Fabrication of Cr-ZnFe2O4/S-g-C3N4 Heterojunction Enriched Charge Separation for Sunlight Responsive Photocatalytic Performance and Antibacterial Study

There has been a lot of interest in the manufacture of stable, high-efficiency photocatalysts. In this study, initially Cr doped ZnFe2O4 nanoparticles (NPs) were made via surfactant-assisted hydrothermal technique. Then Cr-ZnFe2O4 NPs were modified by incorporating S-g-C3N4 to enhance their photocatalytic efficiency. The morphological, structural, and bonding aspects were analyzed by XRD, FTIR, and SEM techniques. The photocatalytic efficiency of the functional Cr-ZnFe2O4/S-g-C3N4 (ZFG) heterostructure photocatalysts was examined against MB under sunlight. The produced ZFG-50 composite has the best photocatalytic performance, which is 2.4 and 3.5 times better than that of ZnFe2O4 and S-g-C3N4, respectively. Experiments revealed that the enhanced photocatalytic activity of the ZFG nanocomposite was caused by a more effective transfer and separation of photo-induced charges. The ZFG photocatalyst can use sunlight for treating polluted water, and the proposed modification of ZnFe2O4 using Cr and S-g-C3N4 is efficient, affordable, and environmentally benign. Under visible light, Gram-positive and Gram-negative bacteria were employed to ZFG-50 NCs’ antimicrobial activity. These ZFG-50 NCs also exhibit excellent antibacterial potential.


Introduction
Pollution is one of the most serious dangers that humans face. Pollution of various aquatic ecosystems is the most widespread type that has a significant impact on living things. Industrial dyes are known to be dangerous to people, especially when soluble in water [1]. Methylene blue (MB) dye is one of the most popular dyes that pollute aquatic habitats. Therefore, developing a good method for the cleanup of wastewater is critical. To remove organic dyes from wastewater, many methods have been used, including biodegradation, adsorption, filtering, sedimentation, and coagulation [2,3]. However, these procedures did not produce good results in terms of dye degradation. Scientists have demonstrated that photocatalytic decomposition is an appropriate alternative technique for the enhanced decomposition of numerous contaminants due to its high efficiency and low cost. Moreover, endorsing photocatalysis does not necessitate the use of other methods to remove the byproducts [1,[4][5][6].
The g-C 3 N 4 semiconductor has shown significant photocatalytic proficiency under visible light, as a result of its favorable characteristics such as high stability and a reduced band gap energy, which improves its capacity to absorb visible radiations [7][8][9][10]. However, the quick recombination of photoinduced e − /h + pairs in the g-C 3 N 4 makes it unsuitable for use as a photocatalyst [11][12][13]. As a result, numerous attempts to remove this limitation have been made, including vacancy, heterojunction formation, and mixing the g-C 3 N 4 with some other metal oxide and nonmetals such as S [14,15]. By stacking its 2p orbitals on the VB of bulk g-C 3 N 4 , S-doping alters the bandgap of g-C 3 N 4 and enhances the mobility and separation of the e-h pairs. Hong et al. reported that the photocatalytic H 2 production efficiency of mesoporous S-g-C 3 N 4 is 30 times more than pure g-C 3 N 4 [16]. Similarly, S-g-C 3 N 4 had an approximately 1.38 times greater photocatalytic CO 2 reduction rate than pure g-C 3 N 4 [14]. Under visible light, porous S-g-C 3 N 4 had better adsorption and photocatalytic degradation of Rhodamine B dye than pure g-C 3 N 4 [17].
S-doping has been shown to change the structural properties of g-C 3 N 4 , reduce its Eg value, and enhance the e − /h + pair separation efficiency both theoretically and empirically [18]. The heterogeneous photocatalyst's nanosheet structure, on the other hand, provides a large number of active sites for the reaction, along with increased surface area and reduced recombination between photoinduced charges. The separation efficiency of photo-produced charges on the g-C 3 N 4 can be expanded by combining it with another good semiconductor like ZnFe 2 O 4 , and the resulting heterojunction can be used for wastewater treatment [19][20][21][22].
Zinc ferrite is a spinel ferrite with all Fe 3+ ions in the octahedral sites and Zn 2+ ions in the tetrahedral sites. Because of its unusual catalytic and magnetic capabilities, it is a promising material. Many studies have shown that doping ZnFe 2 O 4 with appropriate metal ions improves optical and photocatalytic characteristics [23,24]. Patil et al. used the co-precipitation approach to manufacture Gd 3+ doped ZnFe 2 O 4 nanoparticles, which demonstrated improved MB degradation of roughly 99% as compared to pure ZnFe 2 O 4 (95% degradation in 240 min) [25]. According to Ajithkumar et al., yttrium-doped zinc ferrite made by solution combustion technique showed 95% MB degradation in 180 minutes [26]. Y-ZnFe 2 O 4 has higher photocatalytic effectiveness than pure zinc ferrite. Under visible light, cobalt-doped zinc ferrite decomposed methylene blue more efficiently than ZnFe 2 O 4 . Many researchers have concluded that ZnFe 2 O 4 has finite band gap energy and hence might form an effective heterojunction when combined with g-C 3 N 4 [27].
Moreover, the advanced ZnFe 2 O 4 /g-C 3 N 4 nanocomposite, which plays a role in increasing photocatalytic efficiency, may achieve longer separation between photoexcited charges [28]. Owing to the improved charge separation abilities, it is suggested to produce M-ZnFe 2 O 4 /S-g-C 3 N 4 heterojunction to realize significant photocatalytic performance [29]. In this probe, hybrid ZFG-50 nanocomposites have been synthesized successfully via a surfactant (PEG) assisted hydrothermal process. The photocatalytic characteristics of synthesized materials were investigated using MB, an organic pollutant. In step one, the series of chromium-doped zinc ferrite (Cr-ZnFe 2 O 4 ) nanoparticles were synthesized with varying chromium percentages (0.5, 1, 3, 5, 7, and 9 wt. %). The effect of Cr 3+ substitution on photocatalytic properties of zinc ferrite was observed. The 7% Cr-ZnFe 2 O 4 sample manifested the best absorption of solar light and degradation efficiency. In step two, the 7% Cr-ZnFe 2 O 4 nanoparticles were homogenized with diverse concentrations of S-g-C 3 N 4 (10, 30, 50, and 70 wt. %) to produce ZFG-50 with enhanced photocatalytic activity. The 7% Cr-ZnFe 2 O 4 /50% S-g-C 3 N 4 nanocomposite executed the best photocatalytic activity as compared to pure ZnFe 2 O 4 , 7% Cr-ZnFe 2 O 4 , and S-g-C 3 N 4 . Results depicted that the enhanced photocatalytic activity of 7% Cr-ZnFe 2 O 4 /50% S-g-C 3 N 4 nanocomposite was because of the enhanced absorption of sunlight and better separation of e − /h + pairs between Cr-ZnFe 2 O 4 and S-g-C 3 N 4 . To the best of our knowledge, the synthesis of ZFG-50 heterojunctions via the hydrothermal approach has never been used. The precursors used for the synthesis are low-cost, and the synthesized ZnFe 2 O 4 /S-g-C 3 N 4 heterojunctions are not reported to be used as photocatalysts. The synthesized material may have potential applications in the field of water purification.

Synthesis of Chromium Doped Zinc Ferrites
A surfactant-assisted hydrothermal technique was employed to fabricate, a set of chromium doped zinc ferrites (Cr-ZnFe 2 O 4 ) with different chromium percentages (0.5, 1, 3, 5, 7, and 9 wt. %) [12]. For the preparation of 0.5% Cr-ZnFe 2 O 4 three solutions A, B and C were made before synthesis. Solution A: 40 mL of deionized water were mixed with 0.0169 g of CrCl 3 ·6H 2 O. Solution B: 40 mL of deionized water were used to emulsify 2.8624 g of ZnSO 4 .7H2O. Solution C: In 40 mL of deionized water, 3.244 g of FeCl 3 was dissolved. Then, 10 mL of PEG-400 was added as a surfactant to the mixture of solutions A, B, and C in order to prevent the agglomeration of nanoparticles. The suspensions were then moved to a Teflon-lined autoclave after the pH of the resulting solution was adjusted to 11 by adding a 6 M NaOH solution. The autoclave was placed in a 175 • C oven for ten hours before being removed to cool to room temperature. The resulting precipitates were then filtered off and washed with deionized H 2 O and absolute ethanol and then, finally dried at 85 • C in an oven. The same process was applied to synthesize other percentages (0, 1, 3, 5, 7, and 9 wt. %) of Cr-ZnFe 2 O 4 .

Synthesis of S-g-C 3 N 4
S-g-C 3 N 4 was produced via thermal polycondensation of thiourea to 570 • C for 5 h at 5 • C min −1 in a muffle furnace. It was then allowed to cool to ambient temperature and stored the resulting yellowish S-g-C 3 N 4 [14].

Synthesis of Cr-ZnFe 2 O 4 /S-g-C 3 N 4
A range of ZFG-50 nanocomposites was made by incorporating 7% Cr-ZnFe 2 O 4 with different concentrations of S-g-C 3 N 4 (10, 30, 50, 60, and 70 wt. %) via surfactantassisted hydrothermal process [30,31]. For the preparation of 7%Cr-ZnFe 2 O 4 /10%S-g-C 3 N 4 , four solutions A, B, C, and D were made before synthesis. Mixtures of 0.2346 g of CrCl 3 .6H 2 O in 30mL of water (Solution A), 2.6742 g of ZnSO 4 ·7H 2 O in 30mL of water (Solution B), 3.244 g of FeCl 3 in 30mL of water (Solution C), and 0.18 g of S-g-C 3 N 4 in 30mL of water (Solution D) were dissolved in separate beakers and stirred. The solutions A, B, and C were added to solution D and homogenized for 45 minutes along with the addition of 10 mL of polyethylene glycol (PEG-400) as a surfactant. The next steps were the same as for the synthesis of Cr-ZnFe 2 O 4 NPs. Moreover, the same process was repeated to synthesize the 7% ZFG-50 containing the (30, 50, 60, and 70 wt. %) of S-g-C 3 N 4 . The schematic diagram (Figure 1) depicts the synthesis procedure for ZFG-50 NCs, and Table 1 lists the precise composition.

Photocatalytic Activity
The photocatalyzed dye degradation activity of all synthesized photocatalysts evaluated under the irradiation of solar light. The reference contaminant was an aque solution of the organic dye methylene blue (MB). A 100 mL solution of MB was diffu with 0.2 g of each photocatalyst (10 mg L −1 ). To achieve the adsorption-desorp equilibrium, the suspension was sonicated for 15 min, followed by 30 min of darkn After that, the suspension was placed in an open space with sun light, and aliquots mL were taken every 30 min. After centrifugation, the sample's photocatalytic activity assessed using a UV-vis spectrophotometer. Figure 2 shows the X-rays diffractogram of ZF, 7% Cr-ZnFe2O4, SG, and ZF samples. Seven peaks were observed in the case of pure ZnFe2O4 with crystal facets (2 (311), (400), (422), (333), (440), and (533) at 2θ = 29.8°, 35.1°, 42.7°, 53°, 56.7°, 62.2°, 73.8° that fitted well with the pattern of standard ZnFe2O4 with JCPDS file 01-077- [32]. Two characteristic peaks were observed in the XRD pattern of SG, the crystal p (002) was attributed to the interlayer assembling of aromatic systems and the plane (

Photocatalytic Activity
The photocatalyzed dye degradation activity of all synthesized photocatalysts was evaluated under the irradiation of solar light. The reference contaminant was an aqueous solution of the organic dye methylene blue (MB). A 100 mL solution of MB was diffused with 0.2 g of each photocatalyst (10 mg L −1 ). To achieve the adsorption-desorption equilibrium, the suspension was sonicated for 15 min, followed by 30 min of darkness. After that, the suspension was placed in an open space with sun light, and aliquots of 5 mL were taken every 30 min. After centrifugation, the sample's photocatalytic activity was assessed using a UV-vis spectrophotometer.  [32]. Two characteristic peaks were observed in the XRD pattern of SG, 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 [33,34]. After coupling with SG, the crystal phase of Cr-ZnFe 2 O 4 stays unchanged, and the (002) crystal plane of the SG (weak) was indicated in the composite systems. Moreover, the XRD pattern shows no other impurity phase, indicating that ZFG50 is a two-phase nanocomposite. In 7%Cr-ZnFe 2 O 4 /50%S-g-C 3 N 4 composites, owing to high crystallinity of Cr-ZnFe 2 O 4 and low concentration of SG the characteristic peaks of Cr-ZnFe 2 O 4 are prominent. Further, the crystal structure of Cr-ZnFe 2 O 4 in the ZFG50 composite is unaffected by the addition of SG [35][36][37].

XRD Analysis
Molecules 2022, 27, x FOR PEER REVIEW 5 of 15 was ascribed to the inter-planar arrangement of aromatic systems [33,34]. After coupling with SG, the crystal phase of Cr-ZnFe2O4 stays unchanged, and the (002) crystal plane of the SG (weak) was indicated in the composite systems. Moreover, the XRD pattern shows no other impurity phase, indicating that ZFG50 is a two-phase nanocomposite. In 7%Cr-ZnFe2O4/50%S-g-C3N4 composites, owing to high crystallinity of Cr-ZnFe2O4 and low concentration of SG the characteristic peaks of Cr-ZnFe2O4 are prominent. Further, the crystal structure of Cr-ZnFe2O4 in the ZFG50 composite is unaffected by the addition of SG [35][36][37].

TEM, EDX, and XPS Analyses
To evaluate the morphology of the synthesized photocatalysts, SEM and TEM micrographs were taken. The lamellar sheet-like structure is seen in the SEM and TEM pictures of pure S-g-C3N4 (Figure 3a,b). On the other hand, pure ZnFe2O4 and Cr-ZnFe2O4 that have been doped with Cr reveal very non-uniform spherical-like particles, as illustrated in Figure 3c,d, respectively. TEM was used to verify further how S-g-C3N4 and Cr-ZnFe2O4 nanoparticles interacted. The carbon nitride sheets were seen to be coated by the Cr-ZnFe2O4 nanoparticles in the TEM picture of the ZFG-50 NCs.

TEM, EDX, and XPS Analyses
To evaluate the morphology of the synthesized photocatalysts, SEM and TEM micrographs were taken. The lamellar sheet-like structure is seen in the SEM and TEM pictures of pure S-g-C 3 N 4 (Figure 3a,b). On the other hand, pure ZnFe 2 O 4 and Cr-ZnFe 2 O 4 that have been doped with Cr reveal very non-uniform spherical-like particles, as illustrated in Figure 3c,d, respectively. TEM was used to verify further how S-g-C 3 N 4 and Cr-ZnFe 2 O 4 nanoparticles interacted. The carbon nitride sheets were seen to be coated by the Cr-ZnFe 2 O 4 nanoparticles in the TEM picture of the ZFG-50 NCs. Figure 3e shows the TEM picture of the ZFG-50 NCs with a 7% metal oxide content. The S-g-C 3 N 4 nanosheets' surface has Cr-ZnFe 2 O 4 nanoparticles deposited on it, as seen by the TEM pictures. The surface of the S-g-C 3 N 4 nanosheets had evenly dispersed particles with an average size of 19 nm, according to the TEM pictures. By subjecting the composite to an ultrasonic treatment to prepare TEM samples, it was shown that the contact between the S-g-C 3 N 4 sheet and nanoparticles is quite strong. When exposed to light, the S-g-C 3 N 4 sheets and Cr-ZnFe 2 O 4 particles seem to form a heterojunction, making it easier to boost the nanocomposite's photocatalytic activity and separate the electron-hole in the opposite direction to produce the reactive species needed for dye mineralization. The EDX elemental mapping of the ZFG-50 NCs is also shown in Figure 3f, demonstrating that the principal elements of the ZFG-50 were Cr, Fe, Zn, O, C, and N. As shown in Figure S1, ZFG-50 was examined using XPS to ascertain its chemical composition and the electronic states of each of its constituent parts. Additionally, the XPS analysis supported the TEM and EDX findings that the Cr-ZnFe 2 O 4 / S-g-C 3 N 4 included ZnFe 2 O 4 , S-g-C 3 N 4 and Cr.   Figure 3e shows the TEM picture of the ZFG-50 NCs with a 7% metal oxide content. The S-g-C3N4 nanosheets' surface has Cr-ZnFe2O4 nanoparticles deposited on it, as seen by the TEM pictures. The surface of the S-g-C3N4 nanosheets had evenly dispersed particles with an average size of 19 nm, according to the TEM pictures. By subjecting the composite to an ultrasonic treatment to prepare TEM samples, it was shown that the contact between the S-g-C3N4 sheet and nanoparticles is quite strong. When exposed to light, the S-g-C3N4 sheets and Cr-ZnFe2O4 particles seem to form a heterojunction, making it easier to boost the nanocomposite's photocatalytic activity and separate the electron-hole in the opposite direction to produce the reactive species needed for dye mineralization. The EDX elemental mapping of the ZFG-50 NCs is also shown in Figure 3f, demonstrating that the principal elements of the ZFG-50 were Cr, Fe, Zn, O, C, and N. As shown in Figure S1, ZFG-50 was examined using XPS to ascertain its chemical composition and the electronic states of each of its constituent parts. Additionally, the XPS analysis supported the TEM and EDX findings that the Cr-ZnFe2O4/ S-g-C3N4 included ZnFe2O4, S-g-C3N4 and Cr.

FTIR Analysis
The FTIR spectrum of ZF, 7% Cr-ZnFe2O4, SG and ZFG-50 samples is shown in Figure  4. The two active bands 3355 cm −1 and 834 cm −1 are observed in the FTIR spectra of zinc ferrite and 7% Cr-ZnFe2O4 [38]. These active bands are characteristic of the spinel structure of zinc ferrite nanoparticles. The band at 3355 cm −1 is attributed due to the stretching vibrations of the O-H bond of the free or absorbed water, whereas the band at 834 cm −1 is ascribed due to the stretching vibration of the Zn-O bond [39,40]. The band observed in composites at wavelength range 2800 cm −1 to 3400 cm −1 is attributed to N-H stretching, whereas a sharp peak observed at 870 cm −1 in all samples is due to the out-of-plane bending vibration of the tri-s-triazine ring of SG. The bands at 1600-1200 cm −1 were allocated to CN heterocycles (C=N and C-N) stretching vibrations, confirming the presence of S-g-C3N4 in composite samples [8,15]. Then, using the UV-vis spectra, the

FTIR Analysis
The FTIR spectrum of ZF, 7% Cr-ZnFe 2 O 4 , SG and ZFG-50 samples is shown in Figure 4. The two active bands 3355 cm −1 and 834 cm −1 are observed in the FTIR spectra of zinc ferrite and 7% Cr-ZnFe 2 O 4 [38]. These active bands are characteristic of the spinel structure of zinc ferrite nanoparticles. The band at 3355 cm −1 is attributed due to the stretching vibrations of the O-H bond of the free or absorbed water, whereas the band at 834 cm −1 is ascribed due to the stretching vibration of the Zn-O bond [39,40]. The band observed in composites at wavelength range 2800 cm −1 to 3400 cm −1 is attributed to N-H stretching, whereas a sharp peak observed at 870 cm −1 in all samples is due to the out-of-plane bending vibration of the tri-s-triazine ring of SG. The bands at 1600-1200 cm −1 were allocated to CN heterocycles (C=N and C-N) stretching vibrations, confirming the presence of S-g-C 3 N 4 in composite samples [8,15]. Then, using the UV-vis spectra, the light-absorption of the designed photocatalysts ZnFe 2 O 4 , S-g-C 3 N 4 , and ZFG-50NCs was measured ( Figure S2). The BET surface area was determined to be 9.23, 14.31, 27.11, and 63.78 m 2 /g for all formulations: ZnFe 2 O 4 , S-g-C 3 N 4 , and ZFG-50NCs ( Figure S3). light-absorption of the designed photocatalysts ZnFe2O4, S-g-C3N4, and ZFG-50NCs was measured ( Figure S2). The BET surface area was determined to be 9.23, 14.31, 27.11, and 63.78 m 2 /g for all formulations: ZnFe2O4, S-g-C3N4, and ZFG-50NCs ( Figure S3).

Photocatalytic Degradation Study
Under two phases, the photocatalytic activity of synthesized samples was investigated in the sunshine. The photocatalytic activities of ZnFe2O4 and Cr-ZnFe2O4 NPs (Figure 5a) were first investigated using an aqueous methylene blue solution in the presence of sunlight. A UV-vis spectrophotometer with a wavelength of 200-800 nm was used to track the dye degradation rate (Figure 5b). From the degradation contours ( Figure  S4) and % degradation plots (Figure 5b), the photocatalytic activity of chromium-doped zinc ferrite nanoparticles increased by increasing the Cr +3 doping up to 7 wt. %. Because the Cr +3 doping decreases the bandgap of ZnFe2O4, which facilitates the e − /h + pair generation. 7% Cr +3 doping was the optimal concentration of Cr +3 ions. Increasing Cr +3 ions concentration beyond this (<7 wt. %.) leads to a decrease in photocatalytic activity of Cr-ZnFe2O4 NPs (Figure 6a,b). The observed degradation efficiencies of Cr-ZnFe2O4 catalysts with different chromium percentages (0, 0.5, 1, 3, 5, 7, and 9 wt. %) were 78%, 81%, 83%, 87%, 92%, 95%, and 89%, respectively, after 150 min of sunlight irradiation. Thus, the 7% Cr-ZnFe2O4 NPs exhibited the maximum photocatalytic efficiency as compared to other nanoparticles.

Photocatalytic Degradation Study
Under two phases, the photocatalytic activity of synthesized samples was investigated in the sunshine. The photocatalytic activities of ZnFe 2 O 4 and Cr-ZnFe 2 O 4 NPs (Figure 5a) were first investigated using an aqueous methylene blue solution in the presence of sunlight. A UV-vis spectrophotometer with a wavelength of 200-800 nm was used to track the dye degradation rate (Figure 5b). From the degradation contours ( Figure S4) and % degradation plots (Figure 5b), the photocatalytic activity of chromium-doped zinc ferrite nanoparticles increased by increasing the Cr +3 doping up to 7 wt. %. Because the Cr +3 doping decreases the bandgap of ZnFe 2 O 4 , which facilitates the e − /h + pair generation. 7% Cr +3 doping was the optimal concentration of Cr +3 ions. Increasing Cr +3 ions concentration beyond this (<7 wt. %.) leads to a decrease in photocatalytic activity of Cr-ZnFe 2 O 4 NPs (Figure 6a,b). The observed degradation efficiencies of Cr-ZnFe 2 O 4 catalysts with different chromium percentages (0, 0.5, 1, 3, 5, 7, and 9 wt. %) were 78%, 81%, 83%, 87%, 92%, 95%, and 89%, respectively, after 150 min of sunlight irradiation. Thus, the 7% Cr-ZnFe 2 O 4 NPs exhibited the maximum photocatalytic efficiency as compared to other nanoparticles.
In the next step, the 7% Cr-ZnFe 2 O 4 NPs were homogenized with diverse amounts of S-g-C 3 N 4 (as given in Table 1) to develop ZFG-50(ZFG) NCs and their photocatalytic activity was checked after every 15 min interval. Before sunlight exposure, the fabricated NCs were placed in the dark to establish adsorption-desorption equilibrium between dye and the S-g-C 3 N 4 , ZF, ZFG10, ZFG30, ZFG50, ZFG60, and ZFG70 catalysts and the corresponding adsorbed amounts of MB are displayed in Figure 6c. The graph (Figure 6a) clearly shows that the samples absorbed relatively little amounts of dye. Then samples were exposed to sunlight and the ZFG-50 NCs exhibits maximum dye degradation as compared to other samples (Figure 6a). From the degradation contours ( Figure S5) and % degradation plots (Figure 6b), it could be observed that on enhancing SG contents in the ZFG NCs, the dye degradation was increased up to ZFG50 NCs (containing 50% S-g-C 3 N 4 ) and then decreased for ZFG60 and ZFG70 (<50% S-g-C 3 N 4 ). The observed degradation efficiencies of SG, ZF, ZFG10, ZFG30, ZFG50, ZFG60, and ZFG70 catalysts were 23.47%, 26%, 31%, 51%, 100%, 70%, and 63.28%, respectively, after 90 min of sunlight irradiation. Improved charge separation and transfer via Cr-ZnFe 2 O 4 and S-g-C 3 N 4 coupling, as well as higher visible  [7,38,41]. Figure 6b depicts the % photocatalytic degradation of MB by NCs. The Langmuir-Hinshelwood model was applied to explain the kinetics [42]. It is evident that the dye degradation by the NCs under sunlight is fit to pseudo-first-order kinetics (Figure 6c). The rate constant (k) values are summarized in Table 2 and given in Figure 6d. In the next step, the 7% Cr-ZnFe2O4 NPs were homogenized with dive S-g-C3N4 (as given in Table 1) to develop ZFG-50(ZFG) NCs and their activity was checked after every 15 min interval. Before sunlight exposure NCs were placed in the dark to establish adsorption-desorption equilibrium and the S-g-C3N4, ZF, ZFG10, ZFG30, ZFG50, ZFG60, and ZFG70 cat corresponding adsorbed amounts of MB are displayed in Figure 6c. The gra clearly shows that the samples absorbed relatively little amounts of dye. were exposed to sunlight and the ZFG-50 NCs exhibits maximum dye d compared to other samples (Figure 6a). From the degradation contours (Fi and then drops yonder this concentration (<50%). Thus, inherently, 50% S-g-C3N4 is the ideal concentration for the ZFG NC. Further increase in S-g-C3N4 concentration might produce e-h pair combination centers, which successively decrease the photocatalytic efficiency [43,44]. To further analyse this rationalization, a preliminary investigation is required. As shown in Table 3, the photocatalytic efficiency of ZFG50 NC is significantly higher than various prior reported research. Since the ZFG50 NC was found to be the most efficient photocatalyst and so it was further used in the recycling study. The photocatalyst's durability during repeated photocatalytic activity is crucial for its practical uses. The ZFG-50 catalysts were recycled in five tests, and the material's catalytic activity was tracked. In the recycling research, the ZFG-50 kept up its dye  ZFG50 (0.0058 min −1 ) and SG (0.0021 min −1 ) had the greatest and lowest "k" values, respectively. The ZFG50 NCs completely mineralized the MB in 90 min and its "k" value was 2.4 and 3.5 times more than that of SG and ZF respectively. As the S-g-C 3 N 4 concentration increase from 10% to 50% in the ZFG NC, the dye degradation also enhances and then drops yonder this concentration (<50%). Thus, inherently, 50% S-g-C 3 N 4 is the ideal concentration for the ZFG NC. Further increase in S-g-C 3 N 4 concentration might produce e-h pair combination centers, which successively decrease the photocatalytic efficiency [43,44]. To further analyse this rationalization, a preliminary investigation is required. As shown in Table 3, the photocatalytic efficiency of ZFG50 NC is significantly higher than various prior reported research. Since the ZFG50 NC was found to be the most efficient photocatalyst and so it was further used in the recycling study. The photocatalyst's durability during repeated photocatalytic activity is crucial for its practical uses. The ZFG-50 catalysts were recycled in five tests, and the material's catalytic activity was tracked. In the recycling research, the ZFG-50 kept up its dye degradation rate. The composite's dye degradation efficiency did not significantly decrease. According to the findings, even after the fourth cycle, effective dye degradation remained at over 95% (Figure 7a). The ZFG-50 catalysts might thus function as trustworthy, effective, and reusable photocatalytic materials. The ZFG-50 NCs' crystal phase structure did not change significantly before or after the organic pollutants recycling experiments, according to the results of the XRD stability study, demonstrating chemical structural resilience ( Figure S6). EIS in the dark was used to calculate the heterointerface charge transfer rate at the electrode-electrolyte junction. With a smaller arc radius and lower electron transport barrier, interfacial photoinduced charge transfer and departure efficiency is often faster. The heterointerface contact of the ZFG-50 may considerably help electron transmission, boosting electron consumption and enhancing photocatalytic performance, as shown by Figure 7b, which demonstrates that the ZFG-50 sample had the lowest charge-transmission resistance of all the produced samples. According to the experimental results, a ZFG-50 heterojunction may significantly improve light-collecting efficiency, effective separation of photogenerated e − and h + couples, and heterointerface electron transmission.
Molecules 2022, 27, x FOR PEER REVIEW 10 of 15 degradation rate. The composite's dye degradation efficiency did not significantly decrease. According to the findings, even after the fourth cycle, effective dye degradation remained at over 95% (Figure 7a). The ZFG-50 catalysts might thus function as trustworthy, effective, and reusable photocatalytic materials. The ZFG-50 NCs' crystal phase structure did not change significantly before or after the organic pollutants recycling experiments, according to the results of the XRD stability study, demonstrating chemical structural resilience ( Figure S6). EIS in the dark was used to calculate the heterointerface charge transfer rate at the electrode-electrolyte junction. With a smaller arc radius and lower electron transport barrier, interfacial photoinduced charge transfer and departure efficiency is often faster. The heterointerface contact of the ZFG-50 may considerably help electron transmission, boosting electron consumption and enhancing photocatalytic performance, as shown by Figure 7b, which demonstrates that the ZFG-50 sample had the lowest charge-transmission resistance of all the produced samples. According to the experimental results, a ZFG-50 heterojunction may significantly improve light-collecting efficiency, effective separation of photogenerated e − and h + couples, and heterointerface electron transmission.

Photocatalytic Degradation Mechanism
In the photocatalytic degradation mechanism as purposed in the schematic sketch (Figure 8), the enhanced degradation of methylene blue by photocatalysts may be ascribed due to the generation of e − /h + pairs. EPR spectra of ZFG-50 NCs were explored to further corroborate the validation of functional species ·O2 − and ·OH in the photodegradation mechanism ( Figure S7a,b). When solar light is irradiated on ZFG, both Cr-ZnFe2O4 and Sg-C3N4 are energized and e − /h + pairs are generated on their conduction band (CB) and

Photocatalytic Degradation Mechanism
In the photocatalytic degradation mechanism as purposed in the schematic sketch (Figure 8), the enhanced degradation of methylene blue by photocatalysts may be ascribed due to the generation of e − /h + pairs. EPR spectra of ZFG-50 NCs were explored to further corroborate the validation of functional species ·O 2 − and ·OH in the photodegradation mechanism ( Figure S7a,b). When solar light is irradiated on ZFG, both Cr-ZnFe 2 O 4 and S-g-C 3 N 4 are energized and e − /h + pairs are generated on their conduction band (CB) and valence band (VB), respectively [53]. Based on the CB/VB edge potentials, the photoinduced electrons can be easily migrated from the conduction band (CB) of Cr-ZnFe 2 O 4 to the CB of S-g-C 3 N 4 since the CB of Cr-ZnFe 2 O 4 is lower than that of S-g-C 3 N 4 . At the same time, the holes generated in the VB of S-g-C 3 N 4 could migrate to Cr-ZnFe 2 O 4 [23]. The Cr atoms not only decrease the Eg value but also act as facilitators to transport e − from S-g-C 3 N 4 to ZnFe 2 O 4 in the hybrid composite. Thus, doping could considerably reduce the possibility of photogenerated charge recombination by improving the separation of photogenerated e − /h + pairs. The generated e − & h + reacts with the water and oxygen molecules absorbed on the surface of the photocatalyst and produce radicals (·OH and ·O −2 ) [8]. These radicals are utilized to break down MB by transforming it into low molecular weight intermediates, which are then changed into H 2 O, CO 2 , and inorganic ions via an oxidative mechanism. Equations (1)- (7) show the reductive and oxidative reactions involved in the photo-degradation of MB by ZFG NC.
Molecules 2022, 27, x FOR PEER REVIEW 11 of 15 valence band (VB), respectively [53]. Based on the CB/VB edge potentials, the photoinduced electrons can be easily migrated from the conduction band (CB) of Cr-ZnFe2O4 to the CB of S-g-C3N4 since the CB of Cr-ZnFe2O4 is lower than that of S-g-C3N4. At the same time, the holes generated in the VB of S-g-C3N4 could migrate to Cr-ZnFe2O4 [23]. The Cr atoms not only decrease the Eg value but also act as facilitators to transport e − from S-g-C3N4 to ZnFe2O4 in the hybrid composite. Thus, doping could considerably reduce the possibility of photogenerated charge recombination by improving the separation of photogenerated e − /h + pairs. The generated e − & h + reacts with the water and oxygen molecules absorbed on the surface of the photocatalyst and produce radicals (·OH and ·O −2 ) [8]. These radicals are utilized to break down MB by transforming it into low molecular weight intermediates, which are then changed into H2O, CO2, and inorganic ions via an oxidative mechanism. Equations 1-7 show the reductive and oxidative reactions involved in the photo-degradation of MB by ZFG NC.

Antibacterial Study
Both Gram-positive and Gram-negative bacteria were used to examine the antibacterial properties of ZnFe2O4, Cr-ZnFe2O4, and ZFG-50 NCs. Using the standard agar diffusion techniques, the antibacterial activity was carried out. Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Streptococcus salivarius were the four different bacterial strains used in the antibacterial tests. The Petri plates were taken out after the incubation period and placed under a laminar flow hood. Measurements and records of the zones of inhibition are provided in Table 4 for each sample, including the positive and negative

Antibacterial Study
Both Gram-positive and Gram-negative bacteria were used to examine the antibacterial properties of ZnFe 2 O 4 , Cr-ZnFe 2 O4, and ZFG-50 NCs. Using the standard agar diffusion techniques, the antibacterial activity was carried out. Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Streptococcus salivarius were the four different bacterial strains used in the antibacterial tests. The Petri plates were taken out after the incubation period and  Table 4 for each sample, including the positive and negative controls. The zones of inhibition for each of the four bacterial strains against each of the four nanomaterials were measured and reported using the same method. When exposed to the nanomaterials ZnFe 2 O 4 , Cr-ZnFe 2 O 4 , and ZFG-50 NCs, it was found that all four bacterial strains exhibited a zone of inhibition. While ZnO had the lowest bacterial inhibition zones, ZFG-50 NCs had the greatest. The increased surface area that the 7 percent Cr-ZnFe 2 O 4 NPs allowed for surface contact NCs with bacterial membranes and the increased ROS generation brought on by the narrowing of the ZnFe 2 O 4 bandgap may have contributed to the maximum antibacterial activity of the ZFG-50 NCs. All generated samples were examined for zones of inhibition against the four bacterial strains shown in Figure 9 and Table 4 below. The ternary composite has more antibacterial activity than the other synthetic nanomaterials, as seen in the bar graph below. controls. The zones of inhibition for each of the four bacterial strains against each of the four nanomaterials were measured and reported using the same method. Figure 9. Zones of inhibition of the ZnFe2O4, Cr-ZnFe2O4, and ZFG-50 against the employed bacterial strains.
When exposed to the nanomaterials ZnFe2O4, Cr-ZnFe2O4, and ZFG-50 NCs, it was found that all four bacterial strains exhibited a zone of inhibition. While ZnO had the lowest bacterial inhibition zones, ZFG-50 NCs had the greatest. The increased surface area that the 7 percent Cr-ZnFe2O4 NPs allowed for surface contact NCs with bacterial membranes and the increased ROS generation brought on by the narrowing of the ZnFe2O4 bandgap may have contributed to the maximum antibacterial activity of the ZFG-50 NCs. All generated samples were examined for zones of inhibition against the four bacterial strains shown in Figure 9 and Table 4 below. The ternary composite has more antibacterial activity than the other synthetic nanomaterials, as seen in the bar graph below.

Conclusions
In conclusion, we have developed ZnFe 2 O 4 , Cr-ZnFe 2 O 4 nanoparticles and a series of ZFG-50nanocomposites using a straightforward hydrothermal technique. The assembly and purity of samples were examined using XRD, EDX, and FTIR methods. ZnFe 2 O 4 , Cr-ZnFe 2 O 4 , and ZFG were used to degrade MB at ambient temperature. In a comparison photocatalytic investigation of the synthesized samples against MB, the ZFG-50 was found to have very high catalytic efficiency. A rate constant for the dye reduction reaction was discovered to be pseudo-first order both for NPs and NCs. Moreover, ternary composite ZFG-50 possesses significantly higher antibacterial activity compared to the other synthetic nanomaterials. Thus, ZFG-50 heterojunction is a promising candidate and has potential applications in the purification and disinfection of water by photocatalytic degradation of organic contaminants.