Core-Shell Nanostructure of α-Fe 2 O 3 / Fe 3 O 4 : Synthesis and Photocatalysis for Methyl Orange

Fe3O4 nanoparticle was synthesized in the solution involving water and ethanol. Then, α-Fe2O3 shell was produced in situ on the surface of the Fe3O4 nanoparticle by surface oxidation in molten salts, forming α-Fe2O3/Fe3O4 core-shell nanostructure. It was showed that the magnetic properties transformed from ferromagnetism to superparamagnetism after the primary Fe3O4 nanoparticles were oxidized. Furthermore, the obtained α-Fe2O3/Fe3O4 core-shell nanoparticles were used to photocatalyse solution of methyl orange, and the results revealed that α-Fe2O3/Fe3O4 nanoparticles were more efficient than the self-prepared α-Fe2O3 nanoparticles. At the same time, the photocatalyzer was recyclable by applying an appropriate magnetic field.


Introduction
Owing to their tunable properties and large surface-tovolume ratios, nanocrystals (NCs) are being considered for a wide range of applications including photovoltaics, biomedicalimaging, photocatalysts, and optoelectronics [1][2][3][4].Nanocrystal heterostructures (NCHs) are an emerging subclass of NCs where two or more chemically distinct components are brought together epitaxially [5].The best known example is the core/shell structure where the outer shell enhances the properties of the core (e.g., increasing photoluminescence quantum yields of CdSe NCs with ZnS or CdS shell [6,7].These core-shell nanostructures expand single-component nanoparticles to hybrid nanostructures with discrete domains of different materials arranged in a controlled fashion.Thus, different functionalities can be integrated, with the dimension and material parameters of the individual components optimized independently even providing entirely novel properties via the coupling between different components.In view of the scientific importance of these materials, it is not surprising that a wide variety of approaches for their synthesis have been reported [8][9][10][11][12][13].Specifically, the surface oxidation method has attracted much attention because it could be utilized as a simplistic route to obtain core/shell nanostructures [11,12]. Magnetic separation provides a convenient method for removing and recycling magnetized species by applying an appropriate magnetic field.So, magnetic nanoparticles combining with catalysts could not only increase the durability of the catalysts but also help to separate and recycle the catalyst particles.For example, magnetic nanoparticles have been immobilized in a mesoporous silica shell with molybdenum oxide forming a magnetically recyclable epoxidation catalyst [14].A core-shell structure of TiO 2 /BaFe 12 O 19 composite nanoparticle that can photodegrade organic pollutants in the dispersion system effectively and can be recycled easily by a magnetic field was also reported [15].Another magnetically separable photocatalyst TiO 2 /SiO 2 /NiFe 2 O 4 (TSN) nanosphere with egg-like structure was prepared by Xu et al. [16].However, there are few reports on the magnetic nanoparticles integrating with lower cost photocatalysis, α-Fe 2 O 3 for the photodegradation [17][18][19].Here, we fabricated α-Fe 2 O 3 /Fe 3 O 4 core-shell nanostructures by a surface oxidation method on the surface of primary Fe 3 O 4 nanoparticles in molten salts, which showed high efficiency and good recycling for photocatalysis of methyl orange (MO).minutes in a vacuum surround under strong stirring until the white precipitate became black.The obtained black precipitate was collected by centrifugation separating and then was washed with distilled water for 4 times.Finally the precipitate was dried at 80   To characterize phase and crystallization of the products, powder X-ray diffraction (XRD) was performed from the obtained α-Fe 2 O 3 /Fe 3 O 4 and Fe 3 O 4 samples.The XRD pattern for Fe 3 O 4 nanoparticles (Figure 1(b)) shows that all the peaks are in good agreement with the cubic structure [space group: Fd-3m] known from Fe 3 O 4 crystal (JCPDS Card 65-3107), meaning its high crystallization and few impurities.The crystal cell dimension of the sample is calculated to be a = 0.8396 nm by Jade 5, which is accorded with the  The magnetic properties of the as-prepared α-Fe 2 O 3 / Fe 3 O 4 and Fe 3 O 4 nanoparticles were investigated with a Quantum magnetometer at room temperature.The magnetic hysteresis loop (curve a in Figure 3) for the Fe 3 O 4 nanoparticles indicates their ferromagnetism at room temperature.The coercivity (Hc) is shown to be about 2330 Oe, the saturation magnetization (Ms) is 22.6 emu/g, and the remnant magnetization (Mr) is 18.9 emu/g.However, the hysteresis loop of the obtained α-Fe 2 O 3 /Fe 3 O 4 nanostructure shows near superparamagnetism at room temperature, which is different from the primary Fe 3 O 4 nanoparticles distinctly.It is well known that the surface spin disorder enhancing caused by the decreasing of particles size [20] and the coercivity would approach zero under a short thermal fluctuation (so-called super paramagnetism) if the crystal size is small enough.Thus, the transformation from ferromagnetism to superparamagnetism indicates the average size decrease when the primary Fe To evaluate the potential application in water treatment of the obtained α-Fe 2 O 3 /Fe 3 O 4 complex nanostructure, the photocatalysis capacities for the organic pollutant were investigated.Here, MO was chosen as the model organic pollutant.The initial concentration of the MO solution was set to be 20 mg/L. Figure 4(a) shows the visible absorption spectra of for MO solution by the degradation of complex nanostructured α-Fe 2 O 3 /Fe 3 O 4 with different time.The spectra show that the concentration of MO decreased to 54.4% after reacted for 60 min comparing the original concentration, and the maximum removal capacity of the photocatalysis reached about 90.1% in a time period of 120 min.At the same time, additional experiments were made to compare the photocatalysis performance between the core-shell sample (S3 of Table 1) and α-Fe 2 O 3 sample (S2 of Table 1) at the entirely same photocatalysis conditions.As shown in Figure 4 Figure 5(a) is the optical image for the MO solution with different photocatalysis time, which shows the solution became clear gradually from I to III by photocatalysis.Furthermore, the used catalyst of α-Fe 2 O 3 /Fe 3 O 4 nanostructure could be recycled by a magnet.Figure 5(b) and 5(c) shows the optical image of the catalyst being separated by a magnet.So, the photocatalyst could be recycled by a magnetic field, which would be assistance to the recycle of the photocatalyst.Furthermore, to study the recyclability of the photocatalyzer, experiments in multiple cycles of photocatalysis plus magnetic separation were performed, and it revealed that the photocatalyzer owned good recyclability as shown in Figure 5(d).

Conclusion
In summary, we fabricated Fe 3 O 4 nanoparticles in a mixture solution of water and ethanol, and then α-Fe 2 O 3 shell was produced in situ on the surface of the Fe 3 O 4 nanoparticle by surface oxidation to form α-Fe 2 O 3 /Fe 3 O 4 core-shell nanostructure in molten salts.To remove MO in the water by our α-Fe 2 O 3 /Fe 3 O 4 core-shell nanostructure showed better photocatalysis property.At the same time, the photocatalyst could be recycled by the magnetic field, which would be assistance to recycle of the photocatalyst.These studies not only enrich the contents of core-shell nanostructure chemistry but also are beneficial to investigate their potential application in photocatalysis.

2. 4 .
Characterization.The X-ray diffraction (XRD) patterns of the samples were collected using a diffractometer (Rigaku D/Max 2200PC) with CuKα radiation (λ = 1.5418Å) and graphite monochromator from 10 to 80 • at a scanning rate 5.0 • /min.Unit cell dimensions were determined in the JADE 5 program for X-ray diffraction pattern processing, identification, and quantification.The size and morphology of the products were characterized by transmission electron microscopy (TEM, JEM100-CXII) with the potential of performing selected-area electron diffraction (SAED).Magnetometry measurements were taken with a Quantum Design PPMS SQUID magnetometer.The absorption spectra of MO solution were obtained by a UV-vis spectrophotometer (Shimadzu UV-vis 2550).

Figure 2 (
a) shows the representative TEM image of the obtained Fe 3 O 4 nanoparticles.It shows that the sizes of nanocrystals are about 20-50 nm but not uniform, which is roughly closed to the average size resulted from the Debye-Scherrer formula.The inset of Figure 2(a) is corresponding SAED pattern, exhibiting the high crystalline nature of cubic-phase Fe 3 O 4 .Figure 2(b) is TEM image of the α-Fe 2 O 3 /Fe 3 O 4 , which shows clear core-shell nanostructure for the sample.Furthermore, it shows the size of Fe 3 O 4 core decrease to be about 15-30 nm.
3 O 4 nanoparticles became into the Fe 3 O 4 cores of α-Fe 2 O 3 /Fe 3 O 4 , which is accorded with the result of TEM.At the same time, it reveals that the saturation magnetization (Ms) for Fe 3 O 4 cores of α-Fe 2 O 3 /Fe 3 O 4 nanoparticles is about 48.7 emu/g, and the remnant magnetization (Mr) is about 8.6 emu/g.
(b), the α-Fe 2 O 3 /Fe 3 O 4 core-shell nanostructure behaved higher photocatalysis efficiency for MO.The rapid removal of MO may be associated with the electrostatic attraction between the α-Fe 2 O 3 shell and Fe 3 O 4 core of the complex nanostructure.Under the ultraviolet radiation, electrons in the valence band (CB) of α-Fe 2 O 3 were excited to its conduction band (CB), with same amount of holes left in VB.Driven by the decreased potential energy band gap of α-Fe 2 O 3 is ∼2.20 eV and band gap of Fe 3 O 4 is ∼0.10 eV), the photogenerated electrons in CB of α-Fe 2 O 3 tended to transfer to that of Fe 3 O 4 .So, the photogenerated electrons and holes were separated at the α-Fe 2 O 3 /Fe 3 O 4 interfaces, which reduced their recombination probability, and enhanced the efficiency of generating hydroxyl radicals.
The obtained Fe 3 O 4 nanoparticles were put into the crucible to react for 30 minutes, and then the molten salt was cooled to room temperature.The product was obtained by washing the fusion with deionized water to remove the nitrates and filtration for three times.Finally, the precipitate was dried at 80 • C for 12 hours to obtain the α-Fe 2 O 3 /Fe 3 O 4 nanoparticles.
• C for 12 hours to obtain the Fe 3 O 4 nanoparticles.2.2.Synthesis of α-Fe2 O 3 /Fe 3 O 4 .Sodium nitrate (4.2 g) and potassium nitrate (2.5 g) were mixed in a crucible (30 mL) and heated up to 310 • C to melt.orizing MO aqueous solution was performed at ambient temperature.The obtained α-Fe 2 O 3 /Fe 3 O 4 core-shell catalyst (0.02 g) was placed into a tubular quartz reactor of 100 mL 20 mg/L MO aqueous solution.The reactor was surrounded with a UV lamp (125 W) with stirring.After the reaction began, the mixture was sampled at different times and separated by a magnet to discard any sediment.Then, absorption spectra were obtained through a wavelength scan on a UV-Vis spectrophotometer.
To form α-Fe 2 O 3 /Fe 3 O 4 core-shell nanostructure, the appropriate reaction conditions were explored in our work as shown in Table1.It reveals that when the primary Fe 3 O 4 nanoparticles were obtained in water without ethanol, they were difficultly oxidized into α-Fe 2 O 3 even in molten salts of 380 • C for 30 min; however, when the primary Fe 3 O 4 nanoparticles were obtained in 20 mL H 2 O and 40 mL ethanol (the content of ethanol was 2/3), they were oxidized into α-Fe 2 O 3 completely without Fe 3 O 4 leaving even in 310 • C for only 10 min (FigureS1(a) in Supplementary Material available online at doi: 10.

Table 1 :
Different samples obtained from adjusting reaction conditions: different molar ratios of H 2 O and EtOH, oxidation temperature, and oxidation time.Furthermore, according to the full width at half maximum (FWHM) of (121) reflections, the average size of the crystalline particles for the primary Fe 3 O 4 is calculated to be 30.7 nm based on the Debye-Scherrer formula.Figure1(a) is the XRD pattern for the α-Fe 2 O 3 /Fe 3 O 4 obtained from surface oxidation of Fe 3 O 4 in molten salt.It reveals that these nanoparticles were composed of cubic-structured Fe 3 O 4 and α-Fe 2 O 3 (JCPDS No. 04-0784).