Fe3O4/C/TiO2 Magnetic Photocatalyst via Vapor Phase Hydrolysis

A core/multi-shell-structured Fe3O4/C/TiO2 magnetic photocatalyst is prepared via vapor phase hydrolysis process. The as-synthesized core/multi-shell-structured composite is characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM), N2 adsorption-desorption isotherm analyses, vibrating sample magnetometer (VSM), and ultraviolet-visible (UV-Vis) absorption spectroscopy. TEM and HRTEM show that well-crystallized anatase TiO2 nanocrystals are immobilized on the surface of as-prepared Fe3O4/C microspheres with dimensions around 200 nm. N2 adsorption-desorption isotherm analysis shows that the obtained photocatalyst exists disorderedly mesoporous structure. The photocatalytic efficiency of the catalyst in degradation of methylene blue is evaluated, and the Fe3O4/C/TiO2 photocatalyst with low TiO2 content (37%) has a relatively higher activity than commercial anatase TiO2. The intermediate carbon layer avoids the photodissolution of Fe3O4 effectively, and the recycling property is largely improved due to the existence of magnetic Fe3O4 core.


Introduction
Titanium dioxide (TiO 2 ) nanoparticles have inspired scientists for their potential application in treatment of environmental pollutants in wastewater [1][2][3][4][5][6][7][8][9].However, there have been two obvious deficiencies in their practical application.One is the wide bandgap of TiO 2 (≈3.2 eV), so only a small ultraviolet fraction of solar light can be used.The other is the separation of nanosized photocatalyst from wastewater, which is too difficult and energy consumptive [10].Both deficiencies result in the low treatment efficiency and increase the loss of photocatalyst.Doping TiO 2 with foreign element [11,12] and mixing TiO 2 with sensitizer [13,14] are two most important ways to enhance the utilizing of sunlight.And the related researches on catalyst immobilization have also attracted wide attention in order to solve the separation and recycle of the suspended TiO 2 nanoparticles [15,16].Some researchers have coated TiO 2 over glass, zeolite, and other substrates to avoid the catalyst-recovering step [17][18][19].
Recently, magnetic photocatalysts have been addressed to resolve the separation of TiO 2 nanoparticles [20][21][22][23].Cheng et al. [22] obtained a core/shell-structured bactericidal photocatalyst with Fe 3 O 4 core and TiO 2 shell for bactericidal application.Xuan et al. [23] fabricated Fe 3 O 4 /TiO 2 hollow spheres through poly(styrene-acrylic acid) (PSA) template method and carried out the photodegradation of RhB as a model reaction to investigate the photocatalytic activity of the hollow photocatalyst.Nevertheless, Beydoun et al. [24] indicated that the photodissolution of Fe 3 O 4 would occur during the light irradiation.So the barrier layer between magnetic Fe 3 O 4 and TiO 2 was prepared to reduce the negative effects [25][26][27][28].Song and Gao [8] synthesized the bifunctional titania-/silica-coated magnetic spheres and used MB to evaluate their photocatalytic properties.Ye et al. [28] obtained the core/shell-structured Fe 3 O 4 /SiO 2 /TiO 2 magnetic nanocomposites with enhanced photocatalytic activity by combining sol-gel process with calcinations.Presence of silica coating is essential to reduce the negative effect to magnetite during the photocatalytic process [8,28].However, Wang et al. [29] indicated that the carbon shell coated on Fe 3 O 4 microspheres was more compact than others, so the direct contact can be avoided effectively.Its benefit to protect the magnetic carriers enhance the photocatalytic activity and extend the lifetime of magnetic photocatalyst.
Herein, we develop a novel and controllable strategy to prepare the unique Fe 3 O 4 /C/TiO 2 core/multi-shell-structured photocatalyst via vapor phase hydrolysis (VPH) process.It is crucial that TiO 2 in the shell is perfectly crystallized without calcination.In VPH method, the water molecules are gently contacting with alkoxide in vapor phase with the advantage of hydrothermal treatment to obtain highly crystallized TiO 2 .The size and crystallinity of anatase TiO 2 crystallite in the shell could be tuned by reaction temperature, duration, and the thickness of anatase shell could also be tuned by varying tetrabutyl titanate (TBOT) content.Finally, the photocatalytic degradation of MB was selected as a model reaction to demonstrate the unique advantage in their photocatalytic activity and recycle facet.O, ethylene glycol (EG), sodium acetate (NaAc), polyethylene glycol (PEG), oleic acid, cetyltrimethylammonium bromide (CTAB), glucose, TBOT, and absolute ethanol were analytical grade and purchased from Shanghai Chemical Reagent Co., China.All chemicals were used as received without any purification.The distilled water for all processes was purified by ion exchange and then followed by distillation.The specific surface area of commercial anatase TiO 2 is 19 m 2 /g and the crystallite size is about 80 nm.

Synthesis of Fe
3 O 4 Microspheres.The ferrite core was first fabricated by solvothermal method, as described by others [30][31][32].Typically, 1.35 g FeCl 3 •6H 2 O was dissolved in 50 mL EG to form a clear solution, followed by addition of 3.6 g NaAc and 1.0 g PEG.The mixture was stirred vigorously for 30 min at room temperature and then sealed in a 70 mL Teflon-lined autoclave.The autoclave was heated to 200 • C for 12 h.After it was cooled to room temperature, the black products were collected by magnet and rinsed with distilled water and ethanol.The final product was dried under vacuum at 60 • C for 8 h.

Fabrication of Fe
O 4 /C Nanocomposites.The magnetic Fe 3 O 4 /C nanocomposite was fabricated according to the route of Wang et al. [29].The reactant mixtures were prepared by dissolving glucose in the aqueous solution containing oleic-acid-stabilized Fe 3 O 4 nanoparticles.Typically, 4 g glucose was dissolved in 40 mL oleic-acid-stabilized Fe 3 O 4 suspension (2.5 g/L).After that, the mixture was sealed in a 50 mL Teflon-sealed autoclave and heated to 170 • C for 3 h.Finally, the product isolated from the mixture by external magnetic field was rinsed with distilled water and ethanol.The final product was dried under vacuum at 60 • C for 8 h.solution which was then transferred into the vapor phase hydrolysis device (VPHD, as shown in Figure 1) quickly.

Preparation of Magnetic Fe
3 mL deionized water was located at the bottom of VPHD as liquid phase to produce vapor at elevated temperature, thus the liquid water did not contact with TBOT directly and the fierce hydrolysis was avoided.The sealed VPHD was heated to different temperature for varying duration.The TBOT content, the reaction temperature, and duration were varied as shown in Table 1.The as-synthesized powder was collected by magnet, rinsed with distilled water and ethanol for several times, and subsequently dried under vacuum at 60 • C for 8 h.The morphology and size of the particles were observed by transmission electron microscope (JEM-2100F, JEOL, Japan) equipped with an energy-dispersive X-ray and field emission scanning electron microscope (S-4800, Hatachi, Japan).The nitrogen adsorption-desorption isotherm was obtained at 77 K using a Autosorb-1 MP (Quantachrome, USA) utilizing Barrett-Emmett-Teller (BET) calculations of specific surface area and Barrett-Joyner-Halenda (BJH) calculations of pore volume and pore size (diameter) distributions from the desorption branch of the isotherm.Magnetic characterization was conducted on vibrating sample magnetometer (6000, PPMS, USA).The carbon content in the Fe 3 O 4 /C nanocomposites and Sample B is quantitatively measured by Elementar Analyzer (Vario EL III, Elementar, Germany).In order to investigate the recycle property and long-term photocatalytic activity, Sample B was used to degrade MB solutions five times.After ultraviolet light irradiation for 3 h, the magnetic photocatalyst was recycled by external magnetic field, washed three times with deionized water and dried at 105 • C for 5 h, the recovered photocatalyst was subsequently used to degrade fresh MB solution under the same irradiation conditions for 3 h.Then 3-cycle, 4-cycle and 5-cycle were also carried out in the same process.
The degradation rate (H) of MB was calculated using H = C/C 0 × 100%, where C and C 0 are the concentration of remaining MB at different intervals and primal MB solution.planes of anatase TiO 2 (JCPDS No. 21-1272), respectively.The crystallite size of anatase TiO 2 caculated by Scherrer formula is to be ca.11.2 nm.And the intensity of XRD peaks of Fe 3 O 4 has been decreased because there is some carbon and anatase TiO 2 coated on it.XRD results demonstrate that the crystalline anatase TiO 2 and Fe 3 O 4 coexist in Sample B.

Morphology and EDS Analysis.
The morphologies of the selected samples are revealed by TEM (Figure 3) and SEM (Figure 4).The Fe 3 O 4 crystallites are ultrafine as observed in Figure 3(a) and connect tightly to one another to form spheres.And the diameter of the aggregated particles is around 200 nm (Figure 4(a)).5 gives the magnetization-hysteresis loops of samples with different composition.It shows that the magnetic saturation value of samples is closely related with the thickness of nonmagnetic shells in given external magnetic field [8].The magnetic saturation values are 75.1 emu/g for Fe 3 O 4 [30], 65.1 emu/g for Fe 3 O 4 /C, and 56.1 emu/g for Fe 3 O 4 /C/TiO 2 particles.The decrease of saturation magnetization is most likely attributed to the existence of nonmagnetic C and TiO 2 shells on the surface of Fe 3 O 4 as shown in curves b and c.Because of the magnetic core, it is notable that these magnetic photocatalysts could be easily recycled from solution   after treatment by external magnetic field.The inset is the image of Sample B separated from aqueous solution by an external magnetic field (inset).The magnetic photocatalyst (Sample B) is recycled by attracting towards the magnet located in right-hand side of sample vials over a short period, demonstrating the high magnetic sensitivity of as-synthesized magnetic photocatalyst.

Nitrogen Adsorption-Desorption Analysis.
The N 2 adsorption-desorption isotherm (Figure 6(a)) of all four samples presents type-IV-like curve with an H3 hysteresis loop, characteristic of mesopores in the spherical cores and shells.
Comparing with Sample H, the hysteresis loop of Sample F is shifted to low P/P 0 direction relatively because small crystallite stacking small intraparticles pores on the surface of Fe 3 O 4 /C spheres.The hysteresis loops of Sample B, G are also shifted to low P/P 0 direction, but it is not obvious in the Figure 6(a).The hysteresis loop of Samples F, B, and G illustrates the fact that the larger crystallite stacking larger intraparticles pores at elevated temperature.The pore size of Sample F is smaller than that of Sample B. It can be explained that the anatase crystallite prepared under 100 • C is ultrafine and cumulates small pores.3.5.Photocatalytic Activity .Different catalysts are suspended into the MB solution for 30 min in the dark as the adsorption experiment.Figure 7 indicates that the concentration of MB solution is almost no any change (3.33%) in commercial TiO 2 suspension.In contrast, the adsorption of Sample B reaches 12.5%.This adsorption property could increase the local concentration of MB near the titania layer and enhance the photocatalysis efficiency [33].Figure 8(a) shows the photocatalytic activities of Sample B and commercial anatase TiO 2 in photocatalytic decomposition of MB under ultraviolet light irradiation.As shown in Figure 8(a), the decolorization ratio of MB (control) was less than 20% [34] after 180 min irradiation due to the photodecomposition.Because of the anatase TiO 2 located on the outsurface of Fe 3 O 4 /C/TiO 2 microspheres, it can utilize the light effectively than solid TiO 2 sphere.The TiO 2 content in Sample B is 37%, so the photocatalytic activity of Sample B is relatively higher than that of commercial anatase TiO 2 .If the iron oxide directly contacts with a crystalline titanium dioxide phase, electronic interactions between the titanium dioxide coating and the iron oxide core, when the photogenerated electrons in the titanium dioxide phase are transferred into the lower lying conduction band of the iron oxide phase.It is these injected electrons which lead to the reduction of the iron oxide core, with Fe ions are being formed and migrating to the solution through the porous titanium dioxide coating [24,33].So the protection role of intermediate carbon layer is to avoid the direct contact between Fe 3 O 4 and titania.Due to the compacted coating with intermediate carbon layer, it can be avoided the electron interaction or photodissolution of Fe 3 O 4 probably occurrence of at the point of contact.The long-term photocatalytic activity of Sample B is showed in Figure 8(b).MB is removed completely from the solution in every cycle and photocatalytic degradation efficiency is not varied.Owning to the regenerative property of magnetic photocatalyst and relatively high photocatalytic activity, the Fe 3 O 4 /C/TiO 2 hybrid spheres are well likely to be promising catalyst in the near future.

Conclusions
In summary, the core/multi-shell-structured Fe 3 O 4 /C/TiO 2 magnetic photocatalyst has been prepared via a vapor phase hydrolysis process, and the thickness of TiO 2 shell can be tuned from monolayer to several hundred nanometers by varying the TBOT content.The size and crystallinity of anatase TiO 2 crystallite in the shell could be tuned by temperature and duration.The photocatalytic activity of Fe 3 O 4 / C/TiO 2 photocatalyst has relatively higher activity than commercial anatase TiO 2 in photocatalytic degradation of methylene blue (MB) but its recycling property is improved largely.The intermediate carbon layer can avoid the electron interaction or photodissolution of Fe 3 O 4 probably occurring at the point of contact effectively.

Figure 1 :
Figure 1: The setup of the vapor phase hydrolysis device.

2. 5 .
Characterizations.The powder phase composition was identified by X-ray diffraction (XRD) equipment (D/max 2550 PC, Rigaku Co., Japan), using Cu Kα radiation at 40 kV and 200 mA.The crystallite sizes of Fe 3 O 4 and anatase are estimated from the corresponding XRD peak at 2θ = 35.4• and 2θ = 25.3 • by Scherrer formula (D = kλ/β cos θ), where D is the crystallite size, λ is the wavelength of the X-ray radiation (Cu Kα = 0.15406 nm), k is usually taken as 0.89, β is the line width at half-maximum height of the (311) peak of Fe 3 O 4 and the (101) peak of anatase after subtracting the instrumental line broadening, and θ is the diffraction angle.

3. 1 .
X-Ray Diffraction Analysis.The crystallinity and structure of as-prepared Fe 3 O 4 particle and Fe 3 O 4 /C/TiO 2 nanocomposite are confirmed by XRD.As showed in Figure2, the peak position and relative intensity of Fe 3 O 4 (curve a) match well with standard powder diffraction date (JCPDS No. 74-2402), indicating phase purity of Fe 3 O 4 .Well-resolved diffraction peaks reveal good crystallinity of Fe 3 O 4 .The average crystallite size of as-prepared ferrite powder estimated by Scherrer formula is around 14 nm, which is in agreement with the result of Zhang et al.[31].There is no diffraction peaks of carbon appears in curve b due to its amorphous nature and low content compared with Fe 3 O 4 and anatase TiO 2 .In addition to all diffraction peaks of Fe 3 O 4 , there are other four diffraction peaks appear at 2θ = 25.3 • , 37.8 • , 48.1 • , and 55.1 • which correspond to (101), (004), (200), and (211)

Figure 3 (Figure 3 :
Figure 3: TEM images of (a) Fe 3 O 4 , (b) Sample B, HRTEM image (c), and EDS result (d) of the selected area in image (b).

Figure 4
gives the SEM images of the selected samples.It provides more structural information about the three-dimensional morphology of the product.A large quantity of as-prepared Fe 3 O 4 , carbon-coated Fe 3 O 4 , and core/multi-shell-structured Fe 3 O 4 /C/TiO 2 with narrow distributions has been synthesized.As shown in Figure 4(b), the greater crosslinking degrees and slickness of the Fe 3 O 4 /C surface can be seen compared with Figure 4(a) due to the amorphous carbon.Because the small anatase TiO 2 crystallite is coated on the surface of Fe 3 O 4 /C particles, the result that the surface of Sample B becomes rough can be obtained from Figure 4(c).Up to now, the core/multishell-structured Fe 3 O 4 /C/TiO 2 composite is confirmed by the evidence of SEM and TEM analysis.

3. 3 .
Magnetic Study.The magnetic property of as-prepared Fe 3 O 4 , Fe 3 O 4 /C and Fe 3 O 4 /C/TiO 2 is investigated with vibrating sample magnetometer at room temperature.

Figure 5 :
Figure 5: Room-temperature magnetization curves of a: Fe 3 O 4 ; b: Fe 3 O 4 /C; c: Sample B; Sample B separated from aqueous solution by an external magnetic field (inset).

Figure 6 (
b) gives the pore-size distribution curves of different samples, confirming a preferred porous structure with pore volumes (V p ) 0.36, 0.32, 0.40, and 0.42 cm 3 /g and average pore diameters 68.1, 4.7, 9.5, and 81.5 nm corresponding to Samples H, F, B and G, respectively.The high specific surface area of Sample F (155 m 2 /g) is the result of fine anatase crystalline prepared under this temperature.Such a porous structure can promote more adsorption of pollutants.

Figure 6 :Figure 7 :
Figure 6: N 2 adsorption-desorption isotherm (a) of different samples at 77 K with corresponding pore size distributions (b) calculated by Barrett-Joyner-Halenda (BJH) method from desorption branch.All adsorption-desorption isotherm curves in (a) are added some units along the volume adsorbed axis (Y axis) in order to observe clearly.( ) Sample H, (•) Sample F, ( ) Sample B, and ( ) Sample G.

Figure 8 :
Figure 8: Photocatalytic efficiency of commercial anatase TiO 2 and Sample B to methylene blue (MB); (b) the long-term photocatalytic activity of Sample B. 3 2

Table 1 :
Summary of the preparative parameters and the specific surface areas.
×10 −5 M methylene blue (MB) solution.Oxygen was also bubbled into suspension at a flow rate of 100 mL/min during the whole experiment.Prior to photobleaching, the MB-solution-containing catalyst was stirred and stored in dark for 30 min to obtain the saturated absorption of MB.The MB concentration was determined in the sequence of 15, 30, 60, 90, 120, and 180 min using Lambda 35 UV-Vis spectrophotometer (Perkin-Elmer, Waltham, USA) by collecting the absorbance of MB at wavelength of 665 nm.