Study On Surface Adsorption Properties Of Doped ZnO Materials

ZnO is a multi-functional semiconductor material with a wide band gap of optoelectronic and piezoelectric,and it is also a typical II-VI semiconductor oxide.The width of the band gap at room temperature is 3.37e V.Because of its excellent physical and chemical properties and a wide range of applications,the special properties caused by the reduction of material size from macroscale to nanometer scale make the application of ZnO nanomaterials more popular.ZnO nanostructure is a promising photocatalyst with economy,stability,biosafety and excellent performance.it is widely used in the fields of gas sensors,piezoelectric materials and photocatalytic materials.The introduction of metal ions can change the electrical properties and molecular adsorption properties of the crystal plane and play an important role in regulating the function of the crystal plane.Selecting the crystal plane with high conductivity can avoid the problem of excessive resistance caused by the traditional bulk phase doping method,and can also make full use of the doping effect of the crystal plane.The study of crystal plane adsorption mechanism is helpful to explore the molecular adsorption and optoelectronic properties of materials,guide the synthesis and modification of materials in theory,further adjust the electrical properties of materials,and realize the controllable design of gas sensing materials.In this paper,the molecular adsorption behavior of ZnO（002）crystal plane doped with metal ions was studied,and the gas sensing properties of ZnO materials were further optimized theoretically by exploring the effects of different gas molecules on the adsorption properties and optoelectronic properties.The details are as follows:Firstly,Al³⁺-doped ZnO nanoparticles were prepared.The adsorption behavior of molecules in air（O2,N₂ and H₂O）on Al³⁺-doped ZnO（002）crystal plane and its effect on the electrical properties of the crystal plane were studied by density functional theory（DFT）.The results show that O₂ and H₂O molecules chemically adsorb on the crystal plane and form characteristic adsorption species,and N₂ molecules are adsorbed on the crystal plane in physical form.The adsorption of three kinds of molecules leads to the decrease of crystal plane conductivity.The simulation results of electrical conductivity are consistent with the fitting data of electrochemical impedance spectroscopy.The optimized adsorption model shows that the surface atoms of the crystal surface undergo structural restructuring after being affected by molecules.The introduction of Al³⁺changes the electrical and molecular adsorption properties of crystal plane and plays an important role in the regulation of crystal plane function.Secondly,Ni²⁺-doped ZnO nanoparticles were prepared.The adsorption behavior of air molecules（O₂,N₂ and H₂O）on Ni²⁺-doped ZnO（002）crystal plane was studied by density functional theory（DFT）,and the co-adsorption behavior of HCHO molecules on the crystal plane was studied.The results calculated according to the adsorption model show that O₂ and H₂O are characteristic chemical adsorption species after the Zn atoms on the crystal plane are replaced by Ni atoms,and H₂O molecules further promote the co-adsorption of O₂ molecules due to the hydrogen bond with O₂molecules,which is confirmed by infrared spectroscopy.Molecular adsorption leads to the change of crystal plane conductivity,which is consistent with the experimental results of electrochemical impedance measurement.The structure optimization results of the adsorption model based on HCHO molecular detection show that the co-adsorption of the three molecules leads to the increase of crystal plane conductivity,which is consistent with the experimental results of electrochemical impedance measurement.The introduction of Ni²⁺not only changes the characteristic adsorption species and electrical properties of crystal plane,but also promotes the co-adsorption behavior of molecules,which indicates that the characteristic molecular adsorption and electrical properties of materials can be controlled by crystal plane ion doping,which is helpful to guide the synthesis and modification of materials in theory and further adjust the electrical properties of materials.Thirdly,the change trend of electrical conductivity of crystal plane before and after molecular adsorption was studied by AC impedance spectroscopy,and the sensitivity of the material to gas was shown by the change of electrical conductivity.The molecular adsorption of ZnO materials doped with Al³⁺and Ni²⁺in different atmospheres such as acetone,ethanol,methanol and formaldehyde was studied,and the change of electrical conductivity of crystal surface after adsorption of different volatile compounds was analyzed.The purpose of gas detection was achieved through the difference of responsivity of crystals to different gas molecules.It is found that the resolution of and ZnO doped with Al³⁺（002）crystal plane has a higher degree of discrimination between HCHO molecules.ZnO doped with Ni²⁺（002）crystal plane has higher resolution for C₂H5OH molecules.Using the changing law of optoelectronic properties to complete the detection of other gas molecules has a certain guiding significance for understanding the gas sensor detection technology from the point of view of gas molecules.