Magnetically Separable Fe3O4/AgBr Hybrid Materials: Highly Efficient Photocatalytic Activity and Good Stability
© Cao et al. 2015
Received: 11 March 2015
Accepted: 21 May 2015
Published: 3 June 2015
Magnetically separable Fe3O4/AgBr hybrid materials with highly efficient photocatalytic activity were prepared by the precipitation method. All of them exhibited much higher photocatalytic activity than the pure AgBr in photodegradation of methyl orange (MO) under visible light irradiation. When the loading amount of Fe3O4 was 0.5 %, the hybrid materials displayed the highest photocatalytic activity, and the degradation yield of MO reached 85 % within 12 min. Silver halide often suffers serious photo-corrosion, while the stability of the Fe3O4/AgBr hybrid materials improved apparently than the pure AgBr. Furthermore, depositing Fe3O4 onto the surface of AgBr could facilitate the electron transfer and thereby leading to the elevated photocatalytic activity. The morphology, phase structure, and optical properties of the composites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–visible diffuse reflectance spectra (UV–vis DRS), and photoluminescence (PL) techniques.
Up to now, most of the silver oxide and silver halide have attracted much attention because of their strong visible light absorption performance [1–7]. Particularly, AgBr, which has a band gap of 2.6 eV, is well known as a photosensitive material and has been extensively applied to photographic films, which demonstrated excellent performance in degradation of dye pollutants and decomposition of water [8–10]. For example, Ag/AgBr/TiO2 , Ag–AgBr/TiO2/RGO , AgBr(I)@Ag , Fe(III)/AgBr , and Ag/AgBr/ZnO  have been successfully fabricated by diverse techniques, and their novel and unique photocatalytic properties have been extensively explored.
For the nanosized or microsized photocatalysts, effective separation from the mixed system and recycle using are important problems to restrain their real applications [16, 17]. Immobilizing catalysts on magnetic substrates by feasible methods is proven to be an effective approach for removing and recycling particles [18–21]. Moreover, Fe3O4 has excellent conductivity, so it could act as an electron transfer channel and acceptor, which could suppress the photo-generated carrier recombination. For instance, Ye et al. reported that the hierarchical core–shell-structured Fe3O4/WO3 has a more effective photoconversion capability than pure WO3 or Fe3O4 . The Ag halides such as AgBr and AgI are photoactive to visible light. When they were immobilized on SiO2@Fe3O4 magnetic supports, they exhibited faster degradation rates for 4-chlorophenol than N-TiO2 . However, the Ag halides were easily photoreduced and losed their stability quickly.
The motivation of the present research originated from the idea that Fe3O4 has high conductivity and its CB level (1 V vs. NHE) makes it become a good candidate for coupling with AgBr. Based on the above reason, we prospect their combination could improve the photocatalytic performance by enhancing charge transport. Herein, conductive Fe3O4 particles and visible light active AgBr were coupled together to prepare the magnetically recyclable Fe3O4/AgBr composites with visible light activity. Studies of their photocatalytic performance in the decomposition of methyl orange (MO) indicated that Fe3O4/AgBr photocatalysts exhibited excellent catalytic activity under visible light illumination. Meanwhile, the stability of AgBr was improved when it was coupled with Fe3O4.
Preparation of the Photocatalyst
Synthesis of Fe3O4 Nanospheres
The Fe3O4 nanospheres were prepared according to the literature reported previously . In a typical synthesis, 0.5 g of 1 g FeCl3 · 3H2O, 3.0 g NaAc, and 10 mL oleic acid were added to 30 mL ethylene glycol into a three-necked flask, and then a red solution was formed. The mixture was stirred vigorously at 50 °C for 20 min until all reagents were dissolved completely. Then, the mixture was transferred into a Teflon-lined autoclave and heated at 200 °C for 20 h. The products were cooled down to room temperature, washed with ethanol for several times, and dried under vacuum to give a black solid.
Synthesis of Fe3O4/AgBr Hybrid Materials
Fe3O4 nanospheres (0.01 g) were dispersed in 20 mL deionized water and then ultrasonically dispersed evenly. AgNO3 (1.18 g) was added into the solution, and then NaBr (0.1 mol/L) was added dropwise slowly. The resulting suspensions were filtered, washed several times with distilled water, and finally dried in vacuum. Different Fe3O4/AgBr samples were obtained by adjusting the mass ratio of Fe3O4 and AgBr, and the sample was denoted as Fe3O4/AgBr-x (x means the percentage of Fe3O4).
X-ray diffraction (XRD) patterns were measured on an X’Pert Philips diffractometer (Cu Kα radiation, 2θ range 10°–90°, step size 0.08°, accelerating voltage 40 kV, applied current 40 mA). The morphology of the samples was taken on a Hitachi S-4800 scanning electron microscope (SEM). UV–visible diffuse reflectance spectra (UV–vis DRS) were obtained on a Shimadzu U-3010 spectrometer, using BaSO4 as a reference. The photoluminescence (PL) spectra were recorded on a F-7000 FL spectrophotometer.
Evaluation of the Photocatalytic Activity
MO was selected as the model pollutant to evaluate the photocatalytic activity of the Fe3O4/AgBr hybrid materials. In a typical experiment, 0.1 g of the photocatalyst was put into a 120 mL quartz reactor containing 100 mL MO aqueous suspension (20 mg/L, pH = 7). Prior to irradiation, the suspension was magnetically stirred in the dark for 30 min to establish an adsorption–desorption equilibrium. A 300-W Xe arc lamp with a 420 cutoff filter was used as the light source (λ ≥ 420 nm, I 420 = 8.0 mW/cm2). At 2-min intervals, 5 mL of the suspension was collected and centrifuged for 3 min to remove the catalyst particulates for analysis. The residual MO concentration was detected at 464 nm using a UV–vis spectrophotometer (722, Shanghai Jingke Instrument Plant, China).
Results and Discussion
Phase Structure and Morphology of the Samples
Optical Properties of the Photocatalysts
Photocatalytic Activity for MO Degradation on Fe3O4/AgBr Hybrid Materials
Photoluminescence of the Series of Photocatalysts
Fe3O4/AgBr hybrid materials with high photocatalytic efficiency under visible light were prepared through the precipitation method. The Fe3O4/AgBr samples showed much higher photocatalytic activity than the pure AgBr, which was due to the matched band structure of two components and the higher conductivity of Fe3O4. When the loading amount of Fe3O4 was 0.5 %, the highest photoactivity was obtained, and the degradation yield of MO reached 85 % within 12 min. The PL spectra indicated that Fe3O4/AgBr hybrid materials had the higher separation efficiency of the photo-excited charge carriers, and that was in accordance with the photocatalytic activity very well. In addition, the stability of Fe3O4/AgBr composites was improved comparing with the pure AgBr. The photo-excited electrons would transfer out quickly from the surface Fe3O4, so the self-reduction of AgBr to metallic Ag was prohibited, and as a result, the long-term stability of Fe3O4/AgBr was obtained.
The authors gratefully acknowledge the support of the National Science Foundation of China (Nos. 21103042 and 21471047), Program for Science & Technology Innovation Talents in University of Henan Province (No. 15HASTIT043), and the Natural Science Foundation of Henan University (No. 2012YBZR001).
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