Hydrothermal synthesis of 3D hollow porous Fe3O4 microspheres towards catalytic removal of organic pollutants
© Wang et al.; licensee Springer. 2014
Received: 11 November 2014
Accepted: 20 November 2014
Published: 30 November 2014
Three-dimensional hollow porous superparamagnetic Fe3O4 microspheres were synthesized via a facile hydrothermal process. A series of characterizations done with X-ray diffraction, Brunauer-Emmett-Teller method, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy indicated that the production of Fe3O4 microspheres possessed good monodispersity, uniform size distribution, hollow and porous structural characters, and strong superparamagnetic behavior. The obtained Fe3O4 microspheres have a diameter of ca. 300 nm, which is composed of many interconnected nanoparticles with a size of ca. 20 nm. The saturation magnetization is 80.6 emu·g-1. The as-prepared products had promising applications as novel catalysts to remove organic pollutants (methylene blue) from wastewater in the presence of H2O2 and ultrasound irradiation.
In recent years, porous metal oxides have attracted considerable attention due to their potential applications in the field of lithium-ion batteries, drug delivery carrier, catalysis (including enzyme mimetics), sensors, separation, and magnetic resonance imaging. Among the porous metal oxides, iron oxide (Fe3O4) has become a particularly intriguing research target due to its low cost, good biocompatibility, as well as outstanding stability in physiological conditions[7–10]. Despite the great effort that has been made towards the synthesis of porous Fe3O4 with various sizes and morphologies, it still remains a big challenge to develop controlled and efficient ways for the synthesis of porous Fe3O4 microspheres with uniform size and strong magnetic performances in a large scale[11–13]. The hydrothermal method is one of the widely used methods for preparing functional inorganic nanomaterials[14–16]. It has a series of advantages for the resulting products, such as good crystallinity, uniform sizes, and special morphology, followed by excellent properties. Up to now, the Fe3O4 nanomaterials with different sizes and morphologies have been extensively reported by using this method. However, few reports are associated with the synthesis of 3D hollow porous Fe3O4 microsphere structures with uniform sizes on a large scale. Herein, we report a facile hydrothermal method for the construction of 3D hollow porous Fe3O4 microspheres from nanoparticle building blocks. Synthesis systems based on ethylene glycol (EG) and cetyl-methyl-ammonium bromide (CTAB) have been extensively adopted for the preparation of inorganic nanomaterials. In the whole reaction, EG was used as a solvent and CTAB as a dispersant. The as-prepared Fe3O4 microspheres exhibited porous structure, large surface area, strong superparamagnetic characters, as well as peroxidase-like activity, which are used as efficient enzyme mimetics to degrade organic pollutants (methylene blue).
Synthesis and characterization of 3D hollow porous Fe3O4 microspheres
All chemicals were of analytical grade and used as received without further purification (purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). Typically, FeCl3 (0.8 g) was added into a beaker containing 40 mL ethylene glycol to become a clean yellow-brown solution at room temperature. Then, NaAc (3.6 g) and CTAB were subsequently added. The mixture solution was stirred vigorously for 10 min and was transferred into a Teflon-lined autoclave and heated to 200°C for 24 h. After cooling down to room temperature, the resulting products were collected and washed with deionized water and ethanol three times. The washed products were dried at 60°C for 1 day.
X-ray diffraction (XRD) patterns of the samples were recorded using a Bruker AXS micro-diffractometer (D8 ADVANCE; Bruker AXS GmbH, Karlsruhe, Germany) with Cu-Kα radiation (λ = 1.5406 Å) from 10° to 80° at a scanning speed of 0.33° min-1. The surface chemical groups of Fe3O4 microspheres were recorded by Fourier transform infrared spectroscopy (FTIR; Bruker Vector-22 FTIR spectrometer). The pore sizes and distribution curves were derived from the adsorption isotherm by employing the Barrett-Joyner-Halenda (BJH) method, and the surface areas were calculated through the Brunauer-Emmett-Teller (BET) equation. The magnetization versus magnetic field curves were measured at 300 K by a vibrating sample magnetometer (VSM; PPMS-9 T (EC-II), Quantum Design, San Diego, CA, USA). The surface morphology and structure were observed using a field emission scanning electron microscope (FESEM; Carl Zeiss AG, Oberkochen, Germany) operated at an accelerating voltage of 5.0 kV and a transmission electron microscope (TEM; JEM-2010, JEOL, Tokyo, Japan).
Catalytic degradation of methylene blue
Methylene blue was employed as a model dye pollutant to evaluate the catalytic activity of 3D porous Fe3O4 microspheres for the activation of H2O2 under ultrasonic irradiation. Briefly, 0.5 mL of Fe3O4 microsphere stock solution (with different concentrations) was added into 10 mL aqueous solution of methylene blue (2 μg/mL) at pH 5.0. The mixed solution was put for 10 min to achieve adsorption-desorption equilibrium. Then, the degradation was done by rapidly adding H2O2 (with different concentrations) and was carried out with ultrasonic irradiation for 3 min. The solution was reacted for 20 min. Lastly, the Fe3O4 microspheres were collected using a magnet. The concentration of methylene blue in the clear solution was determined by measuring the absorbance of the solution at 662 nm on a UV-vis spectrophotometer (UV-2450, Shimadzu Co., Kyoto, Japan).
Results and discussion
Based on the above results, a possible formation mechanism of the 3D hollow porous microspheres was put forward as follows: The ethylene glycol acts as both a solvent and a reducer during the solvothermal process. On one hand, it can afford -OH groups to coordinate with Fe3+. With the changes of pH (the addition of NaAc), high temperature, as well as high pressure, the Fe3O4 nuclei are generated; they quickly grow up to become small nanoparticles and aggregate to form the microspheres, owing to the high surface energy. With a longer reaction time, the microspheres continue to grow up and finally form the hollow porous structure, probably owing to the Ostwald ripening. The addition of CTAB is used to control and disperse the resulting products. In fact, the detailed mechanism is rather complex and remains a further discussion to materials chemists.
Based on the above results, a possible catalytic mechanism of 3D hollow porous Fe3O4 as the peroxidase mimetic is proposed as follows: due to the large surface and unique structures, H2O2 and methylene blue molecules can be adsorbed onto the surface of 3D hollow porous Fe3O4 microspheres. Under the influence of ultrasound irradiation, H2O2 molecules are activated by the bound Fe2+ and Fe3+ of Fe3O4 microspheres to generate reactive oxygen species (including · OH, O2-˙, HO2˙). These radicals can further attack organic pollutants to degrade them.
In summary, we have developed a simple one-pot hydrothermal procedure for the synthesis of 3D hollow porous Fe3O4 microspheres composed of lots of nanoparticles assembled on a large scale. The as-prepared microspheres have good dispersibility, large BET surface, strong superparamagnetic performance, as well as peroxidase-like activity, which can be used as a kind of adsorbent and catalytic materials for the removal of organic pollutants. We believe that the easy preparation method, the large-scale output, the unique structure, as well as the outstanding performances endow these 3D microspheres many promising applications ranging from biomedicine to energy materials as well as environmental remediation.
This work was supported by the National Natural Science Foundation of China (NSFC Project Nos. 81472001, 31400851, 21106117, and 21036004).
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