High Photocatalytic Activity of Fe 3 O 4-SiO 2-TiO 2 Functional Particles with Core-Shell Structure

This paper describes a novel method of synthesizing Fe 3 O 4 -SiO 2 -TiO 2 functional nanoparticles with the core-shell structure. The Fe 3 O 4 cores which were mainly superparamagnetic were synthesized through a novel carbon reduction method. The Fe 3 O 4 cores were then modified with SiO 2 and finally encapsulated with TiO 2 by the sol-gel method. The results of characterizations showed that the encapsulated 700 nm Fe 3 O 4 -SiO 2 -TiO 2 particles have a relatively uniform size distribution, an anatase TiO 2 shell, and suitable magnetic properties for allowing collection in a magnetic field. These magnetic properties, large area, relative high saturation intensity, and low retentive magnetism make the particles have high dispersibility in suspension and yet enable them to be recovered well using magnetic fields. The functionality of these particles was tested by measuring the photocatalytic activity of the decolouring of methyl orange (MO) and methylene blue (MB) under ultraviolet light and sunlight. The results showed that the introduction of the Fe 3 O 4 -SiO 2 -TiO 2 functional nanoparticles significantly increased the decoloration rate so that an MO solution at a concentration of 10mg/L could be decoloured completely within 180 minutes. The particles were recovered after utilization, washing, and drying and the primary recovery ratio was 87.5%.


Introduction
Nanoparticles could find their uses in many important industrial processes [1][2][3].One of these could be the treatment of chemicals [4] and biological molecules [5] in wastewater.Specifically titania can catalyse the decomposition of a wide range of chemicals such as azo dyes [6,7], aromatic compounds [8], and endocrine disruptors [9].The properties of titania, which is well known for the high productivity of hydroxyl free radicals when exposed to ultraviolet light, low toxicity, and low cost [10,11] make it a popular choice in wastewater treatment.
One of the main obstacles to the application of nanoparticles in industrial applications is the concern of the release and the fate of the nanoparticles in the environment.It is hence desirable to retain and recover the nanoparticles in such applications, particularly, in wastewater treatment.One approach would be to introduce superparamagnetic properties and hence to recover the nanoparticles using magnetic fields [6,12,13].The magnetic composite photocatalyst can be magnetically agitated by an alternating magnetic field in a suspension system [14].In this way, the superparamagnetism of nanoscale Fe 3 O 4 particles makes it a suitable material.
The technical challenge here is coupling the Fe 3 O 4 to SiO 2 with TiO 2 exposed as the outer surface to provide the catalytic sites and Fe 3 O 4 as the core for magnetic separation and recovery [15].This has proved to be difficult due to the fact that the photocatalytic activity of the nanoparticles shows a decline when the magnetic cores experience photodissolution [16].
In this paper, a novel approach to synthesize three-layer core-shell nanoparticles is described.Between the Fe 3 O 4 core and outside surface TiO 2 layer, an SiO 2 layer is introduced to avoid the interaction between the two layers and inhibit photodissolution of the core.The inert SiO 2 acts as a barrier for both electrons and holes and blocks any photoexcitation effects from the iron oxide, and it prevents the iron oxide from scavenging excited carriers from the titania [13,[17][18][19].The resulting nanoparticles were characterized with TEM, XRD, differential light scattering, ultraviolet visible absorption spectroscopy, and VSM and the activity as a photocatalyst was tested by the decolouring of MO as an assay method.The particles showed significantly higher catalytic activity than that of the pure TiO 2 particles under visible light irradiation.with the core-shell particles was put into a dark place for 48 hours before the photocatalysis.An ultraviolet light (TL-K 40W/05, Philips) is used to irradiate the reactant.After centrifuging the particle suspension, the absorbance () of the solution treated for some time was measured.A highspeed desktop centrifuge (TGL-16K Zhuhai Black Horse Medical Equipment Co., Ltd.) was used to centrifuge the particles and the treated solution to avoid the particles affecting the absorbance of the solution.The absorbance of the solution degenerated with time and the decoloration rate was calculated via the following equation:

Materials and Methods
where  is decoloration rate and  0 and   represent the initial absorbance and the absorbance at particular time.the SiO 2 surface coating.There is a broad envelope between 20 ∘ and 30 ∘ , which is suggestive of the presence of amorphous silica in the sample.

Results and Discussion
The XRD spectrum of Fe 3 O 4 -SiO 2 -TiO 2 particles is shown as curve (c).The magnitude of the characteristic peaks suggests that the amount of the anatase phase of TiO 2 is large.Meanwhile, the crystal form of the magnetite nucleus is still Fe 3 O 4 .Therefore, the particles are still probably superparamagnetic or ferromagnetic in nature.It is clear that the magnitudes of the Fe 3 O 4 characteristic peaks decrease sequentially from curve (a) to curve (c).This is suggestive that the successive coatings around the Fe 3 O 4 shield the cores from the X-rays and this behavior is consistent with our assumption that the Fe Figure 2 shows the TEM images of these three samples.
Figure 2(a) is the TEM image of the Fe 3 O 4 particles.It is shown that cubic particles of about 700 nm dimension were obtained by our procedure.There is still some evidence of carbon around the particles.The carbon can protect Fe 3 O 4 particles from oxidizing to Fe 2 O 3 .This result shows that cubic shaped Fe 3 O 4 particles were obtained through the high temperature reduction method.

Size Distribution and Specific Area.
Figure 3 shows the size distribution spectrum of the final Fe 3 O 4 -SiO 2 -TiO 2 particles.The results demonstrate that the average size is around 700 nm with a very big spread of sizes.The photocatalysis efficiency of the Fe 3 O 4 -SiO 2 -TiO 2 particles is closely related to the specific area.The characterization result shows that the BET surface area of the composite Fe 3 O 4 -SiO 2 -TiO 2 particles is 55 m 2 /g ± 2% while the BET surface area of the P25 is 42 m 2 /g ± 2%.This suggests that, despite the large overall size, the titania formed a rough and high area surface distributed around the particles and this was confirmed in Figure 2(c).That is the reason why our particles have a relatively large specific area and this in turn is expected to lead to higher photocatalysis efficiency.
Magnetic Properties.The magnetization behavior shown in Figure 4 indicates that the saturation intensity of the Fe 3 O 4 -SiO 2 -TiO 2 particles is 46.5 emu/g.The result illustrates that the saturation intensity of the particles is large when compared with the particles size.The retentive magnetism of the Fe 3 O 4 -SiO 2 -TiO 2 particles is 7.7 emu/g.The result shows that the saturation intensity of the Fe 3 O 4 -SiO 2 -TiO 2 particles is large enough to enable the particles to be recovered magnetically.Meanwhile, the retentive magnetism of the particles is small enough, so that the particles can be dispersed without agglomeration.These samples appear to have a mixture of ferrimagnetic and superparamagnetic properties, with only a slight indication of hysteresis.

Absorption Spectra.
Figure 5 shows the ultraviolet visible absorption spectra of the Fe 3 O 4 -SiO 2 -TiO 2 particles and P25 pure TiO 2 particles.The absorption spectrum of P25 has a strong peak between 200 and 400 nm, corresponding to the band edge absorption of light by titania.The absorption spectrum of the Fe 3 O 4 -SiO 2 -TiO 2 particles has a broad absorption across the whole of the visible spectrum.Evidently the introduction of the Fe 3 O 4 core makes the core-shell particles' absorption spectrum better match with the visible spectrum [13].This is consistent with the fact that magnetite has a very small energy gap of around 0.1 eV.This result suggests that the Fe 3 O 4 -SiO 2 -TiO 2 particles obtained in this work might be suitable to have high photocatalysis efficiency in the visible part of the spectrum.
To summarize, the Fe 3 O 4 -SiO 2 -TiO 2 particles (∼700 nm) with core-shell structure have a relatively uniform size distribution and have enough anatase TiO 2 shell thickness and suitable magnetic properties to be used for this project to have a recoverable particle that can be used to photocatalyse the degradation of contaminating molecules.Moreover, the novel structure that the introduction of the Fe 3 O 4 core provides makes the core-shell particles' absorption spectrum match better with the visible spectrum.after 180 minutes of UV light irritation, while the decoloration rate of the MO solution without any particles is 4%.The photocatalytic activity of the Fe 3 O 4 -SiO 2 -TiO 2 functional particles is a little higher than that of P25 under ultraviolet light irradiation, which is illustrated by the curves in Figure 6.The decline after 90 min in the P25 degradation curve can be seen clearly while the activity of the core-shell particles is increasing gradually.This performance suggests that the catalytic persistence of these particles is better than that of P25.

Photocatalytic Activity.
where  app is apparent rate constant,  is the solution-phase absorbance of MO, and  0 is the initial absorbance of the MO solution.The corresponding linear transforms in ln( 0 /) as a function of irradiation time are given in Figure 8(b).
From the figure, we can obtain the apparent rate constant for the degradation process by the particles irradiated by different light.The values are 0.0124 and 0.0120 min −1 for the degradation of the MO by these particles under UV light and visible light, respectively.The results show that the reaction rates are almost the same under UV light and visible light irradiation.
Under normal circumstances, the electrons of TiO 2 cannot be excited under visible light, but composite particles of visible light absorbing phenomenon are found in the experiment and this internal mechanism is described in Figure 9. Figure 9 is the diagram of the energy band of the Fe 3 O 4 -SiO 2 -TiO 2 core-shell nanoparticles, plotted by the relative layer thickness on the horizontal axis and the relative energy band gap on the vertical axis.The TiO 2 band gap is the magnetic core makes the Fe 3 O 4 -SiO 2 -TiO 2 particles active under visible light irradiation.In the SiO 2 -TiO 2 experiment, this point is confirmed.Figure 10(b) demonstrates that the composite particles without magnetic core only obtain 7.9% degradation in visible light.So Figure 10 shows that the visible light absorption is extremely based on the magnetic core.
The specific degradation mechanism is that the MB is reduced by photoexcited electrons via a series of processes.In accordance with the new understanding, the composite structure is rather special, which is formed by three different semiconductors.Visible light passes through the TiO 2 and SiO 2 layer and excites the electrons from the internal magnetite core [20].Then the SiO 2 middle layer is formed after calcining precursor which is prepared by sol-gel method and the layer may crack somewhere or generate uneven film [21], leading to the thickness of SiO 2 layer partly reaching angstroms level.We know that the energy of incident visible light is 1.8 eV and the energy of excited electrons from Fe 3 O 4 is 1.7 eV, while the barrier height of SiO 2 is 8.9 eV.When the conditions of energy and thickness are met, the electrons in Fe 3 O 4 excited by the visible light can eventually tunnel through the SiO 2 layer with a large probability [22,23].Then the electrons drop into the conduction band of the titania and hence escape into the surrounding liquid.Further they interact with the lowest unoccupied molecular orbital (LUMO) of the MO and thus chemically reduce that molecule.

Conclusions
In this paper we have presented a novel method of synthesizing Fe 3 O 4 -SiO 2 -TiO 2 functional nanoparticles with coreshell structure.The Fe 3 O 4 cores were synthesized through a novel carbon reduction method.The recovery problem of titania-based photocatalytic particles can be solved by introducing these superparamagnetic/ferrimagnetic cores.An SiO 2 layer has been introduced successfully between the two layers, which makes the three-layer core-shell nanostructure more stable.The core-shell nanoparticles have been characterized by TEM, XRD, particle size measurement, ultraviolet visible absorption spectroscopy, and magnetic characterizations, respectively.All the results are consistent and show that the 700 nm Fe 3 O 4 -SiO 2 -TiO 2 functional particles with a core-shell structure are photocatalysts and recoverable.The results for the decoloration of methyl orange show that the introduction of the Fe 3 O 4 -SiO 2 -TiO 2 functional particles has a significant photocatalytic effect in breaking down the 10 mg/L MO by 90% and 93% under UV light and visible light over 180 minutes, while the P25 particles have very low activity under visible light irradiation.Moreover, the approximate core-shell functional particles can be recovered after use, and the primary recovery ratio is 87.5%.The mechanism of the visible light is dedicated in this paper, and the experiment results show that the magnetic core plays an irreplaceable role in the photocatalytic processes.The high photocatalytic activity of the Fe 3 O 4 -SiO 2 -TiO 2 functional
Figure 2(b) is the TEM image of Fe 3 O 4 -SiO 2 particles.It can be seen that there is a very thin layer around the dark contrast Fe 3 O 4 particles which can be seen clearly in Figure 2 and this is probably the SiO 2 layer.It seems that the dispersion of the Fe 3 O 4 -SiO 2 particles is better than the Fe 3 O 4 particles.The TEM image of Fe 3 O 4 -SiO 2 -TiO 2 particles is shown as Figure 2(c).It is clear that the Fe 3 O 4 -SiO 2 particles were encapsulated by a TiO 2 layer which is composed of many little spherical particles.The results show that the layer-by-layer Fe 3 O 4 -SiO 2 -TiO 2 functional particles have been made successfully.
Figure 6 shows the relationships between irradiation time and decoloration rate of the methyl orange solution treated by P25 particles, Fe 3 O 4 -SiO 2 -TiO 2 particles and no particles under ultraviolet light.The decoloration rates of the MO solution are 71% and 90%, respectively, for the P25 particles, and Fe 3 O 4 -SiO 2 -TiO 2 particles
particles degrad 10 mg/L MO under visible light 0.2 g P25 particles degrad 10 mg/L MO under visible light 0.2 g core-shell particles degrad 10 mg/L MO under visible light

Figure 7 :
Figure 7: The relationships between irradiation time and decoloration rate of the methyl orange solution treated by P25 particles, Fe 3 O 4 -SiO 2 -TiO 2 particles, and no particles under visible light.

Figure 8 :Figure 9 :Fe 3 O 4 TiO 2 -Figure 10 :
Figure 8: (a) The images of the samples of water, methyl orange solution, methyl orange solution with Fe 3 O 4 -SiO 2 -TiO 2 particles, methyl orange solution after being treated by Fe 3 O 4 -SiO 2 -TiO 2 particles under ultraviolet light, and methyl orange solution after being treated by Fe 3 O 4 -SiO 2 -TiO 2 particles under visible light.(b) The kinetic curves of methyl orange disappearance for Fe 3 O 4 -SiO 2 -TiO 2 particles.
2.1.Synthesis of Core-Shell Structure Fe 3 O 4 -SiO 2 -TiO 2 System which was purchased from Ultrapure Water Visible Ltd.The 10 g of Fe 3 O 4 particles were put into the oven with the SiO 2 solution.After ultrasonication for half an hour, the SiO 2 solution was used to coat around the surface of the Fe 3 O 4 particles.After further reaction for three hours at room temperature, the wet particles were calcined at 500 ∘ C for 2 hours so that the SiO 2 was encapsulated around the Fe 3 O 4 particles.The Fe 3 O 4 -SiO 2 particles were obtained in this way.Then, the Fe 3 O 4 -SiO Ltd.), and then it was stirred for 30 min.The wet particles were heated at 500 ∘ C for 2 h after reacting for three hours.After milling using an agate mortar the Fe 3 O 4 -SiO 2 -TiO 2 particles were finally obtained.Characterization.The particles of Fe 3 O 4 , Fe 3 O 4 -SiO 2 , and Fe 3 O 4 -SiO 2 -TiO 2 were characterized by XRD and TEM.
Functional ParticlesMaterials.Ferric chloride, tetraethoxysilane (TEOS), tetrabutyl orthotitanate (TBOT), ethanol, hydrochloric acid, nitric acid, MO, and MB (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China).P25 is a mixture of anatase (80%) and rutile (20% TiO 2 ) which was purchased from Evonik Degussa (Germany).Protocols.Ferric chloride and carbon were used as reactants in a ratio of 1 : 3 to obtain the Fe 3 O 4 superparamagnetic particles after heating at 400 ∘ C for 3 hours under 0.2 torr vacuum conditions.A tube furnace (OTF-1200X-III) from KJ Group (Hefei, China) was used to keep the reactants at this temperature.After the solid product was milled using an agate mortar 700 nm Fe 3 O 4 particles were obtained.The carbon acts as a reducing agent and protector in this reaction.A part of the Fe 3+ of ferric chloride is reduced to Fe 2+ , and Fe 3 O 4 is generated under low oxygen conditions in this way.An excessive amount of carbon produces a carbon coating around the Fe 3 O 4 particles; the Fe 3 O 4 particles can be protected from being oxidized to Fe 2 O 3 in this way.TEOS, hydrochloric acid, ethanol, and deionized water were mixed in a ratio of 4.6 : 0.5 : 12.3 : 1 to obtain an SiO 2 suspension.The solution was mixed with fast stirring and the water must be added last.Deionized water of 18.25 MΩ was purified through Ultrapure (UPR)