High Photocatalytic Hydrogen Production Performance Of G-C₃N₄ Based Heterojunctions By First Principles

In the field of photocatalytic hydrogen production,graphite-phase carbon nitride（g-C₃N₄）is currently the most concerned two-dimensional material due to its non-toxic,non-polluting,low preparation cost,and high chemical and thermal stability,which has a milestone significance for the application of metal-free photocatalysts in the field of photolysis of water and hydrogen production.However,the high recombination rate of photogenerated carriers of g-C₃N₄ and the low light utilization efficiency limit its application in the field of photocatalytic hydrogen production.In order to solve the shortcomings of g-C₃N₄ in the application of photocatalysis,the paper used WS₂,MoS₂,WSe₂,MoSe₂and g-C₃N₄ to construct heterojunctions.And through the three modification methods of doping,application of external electric field and biaxial strain,effective and feasible schemes to improve the performance of g-C₃N₄/XY₂（X=W,Mo,Y=S,Se）heterojunction photocatalytic hydrogen production were further explored,the main research results of the thesis are as follows:（1）Based on the first-principles method,the HSE06 hybrid functional was used in the CASTEP module of the Material Studio software to calculate the band structure,density of state,work function,plane of average differential charge density,absorption spectrum and other properties of single-layer g-C₃N₄ and XY₂（X=W,Mo,Y=S,Se）.Through the DOS of g-C₃N₄,it was found that the reason for the high carrier recombination rate of g-C₃N₄ is that"bridging nitrogen"hinders the migration of carriers.The large band gap（3.06e V）of g-C₃N₄ causes most of the absorption spectrum to be in the ultraviolet region,resulting in low light utilization efficiency of g-C₃N₄.According to the band edge position diagram of g-C₃N₄ and TMDs,it was found that WS₂,MoS₂ and g-C₃N₄ form typeⅡheterostructure,which is beneficial to reduce the carrier recombination rate.At the same time,the absorption spectrum showed that TMDs can absorb visible light and even infrared light,confirming the excellent optical performance of TMDs.The calculation results showed that:XY₂（X=W,Mo,Y=S,Se）can inhibit carrier recombination by forming a typeⅡconfiguration with g-C₃N₄,and at the same time,it can make use of the excellent optical properties of TMDs to compensate for the lack of g-C₃N₄ in photocatalytic utilization.（2）The HSE06 hybrid functional was also used in the CASTEP module to calculate the electrical and the optical properties of g-C₃N₄/XY₂（X=W,Mo,Y=S,Se）heterojunction.The work function and DOS diagrams showed that g-C₃N₄/WS₂ and g-C₃N₄/MoS₂ are typeⅡheterojunctions,and g-C₃N₄/WSe₂ and g-C₃N₄/MoSe₂ are typeⅠheterojunctions.The photocatalytic charge transfer mechanism（Z-scheme）was described in detail based on the band edge position diagram before and after the two layers of materials are in contact.Due to the combined action of the built-in electric field and the interface barrier,the photogenerated carriers are transferred to different layers,thereby effectively reducing the carrier recombination rate.It was worth noting that even if g-C₃N₄/MoSe₂ and WSe₂ are of type I,the Z-scheme charge transfer mechanism is also applicable,but the pH needs to be adjusted within the range of 3.7～14 and 3.2～12.2 respectively.Finally,according to the absorption spectrum of g-C₃N₄/XY₂（X=W,Mo,Y=S,Se）,it was found that the absorption edge of the heterojunction can reach up to about 900nm.The calculation results showed that:After g-C₃N₄ and XY₂（X=W,Mo,Y=S,Se）construct a heterojunction,the photogenerated carrier recombination rate was reduced by forming a built-in electric field and interface barriers.And TMDs made up for the lack of low light utilization efficiency of g-C₃N₄ by virtue of its excellent optical properties.Therefore,the heterojunctions of g-C₃N₄ and XY₂（X=W,Mo,Y=S,Se）can effectively improve the performance of photocatalytic hydrogen production.（3）By comparing the absorption spectrum of g-C₃N₄/WS₂,the paper supposed that WS₂determines the optical properties of the heterojunction and we considered doping WS₂ to improve the optical performance of the heterojunction.So as to ensure that the photocatalytic mechanism remains unchanged while broadening the absorption spectrum of the g-C₃N₄/WS₂ heterojunction,the paper screened out three doping schemes based on the relative band edge positions:Mn-B co-doping,Mn-F co-doping and W Vacancy doping.Through the plane average differential charge density curve,it was found that the charge transfer amount of the three heterojunctions after doping had increased.According to the calculation of work function and band gap,the photogenerated electron potentials of Mn-B co-doping,Mn-F co-doping and W vacancy doping were-0.59V,-1.06V and-0.55V,respectively,compared with those before doping（-0.535V）have improved.Finally,according to the absorption spectrum,it was found that the three doping schemes of g-C₃N₄/WS₂ broaden the absorption spectrum as expected.The calculation results showed that:The three doping schemes used in the paper can broaden the absorption spectrum of the g-C₃N₄/WS₂ heterojunction,which confirms the previous conjecture that WS₂ determines the optical properties of the heterojunction.（4）So as to further explore the scheme of improving the performance of heterojunction photocatalytic hydrogen production,the paper modified and studied g-C₃N₄/XY₂（X=W,Mo,Y=S,Se）with applied electric field and biaxial strain respectively.Based on the first-principles method under density functional theory,the energy band structure,work function,plane average differential charge density,absorption spectrum and other properties of the heterojunction under different applied electric fields and biaxial strains were calculated.According to the band edge position diagram,it was found that applying an external electric field to g-C₃N₄/WS₂ and g-C₃N₄/MoS₂ cannot meet the redox potential of water.For g-C₃N₄/WSe₂ and g-C₃N₄/MoSe₂,when a forward electric field of 0.1V/（?）is applied,the pH can be adjusted to meet the potential requirements of photolysis water.The plane average differential charge density plot showed that applying a forward electric field of 0.1V/（?）would sacrifice some photogenerated carriers.The absorption spectra of the last four heterojunctions showed that the optical performance was hardly affected by the applied electric field.By studying the energy band structure diagram under different strains,it was found that the energy band structure type of the heterojunction changed when the biaxial strain was applied.In addition,according to the band edge position diagram,the differential charge density diagram and the absorption spectrum,it was found that the band gap,CBM,VBM,built-in electric field and optical properties were significantly affected by the biaxial strain.The research results showed that:Applying an external electric field to g-C₃N₄/XY₂（X=W,Mo,Y=S,Se）heterojunctions is very limited in improving its photocatalytic hydrogen production performance,while the application of biaxial strain is a feasible and effective solution to improve photocatalytic performance.