The transition metal dichalcogenide (TMDC) attracted great attention due to their excellent chemical, electrical and optical properties over the past decade    . Graphene was the first 2D material to be found   . Graphene has very high carrier mobility, strength, surface area and thermal conduction capability      , so graphene has great potential applicant in many fields. But graphene’s band gap is zero, limiting its electronic applications. Molybdenum disulfide (MoS2) is the most studied TMDC, which has a special microstructure like sandwich (S-Mo-S), Mo-layer between two sulfur layers by covalent forces, and is relatively abundant as a mineral and well-known for tribological, petroleum desulfurization and catalytic applications  .
Because of its high surface ratio, micro-size MoS2 plays a crucial role in battery cathodes domain  . The devices of energy-storage play an important role in reducing the emission of greenhouse gas, wasting of resources and the environmental pollution      . MoS2 could be used in these devices of energy-storage. L. X. Chen etc. improved the capacity retention rate of the composite electrode up to 50.5% at the current density of 3000 mAg−1 when discharging through coating MoS2 on the surface of hydrogen storage alloys  .
The gas molecules can infiltrate and diffuse freely between the vertically stacked S-Mo-S layers. After the adsorption and diffusion of gas molecules between the S-Mo-S layers, the resistance of MoS2 will change prominently; because of that tremendous research has been made in gas sensing application, such as the detectors for H2O, NH3, NO and many other chemical vapors  -  , but there is little research on systematic study about improving the surface ratio of MoS2.
Here, we report the synthesis of the micro-MoS2 composed of MoS2 nanosheet with high surface ratio.
Through regulating the concentration of the reactant and the temperature, the morphology and the size of the MoS2 grain will be different. This work shows that the surface ratio of micro-MoS2 grains can be improved by regulating reactant concentration and temperature.
2.1. Preparation of Samples
The (NH4)2MoO4, N2H4 and S (All purchased from the Sinopharm Chemical Reagent Co., Ltd.) are the reagents used in the experiment, and they are all analytical purity. The reaction vessels (autoclaves) are 25 ml polytetrafluoroethylene liner, which must be wrapped in the stainless hydrothermal reactor. Nine autoclaves were divided into 3 groups (A, B, C). The reaction temperature of and the reactant concentration of group A, B, C are different, the details are shown in Table 1.
2.2. Experimental Methods
The deionized water, S, N2H4 and (NH4)2MoO4 (details shown in Table 1) were dispersed in hydrazine hydrate, then the solution was transferred to the 25 ml polytetrafluoroethylene liner, the polytetrafluoroethylene liner was put into stainless steel hydrothermal reactor. After tightening the reactor, it was placed into a calorstat, the temperature of the calorstat was kept at a constant value in 24 h, Figure 1 shows this process, after that the reactor was naturally cooled to room temperature, then the solutions were filtered, distilled and dried to obtain reaction production.
Table 1. Reactant quantity and temperature of group A, B, C.
Figure 1. Schematic diagram of processes of the hydrothermal synthesis (A: (NH4)2MoO4, B: S, C: N2H4 and deionized water).
2.3. Characterization of the Composition, Structure and Morphology of As-Prepared MoS2/MoO3 and MoS2 Samples
Laser confocal Raman spectroscopy (Horiba JobinYvon LabRAM HR800 532 nm 200 - 1100), Crystal structure was analyzed by X-ray diffraction (PANalytical PW3040/60, voltage 40 kV, current 40 mA, Cu Kα radiation (λ = 1.5406), 2θ = 10˚ - 80˚). Morphologies and compositions of the samples were characterized using scanning electron microscopy (SEM, HITACHI S-4800, 5.0 kV).
3. Result and Discussion
Figure 2 shows the Raman spectra of group A, B and C. the picture (1) is the Raman spectra of group A, the picture (2) is the Raman spectra of group B, the picture (3) is the Raman spectra of group C. It can be observed that all groups have three main peaks at 285 cm−1, 823 cm−1 and 996 cm−1 of α-MoO3, this match well with the results in previous work  , but Raman peak intensity of group A (180˚C) is the highest and group C (220˚C) is the lowest no matter how much the reactant concentration is, so temperature is the main fact impacting the hydrothermal synthesis and the higher the temperature, the less MoO3 come into being. When the temperature rises to 200˚C or 220˚C it will pass 180˚C not immediately, so we can conclude that (NH4)2MoO4 will becoming MoO3 first and then the MoO3 react with S and N2H4 to produce MoS2 in the condition of group B and C. We can see that the peak intensity of 405 (A1g mode) and 385 (E12g
Figure 2. Raman spectra of group A(1), B(2) and C(3).
mode) appear which approved the existence of MoS2  .
Figure 3 shows the XRD patterns of groups A, B, C, the picture (1), (2) and (3) are the XRD patterns of group A, B, C. There are there obvious diffraction peaks at 2˚ of 14.1, 32.9˚, 35.9˚, that corresponding to (0 0 2), (1 0 0) and (1 0 2) planes of MoS2 in every group, which suggests that MoS2 is synthesized (JCPDS 75-1539). There are three diffraction peaks at 2˚ of 12.0˚, 12.8˚and 17.6˚, that corresponding to (0 0 1), (0 1 0) and (0 1 1) planes of MoO3 in group A, which suggests that the MoS2 synthesized in group A contain MoO3 (JCPDS 73-1544)
Figure 4 shows the SEM images of groups A, B, C. We can see that every group consists of MoS2 nanosheet, the MoS2 nanosheet is about 14 nm thickness, but the size of the nanosheet is different, temperature and concentration impact the morphology of MoS2, the higher the concentration of reactant the bigger the nanosheet will be produced, the higher the temperature the bigger the nanosheet will be produced. A3 generated microsphere structure and its diameter is 1.215 µm, when the concentration increase from A3 to A2 and A1 the microsphere structure disappeared, so low concentration of reactant is good for generating microsphere structure. From the comparison of A3, B3 and C3 we can see that microsphere structure did not appear at high temperature (200˚C, 220˚C), so high temperature (>200˚C) will restrain the generating of microsphere structure.
Because of van der Waals forces, the nearby MoS2 nanosheet trend to gather together forming MoS2 grains. The distance between the nearby MoS2 nanosheet
Figure 3. XRD patterns of group A, B and C, typical patterns of MoS2 and MoO3 (JCPDS 75-1539, JCPDS 73-1544).
Figure 4. SEM images of group A, B and C (20 K). The rest of figures share the scale in figure A.
will determine the size and amount of nanosheet in a unit of the grain (surface ratio), so the surface of MoS2 nanosheets will be different in surface ratio. From Figure 4 it can be easily found that when the concentration increase from A3 to A2 and A1 the distance between the nearby MoS2 nanosheets increasing, when the temperature increase from 180˚C to 200˚C and 220˚C the distance between the nearby MoS2 nanosheet increasing.
From Figure 4 to get 20 points, in order to get the average distance of the nearby MoS2 nanosheet as shown in the Table 2. To simplify the calculation, it is assumed that all the morphology of each group are sphere and only one surface across the body-center of the sphere as shown in Figure 5, the simplified calculation results show in the Table 2. From the result, it can be seen that when the concentration increase from A3 to A2 and A1 the surface ratio of the nearby MoS2 nanosheet decrease, when the temperature increase from 180˚C to 200˚C and 220˚C the surface ratio of the nearby MoS2 nanosheet increasing andA3 has the highest surface ratio 0.0218 nm/nm2.
On the micro level, the kinetic energy of the molecule is proportional to the temperature. It is obvious that the temperature gradient exist in the reaction vessels, so the kinetic energy of the H2O molecule are different in the direction of
Table 2. The approximate value of the distance between nearby MoS2 nanosheet of group A, B, C.
Figure 5. The plan sketch assumed in the calculation of each group.
the temperature gradient, so when temperature increasing, the kinetic energy of the H2O molecule increase and there will be more H2O molecule knock the MoS2 nanosheet, when its impact is bigger than the Van de Waals force, the Van de Waals force cannot to draw the nearby MoS2 nanosheet to get closer, so the distance of the nearby MoS2 nanosheet will be far. At the beginning of the reaction, MoS2 tend to form nanosheet to different direction, when the concentration of reactant is small, the rapid of forming the MoS2 nanosheet is relatively slow, the Van de Waals force have enough time to draw the nearby MoS2 nanosheet to get close, so the distance of the nearby MoS2 nanosheet is small, on the contrary when the concentration of reactant is high, the rapid of forming the MoS2 nanosheet is relatively fast, before the Van de Waals force drawing the nearby MoS2 nanosheet to get more close the MoS2 nanosheet have formed a steady structure that can balance it, so the distance of the nearby MoS2 nanosheet is far, but as the reaction going on the concentration of reactant will become small, so near the last formed nearby MoS2 nanosheet their distance will be small, that explained why in the high concentration situation the distance of the nearby MoS2 nanosheet is inconformity so high concentration of reactant and temperature will increase the distance between the nearby MoS2 nanosheet and decrease the surface ratio of MoS2. In other words, low concentration of reactant and temperature will decrease the distance between the nearby MoS2 nanosheet and increase the surface ratio of MoS2, which could improve MoS2 to absorb and storage gases.
The micro-MoS2 grains were synthesized with one-step hydrothermal synthesis. The micro-MoS2 grains are made of MoS2 nanosheet. Low concentration of reactant and temperature will be good for forming nanosheet and increase the distance between the nearby MoS2 nanosheet. Low concentration of reactant and temperature will increase the surface ratio of MoS2.
This work is supported by the National Natural Science Foundation of China (Grant No.51472096), the R&D Program of Ministry of Education of China (No.62501040202). The authors acknowledge the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for providing Raman measurements. The authors acknowledge the Advanced Manufacturing and Technology Experiment Center of Mechanical College for providing the field emission scanning electron microscope measurements.