Controllable Synthesis And Capacitive Properties Of Two-Dimensional Molybdenum Nitride Nanosheets

Two dimensional molybdenum nitride（2D MoN）has high metal-like conductivity,large specific surface area,abundant active sites and good chemical stability,which is a promising electrode material for advanced supercapacitor（SCs）.However,unlike layered MoS₂ is connected by weak van der Waals forces,MoN is a kind of non-layered material,and its lattice is connected by strong three-dimensional（3D）chemical bonds.These hard-break bonds make synthesizing of 2D structure difficult,which limit the application of 2D MoN in electrochemical energy storage.Despite the high rate performance is exhibited by MoN,the mass-specific capacity still needs to be further improved when applied to SCs.In this thesis,we design a novel method to prepare 2D MoN nanosheets with alkali metal compound assisted.Moreover,the reaction mechanism of the transformation of layered MoS₂ bulk material into 2D non-layered MoN material is revealed.Besides,through in-situ controlled oxidation,the 2D MoN/MoO₃ composite nanomaterials with high specific capacitance are prepared and the corresponding high energy density SCs are assembled.The main research contents and innovations are summarized as follows:1.Developing a new method for preparing 2D MoN nanosheets assisted by alkali metal compounds.By using the natural molybdenite concentrate（bulk MoS₂）as precursor,the bulk MoS₂ is mixed with alkali metal compounds（Na₂CO₃,K₂CO₃ or Na OH）;then the 2D MoN nanosheets can be obtained by nitriding in NH₃ and acid washing.During the reaction,the alkali metal compounds will immerse into the MoS₂layers,and then form a molten mixture in the process of thermal nitridation.Meanwhile,the molting parts will in-situ decompose into alkali metal oxides（Na₂O,K₂O）,which could assist the breakdown of the Mo-S bond in the MoS₂ at the desulfurization reaction,forming the MoN and intermediate materials（Na₂S or K₂S）.Meanwhile,the intermediate materials could still hold onto the 2D MoN nanosheets,which hinder their aggregation and stacking.Then,simply with acid washing,the 2D MoN nanosheets are finally obtained.This method provides a new technical route for low-cost,green,and efficient large-scale preparation of 2D MoN nanosheets.This strategy could also be extended for the preparation of 2D W₂N nanosheets.2.Designing 2D MoN-based capacitive materials with high specific capacitance and assembling a high-energy-density SC.Since the capacitive properties of 2D MoN nanosheets can not fit the demands of the current application,the partial oxide is involved to form a composite structure.By in-situ controlled oxidation from 300oC to 400oC in air,the 2D MoN/MoO₃ composite nanosheets are synthesized.The higher the oxidation temperature is,the more content MoO₃ can be obtained in the composite,and the specific capacitance of MoN/MoO₃ nanosheets shows a parabolic trend.Among them,the material prepared under 350oC（MoN/MoO₃-350）has better hydrophilicity than MoN,which is conducive to ion adsorption.Moreover,in the MoN/MoO₃-350 heterogeneous composite structure,strong electron interaction exists between MoN and MoO₃,which increases the active site of the reaction and makes it exhibit the best capacitance performance.Specifically,the corresponding electrode shows 265.7 F g^-1 at 1 A g^-1,which is 1.85 times the pristine MoN nanosheets,with average decay of only 0.14%per cycle even after 10,000 cycles.The assembled all-solid symmetric SCs can deliver the specific mass capacity of 74.3 F g^-1 and energy/power density of 5.06 Wh kg^-1 and 455.31W kg^-1,respectively.This work provides a useful strategy for high-performance MoN based electrode materials.