500
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.8 Nanostructures Prepared with Anodization
of Miscellaneous Other Metals
13.8.1 Iron
Nanoporous iron oxide layers became important recently for a number of applications like supercapacitors, electrodes for enzyme-free electroanalytical detection
of organic molecules, photoelectrochemical reactions and host materials for electrodes in lithium ion batteries. The varying oxidation state of ion and the diversity
of the crystalline forms of the oxides make it a versatile tool in various fields of
electrochemistry.
The purity of the anodized species was typically at least 99.9 wt% or better. The
anodization solutions that offer to prepare nanostructured oxide layer on iron are
versatile and so are the resulting nanostructures. The majority of works report the
application of viscous organic solvents with a well-controlled water concentration
and ammonium fluoride as solute [228–235]. While the organic baths analogous to
those applied for titanium anodization proved to be efficient, aqueous buffer solutions also allowed the preparation of nanostructured surface layer with a thickness
not larger than a few dozens of nanometers [230]. This limitation was attributed
to the dissolution–reprecipitation nature of the oxide formation in aqueous baths.
For organic baths, an NH 4 F concentration lower than 0.5 wt% was favourable for
vertical pore and nanotube formation, but a higher fluoride salt concentration led to
disordered nanoporous structure [228]. Studies dealing with the impact of the water
concentration of the organic baths agree in that a 0.1–2.0 mol dm
−3 water content
makes it possible to obtain nanotubes with anodization, and the optimal anodization
voltage decreases with the increase of the water concentration [231, 232, 234]. The
higher the water content, the thicker barrier layer is produced and the smaller the
maximum total layer thickness achieved [234]. The anodization voltage ranges to
the 10–100 V interval, and the corresponding anodization current density is between
8 and 50 mA cm
−2 . The interpore distance increases with the anodization voltage,
similar to both Al and Ti. However, the cell size per anodization voltage coefficient
strongly depends on the water content of the bath. This parameter was rated as 1.7
and 3.7 nm V
−1 for ethylene glycol-based solution containing 0.1 mol dm
−3 NH 4 F
plus 0.3 and 1.5 mol dm
−3 H 2 O, respectively [234].
When a fluoride salt is applied as solute of the anodizing solution, the as-received
nanoporous/nanotubular iron oxide contains not only a detectable but rather high
fluorine incorporation [228, 229, 235]. The fluorine content of the porous layer
has a maximum in the barrier layer [231, 234]. Since the as-received structure of
the nanostructured iron oxide is amorphous, an annealing is commonly applied for
achieving the crystalline form. During the annealing process, the fluorine incorporation decreases significantly [228]. The crystal structure of the annealed samples is
usually not uniform, but the XRD pattern indicates the presence of various phases
[228–230, 233, 235] such as magnetite and maghemite as major crystalline forms
and haematite observed occasionally only.
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.8 Nanostructures Prepared with Anodization
of Miscellaneous Other Metals
13.8.1 Iron
Nanoporous iron oxide layers became important recently for a number of applications like supercapacitors, electrodes for enzyme-free electroanalytical detection
of organic molecules, photoelectrochemical reactions and host materials for electrodes in lithium ion batteries. The varying oxidation state of ion and the diversity
of the crystalline forms of the oxides make it a versatile tool in various fields of
electrochemistry.
The purity of the anodized species was typically at least 99.9 wt% or better. The
anodization solutions that offer to prepare nanostructured oxide layer on iron are
versatile and so are the resulting nanostructures. The majority of works report the
application of viscous organic solvents with a well-controlled water concentration
and ammonium fluoride as solute [228–235]. While the organic baths analogous to
those applied for titanium anodization proved to be efficient, aqueous buffer solutions also allowed the preparation of nanostructured surface layer with a thickness
not larger than a few dozens of nanometers [230]. This limitation was attributed
to the dissolution–reprecipitation nature of the oxide formation in aqueous baths.
For organic baths, an NH 4 F concentration lower than 0.5 wt% was favourable for
vertical pore and nanotube formation, but a higher fluoride salt concentration led to
disordered nanoporous structure [228]. Studies dealing with the impact of the water
concentration of the organic baths agree in that a 0.1–2.0 mol dm
−3 water content
makes it possible to obtain nanotubes with anodization, and the optimal anodization
voltage decreases with the increase of the water concentration [231, 232, 234]. The
higher the water content, the thicker barrier layer is produced and the smaller the
maximum total layer thickness achieved [234]. The anodization voltage ranges to
the 10–100 V interval, and the corresponding anodization current density is between
8 and 50 mA cm
−2 . The interpore distance increases with the anodization voltage,
similar to both Al and Ti. However, the cell size per anodization voltage coefficient
strongly depends on the water content of the bath. This parameter was rated as 1.7
and 3.7 nm V
−1 for ethylene glycol-based solution containing 0.1 mol dm
−3 NH 4 F
plus 0.3 and 1.5 mol dm
−3 H 2 O, respectively [234].
When a fluoride salt is applied as solute of the anodizing solution, the as-received
nanoporous/nanotubular iron oxide contains not only a detectable but rather high
fluorine incorporation [228, 229, 235]. The fluorine content of the porous layer
has a maximum in the barrier layer [231, 234]. Since the as-received structure of
the nanostructured iron oxide is amorphous, an annealing is commonly applied for
achieving the crystalline form. During the annealing process, the fluorine incorporation decreases significantly [228]. The crystal structure of the annealed samples is
usually not uniform, but the XRD pattern indicates the presence of various phases
[228–230, 233, 235] such as magnetite and maghemite as major crystalline forms
and haematite observed occasionally only.