First Principles Study On The Heterostructure,Doping And Nanotube Structure Of G-C₃N₄ Photocatalyst

g-C₃N₄ is widely applied in the field of photocatalysis due to its unique electronic structure,excellent thermal and chemical stability.However,g-C3N4 also has some disadvantages,such as small specific surface area,high recombination rate of photogenerated electrons and holes,and narrower range of visible light absorption.Therefore,modifications of g-C3N4 have become the focus of current research.Clarifying the relationship between the geometrical structure,the band structure and the photocatalytic properties can better promote modifications of g-C3N4.In the paper,the electronic structures and photocatalytic properties of the different g-C₃N₄ system were investigated using the first principles,including g-C3N4 based nano heterostructures,doping g-C₃N₄ system and g-C₃N₄ nanotubes.Through theoretical calculation,the experimental phenomena can be explained reasonably.The main results of this paper are as follows:?1?The energy band structures,carrier mobility and photocatalytic properties of bulk and monolayer g-C3N4 were calculated using the hybrid density functional method.The calculated band gaps of bulk and monolayer g-C₃N₄ were 2.71 and 2.76eV,which were very close to the experimental values.The calculated effective mass and mobility of the carriers indicated that large electron mobility of monolayer g-C3N4 contributed to the separation and migration of electrons and holes,and thus reduced the recombination rate of photogenerated carriers.This is the reason that the photocatalytic activity of monolayer g-C₃N₄ is better than that of bulk g-C₃N₄.The calculated absolute band edge potential pointed out that compared with bulk g-C₃N₄,the conduction band edge potential of monolayer g-C₃N₄ remained unchanged,while the valence band edge potential becomes more positive,which can improve its oxidation ability.?2?The band structure and charge transfer of g-C₃N₄/CdS heterostructure were calculated using the hybrid density functional approach.The interaction between g-C3N4?001?and the CdS?110?surface was explored.The results pointed out that the valence and conduction band edge positions of g-C3N4 and CdS changed with the Fermi level and formed a standard type-II heterostructure.Furthermore,density of states,Bader charge,and charge density difference indicated that the internal electric field facilitated the separation of electron-hole pair in the g-C₃N₄/CdS interface and restrained carrier recombination.These results demonstrated that the band structure of the g-C3N4/CdS heterojunction had significant advantages to improve photocatalytic efficiency under visible-light irradiation,and g-C₃N₄/CdS heterostructure exhibited higher photocatalytic properties than g-C₃N₄ and CdS.?3?The density of states,charge distribution,and the band offset of g-C₃N₄/TiO₂ heterojunction are systematically investigated through the hybrid DFT method.Results indicated that the valence band offset and the conduction band offset of the g-C₃N₄/TiO₂ heterostructure were 0.40 and 0.18 eV,respectively.Interfacial interaction made the TiO₂ surface with negative charge,whereas the g-C₃N₄ surface with positive charge,which led to the formation of a built-in electric field at the interface.Under illumination,the built-in electric field accelerates the transfer of photoexcited electrons in the CB of TiO₂ into the VB of g-C₃N₄,thus resulting in the photoexcited electrons and holes naturally accumulating in the CB of g-C₃N₄ and the VB of TiO₂,respectively.The photoexcited electrons and holes gathering in different surface regions efficiently prolonged the lifetime of photogenerated carriers.Meanwhile,electrons in the CB of g-C₃N₄ and holes in the VB of TiO₂ had stronger redox ability.g-C₃N₄/TiO₂ heterostructure exhibited a direct Z type photocatalytic reaction mechanism.This Z type mechanism can well explain the improvement of photocatalytic activity of g-C3N4/TiO2 heterostructure.?4?The band structure,electronic state density and electrostatic potential of monolayer g-C₃N₄/SnS₂ heterostructure were calculated by the hybrid density functional approach.The results indicated that the g-C₃N₄/SnS₂ heterostructure was a staggered band alignment structure.Interfacial interaction made the g-C3N4 surface with positive charge,whereas the SnS₂ surface with negative charge,and built-in electric field from g-C₃N₄ surface to the SnS₂ surface were formed in the interface.The band bending was aslo occured at the interface.When visible light irradiating,excited electrons in the conduction band of SnS₂ were easy to recombination with the holes in the valence band of g-C3N4 under the effect of band bending and internal electric field,which resulted in the photoexcited electrons and holes naturally accumulating in the conduction band of g-C₃N₄ and the valence band of SnS₂,respectively.The separation of photogenerated holes and electrons in space contributed them to participate in the photocatalytic reaction on the surface,which formed the direct Z type photocatalytic reaction mechanism.The more postive of valence band and more negative of conduction band of g-C₃N₄/SnS₂ heterostructure enhanced its oxidation and reduction ability.?5?The electronic structure and optical properties of phosphorus doped g-C3N4system were calculated by hybrid density functional method.The results indicated that compared with pure g-C3N4,The band gap of phosphorus doping N2 and C1 site in g-C3N4 reduces to 2.03 and 2.22 eV,respectively.The CBM and VBM potentials of g-C3N4 with P@N2 and P@C1 shifted downward compared with that of pristine g-C3N4.It indicates that oxidation ability of VBM of two doping system increases.The driving force of CBM for reduction process decreases,but the CBM positions of g-C₃N₄ with P@N2 and P@C1 are still more negative than H⁺/H₂?0 V?,and can reduce H⁺to H₂.In addition,optical absorption coefficients were enhanced and the absorption energy ranges were largely extended in the P doped g-C3N4 systems.?6?The geometrical and electronic structures and photocatalytic properties of the zigzag?n,0?and armchair?n,n?g-C3N4 nanotubes?n=6,9,12?were systematically investigated using the hybrid density functional method.Band calculations show that?9,0?g-C3N4 nanotube was indirect band gap semiconductor,?6,0?,?12,0?,?6,6?,?9,9?and?12,12?g-C3N4 nanotubes were direct band gap semiconductors.The band gap of?n,0?g-C3N4 nanotubes?n=6,9,12?increased with the increasement of diameter.However,the band gap of?n,n?g-C3N4 nanotubes?n=6,9,12?decreased with the increasement of diameter.The effective mass of the electrons of?n,0?g-C3N4nanotubes?n=6,9,12?was smaller than that of the holes,which was favorable for the migration of the electrons.However,the effective mass of the holes of?n,n?g-C3N4nanotubes?n=6,9,12?was smaller than that of the electrons,which was beneficial to the migration of the holes.The calculation of the absolute band edge potential of?n,0?and?n,n?g-C3N4 nanotubes?n=6,9,12?had the ability to split water for hydrogen production.Except for the zigzag?6,0?g-C3N4 nanotube,other g-C3N4 nanotubes had the ability to split water for oxygen production.