Nanoporous anodic alumina (NAA) prepared in different electrolytes with different pore sizes for humidity sensing

The aim of present study is to design a simple and reliable capacitive humidity sensor based on the nanoporous alumina prepared by a two-step anodization technique. Owing to long-range-ordered structure of porous alumina, its electrical properties are sensitive to moisture; with the result, it finds potential applications as humidity sensor. Anodization parameters have been optimized to prepare a series of porous alumina samples in different acidic electrolytes, and the effect of pore size, porosity, and uniformity of pores on humidity sensing was investigated. The sensitivity of the sensor fabricated in oxalic acid at 40 V is 2–3 times higher than the sensors fabricated in sulphuric acid at 25 V and phosphoric acid at 98 V. Its enhanced sensitivity is justified by small porosity and uniformity in pore distribution of porous alumina. The sensitivity of the sensor fabricated in oxalic acid solution with pore diameter 34 ± 1 nm and porosity 10 ± 0.5% reaches up to 0.21 (pF/RH%).