English

• Aromatic organic compounds have been detected in surface and ground waters, and they pose a great threat to the environment and human health [1-6]. As one kind of advanced oxidation process (AOP), the Fenton reaction has attracted extensive attention due to the production of hydroxyl radicals (^•OH) with high redox potential (E⁰(^•OH/H₂O) = 2.73 V), which can attack most aqueous organic contaminants in a nonselective way [7-10]. During the Fenton reaction, ^•OH can be generated by decomposing H₂O₂ under the activation of homogeneous (Fe²⁺) or heterogeneous (Fe(Ⅲ)) catalysts. However, the slow regeneration of Fe³⁺/Fe(Ⅲ) by H₂O₂ are always recognized as the rate-limiting steps. Therefore, many strategies are employed to accelerate the redox cycling of Fe³⁺/Fe²⁺ and develop more effective catalysts with a high ^•OH yield efficiency [11-17].

  Molybdenum disulfide (MoS₂), a type of layered transition metal dichalcogenide, exhibits extraordinary physical properties that make it very attractive in many fields [18-21]. Very recently, it was reported that MoS₂ can be used as a cocatalyst to promote Fe²⁺/Fe³⁺ conversion and subsequently accelerate the generation of ^•OH [8,22]. Specifically, the exposed Mo⁴⁺ could be oxidized to Mo⁶⁺, accompanying the reduction of Fe³⁺ to Fe²⁺. In addition, MoS₂ can activate H₂O₂ to produce ^•OH directly through a heterogeneous Fenton-like reaction [11]. The conversion between Mo⁴⁺ and Mo⁵⁺ triggers this reaction [23,24]. Although MoS₂ performs remarkably as a cocatalyst and Fenton-like catalyst, the difference between the two systems (MoS₂/H₂O₂ and Fe²⁺/MoS₂/H₂O₂) has not been elucidated in depth.

  Generally, MoS₂ has two main phases, the semiconducting 2H phase and metallic 1T phase [25-29]. As previously reported, 1T-MoS₂ provides dense active sites and enhanced intrinsic catalytic activity compared with 2H-MoS₂, therefore facilitating charge transfer kinetics to achieve superior hydrogen evolution reaction (HER) activity [30]. In addition, Luo et al. demonstrated that 1T-MoS₂ showed higher adsorption capacities toward Pb(Ⅱ) and Cu(Ⅱ) due to its lower adsorption energy [31]. However, systematic research on the influence of the MoS₂ phase on different types of Fenton reactions is still limited. In addition, introducing external energy, such as UV–vis light, can assist the Fenton reaction [1,4,32]. On the one hand, light (λ < 310 nm) can directly decompose H₂O₂ to generate additional ^•OH. On the other hand, 2H-MoS₂ and 1T-MoS₂ are also semiconductors. They can be excited by light to produce photogenerated electrons and holes and then further form ^•OH and other active radicals, which affects the AOPs. In view of the abovementioned considerations, it is necessary to explore the influences of MoS₂ phases and light on the MoS₂/H₂O₂ and Fe²⁺/MoS₂/H₂O₂ systems.

  In this work, 1T-MoS₂ was prepared by a simple hydrothermal synthesis method, and 2H-MoS₂ was obtained by calcining 1T-MoS₂ in a tubular furnace. Methylene blue (MB) was chosen as the target pollutant to evaluate different Fenton systems. Our findings showed that the removal rate was very low using MoS₂ as a Fenton-like reagent in the dark, and the phase had little effect on the relative reaction rate. Upon irradiation with light, the degradation rate was greatly improved, especially for 1T-MoS₂. In homogeneous Fenton systems, 1T-MoS₂ exhibited a superior cocatalytic effect regardless of darkness or light compared to 2H-MoS₂. It was difficult for the surface ^•OH_ads generated in the MoS₂/H₂O₂ system to diffuse into solution, while freely diffusible ^•OH_free was directly generated in the Fe²⁺/MoS₂/H₂O₂ system, which can explain the difference between the MoS₂/H₂O₂ and Fe²⁺/MoS₂/H₂O₂ systems. Furthermore, the synergistic effects and mechanism of the photoassisted Fenton-like system were systematically illustrated. This work is expected to provide useful insight for the design of more efficient AOPs and a deeper understanding of their catalytic mechanisms.

  The SEM images (Fig. S1 in Supporting information) demonstrate that the morphology of 2H-MoS₂ is similar to that of 1T-MoS₂. The microstructure of the 1T-MoS₂ was further investigated by TEM and high-resolution TEM (HRTEM). Clearly, the 1T-MoS₂ nanosheets have obvious corrugations with layer-like structures (Fig. 1a). The inset shows that the interlayer spacing is 0.62 nm, which matches well with the (002) diffraction of MoS₂ [33]. Meanwhile, Fig. 1b indicates two distinct lattice matrices in the basal plane. The areas marked by the yellow box (Fig. 1b) exhibit honeycomb cells, corresponding to the 2H phase (Fig. 1c) [29]. The selected red box (Fig. 1b) is ascribed to the 1T phase due to the trigonal lattice structure (Fig. 1d) [29]. Figs. 1e and f show the element mapping of Mo and S, revealing the homogeneous dispersion of S or Mo based on the uniform color and luster.

  Figure 1

  

  Figure 1.  (a) TEM images of 1T-MoS₂ nanosheets; the inset shows the layers and interlayer spacing of the MoS₂ nanosheets. (b) HRTEM image of the 1T-MoS₂ nanosheets. (c, d) Diagrammatic representation of the 2H phase and 1T phase. (e, f) Element mapping of 1T-MoS₂ nanosheets. (g) Raman spectra and (h, i) XPS of 1T-MoS₂ and 2H-MoS₂ nanosheets.

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  Raman spectra in Fig. 1g show the characteristic peaks are assigned to 2H-MoS₂ and 1T-MoS₂ [31]. The relative content of the 1T phase was estimated to be approximately 75% by XPS (Figs. 1h and i). More than 70% of the nanosheets could be determined to be 1T-MoS₂ owing to the high purity of 1T-MoS₂ [31]. In XRD patterns (Fig. S2 in Supporting information), the wider peaks of 1T-MoS₂ compared with the 2H-MoS₂ suggest smaller nanodomains [34]. The DRS measurement in Fig. S3 (Supporting information) demonstrates that the 1T-MoS₂ have a stronger absorption intensity than the 2H-MoS₂.

  The H₂O₂ activation activities of two phases of MoS₂ in different systems were evaluated by degradation of MB as a model reaction. First, the influence of various factors was systematically investigated in photo-Fenton-like processes, such as pH value and doses of 1T-MoS₂ and H₂O₂. As shown in Fig. S4 (Supporting information), MB degradation was closely dependent on H₂O₂ concentration, 1T-MoS₂ dosage and pH. The highest kinetics constant (k) values were obtained at H₂O₂ concentration = 1.0 mmol/L, pH 3.0 and 1T-MoS₂ dosage = 0.050 g/L (Figs. S5–S7 in Supporting information).

  The Fenton-like performances of 1T-MoS₂ and 2H-MoS₂ were compared under light and dark conditions (Fig. 2a). In the dark, the degradation efficiency was low, indicating that MoS₂ cannot effectively decompose H₂O₂ to generate ^•OH under dark conditions. Upon illumination, the degradation efficiency of both MoS₂ systems increased rapidly. Obviously, in Fig. 2b, the corresponding k value in the light/1T-MoS₂/H₂O₂ system (0.0301 min⁻¹) was much higher than that of the light/2H-MoS₂/H₂O₂ system (0.0055 min⁻¹). The results indicated that 1T-MoS₂ exhibited better performance than 2H-MoS₂ in activating H₂O₂ under light irradiation. The similar specific surface areas indicate that the difference in degradation rates is caused by MoS₂ phase but not the specific surface area (Fig. S8 in Supporting information).

  Figure 2

  

  Figure 2.  (a) Degradation efficiency of MB using different MoS₂ samples with and without light irradiation. (b) Corresponding kinetic constant values of MB degradation in different systems. (c) Fluorescence spectra for detecting ^•OH by quenching with TA at 40 min. (d) Degradation of MB by 1T-MoS₂ in different systems. Conditions: [MB] = 10 mg/L, pH₀ 3.0, [MoS₂] = 0.05 g/L, [H₂O₂] = 1.0 mmol/L.

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  The ability of these systems to produce ^•OH was investigated by using terephthalic acid as a tracking probe [18,35]. Fig. S9 (Supporting information) shows real-time monitoring of ^•OH in different systems under the optimized conditions. As shown in Fig. S10 (Supporting information), the PL signals intensified faster with time under light than under dark conditions. In particular, the light/1T-MoS₂/H₂O₂ system exhibited the strongest PL signal intensity when choosing 40 min for comparison (Fig. 2c). Thus, it can be concluded that light played a vital role in decomposing H₂O₂ when MoS₂ was added as a Fenton-like reagent, especially for 1T MoS₂.

  Control experiments were conducted to study the synergistic effects of 1T-MoS₂, H₂O₂ and light on the production of ^•OH (Fig. 2d). When using 1T-MoS₂, H₂O₂ and light alone, negligible removal efficiencies were obtained. For light/H₂O₂, 1T-MoS₂/H₂O₂, light/1T-MoS₂ and light/1T-MoS₂/H₂O₂ systems, the MB removal efficiencies improved to 11.0%, 11.8%, 23.0% and 90.8%, respectively. Obviously, the synergistic effects of the three components were much greater than those of only two components. H₂O₂ can be directly activated by light (λ < 310 nm) to produce ^•OH [32]. The 1T-MoS₂ semiconductor can also produce ^•OH and other active species by photocatalytic effect. By comparison, the contribution of 1T-MoS₂ to decompose H₂O₂ is obvious under light.

  The active species in the light/1T-MoS₂/H₂O₂ system were further identified by quenching experiments. Generally, N₂, AO, FFA, IBA and TBA can be utilized to scavenge the free radicals of ^•O₂⁻, h⁺, ¹O₂, ^•OH_free and ^•OH_free+ads [1,16]. As depicted in Figs. 3a and b and Fig. S11 (Supporting information), IBA and TBA suppressed the degradation maximumly, N₂ and AO followed, and FFA had almost no inhibition. These results indicated that ^•OH played a major role, h⁺ and ^•O₂⁻ played a minor role. Notably, the similar inhibition degrees of IBA and TBA suggested that ^•OH_free was the dominant active species rather than the ^•OH_ads radical. To further investigate this process, XPS was performed to analyze the Mo 3d spectra of 1T-MoS₂ before and after the reaction (Fig. 3c). The relative content of Mo⁵⁺ increased from 6.0% to 8.5%, while that of Mo⁴⁺ decreased from 94.0% to 91.5%, demonstrating that ^•OH generation was induced by polyvalent Mo (Mo⁴⁺/Mo⁵⁺).

  Figure 3

  

  Figure 3.  (a, b) Effects of radical scavengers on MB degradation by 1T-MoS₂ in light. (c) XPS spectra of Mo 3d before and after the reaction. (d) Effects of NaF in dark and light systems. (e) Proposed Fenton-like mechanism of MoS₂/H₂O₂ systems with or without light irradiation. Conditions: [MB] = 10 mg/L, pH₀ 3.0, [MoS₂] = 0.05 g/L, [H₂O₂] = 1.0 mmol/L.

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  Thus, an overall mechanism of H₂O₂ activation by MoS₂ in illuminated and dark conditions was proposed (Fig. 3d). First, H₂O₂ was adsorbed on the surface of MoS₂ and subsequently decomposed into ^•OH_ads by Eq. 1. Then, the ^•OH_ads was separated from the surface to become ^•OH_free for MB degradation (Eq. 2). Simultaneously, Mo⁵⁺ returned to Mo⁴⁺ following Eq. 3. However, in the dark, ^•OH_ads desorption from MoS₂ was difficult because it was an endothermic reaction [15]. Thus, no obvious differences in MB degradation were observed between 2H-MoS₂ and 1T-MoS₂ because ^•OH_ads was not able to enter the solution to oxidize MB. Under light irradiation, it was speculated that the energy of the light may accelerate the desorption of ^•OH_ads. Thus, more ^•OH_free participated in the oxidation [32]. Furthermore, the MoS₂ can be excited by the light to form photoactives of h⁺, ^•OH and ^•O₂⁻ through Eqs. S1-S6 (Supporting information). The photogenerated electrons can also help the recycling of Mo⁴⁺/Mo⁵⁺ (Eq. S7 in Supporting information). All of these led to a higher removal efficiency under light irradiation. The superior performance achieved by 1T-MoS₂ was mainly due to the faster charge transfer rate of the 1T metallic phase than that of 2H-MoS₂.

  (1)

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  (3)

  To further verify the effect of ^•OH_free and ^•OH_ads in dark and light and the role of light. NaF was added to the two systems because NaF can cause ^•OH_ads transform to ^•OH_free [36]. As shown in Fig. 3e, the addition of NaF accelerated the degradation in dark, confirming ^•OH_ads was produced and adsorbed on MoS₂. Herin, NaF desorbed them into ^•OH_free to promote the reaction. On the contrary, the degradation was inhibited to a certain extent in light, indicating the role of ^•OH_free. The inhibition may come from the interference of F⁻ [16]. It should be emphasized that the addition of NaF in dark did not make its degradation rate the same as that of light, because light can activate the catalysts and H₂O₂ to produce more active species to participate the degradation. Meanwhile, the results in turn confirm our hypothesis that light may accelerate ^•OH_ads to ^•OH_free due to the extra energy.

  MoS₂ can also serve as a cocatalyst to increase the efficiency of H₂O₂ decomposition in the Fe²⁺/H₂O₂ system. In the dark, when 1T-MoS₂ and 2H-MoS₂ were added to the solution, the removal rates of MB were 70.5% and 43.6% within 30 min, respectively (Fig. 4a). The higher k value of 1T-MoS₂ indicated its stronger ability to promote the conversion of Fe³⁺ to Fe²⁺ (Fig. 4b). Upon illumination, the time required to totally degrade MB was greatly reduced. Surprisingly, the degradation efficiencies reached 90.4% and 83.6%, in 1 min when employing 1T-MoS₂ and 2H-MoS₂ as cocatalysts, respectively. After 5 min, the removal rates further improved to 97.1% and 89.8%. The k values were 2.3466 min⁻¹ for 1T and 1.8083 min⁻¹ for 2H under light irradiation, which were almost 64.3 and 123.9 times those under dark conditions, respectively (Fig. 4b). The results indicate that light can distinctly improve the cocatalytic effect of MoS₂ in AOPs.

  Figure 4

  

  Figure 4.  (a) MB degradation and (b) corresponding k values in different systems. (c) Fluorescence spectra for detecting ^•OH by quenching with TA at 5 min. (d) XPS spectra of Mo 3d before and after the reaction in the light/1T-MoS₂/Fe²⁺/H₂O₂ system. (e) Proposed cocatalytic mechanism in light/Fe²⁺/MoS₂/H₂O₂ systems. Conditions: [MB] = 10 mg/L, pH₀ 3.0, [MoS₂] = 0.05 g/L, [H₂O₂] = 1.0 mmol/L), [FeSO₄·7H₂O] = 0.02 g/L.

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  The PL intensities at 5 min were selected to compare the ^•OH concentrations. As Fig. 4c shows, the order of PL intensity was consistent with the removal rate in different systems. The cocatalytic effect of 1T-MoS₂ was higher than that of 2H-MoS₂, regardless of whether it was in the dark or under illumination. To further understand the cocatalytic mechanism, the Mo 3d spectra of 1T-MoS₂ before and after the reaction were measured and shown in Fig. 4d. Of note, the used 1T-MoS₂ exhibited a new characteristic peak at 235.54 eV, which corresponded to Mo⁶⁺. This phenomenon indicated that a redox cycle was established between Mo⁴⁺/Mo⁶⁺ and Fe³⁺/Fe²⁺, as shown in Eqs. 4-7. For MoS₂, the exposed Mo⁴⁺, which has reductive properties, could accelerate Fe³⁺/Fe²⁺ conversion, thereby improving the generation of ^•OH_free through Eq. 6. Herein, ^•OH_free was mainly produced by Fe²⁺ and H₂O₂ in solution (Eq. 4) and confirmed by quenching experiments in Fig. S12 (Supporting information), so the desorption process of ^•OH from MoS₂ can be ignored (Fig. 4e). This may be the reason why the MoS₂ phases have little effect on the Fenton-like reaction (Fig. 2a) but a great effect on the cocatalytic system (Fig. 4a) in dark conditions.

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  For practical applications, the long-term stabilities were evaluated. As shown in Figs. S13 and S14 (Supporting information), the MB removal rate is almost unchanged in both systems, highlighting the excellent recycling capability as a Fenton-like regent and cocatalyst. The effect of coexisting anions on degradation was demonstrated in Fig. S15 (Supporting information). KCl has little effect on the degradation of MB, but NaHCO₃, Na₂HPO₄·2H₂O and NaH₂PO₄·2H₂O would inhibit the reaction to a certain degree. This because that the coordination effect affects the reaction between MoS₂ and H₂O₂, reducing the production of free radicals [37]. Other typical aromatic organic pollutants, such as tetracycline hydrochloride (TCH), methyl orange (MO), rhodamine B (RhB) and p-nitrophenol (PNP), were selected for evaluating the applicability of the system (Fig. S16 in Supporting information). Both photo Fenton-like and cocatalytic systems exhibited high removal rates, indicating 1T-MoS₂ has a wide application range for complicated wastewater.

  In conclusion, two different phases of 2H-MoS₂ and 1T-MoS₂ nanosheets were successfully synthesized. The prepared materials demonstrated phase-mediated activation of H₂O₂ and a different mechanism of generating ^•OH in MoS₂/H₂O₂ and MoS₂/Fe²⁺/H₂O₂ systems depending on whether or not light irradiation was applied. In the dark/MoS₂/H₂O₂ system, the removal rate was very low, and the MoS₂ phase had a negligible impact on the degradation rate. When light irradiation was introduced, the k value of 0.0301 min⁻¹ in the light/1T-MoS₂/H₂O₂ system was approximately 5.47 times higher than that of the light/2H-MoS₂/H₂O₂ system. In the dark/Fe²⁺/MoS₂/H₂O₂ and light/Fe²⁺/MoS₂/H₂O₂ systems, 1T-MoS₂ exhibited a superior cocatalytic effect than 2H-MoS₂ in both systems. The main active species were ^•OH_free rather than surface ^•OH_ads. Light irradiation may provide external energy to desorb ^•OH_ads into ^•OH_free. XPS results indicated that the conversion of Mo⁴⁺/Mo⁵⁺ triggers the decomposition of H₂O₂ in the MoS₂/H₂O₂ system, while the cycle of Mo⁴⁺/Mo⁶⁺ promotes the Fe²⁺/Fe³⁺ conversion in the Fe²⁺/MoS₂/H₂O₂ system. In addition, 1T-MoS₂ exhibited long-term stability and the ability to degrade various organics in these two systems.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the Natural Science Foundation of Henan Province (No. 212300410336), Program for Science and Technology Innovation Talent in Universities of Henan Province (No. 20HASTIT016), National Natural Science Foundation of China (No. 51902101).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.107874.

  
  
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  Figure 1  (a) TEM images of 1T-MoS₂ nanosheets; the inset shows the layers and interlayer spacing of the MoS₂ nanosheets. (b) HRTEM image of the 1T-MoS₂ nanosheets. (c, d) Diagrammatic representation of the 2H phase and 1T phase. (e, f) Element mapping of 1T-MoS₂ nanosheets. (g) Raman spectra and (h, i) XPS of 1T-MoS₂ and 2H-MoS₂ nanosheets.

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  Figure 2  (a) Degradation efficiency of MB using different MoS₂ samples with and without light irradiation. (b) Corresponding kinetic constant values of MB degradation in different systems. (c) Fluorescence spectra for detecting ^•OH by quenching with TA at 40 min. (d) Degradation of MB by 1T-MoS₂ in different systems. Conditions: [MB] = 10 mg/L, pH₀ 3.0, [MoS₂] = 0.05 g/L, [H₂O₂] = 1.0 mmol/L.

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  Figure 3  (a, b) Effects of radical scavengers on MB degradation by 1T-MoS₂ in light. (c) XPS spectra of Mo 3d before and after the reaction. (d) Effects of NaF in dark and light systems. (e) Proposed Fenton-like mechanism of MoS₂/H₂O₂ systems with or without light irradiation. Conditions: [MB] = 10 mg/L, pH₀ 3.0, [MoS₂] = 0.05 g/L, [H₂O₂] = 1.0 mmol/L.

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  Figure 4  (a) MB degradation and (b) corresponding k values in different systems. (c) Fluorescence spectra for detecting ^•OH by quenching with TA at 5 min. (d) XPS spectra of Mo 3d before and after the reaction in the light/1T-MoS₂/Fe²⁺/H₂O₂ system. (e) Proposed cocatalytic mechanism in light/Fe²⁺/MoS₂/H₂O₂ systems. Conditions: [MB] = 10 mg/L, pH₀ 3.0, [MoS₂] = 0.05 g/L, [H₂O₂] = 1.0 mmol/L), [FeSO₄·7H₂O] = 0.02 g/L.

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