Construction And Performance Of SrCoO_x-Based FET For PH Detection In Concentrated Alkaline Solution

pH is one of the most notable and significative concepts in chemistry.The measurement range of pH is generally between 1-14.But it is necessary for the pH detection in the strong alkaline solution environments such as metal mining,bleaching processes,photography,and wastewater treatment.Although the development of micro-nano technology has promoted the emergence of various pH-sensitive materials and detection methods,pH measurement of highly concentrated acid-base solutions remains to be a challenging task.This article aims to design and develop a pH sensor for pH detection in high-concentration alkaline environments,which face challenges such as high corrosion,high ion concentration,and low ion migration rates.For this purpose,a perovskite-type metal oxide Sr Co O_x which contains a large number of oxygen vacancies and has a stable structure in alkaline environments was selected as the sensitive material for pH sensor.Combining gate voltage-induced ion injection technology,a Sr Co O_x-based Field-Effect Transistors（FET）sensor that responds to pH was constructed and its response characteristics and functional relationships with OH-ions has been investigated.The experimental results show that the Sr Co O_x-based FET sensor has good response characteristics to high concentrations of OH^-ions.The FET sensor could detect high-concentration Na OH solutions at a concentration of up to 20 mol/L at room temperature.It also has a sensitive linear response to high-concentration alkaline solutions in the pH range of 14.8-17.24 and has good stability and selectivity.Doping of transition metals was adopted to optimize the response time,response range and sensitivity of Sr Co O_x-based FET sensor to improve its performance.Specifically,Sc doping can increase the mobility of oxygen ions in Sr Co O_x,significantly improving the response speed of the Sr Co O_x-based FET sensor.Fe doping can regulate the electronic structure of Sr Co O_x and the content of oxygen vacancies,improving its sensitivity and linear response characteristics.It is noteworthy that Sr Co O_x-based FET sensor doped with 5 mol%Fe shows a good linear relationship between the logarithm of its response resistance（lg（R））and pH in the range of 8.0-17.2.Its sensitivity can reach 0.40 lg（Ω）/pH,and the response time is only 300s,exhibiting excellent pH response performance.The principles and characteristics of the sensitive response of Sr Co O_x-based FET sensors to OH^-ions were studied through XPS/XRD characterization and DFT electronic structure calculation methods.The electronic structure evolution calculation shows that the resistance response of Sr Co O_x-based FET sensors is caused by the change in oxygen vacancy concentration which is a result of the injection of oxygen ions.When the stoichiometry of Sr Co O_x was Sr Co O_2.5,it was in a semiconductor state.However,after oxygen ions were injected into the oxygen ion channel of Sr Co O_x,the Co 3d band would traverse the Fermi level and accumulate in the bandgap.When the stoichiometry changed to Sr Co O₃,it showed a completely conductive electronic structure.XPS test results also proved that the oxygen vacancy concentration was linearly related to lg（R）,indicating that the change in oxygen vacancy concentration directly affects the resistance.This is because the electronic conductivity of Sr Co O_xand similar semiconductors is achieved through the coupling of the 3d orbitals of transition metals and the 2p orbitals of oxygen.In addition,the XRD and XPS tests found that moderate doping of transition metal elements with stronger oxygen binding ability can bind oxygen ions to a certain extent,maintain the cubic perovskite crystal phase of Sr Co O_x,and maintain a three-dimensional oxygen ion diffusion channel,which helps to improve the response rate of the Sr Co O_x FET sensor.In summary,this work designed and constructed a Sr Co O_x-based FET sensor with a highly sensitive response to OH-ions in concentrated alkaline solutions and corresponding pH values,based on the principle and characteristics of Sr Co O_x high-density oxygen vacancies’sensitive response to OH-ions.The findings provide new ideas and technical support for developing pH detection methods in high alkaline environments,with high theoretical and practical application value.