Highly Ordered Nanoporous Alumina Films: Effect of Pore Size and Uniformity on Sensing Performance

The effect of pore size and uniformity on the humidity response of nanoporous alumina, formed on aluminum thick films through an anodization process, is reported. Pore sizes examined range from approximately 13 to 45 nm, with a pore size standard deviations ranging from 2.6 to 7.8 nm. The response of the material to humidity is a strong function of pore size and operating frequency. At 5 kHz an alumina sensor with an average pore size of 13.6 nm (standard deviation 2.6 nm) exhibits a well-behaved change in impedance magnitude of 10³ over 20% to 90% relative humidity. Increasing pore size decreases the humidity range over which the sensors have high sensitivity and shifts the operating range to higher humidity values. Cole–Cole plots of 5 to 13 MHz measured impedance spectra, modeled using equivalent circuits, are used to resolve the effects of water adsorption and ion migration within the adsorbed water layer. The presence of impurity ions within the highly ordered nano-dimensional pores, accumulated during the anodization process, appear highly beneficial for obtaining a substantial variation in measured impedance over a wide range of humidity values.