Study On Preparation And Photocatalytic Properties Of Carbon Nitride By High-energy Microwave Method

Semiconductor photocatalysts can effectively degrade organic pollutants under sunlight,which makes them have a great application prospect in solving environmental pollution.Among various semiconductor photocatalysts,novel metal-free graphitic carbon nitride(g-C3N4)has attracted considerable attention worldwide due to its medium band gap,excellent physical and chemical properties,good biocompatibility and photocatalytic activity.In this paper,the g-C3N4 with different morphologies and properties were synthesized via high-energy microwave heating technology.Meanwhile,the structure and optical properties of g-C3N4 were characterized by various analysis techniques.Then the photocatalytic activities were evaluated by the degradation of aqueous rhodamine B(RhB)and methyl orange(MO),and the enhancement mechanism of photocatalytic activity was studied.Besides,the field emission performance of two-dimensional g-C3N4(2D-g-C3N4)nanosheets synthesized via microwave heating method was preliminarily studied,and the mechanism of field emission was discussed as well.The main research contents are as follows:(1)g-C3N4 with large specific surface area was rapidly prepared via high-energy microwave method using the low-cost melamine(precursor)and graphite(microwave absorber)as raw materials.Meanwhile,the microstructure,morphology and optical property were systematically investigated.Compared with the g-C3N4 synthesized via muffle heating(28 m2/g,2.70 eV),the specific surface areas(84～142 m2/g)and band gaps(2.72～2.83 eV)of the g-C3N4 prepared via microwave heating were obviously increased.In the experiments of photocatalytic degradation of aqueous RhB and MO,g-C3N4 synthesized via microwave heating displayed excellent photocatalytic properties.For example,after the visible-light photocatalytic reaction for 40 min,the RhB and MO decolorization ratios for g-C3N4 synthesized via microwave heating with the irradiation power of 4.0 kW reached up to 100%and 94.2%,respectively.The corresponding pseudo-first order kinetics rate constants were 0.10333 and 0.06799 min-1,respectively.However,the RhB and MO decolorization ratios for g-C3N4 synthesized via muffle heating were 85.9%and 67.4%after 120 min under the same condition,respectively,corresponding to 0.01657 and 0.00937 min-1 for pseudo-first order kinetics rate constants.The g-C3N4 synthesized via microwave heating possessed large specific surface area and rich photocataltic activity sites,which will be favor of enhancing photocatalytic activities.(2)Fe element was successfully incorporated into g-C3N4 by high-energy microwave method using Fe powder(dopant),melamine(precursor)and graphite(microwave absorber)as raw materials.When Fe content was 1.0 wt.%,the as-synthesized sample was characterized as a unique hollow structure.Compared with g-C3N4(2.81 eV),when Fe content was 0.5 wt.%,1.0 wt.%and 2.0 wt.%,respectively,corresponding to 2.75,2.71 and 2.66 eV of band gaps.This result showed that Fe doping could increase the range of absorption in the visible-light.In the experiment of photocatalytic degradation,1.0%Fe-doped g-C3N4 with hollow structure just took 30 min to degrade 100%of RhB and 94.1%of MO,respectively,suggesting that 1.0%Fe-doped g-C3N4 possessed outstanding photocatalytic activities.(3)2D-g-C3N4 nanosheets could be obtained by high-energy microwave approach using melamine and carbon fibers instead of graphite as raw materials.The thickness of 2D-g-C3N4 nanosheets could be decreased with the increase of microwave power.With the increase of microwave from 4.0 to 5.0 kW,the specific surface areas were inceased from 105 to 239 m3/g,and band gaps were also increased from 2.78 to 2.88 eV.In the experiment of photocatalytic degradation,2D-g-C3N4 nanosheets synthesized via microwave heating with the irradiation power of 5.0 kW possessed higher photocatalytic properties.After the visible-light photocatalytic reaction for 35 min,the decolorization ratios of RhB and MO reached up to 100%and 93.3%,respectively.The corresponding pseudo-first order kinetics rate constants for 0.09182 and 0.07476 min-1 were calculated by the analysis of pseudo-first order kinetics.The improvement of the photocatalytic activity was mainly ascribed to stronger adsorption performance,more active sites and shorter carrier thransfer distance of 2D-g-C3N4 nanosheets with large specific surface area.After consecutive 4 times recycling,the decolorization ratios of RhB still reached up to 96.7%,indicating 2D-g-C3N4 nanosheets had very stable photocatalytic activities.(4)2D-g-C3N4 nanosheets with large specific surface area(239 m2/g),few stacking layers(～5 atomic layers),enlarged band gap(2.88 eV)and large aspect ratio(planar size:～8 μm,thickness:～1.6 nm)were investigated by field emission measurement.The 2D-g-C3N4 nanosheets possessed extremely low turn-on fields of around 0.5 V/μm-the highest current density of 4.5 mA/cm2 in the range of testing and a large field-emission factor of～2.1×104.This result indicated that 2D-g-C3N4 nanosheets had excellent field emission properties.