In Operando Impedance Spectroscopic Analysis on NiO-WO₃ Nanorod Heterojunction Random Networks for Room-Temperature...

The use of metal-oxide sensors for effectively detecting hydrogen sulfide (H₂S) gas at room temperature is currently hindered by their inadequate sensitivity and selectivity. Using a lucid fabrication strategy, we report a room-temperature, highly sensitive, and selective H₂S gas sensor using NiO-modified WO₃ nanorod (one-dimensional-one-dimensional) random networks. The observed improvements in gas-sensing sensitivity stem from the synergistic effects of various contributions inside the sensing heterostructure, such as bulk nanorod, p-n heterojunction at the interface of these two dissimilar oxides, and gas-induced conducting species due to sulfurization (WS_2- and NiS_1- ). An in situ impedance measurement during gas exposure was used to investigate the influence of these effects. The analysis revealed that these contributing factors can be either cooperating or competing and lead to either increased or decreased sensitivity, respectively. The presence of semimetallic species (NiS, WS₂) was further confirmed by in situ X-ray diffraction analysis of the heterostructure nanorod sample with H₂S gas exposure. The related sensing mechanism in the heterostructures is presented with a conduction pathway model. The room-temperature-operated nanorod heterostructure sensors showed a lower detection limit of H₂S at ∼0.5 ppm, which is significantly lower than its toxicity limiting value ∼10 ppm, per the Environmental Protection Agency. The nanorod heterostructure sensors can be used for real-time, low-cost, room-temperature alarms in an H₂S monitoring system.