Regulation Of Graphitic Carbon Nitride Toward Photocatalytic And Electrocatalytic Hydrogen Generation

Graphitic carbon nitride?g-C₃N₄?,a graphite-like two-dimensional layered material,is a novel functional material to replace carbon in materials application.It has attracted considerable attention in recent years due to its high nitrogen content,unique electronic and optical peorperty,good chemical stability,eco-friendly and reasonable cost.In this present research,we foucs on the modification of g-C₃N₄ by molecular regulation,morphology control and surface recombination.Based on above results,g-C₃N₄ and its composites have been applied into the visible-light photocatalytic water splitting and electrocatalytic hydrogen evolution.1.Molecular regulation of g-C₃N₄1)Tailoring of molecular structureUltrathin graphitic carbon nitride nanoplatelets?UGCNPs?are synthesized by a facile manner via ball milling approach.The obtained UGCNPs are less than 5 nm in size and 0.7nm in thickness.Photochemical experiments show that the UGCNPs are highly active in visible-light water splitting,13.7-fold greater than that of their bulk counterparts.The notable improvement in the hydrogen evolution rate observed with UGCNPs is due to the synergistic effects derived from the 10-fold increase in specific surface area and a negative shift of 0.31 eV in the conduction band.The calculated apparent quantum efficiency of UGCNPs is 3.5%at 420 nm.In addition to metal-free visible-light-driven photocatalytic hydrogen production,the UGCNPs find attractive applications in biomedical imaging and optoelectronics because of their superior luminescence characteristics.2)Element dopingIodine-doped graphitic carbon nitride nanosheets?IGCNSs?have been prepared by simply ball-milling technique.The IGCNSs with an iodine atom content of 0.34 at%are 6nm in thickness,which show enhanced visible-light harvesting,increased specific surface area and more than 9.1-fold increase in the hydrogen evolution rate,the calculated apparent quantum efficiency is 3.0%at 420 nm.In addition,this method is organic reagent-free,environment protecting and its product yields reach 90 wt%,which provides for the promotion and application of the g-C₃N₄-based materials.2.Morphology control1)Atomically thin g-C₃N₄ nanomeshMesoporous g-C₃N₄ bulk is fabricated by freeze-drying self-assembly approach.Then g-C₃N₄ nanomeshes with different layers are prepared by solvothermal exfoliation of the mesoporous g-C₃N₄ bulk.The hydrogen evolution rate increases with the reduction of the layer numbers of g-C₃N₄ nanomeshes.Atomically thin nanomesh of g-C₃N₄ shows a specific surface area of 331 m² g^-1,and its conduction band is calculated to be upshifted by0.51 eV.Consequently,it exhibits 24.3-times increase in the hydrogen evolution rate,and an apparent quantum efficiency of 5.1%at 420 nm,the highest of all the 2D g-C₃N₄nanosheets photocatalysts.2)g-C₃N₄“seaweed”Based on the above method of 1)in 2,a seaweed-like g-C₃N₄ architecture consisted of mesoporous fiber structures has been firstly prepared by the freeze-drying self-assembly methodlogy.The seaweed network features 1D mesoporous fibers show an increased concentration of electron-hole pairs,a 10-fold increase in the specific surface area,enhanced light harvesting.This hierarchical nanostructured architecture exhibits 33-fold increase in the hydrogen evolution rate,and a remarkable apparent quantum efficiency of7.8%at 420 nm.This work pioneers new perspectives for synthesis of complex 3D structures.3.g-C₃N₄-based hybrids1)g-C₃N₄/nitrogen-rich carbon nanofibersBased on the above method of 2)in 2,a mesoporous g-C₃N₄ nanofibers merged with in situ incorporated N-rich carbon has been prepared by calcinations of polypyrrole coated precursor fibers.The polypyrrole coating was formed by iodine induced chemical oxidation polymerization of pyrrole.The as-obtained catalyst benefits the separation of photoelectron–hole pairs,the efficient light harvesting and the increase of the available reaction space.It exhibits a remarkable apparent quantum efficiency of 14.3%at 420 nm without Pt cocatalysts,which is the highest reported value for g-C₃N₄-based photocatalysts without the assistance of cocatalysts.2)Mesh-on-mesh g-C₃N₄/grapheneA dual-mesh of mesoporous g-C₃N₄ meshes in-situ formed on mesoporous graphene meshes by a straightforward hydrothermal method.The mesh-on-mesh architecture provides highly expanded surface area,utmost hydrogen adsorption sites,multi-electron transport channels and high electron transfer capability.As a result,the hybrid exhibits excellent hydrogen evolution reaction?HER?performance.4.Application of g-C₃N₄ and g-C₃N₄-based hybridsThe g-C₃N₄ and g-C₃N₄-based composites which were prepared through structure regulation and surface recombination have been applied in the energy conversion including photocatalytic water splitting for hydrogen production and electrochemical hydrogen evolution.1)Photocatalytic water splitting for hydrogen production:The fabricated ultrathin g-C₃N₄ nanoplatelets,iodine-doped g-C₃N₄ nanosheets,atomically thin g-C₃N₄ nanomesh,g-C₃N₄“seaweed”and g-C₃N₄/nitrogen-rich carbon nanofibers have been demonstrated to significantly enhance the photocatalytic performance.2)Electrochemical hydrogen evolution:A 3D architecture of g-C₃N₄ nanomesh with graphene nanomesh has been firstly synthesized,which displays a superior HER activity with lower overpotential,higher current density and smaller Tafel slope.