Application of Hydrothermal and Solvothermal Method in Synthesis of MoS 2

: Hydrothermal and solvothermal method were considered as the effective methods for preparation of MoS 2 nanomaterials. The current researches of MoS 2 mainly concerned on electrical properties, the research of reaction system was relatively less. In this paper, synthesis system of MoS 2 was elaborated from precursor, solution, reductant, sulfurizing agent, additive and pH regulator. The application of multifunctional raw materials can greatly simplify reaction system. This provided a reference for the application of hydrothermal and solvothermal method in preparation of MoS 2 .

Many methods were developed to prepare MoS2, such as solvothermal, hydrothermal, templateassisted [21], chemical vapour deposition [22], liquid exfoliation [23], electrochemical anodization [24], insitu-oxidative polymerization [25] and ultrasonication microwave [26]. MoS2 nanomaterials with different morphologies were synthesized such as nanosphere, nanoflowers, nanowires, nanofibers, nanoparticles and microspheres. The researches of hydrothermal and solvent thermal method focused on the properties and applications of prepared materials. The route and mechanism of design reaction system of hydrothermal and solvothermal method was rarely discussed. The advantages and applications of MoS2 nanostructured materials in the area of energy and environment were reported [27,28].  a. Layered structure of MoS2: trigonal prismatic (2H), 3R (rhombohedral and octahedral (1T)), b. "Rim-Edge" The properties and performances of two-dimensional layered MoS2 in electro-chemical application were covered [29]. In this paper, the design route of hydrothermal and solvothermal method for preparation of MoS2 was studied from six aspects (precursor, solution, reductant, sulfurizing agent, additive and pH regulator), which provided a reference for the application of hydrothermal and solvothermal in future.

Materials and methods
For hydrothermal and slvothermal method, teflonlined autoclave was used as reactor, deionized water or organic solvent were used as reaction solvent. Solvothermal method developed on the basis of hydrothermal method, in which deionized water was replaced with organic solvent. These methods were extensively applied to chemical reaction, compound prepared and waste treatment [30][31][32].
Besides precursor, solution, reductant, sulfurizing agent, additive and pH regulator, temperature is key manipulated factor for reaction process, which can affect the properties and morphology of MoS2 [33]. Only amorphous MoS2 is obtained at 120~150 o C [34], with the increasing of reaction temperature, the diameter of products become larger at 230~260 o C [35], smaller at 300~375 o C [36], monolayer MoS2 is prepared above 400 o C [37]. As the reaction temperature goes up, the crystallinity of MoS2 increases and the disorder of material decreases [38]. Besides, high initial temperature also promotes nucleation, thus nuclei aggregation and growth [13], which leads to shorter slabs, more defects and higher catalytic activity.
Compared with solvothermalm method, hydrothermal products were poor crystallinity with more defects such as pleats and holes, which were beneficial to electrical and catalytic performance [24,39]. But doping effect of solvothermal method was better. MoS2/RGO hybrid materials prepared by hydrothermal and solvothermal method were contrasted [40], the results showed that MoS2 and graphene were well doped under solvothermal condition. When DMF was replaced with H2O, two separated phases of MoS2 particles and RGO sheets were obtained. Hydrothermal and solvothermal method carried out at low concentrations, morphologies and properties of MoS2 were easy to fabricate. For low yield, the products were rarely used in industrial catalysis, only applied to the study of catalytic mechanism. So, the preparation of MoS2 at high concentration and its application in the field of industrial catalysis may be a hot issue in future.

Precursor
MoO3, Na2MoO4 [41], H2MoO4, (NH4)6Mo7O24, (NH4)2MoS4, Mo(CO)6 and organic precursor were used in the synthesis of MoS2 commonly. Precursors play an important role in the reaction process, different precursors obtain difference composition and morphology. The concentration of precursor affected the morphology of MoS2 crystallites [35]. With the increasing of concentration, the diffusion rate of ions increases, which can decrease the interfacial reaction rate. High concentration impedes the formation of crystal nucleus leading to larger particle size.
Compared with other precursors, (NH4)2MoS4 is special which contains S and Mo element [36,39,40,50,61,65]. It can decompose into MoS3 at 573K, and then MoS3 can converted into MoS2 [43,44] at 633K. MoS2 microspheres with uniform morphology were prepared with (NH4)2MoS4 as precursors [50]. Among those precursors, MoO3 was used as bridge to connect other precursors. MoO3 can be easily convert to other precursors such as Na2MoO4, (NH4)6Mo7O24), meanwhile other precursors can become MoO3 by high temperature oxidation. MoS2 was prepared by hydrothermal and solventhermal method with MoO3 as precursor, NaSCN as S resource and reducing agent, HCl as pH regulator (Figure 2 a, b). The reaction mechanism was interpreted [43,45] as showed (1,2). Pan [47] prepared MoS2 by solvothermal method with MoO3 as precursor and ethylene glycol as solvent. Among those precursors, MoO4 2-, Mo7O24 2can transform into each other easily in different environments. 3D flower-like MoS2 microspheres (Figure 2 c, d) was synthesized by hydrothermal method with Na2MoO4 as precursor [46] which was comprised by bent sheets with 600nm diameters. Porous MoS2 microspheres were prepared with (NH4)6Mo7O24) [48,49]. Compared with inorganic precursor, the reaction temperature of organic precursor was lower. Hierarchical MoS2 /Polyaniline Nanowires were prepared by hydrothermal with organic precursor (Mo3O10(C6H8N)2.H2O) [51]. Mesostructured lamellar MoS2 was synthesized mesostructured lamellar MoS2 with Mo(CO)6 at 140℃ [42]. New precursor was synthesised through aging, then prepared poor crystalline MoS2 was obtained by hydrothermal stages, the products displayed excellent hydro-desulphurisation performance [91]. In a word, the species and concentration of precursor have large effects on MoS2 crystallity, composition and morphology. Nowadays, it's easy to convert from precursors to MoS2 but harder to reverse, which limits cyclic utilization of Mo resource. To achieve this process, MoO3 precursor maybe an appropriate choice.

Solvent
Deionized water, ethanol, pyrrolidone, N,N-Dimethylformamide (DMF) [40], alkane, N-methyl -2 pyrrolidone (NMP), ethylene glycol [52], n-dodecylamine [53], pyridine [54] and other organic solvents were used as solvent. During the reaction process, the solvent can act as reaction medium and exfoliation agent. The exfoliation effects of nine organic substances were investigated on MoS2 [55]. The results showed that exfoliation of NMP and cyclohexane was the best and the prepared MoS2 was 2~5 layers, effects of other solutions were not obvious regularity. Besides, the exfoliation of methanol, ethanol, propyl alcohol and butanol on MoS2 was studied [57], the rank of denudation ability was methanol < ethanol < propyl alcohol < butanol. Although some solutions no exfoliation ability, their mixed-solvent have certain exfoliation ability. The exfoliation of different concentration of ethanol was investigated [56]. The concentrations of ethanol was 45%, the exfoliation of MoS2 was the best, which can be explained by Hansen solubility parameters. The results of the experiment were well in agreement with Hansen solubility parameters [94] (Figure 3a,b). Mixed-solvent molecular size has an important role in the exfoliation attributing to the larger steric repulsion and this phenomenon is elaborated by Leonard-Jones.  Figure 4a, b was synthesised with NMP as solvent and reductant [41]. These structures exhibited excellent electrochemical performance in both lithium ion storage and hydrogen evolution reaction, because CLET MoS2 exposed more active edges and sites. Pyrrolidone group degraded to produce CO, which improved the purity of MoS2 for high reducibility of CO [61]. Compared with the hydrothermal method, MoS2 prepared by solvothermal method was smaller and uniform. Mesostructured lamellar MoS2 was obtained with n-dodecylamine as the medium [42]. MoS2 nanoflowers decorated reduced graphene oxide paper were prepared with DMF as solvent [62]. There are two theories were applied to select solution such as Young's equation (3) and Hansen solubility parameters [63]. Young's equation involves solid-liquid, solid-gas, liquid-gas surface tension, this method reflects the influence of solution through angle of interfacial tension. Young's equation can be applied to forecast the optimal co-solvent concentration of exfoliation [57]. Hansen solubility parameters combined the effects of dispersion forces, dipole-permanent dipole and hydrogen bonding forces. Hasan solubility parameter can be used in solvent prediction [64,65], the results were coincident with the experimental results. Hansen solubility parameters was suitable for pure solvent and mixed solvent (5). Hansen solubility parameters was applied to predict the exfoliation of layered compounds in different concentration of ethanol [56]. Besides, steric effect, boiling point and molecular weight should be taken into account when select the reaction solution. So, liquid paraffin, diphenyl ether, oleamine, oleic acid, other high molecular weight and high boiling point alkane solvents also are good potential solution.

Reductant
The reductant can reduce Mo 6+ to Mo 4+ , so the addition of reductant can improve the crystallinity of MoS2. The application of reductant reduce the reaction temperature. Generally, water soluble reductants were used in the hydrothermal method, the oil soluble reductants were applied in solvothermal method.
The species of reductant have differen influence on the chemical composition of products. Chemical composition of prepared MoS2 was studied with various reductant [53]. When the reductants were N2H4, NH2OH, N2H4-NR4Cl, NH2OH-NR4Cl, the products were MoS2.4, MoS3.05, MoS2.1C1.58, MoS2.19C1.62. Small molecule reductants were widely used, the applications of macro-molecular reductants were relatively less. Compared with small molecule reductant, there are two advantages for macromolecular reductant. On the one hand, the reducibility of macromolecule reductants is more stable, on the other hand, macromolecule reductants play a role of coating which can affect the morphology of reaction products. Reductants for hydrothermal and solvothermal method are relatively less, reducing agents for other transition metal materials deserve to study and advocate. For example, H2 [70], lithium borohydride [71], 1-octadecene [72], 1,2-dodecanediol [73], ascorbic acid [74].

Additive
Macromolecular surfactant and inorganic salt were used as additives. Surfactants served as template and anchor, which controlled the morphology of the products. Additives improved the dispersion of precursor in solution by creating microenvironment, which increased the dispersion of products [75]. investigated by hydrothermal method [69] (Figure 6a~f), the results showed that various surfactants led to different morphologies. MoS2 microspheres were synthesized with C16H36BrN as additive, the mechanism was elucidated (Figure 7a) [45]. After adding C16H36BrN, the surface of microspheres became smoother. The application of additives was favourable to fixation and nucleation, MoS2/Fe3S4, CoMoS/Fe3S4, NiMoS/Fe3S4 and MoS2/SiO2/Fe3O4 with CTAB and SDS were synthesized as additives [76,77]. The application of additive make the morphology of products become more homogeneous. uniform C@MoS2 was prepared PVP as additive and carbon source [78]. Flower-like MoS2 was prepared with TOP as additive and MoS2NFs composed of crumpled MoS2NSs with more active edges [62]. Ionic liquid also played a crucial role in the formation of MoS2 micro-spheres.
With adding ionic liquid, ion-covered vesicles can be formed, which provide nucleation domains for the hydrothermal reaction. MoS2 nanosheets grew larger, stacked together and curled on the vesicle surfaces during hydrothermal process, resulting in the formation of MoS2 microspheres. The mechanism of the MoS2 microspheres with ionic liquid was illustrated (Figure 7b). Without ionic liquid the structure of products changed obviously [50,79] (the peak of (002) became weaker). Inorganic salts affected the structures and properties of product through synergistic effect and shielding effect. The effects of CH3COONa and NH4Cl on the synthesis of MoS2 was investigated [35]. CH3COONa and NH4Cl can inhibit the formation of MoS2 crystal nucleus at interface, which increase the particle size of product. Wu [80] deemed that the introduction of NH 4+ can improve the ability in the electrical conductivity. Nowadays, the application of additives mainly concentrated on macromolecular surfactants, the research on inorganic salts was rare. The effects of inorganic salt on the products need to be investigated further.

pH regulator
The acidic environment plays an important role in preparation of MoS2 [86], which can accelerate rate crystallization [87] through adjusting the consumption of H + [88,92]. Park believed that H + catalyzed hydrothermal reaction process [82]. The concentration of H + also affects the existence state of precursors [89,92]. pH varies from 0 to14, Mo precursor changes largely (14). Besides, pH affected the bonds between additives and nanoparticles during MoS2 crystallinity process.
The common used pH regulators were HCl [85], H2SO4, NH3.H2O [56] and NaOH. The effects of pH on products were investigated [34]. pH below 6.0, the products consisted of a little MoO2 and MoS2, pH above 7.5, the products were little MoO2 as well as MoS2 and much MoO3, pure MoS2 powder was obtained only when pH between 6.0 and 7.5. Mo conversion increased as the pH increased [77]. Ultrathin MoS2 films with exposed layered structure were grown on fluorine-doped tin oxide (FTO) at pH=10 [68]. Flower-like MoS2 was synthesized with high purity via hydrothermal at pH=6 [66]. Ammoniated MoS2 was prepared with ammonia as the reaction medium [80]. MoS2 nanorods were successfully prepared with sillicontungstic acid [90], the products were nanoparticle without sillicontungstic acid (hydrochloric acid or nitric acid). once sillicontungstic acid was added, the products were nanorods with uniform morphology. Nowadays, the application of pH regulators was limited to inorganic acids, organic acids were less. Compared with inorganic acids, the molecule of organic acids is larger and contains more functional groups. The application of organic acids affect the morphology of products and lead to the formation of metal dopants. Taking citric acid for example, which were applied to the synthesis of nanomaterial [52].it not only can adjust pH, but also can play a role in reduction process.

Conclusions
The current researches of MoS2 mainly concerned on electrical properties, the research of reaction system was relatively less. In this paper, the hydrothermal and solvothermal reaction system was divided into 6 parts, but not every part needed. The application of multifunctional raw materials can greatly simplify reaction system. Besides, low cost, stable and recyclable MoS2 catalysts are also a direction for future research.