Charge Distribution Of C And G-C_xN_y

The charge distribution in a material is closely related to its optical and catalytic properties.Many phenomena need to be explained by the charge distribution.Therefore,a systematic and quantitative expression of the charge distribution in a material is a necessary and meaningful work.We use carbon as the research system.Oweing to its excellent properties,it has shown good application prospects in the fields of semiconductors,energy storage,biomedicine and so on.The outstanding performance and potential applications of carbon allotropes show a large extent of its unique scientific and technological importance.The electronic properties of nanomaterials play a vital role in understanding the applications of their electrical conductivity,optoelectronics,sensors,catalysis and magnetic properties.Many phenomena need to be explained by charge distribution,so a comprehensive and systematic understanding of carbon materials charge distribution is a very important and meaningful work.This thesis studies the inhomogeneity of the charge distribution of carbon allotropes(from 0D to3D),and analyzes the characteristics of atomic charge transfer in carbon materials,which bases on the first principles and semi-empirical molecular orbital calculation method.We study the influence of edge shape and size effect on the local atomic charge of graphene.We study the charge distribution of graphite-phase carbon nitride g-C_xN_y nanosheets,and discusse the influence of size and morphology on the charge distribution.The work of this thesis has positive significance for understanding the electronic structure of nanoparticles through the charge transfer between single atoms,and promoting the application of nanomaterials'optical and catalytic propertie s.The main results are as follows:(1)The atomic charge distribution of carbon materials in different dimensions(0D to 3D)is systematically studied by uing the first-principles calculation method.The calculation results show that if one atom in carbon has different surrounding environment(number of bond,bond length and bond angle)from another one nearby,they always have electron transfer,showing non-zero charge.In low dimension carbon,when the surrounding environment of the atom is the same,the amount of charge transfer of the atom has a strong dependence on its position(end,edge and surface).The valence by Mulliken population analysis in CASTEP for carbon is not zero ranging from-0.12e to 0.13e.(2)The semi-empirical molecular orbital method were used to study the local charge distribution in different shape graphene sheets,and the influence of size and edge shape on atomic charges is systematically analyzed.On the whole,the symmetry of charge distribution is closely related to structural symmetry of graphene.The double-bonded atoms lied in zigzag edge lose electrons and show a positive charge,which varies from+0.19 to+0.65e,and the amount of charge transfer is closely related to the size of the bond angle.When the bond angle of the atom is about 116degrees,the amount of charge transfers is the smallest(+0.11?0.12 e).When the bond angle is about 148 degrees,the amount of charge transfer is the largest,reaching+0.65e.The charge value of the double-bonded atoms lies in armchair edge gradually increases with the increase of the triangle area.When the area is large enough(about0.6770?²),the atomic charge changes from negative to positive;The charge of the triple-bonded atom at the edge is affected by the standard deviation of the three bond lengths around it,as well as by the position of the atom(armchair or zigzag edge).(3)The electronic structure,energy barrier and charge redistribution of cyclo[18]carbon and cyclocarbon oxides(C₂₀O₂,C₂₂O₄and C₂₄O₆)on Na Cl(001)surface were computationally studied by using semi-empirical molecular orbital method.The HOMO electronics are mainly distributed at the inner and outer circumferential half circle of the cyclo[18]carbon along triple C-C bonds.However,the LUMO electronics are distributed at above and below surfaces of the cyclo[18]carbon circle along single C-C bonds.The energy barrier is different along different direction.The energy barrier of cyclo[18]carbon on the Na Cl surface is the smallest(0.023e V per C atom).When the cyclo[18]carbon and cyclo[18]carbon oxides molecular are put on the Na Cl surface,they show different amounts of positive charge,no longer neutral.(4)The charge distribution in porous graphite phase carbon nitride g-C_xN_ynanosheets is inhomogeneity but has good symmetry in the symmetrical structure.And sheet boundary has significant effect on the charge distribution.All N atoms have negative charge due to electrons enrichment.The absolute charge value of the edge atoms is significantly larger than that of the internal atoms in g-CN and g-C₂N,which is opposite in g-C₃N₄.For the C atom,all C atoms show positive charge in g-CN.However,in g-C₂N,the double-bond and triple-bond C atoms at the boundary have different gain and loss electrons.The internal triple-bond C atoms show the same positive charge value.Compared with g-C₃N₄ nanosheets,the charge distribution of g-CN nanosheets has obvious size effect and parity effect.The above research results show that the charge inhomogeneity in carbon,especially low-dimensional carbon and g-C_xN_y,is universal.This is contrary to the common sense that the valence of elemental substances is often zero(except for ozone)or that the valences in compounds are often the same.This work is great significance to understand the structure and properties of condensed matter,especially carbon,and can provide a theoretical basis for interpreting experimental results.It can also provide theoretical choices for the design of future nanoelectronic devices based on carbon materials and graphite phase nitrogen carbide two-dimensional materials.It can provide theoretical guidance and new ideas for its preparation and application.