Molecular Adsorption Behavior On Doped Tin Dioxide Crystal Plane

Tin dioxide（SnO₂）as the n-type metal oxide semiconductor with a wide band-gap of 3.6 e V.It has been widely applied in the gas sensor research,and has attracted extensive attention in aroused academic field and industrial field,because the great optical properties,conductivity and excellent chemical stability.The gas detection principle of the SnO₂gas sensor is that the electrical conductivity of the surface layer is changed through the adsorption of the target gas.The adsorbed gas reacts with the adsorbed oxygen ions on the surface under high temperature to further change the electrical conductivity of the material.Different gases sometimes have different effects on the surface electrical conductivity of materials,because of the different structure and interaction with the crystal plane.In order to deeply understand the influence mechanism of doping in SnO₂（221）crystal surface on molecular adsorption and electrical properties,and effectively regulate the gas-sensitive performance of the crystal surface.In this work,we had synthesis the SnO₂（221）and the metal ions had doped in the crystal surface,to explore the influence of doping on the surface molecular adsorption of materials.In this work,the electrical performance of doped crystal surface before and after absorbed O₂,N₂and H₂O molecules were simulated by density functional theory（DFT）,and the molecular adsorption was verified by electrochemical impedance spectroscopy（EIS）.Four volatile chemicals easily produced in industrial chemical production and processing were selected to explore the ability of different metal ions doped in crystal surfaces to detect acetone,ethanol,methanol and formaldehyde molecules.Specific research contents are as follows:1.The study of the adsorption performance of Al³⁺doped SnO₂（221）crystal surface.The H₂O molecule is referred to as the characteristic adsorption species（CAS）,and the crystal plane model resulting from synthesis and storage conditions is denoted as the H₂O-SnO₂-Al（221）surface model.The DOS value is used to qualitatively judge the crystal surface electrical conductivity trend,because of the consistency between the Fermi level density of states（DOS）value and crystal surface electrical conductivity.When the H₂O-SnO₂-Al（221）mode further adsorbed O₂,N₂and H₂O molecules,the corresponding DOS values are 7.87→4.08 electrons/e V,7.87→7.51 electrons/e V and 7.87→5.81 electrons/e V,respectively.The decreasing trend of crystal surface conductivity caused by the adsorption of O₂and N₂molecules was confirmed by EIS results.Contrary to the simulation results of the H₂O molecule adsorption,the conductivity increases as the increasing humidity.Further infrared spectroscopy measurements showed that this was caused by the aggregation of H₂O molecules on the crystal plane.2.The study of adsorption performance of Co²⁺doped SnO₂（221）crystal surface.The first-principles simulation results show that Co²⁺-SnO₂（221）material has crystal surface oxygen defects,and after adsorption of O₂,N₂and H₂O molecules,The trend of state density at the Fermi level is 8.52→11.61 electrons/e V,8.52→8.58electrons/e V and 8.52→7.38 electrons/e V,respectively.This means that the adsorption of O₂molecules and N₂molecules will lead to an increase in the conductivity of the material,and the change of the conductivity is more obvious after the adsorption of O₂molecules,and the adsorption of H₂O molecules will eventually lead to the decrease of the conductivity of the material.This simulation result is proved by the EIS results.3.The study of molecular detection performance of Al³⁺and Co²⁺doped SnO₂（221）crystal planes.The ability of the material surface to detect volatile compounds was studied.The experimental results show that the detection ability of SnO₂-Al（221）is acetone<formaldehyde<methanol<ethanol.The detection ability of Co²⁺-SnO₂（221）was acetone<methanol<ethanol<formaldehyde.