Modification And Photocatalytic Properties Of G-C₃N₄ Constructed By Molecular Doping

Semiconductor photocatalytic process driven by solar energy is a green and sustainable technology to solve the problems of energy shortage and environmental pollution.Graphite phase carbon nitride（g-C₃N₄,CN）has been widely regarded in the field of photocatalysis due to its wide source,stable structure and narrow band gap（E_g=2.7 e V）.However,the structure of the CN prepared by conventional method is amorphous,and its photocatalytic activity is not high due to its insufficient photoabsorption capacity and low utilization rate.Molecular doping is considered to be an effective way to regulate the molecular and electronic structure of CN to improve its photoresponse and promote the separation of photogenerated charge.Based on this,this paper intends to improve the light absorption of CN and the separation of photogenerated charge.On the one hand,calcination with molten salt reduces the structural defects of carbon nitriding;on the other hand,a series of molecular-doped CN composite catalysts are prepared by introducing heteroatoms into CN through molecular level polymerization.The structure and composition of composite photocatalyst were characterized and its photocatalytic performance and mechanism were studied:（1）The Ni²⁺and polyvinylpyrrolidone coated melamine-cyanic acid assembly was adsorbed by thermal polymerization,and then calcined with KCl-Li Cl mixture for a second time to prepare K⁺,Ni and N doped carbon comodified crystalline CN（KNi CN）.The obtained strip KNi CN has high charge separation efficiency and good light absorption.Density functional theory calculations show that the Ni-N sites formed by atomic orbital hybridization can be used as charge transfer channels to capture electrons,adsorb O₂and improve the selectivity of 2-electron O₂reduction reactions.Under the synergistic effect of K⁺,Ni and N-doped carbon doping and crystallization treatment,the photocatalytic yield of H₂O₂from KNi CN sample in O₂-saturated pure H₂O is 79.6μM,which is 10 times higher than that of CN.（2）A single atom Ni/S codoped CN（Ni/S-CN-N）catalyst was prepared by thermal polymerization of urea and nickel thiourea nitrate mixture and secondary calcination with NH₄Cl.The photocatalytic degradation of tetracycline（TC）and CO₂reduction were investigated.X-ray absorption fine structure measurements and theoretical calculations show that monatomic Ni is anchored to CN by forming Ni-O/N bonds and acts as an active center for electron aggregation.NH₄Cl assisted secondary calcination can increase the specific surface area of the material.The apparent rate constant of Ni/S-CN-N degradation of TC is 0.031 min^-1,which is 6.0times of CN.Using H₂O as reducing agent,Ni/S-CN-N showed a higher reduction rate of CO₂.The yield of CO and CH₄within 5 h were 58.6μmol g^-1and 18.9μmol g^-1,respectively,which were higher than that of CN(CO:7.9μmol g^-1;CH4:0).（3）Preparation of K⁺,P doped CN（KP/CN）with n→π*electron transition and partial crystallization by two-step calcination.P doping was carried out by calcination of phytic acid and N-containing precursor.After calcination with KSCN to control the crystallinity.The prepared KP/CN has n→π*transition,which extends the absorption band edge to 648 nm and improves the charge mobility.Theoretical calculation shows that K⁺,P doping is beneficial to the adsorption and activation of CO₂.Using 0.5 m L water as reducing agent,KP/CN showed a significant photocatalytic reduction rate of CO₂under simulated light irradiation.The yields of CO and CH₄were 58.7μmol g^-1and 29.0μmol g^-1,respectively.KP/CN still has a high activity of reducing CO₂to CO(yield of 129.7μmol g^-1at 5 h)without water,which is 3.3 times of CN.