A high-and-rapid-response capacitive humidity sensor of nanoporous anodic alumina by one-step anodizing commercial 1050 aluminum alloy and its enhancement mechanism

Abstract

Anodic aluminum oxide (AAO) is a well-known nanoporous material and a template for nanotechnology application. The AAO based humidity sensor is traditionally fabricated using low-temperature (0–10 °C) two-step anodization in oxalic acid at 40 V from the expensive high-purity aluminum of small-area as well with low response and low intensity. Here, one-step anodizing commercial large-area low-purity aluminum 1050 alloy was realized for high-performance humidity sensor by hybrid-pulse anodization in 0.3 M oxalic acid at 20 V at 25 °C that merits low cost, short fabrication time and high-and-rapid capacitive response. The performance enhancement is attributed to the increased total pore area adsorption ratio and the electrode area of AAO with lower thickness to make much more water vapor molecules adsorb onto the wall surface of pores for sensing with high capacitance, response and short response-recovery time. The low-thickness pores make water molecules adsorb onto and desorb from the pores surface more quickly because of shorter diffusion path. Also, a small-area sample of traditional 40 V AAO is used as a reference for comparison. A shorter anodization time of 55 min is also fabricated for understanding the thickness effect on the response-recovery time. The large-area AAO humidity sensor of 16 cm2 at 20 V exhibits high response (above 5000 %, 3.5 times the traditional reference) and quick response-recovery time (9 s). The high-and-rapid-response enhancement mechanism of AAO capacitive humidity sensor is established.