Study On The Design,optimization And Photocatalytic Performance Of Sheet G-C₃N₄/Cobalt Oxide Heterojunction

Graphite phase carbon nitride（g-C₃N₄）is one of the most concerned photocatalytic materials at present.However,the single block g-C₃N₄ has few active sites and high recombination rate of photogenerated carriers,which leads to its low photocatalytic performance and seriously hinders its application in the field of photocatalysis.In order to solve the above problems,three different cobalt oxide heterojunctions were designed and constructed on the sheet g-C₃N₄ by combining the controllable construction of multiple heterojunctions and micro-morphology control.The photocatalytic performance of these three heterojunctions was systematically studied,and the enhancement mechanism of photocatalytic activity was further analyzed.The main research contents are as follows:（1）Using melamine treated with acetic acid as the precursor,the regular sheet g-C₃N₄（CNNS）was synthesized by two-step calcination.Compared with block g-C₃N₄,CNNS exhibits superior photocatalytic performance,which hydrogen evolution rate is 413.01μmol×g×^-1×h^-1.The degradation rate of methylene blue（MB）and tetracycline（TC）is 63.8%and 46.4%for 3h of visible light irradiation,respectively.Besides,CNNS has excellent photocatalytic stability after five-cycle tests.（2）The CNNS/Co₃O₄ p-n single heterojunction material formed by the in-situ growth of Co₃O₄ particles on the CNNS surface.The experiment proves that Co₃O₄ particles are uniformly deposited on the surface of CNNS through the Co-N bond.The heterojunction material exhibited the best photocatalytic effect with 10 wt%Co₃O₄ Its hydrogen evolution rate is 594.47μmol×g×^-1×h^-1,which is 1.4 times that of the single CNNS;the apparent quantum yield（AQY）is 1.6%.After 3h of visible light irradiation,the degradation rates of MB and TC are 99.2%and 64.9%,respectively.The enhanced photocatalytic activity can be attributed to the formation of the strong internal electric field between the interface of CNNS（n-type）and Co₃O₄（p-type）heterojunction,which promotes the effective separation and transfer of charge carriers.（3）By in-situ phosphating method,CNNS/Co₃O₄@CoP p-n/ohm double heterojunction composites were constructed on CNNS,and the effects of Co₃O₄@CoP ratios on the photocatalytic properties of the system were investigated.Among the constructed heterojunctions,CNNS/4%Co₃O₄@7%CoP shows the best photocatalytic performance:its hydrogen evolution rate is 4.1 times that of CNNS/10%Co₃O₄heterojunction,and the AQY value is 4.8%;The degradation rate of TC reach 92.3%after3 h visible light,which is 1.4 times that of CNNS/10%Co₃O₄ heterojunction.Electrochemical tests and preliminary density functional theory analysis show that the p-n/ohmic double heterojunction constructed after the introduction of metal-like CoP can effectively reduce the surface overpotential and activation energy barrier of the main material.In addition,it provides multiple transfer paths for photogenerated carriers,promotes the separation/migration of electrons and holes,and extends the fluorescence life of carriers,thus improving the photocatalytic activity of the material.（4）Combining morphology control and heterojunction controllable construction methods,a complex double Z-type Co₃O₄/CoO-CNNS heterojunction complex was fabricated with three-dimensional flower morphology Co₃O₄/CoO material loaded on lamellar g-C₃N₄ by electrostatic self-assembly technology.The 15wt%（Co₃O₄/CoO）-CNNS sample has the optimal photocatalytic performance.The hydrogen evolution rate and AQY are 4050.6μmol×g×^-1×h^-1 and 9.6%,respectively,which are 2.4 times and 2 times that of CNNS/4%Co₃O₄@7%CoP p-n/ohm double heterojunction.The degradation rate of TC reaches 99.5%for 2.5 h illumination.The degradation reaction rate is 2.3 times that of the p-n/ohm double heterojunction sample.The three-dimensional flower morphology can provide more surface active sites,promote the mass transfer and diffusion of reactant molecules and improve the utilization rate of visible light,which contributes to the enhanced photocatalytic activity.The double Z-type heterojunction can promote the separation/migration of semiconductor photogenerated carriers and ensure the carrier’s strong redox ability.Finally,the photocatalytic degradation of organic pollutants had achieved through the design and optimization of heterojunction structure.These results provide a new for the design and preparation of complex heterojunction photocatalysts in the future.