Enhanced humidity sensitivity of nanoporous alumina films by controlling the concentration and type of impurity in pore wall

Controllable humidity sensitive nanoporous anodic aluminum oxide (AAO) films were prepared by the stable high field anodization in the oxalic acid electrolyte. The sensitivity of this AAO-based sensor can be enhanced at the low humidity region by pore widening and annealing. Exposing more anions to pore wall surface by pore widening assists the surface electron tunneling effect. The hydrolyzation of carbon dioxide generated by decompositing oxalic anions in AAO during annealing facilitates the protonic conduction. These could be responsible for the improvement of sensitivity. Different hysteresis types of AAO sensors prepared by the high field anodization were explained by the capillary condensation and lateral moisture diffusion. This study provides a new approach to fabricate high performance metal oxide humidity sensors by incorporating stable and accessible proton sources into humidity sensing materials.