Influence Of Order Degree Of Coal-Based Graphite On Structure Change Of Graphene

Based on the knowledge of mineralogy,mineral materials and crystal chemistry,this paper studied the structural order degree of coal series cryptocrystalline graphite in Hunan,and put forward the structural evolution law of coal in graphitization process.Then graphene oxide,reduced graphene oxide and liquid phase exfoliated graphene were prepared by oxidation-reduction method and liquid phase exfoliation method,respectively,using the coal-based cryptocrystalline graphite with different graphitization degrees as raw materials.The chemical composition,morphology and degree of structural defects of graphene in various stages of preparation were characterized.Structure evolution of graphite-graphene oxide,graphene oxide-reduced graphene oxide and graphite-liquid phase exfoliated graphene were described in detail.The relationship between the order degree of graphite structure and the yield of graphene and its influence mechanism were analyzed.The mineralogical characteristics such as the chemical composition,mineral composition and structure of coal-measure graphite in Lutang,Chenzhou city,Hunan Province,and Shixiangli,Baichong,Shengli Mining area in Xinhua County,were systematically studied by XRD,XPS,Raman,and TEM.It was found that the closer the distance between graphite and magmatic rocks,the higher the degree of metamorphism.The graphitization degrees of Shengli,Baichong,Shixiangli,Lutang coal-grade graphites were 39.4%,57.8%,68.8%,80.6%,respectively.There C/O ratios were 12.62,13.20,29.01,and 31.76,respectively.The ID/IG ratios were 1.88,1.32,0.75,and 0.46,respectively.It can be seen that the increasing degree of metamorphism represented the increase of graphitization degree and C/O ratio,as well as the decrease of defect rate.In addition,with the increase of graphitization degree,the selected area electron diffraction patterns in TEM images gradually changed from the diffractive rings to the standard hexagonal diffraction spots,which proved that the ordering degree of graphite lattice structure increased obviously.Through in-depth analysis of the structure and composition characteristics of graphite in coal measures with different graphitization degrees,it is concluded that there were five-member rings and seven-member rings scattering randomly in the carbon six-member rings.In addition,some heteroatoms such as O,H,N,S were connected with carbon atoms.It will cause the carbon layer to bend when several five-member rings and seven-member rings are connected and lead to the formation of disordered domains,resulting in the average interval of the whole layers to be larger.The graphitization process of anthracite is accompanied by the removal of heteroatoms such as O,H,N.S and the planarization of warping structure units.That is,the "disordered domain" evolved into the "ordered domain",which is ideal crystal structure of graphite.The graphitization process of coal was divided into four stages by comprehensive analysis of different graphitization degrees by XPS,XRD,Raman,and TEM.Each stage corresponds to one different type of carbon layer structure.(1)completely ungraphitized stage(graphitization degree g?40%),the whole carbon layer appears as irregular "vortex layer structure";(2)low graphitization stage(graphitization degree 40%<g<80%),the carbon layer appears as "wrinkle structure",(3)high graphitization stage(graphitization degree 80%? g<100%),the carbon layer presents a "perturbed structure";(4)the whole graphitization stage(graphitization degree g=100%),the carbon layer is "flat structure".In order to study the effect of graphite structural order on the structural change of graphene,a series of graphene oxide with different degrees of oxidation were prepared by using coal-based cryptocrystalline graphite with different graphitization degrees and reference flake graphite as raw materials.The oxygen functional groups and morphologies of each sample were characterized by XPS,SEM,TEM,Raman,XRD,and so on.It was found that the optimum mass ratio of potassium permanganate to graphite was 4:1.The order degree of graphite structure has an important effect on the preparation and structure of graphene oxide.The graphite with high graphitization degree(g? 80%)can be converted into graphene oxide by 100%.The existence of a small number of "disordered domains" will not affect the oxidation reaction.Although the graphitization degree of Lutang coal-based cryptocrystalline graphite is lower than that of flake graphite,it is easier to be oxidized than flake graphite,and the degree of oxidation is higher than that of flake graphite.The graphitization degrees of Shixiangli and Baichong coal-based cryptocrystalline graphite were low.Due to the existence of a large number of "disordered domains" in the structure,the crystal layer is imperfect and contains a large number of heteroatoms,which hinders the oxidation efficiency.The relationship between the conversion rate of graphene oxide(y)and graphitization degree(x)of graphite with low graphitization degree is y=2.292x-0.883,R2=0.983.When the graphitization degree of the sample is less than 40%(Shengli sample),it is completely ungraphitized.It belongs to the high grade metamorphic anthracite and cannot be oxidized by potassium permanganate.In order to improve the properties of graphene oxide,the green reduction of graphene oxide was carried out with anthocyanin from black bean skin for the first time,and compared with that of hydrazine hydrate.The chemical composition,morphology,and structure of the reduction of graphene oxide were studied by XPS,SEM,TEM,Raman,and XRD.It is found that the reduction efficiency of graphite is significantly affected by the graphitization degree of original graphite.For the reduction of graphene oxide by hydrazine hydrate and anthocyanidins,the higher the order degree of graphite structure is,the easier its oxidation products to be reduced,and the higher the C/O ratio after reduction is.In addition,the coal-based graphite with high graphitization degree(graphitization degree g? 80%)and small particle size is easier to be reduced than flake graphite,and the reduction efficiency(?C/O ratio)is higher than that of flake graphite.To make up this property deficiency of reduced graphene oxide,in this paper,the ultrasonic-assisted liquid-phase exfoliation method was used to prepare the high-purity graphene sheets with the thickness of about 1 nm,and 3 layers,for the first time,using urea as the exfoliation assistant.The addition of urea greatly improved the yield of graphene and the stability of dispersion.Urea molecules can act as "molecular wedges"to enter the graphite interlayer,weaken the van der Waals force between the layers,and separate the lamellae under the action of ultrasonic wave.A portion of urea molecules were attached to the exfoliated graphene sheets to prevent its reaggregation so that it can be stably dispersed in the solvent.Urea-assisted liquid phase exfoliation processe can remove O,N and other heteroatoms,resulting in a large increase in the C/O ratio of the exfoliated products.Whether it is low graphitization degree graphite or high graphitization degree graphite,there is an exponential relationship between the liquid phase exfoliated graphene yield y and graphitization degree x,y=0.003e4 286x,R2=0.992.With the increase of the order of graphite structure,the yield of exfoliated graphene in liquid phase increased exponentially.Graphite with low graphitization degree is not suitable for the preparation of graphene by liquid-phase stripping because of its low yield of graphene.Humans are moving from the "silicon age" to the "carbon age".As a substitute for flake graphite,coal-based cryptocrystalline graphite will draw more attention.Therefore,the study on the preparation and mechanism of graphene produced from coal-based cryptocrystalline graphite is of important significance.