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Visible-light photocatalytic activity of semiconductor composites supportedby 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, Chinab Institute for Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green, 42101, USAc Liaoning Key Laboratory of Applied Chemistry, Bohai University, Jinzhou 121000, China

a r t i c l e i n f o

Article history:Received 22 January 2010Accepted 2 May 2010Available online 19 May 2010

Keywords:A. Polymer–matrix composites (PMCs)B. SynergismD. Scanning/transmission electronmicroscopy (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. Thefluoropolymer nanofiber mats with carboxyl groups were prepared by electrospinning, and then tita-nium and zinc ions were introduced onto the fiber surfaces by the coordinating of carboxyl of fluoro-polymer in solution. The TiO2–ZnS composites with diameters 15 nm to 1 lm were immobilized onthe surface of fluoropolymer electrospun fiber using hydrothermal synthesis. The Fourier transforminfrared spectroscopy and X-ray photoelectron spectroscopy analysis reveal that some chemical interac-tion exists between TiO2–ZnS composites and fluoropolymer fibers, so the semiconductor compositeswere immobilized tightly on the surface of fluoropolymer fibers. The ultraviolet–visible absorption spec-tra show the TiO2–ZnS/fluoropolymer fiber composites have low band gap and good visible-lightresponse ability. The degradation rate of methylene blue in TiO2–ZnS/fluoropolymer fiber compositessystem was considerably higher than that of TiO2 or TiO2–ZnS nanoparticles system under visible-lightirradiation, because the TiO2–ZnS/fluoropolymer fiber composites possess good visible-light responseability, high specific surface areas, and adsorption–migration–photodegradation process. The photocat-alytic activity of TiO2–ZnS/fluoropolymer fiber composites changes indistinctively after 10 repeatingphotocatalysis tests.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Growing concerns over the threat of chemical warfare agentsand exposure to toxic industrial chemicals have drawn muchattention to the challenge of developing new harmless treatmentmethods for the toxic organic materials [1]. Photocatalytic degra-dation of harmful organic pollutants in the air and the water usingsemiconductor particles, such as titanium dioxide (TiO2), is one ofthe most widely studied methods [2]. The semiconductor particlesare able to convert abundant solar energy into effective chemicalenergy, and mineralized the organic pollutants completely [3].However, the photocatalytic degradation of toxic organic pollu-tants using semiconductor is still challenged, in terms of the lowphotocatalytic efficiency under natural sunlight, easy agglomerat-ing and losing in the using.

Immobilization of semiconductor particles on the carrier isone of the best effective methods to prevent the agglomeratingand losing of semiconductor particles in using [4]. The semicon-

ductor particles directly depositing onto polymer electrospun fi-bers are also used to prepare photocatalytic materials [5].However, polymers usually have troubles in compounding withinorganic powders, and easy are degraded in the photocatalyticprocess [6]. Fluoropolymers like poly(vinylidene difluoride)(PVDF), which has excellent weather, radiation, chemical andthermal resistance due to stable –C–F bond in the main chain[7]. The fluoropolymers electrospun fiber mats with micro-sizedporous structure [8] are able to offer high specific area and goodenrichment ability for organic compounds, are suitable as photo-catalyst carrier [9]. The visible-light photocatalytic activity of sol-itary TiO2 is able to improved greatly by doping it with otherelements, and the synthesis of nanocrystalline TiO2 capped ZnSunder hydrothermal conditions is a convent way [10].

In this paper we demonstrate a novel method to prepare visi-ble-light photocatalytic activity TiO2–ZnS particles loaded by flu-oropolymer electrospun fiber with carboxyl groups underhydrothermal condition. The photocatalytic activity and stabilitywere investigated through degradation of methylene blue usingTiO2–ZnS/fluoropolymer fiber composites as photocatalyst undervisible-light radiation. The results show the as-prepared compos-ites have good visible-light photocatalytic activity and stabilityfor the potential applicability in environmental remediation.

0266-3538/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.compscitech.2010.05.001

* Corresponding author. Tel./fax: +86 551 2901455.** Corresponding author. Tel.: +1 270 7452221.

E-mail addresses: [email protected] (W. Xu), [email protected] (W.-P.Pan).

Composites Science and Technology 70 (2010) 1469–1475

Contents lists available at ScienceDirect

Composites Science and Technology

journal homepage: www.elsevier .com/ locate/compsci tech


2. Experimental

Trifluoroethyl acrylate (TFA) was obtained from Xuejia Fluo-rine-silicon Chemical Co., Ltd, Harbin China. Anatase Degussa P25was purchased from Shanghai Haiyi Scientific & Trading Co., Ltd.Poly(vinylidene difluoride) (PVDF), titanium oxo-sulphate (TiOS-O4), methacrylic acid (MAA), zinc sulfate (ZnSO4), thioacetamide(TAA), methylene blue, urea and other chemicals were purchasedfrom Shanghai Chemicals Ltd., and used as received.

Perkin Elmer Spectrum 100 FTIR spectrometer was used towidely scan the synthetic products. JSM-6700F scanning electronmicroscopy (SEM) was utilized to study the surface morphologiesof the products. The specific surface area (BET) analyzed by ASAP2020 M + C. ESCALAB 250 X-ray photoelectron spectroscopy (XPS)was used to study the structure of composites. Transmission elec-tron microscope (TEM) image and the selected area electron dif-fraction (SAED) pattern were taken on JEOL 2010. The crystalstructure was detected through the X-ray diffraction (XRD), RigakuD/max-rB. The thermo-gravimetric analysis (TGA), Netzsch TG-209-F3, was applied to estimate the weight loss of composites.Ultraviolet–visible (UV/VIS) absorption spectra were obtained ona Shimadzu Solidspec-3700 DUV spectrophotometer at roomtemperature.

The synthesis of MAA–TFA random copolymers was performedin an automated reactor system. 30 g MAA, 70 g TFA, and 0.5 g 2,2-azobisisobutyronitrile (AIBN) were added into a three-neckedflask capacity 250 mL equipped with a condenser, a stirrer and aN2 inlet. After polymerizing at 80 �C for 1 h, the reaction mixturewas transferred to a stainless steel plate and placed in an oven at40 �C for 12 h. Then the reaction mixture was maintained at100 �C for 3 h, so that the remaining monomers can polymerize.

Poly(MAA-co-TFA)/PVDF electrospun fiber mats were preparedusing a typical electrospinning process [11]. 10.3 g PVDF and1.7 g poly(MAA-co-TFA) were first dissolved in 88 g N,N-dimethyl-formamide (DMF). The solution was electrospun at 25 kV positivevoltage, 15 cm working distance (the distance between the needletip and the target), and 1.0 mL h�1 flow rate. The collection timewas set to 2.0 h. All manipulations were carried out at roomtemperature. The electrospun fiber mats of fluoropolymers werecut into strips of dimension 2.0 cm � 2.0 cm for the followingexperiments.

The above-mentioned strip of fluoropolymer electrospun fibermats were immersed into 10.0 mL, 0.08 mol L�1 aqueous solutionof titanium oxo-sulphate and 1.0 mL concentrated sulfuric acid ina 50 mL Teflon-lined stainless steel autoclave for 6 h in order toform the complex of carboxylic of fluoropolymer electrospun fibersurface and titanium ion. Then 20.0 mL, 0.08 mol L�1 urea and20 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 wasshown in Table 1. The autoclave was sealed at 150 �C for 8 h, andthen cooled to room temperature. The TiO2–ZnS/fluoropolymer fi-ber composites were washed for three times with distilled waterunder ultrasonic to remove the unreacted precursor and byprod-ucts, and dried in vacuum at 80 �C for 12 h.

Photocatalytic degradation of methylene blue solution was per-formed by photochemical reactor (SGY-1, Stonetech Co., Ltd. Nan-jing, China), light source is 350 W xenon lamp, and reaction systemtemperature was 23 ± 1 �C. The TiO2–ZnS/fluoropolymer fiber com-posites and 300.0 mL 16.0 mg L�1 methylene blue were added tothe quartz tube-500 mL. The TiO2–ZnS/fluoropolymer fiber com-posites can be extended well in methylene blue solution withoutstirring. The Degussa P25 and TiO2–ZnS powders synthesizedaccording to Stengl et al. methods [10] were performed as statedin the previous steps with electromagnetic stirring. Prior to irradi-ation, the photocatalytic reaction system was stirred in a darkcondition for 15 min to establish an adsorption–desorption equi-librium. The photocatalytic reaction system was sampled at regu-lar intervals, and the semiconductor powders suspensions werecentrifuged before measured. The remaining methylene blue con-centration after adsorption–desorption equilibrium (C0) and pho-todegradation (C) was detected by UV/VIS at 665 nm, and thedegradation 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 madeof random nonwoven mesh of fibers, and had an interconnectedopen porous structure, as shown in Fig. 1a. The SEM images ofTiO2–ZnS/fluoropolymer fiber composites prepared by differentproportions for 8 h at 150 �C are compared in Fig. 1b–f and the cor-responding Zn content of composites is presented in Table 1. Thesize distribution of semiconductor particles was about 5 nm to1 lm, and the size and agglomeration of semiconductor particleswere improved with the increasing zinc ion contents in the reac-tion system, as shown in Table 1. The reasons are able to explainedas follows: the sulfide ion was released from TAA at low tempera-ture with high rate [12], but the TiO2 crystal prepared by hydro-thermal hydrolysis of titanium oxo-sulphate with urea needmulti-step reaction [13], therefore the generation and growth ofZnS crystal were faster than that of the TiO2 crystal under the samereaction system. Without zinc added, the TiO2 crystals formationand growth were controlled by carboxyl along the surface of elec-trospun fiber, and the about 5 nm TiO2–fluoropolymer fiber com-posites were achieved, as shown in Fig. 1b. With the zinc ionadded, the ZnS crystals generated on the fiber surface precededTiO2 crystals, and then both semiconductor crystals decomposingand combining, so the TiO2–ZnS mixed crystals generated on thefluoropolymer fiber surface. When the zinc ion content of reactionsystem is low, the TiO2–ZnS particle size is less than 100 nm be-cause of heterogeneous nucleation effect, as shown in Fig. 1c andd. With zinc ion content increase, ZnS homogeneous nucleationplays as a dominant role, plus, the nucleation and growth of ZnSparticles accelerates under hydrothermal conditions, which inhib-its instant decomposing and combining of semiconductor particles,thus semiconductor agglomerations sized over 200 nm were ob-tained, as shown in Fig. 1e and f.

3.2. Characterization of TiO2–ZnS/fluoropolymer fiber composites

The XRD patterns and the corresponding characteristic 2h val-ues of the diffraction peaks were shown in Fig. 2. It is confirmedthat semiconductor composites as-prepared samples is identifiedas 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 inthe XRD patterns and all other high angle peaks have submergedin the background due to large line broadening. The crystal

Table 1Reactants content of hydrothermal system.

Samples ZnSO4 (10�2

mol)TiOSO4 (10�2

mol)EDX of Zn(wt.%)

Crystallite size(nm)

TiZn0 0.0 80 0.0 �5TiZn1 0.5 80 0.24 �15TiZn2 1.0 80 0.91 �100TiZn3 3.0 80 4.73 �200TiZn4 5.0 80 13.06 �1000

1470 T. He et al. / Composites Science and Technology 70 (2010) 1469–1475


structure and figuration of semiconductor composites were furtherdiscussed using TEM analysis.

The TEM images of TiO2–ZnS/fluoropolymer fiber compositesprepared by reactants TiZn2 demonstrate the slightly agglomeratedTiO2–ZnS particles, which are inclusive of nanocrystallites withindistinct polygonal shape of about 100 nm in size, as shown inFig. 2a. The selected area electron diffraction (SAED) patterns of cu-bic ZnS and anatase TiO2 are shown in Fig. 3b and c.

Typical FTIR spectra of poly(MAA-co-TFA)/PVDF electrospun fi-ber and TiO2–ZnS/fluoropolymer fiber composites prepared byreactants TiZn2 are compared in Fig. 4. It is evident that the poly(-MAA-co-TFA)/PVDF electrospun fiber mats have peaks at �3350and �1670 cm�1, corresponding to hydroxyl and carbonyl stretch-ing of the carboxyl groups of poly(MAA-co-TFA). The correspond-ing hydroxyl and carbonyl absorption peaks of TiO2–ZnS/fluoropolymer fiber composites have been broadened and slightly

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.

30 40 50 60

abcd

Inte

nsity

2 Theata (Deg.)

fe

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.

T. He et al. / Composites Science and Technology 70 (2010) 1469–1475 1471


shift to the low wavenumber. This may be due to that the metal ionwas complexation adsorbed by the carboxyl on the surface of flu-oropolymer electrospun [15,16], and then the semiconductor nu-clei formed and grew into compound particles on the surface offluoropolymer fiber by hydrothermal precipitation, so the chemicalinteraction exists between fluoropolymer fiber and semiconductorparticles.

The surface properties of TiO2–ZnS/fluoropolymer fiber com-posites were further investigated by XPS analysis, as shown inFig. 5. The Ti2p3/2 bonding energy is 458.6 and has 0.6 eV shiftcompared with the typical anatase TiO2 (459.2 eV) [17], which re-sulted from the interaction between semiconductor particles andfluoropolymer [18], as shown in Fig. 5a. There are peaks appearedat around 282.3 eV, 286.5 eV, 288.7 eV, in the C1s spectrum and

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 carboxylcoordinated with nonbonding metal ion of semiconductor [19]. Asa result, the semiconductor particles were able to immobilizetightly 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 elec-trospun fiber mats have no evident absorption above 250 nmwavenumbers (Fig. 6a). This reveals the poly(MAA-co-TFA)/PVDFelectrospun fiber mats do not disturb the light absorption of semi-conductor of TiO2/ZnS–fluoropolymer fiber composites during thephotocatalytic process. The UV/Vis absorption spectrum of theTiO2–fluoropolymer fiber composites reflects that the absorptionedge is about 382 nm, as shown in Fig. 6b. The UV/Vis absorptionedge of TiO2–ZnS/fluoropolymer fiber composites have obviouslyshift to the long wavelength, as shown in Fig. 6c–f. It is due tothe S of ZnS surface change the light absorption character ofTiO2–ZnS, reduce the band gap, [20] and result in the improvementof the visible-light response ability of TiO2–ZnS/fluoropolymer fi-ber composites. When the reaction system have lower content zincion, the TiO2 crystals were compounded and mixed very well withZnS through heterogeneous nucleation, and the TiO2–ZnS particleshave strong compound effect, therefore the respectively absorptionedge is about 473 nm and 450 nm, as shown in Fig. 6c and d. How-ever, with the zinc ion content of reaction system increased, theZnS agglomeration generation, and ZnS crystals were hard todecompose for TiO2 crystals combining, so the TiO2 crystals arenot capped very well with ZnS crystals, therefore the compound ef-fect reduces, the respectively absorption edge is about 402 nm and390 nm, as shown in Fig. 6e and f.

TGA curve of poly(MAA-co-TFA)/PVDF electrospun fiber showsseveral thermal decomposition stages, but TiO2–ZnS/fluoropoly-mer fiber composites prepared by TiZn2 does not show thermaldecomposition stage until 450 �C, as shown in Fig. 7. This phenom-enon may be due to that the low-molecular weight substances ofpoly(MAA-co-TFA)/PVDF electrospun fiber mats dissolved or fusedconnected under long-time hydrothermal condition, and the inter-action between semiconductors particles and fluoropolymer fibersmay also improve the thermal stability of TiO2–ZnS/fluoropolymer

Fig. 3. TEM images of (a) TiO2–ZnS/fluoropolymer fiber composites prepared by reactants TiZn2, SAED of the sample (b) ZnS and (c) TiO2.

4000 3000 2000 2000 1500 1000 500

T /

%

Wavenumbers / cm-1

3350

a

b1670

Fig. 4. FTIR spectra of (a) poly(MAA-co-TFA)/PVDF electrospun fiber, (b) TiO2–ZnS/fluoropolymer fiber composites prepared by reactants TiZn2.



fiber composites. Semiconductor particles content of TiO2–ZnS/flu-oropolymer fiber composites was measured though the weight lossafter fluoropolymer electrospun fiber was fully decomposed at700 �C, and the TiO2–ZnS content of TiO2–ZnS/fluoropolymer fibercomposites calculated was 24.9%.

The specific surface area of TiO2–ZnS of TiO2–ZnS/fluoropoly-mer fiber composites prepared by TiZn2 is considerably higher thanthat of Degussa P25 and poly(MAA-co-TFA)/PVDF electrospun fibermats, but is lower than that of TiO2–ZnS powders, as shown in Ta-ble 2.

3.3. Photocatalytic degradation of methylene blue

Photocatalysis of TiO2–ZnS/fluoropolymer fiber composites pre-pared by TiZn2, TiO2–ZnS powders, Degussa P25, fluoropolymer

electrospun fiber mats and blank sample were performed for themethylene blue degradation under visible-light irradiation, asshown in Fig. 8. Near-complete degradation of methylene blue oc-curred in 120 min in the presence of TiO2–ZnS/fluoropolymer fibercomposites, as shown in Fig. 8a. A slight change of the methylene

1000 800 600 400 200

S2p

Zn2p

x106

0.5

1.0

C1s

O1s

F1s

Ti2p

Rel

ativ

e In

tens

ity (c

ps)

Binding Energy (eV)

1.5a

Inte

nsity

(cps

)

Binding Energy (eV)

C1sb

292 290 288 286 284 282 280 534 532 530 528

O1sc

Binding Energy (eV)

Inte

nsity

(cps

)

Fig. 5. XPS spectrum of TiO2–ZnS/fluoropolymer fiber composites prepared by reactants TiZn2 (a) survage, (b) C1s, (c) O1s.

200 400 600 800

0.0

0.4

0.8

1.2

1.6

fe

d

b

c

Wavelength nm

a

Abso

rban

ce

Fig. 6. UV/VIS absorption spectra of (a) poly(MAA-co-TFA)/PVDF electrospun fibermats, and the TiO2–ZnS/fluoropolymer fiber composites prepared by differentreactants (b) TiZn0, (c) TiZn1, (d) TiZn2, (e) TiZn3, (f) TiZn4.

200 400 6000

25

50

75

100

b

Wei

ght (

%)

Temperature ( 0C)

a

Fig. 7. Thermal gravity analytical of (a) poly(MAA-co-TFA)/PVDF electrospun fibermats and (b) TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2.

Table 2Specific surface area.

Sample SBET (m2 g�1)

Degussa P25 50.0TiO2–ZnS powders 115.1(MAA-co-TFA)/PVDF electrospun fiber mats 37.2TiO2–ZnS of TiO2–ZnS/fluoropolymer composites (TiZn2) 96.7



blue concentration was observed for the blank sample, as shown inFig. 8e. The remaining methylene blue is 0.01 wt.% in the presenceof TiO2–ZnS/fluoropolymer fiber composites, and it is 75.6 wt.% inthe presence of Degussa P25 after 110 min visible-light irradiation,as shown in Fig. 8a and c. So the TiO2–ZnS/fluoropolymer fibercomposites exhibited higher photocatalytic efficiency than that ofTiO2 powder in the almost same TiO2 concentration (Table 3).The reason is that the specific surface area and visible-light re-spond ability of TiO2–ZnS/fluoropolymer fiber composites werehigher than that of Degussa P25 (Table 2). The specific surface areaof TiO2–ZnS/fluoropolymer fiber composites prepared by TiZn2 waslower of than that of TiO2–ZnS powder, as shown in Table 2, but theremaining methylene blue is 20.2 wt.% after 110 min visible-lightirradiation in the presence of TiO2–ZnS powders. There may beadsorption–migration–photodegradation [21] exists in the photo-catalysis reaction: methylene blue was first adsorbed onto the sur-face of fluoropolymer fibers because of its hydrophobicity, and

then migrated to semiconductor particles surface, finally was pho-tocatalytic degraded by semiconductor particles, so deduce theTiO2–ZnS/fluoropolymer fiber composites possess higher photocat-alytic efficiency than that of TiO2–ZnS powders for the degradationof methylene blue with the same concentration.

The photocatalytic stability of TiO2–ZnS/fluoropolymer fibercomposites prepared by TiZn2 evaluated by the degradation ofmethylene blue solution under 10 times of repeated visible-lightirradiation for 120 min. The results reveal that the photocatalyticactivity of TiO2–ZnS/fluoropolymer fiber composites changesindistinctively. The SEM image of TiO2–ZnS/fluoropolymer fibercomposites prepared by TiZn2 after 10 times degradation of meth-ylene blue solution shows that the semiconductor particles aretightly immobilized on the surface of fluoropolymer nanofibersafter the degradation tests. As a conclusion, the TiO2–ZnS/fluoro-polymer fiber composites possess high photocatalytic stability forthe photodegradation of organic pollutants Fig. 9.

4. Conclusion

The TiO2–ZnS composites with diameters from 15 nm to 1 lmwere immobilize on the surface of fluoropolymer fiber under differ-ent reaction system, and the chemical interaction existed betweenTiO2–ZnS composites and fluoropolymer fibers. When the molar ra-tio of zinc ion and titanic ion in reaction system was 1:80, the TiO2–ZnS/fluoropolymer fiber composites possess good visible-light pho-tocatalytic activity because of its strong visible-light responseactivity, quite high specific area and synergistic effect. The repeatedphotocatalysis tests show the TiO2–ZnS/fluoropolymer fiber com-posites possess good visible-light photocatalytic stability.

Acknowledgments

This work is supported by the National Natural Science Founda-tion of China (20776034), Doctoral Fund of Ministry of Education ofChina (20070359036).

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Table 3Photocatalyst concentration in solution.

Sample Photocatalyst (mg L�1)

TiO2–ZnS/fluoropolymer fiber composites (TiZn2) 34.2TiO2–ZnS powders 34.7Degussa P25 35.1(MAA-co-TFA)/PVDF electrospun fiber mats –Blank –

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 heach.

0 40 80 120

0

25

50

75

100edc

ba

Time (min)

C/C

0(%)

-15

Fig. 8. Photocatalytic degradation of methylene blue by (a) TiO2–ZnS/fluoropoly-mer fiber composites prepared by TiZn2, (b) TiO2–ZnS powders, (c) Degussa P25, (d)(MAA-co-TFA)/PVDF electrospun fiber mats; (e) blank sample.



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