Dynamic response of ammonia sensors constructed from polyaniline nanofibre films with varying morphology

Polyaniline is acid-doped with HCl, HNO3, HClO4, HBr or H2SO4 to yield thin films that are nanofibrous or dense. The nanofibrous films show a faster response to ammonia gas, and the response is greater when the films are in the moderately conducting green state than when they are in the highly conducting yellow state. The film resistance and optical absorption spectrum are monitored at 20 ms and 500 ms intervals, respectively during and after exposure to ammonia. Three absorption processes with characteristic times of order 10 s, 100 s, and 1000 s are observed in the resistive response. The optical spectrum is fitted with three Gaussians or derived Gaussians centred in the blue, orange and infra-red which exhibit similar response times of order 100 s and 1000 s, corresponding to the longer times in the resistive response. The data are interpreted in terms of three processes, physisorption, chemisorption and structural rearrangement of the polymer chains. Gas selectivity is demonstrated in the optical response by comparing the time-dependent desorption of H2O and NH3. These two gases may be distinguished using inexpensive on-chip two band pass filter optical detection.