Photocatalytic Degradation of Methyl Orange Dyes Using Green Synthesized MoS 2 /Co 3 O 4 Nanohybrids

: In this work, a new binary MoS 2 /Co 3 O 4 nanohybrids was successfully fabricated and the chemical structures, morphologies, electrochemical and optical characterizations were carried out. In addition, heterojunction nanoparticles present in S-scheme structures act as electron traps and promote light absorption capacity for the degradation of Methyl orange (MO) with visible-light activity. MoS 2 /Co 3 O 4 nanohybrids suggested excellent photocatalytic performance compared to bare MoS 2 and Co 3 O 4 , where 95.6% of MO was degraded within 170 min, respectively. The results also showed excellent stability and recyclability over ﬁve consecutive cycles, without noticeable changes in the nanocomposite structure. The boosted photocatalytic degradation and redox activities of MoS 2 /Co 3 O 4 can be attributed to the created S-scheme heterostructure to facilitate the separation of and to delay recombination of photoinduced charge carriers. We believe that this strategy of exploiting nanohybrid photocatalysts has great potential in the ﬁeld of environmental catalysis and diverse applications.


Introduction
Environmental contamination and energy demand represent a worldwide concern. To overcome these problems, nanomaterials are an alternative to address environmental aspects of wastewater treatment contamination and energy generation by organic compounds [1][2][3]. The immoderate emptying of waste toxic organic dyes, such as methyl orange (MO), into soils and water pools counteracts environmental protection and safety. Consequently, it is necessary to treat the dyeing wastewater with practical and harmless methods [4,5]. It is well-known that solar light is a favorite choice of clean energy for decomposing pollutants due to its great redox activity and energy density only makes H 2 O [6]. The utilization of visible light is a rather effective process as it adopts abundantly available solar light. To date, the photocatalytic system, as a green and direct reach, has been found to be effective in environmental treatment, especially in the removal of organic pollutants. Photocatalytic degradation with visible light activity is employed to excite the semiconductor material creating electron (e − ) and hole (h + ) pairs for the degradation of organic compounds. To date, different semiconductors have been adopted as photocatalysts in environmental applications [7]. Photocatalytic removal pollutants have attracted sufficient exploration attention and are considered a hopeful system to solve the energy crisis and environmental issues. However, the low absorption of sunlight, high electron-hole recombination, and photo-decay effects are still limiting to the acquisition of an efficient photocatalyst with visible light activity. [8]. Accordingly, it is necessary to fabricate and acquire nano-heterojunction photocatalysts with great solar-to-redox reaction conversion performance.
Recently, heterostructured photocatalysts have attracted researchers' attention due to superior features such as improved electron-hole pair separation performance [9]. The

Results and Discussion
XRD analysis was further collected to check the crystal phase form and structural composition of the as-fabricated photocatalysts, as exhibited in Figure 1. During the hydrothermal process, the apparent feature peaks of MoS2 were weaker while several new feature peaks appeared at 31.2°, 36.8°, 44.9° and 59.4° of the MoS2/Co3O4 photocatalysts, which are related with the (220), (311), (400) and (511) diffraction face of Co3O4 (JCPDS No. 42-1467) [15], respectively. The MoS2 materials exhibits a series of primary feature peaks at 14.3°, 32.9°, 39.8°, 44.5°, 50.2°, 50.7°, and 60.7°, which assigned to the (002), (100), (103), (006), (105), and (110) crystal planes (JCPDS: , respectively, and no other impurity peaks were perceived [11]. Its value indicating that there is no altered signal in the XRD analysis of the MoS2/Co3O4 nanocomposites compared to the MoS2 and Co3O4 samples, suggesting that the presented Co3O4 content is fewer, and the dispersion is greater, and there are no other impurity peaks or feature peak shift. In addition, when the reaction solutions concentration of Co3O4 was increased from 5 wt% to 20 wt%, the (311) feature peak intensity of Co3O4 became higher, which indicated an enlarge in the dosage of Co3O4 in the heterojunction. In particular, contrast with the XRD analysis of Co3O4, the overall diffraction intensities of MoS2/Co3O4 heterojunctions were altered at 36.8° with (311) diffraction face (see Figure 1 inset). In addition, the feature peaks of MC-X (X = 5 wt%, 10 wt%, 15 wt%, and 20 wt%) are comparable to the feature XRD analysis of MoS2 without additional impurity peaks, which suggests that the nanohybrids photocatalyst further possess coherent crystal structure [25,26]. In order to explore the development step of morphologies after hydrothermal process, SEM and TEM were characterized. As exhibited in Figure 2a,b, apparently large- In order to explore the development step of morphologies after hydrothermal process, SEM and TEM were characterized. As exhibited in Figure 2a,b, apparently large-sized bulk MoS 2 and Co 3 O 4 can be found, respectively. It is obtained that the microstructure of the photocatalyst is sphere-like and plate-like with approximately 5 µm equivalent size. However, the MoS 2 and Co 3 O 4 possess a high grade of collection, which intensely suppress the surface of the active location of the photocatalysts. As displayed in Figure 2c, the MoS 2 /Co 3 O 4 heterojunction presents a nanomaterial with an irregular surface, which gives it a greater specific surface area. Therefore, this reflection was verified in the specific surface area examination test. It is pronounced that despite the size of MC-X being immensely decreased, the crystals of MoS 2 still retain great crystallinity, as presented by the (002) major peak observed in the XRD patterns ( Figure 1). This nanohybrid of MoS 2 /Co 3 O 4 serves a sufficient area for the efficient load of Co 3 O 4 , which can mainly disperse MoS 2 /Co 3 O 4 nanohybrids and decrease aggregation. As displayed in Figure 2d, the MoS 2 /Co 3 O 4 -20% heterojunction displays an aggregated form with a dimension of approximately 4-10 nm. From Figure 2e, the lattice fringe of 0. 26  sized bulk MoS2 and Co3O4 can be found, respectively. It is obtained that the microstructure of the photocatalyst is sphere-like and plate-like with approximately 5 μm equivalent size. However, the MoS2 and Co3O4 possess a high grade of collection, which intensely suppress the surface of the active location of the photocatalysts. As displayed in Figure  2c, the MoS2/Co3O4 heterojunction presents a nanomaterial with an irregular surface, which gives it a greater specific surface area. Therefore, this reflection was verified in the specific surface area examination test. It is pronounced that despite the size of MC-X being immensely decreased, the crystals of MoS2 still retain great crystallinity, as presented by the (002) major peak observed in the XRD patterns ( Figure 1). This nanohybrid of MoS2/Co3O4 serves a sufficient area for the efficient load of Co3O4, which can mainly disperse MoS2/Co3O4 nanohybrids and decrease aggregation. As displayed in Figure 2d, the MoS2/Co3O4-20% heterojunction displays an aggregated form with a dimension of approximately 4-10 nm. From Figure 2e, the lattice fringe of 0.26 nm could be apparently noticed, which corresponds to the (002) plane of hexagonal MoS2. The fringe spacing of 0.32 nm is assigned to the (311) plane of Co3O4. During further testing, it could be found that the MoS2/Co3O4-20% nanohybrids were eventually constructed. The element mappings exhibit that the Mo, S, and Co elements are consistently distributed in the MoS2/Co3O4-20% nanohybrids (Figure 2f-i). The construction of the heterostructure of MoS2/Co3O4 is broadly supposed to possess a definite effect on promoting the charge carrier's separation ability of photocatalytic. The consistent distribution of Mo, S, and Co elements can confirm the effective formation of MoS2/Co3O4 nanohybrids.  The UV-vis absorption spectra are generally adopted to check the visible light absorption capability of the photocatalyst, and the analysis of the as-prepared MoS 2 /Co 3 O 4 heterojunction is exhibited in Figure 3. As can be observed, the two optical band gaps of MoS 2 /Co 3 O 4 -X nanocomposites are 1.56~1.69 eV (Eg1) and 2.31~2.61 eV (Eg2), which are in good agreement with the MoS 2 and Co 3 O 4 energy band gap. As MoS 2 /Co 3 O 4 photocatalyst is a photo-response sample, it possessed a wide wavelength absorption ability. Eg1 is joined to the onset of O(II)-Co(III) excitations [13][14][15]. The maximum response wavelength of MoS 2 /Co 3 O 4 is extended to 550 nm. Then, it presents a narrower band gap (Eg2) of 2.31~2.61 eV after adding Co 3 O 4 , which is favorable to improve visible light application and photoexcited charge carriers. In addition, the UV-vis spectra of the MoS 2 /Co 3 O 4 nanohybrids are relativity close. The construction for this case is that the minor added dosage of Co 3 O 4 does not generate an apparent alteration to the light absorption efficiency of the MoS 2 /Co 3 O 4 nanohybrids. The MC-20% nanohybrids display better light absorption and the absorption range has a minor red shift in the range of 550-680 nm. It could be assigned to the cooperation between MoS 2 and Co 3 O 4 . Hence, the Co 3 O 4 materials may be a significant reason to enlarge the light absorption of the MoS 2 /Co 3 O 4 nanohybrids. The band gap energy (Eg) of the corresponding photocatalyst was received via the transformed Kubelka-Munk function [27,28]. As exhibited in Figure   The UV-vis absorption spectra are generally adopted to check the visible light absorption capability of the photocatalyst, and the analysis of the as-prepared MoS2/Co3O4 heterojunction is exhibited in Figure 3. As can be observed, the two optical band gaps of MoS2/Co3O4-X nanocomposites are 1.56~1.69 eV (Eg1) and 2.31~2.61 eV (Eg2), which are in good agreement with the MoS2 and Co3O4 energy band gap. As MoS2/Co3O4 photocatalyst is a photo-response sample, it possessed a wide wavelength absorption ability. Eg1 is joined to the onset of O(II)-Co(III) excitations [13][14][15]. The maximum response wavelength of MoS2/Co3O4 is extended to 550 nm. Then, it presents a narrower band gap (Eg2) of 2.31~2.61 eV after adding Co3O4, which is favorable to improve visible light application and photoexcited charge carriers. In addition, the UV-vis spectra of the MoS2/Co3O4 nanohybrids are relativity close. The construction for this case is that the minor added dosage of Co3O4 does not generate an apparent alteration to the light absorption efficiency of the MoS2/Co3O4 nanohybrids. The MC-20% nanohybrids display better light absorption and the absorption range has a minor red shift in the range of 550-680 nm. It could be assigned to the cooperation between MoS2 and Co3O4. Hence, the Co3O4 materials may be a significant reason to enlarge the light absorption of the MoS2/Co3O4 nanohybrids. The band gap energy (Eg) of the corresponding photocatalyst was received via the transformed Kubelka-Munk function [27,28]. As exhibited in Figure   Then, X-ray photoelectron spectroscopy (XPS) is validated to recognize surface information of the fabricated samples. Figure 4a displays the survey XPS spectra. As further observed, MC-20% is constructed of Mo, S, Co and O elements, which involve the integral elements of MoS2/Co3O4 nanohybrids. There are no other impurity peaks emerging in survey spectra, suggesting that there are no other contaminate photocatalysts during hydrothermal route. The high-resolution spectrum of Mo 3d (Figure 4b) could be divided into two peaks, which centered are at ∼229.6 eV and ∼232.8 eV, referring to Mo 3d 5/2 and Mo 3d 3/2 [10]. Correspondingly, two feature peaks in the high-resolution S 2p XPS spectra of MC-20% centered at nearly 162.6 eV and nearly 163.6 eV ( Figure 4c) are referring to S 2p3/2 and S 2p1/2, respectively [11]. As shown in Figure 4d, binding energies of ∼780.1 eV and ∼795.5 eV referring to Co 2p 3/2 and Co 2p 1/2 for MC-20% nanohybrids, respectively [14]. The O 1s spectra of MC-20% ( Figure 4e) binding energies at 529.8 eV and 531.8 eV, referring to the O 2− in the Co3O4 and the adsorbed oxygen [15]. These results suggested that the MoS2/Co3O4 nanohybrids were well fabricated with a strong interaction between MoS2 and Co3O4. The elements concerned with the formation of MoS2/Co3O4 nanohybrids were confirmed by the EDS spectra. Four noticeable peaks were obtained at nearly 0.8 keV, which was owing to the presence of oxygen atom, and other three peaks arising at Then, X-ray photoelectron spectroscopy (XPS) is validated to recognize surface information of the fabricated samples. Figure 4a displays the survey XPS spectra. As further observed, MC-20% is constructed of Mo, S, Co and O elements, which involve the integral elements of MoS 2 /Co 3 O 4 nanohybrids. There are no other impurity peaks emerging in survey spectra, suggesting that there are no other contaminate photocatalysts during hydrothermal route. The high-resolution spectrum of Mo 3d (Figure 4b) could be divided into two peaks, which centered are at ∼229.6 eV and ∼232.8 eV, referring to Mo 3d 5/2 and Mo 3d 3/2 [10]. Correspondingly, two feature peaks in the high-resolution S 2p XPS spectra of MC-20% centered at nearly 162.6 eV and nearly 163.6 eV ( Figure 4c) are referring to S 2p3/2 and S 2p1/2, respectively [11]. As shown in Figure 4d, binding energies of ∼780.1 eV and ∼795.5 eV referring to Co 2p 3/2 and Co 2p 1/2 for MC-20% nanohybrids, respectively [14]. The O 1s spectra of MC-20% ( Figure 4e) binding energies at 529.8 eV and 531.8 eV, referring to the O 2− in the Co 3 O 4 and the adsorbed oxygen [15]. These results suggested that the MoS 2 /Co 3 O 4 nanohybrids were well fabricated with a strong interaction between MoS 2 and Co 3 O 4 . The elements concerned with the formation of MoS 2 /Co 3 O 4 nanohybrids were confirmed by the EDS spectra. Four noticeable peaks were obtained at nearly 0.8 keV, which was owing to the presence of oxygen atom, and other three peaks arising at 0.8 keV and 6.9 keV related to Co, and 2.3 keV, are related to Mo and S as exhibited in Figure 4f. The results confirmed the successful fabrication of MoS 2 /Co 3 O 4 nanohybrids. The Brunauer-Emmett-Teller (BET) specific surface areas of MoS 2 , Co 3 O 4 , and MC-20% are evaluated to be~35.2, 24.6, and 68.7 m 2 g −1 , respectively ( Table 1). The main aperture size of MoS 2 , Co 3 O 4 , and MC-20% are~38.4, 48.5 and 8.9 nm, respectively. The pore volumes of MoS 2 , Co 3 O 4 , and MC-20% are~0.193 cm 3 g −1 , 0.124 cm 3 g −1 and 0.372 cm 3 g −1 , respectively. BET of MC-20% was more than 2 times than that of MoS 2 and Co 3 O 4 , demonstrating the introduced Co 3 O 4 had a critical influence on raising the specific surface area of MoS 2 .
Catalysts 2022, 12, 1474 6 of 15 0.8 keV and 6.9 keV related to Co, and 2.3 keV, are related to Mo and S as exhibited in Figure 4f. The results confirmed the successful fabrication of MoS2/Co3O4 nanohybrids. The Brunauer-Emmett-Teller (BET) specific surface areas of MoS2, Co3O4, and MC-20% are evaluated to be ~35.2, 24.6, and 68.7 m 2 g −1 , respectively ( Table 1). The main aperture size of MoS2, Co3O4, and MC-20% are ~38.4, 48.5 and 8.9 nm, respectively. The pore volumes of MoS2, Co3O4, and MC-20% are ~0.193 cm 3 g −1 , 0.124 cm 3 g −1 and 0.372 cm 3 g −1 , respectively. BET of MC-20% was more than 2 times than that of MoS2 and Co3O4, demonstrating the introduced Co3O4 had a critical influence on raising the specific surface area of MoS2.    The photodegradation capability of the photocatalysts followed the sequence: MC-20% nanohybrids > MC-15% nanohybrids > MC-10% nanohybrids > MC-5% nanohybrids > MoS2 > Co3O4. This poor photodegradation performance could be assigned to the high recombination of electrons and holes in the bulk component [29]. Noticeably, MC-20% nanohybrids described excellent photodegradation performance of MO pollutants (~95.6%) than bare MoS2 or Co3O4, which was about 2.3 folds than bare Co3O4. The major process of photocatalytic reaction for the composite magnetic photocatalyst MoS2/Co3O4 based on the basic principles and studies reported in the literature can be summarized as follows:    [29]. Noticeably, MC-20% nanohybrids described excellent photodegradation performance of MO pollutants (~95.6%) than bare MoS 2 or Co 3 O 4 , which was about 2.3 folds than bare Co 3 O 4 . The major process of photocatalytic reaction for the composite magnetic photocatalyst MoS 2 /Co 3 O 4 based on the basic principles and studies reported in the literature can be summarized as follows: Composite h + + MO → MO * (Oxidation) Catalysts 2022, 12, 1474 9 of 15   In addition, the photocatalytic reaction kinetics of an MO removal test was calculated to a pseudo first order equation model in order to check the photocatalytic activity. The linear relationship between ln (Co/C) vs. irradiation time (t) is exhibited in Figure 5c and the removal rate constant (K app ) is displayed in Figure 5d. The MC-20% nanohybrids were measured with a higher K app value compared to the other synthesized samples. For effective utility purposes, the stability and reusability of MC nanohybrids were estimated by cycling runs. After every test, the sample was centrifuged, washed and dried before operating the next run. As shown in Figure 5e, the photodegradation capability of the MC-20% sample was 95.6%, 92.3%, 90.5%, 88.3%, and 86.8% for first, second, third, fourth, and fifth test, respectively. The noticed reduction of the photocatalytic activity of MC-20% can be referred to an adsorbed MO and its evolution products on the surface of MC-20% nanohybrids, thus prohibiting the active sites and visible light from approaching the photocatalyst [30]. The exploration of the photodegradation mechanism of the nanohybrids acted as a critical factor in the effective utilization of the heterojunction photocatalyst. Accordingly, the free radical trapping tests were performed, in which p-BQ (1,4-benzoquinone), IPA (2-propanol) and EDTA-2Na were adopted as scavenging free radical ( • O 2 − , • OH and h + ) agents [31]. It could be observed from the analysis results in Figure 5f that when various scavenging free radical agents were further joined, the photocatalytic efficiency of the MC-20% sample was influenced to altering degrees. The MC-20% sample was highly responsive to p-BQ and EDTA-2Na, implying that • O 2 − and h + were the major active species during the photodegradation activity, and • OH only acted as an assisting active species. Consequently, these results demonstrate that the formation of MoS 2 /Co 3 O 4 with a proper ratio (20 wt%) is able to receive the better photocatalytic efficiency of MoS 2 /Co 3 O 4 in the nano-heterostructure. The stability and repeatability of the sample are important indexes to evaluate the performance of photocatalysts. XRD analysis was performed after the fourth recycling experiment. Taking MC-20% photocatalyst as an example, its activity did not change significantly after five cycles, which can still degrade 86.8% of MO (Figure 5g). This result clearly suggests the stability of the fabricated MoS 2 /Co 3 O 4 NC photocatalyst. In addition, Table 2 displays the photocatalytic performance of relevant heterojunction in previous reports [31][32][33][34][35][36]. By contrast, it is observed that the MoS 2 /Co 3 O 4 nanomaterial we prepared has remarkable benefits for organic compounds of high concentration in the photocatalytic process. For confirm the presented S-scheme heterojunction, electron spin resonance (ESR) is adopted to recognize the free radicals produced in the system of MoS 2 , Co 3 O 4 and MC-20% nanohybrids. Applying DMPO as a spin capture agent, DMPO-• O 2 − and DMPO-• OH are observed with light conditions. It could be found in Figure 6a that the MC-20% photocatalyst displays the higher feature signal of DMPO-• O 2 − , while the feature signal of DMPO-• O 2 − of MoS 2 and Co 3 O 4 is nearly undetectable. It exhibits that the construction of heterojunction is more favorable to the production of • O 2 − , which could be referred to the rapider migration and separation of photoexcited charge carriers in heterojunction catalysts [36]. Furthermore, the DMPO-• OH feature signal peaks of MoS 2 , Co 3 O 4 and MC-20% could be found in Figure 6b, of which MC-20% possess the highest signal peak. Hence, it could be resulted that MC-20% nanohybrids can generate more • O 2 − and • OH. The coincidence of • O 2 − and • OH radicals further verify that the charge carrier separation mechanism is S-scheme. The charge transfer routes are analyzed by photoelectrochemical measurements. Electrochemical impedance spectroscopy (EIS) with visible light activity is exhibited in Figure 6c, and the smaller semi-circular curve in Nyquist plots expresses the lesser charge carrier transport resistance. Apparently, the lowest charge carrier transport resistance in MC-20% imply the greatest efficient charge carrier transport routes [37], which corresponds with the outstanding PHE capability of MC-20% nanohybrids. As exhibited in Figure 6d, all the MC-20% nanohybrids electrodes exhibit a great transient photocurrent response than that bare MoS 2 and Co 3 O 4 behaves at the highest intensity, implying the great efficient charge separation. Room-temperate PL spectra are carried out to check the charge carrier dynamics behavior. As displayed in Figure 6e, the apparently reduced PL intensity of MC-20% also proves the prohibited electrons and holes recombination, which is in good approval with the above results. Furthermore, the performance of the charge carrier is further explored under photocatalytic redox reactions. The efficient separation of photo-induced charge carriers is a required situation for enhancing the capability of photodegradation [38]. It is also found that the poorer the peak PL intensity, the lower the possibility of electron and hole pairs recombination rate, which is more favorable to the development of the pollutant removal reaction. This is owing to the evidence that the formation of nano-heterojunction improves the quick separation and transport of photoexcited charge carriers, hence, greatly suppressing the recombination rate of photo-induced charge carriers and enhancing the performance of charge carrier separation.
aration of photo-induced charge carriers is a required situation for enhancing the capability of photodegradation [38]. It is also found that the poorer the peak PL intensity, the lower the possibility of electron and hole pairs recombination rate, which is more favorable to the development of the pollutant removal reaction. This is owing to the evidence that the formation of nano-heterojunction improves the quick separation and transport of photo-excited charge carriers, hence, greatly suppressing the recombination rate of photoinduced charge carriers and enhancing the performance of charge carrier separation. Based on the above data analysis and results, a photocatalytic activity mechanism of MO removal on MC nanohybrids is recommended, as exhibited in Scheme 1. Following the introduction of Co3O4 materials on the surface of MoS2, a firm heterostructure is organized. During exposure to visible light, photocatalysis activity has various efficiencies to the incident visible light in varied wavelength ranges. In this work, MC nanohybrids can absorb a broad range of incident visible light with the same conditions, thus generating more photoexcited charge carriers. More significantly, the system of photocatalytic reaction of nanohybrids design is higher than that of single-component design, implying in an improved interaction among reactant and photocatalyst. Ultimately, the excellent hydrogen evolution and pollutants removal promote the nanohybrids to have a higher charge carrier separation behavior [39,40]. In the meantime, a built electric field from Co3O4 to MoS2 induces the downward band curving of MoS2 and upward band curving of Co3O4 at the heterojunction interface, respectively. During visible light irradiation, both MoS2 and Co3O4 could be photo-induced to produce electrons and holes. The electric field, Based on the above data analysis and results, a photocatalytic activity mechanism of MO removal on MC nanohybrids is recommended, as exhibited in Scheme 1. Following the introduction of Co 3 O 4 materials on the surface of MoS 2 , a firm heterostructure is organized. During exposure to visible light, photocatalysis activity has various efficiencies to the incident visible light in varied wavelength ranges. In this work, MC nanohybrids can absorb a broad range of incident visible light with the same conditions, thus generating more photoexcited charge carriers. More significantly, the system of photocatalytic reaction of nanohybrids design is higher than that of single-component design, implying in an improved interaction among reactant and photocatalyst. Ultimately, the excellent hydrogen evolution and pollutants removal promote the nanohybrids to have a higher charge carrier separation behavior [39,40]. In the meantime, a built electric field from Co 3 O 4 to MoS 2 induces the downward band curving of MoS 2 and upward band curving of Co 3 O 4 at the heterojunction interface, respectively. During visible light irradiation, both MoS 2 and Co 3 O 4 could be photo-induced to produce electrons and holes. The electric field, curved band edges and the coulomb interaction notably drive the CB electrons of MoS 2 to rebuild with the VB holes of Co 3 O 4 , leading to the practical conservation of the high-oxidative holes in the VB of MoS 2 and high-reductive electrons in the CB of Co 3 O 4 , which significantly promotes the photo-response efficiency of MoS 2 /Co 3 O 4 . This facile S-scheme system can efficiently weaken the recombination rate of photoinduced charge carriers in the heterojunction and improve the recombination of limited photo-generated electron and hole pairs, hence increasing the excellent redox activity of the photocatalyst. Finally, the enhanced photodegradation efficiency could be attributed to the increased visible light absorption ability, more reactive sites and powerful transfer of photo response charge carriers. In addition, the interface influence is larger than the efficiency of the photogenerated hole and electron to recombine. The electrons of the conduction band of MoS 2 contribute to the pollutant removal reduction reaction. Then, the holes of Co 3 O 4 are oxidized in the valence band by the photocatalytic process. This novel S-scheme nanocomposite possesses better redox ability [19,26,27], which apparently enhances the capability of photocatalytic pollutants removal. The CB and VB of MC with visible light activity, are induced to create photoexcited electrons and holes, respectively. On the other hand, the photo-response charge carriers in MoS 2 and Co 3 O 4 will recombine rapidly, implying a weakening in photocatalytic capability. Although, owing to the presence of Co 3 O 4 cocatalyst, photoexcited electrons will rapidly migrate to MoS 2 , and also further water reduction so hydrogen evolution will appear. Furthermore, the presence of MoS 2 can weaken over-potential and serve more active sites for photocatalytic and can further enlarge the visible light absorption of Co 3 O 4 .
Catalysts 2022, 12, 1474 12 of 1 absorption ability, more reactive sites and powerful transfer of photo response charge car riers. In addition, the interface influence is larger than the efficiency of the photogenerated hole and electron to recombine. The electrons of the conduction band of MoS2 contribute to the pollutant removal reduction reaction. Then, the holes of Co3O4 are oxidized in the valence band by the photocatalytic process. This novel S-scheme nanocomposite pos sesses better redox ability [19,26,27], which apparently enhances the capability of photo catalytic pollutants removal. The CB and VB of MC with visible light activity, are induced to create photoexcited electrons and holes, respectively. On the other hand, the photo response charge carriers in MoS2 and Co3O4 will recombine rapidly, implying a weakening in photocatalytic capability. Although, owing to the presence of Co3O4 cocatalyst, photo excited electrons will rapidly migrate to MoS2, and also further water reduction so hydro gen evolution will appear. Furthermore, the presence of MoS2 can weaken over-potentia and serve more active sites for photocatalytic and can further enlarge the visible light ab sorption of Co3O4. Scheme 1. The possible photocatalytic mechanism of nanohybrids.
Typically, 1.2 mmol of Na2MoO4·2H2O and 2.5 mmol the L-cysteine was used to 40 mL of deionized (DI) water to develop a homogeneous solution via stirring and ultrasonic treatment for 30 min. CoCl2·6H2O (2.5 mmol) was added in 40 mL distilled water and a certain amount of surfactant (1%, w/w, Triton X-100) was added subsequently. Subse Scheme 1. The possible photocatalytic mechanism of nanohybrids.
Typically, 1.2 mmol of Na 2 MoO 4 ·2H 2 O and 2.5 mmol the L-cysteine was used to 40 mL of deionized (DI) water to develop a homogeneous solution via stirring and ultrasonic treatment for 30 min. CoCl 2 ·6H 2 O (2.5 mmol) was added in 40 mL distilled water and a certain amount of surfactant (1%, w/w, Triton X-100) was added subsequently. Subsequently, the homogeneous solution was transformed into a Teflon-lined stainless-steel autoclave (100 mL) and reacted at 180 • C for 20 h. After cooling down to room temperature naturally, the pristine dark precipitate was obtained by centrifugation, washed three times with deionized water, NMP, absolute ethanol and acetone, and dried at 70 • C in a vacuum oven overnight. A series of MoS 2 /Co 3 O 4 photocatalysts was provided by increasing various doses of Co 3 O 4 (5 wt%~20 wt%); the individual samples are denoted as MC-X (X = 5 wt%, 10 wt%, 15 wt%, and 20 wt%). Finally, the nanocomposite is prepared via the hydrothermal method, as shown in Scheme 2.
(Munich, Germany) at room temperature under atmospheric conditions. A CH Instruments electrochemical workstation with a standard three-electrode system was recorded to carry out electrochemical measurements in 0.2 M Na2SO4 solution. Furthermore, the saturated Pt electrode and Hg/Hg2Cl2 electrode were used as the counter electrode and reference electrode, respectively. The working electrode was used according to the next procedure: the as-prepared photocatalysts (20 mg) were dispersed in 10 mL of DI water to obtain a homogeneous suspension with a consecutive ultrasonic process. The chemical states and surface species were tested by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA).
Photodegradation of MO pollutants over various MC-X photocatalysts was performed at room temperature with visible light activity. A 350 W Xe lamp was employed as the visible-light source. A total of 20 mg of samples were added in 100 mL MO aqueous solution (20 mg L -1 ). The above reaction solution was further stirred under the dark for 20 min to organize absorption-desorption equilibrium between photocatalysts and MO compounds. Under light activity, a definite quantity of reaction solution was carried out from the combination solution at contributed intervals, followed by centrifugation to eliminate all the materials. The concentration alters of MO pollutants were obtained via UV-vis spectroscopy under photocatalytic activity.

Conclusions
In this study, MoS2/Co3O4 nanohybrids were fabricated adopting a facile hydrothermal process. The prepared nanohybrids exhibited better capability for the degradation of MO dye from aqueous solution. The maximum MO photodegradation performance approached by the nanohybrids was 95.6%, and the reaction rate constants was 0.021. The MoS2/Co3O4 nanohybrids with 20 wt% loading of Co3O4 on MoS2 nanoparticles, displays excellent photocatalytic response for MO dye removal as compared to the bare photocatalysts and other nano-heterojunctions. The 20 wt% MoS2/Co3O4 nanohybrids were the optimal photocatalysts through photoreduction and photodegradation activity greater than that of bare MoS2 and Co3O4. The result demonstrates that the formation of MoS2/Co3O4 nanohybrids is conducive to the production of excellent photo response carriers and the enhancement of visible light absorption. In addition, the fabrication of nanohybrids effec- X-ray diffraction patterns (XRD) were determined by a Bruker D8 Advanced diffractometer (Billerica, MA, USA) at 5-75 • with a scanning step of 20 • /min at room temperature. Hitachi UV-4100 UV-visible spectrophotometer and Edinburgh Analytical Instruments F-7000 (Tokyo, Japan) were adopted to check the UV-vis diffusive reflectance spectra and photoluminescence (PL) spectra with an excitation wavelength of 620 nm. The microstructure and morphology of different samples also were tested by employing scanning electron microscopy (SEM) micrographs (ZEISS AURIGA, Oberkochen, Germany) and transmission electron microscopy (TEM) experiment (JEOL JEM-2100F, Tokyo, Japan). Electron paramagnetic resonance (EPR) spectra were observed on a Bruker A300 (Munich, Germany) at room temperature under atmospheric conditions. A CH Instruments electrochemical workstation with a standard three-electrode system was recorded to carry out electrochemical measurements in 0.2 M Na 2 SO 4 solution. Furthermore, the saturated Pt electrode and Hg/Hg 2 Cl 2 electrode were used as the counter electrode and reference electrode, respectively. The working electrode was used according to the next procedure: the as-prepared photocatalysts (20 mg) were dispersed in 10 mL of DI water to obtain a homogeneous suspension with a consecutive ultrasonic process. The chemical states and surface species were tested by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA).
Photodegradation of MO pollutants over various MC-X photocatalysts was performed at room temperature with visible light activity. A 350 W Xe lamp was employed as the visible-light source. A total of 20 mg of samples were added in 100 mL MO aqueous solution (20 mg L −1 ). The above reaction solution was further stirred under the dark for 20 min to organize absorption-desorption equilibrium between photocatalysts and MO compounds. Under light activity, a definite quantity of reaction solution was carried out from the combination solution at contributed intervals, followed by centrifugation to eliminate all the materials. The concentration alters of MO pollutants were obtained via UV-vis spectroscopy under photocatalytic activity.

Conclusions
In this study, MoS 2 /Co 3 O 4 nanohybrids were fabricated adopting a facile hydrothermal process. The prepared nanohybrids exhibited better capability for the degradation of MO dye from aqueous solution. The maximum MO photodegradation performance approached by the nanohybrids was 95.6%, and the reaction rate constants was 0.021. The MoS 2 /Co 3 O 4 nanohybrids with 20 wt% loading of Co 3 O 4 on MoS 2 nanoparticles, displays excellent photocatalytic response for MO dye removal as compared to the bare photocatalysts and other nano-heterojunctions. The 20 wt% MoS 2 /Co 3 O 4 nanohybrids were the optimal photocatalysts through photoreduction and photodegradation activity greater than that of bare MoS 2 and Co 3 O 4 . The result demonstrates that the formation of MoS 2 /Co 3 O 4 nanohybrids is conducive to the production of excellent photo response carriers and the enhancement of visible light absorption. In addition, the fabrication of nanohybrids effectively inhibits the recombination rate of photo response electrons and holes in the photocatalytic activity. Thereby, this research paves way for the facile formation of nanohybrids system for diverse environmental fields.