Synthesis and Characterization of MoS2/TiO2 Nanocomposites for Enhanced Photocatalytic Degradation of Methylene Blue under Sunlight Irradiation

2D nanosheets/ nanoparticles based MoS2/TiO2 nanocomposites were prepared in different weight compositions which were further employed to investigate photocatalytic degradation of methylene blue. Anatase TiO2 powder was prepared via sol-gel reflux method using titanium tetraisopropoxide as Ti precursor. MoS2/TiO2 nanocomposites were prepared by in situ addition of exfoliated MoS2 (2D-nanosheets) in different weight ratios of 0.1%, 0.5%, 1%, 2% and 5% in TiO2 sol. Surface morphology, phase analysis, optical properties were studied using SEM, XRD, UV-Vis spectroscopy respectively. SEM results showed that TiO2 nanoparticles were completely adsorbed over the surface of MoS2 sheets as reflux synthesis was employed. Efficient charge carrier separation was achieved which reduced recombination, and hence, enhanced photo-degradation of methylene blue was observed. The hetero-structures showed less operation time in sunlight for photodegradation of methylene blue and a highest rate constant was observed by 2 wt.% loading of MoS2 on TiO2. These composites can also be used commercially as they show promising results.


Introduction
Organic pollutants in groundwater have become a persistent problem, as they affect human beings and other creatures. These organic pollutants can cause eutrophication [1] as they compete for dissolved oxygen present in water. Moreover, the effluents from textile and paper industries which include organic dyes pollute and destroy marine life. In the recent years, many compounds have been synthesized and applied for the degradation of various organic dyes. Many wide-bandgap semiconductors like ZnO [2,3], TiO 2 [4,5], N-doped TiO 2 [6] and ferrites [7] have been exploited to degrade these organic compounds for water purification and remediation [8]. Among these semiconductors, anatase TiO 2 has been widely utilized as it is anti-corrosive, environmentally friendly, cost-effective and widely available. Rutile TiO 2 has a direct band gap of 3.0 eV while anatase TiO 2 has an indirect band gap of 3.26 eV [9]. As less e -/h + pair recombination occurs in anatase TiO 2 , therefore it is the most widely used form. In the presence of UV light electron-hole pair [10] is generated as: Due to the large band gap of TiO 2 , its many heterostructures have been prepared via doping [11,12] and composites formation [13,14]. These structures have been prepared to enhance its photo-generation of electrons and creating a red shift in absorption spectrum [14]. Transition metal dichalcogenides (TMDs) have also been exploited in recent years [15] because of their novel properties. Among TMDs, MoS 2 [16] has been exploited recently as it has a graphene-like 2D structure, is chemically stable, and shows excellent photocatalytic properties. It also shows flexible properties for various optoelectronic devices, and it is anti-corrosive so it can be used in aqueous media as well. Bulk MoS 2 shows absorption spectrum in the near infra-red region at 1040nm, and when it is exfoliated down to monolayer its band gap (E g ) increases from 1.2 eV to 1.9 eV [17]. The thickness reduction from bulk to nanoscale increases the redox potential of MoS 2, and the effective electron transfer takes place from the conduction band of MoS 2 to TiO 2 [16,18] .
In this study, MoS 2 2D nanosheets/ anatase TiO 2 nanoparticles based nanocomposites [8] were prepared for efficient photocatalytic degradation of organic pollutant. The goal was to prepare nanosized MoS 2 sheets with increased band gap so that they can act as a photosensitizer to TiO 2 for improved photocatalytic degradation activities [13,19]. The nanocomposites were prepared with different weight ratios of MoS 2 sheets, i.e. 0.1 wt.%, 0.5 wt.%, 1 wt.%, 2 wt.% and 5 wt.% respectively.

Experimental
Titanium Tetraisopropoxide (TTIP, Sigma Aldrich) was used as a precursor material to form TiO 2 . 11.6 g of TTIP was mixed with 2.4 g of glacial acetic acid (CH 3 COOH, Merck) for initiating the reaction. CH 3 COOH was added as a proton (H + ) donor and as a chelating agent. The chemicals were mixed for 15 minutes for the complete reaction. After that, 74 ml of water was added to the reaction mixture for hydrolysis of TTIP, and it was stirred for 60 minutes. 0.8 ml of HNO 3 was added in the reaction mixture for peptization to prepare TiO 2 sol [20]. The reaction mixture was subjected to reflux condensation for 24 hours and was dried at 80 ºC to prepare pure TiO 2 powder.

grams of powder was obtained as result of this synthesis.
For the synthesis of MoS 2 nanosheets, 20mg/ml of molybdenum (IV) sulphide powder (Sigma Aldrich) was taken in N-methyl-2-pyrrolidone (NMP, Sigma Aldrich). The mixture was sonicated at 1000 rpm for 64 h that resulted in the exfoliation of bulk MoS 2 to 2D nanosheets. These sheets were added in ratios of 0.1, 0.5, 1, 2 and 5 wt.% respectively in TiO 2 sol before reflux, respectively. Then the reaction mixture was subjected to reflux condensation for 24 hours. The product obtained was dried at 80 ºC to obtain a greyish powder of the composites and was further annealed in a tube furnace at 450 ºC in inert argon gas atmosphere. Fig. 1(a) shows complete schematics for preparation of nanocomposites.

Material Characterization
Scanning electron microscope (SEM) of samples was carried out by (JOEL JSM-6490A) to analyze structure and morphology of prepared composites. The MoS 2 sheets were characterized by Atomic Force Microscope (AFM) using (JEOL-SPM 5200) to determine dimensional aspects. Raman spectroscopy was done by (µRamboss DONGWOO OPRTON) and for XRD studies, an Xray diffractometer (STOE Stadi MP) was employed at operating voltage and current of 40 kV and 20 mA, respectively. Cu-kα radiation was used as source. UV-Vis was observed by (Jenway 7315 UV-Vis spectrometer) to observe optical properties and photodegradation of samples.

Results and Discussion
Scanning Electron Microscope (SEM) was used to determine the morphology and lateral dimensions of the MoS 2 nanosheets. MoS 2 /TiO 2 nanocomposites were also analyzed via SEM. Fig.  2(a) shows pure anatase TiO 2 NPs produced as a product of sol-gel reflux synthesis within 24 hours. It can be seen that all the particles have a spherical morphology and particles range from 20 nm to 35 nm. Fig. 2(b) shows MoS 2 nanosheets obtained at 1000 rpm centrifugation speed. Fig. 2(c,d) shows the morphology of the composites. In high resolution images, it can be seen that TiO 2 nanoparticles are completely adsorbed over the surface of the MoS 2 nanosheets as also shown in Fig. 2(e,f). Therefore, it can be concluded that there is good interfacial bonding between MoS 2 /TiO 2 which will further aid photodegradation . Atomic Force Microscope (AFM) was used to determine the thickness of MoS 2 sheets. 2 mg of exfoliated MoS 2 sheets centrifuged at 1000 rpm were dispersed 10 ml of NMP. It was drop cast over glass slide to analyze the sample. As shown in Fig. 3, it was observed that the sheets were around 0.54 nm to 2.69 nm thick which approximately equals to 1-4 layers of MoS 2 , as its monolayer thickness is 0.65 nm [17]. Therefore, by exfoliation at 1000 rpm the thickness of MoS 2 sheets decreased and hence, its band gap increased.
Powdered X-ray Diffraction (PXRD) was done to evaluate the phase of composites. The peaks were observed at 25.06°, 37.9°, 47.8°, 53.77°, 54.77°, 62.6° and 76° which corresponds to tetragonal anatase TiO 2 (JCPDS 21-1272). This confirms that only anatase TiO 2 is formed as shown in Fig. 3(b) as a result of this synthesis with no extra peak corresponding to rutile or brookite phase. No peak of MoS 2 were present due to its relatively very thin layers. UV-Visible spectrum of pure anatase TiO 2 and nanocomposites with MoS 2 are shown in Fig. 4(a). Samples were prepared by dispersing 2 mg of samples in 10 ml of distilled water. With MoS 2 loading, the absorption bands are showing a red shift as shown in zoomed part of Fig. 4(a). This can be attributed to nano-sized MoS 2 sheets which provide more active sites and intermediate energy levels for faster chemical activities [21]. And hence, the photocatalytic degradation in the presence of sunlight should be enhanced.
Raman spectroscopy of MoS 2 /TiO 2 composites was done to determine the coordination of O-Ti-O. Fig. 4(b) shows the Raman spectra for the composite. TiO 2 was also analyzed to ensure the formation of pure anatase phase. All Raman active modes expected by group theory were observed at 120 cm -1 (E g ), 197 cm -1 (E g ), 395 cm -1 (B 1g ), 515 cm -1 (A 1g ) and 635 cm -1 (E g ) respectively.
Key Engineering Materials Vol. 778 Due to external vibration of anatase structure, E g mode at 140 cm -1 is well resolved. Weak E g mode at 197 cm -1 is also well distinct due to the weak repulsion of oxygen atoms bonded with titanium with the large bond length of 3.04 Å. Anatase characteristic peaks were also observed at 395 cm -1 (B 1g ), and 515 cm -1 (A 1g ) are due to stretching modes of O-Ti-O and Ti-O bonds [22] respectively. It was seen that pure anatase TiO 2 is obtained as result of sol-gel synthesis. Due to the low amount of MoS 2 in the samples, no MoS 2 peak was observed. The photocatalytic degradation of methylene blue (MB) dye was evaluated under sunlight irradiation. MB dye was used as it is a hazardous organic water pollutant. Also, it is used in dyes, cosmetics, paper industries and given off in water which affects marine life [23,24]. 10mg of MB dye was added in 1L of distilled water. 50ml of this solution was used in each beaker and 5mg of photocatalyst was added to it. Before irradiation, the samples were added to MB dye solution and sonicated for 30 minutes in the dark for uniform dispersion of photocatalysts. Fig. 5(a) shows that when samples were irradiated in the presence of sunlight, MB dye started to degrade. This happened due to efficient interfacial bonding between nanosheets and nanoparticles. The degradation was highest for 2 wt.% MoS 2 / TiO 2 nanocomposites, and complete degradation was achieved in just 5 hours. The reaction rate constant of the composites increased up to 2 wt.% of MoS 2 addition and then decreased, as MoS 2 sheets start to agglomerate causing the recombination rate of the photo-generated electrons and holes to increase. The degradation rate and irradiation time follow the first-order reaction kinetics [13,25]. This can be expressed as ln (C/C 0 ) = -kt, where k is rate constant and C and C 0 are the initial and final concentration of methylene blue respectively, and 'k' is the apparent reaction rate constant. This is elaborated graphically in figure 5(b). Fig, 5(c) shows the rate constants which various photocatalysts depicted. Table 1 shows that reaction rate constant reached an optimum value of 0.559 (hr -1 ) by 2 wt.% addition and then decreased by further addition of MoS 2 sheets.

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Advanced Materials -XV  Fig. 6 shows photocatalytic degradation mechanism of nanocomposites prepared. In case of pure anatase TiO 2 , degradation occurs solely due to electrons and holes production in its valence band. In case of nanocomposites, when sunlight is irradiated on photocatalyst e -/h + production takes place in MoS 2 as exfoliated MoS 2 has high redox potential level [25]. ) radicals [26,27]. These species formed then degrade organic pollutants like methylene blue.

Conclusions
TiO 2 was synthesized via facile sol-gel reflux method, and nanocomposites with MoS 2 were prepared by in-situ addition of exfoliated MoS 2 nanosheets in different weight ratios. XRD and Raman spectrum confirmed the formation of a pure anatase phase of TiO 2 . SEM images revealed that the TiO 2 nanoparticles were completely adsorbed over MoS 2 ts. AFM analysis showed that the exfoliated MoS 2 were only 1-4 layers thick. The prepared nanocomposites showed increased photogenerated electron-hole pair production and improved charge transfer as more active edges were present in MoS 2 due to exfoliation, so an enhanced photocatalytic activity was observed. UV-Vis results showed that loading of exfoliated MoS 2 leads to an increase in the photodegradation activity due to increase the charge carrier separation in the visible light. The reaction rate constant gradually increased and reached to 0.559 h -1 which was optimum at 2 wt.% addition of MoS 2 and then decreased due to agglomeration of the sheets and fast recombination rate of the charge carriers.