Study On The Structure Modification And Morphology Control Of Graphitic Carbon Nitride In Photocatalytic Hydrogen Production From Water Splitting

Solar and hydrogen energy are ideal clean energy sources,photocatalytic hydrogen evolution from water splitting technology as an intermediate bridge is one of the most promising methods to solve environmental problems and energy shortages.How to obtain efficient,stable,and inexpensive photocatalysts are required for the photocatalytic hydrogen evolution from water splitting.In this paper,graphitic carbon nitride(g-C3N4),a two-dimensional material,which is inexpensive and easy to prepare,was used as the main photocatalyst,and its structure was modified and morphologically adjusted.Then a series of characterizations were performed on the obtained photocatalysts,after that,the performance of photocatalytic hydrogen evolution from water and seawater splitting was investigated.With the help of density functional theory(DFT)calculations,the band energy and electronic structure of the photocatalytic semiconductor were deeply studied.The nitrogen-deficient carbon nitride g-C3Nx-K was prepared by using the KOHassisted method.The optimal photocatalyst g-C3Nx-0.005 was obtained when 0.005 g of KOH was added,and the rate of hydrogen production from pure water splitting could reach 759 μmol·h-1·g-1.After secondary calcination,the thickness of the obtained gC3Nx-NS sample was about 4 nm,and on this condition,the rate of hydrogen production from pure water splitting was 875 μmol·h-1·g-1.Then,the Pt/g-C3Nx-NS sample was obtained via using an improved platinum loading method,the rate of hydrogen production could reach 1143 and 1321.7 μmol·h-1·g-1 from pure water and seawater splitting,and its apparent quantum efficiency(AQE)at 420 nm was 5.4%and 6.2%,respectively.The band gaps of g-C3N4 and g-C3Nx were calculated by using DFT,which was equal to 2.77 eV.The bandgap of Pt/g-C3Nx was reduced to 2.63 eV.The abovecalculated data were similar to the experimental results.The porous hollow hexagonal prism-shaped carbon nitride(UMx)was obtained by using hydrothermal recrystallization of melamine from urea solution with different concentrations.When the molar ratio of urea to melamine was 5:1(UM5),its hydrogen production rate from pure water and seawater splitting were 214.05 and 541.67 μmol·h1·g-1,which were 3 and 2.5 times than that of the bulk g-C3N4,and the AQEs for 420 nm were 1.01%and 2.55%,respectively.By doping non-metallic elements B and P in UM5,the hydrogen bonds in the layer could be effectively destroyed,and the interlayer van der Waals force could be effectively weakened,so as to generated a nanosheet structure on the outer UM5 surface.The catalyst with the best B/P doping amount was B2/P0.5-UM5,and the rates of hydrogen production from pure water and seawater splitting were 373.9 and 682.9μmol·h-1·g-1,which were 5 and 2.7 times than that of the bulk g-C3N4,and the AQEs at 420 nm were 1.76%and 3.22%,respectively.The band gaps of g-C3N4 and B2/P0.5UM5 were calculated by using DFT,and the results were equal to 2.77 eV and 3.28 eV,respectively.Moreover,it should be mentioned that the bandgap of B2/P0.5-UM5 was increased,which was in line with the experimental results.For B2/P0.5-UM5,the valence band edge was mainly composed of C_p and N_p orbitals,and the conduction band edge was mainly composed of N_p,P_p,and B_p orbitals.