Synthesis of MgFe 2 O 4 / Reduced Graphene Oxide Composite and Its Visible-Light Photocatalytic Performance for Organic Pollution

Recently, binary metal oxides have been proven to be the most investigated semiconductors due to their high activity for the removal of organic pollutants. In this paper, to improve the photocatalytic efficiency of MgFe2O4, a MgFe2O4/reduced graphene oxide (MFO/rGO) photocatalyst was synthesized by a facile generalized solvothermal method. The morphology, structure, and photocatalytic activities in the degradation of methyl orange (MO) reaction were systematically characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and UV-vis absorption spectroscopy, respectively. The results showed that the MFO/rGO composite exhibited enhanced photocatalytic performance in the photodegradation of MO under visible-light irradiation and reached a maximum degradation rate of 99% within 60min of irradiation. This excellent photocatalytic performance is attributed to the introduction of rGO in the composite, which can effectively reduce the photoproduction of the electron-hole pair recombination rate. The excellent photocatalytic activity reveals that the MFO/rGO composite photocatalyst is a promising photocatalyst with good visible-light response and has potential applications in the field of water treatment.


Introduction
Recently, environmental pollution has become one of the outstanding social problems.Photocatalytic technology has attracted great interest as a promising pathway for solving energy supply and environmental pollution problems [1,2].However, the traditional photocatalysts, typically TiO 2 (3.2 eV) or ZnO (3.3 eV), with a wide band gap can only exhibit excellent photocatalytic activity under ultraviolet light irradiation, which significantly limits their practical applications [3][4][5].Faster kinetics recombination of photogenerated electrons and holes is another chief reason for lower photocatalytic performance [6].It takes patience to develop efficient visible-light irradiation photocatalysts in order to utilize solar light, of which about 45% is composed of visible light.Thus, the development of visible-light photocatalysts has become one of the most critical topics in photocatalytical research today.
Magnesium ferrite (MgFe 2 O 4 , MFO) is a semiconductor with a spinel structure, which can absorb visible light due to its narrow band gap (approximately 2.0 eV) and is not sensitive to photoanodic corrosion.MFO has been used as a photocatalyst for the degradation of 2-propanol [7].Besides, it was found that MFO/TiO 2 has a high photocatalytic activity for the degradation of the RhB contaminant [8].Therefore, MFO is considered as one of the efficient photocatalyst candidates for air or water treatment.According to previous reports, to reduce the recombination of photogenerated electrons and holes, the surface of photocatalysts coated with carbon nanotubes (CNTs) or carbon nanofibers (CNFs) has been considered as a promising strategy [9].In the process of photocatalysis, CNTs or CNFs can act as an excellent electron-acceptor/transport donor to effectively facilitate the migration of photoinduced electrons and hinder the quick recombination in electron transfer, which enhances photocatalytic performance.
As a rising star in the carbon family, reduced graphene oxide (rGO) has attracted a great deal of attention in recent years due to its excellent electronic properties (zero gap semiconductor where the conduction band and the valence band touch each other), outstanding ability as an electron acceptor and transport, chemical stability, and high surface area, which have been used to obtain hybrid materials with superior photocatalytic performance [10].For instance, tremendous improvement has been reported for metal oxides [11], metal sulfides [12], and nonmetal g-C 3 N 4 [13] by using rGO as a synergistic catalyst material.However, to the best of the authors' knowledge, there has been no report of the modification of MFO with rGO for preparing new photocatalysts.
In this paper, a novel MFO/rGO photocatalyst has been synthesized by a facile solvothermal method for the first time.The as-prepared samples were characterized, and the photocatalytic activity under visible-light irradiation (λ ≥ 420 nm) was estimated by the photodegradation of methyl orange (MO).The results indicated that MFO/rGO nanocomposites exhibited much higher photocatalytic performance than the pure MFO.At last, the mechanism of enhanced photocatalytic activities of MFO/rGO is proposed.

Experimental
2.1.Catalyst Preparation.All reagents were of analytical grade and used without further purification.Commercial graphite power was used as the raw materials for synthesis of graphene oxide (GO) by a modified Hummer's method according to previous reports [14].
1.75 mmol of Mg(NO 3 ) 2 •6H 2 O and 3.5 mmol of Fe(NO 3 ) 3 •9H 2 O were mixed and dissolved in 80 ml of distilled water after being sonicated for 30 min.Next, a certain amount (10 ml) of 10 mg ml −1 GO suspension was added into the above solution under magnetic stirring for 4 h.The pH value of the solution was adjusted by ammonia hydroxide to about 10.The mixture was stirred vigorously for an hour and then sealed in a Teflon lined stainless steel autoclave (100 ml capacity).The autoclave was heated to and maintained at 180 °C for 12 h, and allowed to cool to room temperature.The grayish-black products were washed several times with ethanol and distilled water, and then dried in a vacuum oven at 80 °C overnight.For comparison, bare MFO was also synthesized by a similar route without adding the GO suspension.
2.3.Photocatalytic Tests.The photocatalytic performance was evaluated with the as-prepared sample powder (50 mg) suspended in MO (20 mg l −1 , 100 ml) with constant stirring.A 300 W Xe arc lamp (CEL-HXF 300, Beijing Jinyuan Co. Ltd.) was used as the light source and equipped with an ultraviolet cutoff filter to provide visible light (λ ≥ 420 nm).Prior to irradiation, the suspensions were stirred in the dark for 30 min to ensure the adsorption/desorption equilibrium.At certain time intervals, 2 ml of the suspension was taken and centrifuged to remove the photocatalyst particles.The filtrate was then analyzed using a UV-vis spectrophotometer at 464 nm as a function of irradiation time by measuring the max absorbance of MO.

Results and Discussion
The XRD patterns of the products are shown in Figure 1.The dominant diffraction peaks in the pattern of MFO and MFO/ rGO can be indexed to those from the spinel MgFe 2 O 4 (JCDPS 89-3084).The diffraction peaks of the MFO are sharp and intense, revealing the highly crystalline character of the sample.After combination with rGO, the intensity of the diffraction peaks becomes weak.However, the crystalline spinel of the sample was not changed, indicating that the crystal structure of MFO remains stable.In addition, the characteristic diffraction peaks of GO (12.2 °) are not found in the XRD of MFO/rGO composite samples, which indicates that GO is fully reduced to rGO during the process of reaction.
Figures 2(a) and 2(b) display the SEM images of the MFO and MFO/rGO samples.The MFO are composed of numerous hexahedron particles, each with a particle size of about 80 nm.The MFO/rGO shows ultrathin crumpled three dimensional (3D) nanosheets, and the MFO and GO are uniform and well-attached to each other.Figures 2(c 2 International Journal of Photoenergy 3084).In addition, rGO serves as a 3D conductive support for MFO nanoparticles.
As can be seen from Figure 3(a), MO selfphotodegradation (under light irradiation without the effect of catalysts) can be negligible; however, the MFO/rGO composite exhibits the best visible-light photocatalytic efficiency, which can decompose MO molecules in 60 min.In contrast, the MFO showed a poor degradation efficiency of ~40% after visible-light irradiation for 60 min.Interestingly, rGO also showed a degradation efficiency of about 20% owing to its absorbing ability to visible light [15].Figure 3(b) shows the absorbance spectra of MO under rGO, MFO, and MFO/ rGO photocatalysts at different times.The enhancement of the photocatalytic performance should be ascribed to the prevention in electron-hole pair quick recombination due to the stepwise energy level structure in the composite and the increase in the light absorption with the presence of rGO [16].Therefore, on the basis of the synergistic effect of the electron-acceptor and transport material rGO and the rGO can absorb visible light, and the MFO/rGO composite displays a highly efficient visible-light photocatalytic activity.
The mechanism of the photocatalytic degradation of MO is given as follows: Firstly, the semiconductor materials (MFO) absorb the photon under light irradiation, and the electron-hole pairs are generated at the photocatalyst surface (1).Then, photoinduced electrons are transferred from the MFO to the rGO, which could efficiently separate the photogenerated electrons and hinder the charge recombination in  3 International Journal of Photoenergy the electron transfer processes.Afterwards, the high oxidation potential of the holes causes the direct oxidation of the MO and some reactive intermediates are generated (2).Finally, a hydroxyl radical can also be created by hydroxyl ions (OH − ) with holes (3).Furthermore, the superoxide anions are also formed by the molecular reduction of O 2 , which may take place by the presence of electrons in the conduction band at the surface of the photocatalyst (4).The conduction band electrons are also responsible for the production of • OH and • O 2 − , which have been identified as the main cause of MO degradation.Due to existence of rGO among MFO, which hinder the recombination of photogenerated electrons and holes, the photocatalytic performance is thus enhanced.

Conclusion
In summary, a novel photocatalyst MFO/rGO composite was prepared via a facile generalized solvothermal method.
The results demonstrate that the incorporation of rGO can enhance the photocatalytic activity for the degradation of MO.The reduced photogenerated electron-hole pair recombination and the enhanced visible-light absorbance with the introduction of rGO are mainly responsible for the enhanced photocatalytic activity of the MFO/rGO composite.The present synthetic strategy should be a promising fabrication technique for a simple, rapid, and effective method of the composite photocatalysts for the removal of organic pollutants.

Figure 1 :
Figure 1: The XRD patterns of MFO and MFO/rGO samples.

Figure 2 :
Figure 2: The SEM images of MFO (a) and MFO/rGO (b) and low-magnification (c, d) and high-resolution (e, f) TEM images of MFO/rGO.

Figure 3 :
Figure 3: (a) Photocatalytic degradation profile of MO under visible-light irradiation with and without catalyst versus exposure time, (b) absorption spectra of the MO (20 mg l −1 ) solution at different photocatalytic time under MFO/rGO photocatalytic.