Synthesis, Characterization and Comparison of Photo-catalytic Efficiency of Fe 3 O 4 /SiO 2 /TiO 2 , SrFe 12 O 19 /SiO 2 /TiO 2 and Fe 3 O 4 /SiO 2 /ZnO Core/shell/shell Nano-structures

: In this research work, three magnetic composites of Fe 3 O 4 /SiO 2 /TiO 2 , SrFe 12 O 19 /SiO 2 /TiO 2 and Fe 3 O 4 /SiO 2 /ZnO with core/shell/shell structures were successfully prepared by the sol-gel method. For this purpose, in the first step, soft magnetic and hard magnetic powders of Fe 3 O 4 and SrFe 12 O 19 were synthesized using carbon reduction and co-precipitation routes, respectively. In the second step, silica coating was performed by controlling the hydrolysis and condensation of the tetraethyl orthosilicate (TEOS) precursor on the magnetic cores. In the third step, a coating of TiO 2 or ZnO photo-catalytic shells was made on as-prepared composites using precursors of titanium n -butoxide (TNBT) or zinc nitrate hexahydrate, respectively. The as-prepared magnetically separable photo-catalysts were characterized using XRD, FESEM, TEM and VSM. The results of the FESEM and TEM analyses confirmed the formation of the core/shell/shell structures. The saturation magnetization of the Fe 3 O 4 /SiO 2 /TiO 2 , SrFe 12 O 19 /SiO 2 /TiO 2 and Fe 3 O 4 /SiO 2 /ZnO photo-catalytic materials was 41.5 emu/g, 33 emu/g and 49 emu/g, respectively. Evaluation of the photo-catalytic activity of Fe 3 O 4 /SiO 2 /TiO 2 , SrFe 12 O 19 /SiO 2 /TiO 2 and Fe 3 O 4 /SiO 2 /ZnO composites by using methylene blue (MB) shows degradation percentages of 84%, 80% and 58%, respectively, under UV illumination.


Introduction
In recent years, the coating of particles surface by the different material and development of the core/shell structures have attracted considerable attention from scientific communities. Coating of core particles with a thin layer of a compatible material having different chemical composition can change the structure, size and composition of particles; thereby it can control their magnetic, optical, mechanical, thermal, electrical and catalytic properties [1][2][3]. The nano-composites with core/shell structure can exhibit improved properties that are completely different relative to each of the core or shell materials [4]. Furthermore, this structure can simultaneously have the properties of core and shell materials. Multi-layered core/shell structures made up of a core and several shells around it have been attended recently [5].
Nowadays, due to increasing environmental concerns, many efforts have been conducted around the world to obtain effective materials and processes for the purification of contaminated water [6,7]. Hence, purification processes of organic pollutants by using semiconductor nanoparticles such as ZnO, ZnS, TiO2 and SnO2, etc. have been studied [8][9][10]. In order to solve the problem of separation of photocatalytic powders from purified water and allowing to be reusing them, TiO2 and ZnO particles are coated on hard and soft magnetic powders to facilitate the separation of these photo-catalytic materials from the environment using an external magnetic field [11][12][13]. However, the photo-catalytic activity of TiO2 and ZnO will be reduced in their direct contact with magnetic cores, which is due to the photo-dissolution phenomenon [14,15]. In order to overcome this problem, three-layered core/shell/shell composites can be utilized. Thus, to prevent electrical contact and direct interaction between the magnetic core and the photo-catalytic shell, an intermediate SiO2 shell can be applied between the magnetic core and the photocatalytic shell [12,16]. Typically, the Stöber method is used for the coating of the SiO2 layer on the magnetic particles. Alcohol should be used as a reaction media [17][18][19]. This method involves the multi-stage hydrolysis and condensation of tetraethyl ortho-silicate (TEOS) in an alcoholic media in the presence of ammonia (NH4OH) as a catalyst. The formation mechanism according to the reactions (1) and (2) can be described as: Reducing the concentration of TEOS in the solution will reduce particle size. Silica particles produced by this method are amorphous and porous. Similarly, the change in the concentration of ammonium hydroxide affects the size, porosity and shape of particles [20].

Synthesis of the magnetic cores
In this study, Fe3O4 magnetic particles and strontium ferrite particles were prepared using the carbon reduction and co-precipitation methods, respectively [23][24][25]. In order to obtain Fe3O4 particles, 80 g FeCl3.6H2O was mixed into 150 ml of distilled water for 15 min. Subsequently, 7 g degreased carbon cotton was added to the solution, and it was subjected to the ultrasonic treatment for 10 min. Then, the product was collected and heated at 400 °C for 4 h.
To produce the strontium hexaferrite powder, at first FeCl3.6H2O and SrCl2.6H2O solutions were prepared in a mixture of distilled water/ethanol with a volume ratio of

Coating of silica shell
Coating of silica shell on magnetic cores was done using the Stöber method.
Fe3O4/SiO2 nanostructures were fabricated as follows: At first, 0.05 g of Fe3O4 powder was dispersed into ethanol and distilled water with a ratio of 4 to 1 in an ultrasonic bath. Then, ammonia (25% by weight) and tetraethyl orthosilicate with a volume ratio of 8:1 were added to the mixture. The coated particles were separated from the suspension using a powerful magnet. Then obtained powder was washed with ethanol and distilled water and dried in a vacuum oven at 60 °C for 8 h.
In order to obtain the SrFe12O19/SiO2 composite, 0.03 g of SrFe12O19 powder was dispersed under an ultrasonic bath in a mixture including isopropanol, distilled water, tetraethyl orthosilicate and ammonia solution with values of 250, 4, 1 and 0.15 ml, respectively [11]. The achieved powder was separated from the mixture using a strong magnet. Then, it was washed with isopropanol and distilled water and finally dried in a vacuum oven at 40 °C for 24 h.

Coating of photo-catalytic shells
In order to cover the ZnO photo-catalytic shell on core/shell particles, a solution of 0.6 g ZnNO3.6H2O in 100 mL DMF was made and 0.05 g of the Fe3O4/SiO2 powder was added to it. Then, the NaOH solution (0.02 M) was slowly added to the initial suspension and was placed under stirring for 6 h. The powders were separated from the solution using magnetic separation and washed with distilled water and ethanol several times, and dried in a dryer at 60 °C for 6 h. In the end, it was calcined at 500 °C for 2 h.
The Fe3O4/SiO2 particles were coated by TiO2 through the hydrolysis and condensation of titanium n-butoxide in ethanol solution. 0.03 g of the Fe3O4/SiO2 particles, 0.24 ml distilled water, 0.1 g HPC and 60 ml absolute ethanol were kept under intense stirring. After dispersion, titanium n-butoxide and ethanol were added and kept for 90 minutes at 85°C. The obtained powder was washed several times by using ethanol and distilled water and dried for 4 hours at 60°C. Finally, particles were calcined in an argon atmosphere at 500℃ for 2 h.
In another part of this study, 0.25 g of SrFe12O19/SiO2 powder with ethanol (140ml) and distilled water (3ml) were dispersed in the ultrasonic bath for coating the TiO2 photo-catalytic shell. Then a solution of titanium n-butoxide was added to the solution drop by drop and placed under stirrer at 90 °C for 2h. The composite was separated using magnetic separation and washed several times using water and ethanol. Then it was dried at 60°C for 6 h. In the end, it was calcined at 500°C for 1 h.

Study of photo-catalytic properties
In this study, the decomposition of methylene blue dye using UV light was used as a model for evaluation of the photo-catalytic properties. In this regard, the photocatalytic powders were added to the methylene blue solution (50 mg/l). Afterwards, the as-prepared mixture was under a stirrer for 0.5 h in the dark and then it was exposed under the UV light. At specified intervals, the absorbance changes were obtained by an 1800 UV-Vis spectroscopy in the range of 500 to 800 nm.   [26][27][28]. By the comparison of the intensity of the photo-catalytic materials peaks in Fig. 1 (c) and (d), it is observed that the intensity of the peaks of the ZnO phase is much lower than that of the TiO2 phases, although more time is taken for ZnO shell synthesis, which could be attributed to a more easy formation of the crystallized TiO2 shell. composites are displayed in Fig. 2 (d) and (g), respectively, which indicated that the photo-catalytic coating is uneven and its morphology is completely different from silica coating [26,28,29].

Results and discussion
Also, EDX spectra of Fe3O4/SiO2/TiO2, Fe3O4/SiO2/ZnO and SrFe12O19/SiO2/TiO2 composites are exhibited in Fig. 3 Fig. 5 (a) compares the magnetic properties of Fe3O4 and SrFe12O19 core particles. As can be seen, the fabricated Fe3O4 soft magnetic particles show clear ferromagnetism hysteresis due to the narrow hysteresis loop, while the wide hysteresis curve as a typical characteristic for ferrimagnetism is clearly observed for the SrFe12O19 particles [35]. Furthermore, the saturation magnetization for Fe3O4 powder is more than that for SrFe12O19 powder. confirms the coating of silica and photo-catalytic shells [36,37]. The saturation magnetization of the Fe3O4/SiO2/ZnO, Fe3O4/SiO2/TiO2 and SrFe12O19/SiO2/TiO2 photo-catalytic nanostructures was 49 emu/g, 41.5 emu/g and 33 emu/g, respectively.
Less saturation magnetization of SrFe12O19/SiO2/TiO2 photo-catalyst could be due to less agglomeration. Although the coating process causes some degree of agglomeration in particles (as shown in Fig. 2(b-c-d and f-g)), however visual examination of SrFe12O19/SiO2/TiO2 powders also confirms comparatively less agglomeration of SrFe12O19/SiO2/TiO2 powders. By examination of absorption changes at the maximum wavelength, the destruction percentage of methylene blue dye was obtained using the following equation [38]: where D is the percentage of degradation, A0 and At are absorbance intensity in wavelength of 664 nm at times of 0 and t, respectively. and SrFe12O19/SiO2/TiO2 composites could be due to the thickening of the TiO2 shell than the ZnO shell. The better photo-catalytic properties of these composites relative to Fe3O4/SiO2/ZnO could also be attributed to the smaller band gap of TiO2 (3.2 ev) than that of ZnO (3.37 ev) [16,27]. On the other hand, decreasing the amount of agglomeration is effective in improving photo-catalytic activity. The less agglomeration of photo-catalytic particles cause increased contact surface with the pollutants and consequently the photo-catalytic efficiency increases. Therefore, less agglomeration of SrFe12O19/SiO2/TiO2 powders, as mentioned earlier, could be an effective parameter in increasing its photo-catalytic efficiency. On the other hand, increasing the surface to volume ratio improves catalytic activity [39]. It is well known that the surface to volume ratio of the cubic shape is higher than that of other shapes such as dodecahedron, octahedron, sphere and icosahedron [39,40].
Therefore, the photocatalytic properties of the Fe3O4/SiO2/TiO2 composite are slightly better than those of SrFe12O19/SiO2/TiO2 composite due to the cubic shape of its magnetic core.    formula that kapp is the apparent rate constant of photo-catalytic degradation [43]. The goodness of linear fit on data is evaluated by employing the R 2 value as the correlation coefficient factor. The kapp at the wavelength of 664 nm for three composites is more than that at 617 nm wavelength, which is in accordance with Fig.  6 that showed more absorption intensity at 664 nm relative to that at 617 nm [44]

Conclusions
The Fe3O4 particles as soft magnetic cores of core-shell structured composites were successfully synthesized by the carbon reduction method, and the SrFe12O19 particles as hard magnetic cores were prepared by a co-precipitation method in water-ethanol media. The coating of silica shells was performed on SrFe12O19 and Fe3O4 magnetic particles using the hydrolysis and condensation of TEOS precursor at room temperature. The photo-catalytic composites of Fe3O4/SiO2/TiO2, SrFe12O19/SiO2/TiO2 and Fe3O4/SiO2/ZnO were successfully prepared using titanium n-butoxide (TNBT) or zinc nitrate hexahydrate precursors.
Microscopy characterization by FESEM and TEM indicated that all synthesized core/ /shell/shell composites were covered with a layer composed of photo-catalyst.
However, the TiO2 photo-catalytic shells were thicker and the coating process was done at the lower time, and consequently coating process was easier compared to the formation of the ZnO photo-catalytic shells. The saturation magnetization of Fe3O4/SiO2/ZnO, Fe3O4/SiO2/TiO2 and SrFe12O19/SiO2/TiO2 composites with core/shell/shell structure was 49 emu/g, 41.5 emu/g and 33 emu/g, respectively. Less saturation magnetization of SrFe12O19/SiO2/TiO2 photo-catalyst could be due to less agglomeration and the nature of core material; however, it resulted in an acceptable recovery by magnetic separation.
Photo-catalytic efficiencies of Fe3O4/SiO2/ZnO, SrFe12O19/SiO2/TiO2 and Fe3O4/SiO2/TiO2 composite materials were obtained as 58%, 80% and 84% by the considering degradation of methylene blue dye. It was found that the photo-catalytic properties of titania coating in the composite with core/shell/shell structure can be improved via increasing surface to volume ratio by using the cubic-shape core particles. Thus, the as-prepared Fe3O4/SiO2/TiO2 composite powder has optimum photo-catalytic efficiency compared with SrFe12O19/SiO2/TiO2 and Fe3O4/SiO2/ZnO composites. However, as seen in sequential cycles of the destruction of MB, all synthesized photo-catalytic composites were nearly well recovered.