Experimental And Theoretical Investigations On N/Si/B-doped Flash Nanomaterials

Nanomaterials characterized by small particle sizes,large specific surface areas and high surface atomic ratios show unique optical,electrical,thermal and magnetic properties.They have important potential applications in nano-electronic devices,energy conversions and storages,environmental protections,biomedicines and etc.Atomic doping as a novel modification method has been widely used in the preparation of nanomaterials,rendering them novel structures and properties.In this thesis,the Flash Joule Heating（FJH）apparatus developed in our group is used to prepare nitrogen-doped flash graphenes for high rate supercapacitors and to produce flash Si C nanoballs and nanowires.We also theoretically predicted a series of B/N-doped bilayer graphene sandwiched nanomaterials at density functional theory levels,with their electrical and magnetic properties investigated,aiming to provide theoretical bases for their experimental syntheses.The main contents of this thesis are divided into three parts:1.Flash Nitrogen-Doped Graphene for High-Rate SupercapacitorsNovel structured graphene produced by an environmentally-friendly and high-yielding method together with excellent electrochemical energy storage performance is pursued.A one-pot,solvent-and catalyst-free flash Joule heating approach is developed for synthesizing nitrogen-doped graphene and so named as flash nitrogen-doped graphene（FNG）.The precursors of amorphous carbon black and urea are quickly converted into high-quality FNG in less than 1 s under a short electrical pulse with a bright flash of blackbody radiation.The prepared FNG product features high graphitization with a turbostratic structure.It delivers a high surface-area-normalized capacitance of 152.8μF cm^–2 at 1 A g^–1,an extraordinary rate capability with prominent capacitance retention of86.1%even at 128 A g^–1,and a knockdown relaxation time of 30.2 ms.Besides,the assembled symmetric quasi-solid-state supercapacitor exhibits a high energy density of16.9 Wh kg^–1 and a maximum power density of 16.0 k W kg^–1,as well as desirable cyclic stability（91.2%of initial capacitance is maintained after 10000 cycles）.These outstanding performances show that FNG is a promising candidate for exploiting high-performance supercapacitors.2.Preparation of flash Si C nanomaterialsSilicon carbide（Si C）nanomaterials show promising potential applications in nano-electronic devices,solar cells,luminescent materials,and high temperature catalysts.Therefore,efficient preparations of Si C nanomaterials are of great significance.FJH apparatus was successfully used to prepare silicon carbide nanoballs and silicon carbide nanowires coated with silicon dioxide.Compared with traditional methods such as arc discharge,chemical vapor deposition and carbon thermal reduction,this method is fast,efficient,and simple to operate with low energy consumption.The averaged diameters of the prepared Si C nanoballs lie between 400-500 nm,the lengths of the Si C nanowires are several micrometers,and the diameters of the Si C nanowires fall in the range between 20-70 nm.The microstructures and chemical compositions of Si C nanomaterials were characterized by Raman spectroscopy,X-ray diffraction,scanning electron microscopy,transmission electron microscopy and infrared absorption spectroscopy.3.Theoretical investigations on B/N-doped graphene bilayer sandwich nanomaterialsA series of two-dimensional bilayer sandwich nanomaterials are assembled by bottom-up approaches using the manually constructed D_3h（B₄C₃H₆）₂Cr,D_3h（B₃N₃H₆）₂Cr and C₂（B₂C₂N₂H₆）₂Cr complexes as precursors isovalent with the well-known sandwich dibenzenechromium（C₆H₆）₂Cr,including the metallic（C₆）₂Cr,semiconducting（B₄C₃）₂Cr,metallic（B₃N₃）₂Cr and metallic（B₂C₂N₂）₂Cr.Density functional theory calculations indicate that（C₆）₂Cr,（B₃N₃）₂Cr and（B₂C₂N₂）₂Cr are magnetic metallic materials,while（B₄C₃）₂Cr is a semiconducting nonmagnetic material with the direct band gap of 1.12 e V.