The High Surface Ratio Micro-MoS 2 Grain Composed of MoS 2 Nanosheet Prepared with One-Step Hydrothermal Synthesis

Micro molybdenum disulfide was prepared with one-step hydrothermal method; the influence of reactant concentration and temperature on the surface ratio of micro-MoS2 grain was investigated. Raman spectroscopy (Raman), X-ray diffraction (XRD), and Scanning electron microscopy (SEM) were used to characterize the structure, composition and morphology of MoS2. The results show that micro-MoS2 grains were synthesized with one-step hydrothermal synthesis, and the morphology of micro-MoS2 grains is like flower and sphere. The SEM figures indicate that the surface ratio of micro-MoS2 grains is different and also show that the surface ratio of micro-MoS2 grains can be improved by regulating reactant concentration and temperature. This research showed a method to improve the surface ratio of micro-MoS2 grains.

Because of its high surface ratio, micro-size MoS 2 plays a crucial role in battery cathodes domain [12].The devices of energy-storage play an important role in reducing the emission of greenhouse gas, wasting of resources and the environmental pollution [13] [14] [15] [16] [17].MoS 2 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 MoS 2 on the surface of hydrogen storage alloys [18].
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 MoS 2 will change prominently; because of that tremendous research has been made in gas sensing application, such as the detectors for H 2 O, NH 3 , NO and many other chemical vapors [19]- [29], but there is little research on systematic study about improving the surface ratio of MoS 2 .
Here, we report the synthesis of the micro-MoS 2 composed of MoS 2 nanosheet with high surface ratio.
Through regulating the concentration of the reactant and the temperature, the morphology and the size of the MoS 2 grain will be different.This work shows that the surface ratio of micro-MoS 2 grains can be improved by regulating reactant concentration and temperature.

Preparation of Samples
The (NH 4 ) 2 MoO 4 , N 2 H 4 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.

Experimental Methods
The deionized water, S, N 2 H 4 and (NH 4 ) 2 MoO 4 (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.mode) appear which approved the existence of MoS 2 [31].

Result and Discussion
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 MoS 2 in every group, which suggests that MoS 2 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 MoO 3 in group A, which suggests that the MoS 2 synthesized in group A contain MoO 3 (JCPDS 73-1544) Figure 4 shows the SEM images of groups A, B, C. We can see that every group consists of MoS 2 nanosheet, the MoS 2 nanosheet is about 14 nm thickness, but the size of the nanosheet is different, temperature and concentration impact the morphology of MoS 2 , 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.will determine the size and amount of nanosheet in a unit of the grain (surface ratio), so the surface of MoS 2 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 MoS 2 nanosheets increasing, when the temperature increase from 180˚C to 200˚C and 220˚C the distance between the nearby MoS 2 nanosheet increasing.

Because of van der
From Figure 4 to get 20 points, in order to get the average distance of the nearby MoS 2 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  the temperature gradient, so when temperature increasing, the kinetic energy of the H 2 O molecule increase and there will be more H 2 O molecule knock the MoS 2 nanosheet, when its impact is bigger than the Van de Waals force, the Van de Waals force cannot to draw the nearby MoS 2 nanosheet to get closer, so the distance of the nearby MoS 2 nanosheet will be far.At the beginning of the reaction, MoS 2 tend to form nanosheet to different direction, when the concentration of reactant is small, the rapid of forming the MoS 2 nanosheet is relatively slow, the Van de Waals force have enough time to draw the nearby MoS 2 nanosheet to get close, so the distance of the nearby MoS 2 nanosheet is small, on the contrary when the concentration of reactant is high, the rapid of forming the MoS 2 nanosheet is relatively fast, before the Van de Waals force drawing the nearby MoS 2 nanosheet to get more close the MoS 2 nanosheet have formed a steady structure that can balance it, so the distance of the nearby MoS 2 nanosheet is far, but as the reaction going on the concentration of reactant will become small, so near the last formed nearby MoS 2 nanosheet their distance will be small, that explained why in the high concentration situation the distance of the nearby MoS 2 nanosheet is inconformity so high concentration of reactant and temperature will increase the distance between the nearby MoS 2 nanosheet and decrease the surface ratio of MoS 2 .In other words, low concentration of reactant and temperature will decrease the distance between the nearby MoS 2 nanosheet and increase the surface ratio of MoS 2 , which could improve MoS 2 to absorb and storage gases.

Conclusion
The micro-MoS 2 grains were synthesized with one-step hydrothermal synthesis.The micro-MoS 2 grains are made of MoS 2 nanosheet.Low concentration of reactant and temperature will be good for forming nanosheet and increase the distance between the nearby MoS 2 nanosheet.Low concentration of reactant and temperature will increase the surface ratio of MoS 2 .

Figure 2
Figure2shows 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-MoO 3 , this match well with the results in previous work[30], 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 MoO 3 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 (NH 4 ) 2 MoO 4 will becoming MoO 3 first and then the MoO 3 react with S and N 2 H 4 to produce MoS 2 in the condition of group B and C. We can see that the peak intensity of 405 (A1g mode) and 385 (E12g

Figure 4 .
Figure 4. SEM images of group A, B and C (20 K).The rest of figures share the scale in figure A.

Figure 5 .
Figure 5.The plan sketch assumed in the calculation of each group.

Table 1 .
Reactant quantity and temperature of group A, B, C.

Table 2 .
MoS 2 nanosheet decrease, when the temperature increase from 180˚C to 200˚C and 220˚C the surface ratio of the nearby MoS 2 nanosheet increasing andA3 has the highest surface ratio 0.0218 nm/nm 2 .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 H 2 O molecule are different in the direction of The approximate value of the distance between nearby MoS 2 nanosheet of group A, B, C.