Metal oxide nanohybrids-based low-temperature sensors for NO2 detection: a short review

A gas sensor is a device used to monitor and quantify the leakage or presence of harmful gases in the environment. The NO2 is mainly emitted from vehicle exhausts, industrial chimneys, and combustion of fossil fuels. It is among the harmful gases which are danger to human beings and is the cause of acid rain. Metal oxides (MOs) have been proven to be effective gas sensors, however, their high operating temperature hampers their practical use. Hence, MOs supported upon graphene-based materials tend to have low operating temperatures since graphene provides a large number of active sites for gas adsorption upon MO surface. It also facilitates charge transfer from MO surface to adsorbed gas molecules. On the other hand, graphene-based materials have high selectivity for NO2. Upon functionalization of graphene with -SO3H groups tend to reduce the response and recovery time of the sensor. Also sensing of NO2 by MO depends upon its p-type or n-type nature. The p-type MOs do not have a better response for NO2 than n-type sensors, however, upon compositing them with functionalized graphene, their response enhances and they show better selectivity towards NO2. Also, creating defects like oxygen vacancies tend to lower the operating temperature of MO-based gas sensors and makes them more selective towards NO2. In this minor review, MO-based sensors for room temperature sensing of NO2 have been discussed taking into account their response, recovery time, sensitivity and selectivity.