Study on photodegradation activity of Fe 3 O 4 @SiO 2 @TiO 2 -Ce/rGO magnetic photocatalyst

The magnetic core-shell Fe 3 O 4 @SiO 2 @TiO 2 -Ce/rGO composite nanomaterials were prepared by sol-gel method and hydrothermal method, composite photocatalyst doped with metal and loaded with graphene, the catalytic activity has been greatly improved. Under normal experimental conditions (pH = 7, [MB] = 10mg/L, magnetic composite photocatalyst concentration = 0.1g/50mL), photocatalyst maximum degrade MB reaches 98.2% in 140 minutes.


1.Introduction
With the continuous advancement of the industrial revolution, the problem of water pollution has become increasingly serious. Polluted water sources seriously endanger human health, and it is extremely important to solve the problem of water pollution [1] . There are many types of pollutants in water sources, such as organic pollutants, inorganic pollutants, harmful metal ions, and harmful nitrogen oxide. Traditional sewage treatment methods are low in efficiency, high in cost, and selective to different pollutants. In particular, there are secondary pollution problems. Therefore, sewage treatment has not been well solved [2] . As early as 1972, Fujishima and Honda discovered that TiO2 in photocells could be redoxed by water to release clean energy (H2/O2) when exposed to light [3] ; in 1976, Carey et al. Used TiO2 semiconductors to degrade organic pollutants [4] . Since then, photocatalytic oxidation has entered a stage of rapid development as a new water treatment technology.
TiO2 photocatalytic degradation is the most suitable and available method for the treatment of organic compounds. The TiO2 photocatalyst has the characteristics of good stability, low cost, strong catalytic activity, and is not harmful to the environment. The photocatalytic efficiency of nano titanium dioxide is related to crystal phase, particle size and specific surface area. Anatase has been demonstrated that anatase is the most stable and effective polymorph at nanoscale due to its relatively low surface energy [5][6][7] . However, due to the large band gap of TiO2, high recombination rate of photogenerated carriers, and difficult to recycle characteristics, the application of TiO2 in water treatment is limited. At present, a small amount of doping of TiO2 with transition metal lanthanide series and metal actinide series metals can reduce the band gap width of TiO2, make it also have photoresponse in the visible light region, thereby improving the utilization of sunlight [8] . Studies on doping of transition metals, including Ce, Co, Ni, etc., have been reported, the modification effect of Ce is better as compared to that of other transition metals [9][10][11][12][13] . In addition, TiO2 photocatalyst makes their complete recovery from the wastewater difficult. This presents a major drawback to the application of the photocatalytic processes for treating wastewaters. Although many research groups have developed magnetic core-shell catalysts containing TiO2 that can be quickly separated from sewage [14,15] . However, The magnetic core may be reduce the photocatalytic efficiency [16] , To resolve this problem, SiO2 can be applied as a barrier layer to form Fe3O4@SiO2 which will not allow the interaction between the magnetic core and the TiO2 coating [17][18][19] . Meanwhile, Graphene is a twodimensional nanomaterial with high specific surface area and high conductivity [20,21] , it can improve the migration efficiency of photogenerated carriers, Therefore, the recombination of photogenerated carriers can be effectively suppressed.
In this study, Fe3O4@SiO2@TiO2-Ce/ RGO core-shell nano photocatalyst was prepared by sol-gel method and hydrothermal method, the TiO2 doped with rare earth metal Ce is loaded on graphene oxide to improve photocatalytic activity, and, Ce metal acts as an electron receiver to reduce electron-hole recombination, Graphene has good electrical conductivity and large specific surface area, It can increase the contact area between pollutants and the catalyst and improve the catalytic activity of the catalyst. The principle is shown in Figure 1.

Synthesis of Fe3O4@SiO2 nanoparticles
Weigh Fe3O4 (0.3g) into a 100mL beaker, and dilute HCl (50 mL, 0.1 mol L -1 ) was added for sonication for 15min. Then, the Fe3O4 solid was magnetically separated and washed three times with deionized water. The magnetically separated Fe3O4 solid was put into a 250mL three-necked flask, and deionized water (18mL) and absolute ethanol (80mL) were added. Then, NH3H2O (2mL) and TEOS (0.6mL) was slowly added to the solution under stirring which continued for 12h at room temperature. The Fe3O4@SiO2 were magnetically separated and washed three times with deionized water and absolute ethanol and dried at 60 ℃ under for 12h to obtain Fe3O4@SiO2 powder.

Synthesis of Fe3O4@SiO2@TiO2-Ce nanoparticles
0.1g of Ce(NO3)3 was weighed into a 100 mL beaker, and then A solution was prepared by sequentially adding 1 mL of H2O, 0.2 mL of HNO3, and 20 mL of absolute ethanol. 0.2g of Fe3O4 was weighed in a 100 mL beaker, and 20 mL of absolute ethanol, 0.3mL of glacial acetic acid, and 8 mL of TEOT were gradually added, then mechanically stirred at room temperature for 30 min to prepare a B solution. The solution A was slowly added dropwise to the solution B, stirred well until a gel was formed, and aged at room temperature for 24h. After drying, it was calcined in an N2 atmosphere for 2 h to obtain Fe3O4 @SiO2 @TiO2-Ce powder.

Synthesis of Fe3O4@SiO2@TiO2-Ce/rGO nanoparticles
0.08g of graphene oxide was dispersed in a mixture of deionized water and absolute ethanol, and ultrasonically dispersed for 1h. Then 0.15g SDBS and 0.2g Fe3O4@TiO2-Ce were added in sequence, and ultrasonic dispersion was continued for 1h. The solution was transferred to a polytetrafluoroethylene autoclave and reacted in an oven at 120℃ for 3h. The product was washed three times with anhydrous ethanol and deionized water, respectively, and then dried at 60℃ for 24h to obtain Fe3O4@SiO2@TiO2-Ce/rGO photocatalyst.   . It shows that the introduced TiO2 is mainly anatase phase, As shown in Figure 2(c, d), no characteristic diffraction peaks of rGO and Ce were found, this may be because the amount of Ce and GO is too small and the detection is not obvious. The typical lamellar structure of GO can be clearly seen in Figure 3a; Figure 3b shows that the Fe3O4 particles have a tetragonal structure,The particle size is about 400nn and evenly distributed; Figure 3c can clearly show that the Fe3O4 surface becomes smooth; The edges and corners are no longer clear, at the same time the particle size has increased, indicating that SiO2 has been successfully wrapped on the surface of Fe3O4; Figure 3d shows that the surface becomes rough with granular substances, The EDS spectrum shows that the Ce content is 3%, indicating that Ce metal has been successfully doped on the surface of TiO2. Figure 3e shows that Fe3O4@SiO2@TiO2-Ce has been successfully loaded on rGO. . In order to further clarify the morphology of the sample, Research group had performed TEM image analysis on Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@TiO2-Ce and Fe3O4@SiO2@TiO2-Ce/rGO. Figure 4a shows the tetragonal configuration of Fe3O4 more clearly; Figure 4b shows that a translucent part is formed on the surface of Fe3O4, It shows that Fe3O4 and SiO2 form a core-shell structure; Figure 4c shows that there are more obvious particles on the outer surface of the particles, It further proves that Ce particles have been successfully doped on the surface of Fe3O4@SiO2@TiO2, Figure 4d shows that Fe3O4@SiO2@TiO2-Ce is tightly supported on the surface of rGO, One step further confirmed the successful preparation of Fe3O4@SiO2@TiO2-Ce/rGO composite nanoparticles in this experiment. (antisymmetric stretching vibration); Fe-O characteristic peak is at 573cm -1 (asymmetric stretching vibration) [22] ; The characteristic peak of Ti-O-Ti is at 478 cm -1 (stretching vibration); 807cm -1 (symmetrical stretch) is Ti-O-Si [17] ; C = C (double bond stretching vibration) zone is at 1623cm -1 [23] . The two curves have not changed significantly, There is an obvious C-C impurity peak at 1226cm -1 in Fe3O4@SiO2@TiO2-Ce, With the addition of graphene, the peak at C=C in Fe3O4@SiO2@TiO2-Ce/rGO is more obvious. It can be seen from the ultraviolet-visible diffuse reflectance spectrum of the catalyst (in Figure 6 The adsorption performance of photocatalyst was characterized by N2 physical adsorption experiment, the corresponding N2 adsorption-desorption isotherm is shown in Figure 7. It can be seen that the samples Fe3O4@SiO2@TiO2-Ce ， Fe3O4@SiO2@TiO2-Ce /rGO show the shape of type IV isothermal curve that they are mesoporous structures, electron microscopy test showed that the particle size was about 40nm. It was confirmed by previous electron microscopy tests. The test results show that the specific surface areas of Fe3O4, Fe3O4@SiO2@TiO2-Ce and Fe3O4@SiO2@TiO2-Ce/rGO are 3.7660 m²/g, 40.6405 m²/g and 46.9017 m²/g, The increase in specific surface area is conducive to the adsorption of pollutants, promote the photocatalytic degradation of pollutants. Figure 8. EIS of the TiO2, Fe3O4@SiO2@TiO2-Ce, and Fe3O4@SiO2@TiO2-Ce/rGO EIS spectrum is a test method used to characterize charge transfer. The smaller the curve radius, the smaller the resistance.The Nyquist curve in Figure 9 has the largest radius of TiO2, The second is Fe3O4@SiO2@TiO2-Ce, the smallest is Fe3O4@SiO2@TiO2-Ce/rGO nanocomposite. It shows that Fe3O4@SiO2@TiO2-Ce/rGO nanocomposite has the highest charge transfer efficiency. This is due to the large π-bond structure of the loaded graphene, which promotes photogenerated electron transfer, graphene is also an excellent electron acceptor. Thereby reducing the recombination with holes, It is consistent with the UV test. Figure 9. PL spectra of nano-photocatalyst samples.

PL analysis
The recombination of photo-generated carriers will reduce the activity of TiO2 photocatalyst, Therefore PL analysis is used to measure the luminescence intensity of the fluorescence generated by the recombination of photo-generated carriers. Figure 9 above, It can be clearly seen that the fluorescence intensity of pure TiO2, Fe3O4@SiO2@TiO2-Ce, Fe3O4@SiO2@TiO2-Ce/rGO gradually decreases. It shows that doping Ce and loading rGO reduces the recombination efficiency of photogenerated carriers. This is consistent with the EIS results. Therefore, the photocatalytic activity is improved. The photocatalytic activity of the sample was studied by the photodegradation rate of MB under visible light irradiation.The absorbance of MB after degradation was measured with ultravioletvisible-near-infrared spectrophotometer. The degradation efficiency of the photocatalyst increases with time.The photocatalytic activity of Fe3O4@SiO2 is 0; the photocatalytic activity of TiO2 is very low, and the degradation rate is 8.7%; The photocatalytic activity of Fe3O4@SiO2@TiO2-Ce nanocomposite material obtained after metal Ce doping has been greatly improved, and the degradation rate is 75.4%; Fe3O4@SiO2@TiO2-Ce/rGO nanocomposite is 98.2%. It shows that the supported graphene plays a great role in improving the photocatalytic activity of the photocatalyst. This is because the excitation TiO2 conduction band electrons can be transferred to the Please do not adjust margins Please do not adjust margins graphene sheet, This prevents photogenerated electrons from recombining with holes， it is consistent with PL test results；The UV-Vis DRS test also proved that the modified TiO2 photocatalyst has been greatly improved in the visible light absorption region ； The EIS test results are consistent with UV-Vis DRS test and PL test. Therefore, the photocatalytic activity is improved. As shown in the right picture of Figure 10. Fe3O4@SiO2@TiO2-Ce/rGO nanocomposite materials can be easily aggregated by magnetic substances, So as to achieve the purpose of convenient recycling.

Conclusions
Therefore, the prepared by sol-gel method and hydrothermal method. The morphology and structure were characterized by TEM, SEM, xrd and FT-IR, and Performance tests for UV-Vis DRS, PL, N2 gas adsorption, EIS, etc. All proved that the prepared Fe3O4@SiO2@TiO2-Ce/rGO nanocomposite has high catalytic activity. In addition, the catalytic activity of TiO2 photocatalyst by doped metal cerium and supported reduced graphene oxide has been greatly improved. There are broad application prospects in the treatment of factory wastewater in the future.

Conflicts of interest
There are no conflicts to declare

Availability of data and material
The datasets supporting the conclusions of this article are included within the article.

Funding
The authors received no specifc funding for this work.