First-Principles Investigation Of The Sensing Mechanism In Metal Oxide Semiconductor Gas Sensors

The metal oxide semiconductor gas sensor is a sensor widely utilized in the field of gas detection.Its fundamental operational principle involves the perception of changes in gas concentration through the adsorption and desorption of gas molecules on the material surface.Over the past few decades,numerous experiments and theoretical studies have indicated that the microstructure,surface defects,and doping of metal oxide semiconductor materials significantly impact sensor performance.Volatile organic gases represent a highly diverse category of gaseous pollutants,possessing distinct functional groups.These functional groups’chemical properties and spatial structures influence the interaction between gases and metal oxide semiconductor materials.Therefore,selecting appropriate sensor materials and optimizing designs for different volatile organic gases is essential to achieve efficient and accurate gas detection.In recent years,with advancements in computer capabilities and material calculation methods,an increasing number of researchers have embarked on the study of metal oxide semiconductor gas sensors using methods such as finite element analysis and first-principles calculations.Macro-scale modeling,accomplished through finite element analysis,couples the analysis of gas diffusion processes,chemical reaction mass transfer,and changes in material physical quantities.Simulating the sensing mechanism on the atomic involves first-principles calculations,where density functional theory calculations reveal the impact mechanisms of material surface defects and doping on the adsorption and desorption of volatile organic gases.This encompasses molecular adsorption,charge transfer,and surface reactions.Therefore,this dissertation combines experimental and theoretical calculation methods to delve deeply into the mechanisms by which material microstructure,surface defects,and doping affect sensor performance.This study provides a crucial reference for designing and optimizing sensor materials.The main research work of this dissertation is outlined below:1.The chemical reaction engineering module is employed to elucidate the dynamic equilibrium process of oxygen ions within the sensor.The simulation captures processes such as convection,diffusion,permeation,and conductivity variations using boundary conditions encompassing temperature transfer,conductivity models,and mass transfer.The study simulates the sensor’s response under different temperatures（445-521 K）and varying concentrations of the target gas（1-500 ppm）.Instead of employing traditional direct parameter fitting methods,this dissertation utilizes an oxygen ion dynamics model to bridge gas concentration and sensor response.The simulated results of the metal oxide semiconductor gas sensor,employing surface oxygen ion control and permeation rate control models,align well with actual sensor behavior.This simulation process also introduces a novel perspective for elucidating the principles of metal oxide semiconductor gas sensors.2.Combining experimental analysis with density functional theory calculations,this study compares and analyzes the sensing characteristics of different morphologies and exposed crystal facets of nanomaterials.It identifies and comprehends the corresponding mechanisms at the atomic or ionic level.Initially,Co₃O₄ nanosheets（Co₃O₄-NS）with exposed（111）crystal facets and Co₃O₄ nanorods（Co₃O₄-NR）with exposed（110）crystal facets are grown on alumina ceramic tubes.The research focuses on the ethanol sensing mechanism,and DFT calculations are employed to analyze the adsorption characteristics of ethanol on the sensor surface.Experimental and simulated results reveal that Co₃O₄-NR exhibits superior sensing performance compared to Co₃O₄-NS.The hydroxyl group in ethanol demonstrates better adsorption performance on the surface than the methyl group.When the oxygen and hydrogen atoms of the hydroxyl group in ethanol interact with the（110）surface Co atoms and their indirectly adjacent low-coordinated oxygen atoms,the adsorption energy is minimized(E_ads=-3.44 e V).The exposed surface atoms induce superior sensing performance through cooperative adsorption of the target gas.3.This study utilizes density functional theory calculations to investigate the sensing characteristics of P-doped Co₃O₄（111）surface towards methanol,methane,formic acid,water,and formaldehyde.The research outcomes indicate that P doping alters the surface electron distribution of Co₃O₄（111）.Upon adsorption on the P-doped surface,formaldehyde molecules undergo significant deformation,where the carbon atom in the aldehyde group is connected to the P atom on the doped surface,and the oxygen atom in the aldehyde group is bonded to the metal atom adjacent to the doped surface P atom.P doping enhances the charge transfer quantity for formaldehyde gas,reduces the adsorption energy for other molecules,and decreases charge transfer.This work broadens the application of P-doped Co₃O₄（111）in gas sensing of volatile organic compounds（VOCs）,providing a theoretical foundation for seeking superior metal oxide semiconductor gas sensors.4.The humidity resistance of materials with varying Ni doping ratios is initially meticulously tested.Subsequently,employing high-resolution transmission electron microscopy（HRTEM）,the surfaces of Co₄₈O₆₄（110）and Co₄₈O₆₄（111）are constructed at the atomic scale.Single,double,and triple Ni atom doping on both（110）and（111）surfaces are then constructed based on minimum energy principles.Following this,the adsorption of water and n-butanol molecule on these eight surfaces is calculated.Results indicate that Ni doping enhances the humidity resistance of Co₄₈O₆₄（110）and Co₄₈O₆₄（111）,with dual atom doping exhibiting higher sensing characteristics for n-butanol than other Ni doping quantities.Lastly,nanorod-shaped Co₃O₄ with predominantly exposed（110）crystal facets and Ni-doped Co₃O₄ are synthesized.Experimental results demonstrate that Ni-doped Co₃O₄exhibits superior humidity resistance compared to Co₃O₄.The decrease in n-butanol response may be attributed to changes in Ni doping concentration or different exposed surface facets.