Magnetically Separable ZnO-MnFe 2 O 4 Nanocomposites Synthesized in Organic-free Media for Dye Degradation Under Natural Sunlight

Magnetically separable ZnO-MnFe2O4 nanocomposites were synthesized in organic-free media. The samples were characterized by several techniques. The photocatalytic activities of the nanocomposites were tested by degradation of Rhodamine B and wastewater of batik dye under natural sunlight irradiation. X-ray diffraction (XRD) pattern of composites is well in agreement with the standard of the hexagonal wurtzite phase of ZnO and spinel cubic phase of MnFe2O4. Scanning Electron Microscope (SEM) image illustrates the morphology of the nanocomposites are square plate-like and porous. The magnetic properties of composites were characterized by Vibrating Sample Magnetometer (VSM) show the superparamagnetic properties of all nanocomposites. Optical properties study indicated that addition of MnFe2O4 to ZnO decreases the band gap. The removal of Rhodamine B (10 mg L−1) and wastewater of batik dye solution by ZnO-MnFe2O4 (1:0.1) composite after 3 h reached 96.7% and 95.1 %.


INTRODUCTION
In recent years, some research works have revealed that ZnO nanoparticles as photocatalysts exhibit superior advantages than those of TiO 2 .Owing to their high activities, lower costs, higher quantum efficiency, biodegradation, and environment-friendly features, ZnO has been widely used as photocatalysts.Recently, a number of studies related to the syntheses of ZnO photocatalysts have been reported, including nanoparticles, nanorods, nanocrystals, nanotubes, and composite membrane 1,2 .In fact, many of obtained products have the photocatalytic activities of ZnO are limited to irradiation wavelengths in the UV region which take only about 3-5% of the solar spectrum because they have wide band-gaps so it is not effective to be used in the visible light region of the sun [3][4][5] .Rapid recombination of photo generated electron-hole pairs still remains a problem to be solved in its application.Therefore, in tropical countries which possess water environment problems, particularly by dye pollution, this material offers a promising solution for environmental issues.Some researchers have made various modifications such as doped with metal 6,7 , nonmetallic 8 , metal/carbon 9 , and metal oxides 10 .However, the photocatalysts are still difficult to separate from the liquid for the subsequence process 11 .
The spinel ferrite MFe 2 O 4 (M = Mn, Co, Zn, Mg, Ni, etc.) is a well-known cubic spinel material where oxygen forms a face-centered cubic close packing, and M 2+ and Fe 3+ occupy either tetrahedral or octahedral interstitial sites 12 .Among magnetic nanoparticles, manganese ferrites MnFe 2 O 4 is a group of soft spinel ferrite materials with high magnetic permeability, high chemical stability, high electrical resistance, and special optical.Manganese ferrites are widely used in many areas such as high-density magnetic recording, ferrofluid technology, electronic devices, magnetic resonance imaging (MRI), microwave adsorption, biomedical drug delivery, catalysis and environmental analysis [13][14][15] .Besides that, this ferrite has moderate saturation magnetization at 300 K (Ms about 83 emu/g) and low Curie temperature (Tc about 580 K) 16 .Therefore, various studies on MnFe 2 O 4 nanoparticles properties have been reported in the literature.However, there are few reports on ZnO-MnFe 2 O 4 composite systems.
The aim of this study is synthesizing ZnO-MnFe 2 O 4 nanocomposites with different compositions of ZnO:MnFe 2 O 4 in organic-free media through a two-stage hydrothermal process.The crystallinity, morphology, optical and magnetic of the as-obtained nanocomposites were studied by several techniques.Interestingly, the addition of MnFe 2 O 4 led to decrease the band gap of ZnO and accordingly modified ZnO to be a visible light photocatalysts.Consequently, the introduced MnFe 2 O 4 in all formulations could be used to completely degrade the dye under sunlight irradiation and the sample could be separated from the liquid by external magnetic field and used for the subsequence photocatalytic process.

MATERIALS AND METHODS
The materials used in this work were Zn(NO 3 ) 2 .4H 2 O (Merck), Fe(NO 3 ) 3 .9H 2 O (Merck), Mn(NO 3 ) 2 .4H 2 O (Merck), NaOH (Merck), distilled water, and Rhodamine B. All the obtained chemicals were of analytical reagent grade and were used as received without any further purification.The batik dye wastewater obtained from the washed batik cloth.

Synthesis of manganese ferrite (MnFe 2 O 4 ) nanoparticles
Nanoparticles of manganese ferrite (MnFe 2 O 4 ) synthesized by hydrothermal route in organic-free media as follow; a stoichiometric amount of 10 mmol of Mn(NO 3 ) 2 .4H 2 O and 20 mmol of Fe(NO 3 ) 3 .9H 2 O as starting materials were dissolved in 40 mL distilled water under stirring at 500 rpm in rapid to obtain a mixed solution.The solution pH was adjusted to 12 with NaOH 2 M was added drop wise.The reaction procedure was carried out in an air atmosphere.The dark mixture obtained was placed into an autoclave and heated at 180°C for 3 h in the oven.After that, the autoclave was cooled to room temperature.The obtained precipitation was collected by filtered and washed with distilled water until pH to 7 and then followed by drying in the oven at 100°C for 2 h to form the MnFe 2 O 4 .The resulted nanoparticle was characterized by several instruments.

Synthesis of ZnO-MnFe 2 O 4 nanocomposites
The ZnO-MnFe 2 O 4 nanocomposites were synthesized by the hydrothermal route in organicfree media using MnFe 2 O 4 that was prepared before.The procedure was adopted from the previous study 17 as follow; A total of Zn(NO 3 ) 2 .4H 2 O and MnFe 2 O 4 nanoparticles was mixed with 40 mL of distilled water with the mole ratio of Zn +2 :MnFe 2 O 4 is 1: 0.01, 1: 0.05 and 1: 0 .1 that was signed as NMn-1, NMn-2, and NMn-3, respectively.To adjust the pH to 12, NaOH 2M solution was added drop wise to the mixture.Then the mixture was transferred into a Teflon-lined stainless steel autoclave and heated at the temperatures of 180 ο C for 3 h before being left to cool to room temperature naturally.ZnO-MnFe 2 O 4 powder obtained was filtered and washed with distilled water until the pH to 7 and then was dried at 100 ο C for 2 h.The same procedure was also carried out to prepare ZnO particles as control sample using Zn(NO 3 ) 2 .4H 2 O as starting material.Several techniques were used in order to investigate the structure, morphology, optic and magnetic properties.

Photocatalytic Activity
Photocatalytic activity of the obtained samples was evaluated by degradation of Rhodamine B solution under natural sunlight irradiation.In a typical experiment, 20 mg of ZnO-MnFe 2 O 4 nanocomposites as catalysts was added into 20 mL of Rhodamine B solution with an initial concentration of 10 mg/L at room temperature and then the mixture was exposed to natural sunlight irradiation for 0.5 until 3 h (from 11:00 am to 14:00 pm).After a specified time, the solid and liquid were separated and the absorbance of the solution (5 mL) was measured by UV-visible spectrophotometer at a wavelength of 553 nm to obtain the degradation percentage of Rhodamine B. The same procedures were performed for ZnO, MnFe 2 O 4 and recycled of ZnO-MnFe 2 O 4 nanocomposites.The maximum absorbance wavelength of Rhodamine B was obtained by measuring Rhodamine B absorbance at different concentrations.The photocatalytic activity of the asproduced samples was also evaluated on the degradation of batik dye wastewater.The parameters that influence the photocatalytic efficiency of as-prepared samples (initial concentration of dye and amount of catalyst used) were investigated.The degradation percentage of dyes were calculated according to follow equation, degradation (%) = (A' -A)/A' x100%, where A' and A is initial and degraded dye solution concentration 18 .

Structural analysis
The structure and particle size of resulted samples were analyzed by X-ray diffractometer (XRD Philips X'pert PAN analytical PW3040).For all NMn XRD patterns, there are no other peaks observed indicating no impurities in the samples.ZnO crystal size within nanocomposite could be calculated from the highest peaks using Debye-Scherer formula 19,20 .The average crystallite size of the nanocomposite is 30.9, 34.7, and 34.2 nm for NMn-1, NMn-2, and NMn-3, respectively.

Morphology and element composition analysis
The morphology of the samples was recorded by scanning electron microscopy (SEM JEOL JSM-6360LA).The surface morphology of (a) MnFe The composition of the elements in the ferrite and composite obtained from EDX as shown in Fig. 3.The EDX spectrum for MnFe 2 O 4 (Fig. 3a) shows the presence of Mn, Fe, Zn, and O elements only.The extra gold peak obtained in EDX is due to the thin coating done on the sample to make the sample conducting, which is required to record the SEM.The percentages of Mn, Fe, and O elements in MnFe 2 O 4 particles are 26.26,55.00, and 18.74%, respectively; while the percentage of Zn, Mn, Fe and O in NMn-3 ( Fig. 3b) are 73.95,2.69, 9.17, and 12.14%, respectively.It reveals that the as-prepared samples are pure and there are no other impurities present.

Magnetic and optical properties analysis
Magnetic properties of the MnFe 2 O 4 nanoparticles, NMn-1, NMn-2, and NMn-3 were measured by vibrating sample magnetometer (VSM OXFORD 1.2 H) at room temperature and the results are shown in Fig. 4. The magnetic saturation (Ms) value of MnFe 2 O 4 nanoparticles is 81.3 emu/g (Fig. 4a).Similar Ms value of MnFe 2 O 4 have been reported by Sharifi et al., where the sample synthesized by coprecipitation method 21 .Meanwhile, the Ms value of NMn-1, NMn-2, and NMn-3 is 6.08, 11.83, and 16.03 emu/g, respectively (Fig. 4b).Based on Ms value of all nanocomposites, it can concluded that increasing the composition of MnFe 2 O 4 in nanocomposites tends to increase the magnetic properties.The coercivity (Hc) of ferrite and composite samples is almost zero and the particles are superparamagnetic.The lower Ms of nanocomposites is caused by the diamagnetic properties of ZnO which reduce the magnetic nature of the MnFe 2 O 4 nanoparticles 22 .The nanocomposites containing magnetic behavior have profit due to separate easily from the liquid by the application of external magnetic field after applied and can be used for the next application.absorption edge for pure ZnO is below 400 nm.The adsorption region for NMn-2 and NMn-3 at the wave length of 578 and 635 with the band gap of 2.14 and 1.95 eV, respectively.The band gap energy was calculated using Eg=1240/λ equation 23 .It can be concluded that increasing the ferrite amount in the composites will increase the adsorption of the composites from UV to visible region light.

Photocatalytic activity evaluation
To demonstrate their potential environmental application in the removal of contaminants from wastewater, the photocatalytic activities of the as-synthesized samples were investigated by the degradation of Rhodamine B as a model of contaminant.From Fig. 6, it is clear that the ZnO-MnFe 2 O 4 samples exhibited different photocatalytic activities.The removal of Rhodamine B from solution (10 mg/L) under natural sunlight irradiation after 3 h by NMn-1, NMn-2, NMn-3 composite was reached 91.4,94.5, and 96.7%, respectively.Meanwhile, for ZnO and MnFe 2 O 4 samples, and without catalyst only reached 73, 35.2, and 28.7%, respectively.This indicates that the presence of MnFe 2 O 4 in ZnO can shift the absorption area and increase the activity of this material in the visible light of the sun.The best photocatalytic activity was obtained if using the NMn-3 sample as a catalyst.
Figure .7a revealed the optimum concentration of Rhodamine B that could be degraded by NMn-3 composite under natural sunlight.This research was carried out by 20 mg of catalyst, irradiation time 3 h and the concentration of Rhodamine B was within the range 5-25 mg/L.The optimum absorption was reached at the concentration of Rhodamine B 10 mg/L.At higher concentration, the percentage of degradation decrease due to the higher number of Rhodamine B molecule in solution.This molecule prevented the natural sunlight achieving the catalyst so that the e --h + pair forming was blocked.As known, the formation of e --h + pair plays important role in catalytic process due to their role as a precursor of OH .formation.This hydroxyl radical that be responsible in dyes degradation process 23 .
As already well known that if working with heterogeneous catalysts, the amount catalyst loading in photocatalytic process is important from an economical view.In this present work, the optimum of NMn-3 catalyst applied was 15 mg with Rhodamine B concentration of 20 mg/L as seen in Fig. 7b.After 3 h irradiation, Rhodamine B degradation percentage by catalyst loading of 0.015 mg give almost the same value with 0.02 g, which is 96.0 and 96.7%, respectively.Therefore for the further photocatalytic process, the amount of catalyst used is 0.015 mg so it is more efficient.
The reusability of composites were determined by tested the activity of recycled composite for the subsequent catalytic process.Fig. 8 showed the degradation percentage of Rhodamine B in the presence initial and recycle of NMn-3.It shows that the stability of NMn-3 composite slightly decreased after used in a catalytic process.This phenomenon was caused by the small part agglomeration that can be observed from SEM image which has an impact on the decrease of its activity.However, the recycled NMn-3 performance still higher and closer to the initial NMn-3 after 3 hours.
Furthermore, the evaluation of catalytic activity was also carried out on textile dye wastewater from batik cloth by the same procedure.The degraded batik dye wastewater solution was The kinetic analysis of the batik dye wastewater degradation process using NMn-1, NMn-2, and NMn-3 composites as catalysts is also studied (Fig. 10).The linear plots of corresponding irradiation times (t) versus ln Co/C can be argued that the reaction mechanism of dye degradation follows the first-order pattern in accordance with that proposed by Langmuir-Hinshelwood 8 .The average squared correlation coefficient (R 2 ) is 0. 9718.Because of the high photocatalytic activity and good stability, this material is recommended as catalyst for degrade the dye in the wastewater so that the environmental problems caused by the dye wastewater can be reduced.

CONCLUSION
ZnO-MnFe 2 O 4 nanocomposite photocatalysts with different composition of MnFe 2 O 4 have been synthesized by hydrothermal route in organic-free media.From XRD analysis results, ZnO phase in nanocomposite shows wurtzite hexagonal phase while MnFe 2 O 4 shows cubic spinel phase.Magnetic properties measured by VSM show that nanocomposites have superparamagnetic behavior.
Resulted nanocomposite were applied to the degradation Rhodamine B dye and batik dye wastewater under natural sunlight irradiation, and the results show that the photocatalytic activity of ZnO-MnFe 2 O 4 with the composition of 1:0.1 is 96.7 % for Rhodamine B and 95.1 % for batik dye wastewater after 3 hours.The high photocatalytic activity and good stability suggest that the as-synthesized ZnO-MnFe 2 O 4 may be promising photocatalysts in removing dye pollutants from wastewater.The photocatalysts can be separated easily from the liquid by the application of external magnetic field after applied and can be used for the next application.