CoFe 2 O 4 – Fe 3 O 4 Magnetic Nanocomposites as Photocatalyst for the Degradation of Methyl Orange Dye

We report the investigation of temperature-dependent magnetic properties and photocatalytic activity of CoFe2O4–Fe3O4 magnetic nanocomposites (MNCs) synthesized by hydrothermal process. Room-temperature magnetic hysteresis (M-H) loops result enhanced saturation magnetization of 90 emu/g and coercivity (HC) of 530 Oe for CoFe2O4–Fe3O4 MNCs. With decreasing temperature to 20 K, HC increases from 500 Oe to 6800 Oe, and the M-H loops exhibit exchange coupling feature between CoFe2O4 and Fe3O4. Lowand high-temperature-dependent magnetization measurements confirm that the blocking temperature lies above 300 K and the presence of two magnetic phase transitions corresponding to CoFe2O4 and Fe3O4, respectively. The photocatalytic activity of the MNCs has been examined on the reduction of methyl orange (MO), a colored compound used in dyeing and printing textiles. The observed results suggest that the CoFe2O4–Fe3O4 MNCs act as an excellent photocatalyst on the degradation of organic contaminants and degrade 93% of MO in 5 hours of UV irradiation. The photocatalytic activity of MNCs is attributed to remarkably high band gap energy and small particle size. Also, the MNCs with a reproducible photocatalytic activity are well separable from water media by applying external magnetic field and acts as a promising catalyst for the remediation of textile wastewater.


Introduction
The interest in magnetic nanoparticles (MNPs) and nanocomposites (MNCs) has greatly increased in recent years not only because of their broad applications in several technological fields including ferrofluids, magnetic data storage, magnetooptical, magnetic resonance imaging, medicine, and drug delivery systems, but also due to their relevance from the point of fundamental physics [1][2][3][4][5].MNPs are also used extensively for catalytic purposes [6].Photocatalysts utilize photon energy to carry out oxidation and reduction reactions.When irradiated with light energy, an electron (e − ) is excited from the valence band (VB) to the conduction band (CB) of the photocatalyst, leaving a photogenerated hole (h + ).This creates hydroxyl ions in aqueous solution thus facilitating catalytic activities [6,7].For a material to qualify as a good photocatalyst needs a relatively large band gap, larger surface area, and nonagglomeration of particles.Agglomeration of particles can be avoided by coating the surface with a surfactant.However, it considerably reduces the magnetic moment of the materials and cannot be reused due to low value of magnetization.That is why development of MNP-based catalyst without surfactant requires greater attention.In particular, tremendous progress has been made in the preparation and processing of MNP-supported catalysts, in view of their high surface area resulting in high catalyst loading capacity [8], high dispersion [9], excellent stability [10], and controllable catalyst recycling without loss in their properties.Magnetic separation renders the recovery of catalyst from liquid-phase reactions much easier and thus dramatically reduces the cost.To fulfill the requirements for various applications, these particles must have suitable magnetic properties (large saturation magnetization (M S ), moderate coercivity (H C ), and high blocking temperature) and also controllable size and shape [11].
Dyes are one of the most notorious contaminants in aquatic environments because of their huge volume of production from industries, slow biodegradation and decoloration, and toxicity [6].Methyl orange (MO) is a colored compound used in dyeing and printing textiles, and the release of those colored waste waters in the ecosystem is a source of esthetic pollution in the aquatic life.Therefore, the degradation of MO in cost effective ways without any secondary pollution is very much important for the safety environment [12,13].There are several traditional physical techniques such as adsorption, ultrafiltration, reverse osmosis, coagulation, chlorination, ozonation, and biodegradation for the removal of dye pollutants [14,15].Nevertheless, most of the above techniques transfer organic compound from water to another phase, thus causing secondary pollution.On the other hand, photocatalytic processes are environmentally friendly methods that utilize radiation energy to perform catalysis under ambient conditions and thus appear as the most emerging technology [16,17].However, the application of photocatalytic procedures using MNPs/MNCs for remediation of textile waste water is rather limited to only a few investigations.
Among other MNP materials, fine Cobalt ferrite particles have been used as MNP-based catalysts due to their strong interparticle interactions and nonagglomeration behaviors [19].Also, it has strong anisotropy, high H C , and moderate M S along with the good mechanical hardness and chemical stability.The catalytic activities of MNPs can further be enhanced by preparing a suitable hybrid nanocomposite with other MNPs.This facilitates a new composite material with novel magnetic, electrical, and optical properties.Various preparation methods such as sonochemical reactions, mechanochemical synthesis, hydrolysis, hydrothermal, and aqueous coprecipitation have generally been used to make nanocomposites with desired properties [18,20,21].In this study, we have employed hydrothermal process to prepare CoFe 2 O 4 -Fe 3 O 4 nanocomposites and investigated their temperature-dependent magnetic properties.Also, the photocatalytic activity of the MNC on the degradation of environmental pollutants (MO) was explored for the first time.

Experimental Details
Magnetic nanocomposites of CoFe 2 O 4 -Fe 3 O 4 were prepared by hydrothermal method.All the reactants and the reagents used for the preparation of MNCs were high-purity commercially available CoCl 2 •6H 2 O (99%), FeCl 3 •6H 2 O (99%), NaOH (96%), and PEG-400 (99%) and used as received.In a typical procedure, CoCl 2 •6H 2 O (1 g in 50 mL PEG-400, 4.2 mmol) and FeCl 3 (1.6 g in 50 mL PEG-400, 10.3 mmol) solutions were mixed and stirred in a magnetic stirrer.Subsequently, NaOH (3 M, 25 mL distilled deionized water) solution was added dropwise and stirred for half an hour at 80 • C.Then, the solution (dark brown) was transferred into an autoclave and heated at 200 • C for 12 hours.After this, the resulting black precipitate was first washed with alcohols to remove the PEG and with water for the complete removal of NaOH content.Eventually, the product was washed with dried ethanol and dried at 200 • C under vacuum.Crystal structure of the as-prepared  and annealed samples were examined by X-ray diffraction (XRD) using a Rigaku TTRAX diffractometer with Cu-K α radiation and transmission electron microscopy (TEM) using a JEOL 2100 microscopy.Room temperature and temperature-dependent magnetic properties were measured by vibrating sample magnetometer (VSM, Lake Shore Model 7410).Optical properties were measured by PerkinElmer RXI FT-IR spectrometer in KBr pellet and Varian Cary 50 UV spectrophotometer.The BET surface area of the MNCs was analyzed by the Chemisorb surface area analyzer (Make: Micromeritics, Model: Chemisorb 2720) with pretreatment of 0.03 g of the sample by degassing at 200 • C for 3 minutes under helium gas.

Results and Discussion
Figure 1 shows bright-field (BF) TEM micrographs and selected area electron diffraction (SAED) pattern of asprepared CoFe 2 O 4 -Fe 3 O 4 MNC.The micrographs show the fine nonagglomerated particles with the average particle size of about 20 nm, which are highly desirable for the catalytic activity.SAED pattern shows that MNCs are crystalline in nature, and the peak profile represents the cubic spinel structure of CoFe 2 O 4 and Fe 3 O 4 , which is consistent with the XRD analysis (not shown here).To confirm the spinel structure of the MNCs, infrared (IR) absorption spectra were obtained in the range between 350 cm −1 are 1000 cm −1 , and the same are depicted in Figure 2(a).It is found that the spectra consist of two significant absorption bands, first at about 590 cm −1 (ν 1 ) and second at about 410 cm −1 (ν 2 ).Absorption bands observed in this range reveal the formation of single phase spinel structure having two sublattices, which are assigned to tetrahedral site (ν 1 ) and octahedral site (ν 2 ) [22].In addition, the formation of soft Fe 3 O 4 phase in the nanocomposite has been confirmed by the UV analysis as shown in Figure 2(b).In case of magnetic nanoparticles, the UV absorption band is observed in the region 330-500 nm, which originates primarily from the absorption and scattering of light by the magnetic nanoparticles.The band gap energy was determined from the absorption spectra using Tauc relation [23], as shown in the inset of Figure 2(b), and found to be around 2.8 eV.It should be mentioned here that with higher band gap energy, the recombination rate of electrons and hole pairs are retarded, and photocatalytic properties are enhanced.
To understand the magnetic properties of CoFe 2 O 4 -Fe 3 O 4 MNCs, temperature, dependent magnetic hysteresis (M-H) loops were measured in the temperature range from   investigated samples [24].Such a ferromagnetic exchange coupling is desirable for higher magnetization value of composites.Temperature-dependent magnetization measurement measured at a magnetic field of 100 Oe under zero-field-cooled (ZFC) and field-cooled (FC) conditions as shown in Figure 4(a) confirms that the blocking temperature lies about room temperature.On the other hand, hightemperature magnetization data (Figure 4 in dark to obtain adsorption/desorption equilibrium until the concentration of MO was constant and then illuminated with a UV lamp.The absorption spectra, obtained using UV-vis spectrophotometer, are illustrated in Figure 5.It can be seen that a maximum absorbance was observed around 465 nm for two cases: (i) for the mixture of MO and water in as-mixed state and (ii) for the mixture of MO and water exposed to UV light for five hours.Also, the as-mixed mixture of MO, water, and MNCs without irradiation showed a maximum absorbance around 465 nm.These results suggest that no MO degradation occurs in these mixtures.However, a typical run performed for the mixture containing MO, water, and the MNCs without UV radiation in dark for 5 hours has revealed a minor change in the maximum absorption around 465 nm (less than 5%), as compared to the initial absorbance of the as-mixed mixture of MO, water, and the MNCs.This is mainly due to the sorption of dye molecules by the MNCs.On the other hand, the maximum absorbance almost disappears for the mixture containing MO, water, and MNCs subjected to UV irradiation for 5 hours.
where A 0 is the initial absorbance of MO before degradation, and A is the absorbance after time t.The degradation percentage of MO by MNC is 93% in 5 h of UV irradiation at a pH range of 5 to 6.This can be attributed to (a) high band gap energy (2.8 eV) for the MNCs as the photocatalytic effect depends on the enhancement in electronhole separation [28], and (b) small particle size which is associated with high surface area.The BET surface area of the CoFe 2 O 4 -Fe 3 O 4 MNCs analyzed using chemisorb surface area analyzer was found to be 112 m 2 /g with a total pore volume of 0.0565 m 2 /g.This is significantly larger for this type of magnetic materials, and hence it accounts for one of the reasons for the presently observed high photocatalytic activities.Also, the pH value of the dye solution plays a major role in the photodegradation of the dye, as the adsorption of the dye molecules on the catalyst surface is pH dependent [29,30].For the presently studied system, we have obtained the optimum degradation of the dye in the low acidic pH (pH ∼5- 6).The degradation of MO using TiO 2 -, and ZnS-based semiconductors has also been reported [31,32].However, the shape of the nanoparticles poses a limit in the photocatalytic activity, and an efficient technique is needed for the facile separation of the catalyst.On the other hand, the MNCs can be synthesized easily with the better control of size and shape, and well separated from the media by applying the external magnetic field.

Conclusion
CoFe 2 O 4 -Fe 3 O 4 magnetic nanocomposites with the average particle size of 20 nm were synthesized by hydrothermal process, and the formation of nanocomposites was confirmed through TEM, XRD, IR, and UV-vis spectroscopy studies.Enhanced magnetization of 90 emu/g and a coercivity of 500 Oe were obtained at room temperature.The blocking temperature of the nanocomposites was found to be above room temperature.Low-and high-temperature magnetization measurements confirmed the existence of magnetic exchange coupling between CoFe 2 O 4 and Fe 3 O 4 and their phase transitions, respectively.Photocatalytic activity of the MNCs was confirmed on methyl orange under UV irradiation and found to be effective on degrading the methyl orange at ambient temperature.MNCs could also be well separated magnetically without losing its photocatalytic activity.

Figure 3 :
Figure 3: Magnetic hysteresis of as-prepared CoFe 2 O 4 -Fe 2 O 3 magnetic nanocomposites obtained at different temperature between 20 K and 300 K. Inset: Variation of coercivity with temperature.

Figure 4 :
Figure 4: Temperature dependent magnetization curves measured at an applied field of 200 Oe (a) at low temperatures between 20 and 300 K under ZFC and FC conditions and (b) at high temperatures for as-prepared CoFe 2 O 4 -Fe 2 O 3 magnetic nanoparticles.Inset: Plot of thermal derivative of magnetization with temperature.

Figure 5 :
Figure 5: Absorbance spectra of methyl orange loaded with and without MNCs/water and UV irradiation for 5 hours.The irradiation was carried out with a 12 mW Xenon lamp with a main wavelength of 400 nm.
These results confirm that the effective degradation of the dye occurs only in the presence of UV irradiation, and this is the first time we are reporting CoFe 2 O 4 -Fe 3 O 4 MNC as a new photocatalyst to degrade MO organic contaminant.The photocatalytic degradation percentage of MO was calculated using Degradation