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

Graphitic phase carbon nitride（g-C₃N₄）has become an emerging and promising field emission cathode material due to its absence of metal components,excellent physicochemical stability and good thermal stability.g-C₃N₄’s field emission performance is influenced by its morphology,specific surface area,defects and micro/nanostructure.In general,g-C₃N₄ is prepared by thermal polycondensation of nitrogen-rich raw materials（e.g.melamine,dicyandiamide,cyanamide and urea）at high temperatures of 520-580°C.This synthetic method usually results in a small specific surface area and few emission sites for the produced samples,which leads to poor field emission performance.The construction of porous nanostructures can increase the sample field enhancement factor and thus improve the field emission performance.In addition,compounding g-C₃N₄ with Zn O can effectively promote the accumulation of electrons in g-C₃N₄,which increases the chance of electron emission under the applied electric field,thus enhancing the field emission performance.The granular Zn O dispersed between g-C₃N₄ can effectively prevent the electric field shielding effect and provide more effective emission sites for the system.The experimental content and main conclusions of this paper are as follows:（1）Supramolecular precursors were obtained by hydrothermal treatment using melamine and hydrochloric acid solutions as raw materials.Carbon nitride nanosheets（ACN）with a more regular and homogeneous morphology were prepared by direct thermal polycondensation of the supramolecular precursors,and bulk phase carbon nitride（BCN）was also prepared by thermal polycondensation of melamine.In this experiment,ACN-0.25,ACN-0.5 and ACN-1.0samples were prepared using hydrochloric acid solutions at concentrations of 0.25,0.5 and 1.0mol/L,respectively,as well as bulk-phase carbon nitride（BCN）obtained by direct heat-condensation of melamine.The experimental results show that the band gap of ACN has increased from 2.7 e V for BCN to 2.76 e V for ACN-1.0.In addition,the field emission test results show that the electric field strengths（Eto）of ACN-0.25,ACN-0.5 and ACN-1.0 are 1.51,0.59 and 0.78 V/μm,respectively,when the applied electric field is 2.44,1.72 and 1.86 V/μm,respectively.The maximum field emission current densities of ACN-0.25,ACN-0.5 and ACN-1.0 were 1.69,3.04 and 2.72 m A/cm2 when the applied electric field was 2.44,1.72 and 1.86V/μm,respectively,with the ACN-0.5 sample having the best field emission performance.The good field emission performance of the ACN-0.5 sample is mainly due to the fact that the edges of the nanosheets can weaken the potential barrier on the surface of the material,which increases the probability of internal electron escape and the field emission current density.（2）Supramolecular precursors with a columnar structure were obtained by hydrothermal treatment using melamine,melinic acid and acetic acid solutions as raw materials,and carbon nitride hexagonal prisms（ACNH）were successfully prepared by direct thermal polycondensation of the columnar supramolecular precursors,in addition to the preparation of bulk phase carbon nitride（BCNH）by thermal polycondensation of melamine.In this experiment,ACNH-1,ACNH-2 and ACNH-3 samples were prepared using different concentrations of acetic acid solutions（HAc:H2O 1:3,1:1 and 3:1,respectively）,as well as the control BCNH obtained by direct heat-condensation of melamine.The results of the field emission tests showed that ACNH-1,ACNH-2 and ACNH-3 had an electric field strength（Eto）of 1.36,0.47 and 0.7 V/μm,respectively,and a threshold electric field strength（Ethr）of 2.58,1.22 and 1.63 V/μm,respectively.The excellent field emission performance of the ACNH-2sample is mainly due to its one-dimensional mesoporous prismatic structure,which has a larger aspect ratio and specific surface area compared to the conventional bulk structure,which can help to improve the field enhancement factor.（3）The corresponding carbon nitride colloids were obtained by liquid phase sonication of ACNH samples,which were used as raw materials for the hydrothermal preparation of ACNH/Zn O composite products with different composite ratios,ACNH/Zn O-4%,ACNH/Zn O-7%and ACNH/Zn O-10%,respectively,and a comparison sample ACNH-T without Zn O.The results show that the shape of the samples remained essentially prismatic after hydrothermal treatment,with Zn O in close contact with the surface of ACNH and in granular form.The results of the field emission tests show that the electric field strengths(E_to)of ACNH-T,ACNH/Zn O-4%and ACNH/Zn O-7%are 1.49,0.86 and 0.72 V/μm respectively,which are superior to those of ACNH-T.