Fig. 1.

IFONFs fabricated from commercial Fe foil with anodization/fluorination process. a Schematic of the fabrication process for IFONFs, starting with a Fe foil. SEM top-view images of b Fe-oxide PTF and c IFONFs-45. Scale bar, 100 nm. d High-magnification SEM image of IFONFs-45. Scale bar, 100 nm. e SEM cross-sectional image of IFONFs-45. Scale bar, 500 nm. f XRD spectrum of Fe-oxide PTF and IFONFs synthesized with various T_fluorinated. For Fe-oxide PTF, the diffraction peaks at 2θ = 11.9°, 16.9°, 26.9°, 35.3°, 39.4°, 46.6°, 53.1°, and 56.2° suggests the formation of FeO(OH) (PDF#75–1594) with anodization. The Fe-oxide PTF anodically grown on Fe substrates consists of FeO(OH) and Fe, while the Fe peaks are from Fe foil. With short T_fluorinated of 15 min, the diffraction peaks of both Fe₂O₃ and FeF₂ are sharp and intense, and Fe₂O₃ is obtained by annealing FeO(OH) in an argon atmosphere during fluorination⁴⁷. With T_fluorinated of 45 min, the primary diffraction peaks associated with Fe₂O₃ are declined while both peaks for FeF₂ (110) and (101) are distinguished, indicating a T_fluorinated-dependent phase transformation for iron fluoride-oxide in fluorination condition. Fe₂O₃ and FeF₂ could coexist with fluorination, and FeF₂ phase has a tetragonal structure with a P42/mnm space group (PDF#81–2271). High-resolution XPS in g Fe 2p, h O 1s, and i F 2p regions for Fe-oxide PTF and IFONFs-45, respectively. For IFONFs-45 in g, two core-level signals of Fe 2p located at ∼710.9 and 724.2 eV are attributed to Fe 2p_3/2 and Fe 2p_1/2, which are negatively shift ~0.6 and ~0.8 eV relative to those in the raw Fe oxide, respectively, owing to the bond formation of Fe and F elements. The satellite peaks at ∼719.9 and 733.3 eV are attributed to Fe⁰, suggesting the presence of elemental Fe on the surface. Moreover, the O 1s peak of IFONFs-45 in h is shifted to lower binding energy with respect to the Fe-oxide PTF, which attributes to the formation of FeF₂ phase