Ex Situ Evidence for the Role of a Fluoride-Rich Layer Switching the Growth of Nanopores to Nanotubes: A Missing Piece of the Anodizing Puzzle

Anodizing involves a high-voltage electrochemical conversion process that forms barrier-type oxide layers or self-organized nanoporous/nanotubular structures. So far, Al2O3-like nanopores and TiO2-like nanotubes could be successfully synthesized on many metals and alloys. The proposed models of anodic oxide nanotubes growth, however, sacrifice from lack of evidence of the transition from nanopores to nanotubes. The present study demonstrates a missing piece of this anodizing puzzle, which is responsible for the formation of nanotubes in fluoride-containing organic electrolytes. For this purpose, we choose an anodic oxide formed on iron, as a model case, because both nanotubes and nanopores can be formed and slow kinetics of transition between those two forms allows us to observe, ex situ, a fluoride-rich-layer upon nanopores/nanotubes transition. The compositional fingerprints of this transition shed a light on the general mechanism of nanotubes growth in fluoride-containing electrolytes.