Effect Of Co-doped Rutile TiO₂ Surface On Optical Gas Sensing Characteristics Of CO Gas

In today’s world,human beings are facing enormous pressure from environmental pollution.With the rapid development of industrialization,the reduction of forest vegetation and the increase of toxic and harmful substances in the air have brought a huge threat to human production and life.When the CO gas in the air reaches a certain concentration,it will cause a huge injury,and the serious one will suffocate and die.How to detect the composition and concentration of toxic gas in gas phase system efficiently and accurately has become a research hotspot of gas sensing materials in the world.Semiconductor optical gas-sensing materials will exhibit certain optical characteristics when exposed to light in a certain wavelength range.The reason is that after the gas is adsorbed on the surface of the optical gas sensing material,the oxygen vacancy interaction between the gas molecules and the surface of the material results in the change of the electronic properties of the material.After being exposed to light,the optical properties of the material will change significantly to achieve the detection of the adsorbed gas composition And the purpose of concentration,this is the working principle of optical gas sensor.The traditional optical gas sensor has the advantages of high sensitivity,high response rate,short response time but high price.At present,TiO₂ is used in the production of optical gas-sensing materials because of its stable chemical properties,strong redox on the surface,high sensitivity,non-toxicity,and low cost,mainly because the TiO₂ surface has more oxygen vacancies and Oxygen vacancies have certain oxidizability.When the oxygen vacancies undergo redox reaction with the gas in the environment,the charge on the surface of TiO₂ will transfer with the gas molecules.The characteristics of the electrons near the Fermi surface will change,which will be reflected in the change of the optical properties of the material.In other words,the concentration of the gas is reflected by the change of the optical properties of the absorption or reflection of the incident light of the material,and the oxidizability of the oxygen vacancies on the surface of TiO₂ is the decisive factor for the sensitivity and response effect of the sensing material to the ambient gas.Therefore,we often improve the activity of oxygen vacancies on the surface of TiO₂ through element doping,which is generally achieved by co-doped with metals and non-metals.This paper mainly studies the optical gas-sensing characteristics of the optical gas-sensing sensor material TiO₂ for reducing gas CO,so we choose the elements that can enhance the oxygen vacancy oxidizability on the surface of TiO₂ for doping.In the periodic table of elements,N and S are elements with the same period and the same main group as O,and they are all close to each other;while Rh and Cu have the outermost s1 electron（Cu 4s1,Rh 5s1）structure,which is more active and easy Electron exchange occurs with the outside world,so in theory,doping of four elements will enhance the activity of oxygen vacancies on the surface of TiO₂.In this paper,the first-principles plane wave super-soft pseudopotential method is used to calculate the adsorption of CO molecules on doped S,Cu,S-Cu co-doped,and doped N,based on the density functional theory（DTF-D）system.The surface optical gas sensing characteristics of Rh,N-Rh co-doped and N-Cu co-doped rutile TiO₂（110） were studied.Through the structural optimization of various non doping,single doping and co doping models,the metal adsorption characteristics,electrical properties and optical properties were comprehensively analyzed,the micro mechanism was explained,and the best doping method was explored to improve the optical gas sensing characteristics of CO gas on TiO₂ surface for the future experimental research and practical application provide some theoretical guidance.It includes the following three aspects:1.Adsorption characteristics.The study found that the doped TiO₂ surface showed good adsorption characteristics after adsorbing CO gas molecules.However,the co-doped system has the advantages of shorter adsorption distance,greater adsorption energy,better stability after adsorption,and easier realization.2.Electronic properties.The study found that doping increases the number of electrons on the surface of TiO₂ and improves the oxidizability of oxygen vacancies on the surface,and the co-doping system has the largest number of electrons,indicating that co-doping improves the surface oxygen vacancy activity the most.The band gap width of TiO₂ is reduced,and its response range to light is changed.At the same time,doping also introduces impurity levels at and near the TiO₂ Fermi level,the probability of electron transition is increased,and the utilization ratio of TiO₂ to light is increased.3.Optical properties.The study found that doping significantly improved the response of TiO₂ to visible light,and the co-doped system has the greatest improvement.From the absorption of absorption coefficient and reflectivity,the single doping has little effect on the utilization of visible light,and the co-doped system shows the optical phenomenon of"high absorption",and"low reflection"is in the visible light area,and the utilization of visible light is the best.This article compares the three aspects of adsorption characteristics,electronic properties and optical properties.The study found that the N-Cu co-doped system has the largest adsorption energy and the shortest adsorption distance;it has the most improvement in the oxygen vacancy oxidation on the surface of TiO₂,the greatest influence on the light of the whole visible light region,and the greatest absorption energy under the same energy,making the electronic transition probability greater and greater.Especially in the high energy region,the optical characteristics of"high absorption and low reflection"are more significant than that of N-Rh co-doped and S-Cu co-dopde.Therefore,the N-Cu co-doped system is one of the three best ways to improve the effect of TiO₂ optical gas sensing.