Synthesis And Structure-Activity Relationship Of Carbon Nitride Composites With Hierarchical Structure

Graphitic carbon nitride?g-C₃N₄?,which can be regarded as an N-substituted graphite,consists of the p-conjugated graphitic planes framework structure formed via sp² hybridization of carbon and nitrogen atoms.The band-gap energy of g-C₃N₄ is about2.7 eV.Due to the unique electronic properties and very stable physical-chemical properties?such as:heat-,acid-and alkali-resistant?,it has been widely applied in fields of photocatalysis,CO₂ capture,energy storage,and so on.However,the separation efficiency of the photo-generated electrons and holes is low,and the specific surface area?BET?of pure g-C₃N₄ is small,which restricts its practical applications.Chemical modification,atomic doping and the formation of composites can all improve the separation efficiency of photo-generated electrons and holes.In this dissertation,the separation efficiency of photo-generated electrons and holes were improved by the fabrication of the g-C₃N₄ composites with multi-components and multi-stage morphologies,and the g-C₃N₄ ultrathin flakes prepared by a liquid nitrogen exfoliation,leading to enhance the photo-,electro-catalytic activities.The main content of this dissertation is as following:?1?Ternary composites g-C₃N₄/ZnS/CuS have been prepared via a facile three-step process:the g-C₃N₄ prepared by the polymerization of melamine,the loading of ZnS nanosheets by a solvothermal method,and further deposition of hexagonal CuS nanosheets by a microwave-assisted hydrothermal method.The characterizations showed that the fluorescence emission?PL?peak intensity of g-C₃N₄/ZnS/CuS was the lowest,the corresponding impedance was the smallest,and the photocurrent?0.54?A?was about8 times that of pure g-C₃N₄,which indicated that the ternary composites g-C₃N₄/ZnS/CuS possessed the highest separation efficiency of photo-generated electrons and holes,the lowest interface charge transfer resistance and high photoconductivity.Under the optimal experimental conditions,the photocatalytic degradation rate constant of Rhodamine B?RhB?is about 10 times that of g-C₃N₄/ZnS and 20 times that of g-C₃N₄.Therefore,the ternary composites g-C₃N₄/ZnS/CuS could be as an efficient photocatalyst and excellent photoconductor device.?2?Ternary composites g-C₃N₄/ZnS/CuS with different morphologies have been fabricated by a facile three-step process:the g-C₃N₄ prepared by the polymerization of melamine,then the loading of ZnS nanorods?nr?,nanosheets?ns?or hollow spheres?hs?by a solvothermal method with varying ligands and sources of zinc,and further deposition of CuS nanoparticles?np?,nanorods?nr?,or hexagonal nanosheets?ns?by a microwave-assisted hydrothermal method or solvothermal method with varying ligands and sources of copper.It was found that the morphology of ZnS has little effect on the photocatalytic activity of ternary composites,while the morphology of CuS has a great influence on the photocatalytic activity.Therefore,we chose the ZnS?ns?with the largest BET,g-C₃N₄ and CuS with three different morphologies to fabricate the ternary composites.The photocurrent test results showed that g-C₃N₄/ZnS-ns/CuS-ns exhibited the largest photocurrent(0.54?A·cm^-2),which was about 8-,7-,and 4-fold of g-C₃N₄,g-C₃N₄/ZnS-ns/CuS-np and g-C₃N₄/ZnS-ns/CuS-nr,respectively.The results of photocatalytic degradation showed that g-C₃N₄/ZnS-ns/CuS-ns possessed the best photocatalytic activity,and its rate constant for photocatalytic degradation of RhB was about 30-,5-,and 2-fold of g-C₃N₄,g-C₃N₄/ZnS-ns/CuS-np and g-C₃N₄/ZnS-ns/CuS-nr.The results of electrocatalytic experiments showed that the electrochemical response of g-C₃N₄/ZnS-ns/CuS-nswasabout1.7-,1.6-,and1.3-foldofg-C₃N₄,g-C₃N₄/ZnS-ns/CuS-nr and g-C₃N₄/ZnS-ns/CuS-np,respectively.These results indicated that the g-C₃N₄/ZnS-ns/CuS-ns ternary composites can be used as an excellent photoconductor device,a high-efficiency photo-,electro-catalyst.Moreover,these results also revealed the influence discipline of different morphologies or different dimensions on the photoconductivity,photo-and electro-catalyticactivity of the ternary composites g-C₃N₄/ZnS/CuS,and the order was two-dimensional nanoplates>one-dimensional nanorods>zero-dimensional nanoparticles.The mechanism of enhanced photocatalytic activity of g-C₃N₄/ZnS/CuS?2D/2D/2D?was also proposed.?3?Taking advantage of liquid nitrogen gasification endothermic and rapid expansion characteristics,the ultrathin g-C₃N₄ flakes with a thickness of about 0.68 nm were successfully prepared by an exfoliation of bulk g-C₃N₄ under high temperature with liquid nitrogen as the exfoliation agent.The characterizations showed that the specific surface area was as high as 129.0 m²·g^-11 and the band-gap energy was about 2.96 eV.Meanwhile,it was found that the photocurrent was about 0.25?A·m^-22 which was about 5times that of bulk g-C₃N₄,and the PL intensity was weak.These results indicated that compared to the bulk g-C₃N₄,the separation efficiency of photo-generated electrons and holes of the ultrathin g-C₃N₄ flakes was significantly enhanced.The experimental results of photocatalytic degradation of RhB showed that the degradation rate constant of the ultrathin g-C₃N₄ flakes was about 100 times that of bulk g-C₃N₄.Electrochemical sensing experimental results showed that the ultrathin g-C₃N₄ flakes exhibited a strong electrochemical response for environmental hormone tetrabromobis phenol A?TBBPA?,and the response current was about 1.5-fold of bulk g-C₃N₄.Based on these above results,it is promising to establish an electrochemical sensing method for detecting the environmental hormone.