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Electrospun magnetically separable calcium ferrite nanofibers for photocatalytic water purification
Journal of Magnetism and Magnetic Materials 428 (2017) 92–98
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Electrospun magnetically separable calcium ferrite nanofibers for photocatalytic water purification
MARK
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A.M. EL-Rafeia, , Amer S. El-Kallinyb, Tarek A. Gad-Allahb a Refractories, Ceramics and Building Materials Department, National Research Centre, 33 EL Bohouth St. (former EL Tahrir St.), P.O. 12622, Dokki, Giza, Egypt b Water Pollution Research Department, National Research Centre, 33 EL Bohouth St. (former EL Tahrir St.), P.O. 12622, Dokki, Giza, Egypt
A R T I C L E I N F O
A BS T RAC T
Keywords: Electrospinning Nanofibers CaFe2O4 Magnetically separable materials Photocatalyst
Three-dimensional random calcium ferrite, CaFe2O4, nanofibers (NFs) were successfully prepared via the electrospinning method. The effect of calcination temperature on the characteristics of the as-spun NFs was investigated. X-ray diffraction analysis showed that CaFe2O4 phase crystallized as a main phase at 700 °C and as a sole phase at 1000 °C. Field emission scanning electron microscopy emphasized that CaFe2O4 NFs were fabricated with diameters in the range of 50–150 nm and each fiber was composed of 20–50 nm grains. Magnetic hysteresis loops revealed superparamagnetic behavior for the prepared NFs. These NFs produced active hydroxyl radicals under simulated solar light irradiation making them recommendable for photocatalysis applications in water purification. In the meantime, these NFs can be easily separated from the treated water by applying an external magnetic field.
1. Introduction The most serious problem facing photocatalytic treatment processes of water and wastewater is the separation and recycling of the photocatalyst [1]. Many attempts were done to solve this problem through the immobilization of the photocatalyst on different types of substrates [2–4]. However, the efficiency of the photocatalyst usually decreases after immobilization due to mass and light transfer limitations caused by the decreased surface-to-volume ratio, absorbance of light by the substrate, and lack in movement of the photocatalyst particles [5]. Recently, magnetically separable photocatalysts were taking a lot of interest as they can be effectively recycled by applying an external magnetic field, so they could overcome the limitation of separation from the treated effluent [6–8]. Moreover, in such a kind of photocatalysts, reduction in the active surface area is much lower than in the case of immobilization over substrates. Thus, using magnetically separable materials with photocatalysts keeps their photoactivity close to that obtained when using the photocatalysts alone in slurry system [9]. The spinel ferrites (MFe2O4, M = Ca, Mg, Zn, Co, Ni, Cd, etc.) are the most important magnetic materials which have been widely used in electronic devices, information storage, magnetic resonance imaging, and drug-delivery technology [10]. They also have attracted much attention due to their importance in environmental remediation [11].
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Among the ferrite family, calcium ferrite has attracted much interest due to its interesting catalytic, optical, and magnetic characteristics [11,12]. Presence of ferrite particles in a nanoscale form provides more advantages over the bulk ferrites [13]. Among different nanoshapes, Nanofibers (NFs) were selected to study their magnetic and photocatalytic behaviors in this study. For that purpose, electrospinning technique was selected for fabricating these functional NFs as it is a simple, versatile, and relatively inexpensive technique [14]. In addition, it has the advantage of being a single-step methodology, performing synthesis of continuous fibers, and producing uniform NFs. This technique is based on inducing static electrical charges on a polymeric solution at a certain viscosity. The self-repulsion of the charges causes the solution to stretch into a fiber with a simultaneous solvent evaporation by the action of an electric field [15]. In this paper, CaFe2O4 NFs with diameters in the range of 50– 150 nm were synthesized via electrospinning method. The prepared materials showed good magnetization with noticeable photocatalytic activity under simulated sunlight, which makes them recommendable to be used as magnetically separable photocatalysts.
Corresponding author. E-mail addresses: [email protected], [email protected] (A.M. EL-Rafei).
http://dx.doi.org/10.1016/j.jmmm.2016.12.020 Received 10 November 2016; Received in revised form 10 December 2016; Accepted 10 December 2016 Available online 11 December 2016 0304-8853/ © 2016 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 428 (2017) 92–98
A.M. EL-Rafei et al.
dried as-prepared Ca(NO3)2/ Fe(NO3)3/PVP NFs mats utilizing SDT Q600 V20.9 Build 20 equipment (TA Company, USA). The sample holder was heated in air at a rate of 10 °C/min, in the temperature range from ambient temperature up to 1000 °C. The compositions of the formed crystals were determined using energy dispersive X-ray spectrometer (EDX, TEAM™ EDS Analysis System) connected to the FE-SEM. X-ray diffraction (XRD) analyses of the powders were performed using BRUKUR D8 ADVANCE with secondary monochromatic beam CuKα radiation at 40 kV and 40 mA. The magnetization hysteresis loop was measured by Riken Denshi BH-55 vibrating sample magnetometer (VSM) at room temperature. The diffuse reflectance spectra in the UV–visible range were measured in the wavelength range 200–900 nm using JASCO spectrophotometer (model V-570, Japan) equipped with an integrating sphere attachment. BaSO4 was used as a reference material in the diffuse reflectance spectra measurements.
2. Experimental section 2.1. Experimental approach Electrospinning technique was used to prepare calcium ferrite NFs. The produced as-spun fibers were examined by Field Emission Scanning Electron Microscope (FE-SEM) in order to investigate their microstructure. Then, their weight loss by temperature was determined to select the range of the calcination temperature. Full characterization, including the formed crystalline phases, the morphology, and the composition of the calcined NFs was done. Besides, the magnetic behavior and the optical properties were determined in order to select the best calcination temperature. Finally, the evaluation of the photocatalytic activity was done by Photoluminescence (PL) technique. 2.2. Preparation of calcium ferrite NFs
2.4. Assessment of NFs photocatalytic activity Various combinations of the factors that control the quality of the electrospun fibers (e.g., composition of the electrospinning solution and its viscosity, applied voltage, and distance between collector and nozzle) were investigated by try-and-error method. The reported conditions are the optimal ones that gave fibers of homogeneous structure and high quality. The fibers were prepared by the dissolution of 2.25 g of Polyvinylpyrrolidone (PVP), (Mw ≈1,300,000, Aldrich) in a mixed solvent composed of 18 mL of ethanol and 8 mL deionized water, followed by magnetic stirring for 1 h to ensure the dissolution of PVP. Then 0.9564g of Ca(NO3)2·4H2O and 3.256g of Fe(NO3)3·9H2O were added into the above solution and were further magnetically stirred for about 1 h at room temperature to form a homogeneous solution. The electrospinning process was performed by using Na-Bond Electrospinning device (China). The formed solution was transferred into a syringe equipped with a metallic capillary nozzle connected to a high power supply. The voltage was adjusted at 30 kV. The inner diameter of the used nozzle was 0.49 mm and its height from the collector was set at 11 cm. The selected flow rate was 1 mL/h. The electrospun fibers were collected on a rotating drum working at 750 rpm. Then, the electrospun NFs samples were dried at 105 °C for 24 h and finally calcined for 1 h at different temperatures; 500 °C, 700 °C, 800 °C, 900 °C, and 1000 °C in the ambient atmosphere. A flowchart of the methodology for preparing the NFs is given in Fig. 1.
Photocatalytic activities of the calcined NFs were evaluated by detection of hydroxyl radicals (•OH) using PL technique [5]. Terephthalic acid (TPA) was used as a probe molecule for •OH. TPA (5 mM) solution was prepared as follows: 830.7 mg TPA was dissolved in 35 mL 0.5 M NaOH, then 60 mL of KH2PO4 buffer was added till complete dissolution and finally the solution was diluted to 1000 mL. This ensures fixing the pH of the reaction at 6.5. In each experiment, 10 mg of CaFe2O4 NFs were dispersed for 5 min in 50 mL of TPA solution using ultrasonication (Daihan Wisd WUC-D06H, Korea). Later on, irradiation was carried out for 4 h using UVA CUBE 2000 solar simulator (Dr. Hönle AG UV Technology, Germany) equipped with halogenide high-pressure lamp (model: SOL 500) that emits radiation simulated to the natural sunlight. The suspensions were magnetically stirred and 3 mL were filtered by 0.2 µm syringe-driven filter unit (Thermo) before analysis. The hydroxyl radicals generated from the irradiated CaFe2O4 NFs react with TPA (nonfluorescent compound) and give a single, fluorescent product, 2-hydroxyterephthalic acid (HTPA). The determination of fluorescent HTPA was carried out according to the method described by Linxiang et al. [16] using high-performance liquid chromatograph (HPLC, Agilent 1260, USA). 3. Results and discussion 3.1. Microstructure of the as-spun NFs
2.3. Characterization of the prepared materials The as-spun calcium and iron nitrates/PVP mat has been examined under the FE-SEM in order to examine its morphology and microstructure. FE-SEM images of the as-spun NFs are shown in Fig. 2. It is clear that the fibers are distributed as a continuous structure with a random orientation over the aluminum substrate as shown in Fig. 2(a).
The microstructure of as-spun NFs and the calcined NFs were examined under FE-SEM (Philips XL30 model) and high-resolution transmission electron microscope (HR-TEM, JEOL, JEM-2100) using a working voltage of 200 kV. Thermal analysis was carried out for the
Fig. 1. Flowchart of the preparation method of calcium ferrite NFs.
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Fig. 2. FE-SEM images of the as-spun mats obtained at 30 kV applied electrical potential at (a) 5000 x and (b) 50,000 x magnifications.
The FE-SEM image of higher magnification (Fig. 2(b)) revealed that the surfaces of the NFs are smooth. This might be due to the amorphous nature of PVP [14]. Diameters of the prepared NFs were found to be in the range of 50–100 nm. 3.2. Thermal analysis The thermal behavior of the dried as-spun Ca(NO3)2/Fe(NO3)3/ PVP mat was evaluated from the curves of thermo-gravimetric (TG) analysis and its differential form (DTG) presented in Fig. 3. DTG curve reveals two clear peaks ranging from 230 °C to 350 °C and from 420 °C to 480 °C, which correspond to 45% and 13% weight loss steps, respectively, in TG curve. The first weight loss step may be attributed to the decomposition of PVP and nitrates, and the second one is probably due to the loss of the remaining PVP. Above 480 °C, sample weight decreased only slightly, indicating that most of the organic moieties decomposed below this temperature. This leads us to select the calcination temperatures to start from 500 °C in order to guarantee that all the carbon contents in PVP are removed and all the remaining is inorganic oxides.
Fig. 4. XRD patterns of the obtained CaFe2O4 NFs after calcination at different temperatures for 1 h in air.
amorphous nature of the sample or to the undetectable tiny crystals. At 700 °C, some peaks of lower intensities belonging to CaFe2O4 phase start to appear. At 800 °C, all of the diffraction peaks can be assigned to the pure orthorhombic structure of CaFe2O4 according to the JCPDS card 72–1199. As the temperature increased to 900 °C and 1000 °C, XRD patterns showed enhancement of CaFe2O4 peaks due to grain growth of the formed crystallites.
3.3. Phase composition of the calcined NFs Crystalline structures of the precursor NFs after calcination at different temperatures were identified from their XRD patterns presented in Fig. 4. No well-defined peaks are observed in the XRD spectrum of the NFs calcined at 500 °C. This can be ascribed to the
3.4. Microstructure of NFs calcined at different temperatures The morphologies of the NFs calcined at different temperatures are shown in Figs. 5(a)–(f). FE-SEM of the NFs calcined at 500 °C (Fig. 5(a)) shows grainy NFs due to evaporation of the polymer with no obvious grains of calcium ferrite. In those calcined at 700 °C, FESEM image demonstrated crystalline calcium ferrites NFs with a retained 3D porous structure. The average diameter of CaFe2O4 NFs was about 70 nm (Fig. 5(b)). A higher magnification (Fig. 5(c)) depicts a single fiber composed of 20–50 nm CaFe2O4 grains. Calcination at 800 °C and 900 °C results in sintering of the NFs and the diameter of the fibers increased to reach 70–300 nm (Figs. 5(d) and (e)). The NFs calcined at 1000 °C, as shown in (Fig. 5(f)), were randomly distributed and grain growth takes place, but still each fiber was individually separated from the other. According to the EDX results shown in Table 1, NFs calcined at 500 °C and 700 °C were composed of Ca/Fe atomic ratios of 1/1.6 and 1/2.1, respectively, supporting the formation of CaFe2O4 phase.
Fig. 3. TG-DTG curves of the as-spun Ca(NO3)2/ Fe(NO3)3/PVP mat.
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Fig. 5. FE-SEM images of NFs calcined at (a) 500 °C, (b), (c) 700 °C, (d) 800 °C, (e) 900 °C, and (f) 1000 °C.
HR-TEM testing was performed to investigate the shape, size, crystalline, and orientation growth features of the sample calcined at 500 °C and 700 °C specifically no diffraction peaks during XRD analysis was observed in case of calcination at 500 °C (see Fig. 4). HR-TEM image of the sample calcined at 500 °C (Fig. 6(a)) displays NFs having a narrow diameter distribution with an average value of 30 nm. These NFs are made from nanoparticles as shown in Fig. 6(b). Selected area electron diffraction (SAED) analysis (Fig. 6(c)) confirms the polycrystalline nature of these nanoparticles [17]. Generally, HRTEM reveals that NFs calcined at 500 °C are crystalline in nature, even
Table 1 Chemical compositions of the prepared NFs. Element
OK Ca K Fe K
Calcined at 500 °C
Calcined at 700 °C
Weight (%)
Weight (%)
Atomic (%)
Atomic (%)
34.03 25.38 40.60
39.30 19.47 41.23
66.74 13.20 20.06
60.99 18.16 20.85
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Fig. 6. HR-TEM images with different magnifications of NFs calcined at: (a, b) 500 °C and (d, e and f) 700 °C. SAED images of NFs calcined at: (c) 500 °C and (g) 700 °C.
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though no distinctive XRD peaks were observed for the sample calcined at 500 °C in Fig. 4. This supports that this sample is composed of very tiny CaFe2O4 crystals which are not large enough to be detected by XRD technique. For the sample calcined at 700 °C, the diameters of NFs range from 50 nm to 100 nm (Fig. 6(d)) and each single fiber is composed of many smaller grains of ~15 to 30 nm (Fig. 6(e)). These results are comparable to that observed in FE-SEM images. Fig. 6(f) shows that NFs calcined at 700 °C are in a crystalline state and have clear lattice fringes of CaFe2O4. The spacing between neighbor lattices in this sample was 2.6 Å, corresponding to (320) plane, which agrees well with CaFe2O4 (PDF # 72-1199). Furthermore, it was observed that this sample demonstrated well-defined dotted-diffraction circle (Fig. 6(g)) indicating its polycrystalline nature [17]. The dots look like a multiple circle pattern because HR-TEM was performed on a suspension solution of calcium ferrite and not on a thin film.
Table 2 Magnetic properties of the obtained CaFe2O4 NFs at different calcination temperatures. Calcination temperatures (°C) 500 700 800 900 1000
Coercivity (mT) 7.940 3.712 12.554 19.676 19.232
Magnetization (emu/g)
Retentivity (emu/g)
8.100 10.557 3.153 1.429 0.924
0.202 0.196 0.207 0.143 0.029
3.5. Magnetic and optical properties of CaFe2O4 NFs Magnetic properties of the prepared CaFe2O4 NFs at different calcination temperatures were investigated at room temperature using VSM. Fig. 7 represents the magnetization curves of these NFs and the extracted values are listed in Table 2. CaFe2O4 NFs have a typical superparamagnetic behavior for the ferromagnetic materials as the coercivity is small with a saturation magnetization depending on the calcination temperature. This superparamagnetic behavior is attributed to the effects of small grain size composing the NFs [18]. In Fig. 7, a low retentivity demeanor for NFs appears clearly which is preferred in water treatment applications as CaFe2O4 NFs aggregate only by the action of external magnetic field. This allows good dispersion for the NFs, in absence of external magnetic field, without losing their surface area during the course of treatment. It is striking to notice that the grain size and morphology of the magnetic materials have a great effect on their magnetic properties at the nanoscale level [19]. For the samples calcined at 500 °C and 700 °C, a perfect nanofibers morphology is obtained and the size of the nanoparticles, making the nanofibers in case of the sample calcined at 700 °C is larger than that of the sample calcined at 500 °C, which means enhancement in magnetization for the sample calcined at 700 °C [19]. At ≥800 °C, it seems that the more dominant factor affecting the magnetic properties is the morphology of the materials rather than the particle size. Since the morphology of the materials is no longer a perfect nanofiber but rather a necklace-like structure (as indicated in FE-SEM images). This caused losing of some dipole polarization, interfacial polarization and shape anisotropy which results in a decrease in magnetization [18]. It is worth mentioning that the saturation magnetization of NFs
Fig. 8. UV–vis DRS of CaFe2O4 NFs calcined at 500 °C and 700 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
those calcined at 700 °C is higher than CaFe2O4 nanoparticles that were prepared by Ghanbari et. al. [20]. This could be attributed to the NFs structure that enhances the magnetization via higher aspect ratio in geometric shapes (i.e. NFs, denoting a higher shape anisotropy, will favor the increase of coercivity) [21]. With the purpose of using the prepared NFs as a magnetic photocatalyst, it is useful to get information about their optical properties. UV–vis diffuse reflectance spectra of the NFs calcined at 500 °C and 700 °C, as they showed the highest saturation magnetization, are depicted in Fig. 8. Both samples showed a significant absorption in the visible region with intense absorption in the UV region. The absorption edge for samples calcined at 500 °C and 700 °C were around 780 nm and 770 nm, respectively, due to the deep brown color of calcium ferrite NFs. The band-gap energy of the NFs were calculated using the equation Eg = 1239.8/ λ , where λ is the absorption cutoff and Eg is the band-gap of the semiconductor [22]. Calculated Eg values for both samples were found to be ~1.6 eV revealing that calcination temperature did not affect significantly the band-gap energy of NFs. This band-gap energy is slightly lower than that observed in earlier studies [22,23] which can be attributed to the difference in morphology. The low band-gap energy of the prepared NFs allows the utilization of a wide range of the solar spectrum during their photocatalytic reactions.
3.6. Photocatalytic performance Photocatalytic activities of the NFs heat treated at 500 °C, 700 °C, and 800 °C were assessed based on their ability for the production of the active •OH. These radicals have high ability for oxidation of organic pollutants in water and wastewater as they are very strong oxidizing agents. Fig. 9 shows the HPLC chromatograms of the irradiated TPA with NFs by solar light for 4 h. The recorded peaks are for the fluorescent HTPA that is formed by the reaction between •OH with nonflorescent TPA. It is obvious that all the investigated NFs produced the fluorescent HTPA in presence of the nonflorescent TPA revealing
Fig. 7. Hysteresis loops for CaFe2O4 NFs after calcination at different temperatures for 1 h in air.
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References [1] Y. Fan, C. Ma, W. Li, Y. Yin, Synthesis and properties of Fe3O4/SiO2/TiO2 nanocomposites by hydrothermal synthetic method, Mater. Sci. Semicond. Process. 15 (2012) 582–585. [2] W. Zhang, Y. Li, Q. Wu, H. Hu, Removal of endocrine-disrupting compounds, estrogenic activity, and Escherichia coliform from secondary effluents in a TiO2coated photocatalytic reactor, Environ. Eng. Sci. 29 (2012) 195–201. [3] V. Goetz, J. Cambon, D. Sacco, G. Plantard, Modeling aqueous heterogeneous photocatalytic degradation of organic pollutants with immobilized TiO2, Chem. Eng. Process.: Process Intensif. 48 (2009) 532–537. [4] Y. Chen, D.D. Dionysiou, A comparative study on physicochemical properties and photocatalytic behavior of macroporous TiO2-P25 composite films and macroporous TiO2 films coated on stainless steel substrate, Appl. Catal. A: Gen. 317 (2007) 129–137. [5] A.S. El-Kalliny, S.F. Ahmed, L.C. Rietveld, P.W. Appel, Immobilized photocatalyst on stainless steel woven meshes assuring efficient light distribution in a solar reactor, Drink. Water Eng. Sci. 7 (2014) 41–52. [6] S. Watson, D. Beydoun, R. Amal, Synthesis of a novel magnetic photocatalyst by direct deposition of nanosized TiO2 crystals onto a magnetic core, J. Photochem. Photobiol. A 148 (2002) 303–310. [7] Z. Zhang, W. Wang, Solution combustion synthesis of CaFe2O4 nanocrystal as amagnetically separable photocatalyst, Mater. Lett. 133 (2014) 212–215. [8] L. William IV, I. Kostedt, J. Drwiega, D.W. Mazyck, S.-W. Lee, W. Sigmund, C.Y. Wu, P. Chadik, Magnetically agitated photocatalytic reactor for photocatalytic oxidation of aqueous phase organic pollutants, Environ. Sci. Technol. 39 (2005) 8052–8056. [9] H. Yao, M. Fan, Y. Wang, G. Luo, W. Fei, Magnetic titanium dioxide based nanomaterials: synthesis, characteristics, and photocatalytic application in pollutant degradation, J. Mater. Chem. A 3 (2015) 17511–17524. [10] X.L. Zhenmin Li, Hong Wang, Dan Mao, Chaojian Xing, Dan Wang, General Synthesis of Homogeneous Hollow Core-Shell Ferrite Microspheres, J. Phys. Chem. C 113 (2009) 2792–2797. [11] W.W. Zhijie Zhang, Solution combustion synthesis of CaFe2O4 nanocrystal as amagnetically separable photocatalyst, Mater. Lett. 133 (2014) 212–215. [12] S.K. Pardeshi, R.Y. Pawar, Optimization of reaction conditions in selective oxidation of styrene over fine crystallite spinel-type CaFe2O4 complex oxide catalyst, Mater. Res. Bull. 45 (2010) 609–615. [13] L. Khanna, N.K. Verma, Size-dependent magnetic properties of calcium ferrite nanoparticles, J. Magn. Magn. Mater. 336 (2013) 1–7. [14] J. Xiang, X. Shen, F. Song, M. Liu, G. Zhou, Y. Chu, Fabrication and characterization of Fe–Ni alloy/nickel ferrite composite nanofibers by electrospinning and partial reduction, Mater. Res. Bull. 46 (2011) 258–261. [15] A.M. EL-Rafei, Optimization of the electrospinning parameters of Mn2O3 and Mn3O4 nanofibers, Ceram. Int. 41 (2015) 12065–12072. [16] L. Linxiang, Y. Abe, Y. Nagasawa, R. Kudo, N. Usui, K. Imai, T. Mashino, M. Mochizuki, N. Miyata, An HPLC assay of hydroxyl radicals by the hydroxylation reaction of terephthalic acid, Biomed. Chromatogr. 18 (2004) 470–474. [17] X. Shen, L.-J. Garces, Y. Ding, K. Laubernds, R.P. Zerger, M. Aindow, E.J. Neth, S.L. Suib, Behavior of H2 chemisorption on Ru/TiO2 surface and its application in evaluation of Ru particle sizes compared with TEM and XRD analyses, Appl. Catal. A: Gen. 335 (2008) 187–195. [18] X. Huang, J. Zhang, S. Xiao, T. Sang, G. Chen, Unique electromagnetic properties of the zinc ferrite nanofiber, Mater. Lett. 124 (2014) 126–128. [19] R. Lu, K. Chang, B. Fu, Y. Shen, M. Xu, S. Yang, X. Song, M. Liu, Y. Yang, Magnetic properties of different CoFe2O4 nanostructures: nanofibers versus nanoparticles, J. Mater. Chem. C 2 (2014) 8578–8584. [20] D. Ghanbari, M. Salavati-Niasari, F. Beshkar, O. Amiri, Electro-spinning of cellulose acetate nanofibers: microwave synthesize of calcium ferrite nanoparticles and CA–Ag–CaFe2O4 nanocomposites, J. Mater. Sci.: Mater. Electron. (2015) 1–9. http://dx.doi.org/10.1007/s10854-015-3502-5. [21] J. Xiang, Y. Chu, X. Shen, G. Zhou, Y. Guo, Electrospinning preparation, characterization and magnetic properties of cobalt–nickel ferrite (Co1−xNixFe2O4) nanofibers, J. Colloid Interface Sci. 376 (2012) 57–61. [22] A.K.R. Police, S. Basavaraju, D.K. Valluri, S. Muthukonda V, S. Machiraju, J.S. Lee, CaFe2O4 sensitized hierarchical TiO2 photo composite for hydrogen production under solar light irradiation, Chem. Eng. J. 247 (2014) 152–160. [23] C. Shifu, Z. Wei, L. Wei, Z. Huaye, Y. Xiaoling, C. Yinghao, Preparation, characterization and activity evaluation of p–n junction photocatalyst p-CaFe2O4/ n-Ag3VO4 under visible light irradiation, J. Hazard. Mater. 172 (2009) 1415–1423. [24] K. Vignesh, A. Suganthi, B.-K. Min, M. Kang, Photocatalytic activity of magnetically recoverable MnFe2O4/g-C3N4/TiO2 nanocomposite under simulated solar light irradiation, J. Mol. Catal. A: Chem. 395 (2014) 373–383.
Fig. 9. HPLC chromatograms for the formation of HTPA during the irradiation of CaFe2O4 NFs under simulated solar light for 4 h. Insets: photographs showing the magnetic separation of CaFe2O4 NFs.
that these samples can produce the active •OH under solar light. This result supports using of the prepared materials as a separable photocatalyst for the treatment of polluted water or wastewater. The quantity of the •OH formed by NFs calcined at 500 °C is higher than that formed by NFs calcined at 700 °C at the same time of irradiation. This behavior may be attributed to the smaller grain size of the sample calcined at 500 °C and hence more active sites are available for the •OH production. However, NFs calcined at 700 °C exhibited 20% higher magnetization saturation than that calcined at 500 °C. Therefore, from the practical point of view, NFs calcined at 700 °C can be considered as the optimal sample for water purification as it can be separated from the effluent easily with reasonable photocatalytic activity. The inset of Fig. 9 showed that the magnetic CaFe2O4 NFs could be separated from the solution by the action of a magnet (STM-30×50-N magnet, magnets4you Co., Germany). The prepared NFs are characterized by low band-gap which facilitates the recombination of photogenerated charge carriers [24]. Therefore, the performance of the prepared CaFe2O4 NFs can be improved by using them in a composite containing another photocatalyst. This will be more effective in the treatment of contaminated water or wastewater. 4. Conclusions Three-dimensional porous random CaFe2O4 NFs were successfully produced by the electrospinning technique followed by a calcination step. The single phase orthorhombic structure of CaFe2O4 was obtained. FE-SEM analysis shows NFs morphology, while the elemental detection from EDX matched well with the CaFe2O4 composition. CaFe2O4 NFs exhibited superferromagnetic behavior which is dependent on the calcination temperature. In addition, these NFs can produce •OH in the presence of solar light. They can be activated not only by UV part in solar light, but also by visible light region as the recorded band gap is 1.6 eV. NFs calcined at 700 °C showed lower photocatalytic activity than that calcined at 500 °C, but from the practical point of view NFs at 700 °C are the best as they are separated easily from the treated water due to its highest magnetization. The photoactivity can be improved by using these magnetic NFs in a composite with another photocatalyst.
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Electrospun magnetically separable calcium ferrite nanofibers for photocatalytic water purification
MARK
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A.M. EL-Rafeia, , Amer S. El-Kallinyb, Tarek A. Gad-Allahb a Refractories, Ceramics and Building Materials Department, National Research Centre, 33 EL Bohouth St. (former EL Tahrir St.), P.O. 12622, Dokki, Giza, Egypt b Water Pollution Research Department, National Research Centre, 33 EL Bohouth St. (former EL Tahrir St.), P.O. 12622, Dokki, Giza, Egypt
A R T I C L E I N F O
A BS T RAC T
Keywords: Electrospinning Nanofibers CaFe2O4 Magnetically separable materials Photocatalyst
Three-dimensional random calcium ferrite, CaFe2O4, nanofibers (NFs) were successfully prepared via the electrospinning method. The effect of calcination temperature on the characteristics of the as-spun NFs was investigated. X-ray diffraction analysis showed that CaFe2O4 phase crystallized as a main phase at 700 °C and as a sole phase at 1000 °C. Field emission scanning electron microscopy emphasized that CaFe2O4 NFs were fabricated with diameters in the range of 50–150 nm and each fiber was composed of 20–50 nm grains. Magnetic hysteresis loops revealed superparamagnetic behavior for the prepared NFs. These NFs produced active hydroxyl radicals under simulated solar light irradiation making them recommendable for photocatalysis applications in water purification. In the meantime, these NFs can be easily separated from the treated water by applying an external magnetic field.
1. Introduction The most serious problem facing photocatalytic treatment processes of water and wastewater is the separation and recycling of the photocatalyst [1]. Many attempts were done to solve this problem through the immobilization of the photocatalyst on different types of substrates [2–4]. However, the efficiency of the photocatalyst usually decreases after immobilization due to mass and light transfer limitations caused by the decreased surface-to-volume ratio, absorbance of light by the substrate, and lack in movement of the photocatalyst particles [5]. Recently, magnetically separable photocatalysts were taking a lot of interest as they can be effectively recycled by applying an external magnetic field, so they could overcome the limitation of separation from the treated effluent [6–8]. Moreover, in such a kind of photocatalysts, reduction in the active surface area is much lower than in the case of immobilization over substrates. Thus, using magnetically separable materials with photocatalysts keeps their photoactivity close to that obtained when using the photocatalysts alone in slurry system [9]. The spinel ferrites (MFe2O4, M = Ca, Mg, Zn, Co, Ni, Cd, etc.) are the most important magnetic materials which have been widely used in electronic devices, information storage, magnetic resonance imaging, and drug-delivery technology [10]. They also have attracted much attention due to their importance in environmental remediation [11].
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Among the ferrite family, calcium ferrite has attracted much interest due to its interesting catalytic, optical, and magnetic characteristics [11,12]. Presence of ferrite particles in a nanoscale form provides more advantages over the bulk ferrites [13]. Among different nanoshapes, Nanofibers (NFs) were selected to study their magnetic and photocatalytic behaviors in this study. For that purpose, electrospinning technique was selected for fabricating these functional NFs as it is a simple, versatile, and relatively inexpensive technique [14]. In addition, it has the advantage of being a single-step methodology, performing synthesis of continuous fibers, and producing uniform NFs. This technique is based on inducing static electrical charges on a polymeric solution at a certain viscosity. The self-repulsion of the charges causes the solution to stretch into a fiber with a simultaneous solvent evaporation by the action of an electric field [15]. In this paper, CaFe2O4 NFs with diameters in the range of 50– 150 nm were synthesized via electrospinning method. The prepared materials showed good magnetization with noticeable photocatalytic activity under simulated sunlight, which makes them recommendable to be used as magnetically separable photocatalysts.
Corresponding author. E-mail addresses: [email protected], [email protected] (A.M. EL-Rafei).
http://dx.doi.org/10.1016/j.jmmm.2016.12.020 Received 10 November 2016; Received in revised form 10 December 2016; Accepted 10 December 2016 Available online 11 December 2016 0304-8853/ © 2016 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 428 (2017) 92–98
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dried as-prepared Ca(NO3)2/ Fe(NO3)3/PVP NFs mats utilizing SDT Q600 V20.9 Build 20 equipment (TA Company, USA). The sample holder was heated in air at a rate of 10 °C/min, in the temperature range from ambient temperature up to 1000 °C. The compositions of the formed crystals were determined using energy dispersive X-ray spectrometer (EDX, TEAM™ EDS Analysis System) connected to the FE-SEM. X-ray diffraction (XRD) analyses of the powders were performed using BRUKUR D8 ADVANCE with secondary monochromatic beam CuKα radiation at 40 kV and 40 mA. The magnetization hysteresis loop was measured by Riken Denshi BH-55 vibrating sample magnetometer (VSM) at room temperature. The diffuse reflectance spectra in the UV–visible range were measured in the wavelength range 200–900 nm using JASCO spectrophotometer (model V-570, Japan) equipped with an integrating sphere attachment. BaSO4 was used as a reference material in the diffuse reflectance spectra measurements.
2. Experimental section 2.1. Experimental approach Electrospinning technique was used to prepare calcium ferrite NFs. The produced as-spun fibers were examined by Field Emission Scanning Electron Microscope (FE-SEM) in order to investigate their microstructure. Then, their weight loss by temperature was determined to select the range of the calcination temperature. Full characterization, including the formed crystalline phases, the morphology, and the composition of the calcined NFs was done. Besides, the magnetic behavior and the optical properties were determined in order to select the best calcination temperature. Finally, the evaluation of the photocatalytic activity was done by Photoluminescence (PL) technique. 2.2. Preparation of calcium ferrite NFs
2.4. Assessment of NFs photocatalytic activity Various combinations of the factors that control the quality of the electrospun fibers (e.g., composition of the electrospinning solution and its viscosity, applied voltage, and distance between collector and nozzle) were investigated by try-and-error method. The reported conditions are the optimal ones that gave fibers of homogeneous structure and high quality. The fibers were prepared by the dissolution of 2.25 g of Polyvinylpyrrolidone (PVP), (Mw ≈1,300,000, Aldrich) in a mixed solvent composed of 18 mL of ethanol and 8 mL deionized water, followed by magnetic stirring for 1 h to ensure the dissolution of PVP. Then 0.9564g of Ca(NO3)2·4H2O and 3.256g of Fe(NO3)3·9H2O were added into the above solution and were further magnetically stirred for about 1 h at room temperature to form a homogeneous solution. The electrospinning process was performed by using Na-Bond Electrospinning device (China). The formed solution was transferred into a syringe equipped with a metallic capillary nozzle connected to a high power supply. The voltage was adjusted at 30 kV. The inner diameter of the used nozzle was 0.49 mm and its height from the collector was set at 11 cm. The selected flow rate was 1 mL/h. The electrospun fibers were collected on a rotating drum working at 750 rpm. Then, the electrospun NFs samples were dried at 105 °C for 24 h and finally calcined for 1 h at different temperatures; 500 °C, 700 °C, 800 °C, 900 °C, and 1000 °C in the ambient atmosphere. A flowchart of the methodology for preparing the NFs is given in Fig. 1.
Photocatalytic activities of the calcined NFs were evaluated by detection of hydroxyl radicals (•OH) using PL technique [5]. Terephthalic acid (TPA) was used as a probe molecule for •OH. TPA (5 mM) solution was prepared as follows: 830.7 mg TPA was dissolved in 35 mL 0.5 M NaOH, then 60 mL of KH2PO4 buffer was added till complete dissolution and finally the solution was diluted to 1000 mL. This ensures fixing the pH of the reaction at 6.5. In each experiment, 10 mg of CaFe2O4 NFs were dispersed for 5 min in 50 mL of TPA solution using ultrasonication (Daihan Wisd WUC-D06H, Korea). Later on, irradiation was carried out for 4 h using UVA CUBE 2000 solar simulator (Dr. Hönle AG UV Technology, Germany) equipped with halogenide high-pressure lamp (model: SOL 500) that emits radiation simulated to the natural sunlight. The suspensions were magnetically stirred and 3 mL were filtered by 0.2 µm syringe-driven filter unit (Thermo) before analysis. The hydroxyl radicals generated from the irradiated CaFe2O4 NFs react with TPA (nonfluorescent compound) and give a single, fluorescent product, 2-hydroxyterephthalic acid (HTPA). The determination of fluorescent HTPA was carried out according to the method described by Linxiang et al. [16] using high-performance liquid chromatograph (HPLC, Agilent 1260, USA). 3. Results and discussion 3.1. Microstructure of the as-spun NFs
2.3. Characterization of the prepared materials The as-spun calcium and iron nitrates/PVP mat has been examined under the FE-SEM in order to examine its morphology and microstructure. FE-SEM images of the as-spun NFs are shown in Fig. 2. It is clear that the fibers are distributed as a continuous structure with a random orientation over the aluminum substrate as shown in Fig. 2(a).
The microstructure of as-spun NFs and the calcined NFs were examined under FE-SEM (Philips XL30 model) and high-resolution transmission electron microscope (HR-TEM, JEOL, JEM-2100) using a working voltage of 200 kV. Thermal analysis was carried out for the
Fig. 1. Flowchart of the preparation method of calcium ferrite NFs.
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Fig. 2. FE-SEM images of the as-spun mats obtained at 30 kV applied electrical potential at (a) 5000 x and (b) 50,000 x magnifications.
The FE-SEM image of higher magnification (Fig. 2(b)) revealed that the surfaces of the NFs are smooth. This might be due to the amorphous nature of PVP [14]. Diameters of the prepared NFs were found to be in the range of 50–100 nm. 3.2. Thermal analysis The thermal behavior of the dried as-spun Ca(NO3)2/Fe(NO3)3/ PVP mat was evaluated from the curves of thermo-gravimetric (TG) analysis and its differential form (DTG) presented in Fig. 3. DTG curve reveals two clear peaks ranging from 230 °C to 350 °C and from 420 °C to 480 °C, which correspond to 45% and 13% weight loss steps, respectively, in TG curve. The first weight loss step may be attributed to the decomposition of PVP and nitrates, and the second one is probably due to the loss of the remaining PVP. Above 480 °C, sample weight decreased only slightly, indicating that most of the organic moieties decomposed below this temperature. This leads us to select the calcination temperatures to start from 500 °C in order to guarantee that all the carbon contents in PVP are removed and all the remaining is inorganic oxides.
Fig. 4. XRD patterns of the obtained CaFe2O4 NFs after calcination at different temperatures for 1 h in air.
amorphous nature of the sample or to the undetectable tiny crystals. At 700 °C, some peaks of lower intensities belonging to CaFe2O4 phase start to appear. At 800 °C, all of the diffraction peaks can be assigned to the pure orthorhombic structure of CaFe2O4 according to the JCPDS card 72–1199. As the temperature increased to 900 °C and 1000 °C, XRD patterns showed enhancement of CaFe2O4 peaks due to grain growth of the formed crystallites.
3.3. Phase composition of the calcined NFs Crystalline structures of the precursor NFs after calcination at different temperatures were identified from their XRD patterns presented in Fig. 4. No well-defined peaks are observed in the XRD spectrum of the NFs calcined at 500 °C. This can be ascribed to the
3.4. Microstructure of NFs calcined at different temperatures The morphologies of the NFs calcined at different temperatures are shown in Figs. 5(a)–(f). FE-SEM of the NFs calcined at 500 °C (Fig. 5(a)) shows grainy NFs due to evaporation of the polymer with no obvious grains of calcium ferrite. In those calcined at 700 °C, FESEM image demonstrated crystalline calcium ferrites NFs with a retained 3D porous structure. The average diameter of CaFe2O4 NFs was about 70 nm (Fig. 5(b)). A higher magnification (Fig. 5(c)) depicts a single fiber composed of 20–50 nm CaFe2O4 grains. Calcination at 800 °C and 900 °C results in sintering of the NFs and the diameter of the fibers increased to reach 70–300 nm (Figs. 5(d) and (e)). The NFs calcined at 1000 °C, as shown in (Fig. 5(f)), were randomly distributed and grain growth takes place, but still each fiber was individually separated from the other. According to the EDX results shown in Table 1, NFs calcined at 500 °C and 700 °C were composed of Ca/Fe atomic ratios of 1/1.6 and 1/2.1, respectively, supporting the formation of CaFe2O4 phase.
Fig. 3. TG-DTG curves of the as-spun Ca(NO3)2/ Fe(NO3)3/PVP mat.
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Fig. 5. FE-SEM images of NFs calcined at (a) 500 °C, (b), (c) 700 °C, (d) 800 °C, (e) 900 °C, and (f) 1000 °C.
HR-TEM testing was performed to investigate the shape, size, crystalline, and orientation growth features of the sample calcined at 500 °C and 700 °C specifically no diffraction peaks during XRD analysis was observed in case of calcination at 500 °C (see Fig. 4). HR-TEM image of the sample calcined at 500 °C (Fig. 6(a)) displays NFs having a narrow diameter distribution with an average value of 30 nm. These NFs are made from nanoparticles as shown in Fig. 6(b). Selected area electron diffraction (SAED) analysis (Fig. 6(c)) confirms the polycrystalline nature of these nanoparticles [17]. Generally, HRTEM reveals that NFs calcined at 500 °C are crystalline in nature, even
Table 1 Chemical compositions of the prepared NFs. Element
OK Ca K Fe K
Calcined at 500 °C
Calcined at 700 °C
Weight (%)
Weight (%)
Atomic (%)
Atomic (%)
34.03 25.38 40.60
39.30 19.47 41.23
66.74 13.20 20.06
60.99 18.16 20.85
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Fig. 6. HR-TEM images with different magnifications of NFs calcined at: (a, b) 500 °C and (d, e and f) 700 °C. SAED images of NFs calcined at: (c) 500 °C and (g) 700 °C.
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though no distinctive XRD peaks were observed for the sample calcined at 500 °C in Fig. 4. This supports that this sample is composed of very tiny CaFe2O4 crystals which are not large enough to be detected by XRD technique. For the sample calcined at 700 °C, the diameters of NFs range from 50 nm to 100 nm (Fig. 6(d)) and each single fiber is composed of many smaller grains of ~15 to 30 nm (Fig. 6(e)). These results are comparable to that observed in FE-SEM images. Fig. 6(f) shows that NFs calcined at 700 °C are in a crystalline state and have clear lattice fringes of CaFe2O4. The spacing between neighbor lattices in this sample was 2.6 Å, corresponding to (320) plane, which agrees well with CaFe2O4 (PDF # 72-1199). Furthermore, it was observed that this sample demonstrated well-defined dotted-diffraction circle (Fig. 6(g)) indicating its polycrystalline nature [17]. The dots look like a multiple circle pattern because HR-TEM was performed on a suspension solution of calcium ferrite and not on a thin film.
Table 2 Magnetic properties of the obtained CaFe2O4 NFs at different calcination temperatures. Calcination temperatures (°C) 500 700 800 900 1000
Coercivity (mT) 7.940 3.712 12.554 19.676 19.232
Magnetization (emu/g)
Retentivity (emu/g)
8.100 10.557 3.153 1.429 0.924
0.202 0.196 0.207 0.143 0.029
3.5. Magnetic and optical properties of CaFe2O4 NFs Magnetic properties of the prepared CaFe2O4 NFs at different calcination temperatures were investigated at room temperature using VSM. Fig. 7 represents the magnetization curves of these NFs and the extracted values are listed in Table 2. CaFe2O4 NFs have a typical superparamagnetic behavior for the ferromagnetic materials as the coercivity is small with a saturation magnetization depending on the calcination temperature. This superparamagnetic behavior is attributed to the effects of small grain size composing the NFs [18]. In Fig. 7, a low retentivity demeanor for NFs appears clearly which is preferred in water treatment applications as CaFe2O4 NFs aggregate only by the action of external magnetic field. This allows good dispersion for the NFs, in absence of external magnetic field, without losing their surface area during the course of treatment. It is striking to notice that the grain size and morphology of the magnetic materials have a great effect on their magnetic properties at the nanoscale level [19]. For the samples calcined at 500 °C and 700 °C, a perfect nanofibers morphology is obtained and the size of the nanoparticles, making the nanofibers in case of the sample calcined at 700 °C is larger than that of the sample calcined at 500 °C, which means enhancement in magnetization for the sample calcined at 700 °C [19]. At ≥800 °C, it seems that the more dominant factor affecting the magnetic properties is the morphology of the materials rather than the particle size. Since the morphology of the materials is no longer a perfect nanofiber but rather a necklace-like structure (as indicated in FE-SEM images). This caused losing of some dipole polarization, interfacial polarization and shape anisotropy which results in a decrease in magnetization [18]. It is worth mentioning that the saturation magnetization of NFs
Fig. 8. UV–vis DRS of CaFe2O4 NFs calcined at 500 °C and 700 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
those calcined at 700 °C is higher than CaFe2O4 nanoparticles that were prepared by Ghanbari et. al. [20]. This could be attributed to the NFs structure that enhances the magnetization via higher aspect ratio in geometric shapes (i.e. NFs, denoting a higher shape anisotropy, will favor the increase of coercivity) [21]. With the purpose of using the prepared NFs as a magnetic photocatalyst, it is useful to get information about their optical properties. UV–vis diffuse reflectance spectra of the NFs calcined at 500 °C and 700 °C, as they showed the highest saturation magnetization, are depicted in Fig. 8. Both samples showed a significant absorption in the visible region with intense absorption in the UV region. The absorption edge for samples calcined at 500 °C and 700 °C were around 780 nm and 770 nm, respectively, due to the deep brown color of calcium ferrite NFs. The band-gap energy of the NFs were calculated using the equation Eg = 1239.8/ λ , where λ is the absorption cutoff and Eg is the band-gap of the semiconductor [22]. Calculated Eg values for both samples were found to be ~1.6 eV revealing that calcination temperature did not affect significantly the band-gap energy of NFs. This band-gap energy is slightly lower than that observed in earlier studies [22,23] which can be attributed to the difference in morphology. The low band-gap energy of the prepared NFs allows the utilization of a wide range of the solar spectrum during their photocatalytic reactions.
3.6. Photocatalytic performance Photocatalytic activities of the NFs heat treated at 500 °C, 700 °C, and 800 °C were assessed based on their ability for the production of the active •OH. These radicals have high ability for oxidation of organic pollutants in water and wastewater as they are very strong oxidizing agents. Fig. 9 shows the HPLC chromatograms of the irradiated TPA with NFs by solar light for 4 h. The recorded peaks are for the fluorescent HTPA that is formed by the reaction between •OH with nonflorescent TPA. It is obvious that all the investigated NFs produced the fluorescent HTPA in presence of the nonflorescent TPA revealing
Fig. 7. Hysteresis loops for CaFe2O4 NFs after calcination at different temperatures for 1 h in air.
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References [1] Y. Fan, C. Ma, W. Li, Y. Yin, Synthesis and properties of Fe3O4/SiO2/TiO2 nanocomposites by hydrothermal synthetic method, Mater. Sci. Semicond. Process. 15 (2012) 582–585. [2] W. Zhang, Y. Li, Q. Wu, H. Hu, Removal of endocrine-disrupting compounds, estrogenic activity, and Escherichia coliform from secondary effluents in a TiO2coated photocatalytic reactor, Environ. Eng. Sci. 29 (2012) 195–201. [3] V. Goetz, J. Cambon, D. Sacco, G. Plantard, Modeling aqueous heterogeneous photocatalytic degradation of organic pollutants with immobilized TiO2, Chem. Eng. Process.: Process Intensif. 48 (2009) 532–537. [4] Y. Chen, D.D. Dionysiou, A comparative study on physicochemical properties and photocatalytic behavior of macroporous TiO2-P25 composite films and macroporous TiO2 films coated on stainless steel substrate, Appl. Catal. A: Gen. 317 (2007) 129–137. [5] A.S. El-Kalliny, S.F. Ahmed, L.C. Rietveld, P.W. Appel, Immobilized photocatalyst on stainless steel woven meshes assuring efficient light distribution in a solar reactor, Drink. Water Eng. Sci. 7 (2014) 41–52. [6] S. Watson, D. Beydoun, R. Amal, Synthesis of a novel magnetic photocatalyst by direct deposition of nanosized TiO2 crystals onto a magnetic core, J. Photochem. Photobiol. A 148 (2002) 303–310. [7] Z. Zhang, W. Wang, Solution combustion synthesis of CaFe2O4 nanocrystal as amagnetically separable photocatalyst, Mater. Lett. 133 (2014) 212–215. [8] L. William IV, I. Kostedt, J. Drwiega, D.W. Mazyck, S.-W. Lee, W. Sigmund, C.Y. Wu, P. Chadik, Magnetically agitated photocatalytic reactor for photocatalytic oxidation of aqueous phase organic pollutants, Environ. Sci. Technol. 39 (2005) 8052–8056. [9] H. Yao, M. Fan, Y. Wang, G. Luo, W. Fei, Magnetic titanium dioxide based nanomaterials: synthesis, characteristics, and photocatalytic application in pollutant degradation, J. Mater. Chem. A 3 (2015) 17511–17524. [10] X.L. Zhenmin Li, Hong Wang, Dan Mao, Chaojian Xing, Dan Wang, General Synthesis of Homogeneous Hollow Core-Shell Ferrite Microspheres, J. Phys. Chem. C 113 (2009) 2792–2797. [11] W.W. Zhijie Zhang, Solution combustion synthesis of CaFe2O4 nanocrystal as amagnetically separable photocatalyst, Mater. Lett. 133 (2014) 212–215. [12] S.K. Pardeshi, R.Y. Pawar, Optimization of reaction conditions in selective oxidation of styrene over fine crystallite spinel-type CaFe2O4 complex oxide catalyst, Mater. Res. Bull. 45 (2010) 609–615. [13] L. Khanna, N.K. Verma, Size-dependent magnetic properties of calcium ferrite nanoparticles, J. Magn. Magn. Mater. 336 (2013) 1–7. [14] J. Xiang, X. Shen, F. Song, M. Liu, G. Zhou, Y. Chu, Fabrication and characterization of Fe–Ni alloy/nickel ferrite composite nanofibers by electrospinning and partial reduction, Mater. Res. Bull. 46 (2011) 258–261. [15] A.M. EL-Rafei, Optimization of the electrospinning parameters of Mn2O3 and Mn3O4 nanofibers, Ceram. Int. 41 (2015) 12065–12072. [16] L. Linxiang, Y. Abe, Y. Nagasawa, R. Kudo, N. Usui, K. Imai, T. Mashino, M. Mochizuki, N. Miyata, An HPLC assay of hydroxyl radicals by the hydroxylation reaction of terephthalic acid, Biomed. Chromatogr. 18 (2004) 470–474. [17] X. Shen, L.-J. Garces, Y. Ding, K. Laubernds, R.P. Zerger, M. Aindow, E.J. Neth, S.L. Suib, Behavior of H2 chemisorption on Ru/TiO2 surface and its application in evaluation of Ru particle sizes compared with TEM and XRD analyses, Appl. Catal. A: Gen. 335 (2008) 187–195. [18] X. Huang, J. Zhang, S. Xiao, T. Sang, G. Chen, Unique electromagnetic properties of the zinc ferrite nanofiber, Mater. Lett. 124 (2014) 126–128. [19] R. Lu, K. Chang, B. Fu, Y. Shen, M. Xu, S. Yang, X. Song, M. Liu, Y. Yang, Magnetic properties of different CoFe2O4 nanostructures: nanofibers versus nanoparticles, J. Mater. Chem. C 2 (2014) 8578–8584. [20] D. Ghanbari, M. Salavati-Niasari, F. Beshkar, O. Amiri, Electro-spinning of cellulose acetate nanofibers: microwave synthesize of calcium ferrite nanoparticles and CA–Ag–CaFe2O4 nanocomposites, J. Mater. Sci.: Mater. Electron. (2015) 1–9. http://dx.doi.org/10.1007/s10854-015-3502-5. [21] J. Xiang, Y. Chu, X. Shen, G. Zhou, Y. Guo, Electrospinning preparation, characterization and magnetic properties of cobalt–nickel ferrite (Co1−xNixFe2O4) nanofibers, J. Colloid Interface Sci. 376 (2012) 57–61. [22] A.K.R. Police, S. Basavaraju, D.K. Valluri, S. Muthukonda V, S. Machiraju, J.S. Lee, CaFe2O4 sensitized hierarchical TiO2 photo composite for hydrogen production under solar light irradiation, Chem. Eng. J. 247 (2014) 152–160. [23] C. Shifu, Z. Wei, L. Wei, Z. Huaye, Y. Xiaoling, C. Yinghao, Preparation, characterization and activity evaluation of p–n junction photocatalyst p-CaFe2O4/ n-Ag3VO4 under visible light irradiation, J. Hazard. Mater. 172 (2009) 1415–1423. [24] K. Vignesh, A. Suganthi, B.-K. Min, M. Kang, Photocatalytic activity of magnetically recoverable MnFe2O4/g-C3N4/TiO2 nanocomposite under simulated solar light irradiation, J. Mol. Catal. A: Chem. 395 (2014) 373–383.
Fig. 9. HPLC chromatograms for the formation of HTPA during the irradiation of CaFe2O4 NFs under simulated solar light for 4 h. Insets: photographs showing the magnetic separation of CaFe2O4 NFs.
that these samples can produce the active •OH under solar light. This result supports using of the prepared materials as a separable photocatalyst for the treatment of polluted water or wastewater. The quantity of the •OH formed by NFs calcined at 500 °C is higher than that formed by NFs calcined at 700 °C at the same time of irradiation. This behavior may be attributed to the smaller grain size of the sample calcined at 500 °C and hence more active sites are available for the •OH production. However, NFs calcined at 700 °C exhibited 20% higher magnetization saturation than that calcined at 500 °C. Therefore, from the practical point of view, NFs calcined at 700 °C can be considered as the optimal sample for water purification as it can be separated from the effluent easily with reasonable photocatalytic activity. The inset of Fig. 9 showed that the magnetic CaFe2O4 NFs could be separated from the solution by the action of a magnet (STM-30×50-N magnet, magnets4you Co., Germany). The prepared NFs are characterized by low band-gap which facilitates the recombination of photogenerated charge carriers [24]. Therefore, the performance of the prepared CaFe2O4 NFs can be improved by using them in a composite containing another photocatalyst. This will be more effective in the treatment of contaminated water or wastewater. 4. Conclusions Three-dimensional porous random CaFe2O4 NFs were successfully produced by the electrospinning technique followed by a calcination step. The single phase orthorhombic structure of CaFe2O4 was obtained. FE-SEM analysis shows NFs morphology, while the elemental detection from EDX matched well with the CaFe2O4 composition. CaFe2O4 NFs exhibited superferromagnetic behavior which is dependent on the calcination temperature. In addition, these NFs can produce •OH in the presence of solar light. They can be activated not only by UV part in solar light, but also by visible light region as the recorded band gap is 1.6 eV. NFs calcined at 700 °C showed lower photocatalytic activity than that calcined at 500 °C, but from the practical point of view NFs at 700 °C are the best as they are separated easily from the treated water due to its highest magnetization. The photoactivity can be improved by using these magnetic NFs in a composite with another photocatalyst.
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