Research On The Modification And Photocatalytic Properties Of Graphite Carbon Nitride

Over-consumption of energy and environmental pollution are now serious problems that all human is facing and needing to solve urgently.In recent years,the photocatalytic technology using solar energy has been widely studied and concerned in hydrogen production from water splitting and organic pollution degradation,showing potential application prospects in obtaining green hydrogen energy and water environment treatment.Graphite carbon nitride（g-C₃N₄）,as an excellent non-metallic semiconductor photocatalyst,has an appropriate band gap value（about 2.7 eV）and can respond to visible light.At the same time,the advantages of cheap raw materials,simple preparation process and stable properties are obvious.However,the practical application of g-C₃N₄ in the field of photocatalysis is not as prevalent as imagined.The problems of small specific surface area and high photogenic carrier recombination rate restrict its development.Therefore,there is still the key research topics of g-C₃N₄ in increasing surface active sites,improving charge transfer rate and ameliorating carrier separation efficiency.In summary,this paper conduct the modifition research on g-C₃N₄ through the microstructure controlling,modifying co-catalysts and doping metal/nonmetal elements,g-C₃N₄ with ultra-thin and mesoporous,RhP_x/g-C₃N₄ and Cu/Cl-g-C₃N₄ photocatalysts were prepared,and advanced structure characterization techniques were used to discuss in detail the microstructure and morphology,physical and chemical properties,photoelectricity of these photocatalysts in detail characteristics and photocatalytic performance,carefully studied their charge behavior mechanism and reaction mechanism during the photocatalytic reaction,as follows:（1）The ultrathin mesoporous g-C₃N₄ photocatalyst was prepared,which shows excellent photocatalytic degradation and water decomposition hydrogen production activity.Research have shown that ultra-thin mesoporous carbon nitride（umpg-C₃N₄）prepared by one-step calcination method with ammonium carbonate and urea as precursors significantly increased the specific surface area,more reactive sites and abundants molecular transport channels compared to g-C₃N₄,so it effectively inhibited the recombination of photo-generated carriers and accelerated the charge transport,thereby greatly enhancing the photocatalytic degradation of bisphenol A（BPA）and the activity of producing hydrogen from splitting water.The best photocatalytic degradation rate for BPA of 7:10 umpg-C₃N₄ could reach 56.9%within60 min,it was significantly higher than the degradation rate of g-C₃N₄ 38.2%;and the photocatalytic hydrogen production rate of umpg-C₃N₄ reached 908 mmol g^-1 h^-1,which was about 2.1 times of g-C₃N₄(426 mmol g^-1 h^-1),and the apparent quantum efficiency（AQE）at 420 nm reached 4.4%.Photocatalytic degradation of BPA and photocatalytic hydrogen production cycle experiments show that umpg-C₃N₄ has high photocatalytic stability and reusability.Researches on the mechanism of photocatalytic degradation of BPA have shown that holes（h⁺）and superoxide radicals（·O₂^-）play an important role in the degradation of BPA,while hydroxyl radicals（·OH）have a relatively small effect.The order of influence on degradation of BPA is h⁺>·O₂^->·OH.（2）The RhP_x/g-C₃N₄ composite photocatalyst prepared with RhP_x nanospecies modified 2D g-C₃N₄ nanosheets can be controlled,the photocatalytic hydrogen production activity and stability of RhP_x/g-C₃N₄ were excellent.The results show that the RhP_x nanospecies as a co-catalyst is strongly supported on the surface of 2D g-C₃N₄ to produce strong interfacial interactions,improve the charge transfer ability and carrier separation efficiency,and thus show excellent photocatalytic hydrogen production activity and stability.The hydrogen production rate of RhP_x/g-C₃N₄-5%composite photocatalyst was 3055.9μmol h^-1 g^-1,which was 5.6 times higher than that of Pt/g-C₃N₄(547.0μmol h^-1 g^-1).The apparent quantum efficiency（AQE）reached 18.4%at 420 nm,compared with the reported g-C₃N₄ composite photocatalyst modified by a single transition metal phosphide,it was much higher than them.At the same time,the RhP_x/g-C₃N₄-5%composite photocatalyst can maintain a stable photocatalytic hydrogen production rate after 25 cycles of cumulative operation for more than 100 h of continuous reaction,so there was excellent stability and retrievability.Research on photocatalytic mechanism shows that the Rh-C bond formed on the surface can effectively move photogenerated electrons faster from 2D g-C₃N₄ nanosheets to RhP_x nanospecies and enhance the stability of RhP_x nanospecies on the surface of 2D g-C₃N₄ nanosheets.The Rh and P atoms in the P-Rh bond formed on the surface can act as diproton active sites to produce synergistic effects to enhance hydrogen production.（3）Double-doped Cu/Cl-g-C₃N₄ photocatalyst on metal/nonmetal surface was prepared and possessed excellent photocatalytic degradation activity,universality and stability.Researchs have shown that the double-doped effect of Cu/Cl element significantly improves the optical absorption capacity,reduction strength and Electron transmission rate of g-C₃N₄,thus the photocatalytic performance was greatly enhancd.Photocatalysis experiments show that Cu/Cl-g-C₃N₄-5%photocatalyst can not only efficiently degrade TC-HCl,its rate constant(0.0271 min^-1)is about 4 times that of pure g-C₃N₄(0.0067 min^-1),Moreover,it shows high-efficiency degradation ability for various persistent organic pollutants.The degradation rate of o-chlorophenol,bisphenol A and 2-mercaptobenzothiazole in 60 min is as high as 48.7%,50.4%and73.6%,which is far higher than pure g-C₃N₄ 5.5%,9.8%and 25.9%.Research on photocatalytic degradation mechanism shows that hole and·O₂^-are the main active species produced in the process of TC-HCl degradation,and·O₂^-plays a major role,while a small amount of·OH is generated by the further reaction between·O₂^-and h⁺,it has a smaller impact.