Synthesis and Structural Characterization of Al 2 O 3-Coated MoS 2 Spheres for Photocatalysis Applications

This paper reports the synthesis of novel monodisperse Al 2 O 3 -coated molybdenum disulfide nanospheres (i.e., core-shell structures) using a one-step facile hydrothermal method. XPS analysis confirmed the purity and stable structure of the Al 2 O 3 coated MoS 2 nanospheres. A possible growth mechanism of the core-shell structure is also reported, along with their influence on the photodegradation process of rhodamine B (RhB). The Al 2 O 3 -coated MoS 2 nanospheres demonstrate good photocatalytic activity and chemical stability compared to MoS 2 spheres. TG-DTA analysis provided insight into the decomposition process of the precursor solution and the stability of the nanoparticles. The enhanced photocatalytic activity makes the Al 2 O 3 -coated MoS 2 nanospheres a promising candidate as a photocatalyst that could be used in place of traditional Al 2 O 3 /MoS 2 photocatalyst for the removal of pollutants from waste water.

In a recent report, Wang et al. demonstrated that carbondecorated MoS 2 nanospheres have better cycling performance with good capacity as a Na-iron battery anode [12].MoS 2 is clearly one of the most significantly and broadly used TMDs for transistors due to its favorable band gap compared to graphene.In addition, MoS 2 is also a suitable candidate for photocatalytic materials.MoS 2 is an indirect narrow-band-gap semiconductor with good stability against photocorrosion in solution [13].General issues with semiconductor catalysts in the conversion of solar energy to hydrogen are poor charge transport ability, slow kinetics for evolution reactions, poor stability, and the hydrophobic nature of the catalyst [14,15].On the other hand, individual MoS 2 catalyst has lower charge separation due to its poor crystallinity.
Despite previous efforts, there has been no material system that can simultaneously satisfy all the criteria for costeffective photoelectrochemical hydrogen production, and new materials with new properties are needed.To overcome these problems, core-shell structures of MoS 2 coupled with another material with different activity are promising.Such structures could enable charge separation by gathering electrons and holes.The major parameters for the selection of shell materials are band alignment and small lattice mismatch between the core and shell materials [16].There are very few studies that have focused on preparing MoS 2 nanosphere structures.Wu et al. [17] synthesized MoS 2 microspheres (with diameter up to 2 m) using a solvothermal method with the addition of SUDEI.Wu et al. [18] prepared MoS 2 nanospheres (with average diameter of 100 nm) using HCl as a surfactant.Park et al. [19] synthesized MoS 2 nanospheres with high capacity and cycle stability for lithium ion batteries using L-cysteine in a surfactant-assisted solvothermal route.
Common ways to synthesize core-shell structures are decorating the core particles with a surface coating [5] or shell formation using surface modification processes [20].In this study, we report the influence of Al 2 O 3 as a shell material on the photocatalytic activity of MoS 2 nanosphere coreshell structures under UV light irradiation.We studied the variations of the activity and selectivity in the degradation of rhodamine B (RhB) using Al 2 O 3 -coated MoS 2 nanospheres as a catalyst.

Experimental Procedure
A schematic illustration of the methodology for the formation of Al 2 O 3 -coated MoS 2 nanospheres is shown in Figure 1.For the synthesis, 0.3 g of ammonium heptamolybdate tetrahydrate and 0.17 g of L-cysteine were dissolved in 30 mL of deionized water.This solution was stirred vigorously for 1 h at 80 ∘ C. The suspension was continuously stirred and refluxed near pH 1.Then, 1.2 mmol of Al(NO 3 ) 3 ⋅9H 2 O and 0.3 mmol of trisodium citrate dehydrate (TSC) were added to the stirred solution and again stirred for 30 min at 80 ∘ C.Then, the solution was transferred to a Teflon-lined autoclave and heated at 230 ∘ C for 24 h.Finally, the resulting precipitates were collected by centrifugation and then the precipitates were washed three times with acetone and water.The obtained precipitates were dried at 250 ∘ C for 6 h and sintering at 450 ∘ C for 2 h.
The structural properties of the obtained precipitates were characterized by powder X-ray diffraction (XRD) with a Shimadzu Labx XRD 6100 using Cu-K radiation ( = 0.14056 nm).The scan range was 10-80 ∘ , and the scan speed was 3 deg/min.The nanoparticles were analyzed with a transmission electron microscope (TEM, Hitachi H-7000) at 100 kV and a high-resolution TEM (HRTEM, Tecnai G 2 F 20 ) S-Twin TEM) at an accelerating voltage of 210 kV.The optical properties of the nanoparticles were studied using UVvisible spectroscopy (Cary 5000 UV-Vis spectrophotometry).Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out on a SDT Q600 thermogravimetric analyzer under N 2 flow at a rate of 30 cm 3 /min.The furnace temperature was increased from room temperature to 900 ∘ C at a heating rate of 10 ∘ C per minute.The purity of the final product was examined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-alpha surface analysis instrument).
The photocatalytic experiments were carried out at the natural pH of the RhB organic pollutant solution.The photoreactor has a 150-W mercury lamp with a main emission wavelength of 254 nm as an internal light source, which is surrounded by a quartz vessel.The suspension includes the Al 2 O 3 -decorated MoS 2 nanosphere catalyst and aqueous RhB (100 mL, 10 mg/L), which completely surrounds the light source.Before irradiation, the suspension was stirred in the dark for 30 min to obtain a good dispersion and to ensure adsorption-desorption equilibrium between the organic pollutant molecules and the catalyst.During light irradiation, the samples of the reaction solution were collected at given intervals and examined using an optical spectrophotometer.

Results and Discussions
The XRD patterns of MoS Thermogravimetric and differential thermal analysis (TG-DTA) was conducted to quantitatively determine the Al 2 O 3 content present in the Al 2 O 3 -coated MoS 2 nanospheres, as shown in Figure 6.The initial weight loss below 220 ∘ C is ascribed to the evaporation of physically absorbed water from the product, whereas the weight loss between 550 and 680 ∘ C that occurred with an exothermic peak at 650 ∘ C could essentially be attributed to the decomposition and separation of the Al 2 O 3 layer.The Al 2 O 3 content was estimated to be approximately 47.25% by weight.
The electronic states of the metals and sulfur in the Al 2 O 3coated MoS 2 nanospheres were tested using XPS, as shown in Figures 7(a as a precursor.The Al2p and O1s peaks were centered at 74.45 eV and 532.36 eV, as described elsewhere [23]. The S2p spectrum shows a supplementary peak at 164.58 eV coexisting with an O1s peak, which is ascribed to the oxidation of sulfur.The formation of covalent S-O bonding without breakage of the Mo-S bond is likely due to the oxidation state of sulfur.No S-O bond is observed in the S2s spectrum, which suggests that only the top surface of sulfur atoms of MoS 2 are oxygen functionalized.This is good evidence that the Al shows two absorption edges at 603 and 660 nm.These are attributed to excitonic transitions of the Brillouin region at the  point, which is consistent with an earlier report [24].The energy separation between the two absorption peaks (at 603 and 660 nm) is 0.15 eV due to the spin-orbit splitting at the  point at the surface of the valence band [24].Moreover, there is weak absorbance in the visible region at a wavelength of 425 nm.The UV-absorption behavior of MoS 2 strongly depends on its size due to quantum effects [25].For example, the absorption edges of MoS 2 nanoparticles with average diameters of about 4.5 and 9 nm have edges at 470 and 700 nm, respectively, in the visible light region [26].In contrast, bulk MoS 2 (with a band gap of 1.23 eV) has an absorption peak at around 1040 nm [25].MoS 2 -based composites have diverse absorption edges with respect to their dimensional parameters [26].
The indirect band gap is estimated using the Tauc equation with optical absorption data for near the band edge [24]: The band gaps (  ) are determined from extrapolation of a linear fit onto the -axis.A plot of (ℎ]) 1/2 versus the photon energy (ℎ]) and the intercept of the tangent to the -axis gives the band gap of the Al 2 O 3coated MoS 2 nanospheres, as shown in Figure 8(b).The band gap energy of Al 2 O 3 -coated MoS 2 nanospheres was found to be 2.42 eV.
Figure 9 shows the progressive changes of the UV-Vis absorption spectra of RhB solution in the presence of Al 2 O 3 -coated MoS 2 nanosphere catalyst under UV light as a function of time.The strong absorption peak of the RhB solution at 564 nm gradually decreases from dark conditions to 60 min, and the color of the solution turns from pink to colorless at the end of the photodegradation process.Figure 10 shows the photodegradation efficiency of MoS 2 nanospheres and Al 2 O 3 -coated MoS 2 nanosphere catalysts under UV light in RhB solution.The results are shown as the relative concentration (/ 0 ) as a function of irradiation time, where  0 and  (mg/L) are the initial and final concentrations of the pollutant solution.
A blank experiment was carried out in the absence of photocatalyst for comparison, which showed no obvious change in the RhB concentration within 60 min.The introduction of MoS 2 nanospheres and Al 2 O 3 -coated MoS 2 nanosphere catalysts can greatly enhance the photocatalytic activity under UV light.Interestingly, the Al 2 O 3 -coated MoS 2 nanosphere photocatalysts displayed much higher photodegradation performance than the MoS 2 nanospheres, and more than 97% of the RhB was degraded within 60 min.The presence of Al 2 O 3coated MoS 2 nanospheres plays a key role in the photocatalytic degradation process.The significant enhancement in photoactivity can be ascribed to the favorable van der Waals surfaces of the Al 2 O 3 -coated MoS 2 nanospheres.
Figure 11 shows a kinetic plot of the photocatalytic degradation of RhB over time under UV light irradiation as ln(/ 0 ).The removal efficiency of the Al 2 O 3 -coated MoS 2 nanospheres was much faster than that of the MoS 2 nanosphere catalyst.To understand the photostability and reusability of the photocatalyst, four successive recycling tests of the photocatalysts were done for the degradation of RhB under UV light, as shown in Figure 12.There were no significant changes in photocatalytic activity, which shows the steadiness of the degradation efficiency of RhB solution.This result implies that the Al 2 O 3 -coated MoS 2 nanosphere photocatalysts have high stability during the photocatalytic oxidation of the pollutant molecules and are reusable.
The catalytic activity increased with the Al 2 O 3 -coated MoS 2 nanospheres.Figure 13 shows the RhB removal efficiency of the Al 2 O 3 -coated MoS 2 photocatalytic nanospheres with 5 mg of catalyst and 50 mL of 10 mgL −1 RhB solution.The pollutant removal efficiency and the adsorption amount at equilibrium (qe) were calculated using the following equation: Pollutant removal efficiency (PR%) = 100 ( 0 − )  0 , where  0 and  (mg/L) are the initial and final concentrations of the pollutant, respectively,  is the volume of the pollutant solution, and  (g) is the mass of the catalyst.A possible mechanism for formation of MoS 2 @Al 2 O 3 core-shell structure is suggested involving MoS 2 spheres acting as a template for the formation of the coated nanospheres.Due to hydrothermal reactions between the metal-oxoanions and surfactant, cations form a composite phase at the surfactant/inorganic interfaces.Thus, nucleation domains were formed during the hydrothermal reaction between MoO 4 2− and S 2− and formed MoS 2 spheres.In this manner, the diameter of the nanospheres is no longer limited by the micelle dimensions.A spherical phase of MoS 2 is formed during hydrothermal treatment under the synthetic conditions, in which L-cysteine acts as a sulfur source to stabilize the spherical organization of Mo and S species.The Al species of the precursor solution can absorb to the surface of MoS 2 spheres, and layer upon layer is formed by the electrostatic interaction.The TSC could be considered a crucial component for the growth mechanism of Al 2 O 3 coating because it acts as a capping agent for the formation of the coating surface on the MoS 2 nanospheres.The complete synthetic mechanism is expressed in Figure 1.

Conclusions
Al 2 O 3 -coated MoS 2 nanospheres were successfully synthesized using a simple hydrothermal method.The Al 2 O 3 shell materials serve as additional electron sources that can significantly recover the electron conduction in MoS 2 , which favors the photodegradation of the pollutant.We revealed the appropriate selection of surfactants that could facilitate the adherence of Al species to the surfaces of the cores.Hydrothermally synthesized Al 2 O 3 -coated MoS 2 nanosphere catalysts show photocatalytic activity higher than that of the MoS 2 nanosphere catalyst due to the enhanced crystallites with high metal content, which minimize the poisoning effect of sulfur by the chemisorption process.

Figure 1 :
Figure 1: Schematic illustration of the methodology for the formation of Al 2 O 3 -coated MoS 2 nanospheres.
)-7(e).The XPS survey spectra of the Al 2 O 3coated MoS 2 nanospheres are shown in Figure 7(a).The Al2p XPS spectra were estimated for the Al 2 O 3 -coated MoS 2 nanospheres to examine the chemical state of Al.The Mo3d, S2P, Al2p, and O1s peaks from the Al 2 O 3 -coated MoS 2 sphere sample (Figures 7(b)-7(e)) show no presence of addition chemical states.The binding energy difference Δ between the Al2p and 2s levels is 53.34 eV for the Al 2 O 3 -coated MoS 2 nanospheres.The XPS results strongly indicate that Al species interacted with the MoS 2 nanospheres and are preferentially formed in the Al 2 O 3 -MoS 2 composite using Al(NO 3 ) 3 ⋅9H 2 O

2 O 3
formed a bond with MoS 2 nanospheres, resulting in the formation of the core-shell structure.The XPS binding energies Δ1 (Mo 2p 3/2 − S 2p 3/2 ) and Δ2 (Mo 3d 5/2 − S 2p 3/2 ) of the Al 2 O 3 -coated MoS 2 nanospheres are 70.3 and 67.09 eV, respectively.The UV-Vis spectrum of the synthesized Al 2 O 3 -coated MoS 2 nanospheres is shown in Figure 8(a).The absorption edge at 275 nm could be attributed to the absorption of Al 2 O 3 -MoS 2 in the UV region.The absorption spectrum

Figure 11 :
Figure 11: The kinetic plot of Al 2 O 3 -coated MoS 2 nanospheres photocatalytic degradation of rhodamine B under UV light irradiation.

Figure 12 :Figure 13 :
Figure 12: Recycling photocatalytic degradation of rhodamine B in the presence of Al 2 O 3 -coated MoS 2 nanospheres photocatalytic under UV light irradiation.
The Al 2 O 3 -coated MoS 2 nanospheres exhibit high RhB removal efficiency (95%), in contrast to the (69%) MoS 2 nanosphere sample.It is well known that catalytic ability increases withAl 2 O 3 -coatedFigure 10: Photodegradation rate of the rhodamine B under UV light and light irradiation time for without catalyst, MoS 2 nanospheres, and Al 2 O 3 -coated MoS 2 nanospheres.increasingcontent of the catalyst with respect to processing time and other environmental conditions like temperature and pH.In this work, the degradation rate of the RhB solution also increased with increasing catalyst content due to the small particle size and good dispersibility of the Al 2 O 3 -coated MoS 2 nanoparticles, which facilitate electron migration between the catalyst and pollutant.The Al 2 O 3coated MoS 2 nanospheres exhibit high catalytic activity due to the high dispersibility of the Al 2 O 3 coating around the MoS 2 nanospheres, which secured more efficient charge transfer between the MoS 2 and Al 2 O 3 .The MoS 2 behaves as the photoactive center, which is generating excited photoelectron pairs under UV irradiation,