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Article

Integration of Mn-ZnFe2O4 with S-g-C3N4 for Boosting Spatial Charge Generation and Separation as an Efficient Photocatalyst

by
Mohsin Javed
1,
Waleed Bin Khalid
1,
Shahid Iqbal
2,*,
Muhammad Azam Qamar
1,
Hamad Alrbyawi
3,
Nasser S. Awwad
4,
Hala A. Ibrahium
5,6,
Murefah Mana Al-Anazy
7,
Eslam B. Elkaeed
8,
Rami Adel Pashameah
9,
Eman Alzahrani
10 and
Abd-ElAziem Farouk
11
1
Department of Chemistry, School of Science, University of Management and Technology, Lahore 54770, Pakistan
2
Department of Chemistry, School of Natural Sciences (SNS), National University of Science and Technology (NUST), H-12, Islamabad 46000, Pakistan
3
Pharmaceutics and Pharmaceutical Technology Department, College of Pharmacy, Taibah University, Medina 42353, Saudi Arabia
4
Chemistry Department, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
5
Biology Department, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
6
Department of Semi Pilot Plant, Nuclear Materials Authority, El Maadi P.O. Box 530, Egypt
7
Department of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, P.O. Box 84428, Riyadh 11671, Saudi Arabia
8
Department of Pharmaceutical Sciences, College of Pharmacy, AlMaarefa University, Riyadh 13713, Saudi Arabia
9
Department of Chemistry, Faculty of Applied Science, Umm Al-Qura University, Makkah 24230, Saudi Arabia
10
Department of Chemistry, College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
11
Department of Biotechnology College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
 
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Author to whom correspondence should be addressed.
Molecules 2022, 27(20), 6925; https://doi.org/10.3390/molecules27206925
Submission received: 18 September 2022 / Revised: 5 October 2022 / Accepted: 12 October 2022 / Published: 15 October 2022
(This article belongs to the Special Issue Preparation of Innovative Nanomaterials for Water/Air Purification)
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Abstract

:
The disposal of dyes and organic matter into water bodies has become a significant source of pollution, posing health risks to humans worldwide. With rising water demands and dwindling supplies, these harmful compounds must be isolated from wastewater and kept out of the aquatic environment. In the research presented here, hydrothermal synthesis of manganese-doped zinc ferrites’ (Mn-ZnFe2O4) nanoparticles (NPs) and their nanocomposites (NCs) with sulfur-doped graphitic carbon nitride (Mn-ZnFe2O4/S-g-C3N4) are described. The samples’ morphological, structural, and bonding features were investigated using SEM, XRD, and FTIR techniques. A two-phase photocatalytic degradation study of (0.5, 1, 3, 5, 7, 9, and 11 wt.%) Mn-doped ZnFe2O4 NPs and Mn-ZnFe2O4/(10, 30, 50, 60, and 70 wt.%) S-g-C3N4 NCs against MB was carried out to find the photocatalyst with maximum efficiency. The 9% Mn-ZnFe2O4 NPs and Mn-ZnFe2O4/50% S-g-C3N4 NCs exhibited the best photocatalyst efficiency in phase one and phased two, respectively. The enhanced photocatalytic activity of the Mn-ZnFe2O4/50% S-g-C3N4 NCs could be attributed to synergistic interactions at the Mn-ZnFe2O4/50% S-g-C3N4 NCs interface that resulted in a more effective transfer and separation of photo-induced charges. Therefore, it is efficient, affordable, and ecologically secure to modify ZnFe2O4 by doping with Mn and homogenizing with S-g-C3N4. As a result, our current research suggests that the synthetic ternary hybrid Mn-ZnFe2O4/50% S-g-C3N4 NCs may be an effective photocatalytic system for degrading organic pollutants from wastewater.

1. Introduction

Water contamination has attracted much attention from researchers since dye emissions from various sectors pose dangers to the public’s health and environment. Organic compounds, dissolved and suspended particles, and heavy metals are present in the complex effluents produced by the printing and textile dyeing industries [1]. About 15–50% of the azo dyes used during the dyeing process do not attach to the fabric and are washed away with the wastewater. This effluent is employed in irrigation processes, although it is terrible for crop growth and germination. Methylene blue (MB) is a basic cationic dye used in different sectors. It makes people more prone to cyanosis, tissue necrosis, shock, jaundice, vomiting, and a faster heartbeat. [2,3]. The wastewater must, therefore, unquestionably be cleaned of these colors. To treat contaminated effluents, a variety of chemical, biological, and physical methods have been used [4]. One of the green technologies for the treatment of industrial wastewater is the photocatalytic destruction of organic contaminants. Due to its incredible effectiveness and low cost, scientists have shown that photocatalytic decomposition is a suitable alternative strategy for the effective decomposition of pollutants [5,6]. The employment of alternative techniques to eliminate by-products is not required in photocatalysis procedures. Numerous nanostructured semiconductor photocatalysts have been explored, including TiO2, ZnO, CuO, NiS, SnO2, ZrO2, and WO3. Due to their significant band gaps and quick electron-hole recombination, these photocatalysts have limitations in visible-light photocatalysis [7,8].
Therefore, creating innovative visible-light-induced photocatalysts with higher activity has been a hot topic for a long time. Due to the superior cation and anion adsorption abilities, low band gap energy, and reduced electron-hole recombination, ferrite (Fe2O4) nanoparticles have particular relevance among photocatalysts [9]. These ferrite NPs have prospective applications in wastewater treatment, biomedicine, electrical devices, energy storage, EMR shielding, and the recording medium. They have a band gap of about 1.9 eV and are highly stable. They are also less harmful and inexpensive, have strong electronic conduction, are recyclable, and are environmentally benign. Their photocatalytic activity is unfortunately constrained for practical use by low quantum efficiency [9,10,11].
Many studies have shown that doping ZnFe2O4 with appropriate metal ions and combining it with a suitable semiconductor material improve optical and photocatalytic characteristics. Patil et al. used the co-precipitation approach to manufacture Gd3+-doped ZnFe2O4 nanoparticles, demonstrating enhanced MB degradation of roughly 99% compared to pure ZnFe2O4 (95% degradation in 240 min.) [12]. According to Ajithkumar et al., yttrium-doped zinc ferrite made by solution combustion showed 95% MB degradation in 180 min. The photocatalytic efficiency of Y-doped ZnFe2O4 is greater than that of pure zinc ferrite [13]. Compared to ZnFe2O4, cobalt-doped zinc ferrite more effectively oxidized methylene blue under visible light. Numerous studies have found that ZnFe2O4 has a finite band gap energy and, as a result, may combine with S-g-C3N4 to create an efficient heterojunction [14]. Similarly, Savunthari et al. constructed (Cu, Bi) codoped ZnFe2O4 nanoparticles via the solution combustion method. The codoped NPs showed an enhanced degradation of bisphenol A compared to undoped NPs [15].
The g-C3N4 semiconductor has demonstrated remarkable photocatalytic competency under visible light due to its advantageous traits, such as excellent stability and a lowered band gap energy that boosts its capacity to absorb visible radiations [16,17]. However, the rapid recombination of photoinduced e−/h+ pairs in g-C3N4 makes it inappropriate for use as a photocatalyst [18]. Consequently, many attempts have been undertaken to overcome this constraint, including vacancy, heterojunction creation, and combining the g-C3N4 with other metal oxides and nonmetals such as sulphur [19,20,21,22]. S-doping modifies the band gap of g-C3N4 and improves the mobility and separation of the e-h pairs by stacking its 2p orbitals on the VB of bulk g-C3N4 [23]. A simple molten salt approach was successfully used by Keke et al. to produce sulfur-doped g-C3N4. The photocatalytic performance of S-doped g-C3N4 toward methylene blue and tetracycline was 10 and 20 times that of bulk g-C3N4 [24]. Similarly, Xin et al. successfully synthesized highly active S-doped g-C3N4 by employing thiourea and melamine as the precursors. The synthesized S-doped g-C3N4 exhibited remarkable photocatalytic efficacy against rhodamine B (RhB) [25]. Basaleh used soft and hard templates to create ZnFe2O4/S-g-C3N4. The photocatalytic efficiency of ZnFe2O4/S-g-C3N4 against acridine orange was 4.4 and 6.3 fold that of ZnFe2O4 and bulk g-C3N4, respectively [26].
Thus, owing to the improved charge separation abilities, it is suggested to produce a metal-ZnFe2O4/S-g-C3N4 heterojunction to realize a significant photocatalytic performance. In this study, hybrid Mn-ZnFe2O4/S-g-C3N4 nanocomposites were synthesized successfully via a surfactant (PEG)-assisted hydrothermal process, and its efficiency for removing MB under sunlight was investigated. In step one, the manganese-doped zinc ferrite (Mn-ZnFe2O4) nanoparticles were synthesized with varying chromium percentages (0.5, 1, 3, 5, 7, 9, and 11 wt.%). The effect of Mn2+ substitution on the photocatalytic properties of zinc ferrite was observed. The 9% Mn-ZnFe2O4 sample manifested the best absorption of solar light and degradation efficiency. In step two, the 9% Mn-ZnFe2O4 nanoparticles were homogenized with diverse concentrations of S-g-C3N4 (10, 30, 50, and 70 wt.%) to produce Mn-ZnFe2O4/S-g-C3N4 with enhanced photocatalytic activity. The 9% Mn-ZnFe2O4/50% S-g-C3N4 nanocomposite executed the best photocatalytic activity compared to pure ZnFe2O4, 9% Mn-ZnFe2O4, and S-g-C3N4. The results depicted that the enhanced photocatalytic activity of the 9% Mn-ZnFe2O4/50% S-g-C3N4 nanocomposite was because of the improved absorption of sunlight and better separation of e-/h+ pairs between Mn-ZnFe2O4 and S-g-C3N4. To our knowledge, the synthesis of Mn-ZnFe2O4/S-g-C3N4 heterojunctions via the hydrothermal approach has never been used.

2. Experimental

2.1. Chemicals

Thiourea (CH4N2S), polyvinyl pyrrolidone, methylene blue (C16H18ClN3S), zinc sulphate heptahydrate (ZnSO4·7H2O), iron (III) chloride anhydrous (FeCl3), manganese (II) chloride (MnCl2), and sodium sydroxide (NaOH) were acquired from Merck and used.

2.2. Synthesis of Chromium-Doped Zinc Ferrites

A series of Mn-doped zinc ferrites (Mn-ZnFe2O4) with varying manganese percentages (0.5, 1, 3, 5, 7, and 9 wt.%) were produced using a surfactant-assisted hydrothermal method [6]. For the preparation of 0.5% Mn-ZnFe2O4, three solutions, A, B, and C, were made preceding the synthesis. The solutions, A, B, and C, were made by dispersing 0.010 g of MnCl2·4H2O, 1.865 g of ZnCl2·7H2O, and 2.435 g of FeCl3 in 50 mL of deionized water. Then, these solutions (A, B, and C) were intermixed, and 10 mL of PVP was added as a surfactant to prevent the aggregation of nanoparticles. The pH of the solution was adjusted to 11 by adding NaOH; next, the suspension was loaded to a Teflon-lined autoclave. The autoclave was heated to 175 °C for 10 h and then cooled to ambient temperature. The resultant precipitates were separated by filtering, rinsed with deionized water and pure ethanol, and then dried at 85 °C in an oven. Other percentages of Mn-ZnFe2O4 (0, 1, 3, 5, 7, and 9 wt.%) were also synthesized using the same method [27].

2.3. Synthesis of S-g-C3N4

Thiourea was heated to 560 °C for 6 h at a rate of 5 °C per minute in a muffle furnace to form S-g-C3N4. After cooling to ambient temperature, the resulting yellowish S-g-C3N4 was stored [17,28].

2.4. Synthesis of Mn-ZnFe2O4/S-g-C3N4

Using a surfactant-assisted hydrothermal technique, 9% Mn-ZnFe2O4 was combined with various amounts of S-g-C3N4 (10, 30, 50, 60, and 70 wt.%) to produce a range of Mn-ZnFe2O4/S-g-C3N4 nanocomposites. For the preparation of 9% Mn-ZnFe2O4/10%S-g-C3N4, firstly, four solutions, A, B, C, and D, were made by dispersing 0.185 g of manganese chloride in 50 mL of water (Solution A), 1.706 g of zinc chloride in 50 mL of water (Solution B), 2.435 g of FeCl3 in 50 mL of water (Solution C), and 0.18 g of S-g-C3N4 in 50 mL of water (Solution D). Then, three solutions, A, B, and C, were added to solution D and homogenized for 1 h along with the addition of 10 mL of PVP as a surfactant. The following steps were the same as for the synthesis of Mn-ZnFe2O4 NPs. Moreover, the same process was repeated to synthesize the 9% Mn-ZnFe2O4/S-g-C3N4 containing 30, 50, 60, and 70 wt.% of S-g-C3N4. The schematic diagram (Scheme 1) depicts the synthesis procedure for Mn-ZnFe2O4/S-g-C3N4 NCs, and Table 1 lists the precise composition.

2.5. Photocatalytic Activity

The photocatalytic activity of zinc ferrites, Mn-doped zinc ferrites, sulphur-doped graphitic carbon nitride, and nanocomposites was studied under solar light irradiation. The aqueous solution of an organic dye, methylene blue, was used as the standard contaminant. Then, 0.2 g of each photocatalyst was added to the beaker in which 100 mL of MB solution (10 mg L−1) was added and allowed to stir in the dark for about 45 min to attain the adsorption–desorption equilibrium. After that, the suspension was positioned under solar light in an open atmosphere, and aliquots of 5 mL were collected after every 30 min. A UV-Vis spectrophotometer was used to evaluate the photocatalytic activity of the collected samples after centrifugation [29].

2.6. Characterization

The structure of the synthesized catalysts was determined by applying XRD (Bruker AXS, D8-S4, Madison WI, USA) using Cu Kα radiation (k = 1.54056 Å) at 40 kV and 30 mA at room temperature, whereas elemental content and morphology were found using SEM-EDS (Hitachi, S-4800, Tokyo, Japan). The UV-visible and photocatalytic absorption spectra were measured using a UV-Vis-NIR spectrophotometer (UV-770, Jasco, Tokyo, Japan) from 800 nm to 200 nm wavelengths. Using a transmission electron microscope, the surface morphologies of the photocatalysts were examined (TEM, JEOL-JEM-1230, Peabody, MA, USA). FTIR spectrometers measured functional groups in the 4000–400 cm−1 range with a resolution of 1 cm−1 (Perkin 400 FTIR, Waltham, MA, USA).

3. Results and Discussion

3.1. TEM and EDX Analyses

The size, shape, and distribution of the particles and the elemental components of the as-prepared materials were disclosed by TEM, SEM, and EDX spectra. Due to their large surface area due to their nanoscale size and solid magnetic interaction, undoped ZnFe2O4 and Mn-doped ZnFe2O4 nanoparticles were observed to be crystalline and included agglomerated, irregular, spherical nanoparticles with an average size of 20–38 nm (Figure 1a,b). The proper exposure to different hosts made possible by the efficient synthesis of Mn-ZnFe2O4 at the nanoscale also guaranteed the single-domain character of the particles for the increase in magnetic remanence and decrease in magnetic coercivity. The S-g-C3N4 specimen had some creases and a beehive-like shape (Figure 1c,d). This supramolecular complex was synthesized using a mixture of three different precursors, and the pyrolysis of the complex resulted in the formation of wrinkles. These wrinkles facilitated high interfacial contact between S-g-C3N4 and other components, which improved the transfer and separation of charge carriers in the NCs. The 9% Mn-ZnFe2O4/50% S-g-C3N4 NCs’ TEM pictures showed that several Mn-ZnFe2O4 nanoparticles seemed to be incorporated in the S-g-C3N4 matrix. As shown in Figure 1e, it was clear that S-g-C3N4 had firmly encircled several Mn-ZnFe2O4 NPs. The 9% Mn-ZnFe2O4/50% S-g-C3N4 NCs had sheet-like clusters with O, Zn, Fe, C, N, and Mn as their primary components, according to EDX mapping in Figure 1f, for the 9% Mn-ZnFe2O4/50 % S-g-C3N4 binary hybrid photocatalyst.

3.2. FTIR Analysis

In the wave number range of 4000–450 cm−1, the FTIR spectra of the produced photocatalysts were compared (Figure 2). The observed FTIR spectrum of ZnFe2O4 confirmed the presence of the M-O bond at 879 cm−1 and the absence of all functional groups. The peak at 3335 cm−1 was due to the O-H bond stretching while the peak at 1696 cm−1 was due to the bending of the O-H bond (Figure 2a) [5]. When the FTIR spectrum of 9% Mn-ZnFe2O4 was compared to pure ZnFe2O4, there was little difference in the peak positions. The effective production of Mn-doped ZnFe2O4 was confirmed by the modest shift in peak positions (Figure 2b) [30,31]. The FTIR spectrum of S-g-C3N4 exhibited a broad band at 3178 cm−1 due to O-H bond stretching; the peaks between 1500 and 2000 cm−1 were due to the stretching vibrations of C=N, and peaks between 1500 and 1000 cm−1 were due to C-N bond stretching, as shown in Figure 2c [32]. A peak at 805 cm−1 indicated the triazine unit. The FTIR spectrum of the composite contained the peaks corresponding to both Mn-ZnFe2O4 and S-gC3N4, signifying that the Mn-ZnFe2O4/S-g-C3N4 composite was formed successfully (Figure 2d) [33,34,35,36].

3.3. XRD Analysis

The X-ray diffractogram of the samples of ZnFe2O4, 9% Mn-ZnFe2O4, S-g-C3N4, and 9% Mn-ZnFe2O4/50% S-g-C3N4 is shown in Figure 3. In the XRD spectra of pure ZnFe2O4, seven peaks were observed with crystal facets (220), (311), (400), (422), (511), (440), and (533) at 2θ = 30°, 34.8°, 42.6°, 53°, 56°, 61.8°, and 73.2°, which fitted well with the pattern of standard zinc ferrites with JCPDS file 22-1012 [12,30,31,37,38]. The distinctive peaks of zinc ferrites in the XRD spectra of Mn-ZnFe2O4 showed that the structure of zinc ferrite does not significantly alter when Mn metal is doped into it. Two characteristic peaks were detected in the XRD pattern of S-g-C3N4; the crystal plane (002) was attributed to the interlayer assembling of aromatic systems, and the plane (100) was ascribed to the inter-planar arrangement of aromatic systems (JCPDS file # 00-087-1526) [39,40]. The crystal phase of Mn-ZnFe2O4 remained intact after coupling with S-g-C3N4, and the (002) crystal plane of S-g-C3N4 was indicated in the composite systems. In 9%Mn-ZnFe2O4/50% S-g-C3N4 composites, owing to the high crystallinity of Mn-ZnFe2O4, the characteristic peaks of Mn-ZnFe2O4 were prominent. The emergence of the distinct peaks of Mn-ZnFe2O4 and S-g-C3N4 in composites demonstrated the successful production of Mn-ZnFe2O4/S-g-C3N4 composites [34,35,41,42].

3.4. Photocatalytic Degradation Study

The photocatalytic activity of synthesized samples was detected in two phases. The photocatalytic activities of ZnFe2O4 and Mn-ZnFe2O4 NPs were first examined in the presence of sunlight using an aqueous methylene blue solution (Figure 4a). The rate of dye disintegration was monitored using a UV-Vis spectrophotometer with a wavelength range of 200–800 nm (Figure 4a). According to the degradation contours (Figure 4b) and percentile degradation graphs (Figure 4c), when we increased the Mn+2 doping (0.5 to 9 wt.%), there was a gradual improvement in the photocatalytic activity of Mn-doped zinc ferrite nanoparticles. Since the Mn+2 doping decreased the band gap of ZnFe2O4 and facilitated the enhanced generation of the e−/h+ pair, the photocatalytic efficiency of Mn-ZnFe2O4 was better than ZnFe2O4. It was observed that the insertion of various metal ions into the pure ferrite can affect its optical and structural properties, which can enhance the ferrite’s photocatalytic ability [43]. When the concentration of doped Mn+2 ions was increased beyond this point (<9 wt.%), the photocatalytic activity of MnxZn1−xFe2O4 NPs was reduced (Figure 5a,b). This Mn (9 wt.%) is the optimum doping concentration for Mn-ZnFe2O4 NPs. The observed degradation efficiencies of Mn-ZnFe2O4 catalysts with different manganese concentrations (0, 0.5, 1, 3, 5, 7, and 9 wt.%) were 71%, 78%, 81%, 86%, 92%, 91%, 95%, and 89%, respectively, after 210 min of sunlight irradiation. Thus, the 9% Mn-ZnFe2O4 NPs exhibited the maximum photocatalytic efficiency compared to other nanoparticles (Figure 4c).
In the next phase, the Mn-ZnFe2O4/S-g-C3N4 NCs were produced by mixing 9% Mn-ZnFe2O4 NPs with diverse amounts of S-g-C3N4 (as given in Table 1. Then, the photocatalytic activity of the produced NCs was checked every 15 min. The samples were placed in the dark to establish adsorption–desorption equilibrium between the dye and the fabricated NCs before sunlight exposure, as described by Mudassar et al. [44]. According to Figure 5b, the photocatalysts absorbed modest amounts of MB. After that, the samples were exposed to sunlight, and the 9% Mn-ZnFe2O4/50S-g-C3N4 NCs exhibited the largest dye degradation relative to the other samples (Figure 5a). It was evident from the degradation contours (Figure 5a) and percent degradation plots (Figure 5b) that the dye degradation increased with an increasing S-g-C3N4 concentration in the Mn-ZnFe2O4/S-g-C3N4 NCs up to 50% and then dropped for ZnFe2O4/S-g-C3N4 NCs containing S-g-C3N4 contents >50%. After 120 min of exposure to sunlight, the measured degradation efficiencies of S-g-C3N4 and 9% Mn-ZnFe2O4/(0, 10, 30, 50, 60, and 70 wt.%) S-g-C3N4 NCs were 51%, 54%, 65%, 89%, 100%, and 85%, respectively. Better charge separation and transportation via Mn-ZnFe2O4 and S-g-C3N4 coupling, as well as improved visible light absorption due to Mn doping in ZnFe2O4, might be responsible for the enhanced photocatalytic efficiency of 9% Mn-ZnFe2O4/50% S-g-C3N4 NCs [41,42,45]. Figure 5b depicts the percentage of photocatalytic degradation of MB by the corresponding NCs.
The Langmuir–Hinshelwood model was applied to understand the kinetics of Mn-ZnFe2O4 (Supplementary Materials: Figure S1) and Mn-ZnFe2O4/S-g-C3N4 NCs, and Figure 5c shows the resultant graph [38]. Figure 5c exhibits that the dye degradation by the NCs under sunlight was fit to pseudo-first-order kinetics. The rate constant (k) values of Mn-ZnFe2O4/S-g-C3N4 NCs are summarized in Table 2.
Therefore, the highest and lowest calculated “k” values were found for ZnFe2O4/50% S-g-C3N4 NCs (0.0142 min−1) and S-g-C3N4 (0.00515 min−1), respectively. The ZnFe2O4/50% S-g-C3N4 NCs completely decolorized the MB in 120 min, and its “k” value was 2.1 and 2.75 times more than that of ZnFe2O4 and S-g-C3N4, respectively. The rate of dye degradation increased as the concentration of S-g-C3N4 increased from 10% to 50% in ZnFe2O4/50% S-g-C3N4 NCs but then declined for NCs with a greater concentration of S-g-C3N4 (<50%). Accordingly, the observed optimal concentration for ZnFe2O4/S-g-C3N4 NCs is 50% S-g-C3N4. A further rise in the concentration of S-g-C3N4 could result in the formation of e-h pair combination centers, which would gradually reduce the photocatalytic efficiency of NCs [32,44]. A preliminary study is required to conduct a more in-depth analysis of this justification. According to Table 3, the photocatalytic efficiency of ZnFe2O4/50% S-g-C3N4 NC was noticeably superior to that of several previously reported studies [46,47,48]. The better photocatalytic efficiency of the NC might be due to the development of good heterojunctions between ZnFe2O4 and S-g-C3N4 as compared to the previously reported composites. The Mn atoms may also facilitate the transportation and separation of the e−/h+ in the composite [21,28,29]. Because the ZnFe2O4/50% S-g-C3N4 NC was the most efficient photocatalyst, it was used in the recycling study.
The produced Mn-ZnFe2O4/50% S-g-C3N4 NCs are shown in Figure 6a throughout their stability cycles and photocatalyst reuse. The recycling and stability test was used to gauge how well the material as prepared performed in real-world applications. Six sequential cycles of MB dye degradation under direct sunshine irradiation were used to test the photocatalyst’s stability. After six cycles of the photocatalytic MB elimination test, Mn-ZnFe2O4/50% S-g-C3N4 NCs’ crystallographic alterations were also examined using XRD, and the relevant findings are shown in Figure 6b. The stability of the crystal structure of Mn-ZnFe2O4/50% S-g-C3N4 NCs is shown by the lack of noticeable change in the XRD curves before and after the photocatalytic test, as shown in Figure 6b.
The improvements in the Mn-ZnFe2O4/50% S-g-C3N4 photocatalyst’s photocatalytic MB removal ability were further examined using Figure 6c. The transient photocurrent responses of the ready photocatalysts are shown in Figure 6c. The law of transient photocurrent responses and the photocatalytic MB elimination efficiency of the various photocatalysts may be compared using the photocatalytic test results shown in Figure 6c. The photocurrent density of Mn-ZnFe2O4/50% S-g-C3N4 NCs is was highest, indicating the material’s best optical responsiveness and output capacity of photogenerated carriers during the photocatalytic reaction. ZnFe2O4, Mn-ZnFe2O4, and Mn-ZnFe2O4/50% S-g-C3N4 NCs’ EIS spectra are shown in Figure 6d. The outstanding photogenerated carrier migration and separation efficiency of Mn-ZnFe2O4/50% S-g-C3N4 NCs is shown by the fact that their EIS curve’s arc is noticeably lower than that of ZnFe2O4. Furthermore, the PL was measured using a 330 nm excitation wavelength to assess the charge transfer and separation effectiveness of ZnFe2O4, Mn-ZnFe2O4, and Mn-ZnFe2O4/S-g-C3N4 heterojunctions, as shown in Figure S3.

3.5. Scavenging Activity

The primary active species for the decomposition of organic pollutants in water are typically superoxide radicals (•O2), hydroxyl radicals (•OH), and photogenerated holes (h+) [46]. Therefore, the active species-generating capacity was examined by incorporating the appropriate scavengers into the suspensions of the MB degradation in the presence of the Mn-ZnFe2O4/50% S-g-C3N4 photocatalyst. In particular, isopropanol (IPA) was specifically utilized to trap •OH, EDTA-2Na to trap holes (h+), and benzoquinone (BQ) to trap •O2. After adding benzoquinone, the efficiency of MB degradation was lowered by 90%. IPA and EDTA-2Na, on the other hand, only suppressed the degradation rates of MB by 61% and 37%, respectively. The effect of trapping chemicals on the dye degradation reaction is depicted in Figure 7. The outcomes demonstrated that •OH and •O−2 are the main reactive species involved in photocatalytic dye degradation rather than the holes (h+).

4. Photocatalytic Degradation Mechanism

The accelerated degradation of methylene blue by photocatalysts may be attributed to the production of e−/h+ pairs in the synthesized photocatalysts as predicted via a schematic sketch (Figure 8). Both Mn-ZnFe2O4 and S-g-C3N4 were excited and e−/h+ pairs were produced on their respective conduction bands (CB) and valence bands (VB) when solar light was irradiated on Mn-ZnFe2O4/S-g-C3N4 [47]. Based on the CB/VB edge potentials, the photo-induced electrons easily migrated from the conduction band (CB) of Mn-ZnFe2O4 to the CB of S-g-C3N4 since the CB of Mn-ZnFe2O4 was lower in potential than that of S-g-C3N4. In addition, the holes that were created in the VB of S-g-C3N4 had the potential to migrate to Mn-ZnFe2O4 [48]. In the hybrid composite, the presence of Mn atoms not only lowered the value of Eg but also served as the facilitator for the transit of electrons from S-g-C3N4 to ZnFe2O4. Therefore, by increasing the separation of photoexcited e−/h+ pairs, doping could significantly lower the chance of charge recombination. The produced e−/h+ pairs combined with the oxygen and water molecules taken up on the surface of the photocatalyst to generate (•OH and •O−2) the reactive oxygen species (ROS) [1]. Then, the generated radicals were consumed in the degradation of MB via an oxidative mechanism.

5. Conclusions

In summary, ZnFe2O4, Mn-ZnFe2O4 nanoparticles, and a series of Mn-ZnFe2O4/S-g-C3N4 nanocomposites were developed via a straightforward hydrothermal technique. XRD, TEM, EDX, and FTIR techniques were used to investigate the structure and purity of the samples. The degradation of MB at room temperature was carried out using ZnFe2O4, Mn-ZnFe2O4, and Mn-ZnFe2O4/S-g-C3N4. The synthesized samples were tested photocatalytically against MB, and it was discovered that the 9%Mn-ZnFe2O4/50% S-g-C3N4 had a very high catalytic efficiency. It was established through the radical scavenging experiment that the 9%Mn-ZnFe2O4/50% S-g-C3N4 utilized electrons, holes, and ROS for MB degradation. For six sequential catalytic cycles, the nanocomposites shown exceptional stability and continuously high levels of MB degradation. The separation and mobility of photoinduced e−/h+ pairs in 9%Mn-ZnFe2O4/50% S-g-C3N4 may be considerably enhanced by the fine interfaces produced and the synergistic effect between Mn and ZnO as supported by transient photocurrent responses. Both for NPs and NCs, it was found that a rate constant for the dye reduction reaction was pseudo-first order. As a result, the 9% Mn-ZnFe2O4/50% S-g-C3N4 heterojunction is a promising contender and may find use in the photocatalytic destruction of organic pollutants to purify water.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27206925/s1.

Author Contributions

Conceptualization, writing-original draft Preparation, M.J., W.B.K. and S.I.; methodology, writing-original draft Preparation, M.A.Q. and H.A.; software, writing review and editing, funding, N.S.A. and H.A.I.; validation, project administration, critical revision, M.M.A.-A., E.B.E. and R.A.P.; writing review and editing, resources, funding, E.A. and A.-E.F. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to express their gratitude to King Khalid University, Saudi Arabia, for providing administrative and technical support. The authors would like to thank the Deanship of Scientific Research at Umm Al-Qura University for supporting this work by Grant Code: (22UQU4320141DSR44). This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2022R7), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors would like to express their gratitude to King Khalid University, Saudi Arabia, for providing administrative and technical support. The authors would like to thank the Deanship of Scientific Research at Umm Al-Qura University for supporting this work by Grant Code: (22UQU4320141DSR44). This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2022R7), Princess Nourah bint Adulrahman University, Riyadh, Saudi Arabia.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 27 06925 sch001 550
Scheme 1. Schematic depiction for the production of Mn-ZnFe2O4/S-g-C3N4.
Scheme 1. Schematic depiction for the production of Mn-ZnFe2O4/S-g-C3N4.
Molecules 27 06925 sch001
Molecules 27 06925 g001 550
Figure 1. TEM profiles of (a) ZnFe2O4, (b) 9% Mn-ZnFe2O4, (c) SEM profile of S-g-C3N4, (d) TEM image of S-g-C3N4, and (e) 9% Mn-ZnFe2O4/50% S-g-C3N4 NCs. (f) EDX of 9% Mn-ZnFe2O4/50% S-g-C3N4 NCs.
Figure 1. TEM profiles of (a) ZnFe2O4, (b) 9% Mn-ZnFe2O4, (c) SEM profile of S-g-C3N4, (d) TEM image of S-g-C3N4, and (e) 9% Mn-ZnFe2O4/50% S-g-C3N4 NCs. (f) EDX of 9% Mn-ZnFe2O4/50% S-g-C3N4 NCs.
Molecules 27 06925 g001
Molecules 27 06925 g002 550
Figure 2. FTIR spectrum of composites of ZnFe2O4 (a), S-g-C3N4 (b), 9% Mn-ZnFe2O4 (c), and 9% Mn-ZnFe2O4/50S-gC3N4 (d).
Figure 2. FTIR spectrum of composites of ZnFe2O4 (a), S-g-C3N4 (b), 9% Mn-ZnFe2O4 (c), and 9% Mn-ZnFe2O4/50S-gC3N4 (d).
Molecules 27 06925 g002
Molecules 27 06925 g003 550
Figure 3. XRD spectrum of ZnFe2O4, 9% Mn-ZnFe2O4, S-g-C3N4, and 9% Mn-ZnFe2O4/50% S-g-C3N4.
Figure 3. XRD spectrum of ZnFe2O4, 9% Mn-ZnFe2O4, S-g-C3N4, and 9% Mn-ZnFe2O4/50% S-g-C3N4.
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Molecules 27 06925 g004 550
Figure 4. Degradation rate (a), degradation contours (b), and % degradation of MB (c) by Mn-ZnFe2O4 NPs.
Figure 4. Degradation rate (a), degradation contours (b), and % degradation of MB (c) by Mn-ZnFe2O4 NPs.
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Molecules 27 06925 g005 550
Figure 5. Photocatalytic degradation rate (a), % degradation (b), and kinetic characteristics of MB by Mn-ZnFe2O4/S-g-C3N4 NCs (c).
Figure 5. Photocatalytic degradation rate (a), % degradation (b), and kinetic characteristics of MB by Mn-ZnFe2O4/S-g-C3N4 NCs (c).
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Molecules 27 06925 g006 550
Figure 6. (a) Cyclic stability of the Mn-ZnFe2O4/50% S-g-C3N4 NCs through the sixth cycle. (b) The structural stability of Mn-ZnFe2O4/50% S-g-C3N4 NCs was determined by comparing XRD patterns obtained before the first cycle and after the sixth recycling experiment. (c) EIS Nyquist plots of ZnFe2O4, Mn-ZnFe2O4, and Mn-ZnFe2O4/50% S-g-C3N4 NCs and (d) transient photocurrent responses of ZnFe2O4, S-g-C3N4, Mn-ZnFe2O4, and Mn-ZnFe2O4/50% S-g-C3N4 NCs in visible-light irradiation.
Figure 6. (a) Cyclic stability of the Mn-ZnFe2O4/50% S-g-C3N4 NCs through the sixth cycle. (b) The structural stability of Mn-ZnFe2O4/50% S-g-C3N4 NCs was determined by comparing XRD patterns obtained before the first cycle and after the sixth recycling experiment. (c) EIS Nyquist plots of ZnFe2O4, Mn-ZnFe2O4, and Mn-ZnFe2O4/50% S-g-C3N4 NCs and (d) transient photocurrent responses of ZnFe2O4, S-g-C3N4, Mn-ZnFe2O4, and Mn-ZnFe2O4/50% S-g-C3N4 NCs in visible-light irradiation.
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Molecules 27 06925 g007 550
Figure 7. Role of radical scavengers in the photocatalytic degradation of MB with Mn-ZnFe2O4/50% S-g-C3N4.
Figure 7. Role of radical scavengers in the photocatalytic degradation of MB with Mn-ZnFe2O4/50% S-g-C3N4.
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Molecules 27 06925 g008 550
Figure 8. A schematic MB sunlight catalytic degradation mechanism over the Mn-ZnFe2O4/S-g-C3N4 NCs.
Figure 8. A schematic MB sunlight catalytic degradation mechanism over the Mn-ZnFe2O4/S-g-C3N4 NCs.
Molecules 27 06925 g008
Table
Table 1. Composition of the synthesized Mn-ZnFe2O4/S-g-C3N4 composites.
Table 1. Composition of the synthesized Mn-ZnFe2O4/S-g-C3N4 composites.
Sr. No.NanocompositesManganese ChlorideZinc ChlorideFerric ChlorideS-Doped g-C3N4
1Mn-ZnFe2O40.185 g1.706 g2.435 g-
2S-g-C3N4---0.52 g
39% Mn-ZnFe2O4/10S-g-C3N40.185 g1.706 g2.435 g0.17 g
49% Mn-ZnFe2O4/30S-g-C3N40.185 g1.706 g2.435 g0.52 g
59% Mn-ZnFe2O4/50S-g-C3N40.185 g1.706 g2.435 g0.87 g
69% Mn-ZnFe2O4/70S-g-C3N40.185 g1.706 g2.435 g0.94 g
Table
Table 2. The rate constant (k) values of the 9% Mn-ZnFe2O4/50% S-g-C3N4 nanocomposites.
Table 2. The rate constant (k) values of the 9% Mn-ZnFe2O4/50% S-g-C3N4 nanocomposites.
Sr. No.NanocompositesS-g-C3N4
(wt.%)		% Degradationk (min−1)
1S-g-C3N4100510.00515
2ZnFe2O4-540.00673
39% Mn-ZnFe2O4/10%S-g-C3N410650.00721
49% Mn-ZnFe2O4/30%S-g-C3N430890.00838
59% Mn-ZnFe2O4/50% S-g-C3N4501000.0142
79% Mn-ZnFe2O4/70% S-g-C3N470850.0096
Table
Table 3. Comparison of photocatalytic efficiency of the Mn-ZnFe2O4/S-g-C3N4 NCs with some previous works.
Table 3. Comparison of photocatalytic efficiency of the Mn-ZnFe2O4/S-g-C3N4 NCs with some previous works.
Sr. No.PhotocatalystContaminantLight SourceRadiation Time (min.)Degradation %Ref.
1ZnFe2O4@metyle celluloseMetronidazoleXe lamp12092.65[49]
2Bi2WO6/CoFe2O4Bisphenol ASolar12092[50]
3ZnNdxFe2−xO4Rhodamine BXe lamp18098[51]
4ZnFe2O4TolueneXe lamp30057.2[52]
5ZnO/Fe3O4/g-C3N4MOVisible15097.87[53]
5Pt-BiFeO3MGSolar24096[54]
7Mn–ZnO/RGORhBVisible14099[55]
8ZnFe2O4@ZnOMOVisible240240[56]
9Mn-ZnFe2O4/S-g-C3N4MBSolar120100Present Work
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Javed, M.; Khalid, W.B.; Iqbal, S.; Qamar, M.A.; Alrbyawi, H.; Awwad, N.S.; Ibrahium, H.A.; Al-Anazy, M.M.; Elkaeed, E.B.; Pashameah, R.A.; et al. Integration of Mn-ZnFe2O4 with S-g-C3N4 for Boosting Spatial Charge Generation and Separation as an Efficient Photocatalyst. Molecules 2022, 27, 6925. https://doi.org/10.3390/molecules27206925

AMA Style

Javed M, Khalid WB, Iqbal S, Qamar MA, Alrbyawi H, Awwad NS, Ibrahium HA, Al-Anazy MM, Elkaeed EB, Pashameah RA, et al. Integration of Mn-ZnFe2O4 with S-g-C3N4 for Boosting Spatial Charge Generation and Separation as an Efficient Photocatalyst. Molecules. 2022; 27(20):6925. https://doi.org/10.3390/molecules27206925

Chicago/Turabian Style

Javed, Mohsin, Waleed Bin Khalid, Shahid Iqbal, Muhammad Azam Qamar, Hamad Alrbyawi, Nasser S. Awwad, Hala A. Ibrahium, Murefah Mana Al-Anazy, Eslam B. Elkaeed, Rami Adel Pashameah, and et al. 2022. "Integration of Mn-ZnFe2O4 with S-g-C3N4 for Boosting Spatial Charge Generation and Separation as an Efficient Photocatalyst" Molecules 27, no. 20: 6925. https://doi.org/10.3390/molecules27206925

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