Boosting of Surface Anion Density on Porous Anodic Alumina Surface—A Key Factor to Enhance Moisture Sensor Operation to Critical Sub-RH Level

Humidity sensing at low RH%, its mechanism, and even lowering further minimum detection limit has no clear answer as of now. Porous anodic alumina (PAA), though an excellent hygroscopic material; still lacks a comprehensive theory on how to lower the minimum detection limit (LOD) to the sub-RH level. In this report, in addition to the pore morphology design, the effect of electrolyte driven anion concentration on PAA surface has been suitably fine-tuned by increasing the attachment of hydroxyl bond density. The organic solvent like glycerol mixed with the anodizing electrolyte has been chosen in proper concentration; leading to enhance the degree of ionic attachment to the PAA surface, hence increase in the sensitivity and the lower detection limit. A lower detection limit of 0.5 RH% is obtained with optimized glycerol concentration, examined through FTIR spectroscopy, which is a direct consequence of increased surface anion concentration that facilitates the adsorption of water vapor molecules on the surface. The developed sensor shows wide detection range 0.5-99 RH%, ultrahigh sensitivity of 275.4 pF/RH%, very fast response- and recovery time of 3 s and 15 s and negligible drift over cycling over 30 times for 0-11.4 RH%. The sensor provides a low-cost alternative solution specifically, to industries having low moisture monitoring requirements e.g., tea and lithium battery manufacturing industry.