Synthesis, characterization, and optical sensing of hydrophilic anodic alumina films

Herein, the anodization of 2.6 mu m Al-0.2 at. % Cu (Al-0.2Cu) films on TiN/p-type Si substrate were performed using common acidic mediums at different ranges of anodization voltage (Va) to produce porous anodic alumina (PAA) nanostructures with high porosity. The anodization of Al-0.2Cu in 1 M H2SO4 at Va range of 10-30 V produced a higher oxide thickness, and hence, a higher volume expansion factor, compared to the anodization in 0.75 M H3PO4 at Va range of 100-140 V. The increase in Va, as expected, increases the inter-pore distance, pore size, and porosity of the PAA and improves the anodization rate. The optical sensing performance of the synthesized PAA under different conditions was investigated. The maximum surface wettability was attained for PAA anodized in 1 M H2SO4 at 20 and 30 V. Moreover, the Vickers hardness (HV) of PAA was improved by forming a thin alumina layer on the Al-0.2Cu surface, and the anodized Al-0.2Cu in 0.3 M C2H2O4 revealed the highest HV compared to other acids. The synthesized PAA was tested as an optical sensor for some liquids by monitoring the refractive index. The synthesized PAA structure demonstrated a sensitivity of 180 nm/RIU. The anodization of Al-0.2Cu films using a one-step anodization process in different acidic mediums, and the resulting multifunctional properties (improved hardness, tunable wettability, and competitive sensitivity) existing a novel and potentially impactful approach.