Microwave-Assisted Synthesis of MoS2/BiVO4 Heterojunction for Photocatalytic Degradation of Tetracycline Hydrochloride

Compared with traditional hydrothermal synthesis, microwave-assisted synthesis has the advantages of being faster and more energy efficient. In this work, the MoS2/BiVO4 heterojunction photocatalyst was synthesized by the microwave-assisted hydrothermal method within 30 min. The morphology, structure and chemical composition were characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and high-resolution transmission electron microscopy (HRTEM). The results of characterizations demonstrated that the synthesized MoS2/BiVO4 heterojunction was a spherical structure with dimensions in the nanorange. In addition, the photocatalytic activity of the samples was investigated by degrading tetracycline hydrochloride (TC) under visible light irradiation. Results indicated that the MoS2/BiVO4 heterojunction significantly improved the photocatalytic performance compared with BiVO4 and MoS2, in which the degradation rate of TC (5 mg L−1) by compound where the mass ratio of MoS2/BiVO4 was 5 wt% (MB5) was 93.7% in 90 min, which was 2.36 times of BiVO4. The active species capture experiments indicated that •OH, •O2− and h+ active species play a major role in the degradation of TC. The degradation mechanism and pathway of the photocatalysts were proposed through the analysis of the band structure and element valence state. Therefore, microwave technology provided a quick and efficient way to prepare MoS2/BiVO4 heterojunction photocatalytic efficiently.


Introduction
The 2022 Global Environmental Performance Index (EPI) issued in June 2022 pointed out that two billion people in the world could not get clean drinking water due to the shortage of water resources and water pollution [1]. Wastewater treatment is a major global problem. Antibiotics from livestock raising [2], sewage treatment plants [3] and other channels are rampant in the water environment [4]. Tetracycline hydrochloride (TC) is a major broad-spectrum antibiotic, cannot be removed through spontaneous degradation because of its high structural stability, and its existence can bring potential risks to plants and human health [5]. Many methods have been used to remove TC from water, including photocatalysis [6], adsorption [7], biodegradation [8], and efficient activation of peroxydisulfate (PS) [9]. In comparison, photocatalytic technology is one of the practical techniques for the  4 ), and p-benzoquinone (PBQ, C 6 H 4 O 2 ) were obtained from Macklin Biochemical Co., Ltd., (Shanghai, China). The reagents used were all analytically pure and all water used for experiments was ultrapure water.

Synthesis of MoS 2 Nanosphere
The hydrothermal treatment was used to manufacture the MoS 2 nanoflower. Na 2 MoO 4 (0.6096 g) and CH 4 N 2 S (0.9104 g) were dissolved in 60 mL ultrapure water. Subsequently, the mixing solution was transferred into a 100 mL Teflon-lined autoclave and heated under 200 • C for 24 h. The product was cooled to room temperature and washed with ethanol and ultrapure water. The obtained black compound was dried at 60 • C for 12 h.

Preparation of MoS 2 /BiVO 4 Heterojunction
Bi(NO 3 ) 3 ·5H 2 O (3.1920 g) and NH 4 VO 3 (0.9453 g) were added to 80 mL (CH 2 OH) 2 , then stirred until fully dissolution. Then, a certain mass of MoS 2 (0.0099, 0.0168, 0.0241 g) was added to the aforementioned solution and ultrasound dispersion. The above mixing solution was transferred into a reaction bottle and reacted at 300 W and 120 • C reacted for 30 min in a microwave reactor (CEM-Discover SP, CEM, Matthews, NC, USA). The product was cooled to room temperature and washed with ethanol and ultrapure water several times. Obtained yellow compound was dried at 60 • C for 12 h. Composites with different loading amounts of MoS 2 were prepared by adding different quality of MoS 2 , which were denoted as MB3, MB5 and MB7, respectively ("3" meant the mass ratio of MoS 2 /BiVO 4 was 3 wt%).

Characterization and Analysis Methods
The surface morphology of the prepared materials was characterized by a scanning electron microscope (SEM, SUPRA 55 Sapphire, Zeiss, Oberkochen, Germany). X-ray diffractometer (XRD, D8 advance, Bruker, Karlsruhe, Germany) was employed to test physical phase and crystalline size of the materials at the 2θ range of 10-80 • at a step size of 0.02 • and a scan rate of 6 • /min (Cu Ka radiation, λ = 0.15814 nm). Raman spectroscopy (Raman, inVia Reflex, Renishaw, Gloucestershire, UK) was used to investigate molecular structure of the samples with an emission wavelength of 532 nm, 10% laser power, and the scanning range was 200-1000 cm −1 . Using FT-IR spectrometer (IRAffinity-1s, Shimadzu, kyoto, Japan) with the scan range from 2000 to 400 cm −1 and the resolution of 4 cm −1 to test the chemical bond type and structure of materials. A UV-vis spectroscopy (UV-2700, Shimadzu, Japan) with a range of 200-800 nm was used to examine UV-visible absorption spectra of catalysts. The specific surface area of the samples was studied by Brunner-Emmet-Teller (BET) measurements (ASAP2460, Micromeritics, Norcross, GA, USA). The elemental composition and changes of the sample surface were investigated by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD Kratos AXIS SUPRA, Shimadzu, Japan).

Photocatalytic Activity Tests
The visible-light source was generated by using a Xenon lamp (300 W, China Education AU-Light, Beijing, China) with a 420 nm cut-off filter to remove light of λ < 420 nm. The photocatalytic degradation of TC was used to test photocatalytic performance of the prepared samples. A double-layer beaker with a water circulation cooling system was used as the container. Specifically, 50 mg of photocatalyst was added to 100 mL of TC solution (5 mg L −1 ) was stirred in dark for 30 min to achieve the adsorption-desorption equilibrium. And then turned on the light illuminated 90 min for photocatalytic reaction. The absorption at 356 nm was used to analyze the TC concentration [35]. The active species capture experiments were performed to explore the major active species in the MB5 photocatalyst and the mechanism of photocatalytic degradation of TC. The active species capture agents were TBA (1.8 mL), AO (0.3 mmol), and PBQ (0.3 mmol), which were used to capture •OH, h + and •O 2 − , respectively.

PEC Spectra Measurements
Electrochemical tests were performed in a standard three-electrode system using a CHI 760E electrochemical workstation (CH Instruments Ins., Shanghai, China). A Pt electrode was used as a counter electrode, and an Ag/AgCl electrode was used as a reference electrode. A total of 50 mg of photocatalyst was dispersed in 2 mL of ultrapure water by ultrasound, then dropped 100 µL of the mixed solution on to FTO glass, where the resultant electrode used as a working electrode [21]. In this study, Na 2 SO 4 (0.1 mol L −1 ) solution was employed as the electrolyte. A Xenon lamp light source (300 W) with a 420 nm cut-off filter to remove light of λ < 420 nm was used as the visible-light source. Nyquist plots tested by scanning the frequency range were 100 mHz-10 kHz with a voltage amplitude of 5 mV. The applied voltage is 1 V vs. Ag/AgCl during current versus time curve (i-t) for testing photocurrent density.

Characterization
The crystal phase of the samples was characterized by XRD. As shown in Figure 1, the diffraction peaks of BiVO 4 , and all MoS 2 /BiVO 4 heterojunctions had a good corresponding relationship with the monoclinic scheelite BiVO 4 (JCPDS No. 14-0688) [36]. The diffraction peaks at 2θ = 14.0 • , 33.0 • and 58.3 • of MoS 2 are derived from the contributions of the (002), (100) and (110) crystal planes of hexagonal MoS 2 (JCPDS No.37-1492) [37]. Remarkably, when the loading amount of MoS 2 was low, the diffraction peaks of MoS 2 were absent in the XRD spectra of MB3 and MB5. As the amount of MoS 2 continued to increase, the diffraction peak located at 33.0 • corresponds to the (100) crystal plane of MoS 2 which appeared in MB7 composites. Moreover, it proved the successful preparation of MoS 2 /BiVO 4 [38]. The enhancement of the diffraction peak intensity of the (121) crystal plane in MB5 and MB7 was significantly greater than the other diffraction peaks, and this probably attributed to the addition of MoS 2 affected the crystal structure and the growth direction of BiVO 4 [39,40] The grain size can be calculated by the Debye-Scherrer equation [41] (Equation (1)), where D is the average grain size, K = 0.89, λ = 1.5406 nm, B is the full width at half maximum of the diffraction peaks, θ is the Bragg's angle, which in this study is the diffraction angle corresponding to the (121) crystal plane. The average crystallite sizes of BiVO 4 , MB3, MB5 and MB7 were calculated as 6.9, 2.9, 5.5, 17.2 nm, respectively. The addition of MoS 2 influenced the particle size of MoS 2 /BiVO 4 composites, where the smallest size was MB3. Raman spectroscopy analysis ( Figure 2a) and FT-IR spectra (Figure 2b) could be helpful to further validate the structure of samples. As shown in Figure 2a, BiVO 4 and MoS 2 /BiVO 4 composites have a strong peak at 820 cm −1 which is caused by V-O bond tensile vibration, and the peak at 340 cm −1 is the asymmetric and symmetric bending vibration of VO 4 3− [42]. Further, the characteristic peaks of MoS 2 at 378 and 406 cm −1 correspond to the E 1 2g and A 1g vibrational modes [43]. In Raman spectra, red shifts in the MoS 2 /BiVO 4 spectral band can be clearly seen, the peak at 815 cm −1 has an increase of the intensity, and along with the stretching vibration of the V-O bond and the bending vibration of VO 4 3− become irregular, which can be attributed to the interaction between MoS 2 and BiVO 4 . According to the available literature [44,45], such a red shift and increase in peak intensity of MB5 were due to successful preparation of heterojunction. FT-IR spectra of MoS 2 , BiVO 4  symmetric and tensile vibration peak [46]. For MoS 2 , the peaks at 420, and 1401 cm −1 were allocated to S-S and S-Mo-S bond vibration and tensile vibration [47]. For MoS 2 /BiVO 4 composites, all single-phase vibrations corresponded well. Raman spectroscopy analysis ( Figure 2a) and FT-IR spectra ( Figure 2b) could be helpful to further validate the structure of samples. As shown in Figure 2a, BiVO4 and MoS2/BiVO4 composites have a strong peak at 820 cm −1 which is caused by V-O bond tensile vibration, and the peak at 340 cm −1 is the asymmetric and symmetric bending vibration of VO4 3− [42]. Further, the characteristic peaks of MoS2 at 378 and 406 cm −1 correspond to the E 1 2g and A1g vibrational modes [43]. In Raman spectra, red shifts in the MoS2/BiVO4 spectral band can be clearly seen, the peak at 815 cm −1 has an increase of the intensity, and along with the stretching vibration of the V-O bond and the bending vibration of VO4 3− become irregular, which can be attributed to the interaction between MoS2 and BiVO4. According to the available literature [44,45], such a red shift and increase in peak intensity of MB5 were due to successful preparation of heterojunction. FT-IR spectra of MoS2, BiVO4 and MB5 were shown in Figure 2b. For BiVO4, the peak at 416 cm −1 corresponds to the chemical stretching of Bi-O. The absorption band observed at 745 cm −1 was attributed to VO4 3− symmetric and tensile vibration peak [46]. For MoS2, the peaks at 420, and 1401 cm −1 were allocated to S-S and S-Mo-S bond vibration and tensile vibration [47]. For MoS2/BiVO4 composites, all single-phase vibrations corresponded well. SEM was employed to observe the micro morphology of the photocatalyst (Figure 3). Figure 3a indicated that MoS2 is a nanosphere structure with an average diameter of about  Raman spectroscopy analysis ( Figure 2a) and FT-IR spectra (Figure 2b) could be helpful to further validate the structure of samples. As shown in Figure 2a, BiVO4 and MoS2/BiVO4 composites have a strong peak at 820 cm −1 which is caused by V-O bond tensile vibration, and the peak at 340 cm −1 is the asymmetric and symmetric bending vibration of VO4 3− [42]. Further, the characteristic peaks of MoS2 at 378 and 406 cm −1 correspond to the E 1 2g and A1g vibrational modes [43]. In Raman spectra, red shifts in the MoS2/BiVO4 spectral band can be clearly seen, the peak at 815 cm −1 has an increase of the intensity, and along with the stretching vibration of the V-O bond and the bending vibration of VO4 3− become irregular, which can be attributed to the interaction between MoS2 and BiVO4. According to the available literature [44,45], such a red shift and increase in peak intensity of MB5 were due to successful preparation of heterojunction. FT-IR spectra of MoS2, BiVO4 and MB5 were shown in Figure 2b. For BiVO4, the peak at 416 cm −1 corresponds to the chemical stretching of Bi-O. The absorption band observed at 745 cm −1 was attributed to VO4 3− symmetric and tensile vibration peak [46]. For MoS2, the peaks at 420, and 1401 cm −1 were allocated to S-S and S-Mo-S bond vibration and tensile vibration [47]. For MoS2/BiVO4 composites, all single-phase vibrations corresponded well. SEM was employed to observe the micro morphology of the photocatalyst ( Figure 3). Figure 3a indicated that MoS2 is a nanosphere structure with an average diameter of about SEM was employed to observe the micro morphology of the photocatalyst (Figure 3). Figure 3a indicated that MoS 2 is a nanosphere structure with an average diameter of about 4 µm. Figure 3b showed BiVO 4 as a nanosphere structure with the size of about 0.5 um. The plots of MoS 2 /BiVO 4 with different ratios were placed in Figure 3c-e, when MoS 2 nanospheres were added to the microwave reactor, they were separated into nanomicrospheres of smaller size by microwave radiation. It can be observed that the surface morphology of the MoS 2 /BiVO 4 heterojunction with a different mass ratio of MoS 2 and BiVO 4 is different. First, MB3 was similar to BiVO 4 nanospheres, and the surface of MB3 that MoS 2 was completely covered by BiVO 4 (Figure 3c). In the second place, some irregular depressions could be observed on the surface of MB5 (Figure 3d). Finally, with MB7 it could be seen that there were many MoS 2 exposed on the surface, which was due to the fact that as the added MoS 2 increases MoS 2, it was not completely covered by BiVO 4 (Figure 3e). The results showed that the growth process and the structure are similar to MoS 2 /BiVO 4 heterojunction prepared by Peng et al. [25]. During the reaction, with MoS 2 as a substrate, Nanomaterials 2023, 13, 1522 6 of 15 BiVO 4 would nucleate and continue to grow on the surface of MoS 2 nano-microspheres, eventually covered the entire surface of MoS 2 , and grew into a nanosphere structure.
BiVO4 is different. First, MB3 was similar to BiVO4 nanospheres, and the surface of MB3 that MoS2 was completely covered by BiVO4 (Figure 3c). In the second place, some irregular depressions could be observed on the surface of MB5 (Figure 3d). Finally, with MB7 it could be seen that there were many MoS2 exposed on the surface, which was due to the fact that as the added MoS2 increases MoS2, it was not completely covered by BiVO4 (Figure 3e). The results showed that the growth process and the structure are similar to MoS2/BiVO4 heterojunction prepared by Peng et al. [25]. During the reaction, with MoS2 as a substrate, BiVO4 would nucleate and continue to grow on the surface of MoS2 nanomicrospheres, eventually covered the entire surface of MoS2, and grew into a nanosphere structure.   Figure 4b, which were correspond to the (121) plane of BiVO 4 [48] and the (100) plane of MoS 2 [49], respectively. In addition, the chemical composition and elemental distribution of MB5 were analyzed by SEM-EDS. As shown in Figure 4c-g, the MB5 composites photocatalyst was composed of five elements (Bi, V, O, Mo, and S). The above images could be clearly observed that BiVO 4 and MoS 2 formed an interface in close contact further provided evidence for the successful preparation of MoS 2 /BiVO 4 heterostructures.
With the use of XPS, the surface molecular structure and chemical states of MoS 2 , BiVO 4 , and MB5 were identified. The presence of Bi, V, O, Mo, and S element in the MB5 were revealed by the full scan XPS spectrum (Figure 5a). Figure 5b shows the Mo 3d high resolution XPS spectra. Mo 3d 5/2 and Mo 3d 3/2 of MB5 were matched with two peaks at 232.1 eV and 235.2 eV. The energy gap of these two peaks is approximately 3.1 eV, indicating that molybdenum ions are Mo 4+ in a lower oxidation state [25]. There is no characteristic peak of Mo3d in BiVO 4 , while it is present in MB5, which proves the presence of MoS 2 in the MoS 2 /BiVO 4 heterojunction. In Figure 5c, two significant peaks that occurred around 164.3 eV and 159.0 eV were identified to the Bi 4f 5/2 and Bi 4f 7/2 ; the energy gap of 5.3 eV confirmed that the bismuth species occurred as Bi 3+ in MB5. Vanadium ions in MB5 existed in the shape of V 5+ . Two significant peaks near 524.2 eV and 516.6 eV were attributed to the V 2p split signals (V 2p 1/2 and V 2p 3/2 ) clearly seen in Figure 5e [24]. Likewise, the main peak of O 1s in the high resolution XPS spectra could be divided into two bands, implying two different types of oxygen existed on the surface of MB5 [23]. The XPS results indicate a strong interaction between MoS 2 and BiVO 4 and the successful preparation of a heterostructure. Nanomaterials 2023, 13, x FOR PEER REVIEW 7 of 16 and the (100) plane of MoS2 [49], respectively. In addition, the chemical composition and elemental distribution of MB5 were analyzed by SEM-EDS. As shown in Figure 4c-g, the MB5 composites photocatalyst was composed of five elements (Bi, V, O, Mo, and S). The above images could be clearly observed that BiVO4 and MoS2 formed an interface in close contact further provided evidence for the successful preparation of MoS2/BiVO4 heterostructures. With the use of XPS, the surface molecular structure and chemical states of MoS2, BiVO4, and MB5 were identified. The presence of Bi, V, O, Mo, and S element in the MB5 were revealed by the full scan XPS spectrum (Figure 5a). Figure 5b shows the Mo 3d high resolution XPS spectra. Mo 3d5/2 and Mo 3d3/2 of MB5 were matched with two peaks at 232.1 eV and 235.2 eV. The energy gap of these two peaks is approximately 3.1 eV, indicating that molybdenum ions are Mo 4+ in a lower oxidation state [25]. There is no characteristic peak of Mo3d in BiVO4, while it is present in MB5, which proves the presence of MoS2 in the MoS2/BiVO4 heterojunction. In Figure 5c, two significant peaks that occurred around 164.3 eV and 159.0 eV were identified to the Bi 4f5/2 and Bi 4f7/2; the energy gap of 5.3 eV confirmed that the bismuth species occurred as Bi 3+ in MB5. Vanadium ions in MB5 existed in the shape of V 5+ . Two significant peaks near 524.2 eV and 516.6 eV were attributed to the V 2p split signals (V 2p1/2 and V 2p3/2) clearly seen in Figure 5e [24]. Likewise, the main peak of O 1s in the high resolution XPS spectra could be divided into two bands, implying two different types of oxygen existed on the surface of MB5 [23]. The XPS results indicate a strong interaction between MoS2 and BiVO4 and the successful preparation of a heterostructure. Specific surface area is an important factor affecting the catalytic efficiency of photocatalysts. Figure 6 is the N 2 adsorption-desorption curve and the pore size distribution map of MoS 2 , BiVO 4 and MB5 measured by the Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH), respectively. As shown in Figure 6a, all photocatalysts showed type-IV isotherms with H 3 hysteresis loops [50]. The specific surface areas of 9.79 m 2 g −1 , 5.07 m 2 g −1 , and 11.02 m 2 g −1 were for MoS 2 , BiVO 4 , and MB5, respectively. The findings indicated that MB5 had the largest specific surface area. This was because in the synthesis process of MB5, where MoS 2 were broken down into smaller-sized MoS 2 by microwave energy. Through the microwave reaction, the increased surface area of the MoS 2 led to an overall increase in the specific surface area of MB5 [51]. The large specific surface area provides more active sites and higher adsorption capacity. In addition, Figure 6b showed the presence of a mesoporous structure, which could serve as a fast transfer path for photogenerated electrons [52]. Specific surface area is an important factor affecting the catalytic efficiency of photocatalysts. Figure 6 is the N2 adsorption-desorption curve and the pore size distribution map of MoS2, BiVO4 and MB5 measured by the Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH), respectively. As shown in Figure 6a, all photocatalysts showed type-IV isotherms with H3 hysteresis loops [50]. The specific surface areas of 9.79 m 2 g −1 , 5.07 m 2 g −1 , and 11.02 m 2 g −1 were for MoS2, BiVO4, and MB5, respectively. The findings indicated that MB5 had the largest specific surface area. This was because in the synthesis process of MB5, where MoS2 were broken down into smallersized MoS2 by microwave energy. Through the microwave reaction, the increased surface area of the MoS2 led to an overall increase in the specific surface area of MB5 [51]. The large specific surface area provides more active sites and higher adsorption capacity. In addition, Figure 6b showed the presence of a mesoporous structure, which could serve as a fast transfer path for photogenerated electrons [52].

Light Absorption and Charge Transfer Performance
The optical features of the photocatalysts could be assessed by using UV-visible diffuse reflectance spectroscopy (UV-vis DRS). In Figure 7a, BiVO4 had an absorption band at 500 nm. MoS2 showed a response to ultraviolet and visible light region, indicating its wide absorption rage [53]. MB5 had an increased absorption capacity in both the ultraviolet and visible light region compared with BiVO4, contributing to the higher

Light Absorption and Charge Transfer Performance
The optical features of the photocatalysts could be assessed by using UV-visible diffuse reflectance spectroscopy (UV-vis DRS). In Figure 7a, BiVO 4 had an absorption band at 500 nm. MoS 2 showed a response to ultraviolet and visible light region, indicating its wide absorption rage [53]. MB5 had an increased absorption capacity in both the ultraviolet and visible light region compared with BiVO 4 , contributing to the higher utilization of solar light. Tauc plot (Figure 7b) showed the band gaps of 1.29 eV, 2.31 eV and 2.25 eV for MoS 2 , BiVO 4 and MB5, respectively. Compared to BiVO 4 , MB5 had a reduced band gap. According to the above analysis, compared with MoS 2 and BiVO 4 , electrons in the MB5 were more easily excited from the valence band to the conduction band due to the stronger optical absorption capability and the reduced band gap width. In order to reveal the photogenerated charge divorce process of the heterojunction in the photocatalysis, the conduction band (CB) and valence band (VB) of BiVO 4 and MoS 2 were calculated by the following formula [54] (Equations (2) and (3)): where, E e is the energy of free electrons at hydrogen standard potential, 4.5 eV. E g is band gap. X is the absolute electronegativity of the semiconductor. The X values of BiVO 4 and MoS 2 are 6.16 eV and 5.32 eV, which are calculated by the following formula (Equation (4)): To further investigate the charge separation of MoS2/BiVO4 heterojunction, the photocurrent response and electrochemical impedance spectroscopy (EIS) of MoS2, BiVO4 and MB5 were measured (Figure 8). Figure 8a showed that, MB5 displayed the highest photocurrent response (1.3 µA cm −2 ) under visible light (λ > 420 nm) illumination, which was 3.25 times that of MoS2 and 1.85 times that of BiVO4. A higher photocurrent response of the MB5 suggests the increased charges separation efficiency, which increased the lifetime of the photogenerated charge and thus improved the photocatalytic rate [56,57]. In the EIS curve, when compared with MoS2 and BiVO4, it was evident that MB5 had the smallest arc radius (Figure 8b), which was consistent with the results of the photocurrent density test. The minimum arc radius implies that MB5 has the lowest resistance and highest charge transfer efficiency. The results suggested that the separation efficiency and transport performance of photogenerated carriers in heterojunction were hugely To further investigate the charge separation of MoS 2 /BiVO 4 heterojunction, the photocurrent response and electrochemical impedance spectroscopy (EIS) of MoS 2 , BiVO 4 and MB5 were measured (Figure 8). Figure 8a showed that, MB5 displayed the highest photocurrent response (1.3 µA cm −2 ) under visible light (λ > 420 nm) illumination, which was 3.25 times that of MoS 2 and 1.85 times that of BiVO 4 . A higher photocurrent response of the MB5 suggests the increased charges separation efficiency, which increased the lifetime of the photogenerated charge and thus improved the photocatalytic rate [56,57]. In the EIS curve, when compared with MoS 2 and BiVO 4 , it was evident that MB5 had the smallest arc radius (Figure 8b), which was consistent with the results of the photocurrent density test. The minimum arc radius implies that MB5 has the lowest resistance and highest charge transfer efficiency. The results suggested that the separation efficiency and transport performance of photogenerated carriers in heterojunction were hugely enhanced.
To further investigate the charge separation of MoS2/BiVO4 heterojunction, the photocurrent response and electrochemical impedance spectroscopy (EIS) of MoS2, BiVO4 and MB5 were measured (Figure 8). Figure 8a showed that, MB5 displayed the highest photocurrent response (1.3 µA cm −2 ) under visible light (λ > 420 nm) illumination, which was 3.25 times that of MoS2 and 1.85 times that of BiVO4. A higher photocurrent response of the MB5 suggests the increased charges separation efficiency, which increased the lifetime of the photogenerated charge and thus improved the photocatalytic rate [56,57]. In the EIS curve, when compared with MoS2 and BiVO4, it was evident that MB5 had the smallest arc radius (Figure 8b), which was consistent with the results of the photocurrent density test. The minimum arc radius implies that MB5 has the lowest resistance and highest charge transfer efficiency. The results suggested that the separation efficiency and transport performance of photogenerated carriers in heterojunction were hugely enhanced.

Catalytic Capacity of MoS2/BiVO4
To assess the visible light photocatalytic activity of BiVO4, MB3, MB5, and MB7, TC degradation experiments were carried out. Figure 9a indicated that 39.7% of TC could be degraded in 90 min by BiVO4. In the same conditions, 93.7% of TC could be degraded by

Catalytic Capacity of MoS 2 /BiVO 4
To assess the visible light photocatalytic activity of BiVO 4 , MB3, MB5, and MB7, TC degradation experiments were carried out. Figure 9a indicated that 39.7% of TC could be degraded in 90 min by BiVO 4 . In the same conditions, 93.7% of TC could be degraded by MB5, it is 2.36 times greater than BiVO 4 . Moreover, with the increase of MoS 2 compounded in MoS 2 /BiVO 4 , the degradation rate of TC by MoS 2 /BiVO 4 photocatalyst showed a trend of increasing and then decreasing, and MB5 showed the best enhancement of photocatalytic activity. Compared with BiVO 4 , there may be three reasons for the enhanced photocatalytic performance of MB5: firstly, the larger specific surface area signifies an increased adsorption capacity, secondly, the improved utilization of visible light and the narrower band gap boosts the excitation of electrons, and thirdly, the generation of transport channels between MoS 2 and BiVO 4 are beneficial to the rapid transfer and separation of photogenerated electron-hole pairs. Figure 9b showed the result of the TC in three cyclic degradation experiments. Due to the loss of photocatalytic materials in the recycling process, the degradation rate decreases with the number of uses. After the catalyst was reused three times, the degradation efficiency remained above 80%. By a pseudo first-order model (lnC 0 /C = kt) [58], the degradation processes were fitted. Figure 9c showed that all fitted curves are nearly linear. As shown in Figure 9d, the kinetic constants k for the synthesized BiVO 4 , MB3, MB5 and MB7 are 0.0015, 0.0031, 0.0215 and 0.0012 min −1 , respectively. The kinetic constants k of MB5 is much higher than other photocatalysts, which was 14.3 times that of BiVO 4 . The above results illustrated the effectiveness and stability of MoS 2 /BiVO 4 heterojunction as a photocatalyst.
After the catalyst was reused three times, the degradation efficiency remained above 80%. By a pseudo first-order model (lnC0/C = kt) [58], the degradation processes were fitted. Figure 9c showed that all fitted curves are nearly linear. As shown in Figure 9d, the kinetic constants k for the synthesized BiVO4, MB3, MB5 and MB7 are 0.0015, 0.0031, 0.0215 and 0.0012 min −1 , respectively. The kinetic constants k of MB5 is much higher than other photocatalysts, which was 14.3 times that of BiVO4. The above results illustrated the effectiveness and stability of MoS2/BiVO4 heterojunction as a photocatalyst.

Photocatalytic Mechanism Study
A radical capture test was used to explore the photocatalytic mechanism. Under the same experimental conditions, TBA, AO and PBQ were added as radical trapping agents for •OH, h + and •O2 − , respectively [59,60]. The outcomes in Figure 10 revealed that after the addition of the three catchers TBA, AO, and PBQ, respectively, the degradation

Photocatalytic Mechanism Study
A radical capture test was used to explore the photocatalytic mechanism. Under the same experimental conditions, TBA, AO and PBQ were added as radical trapping agents for •OH, h + and •O 2 − , respectively [59,60]. The outcomes in Figure 10 revealed that after the addition of the three catchers TBA, AO, and PBQ, respectively, the degradation efficiency reduced to 54%, 13% and 29%. It could be concluded from the test results that the efficient degradation of TC benefited from the synergistic effect of •OH, h + , and •O 2 − . Among them, h + had two working modes, one was the direct oxidative degradation of TC, and the other was reaction with the water adsorbed on the surface of the material to form •OH, which then oxidizes and degrades TC. Owing to the addition of AO had the most pronounced inhibitory effect on the degradation of TC, which indicated that the primary active species for the oxidative degradation TC was h + in the photocatalytic reaction for MB5.
Hence, based on the relative positions of energy bands, and the above radical catch test results, possible photodegradation mechanisms of MoS 2 /BiVO 4 photocatalysis is posited ( Figure 11). The p-type MoS 2 and the n-type BiVO 4 form a p-n heterojunction, and MoS 2 had higher CB and VB than BiVO 4 based on the energy band structure. Firstly, because the adsorption property of MB5 was enhanced by the addition of MoS 2 , the large amount of TC was adsorbed on the surface of the photocatalytic materials. Under visible light irradiation, MoS 2 and BiVO 4 were excited simultaneously. The photogenerated holes of BiVO 4 and MoS 2 degraded TC as h + radicals. And some of the photogenerated holes in the VB of BiVO 4 migrated to the VB of MoS 2 and then formed •OH with water or hydroxyl groups which was adsorbed on the surface of the material. MoS 2 and BiVO 4 originally had different Fermi levels, and the Fermi levels became the same after the heterojunction was formed, which moved the CB of MoS 2 up to a higher level [61]. Therefore, in the heterojunction, the e − in the CB of MoS 2 had a stronger redox ability to react with dissolved oxygen to form •O 2 − . The matching energy band position between MoS 2 and BiVO 4 allows the generation of fast separation channels for photogenerated carriers at the heterojunction interface, which contributes to the generation of •OH, h + and •O 2 − . Moreover, h + played the most dominant role in the degradation of TC. Finally, the three active substances synergize with each other to oxidize and degrade the pollutants which are adsorbed on the surface of the photocatalytic materials into H 2 O and CO 2 .
Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of 16 efficiency reduced to 54%, 13% and 29%. It could be concluded from the test results that the efficient degradation of TC benefited from the synergistic effect of •OH, h + , and •O2 − . Among them, h + had two working modes, one was the direct oxidative degradation of TC, and the other was reaction with the water adsorbed on the surface of the material to form •OH, which then oxidizes and degrades TC. Owing to the addition of AO had the most pronounced inhibitory effect on the degradation of TC, which indicated that the primary active species for the oxidative degradation TC was h + in the photocatalytic reaction for MB5. Hence, based on the relative positions of energy bands, and the above radical catch test results, possible photodegradation mechanisms of MoS2/BiVO4 photocatalysis is posited ( Figure 11). The p-type MoS2 and the n-type BiVO4 form a p-n heterojunction, and MoS2 had higher CB and VB than BiVO4 based on the energy band structure. Firstly, because the adsorption property of MB5 was enhanced by the addition of MoS2, the large amount of TC was adsorbed on the surface of the photocatalytic materials. Under visible light irradiation, MoS2 and BiVO4 were excited simultaneously. The photogenerated holes of BiVO4 and MoS2 degraded TC as h + radicals. And some of the photogenerated holes in the VB of BiVO4 migrated to the VB of MoS2 and then formed •OH with water or hydroxyl groups which was adsorbed on the surface of the material. MoS2 and BiVO4 originally had different Fermi levels, and the Fermi levels became the same after the heterojunction was formed, which moved the CB of MoS2 up to a higher level [61]. Therefore, in the heterojunction, the e − in the CB of MoS2 had a stronger redox ability to react with dissolved oxygen to form •O2 − . The matching energy band position between MoS2 and BiVO4 allows the generation of fast separation channels for photogenerated carriers at the heterojunction interface, which contributes to the generation of •OH, h + and •O2 − . Moreover, h + played the most dominant role in the degradation of TC. Finally, the three active substances synergize with each other to oxidize and degrade the pollutants which are adsorbed on the surface of the photocatalytic materials into H2O and CO2.

Conclusions
Herein, a p-n heterojunction photocatalyst MoS2/BiVO4 was obtained by microwaveassisted synthesis of BiVO4 crystals that were grown on the surface of MoS2 nanomicrospheres. Compared with BiVO4, the heterojunction structure formed by MoS2/BiVO4 has a stronger adsorption capacity, greater visible light utilization, smaller band gap and higher separation of photogenerated carriers, and these advantages synergistically lead to the enhanced photocatalytic activity of the MoS2/BiVO4. For the above reasons, MB5 showed better photocatalytic activity under visible light, and the degradation rate of TC reached 93.7% within 90 min, and the first-order kinetic constant of MB5 was 0.0215 min −1 , which was 14.3 times higher over BiVO4. Cyclic degradation experiments demonstrated the reusability and stability of the MoS2/BiVO4 photocatalyst. This study provides an

Conclusions
Herein, a p-n heterojunction photocatalyst MoS 2 /BiVO 4 was obtained by microwaveassisted synthesis of BiVO 4 crystals that were grown on the surface of MoS 2 nano-microspheres. Compared with BiVO 4 , the heterojunction structure formed by MoS 2 /BiVO 4 has a stronger adsorption capacity, greater visible light utilization, smaller band gap and higher separation of photogenerated carriers, and these advantages synergistically lead to the enhanced photocatalytic activity of the MoS 2 /BiVO 4 . For the above reasons, MB5 showed better