Study On Gas-sensitive Detection Mechanism Of Dissolved Gases In Oil (CO And C₂H₂) By Modified TiO₂ Nanotubes With Copper Oxide

In oil-immersed transformers,insulating oil often decomposes due to various defects and faults in the operation of the transformer,and then produces a variety of characteristic decomposed gases.Through the effective detection of these characteristic gases,we can judge the fault types and severity of oil-immersed transformer.On the one hand,CO gas can be accumulated in the oxidation process of insulating oil,and CO characteristic gas is often generated when insulating materials such as insulating paper are decomposed.On the other hand,C₂H₂ is the decomposition gas produced by insulating oil under high temperature conditions(such as arc discharge and other faults),and high temperature faults account for the largest proportion of the current oil-immersed transformer fault types.Therefore,it is of great significance for transformer fault monitoring and identification to take C₂H₂ and CO as the main characteristic gases under specific insulation fault.Due to the advantage of large comparative area,TiO₂ nanotubes have been widely used in catalysis and other fields.However,in the field of gas sensitive detection,they are limited by their poor electrical conductivity,so they cannot achieve good application effect.In this paper,TiO₂ nanotubes were modified by CuO to improve their electrical conductivity while retaining the advantage of large specific surface area,so as to give full play to the potential of gas sensitive detection of TiO₂ nanotubes.(1)This paper explores the mechanism of CuO modification,and then studies the gas-sensitive response mechanism of the modified TiO₂ nanotubes to C₂H₂ and CO on this basis.Firstly,the electronic properties of intrinsic and CuO modified TiO₂ nanotubes were analyzed by density functional theory calculation.The analysis of the electronic structure of intrinsic TiO₂ nanotubes,especially the distribution of electrostatic potential,shows that there are a lot of areas with positive electrostatic potential on the surface of TiO₂(101)and areas with negative electrostatic potential around O atom,which makes it easier to combine with nucleophile.As CuO is a typical p-type semiconductor,the surface of TiO₂(101)is conducive to the binding of CuO,and has been confirmed in the subsequent analysis results.(2)By analyzing the electronic structure of CuO modified nanotubes,this paper obtained the binding mechanism of CuO nanoparticles on TiO₂ nanotubes,as well as the reasons and influencing factors for CuO nanoparticles to improve the electrical conductivity of intrinsic TiO₂ nanotubes.With the introduction of CuO,Cu-2p will tend to reduce the energy level of the bottom of the intrinsic conduction band,makes the transition more easily;Meanwhile,O-2p tends to increase the density of states near the bottom energy level of the conduction band,which is conducive to more electrons filling into the bottom of the conduction band by transition from valence band.These factors lead to the increase of the conductivity of TiO₂(101)after modification.In addition,since HOMO and LUMO of CuO-TiO₂ are mainly distributed on CuO,this workpredicted and confirmed the location where gas molecules will be adsorbed:around CuO or near the TiO₂ surface of CuO.On the other hand,it has been confirmed that the reactive region of CuO modified TiO₂(101)is around CuO and TiO₂ surface near CuO.(3)This work also reveals the adsorption mechanism between CO and CuO modified TiO₂(101)surface,and it is found that the existence of anti-bonding states between CO gas molecule and TiO₂(101)surface weakens the possibility of forming covalent bonds between Ti and C atoms.Furthermore,by calculating the data of adsorption energy,charge transfer amount and adsorption distance,the results show that the adsorption between the surface CO molecules and TiO₂(101)is mainly due to the electrostatic adsorption caused by the weak dipole moment in the CO gas.The trend of electronic properties and electrical conductivity after adsorption was analyzed.Three typical adsorption configurations of modified TiO₂(101)for C₂H₂ molecules are also given.The comparison of adsorption capacity among the three adsorption configurations revealed the different adsorption types of C₂H₂ molecules on CuO-TiO₂ surface with the gradual increase of temperature.By analyzing the data of density of states,the microcosmic mechanism of gas-sensitive response and the changing trend of electrical conductivity are proposed.(4)TiO₂ nanotube arrays and CuO-modified TiO₂ nanotube arrays were prepared by anodic oxidation method,and the theoretical analysis results were preliminarily verified.The results of SEM and XRD showed that the prepared samples were orderly and successful.The samples were tested by electrochemical workstation.The test results show that the electrical conductivity of the modified TiO₂ nanotubes is significantly increased,and the defects of the intrinsic TiO₂ nanotubes in conductivity are improved,which are consistent with the theoretical results.In addition,the gas-sensitive response of CuO-modified TiO₂ nanotube array was tested under typical operating temperature.The gas-sensitive response mechanism of CuO-modified TiO₂ nanotubes to two characteristic gases proposed in the theoretical part was supplemented and verified.In addition,it was also found that the response capacity of CuO-modified TiO₂ nanotubes to C₂H₂ was significantly increased after increasing the temperature appropriately,which was consistent with the research of others and the theoretical prediction in this paper.