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Preparation of nanostructured and nanosheets of MoS2 oxide using oxidation method
Accepted Manuscript Preparation of nanostructured and Nanosheets of MoS2 oxide using oxidation method Majed Amini, Ahmad Ramazani S.A, Morteza Faghihi, Seyyedfaridoddin Fattahpour PII: DOI: Reference:
S1350-4177(17)30189-X http://dx.doi.org/10.1016/j.ultsonch.2017.04.024 ULTSON 3659
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Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
4 February 2017 18 April 2017 19 April 2017
Please cite this article as: M. Amini, A.R. S.A, M. Faghihi, S. Fattahpour, Preparation of nanostructured and Nanosheets of MoS2 oxide using oxidation method, Ultrasonics Sonochemistry (2017), doi: http://dx.doi.org/ 10.1016/j.ultsonch.2017.04.024
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Preparation of nanostructured and Nanosheets of MoS2 oxide using oxidation method Majed Amini, Ahmad Ramazani S.A*, Morteza Faghihi, Seyyedfaridoddin Fattahpour
Department of Chemical and Petroleum Engineering, Sharif University of technology, Tehran, Iran
Abstract Molybdenum disulfide (MoS2), a two-dimensional transition metal has a 2D layered structure and has recently attracted attention due to its novel catalytic properties. In this study, MoS2 has been successfully intercalated using chemical and physical intercalation techniques, while enhancing its surface properties. The final intercalated MoS2 is of many interests because of its low-dimensional and potential properties in in-situ catalysis. In this research, we report different methods to intercalate the layers of MoS2 successfully using acid-treatment, ultrasonication, oxidation and thermal shocking. The other goal of this study is to form S=O bonds mainly because of expected enhanced in-situ catalytic operations. The intercalated MoS2 is further characterized using analyses such as Fourier Transform Infrared Spectroscopy (FTIR), Raman, Contact Angle, X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-Ray Microanalysis (EDAX), Transmission electron microscopy (TEM), and BET. Keywords: MoS2, Intercalated, Oxidization, Thermal shocking
*
Corresponding author: Polymer Research Centre, Sharif University of Technology , Azadi Avenue, P.O. Box 11365-9465, Tehran, Iran. Email: [email protected], Tel: +98-21-66166405
Introduction MoS2 is a typical and inorganic layered compound, with strong covalent Mo−S in-plane bonds and weak van der Waals bonding between the sheets [1]. The latter enables mechanical or chemical intercalation [2]. Accordingly, MoS2 has been widely studied due to its potential applications as catalysts [3,4], solid lubricants[5,6] and hydrogen storage materials [7]. Recently, due to the need for more significant performance in catalytic reactions some research works have been devoted to reduce the lateral thickness or increase the interlayer distance to produce MoS2 with improved properties. Joensen et al.[8], suggested a chemical route which was reported to dismantle bulk MoS2 partially into platelets that were nano-sized. Furthermore, preparation of a pure thin layer of MoS2 have been reported using physical vapor deposition [9], chemical bath deposition [10], chemical vapor deposition [11], laser ablation [12] and ion-beam-assisted deposition [13]. Despite using some traditional chemical and mechanical exfoliation methods of MoS2 for the production of layered material, still, these methods are conducted to obtain the MoS2 with more interlayer distance and less lateral thickness to promote its applications in commercially viable devices [14]. Such methods tend to yield exfoliated materials with a relatively low active surface area of the 2D structure. The chemical route, [8, 15-22] normally comprises of non-aqueous lithium intercalation and water exfoliation technique, which demands long hours of intercalation [8,15,23] and additional pre-expanding steps [24]. Exfoliation process has been also done with some metal ions such as lithium ion, which leads to intercalated MoS2 with interesting applications [25]. Accordingly, Bockrath and Parfitt [26] have explained the application of exfoliated MoS2 in coal liquefaction, while Miremadi and Morrison [27] have implemented this method to prepare catalysts for methanation. MoS2 is vastly known for its applications as a catalyst in an industry [28]. Taking its catalytic properties into account, MoS2 has been widely used for the extraction of S and N from crude oil [29-31]. MoS2 as a catalyst has a relatively high activity 1
and also resistance to Sulfur poisoning [32]. In refinery operations, supported catalysts with Mo components are the most widely used catalysts, as a basic active ingredient [33]. The catalytic activity of MoS2 is focused on rare surface sites, while the bulk material is relatively inert, mainly because nano MoS2 has particular size and structure [34-36]. It has been widely known that S-ions at the edges of MoS2 is responsible for the importance of the catalytic activity of MoS2 [33]. As a result, it seems that expansion of MoS2 and formation of extra S=O bonds on the surface of MoS2 layers could improve the catalytic activity of MoS2, significantly. These produced highly active sites could be taken for support of other catalysis and active species for in-situ catalytic processes [33]. In this study, we report a tentative attempt for the preparation of intercalated and exfoliated modified MoS2 using treatments as follow (See Figure 1): acid-treatment, acid-treatment and ultrasonication, acid-treatment and Oxidization, acid-treatment and Oxidization and thermally shocked. In this regard, single-layered and expanded few-layered MoS2 sheets are prepared by above-mentioned methods. In comparison with previous studies, such as mechanical exfoliation techniques, which were able to reduce lateral thickness of MoS2 to 90 nm, the methods in this study were able to produce expanded MoS2 with the lateral thickness less than 40 nm. Moreover, further evidence for the formation of S=O and Mo—O bonds have been illustrated using FTIR analysis.
1 Materials and Methods 1.1 Materials Purified Mineral MoS2 (in bulk form) was purchased from Semnan Deposit, Iran. Sulfuric acid (98%, Merck) was employed for oxidation and acid treatment. Acid Nitric (HNO3) was purchased from Aldrich. The aqueous solutions H2 O2 (hydrogen peroxide, H2O2, 30 wt %) was purchased from Dr. Mojalali Company, Iran. Dimethylformamide (DMF), an organic 2
solvent, was purchased from Merck, Germany, 99% for ultrasonication. All the raw materials were used as purchased with maximum storage time of 2 weeks.
1.2 Nanosheets Preparation and Procedures In this study, five samples have been investigated, in which one of them (MoS2) is in bulk natural form and the other four are orderly acid-treated MoS2 (A-MoS2), acid-treated and ultrasonicated MoS2 (A-U-MoS2), acid-treated and Oxidized MoS2 (A-O-MoS2), acid-treated, oxidized and thermally shocked MoS2 (A-O-T-MoS2). 1.2.1 Preparation of A-MoS2: Firstly, purified naturally occurred MoS2 was crushed and sieved. The particles with the mesh size of 63-74 µm have been used for further works. The crushed and sieved samples washed with deionized water and then dried for 24 hours in vacuum oven at 100 ºC. Moreover, untreated sample underwent acid leaching with a 1M HNO3 for 2h at 80 °C, and the filtrated samples were washed with distilled water. The resulting sample was denoted as A-MoS2. The final treated natural MoS2 sample was dried at 100 ºC for 24h. 1.2.2 Preparation of A-U-MoS2: The abovementioned sample (A-MoS2) was dissolved in DMF solution (0.5 wt/v %) and sonicated using ultrasonic homogenizer (Addeco, Iran), under 250 watts for 15 minutes. Then the result was washed with distilled water and filtered respectively, after which, it was dried at 100 ºC for 24 h in the vacuum oven. 1.2.3 Preparation of A-O-MoS2: The acid treated MoS2 was oxidized using a mixture of two acids and few oxidizing agents. Which can be explained as follow: 3.0 g of A-MoS2 was mixed with 50 mL concentrated sulfuric acid (98%) using stirrer at room temperature, then 1.0 g of sodium nitrate was added to form a well-mixed mixture. Potassium permanganate (6.0 g) was added slowly, under 3
intense stirring, within 5 minutes to keep the temperature of the suspension lower than 20 ºC. Furthermore, the reaction set-up was transferred into an ice bath (0 ºC) for about 30 minutes, forming a brown thick paste. Then, the solution was moved to an oil bath (35 °C) and mixed for 3 hours until the final solution would be in brown colour. Moreover, the solution was immediately transferred back to the ice bath (0 °C) to add 50 mL of distilled water. It is vital to keep the temperature below 60 °C. After half an hour, the solution was moved out of the ice bath and left off until it reached ambient temperature. Moreover, 100 mL distilled water was added slowly, after which the temperature reached 40 °C. Furthermore, 8 mL of H2O2 (30%) was slowly added, that turned the colour of the solution from brown into yellow. In order to eliminate metal ions (Potassium, Sodium, etc.) from the final solution, filtering and washing with 1:10 wt/wt hydrochloric acid aqueous solution (250 mL) was performed, followed by several washings with distilled water. The final nanosheets of MoS2 were dried using vacuum oven at 60 °C for 90 minutes. For further treatment, 0.5 g of the final solid was dispersed in 100 mL DMF using intense stirring. The dispersion was carried out in 15 minutes, using probe ultrasonic homogenizer instrument under powers of 250 watts. Finally, the dispersed solution was subjected to centrifugation at 3000 rpm for 30 min to remove any potential aggregates. The concentration of the MoS2 in the centrifuged suspension was determined by drying a certain volume of suspension and weighing the solid residual. 1.2.4 Preparation of A-O-T-MoS2: Mass amounts of molybdenum disulfide sheets prepared with oxidation method (see section 2.1.3), at temperatures 600 and 400 ° C for 10 minutes, under inert gas (argon with a purity of 99.99 percent) was thermal exfoliation. The abovementioned sample (A-O-MoS2), was thermal-shocked using furnace (AzmaGostar, Iran).
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1.3 Nanosheets Characterization Methods The morphology of the nano MoS2 was characterized by a Field emission scanning electron microscopy (SEM; TESCAN – MIRA iii). The thickness of layers of MoS2 samples were estimated by ImageJ software. Before the FESEM test, samples of MoS2 were coated with gold. The morphology of crystalline structure of MoS2 samples were investigated using transmission electron microscopy (TEM) analysis (TEM, Zeiss EM900 under 200 kV). The XRD patterns of MoS2 samples were taken via INEL EQUINOX 3000 X-ray diffractometer, using Cu Kα radiation, to determine structural changes and d-spacing. The diffractograms were recorded at a voltage of 40 kV and a current of 50 mA using Cu K radiation (λ= 0.154 nm) with the scanning rate of 1◦/min from 3° to 60°.Chemical structure of MoS2 samples were studied using FTIR spectrometer (ABB Bomem MB-100). The samples were compressed into KBr pellets for each analysis. BET analysis was carried out to determine the specific surface area of the samples, calculated from N2 adsorption result via a surface area analyser (Quantachrome ChemBET-3000) based on the BET equation. EDAX analysis was performed using MIRAPLMU for elemental analysis. Raman spectra of MoS2 samples were measured via Raman analyser (Almega Thermo Nicolet Dispersive Raman Spectrometer).
2 Results and Discussions 2.1 Nanosheets Characterization 2.1.1 FTIR Analysis Figure 2 shows FTIR spectra of MoS2 for three modified samples, (A-O-MoS2), thermally shocked MoS2 at 400 °C (A-O-T-MoS2) and 600 °C ((A-O-T-MoS2)'). It can be seen in spectra of MoS2, there were no characteristic peaks for MoS2. The wide absorbance bands at 3430 cm-1 can be attributed to the adsorbed hydroxyl groups on the surface of MoS2 [37]. Moreover, after modification of untreated MoS2 via acid treatment and oxidation method, 5
absorbance peaks of S=O and Mo-O observed at wavenumbers of 1119.41 and 689.12 cm-1, respectively. The FTIR spectrums of treated MoS2 after thermal shock at 400 ºC clearly illustrated that thermal shocking of the oxidized MoS2 leads to the formation of S=O bonds in (A-O-T-MoS2) sample. According to the spectra of (A-O-T-MoS2)' sample, thermal shock at 600 ºC changed the total physical and chemical structures of A-O-T-MoS2 [37-38]. The FTIR results demonstrated that S=O bond can be successfully formed on the MoS2 structure through acid modification and thermal shock at around 400 ºC. It should be mentioned that S=O bond is vital for in-situ polymerization for our future studies [39-40].
2.1.2 Raman Analysis Raman spectroscopy has been widely used to determine the number of layers. Figure 3 shows the Raman spectra of samples MoS2. This Figure shows Ag1 (out of plane) and in-plane E2g1 bands of MoS2 occur in 356 cm-1, which is attributed to Mo—S vibrations [42] and 405 cm-1, which is assigned to out-of-plane vibrations (Ag1 mode) [43, 44]. However, the intensity of these bonds slightly changed for A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2. Actually, the peak at 356 cm-1 shifted to the further right for example about A-U-MoS2 shifted to (365 cm-1) and the one at 405 cm-1 shifted to the left to 397 cm-1. These results are in agreement with those previously presented in the literature [45, 46]. These peaks were demonstrated the thickness of layers and the shifts in Raman spectrum show that the treatment used in curve attributed to (A-U-MoS2) sample has changed the thickness of layers. We have also observed a distinctive thickness dependence of the intensities and line widths of both Raman modes [47]. It is evident that Ag1 mode is amplified due to formation of interlayer forces as a result of formation of S—O chemical bonds (see Figure 3), which is confirmed by FTIR analysis as well (see section 3.1.1). It is worthy to note that the intensity
6
ratio between the Ag1 and E2g1 modes (Ag1 / E2g1) remained nearly a constant (∼1.88) in the MoS2, consistent with the previous study [48].
2.1.3 BET Analysis Effects of different treatments and intercalating processes on the accessible surface area of MoS2 samples were presented in Table 1. The acid-treatment, ultrasonication process, oxidation processes and thermal shocking have increased the BET area significantly by almost 113 percent for A-O-T-MoS2. Comparison of obtained results with those presented in the literature [49] shows that the process of acid-treatment, oxidation, and thermal shocking of MoS2 (A-O-T-MoS2) is the most appropriate method amongst all others, in terms of higher surface area. The latter further proves the plausibility of the method for intercalation of layers as well. Furthermore, sample (A-O-T-MoS2) has a BET area of almost 2.15 times more than that of the base sample (see Table 1, MoS2). However in previous study by Ross et al.[50], the BET area only showed an increase from 14.2 to 17 m2/g for thermally shocked sample. It is quite tangible that the present study suggests tentative approaches that showed significant improvement in increasing the d-spacing and specific area. 2.1.4 XRD Analysis Figure 4 showed the normalized X-ray diffractograms of different MoS2 samples. Peaks located at 2θ=14° was related to the interlayer distance of MoS2. Although this peak can be observed for all samples, its intensity was significantly decresaed for treated samples which could be attributed to change in interlayer distance of treated and oxidized samples. The presence of the (00l) peaks, located below 10°, in the XRD pattern of treated and oxidized samples (Figure 4, A-O-T-MoS2) clearly demonstrates the increase in d-spacing of these samples MoS2. As oppose to XRD patern of (MoS2) and (A-MoS2), the existance of (001) peaks in the paterns of (A-U-MoS2), (A-O-MoS2) and (A-O-T-MoS2) proved that these 7
treatments were successful at making increase in inter-layer distances. This proved that the treatments used in this study were successful at increasing d-spacing and forming exfoliation structure of MoS2. The significant decrease in intensity observed in (002) with the observation of slight intensity increase in (001) peak of the XRD pattern in the MoS2 suggests a more disordered phase and is agrrement with previous reports [51, 52]. As could be seen from the curve (MoS2), curve (A-MoS2) shows a (001) peak lower which indicates that acid-treated and ultrasound was more effective than acid-treatments only. Other characteristic peaks at 28.8, 33, 39.6, 43.9, 49.2 and 58 correspond to the crystal plane of (100), (103), (006), (105) and (110). These peaks could be ascribed to hexagonal shape of MoS2 [53, 54]. Strong (00l) reflections in the XRD pattern illustrated the crystallinity and ordered stacking of 2D layers in the as-grown MoS2 nanostructures. Regarding layer stacking, the XRD patterns of modified MoS2 samples revealed that the reflection peaks are, in general, broader after chemical exfoliation treatment. Joensen et al [55] sugested that distroying layers along the (00l) axis reduces particle size in the same plane, which leads to broadening of the (002) peak, meanwhile other peaks also show broadening due to rotation of basal planes that reduce he wide order of the corresponding planes. For modified MoS2 samples in this study, the average stacking height, using analysis of (002) reflection X-ray peak broadening, decreases from about 100 to 20 nm after exfoliation. Due to the exfoliation process, which consequently leads to dismantling of layers, reduces the stacking height.
2.1.5 Contact Angle Analysis Contact angle measurements were implemented to investigate the effect of oxidation and thermal shocking on MoS2. Water and Methanol (methyl alcohol) shapes of droplets on the surface of the modified MoS2 discs are illustrated in Figure 5.
8
The values of water and Methanol (methyl alcohol) contact angles on the surface of MoS2, AO-MoS2 and A-O-T-MoS2 were tabulated in Table (2). Surface free energy ( ) values were calculated using Owens-Wendt (Eq. (1) and (2)) equation, orderly [51, 52]. (1) (2) Where,
is surface tension of liquid,
tension of the both liquids and
and
and
are dispersive and polar parts of the surface
are the polar and dispersive parts of the surface free
energy of the samples’ disks, respectively. Contact angle and total surface free energy of the discs are presented by
and
, orderly [52]. Summarized values of the dispersive and polar
parts of the water and Methanol (methyl alcohol) surface tension were shown in Table (3) [53]. According to Figure 5 and Table 4, it can be found that acid treatment and oxidation of MoS2 decreased surface tension and increased surface polarity due to the formation of S-O and Mo-O bonds on the surface of the treated MoS2.
2.1.6 FESEM Analysis Scanning electron microscopy was used to determine the morphology and nanoparticle dispersion and distribution after different modifications. Presented FESEM images in Figure 6 presented somehow layered structure of MoS2 from the cross-sectional view. These FESEM images revealed that MoS2 samples were distributed as the combination of randomly stacked thin nanosheets with a thickness of several nanometres (see Figure 6). It is fairly obvious that the gap space between MoS2 nanosheets was relatively extensive with lamella morphology [56] and the thickness of the MoS2 stacked layers was estimated to be about 50 nm obtaining by image analysing using image j software.
9
These images are prepared in two magnifications: 1) the ones on the left hand side with 1 micro scale bar and 2) the ones on the right hand side with 500 nm scale bar. It is evident that using certain modifications, with more attention on thermal shocking, we have successfully changed the tangled structure (see Figure 6, MoS2) to Figure (6, A-O-T-MoS2) with significant reduction in stack layers. Comparing Figure 6 (MoS2) with (A-O-T-MoS2) illustrated that oxidation and thermal shocking were efficient at exfoliating the layers and making gaps in between them, in which the d-spacing has increased from 50 to 20 nm. It is worthy to mention that structure of MoS2 has not lost its uniformity. In other words, after using three different modifications, the structure of MoS2 is quite ordered (Figure 6, (A-O-TMoS2)). As a result, the increase in d-spacing of MoS2 multi-layered structure was controlled using three different exfoliation process in order to produce single-layered structure, which has not resulted in deformation of layers or any other defective layout. The latter is evident by the result of XRD analysis, in which the peak at 14 is omnipresent in all samples. 2.1.7 EDX Analysis In order to confirm the considered modification of MoS2 nanosheets and determine the change in amount and dispersion of oxygen element, EDX-Dot mapping was implemented. Figure 7 illustrated the EDX mapping micrograph of samples prepared by acid-treatment (Figure 7, A-MoS2), acid-treatment and ultrasound (Figure 7, A-U-MoS2), acid-treatment and oxidization (Figure 7, A-O-MoS2) and acid-treatment and oxidation and thermal shocking (Figure 7, A-O-T-MoS2) samples. In the all samples oxygen and Sulfur were observed. Except for Mo and S and an insignificant amount of oxygen and copper in untreated sample (Figure 7, MoS2), there were not any other components on the EDX mapping micrograph, which further indicates the purity of all prepared samples. It is evident from the micrograph related to the samples MoS2, A-MoS2 and A-U-MoS2, the amount of oxygen is not so high, compared to that of samples A-O-MoS2 and A-O-T-MoS2. This evidence is expected due to 10
nature of oxidation (see section 2.1.3) and thermal shock process. Based on the results of EDAX analysis, the latter methods had the tremendous effect on increasing the amount of oxygen. It is worthy to mention that existence of Ca within samples A-O-MoS2 and A-O-TMoS2 are due to several oxidants used in the oxidized method. Considering the fact that sample A-O-MoS2 increases the oxygen level, comparing to that one of sample A-U-MoS2, EDX dot-mapping micrographs of oxygen shows an acceptable dispersion, however, the distribution of oxygen is not perfectly uniform. In sample A-O-T-MoS2, in which thermal shocking method was implemented to prepare the sample, EDX dot-mapping clearly suggests for bulk motions of oxygen throughout the sample. To the best of our knowledge, the latter might be due to thermal shock and hence, extending the distance between sheets of this sample, which is also approved by FESEM images as well (See section 3.1.6). Figure 7 revealed that acid-treatment and oxidation and thermal shocking of MoS2 are an effective procedure to produce intercalated materials, as it increases the oxygen level. Interestingly, it discussed that distance between sheets of the above-mentioned samples are higher among all other samples (See section 3.1.6 and 3.1.4). 2.1.8 TEM Angle Analysis For nanostructure analysis, transmission electron microscopy (TEM) experiments were performed and the images (Figure 8) provide more detailed structural information of MoS2 prepared by different treatments (A-O-MoS2 and A-O-T-MoS2). The images clearly cited for the intercalated structure of modified MoS2, and the inset shows distinct layered patterns of selected-area. After exfoliation, the randomly stacked MoS2 sheets were clearly visible with an expanded d-spacing, consistent with the XRD pattern. This observation confirms our explanation for the combination of (001) and (002) peaks in Figure 4 (See section XRD analysis), suggesting that it is possible to further intercalated the layers by further optimizing the synthesis condition. We also examine the edges, as shown in Figure (8, A-O-T-MoS2, 11
50nm). The image illustrated that sample A-O-MoS2 and A-O-T-MoS2 have double-layered (2L) and Four-Layered (4L) structure, respectively. The latter is confirmed by the XRD analysis (see Figure 4). As seen in Figure 8, despite the change in method from A-O-MoS2 to A-O-T-MoS2, the grain sizes have not changed significantly. Yet, the difference contrasts in the image suggests that the layers of sample A-O-T-MoS2 have become more dispersed in comparison with Figure 8, A-O-MoS2. 3 Conclusion Molybdenum disulfide with layered structure could be provided by intercalating and exfoliating of Molybdenum disulfide structures via introduced oxidation methods in this study. EDAX results show that oxidation of Molybdenum disulfide has been successfully conducted using a combination of different Acids. Furthermore, XRD patterns and morphological studies by FESEM and TEM revealed that oxidation of molybdenum disulphide could significantly increase interlayer distance and reduce the number of layers in molybdenum disulphide stacks which could be further increased by appropriate ultrasonication and thermal shocking processes. It is worthy of mention, that this intercalated and exfoliated Molybdenum disulfide could be used in many new application including catalyst support and lubrication applications.
ACKNOWLEDGMENTS The Authors would like to thank the Iran National Science Foundation (INSF) for support of this project under, Proposal number 94027859.
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Helmly, B.C., W.E. Lynch, and D.A. Nivens, Preparation and Spectroscopic Characterization of MoS2 and MoSe2 Nanoparticles. Spectroscopy Letters, 2007. 40(3): p. 483-492. Batista Mancinelli, K.C., et al., Poly(vinyl alcohol) nanocomposite films containing chemically exfoliated molybdenum disulfide. Materials Chemistry and Physics, 2013. 137(3): p. 764-771. Lee, C., et al., Anomalous Lattice Vibrations of Single- and Few-Layer MoS2. ACS Nano, 2010. 4(5): p. 2695-2700.
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Figures Caption Figure 1. Modify and intercalate stages of MoS2 nanolayers. ...............................................18 Figure 2. FTIR spectra of MoS2, A-O-MoS2, A-O-T-MoS2 and (A-O-T-MoS2)'. ...................3 Figure 3. Raman spectrum of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2 .......3 Figure 4. XRD patterns of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2 ...........4 Figure 5. Contact Angle of water droplet on the surface of (MoS2)2, (A-O-MoS2)2 and (AO-T-MoS2)2. Methanol (methyl alcohol) droplet on the surface of (MoS2)1, (AO-MoS2)1 and (A-O-T-MoS2)1, ...........................................................................5 Figure 6. FESEM images of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2. .......7 Figure 7. EDX dot mapping analysis of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-OT-MoS2. ...............................................................................................................9 Figure 8. TEM images of A-O-MoS2 and A-O-T-MoS2. .......................................................9
17
Figures
Figure 1. Modify and intercalate stages of MoS2 nanolayers.
18
Figure 2. FTIR spectra of MoS2, A-O-MoS2, A-O-T-MoS2 and (A-O-T-MoS2)'.
Figure 3. Raman spectrum of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2.
19
Figure 4. XRD patterns of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2
20
Figure 5. Contact Angle of water droplet on the surface of (MoS2)2 and (A-O-MoS2)2 . Methanol (methyl alcohol) droplet on the surface of (MoS2)1 and (A-O-MoS2)1
21
22
Figure 6. FESEM images of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2 .
23
24
Figure 7. EDX dot mapping analysis of MoS2, A-MoS2 , A-U-MoS2, A-O-MoS2 and A-O-T-MoS2.
Figure 8. TEM images of A-O-MoS2 and A-O-T-MoS2.
25
Table 1. BET surface area of samples Sample MoS2 A-MoS2 A-U-MoS2 A-O-MoS2 A-O-T-MoS2
Sonication Conditions Power Time (min) (W) 15 250 5 250 5 250
SBET (m2/g) 8.32 9.79 12.54 17.78 17.95
Table 2: values of equilibrium contact angle for modified MoS2 samples 26
Sample (MoS2) (A-O-MoS2) (A-O-T-MoS2)
Water contact angle (°) 71.20 42.88 41.04
Methanol (methyl alcohol) contact angle (°) 69.15 29.29 27.87
Table 3: The values of the dispersive and polar parts of the surface tension of water and Methanol (methyl alcohol)
27
Liquid Methanol (methyl alcohol) Water
ࢽ࢜ /mN m-1
-1 ࢽࡰ ࡸ / mN m
ࢽࡼࡸ / mN m-1
22.5 72.8
18.2 21.8
4.3 51
Table 4: The values of surface free energy of MoS2 and modified A-O-MoS2 and A-O-T-MoS2 discs
Sample (MoS2)
ࢽࡰ ࡿ/
-1
mN m 0.19
ࢽࡼࡿ / mN m-1
ࢽࡿ / mN m-1
41.60
41.79 28
(A-O-MoS2) (A-O-T-MoS2)
0.90 0.94
67.40 69.57
68.30 70.38
29
• Intercalation of Molybdenum disulfide nanosheets using different methods including ultrasonication, acid treatment, oxidation and thermal shocking • Oxidation of molybdenum disulfide via different novel oxidants • Formation of S=O band as a result of oxidation methods
30
S1350-4177(17)30189-X http://dx.doi.org/10.1016/j.ultsonch.2017.04.024 ULTSON 3659
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
4 February 2017 18 April 2017 19 April 2017
Please cite this article as: M. Amini, A.R. S.A, M. Faghihi, S. Fattahpour, Preparation of nanostructured and Nanosheets of MoS2 oxide using oxidation method, Ultrasonics Sonochemistry (2017), doi: http://dx.doi.org/ 10.1016/j.ultsonch.2017.04.024
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Preparation of nanostructured and Nanosheets of MoS2 oxide using oxidation method Majed Amini, Ahmad Ramazani S.A*, Morteza Faghihi, Seyyedfaridoddin Fattahpour
Department of Chemical and Petroleum Engineering, Sharif University of technology, Tehran, Iran
Abstract Molybdenum disulfide (MoS2), a two-dimensional transition metal has a 2D layered structure and has recently attracted attention due to its novel catalytic properties. In this study, MoS2 has been successfully intercalated using chemical and physical intercalation techniques, while enhancing its surface properties. The final intercalated MoS2 is of many interests because of its low-dimensional and potential properties in in-situ catalysis. In this research, we report different methods to intercalate the layers of MoS2 successfully using acid-treatment, ultrasonication, oxidation and thermal shocking. The other goal of this study is to form S=O bonds mainly because of expected enhanced in-situ catalytic operations. The intercalated MoS2 is further characterized using analyses such as Fourier Transform Infrared Spectroscopy (FTIR), Raman, Contact Angle, X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-Ray Microanalysis (EDAX), Transmission electron microscopy (TEM), and BET. Keywords: MoS2, Intercalated, Oxidization, Thermal shocking
*
Corresponding author: Polymer Research Centre, Sharif University of Technology , Azadi Avenue, P.O. Box 11365-9465, Tehran, Iran. Email: [email protected], Tel: +98-21-66166405
Introduction MoS2 is a typical and inorganic layered compound, with strong covalent Mo−S in-plane bonds and weak van der Waals bonding between the sheets [1]. The latter enables mechanical or chemical intercalation [2]. Accordingly, MoS2 has been widely studied due to its potential applications as catalysts [3,4], solid lubricants[5,6] and hydrogen storage materials [7]. Recently, due to the need for more significant performance in catalytic reactions some research works have been devoted to reduce the lateral thickness or increase the interlayer distance to produce MoS2 with improved properties. Joensen et al.[8], suggested a chemical route which was reported to dismantle bulk MoS2 partially into platelets that were nano-sized. Furthermore, preparation of a pure thin layer of MoS2 have been reported using physical vapor deposition [9], chemical bath deposition [10], chemical vapor deposition [11], laser ablation [12] and ion-beam-assisted deposition [13]. Despite using some traditional chemical and mechanical exfoliation methods of MoS2 for the production of layered material, still, these methods are conducted to obtain the MoS2 with more interlayer distance and less lateral thickness to promote its applications in commercially viable devices [14]. Such methods tend to yield exfoliated materials with a relatively low active surface area of the 2D structure. The chemical route, [8, 15-22] normally comprises of non-aqueous lithium intercalation and water exfoliation technique, which demands long hours of intercalation [8,15,23] and additional pre-expanding steps [24]. Exfoliation process has been also done with some metal ions such as lithium ion, which leads to intercalated MoS2 with interesting applications [25]. Accordingly, Bockrath and Parfitt [26] have explained the application of exfoliated MoS2 in coal liquefaction, while Miremadi and Morrison [27] have implemented this method to prepare catalysts for methanation. MoS2 is vastly known for its applications as a catalyst in an industry [28]. Taking its catalytic properties into account, MoS2 has been widely used for the extraction of S and N from crude oil [29-31]. MoS2 as a catalyst has a relatively high activity 1
and also resistance to Sulfur poisoning [32]. In refinery operations, supported catalysts with Mo components are the most widely used catalysts, as a basic active ingredient [33]. The catalytic activity of MoS2 is focused on rare surface sites, while the bulk material is relatively inert, mainly because nano MoS2 has particular size and structure [34-36]. It has been widely known that S-ions at the edges of MoS2 is responsible for the importance of the catalytic activity of MoS2 [33]. As a result, it seems that expansion of MoS2 and formation of extra S=O bonds on the surface of MoS2 layers could improve the catalytic activity of MoS2, significantly. These produced highly active sites could be taken for support of other catalysis and active species for in-situ catalytic processes [33]. In this study, we report a tentative attempt for the preparation of intercalated and exfoliated modified MoS2 using treatments as follow (See Figure 1): acid-treatment, acid-treatment and ultrasonication, acid-treatment and Oxidization, acid-treatment and Oxidization and thermally shocked. In this regard, single-layered and expanded few-layered MoS2 sheets are prepared by above-mentioned methods. In comparison with previous studies, such as mechanical exfoliation techniques, which were able to reduce lateral thickness of MoS2 to 90 nm, the methods in this study were able to produce expanded MoS2 with the lateral thickness less than 40 nm. Moreover, further evidence for the formation of S=O and Mo—O bonds have been illustrated using FTIR analysis.
1 Materials and Methods 1.1 Materials Purified Mineral MoS2 (in bulk form) was purchased from Semnan Deposit, Iran. Sulfuric acid (98%, Merck) was employed for oxidation and acid treatment. Acid Nitric (HNO3) was purchased from Aldrich. The aqueous solutions H2 O2 (hydrogen peroxide, H2O2, 30 wt %) was purchased from Dr. Mojalali Company, Iran. Dimethylformamide (DMF), an organic 2
solvent, was purchased from Merck, Germany, 99% for ultrasonication. All the raw materials were used as purchased with maximum storage time of 2 weeks.
1.2 Nanosheets Preparation and Procedures In this study, five samples have been investigated, in which one of them (MoS2) is in bulk natural form and the other four are orderly acid-treated MoS2 (A-MoS2), acid-treated and ultrasonicated MoS2 (A-U-MoS2), acid-treated and Oxidized MoS2 (A-O-MoS2), acid-treated, oxidized and thermally shocked MoS2 (A-O-T-MoS2). 1.2.1 Preparation of A-MoS2: Firstly, purified naturally occurred MoS2 was crushed and sieved. The particles with the mesh size of 63-74 µm have been used for further works. The crushed and sieved samples washed with deionized water and then dried for 24 hours in vacuum oven at 100 ºC. Moreover, untreated sample underwent acid leaching with a 1M HNO3 for 2h at 80 °C, and the filtrated samples were washed with distilled water. The resulting sample was denoted as A-MoS2. The final treated natural MoS2 sample was dried at 100 ºC for 24h. 1.2.2 Preparation of A-U-MoS2: The abovementioned sample (A-MoS2) was dissolved in DMF solution (0.5 wt/v %) and sonicated using ultrasonic homogenizer (Addeco, Iran), under 250 watts for 15 minutes. Then the result was washed with distilled water and filtered respectively, after which, it was dried at 100 ºC for 24 h in the vacuum oven. 1.2.3 Preparation of A-O-MoS2: The acid treated MoS2 was oxidized using a mixture of two acids and few oxidizing agents. Which can be explained as follow: 3.0 g of A-MoS2 was mixed with 50 mL concentrated sulfuric acid (98%) using stirrer at room temperature, then 1.0 g of sodium nitrate was added to form a well-mixed mixture. Potassium permanganate (6.0 g) was added slowly, under 3
intense stirring, within 5 minutes to keep the temperature of the suspension lower than 20 ºC. Furthermore, the reaction set-up was transferred into an ice bath (0 ºC) for about 30 minutes, forming a brown thick paste. Then, the solution was moved to an oil bath (35 °C) and mixed for 3 hours until the final solution would be in brown colour. Moreover, the solution was immediately transferred back to the ice bath (0 °C) to add 50 mL of distilled water. It is vital to keep the temperature below 60 °C. After half an hour, the solution was moved out of the ice bath and left off until it reached ambient temperature. Moreover, 100 mL distilled water was added slowly, after which the temperature reached 40 °C. Furthermore, 8 mL of H2O2 (30%) was slowly added, that turned the colour of the solution from brown into yellow. In order to eliminate metal ions (Potassium, Sodium, etc.) from the final solution, filtering and washing with 1:10 wt/wt hydrochloric acid aqueous solution (250 mL) was performed, followed by several washings with distilled water. The final nanosheets of MoS2 were dried using vacuum oven at 60 °C for 90 minutes. For further treatment, 0.5 g of the final solid was dispersed in 100 mL DMF using intense stirring. The dispersion was carried out in 15 minutes, using probe ultrasonic homogenizer instrument under powers of 250 watts. Finally, the dispersed solution was subjected to centrifugation at 3000 rpm for 30 min to remove any potential aggregates. The concentration of the MoS2 in the centrifuged suspension was determined by drying a certain volume of suspension and weighing the solid residual. 1.2.4 Preparation of A-O-T-MoS2: Mass amounts of molybdenum disulfide sheets prepared with oxidation method (see section 2.1.3), at temperatures 600 and 400 ° C for 10 minutes, under inert gas (argon with a purity of 99.99 percent) was thermal exfoliation. The abovementioned sample (A-O-MoS2), was thermal-shocked using furnace (AzmaGostar, Iran).
4
1.3 Nanosheets Characterization Methods The morphology of the nano MoS2 was characterized by a Field emission scanning electron microscopy (SEM; TESCAN – MIRA iii). The thickness of layers of MoS2 samples were estimated by ImageJ software. Before the FESEM test, samples of MoS2 were coated with gold. The morphology of crystalline structure of MoS2 samples were investigated using transmission electron microscopy (TEM) analysis (TEM, Zeiss EM900 under 200 kV). The XRD patterns of MoS2 samples were taken via INEL EQUINOX 3000 X-ray diffractometer, using Cu Kα radiation, to determine structural changes and d-spacing. The diffractograms were recorded at a voltage of 40 kV and a current of 50 mA using Cu K radiation (λ= 0.154 nm) with the scanning rate of 1◦/min from 3° to 60°.Chemical structure of MoS2 samples were studied using FTIR spectrometer (ABB Bomem MB-100). The samples were compressed into KBr pellets for each analysis. BET analysis was carried out to determine the specific surface area of the samples, calculated from N2 adsorption result via a surface area analyser (Quantachrome ChemBET-3000) based on the BET equation. EDAX analysis was performed using MIRAPLMU for elemental analysis. Raman spectra of MoS2 samples were measured via Raman analyser (Almega Thermo Nicolet Dispersive Raman Spectrometer).
2 Results and Discussions 2.1 Nanosheets Characterization 2.1.1 FTIR Analysis Figure 2 shows FTIR spectra of MoS2 for three modified samples, (A-O-MoS2), thermally shocked MoS2 at 400 °C (A-O-T-MoS2) and 600 °C ((A-O-T-MoS2)'). It can be seen in spectra of MoS2, there were no characteristic peaks for MoS2. The wide absorbance bands at 3430 cm-1 can be attributed to the adsorbed hydroxyl groups on the surface of MoS2 [37]. Moreover, after modification of untreated MoS2 via acid treatment and oxidation method, 5
absorbance peaks of S=O and Mo-O observed at wavenumbers of 1119.41 and 689.12 cm-1, respectively. The FTIR spectrums of treated MoS2 after thermal shock at 400 ºC clearly illustrated that thermal shocking of the oxidized MoS2 leads to the formation of S=O bonds in (A-O-T-MoS2) sample. According to the spectra of (A-O-T-MoS2)' sample, thermal shock at 600 ºC changed the total physical and chemical structures of A-O-T-MoS2 [37-38]. The FTIR results demonstrated that S=O bond can be successfully formed on the MoS2 structure through acid modification and thermal shock at around 400 ºC. It should be mentioned that S=O bond is vital for in-situ polymerization for our future studies [39-40].
2.1.2 Raman Analysis Raman spectroscopy has been widely used to determine the number of layers. Figure 3 shows the Raman spectra of samples MoS2. This Figure shows Ag1 (out of plane) and in-plane E2g1 bands of MoS2 occur in 356 cm-1, which is attributed to Mo—S vibrations [42] and 405 cm-1, which is assigned to out-of-plane vibrations (Ag1 mode) [43, 44]. However, the intensity of these bonds slightly changed for A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2. Actually, the peak at 356 cm-1 shifted to the further right for example about A-U-MoS2 shifted to (365 cm-1) and the one at 405 cm-1 shifted to the left to 397 cm-1. These results are in agreement with those previously presented in the literature [45, 46]. These peaks were demonstrated the thickness of layers and the shifts in Raman spectrum show that the treatment used in curve attributed to (A-U-MoS2) sample has changed the thickness of layers. We have also observed a distinctive thickness dependence of the intensities and line widths of both Raman modes [47]. It is evident that Ag1 mode is amplified due to formation of interlayer forces as a result of formation of S—O chemical bonds (see Figure 3), which is confirmed by FTIR analysis as well (see section 3.1.1). It is worthy to note that the intensity
6
ratio between the Ag1 and E2g1 modes (Ag1 / E2g1) remained nearly a constant (∼1.88) in the MoS2, consistent with the previous study [48].
2.1.3 BET Analysis Effects of different treatments and intercalating processes on the accessible surface area of MoS2 samples were presented in Table 1. The acid-treatment, ultrasonication process, oxidation processes and thermal shocking have increased the BET area significantly by almost 113 percent for A-O-T-MoS2. Comparison of obtained results with those presented in the literature [49] shows that the process of acid-treatment, oxidation, and thermal shocking of MoS2 (A-O-T-MoS2) is the most appropriate method amongst all others, in terms of higher surface area. The latter further proves the plausibility of the method for intercalation of layers as well. Furthermore, sample (A-O-T-MoS2) has a BET area of almost 2.15 times more than that of the base sample (see Table 1, MoS2). However in previous study by Ross et al.[50], the BET area only showed an increase from 14.2 to 17 m2/g for thermally shocked sample. It is quite tangible that the present study suggests tentative approaches that showed significant improvement in increasing the d-spacing and specific area. 2.1.4 XRD Analysis Figure 4 showed the normalized X-ray diffractograms of different MoS2 samples. Peaks located at 2θ=14° was related to the interlayer distance of MoS2. Although this peak can be observed for all samples, its intensity was significantly decresaed for treated samples which could be attributed to change in interlayer distance of treated and oxidized samples. The presence of the (00l) peaks, located below 10°, in the XRD pattern of treated and oxidized samples (Figure 4, A-O-T-MoS2) clearly demonstrates the increase in d-spacing of these samples MoS2. As oppose to XRD patern of (MoS2) and (A-MoS2), the existance of (001) peaks in the paterns of (A-U-MoS2), (A-O-MoS2) and (A-O-T-MoS2) proved that these 7
treatments were successful at making increase in inter-layer distances. This proved that the treatments used in this study were successful at increasing d-spacing and forming exfoliation structure of MoS2. The significant decrease in intensity observed in (002) with the observation of slight intensity increase in (001) peak of the XRD pattern in the MoS2 suggests a more disordered phase and is agrrement with previous reports [51, 52]. As could be seen from the curve (MoS2), curve (A-MoS2) shows a (001) peak lower which indicates that acid-treated and ultrasound was more effective than acid-treatments only. Other characteristic peaks at 28.8, 33, 39.6, 43.9, 49.2 and 58 correspond to the crystal plane of (100), (103), (006), (105) and (110). These peaks could be ascribed to hexagonal shape of MoS2 [53, 54]. Strong (00l) reflections in the XRD pattern illustrated the crystallinity and ordered stacking of 2D layers in the as-grown MoS2 nanostructures. Regarding layer stacking, the XRD patterns of modified MoS2 samples revealed that the reflection peaks are, in general, broader after chemical exfoliation treatment. Joensen et al [55] sugested that distroying layers along the (00l) axis reduces particle size in the same plane, which leads to broadening of the (002) peak, meanwhile other peaks also show broadening due to rotation of basal planes that reduce he wide order of the corresponding planes. For modified MoS2 samples in this study, the average stacking height, using analysis of (002) reflection X-ray peak broadening, decreases from about 100 to 20 nm after exfoliation. Due to the exfoliation process, which consequently leads to dismantling of layers, reduces the stacking height.
2.1.5 Contact Angle Analysis Contact angle measurements were implemented to investigate the effect of oxidation and thermal shocking on MoS2. Water and Methanol (methyl alcohol) shapes of droplets on the surface of the modified MoS2 discs are illustrated in Figure 5.
8
The values of water and Methanol (methyl alcohol) contact angles on the surface of MoS2, AO-MoS2 and A-O-T-MoS2 were tabulated in Table (2). Surface free energy ( ) values were calculated using Owens-Wendt (Eq. (1) and (2)) equation, orderly [51, 52]. (1) (2) Where,
is surface tension of liquid,
tension of the both liquids and
and
and
are dispersive and polar parts of the surface
are the polar and dispersive parts of the surface free
energy of the samples’ disks, respectively. Contact angle and total surface free energy of the discs are presented by
and
, orderly [52]. Summarized values of the dispersive and polar
parts of the water and Methanol (methyl alcohol) surface tension were shown in Table (3) [53]. According to Figure 5 and Table 4, it can be found that acid treatment and oxidation of MoS2 decreased surface tension and increased surface polarity due to the formation of S-O and Mo-O bonds on the surface of the treated MoS2.
2.1.6 FESEM Analysis Scanning electron microscopy was used to determine the morphology and nanoparticle dispersion and distribution after different modifications. Presented FESEM images in Figure 6 presented somehow layered structure of MoS2 from the cross-sectional view. These FESEM images revealed that MoS2 samples were distributed as the combination of randomly stacked thin nanosheets with a thickness of several nanometres (see Figure 6). It is fairly obvious that the gap space between MoS2 nanosheets was relatively extensive with lamella morphology [56] and the thickness of the MoS2 stacked layers was estimated to be about 50 nm obtaining by image analysing using image j software.
9
These images are prepared in two magnifications: 1) the ones on the left hand side with 1 micro scale bar and 2) the ones on the right hand side with 500 nm scale bar. It is evident that using certain modifications, with more attention on thermal shocking, we have successfully changed the tangled structure (see Figure 6, MoS2) to Figure (6, A-O-T-MoS2) with significant reduction in stack layers. Comparing Figure 6 (MoS2) with (A-O-T-MoS2) illustrated that oxidation and thermal shocking were efficient at exfoliating the layers and making gaps in between them, in which the d-spacing has increased from 50 to 20 nm. It is worthy to mention that structure of MoS2 has not lost its uniformity. In other words, after using three different modifications, the structure of MoS2 is quite ordered (Figure 6, (A-O-TMoS2)). As a result, the increase in d-spacing of MoS2 multi-layered structure was controlled using three different exfoliation process in order to produce single-layered structure, which has not resulted in deformation of layers or any other defective layout. The latter is evident by the result of XRD analysis, in which the peak at 14 is omnipresent in all samples. 2.1.7 EDX Analysis In order to confirm the considered modification of MoS2 nanosheets and determine the change in amount and dispersion of oxygen element, EDX-Dot mapping was implemented. Figure 7 illustrated the EDX mapping micrograph of samples prepared by acid-treatment (Figure 7, A-MoS2), acid-treatment and ultrasound (Figure 7, A-U-MoS2), acid-treatment and oxidization (Figure 7, A-O-MoS2) and acid-treatment and oxidation and thermal shocking (Figure 7, A-O-T-MoS2) samples. In the all samples oxygen and Sulfur were observed. Except for Mo and S and an insignificant amount of oxygen and copper in untreated sample (Figure 7, MoS2), there were not any other components on the EDX mapping micrograph, which further indicates the purity of all prepared samples. It is evident from the micrograph related to the samples MoS2, A-MoS2 and A-U-MoS2, the amount of oxygen is not so high, compared to that of samples A-O-MoS2 and A-O-T-MoS2. This evidence is expected due to 10
nature of oxidation (see section 2.1.3) and thermal shock process. Based on the results of EDAX analysis, the latter methods had the tremendous effect on increasing the amount of oxygen. It is worthy to mention that existence of Ca within samples A-O-MoS2 and A-O-TMoS2 are due to several oxidants used in the oxidized method. Considering the fact that sample A-O-MoS2 increases the oxygen level, comparing to that one of sample A-U-MoS2, EDX dot-mapping micrographs of oxygen shows an acceptable dispersion, however, the distribution of oxygen is not perfectly uniform. In sample A-O-T-MoS2, in which thermal shocking method was implemented to prepare the sample, EDX dot-mapping clearly suggests for bulk motions of oxygen throughout the sample. To the best of our knowledge, the latter might be due to thermal shock and hence, extending the distance between sheets of this sample, which is also approved by FESEM images as well (See section 3.1.6). Figure 7 revealed that acid-treatment and oxidation and thermal shocking of MoS2 are an effective procedure to produce intercalated materials, as it increases the oxygen level. Interestingly, it discussed that distance between sheets of the above-mentioned samples are higher among all other samples (See section 3.1.6 and 3.1.4). 2.1.8 TEM Angle Analysis For nanostructure analysis, transmission electron microscopy (TEM) experiments were performed and the images (Figure 8) provide more detailed structural information of MoS2 prepared by different treatments (A-O-MoS2 and A-O-T-MoS2). The images clearly cited for the intercalated structure of modified MoS2, and the inset shows distinct layered patterns of selected-area. After exfoliation, the randomly stacked MoS2 sheets were clearly visible with an expanded d-spacing, consistent with the XRD pattern. This observation confirms our explanation for the combination of (001) and (002) peaks in Figure 4 (See section XRD analysis), suggesting that it is possible to further intercalated the layers by further optimizing the synthesis condition. We also examine the edges, as shown in Figure (8, A-O-T-MoS2, 11
50nm). The image illustrated that sample A-O-MoS2 and A-O-T-MoS2 have double-layered (2L) and Four-Layered (4L) structure, respectively. The latter is confirmed by the XRD analysis (see Figure 4). As seen in Figure 8, despite the change in method from A-O-MoS2 to A-O-T-MoS2, the grain sizes have not changed significantly. Yet, the difference contrasts in the image suggests that the layers of sample A-O-T-MoS2 have become more dispersed in comparison with Figure 8, A-O-MoS2. 3 Conclusion Molybdenum disulfide with layered structure could be provided by intercalating and exfoliating of Molybdenum disulfide structures via introduced oxidation methods in this study. EDAX results show that oxidation of Molybdenum disulfide has been successfully conducted using a combination of different Acids. Furthermore, XRD patterns and morphological studies by FESEM and TEM revealed that oxidation of molybdenum disulphide could significantly increase interlayer distance and reduce the number of layers in molybdenum disulphide stacks which could be further increased by appropriate ultrasonication and thermal shocking processes. It is worthy of mention, that this intercalated and exfoliated Molybdenum disulfide could be used in many new application including catalyst support and lubrication applications.
ACKNOWLEDGMENTS The Authors would like to thank the Iran National Science Foundation (INSF) for support of this project under, Proposal number 94027859.
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Figures Caption Figure 1. Modify and intercalate stages of MoS2 nanolayers. ...............................................18 Figure 2. FTIR spectra of MoS2, A-O-MoS2, A-O-T-MoS2 and (A-O-T-MoS2)'. ...................3 Figure 3. Raman spectrum of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2 .......3 Figure 4. XRD patterns of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2 ...........4 Figure 5. Contact Angle of water droplet on the surface of (MoS2)2, (A-O-MoS2)2 and (AO-T-MoS2)2. Methanol (methyl alcohol) droplet on the surface of (MoS2)1, (AO-MoS2)1 and (A-O-T-MoS2)1, ...........................................................................5 Figure 6. FESEM images of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2. .......7 Figure 7. EDX dot mapping analysis of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-OT-MoS2. ...............................................................................................................9 Figure 8. TEM images of A-O-MoS2 and A-O-T-MoS2. .......................................................9
17
Figures
Figure 1. Modify and intercalate stages of MoS2 nanolayers.
18
Figure 2. FTIR spectra of MoS2, A-O-MoS2, A-O-T-MoS2 and (A-O-T-MoS2)'.
Figure 3. Raman spectrum of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2.
19
Figure 4. XRD patterns of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2
20
Figure 5. Contact Angle of water droplet on the surface of (MoS2)2 and (A-O-MoS2)2 . Methanol (methyl alcohol) droplet on the surface of (MoS2)1 and (A-O-MoS2)1
21
22
Figure 6. FESEM images of MoS2, A-MoS2, A-U-MoS2, A-O-MoS2 and A-O-T-MoS2 .
23
24
Figure 7. EDX dot mapping analysis of MoS2, A-MoS2 , A-U-MoS2, A-O-MoS2 and A-O-T-MoS2.
Figure 8. TEM images of A-O-MoS2 and A-O-T-MoS2.
25
Table 1. BET surface area of samples Sample MoS2 A-MoS2 A-U-MoS2 A-O-MoS2 A-O-T-MoS2
Sonication Conditions Power Time (min) (W) 15 250 5 250 5 250
SBET (m2/g) 8.32 9.79 12.54 17.78 17.95
Table 2: values of equilibrium contact angle for modified MoS2 samples 26
Sample (MoS2) (A-O-MoS2) (A-O-T-MoS2)
Water contact angle (°) 71.20 42.88 41.04
Methanol (methyl alcohol) contact angle (°) 69.15 29.29 27.87
Table 3: The values of the dispersive and polar parts of the surface tension of water and Methanol (methyl alcohol)
27
Liquid Methanol (methyl alcohol) Water
ࢽ࢜ /mN m-1
-1 ࢽࡰ ࡸ / mN m
ࢽࡼࡸ / mN m-1
22.5 72.8
18.2 21.8
4.3 51
Table 4: The values of surface free energy of MoS2 and modified A-O-MoS2 and A-O-T-MoS2 discs
Sample (MoS2)
ࢽࡰ ࡿ/
-1
mN m 0.19
ࢽࡼࡿ / mN m-1
ࢽࡿ / mN m-1
41.60
41.79 28
(A-O-MoS2) (A-O-T-MoS2)
0.90 0.94
67.40 69.57
68.30 70.38
29
• Intercalation of Molybdenum disulfide nanosheets using different methods including ultrasonication, acid treatment, oxidation and thermal shocking • Oxidation of molybdenum disulfide via different novel oxidants • Formation of S=O band as a result of oxidation methods
30