Front. Mater. Sci. 2013, 7(3): 308–311 DOI 10.1007/s11706-013-0213-9
COMMUNICATION
Large scale synthesis of FeS coated Fe nanoparticles as reusable magnetic photocatalysts He FENG, Ping-Zhan SI (✉), Xiao-Fei XIAO, Chen-Hao JIN, Sen-Jiang YU, Zheng-Fa LI, and Hong-Liang GE Zhejiang Key Lab of Magnetic Materials, China Jiliang University, Hangzhou 310018, China
© Higher Education Press and Springer-Verlag Berlin Heidelberg 2013
ABSTRACT: The FeS coated Fe nanoparticles were prepared by using high temperature reactions between the commercial Fe nanoparticles and the S powders in a sealed quartz tube. The simple method developed in this work is effective for large scale synthesis of FeS/Fe nanoparticles with tunable shell/core structures, which can be obtained by controlling the atomic ratio of Fe to S. The structural, magnetic and photocatalytic properties of the nanoparticles were investigated systematically. The good photocatalytic performance originating from the FeS shell in degradation of methylene blue under visible light and the high saturation magnetization originating from the ferromagnetic Fe core make the FeS/Fe nanoparticles a good photocatalyst that can be collected and recycled easily with a magnet. An exchange bias up to 11 mT induced in Fe by FeS was observed in the Fe/FeS nanoparticles with ferro/antiferromagnetic interfaces. The enhanced coercivity up to 32 mT was ascribed to the size effect of Fe core. KEYWORDS:
magnetic photocatalyst; exchange bias; FeS coated Fe nanoparticle
Magnetic photocatalysts were originally prepared by Hiroshi et al. through the deposition of titanium dioxide onto a magnetite and have attracted intensive research interests for their magnetically separable properties for about twenty years [1–2]. To date, most magnetic photocatalysts were designed to have a TiO2 photocatalytic shell and a ferrite magnetic core. Zhang et al. prepared the magnetic TiO2/ZnFe2O4 photocatalysts by sol–gel method [3]. Fu et al. synthesized the SrFe12O19 nanoparticles as the magnetic core by citrate precursor technique and the TiO2 nanocrystals shell by sol–gel technology [4]. Photoactive anatase TiO2 nanoparticles were coated on barium ferrite by Lee et al. through the hydrolysis and condensation of titanium bis-ammonium lactato dihydroxide in the presence of polyethyleneimine at 95°C [5]. However, titanium oxide is catalytically active only under UV irradiation Received June 9, 2013; accepted July 17, 2013 E-mail:
[email protected]
(l < 387 nm) due to its wide band gap energy (Eg = 3.2 eV). Recently, metal chalcogenides such as Bi2S3, Sb2S3, CdS and FeS have become promising alternatives because of their catalytic properties in the visible-light region [6–9]. Nano-sized FeS particles obtained from single precursor or dithiocarboxylate precursor complex were recently reported to be active under visible light as photocatalysts [9–10]. By using a modified borohydride reduction method with dithionite, Kim et al. prepared Fe/FeS nanoparticles, which shows a high reactivity toward contaminants and thus were applied for removal of trichloroethylene from water [11]. Most of the current methods belong to wet process in which solvents and chemical wastes are involved. In this communication, we developed a novel method for large scale synthesis of the FeS coated Fe nanoparticles with controllable structures as magnetic photocatalyst. The synthesis method developed in this work is simple, low-cost, and green for free of any byproducts or wastes.
He FENG et al. Large scale synthesis of FeS coated Fe nanoparticles as reusable magnetic photocatalysts
The exchange bias phenomenon was first discovered in CoO/Co system by Meiklejohn and Bean in 1957 [12]. To date, many exchange bias systems, including metals, oxides, nitrides, fluorides, and sulfides, have been prepared and investigated systematically for exchange bias provides the ability to control and manipulate magnetic properties in nanoscale. For the system of Fe/FeS, Greiner et al. first reported the ferromagnetic–antiferromagnetic interaction in sulfide coated iron particles prepared by reacting the iron particles with H2S [13]. Subsequently, Greiner also studied the exchange anisotropy properties in the sulfide iron films prepared by sulfiding the surface of Fe film [14]. Geoghegan et al. synthesized the Fe/FeS nano-scale particles by mechanical alloying and heat-treatments, obtaining the crystallite size of 111 nm [15]. The exchange bias phenomenon is usually affected by the interfacial configurations and preparation methods. In this work, the exchange bias in the FeS coated Fe nanoparticles prepared by direct reactions was investigated. The FeS coated Fe nanoparticles were prepared by direct reactions of Fe nanoparticles and sulfur at high temperatures. The commercially bought Fe nanoparticles with 50 nm in average diameter were produced by electrical explosion of Fe wires. The Fe nanoparticles and sulfur powders in atomic ratio of 2∶1 were mixed by using ball milling for 30 min. Then the mixture was sealed in an evacuated quartz tube with a subsequent heat-treatment at 1023 K for 2 h. The structure of the samples was measured by using a Thermo X’ TRA X-ray diffractometer with Cu Kα radiation. Morphological and compositional analysis was performed by using transmission electron microscopy (TEM). Magnetic measurements were carried out by using a Lakeshore 7407 vibrating sample magnetometer (VSM). The field-cooling (FC) hysteresis loops were recorded at 230 and 78 K with a cooling field of 1.5 T from room temperature. The photocatalytic properties of the Fe/FeS nanoparticles were measured by the degradation of methylene blue (MB) at room temperature using 40 mL MB aqueous solution (1.6510–5 mol/L) with 20 mg of the synthesized nanoparticles in the flask. A Xe lamp placed vertically at a distance of 15 cm was utilized to irradiate on the solution. After a given time interval, 3 mL of the solution was taken from the mixture for analysis with an Shimadzu UV3600 UV-VIS-NIR spectrometer by recording variations of the absorption band maximum (658 nm) in the UV-vis spectrum of MB. A permanent magnet was used for separation of the nanoparticles from the photoreacted solution.
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As shown in Fig. 1, the X-ray diffraction (XRD) patterns of the as-prepared nanoparticles could be indexed with FeS as the major phase and α-Fe as the minor phase. A trace amount of Fe3O4 was also detected in the XRD patterns. We attributed the presence of Fe3O4 to the limited vacuum in the quartz tube and a slight partial oxidation of the original Fe nanoparticles. Since the atomic ratio of Fe to S is 2:1, we can conclude that approximately half of the Fe in the nanoparticles would react with S to form FeS.
Fig. 1 The XRD pattern of the Fe/FeS nanoparticles, which could be indexed with Fe, FeS, and trace amount of Fe3O4.
Since the reaction between S and Fe goes from the surface to the centre of the Fe nanoparticles, therefore a FeS coated Fe structure is expected. Figure 2 shows the TEM images of the FeS/Fe nanoparticles. The obvious contrast between the surface and the central part of the nanoparticle shown in Fig. 2(a) was ascribed to the compositional difference of the FeS shell and the Fe core. A high resolution analysis on the surface layer of the FeS/Fe nanoparticle is shown in Fig. 2(b), in which we observed an inter-planer lattices distance of 0.298 nm, in good agreement with the spacing of (110) planes of FeS. The compositional analysis on an interfacial spot (marked by red circle) as shown in Fig. 2(c) indicates that the atomic content of Fe was nearly twice that of sulfur (Fe: 59.1 at.%; S: 36.2 at.%), indicating the coexistence of FeS and Fe in this area. The copper peak was ascribed to the effect from copper sample holder. Figure 3 shows the magnetic hysteresis loops of the Fe/ FeS nanoparticles at different temperatures. The saturation magnetization of the sample is approximately 33.43 A$m2/ kg at 78 K, which is slightly higher than that at 230 K. The
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Front. Mater. Sci. 2013, 7(3): 308–311
Fig. 2 (a) TEM image of a Fe/FeS nanoparticle. (b) The high resolution image of the surface area shows an inter-planar distance of 0.298 nm of the FeS. (c) The compositional analysis indicates the coexistence of FeS and Fe in the interfacial area. (d) The energy dispersive X-ray analysis indicates that the interfacial spot is mainly composed of Fe and S with an atomic ratio of ~1.6:1.
Fig. 3 The magnetic hysteresis loops of the FeS/Fe nanoparticles measured at 78 and 230 K. The central portion was enlarged as shown in the fourth quadrant as an insert.
coercivity of the nanoparticles is 32 mT at 78 K and 30 mT at 230 K. We also note that there is a shift in the hysteresis loops of the sample, indicating the presence of an exchange bias in the system. We attributed the exchange bias to the interfacial exchange interactions between the ferromagnetic α-Fe and the antiferromagnetic FeS. The exchange bias field of the Fe/FeS nanoparticles is 11 mT at 78 K and 7 mT at 230 K. Figure 4 shows the photocatalytic decomposition of MB solution in the presence of Fe/FeS nanoparticles under Xe lamp irradiation at given time intervals. The photocatalytic activity of the Fe/FeS nanoparticles was determined by recording the absorbance of the MB solution at 658 nm and
Fig. 4 The absorbance spectral change of MB aqueous solution degraded by Fe/FeS nanoparticles under Xe lamp irradiation. The insets: (a) the time dependence of the concentration of the MB solution, (b) the color of the MB aqueous solution faded after degradation by Fe/FeS nanoparticles.
this characteristic peak gradually decreased with irradiation time and MB concentration. The relative concentration (Ct/C0) of the aqueous solution of MB decreased with increasing the irradiation time, as shown in Fig. 4(a) and the degradations were up to 35%. Figure 4(b) displayed the decolorization of MB solution catalyzed by Fe/FeS nanoparticles under the irradiation of Xe lamp for 195 min. Recently Maji et al. has reported the photocatalytic behavior of FeS nanoparticles and obtained excellent degradations owing to the high specific surface area of the nanoparticles and better crystalline nature [10]. We
He FENG et al. Large scale synthesis of FeS coated Fe nanoparticles as reusable magnetic photocatalysts
ascribed the effective decomposition of the methylene blue solution to the large areas of the FeS surface layers. The as prepared nanoparticles could be removed easily with a permanent magnet for the presence of the un-reacted ferromagnetic Fe core. In summary, we have prepared the FeS coated Fe nanoparticles by the direct reaction process, which is useful in large scale synthesis and structural controlling of the Fe/ FeS magnetic nano-photocatalysts. The exchange bias in the Fe/FeS nanoparticles reaches 11 mT at 78 K. The Fe/ FeS nanoparticles were effective in degradation of MB in the visible-light region. The Fe/FeS nano-photocatalysts with large surface areas could be reused and collected with a magnet.
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dihydroxide and its use as a magnetic photocatalyst. Chemistry of Materials, 2004, 16(6): 1160–1164 [6] Bessekhouad Y, Mohammedi M, Trari M. Hydrogen photoproduction from hydrogen sulfide on Bi2S3 catalyst. Solar Energy Materials and Solar Cells, 2002, 73(3): 339–350 [7] Sun M, Li D Z, Li W J, et al. New photocatalyst, Sb2S3, for degradation of methyl orange under visible-light irradiation. Journal of Physical Chemistry C, 2008, 112(46): 18076–18081 [8] Karunakaran C, Senthilvelan S. Solar photocatalysis: oxidation of aniline on CdS. Solar Energy, 2005, 79(5): 505–512 [9] Dutta A K, Maji S K, Srivastava D N, et al. Synthesis of FeS and FeSe nanoparticles from a single source precursor: a study of their photocatalytic activity, peroxidase-like behavior, and electrochemical sensing of H2O2. ACS Applied Materials & Interfaces, 2012, 4(4): 1919–1927
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 10874159, 11074227 and 11204283) and Zhejiang Provincial Natural Science Foundation (Grant No. R6110362 and Y4110420).
[10] Maji S K, Dutta A K, Biswas P, et al. Synthesis and characterization of FeS nanoparticles obtained from a dithiocarboxylate precursor complex and their photocatalytic, electrocatalytic and biomimic peroxidase behavior. Applied Catalysis A,
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