First-principles Study The Photoelectric Properties And Hydrogen Storage Performance Of Graphyne Type BC_XN (x=1, 2)

?-graphyne,?-graphyne and ?-graphyne are two-dimensional carbon materials composed of sp² and sp hybrid carbon atoms.They are intrinsic semiconductors with natural band gaps.Moreover,these graphyne have the advantages of large specific surface areas,no cis-trans isomerization,high conjugation and high carrier mobility.Theoretical predictions of ?-graphyne,?-graphyne and ?-graphyne have broad application prospects in the fields of optoelectronic devices,catalysts,energy storage,and battery anode materials.The band gap values of the three structures are relatively small,ranging from 0eV to 0.46eV.Therefore,regulating band gap of graphyne is an effective way to broaden their application fields.?-graphdiyne has been synthesized successfully.?-graphdiyne can be better applied to solar cells,lithium storage,sensors when it is doped by B and N elements.This phenomenon confirms that the performance of graphyne can be improved after other elements.The current research indicates that?,?,?-graphyne-type BN are wide band gap semiconductors with band gap values greater than 4eV,showing a strong optical response in the ultraviolet region.This phenomenon proves that the B and N elements can regulate the photoelectric property of ?,?,?-graphyne effectively.?,?,?-graphyne-type-BN have been studied extensively,but the ?,?,?-graphyne-type-BC_xN structures which retain carbon elements have not been reported so far.Therefore,based on the first principle of density functional theory,we design ?,?,?-graphyne-type-BC_xN.Beacuse carbon atoms in ?,?,?-graphyne have both sp hybridization and sp² hybridization,three methods are used to replace C atoms in graphyne.That is,sp² hybridization carbon atoms,sp hybridization carbon atoms and carbon chain of ?,?,?-graphyne are replaced by an equal number of B and N atoms individually.After geometric optimization of various possible models,?-BC₂N,?-BCN and ?-BC₂N structures are obtained according to the principle of minimum energy.The geometrical optimization of ?-BC₂N,?-BCN and ?-BC₂N are carried out by CASTEP package.Then,the thermodynamic stability and dynamic stability of ?-BC₂N,?-BCN and ?-BC₂N are verified by calculating the cohesive energy and phonon spectrum.Subsequently,the electronic structure,optical properties,mechanical properties,thermal properties and hydrogen storage properties of the three structures are systematically calculated.Considering the importance of band gap for semiconductor,the regulation of the strain on the band gaps of ?-BC₂N,?-BCN and ?-BC₂N are also calculated.The calculation of the electronic structure shows that the band gap values of ?-BC₂N,?-BCN and ?-BC₂N are 2.541eV,0.241 eV and 1.711 eV.They are direct band-gap semiconductors,because the minimum values of conduction band and the maximum values of valence band are at the same high symmetry point.Compared with the band gap values of pure graphyne,the intervention of B and N atoms breaks the original structure symmetry,and the band gap value increases significantly.The chemical bonds are elongated under tensile strain,and the interaction between electrons is weakened.Therefore,the band gap values of the three structures decrease with the increase of strain.Optical properties are calculated through absorption spectra,reflectance,refractive index,extinction coefficient,dielectric function and loss function of ?-BC₂N,?-BCN and ?-BC₂N.The reflectivity of ?-BC₂N,?-BCN and ?-BC₂N are only 0.04,0.1 and 0.18.So,they can be used as absorbing materials.At the same time,their optical properties exhibit anisotropy.The minimum thermal conductivity of ?-BC₂N and ?-BCN is less than that of ZrO₂,which can be used as thermal barrier material.Young's modulus of?-BC₂N is close to half of grapheme,and it has good incompressibility.For hydrogen storage performance,?-BC₂N,?-BCN and ?-BC₂N can adsorb up to 5 hydrogen molecules,12 hydrogen molecules and 10 hydrogen molecules respectively,and adsorption type is physical adsorption.