Article pubs.acs.org/IECR
Controllable Synthesis of Bi2WO6 Nanofibrous Mat by Electrospinning and Enhanced Visible Photocatalytic Degradation Performances Gang Zhao, Suwen Liu,* Qifang Lu, and Lingjun Song Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics, Shandong Polytechnic University, Jinan 250353, China ABSTRACT: Bi2WO6 nanofibrous mat has been successfully synthesized by a simple electrospinning process. TG-DTA, FT-IR, XRD, SEM, and UV−visible diffuse reflectance spectra were used to characterize the mat. The results indicated that the mat was composed of one-dimensional nanofibers, whose diameter was about 300 nm. The one-dimensional nanofibers consisted of Bi2WO6 nanoparticles with diameter 112 nm. Also, the Bi2WO6 nanofibrous mat exhibited excellent visible photocatalytic property in the photodegradation of methylene blue. Meanwhile, the photocatalyst synthesized by electrospinning is helpful for separation and recycling. What’s more, the preparation method is suitable for large-scale commercial production. So, the work has great application value in the future.
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and nanoparticles composed of the fibers are large. Because the initial Bi2WO6 particles were obtained by a hydrothermal method, crystal growth underwent hydrothermal and calcining processes. Besides, the three steps are finicky. Therefore, synthesizing 1D Bi2WO6 nanostructure by a simple and facile route is a great challenge. In our preliminary work,30 we have prepared 1D Bi2WO6 sheet structure, but some Bi2WO6 nanoparticles mixed in it. Through painstaking trial and error, we discovered how to prepare a kind of clear precursor sol that impedes 1D Bi2WO6 nanofibers electrospinning preparation. On the basis of our former results, we optimized the system and prepared Bi2WO6 nanofibrous mat by a simple electrospinning method. The formation of Bi2WO6 nanofibers was discussed. The photocatalytic activities were evaluated by the degradation of methylene blue (MB) under visible light (λ ≥ 400 nm). The as-prepared Bi2WO6 nanofibrous mat presented better visible photocatalytic property than Bi2WO6 powders synthesized by solid-state reaction (SSR). In addition, it can guide the industrial production of Bi2WO6 photocatalyst by electrospinning in the future. Therefore, this experiment was worthy of being researched.
INTRODUCTION Nanofibers have received widespread attention for their high specific surface area, porous structure,1,2 and easy recycling. Electrospinning3 is a simple and versatile route for the production of nanofibers in recent years, and electrospinning combined with hydrothermal,4 sol−gel,5−7 and calcination techniques8 is used to prepare oxides. Electrospinning can assemble and organize inorganic, organic, and even biological components in one-dimentional nanomaterials, which is an exciting direction for developing novel multifunctional materials.9−11 One-dimensional (1D) nanomaterials12−14have versatile morphologies and excellent physical and chemical properties superior to the corresponding bulk counterparts, so they are widely investigated not only for their fundamental scientific significance but also for their diverse technological applications. As an important photocatalyst, bismuth tungstate (Bi2WO6) has become a research hot spot in recent years, and the photocatalytic activities are closely related to the structure of the photocatalysts.15−19 Hence, controlling the structure of Bi2WO6 can effectively improve its catalytic performance.20 For example, Wu21 et al. designed hierarchical Bi2WO6 nestlike structures, and Ma22 et al. synthesized three-dimensional (3D) hierarchical umbilicate Bi2WO6 microspheres, and both of them exhibit excellent photocatalytic properties under visible light irradiation compared to zero-dimensional Bi2WO6 powder.23−26 However, the photocatalysts cannot be recycled due to their small size, which hinders their industrial application. By an amazing coincidence, one-dimensional nanofibers by electrospinning are not only easy to recycle but also retain superior catalytic performance, for the building blocks of the fibers are nanoscaled. All this exciting research indicates that design of 1D novel Bi2WO6 nanostructures by electrospinning27,28 opens up a new wide opportunity. Sheng29 et al. designed Bi2WO6 nanofibrous mat by combining hydrothermal synthesis with electrospinning and calcination. However, the nanofibers are discontinuous, © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Preparation of Clear Precursor Sols. All reagents were of analytical purity, received from Shanghai Chemical Company, and used without further purification. In a typical synthesis, 2.52 g (0.012 mol) of citric acid was dissolved in 7 mL of distilled water, 3 mL of HCl solution was added into the above solution, and then 1.942 g (0.004 mol) of Bi(NO3)3·5H2O was put into the mixture with magnetic stirring at room temperature. Meanwhile, 0.329 g (0.001 mol) of Na2WO4·2H2O was dissolved in 10 mL of distilled water. Received: Revised: Accepted: Published: 10307
April 15, 2012 July 8, 2012 July 17, 2012 July 17, 2012 dx.doi.org/10.1021/ie300988z | Ind. Eng. Chem. Res. 2012, 51, 10307−10312
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3. RESULTS AND DISCUSSION TG-DTA curves of the as-spun precursor are displayed in Figure 1A, which gives a four-step combustion process. The
Subsequently, Na2WO4 aqueous solution was dropped slowly to the Bi(NO3)3 solution while the mixture was still clear. Afterward, 3 mL of the above mixture was joined into a certain concentration of PVP (poly(vinylpyrrolidone), K-90) ethanol solution (1.0 g of PVP was dissolved in 10 mL of ethanol) with continuous stirring. Thus the spinnable precursor sols were obtained. 2.2. Electrospinning. All the precursor sols were transferred into a 10 mL syringe which was attached to a stainless steel needle with inner diameter of 0.5 mm and then were ejected from the needle with a voltage of 32 kV. The distance between the needle and collector was 25 cm. The sols were spun at about 28 °C in air by electricity. The composite fibers in the form of nonwoven mats were collected from a collector plate (Alfoil) and dried at 80 °C for 12 h. According to the TG, XRD, and FT-IR results, the dry fibers were put into an air-atmosphere programmable tube furnace for heat treatment. The fibers were calcined from room temperature to 500 °C at a rate of 1 °C/min and kept for 1 h. The products were naturally cooled to room temperature in the furnace to obtain the final products. Then the obtained nanomaterials were given characterization. 2.3. Characterization. The thermogravimetric (TG) curve and differential thermal analysis (DTA) curve were obtained on a TGA/SDTA851 (Mettler Toledo). The X-ray diffraction (XRD) patterns of the samples were measured on a D8 Advance X-ray diffractometer (Bruker, German), using monochromatized Cu Ka (λ = 0.154 18 nm) radiation at a scan range from 25 up to 90 deg. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. FT-IR spectra were recorded with a Bruker Vertex 70 FTIR spectrometer in the range of 400−4000 cm −1 . The morphologies and microstructures of as-prepared samples were analyzed by a FESEM-4800 field emission scanning electron microscope (Hitachi). UV−visible diffuse reflectance spectra of the samples were taken with a UV-2550 spectrophotometer (Shimadzu) in the wavelength range of 200−700 nm. 2.4. Photocatalytic Degradation of MB. The photocatalytic activities of the samples were evaluated by the degradation of MB under simulated sunlight irradiation by using a 500 W Xe lamp with a cutoff filter (λ ≥ 400 nm). The MB initial concentration was 20 mg/L. A 0.06 g amount of photocatalysts was put into 40 mL of MB solution. Before the photodegradation experiment was initiated, the suspension was magnetically stirred in the dark for 30 min. Once the photodegradation experiment started, at given time intervals, 4 mL aliquots solution were sampled and centrifuged to remove the photocatalysts. The filtrates were analyzed by the variations of the absorption-band maximum (664 nm) in the UV−vis spectrum of MB with a UV-2550 spectrophotometer. The filtrates were not returned to the exposed solution, and the photocatalyst was washed and collected for recycling use. The photodegradation efficiency of the MB was estimated according to the following formula: η = [(A0 − At)/A0]*100%,31 where A0 and At are the absorbance of the pre- and postirradiation of the MB solution, respectively. For photocatalytic activity comparison, the Bi2WO6 nanoparticles were synthesized via the solid-state reaction (SSR) method.
Figure 1. (A) TG-DTA curves of the as-spun precursor; (B) XRD patterns of the Bi2WO6 nanofibers (T = 500, 600, and 700 °C); (C) FT-IR of PVP/Bi2WO6 composite fibers and Bi2WO6 nanofibers calcined at 500 °C.
curves are performed in the temperature range of 50−800 °C to investigate the amounts of organics in the precursor. From the TG-DTA curves, all the volatiles (H2O, HCl, and ethanol), organic components (PVP, citric acid), and NO3− groups are removed completely below 500 °C, and a metal oxide phase was obtained. An initial weight loss (∼9.2%) step occurs at around 200 °C. Thereinto, the weight loss step in the range of 50−100 °C is due to the evaporation of absorbed water and trapped HCl and ethanol. Removal of the crystal water molecules of the nitrates and melting organic component take place between 100 and 200 °C. These processes are endothermic, which is proved in the DTA curve. Then, the side chain of PVP oxidative decomposition and the complete decomposition of citric 10308
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Figure 2. (a) SEM image of PVP/Bi2WO6 composite nanofibers; (b) Bi2WO6 nanofibers calcined at 500 °C for 1 h.
Figure 3. SEM images of Bi2WO6 nanofibers prepared with different heating rates: (a) 3; (b) 5; (c) 7 °C min−1 annealing at 500 °C for 1 h.
Figure 4. SEM images of Bi2WO6 with heating rates 1 °C min−1: (a) T = 100 °C for 0.5 h, then calcined to 500 °C for 1 h; (b) T = 300 °C for 0.5 h, then calcined to 500 °C for 1 h.
acid32,33 mainly occur over 200−360 °C. As the decomposition process is an exothermic chemical reaction, there is an obvious exothermic peak between 200 and 360 °C from the DTA curve, while the weight loss is about 29.8%. The NO3− ions decomposed at 360−410 °C, which is also an exothermic chemical process, and the weight loss is about 9%. The significant weight loss of approximately 36.3% between 410 and 500 °C is attributed to the complete decomposition of the main polymer chain of PVP, after which the weight of the samples remains constant. The total weight loss amounts to 84.3%. There is a relaxative exothermic peak between 500 and 800 °C from the DTA curve, but the weight of the samples remains constant, which indicates the phase change of metal oxide. According to the XRD patterns shown in Figure 1B, the crystal change is confirmed. After the fibers calcined at 500 °C, the diffraction peaks can be indexed to pure orthorhombic Bi2WO6 (JCPDS, No. 39-0256) with the lattice constants of a = 5.45 Å, b = 16.43 Å, and c = 5.44 Å. When calcined at 600 °C, the products are composed of Bi2WO6 and Bi3.84W0.16O6.24. When calcined at 700 °C, the diffraction peaks can be indexed to
Bi3.84W0.16O6.24 (JCPDS, No. 43-0447), and the lattice constants of Bi3.84W0.16O6.24 are a = 5.57 Å, b = 5.57 Å, and c = 5.56 Å. Figure 1C compares the FT-IR spectra of composite fibers and Bi2WO6 nanofibers calcined at 500 °C. For the composite fibers, they consist of PVP, a little citric acid, ethanol, and some inorganics. As shown in Figure 1C, the main peaks belong to PVP. The band located at 3400 cm−1 can be attributed to the symmetric vibration of −OH groups of PVP, and the CO stretching vibration characteristic peak locates at 1656 cm−1 in the PVP. The bands at 2956 and 1464 cm−1 correspond to the −CH2 absorption of PVP. In the composite fibers, the band located at 844.8 cm−1 is due to the Bi−O band. After calcination at 500 °C, the peak at 844.8 cm−1 becomes sharp. Meanwhile, the band located at 821.6 cm−1 belongs to the W− O band. Therefore, combining FT-IR spectra and XRD patterns of the products calcination at 500 °C, a reasonable conjecture may be drawn that pure Bi2WO6 has been obtained. The morphologies of the electrospun composite fibers and the nanofibers calcined at 500 °C are shown in Figure 2. The 10309
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Scheme 1. Bi2WO6 Nanofibers Formation Mechanism by Electrospinning
PVP/Bi2WO6 composite fibers have a one-dimensional texture structure and are randomly oriented to form nonwoven mats (Figure 2a). The diameter of these composite fibers is about 2 μm, and the length of individual fibers is up to tens of micrometers. After calcination at 500 °C, the mats with welldefined fiber texture still retained, indicating that the removal of PVP and calcination at high temperature cannot destroy onedimensional morphology of fibers. The diameter of fibers has drastically decreased to about 300 nm due to the removal of PVP and calcination at high temperature, and the onedimensional nanofibers consist of Bi2WO6 nanoparticles with diameter of 112 nm (Figure 2b). To investigate the effect of heating rate on the morphology of Bi2WO6 fibers, the fibers were calcined at 500 °C for 1 h with different heating rates. Figure 2b and Figure 3 show SEM images of the Bi2WO6 nanofibers prepared with different heating rates. When the heating rate was 1 °C· min−1, a nanofibrous mat composed of Bi2WO6 fibers was formed (Figure 2b). Increasing the heating rate to 3 °C·min−1, the diameter of Bi2WO6 fibers became large (Figure 3a), whereas further increasing to 5 °C·min−1 resulted in the formation of Bi2WO6 belts and the discontinuity of the 1D nanostructures (Figure 3b). Even as the heating rate increased to 7 °C·min−1, Bi2WO6 nanobelts were also destroyed. To research the effect of PVP on the calcination process, the fibers were calcined for two steps with the heating rate of 1 °C·min−1 and the second step remaining the same. The fibers were first calcined to 100 °C when PVP does not melt and kept at that temperature for 0.5 h. With the heat treatment, the composite fibers shrank. After being calcined at 500 °C for 1 h, Bi2WO6 fibers became hollow structures (Figure 4a). When first calcined to 300 °C for 0.5 h, PVP completely melted, and the self-assembly of Bi2WO6 formed 1D sheet structures (Figure 4b). Based upon the above experiment results, a possible formation mechanism of Bi2WO6 nanofibrous mat is described in Scheme 1. In the preparation of the initial precursor solution, bismuth(III) nitrate pentahydrate first dissolved in citric acid solution, and the addition of HCl solution provided an acidic condition to inhibit the hydrolysis of Bi3+ ions, while citric acid could completely complex with Bi3+ ions. Sodium tungstate solution is dropped slowly into the above solution, [WO4]2− adhering on the Bi3+ ion surface. The precursor solution is transparent due to the complexation action of citric acid (Scheme 1, part 1).
In the electrospinning process, the precursor sols form beetling liquid drops in the high-voltage electrostatic field. When the charge repulsion is larger than surface tension of the liquid drop, the composite nanofibrous mat formed (Scheme 1, part 3). PVP is not only the adhesive but also the excellent template in this system. In the calcination process, the morphology of Bi2WO6 nanofibrous mat is influenced greatly by the heating rate (removal rate of PVP). When the heating rate is fast, the Bi2WO6 crystalline structure is broken. But at a slow heating rate, the 1D Bi2WO6 nanostructure is completely destroyed. So the removal rate of PVP plays an important role in the formation of Bi2WO6 nanofibrous mat (Scheme 1, part 5). Diffuse reflectance spectroscopy is a useful tool to characterize the electronic states of optical materials. Figure 5 shows the
Figure 5. Diffuse reflectance spectra of Bi2WO6 nanofibers. Inset is the plot of (AEphpton)2 ∼ Ephpton.
UV−visible diffuse reflectance spectrum of Bi2WO6 nanofibers. It can be seen that the Bi2WO6 nanofibers have a steep absorption edge in the visible range, indicating that the absorption relevant to the band gap is due to the intrinsic transition of the nanomaterials rather than the transition from impurity levels. The plot of (AEphoton)2 ∼ Ephoton is shown in the inset of Figure 5. The extrapolated value (the straight line to the X-axis) of Ephoton at A = 0 gives an absorption edge energy corresponding to Eg = 2.727 eV, which is smaller than that of powder (SSR) obtained at high temperature,34 indicating that the activity of Bi2WO6 nanofibers is higher than that of the powder (SSR). 10310
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Figure 6. (a) Adsorptivity of Bi2WO6 nanofibers for MB; (b) temporal evolution of the spectra during the photodegradation of MB mediated by the Bi2WO6 nanofibers under visible light illumination (λ ≥ 400 nm). Inset shows the photodegradation rates between Bi2WO6 nanofibers and Bi2WO6 (SSR).
Figure 6a shows the adsorptivity of Bi2WO6 nanofibers for MB. The equilibrium between adsorption and desorption is established in dark, and 60.0% of MB was adsorbed by Bi2WO6 nanofibers for 30 min. According to the degradation test of MB under visible light (λ ≥ 400 nm) shown in Figure 6b, photodegradation efficiency (η) of Bi2WO6 nanofibers for MB reaches 94.8% for 3.5 h. In the same experimental conditions, the photodegradation efficiency of Bi2WO6 powder (SSR) for MB is 44.1%. The photocatalytic mechanism of Bi2WO6 nanofibers is that Bi2WO6 is a strong acid oxide.35 Bi2WO6 nanofibers, prepared in strongly acidic system, possess many strong acid sites, which could adsorb hydroxide free radical of MB. So Bi2WO6 nanofibers exhibit strong adsorption ability for MB. Meanwhile, the short distance between the MB and the photocatalyst surface allows the photogenerated electrons, holes, or radicals to reach the pollutant easily. Additional, the 1D structure leads to the band gap narrowing and the absorption edges of Bi2WO6 nanofibers shifting obviously to the infrared region; therefore, Bi2WO6 nanofibers present a strong absorption ability in the visble light region. According to the degradation test of MB, the prepared Bi2WO6 nanofibers by electrospinning method exhibit enhanced photocatalytic activity under visible light. Besides, the Bi2WO6 nanofibers are also repeatedly used to investigate photodegradative stability. After four recycles for photodegradation of MB, the sample does not exhibit obvious loss of activity, indicating that the catalyst is very stable in the cycling degradation process under visible light irradiation (Figure 7).
Figure 7. Cycling degradation curve of MB mediated by Bi2WO6 nanofibers under visible light illumination.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: +86-531-89631231. Fax: +86-531-89631227. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51172133), the Key Project of Chinese Ministry of Education (Grant No. 211098), the Ministry of Education of Shandong Province (Grant No. J09LD23), and the science and technology development plan of Shandong Province (2011SJGZ13). The authors also thank the Analytical Center of Shandong Polytechnic University for technological support.
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4. CONCLUSIONS The Bi2WO6 nanofibrous mat is fabricated by a facile electrospinning method. The heating rate (PVP removing rate) plays an important role in the formation of nanofibrous mat. Also, the as-prepared Bi2WO6 nanofibrous mat exhibits excellent adsorption ability and visible photocatalytic property in the degradation of MB compared with Bi2WO6 powders. In addition, the nanofibrous mat favors recycling, which is an obvious advantage for liquid photocatalysis in practical applications.
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