Magnetically Separable Fe3O4/AgBr Hybrid Materials: Highly Efficient Photocatalytic Activity and Good Stability

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.

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][19][20][21]. Moreover, Fe 3 O 4 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 coreshell-structured Fe 3 O 4 /WO 3 has a more effective photoconversion capability than pure WO 3 or Fe 3 O 4 [22]. The Ag halides such as AgBr and AgI are photoactive to visible light. When they were immobilized on SiO 2 @Fe 3 O 4 magnetic supports, they exhibited faster degradation rates for 4-chlorophenol than N-TiO 2 [23]. However, the Ag halides were easily photoreduced and losed their stability quickly.
The motivation of the present research originated from the idea that Fe 3 O 4 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 Fe 3

Synthesis of Fe 3 O 4 Nanospheres
The Fe 3 O 4 nanospheres were prepared according to the literature reported previously [24]. In a typical synthesis, 0.5 g of 1 g FeCl 3 · 3H 2 O, 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. Characterization 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 BaSO 4 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 Fe 3 O 4 /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/ cm 2 ). 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 Figure 1a shows that the size of Fe 3 O 4 nanospheres was about 100~200 nm. The surface of Fe 3 O 4 particles was rough, and each magnetic microsphere was constructed with many small magnetic grains. From Fig. 1b, we can clearly see that the obtained AgBr particles by the precipitation method easily agglomerate to large particles and their size was more than 300 nm. Figure 1c displays that when Fe 3 O 4 was coupled with AgBr, the particle size of the composite increased apparently than the pure AgBr particles. The magnetic property of the surface Fe 3 O 4 would result in the agglomeration of the particles. The EDS spectrum of Fe 3 O 4 /AgBr-0.5 hybrid materials indicates that the atomic ratio of Fe and Ag is approximately 1:134, which is a little larger than the designed value. Figure 2 shows the typical XRD patterns of the asprepared  In order to clarify the reasons for this result, the active species in photodegradation process of MO were detected. Methanol, silver nitrate, and terephthalic acid solution were added into MO dye solution to capture electrons, holes, and · OH, respectively. As can be seen from Fig. 6, when the active species of electrons, holes, and · OH were captured, the degradation yield of MO decreased from 85 % to 68 %, 74 %, and 51 %, respectively. That indicated · OH and electrons played more important roles comparing the holes in the photodegradation of MO.
As well known, AgBr is not stable, and it often suffers photo-corrosion. So, the stability of AgBr and Fe 3 O 4 / AgBr-0.5 was evaluated. As shown in Fig. 7, the photocatalytic activity on the pure AgBr decreased sharply in the consecutive three cycles. The degradation yield of MO on the pure AgBr particles in the tree cycles was 0.52, 0.33, and 0.12, respectively. The photo-excited electrons on AgBr would reduce Ag + to the metallic Ag, and the small Ag nanoparticles would cover on the surface of AgBr. And the surface Ag nanoparticles would prohibit the photo-absorption of the inner AgBr. When the amount of Ag was enough, the photo-excitation of the inner AgBr would be hold back, and as a result, the photocatalytic activity decreased remarkably as the reaction proceeding. However, for the Fe 3 O 4 /AgBr hybrid

Photoluminescence of the Series of Photocatalysts
The fluorescence spectrum can provide much more information about carrier capture, migration, conversion, separation, etc., so it has been used for measuring the separation of the photo-generated electron-hole pairs [26]. The emission signals in the fluorescence spectrum are mainly from the recombination of the photo-generated electron-hole pairs, and the lower fluorescence intensity often implies the higher separation efficiency of the charge carriers. Figure 8 shows the fluorescence spectra of the samples in a wavelength range of 400-700 nm. It