The operation of gas sensors – devices for detecting various gases – is based on the use of metal oxides in a nanoscale state. This ensures the sensors are sensitive to air components, although it makes them unstable at high temperatures. One hypothesis posits that small crystals merge together under these conditions, resulting a decrease in sensor properties.
In order to test this hypothesis, the scientists from Lomonosov Moscow State University continuously tested sensors for more than a month, as well as simulated the aging of the properties of the powders of sensor materials. To track the changes, they used the electron paramagnetic resonance method, which is considered as the ancestor of spectroscopy.
The study showed that the deterioration of the sensor properties of powders is caused by the correction of defects in their crystal structure. This refers to oxygen vacancies, i.e., empty positions in the crystal lattice that are normally occupied by oxygen atoms. These vacancies, which are formed during the synthesis of highly sensitive materials, determine the concentration of free electrons – charge carriers responsible for semiconductor properties. When the material is exposed to high-temperature conditions, oxygen vacancies get filled with atmospheric oxygen.
The number of oxygen vacancies can continuously decrease for more than a month, causing the efficiency of gas detection devices to fall gradually rather than at once. The authors of the study attempted to solve this problem in a counterintuitive manner, increasing the number of oxygen vacancies at the stage of synthesis of the powder material.
“In order to offset the detected effect, we introduced artificially created oxygen vacancies into the oxide structure by adding an electron-capturing impurity. To prevent this replacement from causing a loss of free charge carriers and the resulting loss of semiconductor properties, we also introduced the required amount of an impurity with a relative excess of electrons into the system. To summarize, this approach represents a replacement of intrinsic charge carriers with those generated by the impurity,” postgraduate student Alina Sagitova is quoted as saying by Lomonosov Moscow State University.
