Preparation and Photocatalytic Performance of MoS2/MoO2 Composite Catalyst

Solar energy is an inexhaustible clean energy providing a key solution to the dual challenges of energy and environmental crises. Graphite-like layered molybdenum disulfide (MoS2) is a promising photocatalytic material with three different crystal structures, 1T, 2H and 3R, each with distinct photoelectric properties. In this paper, 1T-MoS2 and 2H-MoS2, which are widely used in photocatalytic hydrogen evolution, were combined with MoO2 to form composite catalysts using a bottom-up one-step hydrothermal method. The microstructure and morphology of the composite catalysts were studied by XRD, SEM, BET, XPS and EIS. The prepared catalysts were used in the photocatalytic hydrogen evolution of formic acid. The results show that MoS2/MoO2 composite catalysts have an excellent catalytic effect on hydrogen evolution from formic acid. By analyzing the photocatalytic hydrogen production performance of composite catalysts, it suggests that the properties of MoS2 composite catalysts with different polymorphs are distinct, and different content of MoO2 also bring differences. Among the composite catalysts, 2H-MoS2/MoO2 composite catalysts with 48% MoO2 content show the best performance. The hydrogen yield is 960 µmol/h, which is 1.2 times pure 2H-MoS2 and two times pure MoO2. The hydrogen selectivity reaches 75%, which is 22% times higher than that of pure 2H-MoS2 and 30% higher than that of MoO2. The excellent performance of the 2H-MoS2/MoO2 composite catalyst is mainly due to the formation of the heterogeneous structure between MoS2 and MoO2, which improves the migration of photogenerated carriers and reduces the possibilities of recombination through the internal electric field. MoS2/MoO2 composite catalyst provides a cheap and efficient solution for photocatalytic hydrogen production from formic acid.


Introduction
With the progress of science and technology, while productivity has developed rapidly, problems such as environmental damage and energy shortage have also deteriorated. In order to relieve the dependence on fossil energy, cheap and clean energy is desirable. Many new energy technologies have emerged, but some of them will be severely limited by factors such as geography and policy. Among them, solar photocatalysis is a new promising sustainable technology [1,2]. Solar energy is an inexhaustible source of energy in nature. However, it has fluctuations in supply time and is not convenient to use. Therefore, we can convert solar energy into other forms that are easy to use and store-hydrogen energy. Small-molecule hydrogen storage materials, such as hydrazine hydrate [3,4], sodium borohydride [5,6], formic acid and other materials, have been extensively studied as hydrogen storage materials with high hydrogen content and stable properties. Formic acid, as a by-product of the organic chemical industry, can be prepared as a product in the catalytic reaction of carbon dioxide hydrogenation [7]. Besides having the advantages common to small molecule hydrogen storage materials, formic acid also has many distinct properties.
with MoO 2 , hydrogen production and hydrogen selectivity were significantly improved. Though the hydrogen production rate and selectivity of the catalysts used in this work are much higher than those reported in the literature, the energy to irradiate the reaction was from the Unfiltered Hg lamp which contains a much higher amount of ultraviolet light. The catalysts studied in this work are applicable in high-concentration pollutant control in water source and hydrogen production under high irradiation.

Synthesis of Photocatalysts
MoS 2 particles: 1T-MoS 2 was synthesized by hydrothermal method. A total of 1.059 g of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and 2.284 g of CH 4 N 2 S(thiourea) were added in 60 mL deionized water at room temperature and was stirred until a clear solution was obtained. The solution was then transferred to a 100 mL Teflon-lined autoclave and held at 200 • C for 3 h. The resulting black precipitate was then dried at 40 • C for 8 h. For the preparation of 2H-MoS 2 , a similar procedure was applied except the hydrothermal temperature was increased to 240 • C and the duration was increased to 8 h.
MoO 2 Flakes: 0.288 g of molybdenum powders and 0.384 g of molybdenum trioxide were added into a quartz boat and then calcined in a tubular furnace with an N 2 atmosphere. The powder was heated to 700 • C at a rate of 5 • C/min and kept at 700 • C for 10 h. A black powder was obtained after cooling down to room temperature naturally.
MoS 2 /MoO 2 composite catalyst: Since hydrothermal temperature is a crucial parameter to make the desired polymorph of MoS 2 , the same strategy was applied to prepare the MoS 2 /MoO 2 composite catalyst with different polymorphs of MoS 2 . For the preparation of 1T-MoS 2 /MoO 2 , 1.0593 g of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and 0.4568 g of CH 4 N 2 S were dissolved in 40 mL deionized water at room temperature. After continuous stirring, an appropriate amount of 5% HCl was added to the solution. Then, the solution was moved into a 100 mL Teflon-lined autoclave and kept at 220 • C for 20 h. The HCl solution was used to control the amount of MoO 2 produced and thus the MoS 2 /MoO 2 ratio can be changed accordingly. Three 1T-MoS 2 /MoO 2 samples with different ratios of MoS 2 /MoO 2 were obtained using 18 mL, 20 mL and 22 mL of 5% HCl solution and the resulting products were denoted as 1T-18HCl, 1T-20HCl and 1T-22HCl, respectively. For the preparation of 2H-MoS 2 /MoO 2 with different MoS 2 /MoO 2 ratios 18 mL, 20 mL, 22 mL and 30 mL of 5% HCl solution was used, and the hydrothermal synthesis was carried out at 240 • C for 20 h. The resulting 2H-MoS 2 /MoO 2 composites were named 2H-18HCl, 2H-20HCl, 2H-22HCl and 2H-30HCl.

Characterization
Powder X-ray diffraction patterns were obtained by Rigaku SmartLab diffractometer (Tokyo, Japan) using Cu Kα (λ = 0.154178 nm) as a radiation source. Rietveld refinement of XRD data was performed using GSAS-II (version 5511). The morphology of the samples was measured by scanning electron microscope (FESEM, Ultra-55, Zeiss, Oberkochen, Germany). Nitrogen adsorption-desorption isotherms were measured at −196 • C using a specific surface area and pore analyzer V-Sorb 1800 (NETZSCH-Gerätebau GmbH, Selb, Germany). The samples were pretreated at 105 • C for 12 h prior to measurement. The electrochemical impedance was measured at the electrochemical workstation CH1660E. X-ray photoelectron spectroscopy was recorded using Kratos AXIS Supra (Shimadzu, Kyoto, Japan).

Photocatalytic Degradation and Photoelectrochemical Test
The photocatalytic performance of the composite catalyst was evaluated by catalytic hydrogen evolution from formic acid under ultraviolet light using a 500 UV Hg lamp as the light source. For each test, 50 mg of the catalyst was dispersed in 500 mL of 10% formic acid solution. Prior to the test, the suspension was stirred in the dark for 30 min and the photocatalyst reactor was vacuumed to eliminate the air in the reactor. The produced gas was extracted every 5 min using a 500 µL gas sampler and the composition was analyzed using a gas chromatograph (SP-6890,Shandong Lunan Ruihong Chemical Instrument Co., Ltd., Tengzhou City, China). The amount of gas produced by photocatalysis was evaluated using the drainage method.
Electrochemical impedance measurement was conducted to study the electrochemical characteristics. A total of 20 mg of the catalyst was dispersed into 40 µL of a mixture of 5 wt% Nafion and 0.5 mL of anhydrous ethanol under ultrasound, and then 200 µL of the suspension was spread onto the surface of the conductive glass to prepare the working electrode. A total of 0.1 M Na 2 SO 4 solution is used as the electrolyte, platinum electrode as the counter electrode and saturated glycerol electrode as the reference electrode Electrochemical impedance measurement was carried out using CH1660E electrochemical workstation. The samples were pretreated at 105 °C for 12 h prior to measurement. The electrochemical impedance was measured at the electrochemical workstation CH1660E. X-ray photoelectron spectroscopy was recorded using Kratos AXIS Supra (Shimadzu, Japan).

Photocatalytic Degradation and Photoelectrochemical Test
The photocatalytic performance of the composite catalyst was evaluated by catalytic hydrogen evolution from formic acid under ultraviolet light using a 500 UV Hg lamp as the light source. For each test, 50 mg of the catalyst was dispersed in 500 mL of 10% formic acid solution. Prior to the test, the suspension was stirred in the dark for 30 min and the photocatalyst reactor was vacuumed to eliminate the air in the reactor. The produced gas was extracted every 5 min using a 500 µL gas sampler and the composition was analyzed using a gas chromatograph (SP-6890,Shandong Lunan Ruihong Chemical Instrument Co., Ltd., Tengzhou City, China). The amount of gas produced by photocatalysis was evaluated using the drainage method.
Electrochemical impedance measurement was conducted to study the electrochemical characteristics. A total of 20 mg of the catalyst was dispersed into 40 µL of a mixture of 5 wt% Nafion and 0.5 mL of anhydrous ethanol under ultrasound, and then 200 µL of the suspension was spread onto the surface of the conductive glass to prepare the working electrode. A total of 0.1 M Na2SO4 solution is used as the electrolyte, platinum electrode as the counter electrode and saturated glycerol electrode as the reference electrode Electrochemical impedance measurement was carried out using CH1660E electrochemical workstation.  Rietveld quantitative analysis was carried out using the GSAS-II software package (version 5511) [30,31]. Figure 1b-d are the Rietveld plots of 1T-MoS2/MoO2 with different MoO2 contents. The blue line represents the raw XRD data obtained from the actual test, the red line is the data calculated by the software, the green line is the difference between the actual data and the fitted data, and the different colored tick marks represent the Bragg peak positions of different substances. The Rwp after refinement are 2.16%, 1.97%, and 1.93% for 1T-18HCl, 1T-20HCl and 1T-22HCl, respectively, indicating a good fit was obtained. The contents for each substance in 1T-MoS2/MoO2 are listed in Table 1. Rietveld quantitative analysis suggests that the content of MoO2 is positively correlated with the amount of dilute hydrochloric acid used.

Morphology Analysis
Different additions of 5% HCl solution will cause changes in the composition of the composite catalyst as described in the preceding section. Further characterization of morphology is carried out and Figure 2 shows scanning electron microscope images of three 1T-MoS2/MoO2 composite catalysts together with the images of 1T-MoS2 and MoO2 for comparison. As shown in Figure 2a, MoO2 exhibits a regular shape with a size of about 1µm, while 1T-MoS2 presents a nano-flower-like morphology with abundant petals shown in Figure 2b. Figure 2c shows the SEM image of 1T-18HCl, which is basically dominated by 1T-MoS2 nanoflowers. In Figure 2d, more MoO2 can be seen, while nano-flowerlike MoS2 grows on block MoO2. In Figure 2e, the flower-like 1T-MoS2 still grows on the block MoO2, but more block MoO2 can be seen. It can be seen from Figure 2 that HCl promotes the growth of MoO2 in the composite catalyst. The SEM study is consistent with the results of the Rietveld quantitative analysis using GSAS-II. Rietveld quantitative analysis was carried out using the GSAS-II software package (version 5511) [30,31]. Figure 1b-d are the Rietveld plots of 1T-MoS 2 /MoO 2 with different MoO 2 contents. The blue line represents the raw XRD data obtained from the actual test, the red line is the data calculated by the software, the green line is the difference between the actual data and the fitted data, and the different colored tick marks represent the Bragg peak positions of different substances. The R wp after refinement are 2.16%, 1.97%, and 1.93% for 1T-18HCl, 1T-20HCl and 1T-22HCl, respectively, indicating a good fit was obtained. The contents for each substance in 1T-MoS 2 /MoO 2 are listed in Table 1. Rietveld quantitative analysis suggests that the content of MoO 2 is positively correlated with the amount of dilute hydrochloric acid used.

Morphology Analysis
Different additions of 5% HCl solution will cause changes in the composition of the composite catalyst as described in the preceding section. Further characterization of morphology is carried out and Figure 2 shows scanning electron microscope images of three 1T-MoS 2 /MoO 2 composite catalysts together with the images of 1T-MoS 2 and MoO 2 for comparison. As shown in Figure 2a, MoO 2 exhibits a regular shape with a size of about 1µm, while 1T-MoS 2 presents a nano-flower-like morphology with abundant petals shown in Figure 2b. Figure 2c shows the SEM image of 1T-18HCl, which is basically dominated by 1T-MoS 2 nanoflowers. In Figure 2d, more MoO 2 can be seen, while nano-flower-like MoS 2 grows on block MoO 2 . In Figure 2e, the flower-like 1T-MoS 2 still grows on the block MoO 2 , but more block MoO 2 can be seen. It can be seen from Figure 2 Figure 3 shows the electrochemical impedance measurements of the 1T-18HCl, 1T-20HCl, and 1T-22HCl composite catalysts. The arc radius of the high-frequency region of the measured electrochemical impedance represents the photoexcited carrier transfer efficiency of the material. The smaller the curvature radius, the lower the electrical impedance of the catalyst and the higher the transfer efficiency of the electron-hole pair. The results show that the impedance radius of 1T-18HCl is large, indicating that the transfer efficiency of electron-hole pairs is low. The radius of 1T-20HCl is smaller than that of 1T-18HCl, while the radius of 1T-22HCl is significantly smaller than that of 1T-20HCl and 1T-18HCl. The electrochemical impedance measurement showed that the electron transfer efficiency of the composite catalyst increased with the increase in MoO2 content.  Figure 3 shows the electrochemical impedance measurements of the 1T-18HCl, 1T-20HCl, and 1T-22HCl composite catalysts. The arc radius of the high-frequency region of the measured electrochemical impedance represents the photoexcited carrier transfer efficiency of the material. The smaller the curvature radius, the lower the electrical impedance of the catalyst and the higher the transfer efficiency of the electron-hole pair. The results show that the impedance radius of 1T-18HCl is large, indicating that the transfer efficiency of electron-hole pairs is low. The radius of 1T-20HCl is smaller than that of 1T-18HCl, while the radius of 1T-22HCl is significantly smaller than that of 1T-20HCl and 1T-18HCl. The electrochemical impedance measurement showed that the electron transfer efficiency of the composite catalyst increased with the increase in MoO 2 content.

Photocatalytic Performance
The photocatalytic performance of hydrogen evolution from formic acid using 1T-MoS2/MoO2 composite catalysts is summarized in Figure 4. It can be seen from Figure 4 that in the composite catalyst, the hydrogen production and hydrogen selectivity will increase with the increase in MoO2 content. 1T-22HCl shows the best hydrogen selectivity (73%) which is increased by 7% and 27% compared with that of pure 1T-MoS2 and MoO2, respectively. However, in terms of hydrogen production per unit time, the composite catalyst is inferior to pure 1T-MoS2.

Photocatalytic Performance
The photocatalytic performance of hydrogen evolution from formic acid using 1T-MoS 2 /MoO 2 composite catalysts is summarized in Figure 4. It can be seen from Figure 4 that in the composite catalyst, the hydrogen production and hydrogen selectivity will increase with the increase in MoO 2 content. 1T-22HCl shows the best hydrogen selectivity (73%) which is increased by 7% and 27% compared with that of pure 1T-MoS 2 and MoO 2 , respectively. However, in terms of hydrogen production per unit time, the composite catalyst is inferior to pure 1T-MoS 2 .

Photocatalytic Performance
The photocatalytic performance of hydrogen evolution from formic acid using 1T-MoS2/MoO2 composite catalysts is summarized in Figure 4. It can be seen from Figure 4 that in the composite catalyst, the hydrogen production and hydrogen selectivity will increase with the increase in MoO2 content. 1T-22HCl shows the best hydrogen selectivity (73%) which is increased by 7% and 27% compared with that of pure 1T-MoS2 and MoO2, respectively. However, in terms of hydrogen production per unit time, the composite catalyst is inferior to pure 1T-MoS2.  Figure 5a shows the XRD patterns of the 2H-MoS2/MoO2 composite catalysts prepared at 240 °C for 20 h, and the addition of 5% dilute HCl was 18, 20, 24, and 30 mL. It can be seen from Figure 5a shows the XRD patterns of 2H-MoS2/MoO2 composite catalysts with different MoO2 contents. It can be seen from Figure 5a    Rietveld quantitative analysis was carried out between the differentiated 2H-30HCl and the intermediate value of 2H-20HCl. The blue line represents the raw XRD data obtained from the actual test, the red line is the data calculated by the software, the green line is the difference between the actual data and the fitted data, and the different colored tick marks represent the Bragg peak positions of different substances. The smaller Rwp after refining were 4.2% and 3.1%, respectively, indicating a good fit. Table 2 lists the content of each substance in 2H-MoS2/MoO2. Rietveld quantitative analysis showed that the content of MoO2 was positively correlated with the amount of dilute chloric acid. The Rwp of Rietveld quantitative analysis for 2H-20HCl and 2H-30HCl are 4.2% and 3.1%, respectively, indicating that the fittings are reliable. The results of the analysis illustrate that the content of MoO2 in 2H-30HCl is significantly more than that in 2H-20HCl. This agrees well with the fact that HCl can promote the formation of MoO2 as indicated by the preparation for 1T-MoS2/MoO2.

Morphology Analysis of 2H-MoS2/MoO2
The scanning electron microscopy images of 2H-20HCl and 2H-30HCl are shown in Figure 6. Due to a longer period of high temperature in hydrothermal treatment, it is difficult for MoS2 to maintain the flower-like morphology, and the agglomeration of crystal Rietveld quantitative analysis was carried out between the differentiated 2H-30HCl and the intermediate value of 2H-20HCl. The blue line represents the raw XRD data obtained from the actual test, the red line is the data calculated by the software, the green line is the difference between the actual data and the fitted data, and the different colored tick marks represent the Bragg peak positions of different substances. The smaller R wp after refining were 4.2% and 3.1%, respectively, indicating a good fit. Table 2 lists the content of each substance in 2H-MoS 2 /MoO 2 . Rietveld quantitative analysis showed that the content of MoO 2 was positively correlated with the amount of dilute chloric acid. The R wp of Rietveld quantitative analysis for 2H-20HCl and 2H-30HCl are 4.2% and 3.1%, respectively, indicating that the fittings are reliable. The results of the analysis illustrate that the content of MoO 2 in 2H-30HCl is significantly more than that in 2H-20HCl. This agrees well with the fact that HCl can promote the formation of MoO 2 as indicated by the preparation for 1T-MoS 2 /MoO 2 .

Morphology Analysis of 2H-MoS 2 /MoO 2
The scanning electron microscopy images of 2H-20HCl and 2H-30HCl are shown in Figure 6. Due to a longer period of high temperature in hydrothermal treatment, it is difficult for MoS 2 to maintain the flower-like morphology, and the agglomeration of crystal grains is intensified. Instead, MoS 2 nanospheres are observed. Figure 6b shows that MoO 2 transforms into a rod-like morphology when the temperature is 240 • C. Figure 6c shows a similar rod-like shape for MoO 2 in 2H-30HCl compared with that of 2H-20HCl, indicating excess HCl have little effect on the morphology of MoO 2 . 2H-20HCl and 2H-30HCl exhibit similar morphology for MoS 2 .
Materials 2023, 16, x FOR PEER REVIEW 9 of 14 grains is intensified. Instead, MoS2 nanospheres are observed. Figure 6b shows that MoO2 transforms into a rod-like morphology when the temperature is 240 °C. Figure 6c shows a similar rod-like shape for MoO2 in 2H-30HCl compared with that of 2H-20HCl, indicating excess HCl have little effect on the morphology of MoO2. 2H-20HCl and 2H-30HCl exhibit similar morphology for MoS2.  Figure 6d,e is the EDS elemental mapping of 2H-30HCl, it can be seen that S, Mo, and O elements are evenly distributed on the surface of the catalyst material. The elemental analysis using EDS is listed in Table 3, which is approximately consistent with the quantitative analysis using the Rietveld method.  Table 4 shows the specific surface area results obtained by the catalyst after nitrogen adsorption-desorption, and it can be seen from the data in Table 4 that the specific surface  Figure 6d,e is the EDS elemental mapping of 2H-30HCl, it can be seen that S, Mo, and O elements are evenly distributed on the surface of the catalyst material. The elemental analysis using EDS is listed in Table 3, which is approximately consistent with the quantitative analysis using the Rietveld method.  Table 4 shows the specific surface area results obtained by the catalyst after nitrogen adsorption-desorption, and it can be seen from the data in Table 4 that the specific surface area of 2H-20HCl is 10% larger than that of 2H-30HCl. According to the calculated average pore size, both 2H-20HCl and 2H-30HCl belong to mesoporous materials.

Electrochemical Impedance Measurement
The electrochemical impedance measured results of the 2H-MoS 2 /MoO 2 composite catalyst are shown in Figure 7. Compared with 2H-30HCl, the curve of 2H-20HCl is closer to a straight line, indicating that its radius of curvature is greater than that of 2H-30HCl, that is, the charge transfer resistance is greater than 2H-30HCl, and the charge transfer resistance and the photocatalytic performance of the material often show a negative correlation. It is speculated that the photocatalytic performance of 2H-20HCl is not as good as that of 2H-30HCl. In summary, the decrease in pH caused by the addition of dilute hydrochloric acid can increase the MoO 2 content in the 2H composite catalyst, and with the increase in MoO 2 content, the charge transfer resistance of the material decreases. area of 2H-20HCl is 10% larger than that of 2H-30HCl. According to the calculated average pore size, both 2H-20HCl and 2H-30HCl belong to mesoporous materials. The electrochemical impedance measured results of the 2H-MoS2/MoO2 composite catalyst are shown in Figure 7. Compared with 2H-30HCl, the curve of 2H-20HCl is closer to a straight line, indicating that its radius of curvature is greater than that of 2H-30HCl, that is, the charge transfer resistance is greater than 2H-30HCl, and the charge transfer resistance and the photocatalytic performance of the material often show a negative correlation. It is speculated that the photocatalytic performance of 2H-20HCl is not as good as that of 2H-30HCl. In summary, the decrease in pH caused by the addition of dilute hydrochloric acid can increase the MoO2 content in the 2H composite catalyst, and with the increase in MoO2 content, the charge transfer resistance of the material decreases.

X-ray Photoelectronic Spectrum Analysis of 2H-MoS2/MoO2
XPS analysis was performed for 2H-20HCl and 2H-30HCl. The spectrum for each sample shows similar characteristics and 2H-30HCl is discussed as an example. Figure 8 shows a narrow spectrum. (Please view the Figure S1 for the full spectrum) The peak fitting of Mo 3d and S 2p are shown in Figure 8a,b. As to the S 2p spec, the characteristic doublets at 162.4 eV, 163.6 eV,163.8 eV and 166.0 eV correspond to S 2− [32], while the band at 169.6 eV indicates that S is partially oxidized [33]. Figure 8a, the band corresponding to 233.3 eV is formed by Mo 6+ spin orbit, which can be attributed to the slight oxidation of the sample surface [34,35]. The band at other positions such as 229.6 eV and 233.2 eV conform to the characteristic peak regularity of Mo 4+ 3d5/2 and Mo 4+ 3d3/2, and a characteristic band belonging to Mo 6+ 3d3/2 appears near 236.5 eV. The small band at 226.7 eV is the interference peak of S 2 s, which indicates that MoS2 exists on the surface of the detected substance. Calculated by the XPS data, the molar ratio of Mo: S is 1:1.50, larger than the stoichiometric ratio of pure MoS2 (1:2), providing direct evidence for the incomplete sulfurization of MoO2. 3.2.5. X-ray Photoelectronic Spectrum Analysis of 2H-MoS 2 /MoO 2 XPS analysis was performed for 2H-20HCl and 2H-30HCl. The spectrum for each sample shows similar characteristics and 2H-30HCl is discussed as an example. Figure 8 shows a narrow spectrum. (Please view the Figure S1 for the full spectrum) The peak fitting of Mo 3d and S 2p are shown in Figure 8a,b. As to the S 2p spec, the characteristic doublets at 162.4 eV, 163.6 eV,163.8 eV and 166.0 eV correspond to S 2− [32], while the band at 169.6 eV indicates that S is partially oxidized [33]. Figure 8a, the band corresponding to 233.3 eV is formed by Mo 6+ spin orbit, which can be attributed to the slight oxidation of the sample surface [34,35]. The band at other positions such as 229.6 eV and 233.2 eV conform to the characteristic peak regularity of Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 , and a characteristic band belonging to Mo 6+ 3d 3/2 appears near 236.5 eV. The small band at 226.7 eV is the interference peak of S 2 s, which indicates that MoS 2 exists on the surface of the detected substance. Calculated by the XPS data, the molar ratio of Mo: S is 1:1.50, larger than the stoichiometric ratio of pure MoS 2 (1:2), providing direct evidence for the incomplete sulfurization of MoO 2 . Materials 2023, 16 3.2.6. Photocatalytic Performance Figure 9 is a performance of the photocatalytic hydrogen production from formic acid using a 2H-MoS2/MoO2 composite catalyst. When the ratio of MoO2 to 2H-MoS2 in the sample changes, the selectivity of hydrogen increases. For 2H-30HCl, hydrogen selectivity reaches 75%, which is 2.14 times that of catalyst-free conditions and 1.42 times that of pure 2H-MoS2. The hydrogen production per unit time of the 2H composite catalyst is also higher than that of pure 2H-MoS2. Taking 2H-30HCl as an example, its hydrogen production has reached 960µmol/h, which is not only twice that of pure MoO2, but also higher than that of pure 2H-MoS2.

Conclusions
MoS2 of different polymorphs can be synthesized under appropriate hydrothermal conditions. 1T-MoS2 can be obtained at 200 °C for 3 h while 2H-MoS2 is formed at 240 °C for 8 h. MoS2/MoO2 composite catalysts can be synthesized by a one-step hydrothermal method. The polymorph of MoS2 in the composite can be adjusted using a similar strategy for the synthesis of pure 1T-MoS2 and 2H-MoS2.
Hydrochloric acid promotes the formation of MoO2 in the process of preparing MoS2/MoO2 composite catalysts. In the process of composite catalyst generation, an acidic environment promotes the formation of MoO3 from ammonium molybdate in aqueous solution. MoO3 is then reduced by thiourea to form MoO2, and then MoO2 is further vulcanized to produce MoS2. When the amount of hydrochloric acid is low, the MoOx produced will be completely vulcanized to MoS2. With the gradual increase in the dosage of hydrochloric acid, the thiourea involved in sulfide MoOx was gradually reduced, and the 3.2.6. Photocatalytic Performance Figure 9 is a performance of the photocatalytic hydrogen production from formic acid using a 2H-MoS 2 /MoO 2 composite catalyst. When the ratio of MoO 2 to 2H-MoS 2 in the sample changes, the selectivity of hydrogen increases. For 2H-30HCl, hydrogen selectivity reaches 75%, which is 2.14 times that of catalyst-free conditions and 1.42 times that of pure 2H-MoS 2 . The hydrogen production per unit time of the 2H composite catalyst is also higher than that of pure 2H-MoS 2 . Taking 2H-30HCl as an example, its hydrogen production has reached 960µmol/h, which is not only twice that of pure MoO 2 , but also higher than that of pure 2H-MoS 2 . 3.2.6. Photocatalytic Performance Figure 9 is a performance of the photocatalytic hydrogen production from formic acid using a 2H-MoS2/MoO2 composite catalyst. When the ratio of MoO2 to 2H-MoS2 in the sample changes, the selectivity of hydrogen increases. For 2H-30HCl, hydrogen selectivity reaches 75%, which is 2.14 times that of catalyst-free conditions and 1.42 times that of pure 2H-MoS2. The hydrogen production per unit time of the 2H composite catalyst is also higher than that of pure 2H-MoS2. Taking 2H-30HCl as an example, its hydrogen production has reached 960µmol/h, which is not only twice that of pure MoO2, but also higher than that of pure 2H-MoS2.

Conclusions
MoS2 of different polymorphs can be synthesized under appropriate hydrothermal conditions. 1T-MoS2 can be obtained at 200 °C for 3 h while 2H-MoS2 is formed at 240 °C for 8 h. MoS2/MoO2 composite catalysts can be synthesized by a one-step hydrothermal method. The polymorph of MoS2 in the composite can be adjusted using a similar strategy for the synthesis of pure 1T-MoS2 and 2H-MoS2.
Hydrochloric acid promotes the formation of MoO2 in the process of preparing MoS2/MoO2 composite catalysts. In the process of composite catalyst generation, an acidic environment promotes the formation of MoO3 from ammonium molybdate in aqueous solution. MoO3 is then reduced by thiourea to form MoO2, and then MoO2 is further vulcanized to produce MoS2. When the amount of hydrochloric acid is low, the MoOx produced will be completely vulcanized to MoS2. With the gradual increase in the dosage of hydrochloric acid, the thiourea involved in sulfide MoOx was gradually reduced, and the

Conclusions
MoS 2 of different polymorphs can be synthesized under appropriate hydrothermal conditions. 1T-MoS 2 can be obtained at 200 • C for 3 h while 2H-MoS 2 is formed at 240 • C for 8 h. MoS 2 /MoO 2 composite catalysts can be synthesized by a one-step hydrothermal method. The polymorph of MoS 2 in the composite can be adjusted using a similar strategy for the synthesis of pure 1T-MoS 2 and 2H-MoS 2 .
Hydrochloric acid promotes the formation of MoO 2 in the process of preparing MoS 2 /MoO 2 composite catalysts. In the process of composite catalyst generation, an acidic environment promotes the formation of MoO 3 from ammonium molybdate in aqueous solution. MoO 3 is then reduced by thiourea to form MoO 2 , and then MoO 2 is further vulcanized to produce MoS 2 . When the amount of hydrochloric acid is low, the MoO x produced will be completely vulcanized to MoS 2 . With the gradual increase in the dosage of hydrochloric acid, the thiourea involved in sulfide MoO x was gradually reduced, and the content of MoO 2 phase in the product was gradually increased. Therefore, the ratio of MoS 2 to MoO 2 in the composite catalysts can be modified using different amounts of hydrochloric acid.
Comparing 1T-MoS 2 /MoO 2 and 2H-MoS 2 /MoO 2 , it can be found that each has its own advantages. Based on the relatively high hydrogen production of 1T-MoS 2 , the overall hydrogen production per unit time of 1T-MoS 2 /MoO 2 is higher than that of 2H-MoS 2 /MoO 2 , and hydrogen production on 1T-22HCl reached 1150 µmol/h. The advantage of 2H-MoS 2 /MoO 2 is presented in its hydrogen selectivity, with a hydrogen selectivity of 75% for 2H-30HCl.
Due to the synergistic effect, the hydrogen selectivity of composite catalyst 1T-MoS 2 /MoO 2 is improved compared to pure 1T-MoS 2 and MoO 2 . The hydrogen selectivity also increases with the increase in MoO 2 content, though the hydrogen yield on 1T-MoS 2 /MoO 2 is lower than that of pure 1T-MoS 2 , which may be caused by the reduction in active sites from 1T-MoS 2 with the addition of MoO 2 [27]. However, with the increase in the amount of composite MoO 2 , the synergistic effect increases, and the hydrogen yield of the composite catalyst increases gradually. P-type semiconductor 2H-MoS 2 has a similar crystal structure matching lattice constant and crossed band gaps to N-type semiconductor MoO 2 . A heterostructure of 2H-MoS 2 and MoO 2 is formed [36,37]. Due to the formation of heterojunctions, oxidation and reduction occur in different components of the composite catalyst, which greatly reduced the recombination rate of charge carriers. Moreover, due to the presence of a junction electric field, the migration efficiency of charge carriers has also been significantly improved. Therefore, the photocatalytic performance of 2H-MoS 2 /MoO 2 is better than that of pure 2H-MoS 2 and MoO 2 .

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