| Graphite carbon nitride(g-C3N4)not only shows good photoelectric characteristics in a series of photocatalytic REDOX reactions such as hydrogen production,pollutant degradation,fuel cell,biosensor,etc.,but also has excellent thermal and chemical stability.However,its catalytic activity is hindered by the disadvantages of small specific surface area,low visible light response and fast binding of photogenerated electron hole pairs.Studies have shown that in order to improve the photocatalytic efficiency of g-C3N4 and broaden its application in the field of photocatalysis,vacancy,doping,adsorption and construction of heterojunction can be used to improve the photoelectric characteristics of g-C3N4.In this paper,the first principle calculation method in density functional theory(DFT)is used to construct the models of single vacancy in g-C3N4 and g-C3N4/h-BN,g-C3N4/ZrS2 and g-C3N4/ZrS2/g-C3N4 by using Materials Studio(MS)software.Vienna ab-initio Simulation Package(VASP)is used to complete the calculation and analysis of a series of properties such as crystal structure and optoelectronic properties of Materials.At first,the crystal structure,photoelectronic properties and work functions of the C/N single vacancy of g-C3N4 with two different structures(g-h-C3N4 and g-t-C3N4)are studied.The different vacancy structures of g-h-C3N4 are designated as:V-N11,V-N12,V-N13,V-C11 and V-C12.The results show that the single-vacancy defect structure of g-h-C3N4 has some deformation compared with the original structure,and both are stable.The single-layer g-h-C3N4 is an indirect bandgap semiconductor,the C vacancy is a direct bandgap semiconductor and the N vacancy is an indirect bandgap semiconductor.Expect the V-N13 structure,the bandgaps of all structurals have been reduced to a certain extent,and the optical response of the all structures has been enhanced in the visible region.The different vacancies in g-t-C3N4 are denoted as:V-N21?V-N22 and V-C21.The monolayer g-t-C3N4 is a direct bandgap semiconductor,the N vacancy is an indirect bandgap semiconductor and the C vacancy is a direct bandgap semiconductor.All of the vacancy defect materials have lower bandgap values than the single-layer g-t-C3N4.In the visible region,they also have higher absorption peaks than the monolayer g-t-C3N4,and the optical absorption peaks have an obvious red shift phenomenon.Then,the crystal structure,photoelectric characteristics,charge differential density and band edge position of g-C3N4/h-BN are studied.It is shown that g-C3N4/h-BN is a good stability type-? heterojunction,and it is two-dimensional heterogeneous structure through the van der Waals(vdW)interactions.The band gap is 2.89ev,which is reduced 2.22ev than the monolayer h-BN(5.11ev).The valence band maximum is contributed by the N 2p orbitals in the h-BN,and the conduction band minimum is contributed by the C 2p and N 2p orbital in the g-C3N4.The spectrum of light absorption shows that the g-C3N4/h-BN has stronger light response capacity under visible light than the monolayer g-C3N4 and h-BN heterojunction.g-C3N4/h-BN heterojunction has a conduction band edge greater than-4.44eV and valence band edge less than-5.67eV,which is prone to h+ and reduction of O2-oxidation for improving the redox capacity and photocatalytic efficiency.Finally,the g-C3N4/ZrS2 and g-C3N4/ZrS2/g-C3N4 are preliminarily predicted by simple calculations.According to the result analysis,the band gaps of the two structures are 1.63eV and 1.56eV,respectively.Their conduction band bottom and valence band top are contributed by Zr atoms in ZrS2 and N atoms in g-C3N4.In addition,both kinds of heterojunction have two absorption peaks in the visible light range.The light response capacity of the two-layer heterojunction is obviously weaker than that of the three-layer heterojunction in the light absorption spectrum,and the two absorption peaks are also significantly lower than that of the three-layer heterojunction.Therefore,g-C3N4/ZrS2/g-C3N4 has strong optical responsiveness. |
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