Series Of Novel Morphology N-doped Graphene Tubes: Synthesis, Mechanism And Field Emission Properties Research

Nitrogen-doped graphene tubes(N-doped GTs),a combination of the nitrogen-doping graphene inherent physical and chemical properties and one-dimensional hollow structure feature,exhibit their extensive application in field emission cathode,new energy battery and supercapacitor electrode materials,ultralight material,et al.N-doped GTs structure is gradually becoming an important member of the graphene nanostructures family.So far,the research on N-doped GTs was still in the start stage,and the preparation method was restricted to the template method.So that,to develop a relatively simple,effective,template-free approach for synthesizing a series of novel N-doped graphene one-dimensional(1D)hollow nanostructures and further to explore their various innovative properties have became a very challenging and urgent research subject.In this paper,a facile template-free,one-step CVR method was developed to obtain a series of novel N-doped graphene 1D hollow nanostructures(nanotubes with different degrees of wrinkles,heterojunction coating nanotubes and helical nanotubes).The study on the preparation technology,growth mechanism,field emission(FE)properties and the properties optimized mechanisms was innovatively carried out.Furtherly,the in-situ mechanics performance and electricity property of the individual helical N-doped GTs were explored.The main research contents and conclusions are as following:(1)The in-situ N-doped GTs were synthesized by a template-free,one-step CVR method using melamine as raw material,Si-Si O2 mixed powder as auxiliary material,methane as supplementary carbon source.The influence rules of the technological parameters(such as reaction temperature,raw material ratio and methane feeding time)on the morphology and field emission properties of N-doped GTs were studied in detail.The reasonable growth mechanism model and the properties optimized mechanism had been established.The research results showed that the in-situ N-doped GTs with smooth surface and wrinkle surface can be respectively obtained by adjusting the technological parameters.The as-prepared in-situ N-doped GTs,especially the products with rich-wrinkles surface,exhibited excellent field emission properties with Eto and Ethr of 0.5V·?m-1 and 1.43V·?m-1,respectively.The enhanced field emission properties was attributed to a synergistic effect of the unique morphology,a high defect density induced by N-doped and a high ratio of graphitic-N bonding configuration in products.(2)The novel in-situ N-doped GTs@Si O2 coaxial nanotubes was constructed.The optimized process parameters were as follows: the raw material ratio n(C3N6H6): n(Si-Si O2)was 1:1,the reaction temperature was 1250?,methane feeding time was 20 min,the heat preservation period before feeding methane was 25 min and the heat preservation period after feeding methane was 20 min.The core of the as-prepared in-situ N-doped GTs@Si O2 was the N-doped GTs with diameter of about 200 nm and the shell was the amorphous Si O2 layer with 8~10 nm thickness.Compared to the smooth surface in-situ N-doped GTs without coating,N-doped GTs@Si O2 revealed superior field emission properties with the Eto dropping from 1.2V·?m-1 to 0.5 V·?m-1 and the current density fluctuation dropping from 3.9% to 2.8%.Based on the systemic characterization results,the properties optimized synergistic mechanism was put forward.The enhanced field emission properties was attributed to a synergistic effect of the unique morphology,the higher ratio of graphitic-N bonding configuration,excellent electroconductivity and the interface effect between N-doped GTs and Si O2 coating.The lower current density fluctuation could primarily be attributed to the protection afforded by Si O2 layer for the N-doped GTs core.(3)With the Ar/H2 atmosphere,helical in-situ N-doped GTs was synthesized.The growth mechanism model was established.The optimized process parameters were obtained as follows: the raw material ratio n(C3N6H6): n(Si-Si O2)was 2:1,the reaction temperature was 1250?,the heat preservation time was 25 min.The outer helix diameters of the products were 400~650nm,the helix pitches were 400~600nm.And the as-prepared helical in-situ N-doped GTs consisted of double walls,the thickness of outer wall was 8~10nm.Compared to ordinary in-situ N-doped GTs,helical products showed superior field emission properties with the Eto dropping from 1.2V·?m-1 to 0.7 V·?m-1,which was attributed to the more local structure defects of the helical structure.(4)For individual helical N-doped GTs,by applying in situ electron microscope technique and real-time electrical measurements,the mechanical and electrical properties subjected to uniaxial tensile strain were investigated.The helical in situ N-doped GTs with different structure parameters revealed different mechanical properties deduced by uniaxial tensile strain.For helical in situ N-doped GTs with larger helix pitch,a 10% elastic deformation before a typical brittle fracture occurred.For helical in situ N-doped GTs with smaller helix pitch,a two-step mechanical feature: linear-elastic followed by a plastic process had been observed.The elastic deformation was about 4.0% and the plastic deformation was about 9.2% before fracture.The in situ electrical measurement experiments showed that the enhancement in conductivity was achieved under uniaxial tensile.When tensile strain reached to 12.52% before fracture,a 14.37% enhancement in conductivity had been achieved.(5)On the basis of the above results,a new raw material system using single Si powder as auxiliary material was designed.The influence rules of the technological parameters(such as raw material ratio,reaction temperature,the heat preservation time and methane feeding time)on the morphology and field emission properties of N-doped GTs were studied in detail.The optimized process parameters were obtained as follows: the raw material ratio n(C3N6H6): n(Si-Si O2)was 2:1,the reaction temperature was 1300?,methane feeding time was 30 min,the heat preservation period was 25 min.The obtained in-situ N-doped GTs revealed the morphology with rich-wrinkles surface.The diameters of the as-prepared products were about 500 nm.The nitrogen-doped bonding configuration in the products was mainly in the form of “graphitic-N”,whose content was up to 87.56%.The synergistic effect of these factors endowed the products excellent field emission properties with the lower Eto and Ethr of 0.6 V·?m-1 and 1.05 V·?m-1,repectively.Compared to the in-situ N-doped GTs with wrinkles prepared in the original Si-Si O2 raw material system,the value of Ethr had a further reduction from 1.43V·?m-1 to 1.05V·?m-1.(6)Utilizing ammonia gas as nitrogen source,secondary nitriding treatment was performed on the prepared in-situ N-doped GTs with rich-wrinkle surface.The influence rules of the technological parameters(such as nitriding temperature and times)on the field emission properties of N-doped GTs were studied in detail.The optimized process parameters were obtained as follows: the secondary nitriding temperature was 900? and the nitriding time was 180 min.In comparison to the pristine in-situ N-doped GTs,the products under secondary nitriding treatment displayed dramatically enhanced FE properties.The Eto was reduced from 0.60V·?m-1 to 0.33V·?m-1.Based on the experiments results,Density Functional Theory(DFT)simulations were performed.The model of pristine large diameter GTs(15,0)was established.The effects on the electronic structures of GTs caused by the different nitrogen bonding configurations(“graphitic-N”,“pyridinic-N” and “pyrrolic-N”)and various contents had been investigated.The results showed that the electronegativity of “graphitic-N” was the strongest and the calculated total densities of states resulted from the “graphitic-N” bonding configurations was greatest.These simulations results gave the theoretical explanation about the outstanding field emission properties of N-doped GTs with higher ratio of graphitic-N bonding configuration.