Corn-like, recoverable γ-Fe2O3@SiO2@TiO2 photocatalyst induced by magnetic dipole interactions

Corn-like, γ-Fe2O3@SiO2@TiO2 core/shell heterostructures were synthesized by a modified solvothermal reduction combined with a sol-gel method. SiO2 shells were first deposited on monodisperse Fe3O4 microspheres by a sol-gel method. Fe3O4@SiO2@TiO2 corn-like heterostructures were then obtained by sequential TiO2 coating, during which the magnetic dipolar interactions induced the anisotropic self-assembly process. After annealing at 350 °C, the crystalized TiO2 enhanced photocatalytic activity, while Fe3O4 was converted to γ-Fe2O3. The corn-like γ-Fe2O3@SiO2@TiO2 photocatalyst can be recycled and reused by magnet extraction. Despite the photocatalytic activity decreased with each cycle, it can be completely recovered by moderate heating at 200 °C.

high magnetization are chosen as magnetic cores in magnetically separable photocatalysts 25 . It is reported by Li et al. that the direct contact between magnetic Fe 3 O 4 and TiO 2 photocatalyst usually results in an increase in electron-hole recombination and photodissolution 35 . In order to overcome the problem of charge recombination, mesoporous TiO 2 /SiO 2 /Fe 2 O 3 38 , (γ-Fe 2 O 3 @SiO 2 )n@TiO 2 hybrid nanoparticles with γ-Fe 2 O 3 @SiO 2 fine particles dispersed in a TiO 2 matrix 39 , and γ-Fe 2 O 3 @SiO 2 @TiO 2 composite microspheres with SiO 2 barrier layers 40 were synthesized. Compared with Fe 3 O 4 @TiO 2 , the insert of SiO 2 shell between γ-Fe 2 O 3 core and TiO 2 shell exhibits two positive effects to enhance the photocatalytic activity. One is to block the electron injection from TiO 2 to γ-Fe 2 O 3 at the interface, the other is to provide a porous surface with large surface-to-volume ratio for catalytic reactions 40 . However, the photogenerated electrons can still transfer if the thickness of the SiO 2 is less than 5 nm 41 . Therefore, the thickness of SiO 2 is a key factor responsible for the photocatalytic performance of iron oxide/SiO 2 / semiconductor systems. While most works focus on the recoverability of magnetic core-shell photocatalysts, surprisingly little has been reported on the regeneration and reuse of such photocatalysts.
Here, we report the synthesis and characterization of a corn-like, anisotropic γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructure. The formation mechanism of the anisotropic heterostructure is proposed, revealing the importance of magnetostatic interaction as a tuning knob for morphological control. The heterostructure demonstrates photocatalytic activity for degradation of Rhodamine B and can be magnetically recycled and reused, Moreover, the lost photocatalytic activity of the used γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructure can be fully recovered by heating at 200 °C for 30 min. Such magnetically recyclable, easily regenerated γ-Fe 2 O 3 @SiO 2 @TiO 2 composite provides a design paradigm for low cost photocatalysts for renewable energy and environmental applications.

Results and Discussion
Structural and morphology characterizations. Figure 1 shows the XRD patterns of Fe 3 O 4 and Fe 3 O 4 @ SiO 2 @TiO 2 heterostructures before and after annealing. For the Fe 3 O 4 microspheres, the characteristic XRD peaks of magnetite with inverse spinel structure were observed, indicating that Fe 3 O 4 phase is obtained by the solvothermal method. However, a minor amount of γ-Fe 2 O 3 was present in the microspheres. As-synthesized Fe 3 O 4 @SiO 2 @TiO 2 shows only the similar peaks of Fe 3 O 4 microspheres without any characteristic peaks of SiO 2 and TiO 2 . In contrast, when Fe 3 O 4 @SiO 2 @TiO 2 was annealed at 350 °C for 2 h in air, a clear XRD peak at 25.5° corresponding to the (101) crystal planes of anatase TiO 2 emerged. Meanwhile, a weak TiO 2 (200) was also observed after annealing. Thus, TiO 2 should be an amorphous phase in the as-synthesized Fe 3 O 4 @SiO 2 @ TiO 2 composite. As for SiO 2 , no XRD peaks is seen after annealing at 350 °C for 2 h, suggesting that it remains amorphous despite the heat treatment. However, all the original peaks of Fe 3 O 4 show a shift to higher 2θ values, which correspond to the peaks of γ-Fe 2 O 3 42 . Moreover, the change in color of the heterostructures from black to brownish-red also suggests the oxidation of Fe 3 O 4 into γ-Fe 2 O 3 . Thus, γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructures can be obtained by such heat treatment in air. In order to further confirm the coating of SiO 2 and oxidation of Fe 3 O 4 , the elemental and valence state analysis were performed using XPS. Here the chemical states of the constituent elements were calibrated by C 1s peak (284.6 eV). Figure 2(a) shows the survey scan XPS spectrum of γ-Fe 2 O 3 @SiO 2 @TiO 2 , which contains O 1s, Ti 2p, Fe 2p, Si 2p and Si 2s peaks. This indicates the existence of SiO 2 and TiO 2 in γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructure. Figures 2(b) and (c) show the core-level XPS spectra of Fe 2p in Fe 3 O 4 microspheres and γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructures, respectively. For the Fe 3 O 4 in Fig. 2(b), the main-peaks located at 711.0 eV and 724.6 eV are attributed to Fe 3+ 2p3/2 and Fe 3+ 2p1/2, respectively. However, there is a significant asymmetry for both of them. A double peak fitting yields peak positions at 709.3 eV and 722.5 eV, which belong to Fe 2+ 2p3/2 and Fe 2+ 2p1/2, respectively. These results are in agreement with the reported Fe 2p XPS spectrum of Fe 3 O 4 43 . In addition, the area ratio of Fe 3+ /Fe 2+ is about 3.6, which deviates from the stoichiometry of Fe 3 O 4 . Such a deviation indicates that Fe 2+ ions have been partially oxidized to Fe 3+ ions, which is in agreement with the XRD results in Fig. 1, which show trace amount of γ-Fe 2 O 3 . Here the FeO x can reflect the coexistence of Fe 3 O 4 and γ-Fe 2 O 3 . As for the Fe 2p core-level XPS spectrum of γ-Fe 2 O 3 @SiO 2 @TiO 2 [ Fig. 2(c)], it presents only the peaks of Fe 3+ at 710.8 eV and 724.7 eV. But, it is hard to observe the peak of Fe 2+ at about 709.0 eV and 723 eV. Such results suggest that Fe 3 O 4 in the Fe 3 O 4 @SiO 2 @TiO 2 heterostructure has been oxidized to γ-Fe 2 O 3 after annealing at 350 °C for 2 h in air. The difference in color further provides the evidence for oxidation. Figure 3 shows the SEM images of Fe 3 O 4 microspheres synthesized by different surfactants and γ-Fe 2 O 3 @ SiO 2 @TiO 2 corn-like heterostructures. As seen from Fig. 3(a)-(c), the surfactants play an important role in controlling the surface morphology of Fe 3 O 4 microspheres. For the ED surfactant, it can be seen that the spherical morphology is not completely formed, and Fe 3 O 4 shows some irregular shape [ Fig. 3(a)]. When PEG is chosen as the surfactant, Fe 3 O 4 microspheres with a smooth surface can be obtained [ Fig. 3(b)]. On the other hand, as NaPAA is chosen as the surfactant, the Fe 3 O 4 microspheres demonstrate rough surface. The surface consists of uniformly sized nanoparticles, and the mean diameter of the microspheres is about 450 nm [ Fig. 3(c)]. These well-dispersed, rough Fe 3 O 4 microspheres with large surface-to-volume ratio were chosen as templates for coating the SiO 2 coupling layer and subsequently the TiO 2 functional shell. It can be seen from Fig. 3(d)-(f) that γ-Fe 3 O 4 @SiO 2 @TiO 2 shows anisotropic corn-like structure after TiO 2 coating. The discrete Fe 3 O 4 microspheres are now linked by the SiO 2 /TiO 2 coating to self-assemble into corn-like heterostructures with length exceeding 10 μm. The newly formed TiO 2 shell is uniformly coated onto the magnetic Fe 3 O 4 core, leading to a smoother surface than that of the starting Fe 3 O 4 microspheres. Interestingly, this corn-like heterostructure is only found after coating with TiO 2 shell, while not observed in the Fe 3 O 4 @SiO 2 composite. Therefore, the TiO 2 shell should play a crucial role in forming the corn-like heterostructure. In order to reveal the structural characteristics of corn-like Fe 3 O 4 @SiO 2 @TiO 2 heterostructure, HRTEM images were done on Fe 3 O 4 and Fe 3 O 4 @SiO 2 @TiO 2 before and after annealing. As seen from Fig. 4(a), the diameter of Fe 3 O 4 is about 450 nm, and Fe 3 O 4 porous microspheres possess a hierarchical structure consisting of densely packed nanoparticles with sizes of about 20 nm. Such morphology is beneficial for the subsequent surface coating. In Fig. 4(b), the uniform SiO 2 and TiO 2 shells are coated on Fe 3 O 4 spheres to form one-dimensional chain-like  SCIeNtIfIC REPORTS | 7: 6960 | DOI:10.1038/s41598-017-07417-z Fe 3 O 4 @SiO 2 @TiO 2 structures, which act as basic units to assemble into corn-like heterostructures. As shown in Fig. 4(c), one can clearly distinguish SiO 2 and TiO 2 shells from Fe 3 O 4 cores in the as-prepared Fe 3 O 4 @SiO 2 @TiO 2 microspheres. The thickness of SiO 2 and TiO 2 shells are close to 60 nm and 15 nm, respectively. The 60 nm SiO 2 shell can not only act as a coupling layer for TiO 2 coating, but also preserve the photocatalytic activity of TiO 2 by inhibiting the electron transfer between the magnetic core and TiO 2 shell. After annealing, the chain-like structural unit and a thinner TiO 2 shell can be clearly observed in Fig. 4(d) and (e). Furthermore, the (103) and (200) lattice fringes of anatase TiO 2 shown in Fig. 4(f) indicates that the TiO 2 shell has been well-crystallized. This is in agreement with the XRD results of γ-Fe 2 O 3 @SiO 2 @TiO 2 (Fig. 1). The average grain size of TiO 2 is about 5 nm. @SiO 2 @TiO 2 heterostructure decreases to 45 emu/g due to the increased volume fraction of nonmagnetic materials. After annealing at 350 °C for 2 h in air, M s of the γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructure is further reduced to 37 emu/g, which is about 18% lower than that of Fe 3 O 4 @SiO 2 @TiO 2 . Such reduction in M s is mainly due to the lower M s of γ-Fe 2 O 3 than that of Fe 3 O 4 . Nevertheless, the anisotropic corn-like heterostructures respond strongly to an external magnetic field, and can be efficiently extracted, as shown in the upper left inset. Such efficient separation is necessary for recyclable photocatalysts. For example, the collection time of 0.1 g γ-Fe 2 O 3 @SiO 2 @TiO 2 in 20 ml ethanol is less than 10 seconds.
As seen from the lower right inset of Fig. 5, the isolated Fe 3 O 4 microspheres show typical ferrimagnetic behavior but with insignificant remanent magnetization (M r ). The low M r is attributed to the multi-domain structure of the 450 nm-sized microspheres with small anisotropy. However, a higher M r of about 5 emu/g is found for Fe 3 O 4 @ SiO 2 @TiO 2 which changes very little after annealing. The enhanced remanent magnetization of the heterostructure, on the other hand, is due to the formation of the anisotropic shape. As will be further discussed below, it is the magnetic dipole interactions between the microspheres lead to their self-assembly into chain-like structures and further corn-like heterostructure; while the chain formation in turn changes the magnetic behavior of the heterostructure. In the chain-like unit of the heterostructure, when the magnetization lies along the chain, there is a reduction of the stray field. This effective shape anisotropy results in the magnetic easy axis to lie along the chain. While this anisotropy is generally not large, it does increase both the remanent magnetization and coercivity of the assembly. Upon annealing, Fe 3 O 4 is oxidized into γ-Fe 2 O 3 with higher magnetocrystalline anisotropy, which further increases the coercivity of the γ-Fe 2 O 3 @SiO 2 @TiO 2 corn-like assembly. The enhanced remanence is responsible for the enhanced magnetic recyclability under weak magnetic fields.
Formation mechanism. It is interesting to note that the corn-like heterostructure is formed only in the Fe 3 O 4 @SiO 2 @TiO 2 , while in neither the Fe 3 O 4 microspheres nor the Fe 3 O 4 @SiO 2 core-shell structures. The chain-like structure is the basic component of these corn-like heterostructures. The formation is primarily a consequence of the competition between the magnetic dipole interactions, which favor chain-formation, and Brownian motion, which tends to randomize the assembly. We propose the following formation mechanism, as shown schematically in Fig. 6. During the first stage of synthesis [ Fig. 6(a)], each isolated Fe 3 O 4 microspheres can be regarded as a magnetic dipole. The dipole-dipole interactions between the microspheres tend to arrange the particles head-to-tail, forming a linear chain. However, the dipole moment in each microsphere is relatively small, due to the multidomain configuration. The random Brownian motion thus disrupts the chain formation. Moreover, the NaPAA ligand exerts steric forces, increasing the spacing between Fe 3 O 4 microspheres, further reducing the dipole interactions. Therefore, Fe 3 O 4 microspheres are dispersed as isolated entities at this stage. After SiO 2 coating, the core/shell Fe 3 O 4 @SiO 2 still maintains good dispersibility due to the long-chain capping groups of TEOS precursor on the microspheres [ Fig. 6(b)] 44 .
However, during the TiO 2 coating, the TiO 2 colloidal sol caused by the hydrolysis of TTIP fill in the voids between Fe 3 O 4 @SiO 2 microspheres, acting as a binder to fuse the neighboring microspheres. Once a Fe 3 O 4 @SiO 2 dimer is formed, the symmetry is broken, and the axis along the dimer defines the magnetic easy axis, as shown in Fig. 6(c). The magnetization now tends to align along the dimer axis, increasing the effective dipole moment of the dimer. It is now energetically more favorable to connect more particles head-to-tail along the long axis to form a linear chain [ Fig. 6(d)]. Thus, the chain-formation is a self-assembly process induced by magnetic dipole interactions, and assisted by the fusing effect of the TiO 2 shell. Klokkenburg et al. first directly observed this dipolar chain formation in zero fields in a ferrofluid containing the largest synthetic single-domain magnetite particles 45 . The dipole-dipole interactions induced the anisotropic features, favoring a head-to-tail orientation. Moreover, Zhang et al. found that the Fe 3 O 4 nanoparticles can self-assemble into one-dimensional chains in a colloidal dispersion through magnetic dipolar interaction without the help of an applied magnetic field 46 . As more chains are nucleated in the solution, when they touch by random motion, they tend to align side-by-side. This is akin to two bar magnets sticking together by aligning their north-and south-poles in opposite directions for flux closure to minimize magnetostatic interactions [ Fig. 6(e)]. The flux closure is in Co particles simulated by Chantrell et al. 47 We postulate that with increase in the numbers of chains, they order into tight bundles, forming corn-like heterostructures as depicted in the SEM image of Fig. 6. Moreover, the formation of corn-like heterostructures may be aided by Van der Waals interactions between the chains, as was reported in Fe 3 C microfiber assemblies 48 .

Photocatalytic and magnetic recovery properties. The photocatalytic activity of Fe 3 O 4 , corn-like
Fe 3 O 4 @SiO 2 @TiO 2 and γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructures were tested by measuring the photocatalytic degradation of RhB in water (10 mg/L) under the illumination of a Xe lamp (300 W). In order to ensure the reliability of the experiments, two control experiments, namely Blank I with catalysts only without light illumination (dark adsorption) and Blank II with light illumination only without catalysts (pure photolysis), were also performed. Figure 7(a) shows the normalized concentration of RhB (C t /C 0 ) as a function of irradiation time for different photocatalyst and blank samples, in which C t and C 0 denotes the concentration of RhB aqueous solution at the irradiation time of t and t = 0 h, respectively. It can be seen that there is no measurable RhB degradation up to 5 h under light irradiation with Fe 3 O 4 alone. However, RhB can be degraded rapidly in the presence of Fe 3 O 4 @SiO 2 @ TiO 2 and γ-Fe 2 O 3 @SiO 2 @TiO 2 catalysts under light illumination. The normalized concentration C t /C 0 reaches to nearly zero with the illumination time of 5 h for γ-Fe 2 O 3 @SiO 2 @TiO 2 , while it is only 0.55 for Fe 3 O 4 @SiO 2 @ TiO 2 . It has been reported that the decrease in the bulk defects of TiO 2 can enhance the separation of photogenerated electrons and holes, which results in improved photocatalytic activity. Kong et al. have also found that TiO 2 synthesized at 120 °C exhibits the lower photocatalytic efficiency than the one prepared at 180 °C and calcined at 480 °C for 3 h, which is attributed to the recombination of most photogenerated charge carriers in the bulk defects for the former 49 . Moreover, Guo et al. have shown that the photocatalytic activity can be enhanced in ZnO tetrapods with less nonradiative defects 50 . Here the lower photocatalytic efficiency of Fe 3 O 4 @SiO 2 @TiO 2 is attributed to the amorphous TiO 2 shell. Amorphous TiO 2 has high concentration of bulk defects, which may act as charge trapping centers, preventing the photo-generated carriers to be used for reactions. After annealing, TiO 2 in the assembly is converted into highly crystalline anatase structure with low concentration of bulk defects, which SCIeNtIfIC REPORTS | 7: 6960 | DOI:10.1038/s41598-017-07417-z possesses high photocatalytic activity. Thus the corn-like γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructure demonstrates the highest degradation rate for RhB dye. Figure 7(b) shows the corresponding evolution of UV-vis spectra for RhB as a function of time for γ-Fe 2 O 3 @ SiO 2 @TiO 2 . The relative concentration of RhB (C t /C 0 ) is extracted from the integrated UV-vis peak intensity. The decrease and shift in the maximum of the absorption peak suggest the reduction of the chromophoric group and thus the degradation of RhB molecules. As shown in Fig. 7(b), the maximum absorbance of RhB shifts gradually from 553 nm to 510 nm, and it remarkably fade away at the illumination time of 5 h. The final product has an absorption peak at 510 nm, which can be identified as an incompletely N-deethylated outcome of RhB, N-ethyl rhodamine (MER). Such a process indicates that RhB has been degraded in the presence of γ-Fe 2 O 3 @SiO 2 @TiO 2 sample. The similar photodegradation process of RhB by TiO 2 has been studied in detail earlier 51,52 .
To assess the magnetic recyclability and photoactivity after recycling of the γ-Fe 2 O 3 @SiO 2 @TiO 2 catalysts, the above experiment was performed, after which the catalysts were recycled by a magnet and re-used for the same experiment. This procedure was repeated for ten times. The percentage of degradation of RhB (at 5 h) after each experiment was calculated using (1 - C t /C 0 ) × 100%. As shown in Fig. 8, a degradation percentage of 65% is retained after eight cycles, with an average loss of ~5% activity after each cycle. There is a large drop of the activity at the 9 th and 10 th cycles. We attribute the reduced activity to the adsorbed by-product from RhB photodegradation, which blocks the active sites of TiO 2 . We thus postulate that the photoactivity can be recovered by heating to remove the adsorbates. To ensure dispersibility of the catalysts, and prevent aggregation (reduces active surface area) and oxidation of γ-Fe 2 O 3 to antiferromagnetic α-Fe 2 O 3 , the temperature should be as low as possible. We found that moderate heat treatment at 200 °C for 30 min in air is the optimal condition for regeneration of the photocatalysts. As can be seen in Fig. 8, after the recycled γ-Fe 2 O 3 @SiO 2 @TiO 2 sample was subjected to the treatment, 100% RhB degradation was achieved at 5 h again, suggesting that the photocatalytic activity was fully recovered. The photocatalytic activity does not decrease after repeated regeneration (11 th and 12 th cycles in Fig. 8), demonstrating the reusability and durability of the catalysts.

Conclusions
In summary, we have successfully synthesized anisotropic corn-like γ-Fe 2 O 3 @SiO 2 @TiO 2 core/shell heterostructures by modified solvothermal and sol-gel methods. The formation mechanism of the hierarchical heterostructures is attributed to a self-assembly process induced by magnetic dipole interactions assisted by fusing effect of TiO 2 . The magnetic remanence is enhanced in the chain-like structure, facilitating magnetic recovery. The assembly demonstrates good photocatalytic activity and magnetic recyclability. While the catalytic activity decreases after each cycle due to adsorption of contaminants, it can be completely recovered by moderate heating. Therefore, our material is a robust and durable photocatalyst with excellent magnetic recyclability, and does not suffer from reduced surface area in supported catalysts.

Methods
Synthesis of Fe 3 O 4 microspheres. All the reagents were analytical grade without further purification.
Deionized water was used for all synthesis and post-treatment processes. Fe 3 O 4 microspheres were synthesized by a modified solvothermal method 53 . Ferric chloride (FeCl 3 , 0.4 g) was dissolved in ethylene glycol (EG, 20 mL) to form a clear solution, and then sodium acetate (NaAc, 1.8 g) and sodium polyacrylate (NaPAA, 0.07 g) were added into it to obtain a mixture. The mixture was stirred vigorously for 30 min and then sealed in a Teflon-lined stainless-steel autoclave (20 mL capacity). The autoclave was maintained at 200 °C for 12 h, and allowed to cool to room temperature naturally. Fe 3 O 4 microspheres were obtained. They were rinsed with ethanol, and then suspended in ethanol for later use. In addition, polyethylene glycol (PEG) and ethylene diamine (ED) were also tried as surfactants to control the morphology of Fe 3 O 4 .
Synthesis of anisotropic, corn-like γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructure. To obtain the anisotropic γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructure, a SiO 2 coupling layer was first coated on Fe 3 O 4 by a sol-gel process 44 . Fe 3 O 4 solution was maintained in a water bath at 40 °C, and then ammonia (25%, 89.5 mL), water (75 mL), and tetraethyl orthosilicate (TEOS, 1.5 mL) were added sequentially into the above Fe 3 O 4 solution. The mixture was stirred vigorously for 2 h and followed by sonication for 1 h. Thus, Fe 3 O 4 microspheres with a SiO 2 shell can be obtained. The microspheres were rinsed with ethanol, and then resuspended in ethanol. This suspension was heated up to 60 °C for 12 h to strengthen the Si-O-Fe cross-linking on the surface of the Fe 3 O 4 microspheres. The Fe 3 O 4 @SiO 2 magnetic microspheres were then flocculated by applying a magnetic field to allow convenient removal of the free silica.
For TiO 2 shell coating, 0.15 mL water and 5 mL ethanol were added into the suspension of Fe 3 O 4 @SiO 2 microspheres, and then subjected to vortex-mixing for 15 min. Subsequently, a solution of titanium (IV) isopropoxide (TTIP, 0.5 mL) in 10 mL ethanol was added into the mixture, and then stirred vigorously in a 70 °C water bath for 4 h. The mixture was then sealed in an autoclave, heated at 120 °C for 5 h, and cooled to room temperature. The product was then rinsed and dried. To improve the crystallinity of TiO 2 for better photocatalytic properties, the Fe 3 O 4 @SiO 2 @TiO 2 heterostructures were annealed at 350 °C for 2 h in air. Upon annealing, TiO 2 crystalized and Fe 3 O 4 was oxidized into γ-Fe 2 O 3 , and as a result, the color turned from black for Fe 3 O 4 @SiO 2 @TiO 2 to brown-red for γ-Fe 2 O 3 @SiO 2 @TiO 2 heterostructures.
Photocatalytic activity measurement. The photocatalytic activity of the samples was evaluated by the degradation of Rhodamine B (RhB) under the irradiation of a Xe lamp (300 W). 100 mL RhB aqueous solution with a concentration of 10 mg/L was mixed with 0.1 g catalysts in an optically matched Pyrex vessel. The reactor is tightly sealed and water cooled to a temperature of 25 °C to avoid heating effects under irradiation. The distance between the lamp and RhB solution is 10 cm. Before the irradiation, the suspension was stirred in dark condition until an adsorption-desorption equilibrium was established. Samples of the solution were taken out from the reactor every one hour, and the concentration of RhB was analyzed by UV-vis spectra. At the end of each photocatalytic cycle, the photocatalysts were magnetically collected, and then rinsed with water for the next cycle use.
Characterizations. X-ray diffraction (XRD) patterns were recorded with a powder X-ray diffraction instrument with Cu Kα radiation (λ = 1.54 Å). The elements and valence states were characterized using X-ray photoelectron spectroscopy (XPS). The morphologies were observed on a field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). Magnetic properties were carried out in a vibrating sample magnetometer (VSM) at room temperature.