Optical Gas Sensing Characteristics And Modification Of Rutile TiO₂,SnO₂,GeO₂ To HCl

Sulfides,nitrogen oxides and halides not only pollute the environment,but also harm the human respiratory system.Therefore,it is particularly important to detect various harmful gas components in the air and their concentrations.Although semiconductor sensing materials are widely used in gas detection and other fields due to their stable performance,low cost and good repeatability,different sensing materials often have different sensitivities to the same gas and the same sensing materials will also have different sensing properties after modification.TiO₂,SnO₂,and GeO₂ are metal oxide gas-sensitive materials with good gas-sensing properties,and have the same rutile phase structure.The principle of gas sensing can be summarized as when the surface of the semiconductor undergoes an oxidation-reduction reaction with gas molecules,due to the transfer of charge,the charge distribution on the surface of the material changes,thereby changing the electrical properties of the material that we can know the type and concentration of gas detected by detecting changes in electrical properties.Optical gas sensing is based on the change of optical properties caused by the redox reaction of surface oxygen vacancies and environmental gas molecules to reflect the sensing characteristics,so the research on the adsorption characteristics of surface oxygen vacancies is an important aspect.In order to find gas-sensitive materials with high sensitivity to hydrogen chloride?HCl?,the surface oxidization,adsorption energy,charge density,density of States,and optical properties of rutile TiO₂,SnO₂ and GeO₂?110?plane adsorbing HCl are studied by using the plane-wave ultrasoft pseudopotential method under the first principle of density functional?DFT-D?system.The results showed that:By comparing the adsorption distance and energy of the three systems,it is found that the TiO₂ system is the most unstable in all four cases.On the other hand,the GeO₂ system is most stable when undoped,Cu-doped and Cu-Cr co-doped,and the SnO₂ system is most stable when Cr-doped.According to the number of HCl molecular charge transfers,On the surface of undoped,Cu-doped,and Cr-doped TiO₂,the number of HCl molecules lost charge is 0.02e,0e,0.02e,and the number of charges obtained on the surface oxygen vacancies are more than that of the other two systems,so the oxidation relationship is:TiO₂>SnO₂>GeO₂.In the case of Cu-Cr co-doped,when HCl molecules are adsorbed on the surface of GeO₂,the oxygen vacancies get the most charge?0.04e?.At this time,the oxidation relationship is:GeO₂>SnO₂>TiO₂.From the analysis of electronic density of states,the SnO₂ system has the smallest band gap width?1.2eV?when undoped and Cr doped.When Cudoped and Cu-Cr co-doped,the electrical properties of SnO₂ and GeO₂ change to metallicity due to the influence of Cu 4s orbital,and this metallicity is more obvious when Cu-Cr codoped,so it is conducive to the selective detection of HCl gas.On the other hand,TiO₂ maintains good semiconductor performance,and the valence band moves down across the Fermi surface,which is more conducive to the transition of valence band electrons.In all four cases,SnO₂ has better optical properties than other systems because of its high net absorption of light in some bands.The light absorption coefficient for the 350nm?450nm band is greater than 10000cm^-1 at no doped surface.When Cuis doped,the light absorption coefficient increases with the decrease of wavelength,and the absorption coefficient reaches 14000cm^-1.And when Cr is doped,the absorption peak of SnO₂ reaches 45000cm^-1at the 600nm.When Cu-Cr is co-doped,the peak absorption coefficient of SnO₂ system reaches 36000cm^-1.In short,the SnO₂ system has the best optical gas-sensitive sensing performance for HCl when Cr is doped.This is because the state density peak formed by the Cr 3d orbital extends the valence band of SnO₂ to near the fermi surface and reduces the forbidden band width that reducing the transition energy of valence band electrons.This also provides a certain reference for the study on the preparation and modification of HCl sensing materials.