Gas-sensitive Property Study Of Metal Oxide Under The Optical/Thermal Field

Metal oxide,as one of the widely studied and used gas-sensitive materials,has received great attention.After years of research,metal oxide used as the gas sensor is still facing some technical problems:?a?Lack of high selectivity.?b?Unable to detect the ppb-level concentration of harmful gas.These problems are not only related to the nature of the material itself,but also associated with the working conditions of material,namely the working temperature and optical excitation.The regulation and control of the working temperature can not only enhance selectivity by changing the reaction activity of metal oxide,but also affect sensitivity by changing the type and amount of the adsorbed oxygen at surface.Similarly,the regulation and control of the optical excitation can not only change the quantity of surface active centers?enhance sensitivity?by photon-generated carriers,but also improve selectivity through capturing the transient signals in the process of optical modulation.Compared with the research of material itself,the gas sensitive properties under these field conditions are rarely concerned.Especially the optical modulation and the excitation/modulation of optical/thermal field combination are neglected.In this article,we studied the gas-sensitive reaction processes of metal oxide by regulating the optical/thermal field with the help of the self-developed high-throughput test system and the miniaturization module.Then the gas sensitive characteristics and the relevant gas sensitive mechanism under the field control were found out.Based on these gas sensitive characteristics,we put forward some effective technical methods and new research ideas to improve gas sensitive properties and also solve the above problems.Firstly,this paper studied the gas-sensitive property of TiO2 for formaldehyde detection under excitation of the optical/thermal field combination.Inspired by the photocatalysis,TiO2 under optical excitation is used in gas sensing by the researchers.Due to the super hydrophilicity of TiO2 under optical excitation,the gas sensitive response of TiO2 under optical excitation at room temperature is easily affected by humidity.With the increase of the humidity from 0 mg/L to 18 mg/L,the gas sensitive response of TiO2 under UV excitation for sensing 100 ppm formaldehyde reduced from 9358.5 to 4.5.In order to eliminate the physical adsorption of water on the surface and keep the chemical adsorption of water?hydroxyl radical?,we applied low temperature?60°C?thermal excitation to TiO2 under UV excitation.The excitation of optical/thermal field combination with low temperature retained the beneficial effects of optical excitation.It made TiO2 be sensitive,recovering fast,and humidity less sensitive for gas sensing.Whereafter based on the excitation of optical/thermal field combination,we studied the influence of electrode material on the stability of TiO2 for sensing formaldehyde.For the TiO2 film on Au electrode,it would occur the serious chameleon phenomenon in moisture circumstances.The TiO2 changed?Ti4+ transform to Ti3+?resulting in signal drift.While the TiO2 film on Pt electrode showed the outstanding stability in high-moisture environments,because there was the reverse reaction?Ti3+ transform to Ti4+?accompanied by the above reaction.Besides,as the co-catalyst of TiO2,Pt made the catalytic oxidation reaction of formaldehyde easier.Therefore,it could obtain larger gas sensitive response for sensing formaldehyde.Then take Pt-WO₃ for instance,the optical excitation was used as a modulation method.We studied the selectivity of metal oxide gas sensor under optical modulation.The gas-solid reaction on the surface of metal oxide would change under optical modulation.It could affect the photoresponse and the photorelaxation time,thus producing different electrical signals.We tested the four kinds of model gases?H2,CO,ethyl alcohol,acetic acid?based on the optical modulation and compared the characteristic parameters?the photoresponse and the photogenerated carrier lifetime?in these four gases.Combined with analysis of the two characteristic parameters and the gas sensitive response,we could identify CO among them.Besides,high working temperature can enhance activity of metal oxide and speed up the gas sensing.Combined with the above gas sensitive property,we used the fast optical modulation to improve the recognition speed of gas sensor.Afterwards,in view of the multiple types of chemical adsorption oxygen under temperature modulation,we proposed the proper adsorption potential energy curves and the adsorption isobar for oxygen adsorption.Meanwhile,we studied the oxygen adsorption behavior of metal oxide?set SnO2 as an example?under the programmed temperature cycle.In this process,the type and amount of adsorption oxygen on the surface would change with the temperature change.Thus the resistance signals showed differences in the heating and cooling periods.When the rate of programmed temperature became high enough,there was no time for the transformation of the high reactive oxygen species.Thereby they could largely preserve on the surface of metal oxide.Based on the above theory and experiment research,we proposed the way of fast temperature programmed cooling to improve the amount and activity of adsorbed oxygen on the surface of metal oxide under low temperature.Inspired by the above research results,temperature programmed cooling was used as a standard method of dynamic gas sensitive test.We studied the sensitivity of metal oxide?take SnO2 and In2O₃ for instance?under thermal modulation.The gas sensitive responses of the constant temperature and the temperature programmed cooling test were compared through a lot of experiments.It was found that the curve of gas sensitive response with temperature?S-T curve?was consistent with the report above.Its shape was similar to the skewed distribution.However the S-T curve monotonously increased under temperature programmed cooling.The largest gas sensitive response was obtained at low temperature.For the same test gas,the dynamic responses?temperature programmed cooling?were larger than the static responses?constant temperature?by more than one or two orders of magnitude.Thus it can be seen that the temperature programmed cooling?thermal modulation?contributed to the sensitivity improvement of metal oxide.In conclusion,we studied the gas sensitive properties of metal oxide under the control of optical/thermal field.Based on the superficial phenomena,we discussed the inherent mechanism of gas sensing and put forward several effective methods to improve the gas sensitive property of metal oxide.The researches in this paper had a certain guiding role on the basic research and practical application of metal oxide gas sensor.