Photocatalytic Property of Fe3O4/SiO2/TiO2 Core-Shell Nanoparticle with Different Functional Layer Thicknesses

1 Key Laboratory of Instrumentation Science and Dynamic Measurement of Ministry of Education, North University of China, Taiyuan, Shanxi 030051, China 2 Science and Technology on Electronic Test & Meaurement Laboratory, North University of China, Taiyuan, Shanxi 030051, China 3 College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030051, China 4Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong

Numerous studies on the photocatalytic effect of coreshell structures based on titanium dioxide, silica layer, and magnetic core have been reported in the literature [16][17][18][19][20][21][22].A study [16] showed that core-shell structure TiO 2 /BaFe 12 O 19 composite nanoparticles can effectively photodegrade organic pollutants in the dispersion system.As reported in [17], the SiO 2 /TiO 2 particles exhibit higher photocatalytic activity than pure TiO 2 .To optimize photocatalytic performance, several studies [18][19][20] synthesized Fe 3 O 4 /SiO 2 /TiO 2 (FST) particles for the decomposition of acid [19,20] or methyl orange (MO) dye [21], which can be recycled easily using a magnetic field method because of the existence of Fe 3 O 4 .The amount of TiO 2 is closely related to the functional layer and serves a key function in the photocatalytic system.The studies [22,23] analyzed the SiO 2 /TiO 2 composite nanoparticles for multistep shell coating of TiO 2 and found that the number of coating steps is not more than three.A study [19] reported that FST particles maintain high degradation rate and catalyst recovery even after being used for eight times.All these results indicated that the properties of FST core-shell nanoparticles can be optimized through TiO 2 coating and recovery for a suitable number of times, without introducing any additional doping materials that may result in secondary pollution.
This paper presents our research on the properties of FST core-shell nanoparticles by coating TiO 2 functional layers at different times.The coating activity aimed to enhance the photocatalytic capacity of the functional nanoparticles.To characterize the properties of FST nanoparticles, transmission electron microscopy (TEM) was used to test the morphology, and X-ray diffraction (XRD) was used to verify the components of these particles.Through vibratory sample magnetometer (VSM) and Brunauer-Emmett-Teller (BET) analysis, the magnetic property and specific surface area of these FST particles were successfully observed.The recycle experiment was performed to demonstrate the recyclability of the FST particles.The experimental results illustrate that the degradation of MO was in agreement with the apparent firstorder kinetic equation, thereby verifying the photocatalytic capability and stability of the FST particles.

Methodology
2.1.Reagents and Apparatus.Ferric chloride, tetraethoxysilane (TEOS), tetrabutyl orthotitanate (TBOT), alcohol, hydrochloric acid, and nitric acid were all provided by Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China).Titanium dioxide (TiO 2 ) powder sample, used as a contrast photocatalyst in our study, was commercial Degussa P25 with 80% anatase and 20% rutile produced by the Degussa AG Company in Germany.MO ( max = 462.5 nm) obtained from Sigma was used as a model pollutant.Deionized water with 18.25 MΩ was purified using an ultrapure system.With a JEOL JEM-1200EX TEM, measurements were performed with an accelerating voltage of 200 kV.The surface morphology of the composite nanomaterials was observed using a S4700 scanning electron microscope (SEM).XRD measurements were also performed by a Bruker D8 Advance XRD system.Magnetic properties were characterized by a Lake shore 7307 VSM.The average size of particles was tested by a Mastersize 2000 Laser Granularity Apparatus.The specific surface area was measured using a QUADRASORB SI specific surface area instrument.The absorbance of the degraded solution was tested by a 721G visible spectrophotometer.

Synthesis of Core-Shell Structure FST Magnetic Particles.
The Fe 3 O 4 magnetic particles were obtained using the carbon reduction method [13] and modified with SiO 2 .The Fe 3 O 4 particles were placed in a beaker with 100 mL of alcohol, to which 15 mL of TEOS was added after the temperature reached 50 ∘ C. Thereafter, 0.97 mL of hydrochloric acid and a small amount of deionized water were added into the collosol.After 3 h, the residual collosol was poured out, and the particles at the bottom of the beaker were attracted by a magnet.The wet particles were placed in a quartz Petri dish and heated at 500 ∘ C for 2 h under −0.2 Trom vacuum conditions.After milling, Fe 3 O 4 /SiO 2 particles were finally obtained.
The Fe 3 O 4 /SiO 2 particles were then encapsulated with different TiO 2 shell thicknesses.First, the particles were placed in a beaker with TBOT and alcohol.The beaker with the collosol and particles was treated under ultrasound for 10 min and stirred for 20 min.Second, deionized water was slowly added with continuous stirring.The ratios of ester and alcohol over water were 30.3 : 1 and 12.5 : 1, respectively.Third, the particles were heated at 500 ∘ C for 2 h under −0.2 Trom vacuum conditions.After milling, FST1 was obtained.The other types of nanoparticles (e.g., FST2, FST3, FST4, and FST5) with different TiO 2 shell thicknesses were also obtained by following the same procedures.

Results and Discussion
3.1.Characterization of FST Particles.The morphologies of SiO 2 /TiO 2 core-shell particles with various coating steps are shown in Figure 1. Figure 1(a) shows that the FST1 particles were successfully encapsulated by a TiO 2 layer even though the thickness of the layer was extremely thin.This result may be caused by particles with low photocatalytic performance.Figure 1(b) shows that the TiO 2 layer of FST2 was thicker and more uniform than that of FST1.The TEM image of the distribution of FST2 particles is shown in Figure 1(c), which shows small clusters of FST particles with good dispersion.The TEM image of FST3 in Figure 1(d) reveals that FST3 also had a uniform coating layer and the amount of TiO 2 in FST4 (Figure 1(e)) was slightly less than that in FST3.The TEM image of FST5 in Figure 1(f) shows that the amount of TiO 2 in FST5 was similar to that in FST4 with a shell thickness of approximately 60 nm.More importantly, the TiO 2 layer of one FST5 particle had no magnetic core, which indicates that the nonmagnetic core particles could be generated as the coating times increased.
The morphology of the as-prepared FST nanoparticles was observed by SEM. Figure 2 illustrates the panoramic morphologies of the as-obtained products, which were mainly uniform nanoparticles, with diameters ranging from 400 nm to 500 nm.Figures 2(a) and 2(b) show that the surfaces of the FST1 and FST2 nanomaterials were rough, which resulted in larger specific surface areas, as specified in Table 1.The FST2 particles in Figure 2(c) displayed a certain level of homogeneity in shape and size.The surfaces of the other three nanoparticles, as shown in Figures 2(d), 2(e), and 2(f), became increasingly smooth as the coating times increased, which resulted in a sharp drop in specific surface area.
The structure and phase purity of synthesized Fe Notably, the relative intensity of FST2 peaks at 25.4 ∘ (101) was larger than that of FST1 but smaller than those of FST3 and FST4, and the peak of FST5 was the largest.We hypothesize that the TiO 2 content can be enhanced as the coating time increases.Figure 4 and Table 1 show the size distribution spectrum and specific surface areas of the five FST particles and pure TiO 2 .The results demonstrate that the average size was 500 nm, as shown in curves (a) and (b) of Figure 4 and the average size of FST2 particles (550 nm) increased by approximately 150 nm compared with that of FST1 (330 nm) as the coating times increased.After titania coating for three   times, the size of the FST3 (517 nm) and FST4 (528 nm) particles did not increase as significantly as those of FST2.Instead, as the titania coating steps increased, the specific surface area of FST particles sharply declined (e.g., from FST2 (48 m 2 /g ± 2%) to FST5 (4.58 m 2 /g ± 2%)), as specified in Table 1.This finding indicates that the outermost functional layer became tighter as the titania coating times increased, thereby resulting in a smaller surface area [23].As shown in the SEM micrographs, the surface of the particles became increasingly smooth because of the reduced photocatalytic degradation capability of FST3, FST4, and FST5 particles.The magnetization behavior shown in Figure 5 indicates that the saturation intensity of the five FST particles progressively decreased from FST1 (46.5 emu/g) to FST5 (32.2 emu/g).Meanwhile, the remanence of the five types of particles was nearly free so that the particles could not agglomerate.These samples appeared to have a mixture of ferrimagnetic and superparamagnetic properties, with only a minor indication of hysteresis.Although the magnetism of FST5 remained sufficiently high to be magnetically segregated by applying a magnetic field, this property could accelerate the separation of photocatalysts from MO solutions.These recycled materials may be further used in the water treatment process, thereby reducing secondary pollution.

Photocatalytic Activity.
The photocatalytic activity of FST functional particles with different coating times was studied using the decolorization of MO solution as a model pollutant.The experiments were conducted in a 150 mL cylindrical glass reactor equipped with a UV lamp (365 nm, 8 W).The reactor contained 40 mL of MO solution with an initial concentration of 10 mg/L and 0.4 g of FST functional particles.The standard P25 was used as a reference under the same experimental conditions.
Figure 6 shows that the decolorization of MO solution for FST1 reached 25.10%.Previous characterization results showed that the thickness of the TiO 2 functional layer of FST1 was small, which may result in a low degree of decolorization of the MO solution for FST1.Furthermore, the decolorization of MO solution for FST3 during photocatalytic degradation was only 12.25%, whereas those for FST4, FST5, and blank were less than 1%.These results indicate that the photocatalytic degradation of MO solution may be induced by the titania content and specific surface area.Our experimental results reveal that the decoloration of MO solution reached 71% and 85% for P25 and FST2 particles, respectively, after 25 h of UV light irradiation.FST2 exhibited the highest photocatalytic efficiency among the five FST nanoparticles.Meanwhile, the photocatalytic activity of the FST2 particles was slightly higher than that of P25 under UV light irradiation, as illustrated by the curves in Figure 6.The decolorization of the MO solution of P25 started to decrease after 18 h, whereas the activity of the FST2 particles gradually increased.This finding further suggests that the catalytic persistence of FST2 particles was better than that of P25.In summary, Figure 6 illustrates the photodegradation rates of MO solution in the following order: FST2 > P25 > FST1 > FST3 > FST4 > FST5 > blank.
The variation in photocatalytic activity among the different types of particles could be ascribed to the following reasons.In the photocatalytic decomposition of MO solution under UV irradiation, photocatalytic degradation by activated TiO 2 and photosensitization process of MO are the two causes of MO solution degradation [24,25].Despite the existence of the photosensitization process, titanium dioxide, not the MO molecule, primarily absorbs the photon to generate electrons and holes in the conduction and valence bands, respectively.The photogenerated holes can react with H 2 O or adsorbed hydroxyl to form hydroxy radicals, which can effectively oxidize the MO molecule.Moreover, the photogenerated electron can react with the adsorbed oxygen to produce a superoxide radical anion, which can also oxidize the MO molecule to a great extent.Thus, the adsorption of MO molecules by the photocatalyst is a key reason for degradation, and the adsorption condition is dependent on the size of the specific surface area of the photocatalyst [26].Meanwhile, titania is a key factor in the degradation of MO solution, which further explains why FST2 exhibited high photocatalytic activity based on the results of XRD (see Figure 3(a)) and TEM (see Figure 1(a)).

FST2 Recycling Capacity.
We further examined the cyclic utilization rate in the photocatalytic activity of FST2 to  verify the recycle activity.The photocatalytic degradation of MO solution, which is a function of irradiation time in the presence of different recycling samples, was observed to follow the first-order kinetic reaction as follows: where  app is the apparent rate constant and can be selected as the basic kinetic parameter for different photocatalysts,  0 is the initial absorbance of the MO solution, and  is the solution-phase absorbance of MO solution.Using (1), we can determine the apparent rate constant from the gradient of the graph of ln( 0 /) against the irradiation time.Notably,  app can be selected as the basic kinetic parameter for different photocatalysts, which can aid in determining the photocatalytic activity despite the previous adsorption period in the dark and MO concentration remaining in the solution.
Table 2 provides the degradation kinetic parameters and apparent rate constant of FST2 with different cycles of MO degradation.These results further confirm the stable property of the generated FST2. Figure 7 illustrates that FST2 exhibited the highest value of  app , that is, the highest photocatalytic capability, in the first cycle.The reduction in  app after the first cycle may be attributed to the intermediates adsorbed on the surface of FST2 (carbonization) during the recycling process [27].

Figure 6 :
Figure 6: Curves of photocatalytic decoloration of MO solution treated by FST catalysts with various coating steps under UV light.

Table 1 :
Specific surface area of the FST nanoparticles.

Table 2 :
Photocatalytic capability of FST2 with different cycles of MO degradation.