Size-Dependent Surface Properties of Low-Reflectivity Nanoporous Alumina Thin-Film on Glass Substrate

Abstract

The size-dependent surface properties of low-reflectivity porous alumina thin film on glass substrate produced by two-step self-organized anodization of aluminum are observed. By electro-polishing the aluminum coating, a minimum roughened surface with an Rrms of 3.91 nm is obtained. After adjusting the applied potential, an optimum regular structure of anodic aluminum oxide (AAO) is obtained, corresponding to the pore diameter and density of 23 nm and 8.67×1010 cm−2. In particular, nanoporous AAO with an air/solid ratio of 58% exhibits a maximum water contact angle of 73.4° corresponding to surface energy of 40.1×10−5 N cm−1. Furthermore, the TM-mode reflection analysis shows a diminishing Brewster angle shifted from 60°-54° with an increasing air/solid ratio from 37%-58% at 532 nm. The greatly reduced small-angle reflectance and surface energy reveals a nonlinear trend with an enlarging pore size and air/solid ratio, leading to a minimum surface reflectance and maximum water contact angle at the nanoporous AAO prepared with 60V.