From conventional SnO2 to novel pyrochlore metal oxides: a comparative review of gas sensing materials

Nitrogen dioxide (NO 2 ) and ammonia (NH 3 ) are among the most hazardous gaseous pollutants, necessitating highly sensitive and reliable detection technologies. Conventional metal-oxide gas sensors, particularly those based on SnO 2 , exhibit strong sensing responses toward NO 2 and NH 3 ; however, their practical application is often limited by high operating temperatures and suboptimal stability, repeatability, and response–recovery kinetics. Although strategies such as elemental doping, composite formation, and heterojunction engineering have improved SnO 2 -based sensors across a wide temperature range, fundamental challenges remain. Recently, pyrochlore metal oxides with the general formula A₂B₂O₇ have emerged as promising defect-engineered materials for chemiresistive gas sensing. Their open crystal framework, compositional flexibility, intrinsic oxygen vacancies, mixed-valence cations, and high chemical stability provide new opportunities for regulating gas adsorption, surface redox reactions, and charge transport. This review critically compares SnO 2 -based sensing materials with representative pyrochlore systems, represented by Ce 2 Sn 2 O 7 . Particular attention is given to the role of defect chemistry, oxygen-vacancy engineering, and interfacial charge modulation in improving sensing performance at low or room temperature (RT). This review highlights pyrochlore metal oxides as a tunable materials platform for next-generation low-power, stable, and selective gas sensors.