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Optical and photocatalytic application of ZnFe2O4SmFeO3 nanocomposites

The xZnFe2O4–(1−x)SmFeO3 (x = 0.1, 0.2 and 0.3) nanocomposite were synthesised through the thermal decomposition technique and calcinated at 873 K. The obtained nanocomposite was examined using XRD, SEM with EDS, UV-DRS, and TEM with SAED. XRD exhibits a spinel-perovskite mixed structure. The 0.3ZnFe2O4-0.7SmFeO3 nanocomposite showed a substantial red shift in absorption edge with narrow band gap of ~1.8 eV. Further, photodegradation of prepared nanocomposite was assessed for the degradation of Methyl orange (MO). The 0.3ZnFe2O4-0.7SmFeO3 photocatalyst showed effective photodegradation for MO under direct sun light irradiation.


Introduction
The contamination of environment with synthetic dyes from industrial wastewater has received increasing attention due to their severe consequences, such as slow biodegradation, aesthetic pollution, toxicity and perturbation to aquatic life [1,2]. The presence of organic dyes is of major group of water pollutants, may lead to irreversible environmental hazards. To solve these difficulties, more and more research attentions have been focused on development of catalytic materials. With concerns about a clean environment, semiconductor photocatalysis have been the centre of many studies due to their economic and ecologically safe option for solving energy and environment crises [3]. The use of solar energy to eradicate organic pollutants from industrial wastewater makes photocatalysis interesting [4]. Spinel ZnFe 2 O 4 is a well-known nano-photocatalyst which has been extensively studied due to its narrow band gap and remarkable chemical stability [5]. Nonetheless, the reports on the photodegradation of organic dyes using zinc ferrite as photocatalyst is limited as the activity of pure ZnFe 2 O 4 is not high enough due to the fast recombination of photogenerated electron-hole pairs [6]. An effective approach to overcome this barrier is to couple ZnFe 2 O 4 with another semiconductor material since coupling two semiconductors with different energy levels results in mutual transfer of photogenerated electrons and holes from one semiconductor to the other, resulting in an efficient charge separation, an increased life time of the charge carriers, and an enhanced interfacial charge transfer to adsorbed substrates. Coupling of semiconductors exert the shared benefits of two materials and also accelerate the separation of electron-hole pairs by keeping the reduction and oxidation reactions at two different sites [7,8].
Xian Ming Liu et al. [9] reported on CoFe 2 O 4 -BiFeO 3 nanocomposites and found that this spinel-pervokite system exhibited a strong magnetic property. REFeO 3 perovskite-type oxides, rare earth (RE) are very vital materials for composite partner in advanced technologies. SmFeO 3 is reported to have a perovskite structure with orthorhombic phases and have been studied extensively as promising photocatalysts [10,11]. Based on the above facts, in the present paper, we have coupled ZnFe 2 O 4 with SmFeO 3 , in the purpose of achieving enhanced photocatalytic activity toward organic dyes.
In this article, spinel-perovskite xZnFe 2 O 4 -(1−x)SmFeO 3 (x = 0.1, 0.2 and 0.3) nanocomposites were synthesised by the thermal decomposition technique. The structure of the spinel-perovskite nanocomposites were characterized by means of an X-ray diffractometer and transmission electron microscope. The optical properties of the novel xZnFe 2 O 4 -(1−x)SmFeO 3 nanocomposites were studied. These samples were used as photocatalyst in the photodegradation of Methyl Orange. [

Preparation of nano-photocatalyst
The xZnFe 2 O 4 -(1−x)SmFeO 3 samples were synthesised by thermal decomposition of precursor gels [10]. The citric acid was added into deionized water with constant stirring and kept at 333K-343 K. Then, ferric nitrate, zinc nitrate and samarium chloride were added in turn. Appropriate molar portions of metal nitrates were fixed at Zn:Sm:Fe ratio of 1:9:11, 1:4:6, and 3:7:13. After the mixing of the salts to their total dissolution, ethylene glycol was added into the solution with a proportion of citric acid to ethylene glycol ratio of 60:40. The temperature was increased up to 363 K. The gels were dried overnight at 373 K in order to remove excess water. During the process of drying, the gel swelled into the fluffy mass and eventually broke into brittle flakes. The samples were grinded and calcinated at 873 K for 4 hours in order to get the oxide materials.

Analytical and testing instruments
The identification of phase composition and crystalline structure of obtained samples were done by PXRD using PANalytical Xpert Pro X-ray diffractometer with Cu-Kα radiation (λ = 1.

Photocatalytic experiment
The photocatalytic activities of the xZnFe 2 O 4 -(1−x)SmFeO 3 nanocomposites were evaluated by the degradation of Methyl Orange (MO) solution under direct sunlight irradiation. In every experiment, 0.1 g of photocatalyst was added to 100 ml of MO solution (10 ppm). Prior to irradiation, the suspension was magnetically stirred for 30 min in dark to reach the establishment of an adsorption/desorption equilibrium between MO and photocatalyst. Then, at certain time intervals, 5 ml of samples were collected and removed the catalyst particles for subsequent analysis. The change in the concentration (absorbance) of MO solution was monitored on UV-Vis spectrophotometer by measuring the absorbance in 200-800 nm range using deionised water as reference. Blank tests were also carried out on an aqueous MO solution without photocatalyst under the same condition. In addition, the recyclability of the photocatalyst was also studied by washing and drying the catalyst before the next cycle.  Fig.1. The pure zinc ferrite shows strong diffraction peak at 35 which corresponding to the (311) plane of the spinel phase. It was clear that all the diffraction peaks of ZnFe 2 O 4 can be perfectly indexed as cubic phase with Fd3m (227) space group and match well with the standard card JCPDS card no. 82-1048 (a= 8.442 Å). The SmFeO 3 shows the diffraction peak at 33.1 corresponds to the (121) plane of the orthorhombic phase. It was obvious that all the patterns of SmFeO 3 express pure perovskite structure, which are accordance with JCPDS card no. 74-1474 (space group = Pbnm(62) a=5.41 Å, b= 5.59 Å, c= 7.71 Å). Furthermore, the distinct diffraction peak displays the formation of the spinel-perovskite mixed structure. The xZnFe 2 O 4 -(1−x)SmFeO 3 (x = 0.1, 0.2, 0.3) nanocomposite shows the XRD patterns of ZnFe 2 O 4 and SmFeO 3 which suggest the coexistence of the both ZnFe 2 O 4 and SmFeO 3 in the nanocomposite [12]. The average crystallite size of the ZnFe 2 O 4 , SmFeO 3 were calculated using scherrer formula [13] and are 38 and 32 nm, respectively. The morphology of 0.3ZnFe 2 O 4 -0.7SmFeO 3 sample are shown in Fig.2a-b. It was clearly seen that the sample present in irregular shape and most of them are agglomerated. Additionally, the 0.3ZnFe 2 O 4 -0.7SmFeO 3 nanocomposite is interconnected and fused together to form open structure and that would be a better for dye adsorption [14][15][16][17][18][19]. The composition and purity of nanocomposite were investigated using EDS and are shown in Fig.3   Photocatalytic activity of the 0.3ZnFe 2 O 4 -0.7SmFeO 3 nanocomposite was evaluated by photodegradation of MO, are shown in Fig.6. The MO is a typical anionic dye with a maximum absorption at 463 nm. Blank experiment was performed under same condition without photocatalyst the degradation of MO was negligible. Prior to the photocatalytic reaction, the suspension was stirred to attain adsorption/desorption equilibrium in the presence of photocatalyst without irradiation was achieved (dark reaction). The absorption rates were 11%, which clearly specified that the photocatalyst and sunlight are essential for photodegradations of MO. The suspension was exposed to sunlight irradiation at different interval of time, absorption results were recorded for another 80 min. The gradual decrease in the intensity of MO absorption peak at 463 under solar light irradiation over 0.3ZnFe 2 O 4 -0.7SmFeO 3 nanocomposite photocatalyst with the color changing from initial orange to colorless, indicating the photodegradation of MO are shown in Fig. 6. The degradation of MO over 0.3ZnFe 2 O 4 -0.7SmFeO 3 reaches 91% in 110 min. The greater photodegradation effeciency of 0.3ZnFe 2 O 4 -0.7SmFeO 3 sample makes it efficient photocatalyst in the removal of dye. The percentage photodegradation was calculated by following relation. Photodegradation(%) = (C 0 -C t )/C t 100

FESEM and EDS analysis
(1) The higher photocatalytic activity of the 0.3ZnFe 2 O 4 -0.7SmFeO 3 nanocomposites showed the effective semiconductor coupling, which enhance the photocatalytic activity. The coupling of ZnFe 2 O 4 and SmFeO 3 , results in an efficient charge separation, an increased lifetime of the charge carriers, and an enhanced interfacial charge transfer to adsorbed substrates [15].

Mechanism of photocatalytic activity enhancement
The possible mechanism is proposed to explain the enhanced photocatalytic activity of the xZnFe 2 O 4 -(1−x)SmFeO 3 nanocomposites, are demonstrated in Scheme.1.

Scheme.1. Possible photocatalytic mechanism of MO degradation over the xZnFe 2 O 4 -(1−x)SmFeO 3 nanocomposite under Solar light irradiation
In order to know the separation of photogenerated electron-hole pairs over photocatalytic process. It was necessary to find out the energy level of the ZnFe 2 O 4 and SmFeO 3 , their conduction band (CB) and valence band (VB) edge potentials of the components were estimated using the following equations [16,17]: (3) Where  and Eg denotes the absolute electronegativity and band gap, respectively. E e is the energy of free electrons on the hydrogen scale (~ 4.5 eV).  is the electronegativity of semiconductor and it was calculated by the following equation: (4) Where A, B, and C are the different atoms in compound and a, b, and c are number of atoms in the compound [18] respectively, preventing fast recombination of the charge carriers. These interfacial charge transfers prolong lifetime of the charge carrier by suppressing the recombination of electron-hole pairs, which is more encouraging to enhance the photocatalytic activity of the xZnFe 2 O 4 -(1−x)SmFeO 3 nanocomposites, leading to production of more reactive species and has vital role in the degradation reaction. Then, the photogenerated electrons react with adsorbed oxygen and water molecules on the surface of the nanocomposite to produce reactive species of O 2  and OH  [19]. Meanwhile, the photogenerated holes are trapped by adsorbed H 2 O and OH  to produce OH  species. Therefore, the photocatalytic activity was greatly enhanced in xZnFe 2 O 4 -(1−x)SmFeO 3 nanocomposites. 3) nanocomposite for long time performance, MO degradation reactions over this photocatalyst were carried out for three successive runs. In each test, the photocatalyst were gathered by external magnet and reused after washing with water while other factors were kept same. It showed no changes in activity of photodegradation of MO during the three cycles, indicating that the xZnFe 2 O 4 -(1−x)SmFeO 3 possessed good stability as photocatalyst. Therefore, it can be concluded that the xZnFe 2 O 4 -(1−x)SmFeO 3 was an outstanding photocatalyst in photodegradation of MO due its reutilization. Further, it could also be used in treatment of wastewaters containing various dye pollutants.

Conclusion
In this article, we have reported thermal decomposition technique for synthesis of mixed spinel-perovskite xZnFe 2 O 4 -(1−x)SmFeO 3 (x = 0.1, 0.2 and 0.3) nanocomposite. The 0.3ZnFe 2 O 4 -0.7SmFeO 3 nanocomposite showed a substantial red shift in absorption edge with narrow band gap of ~1.8 eV. The enhanced photocatalytic activity was mainly attributed to the more harvesting ability of visible light and efficient charge separation on the surface of the xZnFe 2 O 4 -(1−x)SmFeO 3 nanocomposite. The optimal concentration to achieve the higher photocatalytic activity is x=0.3, due to perfect matching of the band energies for the counterparts. The photodegradation of MO under direct sunlight irradiation was accomplished over 0.3ZnFe 2 O 4 -0.7SmFeO 3 photocatalyst in 110 min. The nanocomposite displays significant photostability and reusability. Hence it was confirmed that the 0.3ZnFe 2 O 4 -0.7SmFeO 3 nanocomposite could offer a new solar light induced photocatalyst for further degradation of organic pollutants.