Controllable Preparation,Heterostructure Construction And Photocatalytic Properties Of Novel Nanoporous G-C₃N₄

Photocatalytic technology is becoming more and more widely used in the treatment of environmental pollution,the preparation of clean energy,the construction of environmental protection buildings and the medical field.Graphitic carbon nitride?g-C₃N₄?,as the most commonly used photocatalyst nowadays,has many excellent properties.G-C₃N₄ is an infinite planar network consisting of C and N rings.It has excellent photoelectric properties and high thermodynamic stability.Its narrow band gap structure has a certain visible light response ability,and the secondary pollution probability is low,therefore it is an ideal candidate for photocatalyst.However,the g-C₃N₄ prepared by the traditional method has problems such as small specific surface area,narrow visible light response range,short lifetime of photogenerated carriers,and low photocatalytic activity.It is an effective way to enhance the photocatalytic activity by improving the specific surface area,adjusting the band structure and improving the carrier transmission mechanism by modifying the g-C₃N₄ photocatalyst.In this dissertation,a two-dimensional nanoporous graphene g-C₃N₄ was synthesized by aqueous solution assisted pyrolysis.Based on these,novel heterojunction photocatalysts were synthesized by combining nanoporous g-C₃N₄ and titanate?ZnTiO₃,MnTiO₃?nanocrystalline.The optical absorption and response ability of the composite can be enhanced,the recombination rate of photogenerated carriers is reduced,the survival rate of strong oxidation cavitation and strong reductive electrons is improved,and the photocatalytic activity of g-C₃N₄ is significantly improved.The main contents and conclusions are as follows:1.Nanoporous graphene-like g-C₃N₄ with mesoporous morphology was prepared by aqueous solution assisted pyrolysis.Through the change of heating temperature,the micro-morphology,energy band structure,and photoelectric property of g-C₃N₄were regulated,and two-dimensional nanoporous graphene-like g-C₃N₄ with high photoelectric conversion ability,good adsorption performance,wide light absorption range and high photocatalytic activity was prepared,which significantly improved the visible light photocatalytic activity of g-C₃N₄.The BET test and calculations showed that the synthesized g-C₃N₄ pore size was mainly concentrated in the range of 20-50nm,and the specific surface area was 197 m²/g.The photocatalytic degradation of methylene blue?MB?solution showed that the degradation rate reached 98.5%.Combined with the first order kinetic behavior analysis,g-C₃N₄ showed a high degradation kinetics rate constant.Meanwhile,the adsorption and mineralizing properties of g-C₃N₄ on MB were studied,and the main active groups in the photocatalytic process were tested to explore the photocatalytic mechanism of g-C₃N₄.2.Interface tightly integrated g-C₃N₄/MnTiO₃?HS-CN/MT?heterojunction photocatalyst was firstly obtained by hydrothermal method on the basis of two-dimensional nanoporous graphene-like g-C₃N₄.The TEM and EDS analysis showed that MnTiO₃ was uniformly distributed on the nanoporous g-C₃N₄.Good dispersion promoted the interaction between electronic effect and volume effect,and had a significant effect on the change of electronic structure and band structure.The two closely combined to form a heterostructure.It was found that the hydrothermal treatment promoted the formation of the heterojunction by comparing the two treatments of water heat and ball milling.UV-vis DRS showed that the light absorption band edge of HS-CN/MT heterojunction photocatalyst was red shift than that of g-C₃N₄ and MnTiO₃,and the absorption range and absorption strength were enhanced.Photocatalytic activity showed that HS-65CN/MT had the highest photocatalytic degradation rate,which was 30.9,14.5 and 2.4 times higher than that of MnTiO₃,bulk g-C₃N₄ and nanoporous g-C₃N₄,respectively.The kinetic behavior analysis showed that the apparent kinetic constant was 0.0411min^-1.The electrochemical impedance spectroscopy?EIS?,photoluminescence?PL?and active group detection were used to analyze the type of photocatalytic active group of HS-CN/MT heterojunction and the mechanism of electron hole transfer.It was found that HS-CN/MT had a Z-type heterojunction mechanism.Under the action of the interface electric field of heterojunction,the charge is effectively migrated,prolonging the survival time of electrons and holes,and enhancing the optical quantum efficiency.3.Through the combination of nanoporous g-C₃N₄ and hexagonal-phase ZnTiO₃composited with anatase-phase TiO₂?h'ZnTiO₃-a'TiO₂ or h'ZT-a'T?,novel g-C₃N₄/h'ZnTiO₃-a'TiO₂?CN/h'ZT-a'T?ternary Z-type heterojunction photocatalyst was constructed to extend carrier transport path,expand the optical absorption range and enhanced electron-hole reduction and oxidation capacity and photocatalytic activity.The composites were characterized by XRD,X ray photoelectron spectroscopy?XPS?,TEM,BET specific surface area,infrared spectrum?IR?,UV-vis DRS,PL and EPR.The crystal structure,microstructure,band structure and photoelectric properties were analyzed.TEM results showed that anatase-phase TiO₂and layered g-C₃N₄ were connected by h'ZnTiO₃ to form a CN/h'ZT-a'T ternary heterojunction structure.UV-vis DRS showed that the band edge of the optical absorption band of 50CN/h'ZT-a'T composite had a red shift than that of g-C₃N₄ and h'ZT-a'T,which enhanced the absorption range and strength of visible light.The PL,transient photocurrent response and EPR test showed that the 50CN/h'ZT-a'T heterojunction enhanced the separation and transfer capacity of the photogenerated carrier.The photocatalytic performance showed that the kinetic constant of photocatalytic degradation of 50CN/h'ZT-a'T reached 2.921h^-1,which was 3 times as high as that of two-dimensional nanoporous g-C₃N₄.In the catalytic mechanism analysis,the photoinduced electrons in the CB of TiO₂ were transferred to h'ZnTiO₃and then directly injected into the HOMO of g-C₃N₄,becoming trapped by the holes there.H'ZnTiO₃ with high electron mobility in the direct Z-scheme heterojunction played a important role of electronic transfer station.This would be very beneficial for reducing the recombination of photogenerated electron–hole pairs while retaining the reduction and oxidation capacity of electrons and holes in the heterojunction with high electrochemical potential,which is theoretically suitable for the degradation of refractory organic pollutants and photocatalytically splitting water to produce hydrogen.