Review—Metal Oxide Chemoresistive Gas Sensing Mechanism, Parameters, and Applications

The economic growth of any country depends upon the MSMEs as it plays a vital role in GDP and employment. The transportation is considered as the lifeline of the country. Hence due to developing countries, the industries and vehicles are continuously increasing to fulfil industrial or domestic requirements. But unfortunately, industries and vehicles emit harmful gases as exhaust to the environment. Which directly or indirectly impact the human health. Fresh and clean air is the prime need of the society. Hence the monitoring of different gas concentrations in the environment is very essential to take preventive steps to control air pollution. The traditional method of monitoring the air quality is very expensive, hence most of the countries have limited air monitoring stations. In the field of nanotechnology, scientists have developed different types of soft metal oxide materials that are capable of sensing different gases at low concentrations and can work in different environmental conditions. For the last 10 years, ferrite-based sensors have the primarily used to detect harmful gases, and pollutants from vehicle exhaust, and environmental pollution monitoring. These soft ferrites have excellent electrical and magnetic properties that can also be tuned according to the requirement of the sensor to increase sensitivity and selectivity. The tuning of ferrite sensors depends upon synthesis technique, optimizing preparation conditions, sintering temperatures, operating temperatures, dopant concentration, etc This paper is based on a deep study of the synthesis techniques of nano-ferrites, different types of gas sensors, gas sensing mechanisms, parameters, and application of chemo-resistive metal oxide gas sensors. The key parameters for the ferrite gas sensors are phase formation, crystallite size, grain size, surface area, selectivity, dopants, sensitivity, gas concentration, operating temperature, and response/recovery time. This review paper also includes the study of different researchers to find the impact of high concentrations of gases like hydrogen (H _2 ), carbon monoxide (CO), carbon dioxide (CO _2 ), oxygen (O _2 ), ethylene glycol ${{({CH}}_{2}{OH})}_{2},$ methane (CH _4 ), ammonia (NH _3 ) liquid petroleum gas (LPG), acetylene (C _2 H _2 ), and nitrogen oxides (NOx) in the environment and the metal oxide materials selected for the sensor application.