Anodizing is a high voltage electrochemical conversion process that forms barrier-type oxide layers or self-organized nanoporous/nanotubular structures. So far, the Al₂O₃-like nanopores and TiO₂-like nanotubes could be successfully synthesized on many valve metals and alloys. The proposed models of anodic oxide nanotubes growth, however, sacrifice from lack of evidence on the transition from nanopores to nanotubes. Recently, anodizing become possible for more oxidizable metals such as iron and its alloys. The challenge in anodizing of iron lies in the proper control of corrosion/passivity of iron which can be successfully realized in organic electrolytes containing small amount of water. ^1-3

Herein, we investigate the origin of the growth of nanotubes on iron in ethylene glycol electrolyte containing ammonium fluoride and desired amount of water. The composition of the nanoporous/ nanotubular anodic films formed on iron at various water concentrations is examined by high resolution electron microscopy/EDS elemental mapping.

The anodizing of iron in organic electrolytes containing fluorides typically leads to formation of nanopores and nanotubes depending on anodizing voltage. The kinetics of transition from nanopores to nanotubes is relatively slow for iron oxide and therefore allows for ex-situ observation of metal-fluoride layer upon nanopores/nanotubes transition. The TEM image shows pseudoporous structure in which anodic oxide is surprisingly in the form nanotubes as demonstrated by corresponding compositional TEM/EDS mapping. ⁴ The oxide nanotubes are separated by metal fluoride matrix which dissolves during anodizing process. The presented compositional fingerprints of the transition of nanopores into nanotubes are an evidence on the role of fluorides in the growth of anodic nanotubes.

References:

1. H. E. Prakasam, O. K. Varghese, M. Paulose, G. K. Mor and C. A. Grimes, Nanotechnology, 2006, 17, 4285-4291.
2. H. Habazaki, Y. Konno, Y. Aoki, P. Skeldon and G. E. Thompson, Journal of Physical Chemistry C, 2010, 114, 18853-18859.
3. Y. Konno, E. Tsuji, P. Skeldon, G. E. Thompson and H. Habazaki, J. Solid State Electrochem., 2012, 16, 3887-3896.
4. K. Shahzad, D. Kowalski, C. Y. Zhu, Y. Aoki and H. Habazaki, Chemelectrochem, 2018, 5, 610-618.

Figure 1 Bright field TEM micrograph for FIB cross-section of anodic iron formed at 100V in ethylene glycol electrolyte containing 1.5M H₂O and 0.1M NH₄F. The corresponding EDS maps show the elemental distribution of oxygen (green) and superimposed fluorine (red) and oxygen (green).