Highly sensitive humidity sensor based on amorphous Al2O3 nanotubes

Amorphous Al2O3 nanotubes were obtained by the transformation of NH4Al(OH)(2)CO3 nanotubes annealed at 600 degrees C. The tube-like nanostructures not only increase efficient sites for gas adsorption, but also promote the dissociation of water absorbed onto the surfaces of the nanotube walls. Moreover, they also provide effective and fast channels for vapor and liquid transport. Therefore, the sensors based on Al2O3 nanotubes show high sensitivity and fast response/recovery time to humidity. The impedance changes approximately four orders of magnitude as relative humidity (RH) varies from 11% to 95% at the measured frequency of 40 Hz. Additionally, the sensor also presents relatively small hysteresis and long-term stability. For low RH levels, the protonic conductor is dominant, whereas for increasing RH levels the ionic contribution becomes prevalent. This study demonstrates that Al2O3 nanotubes have promising applications in environmental monitoring.