Formation of Nanoporous Mixed Aluminum-Iron Oxides by Self-Organized Anodizing of FeAl₃ Intermetallic Alloy

Abstract

Nanostructured anodic oxide layers on an FeAl3 intermetallic alloy were prepared by two-step anodization in 20 wt.% H2SO4 at 0°C. The voltage range was 10.0 – 22.5 V with a step of 2.5 V. The structural and morphological characterizations of the received anodic oxide layers were performed by FE-SEM. Therefore, the formed anodic oxide was found to be highly porous with a high surface area, as indicated by the FE-SEM studies. It has been shown that the morphology of fabricated nanoporous oxide layers is strongly affected by the anodization potential. The oxide growth rate first increased slowly (from 0.010 μm/s for 10 V to 0.02 μm/s for 15 V) and then very rapidly (from 0.04 μm/s for 17.5 V up to 0.13 μm/s for 22.5 V). The same trend was observed for the change in the oxide thickness. Moreover, for all investigated anodizing voltages, the structural features of the anodic oxide layers, such as the pore diameter and interpore distance, increase with increasing anodizing potential. The obtained anodic oxide layer was identified as a crystalline FeAl2O4, Fe2O3 and Al2O3 oxide mixture.

Nanoporous Anodic Aluminum-Iron Oxide with a Tunable Band Gap Formed on the FeAl3 Intermetallic Phase

“…The thickness of the coating varies depending on the voltage applied. These values are between 0.62 μm and 7.47 μm in extreme cases for 10 V and 22.5 V. This is precisely described in the previously published work [ 18 ]. Observed peaks coming from the substrate material may also provide the evidence of potential discontinuities in the resulting coating.…”

“…In addition, an increase in the morphological parameters of nanopores in the oxide structure obtained on the FeAl intermetallic alloy was observed with an increase in the applied voltage [ 15 ]. Similar effects were observed for the structures produced by anodizing aluminum at various temperatures using 20% H 2 SO 4 as well as 0.3 M oxalic acid, where in both cases, an increase in pore diameter and distance between the pores was observed along with an increase in the potential [ 18 , 19 ].…”

“…In addition, a decrease in the band gap of the produced oxide structure was noted along with an increase in the anodization potential. The oxide coating produced at a voltage of 5 V has a bandgap of approximately 5.0 eV, while the band gap for the coating manufactured at 17.5 V is 2.09 eV [ 15 , 18 ]. This can prove the possibility of controlling the value of the band gap through the selection of technical process parameters.…”

“…In turn, this is not only the result of the structure and composition of the base material but also the result of the parameters of the anodization process. Considering that alumina produced on pure Al has a satisfactory durability but a high band gap, it seems reasonable to analyze the ability to produce anodic oxide coatings on a substrate based on intermetallic alloys from the Fe-Al system with a reduced iron concentration [ 18 ]. An example of such a material is an alloy based on the FeAl 3 intermetallic.…”

“…Literature studies indicate an almost complete lack of work dedicated to FeAl 3 anodization. Therefore, the need to thoroughly investigate the possibility of producing a nanoporous anode layer on the FeAl 3 intermetallic alloy substrate as a potential new photocatalytic material is justified [ 18 ]. Recently, we published a paper on the structure of anodic oxides produced on an FeAl 3 substrate.…”