Theoretical Study On The NO₂ Gas Sensing Reaction Mechanism Of Nickel- And Copper-doped Zinc Oxide And Their Composite

Industrial production and human activity have led to significant consumption of fossil energy.As a result,the content of nitrogen oxides(NOx)in the environment has seriously exceeded the standard.NOx has caused serious harm to the ecological environment and human health.NOx-gas sensitive materials currently used in the market suffer problems including unclear structure-property relationship,long response/recovery time,high working temperature and unclear gas sensing mechanism.Thus,in this thesis a series of zinc oxide-based sensitive materials have been investigated using density functional theory for their structural properties and sensing mechanism towards NO2.Based on the finite cluster model,adsorption behaviors of the sensitive materials ZnO to nitrogen dioxide(NO2),O2 and H2O were studied.Calculations revealed the mechanism that ZnO senses NO2 and generates nitrate intermediate.ZnO favors adsorbing NO2 over O2.And it chemically holds water via dative bonds,in line with the experimental result that the humidity greatly affects the gas-sensing performance.We further explored sensitive materials of ZnO in combination with graphene(Gr),graphitic carbon nitride(g-C3N4),and cellulose(Cel).Geometric/electronic structures,interfacial interaction and thermodynamic calculations show that the composites themselves are stable,supported by strong chemical interfacial interactions.Of them,ZnO/Gr has the best NO2 gas-sensing performance.Divalent Ni was used to dope ZnO to construct a heterobimetallic Ni-Zn sensitive material ZnO(Ni),and it was further combined with materials of Gr,g-C3N4 and Cel.It is shown that ZnO(Ni)possesses bimetallic adsorption active sites with d8-d10 electronic configuration.The orbitals contributed by Ni composition right inserted between HOMO and LUMO of the original ZnO substrate.Consequently,the Ni site captures NO2 via donation and back donation interactions,which makes the adsorption free energy 1.03 eV larger than that of ZnO.This result well explains the experimental finding that Ni-doped ZnO has much faster response time to NO2 than ZnO.Our calculations show that ZnO(Ni)/g-C3N4 has better NO2 gas-sensing performance,which is corroborated by the largest adsorption free energy for NO2,the smallest uphill energy for capturing the second NO2 and the modest downhill energy for producing nitrate.Divalent copper doping into ZnO has fabricated the Cu-Zn sensitive material ZnO(Cu),which has the d9-d10 electronic configuration.Compared with pure ZnO gassensing materials,copper doping and further combining g-C3N4 all promote the adsorption towards NO2,but they are slightly weaker than ZnO(Ni).Comparatively,the simultaneously doping Cu and combining graphene can enhance NO2 adsorption,but the enhancement extent is relatively small.The analyses of energetics,bondings and electronic structures demonstrate that ZnO(Cu)/g-C3N4 has the best gas sensing performance.