Nitric Oxide Detector Based on WO₃-1wt%In₂O₃-1wt%Nb₂O₅ with State-of-the-Art Selectivity and ppb-Level Sensitivity

Fast, sensitive, and precise detection of nitric oxide (NO) is critical to many applications in environmental monitoring and early disease diagnosis via respiratory testing. An effective detection system requires a sensor to detect NO gas at the parts per billion (ppb) level, and this system should possess a high degree of anti-interference selectivity. To achieve these targets, a series of gas sensor thin films based on intrinsic WO₃, one-additive-doped WO₃ (prepared by doping In₂O₃ or Nb₂O₅), and two-additive-doped WO₃ (synthesized by doping with In₂O₃ and Nb₂O₅) oxides were successfully grown. By analyzing the properties of sensitivity, selectivity, responsiveness, and recovery time of the gas sensors, we found that WO₃-1wt%In₂O₃-1wt%Nb₂O₅ has overwhelming advantages over intrinsic WO₃, WO₃-In₂O₃, and WO₃-Nb₂O₅. A sensing response value of 2.4 was observed for NO concentrations as low as 20 ppb from the WO₃-1wt%In₂O₃-1wt%Nb₂O₅ sensor. With 100 ppb NO gas, the WO₃-1wt%In₂O₃-1wt%Nb₂O₅ sensor achieved a high response of 56.1 at 70 °C, which is a state-of-the-art performance for NO detection at low working temperature settings. WO₃-1wt%In₂O₃-1wt%Nb₂O₅ also yields significantly improved selectivity and stability over intrinsic WO₃, WO₃-In₂O₃, and WO₃-Nb₂O₅. Studies on the sensing mechanism show that the grain size, rather than the n-n heterostructure effect, plays a dominant role in the observed results. By decreasing the grain size so that it is close to the thickness of the space-charge layer, the sensing response is enhanced. Although room remains to further improve the sensing properties, the performance of WO₃-1wt%In₂O₃-1wt%Nb₂O₅ is sufficient for implementation in low-content NO detection devices.