Morphology Regulation Of G-C₃N₄ And G-C₃N₄/BiOI With Enhanced Photocatalytic Performance

Graphite carbon nitride（g-C₃N₄）is a novel non-metallic organic polymer semiconductor,which has been widely used in photocatalysis and photoelectric conversion due to its suitable energy band structure and good physical and chemical stability.However,the g-C₃N₄ prepared by the traditional preparation method has low photocatalytic performance due to the compact accumulation of layered g-C₃N₄,resulting in fewer active sites and serious recombination of photogenerated carriers.The specific surface area of g-C₃N₄ can be increased by means of microscopic morphology modification to optimize the recombination of photogenerated carriers.It is of great significance to increase the specific surface area of g-C₃N₄,reduce the recombination probability of photogenerated carriers,thus improve the photocatalytic performance of g-C₃N₄ by morphology modification.The morphology and structure of g-C₃N₄were controlled by the method of freeze-drying pretreatment and high temperature calcination thermal polycondensation,with melamine and melamine as precursors.A novel g-C₃N₄/BiOI photocatalyst with an island distribution of Z-scheme heterojunction was synthesized by prerecombination of precursor and thermal condensation by calcination at high temperature.Degradation of tetracycline hydrochloride（TC）under visible light was employed as the probe reaction,in order to study the optical performance of catalyst.Through a series of modern analytical technology,the structure characterization and the structure-activity relationship were explored.The mechanism of photocatalytic reaction was elucidated,for a series of meaningful research results and important basic data.（1）A green and efficient morphology control method of g-C₃N₄:combining the freeze-drying pre-treatment with the thermal polycondensation method of high temperature calcination,using melamine and cyanuric acid as raw materials,by controlling the mass ratio of melamine and melamine,the flask g-C₃N₄（FCN-3）with porous structure was successfully prepared.It was found that when the mass ratio of melamine to cyanuric acid was 4:3,g-C₃N₄ showed a flask structure,and the catalytic degradation performance was the best at this time.The results of photoelectric performance showed that the flask FCN-3 with a large specific surface area（42.4m2/g）fully exposed the active site,which effectively reduced the recombination probability of photogenerated carriers,thus improving the photocatalytic activity of g-C₃N₄.Methylene blue（MB）was employed to evaluate the photocatalytic activity of flask FCN-3.The degradation rate constant was 0.0299 min^-1,which was 23.9 times that of bulk CN.Through the capture experiment of active substances,it was confirmed that the main active substances in the photocatalytic degradation of flask g-C₃N₄ were superoxide radical（·O₂^-）and hole（h⁺）.Combined with the active capture experiment and the band structure of flask FCN-3,the mechanism of catalytic degradation of MB by flask FCN-3 under visible light was deduced.（2）Preparation of a novel island distribution Z-scheme heterojunction g-C₃N₄/BiOI photocatalyst:combined with the precursor precomposition and thermal condensation method of high temperature calcination,a novel island distribution Z-scheme heterojunction g-C₃N₄/BiOI photocatalyst（BCN）was successfully prepared with melamine,cyanuric acid,bismuth nitrate pentahydrate and potassium iodide.The effect of preparation temperature on the composite was explored:when the temperature reached 500℃,BCN was formed,and g-C₃N₄ showed an island distribution on BiOI.The specific surface area of BCN is not large（22.7m²/g）,but the novel island distributed Z-scheme heterojunction makes the photogenerated electrons cluster in the g-C₃N₄ conduction band,while the holes cluster in the valence band of BiOI,which effectively reduces the recombination probability of photogenerated carriers.In addition,BCN effectively utilizes the conductivity band of g-C₃N₄ with stronger reducing capacity and the valence band of BiOI with stronger oxidizing capacity,greatly improving the REDOX capacity of BCN.TC was employed to evaluate the photocatalytic activity of BCN under visible light,the degradation rate constant is 0.0309 min^-1,which is 22.1 times of that of g-C₃N₄.BCN has a certain universality to methyl orange（MO）,p-chlorophenol（4-CP）,and it is less affected by environmental factors,showing a good practicability.Finally,combined with the capture experiment of active substance and the energy band structure of BCN,it was explained that the main active substances in the photocatalytic degradation reaction of TC were·O₂^-and h⁺,and the degradation mechanism of BCN was proposed.