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-MgFe2O4–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 MgFe2O4/ZnS NCs were characterized using PXRD, FTIR, UV–Visible spectroscopy, PL and FESEM analysis which reveals the formation of flake like structure with size as to be ~ 50 nm. The photocatalytic experimental result demonstrates that the MgFe2O4/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, MgFe2O4/ZnS NCs has higher photocatalytic performance than that of MgFe2O4, and ZnS due to efficient separation of the photo-induced electron–hole pairs and effective electron–hole generation transfer by the formation of n-MgFe2O4–n-ZnS heterojunction. Hence, photodegradation performance implies that the synthesized MgFe2O4/ZnS NCs can be effectively utilized in waste water purification systems.


Introduction
Interest in research on semiconductor based photocatalysts has attracted much attention in the fields 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, 1 3 have played a crucial role on receiving ceramic powders with preferred shape and size [6].
In  O 19 etc.) has been widely researched as photocatalysts due to their easy magnetic separation, photocorrosion in aqueous solutions, low cost and biocompatibility [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 efficiency 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 efficient separation of photo-exited electron and hole pairs. The photocatalytic activity was investigated by prepared samples of MgFe 2 O 4 , ZnS and MgFe 2 O 4 /ZnS NCs under visible-light illumination by the photodegradation of Methylene Blue (MB) and Crystal Violet (CV) and reported for the first time.

Synthesis of MgFe 2 O 4 Nanoparticles
The utilized chemical reagents were received from Merk and high purity (99.99%). In a chemical co-precipitation process, MgFe 2 O 4 NPs were fabricated, and this step-by-step technique is followed. Mg (NO 3 ) 2 ·6H 2 O (Magnesium (II) nitrate hexahydrate) and Fe (NO 3 ) 3 ·9H 2 O (Ferric (III) nitrate nonahydrate) are taken and gradually mixed in 200 ml of DD water in a 1:2 ratio. The solvent was stirred to produce a homogeneous solution for 30 min. After that, diluted NaOH (4 M in 20 ml of DD water) aqueous solution was added to the above homogenous solution and stirring for 7 h. Finally, precipitate was obtained and filtered through filter paper using ethanol as well as dried in hot air oven at 80 °C for 12 h. The dried particles are smashed and calcined at 700 °C in the muffle furnace for 3 h.

Synthesis of ZnS Nanoparticles
To achieve ZnS nanoparticles, Zn (NO 3 ) 2 ·6H 2 O (Zinc nitrate hexahydrate) was dispersed in 100 ml of DD water. After that, 0.5 M of sodium sulfide (Na 2 S) was added slowly into the above formulation mixture and stirred vigorously for 7 h. The attained precipitate was collected and subsequently rinsed using ethanol. Finally, washed precipitate was dried at 80 °C for 12 h in hot air oven. The substance was grinding with a mortar to produce a fine 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 min. 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, 0.5 M of sodium sulfide was added gradually and vigorously stirring for 7 h. Finally, the collected precipitate was rinsed several times with ethanol and dried for 12 h at 80 °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 400-4000 cm −1 . UV-Visible absorption spectrum of the synthesized nanoparticles were analyzed using Jasco7800 at the wavelength ranging from 200 to 800 nm. 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 reflectors as well as a 200 W tungsten halogen light with range of wavelength from 320 to 850 nm was utilized. Organic dyes were taken as sample contaminants, such as CV and 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 min for up to 2 h. The photocatalytic degradation ratio (DR) was determined for MB and CV dyes by the given equation where C 0 and C t are the initial dye concentration and concentration of dyes at the irradiation time t.
where λ signifies the wavelength of the incident X-ray beam, k is a shape factor, β and θ are full width at half maximum of broadened characteristic peaks and Bragg diffraction angle, respectively [17]. The average particle size of as-synthesized MgFe 2 O 4 , ZnS and MgFe 2 O 4 /ZnS NCs was determined to be 55 nm, 33 nm, and 47 nm.  [7,18]. The peak at 654 cm −1 is attributed owing to symmetric bending vibration of ZnS [19].

Optical Absorption Studies
The UV-Vis absorption spectrum of the synthesized MgFe 2 O 4 , ZnS and MgFe 2 O 4 /ZnS NCs were shown in Fig. 3. The ZnS respond strong UV-light absorption (200-450 nm) and MgFe 2 O 4 act as more visible light region (600-700 nm) [19,20]. The energy band gap (Eg) of the prepared materials was estimated based on the optical absorption edge attained from the UV-Visible absorption spectra by Tauc's relation: where 'A' signifies as constant depending on the type of transition, 'α' and 'hν' signifies as an absorption coefficient and discrete photon energy [21].

Photoluminescence (PL) Studies
The PL spectrum is used to study about the charge transfer, charge carrier trapping efficiency, 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) [19]. Green emission may generate due to radiative recombination of a photo-induced 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 [22].

BET Analysis of MgFe 2 O 4 /ZnS NCs
The BET analysis is used to examine the presence of nitrogen adsorption and desorption of the synthesized material.    [23,24]. Meanwhile, photo-excited holes in VB of ZnS can simply response with surface-bound hydroxyl groups (H 2 O) to generate a · OH radicals [23]. Therefore, superoxide anions and OH radical's are plays a major role for degradation of MB and CV dyes. As can be seen from Fig. 9a and b /ZnS nanocatalyst for degradation of MB and CV dyes obeys the pseudo-first-order reaction kinetics according to Langmuir-Hinshelwood model and its expression is as follows: where C 0 and C t are the initial dye concentration and concentration of dyes at the irradiation time t. Figure 9c and d exhibits the variation of ln (C t /C o ) with time [24]. From  Table 1. As well as, comparative photocatalytic degradation efficiency with different nanocatalysts were shown in Table 2.

Active Species Scavenger Experiment
The active species are act as a crucial role in the dye degradation process. Hence, O2¯ anions scavenger and · OH radicals scavenger was studied by p-benzoquinone (p-BQ) and isopropyl alcohol (IPA). Figure 10 shows active species scavenger experiment for MB and CV. In this experimental studies, 3 ppm concentration of p-BQ and IPA were added to the photocatalytic reaction as O2 ·− (CB) and h + (VB) This result demonstrated that the e − doesn't play a major role in the photobleaching process. However, degradation percentage of dyes was drastically reduced for IPA scavenger compare to p-BQ. Therefore, it is concluded that the large number of · OH radicals were generated by h + when MgFe 2 O 4 /ZnS NCs was irradiated under visible-light and played an important role in the degradation of the dyes active species (Fig. 11).

Stability of MgFe 2 O 4 /ZnS NCs
The degradation rates are tabulated in Table 3 for MB and CV dyes. In the reusability of MgFe 2 O 4 /ZnS NCs, degradation efficiency was slightly decreased in after 4th consecutive rounds respectively for MB and CV dyes which were shown in Fig. 10   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.