S-Scheme System of MoS2/Co3O4 Nanocomposites for Enhanced Photocatalytic Hydrogen Evolution and Methyl Violet Dye Removal under Visible Light Irradiation

Abstract

Photocatalytic hydrogen production joined with simultaneous organic compound removal is a potential but challenging approach for both environmental modification and reusable energy generation. In this study, we designed a nanocomposite method for the fabrication of MoS₂/Co₃O₄ heterojunction with an extremely productive photocatalytic capability. The as-fabricated MoS₂/Co₃O₄ nanocomposites displayed greatly enhanced the hydrogen production (3825 μmol/g/h) and methyl violet dye (MV) contaminant removal (apparent kinetic constant of 0.038 min⁻¹) activity. The nanocomposites’ structures had a better specific surface area, numerous active sites, and enhanced the transport ability of charge carriers to promote the photocatalytic activity. The increase in Co₃O₄ improved the visible-light absorption efficiency and narrowed energy bandgap and served as a highway for charge carriers to facilitate the transfer and separation and inhibit the combination of photoinduced charge carriers. The migration route of the photoexcited charges, the formation pathway, and the function of various reactive oxygen species (such as O²⁻ and •OH) are discussed. The optimized energy band structure and high electron transfer rate of the S-scheme heterojunction nanocomposite promotes the evolution of H₂ and the removal of pollutants, which shows an excellent potential in a stable and efficient photocatalytic hydrogen evolution and environment remediation.

1. Introduction

The typical solar driven-inspired hydrogen production from water splitting adopting extremely active and powerful photocatalysts is one of the major existing systems used to generate renewable hydrogen (H₂) energy [1,2,3]. In this regard, H₂ is a popular selection of green energy in place of fossil fuels owing to its stately combustion and energy production used to establish only H₂O [4]. Photocatalytic hydrogen evolution has attracted much attention and is considered as a promising method to solve environmental problems and the energy crisis. The photocatalytic production of H₂ from water has been regarded as having an acceptable probability approach to clean hydrogen energy, but the critical issue is how to progress the photocatalysts that can productively absorb solar light to conduct a hydrogen evolution reaction (HER) in this developing region [5,6]. In recent decades, a number of reports have attempted to develop semiconductor photocatalysts that can perform the removal of organic pollutants or a hydrogen evolution in the environment through light excitation. Hence, it is essential to build and obtain nanocomposite photocatalysts with a superior solar-to-hydrogen energy transition performance.

Recently, transition metal dichalcogenides (TMDs) have received a great amount of attention due to the consecution of their impressive premium properties [7,8]. Among these TMDs, molybdenum disulfide (MoS₂) has a good charge carrier mobility, a great conductivity, and tunable band gap energy, which is beneficial and practical for developing an environmental treatment and enhancing the performance of the photocatalysis under a visible light response [9]. As a type of promising TMDs, MoS₂ has an energy bandgap of nearly 1.65~2.72 eV and suitable conduction band valence potentials that possess a better photocatalytic capability and could utilize a large region of solar light [10]. Co₃O₄ has been extensively performed in photocatalytic studies attributable to its stupendous benefits such as its low cost, non-toxicity, great quantum efficiency, excellent photosensitivity, and high stability against photo-corrosion. To further enhance the intrinsic activity of MoS₂, constructing MoS₂-based nanocomposites can boost the spatial charge separation, improve the active sites, and promote photocatalytic activities [11]. Compared to conventional composites, the nano-heterojunctions formed by the van der Waals force provide a larger surface for the exciton separation, suggesting an outstanding nanocomposite capability and interface quality [12]. Among these heterostructures, MoS₂ and cobalt oxide (Co₃O₄) nanoparticles both display excellent, yet different, properties for the particular marvelous direct energy bandgaps, premium mechanical properties, and excellent optical properties, including an elevated photocatalytic capability [13,14,15]. Therefore, the low light harvesting efficiency, inactive H₂ production capability, and rapid recombination efficiency of photoexcited electrons and holes are the typical drawbacks that suppress the profitable operation of MoS₂ [16]. On the other hand, bare MoS₂ and Co₃O₄ for conducting photocatalysis or the decomposition of organic pollutants are rather challenging due to their low utilization and higher electron and hole recombination rate [17]. Even though the MoS₂ heterostructure as an effective candidate for the hydrogen production reaction photocatalyst has numerous benefits, such as an outstanding photocatalytic capability, an excellent stability, a large specific surface, and a low-cost MoS₂ heterostructure, there remains a large yet intriguing challenge. The HERs’ adoption of bare photocatalysts is normally poor owing to various limitations, for instance, a weak visible light absorption, the rapid charge in the carriers’ recombination rate, and a low photocatalytic capability.

Although, the bare MoS₂ and Co₃O₄ catalysts displayed a low photocatalytic capability due to their aggregation, insufficient surface-active centers, and a rapid photoexcited charge carriers recombination. However, the Co₃O₄ catalyst absorbs mainly in the solar light response and owns a broad energy band gap, which forbids their photocatalytic performance. To overcome this problem, a typical process is to build a heterojunction among Co₃O₄ and MoS₂ with a visible light response to enhance their visible light operation. Huge specific surface areas could supply a lot of active sites, so nanostructured catalysts are more favorable for enhancing their photocatalytic performance. Particularly, co-catalyst loading and heterostructure building are the two useful processes to promote the separation of the charge carriers of MoS₂ [18]. In addition, Co₃O₄ also exhibits an obvious stability with a photocatalytic activity. In addition, Co₃O₄ receives the coordinated energy band gap potential coupled with MoS₂ to improve a valid step-scheme (S-scheme) heterojunction. The S-scheme heterojunction usually consists of an oxidation photocatalyst with relatively low Fermi levels and conduction bands; a reduction in the photocatalyst is usually accompanied by comparatively high Fermi levels and conduction bands [19]. To solve these issues, a substantial promotion has been made in the practical construction and proposal of MoS₂/Co₃O₄-based nanocomposites with well-established interfaces and nanostructures [20]. Recently, there have been various efforts to build many designs of a joined nanomaterials-based heterojunction and to use these designs for various photocatalytic conversions. Nanocomposites, compared to their bulk components, exhibit improved and excellent characteristics such as unique optical and electronic characteristics, a higher surface-to-volume ratio, compact size, and a quantum confinement effect [21]. Herein, the fabrication and good design of MoS₂/Co₃O₄ nanocomposites is proposed to reveal more surface-active sites, shorten the charge diffusion length, and prompt the redistribution of numerous charge carriers. Consequently, the newly developed heterostructure catalysts with an architectural variation, band gap energy, and great solar-light application are more expressive in this section. Recent investigations have suggested that various heterojunction nanomaterials, which display a proportional or even better photocatalytic efficiency than traditional photocatalysts, could be utilized as cost-effective photocatalysts and are environmentally friendly [22].

In the present study, a new MoS₂/Co₃O₄ nanocomposite photocatalyst was synthesized by the simple hydrothermal process for the potential production of photocatalytic H₂. For the first time, we are reporting on the photocatalytic hydrogen production of MoS₂/Co₃O₄ nanocomposites with a visible light irradiation. All the above results are favorable to the tunable band structure of MoS₂, the create active sites, and build a valid transfer pathway for the charge carriers, then increase the entire photocatalytic efficiency. In addition, a possible S-scheme heterostructure can be created among MoS₂ and Co₃O₄ while the Schottky junction developed at the MoS₂/Co₃O₄ interface, which combined to improve the electrons and the holes’ separation efficiency. The S-scheme heterostructure is primarily constructed of a reduction in the photocatalyst, including a high Fermi level and a low work function, and an oxidation photocatalyst, including a small Fermi level and a high work function for a staggered style, which can mainly achieve the separation of charge carriers under an excellent redox performance. Thus, with the optimized amount of Co₃O₄ loading in the composite, the MoS₂/Co₃O₄ heterojunction displayed a great H₂ production rate of 3825 μmol h⁻¹g⁻¹, which was nearly 11.5- and 13.4-folds higher than that of the bulk MoS₂ and Co₃O₄ samples, respectively. This work proposed a unique path for the designed construction of MoS₂-based nanocomposites and further enhanced their photocatalytic hydrogen production and organic pollutants photocatalytic activity. The promoted surface reactive sites established through the large specific surface area had a better redox capability, suggesting a greater negative conduction performance, and a decreased recombination rate of the photoexcited charge carriers ultimately resulted in an enhanced photocatalytic activity.

2. Materials and Methods

2.1. Materials

Sodium molybdenum oxide dihydrate (Na₂MoO₄·2H₂O) (≥99.5%), L-cysteine (C₃H₇NO₂S), cobalt chloride hexahydrate (CoCl₂·6H₂O, ≥98%), potassium hydroxide (KOH, ≥86%), triton X-100, absolute ethanol (CH₃CH₂OH, ≥96%), 1-Methyl-2-pyrrolidone (NMP, ≥98%), and acetone (CH₃COCH₃, ≥99.5%) were obtained from Sigma-Aldrich (Burlington, MA, USA). Deionized (DI) water (18.2 MΩ cm) was used throughout the experiment. All the chemical agents were used directly without purification.

2.2. Synthesis

Typically, 1.3 mmol/g (0.31 g) of Na₂MoO₄·2H₂O and 2.6 mmol/g (0.31 g) of the L-cysteine was added to 50 mL of deionized (DI) water to form a uniform solution by stirred and ultrasonic operation for 40 min. 0 wt% (bare MoS₂), 5 wt% (0.03 g), 10 wt% (0.06 g), 15 wt% (0.09 g), and 20 wt% (0.12 g) of CoCl₂·6H₂O were used in 50 mL of DI water and a particular amount of Triton X-100 surfactant (1%, w/w) was used subsequently, respectively. In addition, bare Co₃O₄ photocatalysts were synthesized according to the literature [13]. Then, the homogeneous solution was transformed into 100 mL Teflon-lined stainless-steel autoclaves and heated at 180 °C for 20 h in the oven. The autoclaves were accordingly cooled down to room temperature in an oven, after which the resultant products were separated from the solution by centrifugation several times through DI water, absolute ethanol, NMP, and acetone. Then, they were dried at 80 °C in a vacuum oven for 12 h. A set of MoS₂/Co₃O₄ photocatalysts was provided by increasing the various doses of Co₃O₄ (5 wt%, 10 wt%, 15 wt%, and 20 wt%); the individual products are denoted as MC-X (X = 1, 2, 3, and 4). Eventually, the nano-heterojunction was fabricated by the hydrothermal route as displayed in Scheme 1.

2.3. Characterization

The morphology and microstructure of various materials were carried out via using scanning electron microscopy micrographs (SEM, ZEISS AURIGA, Stuttgart, Germany) and transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan). X-ray diffraction patterns (XRD) were adopted by a Bruker D8 Advanced (Karlsruhe, Germany) diffractometer at 5°–70° at room temperature with a scanning step of 20°/min. The electron spin resonance (ESR) measurement was performed on a Bruker A300 (ESR/EPR) spectrometer in the visible-light mode (Karlsruhe, Germany). A UV–visible spectrophotometer (Hitachi UV-4100, Tokyo, Japan) and F-7000 Edinburgh Analytical Instruments (Tokyo, Japan) were determined to confirm the UV–vis spectrum and photoluminescence spectrum (PL) under an excitation wavelength of 620 nm. The chemical and surface states were characterized through Thermo Fisher Scientific X-ray photoelectron (XPS) spectroscopy (Waltham, MA, USA). The CH Instruments electrochemical workstation for a standard three-electrode mode was observed to use electrochemical analysis in 0.2 M of Na₂SO₄ solution. In addition, the saturated Pt electrode and Hg/Hg₂Cl₂ electrode were carried out as the counter electrode and reference electrode, respectively. Then, the working electrode was adopted following to the afterward step: the as-fabricated nanocomposites of 20 mg were spread in DI water of 20 mL to receive a uniform suspension with the following ultrasonic route.

2.4. Photocatalytic Hydrogen Production

A photocatalytic hydrogen evolution was achieved in the presence of the samples and was evaluated by adopting gas chromatography (GC-7890, Agilent, USA). The visible light source was a 350 W Xe lamp. A total of 30 mg of the as-fabricated samples were added in 200 mL of the lactate solution (10 vol%, 50 mL). Prior to the light activity, the system was vacuumed to hold it stable in a vacuum condition. Hydrogen (H₂) products were carried out at regular intervals and were screened by GC provided with a thermal conductivity detector (TCD) during the photoreaction.

2.5. Photocatalytic MV Degradation Activity

The photodegradation of MV pollutants over different MC-X samples was used with the visible light activity at room temperature. Xe lamp of 350 W was performed as the solar-light source. Then, 20 mg of the photocatalysts were used under MV aqueous solution (20 ppm) of 100 mL. The above mixture solution was also stirred with the darkness condition for 20 min to establish an absorption-desorption equilibrium among nanocomposites and MV pollutants. With visible light irradiation, an evident quantity of the reaction solution was employed due to the mixture solution at the provided intervals, then performed by centrifugation to remove all of the photocatalysts. The concentration change in the MV compounds was recorded by UV–vis spectroscopy with the photocatalytic system.

3. Results

3.1. XRD Analysis

XRD patterns were recorded to confirm the structural composition and crystal phase form of the as-synthesized heterojunctions, as displayed in Figure 1. After the hydrothermal process, the obvious major peaks of MoS₂ were lower while several new characteristic peaks arose at 31.4°, 36.6°, 44.7°, and 59.5° of the MoS₂/Co₃O₄ samples, which are corresponding with the (220), (311), (400), and (511) diffraction face of Co₃O₄ (JCPDS No. 42–1467) [23], respectively. The MoS₂ photocatalysts display a series of major characteristic peaks at 14.4°, 32.7°, 39.6°, 44.4°, 50.1°, 50.8°, and 60.8°, which correspond to the (002), (100), (103), (006), (105), and (110) crystal planes of MoS₂ (JCPDS Card no: 37–1492), respectively, and no secondary peaks were obtained [24]. It is noted that there is no signal change in the XRD patterns of the MoS₂/Co₃O₄ nanocomposites compared to the MoS₂ and Co₃O₄ samples, suggesting that the presented Co₃O₄ content is lower and the dispersion is better, and there are no further impurity characteristic peak signals or major peak signal shift. Especially, compared with the XRD pattern of Co₃O₄, the overall feature peak intensities of the MoS₂/Co₃O₄ nanocomposite were adjusted at 36.8^o with the (311) diffraction face phase (shown in Figure 1 inset). Additionally, the diffraction phases of MC- X correspond to the characteristic XRD pattern of MoS₂ without further impurity diffraction peaks, which implies that the nanocomposites maintain a consistent crystal phase and structure. Moreover, when the reaction mixture solutions concentration of Co₃O₄ was enlarged from 5 wt% to 20 wt%, the characteristic peak intensity of Co₃O₄ became higher, which exhibited an extent in the amount of Co₃O₄ in the heterojunction. In addition, the feature peaks of MC- X (X = 1, 2, 3, and 4) are comparable to the XRD patterns of MoS₂ without increased impurity peaks, which implies that the nanocomposite photocatalyst also possesses a great crystal phase.

3.2. SEM and TEM Analysis

In order to study the morphology and structure by the hydrothermal route, SEM images and TEM images were performed. As displayed in Figure 2a,b, obviously a significant size pristine MoS₂ and Co₃O₄ could be noted. It can be seen that the structure of the sample is plate-like and sphere-like with a relative dimension of around 6 μm. Then, the MoS₂ and Co₃O₄ own a high degree of aggregation, which extremely restrains the surface of the active site of the samples. As shown in Figure 2c, the MoS₂/Co₃O₄ composite reveals a nano-scale with a rough surface, which propose to receive a great specific surface region. Accordingly, this study was confirmed in the specific surface region investigation. It is declared that the dimension of MC-4 was extremely decreased; the crystals phase of MoS₂ also retains a good crystallinity, as noted by the (002) primary peak recorded in the XRD analysis (Figure 1). This nanocomposite of MoS₂/Co₃O₄ serves the sufficient area for the efficient load of Co₃O₄, which can mainly disperse MoS₂/Co₃O₄ nanocomposites and decrease the aggregation. Figure 2d shows that the MoS₂/Co₃O₄-4 nanocomposite exhibits an aggregated form with dimensions of nearly 5–15 nm. The average diameter distribution of the MoS₂/Co₃O₄ nanocomposites after the hydrothermal process is 7.8 ± 1 nm (Figure 2e). According to Figure 2f, the interplanar spacing of 0.26 nm can be obviously recorded, which corresponds to the (002) lattice plane of hexagonal MoS₂. The interplanar spacing of 0.32 nm is attributed to the (311) lattice plane of Co₃O₄. During further testing, it could be found that the MoS₂/Co₃O₄ nanocomposite is ultimately formed. The multiple lattice intersections indicated the successful assembly of MoS₂ and Co₃O₄. Additionally, the elemental dispersion of MoS₂/Co₃O₄-4 nanocomposite was studied using elemental mapping (Figure 2g–j). It can be seen that the composite catalyst contains Mo, S, and Co, resulting in the formation of a larger contact surface, beneficial to the charge transfer and separation [21]. The aforementioned results show that the preparation of the MoS₂/Co₃O₄ nanocomposite photocatalyst was successful.

3.3. UV–vis Spectra Analysis

As shown in Figure 3, the UV–vis absorption spectrum is mainly performed to confirm the light absorption ability of the sample, and the measurement of the as-fabricated MoS₂/Co₃O₄ nanocomposite. As could be noted, the two energy band gaps of MoS₂/Co₃O₄ photocatalysts are 1.55−1.68 eV (Eg1) and 2.32−2.63 eV (Eg2), which are in a good agreement with the MoS₂ and Co₃O₄ energy band [17,18]. As MoS₂/Co₃O₄ is a photo-excited photocatalyst, it owned a broad absorption capability of wavelength. Eg1 is connected to the approach of the O(II)-Co(III) excitations [25]. The largest signal wavelength of MoS₂/Co₃O₄ increased to 560 nm. Therefore, it implies a narrower energy band gap (Eg2) of 2.32−2.63 eV after the increasing of Co₃O₄, which is valuable to enhance the visible light utilization and photo-response charge carriers. As for the MoS₂/Co₃O₄ series nanocomposites, the light absorption is all significantly enhanced in the region of visible light; this result suggests that the heterojunction structure in the MoS₂/Co₃O₄ nanocomposites may broaden the spectral scope of the light absorption, which will improve the photon harvesting performance in the visible light region. In addition, the MC-4 nanocomposites exhibit a great visible light absorption ability and the absorption has a lesser red shift in the range of 540–670 nm. This can be obtained from the combination among MoS₂ and Co₃O₄. After the addition of the Co₃O₄ nanoparticles, the light absorption of the MoS₂/Co₃O₄ nanocomposites is significantly increased in the visible light region compared with that of the nanocomposites, suggesting the improved photoabsorption ability, which is attributed to the black color of the Co₃O₄ nanoparticles. The energy bandgap Eg of the consistent materials was approached through the converted Kubelka–Munk function [26]. As shown in Figure 3b, the Eg2 (Eg1) of MC-1, MC-2, MC-3, and MC-4 are approximately 2.62 (1.57) eV, 2.52 (1.62) eV, 2.46 (1.65) eV, and 2.32 (1.68) eV.

3.4. XPS Analysis

X-ray photoelectron spectroscopy (XPS) was performed to elucidate the chemical composition and elemental valence state of the representative samples. The survey spectra (Figure 4a) displayed the different characteristic peaks of the elements Mo, S, O, and Co, implying their dispersed in the MoS₂/Co₃O₄ heterojunction, which could be further confirmed by the mapping images, as exhibited in Figure 2g–i. There are no extra impurity signal peaks appearing in the survey spectrum, indicating the successful synthesis of the nanocomposite after the hydrothermal process. The high-resolution XPS spectrum of Mo 3d (Figure 4b) could be split into two feature peaks, which appeared at ∼232.8 eV and ∼229.6 eV, corresponding to Mo 3d_3/2 and Mo 3d_5/2 [11]. Accordingly, two characteristic peaks in the high-resolution XPS spectrum of S 2p of the MC-4 sample centered at approximately 163.6 eV and 162.6 eV (Figure 4c) correspond to S 2p_1/2 and S 2p_3/2, respectively [12]. As displayed in Figure 4d, the binding energy of ∼795.5 eV and ∼780.1 eV correspond to Co 2p_1/2 and Co 2p_3/2 for the MC-4 samples, respectively [13]. In addition, the O 1s spectrum of the MC-4 samples’ (Figure 4e) binding energy at 531.8 eV and 529.8 eV correspond to the O²⁻ in the adsorbed oxygen and the Co₃O₄ samples [14]. The above results from the analysis imply that the MoS₂/Co₃O₄ nanocomposites were successfully prepared with a vigorous interaction among MoS₂ and Co₃O₄.

3.5. Photocatalytic Hydrogen Evolution and Stability

The photocatalytic hydrogen production efficiency of the MoS₂, Co₃O₄, and MC-X (X = 1, 2, 3 and 4) samples in the Na₂SO₃ aqueous solution were performed with visible light activity. The hydrogen production of MoS₂ and Co₃O₄ for 5 h are displayed in Figure 5a, which are 286 μmol and 334 μmol, respectively. Apparently, with the same situations, the hydrogen production performance of MC-4 approached 3825 μmol. In addition, the MC-4 nanocomposites exhibited an evident benefit in the production of hydrogen, which was 13.4 times and 11.5 times that of the bulk MoS₂ and Co₃O₄. This suggests that the heterojunction nanocomposites formed by MoS₂ and Co₃O₄ significantly enhance the hydrogen production capability of the bulk photocatalyst. This great hydrogen production efficiency could be referred to the apparent visible light catching capacity of the heterojunction catalysts and the huge number of active sites on the surface [27]. The MC-X nanocomposite improves the hydrogen evolution reaction (HER) with various Co₃O₄ adding levels, and with the hydrogen production efficiency of MC-4 preferably, it can approach 765.5 μmol/h (Figure 5b). This result demonstrates that various additions of Co₃O₄ have a critical impact on the photocatalysis capability of the nanocomposite samples. With the purpose of verifying the preferred dosage of MC to the hydrogen evolution, the HER activities of the heterojunction photocatalysts with the various dosage of MC nanocomposite have been analyzed. As exhibited in Figure 5c, with the extended amount of MC photocatalysts from 10 mg to 50 mg, the HER rate of the heterojunction system first increased then decreased. Under a photocatalytic HER reaction, owing to the charge carrier production step, both charge carrier transporting and trapping are required situations [28]. Redundant MC-4 samples arise to be significantly agglomerated and appear to be further from the crystal boundary, ultimately affecting the serious electrons and holes’ recombination rate, which reduces the operation of the photocatalysts. The maximum efficiency was approached at the MC-4 nanocomposite content of 30 mg, suggesting that this is the optimal condition for photocatalytic water splitting. Therefore, the following tests of the photocatalytic activity were managed with the fixed dosage (30 mg) of the MC nanocomposite. As shown in Figure 5d, after 25 h of consecutive experiments, the HER of the MC-4 samples did not decay significantly, but also kept 90% of the initial efficiency from 3825 μmol h⁻¹ (first run) to 3125 μmol h⁻¹ in the fifth runs. The remarkable enhancement of the hydrogen production rate exhibits that the nano-heterojunction can improve the photocatalytic HER, which further demonstrates the requirement of this study. As shown in Table 1 [29,30,31,32,33], the composite catalyst MoS₂/Co₃O₄ exhibits an excellent photocatalytic degradation and hydrogen evolution activity compared with other catalysts.

3.6. Photocatalytic Degradation of MV

Figure 6a displays the absorbance signal peak intensity of 562 nm with an increasing light irradiation time. Clearly, it exhibits that the visible light absorbance efficiency of MV was consecutively degraded under visible light action over the as-synthesized nanocomposites. Furthermore, the absorbance ability of MV decreased obviously with the enlargement of the light irradiation time via adopting the MC-4 nanocomposite as the primary photocatalyst with the visible light activity, which was performed through UV–vis spectroscopy, which confirmed the decomposition of the MV pollutants. The photocatalysis efficiency of MV by the as-prepared MoS₂, Co₃O₄, and MC-X samples was recorded under the visible light response within a 70 min irradiation time (Figure 6b). The control experiments have shown the performance for the efficient photocatalytic of the MV molecules with the same test condition. These results imply that the removal of the MV molecules is due to the photocatalytic activity in the as-prepared nanocomposites. Additionally, the individual MoS₂ and Co₃O₄ photocatalysts can remove almost 44.6 and 42.3% of MV for 70 min under visible light activity. The photocatalytic performance of the samples is in the following order: MC-4 nanocomposite > MC-3 nanocomposite > MC-2 nanocomposite > MC-1 nanocomposite > MoS₂ > Co₃O₄. This low photocatalytic efficiency can be attributed to the great recombination rate of the charge carriers in the pristine materials [34]. Apparently, MC-4 nanocomposites presented the excellent photocatalytic activity of the MV molecules (~93.5%) than pristine MoS₂ or Co₃O₄, which was about 2.2-folds than bare Co₃O₄. The primary route of the photocatalytic activity for the heterojunction photocatalyst MoS₂/Co₃O₄ samples on the fundamental basis and investigation described in the literature could be described as observed:

(1)${\mathrm{MoS}}_2+\mathrm{h}\mathsf{\nu }\to {\mathrm{MoS}}_2\left({\mathrm{e}}_{\mathrm{CB}}^{-}+{\mathrm{h}}_{\mathrm{VB}}^{+}\right)$

(2)${\mathrm{Co}}_3{\mathrm{O}}_4+\mathrm{h}\mathsf{\nu }\to {\mathrm{Co}}_3{\mathrm{O}}_4\left({\mathrm{e}}_{\mathrm{CB}}^{-}+{\mathrm{h}}_{\mathrm{VB}}^{+}\right)$

(3)${\mathrm{O}}_2+{\mathrm{e}}_{\mathrm{CB}}^{-}\to •{\mathrm{O}}_2^{-}$

(4)${\mathrm{MoS}}_2\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)\to {\mathrm{Co}}_3{\mathrm{O}}_4\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)$

(5)$2{\mathrm{H}}^{+}+•{\mathrm{O}}_2^{-}+2{\mathrm{e}}_{\mathrm{CB}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$

(6)${\mathrm{Co}}_3{\mathrm{O}}_4\left({\mathrm{h}}_{\mathrm{VB}}^{+}\right)\to {\mathrm{MoS}}_2\left({\mathrm{h}}_{\mathrm{VB}}^{+}\right)$

(7)$2{\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}_{\mathrm{VB}}^{+}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}$

(8)${\mathrm{Composite}\mathrm{h}}^{+}+\mathrm{MV}\to {\mathrm{MV}}^{*}\left(\mathrm{Oxidation}\right)$

(9)$\mathrm{MV}+{\mathrm{Composite}\mathrm{h}}^{+}\mathrm{or}•{\mathrm{O}}_2^{-}\to \mathrm{Degradation}\mathrm{products}$

(10)$\mathrm{MV}+\mathrm{HO}•\to \mathrm{MV}\left(\mathrm{HO}•\right)\to \mathrm{Intermediates}+\mathrm{HO}•\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$

Additionally, the kinetics of the photocatalytic activities of an MV decomposition analysis were estimated to a pseudo first order equation standard to confirm the photocatalysis’ performance. The linear relationship among ln (Co/C) vs. the visible light illumination time (t) is displayed in Figure 6c and the decomposition rate constant (K_app) is shown in Figure 6d. Similarly, the MC-4 nanocomposites have tested with a greater K_app value in contrast to the other fabricated photocatalysts. For the useful profit function, the reusability and stability of the MC nanocomposites were performed via cycling tests. Per run, the photocatalyst was performed by centrifugation, washed and dried before performing the next test. As exhibited in Figure 6e, the photocatalytic efficiency of the MC-4 nanocomposite was 93.5%, 90.6%, 88.4%, 86.3%, and 85.7% for the 1st, 2nd, 3rd, 4th, and 5th run, respectively. The observed decrease in the photodegradation efficiency of the MC-4 samples could be recommended to a degraded MV and its boost products on the surface of the MC-4 nanocomposite, hence increasing the active sites, absorption ability, and visible light utilization from reaching the samples [35]. The recommendation of the photocatalytic mechanism of the nanocomposite played a significant part in the successful application of the heterostructure photocatalyst. Therefore, the reactive species trapping measurements were conducted, in which 1,4-benzoquinone (p-BQ), 2-propanol (IPA), and EDTA-2Na were used as the yielding scavengers for •O₂⁻, •OH, and h⁺ free radical agents [36]. It can be noted, as shown in Figure 6f, that the main reactive species for the photodegradation of organic compounds over the nanocomposites, the photodegradation activities of the MC-4 nanocomposites, was affected to changing grades. The MC-4 photocatalysts were more sensitive to EDTA-2Na and p-BQ, suggesting that h⁺ and •O₂⁻ were the primary active species under the photocatalytic system, and •OH only played as a serving active species. Therefore, these results illustrate that the development of MoS₂/Co₃O₄ with a fitting proportion (20 wt%) could be the approach to the higher photocatalysis capability of MoS₂/Co₃O₄ in the nano-heterostructure. Additionally, Table 2 shows the photocatalytic activities of the relevant composites in previous reports [35,36,37,38,39]. By contrast, it is observed that the MoS₂/Co₃O₄ nanocomposites have notable profits for organic pollutants in the photocatalytic system.

To check the designed S-scheme’s heterostructure, electron spin resonance (ESR) was performed to confirm the free radicals generated in the process of MoS₂, Co₃O₄, and MC-4 nanocomposites. Noticeably, DMPO as spin capture agents (DMPO- •OH and DMPO- •O₂⁻) are recorded under the same light condition. It can be noted, as shown in Figure 7a, that the MC-4 nanocomposites show the great characteristic signal peak of DMPO- •O₂⁻, while the characteristic signal peak of DMPO- •O₂⁻of MoS₂, and Co₃O₄ is nearly undetectable. It displays that the formation of the heterostructure is a further value to the generation of •O₂⁻, which corresponds to the separation of the photoinduced charge carriers and quicker migration in the heterostructure’s photocatalysts [40]. In addition, the DMPO- •OH characteristic peaks of MoS₂, Co₃O₄, and MC-4 can be observed in Figure 7b, of which MC-4 nanocomposites own the better characteristic signal peak. Therefore, it can be conducted that the MC-4 nanocomposite can produce further •O₂⁻ and •OH. The conjunction of •O₂⁻ and •OH radicals also proves the carries separation mode for the S-scheme heterojunction. The charge carriers’ delivery process is recorded via electrochemical impedance spectroscopy (EIS) tests. The EIS analysis under visible light action is displayed in Figure 7c, and the lower arc radius in the EIS Nyquist plots presents the less charge carries transfer resistance. Obviously, the poor charge carries transfer resistance of the MC-4 samples suggests the greater validity charge carries transfer paths, which correspond with the excellent PHE performance of the MC-4 nanocomposite. As shown in Figure 7d, the whole of the MC-4 nanocomposites electrodes display a superior transient photocurrent response than that of pristine MoS₂ and Co₃O₄ behave with the highest intensity response, suggesting the marvelous effective electrons and holes separation. On the other hand, the PL spectrum are achieved to confirm the electrons and the holes migration dynamics’ action. As exhibited in Figure 7e, the obviously decreased PL intensity of the MC-4 nanocomposites further prove the suppressed charge carriers’ recombination rate, which is in a fine agreement with the above discussion. In addition, the capability of the electron and hole is also investigated with the photo-response redox activities. The potential separation ability of the photo-excited electrons and holes is a necessary condition for improving the efficiency of the photocatalytic activity and hydrogen production [41]. It is further observed that the poorer the PL intensity of the feature peak, the poorer the probability of the charge carriers’ recombination rate, which is a greater benefit to the evolution of the organic compounds’ decomposition and hydrogen production action [42]. This is due to the evidence that the construction of the nanocomposites enhances the rapid transport and separation ability of the photo-response charge carriers, therefore, excellently inhibiting the recombination rate of the photo-response charge carriers and improving the capability of the charge carries’ separation.

3.7. Schematic of Degradation

According to the above analysis and results, a probably photocatalysis mechanism of the hydrogen production reaction and MV decomposition over the MC nanocomposites is proposed, as exhibited in Scheme 2. Based on the introduction of the Co₃O₄ samples on the surface of MoS₂, a strong heterostructure is obtained. Under visible light, the photocatalytic activity has an outstanding efficiency to the visible light activity in different visible light ranges. At this time, the MC nanocomposite can enhance an absorption of visible light under the same states, hence creating further photoexcited electrons and holes. Consequently, in the route of the photocatalytic activity, the hydrogen production of the nanocomposite purpose is greater than that of the pristine component, suggesting an improved interaction between the photocatalyst and pollutants. Eventually, a better hydrogen evolution and the decomposition of the organic compounds promotes the nanocomposite to possess greater electrons and holes separation behavior [41]. When the two samples are in connection, the electrons will promptly migrate from MoS₂ to Co₃O₄ until the two Fermi levels approach a balance. While the Fermi level of MoS₂ is higher than the Co₃O₄ sample, the consistent energy band gap is driven upper and lower, respectively. Then, Co₃O₄ is negatively charged and MoS₂ is positively charged, through developing an inherent electric field at the interface. MoS₂ and Co₃O₄ are induced to produce electrons and holes with a visible light irradiation. Because of the jointed effect of the inherent energy band bending and electric field, the electrons with a smaller reducing capacity in the conduction band of Co₃O₄ merge with the holes in the valence band of MoS₂, mainly decreasing the secondary recombination of the MoS₂ electron and hole pairs. The visible light activity plays a role as an external driving force to diminish the photogenerated holes and electrons recombination rate of Co₃O₄ itself. In addition, the interface effect is more than the capability of the photoinduced electron and hole to be coupled. Therefore, the electrons of the conduction band of MoS₂ provide the hydrogen production and organic compounds decomposition reaction. Additionally, the holes of Co₃O₄ are oxidized from the valence band via the photocatalytic activity. This design S-scheme heterojunction owns to a high redox ability, which obviously enhances the capability of the photocatalytic hydrogen production and organic compounds photodegradation. Clearly, the CB and VB of the MoS₂/Co₃O₄ photocatalysts under visible light irradiation are excited to generate the photo-response charge carriers. Furthermore, the photo-induced electrons and holes in MoS₂ and Co₃O₄ will recombine quickly, suggesting a weakening in the photocatalytic activity. In addition, due to the presence of the Co₃O₄ cocatalyst, the photoexcited electrons will quickly transfer to MoS₂ and a further H₂O reduction in the evolution of H₂ will arise. Moreover, the presence of MoS₂ can make the over-potential weaker and supply greater active sites for the HER and could enhance the solar light application of Co₃O₄.

4. Conclusions

In summary, the well-designed MoS₂/Co₃O₄ nanocomposite with visible light activity displays an outstanding photocatalytic efficiency, which is primarily owing to the novel transport of the S-scheme charge carriers. Based on the photocatalytic activity, it could be observed that MoS₂/Co₃O₄ nanocomposite has a stronger photocatalytic efficiency than bare MoS₂ and Co₃O₄. The Co₃O₄ nanoparticles not only increased in the photoabsorption ability but also enhanced the spatial charge separation and electron-hole pairs with a strong redox capacity. Compared with the bare MoS₂ and Co₃O₄, the synthesized MoS₂/Co₃O₄ nanocomposite photocatalyst has an excellent light absorption ability. At the time, the novel structure and large specific surface area of MoS₂/Co₃O₄ offer sufficient active sites and space for the productive load of Co₃O₄, which makes MoS₂ display a greater disperse ability and reduces the agglomeration effect of MoS₂. The optimized fabricated MoS₂/Co₃O₄ has an excellent photocatalytic hydrogen evolution rate of 3825 μmol h⁻¹ with visible light activity, which is 11~13-folds higher than bare MoS₂ and Co₃O₄. Finally, the experimental results demonstrated that MoS₂/Co₃O₄ displays an optimized photoexcited charge carrier efficiency conduct about its excellent hydrogen production capability. This study supplies an important concept to be explored for the build-up of the nanocomposite and the potential application of solar energy. In the future, many nanocomposites, such as the MoS₂/Co₃O₄ S-scheme heterojunction, will be widely used in the field of environment and energy.

Author Contributions

Conceptualization, E.L.C.; Methodology, T.-M.T.; Software, E.L.C.; Investigation, T.-M.T.; Data curation, E.L.C.; Writing—original draft, T.-M.T.; Writing—review & editing, E.L.C.; Supervision, E.L.C.; Funding acquisition, T.-M.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This work is financially supported by the MSTC and the NKUST of Taiwan. The authors gratefully acknowledge the measurement of HRTEM equipment belonging to the Instrument Center of National Cheng Kung University.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures, Schemes and Tables

Scheme 1. Procedures of the MoS2/Co3O4 nanocomposites.

Figure 1. XRD patterns of MoS2, Co3O4, MC-1, MC-2, MC-3, and MC-4 samples.

Figure 2. (a–d) TEM and (e) the average diameter distribution, (f) HRTEM images of MoS2/Co3O4. (g–j) Elemental mapping images of MoS2/Co3O4 nanocomposites.

Figure 3. (a) UV–vis spectrum and (b) the plots of (αE)2 versus E of as-fabricated samples.

Figure 4. (a) Survey spectrum of XPS spectrum analysis, high resolution spectrum of (b) Mo 3d, (c) S 2p, (d) Co 2p, and (e) O 1s spectra of MC-4 samples.

Figure 5. (a,b) The hydrogen evolution rate and (c) hydrogen evolution rates of the photocatalysts with various dosages, (d) Cycling tests of MC-4 with visible-light irradiation (30 mg photocatalyst), (e) Comparison of XRD patterns of MC-4 photocatalysts before and after hydrogen production.

Figure 5. (a,b) The hydrogen evolution rate and (c) hydrogen evolution rates of the photocatalysts with various dosages, (d) Cycling tests of MC-4 with visible-light irradiation (30 mg photocatalyst), (e) Comparison of XRD patterns of MC-4 photocatalysts before and after hydrogen production.

Figure 6. Photocatalytic activity of MV pollutant removal, (a) UV–vis absorption spectrum, (b) activities of MV by MC-4 nanocomposite catalyst, (c) kinetics analysis of the photodegradation, (d) removal rate constant of prepared photocatalysts, (e) Cycling tests and (f) different reactive species scavenging activity during the photocatalytic of MV over the MC-4 nanocomposites with visible light irradiation.

Figure 7. DMPO spin-trapping ESR spectrum for (a) •O2−and (b) •OH under visible light activity for MoS2, Co3O4, and MC-4, (c) electrochemical impedance spectra (EIS) spectrum of MoS2, Co3O4, and MoS2/Co3O4 in 0.5 M Na2SO4 solution under visible light condition, and (d) transient photocurrent densities of MoS2, Co3O4, and MoS2/Co3O4 electrodes in 0.5 M Na2SO4 solution with visible light.

Scheme 2. The possible photocatalytic mechanism of nanocomposite.

Table 1

Comparison of photocatalytic hydrogen production between MoS₂/Co₃O₄ and other reported composited photocatalysts.

Catalysts	Light Source	H2 Production Rate	Sacrificial Reagent	Reference
ZnS/g-C3N4	Visible light	714 µmol/h · g	Na2S/Na2SO3	[29]
ZnS/Cu3P	Visible light	729 µmol/h · g	Na2S/Na2SO3	[30]
ZnIn2S4	UV–vis light	1750 µmol/h · g	Na2S/Na2SO3	[31]
ZnO/ZnS	Visible light	126 µmol/h · g	Methanol (10 vol%)	[32]
M− ZnS@gC3N4/TiO2	Visible light	459 μmol/h · g	TEOA (10 vol%)	[33]
MoS2/Co3O4	Visible light	766 µmol/h · g	Lactate solution (10 vol%)	Present work

Table 2

Comparison of photocatalytic activity between MoS₂/Co₃O₄ and other reported composited photocatalysts.

Catalysts	Pollutants	Concentrations	LampPower	Mass ofCatalysts	Time (min)	Degradation(%)	Reference
Zn(II)-MOFs	MV	10 mg L−1	100 W	40 mg	40	55	[35]
HoVO4	MV	5 mg L−1	400 W	50 mg	90	67	[36]
ZnO	MV	10 mg L−1	300 W	50 mg	210	78	[37]
TiSiW12O40/TiO2	MV	20 mg L−1	300 W	30 mg	180	84	[38]
KCoPO4	MV	10 mg L−1	200 W	10 mg	300	90	[39]
MoS2/Co3O4	MV	20 mg L−1	350 W	30 mg	70	93	Present work

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