Graphitic Carbon Nitride-based Photocatalytic Materials:Controllable Preparation,Modification,and Mechanism

Semiconductor photocatalysis is one of the most promising solutions to problems of the energy and environment.The core of photocatalysis is photocatalyst.A suitable photocatalyst requires not only high catalytic activity and visible-light response but also low cost and facile preparation.As a new non-metallic semiconductor photocatalyst,graphitic carbon nitride(g-C₃N_4,CN)meets the above requirements.CN has potential applications in photocatalytic reduction of O₂ to produce H₂O₂ and environmental remediation.However,the photocatalytic performance of bulk g-C₃N₄(BCN)has been reduced by its low active sites,low electron-hole carriers transport rate,and low utilization of visible light.Besides,the traditional method of preparing CN has high energy consumption and a long cycle,which does not meet the concept of green production.In this paper,based on the advantage of nonmetallic CN,the photocatalytic performance of CN is improved through morphological regulation,element doping,and heterogeneous structure construction.The microwave heating method is used to reduce energy consumption and save resources.The controlled preparation and modification of CN are realized,and the mechanism is studied systematically.The main results are as follows:(1)Using melamine and cyanuric acid as raw materials,the MCS with network structure was obtained by their self-assembly.Those network MCS will grow along the axis to form a rod-like structure by competing with water for hydrogen bonds.Then,the MCS crystals with tighter structures and higher crystallinity can be obtained by hydrothermal treatment.The effect of hydrothermal treatment temperature,time,and concentration of MCS complex on the yield and morphology of the MCS crystals was investigated.This makes a great contribution to the formation of g-C₃N₄ tubes(CNT)with uniform size and high yield.Compared with the CNT obtained by the same preparation method,the yield of CNT increased by 196.5 times.(2)To further improve the photocatalytic performance of CNT,the novel non-metal elements(B,P,and S)doped CNT(B-CNT,P-CNT,and S-CNT)were prepared.The as-prepared B-CNT photocatalysts showed the highest photocatalytic H₂O₂production with the formation rate constants values of 42.31?M·min^-1,which is 2.6times higher than that of BCN(16.36?M·min^-1).The synergistic effects of superior morphology and molecular structure lead to the remarkable enhancement in photocatalysis.The·O₂^-are dominant active species in photocatalysis and the H₂O₂ is generated from two-step single-electron O₂ reduction.The result has provided a new way of realizing the practical application of CNT photocatalysts.(3)To improve the separation rate of electron-hole charge carriers,a CN tube/CN nanoparticle heterostructure material was constructed.The MUCN could be obtained by calcination of the mixed precursors of MCS rods and urea granules.Due to the potential difference in the conduction band and valence band between the CN prepared by the above two precursors,the internal driving force is formed,which can change the migration path of carriers,restrain their self-recombination and improve their separation rate.The as-prepared MUCN photocatalysts showed 36.6 times higher kinetic constant for Rhodamine B(Rh B)degradation than that of BCN.(4)To overcome the problems of high energy consumption and low utilization in the traditional muffle furnace heating method,microwave synthesis was used to prepare CN.The molecular structure of the MCS precursor is similar to that of CN,which can help the CN form at a lower temperature.A novel CN nanoribbon(CNNR)assembled seaweed-like architecture by microwave synthesis using the melamine-cyanuric acid supramolecular(MCS)as the precursor.The preparation process is facile,fast(<30 mins),and green.The as-prepared CNNR is stable with a large specific area and exhibits a decreased bandgap of 2.5e V as well as 52.2 times higher kinetic constant for Rhodamine B(Rh B)degradation than that of bulk CN.The CNNR shows highly efficient degradation for methylene blue(MB),methyl orange(MO),and tetracycline(TC).The CNNR showed a higher photocatalytic H₂O₂production than the CN prepared by the traditional muffle furnace heating method.The significantly enhanced photocatalytic property of CNNR is mainly attributed to the synergistic effects of superior structure,O doping,and proper N defects.This provides a new idea for the rapid preparation of high-performance CN materials.