Nanostructured metal oxide semiconductors and composites for reliable trace gas sensing at room temperature

Nanocrystalline metal oxide thin films offer toxic gas sensing with higher sensitivity at lower temperature compared to their bulk counter parts leading to miniaturization of the sensors, making them wearable and easily portable for field trials without compromising on the sensitivity, but aided with improved selectivity. Nanomaterial of different size, morphology, geometry, preparation methods and composition play an important role in sensitivity, selectivity, response time and stability of the sensor. Although there are many reports about nanostructured materials for toxic gas sensing, they lack commercialization due to reliability issues. While some reviews have focused on toxic gas sensors based on macro and nanostructured materials which work at elevated temperatures and/or for a specific gas species, this review presents the systematic advances of room temperature operating sensors and their toxic gas sensing mechanism based on design of nanostructured metal oxide semiconductors and heterostructures, with insights into their high sensitivity, selectivity and reliability. Recent studies on room temperature operating trace gas sensors (for NH3, H2S, H2, NO2 and SO2) based on nanostructured semiconducting materials and their composites in the chemiresistive mode are discussed. The roles of nanocrystallite size, morphology, surface adsorbed species, surface charge depletion layers and heterostructure interfaces of metal oxide semiconductors and their composite materials for reliable room temperature gas sensing are discussed. The article concludes with current status and future scope for optimizing the nanostructures and their heterostructure interfaces for specific gas sensing at room temperature with an understanding of the various physicochemical properties involved in enhancing the sensitivity, selectivity, stability and commercial viability.