One-step hydrothermal synthesis of 2H-MoS 2 nanoflowers for efficient degradation of methylene blue and rhodamine B dyes under UV and visible-light irradiation: A comparative study

Molybdenum disulfide (MoS 2 ), with its low energy bandgap, plays an essential role in removing organic pollutants from wastewater via the mechanism of photocatalysis. In this paper, the 2H phase of MoS 2 nanoflowers (NFs) as a photocatalyst was synthesized by the facial one-step hydrothermal method. Various characterization techniques, such as X-ray diffraction


INTRODUCTION
In the last few years, the issue of water pollution has emerged as a significant worry for the survival of both humans and the natural environment [1].The rise in industrial development has resulted in the discharge of untreated, dangerous organic Over the past few decades, researchers have shown a high level of intrigue and interest in the field of environmental advancement with regard to semiconductor nanomaterials.These innovative materials have sparked considerable attention, especially when they are used for water purification, due to their exceptional characteristics, such as remarkable physical and chemical properties, minimal harm to living organisms, strong resistance to electrochemical processes, unique dimensions, and expansive surface coverage.Currently, transition metal dichalcogenides (TMDs), with their structural similarity to graphene, have sparked significant attention because of their structural, electronic, optical, and mechanical characteristics.Particularly, the electronic, catalytic, and resistance to photo-corrosion properties of these materials make them well-suited for numerous applications, including energy preservation and transformation, catalysis, and lubrication [18].These unique characteristics are related to its fundamental layered structure.MX 2 is the general formula for TMDs, where M represents a transition metal element from group IV (Ti, Zr, or Hf), group V (V, Nb, or Ta), or group VI (Mo or W) in the periodic table, and X represents a chalcogen element from group VI (S, Se, or Te) [19,20].
In spite of its structural similarity to graphene, MoS 2 exhibits unique semiconducting characteristics.In particular, the monolayer form of it possesses an energy bandgap of ~1.9 eV, which is considered a direct bandgap.In contrast, the bulk form of MoS 2 has an energy bandgap of 1.3 eV, which is classified as an indirect bandgap.This is in stark contrast to graphene, which does not possess any bandgaps [44].This change from an indirect to a direct bandgap is caused by the quantum confinement effect.Due to its bandgap, MoS 2 exhibits photoluminescence, making it an active photocatalyst in the visible light region.The as-prepared MoS 2 nanoparticles have exhibited impressive ability to photodegrade organic dyes because of several active edge locations, a high surface-to-volume ratio, and low energy bandgap.
MoS 2 nanoparticles can be synthesized by various techniques, such as the mechanical exfoliation method, liquid exfoliation method [45,46], chemical vapor deposition (CVD) method [47][48][49], sulfurization method [50], and hydrothermal method [48,51,52].However, the hydrothermal method is still one of the most effective methods for growing MoS 2 because it allows for the growth of various morphologies and the exposure of rich edge states [53,54].
In this work, pure MoS 2 NFs were prepared via a straightforward and effective hydrothermal route without using any surfactant or heat treatment and investigated with different characterization techniques.The MB and RhB dyes are degraded through photocatalysis using MoS 2 NFs photocatalyst under the illumination of two different light sources: ultraviolet (UV) and visible light.This research demonstrates, in a comparative way, the potential of MoS 2 as a versatile and effective photocatalyst against different dyes for environmental remediation.This provides empirical evidence that the photocatalytic activity of MoS 2 NFs depends on the wavelength of the light source (UV or visible) while keeping all the other experimental factors constant, such as the type and concentration of pollutant, amount of catalyst, intensity of the light source, and illumination exposure duration through the experiment.

Materials
All the chemicals used in these experiments were obtained from the ALPHA CHEMIKA company and utilized without any additional purification.Ultrapure distilled water was used throughout the whole experiment.Ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 •4H 2 O) was used as a source of Mo, and thiourea (CH 4 N 2 S) was used as a source of S. Two dyes were used in the photocatalysis experiments: MB dye (C 16 H 18 CIN 3 S) and RhB dye (C 28 H 31 ClN 2 O 3 ).The cleaning elements used are ethyl alcohol (CH 3 CH 2 OH) and diluted nitric acid (HNO 3 ).

Synthesis method of MoS 2 nanoflowers
MoS 2 NFs were synthesized using the facial hydrothermal route.In a typical synthetization, 1.76 g of (NH 4 ) 6 Mo 7 O 24 •4H 2 O and 1.52 g of CH 4 N 2 S were dissolved in 35 mL of ultrapure deionized water, and then the aqueous solution was kept stirring for 1 hour using a magnetic stirrer.Then, the solution is transferred to a 50-mL Teflon-lined hydrothermal reactor (autoclave) that is heated at 200 °C for 24 hours.After that, the autoclave cooled to ambient temperature, and the resulting black powder was washed a few times with CH 3 CH 2 OH and distilled water.Finally, the sample was dried at 80 °C for 48 hours.

Photodegradation activity measurements
The photocatalytic activities of the as-synthesized MoS 2 nanoparticles were carried out by monitoring the degradation of MB and RhB dyes under UV (8-watt with 365 nm wavelength) and visible light (150-watt halogen lamp) irradiation.The experiments took place in a 250-mL beaker at ambient temperature.In each experiment, 10 mg of the prepared MoS 2 photocatalyst powder was dispersed in 100 mL of MB or RhB aqueous solution, where the concentration was maintained at 10 -5 M for both dyes.The suspension was stirred for 30 minutes in the dark in order to achieve the adsorptiondesorption equilibrium between the catalyst and the used dye.Then, the solution was maintained under the selected light source (UV or visible light) while the mixture was stirred continuously.The experiments were conducted over a 180-minute period for the sample under UV and over a 60-minute period for the sample under visible light, where 3 mL of the mixture were collected at regular intervals in each case, centrifuged, and examined using a UV-vis spectrometer ranging from 400 to 750 nm.

Sample characterization
The XRD pattern of the prepared sample was detected using a Philips X-ray diffractometer of the 1710 PW model, which is dependent on CuK α radiation (wavelength =1.54184 Å) and operates at an applied voltage of 40000 V. Angles ranging from 5° to 80° were measured with a constant angle-scanning speed of 0.06° min -1 .FT-IR transmittance spectra were conducted with a NICOLET FT-IR 6700 spectrometer across the range of 400-4000 cm −1 for pellets samples mixed with KBr.To explore the morphological characteristics of the produced specimen, FE-SEM images were acquired using a ZIESS Sigma 500 VP scanning electron microscope.Optical absorbance and catalytic data were recorded for the suspension NFs using a Thermo Evolution 300 UVvisible spectrophotometer within a range of 300 to 800 nm.Estimation of the optical energy bandgap was carried out using the Tauc plot based on the UV-visible data.The elemental composition of the synthesized sample was determined using an EDX unit connected to a scanning electron microscope model, ULTRADRY QUANTA FEG 250 (Field Emission Gun).

X-ray diffraction
The phase and crystalline nature of the as-prepared sample without any heat treatment were analyzed using powder X-ray diffractometer.As shown in Figure 1, the pattern reveals reflections located at 2θ = 13.9º,29º, 32.6º, 39.46º, 49.78º, and 58.34º that were confirmed as (002), ( 004), ( 100), ( 103), (105), and (110) planes, respectively.As indexed in JCPDS file no.37-1492, these diffraction peaks in the XRD pattern are highly matched with those of the hexagonal structure of pure MoS 2 (2H-MoS 2 ) with space group p6 3 /mmc and a trigonal prismatic coordination between Mo and S atoms [55][56][57].No additional peaks were noticed in the XRD pattern, which depicts the high purity of the synthesized specimen.Additionally, the diffraction peaks' broadening suggests the existence of smaller nanodomains [58].The intense peak (002) that is the main peak of pristine MoS 2 , corresponding to 13.9º, indicates that the S-Mo-S layers are well stacked along the C axis [59].To gain a deeper understanding of the lattice parameters of MoS 2 , the Williamson-Hall (W-H) model displayed in Equation 1 was employed to determine the average grain size (D) and the microstrain (ε) of the nanocrystals [60].According to W-H, the diffraction line broadening is because of the crystallite size and strain contribution of the MoS 2 NFs (Equation 2), as the strain-induced broadening in MoS 2 NFs results from crystal imperfections and distortions.The peak broadening of the XRD reflections of the prepared sample includes the instrumental broadening and the particle size broadening, which can be expressed by Equation 3 [60]. (1) Where β is the full-width at half maximum of the diffracted peak measured in radians, k s is the shape factor assigned to be 0.94, λ is the wavelength of CuK α radiation, and θ is Bragg's angle in radians.
The strain in the aforementioned equation is known as a uniform deformation model (UDM), as it is thought to be uniform in all crystallographic directions.This model assumes that the crystal is naturally isotropic and that its characteristics are independent on the size of the crystallite (D).By plotting the values of β hkl cos(θ) on the Y-axis as a function of 4sin(θ) on the X-axis and using linear fitting, the Y-intercept was used to estimate D, and the slope of the linear fit was used to determine ε, as shown in Figure 2 [61].The expansion of the lattice, which causes intrinsic strain in the MoS 2 NFs, is indicated by the positive slope of the carve.This intrinsic strain determined from the slope is very low (0.070), which gives another confirmation of the smaller strain in the synthesized MoS 2 .5) and ( 6) can be used to calculate both dislocation density (δ) and theoretical density (ρ), respectively [63].All the XRD-obtained parameters are declared in Table 1.

Scanning electron microscopy
The surface morphology of the as-synthesized MoS 2 photocatalyst was investigated at different magnifications using the FE-SEM technique.As shown in Figure 3a, a typical low-magnification (3 k x) recorded image shows that pristine MoS 2 exhibits a flower-like architecture that is assembled from MoS 2 nanosheets.A highermagnification (40 k x) image is recorded, as shown in Figure 3b, which confirms that each flower-like structure is composed of thin sheets that are slightly curved.The observed average width of the MoS 2 nanosheets is about several hundred nanometers.
Vattikuti et al. [64] reported in their previous work that the MoS 2 sample synthesized by using thiourea as a source of sulfur has multilayer nanosheets of a few nanometers in size.The use of precursor materials, specifically ammonium molybdate and thiourea, is crucial in determining the shape of the MoS 2 through this reaction.In the hydrothermal process, ammonium molybdate produces MoO 4 -and ammonium ions, while thiourea serves as the source of sulfur.As MoO 4 -ions react with sulfur ions, MoS 2 is formed.The presence of remaining ammonia minimizes the stacking of MoS 2 nanostructures, resulting in a spherical, flower-like morphology through self-assembly [65].This particular morphology is frequently observed for MoS 2 [66].Ghaleghafi et al. [67] were able to synthesize MoS 2 in the form of flower-like microspheres composed of 2D nanosheets using the simple hydrothermal method, with a nanosheet thickness of about 8-62 nm.

Fourier Transform Infra-Red Spectroscopy
FT-IR spectroscopy, which is a non-destructive technique, was employed to acquire information about the bending and stretching vibrations of the functional groups found in the as-prepared MoS 2 NFs. Figure 4 describes the FT-IR spectra of the sample being examined, where there are eight absorption bands at 490 cm -1 , 570 cm -1 , 613 cm -1 , 915 cm -1 , 1090 cm -1 , 1400 cm -1 , 1630 cm -1 , and 3445 cm -1 .The absorption characteristic bands at 490 cm -1 , 570 cm -1 , and 613 cm -1 are due to the Mo-S bond stretching vibrations, and the characteristic weak band shown at 915 cm -1 is attributed to the S-S stretching bond [68][69][70].The stretching vibrations of the hydroxyl-group and Mo-O vibrations are accountable for the absorption bands located at 1090 cm -1 , 1400 cm -1 , and 1630 cm -1 , while the broad band centered around 3445 cm -1 is caused by the symmetrical stretching vibration of hydroxyls (O-H) [69][70][71][72].

Energy Dispersive X-Ray Spectroscopy
The high spatial resolution EDX spectra of the as-synthesized MoS 2 , depicted in Figure 5, provide clear evidence of the existence of Mo and S atoms as expected.These peaks show that there are no foreign impurities.Based on the EDX spectra, the peaks of Mo are observed at 2.47 keV, indicating L α and L β characteristic lines, while sulfur exhibits its peak at 2.48 keV, representing the K α characteristic line.These peaks overlap due to the close proximity of the energy levels of S and Mo [63,73].Also, as shown in the inset table of Figure 5, the EDX quantitative analysis depicts that the composition of Mo and S in the MoS 2 NFs is determined to be close to 1:2, which is very close to the theoretical value [74].A tiny quantity of oxygen (K α , 0.523 keV), if it exists at all, is indicated in the elemental map displayed in the detected spectra [75].

Ultraviolet Visible Spectroscopy
UV-visible absorption spectroscopy is an essential method of measurement that is necessary for characterizing the electronic band structures of different materials.The optical properties of pristine MoS 2 NFs suspended in distilled water have been analyzed using UV-vis absorption spectroscopy in the range of 300-800 nm. Figure 6a provides a visual representation of the absorbance spectra of the as-prepared MoS 2 .It can be observed that there are three distinct bands.The first band, located within the wavelength 2 or d xz-yz orbitals).This transition is assigned as C exciton, while the additional two absorption bands, seen at approximately 630 nm and 680 nm, occur due to the excitonic transitions of the first and second Brillouin zones at the K/K' points [67,69,[76][77][78].These two distinct optical transitions are assigned as A and B excitons [67].This suggests that MoS 2 nanoflowers exhibit a narrow optical band gap as a result of the quantum confinement effect [79].Equation 7 can be utilized to compute the absorption coefficient α [80].(7) Where the symbols ρ and A represent the theoretical density and the absorbance of the sample, respectively, while L, c, and m represent the distance light passing through the solution (assumed to be 1 cm), the molar mass, and the molar concentration, respectively.The E g of the MoS 2 photocatalyst can be determined through the utilization of the Tauc formula [81]: (8) Where α represents the absorption coefficient measured in cm -1 , h is Planck's constant measured in Joule second, ν is the photon's frequency measured in Hz, B is a constant that does not depend on the energy of the photons, and n is a constant with four possible values: 2, 1/2, 3, or 1/3.These four values correspond to direct allowed transitions, indirect allowed transitions, direct forbidden transitions, and indirect forbidden transitions, respectively.The value of E g can be determined by extending the linear portion of the graph where (αhν)² is plotted on the Y-axis against the photon energy (hν) on the X-axis to the X-axis itself (Figure 6b).For pure MoS₂ , E g is calculated to be 1.82 eV, showing strong consistency with earlier studies [82][83][84][85].This calculated bandgap makes the produced MoS 2 NFs very responsive to visible light and can be utilized as photocatalysts in the visible light region.

Photocatalytic study 3.6.1 Photocatalytic activity evaluation
To confirm its potential for ecological application in the removal of contaminants from wastewater, the degradation activity of the as-prepared MoS 2 NFs was assessed using MB and RhB dyes under UV and visible-light irradiation.The degradation process was observed by tracking the variance in the absorption intensity of the main peak of each dye as a function of the exposure time.The main absorption peak of RhB dye exists around 554 nm, while the main absorption peak exists around 664 nm for MB dye [86][87][88].As shown in Figure 7a, MB and RhB are very hard to degrade in the absence of the MoS 2 photocatalyst, which proves that the photolysis of MB and RhB can be neglected.Figures 7b-e  Where C 0 and C t represent the absorption values detected by the UV-visible spectrophotometer at 0 and t minutes, respectively.It is noted from the calculated degradation efficiency values as shown in Figure 8 that MoS 2 NFs are more efficient to degrade RhB than MB under UV illumination, where the efficiency was 84.31% in the case of RhB dye while it was 75.97% in the case of MB dye through 180 minutes However, when using visible light as a source of illumination, an opposite behavior was detected, and MoS 2 was more efficient to degrade MB (84.25%) than RhB (73.99%) through 60 minutes of irradiation.These results give a very strong empirical evidence that MoS 2 as a photocatalyst can have different efficiencies in removing specific types of pollutants depending upon the wavelength of the light source utilized, keeping all the other experimental factors constant.
Relative degradation is presented by the C t /C 0 ratio, as shown in Figure 9, when using UV and visible light as sources of irradiation, respectively.It is clear from Figure 9a that, in the case of using the UV lamp, the photocatalytic degradation speed of the MB dye is very close to that of the RhB dye, while the RhB dye is still faster, especially in the first 20 minutes of the irradiation period.Moreover, in the case of visible light, the MB dye degraded faster than that of RhB, as shown in Figure 9b.In addition, the Langmiur-Henshelwood (L-H) model [89], which is considered a pseudo first-order kinetic mode, is used to determine the recombination rate constant (γ) of the MB and RhB dyes using the given Equation [89]. (10)

Photocatalytic mechanism
The UV and visible light photocatalytic degradation activity of MB and RhB in the presence of MoS 2 NFs can be most effectively explained by the mechanism of the process, as shown in Figure 11.Firstly, due to the narrow energy bandgap of the MoS 2 NFs, both electrons (e -) and holes (h + ) are produced.Electrons in the CB interact with the available oxygen to form superoxide radicals ( • O 2 -), which react with H + ions to form H 2 O 2 , and finally H 2 O 2 decomposes into hydroxyl radicals ( • OH) [90].In the same manner, • OH radicals are created when holes in the VB react with water.Consequently, the • OH radicals produced effectively degrade the absorbed dyes at normal room temperature through a straightforward process of magnetic stirring.The generation of radicals, degradation of dyes, and production of products occur in the reactions represented in Equations 11 to 17 [69,71].In general, these pairs of electrons and holes tend to recombine together again, and only a small number of charges are moved to the reaction sites in order to convert H + to H 2 during the process of photocatalysis.Therefore, it is necessary to prevent the recombination of the charge pairs generated by light in order to increase the efficiency of photocatalysis [91].The position of the energy bandgap (CB and VB) in MoS 2 was determined by utilizing the subsequent mathematical formulas [92,93]: (18) (19) where E CB and E VB represent the CB and VB edge potentials, respectively, X represents the electronegativity of the semiconductor, which is equal to 5.32 eV in the case of MoS 2 , E e is the energy of unbound electrons on the hydrogen scale (4.50 eV), and E g is the energy bandgap of the semiconductor (1.85 eV for MoS 2 ) as calculated from diffuse reflectance spectroscopy (DRS) data.By using the above-mentioned equation, the E CB obtained for MoS 2 was determined to be -0.11eV [91,93].

Reusability of MoS 2 Photocatalyst
The stability of MoS 2 NFs photocatalyst for the photodegradation of MB under UV light irradiation was examined.Figure 12 displays the reusability of the MoS 2 NFs over three consecutive cycles using identical experimental conditions.It is evident from the graph that there was only a minimal decrease (from 73.50% in the 1 st cycle to 71.36% in the 3 rd cycle) in the effectiveness of photocatalytic activity for MB removal.The slight decline in photocatalytic activity may be attributed to the loss of catalyst during the recovery process, which includes centrifugation, washing, and subsequent drying.

CONCLUSION
In summary, MoS 2 photocatalyst was successfully synthesized via the facile hydrothermal method.The XRD analysis reveals the formation of a pure semiconducting 2H-MoS 2 phase with a space group of p6 3 /mmc.The W-H model was employed to calculate the average grain size and the lattice strain, which were found to be 6.84 nm and 0.007, respectively.The FE-SEM images clearly confirmed the formation of MoS 2 NFs composed of thin nanosheets that are slightly curved, which provide more essential active sites for the photocatalytic reactions.The appearance of the absorption characteristic bands between 490 cm -1 and 615 cm -1 owing to the Mo-S bond and at 915 cm -1 due to the S-S bond through FT-IR analysis also confirmed the formation of the MoS 2 nanostructure.The detection of Mo and S elements using EDX analysis confirmed the proposed formation of pure MoS 2 nanocrystals.The as-prepared MoS 2 was utilized as a potential photocatalyst for the degradation of MB and RhB under the irradiation of UV and visible light in a comparative way.The results showed that the photocatalytic degradation of RhB dye was faster and more efficient, with a kinetic rate constant and efficiency of 0.00879 min -1 and 84.31%, respectively, than the degradation of MB under UV irradiation.However, the photodegradation of MB was faster and more efficient, with a kinetic rate constant and efficiency of 0.0219 min -1 and 84.25, respectively, than that of RhB under visible light illumination.The degradation of these two dyes under the irradiation sources was attributed to the characteristics of the MoS 2 NFs, namely the narrow E g (1.82 eV) as estimated via Tauc's plot through UV-vis spectroscopy and the relatively large number of active edge sites.Furthermore, the photocatalytic mechanism has been discussed in detail, and it is reported that just the • OH radicals are produced by the MoS 2 NFs, which are the main factors responsible for the MB and RhB degradation.Finally, MoS 2 NFs have shown excellent stability when used in degrading MB dye under UV light over three complete cycles, where the experiment reflected a minimal decrease from 73.50% in the first cycle to 71.36% in the last cycle.
the number of molecules per unit cell (Z = 2 for MoS 2 ), M represents the molecular weight, and N a represents Avogadro's number.

Figure 3 .
Figure 3. FE-SEM images of pure MoS 2 nanoparticles at the magnification of (a) 3 k x, (b) 40 k x.
display the time-dependent UV-visible absorption spectra of MB and RhB solutions in the presence of MoS 2 NFs under UV irradiation up to 180 minutes and visible light up to 60 minutes.The photocatalytic degradation increases with increasing irradiation time, and the dye's degradation percentage is represented by Equation (9) [69, 73]: (9)

Figure 7 .
Figure 7. Time-dependent UV-visible absorption spectra of (a) only MB or RhB without photocatalyst under UV-light, (b) MB in the presence of MoS 2 under UV-light, (c) RhB in the presence of MoS 2 under UV-light, (d) MB in the presence of MoS 2 under visiblelight, and (e) RhB in the presence of MoS 2 under visible-light.

Figure 8 .
Figure 8. Photocatalytic degradation efficiency of MB and RhB using MoS 2 nanostructure under UV for 180 minutes and visible light irradiation for 60 minutes.

Figure 9 .
Figure 9. Photocatalytic activity representation through the change in the C t /C 0 ratio versus the light irradiation time of MoS 2 nanoflowers against MB and RhB dyes under (a) UV light for 180 minutes and (b) visible light for 60 minutes.

Figure 10 .
Figure 10.The pseudo first order kinetics fitted data for MB and RhB photodegradation by MoS 2 under the illumination of (a) UV light for 180 minutes and (b) visible light for 60 minutes.

Figure 11 .
Figure 11.Schematic representation for the dye degradation mechanism using MoS 2 nanostructure photocatalyst.

Figure 12 .
Figure 12.Recyclability in the presence of MoS 2 nanostructure for photocatalytic degradation of MB dye under UV irradiation.