Study On The Preparation,Photocatalysis And Field Emission Properties Of Graphitic Carbon Nitride And Its Composites

Graphitic carbon nitride（g-C₃N₄）possesses many merits such as excellent physical and chemical properties and good biocompatibility.In addition,it also has suitable conduction band and valence band positions.Due to these advantages,the g-C₃N₄shows great potential in photocatalytic fields.However,shortcomings such as small specific surface area,easy recombination of photogenerated carriers,and low utilization efficiency of visible light severely limit its photocatalytic performance.Therefore,g-C₃N₄ nanosheets were prepared to increase the specific surface area and promote the carrier migration in this paper.Besides,composites were prepred to increase the separation efficiency of photogenerated carriers by coupling g-C₃N₄ with Zn O,using hydrothermal and high-energy microwave methods,respectively.In addition,g-C₃N₄ has been found to exhibit excellent field emission performance.However,the ways to reduce the turn-on electric field and increase the emission currents to meet the needs of practical applications remain to be explored.In this paper,hybrid field emitters were formed by compounding g-C₃N₄with Zn O,which can not only increase the field enhancement factor,but also increase the additional emission sites,thereby improving the field emission performance.The specific research contents are as follows:（1）Carbon nitride nanosheets（ACN）was achieved by hydrothermally pretreating and calcining the precursors,using melamine and nitric acid as raw materials.In this paper,nitric acid solutions with different concentration were used to prepare bulk g-C₃N₄（BCN）,ACN-0.25,ACN-0.5 and ACN-1.0.Results demonstrated that the bandgap enlarged from 2.63 e V of BCN to 2.68 e V of ACN-1.0.Besides,the separation efficiency of photogenerated electron-hole pairs in ACN samples was significantly promoted compared with BCN.In the experiment of photocatalytic degradation of Rh B solution,ACN showed the better photocatalytic activity than BCN.Especially for ACN-0.5 sample,the decolorization rate of Rh B reached up to97.2%in 6 min reaction,and its first order kinetic constant rate was 0.5864 min^-1.In the field emission test,ACN showed excellent field emission performance,the turn-on electric fields of ACN-0.25,ACN-0.5 and ACN-1.0 were 1.57,0.33 and 0.79 V/μm,respectively.The corresponding field enhancement factors were 2693,26833 and5689,respectively.The excellent field emission performance of ACN-0.5 mainly came from its uniformly arranged and vertically oriented nanosheet structure.（2）The carbon nitride nanosheets were obtained from ACN samples by liquid-phase ultrasonic-assisted exfoliation.ACN-H samples and ACN/Zn O composites were successfully prepared by the hydrothermal method,using carbon nitride nanosheets and zinc acetate dihydrate as raw materials and adjusting the mass ratio between the two raw materials.ACN-H samples was free of Zn O and used as control samples,and the composites with different ratios were respectively named ACN/Zn O-2%,ACN/Zn O-6%and ACN/Zn O-10%.The results showed that ACN in composites still maintained the morphology of nanosheets after hydrothermal process,and Zn O was granular and formed close contact with the ACN nanosheets.After compounding with Zn O,the in-plane crystallinity of ACN was enhanced,XRD patterns showed that the diffraction peak of（200）crystal plane appeared.As the proportion of Zn O in the ACN/Zn O composites increased,the separation efficiency of photogenerated electron-hole pairs in the composites was significantly promoted,therefore the photocatalytic performance of the composites gradually improved.Especially for ACN/Zn O-10%,its first order kinetic constant rate was twice that of ACN-H.In the field emission tests,the turn-on electric fields of ACN-H,ACN/Zn O-2%and ACN/Zn O-6%were 1.42,0.79 and 0.64 V/μm,respectively,and the threshold electric fields were 3.14,2.01 and 1.39 V/μm,respectively.Compared with ACN-H,the field emission performance of the ACN/Zn O composites was significantly improved.（3）GCN/ZnO composites with different ratios were successfully prepared by the high-energy microwave method,using different ratios of melamine and zinc acetate dihydrate as raw materials and carbon fiber as microwave absorbers.The composites were respectively named GCN,GCN/Zn O-2%,GCN/Zn O-6%and GCN/Zn O-10%.The results showed that GCN in composites presented the morphology of nanosheets,and Zn O was granular and attached up to the GCN.GCN and Zn O contacted closely to form a heterojunction.Greater internal stress was accumulated in GCN because of the addition of Zn O during microwave synthesis,so some fractures were found in GCN,and the band gap of GCN was correspondingly reduced.When the proportion of Zn O in the compsites increased to 10%,a part of block structure appeared.As the proportion of Zn O in the GCN/Zn O composites increased,the separation efficiency of photogenerated electron-hole pairs in the composites was significantly promoted compared with GCN,which was due to the role of heterojunction between GCN and Zn O.Thereore,the photocatalytic performance of the GCN/Zn O composites was significantly improved compared with GCN.In the field emission tests,the field emission performance of the GCN/Zn O composites was significantly improved compared with GCN.GCN/Zn O-6%showed the best field emission performance,and its turn-on electric field and threshold electric field were 0.63 and 1.3 V/μm,respectively.The field enhancement factor of GCN/Zn O-6%was about 12106.