Article pubs.acs.org/IECR
Synthesis of Mesoporous BiPO4 Nanofibers by Electrospinning with Enhanced Photocatalytic Performances Guoshuai Liu, Suwen Liu,* Qifang Lu, Haiyan Sun, and Zhiliang Xiu Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics, School of Material Science and Engineering, Qilu University of Technology, Jinan 250353, People’s Republic of China S Supporting Information *
ABSTRACT: Bismuth phosphate (BiPO4) nanofibers were prepared through a simple electrospinning method and followed by calcination treatment. The prepared samples were characterized with thermogravimeter and differential scanning calorimeter (TG-DSC), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption−desorption measurements, UV−vis absorbance spectroscopy, electrochemical impedance spectra (EIS), and the photoluminescence spectra. The diameter of the as-prepared fibers is about 200 nm. The nanofibers are mesoporous and composed of the linked nanoparticles with sizes of about 15 nm. The rate constant of degradation of APMP pulping effluent for mesoporous nanofibers is 1.55 times of that of the powders prepared by sol-gel method and 3.72 times of that of P25.
1. INTRODUCTION Semiconductor photocatalysts have been widely used in the control of environmental pollution for the degradation of organic pollutants.1,2 In a variety of semiconductor photocatalysts, TiO2 has been widely applied for its nontoxicity, low cost, and availability.3,4 However, the photocatalytic activity of TiO2 is not high enough due to the rapid recombination rate of photogenerated electron hole.5,6 Thus, the research of synthesizing new photocatalysts with high activity is very attractive. As a novel photocatalytic material, BiPO4 is an excellent photocatalyst and has superior activity to P25 (the commercial TiO2);7 therefore, the synthesis of BiPO4 has attracted much attention. Pan et al. synthesized BiPO4 nanocrystals via a high-temperature hydrolysis reaction8 and Li et al. synthesized BiPO4 nanoparticles by a microwave method.9 Generally speaking, the photocatalytic activity is closely related to the size and morphology of the nanomaterials, so the controllable synthesis of BiPO4 with special morphology is meaningful.10,11 One-dimensional semiconductor photocatalysts with high surface area and porosities have aroused much attention.12 Li et al. reported that anatase/TiO2 (B) core−shell nanofiber exhibited the enhanced photocatalytic activity.13 Liu and co-workers fabricated the biocomponent TiO2/SnO2 nanofibers, which showed an excellent photocatalytic activity for the degradation of organic pollutants.14 Zhao et al. synthesized Bi2WO6 nanofibrous mats and exhibited excellent visible photocatalytic property in the photodegradation of methylene blue (MB),15 but the research on the synthesis of BiPO4 nanofibers have not been reported yet. The electrospinning is a simple technique to prepare nanofibers in recent years.16−18 The electrospinning unit for nanofibers is a system for producing the ultrafine fibers with diameter of 20−1000 nm. The nanofiber generally owns the high specific surface area, small diameter, and large porosity.15−18 In addition, one-dimensional nanofibers by electrospinning not only are easy to recycle but also retain © 2014 American Chemical Society
their superior catalytic performance due to the nanoscaled building blocks of the fibers.19 It also appropriates to solve the problems of the recycling of the photocatalysts, which hinders their applications in the industrial application due to their small sizes and inactivation. Therefore, it is a great challenge to design and synthesize BiPO4 nanofibers with high photocatalytic activity and reuse properties. All these exciting researches indicate that the design of 1D novel BiPO4 nanostructures by electrospinning opens up new and wide opportunities. In this work, BiPO4 nanofiber was first prepared by a facile and rapid electrospinning method, and the formation mechanism of nanofibers was discussed. The photocatalytic activities were evaluated by the degradation of alkaline peroxide mechanical pulping (APMP) effluent under UV light irradation, and the BiPO4 nanofibers showed the better photocatalytic property than BiPO4 powders synthesized by sol-gel method. Alkaline peroxide mechanical pulping (APMP) processes are relatively new technologies in the pulp and paper industry, and APMP pulping effluent need to be treated before it can be discharged into the receiving water bodies. These explorations and experiments illustrate that the mesoporous BiPO4 nanofibers owned good prospects in industrial wastewater treatment. In addition, it can be used to guide the industrial production of BiPO4 photocatalysts by electrospinning process in the future.
2. EXPERIMENTAL SECTION 2.1. Preparation of Spinnable Precursor Sols. All reagents were of the analytical purity and used without further treatment. In a typical experimental procedure, 0.195 g of Bi (NO3)3·5H2O (0.4 mmol) and 0.5 g of citrate acid (2.6 mmol) Received: Revised: Accepted: Published: 13023
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electrochemical system with a counter electrode (Pt wire, 70 mm in length with a diameter of 0.4 mm), a working electrode (mass ratio, BiPO4 sample:acetylene black:poly(vinylidene fluoride) = 8:1:1 deposited on a 1 × 3 cm nickel foil, kept 12 h at 30 °C), and a reference electrode (a saturated calomel electrode, SCE). A quartz electrolytic cell was employed, filled with 150 mL of 0.5 M KCl solution. A 20 W Xe lamp was used as irradiation source.
were added into 1.4 mL of deionized water with magnetic stirring to form a suspension, and then 0.0812 g of (NH4)3PO4· 3H2O (0.4 mmol) were dissolved in 2 mL of deionized water to form a solution. The above suspension and the solution were mixed together, and subsequently nitric acid was added to adjust the pH value to 1. Finally, it was mixed with 10 mL polyvinylpyrrolidone (PVP, Mw = 1300 000) solution (0.1 g/ mL), which was obtained by being dissolved in anhydrous ethanol to form the spinnable precursor sols. 2.2. Process of Electrospinning and the Calcination of Samples. The precursor sol was drawn into the needle tube, and the inner diameter of the needle is 0.5 mm. The electrospinning voltage was set as 25 kV with pumping speed of 0.0015 mm/s, whereas the distance between needle and collector was 30 cm. After electrospinning, the mat-like gel fibers were obtained. Then it was heated to a targeted temperature and then cooled to the room temperature naturally in the furnace to obtain the final products. In this paper, BiPO4 powders prepared by sol-gel method were taken as a comparison. 2.3. Characterization. Thermogravimetry and differential scanning calorimeter (TG-DSC) were implemented on Labsys evo STA Simultaneous thermal analyzer (France) under air atmosphere. FT-IR spectrum was tested on infrared spectrometer (IR Prestige-21). The X-ray diffraction (XRD) patterns of the samples were measured on a D8 ADVANCE X-ray diffractometer (Bruker, Germany), using monochromatized Cu Kα (λ = 0.15418 nm) radiation with a scan range from 10 to 60°. The accelerating voltage and the applied current were 40 kV and 40 mA. The morphologies and microstructures of as-prepared samples were analyzed by a FESEM-4800 field emission scanning electron microscope (SEM, Hitachi). The UV−vis spectrum of the samples was recorded on a UV-2550 spectrophotometer (Shimadzu) in the wavelength range of 200−800 nm. The Brunauer−Emmett−Teller (BET) surface area was determined by nitrogen sorption using a Micromeritics ASAP 2020 analyzer. The photoluminescence spectra were obtained on Hitachi Fluorescence spectrophotometer F-7000. 2.4. Photocatalytic Activity Measurement. The photocatalytic activities of the samples were evaluated by the degradation of APMP pulping effluent under irradiation by using a 500 W Xe lamp with a cutoff filter (λ ≤ 400 nm), which was set about 12 cm from the liquid surface of the APMP suspensions, and the irradiation area was approximately 30 cm2. A total of 0.08 g of photocatalysts were put into 60 mL of APMP pulping effluent. 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 of solution were sampled and centrifuged to remove the photocatalysts. The filtrates were analyzed by the variations of the maximum absorbance (305 nm) in the UV−vis spectrum of APMP pulping effluent. The concentration ratio (C/C0) of APMP pulping effluent was calculated by the ratio of the absorbance value of the APMP. (The components of the APMP are complex, and the concentration is proportional to the absorbance of the APMP, so we calculated the C/C0 by the ratio of the maximum absorbance value of the APMP. C0 represents the concentration when the adsorption−desorption equilibrium.) The chemical oxygen demand (COD) of the APMP effluent was determined according to the standard procedures (APHA, 2005). 2.5. Photoelectrochemical Measurement. Photoelectrochemical measurement was performed on a PAR2273
3. RESULTS AND DISCUSSION TG-DSC curves of the gel fibers are shown in Figure 1. In the TG curve, there are four main weight losses. The first loss
Figure 1. TG-DSC curves of the BiPO4/PVP gel fibers at a heating rate of 10 °C/min.
below 260 °C (ca. 24.2%) is mainly due to the removal of ethanol and the absorbed water. The second loss stage (ca. 16.7%) in the range of 260−350 °C is attributed to removal of nitrate and citric acid in the gel fiber, and the third one (ca. 19%) from 350 to 450 °C results from the decomposition of the side chains of PVP.20,21 The fourth one (ca. 30.8%) from 450 to 530 °C is caused by the decomposition of PVP main chain.20 From the corresponding DSC curve, the exothermic peak at 340 °C is attributed to the decomposition of citric acid and nitrate. The exothermic peak around 400 °C corresponds to the decomposition of the side chain of the PVP,20 whereas the exothermic peak around 500 °C is caused by the decomposition of PVP. The total weight loss amounts to 92%. FT-IR spectroscopy is shown in Figure 2. For the gel fibers, the peaks from 950 to 3100 cm−1 are the characteristic vibrations of PVP.22 The vibrations at 1663 and 1292 cm−1 are assigned to the stretching vibration of CO and CN bonds in PVP, and the vibrations at 2957 and 2926 cm−1 are attributed to the vibrations of CH bond.22,23 As to the BiPO4 fibers by electrospinning and powders by sol-gel, the vibrations at 550, 850, 1000, and 1100 cm−1 are characteristic ν1, ν3, and ν4 vibration of PO43−.24 Reference to the previous work, those IR bands were attributed to the characteristic vibrations of BiPO4.25−28 As shown in the Supporting Information Figure S1, the FT-IR curve of the powders sample after 90 °C insulation, the vibrations at 3480 and 1630 cm−1 disappeared in the corresponding IR image. The vibrations at 3480 and 1630 cm−1 of the powders are attributed to the stretching and bending vibrations of adsorbed water.29 Figure 3 shows the XRD patterns of the nanofibers and the powders, which were in good agreement with the standard data 13024
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TEM images provide the further analysis for the microstructure of the products. BiPO4 nanofibers seen in Figure 5a
Figure 5. Typical TEM images of the BiPO4 fibers (a) and the powders (b). Figure 2. FT-IR Spectra of the gel fibers, the BiPO4 fibers, and the BiPO4 pwders.
indicates the typical fibrous morphology and the nanofibers are comprised of small nanopartilces with sizes of about 10−20 nm. Also, the porous structure is clearly observed in the nanofibers. As to the BiPO4 powders by sol-gel process, the TEM image shown in Figure 5b confirmed that the large particles composed of the BiPO4 powders. As is well known, the particle sizes play an important role in the dynamics of electron−hole recombination process and the small particle sizes were expected to exhibit high efficiency in photocatalysis.31 The nitrogen adsorption−desorption isotherms of BiPO4 fibers and BiPO4 powders are shown in Figure 6. The isotherm of BiPO4 fibers belongs to the type IV with a hysteresis loop observed in the range of (0.4−1.0) P/P0. The specific surface areas calculated by the multipoint Brunauer−Emmett−Teller (BET) method are 25.9 m2·g−1 for fibers and 4.2 m2·g−1 for powders. The pore size distribution of BiPO4 nanofibers calculated from the desorption branch of the nitrogen isotherm by the BJH (Barrett−Joyner−Halenda) method (inset in Figure 6a) indicates a relatively narrow range with some mesopores and the most probable pore size is about 8.32 nm, which is consistent with the TEM analysis. But for BiPO4 powders the pore sizes distribution almost has no significance due to its small specific surface. The photocatalytic activity of a material depends greatly on its electron structure, which determines the light absorption and the migration of the photogenerated electrons and holes.32 Diffuse reflectance spectroscopy (DRS) is a useful route to characterize the electronic states of the semiconductor materials. The DRS spectra of the BiPO4 nanofibers and powders are shown in Figure 7. The as-prepared BiPO4 fibers and powders both present the photoabsorption properties in UV light region. For the semiconductor materials, a classical Tauc approach is employed to estimate the Eg value of BiPO4 nanofibers according to the following equation: Ephoton = K(Ephoton − Eg)n/2, where K is a constant, the absorption coefficient, Ephoton is the discrete photon energy, and Eg is the
Figure 3. XRD patterns of the BiPO4 fibers and the BiPO4 powders.
of the pure hexagonal phase BiPO4 (JCPDS No. 15-0767). No impurity peaks were observed, indicating the high purity of the product. And the sharp diffraction peaks imply the good crystallinity.30 FE-SEM was used to characterize the microstructure of the fibers and powders. From Figure 4a, the length of the gel fiber is up to centimeter scale and its diameter is relatively uniform. The surface of the gel nanofibers is smooth, and the diameter is in the range of 300−400 nm. Figure 4b is the FE-SEM image for the fiber samples that were obtained by annealing the asspun gel fibers at 500 °C. Obviously, the as-formed nanofibers were shrinkage and the diameter of fibers has drastically decreased to about 200 nm after the calcination, resulting from the decomposition of PVP, (NH4)3PO4, and Bi(NO3)3.19 Figure 4c is the image of the powders by sol-gel method. The sample is composed of large particles that are in a disordered arrangement.
Figure 4. FESEM images of the gel fibers (a), the BiPO4 fibers (b), and powders (c). 13025
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Figure 6. Nitrogen adsorption−desorption isotherms of BiPO4 fibers (a) and powders (b) and the pore size distribution (inset) of BiPO4 fibers (a) and powders (b).
pulping effluent photodegraded by TiO2 (P25) was also performed. In order to distinguish the portion that the APMP pulping effluent was removed by adsorption and photodegradation, the dark equilibration was carried out. Figure 8a shows the temporal evolution of the APMP pulping effluent concentration. After 30 min of dark equilibration, the percentage of APMP pulping effluent adsorbed on the surface of the BiPO4 nanofibers, BiPO4 powders and P25 is 16%, 8%, and 11%, respectively, which indicates that BiPO4 nanofibers can absorb more molecules of waster paper water on their surface than BiPO4 powders and P25. When the lamp turns on, the concentration starts to decrease. Figure 8a shows the photodegradation rates of APMP pulping effluent by the BiPO4 nanofibers, the BiPO4 powders, P25, and without any photocatalysts. The photodegradation ratio reached 88% after irradation for 120 min in the presence of the BiPO4 nanofiber, whereas that of the P25 and the BiPO4 powder were 41%, 72% under the same conditions. Without any photocatalysts, the photodegradation ratio was about 3% after irradiation for 120 min, indicating BiPO4 nanofiber exhibits high photocatalytic activity. Figure 8c shows the UV−vis spectra of APMP pulping effluent degraded by the BiPO4 nanofibers. With the degradation of APMP pulping effluent, the UV−vis spectra peak position of the APMP pulping effluent shifted toward shorter wavelength, which may be caused by the molecular structure decomposed in photocatalytic process.38 It was found that the photocatalytic degradation process followed pseudo-first-order kinetics. The first-order reaction kinetics, ln(C0/C) = kt, where k is the first-order rate constant and t is the irradiation time.39 A greater k indicates a faster
Figure 7. Diffuse reflectance spectra of the BiPO4 fibers and the powders. The inset is the Kubelka−Munk function of the BiPO4 fibers and powders.
band gap energy.33 Among them, n is determined by the type of optical transition of a semiconductor (i.e., n = 1 for direct transition and n = 4 for indirect transition).34−36 As the hexagon BiPO4 exhibits the characteristic of direct band transition, so the value of n is 1. The band gap energies of BiPO4 could be estimated from plots of (Ephoton)2 versus energy (Ephoton). The intercept of the tangent to the x axis is very close to the band gap energy. As illustrated in Figure 7, the estimated band gap for BiPO4 fibers and powders was about 4.31 and 4.5 eV, which may be caused by the particle size effect.37 Photocatalytic activity was evaluated by degradating the APMP pulping effluent solution. As a comparison, the APMP
Figure 8. (a) Degradation profiles of APMP pulping effluent. (b) Kinetic linear simulation curves of APMP pulping effluent photocatalytic degradation. (c) UV−vis spectra of APMP pulping effluent degraded by the BiPO4 nanofibers. 13026
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be explained that the bleaching of the dye solution goes beyond the inactivation of the chromophore groups in the molecular structure.38 The inactivation of the chromophore groups of APMP leads to the bleach of the solution, but not necessarily to the mineralization of the dye. The recycling measurements of the photocatalysts were carried out to test the stability of the sample during the photocatalytic process. After four recycles for the photodegradation of APMP, the photocatalysts did not exhibit any significant loss of activity, as shown in Supporting Information Figure S2, confirming BiPO4 is not photocorroded during the photocatalytic oxidation of the pollutant molecules. The SEM image of the BiPO4 nanofibers reused for four recycles (Supporting Information Figure S3) shows no obvious change, indicating its good stability. Many factors can affect the photocatalytic activity including the surface state, crystallinity, specific surface area, and so on. The large surface area of the photocatalysts could provide more reaction sites, which favors improving its activity.37−40 Therefore, the porous BiPO4 nanofibers is more conducive for the photocatalytic reaction than the BiPO4 powders, and the mesoporous structure can facilitate the molecular transport between the reactants and the photocatalysts.40−42 Additionally, for the 1D structural samples, some localized P 3p and O 2p related gap states are introduced in BiPO4. The interaction between P and O atoms by integrating the P 3p and O 2p states in the conduction band and valence band could favor the generation and separation of the photoelectron−hole pairs and, thus, enhance the photocatalytic activity.7 Therefore, the porous 1D fibers by electrospinning exhibit the enhanced catalytic activity than the BiPO4 powders prepared by sol-gel process. In order to further understand the different activity for the BiPO4 nanofibers and the powders, the electrochemical impedance spectra (EIS) and the photoluminescence test
reaction rate. The linear relationship of ln(C0/C) versus time for degradation of APMP pulping effluent is shown in Figure 8b. The calculated k values for the BiPO4 nanofibers, the BiPO4 powders obtained by sol-gel method and P25 is 0.01638, 0.00995, and 0.00439 min−1, respectively. The rate constant for the BiPO4 nanofibers is approximately 1.55 times of that of BiPO4 powders and 3.72 times of that of P25, which confirms that the BiPO4 nanofibers presents the higher photocatalytic activity than the corresponding powders and P25. Because the APMP effluent is industrial wastewater, the components are complex. To further investigate the evolution process of BiPO4, the COD experiment was performed. From Figure 9, the COD removal efficiency of the BiPO4 nanofibers
Figure 9. COD removal of APMP in the photocatalytic process.
reaches a value of 58% after 120 min of irradiation, indicating most organic component of the APMP pulping effluent are degraded, whereas the degradation ratio is 88%. This ratio of COD removal is lower than the degradation of dye, which can
Figure 10. Electrochemical impedance spectra of BiPO4 powders (a) and fibers (b) in dark; BiPO4 powders (c) and fibers (d) under UV light illumination. 13027
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Industrial & Engineering Chemistry Research were carried out. The corresponding electrochemical impedance spectra (EIS) are shown in Figure 10. As shown in Figure 10a and b, the EIS plots of BiPO4 powders and nanofibers are almost straight in dark, and their impedance arc radii tend to infinity. From the perspective of the electrochemical reaction, this means two points: (1) reaction on the electrode hardly occurs, and the electrode reaction need overcome the great energy barrier; (2) electrode reaction rate is very slow, so the concentration of the reactants participating in the electrode reaction is low.40 The samples as the work electrode in dark state need overcome the greater energy barrier. When the samples were treated in UV light, the impedance arc radius decreases obviously, as is shown in Figure 9c and d, and it owns the BiPO4 photocatalytic activity reduced electrode reaction energy barrier, accelerating the electrode reaction. Furthermore, the impedance arc radius of the nanofibers is smaller than the powders, and it also shows the BiPO4 nanofiber electrode owned the good charge transport properties, which implies a better photocatalytic activity than that of the BiPO 4 powders.43−45 The photoluminescence (PL) emission spectra have been widely used to investigate the efficiency of charge carrier trapping, immigration, and transfer and to understand the fate of electron/hole pairs in semiconductor particles. The stronger the PL intensity is, the faster electro-hole pairs recombination rate is.46,47 The photoluminescence spectra of the BiPO4 fibers and the powders are shown in Supporting Information Figure S4. The excitation wavelength is 320 nm. The PL spectra reflect the combination rate of electro-hole pairs.48,49 The emission spectra of the BiPO4 powders is stronger than that of the fibers, which indicates the electro-hole pairs recombination rate of the powders is faster than that of the fibers.46−49 Therefore, the mesoporous BiPO4 fibers by electrospinning process exhibits the enhanced the photocatalytic activity comparing to the powders prepared by sol-gel method.
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ASSOCIATED CONTENT
* Supporting Information S
TG and FT-IR tests, cycling runs in photocatalytic degradation, SEM image, photoluminescence spectra. This material is available free of charge via the Internet at http://pubs.acs. org/.
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ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (Grant No. 51172133), Key Project of Chinese Ministry of Education (Grant No. 211098), Project of University Innovation of Jinan (Grant No. 201303063), and the Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J13LA01). The authors also thank the Analytical Center of Qilu University of Technology for technological support.
4. CONCLUSION BiPO4 nanofibers with diameters about 200 nm have been successfully prepared for the first time by the electrospinning method. The porous nanofibrers with high surface area and a relatively narrowed mesopore size distribution showed the highest photocatalytic activity for the photodegradation of APMP pulping effluent. The reasons for the different photocatalytic activity of the BiPO4 fibers and the powders were discussed. This work provides a facile and economical way to fabricate BiPO4 nanofibers, which can be used in the degradation of APMP pulping effluent in the practical application.
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AUTHOR INFORMATION
Corresponding Author
*S. Liu. E-mail:
[email protected]. Tel.: +86-531-8963-1231. Fax: +86-531-8963-1227. Notes
The authors declare no competing financial interest. 13028
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dx.doi.org/10.1021/ie4044357 | Ind. Eng. Chem. Res. 2014, 53, 13023−13029
