Study On The Regulation Of G-C₃N₄ Piezocatalytic Performance And Degradation Of Organic Pollutants

Energy and environmental problems have always been the most important problems in today’s world.Piezoelectric materials can convert mechanical energy from nature（such as tides,vibrations,sound waves,etc.）to chemical energy.When the piezoelectric material is subjected to mechanical stress,it’s going to be polarized internally,resulting in the opposite charges on two sides of the material.This piezoelectric potential generated by the polarized charge can effectively drive the transfer of electrons to trigger the electrochemical reaction.The intrinsic piezoelectricity of graphitic carbon nitride has been proved,and its two-dimensional nanosheet structure has good piezocatalytic activity.Due to the cheap and available raw materials and multiple catalytic sites,g-C₃N₄ has a foreseeable good prospect in piezoelectric catalysis.However,the influencing factors of g-C₃N₄ piezocatalysis are unclear,which confuse the development of g-C₃N₄ based piezocatalyst.In this dissertation,the effects of piezoelectric polarization strength and specific surface area on piezoelectric properties of g-C₃N₄ were studied,and the performance of piezocatalytically degrading organic pollutants was improved by modifying g-C₃N₄.Related work is listed as follows:1.The influence of the specific surface area and piezoelectric polarization on the catalytic performance of g-C₃N₄ was investigated.The bulk g-C₃N₄was exfoliated into g-C₃N₄nanosheets with different sizes and thicknesses by the thermally exfoliated method.The ultrasonic cleaning instrument was used to provide periodic vibration to investigate the effects of exfoliated g-C₃N₄ under different temperatures on the catalytic activity to degrade tetracycline hydrochloride（TC）.The characterization results showed that the specific surface area of bulk g-C₃N₄ was much lower than those of g-C₃N₄ nanosheets,but the bulk g-C₃N₄ had the largest ferroelectric polarization,and the polarization intensity decreased gradually with the increase of exfoliating temperature.Under ultrasonic vibration,the bulk g-C₃N₄ has the highest catalytic degradation efficiency of TC,and the higher the thermal exfoliating temperature,the smaller the piezoelectric polarization of g-C₃N₄,the lower the catalytic activity of g-C₃N₄,which proves that the polarization strength is the dominant factor for g-C₃N₄ piezoelectric catalysis.Electron paramagnetic resonance and radical capture experiments showed that hydroxyl radical,superoxide radical and photogenerated hole were the main reactive oxygen species to degrade TC.A possible catalytic mechanism was proposed based on piezoelectric catalytic experiments and catalyst characterization.Namely,the piezoelectric potential induced the band incline,and then produced reactive oxygen species（ROS）to degrade TC.Finally,LC-MS technique was used to determine the possible TC degradation pathway.2.The effect of chlorine doping on the piezocatalytic performance of g-C₃N₄ was investigated.Chlorine-doped g-C₃N₄ was synthesized by the one-step blowing method,and the doped amount of chlorine was regulated by adjusting the ratio of ammonia chloride to melamine.The piezocatalytic performance of chlorine-doped g-C₃N₄ was investigated by degrading TC.After 90 minutes of ultrasonic vibration,the degradation rate of g-C₃N₄ with the highest doping concentration was significantly increased compared with that of undoped g-C₃N₄,and the kinetic constant was 4.5 times that of undoped g-C₃N₄.Meanwhile,the characterization results showed that with the increase of ammonium chloride ratio,the concentration of doped chlorine increased.Compared with bulk g-C₃N₄,this catalyst has stronger degradation performance and is more stable.Electron paramagnetic resonance（EPR）and free radical capture experiments showed that hydroxyl radicals,superoxide radicals,and holes were the main reactive oxygen species（ROS）.Combined with the above characterization,we believe that the mechanism of chlorine-doped g-C₃N₄degradation of TC is the same as that of bulk g-C₃N₄,which is caused by the formation of ROSs caused by the change of band potential caused by the voltage potential,and ROS can further oxidize TC.The performance improvement is attributed to the enhancement of the dipole moment of g-C₃N₄ by chlorine doping,which leads to the enhancement of piezoelectric polarization and the increase of·O₂^-and·OH concentrations.Finally,we also analyzed the intermediate products in the degradation process according to LC-MS and analyzed the possible TC degradation paths.3.The effect of heterostructure fabrication on the catalytic performance of g-C₃N₄piezoelectricity was investigated.The heterojunction of g-C₃N₄/Cd S with different proportions were prepared.The characterization results show that the heterojunction catalyst has been successfully synthesized,and the synthesized catalyst has good piezoelectric property.After 60 min of ultrasonic vibration,the degradation rate of 2,4-DCP by 0.5g-C₃N₄/0.5Cd S reached 97%,which was higher than 60%of pure g-C₃N₄and 65%of Cd S.The rate constants were 3.6 times and 2.9 times of g-C₃N₄ and Cd S,respectively.In the meantime,the heterojunction catalyst has stable structure,and good degradation performance for pollutants under different p H conditions.The mechanism study shows that the formation of Z-type heterojunction between g-C₃N₄and Cd S can promote the separation of carrier and improve the catalytic performance.Finally,the intermediate of 2,4-DCP degradation was determined by LC-MS,and the possible degradation path of 2,4-DCP was proposed.