Ammonia detection using nanoporous alumina resistive and surface acoustic wave sensors

The effect of pore size and uniformity on the response of nanoporous anodized alumina to ammonia at room temperature, 23 °C, is reported. Pore sizes examined range from 13.6 to 48 nm, with pore size standard deviations ranging from 2.6 to 7.8 nm. The nanoporous alumina films were used in conjunction with 98.5 and 32.5 MHz surface acoustic wave (SAW) devices, and as impedance-based sensors, both of which were found sensitive to ammonia. The SAW devices showed a frequency shift of approximately 0.001% per percentage change in ammonia concentration. Similarly, at 5 kHz an alumina sensor with an average pore size of 13.6 nm, standard deviation 2.6 nm, exhibits a factor of two change in impedance magnitude as it is cycled between an ammonia and argon environment. The nanoporous alumina-based sensors demonstrated a fast response time (≈30–40 s) and recovery time (≈60–80 s), and complete recovery upon removal from an ammonia ambient. We believe the enhanced physisorption of ammonia on alumina films due to the large surface area, provided by the mesoporous topology, is responsible for the sensing behavior.