Direct Z-Scheme CoFe2O4-Loaded g-C3N4 Photocatalyst with High Degradation Efficiency of Methylene Blue under Visible-Light Irradiation

Abstract

Magnetically recyclable direct Z-scheme CoFe₂O₄-loaded g-C₃N₄ photocatalyst material was fabricated using a facile hydrothermal technique and subsequently characterized by XRD, VSM, PL, FT-IR, EDX, DRS, SEM, and BET techniques. The characterization results confirmed that nanoparticles of CoFe₂O₄ are loaded on the surface of g-C₃N₄ sheets. The optical band gap of g-C₃N₄ has been decreased from 2.65 eV to 1.30 eV by means of the loading of CoFe₂O₄ nanoparticles onto the nanosheets of g-C₃N₄. This has enhanced the separation process of electron-hole. Under visible light irradiation, the photocatalytic activity of the developed direct Z-scheme CoFe₂O₄-loaded g-C₃N₄ photocatalyst was evaluated for the photodegradation of methylene blue (MB); during this process the MB decomposed by up to 98.86% in 140 min. Meanwhile, under the same irradiation and time conditions, the g-C₃N₄ and CoFe₂O₄ themselves degraded MB up to 74.92% and 51.53%, respectively. The direct Z-scheme CoFe₂O₄-loaded g-C₃N₄ material was recovered from the solution after the photocatalytic activity using an external magnet and studied to determine its stability. It was shown that the photoactivity did not change significantly after five consecutive cycles.

1. Introduction

With the increasing expansion of global industry, the problem of access to clean water has risen to the forefront of discussion. Every year, a massive volume of industrial dye wastewater is dumped into natural water bodies, posing a risk to both the aquatic ecosystem and people [1]. Nowadays, removal of organic pollutants from wastewater/water through photocatalysis provides a viable method for solving environmental issues [2,3]. In order to eliminate environmental pollution and degrade these harmful organic pollutants, photocatalytic technology has received an increasing amount of interest. So far, TiO₂ is the most widely studied photocatalyst; it was reported by Fujishima and Honda in 1972 for use in photoelectrochemical water splitting [4]. This study was a pioneer in photocatalysis technology. Nevertheless, TiO₂ has high band gap (3.2 eV), which accounts for around 4% of sunlight; this severely restricts its ability to function as a photocatalyst and restricts the range of use of this material [5,6]. These limitations have inspired researchers to develop novel materials with narrow band gaps (E_g) to utilize solar energy in a more efficient way. Due to its chemical inertness, distinctive layered structure, nontoxic nature, a middle band (i.e., band gap between 2.4–2.9 eV), and capacity to absorb visible light, graphitic carbon nitride (g-C₃N₄) is now frequently employed for the photocatalytic degradation of organic contaminants [7,8,9,10,11,12,13,14]. However, inadequate absorption of visible light, small specific surface area, and fast electron-hole pair recombination still limits its photocatalytic activity [15,16].

Thus, numerous techniques were exploited to improve graphitic carbon nitride’s photocatalytic efficiency, including doping [12], morphological modification [17], copolymerization [18], and combining with one or more semiconductor materials [19]. Thanks to its appropriate valence band edge and conduction band edge positions, g-C₃N₄ can produce a heterojunction structure/nanocomposite with other semiconductor photocatalysts. This could improve its photocatalytic activity performance by improving the photogenerated electron-hole separation rate of the heterojunction photocatalyst [20,21,22]. Currently, spinel ferrites with the general formula of MFe₂O₄ (M = Zn, Co, Ni, Cu) have been widely used in photocatalytic applications due to their visible-light absorption, stability, environmentally friendly nature, magnetic properties, and low-cost [23]. Cobalt ferrite (CoFe₂O₄) is one of the inverse spinel structures that attracted significant interest because of its non-toxic nature and its abundant and narrow bandgap (1.2–2.7 eV) [24,25]. Even though pristine CoFe₂O₄ has a poor photocatalytic performance, its photocatalytic activity considerably increases when it is coupled with a π-conjugated semiconductor materials [26]. For instance, CoFe₂O₄ nanoparticles can be coupled with g-C₃N₄ to prepare heterojunction structures, direct Z-schemes, or all-solid-state Z-schemes due to the matching conduction band edge position and valence band edge position of CoFe₂O₄ and g-C₃N₄ semiconductors.

Huang et al. [27] reported on the CoFe₂O₄/g-C₃N₄ composites, which are synthesized using an easy calcination process. The composite containing 41.4 wt% CoFe₂O₄ showed the maximum catalytic activity for the MB degradation under irradiation of visible light in the presence of H₂O₂. However, the use of H₂O₂ may not be economically viable during practical applications. Inbaraj et al. [28] also synthesized CoFe₂O₄/g-C₃N₄ composite via the coupling of a honey-mediated green approach and hydrothermal technique. The MB degradation and the adsorption removal of lead (Pb²⁺) ion from water were both used to test the photocatalytic activity of the composite photocatalyst. However, the hydrothermal process used in the synthesis is time-consuming and labor-intensive; our experimental strategy seems to be simpler. Moreover, the photocatalytic mechanism was a type-II heterojunction, which means the redox reaction takes place in the less negative conduction band/reduction potential and in the less positive valence band/oxidation potential of the semiconductors. This limits the photocatalytic performance of the CoFe₂O₄/g-C₃N₄ composites. Recently, in the presence of peroxymonosulfate (PMS), which is utilized as a sulfate radical-based Fenton-like oxidation reaction, Guo et al. [1] reported that rhodamine B (RhB) was photodegraded by CoFe₂O₄@g-C₃N₄ photocatalyst material. The Fen+/PMS system must be run at an acidic pH because of the precipitation and hydrolysis of iron ions; doing so after the reaction results in additional operational costs [29,30]. These inescapable disadvantages limit the widespread application of the homogeneous iron/PMS approach in wastewater treatment technology.

Even though several research have been widely published about the photocatalytic activity of the CoFe₂O₄/g-C₃N₄ composite and its ability to photodegrade several organic dyes/pollutants, the literature currently available falls significantly short in addressing crucial variables, including (1) construction of the direct Z-scheme structure and (2) the photoactivity of the composite in the absence of a Fenton-like system. Hence, to overcome these challenges, direct Z-scheme CoFe₂O₄-loaded g-C₃N₄ photocatalyst material was successfully fabricated using the facile hydrothermal method.

The photodegradation activity of the obtained direct Z-scheme CoFe₂O₄-loaded g-C₃N₄ photocatalyst was assessed for methylene blue (MB) photodegradation under irradiation of visible light and shows degradation activity 6.6 and 3.6 times higher than pristine CoFe₂O₄ and pure g-C₃N₄ photocatalysts, respectively. Such an extraordinary enhancement of catalytic activity under irradiation of visible-light could be due to the significantly increased electron-hole separation in the direct Z-scheme g-CoFe₂O₄-loaded g-C₃N₄, which improves the oxidation/reduction ability of the photocatalytic reaction. Moreover, the direct Z-scheme g-CoFe₂O₄-loaded g-C₃N₄ photocatalyst was collected from the aqueous solution without significant loss and demonstrated a negligible decline in performance throughout five cycles.

2. Results and Discussion

2.1. Characterization of the Photocatalyst

The FT-IR spectra of CoFe₂O₄, g-C₃N₄, and CoFe₂O₄-loaded g-C₃N₄ are displayed in Figure 1. As shown in Figure 1a, the broad bands in the 3086–3252 cm⁻¹ matches to the -NH stretching mode at the uncondensed sites of the aromatic structures [31]. The peaks at 1572 and 1639 cm⁻¹, and the other four peaks at 1407, 1461, 1316, and 1236 cm⁻¹, represented typical C=N stretching vibration modes and C-N stretching of g-C₃N₄, respectively. The intense peak at 812 cm⁻¹ can be attributed to the tri-s-triazine units’ breathing mode [7,32,33]. In spectrum of CoFe₂O₄, in the range of 587 and 419 cm⁻¹, there were two prominent absorption bands that are ascribed to both the tetrahedral and octahedral metal-oxygen (M-O) vibrational modes links in the spinel lattice of CoFe₂O₄ nanoparticles, as shown in Figure 1b [34]. The H-O-H bending vibration of absorbed or free water molecules is responsible for the peak seen at 3408 cm⁻¹. All the important characteristic peaks of CoFe₂O₄ and g-C₃N₄ appeared in the CoFe₂O₄-loaded g-C₃N₄ material; this confirmed the formation of CoFe₂O₄-loaded g-C₃N₄ photocatalyst material (Figure 1c).

XRD spectra of all the photocatalysts were taken for phase identification. Figure 2 displays the XRD spectra of CoFe₂O₄, g-C₃N₄, and CoFe₂O₄-loaded g-C₃N₄ photocatalysts. The diffraction peaks at 30°, 35.62°, 43.28°, 53.78°, 57°, and 62.27° are willingly attributed to the face centered cubic inverse spinel crystal structure of CoFe₂O₄ (JCPDS file no: 22-1086) [35]. The XRD spectrum from pure g-C₃N₄ exhibited peaks at 27.52° and 12.74°; these can be assigned to the (002) and (100) crystal planes of polymeric g-C₃N₄, respectively. In the spectrum of the CoFe₂O₄-loaded g-C₃N₄, all diffraction peaks of g-C₃N₄ and CoFe₂O₄ are observed, indicating that the CoFe₂O₄-loaded g-C₃N₄ photocatalyst materials were successfully prepared. Moreover, the Scherrer’s formula was used to determine the average crystallite size of CoFe₂O₄-loaded g-C₃N₄ photocatalyst and [36,37]; this was determined to be 13.43 nm.

The surface morphology and texture of the photocatalysts were investigated using SEM; the image is presented Figure 3a–c. The cubic structure of CoFe₂O₄ and crumpled sheets g-C₃N₄ are observed in Figure 3a,b, respectively. It has also been observed that numerous nanoparticles of CoFe₂O₄ were loaded on the g-C₃N₄ sheet surface (Figure 3c), suggesting the formation of CoFe₂O₄-loaded g-C₃N₄ photocatalyst. The elemental composition of the fabricated samples was examined using EDX; the spectra is displayed in Figure 3d–f. As shown in Figure 3a, the EDX peaks of the pristine CoFe₂O₄ sample are accredited to the elements Fe, Co, and O. The peaks in the EDX of neat g-C₃N₄ are linked to the elements C and N, as shown in Figure 3e. Also, the equivalent EDX spectrum of g-C₃N₄ that has CoFe₂O₄ loaded on it exhibits the presence of C, N, Co, Fe, and O, as shown in Figure 3f. This confirms the CoFe₂O₄-loaded g-C₃N₄ photocatalyst was synthesized without any additional impurities.

The magnetic possessions of the CoFe₂O₄-loaded g-C₃N₄ material and pristine CoFe₂O₄ photocatalysts were studied; the magnetization hysteresis curves are shown in Figure 4. The saturation magnetization (Ms) of pristine CoFe₂O₄ and CoFe₂O₄-loaded g-C₃N₄ photocatalyst was determined to be 43.4 emu/g and 41.9 emu/g, respectively. As a result, the CoFe₂O₄-loaded g-C₃N₄ photocatalyst could be easily collected using a magnet.

The pore size distribution, surface area, and pore volume of the CoFe₂O₄-loaded g-C₃N₄ photocatalyst were studied using N₂ adsorption and desorption isotherms. The type IV isotherm and H3 hysteresis loop of the N₂ adsorption-desorption curve are clearly visible in Figure 5a, confirming the mesoporous nature of the CoFe₂O₄-loaded gC₃N₄ photocatalyst [38]. The Barrett–Joyner–Halenda equation and Brunauer–Emmett–Teller (BET) method were used to determine the photocatalyst’s pore size distribution and surface area. The BET specific surface area of CoFe₂O₄-loaded g-C₃N₄ photocatalyst was determined to be 63.632 m²/g. Furthermore, the pore volume and pore size of the CoFe₂O₄-loaded g-C₃N₄ photocatalyst was 4.29 nm and 0.247 cm³/g, respectively, as presented as Figure 5b.

The UV−vis DRS spectra of g-C₃N₄, CoFe₂O₄, and CoFe₂O₄-loaded g-C₃N₄ samples are shown in Figure 6a. The pristine g-C₃N₄ photocatalyst possessed an absorption edge at about 500 nm. The CoFe₂O₄ nanoparticle showed a wide absorption range (200 to 900 nm). The absorption efficiency of the g-C₃N₄ photocatalyst was significantly extended in the visible-light region after loading nanoparticles of CoFe₂O₄ onto the g-C₃N₄ (Figure 6a). As a result, the boosted visible light absorption of the CoFe₂O₄-loaded g-C₃N₄ led to the production of extra electron−hole pairs under irradiation of visible-light, which boosted its photocatalytic activity. Furthermore, the band gap energy (E_g) of the photocatalysts were assessed using Tauc’s equation and the results are displayed in Figure 6b,c. The obtained band gap (E_g) values for pristine CoFe₂O₄ and g-C₃N₄ are 1.30 eV and 2.65 eV, respectively. However, after loading the CoFe₂O₄ on the surface of g-C₃N₄ nanosheets, its band gap energy (E_g) decreased to 1.42 eV. This result confirmed that the visible light absorption efficiency of g-C₃N₄ was significantly improved after modification by CoFe₂O₄ nanoparticles.

The photogenerated produced electron-hole pairs separation efficiency of the g-C₃N₄ and CoFe₂O₄-loaded g-C₃N₄ photocatalysts was examined using PL; the result is presented in Figure 6d. The PL strength of the CoFe₂O₄-loaded g-C₃N₄ is enormously diminished when compared to that of the pristine g-C₃N₄. This result shows that the rate of electron-hole separation efficiency in the CoFe₂O₄-loaded g-C₃N₄ was better than that in the g-C₃N₄ material. This could be because of the transfer of the electron-hole pairs between CoFe₂O₄ and g-C₃N₄ in the CoFe₂O₄-loaded g-C₃N₄.

2.2. Photocatalytic Activity

Figure 7a depicts the MB degradation under irradiation of visible light without photocatalyst as well as with CoFe₂O₄, g-C₃N₄, and CoFe₂O₄-loaded g-C₃N₄ photocatalysts. Figure 7a demonstrates that, without a photocatalyst, the MB is practically stable for 140 min when irradiated using visible light. This demonstrates that the presence of a photocatalyst caused MB degradation. The CoFe₂O₄-loaded g-C₃N₄ photocatalyst degraded 98.86% of the MB within 140 min under irradiation of visible light. However, under the same irradiation and time conditions, the g-C₃N₄ and CoFe₂O₄ degraded MB up to 74.92% and 51.53%, respectively. Therefore, the photodegradation rates of MB by the CoFe₂O₄-loaded g-C₃N₄ photocatalyst was better than that of pure CoFe₂O₄ and pristine g-C₃N₄ photocatalysts. In addition, the rate constant (k_ap) of degradation of MB by the CoFe₂O₄, g-C₃N₄, and CoFe₂O₄-loaded g-C₃N₄ photocatalysts can be expressed by a pseudo-first-order kinetics equation: ln(C_o/C) = k_apt, where C_o is initial concentration, C is concentration after irradiation at a time t, and k_ap is apparent pseudo-first-order rate constant (Figure 7b). As shown in Figure 7b, the k_ap of g-C₃N₄ is higher than that of CoFe₂O₄. However, after loading CoFe₂O₄ on the g-C₃N₄ sheets surface, its k_ap increased significantly. As a result, CoFe₂O₄-loaded g-C₃N₄ photocatalysts showed photodegradation activity with 6.6 and 3.6 times higher k_ap than pristine CoFe₂O₄ and g-C₃N₄, respectively. This might be a result of the components’ synergistic interaction. Table 1 compares the degradation efficiency of different materials from the literature with the fabricated direct Z-scheme CoFe₂O₄-loaded g-C₃N₄ photocatalyst material for the photodegradation of organic dyes in water.

The stability of CoFe₂O₄-loaded g-C₃N₄ photocatalyst was examined by recycling and using it for repeated photocatalytic reactions. The CoFe₂O₄-loaded g-C₃N₄ photocatalyst exhibits nearly the same photocatalytic activity after five consecutive cycles of the degradation reaction, as displayed in Figure 8, demonstrating the produced photocatalyst’s exceptional stability. This could be due to the good adherence of the CoFe₂O₄ nanoparticles on the g-C₃N₄ sheets.

In order to explore the principal active species in the photocatalytic process, reactive species detection tests were also conducted. As superoxide radical (•O₂), hydroxyl radical (•OH), and hole (h⁺) scavengers, p-benzoquinone (BQ), isopropanol (IPA), and formic acid (FA) were introduced at the same concentration (0.5 mmol/L), respectively [39,40]. As shown in Figure 9, the addition of IPA has an insignificant effect on the degradation efficiency, while the addition of BQ and FA diminished the degradation efficiency to 37.8% and 62.54%, respectively. Therefore, •O₂⁻ and h⁺ are the primary reactive species in the photocatalytic process, and •O₂⁻ has a greater influence on the photodegradation process than h⁺.

The boosted photocatalytic efficiency of the synthesized CoFe₂O₄-loaded g-C₃N₄ photocatalyst can be credited to the visible light sensitization of the g-C₃N₄ by the CoFe₂O₄ nanoparticles. To clarify the separation of electron-hole in the CoFe₂O₄-loaded g-C₃N₄ photocatalyst, the valence band (E_VB) edge and conduction band (E_CB) edge positions of the photocatalysts were calculated by Equations (1) and (2) [40].

(1)$E_{CB}=X-E-\frac{1}{2}{E}_g$

(2)$E_{VB}=E_{CB}+E_g$

where E_CB is conduction band potential, E_VB is valence band potential, χ is the absolute electronegativity of the photocatalyst (the χ values of g-C₃N₄ is 4.73 eV and χ values CoFe₂O₄ is 5.81 eV [3,41]), E is the hydrogen scale’s free electron energy, which is 4.5 eV, and E_g is band-gap energy. Hence, it was discovered that CoFe₂O₄ and gC₃N₄ have band-gap energies of 1.30 eV and 2.65 eV, respectively (Figure 6b,c). Then, the values of E_VB and E_CB for CoFe₂O₄ and g-C₃N₄ were calculated using Equations (1) and (2), respectively. The obtained results are shown in Table 2.

Photogeneration electron-hole (e⁻-h⁺) pairs are formed by both g-C₃N₄ and CoFe₂O₄ of the CoFe₂O₄-loaded g-C₃N₄ after irradiation by sufficient photon energy. The E_CB of CoFe₂O₄ (+0.66 eV) is less negative than that of the g-C₃N₄ (−1.09 eV), and the E_VB of CoFe₂O₄ (+1.96 eV) is more positive than the E_VB of g-C₃N₄ (+1.56 eV). In the case of type II heterojunction phase way, the e⁻ in the E_CB of g-C₃N₄ is transferred to the E_CB of CoFe₂O₄ and the h⁺ in the E_VB of CoFe₂O₄ are transferred to E_VB of the g-C₃N₄. These interfacial e⁻-h⁺ transfers suppress the charge carrier recombination and improve its photocatalytic activity. But, the e⁻ at E_CB of CoFe₂O₄ (+0.66 eV) cannot reduce O₂ to •O₂⁻ (E° = −0.33 eV) [16]. Alternatively, the h⁺ at E_VB of g-C₃N₄ (+1.56 eV) has insufficient oxidation capacity to oxidize H₂O or OH⁻ into •OH (•OH/H₂O, E° = +1.99 eV) [16]. This contradicts the experiments on active species trapping. Clearly, the type-II photogenerated electron-hole (e⁻-h⁺) mechanism was not appropriate for the CoFe₂O₄-loaded g-C₃N₄ photocatalytic system. Hence, the e⁻-h⁺ mechanism of the CoFe₂O₄-loaded g-C₃N₄ may be illustrated by the direct Z-scheme system (Figure 10). In the direct Z-scheme system, the e⁻ in the E_CB of CoFe₂O₄ are tending to transfer and recombine with the h⁺ in the E_VB of g-C₃N₄ by driving by the internal electric field at the intersection of surfaces, resulting in the gathering of e⁻ and h⁺ in the E_CB of g-C₃N₄ and the E_VB of the CoFe₂O₄, respectively [42,43]. Consequently, the e⁻ in the E_CB of g-C₃N₄ can easily reduce the O₂ into •O₂⁻ as shown in Figure 10. Even though these h⁺ in the E_VB of the CoFe₂O₄ cannot oxidize H₂O or OH⁻ into •OH, they can directly participate in MB pollutants adsorbed onto the surface of the CoFe₂O₄-loaded g-C₃N₄ photocatalyst [38]. The experiments on active species trapping are compatible with this strategy. As such, the CoFe₂O₄-loaded gC₃N₄ photocatalyst in the present direct Z-scheme photocatalytic system not only improves the transfer and separation efficiency of photogenerated charge carriers but also retains a high degree of redox ability during the photodegradation of MB pollutant.

3. Materials and Methods

3.1. Chemicals

Iron(III) chloride hexahydrate (FeCl₃.6H₂O, 97%), sodium hydroxide (NaOH, 98%), Cobalt(II) chloride hexahydrate (CoCl₂.6H₂O, 98%), Urea (NH₂CONH₂, 99.5%), and nitric acid (HNO₃, 70%) were obtained from Merck (India). All chemicals were utilized directly without further purification, and all experimental work was conducted with distilled water.

3.2. Synthesis of Photocatalysts

Graphitic carbon nitride (g-C₃N₄) was prepared by thermal polycondensation of urea; the detailed procedure has been described in our previously published article [7]. The CoFe₂O₄ loaded g-C₃N₄ photocatalyst was fabricated using the hydrothermal technique. Typically, 0.5 g of g-C₃N₄ was put in 50 mL of distilled water and sonicated for 40 min to form a uniform suspension (solution A). Similarly, 0.491g of FeCl₃·6H₂O (1.82 × 10⁻³ mol) and 21.62 mg of NiCl₂·6H₂O (9.1 × 10⁻⁵ mol) were put in 50 mL of distilled water and sonicated for 20 min (solution B). After that, the two solutions (solution A and solution B) were combined under sonication for 60 min. The resulting mixture was put in the Teflon-lined autoclave at 150 °C for 8 h. Then, the mixture was centrifuged and washed several times using ethanol and distilled water separately. The obtained sample was dried at 60 °C in a hot air oven to obtain the CoFe₂O₄ loaded g-C₃N₄ photocatalyst. Likewise, we prepared CoFe₂O₄-loaded g-C₃N₄ photocatalysts using the same procedure for comparison study.

3.3. Characterization Techniques

The crystallinity of the prepared samples was performed using X-ray Diffraction (D8 XRD, Bruker AXS) with Cu Kα radiation (λ = 0.154060 nm) over 2θ range of 7° to 70°. The molecular structure of the samples was examined using Fourier transform infrared spectroscopy (FT-IR, PerkinElmer–Frontier MIR/FIR) scanned in the range of 4000 to 400 cm⁻¹. The morphology and elemental composition were determined using scanning electron microscopy (SEM, JEOL- JSM 6390LV) with energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments). The UV–vis diffuse reflectance (DRS) was carried out on a UV–VIS spectrophotometer (Shimadzu–UV-2450) in the range of 200–900 nm by using BaSO₄ as the reference. The Kubelka–Munk function (F(R) was then used to determine the optical bandgap energy of the photocatalysts. The recombination rate photoluminescence (PL) spectra were obtained by Fluorescence spectrophotometer (PerkinElmer–LS 55). By employing a BET surface area analyzer (NOVA 1000E, Quantachrom) to conduct a N₂ adsorption-desorption investigation, the BET and pore size distribution of the sample were examined. A vibrating sample magnetometer (VSM, Lakeshore-7410) was used to examine the magnetic characteristics.

3.4. Photocatalytic Experiments

To study the photocatalytic activity of the photocatalysts in the breakdown of methylene blue (MB) under irradiation of visible light, several photocatalytic experiments were carried out. To attain an equilibrium between the photocatalyst and the dye, 30 mg of photocatalyst was suspended in 100 mL of MB aqueous solution (10 mg/L) and swirled in the dark for 60 min. A 10 W LED lamp (Havells, India) was used to irradiate the photocatalytic system and a 3 mL aliquot part; these were withdrawn from the reaction solution at 20 min intervals. Then, the magnetic photocatalysts was separated using a magnet and nonmagnetic photocatalysts were separated using a centrifuge (Universal 320 Hettich). The residual concentration of MB was analyzed using a UV-visible spectrophotometer (Agilent Cary 60) at a 664 nm. The primary active species were trapped using scavengers including formic acid (FA, h⁺ scavenger), benzoquinone (BQ, •O₂⁻ radical scavenger), and 2-propanol (2-PA, •OH radical scavenger).

4. Conclusions

In conclusion, we fabricated magnetically separable direct Z-scheme CoFe₂O₄-loaded g-C₃N₄ using a facile and simple hydrothermal technique. Characterization results confirmed that the CoFe₂O₄-loaded g-C₃N₄ was prepared successfully. The DRS study also showed that the absorption efficiency of the g-C₃N₄ photocatalyst was significantly extended in the visible-light region after loading nanoparticles of CoFe₂O₄ onto the g-C₃N₄ nanosheets. The photocatalytic performance of CoFe₂O₄, g-C₃N₄, and CoFe₂O₄-loaded g-C₃N₄ photocatalysts were examined for the MB degradation under irradiation of visible-light. The direct Z-scheme CoFe₂O₄-loaded g-C₃N₄ shows supreme degradation efficiency, and its activity is about 6.6 and 3.6 times higher than the CoFe₂O₄ and g-C₃N₄ photocatalysts, respectively. Such an extraordinary activity enhancement under irradiation of visible-light was possibly due to the significantly promoted electron-hole separation in the direct Z-scheme g-CoFe₂O₄-loaded g-C₃N₄; this improves the reduction/oxidation ability of the photocatalytic reaction. Mechanisms elucidated by scavenger studies revealed that •O₂⁻ and holes were the primary reactive radicals responsible for the degradation of MB. Moreover, the direct Z-scheme g-CoFe₂O₄-loaded g-C₃N₄ photocatalyst was collected from the aqueous solution without significant loss and demonstrated a negligible catalytic performance decline over five consecutive cycles. Therefore, this study can deliver novel insights for design and preparation of magnetic direct Z-scheme photocatalysts for dye degradation applications.

Author Contributions

The major work for this article, including conceptualization, methodology, software, validation, and writing the original draft, was performed by the first author (G.G.). The second author (M.G.) participated in conceptualization, software, and drawing. The final three authors (M.T., B.L. and W.L.) were responsible for conceptualization, supervising, and editing the article. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

We acknowledge Tezpur University, Assam, India, for some characterization facility support. We are also thankful to Addis Ababa Science and Technology University, Ethiopia, for the financial support from the Internal grant project (Ref. No. EG-51/11-1/20).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. FTIR spectra of (a) g-C3N4, (b) CoFe2O4, (c) CoFe2O4-loaded g-C3N4 photocatalysts.

Figure 2. XRD patterns of g-C3N4, CoFe2O4, and CoFe2O4-loaded g-C3N4 photocatalyst.

Figure 3. SEM images of (a) pure CoFe2O4, (b) pure g-C3N4, and (c) CoFe2O4-loaded g-C3N4 photocatalyst, and EDX pattern of (d) pure CoFe2O4, (e) pure g-C3N4, and (f) CoFe2O4-loaded g-C3N4 photocatalyst.

Figure 4. The magnetic hysteresis loops of pure CoFe2O4 and CoFe2O4-loaded g-C3N4 photocatalyst material.

Figure 5. (a) N2 adsorption–desorption isotherm of CoFe2O4-loaded g-C3N4 photocatalyst, and (b) the pore size distribution curve of CoFe2O4-loaded g-C3N4 photocatalyst.

Figure 6. (a) DRS spectra of g-C3N4, CoFe2O4, and CoFe2O4-loaded g-C3N4 photocatalyst. (b) Tauc plot for g-C3N4 photocatalyst. (c) Tauc plot for CoFe2O4 and CoFe2O4-loaded g-C3N4 photocatalyst. (d) PL spectra of g-C3N4, and CoFe2O4-loaded g-C3N4 photocatalyst.

Figure 7. (a) Photodegradation of MB by CoFe2O4, g-C3N4, and CoFe2O4-loaded g-C3N4 photocatalysts under visible light irradiation. (b) Photodegradation rate constant (kap) of MB over the CoFe2O4, g-C3N4, and CoFe2O4-loaded g-C3N4 photocatalysts.

Figure 8. Reusability of the CoFe2O4-loaded g-C3N4 photocatalyst after five successive runs.

Figure 9. Scavenger effects on the performance of MB degradation over CoFe2O4-loaded g-C3N4 photocatalyst.

Figure 10. The CoFe2O4-loaded g-C3N4 under irradiation of visible-light and the proposed direct Z-scheme photocatalytic process.

Table 1

Comparison of the effectiveness of several magnetic photocatalysts in the degradation of organic dyes.

No.	Photocatalyst	Light Source	Target Dye	Degradation Efficiency	Ref.
1	CoFe2O4/g-C3N4	Sun light	MB	98%/150 min	[28]
2	41.4% CoFe2O4/g-C3N4-H2O2	Xenon lamp	MB	97.3%/180 min	[27]
3	CoFe2O4/g-C3N4-PMS	Halogen tungsten lamp	RhB	96%/30 min	[1]
4	Direct Z-scheme CoFe2O4-loaded g-C3N4	LED lamp	MB	98.86%/140 min	This work

Table 2

Electronegativity (χ), band gap (E_g), and conduction band (E_CB) position and valance band (E_VB) of the photocatalysts on NHE.

Photocatalyst	χ (eV)	Eg (eV)	ECB (eV)	EVB (eV)
CoFe2O4	5.81	1.30	+0.66	+1.96
g-C3N4	4.73	2.65	−1.09	+1.56

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