Photocatalytic Degradation of Methyl Orange Dyes Using Green Synthesized MoS₂/Co₃O₄ Nanohybrids

Abstract

In this work, a new binary MoS₂/Co₃O₄ 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. MoS₂/Co₃O₄ nanohybrids suggested excellent photocatalytic performance compared to bare MoS₂ and Co₃O₄, 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 MoS₂/Co₃O₄ 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.

1. 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 H₂O [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 transition-metal dichalcogenides (TMDs) have obtained large attention owing to a series of interesting excellent properties [10]. Between these materials, molybdenum disulfide (MoS₂) has great charge carrier density, high conductivity, and a controllable energy band gap, which is valuable and effective for optimizing environmental remediation and improving photocatalytic water splitting capability with visible light irradiation [11]. As a kind of normal TMDs, MoS₂ has a bandgap energy of about 1.7~2.7 eV and proper valence conduction band potentials that own great photocatalytic efficiency and can employ an immense section of solar light [12]. Unfortunately, the poor light harvesting ability, inactive removal pollutants performance, and quick recombination rate of photo-excited charge carriers are the natural disadvantages that inhibit the useful utilization of MoS₂ [13]. In addition, Co₃O₄ also exhibits obvious stability under the photocatalytic system. In addition, Co₃O₄ possesses the matched band gap potential with MoS₂ to develop an efficient step-scheme (S-scheme) heterostructure because the valence band (VB) of Co₃O₄ is close to the conduction band (CB) of MoS₂ based on previous works [12]. However, the Co₃O₄ catalyst absorbs primarily visible light and possesses a wide band gap energy, which prohibits their photocatalytic efficiency. Among these heterostructures, MoS₂ and Cobalt oxide (Co₃O₄) photocatalysts exhibit various superior properties for instance remarkable direct bandgaps, excellent mechanical properties, outstanding optical property as well as distinguished photocatalytic efficiency [14,15,16]. On the other hand, bare MoS₂ and Co₃O₄ for direct photocatalytic water splitting or removal of contaminants are quite challenging because of their poor operation and high electron and hole recombination rate [17,18]. In addition, this heterojunction not only improved H⁺ adsorption and promoted migration and transport of H⁺ to the surface reactive sites, but further suppress the stacking or aggregation of materials [19]. To solve this issue, a common method is to develop nano-heterostructure between Co₃O₄ and MoS₂ under visible light irradiation to improve their visible light activity. To overcome these barriers, substantial promotions have been made in the reasonable formation and design of MoS₂/Co₃O₄-based nanohybrids with well-established interfaces and nanostructures [20]. Constructing heterojunction photocatalysts can assist the charge transfer between the diverse photocatalysts and enhance the separation of charge carriers. Therefore, the introduction of new-form nano-heterojunction photocatalysts with structural variety, band gap energy, and better visible-light utilization is superior in this investigate area [21,22].

In the present study, a new MoS₂/Co₃O₄ nanohybrid photocatalyst was fabricated by a facile hydrothermal route for powerful photocatalytic activity. The green–hydrothermal technique is energy-saving, cost-effective, environmentally friendly, and a green process compared with other synthesis methods [23,24]. To our knowledge, there is no report on the photocatalytic degradation of MoS₂/Co₃O₄ nanohybrids with visible light irradiation. A potential S-scheme heterojunction can be constructed between MoS₂ and Co₃O₄ while Schottky junction developed at the MoS₂/Co₃O₄ interface, respectively, which connected to enhance the charge carries separation ability. With the optimized amount of Co₃O₄ loading in the composite, the MoS₂/Co₃O₄ nano-heterojunction displayed a great removal MO rate of 95.6%, which was approximately 2.1 and 2.3 folds higher than that of bulk MoS₂ and Co₃O₄ samples, respectively. MoS₂/Co₃O₄ nanohybrids demonstrated higher photocatalytic efficiency than MoS₂ and Co₃O₄ on the removal of MO dye with visible light irradiation. This system opened up a new route for the controlled fabrication of MoS₂-based heterojunction and also improved their organic compound photodegradation activity. The further the surface reactive positions created by the huger specific surface area, the higher the redox efficiency due to the farther negative conduction capacity, and decreased recombination degree of photogenerated electrons and holes eventually resulted in improved photocatalytic performance.

2. 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: 37-1492), 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-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 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 3b, the Eg2 (Eg1) values of MC-5%, MC-10%, MC-15%, and MC-20% are about 2.61 (1.56) eV, 2.51 (1.61) eV, 2.45 (1.64) eV, and 2.31 (1.69) eV.

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 O²⁻ 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 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 m² g⁻¹, respectively (

Table 1. The physical adsorption parameters of the photocatalysts of MoS₂, Co₃O₄, and MC-20% were measured three times.

Samples	Specific Surface Area (m2/g, BET) a	Total Pore Volume (cm3/g, BET) b	Average Pore Diameter (nm, BJH) b
MoS2	35.2 ± 5	0.193 ± 0.02	38.4 ± 5
Co3O4	24.6 ± 5	0.124 ± 0.02	48.5 ± 5
MC-20%	68.7 ± 5	0.372 ± 0.02	8.9 ± 3

^a Received from BET analysis. ^b Relative pressure (P/P₀) was 0.99.

). 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 cm³ g⁻¹, 0.124 cm³ g⁻¹and 0.372 cm³ 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.

Figure 5a exhibits the decay in absorbance peak intensity of 447 nm with extending irradiation time. It displays that the absorbance ability of MO was continuously removed with visible light activity in the presence of the as-fabricated photocatalysts. In addition, the absorbance of MO decreased significantly with the increase of irradiation time by using MC-20% nanohybrids as primarily photocatalyst under solar irradiation was recorded by UV–vis absorption spectroscopy, which proved the destruction of MO molecules. The photocatalytic activity of MO by as-fabricated MoS₂, Co₃O₄ and MC-X materials was evaluated with and without visible light activities (Figure 5b). The control experiments displayed the capability values for the self-photocatalytic of MO pollutants under same light condition. This result suggests that the degradation of MO pollutants is owing to the photodegradation activity in the as-fabricated samples. On the other hand, the individual MoS₂ and Co₃O₄ samples could degrade about 45.8 and 41.4% of MO under 140 min of visible light radiation. 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:

$$\mathrm{M}\mathrm{o}{\mathrm{S}}_2+\mathrm{h}\mathsf{\nu }\to \mathrm{M}\mathrm{o}{\mathrm{S}}_2\left({\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\right)$$

(1)

$$\mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4+\mathrm{h}\mathsf{\nu }\to \mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4\left({\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\right)$$

(2)

$${\mathrm{O}}_2+{\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}\to •{\mathrm{O}}_2^{-}$$

(3)

$$\mathrm{M}\mathrm{o}{\mathrm{S}}_2\left({\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}\right)\to \mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4\left({\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}\right)$$

(4)

$$2{\mathrm{H}}^{+}+•{\mathrm{O}}_2^{-}+2{\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(5)

$$\mathrm{C}{\mathrm{o}}_3{\mathrm{O}}_4\left({\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\right)\to \mathrm{M}\mathrm{o}{\mathrm{S}}_2\left({\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\right)$$

(6)

$$2{\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}$$

(7)

$$\mathrm{Composite} {\mathrm{h}}^{+}+\mathrm{M}\mathrm{O}\to \mathrm{M}{\mathrm{O}}^{*} \left(\mathrm{O}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\right)$$

(8)

$$\mathrm{M}\mathrm{O}+\mathrm{Composite} {\mathrm{h}}^{+}\mathrm{o}\mathrm{r} •{\mathrm{O}}_2^{-}\to \mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}$$

(9)

$$\mathrm{M}\mathrm{O}+\mathrm{H}{\mathrm{O}}^{•}\to \mathrm{M}\mathrm{O}\left(\mathrm{H}{\mathrm{O}}^{•}\right)\to \mathrm{I}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{s}+\mathrm{H}{\mathrm{O}}^{•}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(10)

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 (^•O₂⁻, ^•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 ^•O₂⁻ 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 MoS₂/Co₃O₄ with a proper ratio (20 wt%) is able to receive the better photocatalytic efficiency of MoS₂/Co₃O₄ 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 MoS₂/Co₃O₄ NC photocatalyst. In addition,

Table 2. Comparison of photocatalytic activity of some photocatalysts for removal MO dye under visible light irradiation.

Catalysts	Pollutants Volume	Concentration	Lamp	Mass of Catalysts	Time (min)	Degradation Rate %	Reference
ZnO-TiO2/SO42−	25 mL	20 mg L−1	halide	20 mg	72	90	[31]
CoOx/g-C3N4	35 mL	10 mg L−1	Xenon	35 mg	180	92	[32]
ZnPcs	3 mL	3.3 × 10−5 M	White LED	10 mg	720	93	[33]
SbSI MRs	50 mL	30 mg L−1	halide	25 mg	10	78	[34]
Sn-ZnO/GO	1000 mL	50 mg L−1	Xenon	100 mg	120	87	[35]
CdS	150 mL	10 mg L−1	Xenon	500 mg	180	94	[36]
MoS2/Co3O4	50 mL	20 mg L−1	halide	10 mg	170	96	Present work

displays the photocatalytic performance of relevant heterojunction in previous reports [31,32,33,34,35,36]. By contrast, it is observed that the MoS₂/Co₃O₄ 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 MoS₂, Co₃O₄ and MC-20% nanohybrids. Applying DMPO as a spin capture agent, DMPO-^•O₂⁻ 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-^•O₂⁻, while the feature signal of DMPO-^•O₂⁻of MoS₂ and Co₃O₄ is nearly undetectable. It exhibits that the construction of heterojunction is more favorable to the production of ^•O₂⁻, 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 MoS₂, Co₃O₄ 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 ^•O₂⁻ and ^•OH. The coincidence of ^•O₂⁻ 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 MoS₂ and Co₃O₄ 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 photo-excited charge carriers, hence, greatly suppressing the recombination rate of photo-induced 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, curved band edges and the coulomb interaction notably drive the CB electrons of MoS₂ to rebuild with the VB holes of Co₃O₄, leading to the practical conservation of the high-oxidative holes in the VB of MoS₂ and high-reductive electrons in the CB of Co₃O₄, which significantly promotes the photo-response efficiency of MoS₂/Co₃O₄. 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 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 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 MoS₂ and Co₃O₄ will recombine rapidly, implying a weakening in photocatalytic capability. Although, owing to the presence of Co₃O₄ cocatalyst, photoexcited electrons will rapidly migrate to MoS₂, and also further water reduction so hydrogen evolution will appear. Furthermore, the presence of MoS₂ can weaken over-potential and serve more active sites for photocatalytic and can further enlarge the visible light absorption of Co₃O₄.

3. Materials and Methods

Sodium molybdenum oxide dehydrates (Na₂MoO₄·2H₂O) (≥99.5%), l-cysteine (C₃H₇NO₂S), Cobalt chloride hexahydrate (CoCl₂·6H₂O, ≥98%), Methyl orange (MO, ≥85%), Triton X-100, Potassium hydroxide (KOH, ≥85%), 1-Methyl-2-pyrrolidone (NMP, ≥99%), absolute ethanol (CH₃CH₂OH, ≥95%), and acetone (CH₃COCH₃, ≥99.5%) were used from Sigma-Aldrich (St. Louis, MO, USA). Deionized water was utilized to produce solutions to be examined. All the chemicals were adopted directly without purification.

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. 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 MoS₂/Co₃O₄ photocatalysts was provided by increasing various doses of Co₃O₄ (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.

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 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.

4. 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 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.