Gas Sensing Mechanism And Performance Tunability Of ZnO Nanorod Arrays Based On DFT Calculation

Gas sensor can be utilized to detect and moniter specific gas specie and concentration,which possesses broad prospects in environmental monitering,rapid medical diagnosis and Internet of Things.Metal oxide semicondutors?MOSs?nanomaterials have drawn extensive attention in gas sensing field due to their high gas response,high integration level and low cost.Based on present understanding of gas sensing mechanism,numerous methods have been applied to improving gas sensing properties of MOSs.However,high operating temperature and poor selectivity are still Achilles'heel.The key to solve the dilemma is to reveal the underlying mechanism of operating temperature and selectivity.First-principle simulation based on DFT?DFT simulation?posseses advantages in revealing gas sensing mechanism in actomic and electronic scale.Nevertheless,most of DFT simulations on gas sensing mechanism still concentrate on absorption process.Little concentration has been paid on surface reactions between gas molecules and absorbed oxygen or lattice oxygen,and instruction in performance tunability from DFT simulation is still rarely reported.In this thesis,ZnO nanorod arrays are chosen as object of study.Absorption and reaction processes of O₂,CH₃OH,NH₃,CO on ZnO surfaces are revealed by combining experimental results and DFT simulation,and charge transfer and electron structure of each process are also reveal.On basis of that,operating temperature mechanism and selectivity mechanism of ZnO nanorod arrays are revealed.DFT simulation is continuously utilized to search for an approach for lowering operating temperature and improving selectivity to CO.The approach is verified through experimental results.ZnO nanorod arrays are prepared through liquid synthesis method,which are then exfoliated and attached on the Al2O3 ceramic tube with Au interdigital electrodes to fabricate gas sensor device.Thermal pulse method is utilized to study the correlation between resistance of ZnO nanorod arrays and temperature.According to the correlation,grain barrier of ZnO nanorod arrays is calculated to be 0.17 eV,and reaction energy for the dissociation is 50.80 kJ mol^-1.The dissociation is slightly endothermic,suggesting that temperature elevation can promote the process.DFT simulation reveals the dissociation from atomic level.And the activation energy is calculated to be 351.71 kJ mol^-1,suggesting that absorbed O₂ molecule is not likely to dissociate at room temperature.O adatom dissociated from absorbed O₂ molecule can extract more electrons from ZnO surface,which is the main reason for resistance enhancement among the 443⁵53 K.DFT simulation is also utilized to study interactions between ethanol molecule and two kinds of absorbed oxygen.It is discovered that ethanol molecule can not react with aborbed O₂ molecule,but can be spontaneously oxidized by O adatom.Combining results of experiment and DFT simulation,the lowest operating temperature for ZnO nanorod arrays detecting ethanol is the temperature at which absorbed O₂ molecule begins to dissociate into O adatoms.Through analyzing electron structures of absorbed oxygen and lattice oxygen,it is demonstrated that there are only O₂^-?i.e absorbed O₂ molecule?and O^-?i.e O atatom?on ZnO?101?0?surface.The strong oxidizing ability of O adatom results from its unoccupied and unstable configuration of valence eletron configuration.Gas responses of ZnO nanorod arrays to various alphalic alcohols are tested at593 K.It demonstrates that selectivity is not merely determined by the quantities of absorbed oxygen that reducing gases consume.Gas responses to CH3OH,NH3 and CO suggest that ZnO nanorod arrays show selectivity to CH₃OH.Reaction processes of the gas molecules on ZnO?101?0?simulated by DFT pinpoints three factors for the selectivity.Firstly,CH₃OH consumes O adatom more rapidly.Secondly,products oxidized from CH₃OH hamper the reabsorption of O₂ molecule in air.Thirdly,H adatoms dissociated from CH₃OH inject more electrons to ZnO?101?0?surface.Based on the mechanisms of operating temperature and selectivity,DFT simulation is continuously utilized to study absorption and reaction processes of O₂,CO,CH3OH and NH3 on PdO?101?.It is discovered that PdO?101?have strong ability to absorb and oxidize CO molecule.CO molecule can sustainedly consume lattice oxygen at PdO?101?surface,and can hamper reabsorption of O₂ molecule.In contrast,CH₃OH and NH₃ cannot consume lattice oxygen.Additionally,CH₃OH and NH₃ show little advantage over CO in absoption ability and electron injection.Therefore,PdO nanoparticles decoration can be an effective approach for lowering operating temperature of ZnO nanorod arrays and improving selectivity to CO.Hence,ZnO nanorod arrays decorated with PdO nanoparticles are prepared through photochemical deposition method,which shows good selecitivity to CO at room temperature.ZnO nanorod arrays co-decorated with PdO and SnO₂ are also prepared.Introduction of SnO₂ improves gas response at room temperature without changing selectivity.