Aging-Improved NH$_3$ Sensing in Polypyrrole Nanotube Chemiresistors

Polypyrrole NH$_3$ chemiresistors in nanotube (NT) and planar film (PF) morphologies are compared on response magnitude, limit of detection (LOD), sensitivity, and response/recovery kinetics, including the performance of the same sensors tracked over 60 days of ambient storage and repeated NH$_{3}$ exposure. In the case of fresh sensors, the time constants for the fast process ($τ_1$) is shorter in the NT than in the PF at every concentration. NT is 38 \% faster on average than PF. The slow process ($τ_2$) overlaps between two morphologies and shows no concentration dependence. The two time constants differentiate the two morphologies through aging: NT $τ_1$ and $τ_2$ decrease substantially and monotonically over 60 days, while the time constants of PF show no consistent trend. All measured sensing parameters, LOD, sensor response and sensitivity improve or at least remain stable with aging in the case of NT, while all parameters degrade in the PF case. These improved sensor performance and aging identify the nanotube morphology as the more robust and manufacturable choice for long-term NH$_3$ sensing applications.