Rapid Solar-Light Driven Superior Photocatalytic Degradation of Methylene Blue Using MoS2-ZnO Heterostructure Nanorods Photocatalyst

Herein, MoS2-ZnO heterostructure nanorods were hydrothermally synthesized and characterized in detail using several compositional, optical, and morphological techniques. The comprehensive characterizations show that the synthesized MoS2/ZnO heterostructure nanorods were composed of wurtzite hexagonal phase of ZnO and rhombohedral phase of MoS2. The synthesized MoS2/ZnO heterostructure nanorods were used as a potent photocatalyst for the decomposition of methylene blue (MB) dye under natural sunlight. The prepared MoS2/ZnO heterostructure nanorods exhibited ~97% removal of MB in the reaction time of 20 min with the catalyst amount of 0.15 g/L. The kinetic study revealed that the photocatalytic removal of MB was found to be in accordance with pseudo first-order reaction kinetics with an obtained rate constant of 0.16262 min−1. The tremendous photocatalytic performance of MoS2-ZnO heterostructure nanorods could be accredited to an effective charge transportation and inhibition in the recombination of photo-excited charge carriers at an interfacial heterojunction. The contribution of active species towards the decomposition of MB using MoS2-ZnO heterostructure nanorods was confirmed from scavenger study and terephthalic acid fluorescence technique.


Introduction
Water pollution is a grave environmental concern and perilous to the exquisite essence of the environment [1]. The release of noxious, persistent, carcinogenic, and mutagenic organic pollutants into the water bodies adversely affects the quality of water [2][3][4]. The contaminants discharged from numerous industries disturb the aquatic ecosystem by blocking the penetration of solar light into water, and thereby diminishing the photosynthetic activity of marine organisms [5][6][7][8]. Methylene blue (MB), a cationic synthetic dye, was invented by Caro in 1878 and it contains a heterocyclic aromatic structure with the molecular formula, C 16 H 18 N 3 SCl. It is extensively utilized for dyeing
ZnO nanoparticles: ZnO nanoparticles were hydrothermally synthesized using Zn(CH 3 COO) 2 ·2H 2 O and NaOH. In the synthesis procedure, 0.25 M (5.487 g) of Zn(CH 3 COO) 2 ·2H 2 O was added to 100 mL of double distilled water and continuously stirred for 30 min. Afterwards, a pH of 11 was maintained using 1 M NaOH solution and the suspension was poured into a Teflon-lined stainless steel autoclave kept at 100 • C for 15 h. The prepared ZnO was filtered and thorough washing was done with water/ethanol mixture and dried in an electric oven.
MoS 2 -ZnO heterostructure nanorods: MoS 2 -ZnO heterostructure nanorods synthesis was performed via the hydrothermal approach. For the preparation of MoS 2 /ZnO heterostructure nanorods, 4 mmol (0.967 g) of Na 2 MoO 4 ·2H 2 O and 10 mmol (0.761 g) of CH 4 N 2 S were added separately to 35 mL of distilled water under continuous stirring. Consequently, 1.6 g of as-formed ZnO nanoparticles was dispersed in it and the suspension was magnetically stirred. After the stirring process, the suspension was transferred to Teflon-lined stainless steel autoclave for hydrothermal treatment, which was heated to 180 • C for 38.5 h. After completing the hydrothermal reaction, the vessel was cooled to room temperature and the obtained product was filtered and washed with alcoholic mixtures. The material was finally dried in an electric oven and then ground in order to obtain the fine powder.
Characterizations: The phase structure was determined by PANalytical X'Pert PRO diffractometer (Malvern Panalytical, Almelo, The Netherlands) and X-ray diffraction (XRD) patterns were obtained in the 2θ range from 20 • -80 • using Cu Kα radiations (λ = 1.54056 Å). The identification of functional groups was performed on a Nicolet iS50 FT-IR (Thermo Scientific, Waltham, MA, USA) spectrophotometer. The fluorescence (FL) spectrophotometer (Hitachi F-7000, Hitachi, Tokyo, Japan) was used for the measurement of FL emission spectrum at a room temperature. The diffuse reflectance spectrum (DRS) was taken on a UV-2600 (Shimadzu, Kyoto, Japan) UV-vis spectrophotometer using BaSO4 as a reference material. The general morphologies were observed by transmission electron microscope (TEM; Hitachi H-7500, Tokyo, Japan). The high resolution transmission electron microscope (HRTEM) and selected area electron diffraction pattern (SAED) of MoS2-ZnO heterostructure were analyzed by FEI Technai F20 microscope (FEI, Eindhoven, The Netherlands). The specific surface area was obtained from nitrogen adsorption-desorption isotherm using a Quantachrome Nova 2000e BET (Boynton Beach, FL, USA) and pore size distribution analyzer. The sample was pre-heated at 150°C prior to BET measurements. The ultraviolet-visible (UV-vis) absorbance spectra were collected on a Systronics-2202 UV-vis spectrophotometer (Uvsar India, Ghaziabad, India).

Photocatalytic Degradation of MB Based on MoS 2 -ZnO Heterostructure Nanorods
The catalytic ability of the MoS 2 -ZnO heterostructure nanorods was explored for the decomposition of MB dye under natural sunlight. The light intensity of 65-75 Klux (Latitude 30 • 45 34 N and Longitude 76 • 46 14 E) was recorded on a CHY-332 digital light meter. For photocatalytic reactions, a specific amount of catalyst was dispersed into 100 mL of MB solution and kept in a dark environment under stirring for 30 min to achieve an adsorption/desorption equilibrium. Then, the solution was placed under solar light irradiation and aliquots (2 mL) were extracted from the beaker at specified time intervals. The catalyst was separated by filtering the solution through a 0.45 µm Chromafil syringe filter and the absorbance of the filtrate was monitored at λ max = 664 nm on a UV-vis spectrophotometer. The degradation (%) was computed from Equation (1): where, C 0 is the MB concentration at the initial state and C is the MB concentration after illuminating with solar light at a specified time.
The role of reactive oxygen species in the removal of MB using MoS 2 /ZnO heterostructure nanorods was ascertained from a scavenger study. For this, 0.01 M of scavengers, such as HCOOH, KI, NaCl, and IPA, were added into the MB solution prior to the addition of the MoS 2 /ZnO heterostructure. The role of hydroxyl radicals (·OH) was validated using the terephthalic acid fluorescence technique. For the photocatalytic reaction, 5 × 10 −4 M of TPA was added to 2 × 10 −3 M NaOH solution. Then, 0.15 g/L of the synthesized catalyst, i.e., MoS 2 /ZnO heterostructure, was then dispersed into 100 mL of TPA solution and it was placed under solar light. At specified time periods, samples were collected, filtered through Chromafil syringe filter, and measured using FL spectrophotometer at λ exc = 315 nm.

Characterization of As-Formed ZnO Nanoparticles and MoS 2 -ZnO Heterostructure Nanorods
The purity and structural characteristics of the synthesized materials were studied by the XRD technique. Figure 1 [44]. The XRD diffractogram of MoS 2 -ZnO heterostructure nanorods clearly revealed the existence of individual components of MoS 2 and ZnO in the synthesized heterostructure nanorods. The crystallite size of ZnO and MoS 2 -ZnO heterostructure were computed from the most intense peak of the XRD pattern using Scherrer's equation (Equation (2)): where, D is the crystallite size, λ refers to the wavelength of X-rays, β belongs to the full width at the half maxima value, and θ is considered as the Bragg's angle. According to the above equation, the calculated crystallite size of pure ZnO nanoparticles and MoS 2 -ZnO heterostructure nanorods werẽ 38.45 nm and 33.52 nm, respectively. The morphological features of synthesized ZnO and MoS2-ZnO heterostructure were investigated using TEM. Figure 2a,b depicts the TEM micrographs of bare ZnO, which clearly demonstrate that the prepared sample possesses particle like morphologies, and, due to nanosize dimensions, it was termed as "nanoparticles". Further, because of the dense growth, some agglomeration in the nanoparticles is also seen in the micrographs.   The morphological features of synthesized ZnO and MoS 2 -ZnO heterostructure were investigated using TEM. Figure 2a,b depicts the TEM micrographs of bare ZnO, which clearly demonstrate that the prepared sample possesses particle like morphologies, and, due to nanosize dimensions, it was termed as "nanoparticles". Further, because of the dense growth, some agglomeration in the nanoparticles is also seen in the micrographs. The morphological features of synthesized ZnO and MoS2-ZnO heterostructure were investigated using TEM. Figure 2a,b depicts the TEM micrographs of bare ZnO, which clearly demonstrate that the prepared sample possesses particle like morphologies, and, due to nanosize dimensions, it was termed as "nanoparticles". Further, because of the dense growth, some agglomeration in the nanoparticles is also seen in the micrographs.        The presence of various functional groups was examined by FT-IR spectroscopy. Figure 5 displays the FT-IR patterns of bare ZnO nanoparticles and MoS 2 /ZnO heterostructures. The FT-IR pattern of ZnO nanoparticles displayed well defined absorption peaks at 539, 886, 1405, 1634, and 3390 cm −1 . The characteristic peaks observed at 539 cm −1 and 886 cm −1 were due to the stretching and bending vibrating modes of Zn-O, respectively [45,46]. A peak at 1405 cm −1 might be accredited to the C=O bonding [47]. The broad peaks at 1634 cm −1 and 3390 cm −1 were related to the bending and stretching frequencies of the hydroxyl group, respectively [48,49]. The distinct FT-IR peaks of the MoS 2 /ZnO heterostructure were observed at 435, 568, 686, 864, 1397, 1502, and 3398 cm −1 . A peak of Mo-S stretching vibration mode was found at 435 cm −1 [47,50]. A peak at 686 cm −1 might be related to the asymmetric vibration of the Mo-O group [51]. The peaks at 568 cm −1 and 864 cm −1 revealed the existence of stretching and bending vibration modes of Zn-O bond, respectively [46,49]. The bands observed at 1397 cm −1 and 1502 cm −1 corresponded to the C=O and C-O absorption, respectively [47,52]. A peak at 3398 cm −1 was due to the presence of surface bounded water molecules [49]. Thus, the FT-IR spectrum of MoS 2 /ZnO heterostructures indicated the successful coupling of MoS 2 and ZnO in the prepared heterostructure. The presence of various functional groups was examined by FT-IR spectroscopy. Figure 5 displays the FT-IR patterns of bare ZnO nanoparticles and MoS2/ZnO heterostructures. The FT-IR pattern of ZnO nanoparticles displayed well defined absorption peaks at 539, 886, 1405, 1634, and 3390 cm −1 . The characteristic peaks observed at 539 cm −1 and 886 cm −1 were due to the stretching and bending vibrating modes of Zn-O, respectively [45,46]. A peak at 1405 cm −1 might be accredited to the C=O bonding [47]. The broad peaks at 1634 cm −1 and 3390 cm −1 were related to the bending and stretching frequencies of the hydroxyl group, respectively [48,49]. The distinct FT-IR peaks of the MoS2/ZnO heterostructure were observed at 435, 568, 686, 864, 1397, 1502, and 3398 cm −1 . A peak of Mo-S stretching vibration mode was found at 435 cm −1 [47,50]. A peak at 686 cm −1 might be related to the asymmetric vibration of the Mo-O group [51]. The peaks at 568 cm −1 and 864 cm −1 revealed the existence of stretching and bending vibration modes of Zn-O bond, respectively [46,49]. The bands observed at 1397 cm −1 and 1502 cm −1 corresponded to the C=O and C-O absorption, respectively [47,52]. A peak at 3398 cm −1 was due to the presence of surface bounded water molecules [49]. Thus, the FT-IR spectrum of MoS2/ZnO heterostructures indicated the successful coupling of MoS2 and ZnO in the prepared heterostructure. The FL technique provides valuable information about the direct recombination of photoinduced charge carriers. Figure 6a illustrates the FL plots of ZnO nanoparticles and MoS2/ZnO heterostructure. Upon excitation at 290 nm, pure ZnO displayed emission peaks at 472 nm (bluegreen band), 498 nm, and 533 nm (green band). An emission peak located at 472 nm was assigned to the band edge bound excitons [53]. The FL emission peaks at 498 nm and 533 nm were related to the defect emissions of ZnO, oxygen interstitials, and surface defects [38,49,54,55]. It was observed that FL emission intensity for the MoS2/ZnO heterostructure was remarkably quenched with respect to bare ZnO nanoparticles, depicting the reduction in the recombination rate of electron and hole pairs and thereby being responsible for the outstanding photocatalytic activity of the prepared MoS2/ZnO heterostructure.
The photo-absorption tendency of as-formed materials was explored using UV-vis DRS spectroscopy. The band gap was measured from the classical Tauc's relation using Equation (3): where α, h, ν, A, and Ebg represent the absorption coefficient, Planck's constant, frequency of light, proportionality constant, and energy band gap, respectively. The energy band gap was found to be 3.22 eV and 3.12 eV, respectively, for ZnO nanoparticles and the MoS2/ZnO heterostructure ( Figure  6b). A decrease in the band gap was observed for the MoS2/ZnO heterostructure with respect to pure ZnO nanoparticles, indicating the better photo-absorption capacity of the prepared heterostructure. The FL technique provides valuable information about the direct recombination of photoinduced charge carriers. Figure 6a illustrates the FL plots of ZnO nanoparticles and MoS 2 /ZnO heterostructure. Upon excitation at 290 nm, pure ZnO displayed emission peaks at 472 nm (blue-green band), 498 nm, and 533 nm (green band). An emission peak located at 472 nm was assigned to the band edge bound excitons [53]. The FL emission peaks at 498 nm and 533 nm were related to the defect emissions of ZnO, oxygen interstitials, and surface defects [38,49,54,55]. It was observed that FL emission intensity for the MoS 2 /ZnO heterostructure was remarkably quenched with respect to bare ZnO nanoparticles, depicting the reduction in the recombination rate of electron and hole pairs and thereby being responsible for the outstanding photocatalytic activity of the prepared MoS 2 /ZnO heterostructure.
The photo-absorption tendency of as-formed materials was explored using UV-vis DRS spectroscopy. The band gap was measured from the classical Tauc's relation using Equation (3): where α, h, ν, A, and E bg represent the absorption coefficient, Planck's constant, frequency of light, proportionality constant, and energy band gap, respectively. The energy band gap was found to be 3.22 eV and 3.12 eV, respectively, for ZnO nanoparticles and the MoS 2 /ZnO heterostructure (Figure 6b).
A decrease in the band gap was observed for the MoS 2 /ZnO heterostructure with respect to pure ZnO nanoparticles, indicating the better photo-absorption capacity of the prepared heterostructure.
The textural properties of the as-synthesized MoS 2 /ZnO heterostructure were studied using N 2 adsorption/desorption isotherm and the pore size distribution (Figure 7a,b). The specific surface area and the total pore volume of the MoS 2 /ZnO heterostructure were estimated to be 8.132 m 2 /g and 6.590 × 10 −2 cm 3 /g, respectively. The density functional theory (DFT) method was opted to find the average pore diameter and it was found to be around 2.74 nm for the synthesized MoS 2 /ZnO heterostructure. The textural properties of the as-synthesized MoS2/ZnO heterostructure were studied using N2 adsorption/desorption isotherm and the pore size distribution (Figure 7a,b). The specific surface area and the total pore volume of the MoS2/ZnO heterostructure were estimated to be 8.132 m 2 /g and 6.590 × 10 −2 cm 3 /g, respectively. The density functional theory (DFT) method was opted to find the average pore diameter and it was found to be around 2.74 nm for the synthesized MoS2/ZnO heterostructure.

Investigation of MoS2/ZnO Heterostructure for the Removal of MB under Natural Sunlight
The catalytic performance of the synthesized MoS2/ZnO heterostructure was measured for the oxidation of MB dye under natural solar light. A chain of experiments was conducted to examine the impact of pH on the decomposition of MB by altering the pH from 3 to 11 using a catalyst dose of 0.15 g/L and substrate concentration of 10 mg/L as shown in Figure 8a. It was found that MB removal was enhanced from 63% to 66% with the increase in pH from 3 to 6. The removal efficiency was enhanced up to 78% after changing the pH to 9. The maximum decomposition rate for MB was found at pH 11 and about 97% decomposition of MB was acquired in the time period of 20 min. The impact of catalyst loading towards oxidation of MB was investigated by performing the reactions with different MoS2/ZnO heterostructure doses (pH 11 and dye concentration 10 mg/L). From Figure 8b, The textural properties of the as-synthesized MoS2/ZnO heterostructure were studied using N2 adsorption/desorption isotherm and the pore size distribution (Figure 7a,b). The specific surface area and the total pore volume of the MoS2/ZnO heterostructure were estimated to be 8.132 m 2 /g and 6.590 × 10 −2 cm 3 /g, respectively. The density functional theory (DFT) method was opted to find the average pore diameter and it was found to be around 2.74 nm for the synthesized MoS2/ZnO heterostructure.

Investigation of MoS2/ZnO Heterostructure for the Removal of MB under Natural Sunlight
The catalytic performance of the synthesized MoS2/ZnO heterostructure was measured for the oxidation of MB dye under natural solar light. A chain of experiments was conducted to examine the impact of pH on the decomposition of MB by altering the pH from 3 to 11 using a catalyst dose of 0.15 g/L and substrate concentration of 10 mg/L as shown in Figure 8a. It was found that MB removal was enhanced from 63% to 66% with the increase in pH from 3 to 6. The removal efficiency was enhanced up to 78% after changing the pH to 9. The maximum decomposition rate for MB was found at pH 11 and about 97% decomposition of MB was acquired in the time period of 20 min. The impact of catalyst loading towards oxidation of MB was investigated by performing the reactions with different MoS2/ZnO heterostructure doses (pH 11 and dye concentration 10 mg/L). From Figure 8b

Investigation of MoS 2 /ZnO Heterostructure for the Removal of MB under Natural Sunlight
The catalytic performance of the synthesized MoS 2 /ZnO heterostructure was measured for the oxidation of MB dye under natural solar light. A chain of experiments was conducted to examine the impact of pH on the decomposition of MB by altering the pH from 3 to 11 using a catalyst dose of 0.15 g/L and substrate concentration of 10 mg/L as shown in Figure 8a. It was found that MB removal was enhanced from 63% to 66% with the increase in pH from 3 to 6. The removal efficiency was enhanced up to 78% after changing the pH to 9. The maximum decomposition rate for MB was found at pH 11 and about 97% decomposition of MB was acquired in the time period of 20 min. The impact of catalyst loading towards oxidation of MB was investigated by performing the reactions with different MoS 2 /ZnO heterostructure doses (pH 11 and dye concentration 10 mg/L). From Figure 8b, it was observed that on increasing the MoS 2 /ZnO heterostructure dose from 0.05 g/L to 0.15 g/L, the removal efficacy was enormously enhanced from 61% to 97%, owing to the existence of more reactive sites on the MoS 2 /ZnO heterostructure surface, resulting in the better adsorption of the dye molecules. it was observed that on increasing the MoS2/ZnO heterostructure dose from 0.05 g/L to 0.15 g/L, the removal efficacy was enormously enhanced from 61% to 97%, owing to the existence of more reactive sites on the MoS2/ZnO heterostructure surface, resulting in the better adsorption of the dye molecules. The degradation rate was decreased to 93% with increased catalyst loading up to 0.25 g/L, depicting the optimum MoS2/ZnO heterostructure amount to be 0.15 g/L for the catalytic reactions. The influence of the initial MB concentration towards the removal of MB was explored at optimized pH and catalyst dose conditions (Figure 8c). The removal rate of 97% was attained at 10 mg/L MB concentration. The degradation rates were diminished to 69% and 50% for 20 and 30 mg/L dye concentrations, respectively. Thus, from the series of experiments performed, it was clearly noticed that the maximum decomposition of MB was found at pH 11, catalyst amount of 0.15 g/L, and substrate concentration of 10 mg/L. Figure 8d depicts the UV-vis absorbance spectra for the oxidation of MB over the MoS2/ZnO heterostructure with respect to time (pH: 11, catalyst dose: 0.15 g/L, dye concentration: 10 mg/L). It was found that with the increase in illumination time, there was a reduction in the absorbance maximum (λmax = 664 nm) of MB and 97% degradation was accomplished in 20 min. A blank experiment was performed under sunlight without the addition of the MoS2/ZnO heterostructure. The effect of photolysis is shown in Figure 9a and insignificant decomposition (8%) of MB was observed in the reaction time of 20 min. An adsorption experiment was performed with the addition of the MoS2/ZnO heterostructure without sunlight illumination and the obtained results are described in Figure 9a. Approximately, a 32% removal of MB was achieved using the MoS2/ZnO The degradation rate was decreased to 93% with increased catalyst loading up to 0.25 g/L, depicting the optimum MoS 2 /ZnO heterostructure amount to be 0.15 g/L for the catalytic reactions. The influence of the initial MB concentration towards the removal of MB was explored at optimized pH and catalyst dose conditions (Figure 8c). The removal rate of 97% was attained at 10 mg/L MB concentration. The degradation rates were diminished to 69% and 50% for 20 and 30 mg/L dye concentrations, respectively. Thus, from the series of experiments performed, it was clearly noticed that the maximum decomposition of MB was found at pH 11, catalyst amount of 0.15 g/L, and substrate concentration of 10 mg/L. Figure 8d depicts the UV-vis absorbance spectra for the oxidation of MB over the MoS 2 /ZnO heterostructure with respect to time (pH: 11, catalyst dose: 0.15 g/L, dye concentration: 10 mg/L). It was found that with the increase in illumination time, there was a reduction in the absorbance maximum (λ max = 664 nm) of MB and 97% degradation was accomplished in 20 min. A blank experiment was performed under sunlight without the addition of the MoS 2 /ZnO heterostructure. The effect of photolysis is shown in Figure 9a and insignificant decomposition (8%) of MB was observed in the reaction time of 20 min. An adsorption experiment was performed with the addition of the MoS 2 /ZnO heterostructure without sunlight illumination and the obtained results are described in Figure 9a. Approximately, a 32% removal of MB was achieved using the MoS 2 /ZnO heterostructure. The kinetics of the reaction for the decomposition of MB over the MoS 2 /ZnO heterostructure was analyzed by a pseudo first-order kinetic model (Equation (4)): ln(C 0 /C) = kt (4) where C 0 and C are the concentrations of MB before and after exposure to natural sunlight, k is the reaction rate constant acquired from the slope of the graph, and t is the irradiation time for the reaction. A graph between ln(C 0 /C) and t described that the oxidation of MB using the MoS 2 /ZnO heterostructure was found to be fitted well with the pseudo first-order reaction kinetics model, and a rate constant of 0.16262 min −1 was computed (Figure 9b). heterostructure. The kinetics of the reaction for the decomposition of MB over the MoS2/ZnO heterostructure was analyzed by a pseudo first-order kinetic model (Equation (4)): ln(C0/C) = kt (4) where C0 and C are the concentrations of MB before and after exposure to natural sunlight, k is the reaction rate constant acquired from the slope of the graph, and t is the irradiation time for the reaction. A graph between ln(C0/C) and t described that the oxidation of MB using the MoS2/ZnO heterostructure was found to be fitted well with the pseudo first-order reaction kinetics model, and a rate constant of 0.16262 min −1 was computed (Figure 9b). The degradation extent of the MoS2/ZnO heterostructure was compared with pure ZnO nanoparticles and commercial TiO2 (PC-50) (Figure 10). The synthesized MoS2/ZnO heterostructure exhibited enhanced photocatalytic performance (97%) than pure ZnO (89%) and TiO2 PC-50 (83%) under identical reaction conditions.  heterostructure. The kinetics of the reaction for the decomposition of MB over the MoS2/ZnO heterostructure was analyzed by a pseudo first-order kinetic model (Equation (4)): ln(C0/C) = kt (4) where C0 and C are the concentrations of MB before and after exposure to natural sunlight, k is the reaction rate constant acquired from the slope of the graph, and t is the irradiation time for the reaction. A graph between ln(C0/C) and t described that the oxidation of MB using the MoS2/ZnO heterostructure was found to be fitted well with the pseudo first-order reaction kinetics model, and a rate constant of 0.16262 min −1 was computed (Figure 9b). The degradation extent of the MoS2/ZnO heterostructure was compared with pure ZnO nanoparticles and commercial TiO2 (PC-50) (Figure 10). The synthesized MoS2/ZnO heterostructure exhibited enhanced photocatalytic performance (97%) than pure ZnO (89%) and TiO2 PC-50 (83%) under identical reaction conditions.

Role of Reactive Oxygen Species for the Removal of MB Using MoS 2 /ZnO Heterostructure
Various quenchers were chosen to determine the role of active species involved in the photocatalytic decomposition of MB. Different quenchers, like KI (quencher for holes, h + , and surface bounded hydroxyl radicals, ·OH s ), NaCl (quencher for h + ), HCOOH (quencher for electrons, e − ), and IPA (quencher for hydroxyl radicals, ·OH), were chosen to study their inhibitory effects. The results for the MB degradation using different scavengers over synthesized MoS 2 /ZnO photocatalyst under solar light are described in Figure 11a. It was found that the photocatalytic efficiency was reduced from 97% (without scavenger) to 16.8%, 86.8%, 87%, and 61.4% upon addition of HCOOH, NaCl, IPA, and KI, respectively, which verified the pivotal contribution of e − , h + , ·OH, and ·OH s in the degradation process. The formation of ·OH during the course of the photocatalytic reaction was verified using the terephthalic acid fluorescence technique. The reaction of TPA with ·OH resulted in the generation of 2-hydroxyterephthalic acid (HTA), which is a highly fluorescent material, and showed an emission peak at around λ max = 425 nm. The FL emission intensity of HTA is directly proportional to the formation of ·OH. As shown in Figure 11b, there was an enhancement in FL emission intensity with the progress in the photocatalytic reaction, verifying the pivotal role of ·OH towards the oxidation of MB dye [43]. bounded hydroxyl radicals, ·OHs), NaCl (quencher for h + ), HCOOH (quencher for electrons, e − ), and IPA (quencher for hydroxyl radicals, ·OH), were chosen to study their inhibitory effects. The results for the MB degradation using different scavengers over synthesized MoS2/ZnO photocatalyst under solar light are described in Figure 11a. It was found that the photocatalytic efficiency was reduced from 97% (without scavenger) to 16.8%, 86.8%, 87%, and 61.4% upon addition of HCOOH, NaCl, IPA, and KI, respectively, which verified the pivotal contribution of e − , h + , ·OH, and ·OHs in the degradation process. The formation of ·OH during the course of the photocatalytic reaction was verified using the terephthalic acid fluorescence technique. The reaction of TPA with ·OH resulted in the generation of 2-hydroxyterephthalic acid (HTA), which is a highly fluorescent material, and showed an emission peak at around λmax = 425 nm. The FL emission intensity of HTA is directly proportional to the formation of ·OH. As shown in Figure 11b, there was an enhancement in FL emission intensity with the progress in the photocatalytic reaction, verifying the pivotal role of • OH towards the oxidation of MB dye [43].

Proposed Photocatalytic Degradation Mechanism
The conduction band (CB) and valence band (VB) potentials of MoS2 and ZnO can be calculated from Equations (5) and (6): where X refers to the absolute electronegativity of the material; Ee represents the energy of free electrons on a hydrogen scale (~4.5 eV); and Eg corresponds to the band gap. The value of X for MoS2 and ZnO was reported to be 5.32 eV and 5.79 eV in the literature [56,57]. Figure 12 depicts the band gap of pure MoS2 and it was measured to be around 1.35 eV. The CB and VB potentials for MoS2 were measured to be 0.14 eV and 1.49 eV, respectively [58]. The corresponding CB and VB potentials for ZnO were found to be −0.32 eV and +2.9 eV, respectively. The plausible mechanism for the catalytic oxidation of MB over the MoS2/ZnO heterostructure under solar light is depicted in Figure 13. When sunlight was irradiated on the MoS2/ZnO heterostructure, both MoS2 and ZnO could be excited to yield e − /h + pairs. The photogenerated e − will shift from the CB of ZnO to the CB of MoS2 as the CB potential of ZnO is more negative than that of MoS2. Also, the photoinduced h + will rapidly migrate from the VB of MoS2 to the VB of ZnO. The photoexcited e − were captured by adsorbed molecular oxygen (O2) to yield superoxide anion radicals (O2 − ). MoS2 can capture e − due to its high conductivity, and thus suppressed the recombination rate of photoexcited charge carriers [59,60]. The photo-generated h + also reacted with OH − or H2O,

Proposed Photocatalytic Degradation Mechanism
The conduction band (CB) and valence band (VB) potentials of MoS 2 and ZnO can be calculated from Equations (5) and (6): where X refers to the absolute electronegativity of the material; E e represents the energy of free electrons on a hydrogen scale (~4.5 eV); and E g corresponds to the band gap. The value of X for MoS 2 and ZnO was reported to be 5.32 eV and 5.79 eV in the literature [56,57]. Figure 12 depicts the band gap of pure MoS 2 and it was measured to be around 1.35 eV. The CB and VB potentials for MoS 2 were measured to be 0.14 eV and 1.49 eV, respectively [58]. The corresponding CB and VB potentials for ZnO were found to be −0.32 eV and +2.9 eV, respectively. The plausible mechanism for the catalytic oxidation of MB over the MoS 2 /ZnO heterostructure under solar light is depicted in Figure 13. When sunlight was irradiated on the MoS 2 /ZnO heterostructure, both MoS 2 and ZnO could be excited to yield e − /h + pairs. The photogenerated e − will shift from the CB of ZnO to the CB of MoS 2 as the CB potential of ZnO is more negative than that of MoS 2 . Also, the photoinduced h + will rapidly migrate from the VB of MoS 2 to the VB of ZnO. The photoexcited e − were captured by adsorbed molecular oxygen (O 2 ) to yield superoxide anion radicals (O 2 − ). MoS 2 can capture e − due to its high conductivity, and thus suppressed the recombination rate of photoexcited charge carriers [59,60]. The photo-generated h + also reacted with OH − or H 2 O, resulting in the production of ·OH, which are accountable for the decomposition of noxious organic contaminants. Thus, the prepared MoS 2 /ZnO heterostructure was utilized as a potent catalyst for the decomposition of MB under natural solar light. Different oxidation and reduction reactions (Equations (7)- (10)) that occurred over the MoS 2 /ZnO heterostructure surface under natural sunlight irradiation for the generation of reactive species are given below: resulting in the production of ·OH, which are accountable for the decomposition of noxious organic contaminants. Thus, the prepared MoS2/ZnO heterostructure was utilized as a potent catalyst for the decomposition of MB under natural solar light. Different oxidation and reduction reactions (Equations (7)- (10)) that occurred over the MoS2/ZnO heterostructure surface under natural sunlight irradiation for the generation of reactive species are given below: MoS2 (e − CB) + O2 → O2 − ZnO (h + VB) + OH − →·OH ·OH + MB → CO2 + H2O + simpler molecules (10)   resulting in the production of ·OH, which are accountable for the decomposition of noxious organic contaminants. Thus, the prepared MoS2/ZnO heterostructure was utilized as a potent catalyst for the decomposition of MB under natural solar light. Different oxidation and reduction reactions (Equations (7)- (10)) that occurred over the MoS2/ZnO heterostructure surface under natural sunlight irradiation for the generation of reactive species are given below: ZnO + hν → e − CB + h + VB (7) MoS2 (e − CB) + O2 → O2 − ZnO (h + VB) + OH − →·OH (9) ·OH + MB → CO2 + H2O + simpler molecules (10)

Conclusions
In summary, MoS 2 /ZnO heterostructure was prepared through a facile hydrothermal route and extensively characterized in detail by spectroscopic and analytical techniques. The synthesized MoS 2 /ZnO heterostructure showed excellent catalytic behavior for the decomposition of MB under natural solar light. Approximately, 97% decomposition of MB was obtained at pH 11 with a catalyst dose of 0.15 g/L. The FL intensity of MoS 2 /ZnO heterostructure was strongly quenched as compared to pure ZnO, thereby increasing the photochemical quantum efficiency of the heterostructure. The superior photocatalytic efficacy of the MoS 2 /ZnO heterostructure could be assigned to the effective charge transportation of photoinduced e − /h + pairs. The present study demonstrated that the MoS 2 /ZnO heterostructure can be employed as a marvelous photocatalytic material for the deterioration of noxious contaminants in terms of environmental restoration.