Construction of Embedded Heterostructured SrZrO3/Flower-like MoS2 with Enhanced Dye Photodegradation under Solar-Simulated Light Illumination.

SrZrO3/flower-like MoS2 composites with an embedded heterostructure were synthesized via a simple two-step hydrothermal method and their performance was evaluated by photodegradation of methylene blue (MB) under solar-simulated light irradiation. The sandwiched flower-like MoS2 with a high Mo(VI) ratio was adopted as the matrix, and SrZrO3 was grown between the MoS2 layers, forming an intense contact interface, which promotes the efficient separation and transport of photoinduced carriers. The enhanced photocatalytic degradation of 99.7% after 80 min of irradiation is exhibited over the MS5 sample (5 wt % SrZrO3 loading amount on the MoS2 matrix). Moreover, the ratio of Mo(VI) and the superoxide radical plays a crucial role in the photodegradation of MB, and the higher the ratio the better the performance. This work provided a strategy to design a new kind of photocatalyst for photocatalysis and indicated that MoS2 is preferably adopted as a matrix rather than as a loading component.

Introduction

The increasingly serious environmental degradation and the continuous growth of energy crisis have become two major global issues over the past few years.[1,2] Among all the solutions, semiconductor photocatalysts have attracted close attention because of their features of inexhaustible solar energy and various “green” applications, such as complete degradation of pollutants,[3,4] hydrogen (H2) production from water splitting,[5,6] conversion of carbon dioxide (CO2) into hydrocarbons, and so forth.[7,8] To date, quite a number of samples have been employed for the photocatalytic test, such as TiO2, CdS, BiVO4, g-C3N4, and so forth.[9−12] However, the facts low conversion of the incident solar light, fast recombination of photoinduced electrons–holes, and inefficient transport of photogenerated carriers still limited the extensive application of photocatalysis.[13,14]

MoS2, as one kind of transition metal dichalcogenide, has drawn considerable attention. MoS2 alternates with a layer of sulfur and molybdenum to form a flower-like sandwich structure. The layers are connected by weak van der Waals forces, and the layers are connected by strong covalent bonds and ionic bonds. The variable atomic coordination structure and electronic structure make its carrier transport speed extremely fast (exceeds 200 cm2·V–1·S–1), and the band gap varies with the layer thickness, nanometer size, and ion doping. It can be adjusted in the range of 1.20–1.90 eV, corresponding to the upper limit of absorption wavelength of 690–1030 nm, which displays a good match with sunlight and high utilization of sunlight.[15] Besides, photocatalysts with flower-like structures were confirmed to display more efficient interfacial transfer and tardier recombination of the photogenerated carriers than nanoparticles.[16,17] The main reason was ascribed to the fact that the mesopores and macropores could be independently controlled by the flower-like superstructures.[18,19] It is well known that the mesopores and macropores are the key transport channels for the reactant molecules to approach the reactive sites, thereby affecting the photocatalytic performance. Moreover, because of the characteristics of large surface area and high thermal stability, flower-like MoS2 is propitious for applying as the matrix to dominate other semiconductors to form a heterogeneous structure,[20,21] which could further enhance the separation of the photoinduced carriers. Fu et al. synthesized NiFeO4/MoS2–Pd nanocomposites and displayed satisfactory photocatalytic rhodamine B (RhB) degradation and Suzuki–Miyaura coupling reaction, caused by the formation of p–n heterojunctions.[15] Lu et al. synthesized a Z-scheme g-C3N4/Ag/MoS2 ternary photocatalyst, and it displayed optimal visible-light photodegradation activity for RhB.[1]

Materials containing d0 and d10 metal ions, such as Ti4+, Zr4+, Nb5+, Ga3+, and Sb5+ have been confirmed to display high photocatalytic performance.[22,23] Among these catalysts, perovskite-type oxides ABO3 are considered as one of the most promising candidates, owing to their stable chemical structure and nontoxicity.[24] In the ABO3 structure, A presents a rare earth metal with a large ionic radius and B presents a transition metal with a small ionic radius. Among the vast members of perovskite oxides, strontium titanate (SrTiO3) and strontium zirconate (SrZrO3, SZO) are two typical photocatalysts.[25] However, SZO has attracted more attention because of its wide band gap of 5.6 eV.[6] In other words, SZO possesses more negative reduction potential and more positive oxidation potential, when compared to other perovskite oxides. Therefore, the perovskite oxide SZO could be considered as a promising semiconductor to be dominated by the MoS2 matrix. Tian et al. prepared a MoS2/SZO heterostructure and showed that 0.05 wt % MoS2 content exhibited the optimized photocatalytic performance.[6] However, SZO was adopted as the matrix and MoS2 was used as the loading component, which could not fully bring out the photocatalytic potential of MoS2, such as the fast transfer capacity of carriers.

Herein, heterostructured SZO/MoS2 photocatalysts with various SZO loading amounts were prepared via a hydrothermal method, and they were evaluated by the photocatalytic degradation of methylene blue (MB) under solar-simulated light. Among all the samples, 5 wt % SZO exhibited the optimal photocatalytic performance, with a degradation rate of 99.7% within 80 min. The reason for the increase was investigated using various kinds of techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis measurements, and photoelectrochemical measurements. Based on the experimental results and analysis, the photodegradation mechanism over heterostructured SZO/MoS2 was also proposed.

Results and Discussion

Morphology and Crystal Structure Analysis

The morphology of all samples was observed by scanning electron microscopy (SEM) characterization, as shown in Figure . The MoS2-only sample displays an apparent flower-like shape, while the SZO-only sample exhibits an obvious cubic structure. The other SZO/MoS2 samples mainly show a flower-like shape, similar to the MoS2-only sample. However, the cubic particles appeared when the SZO loading amount is increased up to 7 wt %.

Figure 1

SEM images of (a) MoS2-only, (b) MS1, (c) MS5, (d) MS7, (e) MS10, and (f), SZO-only.

The crystal structures of all samples were investigated by wide-angle XRD characterization, as shown in Figure a. The peaks at 32.7 and 58.3° are, respectively, indexed to the (1 0 0) and (1 1 0) planes of MoS2, according to JPCDS no. 37-1492. It is noteworthy that the (0 0 2) plane of MoS2 is absent in the spectrum, which generally occurs at 14.4°. A new peak can be observed at 17.6°. There is an obvious decrease of signal intensity at 10–15°, indicating that there may be a peak around <10°. Previous studies revealed that the diffraction peak at 17.6° may have resulted from the spacing between MoS2 and the carbon layer, and the peak at <10° may correspond to the distance between two adjacent MoS2 layers in which a carbon layer is sandwiched.[27] The formation of sandwiched carbon layers can be ascribed to the carbonaceous materials produced by the hydrothermal carbonization of oxalic acid introduced into the MoS2 layers. The SZO-only sample displays five distinct peaks at 30.7, 44.1, 54.8, 64.1, and 72.9°, corresponding to the (2 0 0), (2 0 2), (0 4 2), (2 4 2), and (1 6 1) crystal planes of SZO (JCPDS no. 44-0161). The small peaks at 25.2° in the XRD pattern of SZO are assigned to the (1 1 1) plane of SrCO3 (JCPDS no. 05-0418). The formation of SrCO3 may be ascribed to CO2 in the atmosphere. The MS1, MS3, and MS5 samples mainly display the hexagonal phase MoS2 patterns, which is because of the fact that MoS2 was adopted as the matrix when the loading amount of SZO was relatively low. The peaks corresponding to SZO appear when the loading amount reaches up to 7 wt %, indicating that a high SZO loading amount is not beneficial for the dispersion of SZO on the MoS2 matrix, which is in accord with SEM analysis. Hence, there exists an optimum SZO loading amount for the formation of uniform heterostructured SZO/MoS2. In addition, the (2 0 2) peak in the XRD patterns of MS7 and MS10 shifted to 45°. This shift may be ascribed to the twist of the crystal planes as SZO is grown between the MoS2 layers.[27] The FT-IR characterization was also applied for further explaining the heterostructure between SZO and MoS2, as shown in Figure b. The peak at 912 cm–1 appeared in pure MoS2, but it disappeared after SZO was loaded onto the MoS2 matrix. Moreover, the peaks at 1259 and 857 cm–1, which corresponded to pure SZO, both emerged on the SZO/MoS2 composites. These results indicated that the heterostructure between SZO and MoS2 was formed.

Figure 2

XRD patterns (a) and FT-IR spectrum (b) of SZO-only, MoS2-only, and SZO/MoS2 catalysts.

The elemental distribution of the MS5 sample was investigated by energy-dispersive spectroscopy (EDS) mapping analysis, as shown in Figure a. The results display the existence of Sr, Zr, O, Mo, and S elements and confirm the uniform distributions of Sr, Zr, and O elements on the MoS2 matrix. The different distribution of Sr and Zr in the EDS spectrum of the MS5 sample was ascribed to the low resolution of the EDS instrument and the insufficient scanning times for the separate Sr and Zr. To view the specific morphology of MoS2 in the MS5 sample, TEM and high-resolution TEM (HR-TEM) were performed, as shown in Figure b–d. The MS5 sample still displays the obvious flower-like morphology on a nanometer scale, and the sandwiched layer structure is found in the partial enlarged view of the flower-like shape, which exhibits the lattice spacing with an interplanar distance of 0.52 nm. According to the Bragg formula (2d × sin θ = nλ), the 2θ degree that corresponded to this lattice spacing is close to 17.6°. Therefore, based on the observation of morphology and the result of lattice spacing, the conclusion deduced from XRD analysis is confirmed.

Figure 3

EDS elemental mapping (a), TEM (b), and HR-TEM (c) images of the MS5 sample.

The morphologies of MoS2 and SZO/MoS2 catalysts are illustrated in Scheme . SZO could spread evenly between the layers of MoS2 when the loading amount is small. As the amount of SZO increased, it grew into a large crystal and covered the surface of MoS2 flowers.

Scheme 1

Schematic Illustration of the Morphologies of MoS2 and SZO/MoS2 Catalysts

The N2 adsorption–desorption isotherms and pore size distributions of all samples are shown in Figure S1a,b. All samples exhibit isotherms of type IV with the hysteresis loops of type H3,[28] confirming the existence of slit-like mesopores.[29] As shown in Table , the MoS2-only sample exhibits the highest surface area and pore volume, while the SZO-only sample displays the lowest value. The surface area of the MoS2-only sample obtained here is similar to the value reported by Pujari et al.[30] Besides, with the increase of the SZO loading amount, the surface area and the pore volume of SZO/MoS2 series catalysts decrease while the pore size increases. It is well known that high surface areas promote adsorption of the reactants, thus enhancing the photocatalytic performance.[8]

Table 1

Surface Area and Pore Structure of Pure SZO, Pure MoS2, and SZO/MoS2 Catalysts

samples	BET surface area (m2/g)	pore volume (mm3/g)	pore size (nm)
SZO	0.9	3.6	46.3
MS	17.3	36.1	19.2
MS1	15.4	35.1	21.4
MS3	13.7	28.8	21.7
MS5	12.2	23.5	23.4
MS7	7.9	19.0	27.5
MS10	3.1	16.1	25.9

Band Structure Analysis

Generally, the optical absorption capacity and the band structure of the photocatalyst play a crucial role during the photocatalysis reaction. Therefore, UV–vis DRS spectra of MS, MS5, and SZO samples were profiled, as shown in Figure a. The SZO-only sample displays an absorption peak around 280 nm, which is supposed to be the spontaneous band gap absorption.[6] MoS2-only and SZO/MoS2 samples show a wide adsorption band, which is distinctly different from the spectra of SZO.

Figure 4

UV–vis DRS profiles (a) and Mott–Schottky profiles (b) of SZO-only, MoS2-only, and SZO/MoS2 catalysts.

The light absorption intensity of MS5 is slightly lower than that of MS, owing to the loading of SZO. The band gap (Eg) of SZO sample was calculated based on the equation (Ahν)2 = hν – Eg,[8] exhibiting the value of 5.3 eV, which was smaller than that in the reported work (5.6 eV).[6] This may be ascribed to the particle size of the prepared sample.[31]

The conduction band edge (ECB) of MS, MS5, and SZO samples was obtained from Mott–Schottky measurements, as shown in Figure b. The positive slope of the tangent of profiles over three samples indicates that they are both n-type semiconductors. The flat band positions (Vfb, whose unit is V vs Ag/AgCl) were obtained from the intersection between the tangent and the y = 0 plot. Based on the formula (E(NHE) = E(Ag/AgCl) + 0.197 V),[32] the unit of Vfb was converted from V versus Ag/AgCl to normal hydrogen electrode (NHE) potential. It was reported that for the n-type semiconductors, the conduction band (ECB) was slightly more negative than Vfb.[8] Here, the specific value of 0.2 V versus NHE was used for the acquisition of ECB. Thus, the conduction band edge of MS is located at −0.03 V, and that of SZO is at −0.47 V. Moreover, the ECB value of the MS5 sample falls between the values of MS-only and SZO samples, which can further confirm the formation of a heterostructure between MoS2 and SZO.

MB Photodegradation Performance

The MB photocatalytic degradation performance under solar simulated light over all samples is displayed in Figure a. Compared to SZO-only and MoS2 samples, the SZO/MoS2 composites exhibit higher photocatalytic activity for the degradation of MB under solar-simulated light irradiation. SZO-only and MoS2-only display 59.7 and 67.7% MB degradation rates during a reaction time of 100 min. Among the SZO/MoS2 series composites, the MS5 sample exhibits the optimum performance, which reaches up to 99.7% degradation rate within a reaction time of 80 min. The specific profiles of MB degradation over the MS5 sample are displayed in Figure S2, and the results show that the absorption peak assigned to MB at 664 nm decreased significantly with the increase of time. The comparison of MB photodegradation efficiency between SZO/MoS2 and other MoS2-based photocatalysts is displayed in Table . Compared to other MoS2-based photocatalysts, the synthesized SZO/MoS2 in this work displayed relatively high MB photodegradation efficiency. Moreover, the simple preparation process and the construction of an embedded heterostructure between MoS2 and perovskite could provide a new strategy for the investigation of photocatalysts.

Figure 5

(a) Photocatalytic degradation of MB over SZO-only, MoS2-only, and SZO/MoS2 catalysts under solar-simulated light. (b) Solar-simulated light-driven time-cycle MB degradation of the MS5 sample.

Table 2

Comparison of MB Photodegradation Efficiency for MoS2-Based Photocatalysts

samples	MB degradation efficiency/%	degradation time/min
SZO/MoS2	99.7	80
MoS2/rGO[33]	98.0	10
CF/MoS2/Bi2S3 cloth[34]	91.8	100
LHZnO/MoS2[35]	75.0	300
CDs@MoS2[36]	91.1	35
MoS2/ZnO[37]	81.8	80

The catalytic reaction stability of the catalyst is crucial for practical applications. Therefore, the photocatalytic stability over the MS5 sample was studied by four time-cycle degradation experiments, as shown in Figure b. After four runs of photocatalysis reaction, the MB degradation over the MS5 sample decreases slightly from 97.3% in the first run to 95.5% in the fourth run, indicating its satisfied photocatalytic stability.

Enhanced Photodegradation Investigation

XPS analysis was done to investigate the enhanced photocatalytic MB degradation over SZO/MoS2 composites, as shown in Figure . The survey spectra confirm the existence of the corresponding elements for the corresponding samples. The weak Zr 3d peak and the Sr 3d peak in the MS3 sample are ascribed to the low loading amount of SZO on the MoS2 matrix. Figure b shows the typical XPS spectra of Mo 3d, in which the peaks at 229.0 and 232.2 eV were attributed to Mo(IV) 3d5/2 and 3d3/2, the peaks at 230.3 and 233.0 eV to Mo(V) 3d5/2 and 3d3/2, and the peaks at 233.8 and 235.9 eV to Mo(VI) 3d5/2 and Mo(VI) 3d3/2, respectively.[38] The peak at about 226.3 eV was attributed to S 2s.[3] Based on the deconvolution of the XPS spectra, the ratios of Mo(IV), Mo(V), and Mo(VI) are listed in Table . Compared to the MoS2-only sample, the SZO/MoS2 series samples exhibit a higher Mo(VI) ratio. Moreover, among the SZO/MoS2 series samples, the MS5 sample exhibits the highest Mo(VI) ratio. As for the S 2p spectra, two doublets around 162.0 and 163.3 eV were assigned to the binding energies (BEs) of S(−II) 2p3/2 and 2p1/2,[6] respectively, and the two doublets around 169.0 and 170.2 eV were assigned to the BEs of S(VI) 2p3/2 and 2p1/2, respectively. Mo(VI) and S(VI) on the surface of catalysts may result from the hydrothermal process. The components containing Mo(VI) and S(VI) are difficult to be identified because no corresponding peak appeared in XRD patterns. The amount of Mo(VI) positively correlated with the degradation efficiency, which suggested that the Mo(IV) species on the surface of MoS2 might also behave as active sites in photocatalytic reactions.

Figure 6

XPS spectra of SZO-only, MoS2-only, and SZO/MoS2 catalysts. (a) Survey; (b) Mo 3d; (c) S 2p; (d) Sr 3d; (e) Zr 3d; and (f) O 1s.

Table 3

XPS Elementary Surface Species Concentration of SZO-Only, MoS2-Only, and SZO/MoS2 Catalysts

	Mo 3d			S 2p
samples	Mo(IV) %	Mo(V) %	Mo(VI) %	S(−II) %	S(VI) %
MS	61.3	23.8	14.8	88.2	11.8
MS3	46.4	15.8	37.8	73.2	26.8
MS5	44.9	13.1	42.0	73.2	26.8
MS7	50.9	14.8	34.3	72.7	27.3

Two peaks of Sr 3d at about 134.7 and 133.0 eV were assigned to Sr 3d3/2 and Sr 3d5/2,[6] respectively, and the peaks corresponding to Zr 3d3/2 and Zr 3d5/2 are located at about 183.8 and 181.4 eV, respectively,[39] as shown in Figure d,e. As shown in Figure f, the O 1s spectra were fit to two main peaks. The peak at a low BE was ascribed to the lattice oxygen (Oα), while the high BE peak was attributed to the chemisorbed oxygen (Oβ), such as O2– or O–.[8] Deserved to be mentioned, it can also be found that along with the increase of the SZO loading amount, the BE of the peaks in Mo 3d and S 2p spectra shift to a lower value, while the peaks in the Sr 3d, Zr 3d, and O 1s spectra shift to a higher BE. This phenomenon indicates that the electrons were transferred from SZO to MoS2 when SZO was loaded onto the MoS2 matrix, which can enhance the photocatalytic performance. On the other hand, this phenomenon confirms the formation of a heterojunction between SZO and MoS2.

The separation and transfer efficiencies of photoinduced carriers were investigated by the photoelectrochemical method. It was reported that the small radius of the EIS Nyquist plot and the strong intensity of the photocurrent during on and off cycles of irradiation represented a better migration and transfer efficiency of the photoexcited carriers.[40,41] As shown in Figure , compared to SZO-only and MoS2-only, the heterostructured SZO/MoS2 series samples display a smaller radius and a higher intensity of photocurrent, indicating that the formation of a heterostructure is beneficial for the separation of photogenerated carriers.

Figure 7

EIS spectra (a) and photocurrent potential curves (b) of SZO-only, MoS2-only, and MS5 samples.

Combined with the photocatalytic activity, XPS, and photoelectric test, the obtained results display that the high Mo(VI) ratio and the formation of the heterostructure in the SZO/flower-like MoS2 composites can lead to efficient separation and transport of photoexcited carriers, thus improving the photocatalytic performance.

Photocatalysis Mechanism

In order to investigate the exact active species for the photocatalytic degradation and then find out the photocatalysis mechanism, the active species trapping experiments over the MS5 sample was applied. It is known that p-benzoquinone, triethanolamine, and tert-butanol are usually applied as the trapping agents of superoxide radicals (O2–), holes (h+), and hydroxyl radicals (•OH), respectively. Therefore, it can be seen from Figure a that the photocatalytic activity decreases significantly when p-benzoquinone is present, indicating that O2– is the main active species for the degradation of MB. The photocatalytic activity is enhanced in the presence of triethanolamine, which is because the separation of charge carriers is improved when h+ are trapped by the agent, thus promoting the generation of O2–. This phenomenon further confirms that O2– plays a crucial role in the photocatalytic process. However, the degradation efficiency almost remains unchanged when tert-butanol was added, demonstrating that •OH hardly contributes to the degradation of MB. The rapid and efficient interaction between the dye and O2– may be the reason that •OH radicals hardly act as reactive species.[42]

Figure 8

(a) Photocatalytic degradation activity of the MS5 sample in the absence and presence of p-benzoquinone, triethanolamine, and tert-butanol scavengers. (b) Schematic diagram of the photocatalytic degradation of the MB mechanism over SZO/MoS2 catalysts.

Based on the above analysis, the possible photocatalytic degradation mechanism over heterostructured SZO/flower-like MoS2 was proposed, as shown in Figure b. Under solar-simulated light irradiation, both MoS2 and SZO can be excited, and the photoinduced electrons on the CB of SZO react with the adsorbed oxygen to form O2–, which is confirmed by XPS analysis. Then, the dye of MB is oxidized by the generated O2– to be degraded. Because of the formation of a heterostructure between MoS2 and SZO, the holes on the VB of SZO are transferred to the MoS2 matrix, which can promote the separation efficiency of the photoinduced carriers and generate more electrons on SZO. Therefore, the heterostructured SZO/MoS2 exhibits higher photocatalytic degradation performance than MoS2-only and SZO-only. Besides, as analyzed by XPS, Mo(VI) may also be beneficial to the photocatalytic reaction on the SZO/MoS2 composite.

Conclusions

In summary, an embedded heterostructured SZO/flower-like MoS2 composite was synthesized by a simple two-step hydrothermal method, and the influence of different SZO loading amounts on the photocatalytic degradation of MB under solar-simulated light was investigated. The optimal photocatalytic performance is obtained with 5 wt % SZO loading amount on the MoS2 matrix, which reaches up to 99.7% degradation rate after 80 min of solar-simulated light irradiation. The enhanced photocatalytic performance is ascribed to the fact that MoS2 was adopted as the matrix, which can utilize the advantages of MoS2 adequately, such as the fast transfer capacity of carriers. Based on the results of various techniques, it is concluded that the enhanced photocatalytic performance is because of the uniform SZO distribution, large surface area, and especially intense contact interface. This new kind of photocatalyst can promote the efficient separation and transport of photoinduced carriers, thus enhancing the photocatalytic activity under solar-simulated light. Moreover, the ratio of generated Mo(VI) and superoxide radicals plays a crucial role in the photodegradation of MB, and the results display that the higher the ratio, the better the performance.

Experimental Section

Chemicals and Materials

Sodium molybdate dihydrate (Na2MoO4·2H2O), thioacetamide (C2H5NS), oxalic acid, strontium nitrate (Sr(NO3)2), zirconium(IV) oxychloride octahydrate (ZrOCl2·8H2O), potassium hydroxide (KOH), MB, and ethanol were used in the experiment. All the chemicals were provided by Shanghai Sinopharm Group in analytical grade and were used without further purification. Deionized water (H2O) purified using a Millipore system was used throughout all experiments.

Synthesis of Flower-like MoS2

Flower-like MoS2 was synthesized via a hydrothermal method. First, 1.2 g of Na2MoO4·2H2O, 1.6 g of C2H5NS, and 0.6 g of oxalic acid were dissolved in 80 mL of H2O and stirred for 30 min. Following that, the mixed solution was transferred into a 100 mL Teflon autoclave and then hydrothermally treated at 180 °C for 24 h. After the reactor cooled to room temperature, the as-prepared catalyst was centrifuged and washed with ethanol and H2O several times. The final catalyst was obtained after drying at 60 °C in a vacuum oven overnight and denoted MS.

Synthesis of Pure SZO

The pure SZO sample was also synthesized by the hydrothermal method as reported previously.[26] Typically, 1.164 g of Sr(NO3)2 and 1.608 g of ZrOCl2·8H2O were dissolved in 60 mL of 12 mol·L–1 KOH solution and stirred at room temperature for 1 h. Then, the mixed solution was transferred into a 100 mL Teflon container and sealed in an autoclave. After reacting at 200 °C for 24 h, the container was cooled naturally. Centrifugation and washing with distilled water and ethanol were done several times, and then the sample was transferred to a vacuum oven to be dried at 60 °C for 6 h. The obtained SZO catalyst was named SZO.

Preparation of SZO/MoS2 Composites

The SZO/MoS2 composite was prepared by the following hydrothermal reaction process. The as-prepared MoS2 (0.5 g) was dissolved in 30 mL of ethylene glycol, and then a certain quantity of Sr(NO3)2 and ZrOCl2·8H2O was added to the solution, wherein the molar ratio of Sr(NO3)2 and ZrOCl2·8H2O was 1:1. Subsequently, 30 mL of 12 mol·L–1 KOH solution was added to the mixed solution, and the solution was ultrasonicated for 30 min and stirred for 1 h to enable the components to be dispersed uniformly. The solution was then transferred into a 100 mL Teflon container, sealed in an autoclave, and hydrothermally reacted at 200 °C for 24 h. After cooling naturally, the product was centrifuged and washed with distilled water and ethanol several times. Finally, the product was vacuum-dried at 60 °C for 6 h. The prepared samples denoted MS1, MS3, MS5, MS7, and MS10represent the 1, 3, 5, 7, and 10 wt % SZO to MoS2 ratio, respectively.

Characterization

XRD profiles were recorded on an XD-3 instrument with Cu Kα X-ray radiation. N2 adsorption–desorption isotherms were measured at −196 °C using a Micromeritics ASAP 2020 physisorption instrument. The surface areas and the porosity were obtained by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda methods. Field-emission scanning electron microscopy (FE-SEM) was performed on a Quanta 250F. HR-TEM was conducted on a JEM-2100 instrument equipped with a slow-scan CCD camera at an accelerating voltage of 200 kV. XPS analyses were conducted on an ESCALAB 250 spectrometer. All BEs were referenced to the adventitious C 1s at 284.4 eV. The X-ray source utilized was Al Kα X-ray (hν = 1486.6 eV) radiation. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) was performed on a Shimadzu UV-2550 spectrophotometer. The photoelectrochemical tests were carried out using a CHI760 electrochemical working station with a standard three-electrode system and 0.5 mol·L–1 Na2SO4 as the electrolyte solution. FTO glass was dip-coated with a mixed solution of the photocatalyst, naphthol, deionized water, and alcohol and used as the work electrode.

Photocatalytic Section

Photocatalytic MB degradation over SZO/MoS2 was conducted at room temperature and atmospheric pressure. Typically, 15 mg of photocatalyst was added to 100 mL of MB solution at a concentration of 25 mg/L. The suspension was stirred for 30 min in the dark environment to achieve adsorption equilibrium. The adsorption-balanced suspension was irradiated with a 300 W xenon (Xe) lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd.), and 6 mL of the suspension was taken every 10 min. The suspension was centrifuged to remove the catalyst, and then 4 mL of the supernatant was taken for the following analysis. The MB concentration of the obtained supernatant was measured with the peak intensity at 664 nm.