Synthesis and Characterization of CoFe2O4 Nanoparticles and Its Application in Removal of Reactive Violet 5 from Water

INTRODUCTION
Simultaneously with advance going of human social and economic life, new challenges are emerged in human life that these challenges can face significant risks to human life. One of the main problems in this regard is the problems created for the environment and its destructive effects on the all aspects of human life [1-3]. In recent years, with increasing expansion of the paint and paper industries, various organic pollutants have increasingly entered the environment. The presence of these pollutants can cause a series problems and pose major challenge to the health of drinking water [4-8]. To eliminate these pollutants, various methods such as filtration [9, 10], electrochemical oxidation [11, 12], ozonation [13, 14], and photocatalytic process [15-17] have been applied. Among these process, the photocatalytic method has received a lot of attention because it uses free sunlight to remove organic pollutants. One of the main reasons for this attention lies in the greenness of this method. Because this method use green energy and also produces non-toxic products during the process. This becomes even more important when it is noted that that other methods often produce toxic products during the process that can subsequently cause other containments [18-20]. 
Photocatalysts are semiconductor-based compounds that can trigger specific chemical reactions by absorbing visible and ultraviolet light. In this regard, semiconductors can be used to degradation of water organic pollutants [21, 22]. The photocatalytic activity of semiconductors depends on the optical band gap intensively. So, providing semiconductor with desired optical properties for photocatalytic process is the vital step in the process [23, 24]. In recent years, with growing development of nanotechnology and nanoscience, various nanomaterials have contributed to the photocatalytic process in this regard. Therefore, one of the ways to improve the optical properties of semiconductors is to reduce particle size and reach nanoscale. Transition metal oxide-based semiconductors are attractive candidates that can effectively degrade pollutants [25, 26]. Ferrites with the formula MFe2O4 commonly include metal cations such as barium [27, 28], calcium [29], cobalt [30, 31], copper [32], magnesium [33], manganese [34], and nickel [35]. Ferrites are an important group of these compounds that due to their unique properties have found many applications in the degradation of various water pollutants. Sufficient optical band gap, spinel crystal structure, chemical and thermal stability, high specific surface area and attractive magnetic behavior make them excellent photocatalyst option [36]. Also, the physical and chemical properties of ferrite-based nanostructures depend on the synthesis route intensively [37].
Sneha Singh et al. prepared ruthenium doped CoFe2O4 nanostructures via sol-gel route with different amount of ruthenium. The findings revealed that the ruthenium doping improves the crystalline structure and magnetic behavior of cobalt ferrite. The photocatalytic activity of the prepared CoRuxFe2-xO4 were studied against organic dyes under visible light. The obtained results confirmed that doping of ruthenium into CoFe2O4 nanomaterials make it superior photocatalyst. They reported that the excellent magnetic properties of prepared ruthenium doped cobalt ferrite lead to facilitate separation and reuse of photocatalyst [38]. 
In this work, CoFe2O4 nanoparticles was synthesized via hydrothermal rout at 170 °C for 12 h. The crystalline structure of prepared cobalt ferrite was investigated via XRD pattern. The purity of prepared samples was studied via EDS analysis. Also, the morphological and magnetic properties of samples were studied via SEM and VSM analysis respectively. Finally the photocatalytic activity of prepared CoFe2O4 nanoparticles was investigated via photodegradation of Reactive Violet 5 under visible light irradiation. 

MATERIALS AND METHODS
Precursors and materials
All starting materials and chemicals were purchased from a Sigma-Aldrich with synthesis grade and used without further purification. FT-IR was applied for the investigation of functional groups (Nicolet Magna-550/ KBr pellets). The structural composition of nanocomposites was performed by an X-ray diffractometer device using Ni-filtered Cu Ka radiation (Philips-X’pertpro). Investigation of surface morphology was studied by SEM images (model: LEO-1455VP). The magnetic measurement of samples was obtained by VSM.

Preparation of CoFe2O4 nanoparticles 
First of all, CoFe2O4 nanoparticles were prepared through the hydrothermal method by adding Co(NO3)2.6H2O and Fe(NO3)2.9H2O as precursor materials at 1:1 molar ratios in DI water (60 ml). The hydrothermal synthesis procedure has been explained in detail elsewhere [39]. After that, the as-prepared sodium hydroxide solution [10 M] was added to the above mixture by dropping. Next, the whole mixture was kept in a Teflon-lined stainless steel autoclave for 12 h at 170 °C. Upon completion of the time reaction, the solid was separated by an external magnet and then, washed with ethanol and DI water several times. Finally, the resulting solid was dried at 65 °C overnight.  

Photocatalytic test
Typically, a 30 ppm of Reactive Violet 5 solution was provided. Then 0.1 g of prepared CoFe2O4 nanoparticles was added to the Reactive Violet 5-containing solution and the obtained mixture was transferred to dark box under stirrer for 30 minutes. To provide the solution oxygen-saturated via the reaction, air was introduced into the prepared mixture via a pump. Then CoFe2O4 nanoparticles was filtered from the mixture, and the concentration of Reactive Violet 5 was determined through a UV-Vis spectrophotometer. To calculate the Reactive Violet 5 degradation efficiency, Eq. 1 was utilized:

RESULTS AND DISCUSSION
The CoFe2O4 nanoparticles were investigated in terms of purity and structural composition by applying the XRD approach. The XRD pattern of CoFe2O4 nanoparticles is shown in Fig. 1. The observed peaks corresponded to a standard reference (JCPDS= 00-003-0864), and it confirms the formation of a single-phase with a cubic shape [40]. In addition, CoFe2O4 nanoparticles Miller’s index is observed. According to the Debye-Scherrer equation (D= kλ/βcosθ), the crystallite size was measured at approximately 38 nm.
FT-IR test was used for the detection of functional groups of sample and is shown in Fig. 2. The main peak at 574 cm-1 is related to metal-oxide vibration which is overlap with each other. Besides, two bonds at 3435 cm-1 and 1635 cm-1 related to the stretching and bending absorption of water, respectively.
EDS analysis was carried out to determine the elemental composition of CoFe2O4 nanoparticles (Fig. 3). EDS information shows the percentage of oxygen, iron, and cobalt in the structural composition. So, data approved the formation of CoFe2O4 nanoparticles without any impurities. Elemental mapping reveals the homogenous dispersion of O, Fe, and Co elements into their corresponding structural composition (Fig. 3).
The magnetization property of as-prepared CoFe2O4 nanoparticles was investigated with VSM analysis. The resulting data was shown in Fig. 4. Based on the resulting curve, the magnetic property was reported about 45 (emu/g).  
The morphologies of as-prepared CoFe2O4 nanoparticles were investigated by the FE-SEM technique. FE-SEM images of the sample are shown in Fig. 5. It can be seen that the obtained average particle sizes (78.87 nm) are clearly in the nanostructure range with agglomeration.
The performance of any photocatalyst depends on optical band gap energy of photocatalyst. The UV-Vis analysis is shown in Fig. 6a. Optical band gap energy of prepared CoFe2O4 nanoparticles is shown in Fig. 6b. The optical band gap (Eg) was measured by the Tauc equation (Eq. 2):

where hν is the photon energy; α is absorbance, B is a constant attributed to the photocatalyst; and n equal either 2 or ½ for direct transition and indirect transition, respectively [41]. As well as known, the optical band gap for the absorption peak determined by extrapolating the linear portion of the (αhν)n − hν curve to zero as shown in Fig. 7b. The band gap values of prepared CoFe2O4 nanoparticles was determined 2.41 eV. 
Fig. 7 shows the photocatalytic activity of prepared cobalt ferrite nanoparticles against Reactive Violet 5 under visible light after 75 minutes. As well as seen, the 76.4% of Reactive Violet 5 was degraded after 75 min. CoFe2O4 have been approved to be effective photocatalysts by utilizing light energy to form electron/hole pairs on the CoFe2O4 surface. The electron/hole pairs are then facilities oxidation and reduction reaction, which lead to the make the reactive oxygen species (ROS), such as O2•- and •OH which then further aid in the degradation of Reactive Violet 5. 

CONCLUSION
In conclusion, the CoFe2O4 nanoparticles was applied as a new magnetic nano photocatalyst to removal of the Reactive Violet 5 from waste water. The magnetic nano photocatalyst  was prepared via hydrothermal method at 170 °C for 12 h. Then, the structural properties, shape and size of prepared CoFe2O4 nanoparticles were determined via X-ray powder diffraction (XRD), scanning electron microscope (SEM), vibrating-sample magnetometer (VSM) technique. The optical was calculated 2.41 eV through Tauc equation via assistance of UV-Vis spectroscopy. Results showed that the CoFe2O4 nanoparticles could act as an excellent photocatalyst for degradation of Reactive Violet 5 from the waste water. The degradation mechanism was provided via reactive oxygen species (O2•- and •OH) which then further lead to photodegradation of Reactive Violet 5.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.