Moisture detection at ppm level by anodic aluminum oxide thin film capacitive sensor

Abstract

In the present investigation, the fabrication of nanoporous anodic aluminum oxide (AAO) thin films via a one-step and two-step anodization process was carried out. It utilizes a binder-free method to form nanostructured AAO films on aluminum foil surface. The impact of anodization conditions on the pore structure was examined using scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). The bulk porosity and surface hydrophilicity of the AAO films were evaluated using Brunauer-Emmett-Teller (BET) analysis and contact angle measurements. The nanoporous AAO film synthesized via one-step anodization exhibited comparatively higher bulk porosity (42 percent), smaller average pore diameter (37.0 plus-minus 2.3 nm), and superior hydrophilic contact angle of 84.2 degrees. These morphological characteristics directly enhanced water adsorption and capillary condensation within the nanopores. To evaluate the functional response of the porous network, Ag/AAO/Al capacitive structures were tested for low-ppm moisture sensing (20-127 ppm). The device based on one-step AAO exhibited a linear response (R sup(2) = 0.9950), with response and recovery times of 305 s and 31 s, respectively, which is much better compared to two-step AAO samples. These results demonstrate that the controlled tailoring of the AAO morphology enables tunable adsorption properties, supporting its application in porous sensing platforms and moisture-sensitive devices.