-light radiation. The results show the as-prepared composites have good visible-light photocatalytic activity and stability
for the potential applicability in environmental remediation.
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T. He et al. / Composites Science and Technology 70 (2010) 1469–1475
2. Experimental
Trifluoroethyl acrylate (TFA) was obtained from Xuejia Fluorine-silicon Chemical Co., Ltd, Harbin China. Anatase Degussa P25
was purchased from Shanghai Haiyi Scientific & Trading Co., Ltd.
Poly(vinylidene difluoride) (PVDF), titanium oxo-sulphate (TiOSO4), methacrylic acid (MAA), zinc sulfate (ZnSO4), thioacetamide
(TAA), methylene blue, urea and other chemicals were purchased
from Shanghai Chemicals Ltd., and used as received.
Perkin Elmer Spectrum 100 FTIR spectrometer was used to
widely scan the synthetic products. JSM-6700F scanning electron
microscopy (SEM) was utilized to study the surface morphologies
of the products. The specific surface area (BET) analyzed by ASAP
2020 M + C. ESCALAB 250 X-ray photoelectron spectroscopy (XPS)
was used to study the structure of composites. Transmission electron microscope (TEM) image and the selected area electron diffraction (SAED) pattern were taken on JEOL 2010. The crystal
structure was detected through the X-ray diffraction (XRD), Rigaku
D/max-rB. The thermo-gravimetric analysis (TGA), Netzsch TG209-F3, was applied to estimate the weight loss of composites.
Ultraviolet–visible (UV/VIS) absorption spectra were obtained on
a Shimadzu Solidspec-3700 DUV spectrophotometer at room
temperature.
The synthesis of MAA–TFA random copolymers was performed
in an automated reactor system. 30 g MAA, 70 g TFA, and 0.5 g 2,
2-azobisisobutyronitrile (AIBN) were added into a three-necked
flask capacity 250 mL equipped with a condenser, a stirrer and a
N2 inlet. After polymerizing at 80 °C for 1 h, the reaction mixture
was transferred to a stainless steel plate and placed in an oven at
40 °C for 12 h. Then the reaction mixture was maintained at
100 °C for 3 h, so that the remaining monomers can polymerize.
Poly(MAA-co-TFA)/PVDF electrospun fiber mats were prepared
using a typical electrospinning process [11]. 10.3 g PVDF and
1.7 g poly(MAA-co-TFA) were first dissolved in 88 g N,N-dimethylformamide (DMF). The solution was electrospun at 25 kV positive
voltage, 15 cm working distance (the distance between the needle
tip and the target), and 1.0 mL h 1 flow rate. The collection time
was set to 2.0 h. All manipulations were carried out at room
temperature. The electrospun fiber mats of fluoropolymers were
cut into strips of dimension 2.0 cm 2.0 cm for the following
experiments.
The above-mentioned strip of fluoropolymer electrospun fiber
mats were immersed into 10.0 mL, 0.08 mol L 1 aqueous solution
of titanium oxo-sulphate and 1.0 mL concentrated sulfuric acid in
a 50 mL Teflon-lined stainless steel autoclave for 6 h in order to
form the complex of carboxylic of fluoropolymer electrospun fiber
surface and titanium ion. Then 20.0 mL, 0.08 mol L 1 urea and
20 mL distilled water were added. Then 0.0 mL, 0.5 mL, 1.0 mL,
3.0 mL, 5.0 mL, 0.01 mol L 1 ZnSO4 and corresponding 0.02 mol L 1
TAA were added. The reactant content of hydrothermal system was
shown in Table 1. The autoclave was sealed at 150 °C for 8 h, and
then cooled to room temperature. The TiO2–ZnS/fluoropolymer fiber composites were washed for three times with distilled water
under ultrasonic to remove the unreacted precursor and byproducts, and dried in vacuum at 80 °C for 12 h.
Photocatalytic degradation of methylene blue solution was performed by photochemical reactor (SGY-1, Stonetech Co., Ltd. Nanjing, China), light source is 350 W xenon lamp, and reaction system
temperature was 23 ± 1 °C. The TiO2–ZnS/fluoropolymer fiber composites and 300.0 mL 16.0 mg L 1 methylene blue were added to
the quartz tube-500 mL. The TiO2–ZnS/fluoropolymer fiber composites can be extended well in methylene blue solution without
stirring. The Degussa P25 and TiO2–ZnS powders synthesized
according to Stengl et al. methods [10] were performed as stated
in the previous steps with electromagnetic stirring. Prior to irradiation, the photocatalytic reaction system was stirred in a dark
condition for 15 min to establish an adsorption–desorption equilibrium. The photocatalytic reaction system was sampled at regular intervals, and the semiconductor powders suspensions were
centrifuged before measured. The remaining methylene blue concentration after adsorption–desorption equilibrium (C0) and photodegradation (C) was detected by UV/VIS at 665 nm, and the
degradation efficiency be expressed as (C/C0)%.
3. Results and discussion
3.1. Morphology of TiO2–ZnS/fluoropolymer fiber composites
The poly(MAA-co-TFA)/PVDF electrospun fiber mats were made
of random nonwoven mesh of fibers, and had an interconnected
open porous structure, as shown in Fig. 1a. The SEM images of
TiO2–ZnS/fluoropolymer fiber composites prepared by different
proportions for 8 h at 150 °C are compared in Fig. 1b–f and the corresponding Zn content of composites is presented in Table 1. The
size distribution of semiconductor particles was about 5 nm to
1 lm, and the size and agglomeration of semiconductor particles
were improved with the increasing zinc ion contents in the reaction system, as shown in Table 1. The reasons are able to explained
as follows: the sulfide ion was released from TAA at low temperature with high rate [12], but the TiO2 crystal prepared by hydrothermal hydrolysis of titanium oxo-sulphate with urea need
multi-step reaction [13], therefore the generation and growth of
ZnS crystal were faster than that of the TiO2 crystal under the same
reaction system. Without zinc added, the TiO2 crystals formation
and growth were controlled by carboxyl along the surface of electrospun fiber, and the about 5 nm TiO2–fluoropolymer fiber composites were achieved, as shown in Fig. 1b. With the zinc ion
added, the ZnS crystals generated on the fiber surface preceded
TiO2 crystals, and then both semiconductor crystals decomposing
and combining, so the TiO2–ZnS mixed crystals generated on the
fluoropolymer fiber surface. When the zinc ion content of reaction
system is low, the TiO2–ZnS particle size is less than 100 nm because of heterogeneous nucleation effect, as shown in Fig. 1c and
d. With zinc ion content increase, ZnS homogeneous nucleation
plays as a dominant role, plus, the nucleation and growth of ZnS
particles accelerates under hydrothermal conditions, which inhibits instant decomposing and combining of semiconductor particles,
thus semiconductor agglomerations sized over 200 nm were obtained, as shown in Fig. 1e and f.
3.2. Characterization of TiO2–ZnS/fluoropolymer fiber composites
Table 1
Reactants content of hydrothermal system.
Samples
ZnSO4 (10
mol)
TiZn0
TiZn1
TiZn2
TiZn3
TiZn4
0.0
0.5
1.0
3.0
5.0
2
TiOSO4 (10
mol)
80
80
80
80
80
2
EDX of Zn
(wt.%)
Crystallite size
(nm)
0.0
0.24
0.91
4.73
13.06
5
15
100
200
1000
The XRD patterns and the corresponding characteristic 2h values of the diffraction peaks were shown in Fig. 2. It is confirmed
that semiconductor composites as-prepared samples is identified
as anatase-phase (JCPDS card No. 21-1272), ZnS as cubic-phase
(JCPDS card No. 5-566) and the typical PVDF crystal structure
[14]. Three intensity peaks only of TiO2 or ZnS have appeared in
the XRD patterns and all other high angle peaks have submerged
in the background due to large line broadening. The crystal
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1471
Intensity
Fig. 1. SEM images of (a) poly(MAA-co-TFA)/PVDF electrospun fiber mats, and TiO2–ZnS/fluoropolymer fiber composites prepared by different reactants. (b) TiZn0, (c) TiZn1,
(d) TiZn2, (e) TiZn3, (f) TiZn4.
f
e
d
c
b
a
30
40
50
60
2 Theata (Deg.)
Fig. 2. XRD patterns of (a) poly(MAA-co-TFA)/PVDF electrospun fiber and the TiO2–
ZnS/fluoropolymer fiber composites prepared by different reactants. (b) TiZn0, (c)
TiZn1, (d) TiZn2, (e) TiZn3, (f) TiZn4.
structure and figuration of semiconductor composites were further
discussed using TEM analysis.
The TEM images of TiO2–ZnS/fluoropolymer fiber composites
prepared by reactants TiZn2 demonstrate the slightly agglomerated
TiO2–ZnS particles, which are inclusive of nanocrystallites with
indistinct polygonal shape of about 100 nm in size, as shown in
Fig. 2a. The selected area electron diffraction (SAED) patterns of cubic ZnS and anatase TiO2 are shown in Fig. 3b and c.
Typical FTIR spectra of poly(MAA-co-TFA)/PVDF electrospun fiber and TiO2–ZnS/fluoropolymer fiber composites prepared by
reactants TiZn2 are compared in Fig. 4. It is evident that the poly(MAA-co-TFA)/PVDF electrospun fiber mats have peaks at 3350
and 1670 cm 1, corresponding to hydroxyl and carbonyl stretching of the carboxyl groups of poly(MAA-co-TFA). The corresponding hydroxyl and carbonyl absorption peaks of TiO2–ZnS/
fluoropolymer fiber composites have been broadened and slightly
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Fig. 3. TEM images of (a) TiO2–ZnS/fluoropolymer fiber composites prepared by reactants TiZn2, SAED of the sample (b) ZnS and (c) TiO2.
shift to the low wavenumber. This may be due to that the metal ion
was complexation adsorbed by the carboxyl on the surface of fluoropolymer electrospun [15,16], and then the semiconductor nuclei formed and grew into compound particles on the surface of
fluoropolymer fiber by hydrothermal precipitation, so the chemical
interaction exists between fluoropolymer fiber and semiconductor
particles.
The surface properties of TiO2–ZnS/fluoropolymer fiber composites were further investigated by XPS analysis, as shown in
Fig. 5. The Ti2p3/2 bonding energy is 458.6 and has 0.6 eV shift
compared with the typical anatase TiO2 (459.2 eV) [17], which resulted from the interaction between semiconductor particles and
fluoropolymer [18], as shown in Fig. 5a. There are peaks appeared
at around 282.3 eV, 286.5 eV, 288.7 eV, in the C1s spectrum and
a
3350
b
T/%
1670
4000 3000 2000 2000
1500
Wavenumbers / cm
1000
500
-1
Fig. 4. FTIR spectra of (a) poly(MAA-co-TFA)/PVDF electrospun fiber, (b) TiO2–ZnS/
fluoropolymer fiber composites prepared by reactants TiZn2.
531.7 eV, 532.8 eV in the O1s spectrum, shown in Fig. 5b and c,
and the peaks were also able to ascribe to the influence of carboxyl
coordinated with nonbonding metal ion of semiconductor [19]. As
a result, the semiconductor particles were able to immobilize
tightly on the surface of fluoropolymer fibers.
UV/Vis spectra show the photosensitive properties of TiO2/ZnS–
fluoropolymer fiber composites. The poly(MAA-co-TFA)/PVDF electrospun fiber mats have no evident absorption above 250 nm
wavenumbers (Fig. 6a). This reveals the poly(MAA-co-TFA)/PVDF
electrospun fiber mats do not disturb the light absorption of semiconductor of TiO2/ZnS–fluoropolymer fiber composites during the
photocatalytic process. The UV/Vis absorption spectrum of the
TiO2–fluoropolymer fiber composites reflects that the absorption
edge is about 382 nm, as shown in Fig. 6b. The UV/Vis absorption
edge of TiO2–ZnS/fluoropolymer fiber composites have obviously
shift to the long wavelength, as shown in Fig. 6c–f. It is due to
the S of ZnS surface change the light absorption character of
TiO2–ZnS, reduce the band gap, [20] and result in the improvement
of the visible-light response ability of TiO2–ZnS/fluoropolymer fiber composites. When the reaction system have lower content zinc
ion, the TiO2 crystals were compounded and mixed very well with
ZnS through heterogeneous nucleation, and the TiO2–ZnS particles
have strong compound effect, therefore the respectively absorption
edge is about 473 nm and 450 nm, as shown in Fig. 6c and d. However, with the zinc ion content of reaction system increased, the
ZnS agglomeration generation, and ZnS crystals were hard to
decompose for TiO2 crystals combining, so the TiO2 crystals are
not capped very well with ZnS crystals, therefore the compound effect reduces, the respectively absorption edge is about 402 nm and
390 nm, as shown in Fig. 6e and f.
TGA curve of poly(MAA-co-TFA)/PVDF electrospun fiber shows
several thermal decomposition stages, but TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2 does not show thermal
decomposition stage until 450 °C, as shown in Fig. 7. This phenomenon may be due to that the low-molecular weight substances of
poly(MAA-co-TFA)/PVDF electrospun fiber mats dissolved or fused
connected under long-time hydrothermal condition, and the interaction between semiconductors particles and fluoropolymer fibers
may also improve the thermal stability of TiO2–ZnS/fluoropolymer
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Relative Intensity (cps)
x10
6
1.5
1.0
a
O1s
Zn2p
Ti2p
F1s
C1s
0.5
S2p
1000
800
600
400
200
Binding Energy (eV)
C1s
O1s
c
Intensity (cps)
Intensity (cps)
b
292
290
288
286
284
282
280
534
532
Binding Energy (eV)
530
528
Binding Energy (eV)
Fig. 5. XPS spectrum of TiO2–ZnS/fluoropolymer fiber composites prepared by reactants TiZn2 (a) survage, (b) C1s, (c) O1s.
1.6
100
75
Weight (%)
Absorbance
1.2
0.8
d
c
e
f
b
a
0.4
0.0
200
400
600
50
0
200
Wavelength nm
fiber composites. Semiconductor particles content of TiO2–ZnS/fluoropolymer fiber composites was measured though the weight loss
after fluoropolymer electrospun fiber was fully decomposed at
700 °C, and the TiO2–ZnS content of TiO2–ZnS/fluoropolymer fiber
composites calculated was 24.9%.
The specific surface area of TiO2–ZnS of TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2 is considerably higher than
that of Degussa P25 and poly(MAA-co-TFA)/PVDF electrospun fiber
mats, but is lower than that of TiO2–ZnS powders, as shown in Table 2.
3.3. Photocatalytic degradation of methylene blue
Photocatalysis of TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2, TiO2–ZnS powders, Degussa P25, fluoropolymer
a
25
800
Fig. 6. UV/VIS absorption spectra of (a) poly(MAA-co-TFA)/PVDF electrospun fiber
mats, and the TiO2–ZnS/fluoropolymer fiber composites prepared by different
reactants (b) TiZn0, (c) TiZn1, (d) TiZn2, (e) TiZn3, (f) TiZn4.
b
400
600
0
Temperature ( C)
Fig. 7. Thermal gravity analytical of (a) poly(MAA-co-TFA)/PVDF electrospun fiber
mats and (b) TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2.
Table 2
Specific surface area.
Sample
SBET (m2 g
Degussa P25
TiO2–ZnS powders
(MAA-co-TFA)/PVDF electrospun fiber mats
TiO2–ZnS of TiO2–ZnS/fluoropolymer composites (TiZn2)
50.0
115.1
37.2
96.7
1
)
electrospun fiber mats and blank sample were performed for the
methylene blue degradation under visible-light irradiation, as
shown in Fig. 8. Near-complete degradation of methylene blue occurred in 120 min in the presence of TiO2–ZnS/fluoropolymer fiber
composites, as shown in Fig. 8a. A slight change of the methylene
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T. He et al. / Composites Science and Technology 70 (2010) 1469–1475
100
e
d
c
C/C0(%)
75
50
25
b
a
0
-15
0
40
80
120
Time (min)
Fig. 8. Photocatalytic degradation of methylene blue by (a) TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2, (b) TiO2–ZnS powders, (c) Degussa P25, (d)
(MAA-co-TFA)/PVDF electrospun fiber mats; (e) blank sample.
blue concentration was observed for the blank sample, as shown in
Fig. 8e. The remaining methylene blue is 0.01 wt.% in the presence
of TiO2–ZnS/fluoropolymer fiber composites, and it is 75.6 wt.% in
the presence of Degussa P25 after 110 min visible-light irradiation,
as shown in Fig. 8a and c. So the TiO2–ZnS/fluoropolymer fiber
composites exhibited higher photocatalytic efficiency than that of
TiO2 powder in the almost same TiO2 concentration (Table 3).
The reason is that the specific surface area and visible-light respond ability of TiO2–ZnS/fluoropolymer fiber composites were
higher than that of Degussa P25 (Table 2). The specific surface area
of TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2 was
lower of than that of TiO2–ZnS powder, as shown in Table 2, but the
remaining methylene blue is 20.2 wt.% after 110 min visible-light
irradiation in the presence of TiO2–ZnS powders. There may be
adsorption–migration–photodegradation [21] exists in the photocatalysis reaction: methylene blue was first adsorbed onto the surface of fluoropolymer fibers because of its hydrophobicity, and
Table 3
Photocatalyst concentration in solution.
then migrated to semiconductor particles surface, finally was photocatalytic degraded by semiconductor particles, so deduce the
TiO2–ZnS/fluoropolymer fiber composites possess higher photocatalytic efficiency than that of TiO2–ZnS powders for the degradation
of methylene blue with the same concentration.
The photocatalytic stability of TiO2–ZnS/fluoropolymer fiber
composites prepared by TiZn2 evaluated by the degradation of
methylene blue solution under 10 times of repeated visible-light
irradiation for 120 min. The results reveal that the photocatalytic
activity of TiO2–ZnS/fluoropolymer fiber composites changes
indistinctively. The SEM image of TiO2–ZnS/fluoropolymer fiber
composites prepared by TiZn2 after 10 times degradation of methylene blue solution shows that the semiconductor particles are
tightly immobilized on the surface of fluoropolymer nanofibers
after the degradation tests. As a conclusion, the TiO2–ZnS/fluoropolymer fiber composites possess high photocatalytic stability for
the photodegradation of organic pollutants Fig. 9.
4. Conclusion
The TiO2–ZnS composites with diameters from 15 nm to 1 lm
were immobilize on the surface of fluoropolymer fiber under different reaction system, and the chemical interaction existed between
TiO2–ZnS composites and fluoropolymer fibers. When the molar ratio of zinc ion and titanic ion in reaction system was 1:80, the TiO2–
ZnS/fluoropolymer fiber composites possess good visible-light photocatalytic activity because of its strong visible-light response
activity, quite high specific area and synergistic effect. The repeated
photocatalysis tests show the TiO2–ZnS/fluoropolymer fiber composites possess good visible-light photocatalytic stability.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (20776034), Doctoral Fund of Ministry of Education of
China (20070359036).
References
Sample
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TiO2–ZnS/fluoropolymer fiber composites (TiZn2)
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–
–
1
)
Fig. 9. SEM image of TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2
after 10 times degradation of methylene blue solution under UV irradiation for 2.0 h
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