MnCo2O4/Co3O4 Nanocomposites: Microwave-Assisted Synthesis, Characterization and Photocatalytic Performance

INTRODUCTION
Transition-metal oxides based nanocomposites exhibit novel properties that significantly have different physical and chemical properties than those matrix material and the filler resulting [1-4].  In other hand, nanocomposites have a unique and attractive properties due to small size effect [5, 6]. Magnetic nanocomposites not only have unique size-dependent properties but also get benefits from interesting magnetic properties. The magnetic nanocomposites due to vast variety of different materials have high capability in different application fields, ranging from biomedical to photocatalysis applications [7-9]. Photocatalysis is a type of catalysis that speeding up the rate of a photoreaction - a chemical reaction that involves the absorption of light by one or more reacting species - by adding catalysts that participate in the chemical reaction without being consumed [10, 11]. A photocatalyst is defined as a material that is capable of absorbs Ultraviolet (UV) radiation from sunlight or illuminated, producing electron–hole pairs that enable chemical transformations of the reaction participants and regenerate its chemical composition after each cycle of such interactions. An efficient photocatalyst should benefits desirable optical features, suitable morphological properties and reusability [12-14].
Cobalt oxide (Co3O4) is an important magnetic and P-type semiconductor. Till now, wide range of Co3O4 based nanocomposites have been prepared and applied in photocatalysis field [12, 15, 16]. Yong sheng Yan and et al. prepared carbon modified Co3O4/BiVO4 p-n heterojunction photocatalyst (Co3O4/BiVO4/C) for enhancing light absorption and the facilitating separation of photogenerated charge carriers through forming a p-n heterojunction. They reported that optimum activity of the Co3O4/BiVO4/C p-n heterojunction is higher than that of pure Co3O4 and BiVO4 for the degradation of tetracycline under visible light [17]. In other work, Xinfa Dong and et al. prepared Co3O4/Cd0.9Zn0.1 nanocmposites via solvothermal method. Under visible light, they showed Co3O4/Cd0.9Zn0.1 H2 evolution is 15.88 times higher than that obtained over the bare Cd0.9Zn0.1S [18]. Ashok Kumar Chakraborty synthesized Co3O4/WO3 nanocomposites by dispersing p-type semiconductor Co3O4 on the surface of n-type semiconductor WO3. Results revealed that prepared nanocomposites have higher photocatalytic activity than WO3, Co3O4 nanoparticles for the complete decomposition of 2-propanol in gas phase and phenol in aqueous phase and evolution of CO2 under visible light irradiation [19].  
In this work, MnCo2O4/Co3O4 nanocomposites were prepared via simple and low-cost microwave- assisted method.  The as-prepared products were characterized by different analyses such as XRD, SEM, TEM, FT-IR, and UV-Vis and the photocatalytic performance of the product was investigated by degradation percent of methylene blue as an organic pollutant under UV irradiation.
MATERIALS AND METHOD
Co(CH3COO)2.4H2O and of Mn(CH3COO)2.4H2O  and ethylene glycol was purchased from Merck and all the chemicals were used as received without further purifications. XRD patterns were recorded by a Philips, X-ray diffractometer using Ni-filtered CuKα radiation. Fourier transform infrared (FTIR) spectra were detected by means of Nicolet Magna-550 spectrometer in KBr pellets. The UV–Vis diffuse reflectance analysis of the as-prepared nanocomposite was done by applying a UV–vis spectrophotometer (Shimadzu, UV- 2550, Japan). SEM images were obtained using a TESCAN instrument model Mira3 to taking images, the samples were coated by a very thin layer of Pt to make the sample surface conductor and prevent charge accumulation, and obtaining a better contrast. Transmission electron microscopy (TEM) image was achieved via a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV.

Synthesis MnCo2O4/Co3O4 nanocomposite
Co(CH3COO)2.4H2O and of Mn(CH3COO)2.4H2O with 1:1 molar ratio were dissolved in water/ethylene glycol solvent, which was mixed with a ratio of 2:1. After completely dissolving, the gained transparent solution was transferred to the microwave oven and placed under irradiation at various time and power. The obtained precipitate was washed with distilled water and dry at 85 °C for 5h. Eventually, the product was calcined at 600 °C for 3h. Three samples were prepared at 10 minutes irradiation with power of 900 and 750.

Photocatalytic test
For photocatalyst testing, the amount of 0.05 g of nanocomposites is added to a dye with10 ppm concentration in the quartz reactor. Then the mixture was placed in photoreactor after stirred for 30 min at dark, the UV light was applied. Then MnCo2O4/Co3O4 nanocomposites was separated from the 5 ml samples, taken from the degraded solution at various time intervals, using 5 min centrifuging at 12,000 rpm. The dye concentration was determined with aid of a UV-vis spectrophotometer. The test was performed for 120 minutes.

RESULTS AND DISCUSSION
XRD analysis, which is the most useful and functional technique for both existence and identification of crystalline structure, was hired to investigate the synthesized samples. Fig. 1 shows XRD pattern of MnCo2O4/ Co3O4 nanocomposites. It can be observed Cubic phase of Co3O4 (JCPDS: 43-1003) with space group of Fd3m and cell constants a = b = c = 8.0840 Å and Cubic phase of MnCo2O4 (JCPDS: 23-1237) with space group of Fd3m and cell constants a = b = c = 8.2690 Å were formed. The crystalline size was calculated from Scherrer equation, Dc = Kλ/βCosθ, where β is the width of the observed diffraction peak at its half maximum intensity (FWHM), K is the shape factor, which takes a value of about 0.9, and λ is the X-ray wavelength (CuKα radiation, equals to 0.154 nm) was about 19 nm. 
Fourier transform infrared (FT-IR) spectroscopy has been employed for analysis of the surface functional groups of MnCo2O4/ Co3O4 nanocomposites at 10 min in 900 W after calcination at 600 °C for 3h. As shown in Fig. 2, the most prominent absorption bands at 657 cm-1 and 560 cm-1 are corresponding to metal-oxygen bonds in spinel structure of composite [20]. Due to the calcination of the sample at 600 ° C, no further peaks were observed in the sample. Furthermore, the broad bands observed at 3436 cm-1 were attributed to the OH groups stretching vibrations of the water molecules [21].
The elemental composition analysis of the as-synthesized MnCo2O4/ Co3O4 nanocomposites at 10 min in 900 W were further confirmed by EDS analysis. As can be seen in Fig. 3, the MnCo2O4/ Co3O4 nanocomposites were composed of stoichiometric Co, Mn and O elements, which indicating the high purity of the products. This result is consistent with the results of XRD pattern presented in Fig. 1.
To investigate the effect of the power and type of irradiation on the morphology and particle size of the MnCo2O4/ Co3O4 nanocomposites the samples were fabricated by using the power of 900 and 750 W for 10 min, and 900 W with cyclic reaction (1 min on and 30 sec off). To assess the effect of these conditions SEM images were investigated. As can be seen in Fig. 4, when the power is set to (a) 750 W, larger particles are formed due to the moderate radiation power. At (b) 900 W, Due to the high radiation power, the ratio of nucleation to growth speeds up, resulting in smaller particles. Also in (c) cyclic reaction, when radiation is present, nucleation occurs and the particles are formed but when radiation is cut off, the particles of opportunity grow, resulting in larger particles. According to SEM images, the sample obtained at 900 W and 10 min was selected as the optimum sample.
TEM analysis was applied to in-depth investigation of size and morphological properties of MnCo2O4/ Co3O4 nanocomposites. It is worth bearing in mind that SEM analysis cannot distinguish between Co3O4 and MnCo2O4 in MnCo2O4/ Co3O4 nanocomposites. Fig. 5 illustrated TEM images in different magnification validate uniform nanoparticles of as-prepared nanocomposites via 35 nm in diameter.
Fig. 6 presents the UV-Vis diffuse reflectance spectra (DRS) of the as-prepared MnCo2O4/ Co3O4 nanocomposites (samples). It can be observed that the nanocomposites have strong and broad absorption peaks in the range of 250-400 nm. The value of energy band gap (Eg) of the corresponding nanoparticles is calculated 3.3 eV, based on Tauc’s equation[22], which indicating the nanocomposites can be employed as a potential photocatalyst for degradation water soluble dye as a pollution.
Optical properties of product implies that prepared nanocomposites can be applied in photocatalytic process. Rhodamine B and methyl violet are selected as organic pollutants for investigation of photocatalytic efficiency of MnCo2O4/ Co3O4 nanocomposites under UV irradiation. Results are presented in Fig. 7. For methyl violet case, it can be seen that dye is degraded very faster in early 30 min irradiation. After 30 min, degradation rate was gone down gradually. It can be related to occupying active sites on the MnCo2O4/ Co3O4 nanocomposites (adsorption route). After 120 min, approximately 61% of methyl violet was degraded. For rhodamine B, degradation rate keeps constant after 80 min. After 120 min, approximately 58% of rhodamine B was degraded under UV irradiation. The electronic band structure of prepared MnCo2O4/ Co3O4 nanocomposites make it very good candidate for photocatalytic degradation. As well as mentioned in Fig. 6, the band gap of prepared nanocomposites was calculated 3.3 eV. This means that under UV irradiation, electrons can be moved from valance band to conducting bond in MnCo2O4/ Co3O4 nanocomposites. This movement can be lead to generation of holes (h+). Photogenerated holes can convert water to hydroxide ions (OH-). OH- reacts with h+ and produce hydroxyl radical (OH•). Produced OH• could give rise to oxidation of dyes [24].
hʋ → e- + h+                                                                (1)
h+ +H2O → H+ + OH-                                                    (2)
h+ + OH-→ OH•                                                           (3)
OH• + dye → Oxidation dye                  (4)

In the parallel pathway, photogenerated OH- in reaction (2) can be reacted with dissolved oxygen and produce ozone (O3). Produced ozone can be converted to O3• through reaction with photogenerated electron in conducting bond.  Generated O3• could be degraded dye.
2 OH- + O2 + 2h+ → O3 + H2O                                    (5)
e- + O3 → O3•                                                               (6)
O3•+ dye → Oxidation dye                                       (7)

CONCLUSION
In conclusion, microwave-assisted route was applied for preparation of MnCo2O4/Co3O4 nanocomposites. Prepared products were characterized via XRD, FT-IR, EDS, SEM and TEM analysis. Optical properties of products was investigated via UV-Vis spectroscopy. Results cleared that prepared MnCo2O4/Co3O4 nanocomposites are good candidate for photocatlytic processs. Fot this purpose, MnCo2O4/Co3O4 nanocomposites were applied for photodegradation of rhodamine B and methyl violet. Findings showed that rhodamine B and methyl violet were degraded via 58% and 61% efficiency.

CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.