Article pubs.acs.org/IECR
Fabrication of TiO2/WO3 Composite Nanofibers by Electrospinning and Photocatalystic Performance of the Resultant Fabrics Zhouli Chen,† Jingxin Zhao,† Xuxin Yang,§ Quanlin Ye,§ Keke Huang,# Changmin Hou,# Zhigang Zhao,*,‡ Jichun You,*,† and Yongjin Li† †
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Street, Xiasha High-education Zone, Hangzhou 310036, P.R. China ‡ Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, 398 Ruoshui Road, Advanced Education District of Dushu Lake, Suzhou Industry Park, Suzhou 215125, P.R. China § Department of Physics, Hangzhou Normal University, No. 16 Xuelin Street, Xiasha High-education Zone, Hangzhou 310036, P.R. China # State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P.R. China ABSTRACT: By coupling the self-assembly of polystyrene-block-poly(ethylene oxide)-containing titanium-tetraisopropoxide and tungsten hexaphenoxide (the precursors of TiO2 and WO3, respectively) with electrospinning technique, the hierarchically porous TiO2/WO3 composite nanofibers with inner-bicontinuous and outer-shell structures have been synthesized. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were employed to characterize the structure of the fibers. In these nanofibers, the TiO2 acts as the frames, and WO3 fills the gaps. The UV−vis spectroscopy suggests that the spectral response range has been successfully extended into the visible regime. Furthermore, the resulting fabrics show enhanced performance in the photocatalytic degradation of acetaldehyde relative to neat TiO2 and neat WO3. Our results indicate that electrospinning the blend solution of block copolymer and two kinds of precursor of inorganic material is an effective strategy to fabricate composite nanofibers that can improve the photocatalytic activity of TiO2, especially for visible light.
1. INTRODUCTION Titanium dioxide has been widely used in photocatalysis because of its effectiveness, chemical stability, photostability, inert nature, nontoxicity, and low cost.1−5 However, its photocatalytic efficiency is far too low for practical large-scale applications because of its short wavelength cutoff (∼380 nm for anatase) and band gap of 3.2 eV. Furthermore, the fast recombination of photogenerated electrons (e−) and holes (h+) in the material also limits its efficiency. In the improvement of photocatalytic efficiency of TiO2, the key problem is to extend the spectral response range to visible light and reduce the recombination of the photogenerated carriers. To date, numerous efforts have been made for this purpose.6−8 For instance, the strategy of combining TiO2 with other metal oxides with a narrow band gap has been employed to enhance its photocatalytic activity. WO3 (Eg ≈ 2.7 eV, λ = 443 nm) is an ideal material to dope with TiO2. For instance, Chai et al. prepared WO3/TiO2 nanoparticles by modifying the surface of TiO2 with WO3. The composite nanoparticles can be activated by visible light in the photocatalytic reactions.9 Pan et al. synthesized highly ordered cubic mesoporous WO3/TiO2 thin films by evaporation-induced self-assembly (EISA). The photocatalytic activity of consequent composite materials was 2.2 times that of a mesoporous TiO2 film and 6.1 times that of a nonporous TiO2 film derived from a typical sol−gel method.10 In the results of Reyes-Gil et al., the composite material consisting of TiO2 nanotubes with WO3 electrodeposited on its © XXXX American Chemical Society
surface has been fabricated. The composite WO3/TiO2 nanostructures showed higher ion storage capacity, better stability, and longer memory time compared with the neat WO3 and TiO2.11,12 Some nanocomposite TiO2/WO3 porous materials available to date have been aided by the development of synthetic approaches as discussed above. In the reported strategies, however, few works have focused on the fabrication of composite TiO2/WO3 materials doped on the nanometer scale, although they correspond to higher efficiency in many applications. Furthermore, it is still a significant challenge to obtain hierarchically porous hybrid nanomaterials. Additionally, the reported materials are always too brittle to be prepared into a self-supporting film due to the high porosity and poor resulting mechanical properties.12−14 In our previous work, novel cigarlike TiO2 nanofibers and resulting fabrics have been obtained by coupling “top-down” (electrospinning) and “bottom-up” (self-assembly of block copolymer) strategies. The resulting fabrics exhibited high specific surface area (because of the hierarchical pores among fibers and in fibers) and better mechanical and enhanced electrochemical performances in Li-ion batteries.15 FurtherReceived: September 24, 2015 Revised: December 10, 2015 Accepted: December 14, 2015
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DOI: 10.1021/acs.iecr.5b03578 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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is followed by the adjustment of acetaldehyde concentration at 308 ppm. After the adsorption equilibrium of acetaldehyde was achieved in the dark, the irradiation with an ultraviolet cutoff filter to provide visible light (λ ≥ 420 nm) of the sample was carried out using a 500 W Xe lamp at room temperature. Finally, the mixed gas was transported (by the inert gas of N2) to the gas chromatography (GC-7800, Beijing) to determine the concentration of acetaldehyde.
more, the electrospinning has been validated to be an efficient method to fabricate nanostructures. For instance, Lu et al. prepared Ag@TiO2@Ag NT heterostructures that exhibit excellent electrochemical properties and superior photocatalytic performance.16−18 Szilagyi fabricated WO3/TiO2 core/shell nanofibers by electrospinning. The resulting composite shows remarkably improved performance in photocatalytic reactions.19,20 In this work, therefore, we will synthesize TiO2/ WO3 hierarchical composite nanofibers by electrospinning block copolymer [polystyrene-block-poly(ethylene oxide), i.e., (PS-b-PEO)] containing precursors of TiO2 and WO3. During the electrospinning of this blend solution of copolymer and precursors, the polymer plays a key role. On one hand, the block copolymer provides viscosity to the blend solution, which is the base of electrospinning;21 On the other hand, the microphase separation of it acts as a template to induce the selective distribution of the precursor based on the special interactions between them.22 Upon calcination, the nanodoped TiO2/WO3 composite fibers can be expected. Furthermore, the photocatalytic performance of the resulting fabrics will be investigated in detail.
3. RESULTS AND DISCUSSION Figure 1 shows scanning electron microscope (SEM) images of the as-spun and calcined TiO2/WO3 composite nanofibers of S-
2. EXPERIMENTAL SECTION Electrospinning. Polystyrene-block-poly(ethylene oxide) (Mw ≈ 59000-b-31000, PDI ≈ 1.05, purchased from Aldrich), titanium-tetraisopropoxide (TTIP, Mw = 28422, purchased from TCI) and hexaphenoxide tungsten (the detail of synthetization has been discussed in ref 23) were used in the experiment. The composition ratios have been listed in Table 1.
Figure 1. SEM images of (a) as-spun S-0-1, (b) as-spun S-1-1 (c) asspun S-1-0; (d) calcined S-0-1, (e) calcined S-1-1, and (f) calcined S-10 TiO2/WO3 composite nanofibers.
0-1, S-1-1, and S-1-0. In the as-spun samples, there are some fabrics composed of nanofibers with diameters ranging from 300 nm to 2 μm. In Figure 1a, the specimen of PS-b-PEO and tungsten hexaphenoxide have nanofibers showing larger diameters relative to Figure 1b and c. After calcination, they turn into some “clusters” as shown in Figure 1d, suggesting that they cannot maintain the shape of fibers during the degradation of block copolymer and the transition from tungsten hexaphenoxide to tungsten oxide. Panels c and f in Figure 1 illustrate the as-spun and calcinated nanofibers of copolymer and TTIP, respectively. It is clear that we can obtain the cigarlike nanofibers with inner-bicontinuous and outer-shell structures as discussed in our previous work.15 The block copolymer serves as the structure template and viscosity agent for the TiO2 precursor during electrospinning, whereas the latter acts as the backbone that maintains the shape of the fibers during thermal annealing and calcination at 450 °C. As a result, the TiO2 nanofibers remain stable upon calcination, and the fabrics are self-supporting. In Figure 1b, the SEM image of asspun composite nanofibers with precursors of both titanium dioxide and tungsten oxide, there are some compact polymer/ precursor nanofibers with a diameter of ∼1 μm. Upon calcination, some cigarlike nanofibers with similar diameters are obvious (Figure 1e). Our results indicate that we can fabricate composite nanofibers via the strategy of coupling the self-assembly of block copolymers containing two precursors of inorganic material with the electrospinning technique.24,25 Figure 2 shows the Ti 2p and W 4f XPS spectra of sample S1-1. As shown in Figure 2A, there are two peaks in the Ti 2p result. The Ti 2p3/2 peak is found at 459.2 eV, and the Ti 2p1/2 peak is located at 464.9 eV, suggesting a normal state of Ti4+ in the composite nanofibers. In Figure 2B, the spectra of W 4f with peaks at 36.3 eV (corresponding to W 4f7/2) and 38.3 eV (assigned to W 4f5/2) indicate that the valence state of W is +6.19,20,26−30 X-ray diffraction (XRD) was employed to
Table 1. Composition Ratio of TiO2/WO3 Composited Nanofibers sample
PS-b-PEO (g)
Ti(OiPr)4 (g)
W(OPh)6 (g)
CHCl3 (mL)
S-0-1 S-1-1 S-1-0
0.3 0.3 0.3
0 0.3 0.6
0.6 0.3 0
1 1 1
The samples were named by the weight ratio of TTIP and tungsten hexaphenoxide. For instance, S-1-1 corresponds to the sample with the equal weight fractions. The mixed solution was electronspun from a 5 mL syringe at a feeding rate of 0.1 mL/h by a syringe pump (KDS 200, KD Scientific, USA) upon applying a 15 kV voltage after stirring for 12 h. The as-spun fibers were annealed at 120 °C for 48 h in vacuum and calcined at 450 °C for 2 h. Characterization. A HELIOS NanoLab 600i field emission scanning electron microscope (SEM, FEI, USA) was used for morphology measurements at an accelerating voltage of 5.0 kV. The X-ray diffraction (XRD, smartlab3, Rigaku Japan) data were collected at a scanning speed of 2°/min with a step interval of 0.02°. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) with a monochromatic Al X-ray source (1486.6 eV) was used to determine the valence states of Ti and W in the nanofibers. A high resolution transmission electron microscope (HRTEM, Talos F200A, FEI) operating at 200 kV equipped with a Super X probe for energy dispersive Xray (EDX) spectroscopic analysis was used to obtain the TEM images and element mapping. The absorption spectroscopy of the TiO2/WO3 composite fabrics was detected by ultraviolet− visible spectroscopy (UV−vis, Jasco V-660, Japan). Photocatalytic Performance. Acetaldehyde (0.15 mL) was injected into a reaction chamber and mixed with air, which B
DOI: 10.1021/acs.iecr.5b03578 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. X-ray photoelectron spectroscopy of Ti 2p (A) and W 4f (B) in the composite nanofibers.
investigate the crystallinity of the calcinated composite nanofibers. Figure 3 shows the XRD profile of S-0-1 (blue),
Figure 4. HRTEM image of sample S-1-1 with a scale bar of 20 nm.
on the HRTEM was performed to investigate the distribution of TiO2 and WO3 in the composite nanofibers. The results are shown in Figure 5. In the cross-section (not side view) of the Figure 3. XRD profile of the neat TiO2, neat WO3, and composite nanofibers. The blue, red, and black lines correspond to S-0-1, S-1-1, and S-1-0, respectively.
S-1-1 (red), and S-1-0 (black) after calcination at 450 °C for 2 h. For S-0-1, there are only characteristic peaks (e.g., triple peaks at 23.15−24.35°) of the WO3 crystal.31 The profile of S1-0 reveals strong diffraction peaks at 25, 38, and 48°, suggesting that TiO2 is mainly composed of the anatase phase.32,33 Additionally, the peaks at 34, 42 and 53° can be attributed to the existence of Ti2O3, which has good agreement with the results of YoonKeun Chae.33 For the sake of simplicity, titanium oxide is hereafter designated as TiO2 although there is still a little Ti2O3. The result of TiO2/WO3 composite nanofibers is shown as a red line in Figure 3. For one thing, all diffraction peaks can be indexed as either TiO2 or WO3 crystals, that is to say, the addition of WO3 does not cause any shift of the main peaks of TiO2. This result indicates that TiO2 and WO3 are physically mixed (i.e., doped in the nanometer scale). For another thing, the diffraction peaks of anatase TiO2 are much stronger than those of WO3. This can be attributed to the different mass loss during calcination and distribution of them in the nanofibers, which will be discussed in the following sections. For confirming the existence and distribution of TiO2 and WO3 crystals, high-resolution transmission electron microscopy (HRTEM) was employed.34−36 Before observation, our specimen was embedded with epoxy resin and cut into slices. In the results, shown in Figure 4, the fringe spacing between 0.35 nm (corresponding to the space of the (101) lattice planes of anatase TiO2) to 0.37 nm (the spacing of the (200) lattice planes of WO3) can be found. It is very hard to distinguish TiO2 and WO3 just on the basis of HRTEM analysis because the fringe spacings of them are close to each other. Therefore, energy dispersive X-ray (EDX) spectroscopic analysis equipped
Figure 5. TEM image (A) of the obtained composite nanofiber and EDX mapping of Ti (B) and W (C) elements in the S-1-1 sample with a scale bar of 500 nm.
nanofiber (Figure 5a), there are inner-bicontinuous (continuous air and continuous inorganic material) and outer-shell structures. Panels b and c in Figure 5 illustrate the distribution of Ti and W elements at the same position, respectively. In the outer-shell part, the distribution of Ti agrees well with the morphology in Figure 5a, whereas there is no signal in the EDX mapping of the W element. This result makes it clear that the outer-shell is mainly composed of TiO2. This is also supported by the XPS survey spectra (data now shown here) in which the ratio of Ti/W has a magnitude of 78.4/21.6. Furthermore, the mapping of the Ti element in Figure 5b is continuous, whereas there are only separated spots in Figure 5c (W mapping). This indicates that the anatase TiO2 acts as the frame and WO3 fills the gaps. The formation mechanism of the composite TiO2/WO3 nanofibers is very interesting. It is worth noticing the following issues: First, TTIP, the precursor of TiO2, shows stronger interaction with the PEO block (relative to PS block) before calcination, which has been validated via differential scanning calorimetry (DSC) and discussed in detail in our previous work.15 As a result, the TTIP accompanies PEO during electrospinning and annealing above the glass transition temperature of copolymers. Second, it is reasonable to expect a special interaction between the PS block and tungsten hexaphenoxide (the precursor of WO3) because both have C
DOI: 10.1021/acs.iecr.5b03578 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research benzene rings. Third, the blocks of PS and PEO in the copolymer are immiscible, resulting in a variety of self-assembly structures in the confined environment (nanofiber here).37 Then, we can describe the structure formation as follows: during the electrospinning of the blend solution of block copolymer, TTIP and tungsten hexaphenoxide in chloroform, the special interactions between PEO/TTIP and PS/tungsten hexaphenoxide coupled by the microphase separation of the block copolymer make it possible to obtain the complex structures. Subsequent thermal annealing induces further microphase separation of the block copolymer accompanied by migration of the precursors. Upon calcination at 450 °C (higher than the degradation temperature of PS and PEO blocks), the copolymer can be removed, and the precursors are transformed into TiO2 and WO3. The degradation of copolymer can be validated via thermogravimetric analysis (TGA, data not shown here). During the transition from the precursors to metal oxides, the tungsten hexaphenoxide shows a more obvious mass loss, leading to a lower component in the resulting composite nanofibers. This is the reason for the formation of TiO2 continuous frame and the WO3 filler in the gaps (shown in Figure 5), which agrees well with the stronger TiO2 signals in the XRD profile in Figure 3. The photocatalytic performance of the resulting fabrics depends crucially on its response range, which can be characterized by means of UV−vis spectra. The results of S0-1 (black), S-1-1 (red), and S-1-0 (blue) are shown in Figure 6. The absorption edge of neat TiO2 and WO3 are at roughly
Figure 7. Photocatalytic degradation of acetaldehyde in the presence of nanofibers under Vis light.
composite material has been extended to the visible light region, which has been validated via UV−vis data and the band gap (2.65 and 3.17 eV for TiO2/WO3 composite and neat TiO2, respectively); on the other hand, the existence of outershell TiO2 decreases the recombination possibility of holes and electrons in the composite, resulting in higher photocatalytic activity compared to neat TiO2 and WO3. This conclusion has good agreement with the results from Szilagyi and Bai.19,42 Our results indicate that electrospinning the blend solution of block copolymer and two kinds of precursor of inorganic material is an effective strategy to fabricate composite nanofibers. The fabrics composed of these nanofibers show enhanced photocatalytic activity.
4. CONCLUSIONS In this work, a “one-step” strategy to fabricate TiO2/WO3 composite nanofibers by coupling self-assembly of block copolymer containing the precursors of two kinds of metal oxides and the electrospinning technique has been established. Cigarlike composite nanofibers with inner-bicontinuous and outer-shell structures can be obtained. In these fibers, the TiO2 acts as a frame with WO3 filling the gaps. During the formation of these structures, the block copolymer plays a key role: on one hand, it provides the viscosity for electrospinning, and on the other hand, it can act as the structure template to induce the selective distribution of TiO2 and WO3 based on the special interaction between the copolymer and the precursors. The photocatalytic performance of the resulting fabrics under visible light has been investigated by taking acetaldehyde as an example. The results indicate that the composite TiO2/WO3 fabrics show improved photocatalytic activity compared with neat TiO2 and neat WO3, which can be attributed to the extension of the absorption edge to the visible region and the inhibition of recombination of photogenerated carriers. Our result is significant for the development of a fabrication strategy and improvement of the photocatalytic efficiency of TiO2.
Figure 6. UV−vis diffuse reflectance spectra of nanofibers S-0-1, S-1-1, and S-1-0 after calcining.
380 and 450 nm, respectively. This agrees well with the reported results.38−40 The composite nanofibers, however, exhibit similar absorption behaviors with WO3, that is to say, the absorption edge of the composite nanofibers has been successfully extended to the visible region relative to the neat TiO2. Furthermore, the band-gaps of S-0-1 (WO3), S-1-1 (TiO2/WO3), and S-1-0 (TiO2) obtained via the Tauc relation are 2.65, 2.65, and 3.17 eV, respectively.41 This result confirms that the band gap of the composite material is located in the visible light region. Figure 7 shows the photodegradation of the obtained fabrics under visible light by taking acetaldehyde as an example. The composite nanofibers exhibit more prominent photocatalytic activity relative to neat TiO2 and neat WO3. After irradiation by visible light for 60 min, 40% of the acetaldehyde molecules are decomposed, but 85% of the acetaldehyde molecules are intact when neat TiO2 or neat WO3 nanofibers are used as the photocatalyst. In other words, the performance of the composite nanofibers has been improved remarkably. This improvement can be attributed to the following issues: on one hand, the absorption edge of the
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Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51173036, 21104013, 51102274, D
DOI: 10.1021/acs.iecr.5b03578 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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21374027, 21234007, and 11104054) and Natural Science Foundation of Zhejiang Province (LQ13A040008).
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