Preparation And CO₂ Reduction Performance Of Carbon Nitride-based Hybrid Photocatalysts

Simulating artificial photosynthesis to convert CO2 into high value-added hydrocarbon fuels is an effective way to address energy shortage and global warming.Graphitic phase carbon nitride(g-C3N4)is considered as a promising photocatalyst for photocatalytic CO2 reduction due to its suitable energy band structure,visible light response,low cost and unique electronic structure.However,it still suffers from severe exciton effects,high charge complexation rates and slow surface reaction kinetics,resulting in poor photocatalytic performance.In order to overcome the shortcomings of g-C3N4,we prepared new g-C3N4-based composite photocatalyst by introducing elemental doping,constructing homo-junction,constructing hetero-junction and loading co-catalyst.The mechanism of photocatalytic CO2 reduction was also studied.The details are as follows:1.B/K co-doped BKCN was prepared by thermal polymerization,and then Au/BKCN photocatalysts were synthesized by photodeposition.B and K doping accelerates exciton dissociation and promotes charge transfer,thus effectively improving the separation efficiency of photogenerated electrons and holes.The formation of Schottky junctions between Au and BKCN further promotes interfacial charge transfer,thus synergistically enhancing the photocatalytic CO2 activity.Thanks to the synergistic effect of Au loading and B/K co-doping,the 1%Au/BKCN catalyst had the highest photocatalytic CO2 reduction activity.The CO evolution rate was 11.56 μmol g-1 h-1,which was about 7 times higher than that of pure CN.This work provides a new strategy to enhance the photocatalytic CO2 reduction activity of CN-based photocatalysts through elemental doping and co-catalyst modifications.2.An intramolecular g-C3N4-based ternary homojunction(CN-TH)modified by cyano and cyanamide groups was prepared by using melamine as precursor and KSCN/NH4Cl mixture as liquid reaction medium.BiOBr/CN-TH heterojunction photocatalyst was synthesized by selfassembly method.The introduction of cyano and cyanamide groups accelerates the dissociation of excitons within the layer and greatly facilitates the production and migration of hot carriers.Sscheme heterojunction induces rapid electron-hole separation and migration and maintains good redox properties.In addition,the 2D/2D heterojunction structure greatly reduces the distance and time of photogenerated carrier transport,which is beneficial to the separation of photogenerated carriers.The experimental results show that the CO yield of the best sample 50%BiOBr/CN-TH under visible light irradiation for 5 h reached 131.68 μmol g-1,which was 8.6 times that of the pristine CN.At the same time,the prepared 50%BiOBr/CN-TH photocatalyst showed excellent stability.This study provides a neoteric concept and reference for the construction of the Sscheme g-C3N4-based heterojunction photocatalysts.3.Using CN-TH photocatalyst as matrix,1T-2H MoSe2 containing semiconductor phase 2H and metal phase 1T was prepared by hydrothermal method,then the 1T-2H MoSe2/CN-TH heterojunction photocatalyst was synthesized by self-assembly method.The introduction of cyano and cyanamide groups accelerates the dissociation of excitons within the layer and greatly facilitates the production and migration of hot carriers.Meanwhile S-type heterojunction formed between CN-TH and 1T-2H MoSe2 further promotes photoexcited charge separation.1T-phase MoSe2 has good electrical conductivity and can be used as an electron medium to accelerate the transport of photogenerated charge at the interface and the separation of photogenerated electrons/holes.The 25%1T-2H MoSe2/CN-TH heterojunction photocatalyst showed the best photocatalytic activity of CO2,and the CO yield reached 136.79 μmol g-1,which was 9 times of the original CN.Meanwhile,the prepared 25%1T-2H MoSe2/CN-TH photocatalyst exhibited excellent stability.This work provides a new strategy to construct S-scheme heterojunction photocatalyst with high efficient photocatalytic CO2 reduction performance.