First Principles Study Of Two-dimensional Photocatalytic Materials And Their Heterogeneous Structures

Semiconductor photocatalysts can split water to produce hydrogen under visible light irradiation.which is one of the most promising ways to produce hydrogen.Two-dimensional photocatalytic materials have the advantages of high specific surface area and high carrier mobility,but their disadvantages such as low visible light absorption and easy compounding of photogenerated carriers are not favorable for the application in photocatalysis.Constructing two-dimensional/two-dimensional heterostructures not only can retain the excellent properties of the original monolayer,but also can promote the separation and migration of photogenerated carriers,prolong the lifetime of photogenerated carriers,and improve the photocatalytic performance.Based on this,the research are as follows by using hybrid density functional theory(DFT)method:(1)The electronic structure,optical properties and photocatalytic performance of monolayer g-C3N4,AlN,BSe,SiS and SiS2 were systematically investigated.The results show that monolayers g-C3N4 and AlN are direct band gap semiconductors,while monolayers BSe,SiS,and SiS2 are indirect band gap semiconductors.AlN possesses superior electron and hole mobility than g-C3N4,and the hole mobility of BSe is better than that of SiS(SiS2).The absorption spectra of the five structures are mainly distributed in the UV region,among which only SiS absorbs more strongly in the solar spectrum in the range of 3～4eV.The band edge positions of all five structures crossed the redox potential of water,which satisfied the conditions for water cleavage by photocatalysts.(2)The g-C3N4/AlN heterojunction was constructed,and the electronic,interfacial and optical properties of the heterojunction were systematically investigated,as well as the effects of electric field and biaxial strain on its photocatalytic properties.g-C3N4/AlN is a direct band-gap semiconductor(2.71eV)with a staggered band arrangement structure.g-C3N4 contacts AlN,and electrons flow from AlN to g-C3N4 until the Fermi energy levels of both are the same.The interface region forms a built-in electric field from AlN to g-C3N4 due to the accumulation of net charge,resulting in the bending of the energy band.When the visible light is irradiated,the synergistic effect of the band bending and the built-in electric field accelerates the compounding of the photoelectrons on the conduction band of g-C3N4 with the holes on the valence band of AlN,which eventually leads to the enrichment of photoelectrons and holes on the conduction band of AlN and the valence band of g-C3N4,respectively,and achieves the effective separation of photoelectrons and holes in space,so that g-C3N4/AlN is a typical direct Z-type heterogeneous junction.Compared with monolayer g-C3N4 and AIN,g-C3N4/AlN heterojunctions can effectively enhance the solar energy absorption and broaden the optical response range.Under the effect of applied electric field and biaxial strain,the characteristics of energy band interleaving remain unchanged,but the band edge position of the heterostructure is changed,and its VBM potential and CBM potential still have strong redox capability,which can meet the redox potential conditions of photocatalytic water cracking.Meanwhile,the band gap gradually decreases as the electric field intensity increases between-0.5V/? and 0.5V/?.The 1%compression strain changes the heterogeneous structure from a direct band gap to an indirect band gap.(3)BSe/SiS(SiS2)heterojunctions were constructed,and the electronic structure,optical properties and photocatalytic performance of the heterojunctions were systematically investigated,as well as the effects of electric field and biaxial strain on their photocatalytic performance.The results show that the SiS/BSe and SiS2/BSe heterojunctions are characterized by an indirect band gap with band gap values of 2.04 eV and 1.48 eV,respectively,and have a staggered band arrangement structure,which is a typeⅡ heterostructure.the SiS/BSe and SiS2/BSe heterojunctions have structural and thermodynamic stability(300 K).Under the synergistic effect of energy band shift and built-in electric field,the photogenerated electrons accumulate on the CBM of BSe and the photogenerated holes accumulate on the VBM of SiS(SiS2),which successfully achieve the separation of photogenerated electrons and space holes and prolong the catalyst lifetime.Compared with monolayer BSe,the two heterojunctions can effectively enhance the solar energy absorption and broaden the optical response range.The biaxial strain and electric field can effectively regulate the band gap and band edge positions of the two heterojunctions.Compared with monolayer SiS2,SiS and BSe,SiS/BSe and SiS2/BSe heterojunctions have lower me*on Γ→K and K→M,indicating higher electron mobility and better photocatalytic activity.The STH efficiencies of SiS/BSe and SiS2/BSe heterojunctions are 16.84%and 26.67%,respectively,which are higher than those of monolayer SiS,SiS2 and BSe,respectively,The results of the study provide a theoretical basis for the design of efficient photocatalysts.