Sodium titanate: A proton conduction material for ppb-level NO₂ detection with near-zero power consumption

Constrained by the traditional charge transfer sensing mechanism, it is quite challenging to fabricate NO₂ sensors that simultaneously exhibit high sensitivity, rapid response/recovery, and low power consumption. Herein, sodium titanate (NTO), a layered material with abundant surface-rooted OH groups (OH_R), is demonstrated to be a promising NO₂ sensing material. To understand the sensing behavior of NTO, the influences of operating temperature, applied voltage, and relative humidity are investigated, and a novel OH_R-enabled proton conduction sensing mechanism is proposed. The sensing process mainly involves selective NO₂ adsorption on OH_R, thereby lowering the activation energy for proton transportation along the NTO surface. Meanwhile, the moderate intermolecular interaction makes NO₂ both easily adsorbed and desorbed at room temperature. Hence, NTO exhibits a highly sensitive, rapid, and fully recoverable response (∼5.7-1 ppm NO₂ within 3 s), wide detection range (1 ppb-20 ppm), good stability (>2 months), and near-zero power consumption (0.5 nW). Finally, we demonstrate that NTO has an excellent practical indoor/outdoor NO₂ sensing ability. This work offers a new pathway to resolve the inherent conflicts in available NO₂ sensors by using NTO via the OH_R-enabled proton conduction sensing mechanism, which may also provide insight into designing high-performance sensors for other gases.