Hydrothermal Synthesis of MoS2/SnS2 Photocatalysts with Heterogeneous Structures Enhances Photocatalytic Activity

The use of solar photocatalysts to degrade organic pollutants is not only the most promising and efficient strategy to solve pollution problems today but also helps to alleviate the energy crisis. In this work, MoS2/SnS2 heterogeneous structure catalysts were prepared by a facile hydrothermal method, and the microstructures and morphologies of these catalysts were investigated using XRD, SEM, TEM, BET, XPS and EIS. Eventually, the optimal synthesis conditions of the catalysts were obtained as 180 °C for 14 h, with the molar ratio of molybdenum to tin atoms being 2:1 and the acidity and alkalinity of the solution adjusted by hydrochloric acid. TEM images of the composite catalysts synthesized under these conditions clearly show that the lamellar SnS2 grows on the surface of MoS2 at a smaller size; high-resolution TEM images show lattice stripe distances of 0.68 nm and 0.30 nm for the (002) plane of MoS2 and the (100) plane of SnS2, respectively. Thus, in terms of microstructure, it is confirmed that the MoS2 and SnS2 in the composite catalyst form a tight heterogeneous structure. The degradation efficiency of the best composite catalyst for methylene blue (MB) was 83.0%, which was 8.3 times higher than that of pure MoS2 and 16.6 times higher than that of pure SnS2. After four cycles, the degradation efficiency of the catalyst was 74.7%, indicating a relatively stable catalytic performance. The increase in activity could be attributed to the improved visible light absorption, the increase in active sites introduced at the exposed edges of MoS2 nanoparticles and the construction of heterojunctions opening up photogenerated carrier transfer pathways and effective charge separation and transfer. This unique heterostructure photocatalyst not only has excellent photocatalytic performance but also has good cycling stability, which provides a simple, convenient and low-cost method for the photocatalytic degradation of organic pollutants.


Introduction
Today's industrialized and urbanized world is facing severe energy shortages and environmental pollution problems. The excessive use of organic dye and the indiscriminate release of organic pollutants cause serious damage to ecosystems and have serious effects on future generations [1]. Moreover, the organic ingredients in our living environment are difficult to degrade and toxic in nature. There have been many methods to solve the problem of organic dye contamination in environment, such as adsorption [2,3], membrane separation [4,5], biological decomposition [6], chemical oxidation [7], electrocatalysis [8] and photocatalysis [9,10] decomposition. Among them, the use of solar photocatalysis for the degradation of organic pollutants is considered one of the most promising and efficient strategies [11,12].
Transition metal sulfides have attracted a lot of attention in wastewater treatment because of their high specific surface area, high surface activity and special microstructure. In recent years, MoS 2 has been widely used in organic dye decomposition due to its low cost, high abundance and noble-metal-like activities [13][14][15]. MoS 2 has a graphene-like layered structure with three crystal phases: 1T, 2H and 3R [16]. In a natural state, MoS 2 is usually present in the steady 2H phase, which exhibits semiconducting properties [17,18]. However, the low density of active sites and relatively poor conductivity of 2H-MoS 2 lead to limited photocatalytic activity [19,20]. Compared with 2H-MoS 2 , the metallic 1T-MoS 2 phase has the advantages of significant conductivity and a high density of marginal active sites at room temperature and shows better performance in photocatalysis [21][22][23][24]. So far, most 1T-MoS 2 is fabricated as two-dimensional nanosheets to construct hybrid structures with nanoconjunctions [25][26][27].
Li et al. [28] prepared two-dimensional heterostructured MoS 2 /g-C 3 N 4 (graphite-C 3 N 4 ) photocatalysts using a facile impregnation-calcination method. The experimental results showed that surface MoS 2 nanosheets were successfully loaded horizontally onto g-C 3 N 4 nanosheets. Meanwhile, the two-dimensional heterojunction formed between g-C 3 N 4 nanosheets and MoS 2 nanosheets improved the separation efficiency and charge transfer rate of photogenerated electrons. One of the synthesized samples, MCNNs-3 (3 wt% MoS 2 in MoS 2 /g-C 3 N 4 heterojunction), with a catalyst content of 0.8 g/L, reduced the concentration of rhodamine B (RhB) by about 96% after 20 min of irradiation. Chen et al. [29] prepared MoS 2 /TaON (tantalum oxynitride) hybrid nanostructures by a hydrothermal method. This work showed that the photocatalytic degradation of rhodamine B (RhB) on Ta1Mo1 (mass ratio of TaON:MoS 2 = 1:1) was about 65% after 2 h of visible light irradiation, which was about five times higher than that of pure TaON. In addition, MoS 2 /SiO 2 /TaON ternary photocatalysts were constructed to further improve the photocatalytic performance. When the mass ratio of Ta8Si1 (TaON:SiO 2 = 8:1) to MoS 2 was 1:1, the degradation rate of RhB reached 75% under 2 h of visible light irradiation. Yin et al. [30] synthesized two kinds of MoS 2 and PbBiO 2 Cl nanosheets by the solvothermal method and then prepared a novel 2D/2D MoS 2 /PbBiO 2 Cl photocatalyst by mechanical stirring at room temperature. The resulting experiments showed that 1 wt% of MoS 2 /PbBiO 2 Cl showed stronger photocatalytic performance and 80% of rhodamine B (RhB) could be completely degraded within 120 min, whereas the photocatalytic activity decreased when the content of MoS 2 was higher.
Composites containing MoS 2 with other similar materials are an effective way to enhance the photocatalytic ability of the material. SnS 2 has a narrow band gap of 2.0 to 2.3 eV and is a low-cost, non-toxic CdI 2 -type layered semiconductor [31][32][33]. According to the literature [34][35][36], SnS 2 is a relatively stable visible-light-driven photocatalyst in the degradation of organic compounds. However, like most semiconductor photocatalysts, SnS 2 also has the disadvantage of high recombination rates of photogenerated electrons and holes, resulting in low photocatalytic efficiency [37]. Among the modification strategies explored to improve the photocatalytic efficiency of SnS 2 , the combination with a suitable semiconductor or other components (e.g., graphene) facilitates the separation of photogenerated electrons and holes through interfacial charge transfer [38][39][40]. Zhang et al. [41] prepared 2D/2D-type SnS 2 /g-C 3 N 4 (graphite-C 3 N 4 ) heterojunction photocatalysts using an ultrasonic dispersion method. The electron microscopic characterization analysis showed that a large contact zone was induced at the heterojunction interface due to the lamellar structure of both the SnS 2 and g-C 3 N 4 materials. In the photoluminescence spectra, it can also be shown that the photo-coordination effect of the SnS 2 /g-C 3 N 4 heterojunction effectively enhances the interfacial carrier transfer, leading to enhanced charge separation during the photocatalytic reaction.
Due to the outstanding reactivity of both MoS 2 and SnS 2 in the photocatalytic degradation of organic dyes, the structures of MoS 2 and SnS 2 were coupled to construct a heterogeneous structure to enhance the degradation of organic dyes. Compared with previous work, this experiment is further improved: firstly, by changing the synthesis method of the sample and the synthesis conditions, the high temperature and high energy consumption in the experiment as well as the shortened reaction time are avoided; secondly, the synthesis steps are simpler and less cumbersome; and finally, the reactants are easily available throughout the experiment and the heterogeneous structure is binary, which can efficiently solve the cost problem in the application.
In this work, we used a convenient hydrothermal method to obtain MoS 2 /SnS 2 composite catalysts with SnS 2 nanosheets grown on MoS 2 nanoparticles. By adjusting the hydrothermal time of the reaction (12 h, 14 h and 16 h) and changing the molar ratio of the substances (1:1, 2:1, 3:1 and 4:1 atomic molar ratio of molybdenum-tin), a heterogeneous structure was constructed between MoS 2 and SnS 2 after the hydrothermal reaction, resulting in a semiconducting composite photocatalyst with a narrow band gap. Theoretically, the narrowing of the band gap of the material can effectively improve the absorption of visible light and the catalyst material can produce a large number of electrons and holes when light can be irradiated. In addition, the heterostructure can effectively modulate the electronic structure of the complex system while promoting electron transport between the interfaces more effectively, thus improving the photocatalytic ability. This work highlights that the construction of heterojunctions between two substances may be an attractive method for the removal of pollutants from industrial wastewater. (Tianjin, China). Except for hydrochloric acid, which is superiorly pure, the rest of the chemical reagents are of analytical grade and were used without further purification.

Synthesis of Photocatalysts
The fabrication steps involved in the synthesis of the MoS 2 /SnS 2 composite catalysts are schematically illustrated in Figure 1. First, the synthesis of MoS 2 nanoparticles [42]: MoS 2 nanoparticles were synthesized by the hydrothermal method. Typically, 1.0592 g of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and 1.828 g of CH 4 N 2 S were dissolved in 60 mL of deionized water at room temperature with continuous stirring until complete dissolution. The mixed solution was then transferred to a 100 mL Teflon-lined autoclave and kept at 180 • C for 16 h. Then, the obtained black precipitate was dried at 80 • C for 2 h. synthesis steps are simpler and less cumbersome; and finally, the reactants are easily available throughout the experiment and the heterogeneous structure is binary, which can efficiently solve the cost problem in the application. In this work, we used a convenient hydrothermal method to obtain MoS2/SnS2 composite catalysts with SnS2 nanosheets grown on MoS2 nanoparticles. By adjusting the hydrothermal time of the reaction (12 h, 14 h and 16 h) and changing the molar ratio of the substances (1:1, 2:1, 3:1 and 4:1 atomic molar ratio of molybdenum-tin), a heterogeneous structure was constructed between MoS2 and SnS2 after the hydrothermal reaction, resulting in a semiconducting composite photocatalyst with a narrow band gap. Theoretically, the narrowing of the band gap of the material can effectively improve the absorption of visible light and the catalyst material can produce a large number of electrons and holes when light can be irradiated. In addition, the heterostructure can effectively modulate the electronic structure of the complex system while promoting electron transport between the interfaces more effectively, thus improving the photocatalytic ability. This work highlights that the construction of heterojunctions between two substances may be an attractive method for the removal of pollutants from industrial wastewater. . Except for hydrochloric acid, which is superiorly pure, the rest of the chemical reagents are of analytical grade and were used without further purification.

Synthesis of Photocatalysts
The fabrication steps involved in the synthesis of the MoS2/SnS2 composite catalysts are schematically illustrated in Figure 1. First, the synthesis of MoS2 nanoparticles [42]: MoS2 nanoparticles were synthesized by the hydrothermal method. Typically, 1.0592 g of (NH4)6Mo7O24·4H2O and 1.828 g of CH4N2S were dissolved in 60 mL of deionized water at room temperature with continuous stirring until complete dissolution. The mixed solution was then transferred to a 100 mL Teflon-lined autoclave and kept at 180 °C for 16 h. Then, the obtained black precipitate was dried at 80 °C for 2 h. Preparation of MoS2/SnS2 composite catalysts [43] with different reaction times (12 h,14 h and 16 h): the black MoS2 powder was weighed according to the ratio and dispersed in 60 mL deionized water to form a suspension. Certain proportions of SnCl4·5H2O and CH4N2S were added to the suspension; then, a certain amount of 1 mol/L hydrochloric acid was added to adjust the mixed solution to acidity. Finally, the mixture was transferred to a 100 mL Teflon-lined autoclave and kept at 180 °C for a certain time. After the Preparation of MoS 2 /SnS 2 composite catalysts [43] with different reaction times (12 h,14 h and 16 h): the black MoS 2 powder was weighed according to the ratio and dispersed in 60 mL deionized water to form a suspension. Certain proportions of SnCl 4 ·5H 2 O and CH 4 N 2 S were added to the suspension; then, a certain amount of 1 mol/L hydrochloric acid was added to adjust the mixed solution to acidity. Finally, the mixture was transferred to a 100 mL Teflon-lined autoclave and kept at 180 • C for a certain time. After the reaction was completed, the catalyst was collected by centrifugation and dried at 80 • C for 2 h to obtain the catalyst. The catalysts were named MS12-2-H, MS14-2-H and MS16-2-H according to the reaction time and the atomic molar ratio of molybdenum to tin.

Structural Characterization
Powder X-ray diffraction patterns were obtained using a Rigaku Smart Lab diffractometer with Cu Kα (λ = 0.154178 nm) as the radiation source. The morphology of the samples was measured using a JSM-6510 scanning electron microscope. Nitrogen adsorptiondesorption isotherms were measured at −196 • C using a specific surface area and pore-size analyzer, the V-Sorb 1800. The samples were all pretreated at 105 • C for 12 h prior to measurement. Electrochemical impedance was tested with a CH1660E electrochemical workstation. X-ray photoelectron spectra were obtained with a KRATOS AXIS SUPRA.

Photocatalytic Degradation and Photoelectrochemical Test
The catalytic performances of the composite catalysts were evaluated by their ability to degrade the target pollutant, MB, under visible light using a 300 W xenon lamp as a visible light source. In each test, 30 mg of catalyst was dispersed in 80 mL of MB solution (15 mg/L). The mixed solution was stirred in the dark for 30 min prior to the test to achieve an adsorption-desorption equilibrium between the catalyst and the solution. The 7 mL suspension was removed every 10 min under visible light and centrifuged at 3500 r/min for 5 min. The absorbance of the solution at different reaction times was measured by UV-visible spectrophotometer.
The determination of the concentration of organic dyes can be described by the Beer-Lambert law, and the amount of light absorbed by the solution follows the Beer-Lambert law. The specific equation is as follows [44]: where A is the absorbance, ε is the light absorption coefficient, b is the solution thickness and c is the dye concentration solution at the time of sampling. According to the Beer-Lambert law, the relationship between dye concentration and absorbed light is linear. The electrochemical impedance test was carried out in a CH1660E electrochemical workstation with a platinum electrode as the counter electrode and a saturated glycerol electrode as the reference electrode, and the corresponding open circuit voltage and frequency were set. In this experiment [43], catalyst-coated conductive glass was used as the working electrode, namely 20 mg of the catalyst dispersed into a mixture containing 40 uL of 5 wt% Nafion and 0.5 mL of anhydrous ethanol. After mixing well with ultrasound, 200 uL of the suspension was coated onto the surface of the conductive glass with a pipette gun; this was then dried naturally at room temperature. The working electrode for the electrochemical impedance was tested in 0.1 M Na 2 SO 4 solution.  [46]. In addition, the layer spacing of MoS 2 becomes larger during the reaction process, resulting in a shift of the (002) crystal plane to 10.9 • . The presence of both MoS 2 and SnS 2 components in the synthesized catalysts without spurious peaks in the XRD patterns indicate the successful synthesis of MoS 2 /SnS 2 composite catalysts.

Characterization and Properties of Composite Catalysts Synthesized for Different Reaction Times
correspond to the (001), (100), (101), (102), (110), (111) and (103) SnS2 crystalline pla respectively (JCPDS No. 23-0677) [45]. The SnS2 component is successfully synthesize the MoS2/SnS2 composite catalysts. The characteristic peaks of MoS2 at 2θ equal to 1 32.8° and 57.2° are not clearly shown in the figure because of the unique lamellar struc and small grain size of MoS2 [46]. In addition, the layer spacing of MoS2 becomes la during the reaction process, resulting in a shift of the (002) crystal plane to 10.9°. The p ence of both MoS2 and SnS2 components in the synthesized catalysts without spur peaks in the XRD patterns indicate the successful synthesis of MoS2/SnS2 composite c lysts. With increasing reaction time, the characteristic peak of MoS2 gradually broad and the characteristic peaks of SnS2 gradually narrow, indicating the stronger crystalli of the SnS2 phase, which on the other hand also means that the structure of SnS2 in reaction process is more complete.

Morphology Analysis
Scanning electron micrographs of the catalysts MS12-2-H, MS14-2-H and MS16synthesized at different times are shown in Figure 2. In Figure 3a, the hexagonal S nanosheets are grown on MoS2 nanosphere flowers [1], whereas the hexagonal nanosheets are not uniformly distributed and a large portion of the nanoflake particles not in contact with the nanosheets With increasing reaction time, the characteristic peak of MoS 2 gradually broadens and the characteristic peaks of SnS 2 gradually narrow, indicating the stronger crystallinity of the SnS 2 phase, which on the other hand also means that the structure of SnS 2 in the reaction process is more complete.

Morphology Analysis
Scanning electron micrographs of the catalysts MS12-2-H, MS14-2-H and MS16-2-H synthesized at different times are shown in Figure 2. In Figure 3a, the hexagonal SnS 2 nanosheets are grown on MoS 2 nanosphere flowers [1], whereas the hexagonal SnS 2 nanosheets are not uniformly distributed and a large portion of the nanoflake particles are not in contact with the nanosheets.
The morphology of the catalysts in Figure 3b changed considerably. The growth of SnS 2 nanosheets on the surface of the flower-like morphology of MoS 2 was not only more uniformly distributed but also the agglomeration of SnS 2 nanosheets was slight, which was obviously different from the other catalysts and could expose more active sites. The SnS 2 nanosheets in Figure 3c also have better crystallinity of the grains, although they are more uniformly distributed than in (a), which is consistent with the results for the XRD experiments. Table 1 shows the results of the specific surface area test results. The nitrogen adsorption-desorption isotherms were measured at −196 • C after all the samples were pretreated at 105 • C for 12 h prior to measurement. The specific surface area of the composite catalyst showed a trend of increasing and then decreasing with the increase in the reaction time. The reaction time did not have a great influence on the average pore diameter in the range 2.32-2.34 nm and total pore volume of 0.003 cm 3 /g of the composite catalysts, which were almost negligible. Combined with the analysis of the SEM images of the composite catalysts, the specific surface area of the MoS 2 and SnS 2 materials was greatly enhanced due to the uniform growth of lamellar SnS 2 particles on the surface of the flower-like MoS 2 particles; thus, effectively improving the catalytic performance [47].  The morphology of the catalysts in Figure 3b changed considerably. The growth of SnS2 nanosheets on the surface of the flower-like morphology of MoS2 was not only more uniformly distributed but also the agglomeration of SnS2 nanosheets was slight, which was obviously different from the other catalysts and could expose more active sites. The SnS2 nanosheets in Figure 3c also have better crystallinity of the grains, although they are more uniformly distributed than in (a), which is consistent with the results for the XRD experiments. Table 1 shows the results of the specific surface area test results. The nitrogen adsorption-desorption isotherms were measured at −196 °C after all the samples were pretreated at 105 °C for 12 h prior to measurement. The specific surface area of the composite catalyst showed a trend of increasing and then decreasing with the increase in the reaction time. The reaction time did not have a great influence on the average pore diameter in the range 2.32-2.34 nm and total pore volume of 0.003 cm 3 /g of the composite catalysts, which were almost negligible. Combined with the analysis of the SEM images of the composite catalysts, the specific surface area of the MoS2 and SnS2 materials was greatly enhanced due to the uniform growth of lamellar SnS2 particles on the surface of the flower-like MoS2 particles; thus, effectively improving the catalytic performance [47].    Figure 4 shows the electrochemical impedance plots for the composite catalysts synthesized for different reaction times. In the electrochemical impedance diagram, the radius of the semicircle in the high-frequency region is positively correlated with the charge transfer resistance, reflecting the transfer characteristics of photogenerated electrons and holes in the catalysts under the light conditions. According to the test results, the impedance radius of the MoS 2 /SnS 2 composite catalysts showed a trend of decreasing and then increasing as the reaction time increased, in which the MS14-2-H composite catalyst had the smallest impedance radius and presented a high charge transfer migration efficiency [48]. This also proves that a suitable reaction time has a great impact on the electronic structure of the two components in the catalysts, resulting in the improvement of their charge transfer capacity and thus the photocatalytic degradation performance. the smallest impedance radius and presented a high charge transfer migration efficiency [48]. This also proves that a suitable reaction time has a great impact on the electronic structure of the two components in the catalysts, resulting in the improvement of their charge transfer capacity and thus the photocatalytic degradation performance.

Photocatalytic Performance
The degradation of methylene blue solution by MoS2/SnS2 composite catalysts formed at different reaction times is shown in Figure 5. From the figures, it can be observed that the composite catalyst degraded MB in visible light. Of the three catalysts MS14-2-H exhibited the highest MB degradation rates. With the increasing of the synthesis time of the MoS2/SnS2 composite catalysts, a trend of enhancing and then weakening is observed, which is consistent with the test results of the electrochemical impedance of the composite catalysts. The length of the reaction time has an effect on the electronic structure of the synthesized composite catalyst, thus affecting the migration rate of photogenerated charges in visible light and producing an effect on the performance of the degradation of MB. From the degradation data, it was found that the best catalytic performance was achieved by sample MS14-2-H, which had an 83.0% degradation rate after 80 min of visible light irradiation. This was followed by sample MS12-2-H, which had a 55.9% degradation rate. This corroborates the previous results of the XRD, SEM and EIS analyses. From the degradation data, it was found that the best catalytic performance was achieved by sample MS14-2-H, which had an 83.0% degradation rate after 80 min of visible light irradiation. This was followed by sample MS12-2-H, which had a 55.9% degradation rate. This corroborates the previous results of the XRD, SEM and EIS analyses. [48]. This also proves that a suitable reaction time has a great impact on the electronic structure of the two components in the catalysts, resulting in the improvement of their charge transfer capacity and thus the photocatalytic degradation performance.

Photocatalytic Performance
The degradation of methylene blue solution by MoS2/SnS2 composite catalysts formed at different reaction times is shown in Figure 5. From the figures, it can be observed that the composite catalyst degraded MB in visible light. Of the three catalysts MS14-2-H exhibited the highest MB degradation rates. With the increasing of the synthesis time of the MoS2/SnS2 composite catalysts, a trend of enhancing and then weakening is observed, which is consistent with the test results of the electrochemical impedance of the composite catalysts. The length of the reaction time has an effect on the electronic structure of the synthesized composite catalyst, thus affecting the migration rate of photogenerated charges in visible light and producing an effect on the performance of the degradation of MB. From the degradation data, it was found that the best catalytic performance was achieved by sample MS14-2-H, which had an 83.0% degradation rate after 80 min of visible light irradiation. This was followed by sample MS12-2-H, which had a 55.9% degradation rate. This corroborates the previous results of the XRD, SEM and EIS analyses.  Figure 6 represents the XRD patterns of the composite catalysts synthesized by varying the molybdenum-tin molar ratio in the reactants at a reaction temperature of 180 • C for 14 h. It can be seen from the figures that the characteristic peaks at 2θ equal to 14.9 • , 28.  [45]. In addition, no excess spurious peaks were found in the XRD patterns, indicating that the MoS 2 /SnS 2 composite catalysts were successfully synthesized.  Figure 6 represents the XRD patterns of the composite catalysts synthesized by varying the molybdenum-tin molar ratio in the reactants at a reaction temperature of 180 °C for 14 h. It can be seen from the figures that the characteristic peaks at 2θ equal to 14.9°, 28.2°, 32.1°, 41.8°, 49.9°, 52.4° and 54.9° correspond to the (001), (100), (101), (102), (110), (111) and (103) crystal planes of SnS2 in the composite catalysts of different molybdenumtin molar ratios, respectively [45]. In addition, no excess spurious peaks were found in the XRD patterns, indicating that the MoS2/SnS2 composite catalysts were successfully synthesized. With an increase in the MoS2 fraction, the main change in the morphology is that the SnS2 nanosheets are not uniformly distributed on the MoS2 surface, as shown in Figure 7c,d. At lower Mo/Sn ratios, the scanning images of the catalyst changed more, and the MoS2 morphology was no longer a regular spherical flower shape but a nano-flake shape. In addition, the SnS2 nanosheets were closely distributed on the MoS2 surface, as shown in Figure  7a,b. The close distribution of the SnS2 nanosheets on the MoS2 surface contributes to the close bonding of the two materials, which can effectively change the electronic structure of the catalyst and increase the photocatalytic active sites.  [1]. With an increase in the MoS 2 fraction, the main change in the morphology is that the SnS 2 nanosheets are not uniformly distributed on the MoS 2 surface, as shown in Figure 7c,d. At lower Mo/Sn ratios, the scanning images of the catalyst changed more, and the MoS 2 morphology was no longer a regular spherical flower shape but a nano-flake shape. In addition, the SnS 2 nanosheets were closely distributed on the MoS 2 surface, as shown in Figure 7a,b. The close distribution of the SnS 2 nanosheets on the MoS 2 surface contributes to the close bonding of the two materials, which can effectively change the electronic structure of the catalyst and increase the photocatalytic active sites.   Figure 8b, which could correspond to the (002) plane of MoS2 and the (100) plane of SnS2, respectively [49]. These images objectively further explain the coexistence of SnS2 and MoS2 in the MS14-2-H composite catalyst. In addition to this, the lattice stripe in the (002) plane of MoS2 is larger than the standard spacing value (0.62 nm) according to the Bragg equation:

XRD Characterization
where d is the lattice spacing, θ is the angle between the incident ray, the reflection line and the reflected crystal plane, λ is the wavelength and n is the number of reflection levels.
It is known that under specific conditions, the lattice spacing d is inversely proportional to the angle θ, i.e., at this time, the lattice spacing of the (002) plane of MoS2 is large and the corresponding diffraction peak angle becomes small, which is similar to the diffraction peak of the (002) corresponding to the MoS2 phase of the composite catalyst in the XRD analysis shifting to 10.9°.   Figure 8b, which could correspond to the (002) plane of MoS 2 and the (100) plane of SnS 2 , respectively [49]. These images objectively further explain the coexistence of SnS 2 and MoS 2 in the MS14-2-H composite catalyst. In addition to this, the lattice stripe in the (002) plane of MoS 2 is larger than the standard spacing value (0.62 nm) according to the Bragg equation: where d is the lattice spacing, θ is the angle between the incident ray, the reflection line and the reflected crystal plane, λ is the wavelength and n is the number of reflection levels. It is known that under specific conditions, the lattice spacing d is inversely proportional to the angle θ, i.e., at this time, the lattice spacing of the (002) plane of MoS 2 is large and the corresponding diffraction peak angle becomes small, which is similar to the diffraction peak of the (002) corresponding to the MoS 2 phase of the composite catalyst in the XRD analysis shifting to 10.9 • . Materials 2023, 16, x FOR PEER REVIEW 10 of 16  Table 2 shows the results of the specific surface area tests of the MoS2/SnS2 composite catalysts with different molybdenum-tin molar ratios. From the data in the table, it can be observed that the specific surface area of the composite catalysts shows a trend of increasing and then decreasing when increasing the molybdenum-tin molar ratio from 1:1 to 4:1. In addition, the average pore diameter of the catalyst was 2.30~2.34 nm and the total pore volume was 0.003 cm 3 /g. The effect of molybdenum-tin molar ratio on the average pore diameter and total pore volume of the composite catalysts is almost negligible, and the effect on the catalyst performance is mainly due to the specific surface area, which is 17.10 m 2 /g for MS14-2-H, followed by 15.05 m 2 /g for MS14-1-H. Combined with the analysis of the SEM images of the composite catalysts, the reason for the large differences in the specific surface areas of the catalysts may be due to the size and shape of the particles of MoS2 and SnS2 and the difference in the shape of the particles.

Electrochemical Impedance Measurement
The electrochemical impedance diagrams of the composite catalysts synthesized at different molybdenum-tin molar ratios are shown in Figure 9. According to the test results, the impedance radii of the MoS2/SnS2 composite catalysts showed a trend of decreasing and then increasing as the molybdenum-tin molar ratio increased; the MS14-2-H composite catalyst having the smallest impedance radius. Since the radius of the semicircle in the high-frequency region is positively correlated with the charge transfer resistance in the electrochemical impedance diagram, the sample MS14-2-H has the smallest charge transfer resistance [48].  Table 2 shows the results of the specific surface area tests of the MoS 2 /SnS 2 composite catalysts with different molybdenum-tin molar ratios. From the data in the table, it can be observed that the specific surface area of the composite catalysts shows a trend of increasing and then decreasing when increasing the molybdenum-tin molar ratio from 1:1 to 4:1. In addition, the average pore diameter of the catalyst was 2.30~2.34 nm and the total pore volume was 0.003 cm 3 /g. The effect of molybdenum-tin molar ratio on the average pore diameter and total pore volume of the composite catalysts is almost negligible, and the effect on the catalyst performance is mainly due to the specific surface area, which is 17.10 m 2 /g for MS14-2-H, followed by 15.05 m 2 /g for MS14-1-H. Combined with the analysis of the SEM images of the composite catalysts, the reason for the large differences in the specific surface areas of the catalysts may be due to the size and shape of the particles of MoS 2 and SnS 2 and the difference in the shape of the particles.

Electrochemical Impedance Measurement
The electrochemical impedance diagrams of the composite catalysts synthesized at different molybdenum-tin molar ratios are shown in Figure 9. According to the test results, the impedance radii of the MoS 2 /SnS 2 composite catalysts showed a trend of decreasing and then increasing as the molybdenum-tin molar ratio increased; the MS14-2-H composite catalyst having the smallest impedance radius. Since the radius of the semicircle in the high-frequency region is positively correlated with the charge transfer resistance in the electrochemical impedance diagram, the sample MS14-2-H has the smallest charge transfer resistance [48].      Figure 10b shows the XPS spectra of Mo 3d, in which five different characteristic peaks appear. It is known from the split peak fitting and literature review [50] that peaks at 229.1 eV and 232.2 eV correspond to Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 , respectively, peaks at 233.3 eV and 235.5 eV correspond to Mo 6+ 3d 5/2 and Mo 6+ 3d 3/2 , respectively, and the peak at 225.8 eV corresponds to S 2− 2s. The Mo 4+ species belong to MoS 2 and Mo 6+ signals and may be caused by slight oxidation in air. Only two characteristic peaks appear in Figure 10c, and a review of the literature shows that [51] signals at binding energies of 486.8 eV and 495.3 eV correspond to Sn 4+ 3d 5/2 and Sn 4+ 3d 3/2 , respectively, whereas the Sn 4+ species belong to SnS 2 . Signals of S 2− 2p 3/2 and S 2− 2p 1/2 from the composite catalyst present at binding energies equal to 161.9 eV and 163.1 eV [52]. Therefore, the analysis shows that the composition of the synthesized samples is consistent with the target MoS 2 /SnS 2 catalyst.
3.2.6. Photocatalytic Performance Figure 11a shows the performance of the composite catalysts synthesized at different molybdenum-tin molar ratios in the degradation of methylene blue solutions. From the degradation data in the figure, the visible light degradation performances in methylene blue solution of the composite catalysts synthesized at different molybdenum-tin molar ratios are better than that of either pure MoS 2 or SnS 2 , indicating that the photocatalytic performance can be effectively improved by using a composite of these two materials. In addition, after visible light irradiation for 80 min, the best catalytic performance was achieved for sample MS14-2-H, which had a degradation rate of 83%, whereas the degradation rate of sample MS14-3-H was 44.6% and that of sample MS14-4-H was 24.7%, indicating that the optimal molybdenum-tin ratio that can effectively improve the photocatalytic performance is 2:1 and that the effect on the photocatalytic performance is limited by only increasing the content of MoS 2 in the composite catalyst. Attention should be paid to the reasonable distribution of the two components in the composite catalysts, which is consistent with the previous results of the XRD, SEM and EIS analyses.  Figure 10b shows the XPS spectra of Mo 3d, in which five different characteristic peaks appear. It is known from the split peak fitting and literature review [50] that peaks at 229.1 eV and 232.2 eV correspond to Mo 4+ 3d5/2 and Mo 4+ 3d3/2, respectively, peaks at 233.3 eV and 235.5 eV correspond to Mo 6+ 3d5/2 and Mo 6+ 3d3/2, respectively, and the peak at 225.8 eV corresponds to S 2− 2s. The Mo 4+ species belong to MoS2 and Mo 6+ signals and may be caused by slight oxidation in air. Only two characteristic peaks appear in Figure  10c, and a review of the literature shows that [51] signals at binding energies of 486.8 eV and 495.3 eV correspond to Sn 4+ 3d5/2 and Sn 4+ 3d3/2, respectively, whereas the Sn 4+ species belong to SnS2. Signals of S 2− 2p3/2 and S 2− 2p1/2 from the composite catalyst present at binding energies equal to 161.9 eV and 163.1 eV [52]. Therefore, the analysis shows that the composition of the synthesized samples is consistent with the target MoS2/SnS2 catalyst. Figure 11a shows the performance of the composite catalysts synthesized at different molybdenum-tin molar ratios in the degradation of methylene blue solutions. From the degradation data in the figure, the visible light degradation performances in methylene blue solution of the composite catalysts synthesized at different molybdenum-tin molar ratios are better than that of either pure MoS2 or SnS2, indicating that the photocatalytic performance can be effectively improved by using a composite of these two materials. In addition, after visible light irradiation for 80 min, the best catalytic performance was achieved for sample MS14-2-H, which had a degradation rate of 83%, whereas the degradation rate of sample MS14-3-H was 44.6% and that of sample MS14-4-H was 24.7%, indicating that the optimal molybdenum-tin ratio that can effectively improve the photocatalytic performance is 2:1 and that the effect on the photocatalytic performance is limited by only increasing the content of MoS2 in the composite catalyst. Attention should be paid to the reasonable distribution of the two components in the composite catalysts, which is consistent with the previous results of the XRD, SEM and EIS analyses.    Figure 11b shows the cycling stability test of the composite catalyst MS14-2-H. It can be seen from the figure that the catalytic degradation efficiency of the composite catalyst MS14-2-H in the MB solution decreased from 83.0% to 74.7% after four visible photocatalytic cycle tests and that the loss of photocatalytic activity was 8.3%. This indicates that the stability and repeatability of the composite catalyst MS14-2-H are good, whereas the loss of photocatalytic activity may be caused by the loss of photocatalysis during the cycle test [1].

Photocatalytic Mechanism
The photocatalytic mechanism of the composite catalyst is shown in Figure 12. MoS 2 is a p-type semiconductor material with a narrow band structure (e.g., =1.85 eV), whereas SnS 2 is an n-type semiconductor material with a forbidden band width of 2.08 eV [48]. Because the two semiconductors have opposite conductivity types, the electrons and holes of these two semiconductor materials are transferred when they are in close contact to form a heterojunction until the Fermi energy levels of the two semiconductor materials are equal, at which point the p-n heterojunction is in thermal equilibrium and a stable built-in electric field is formed. of these two semiconductor materials are transferred when they are in close contact to form a heterojunction until the Fermi energy levels of the two semiconductor materials are equal, at which point the p-n heterojunction is in thermal equilibrium and a stable built-in electric field is formed.
In the mechanism diagram of the composite catalyst, the CB and VB of MoS2 are higher than that of SnS2, the energy band structures of both are staggered and the heterogeneous structure of the composite catalyst is of type II. When irradiated by visible light, a large number of photogenerated electrons accumulate in the conduction band and a large number of photogenerated holes accumulate in the valence band of both semiconductor materials. Under the effect of potential difference, electrons in the conduction band of MoS2 are transferred to the conduction band of SnS2, whereas holes in the valence band of SnS2 are transferred to the valence band of MoS2. In this way, the electrons and holes can be separated to the maximum extent. The photocatalytic degradation of MB by composite catalysts under visible light is mainly based on the chemical reaction of the photogenerated electron reduction transferred to the surface of the photocatalyst with dissolved oxygen, which produces strongly oxidizing superoxide radicals (·O 2− ), and the chemical reaction of the strongly oxidizing holes transferred to the surface of the photocatalyst with hydroxyl radicals (OH − ) in water and aqueous solutions, which produces hydroxyl radicals (·OH) [1]. The photocatalytic reaction process is as follows:

Conclusions
In summary, a novel MoS2/SnS2 heterostructure was successfully prepared by growing SnS2 nanosheets on MoS2 nanospheres by a facile multi-step hydrothermal method. In the mechanism diagram of the composite catalyst, the CB and VB of MoS 2 are higher than that of SnS 2 , the energy band structures of both are staggered and the heterogeneous structure of the composite catalyst is of type II. When irradiated by visible light, a large number of photogenerated electrons accumulate in the conduction band and a large number of photogenerated holes accumulate in the valence band of both semiconductor materials. Under the effect of potential difference, electrons in the conduction band of MoS 2 are transferred to the conduction band of SnS 2 , whereas holes in the valence band of SnS 2 are transferred to the valence band of MoS 2 . In this way, the electrons and holes can be separated to the maximum extent.
The photocatalytic degradation of MB by composite catalysts under visible light is mainly based on the chemical reaction of the photogenerated electron reduction transferred to the surface of the photocatalyst with dissolved oxygen, which produces strongly oxidizing superoxide radicals (·O 2− ), and the chemical reaction of the strongly oxidizing holes transferred to the surface of the photocatalyst with hydroxyl radicals (OH − ) in water and aqueous solutions, which produces hydroxyl radicals (·OH) [1]. The photocatalytic reaction process is as follows: MoS 2 /SnS 2 + hν → e − + h +

Conclusions
In summary, a novel MoS 2 /SnS 2 heterostructure was successfully prepared by growing SnS 2 nanosheets on MoS 2 nanospheres by a facile multi-step hydrothermal method. Based on the measurements of the XRD, SEM, HRTEM and XPS analyses, the present composite sample was found to have high crystalline quality and excellent heterojunction formation. By constructing heterojunctions between the two sulfides, an improved photocatalytic performance was achieved, which greatly solved the problems of low visible light utilization and photogenerated charge recombination. Compared with pure MoS 2 or SnS 2 , this easily accessible and simple composition photocatalyst shows higher photocatalytic activity and good photostability; these effects are attributed to the constructed heterostructure, better light trapping and rapid separation and migration of light-induced electron and hole pairs with the assistance of the MoS 2 metal phase. The optimal MoS 2 /SnS 2 photocatalyst (i.e., the one that achieved the best photocatalytic performance) had a degradation efficiency of 83.0% for MB solution, which was 8.3 times higher than the degradation with pure MoS 2 and 16.6 times higher than the degradation with pure SnS 2 . The experimental results indicate that this construction of heterojunctions between semiconductors can effectively improve the photocatalytic ability of MoS 2 /SnS 2 catalysts in terms of MB degradation.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.