Study On Preparation And Performance Of Two Dimensional Molybdenum Tantalum Carbide By Chemical Vapor Deposition

Transition metal carbides have wide range of applications in catalysis and superconductivity because of their dual properties of ceramics and metals.At present,the preparation of two dimensional transition metal carbides mainly uses chemical etching to etch the MAX（M:transition metal,A:aluminum,X:C or N）phase in order to obtain M_n+1X_n type layered materials.But the size of the two dimensional carbides prepared by this method is between 0.1-10μm,which is currently difficult to break the size limit.The two dimensional materials have been grown by precisely controlling the growth parameters of chemical vapor deposition.However,there are relatively few two dimensional transition carbides prepared at present.In this experiment,a new ternary two dimensional molybdenum tantalum carbide was prepared by chemical vapor deposition,and its stability,mechanical properties,electrical properties were studied.Tantalum wire and molybdenum foil are used as tantalum and molybdenum source.The metal sources diffuse on the surface of liquid copper by the stacking method of molybdenum foil-copper foil-tantalum wire.It is found that two different phases are grown by adjusting the process parameters such as hydrogen and annealing time.The polygonal shape is two dimensional orthorhombic(Ta_1/3Mo_2/3)₂C,and the triangular shape is two dimensional cubic(Ta_0.13Mo_0.87)C.When the annealing time is 30 minutes,orthorhombic molybdenum tantalum carbides are grown on the surface,until the proportion of molybdenum atoms on the surface of liquid copper reaches 90%.When the annealing time is 60 minutes,the tantalum atoms on the surface of liquid copper are 80%-90%.Cubic molybdenum carbide tantalum is grown.Graphene is formed on the surface when the hydrogen flow rate is 100 sccm.The orthorhombic molybdenum tantalum carbide is grown when the hydrogen flow rate is 200-300 sccm.The cubic two dimensional molybdenum carbide is formed until the hydrogen flow rate reaches 400-500 sccm.However,if the hydrogen flow continues to rise to 600 sccm,the tantalum atoms diffused from the tantalum wire to the surrounding become one dimensional tantalum nanowires.With the increase of reaction time（30-120 min）,the cubic molybdenum tantalum carbides grow from small scale aggregation along six directions to omnidirectional coverage growth,forming a thin film.Orthorhombic molybdenum tantalum carbide is the opposite,showing disordered growth.When the reaction time increases from 30 min to 120 min,the size of nanosheets decrease from 64μm to 10μm,but the nucleation density increase dramatically,eventually covering the copper surface on a large scale.In terms of chemical stability,cubic and orthorhombic molybdenum tantalum carbide can keep stable for 2 h in acidic,alkaline and organic solutions.Cubic molybdenum tantalum carbides are etched at 600℃,while orthorhombic molybdenum tantalum carbides are etched at 200℃with 2 h.At roughly the same nanosheet thickness（150 nm）,the Young’s modulus of cubic molybdenum tantalum carbide is230 GPa higher than that of orthorhombic molybdenum tantalum carbide.The surface potential difference between the two dimensional molybdenum tantalum carbide and the silicon substrate is 60 me V,and the thickness has no significant effect on the potential of nanosheets.When biased at 2 V,cubic molybdenum-tantalum carbide reaches a limited current of 5 n A,while orthorhombic molybdenum tantalum carbide reaches the same current with a bias voltage of 4 V.The cubic phase shows better electrical conductivity than the orthorhombic phase.