Cobalt–Nitrogen Bonding for Triggering the Low-Power Dual Detection of NO₂ and NH₃ by Mixed-Valence Co²⁺ and Co³⁺

Abstract

The real-time detection of harmful gases like NO2 and NH3 is crucial for smart agriculture and the noninvasive diagnosis of kidney, liver, lung, and heart diseases. However, sensing these gases and understanding the primary sensing mechanisms can be challenging since the nitrogen site of NO2 or NH3 is the primary site that binds the sensing crystallite. Herein, we designed and fabricated low-cost sensors for dual NO2 and NH3 sensing using a nanodevice-on-chips sensor (NDCS) with mixed-valence Co3O4 (Co2+ and Co3+). Co3O4–NDCS detects NO2 and NH3 at low power, room temperature, and 70% relative humidity. It also exhibits stable, recoverable, and reproducible sensing performance for NO2 and NH3, comparable to several high-performance sensors. The dual sensing performances are closely related to two distinct Co sites (Co2+ and Co3+) in the Co3O4 crystal. Co2+ could host the nitrogen atom of either NO2 or NH3 to form Co–N bonding, which is preferable to NO2 detection. Meanwhile, the N atom could replace one of the six oxygen atoms coordinating with Co3+ to form Co–N bonding, favorable to NH3 detection. Looking forward, the strategy proposed here makes it possible to fabricate high-performance gas sensors with the desired nanostructures and known sensing mechanisms.