A facile synthesis of MgFe2O4/ZnS heterojunction with effectively enhanced visible light photocatalytic activity for degradation of methylene blue and crystal violet dyes

A novel n-MgFe 2 O 4 –n-ZnS heterojunction catalyst was employed via two step approach for photodegradation of organic dyes such as methylene blue (MB) and crystal violet (CV) dyes under visible light irradiation. The synthesized MgFe 2 O 4 /ZnS NCs were characterized using PXRD, FTIR, UV-visible spectroscopy, PL and FESEM analysis which reveals the formation of ake like structure with size as to be ~50 nm. The photocatalytic experimental result demonstrates that the MgFe 2 O 4 /ZnS nanocomposites (NCs) improve photodegradation performance with 98.0% and 91% decomposition of MB and CV dyes within 120 min illumination during simulated visible light irradiation. From the result, MgFe 2 O 4 /ZnS NCs has higher photocatalytic performance than that of MgFe 2 O 4, and ZnS due to ecient separation of the photo-induced electron-hole pairs and effective electron–hole generation transfer by the formation of n-MgFe 2 O 4 –n-ZnS heterojunction. Hence, photodegradation performance implies that the synthesized MgFe 2 O 4 /ZnS NCs can be effectively utilized in waste water purication systems.


Introduction
Interest in research on semiconductor based photocatalysts has attracted much attention in the elds of photochemistry, catalysis, electrochemistry, and owing to their high chemical stability and photocatalytic activity [1]. Several semiconductor photocatalysts like ZnO, TiO 2 , WO 3 , CdO, Fe 2 O 3 , ZnS, MoS 2 and CdS etc. have been more apt for the elimination of organic pollutants contaminate in the waste water which release from the paper, leather and textile industries [2]. Among this, ZnS act as a considerable II-VI compound semiconductor material with wide band gap (3.6 eV) and large exciton binding energy (40 meV), has been studied widely as an excellent photocatalyst due to its quick photoexcited electron (e − )hole (h + ) pairs and highly negative reduction potentials of excited electrons [3,4]. Numerous methods have been utilized to develop ZnS nanoparticles (NPs) like electrochemical deposition, sol-gel, solvothermal, hydrothermal, co-precipitation, pyrolysis, microemulsion, laser ablation, combustion synthesis, and vapor deposition [5]. Among them, co-precipitation technique has various reaction time, temperature, and parameters like pH. As well as, initial concentration of the solution and material, have played a crucial role on receiving ceramic powders with preferred shape and size [6].  [7]. Among ferrites, magnesium ferrite (MgFe 2 O 4 , MFO) is an n-type semiconductor with a spinel structure, which endows the absorption of visible light owing to their narrow band gap of 2.0 eV [8]. Fascinatingly, earlier literatures revealed p-n (or) n-n type heterojunction structure for enhance the photocatalytic activity by their charge separation e ciency between photo-induced electron and hole pair's [9]. In this aspect, Su et al. has developed MgFe 2 O 4 --ZnO heterojunction photocatalyst for degradation of Rhodamine B organic dye. The excellent photocatalytic activity has been achieved via interconnected heterojunction of n-MgFe 2 O 4 and n-ZnO nanoparticles [10]. Therefore, in the present investigation focus to form a novel n-MgFe 2 O 4 /n-ZnS heterojunction for improve the photocatalytic activity owing to their e cient separation of photo-exited electron and hole pairs. The

Synthesis of ZnS nanoparticles
To achieve ZnS nanoparticles, Zn (NO ) •6H O (Zinc nitrate hexahydrate) was dispersed in 100 ml of DD water. Subsequently, 0.5 M of sodium sul de (Na 2 S) was added slowly into the above formulation mixture and stirred vigorously for 7 hours. The attained precipitate was collected and subsequently rinsed using ethanol. Finally, washed precipitate was dried at 80 o C for 12 hours in hot air oven. The substance was grinding with a mortar to produce a ne powder.

Synthesis of MgFe 2 O 4 /ZnS nanocomposite
In order to achieve a complete dispersion of nanoparticles, 1 mg of MgFe 2 O 4 NPs was dissolved in 100 ml of ethanol and sonicate for 45 minutes. After that, CTAB (0.15 M) was mixed and vigorously stirred.
Subsequently, Zinc nitrate hexahydrate (0.5 M) salt was gradually mixed with that solution and stirred well. After that, 0.5M of sodium sul de was added gradually and vigorously stirring for 7 hours. Finally, the collected precipitate was rinsed several times with ethanol and dried for 12 hours at 80 o C in a hot air oven.

Characterization of as prepared MgFe 2 O 4 /ZnS NCs
The structural characterization of the synthesized nanomaterials was analyzed using XRD pattern made by Bruker AXS D8 Advance diffractometer. The attained stretching and bending vibrations of prepared MgFe 2 O 4, ZnS and MgFe 2 O 4 /ZnS NCs were recorded using FT-IR (Perkin-Elmer spectrum), in the range of Jasco7800 at the wavelength ranging from 200-800nm. Morphology and topographical view of the synthesized nanomaterials were determined using FESEM analysis (JSM-7600F Japan) equipped with energy dispersive X-ray (XL 30 Philips instruments).

Experimental Setup of Photocatalytic activity
In order to evaluate the photocatalytic function of the prepared nanoparticles and nanocomposites, a commercial photo-reactor unit with built-in re ectors as well as a 200 W tungsten halogen light with range of wavelength from 320 nm to 850 nm was utilized. Organic dyes were taken as sample contaminants, such as Crystal Violet (CV) and Methylene Blue (MB). Each dye was taken by 100 ml of 20 mg/L solution with catalyst (1 mg) was applied to decay the dye solution. The deterioration mechanism of dye solutions was monitored in UV-Visible spectrophotometer by concentration changes at each 20 minutes for up to 2 hours. The photocatalytic degradation ratio (DR) was determined for MB and CV dyes by the given equation Where, C 0 & C t are the initial dye concentration and concentration of dyes at the irradiation time t.

Structural analysis
The Powder X-ray diffraction (PXRD) gives details about the crystallographic structure and phase detection of crystalline materials. The PXRD patterns of as-prepared MgFe 2 O 4, ZnS and MgFe 2 O 4 /ZnS NCs exhibit in Fig. 1 (a-c) [11,12]. The average crystallite size (D) of the prepared materials was evaluated by Scherrer formula [13][14][15].
Where, λ signi es the wavelength of Cu Kα radiation (1.5405Ǻ), β and θ are full width at half maximum of broadened characteristic peaks and Bragg diffraction angle, respectively. The average particle size of assynthesized MgFe 2 O 4, ZnS and MgFe 2 O 4 /ZnS NCs was determined to be 33 nm, 47 nm, and 55 nm. Where, 'A' signi es as constant depending on the type of transition, 'α' and 'hν' signi es as an absorption coe cient and discrete photon energy [19].

Photoluminescence (PL) studies
The PL spectrum is used to study about the charge transfer, charge carrier trapping e ciency, and immigration as well as oxygen vacancies and surface defects. PL spectra demonstrate near band edge emissions peak occur at 359, 362, 368 nm. In addition, blue and green emission peak arising at 412, 415 nm and 577, 583 nm as well as some more blue emission peaks take place at 458 nm and 493 nm could be attributed to the electron move from sulfur vacancies to zinc defects and zinc vacancies for ZnS and MgFe 2 O 4 /ZnS NCs (Fig. 4) [17]. Green emission may generate due to radiative recombination of a photoinduced hole and an electron occupying oxygen vacancy as well as Zn interstitial sites transfer to the deep accepter levels of oxygen vacancies which existence in the ferrite species of MgFe 2 O 4 /ZnS NCs [20].

Morphological Analysis
FESEM analysis is an important tool which provides information about the morphology, topography and composition of the synthesized MgFe 2 O 4 /ZnS NCs. Figure 5 (a-b) shows FESEM images of prepared MgFe 2 O 4 /ZnS NCs material, whereas the synthesized material exhibits ake like structure and average particle size of the MgFe 2 O 4 /ZnS NCs lies between 50 and 100 nm. The EDAX analysis was used to examine the chemical composition of the MgFe 2 O 4 /ZnS NCs and it is reveals in Fig. 5(c). The uniform arrangement of Mg, Zn, Fe, S, and O elements exist in the prepared MgFe 2 O 4 /ZnS NCs.nanocatalysts.

Photocatalytic activity of MgFe 2 O 4, ZnS and MgFe 2 O 4 /ZnS NCs
The photocatalytic activities of the as-prepared samples of MgFe 2 O 4, ZnS and MgFe 2 O 4 /ZnS NCs were evaluated by the degradation of model organic pollutants (MB and CV dyes) under visible-light photocatalytic irradiation, respective photocatalytic results are depicts in Fig. 6 (a & b). The major two absorption peaks are arising at 665 nm and 592 nm correspond to MB and CV dyes. Before illumination, adsorption/desorption experiment was taken under dark condition for 60 min to ensure the equilibrium of the solution. The maximum absorbance intensity peak of MB (665nm) and CV (592 nm) dyes are steadily decreased in 120 min irradiation time by occurrence of MgFe 2 O 4, ZnS and MgFe 2 O 4 /ZnS NCs. In addition, no other new peak was not appear under degradation of both dyes due to absence of photocatalyst there is a negligible amount of MB and CV dyes under visible light illumination. As can be seen from Fig. 7    and effectively reduces e − -h + pair's recombination, which is more helpful to degrading the organic dyes. Therefore, observed result concluded that the present study implying the possibility of its being used in industrial wastewater treatment in the future.  The PL emission spectra of MgFe2O4, ZnS and MgFe2O4/ZnS NCs