Study On Photocatalytic Properties And Mechanism Of G-C₃N₄ Based Systems By Theoretical Chemistry Method

Air is a space that human depend for their survival,which makes people pay more attention to the severe problems of atmospheric environment.VOC s,volatile organic pollutant,is one of the main components of air pollutants,which has the characteristics of high evaporation and low dissolution.Its own toxicity have great harm to human health and DNA,and even lead to cancer.In addition to,it is easy to produce ozone and haze pollution.There is an urgent desire to find a low-cost,high performance semiconductor photocatalyst that can efficiently utilize the cleanest solar energy to photocatalytic degradation the pollutants in the atmosphere.Graphitic carbon nitride（g-C₃N₄）,as a non-metallic two-dimensional photocatalyst,is the most stable configuration of carbon nitride and has been paid much attention at present.The huge surface area can produce more active sites and has a unique electrical,biological properties.It has a huge development space in photocatalytic and energy storage.Based on the DFT calculation of the first principle,we use the CASTEP and DMOL3modules of Materials Studio to modify g-C₃N₄.By functional groups modification,semiconductor composite and metal single atom doping,we hope it has a better apply in photocatalytic.The main results we have achieved are as follows:（1）Effect of oxygen groups on photocatalytic properties of g-C₃N₄ was studied.We found that only N-OH and N=O can be stably attached at the center of CNNR,and all three groups（-COOH,-OH and=O）are stable at the edges.N-COOH or N-OH can significantly reduce the band gap of CNNR and the bandgap would narrow sharply at high concentration of these oxygen groupss.However,attaching=O alone does not change the band gap,unless replacing H atoms at the right edge.The presence of-COOH and-OH can induce new band that locates between the conduction and valence bands,thus reducing the band gap remarkably.Therefore,oxygen groups can tune the band structure of g-C₃N₄,promoting the generation of electron-hole pair.This work provides fundamental guidance to improve g-C₃N₄ performance for the potential photocatalytic applications.（2）We have creatively combined g-C₃N₄ dots with graphene to study its photocatalytic properties.After calculation,we found that the g-C₃N₄ dots/graphene has a lot of potential advantages:（i）Compared with g-C₃N₄ and g-C₃N₄ dots,the band gap of g-C₃N₄ dots/graphene is much smaller,which is more prone to light exci tation and generate electron-hole pairs;（ii）The solar light by photocatalysis is the cleanest and richest natural energy on earth.The g-C₃N₄ dots/graphene can increase visible light utilization,then it will greatly reduce the cost of photocatalytic deg radation of VOCs;（iii）The g-C₃N₄ dots/graphene has low electron recombination rate to better photodegradation;（iv）VOCs and H₂O can be well adsorbed on g-C₃N₄ dots/graphene,that can effectively produce free radicals to act on the degradation of VOCs;（v）Most importantly,the electronic migration path of the g-C₃N₄ dots/graphene is not previously recognized that directly from the valence band to the conduction band of g-C₃N₄,but from the valence band of g-C₃N₄ dots,leap to graphene,and then to the conduction band of g-C₃N₄.This reduces nearly half of the energy required for electronic transitions and is beneficial for photocatalysis.In general,the g-C₃N₄dots/graphene is an effective photocatalyst,and a new transition path of light excitation electron is found.（3）Through the three thoughts that metal single atom doping,visible light catalysis and Fenton reaction,we construct the Fe-g-C₃N₄ dots/graphene system,using the widely existing Fe in nature and g-C₃N₄ dots/graphene.After calculation,we found Fe-g-C₃N₄ dots/graphene has significantly enhanced electrical and optical properties,while also equipped with Fenton reaction.The Fe-g-C₃N₄ dots/graphene is a metal single-atom doped high-efficiency catalyst for VOCs with Fenton reaction and visible light catalytic reaction,which can be used to efficiently deal with the gas pollution problem in environment by the synergy of these two reactions.