Ultra-High Density Single Nanometer-Scale Anodic Alumina Nanofibers Fabricated by Pyrophosphoric Acid Anodizing

Abstract

Anodic oxide fabricated by anodizing has been widely used for nanostructural engineering, but the nanomorphology is limited to only two oxides: anodic barrier and porous oxides. Therefore, the discovery of an additional anodic oxide with a unique nanofeature would expand the applicability of anodizing. Here we demonstrate the fabrication of a third-generation anodic oxide, specifically, anodic alumina nanofibers, by anodizing in a new electrolyte, pyrophosphoric acid. Ultra-high density single nanometer-scale anodic alumina nanofibers (1010 nanofibers/cm2) consisting of an amorphous, pure aluminum oxide were successfully fabricated via pyrophosphoric acid anodizing. The nanomorphologies of the anodic nanofibers can be controlled by the electrochemical conditions. Anodic tungsten oxide nanofibers can also be fabricated by pyrophosphoric acid anodizing. The aluminum surface covered by the anodic alumina nanofibers exhibited ultra-fast superhydrophilic behavior, with a contact angle of less than 1°, within 1 second. Such ultra-narrow nanofibers can be used for various nanoapplications including catalysts, wettability control, and electronic devices.

Highly Ordered Anodic Alumina Nanofibers Fabricated via Two Distinct Anodizing Processes

“…As a result, various nanofibers were tangled and bundled together due to their flexibility, and the interesting nanofeatures that formed on the aluminum substrate were covered by alumina nanofibers having the capability to move. The surface nanofeatures were very similar to those obtained from the single-step pyrophosphoric acid anodizing, 29 and long-term anodizing is inadequate for the fabrication of highly ordered nanofiber structures. It is speculated that the alumina nanofibers float freely in the pyrophosphoric acid solution during anodizing.…”

“…In the previous study, the diameter was measured as less than 10 nm using high-resolution transmission electron microscopy. 29 During the typical two-step anodizing process for the formation of ordered porous alumina, the nanopores are preferentially generated at the bottom of the dimple array and grow downward from the surface (Fig. 1c).…”

“…This rapid growth rate of the alumina nanofibers can be explained by the growth behavior in the initial stage of pyrophosphoric acid anodizing. 29 Anodic alumina nanofibers grow through the formation of barrier and honeycomb oxides and the subsequent selective dissolution of honeycomb oxide. As a result, various nanofibers were tangled and bundled together due to their flexibility, and the interesting nanofeatures that formed on the aluminum substrate were covered by alumina nanofibers having the capability to move.…”

“…29 During this anodizing process, alumina nanofibers measuring less than 10 nm in diameter were formed at the triple points of the honeycomb-shaped anodic oxide on the aluminum substrate. The aluminum surface covered with nanofibers exhibits ultra-fast superhydrophilic properties.…”

Porous Aluminum Oxide Formed by Anodizing in Various Electrolyte Species

“…Moreover, a third generation anodic oxide, different from barrier and porous oxide, anodic alumina nanofibers fabricated by novel anodizing has been investigated by us. In this investigation, ultra-high-density single nanometer scale alumina nanofibers grow on the aluminum substrate during anodizing [178]. Anodizing science and technology will continue to evolve in the future.…”