Enhanced photocatalytic activity of molybdenum disulfide by compositing ZnAl–LDH

Journal Pre-proof Enhanced photocatalytic activity of molybdenum disulfide by compositing ZnAl−LDH Shuang Chen, Fan Yang, Zhanfang Cao, Chao Yu, Shuai Wang, Hong Zhong

PII:

S0927-7757(19)31132-X

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124140

Reference:

COLSUA 124140

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

14 August 2019

Revised Date:

18 October 2019

Accepted Date:

18 October 2019

Please cite this article as: Chen S, Yang F, Cao Z, Yu C, Wang S, Zhong H, Enhanced photocatalytic activity of molybdenum disulfide by compositing ZnAl−LDH, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124140

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Mos Enhanced photocatalytic activity of molybdenum disulfide by compositing ZnAl−LDH Shuang Chen, Fan Yang, Zhanfang Cao*, Chao Yu, Shuai Wang, Hong Zhong College of Chemistry and Chemical Engineering, and Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, Hunan, China

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Abstract

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Graphical abstract

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[email protected]

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MoS2 is a new type of two-dimensional material that can be used in the field of photocatalysis, but its agglomeration characteristics limit its use. So we tried to combine MoS2 with hydrotalcite to improve the morphology and enhance the performance of MoS2. The XRD, XPS, UVVis prove that the material obtained is molybdenum disulfide intercalated zinc-aluminum hydrotalcite, and the SEM shows the particle size of molybdenum disulfide is reduced due to the interlaminar structure of hydrotalcite. Then, in the experiment of degrading methylene blue, the effects of cation ratio, thiourea dosage, pH and catalyst dosage on

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degradation were investigated. After 30 minutes of adsorption and 150 minutes of illumination, the maximum degradation rate of methylene blue can reach 93%. And by analyzing the photocatalytic effect of the physical mixing of the two materials, it can be concluded that the photocatalytic performance of the composite material is better than that of the physical mixing of the two materials, which is close to 60%. Therefore, composite with hydrotalcite enhances photocatalytic performance of MoS2, so the composite of MoS2/LDH has great potential in the field of photocatalytic degradation. Keywords: molybdenum disulfide hydrotalcite intercalation photocatalysis 1. Introduction

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In recent years, wastewater pollution[1, 2] represented by dye wastewater is seriously threatening people's health[3, 4]. How to deal with it[5] properly has become a problem that must be solved. Among different wastewater treatment methods[6-10], photocatalytic degradation[11-15] of dyes has attracted much attention because of its simple operation and pollution-free. It uses semiconductor materials to generate photoproduced electron–hole pairs after absorbing photons[16, 17], which induces redox reaction and achieves the purpose of degradation of pollutants. Molybdenum disulfide is a layered compound with excellent performance in photoelectric properties[18]. Its properties and structure are similar to those of graphene, and it belongs to a new two-dimensional material and can be used in the field of photocatalysis. It can respond to a wide range of wavelengths, absorb more photons of visible light frequency, and has high edge potentials in both bands, which is very advantageous for carrier separation. Unlike the zero bandgap of graphene, the band structure of MoS2 varies dramatically with the thickness of the nanosheets, offer it further opportunities in photocatalysis[19]. The nano-sized molybdenum disulfide material has a large specific surface area and a large catalytic activity point, so it has high reactivity. However, the applications of MoS 2 in optoelectronic devices are impeded by the lack of high-quality p–n junction, low light absorption for mono-/multilayers, and the difficulty for large-scale monolayer growth[20]. Besides, both theoretical and

experimental investigations have also shown that the catalytic active site is along the edge of MoS2, and molybdenum disulfide tends to agglomerate during the preparation process, where the limited area further influences its performance[21]. Hydrotalcite is a layered double metal oxide with good adsorption and catalytic properties. Its general formula is [M12x M x3 (OH )2 ]( Axn/n )  mH2O .M refers to the cation of the laminate, A refers to the anion between the

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laminates having a valence number of n, and X is the ratio of the trivalent metal ions in the structure. The interlayer anions of the hydrotalcite itself

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can be ion-exchanged[22, 23], and introduce ions of different particle sizes,

plate spacing

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achieved. Zhang

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synthesized

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material

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so that the interlayer spacing can be changed, and the change of the

LDH@Co(OH)2 composite as a new sulfur host for Li–S batteries [24],

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Jiang[25] and Gomes Silva[26] synthesized NiCo-LDH and ZnCr-LDH as

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a electrocatalyst for the OER. Compared with traditional photocatalysts,

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this material has narrower band gaps , which makes hydrotalcite materials have good prospects in adsorption and photocatalysis. Researchers found ZnAl−LDH film[27], magnetic Fe3O4/ZnCr−LDH[28] composite and Mg– Zn–Al LDH[29] have good performance in photocataalysis. Among them, the Fe3O4/ZnCr−LDH composite can decompose 95% of methylene blue

(MB) under UV irradiation[28].

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In order to enhance the photocatalytic performance of LDH, the researchers inserted different ions between the layers of LDH, Xu prepared two new polyoxometalate-intercalated ZnAlFe-layered double hydroxides[30], Chen synthesized LDH-Ag2O/Ag composites[31], Ma synthesized BiOCl-NiFe-LDH composites[32], Abazari, Reza prepared gC3N4@Ni–Ti LDH nanocomposites[33]. However, there are some problems in these materials, such as the complex preparation process, low degradation efficiency, high cost and unstable material properties. So we designed a new preparation method which combines the advantages of hydrotalcite and molybdenum disulfide to achieve the purpose of photocatalytic degradation of organic matter. The combination of molybdenum disulfide and hydrotalcite can exert the synergistic effect of the two materials, adjust the band gap width to increase the absorption of visible light. Moreover, since molybdenum disulfide is formed between the hydrotalcite layers and tends to grow in spots or flakes, its agglomeration phenomenon can be effectively solved. The materials were characterized by different means, and the effects of various factors on the degradation of methylene blue by composites were investigated. And the possible electron pathway in composite is also discussed.

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2. Experimental

2.1 Preparation of MoS2/LDH composite

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Molybdenum disulfide and hydrotalcite composites were prepared by coprecipitation and hydrothermal methods[34, 35]. Zinc-aluminium hydrotalcite is prepared by coprecipitation，using zinc nitrate, aluminium nitrate, sodium molybdate and sodium hydroxide. Weighing proper amounts of Zn(NO3)2 ·9H2O, Al(NO3)3 ·6H2O, Na2MoO4 ·2H2O and NaOH ,and the molar ratio of Zn2+:Al3+:MoO42-:OH- is 6：2：1：16. Solution A was prepared by dissolving zinc nitrate and aluminium nitrate in 50 ml deionized water, and solution B was prepared by dissolving sodium

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molybdate and sodium hydroxide in 50 ml deionized water. Add solution A and solution B into three flasks containing 100 ml deionized water by dropping funnel, and the dropping rate was controlled so that the pH of the solution was about 10, and the temperature of the water bath was maintained at 45 ° C, and the dropwise addition was completed at 1.5 hours. After the addition was completed, the mixture was stirred for 1 hour, then it was placed in an environment of 80 ° C for 12 hours. After cooling, it was washed with deionized water, centrifuged to neutrality, and the obtained material was placed in a suspension of 50 ml, 25 ml was taken into the reaction vessel, 5 times of molybdenum amount of thiourea and 2 ml of hydrazine hydrate were added, and the mixture was placed in a muffle. Naturally cooled to room temperature, the obtained black substance was washed with ethanol and deionized water, centrifuged several times to neutral, and then dried in a freeze dryer. The obtained material was molybdenum disulfide intercalated zinc-aluminium hydrotalcite.

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2.2 Photocatalytic experiment

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Methylene blue was used to evaluate the photocatalytic activity of the composites[34, 36], and 20 mg/L methylene blue solution was prepared for use. The absorbance of methylene blue solution was measured at its maximum absorption wavelength of 664 nm. The relationship between absorbance and concentration was measured by ultraviolet spectrophotometer, then fit the standard curve equation.

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A 300 W Xe lamp was used as the light source, the composite material was dispersed in a methylene blue solution, sonicated for 5 min, adsorbed in the dark for 30 min, placed under a xenon lamp, and the absorbance was measured at intervals, and the true concentration of the dye was calculated by a standard curve. The degradation efficiency is expressed in terms of C/C0, C0 is the initial concentration of dye and C is the solution concentration in the experiment 3. Results and discussion 3. Characterization The the structure of samples are analyzed by X-ray diffraction analysis (XRD)( Shimadzu ,XRD-6000),which is operated at the scanning

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speed of 5º/min in the 2θ range from 5º to 80º. The morphology of the samples was observed using Sirion200 Scanning Electron Microscope(SEM) and Tecnai G2 F20 Transmission Electron Microscope(TEM).X-ray photoelectron spectroscopy(XPS) spectra dota was collected based on a K-Alpha+ Photoelectron Spectrometer(Thermo fisher),while the pass energy of full-spectrum scanning is 100 eV and the scanning step is 1 eV ,the pass energy of narrow-spectrum scanning is 30eV and the scanning step is 0.1eV.The optical absorption properties of samples in the 200-800 nm is recorded by Ultraviolet Visible Absorption Spectroscopy(UV–vis) using Angilent CARY 300. The pore size distribution and pore volume of the material are determined by BET surface area using Kubo X1000.

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3.1. XRD

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Fig.1 XRD patterns of different catalyst(A) and LDH with different metal molar ratios(B)

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Fig 1-A shows the XRD patterns of different catalyst. Obviously, XRD pattern of Zn-Al LDH (MoO42-) display characteristic peaks of ZnAl LDH in 2θ=11.98°、23.76°、34.64°、39.38°、47.24°、60.42°、61.74°, corresponding crystal plane (003)、(006)、(012)、(015)、(018)、(110)、 (113).Compared with Zn-Al LDH (CO32-), the Zn-Al LDH (MoO42-) has a lower intensity in characteristic peaks, maybe it’s due to MoO42- has larger molecular mass than CO32-, so the structure of LDH is poorly crystallized. The pure phase MoS2 prepared by hydrothermal method is consistent with the literature, corresponding characteristic diffraction peaks appear at 33.36°, 56.78°, respectively (100), (110) crystal planes. The characteristic

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diffraction peaks of MoS2 and LDH appear in the XRD pattern of MoS2/LDH composites, indicating that the composite consists of MoS2 and LDH, but some diffraction peaks of LDH disappear in the diffraction pattern of MoS2/LDH composites. This is due to the insertion of MoS2 expanded the interlayer space of LDH, the crystal structure is destroyed, and the MoS2 attached to the surface affects the XRD results. And the EDS energy spectrum of Fig 3.3 shows that the atomic ratio of Mo to S is close to 1:2, prove the composites contain MoS2, meet the analysis result of XRD. And Fig 1-B confirmed the structure and the phase purity of the products at different mole ratios of Zn and Al(Zn:Al-2:1,3:1,4:1). The XRD patterns show that LDH with different mole ratios has similar crystal structure, but the intensities of LDH(Zn:Al-4:1) characteristic peaks are weaker compared to LDH(Zn:Al-2:1) and LDH(Zn:Al-3:1), especially in the peaks indexed as (003) and (012). Indicating excessive Zn content will destroy the crystallinity of the material.

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3.2. SEM

Fig 2. SEM images of Zn-Al LDH(a), MoS2(b), MoS2/LDH composite (c, d)

Fig 2 shows the SEM images of Zn-Al LDH, MoS2 and MoS2/LDH composite. The hydrotalcite material (Fig 2-a) prepared by the coprecipitation method has a massive shape, the size is between 10-20 um and stacked by layered structures. Molybdenum disulfide (Fig 2-b) reduced

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by hydrothermal method presents a typical globular shape, and the size of the globule is between 100 and 150 nm. The Fig 2-c shows that the hydrotalcite layer structure is opened and the sheets look like leaves and oriented in different directions. It can be seen in the Fig 2-d that diameter of catalyst structure is between 100 and 200 nm, and its thickness is thinner. There is no characteristic structure of molybdenum disulfide on the surface, which proves that molybdenum disulfide distributes on the surface of hydrotalcite in the form of point or single layer rather than agglomerating into spheres. This structure can provide more active sites, which is conducive to the photocatalytic process.

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Fig 3. SEM-Mapping analysis chart of MoS2/LDH

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By SEM-EDS and SEM-Mapping analysis of the composites, it is found that the surface of the material is uniformly distributed with Zn, Al, Mo and S elements. And as it shows in Fig 3, the surface of the material has a molybdenum-sulfur ratio of 1: 2.16, which basically conforms to molybdenum disulfide, and the excess sulfur atom can contribute to the improvement of its photocatalytic activity. It was proved that molybdenum disulfide was reduced and the composite was successfully prepared.

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3.3. XPS

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Fig. 4. XPS chart of MoS2/LDH nanocomposites

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Fig 4 is the XPS chart of MoS2/LDH nanocomposites. Fig 4-A is the total spectrum of the material, it can be seen in Fig 4-a that the Zn, Al, Mo and S element are obvious present in material, and the presence of C 1s may be caused by CO2 contamination in the air and by the exogenous carbon of the calibration spectrum, which is very common in the XPS of LDH. The peak of Mo 3d is shown at 232.3eV, indicating that MoO42- was successfully inserted between the hydrotalcite layers. Fig 4-B is the spectrum of O element in composite, it can be seen that there are mainly two forms of Mg-O-Al and O2-. Fig 4-C is the spectrum of Mo element in composite, since there are MoS2 in the 1T and 2H phases, there are 3d peaks of two sets of Mo, the peaks at 228.8 and 232.2 eV are assigned to the 1T phase, while the peaks at 230.1 and 233.5 eV are assigned to the 2H phase. The weak peak at 235.8 eV is unreacted MoO42- and the medium strong peak at 226.9 eV is S 2s. And as is shown in Fig 4-D, there are also two 2p peaks of S, the peaks at 161.9eV are assigned to the 1T phase, while the peaks at 162.3eV are assigned to the 2H phase. After the integral calculation, the proportion of 1T-MoS2 is about 68.3%, the reason for the high proportion of 1T phase is the confinement effect between the layers

of hydrotalcite.

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3.4. TEM

Fig. 5. TEM images of LDH (a, b), MoS2(c, d), MoS2/LDH composite (e, f)

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TEM images of different materials are shown in Fig 5. The Fig 5-b and Fig 5-d show that the interlayer spacing of LDH and MoS2 are 0.21nm and 0.62 nm, respectively. Fig 5-e is a low resolution and high resolution TEM image of a MoS2/ZnAl-LDH composite. It can be seen from Fig 5-e that the thickness of the composite material is thin, and the thin layer structure of MoS2 is uniformly dispersed on the surface of the LDH plate layer. It can be seen from the high-resolution TEM photograph (Fig 5-f) that the layer thickness of the unpeeled material is about 0.86 nm, which is larger than the layer spacing of pure MoS2 and ZnAl-LDH. Moreover, it can be seen that during the stripping process of the composite material, the interlayer spacing is significantly larger and dispersed, and more defect sites are formed, and it’s considered to greatly improve the photocatalytic ability.

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3.5. BET and Particle size analysis

Fig. 6. BET spectrum of MoS2/ZnAl-LDH

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In order to further understand the pore structure and specific surface area of the composite material MoS2/ZnAl-LDH, the material was tested by N2 adsorption and desorption. As can be seen from Fig. 6-A, MoS2/ZnAl-LDH conforms to the type IV adsorption isotherm model, and a hysteresis loop of the H3 type appears. The curve of MoS2/ZnAl-LDH increases steadily in the low pressure stage, in which gas molecules are adsorbed on the outer surface of the material. When P/P0 = 0.5~1.0, the adsorption capacity increases sharply, which is due to the capillary phenomenon caused by the internal gap, and the pore size concentrates at 10-30 nm. The specific surface area of ZnAl-LDH is 33.07 m2/g. Figure B shows that the pore structure is small and basically in the mesoporous range. The above results show that after reduction of molybdate by thiourea and hydrazine hydrate, the interlayer of LDH is opened, which exposes more contact surfaces and greatly increases the adsorption capacity of N2. This open structure is undoubtedly a great help for improving the catalytic ability of materials.

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Fig. 7. Average particle size of LDH、MoS2 and MoS2/LDH

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Hydrotalcite materials with different crystallization times, pure phase MoS2 and MoS2/LDH composites were measured by laser particle size analyzer. It can be seen from Fig 7 that as the crystallization time is prolonged, the average particle size of the hydrotalcite material is also increasing, which corresponds to the continuous growth of the crystal in the solution. At the same time, it should be noted that the particle size of MoS2 is much smaller than that of hydrotalcite, while the particle size of MoS2/LDH is the smallest. Based on the previous tests, we analyzed that the multi-layered hydrotalcite was stripped into a few layers or even a single layer hydrotalcite structure due to the growth of molybdenum disulfide in its interlayer. Due to the limited structure of hydrotalcite, molybdenum disulfide attached to the surface of hydrotalcite in a single layer or in a spot shape, so its average particle size is greatly reduced.

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3.6. UV-Vis and PL

Fig 8. UV-Vis diffuse reflectance spectra for different catalysts(A), PL spectroscopy for different catalysts(B)

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Fig 8-A shows the UV-Vis diffuse reflection spectra of three materials. The MoS2 maintain a high absorption during the whole wavelength. The unreduced LDH material has strong absorption in the near-ultraviolet portion, but as the wavelength of light increases, the absorption capacity of the unreduced LDH material gradually decreases, and maintains an equilibrium state when approaching 0. The LDH material absorbs weakly in the visible light range, but after being reduced, the absorption capacity of the MoS2/LDH composite material gradually increases, and finally approaches 1, indicating that it has a strong absorption capacity in the visible light portion. This is because the LDH material is peeled off by MoS2, and since the two are combined bonds, the band structure is changed, so that the photocatalytic ability thereof is greatly improved. The results of PL spectroscopy in Fig 8-B shows that the MoS2/LDH composite has a strong PL emission at about 656 nm, and the bulk MoS2 shows negligible PL emission, indicating that there are more defects in the MoS2/LDH composite.

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The photocatalytic activity of the material was tested by adding 40 mg catalyst and using 200 ml methylene blue solution with 20 mg/L concentration. From Fig 9 we know the LDH material has almost no adsorption property to methylene blue. Due to the positive electrical conductivity of the LDH material, LDH has strong adsorption capacity to the anion, and the weak adsorption performance to the cationic dye. The MoS2 material has good adsorption capacity for cations, so the MoS2/LDH material has a certain adsorption capacity for methylene blue, which is also beneficial to photocatalysis.

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Fig 9 Adsorption of methylene blue by LDH and MoS2/LDH in dark conditions

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The different metal molar ratios also change the structure and composition of the hydrotalcite, thereby affecting its photocatalytic properties. We prepared three different molar ratios of molybdate intercalated hydrotalcites of ZnAl, Zn2Al and Zn3Al. Thiourea: molybdate 5:1, hydrothermal time 12 h, temperature 200 ° C, and the obtained material is used for the degradation of methylene blue. It can be seen from the results of Fig 10 that as the amount of Zn added increases, the degradation ability of the catalyst to methylene blue is enhanced, indicating that the photocatalytic ability is also improved, this is related to the fact that Zn is the active site of photocatalysis in composite materials. But when the mole ratio reaches 4:1, photocatalytic performance will be greatly reduced, implying excessive molar ratio will destroy the crystal structure, the results are consistent with XRD characterization results.

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Fig 10 Effect of metal molar ratio on photocatalytic ability

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It can be found from Fig 11 that the best catalytic ability is obtained when the ratio is 1:5, and the obtained material has a black powder shape and is delicate in hand. When the ratio is 1:2, it also has catalytic ability, but its adsorption and photocatalytic ability are lower than that of 1:5. When the ratio is 1:10, the material has good adsorption performance, but the photocatalytic performance is weak, and the product is more granular. We speculate that the excessive addition of thiourea causes the reaction of Zn and thiourea, and part of ZnS is formed. Although ZnS is also a photocatalytic material, the photodegradability is not as high as MoS2/LDH. Therefore, we choose a ratio of molybdate to thiourea of 1:5.

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Fig 11 Effect of ratio of molybdate to thiourea on photocatalytic ability

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Different effect of initial pH on photocatalytic ability is shown in Fig 12, it is shown that the photocatalytic effect of the composite under alkaline conditions is weaker than under acidic conditions, which may be due to the fact that methylene blue exhibits a quinoid structure in an acidic solution and an azo structure in an alkaline solution. Since ·OH, ·H and eap - can destroy the conjugated system of methylene blue, leading to its degradation and decolorization. In addition, ·H and eap - can destroy the conjugated system of methylene blue more effectively than · OH, and quinone structure is easier to be reduced than azo structure. Therefore, methylene blue is easier to be photocatalytically decomposed in acidic solution than in alkaline solution.

Fig 12 Effect of initial pH on photocatalytic ability

The degradation rate of methylene blue was tested using different

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catalyst amounts. It can be seen from Fig 13 that the slope of the degradation curve increases as the amount of photocatalyst increases. In theory, the more photocatalysts were added, the more photogenerated electrons and holes will be produced, and the more photocatalytic ability will be. However, the excessive amount of photocatalyst will cause light to scatter, thus reducing its degradation efficiency. This leads to the degradation curves of 40 mg and 80 mg are close. Therefore, in this experiment, the optimal photocatalyst was 40 mg. As is shown in Fig 14, by adding 35 mg LDH+5 mg MoS2, MoS2/LDH composites and LDH, the degradation rate of methylene blue increased after doping MoS2, but it was not significant. And the photocatalysis performance of bulk MoS2 is even better than the physical mixing of LDH and MoS2. While the degradation rate of methylene blue was significantly enhanced by MoS2/LDH composites, which proved that MoS2/LDH composites were not pure physical mixtures, but exist chemical bonds, the combination of the two catalyst has played a synergistic role in photocatalysis.

Fig 13 Effect of composites dosage on photocatalytic ability

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Fig 14 Photocatalytic ability of different catalyst

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We examined the cyclic degradation experiments of composite materials. It can be seen from Fig. 15 that after three degradation experiments, the prepared composites still have the ability to adsorb and degrade dyes, but the effect is greatly reduced. It is speculated that the adsorption of the material reached saturation during the experiment, and part of the molybdenum disulfide in the composite was detached from the hydrotalcite due to vigorous stirring, and some active sites were inactivated during the long-term photoreaction reaction.

Fig 15 Cyclic degradation of MoS2/LDH composites

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5. Discussion

Fig. 16. Schematic diagram of photocatalytic degradation mechanism of MoS2/ZnAl-LDH composite

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After characterization and photocatalysis experiments, we consider that the photocatalytic activity of MoS2/LDH composites is mainly due to the formation of a heterostructure between MoS2 and ZnAl-LDH.And hydrotalcite can limits the growth space of MoS2, it is beneficial to form 1T MoS2 (about 68.3%), while 1T MoS2 is a highly conductive and highly active metal phase with more active sites[37]. Schematic diagram of

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photocatalytic degradation mechanism of MoS2/ZnAl-LDH composite can be described as Fig. 16. The band gap of MoS2 and ZnAl-LDH is tested to be 1.17eV and 3.495eV respectively[3840].The conduction band (CB) and valence band (VB) can be obtained formula: 𝐸𝐶𝐵 = χ − 𝐸𝑒 − 0.5𝐸𝑔

and

𝐸𝑉𝐵 = 𝐸𝐶𝐵 +

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𝐸𝑔 ,where ECB is the CB edge potential, χ is the electronegativity of

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semiconductors, Ee is the energy of free electrons on the hydrogen scale (∼4.5 eV),Eg is the band gap and the EVB is the VB edge potential[41]. The χ of MoS2 is 5.32 [37, 41]. So the CB and VB edge potential of MoS2 can be calculated to be 0.235 and 1.405. And the CB and VB edge potential of ZnAl-LDH can be found in the literature

to be -1.12 and 2.375[39]. As the CB and VB of ZnAl-LDH are both different with those of MoS2, the photo-induced electrons at the CB of ZnAl-LDH will transfer into the CB of MoS2 while the photo-

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induced holes at the VB of MoS2 will transfer into the VB of ZnAlLDH. The photoelectrons produced by MoS2 are transferred to the conduction bands of the ZnAl-LDH lamellae through the heterojunction interface, and then migrate to the surface to combine with the adsorbed oxygen molecules to form highly catalytic O2-. The photogenerated holes produced by the two components are also transferred to the valence band of MoS2 nanosheets through heterojunction structure. The photogenerated holes migrate to the surface of MoS2 nanosheets, and form OH· and HO2· free radicals with H2O and OH-. The composite material can increase the lifetime of the photogenerated carriers, and enhances the photocatalytic ability. 5. Conclusion

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We prepared Molybdenum disulfide intercalated hydrotalcite composite photocatalytic materials by coprecipitation-hydrothermal reaction. It is a heterojunction structure with higher specific surface area and smaller size, which can inhibit carrier recombination. XRD, SEM, XPS, UV-Vis and other characterization confirm the formation of this structure. In the photocatalytic experiment, we conclude that the best synthesis condition is Zn to Al is 3:1 and molybdate to thiourea is 1:5. Composite materials have better photodegradation under acidic conditions and can reach 93% when pH is 4. The optimum catalyst dosage is 40mg. Through comparative experiments, it is confirmed that the synthesized composite material has better photodegradation effect than the physical mixing of the two materials, explain that this composite material is indeed beneficial to the improvement of MoS2 performance. And after three cycles of experiments, the material still has obvious photocatalytic properties. Experiments have confirmed that MoS2/LDH composites has a good photodegradation effect on methylene blue. The electron-hole pairs generated under illumination can form active oxygen and active hydrogen, then degrade organic groups. In summary, the composite materials synthesized in this experiment have great potential in the field of photocatalysis, and have good application prospects in wastewater treatment.

Declaration of interests The authors declare that they have no known competing financial

interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which

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may be considered as potential competing interests:

ACKNOWLEDGMENTS

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This research was supported by the National Natural Science Foundation of China (No. 21776320) and Hunan Provincial Natural Science Foundation of China (No. 2018JJ2489), the Hunan Provincial Science and Technology Plan Project (No. 2016TP1007). Referances 1. M. Hua, S.J. Zhang, B.C. Pan, W.M. Zhang, L. Lv, Q.X. Zhang, Heavy metal removal from water/wastewater by nanosized metal oxides: A review. J. Hazard. Mater. 211(2012)317-331. 2. I. Michael, L. Rizzo, C.S. McArdell, C.M. Manaia, C. Schwartz, C. Dagot, D. Fatta-Kassinos, Urban wastewater treatment plants as hotspots for the release of antibiotics in the environment: A review. Water Res. 47(3)(2013)957-995. 3. V.K. Gupta, Suhas, Application of low-cost adsorbents for dye removal-A review. J. Environ. Manage. 90(8)(2009)2313-2342. 4. A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, Adsorptive removal of hazardous anionic dye "Congo red" from wastewater using waste materials and recovery by desorption. J. Colloid Interface Sci. 340(1)(2009)16-26. 5. A. Srinivasan, T. Viraraghavan, Decolorization of dye wastewaters by biosorbents: A review. J. Environ. Manage. 91(10)(2010)1915-1929. 6. A. Ahmad, S.H. Mohd-Setapar, C.S. Chuong, A. Khatoon, W.A. Wani, R. Kumar, M. Rafatullah, Recent advances in new generation dye removal technologies: novel search for approaches to reprocess wastewater.

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