Constructing The Low-layer Crystalline Carbon Nitride And Studying Its Optical/Piezoelectric Catalytic Properties

Photocatalysis and piezoelectric catalysis show wide range of application prospects in the field of water treatment as they could drive the activation of small molecules like oxygen and water to generate·O₂^-,·OH,H₂O₂ and ¹O₂ etc.reactive oxygen species（ROS）,and further utilizing these ROS to completely decompose or kill common pollutants and pathogenic bacteria,with only solar light and mechanical force as the energy source.Up till now,the key to photocatalysis and piezoelectric catalysis is still relying on developing efficient catalysts.Among various photocatalytic and piezoelectric catalytic materials,graphite-phase carbon nitride（g-C₃N₄,abbreviated as GCN）has been widely concerned owing to its photocatalytic and piezoelectric catalytic bi-functional properties.However,the catalytic activity of pristine GCN is still restricted by the rapid charge recombination,insufficient active site exposure,and disturbed in-plane piezoelectricity.In this work,we propose a simple strategy:decreasing the thickness combined with improving crystallinity of GCN that is to prepare few-layer crystalline carbon nitride（FL-CCN）to synchronously solve the aforementioned drawbacks.H₂O₂ is first selected as a representative to investigate the ROS production performance of this material,and then tetracycline（TC）and Escherichia coli（E.coli）,which have been widely studied in the environmental field,are selected as targets to preliminarily explore their application prospects in the field of water treatment.Detailed research contents and achievements are as follows:The FL-CCN is first prepared by a combination process of molten salt etching and ultrasonic stripping with the original GCN as precursor.Moreover,in order to further exploring the contribution of only introduction of fewer layers or crystalline structures to improve the performance of GCN,fewer layers of carbon nitride nanosheets（FL-GCN）and crystalline carbon nitride nanosheets（CCN）are respectively synthesized by thermal etching and molten salt etching GCN precursor as contrast.HRTEM and AFM results show that,compared with the original GCN,obvious lattice fringes about 0.32 nm can be observed in the HRTEM image of CCN,while ultra-thin thickness of 0.9 nm and 0.8 nm are detected in the AFM image of FL-GCN and FL-CCN,respectively.These results indicate the successful preparation of the above four carbon nitride based catalysts.Subsequent photocurrent and piezoelectric coefficient measurements show that the FL-CCN has the best charge separation and piezoelectric properties among the four materials,because its photocurrent value is 1.0μA·cm^-2,which is about 20.0,11.1 and 1.3 times higher than that of GCN,FL-GCN and CCN;while the piezoelectric coefficient of FL-CCN reaches 2.91 pm·V^-1,which is 4.2,2.5 and 2.3 times higher than that of GCN,FL-GCN and CCN,respectively.This can be ascribed to the result of adjusting the crystallinity and thickness of the original GCN.Then,taking the activation of molecular oxygen and water to produce H₂O₂ reaction as an example,the performance of photocatalytic and piezoelectric catalytic production of ROS over FL-CCN is investigated.The related results show that the photocatalytic H₂O₂ production efficiency over FL-CCN reaches 63.95μmol·g^-1·h^-1,which is 7.8,1.6and 1.1 times higher than that of GCN,FL-GCN and CCN,respectively.The piezoelectric catalytic H₂O₂ production efficiency of FL-CCN reaches 491.68μmol·g^-1·h^-1,which is8.6,1.1 and 1.4 times higher than that of GCN,FL-GCN and CCN,respectively.The subsequent mechanism investigation experiments show that the charges involved in photocatalytic production of H₂O₂ over FL-CCN are the photogenerated electron-hole pairs,they drive the oxygen reduction reaction（ORR）and water oxidation reaction（WOR）in the whole plane.While the redox charges used to piezoelectric catalysis production of H₂O₂ over FL-CCN are the shielding charges derived from the surrounding environment,which mainly drive the ORR and WOR processes at the edge of the material.The difference in ROS production mechanism of FL-CCN under photocatalysis and piezoelectric catalysis may lead to its different degradation and sterilization performances under the driven of photocatalysis and piezoelectric catalysisFinally,the application potential of FL-CCN in the field of water treatment is preliminarily explored with TC and E.coli as target pollutants.The results show that 50m L 10 mg·L^-1 of TC is effectively degraded and 50 m L 7.00 lg CFU·m L^-1 of E.coli is effectively inactivated by 10 mg of FL-CCN under 60 min visible light irradiation（400nm<λ<780 nm）.The kinetic rate constant of TC degradation by FL-CCN is0.02488±0.00327 L·mg^-1·min^-1,which is 3.8 times that of GCN(kinetic rate of0.00651±0.00084 L·mg^-1·min^-1).In terms of sterilization efficiency,the killing rate of E.coli by FL-CCN increased from 92.0%to 99.9%as compared with GCN,which fully demonstrated that FL-CCN has better photocatalytic degradation and sterilization performance.Subsequent experiments of piezoelectric catalytic degradation and sterilization also obtained similar conclusions.The kinetic rate constant of FL-CCN is0.00499±0.00068 L·mg^-1·min^-1,and the bactericidal rate is 94.3%,the kinetic rate constant is 2.9 and 1.2 times that of GCN,respectively.