NOx sensors based on semiconducting metal oxide nanostructures: Progress and perspectives

21st century has already seen huge progress in science and technology of small, highly sensitive gas sensors, which can selectively detect environmental toxins like NOx – the oxides of nitrogen – a byproduct of fossil fuel combustion. Into this bargain, public became more health-aware and environmental bodies grew stricter, stimulating analytical and material scientists to find new strategies from material synthesis to fabrication of NOx sensors in order to produce fast and reliable gas detectors. To the scientists, semiconducting metal oxides, owing to their low cost, easy processing, high gas response, good electrical properties and above all tunable structure at the nanoscale, always presented a first-hand choice for sensor fabrication. This article presents an overview of the most recent developments in semiconducting NOx gas sensors based on these metal oxide nanostructures and their applications in vehicle exhaust and environmental monitoring. A strong emphasis is presented on chemiresistor and field effect transistor devices using semiconducting metal oxides as active layers. The performance levels of these NOx sensors are compared to those of other devices as well as other semiconductor materials. Furthermore, keeping in mind the ultimate user demands, limitations of the current sensor technologies and future strategies are discussed.