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25-01-2021 | Ceramics | Issue 12/2021 Open Access

# Exploring (1$$\overline{1}$$2)-related ordered structure in oxidation-synthesized α-Fe2O3 nanowhiskers

Journal:
Journal of Materials Science > Issue 12/2021
Authors:
Ming-Wei Lai, Hiroki Kurata
Important notes
Handling Editor: N. Ravishankar.

## Supplementary Information

The online version of this article (https://​doi.​org/​10.​1007/​s10853-021-05787-4).

## Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Introduction

Hematite (α-Fe 2O 3) has long been regarded as a promising photoanode material for photoelectrochemical (PEC) water splitting because of its superior chemical stability, prominent solar light absorption ability, favorable valence band edge position, abundance, and non-toxicity [ 1, 2]. Despite these attractive qualities, the intrinsically short hole diffusion length of α-Fe 2O 3 impedes the practical performance of α-Fe 2O 3 photoanodes [ 3]. One feasible strategy to overcome this limitation is to construct α-Fe 2O 3 photoanodes from building blocks of aligned one-dimensional (1D) α-Fe 2O 3 nanostructures, which not only reduces the difficulty for the photogenerated holes to reach the semiconductor–electrolyte interface but also improves the majority carrier collection efficiency [ 1, 2].
Large-scale facile synthesis of aligned α-Fe 2O 3 nanowhiskers (NWs) can be achieved through oxidation of an iron-based substrate under a suitable environmental condition [ 48]. α-Fe 2O 3 NWs synthesized via such a means normally exhibit tapered quasi-1D geometries with geometrically basal faces. The NW growth is suggested to be stress-driven, for which stress in the iron oxide scale that forms between the α-Fe 2O 3 NWs and iron-based substrate drives the outward diffusion of iron atoms to the NW growth fronts [ 9, 10]. Possibly connected with stress, ordered axial structures are observed in the α-Fe 2O 3 NWs. As the best-known case, a (3 $$\overline{3}$$0)-related ordered structure has been observed to exist in NWs with basal faces perpendicular to the [001] direction [ 5, 1115]. Furthermore, a (1 $$\overline{1}$$ 2)-related ordered structure was reported to be present in NWs with basal faces perpendicular to the [ $$\overline{1}$$11] direction [ 6, 7, 12, 14].
For oxidation-synthesized α-Fe 2O 3 NWs, understanding the internal ordered structures is meaningful for evaluating their practical performance in PEC reactions. Through both microstructural and microchemical analyses, the (3 $$\overline{3}$$0)-related ordered structure has been verified to be linked to oxygen vacancy ordering [ 5, 11, 15]. Since the introduction of oxygen vacancies into an α-Fe 2O 3 photoanode can increase the photoanode’s major carrier concentration [ 1619], thereby suppressing reverse current flow in the photoanode during PEC reactions [ 20], the (3 $$\overline{3}$$0)-related ordered structure is expected to enhance the PEC performance of the host α-Fe 2O 3 NWs. Similarly, the (1 $$\overline{1}$$2)-related ordered structure is also considered to be associated with oxygen vacancy ordering [ 6, 7, 12, 14]. Although reasonable, this assertion is merely based on conventional HR-TEM observations, the limited resolving power of which is incapable of revealing the microstructural characteristic of this ordered structure. In other words, the origin of this ordered structure’s relation with oxygen deficiency remains unclear. Following the development of spherical aberration (C S)-corrected transmission electron microscopy (TEM), high-resolution (HR)-TEM observations are now available to have an information limit of 1 Å −1 or better [ 21, 22] and much reduced image delocalization. Moreover, the recently realized monochromated electron energy loss spectroscopy (EELS) [ 23, 24] enables the sensitive detection of crystal structure variations via a transmission/scanning transmission electron microscope (TEM/STEM)-based microchemical analysis. These two microscopic advancements provide the opportunity for unequivocally ascertaining the (1 $$\overline{1}$$2)-related ordered structure. In this study, we investigated α-Fe 2O 3 NWs that were synthesized through a thermal oxidation process and exhibited the (1 $$\overline{1}$$2)-related ordered structure. We first revealed that the (1 $$\overline{1}$$2)-related ordered structure actually existed in bicrystalline α-Fe 2O 3 NWs and was associated with the (3 $$\overline{3}$$0)-related ordered structure. Using C S-corrected HR-TEM and monochromated EELS, a link between the (1 $$\overline{1}$$2)-related ordered structure and tensile strain was verified, which explicates the connection between this ordered structure and oxygen vacancy ordering. Finally, through an EELS-based quantitative comparison, we further observed that the degree of oxygen deficiency of the (1 $$\overline{1}$$2)-related ordered structure is smaller than that of the (3 $$\overline{3}$$0)-related ordered structure.

## Experimental details

An atmospheric thermal oxidation process with a reaction temperature and duration of 550 °C and 20 h, respectively, was adopted to synthesize α-Fe 2O 3 NWs on a 45 Permalloy substrate (The Nilaco Corp.). The morphology of the synthesized α-Fe 2O 3 NWs has been shown in our previous study [ 15] and is similar to those of α-Fe 2O 3 NWs synthesized via other thermal oxidation processes [ 5, 2528]. A TEM sample of the α-Fe 2O 3 NWs was prepared by ultrasonically dispersing the NWs in ethanol and then transferring one small drop of the suspension to a holey carbon-coated copper grid. Analyses of the α-Fe 2O 3 NWs were performed utilizing a monochromator-equipped double-Cs-corrector TEM/STEM (JEM-ARM200F, JEOL Ltd.) operated at a 200-kV acceleration voltage. The C S value for the HR-TEM observation of the (1 $$\overline{1}$$2)-related ordered structure was tuned to −14 μm. EELS spectra were acquired via the STEM-EELS method (with the convergence and collection semi-angles of 32.3 and 28.2 mrad, respectively) under the dual-EELS mode [ 29], for which the full-width-at-half-maximum (FWHM) values of the zero-loss peaks were 0.25 eV (monochromated O K-edge EELS) and 1.25 eV (wide-range EELS). Each acquired spectrum was based on summing all spectra within a spectrum image containing at least 9500 spectrum data. Simulated electron diffraction patterns and HR-TEM images were calculated with the xHREM program (HREM Research Inc.); O K-edge near-edge EELS spectrum simulations were performed using the FEFF9 code [ 30].

## Results and discussion

An HR-TEM image of an as-synthesized α-Fe 2O 3 NW is shown in Fig.  1a, in which superstructure modulation is clearly visible. The fast Fourier transform (FFT) pattern of the HR-TEM image (Fig.  1c) shows that the α-Fe 2O 3 NW grows along the [110] direction (as shown in Fig. S1, the [110] NW growth direction was also verified by using the shadow image method), which is the typical growth direction of α-Fe 2O 3 NWs synthesized via oxidation. More importantly, in addition to the prominent spots of Bragg diffraction along the [ $$\overline{1}$$11] zone axis, extra spots occur at one-fourth the reciprocal lattice vector of the (1 $$\overline{1}$$2) planes, indicating that an (1 $$\overline{1}$$2)-related ordered structure with a period of four times the interplanar spacing of the (1 $$\overline{1}$$2) planes is present in the α-Fe 2O 3 NW. This finding agrees with the prior observed periodicity of the (1 $$\overline{1}$$2)-related ordered structure [ 6, 7, 12]. Further, the extra spots exhibit obvious intensity fluctuation, which we have also noticed in prior studies [ 6, 12]. This phenomenon is not common for superstructure-based TEM electron diffraction. After a careful spot identification, as shown in Fig.  2a, we determined that part of the brighter extra spots were associated with Bragg diffraction along the [001] zone axis (i.e., these spots are not only superstructure-based but also associated with Bragg diffraction along the [001] zone axis), meaning that the α-Fe 2O 3 NW possesses a [ $$\overline{1}$$11] ∥ [001] bicrystalline structure. Through this finding, the remaining brighter extra spots were then understood to be relevant to double diffraction [ 31] in the bicrystal (i.e., these spots are both superstructure- and rediffraction-related). It was found that multiple electron diffraction was incapable of reasonably explaining the emergence of the dim extra spots, thereby directly reflecting the existence of the (1 $$\overline{1}$$2)-related ordered structure. Based on our FFT pattern interpretation, a previously overlooked fact about the (1 $$\overline{1}$$2)-related ordered structure was revealed: instead of existing in single-crystalline α-Fe 2O 3 NWs, it actually exists in [ $$\overline{1}$$11]-oriented grains of [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs.
The emergence of [ $$\overline{1}$$11] ∥ [001] bicrystal has already been observed in the synthesis of α-Fe 2O 3 quasi-1D nanostructures via oxidation [ 4, 10, 26, 32], for which Wen et al. have pointed out that the corresponding bicrystal junction does not have a twin boundary [ 4]. Without considering lattice imperfections, we illustrate the lattice registration of the bicrystal junction for the [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs in Fig.  3a, where the (3 $$\overline{3}$$0) planes in the [001]-oriented grain can be seen coupling to the (1 $$\overline{1}$$2) planes in the [ $$\overline{1}$$11]-oriented grain. As indicated, the contraposition relation between these two plane sets is five times the interplanar spacing of the (3 $$\overline{3}$$0) planes (7.3 Å) against twice the interplanar spacing of the (1 $$\overline{1}$$2) planes (7.4 Å). The ~1.4% lattice mismatch implies that this bicrystal junction is accompanied by interfacial stress and is thus not thermodynamically preferred. Therefore, a supporting mechanism should be present that overcomes the anticipated energy barrier of the formation of the [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs. According to Shen et al. [ 33], who synthesized bicrystalline Zn 3P 2 and Cd 3P 2 nanobelts, formation of bicrystalline structures in vapor–solid quasi-1D nanomaterial growth is possible to stem from strain relaxation. Since the growth of the studied bicrystalline α-Fe 2O 3 NWs is stress-driven [ 9, 10, 15], an analogous reason is considered to explain their formation.
For the synthesis of α-Fe 2O 3 NWs via oxidation of iron-based substrates, the layered iron oxide scale forming beneath the synthesized α-Fe 2O 3 NWs has been investigated by Yuan et al., who indicated that the constitution of the iron oxide scale was α-Fe 2O 3/Fe 3O 4/FeO when the process temperature was above 570 °C (i.e., the threshold temperature of forming the FeO phase), and α-Fe 2O 3/Fe 3O 4 when below [ 10]. On the basis of this conclusion, the thermal oxidation condition used in this study would result in the iron oxide scale comprising upper α-Fe 2O 3 and lower Fe 3O 4 layers (Fig.  3b). The topotaxial relation between these two layers, i.e., α-Fe 2O 3 [001] ∥Fe 3O 4 [111] [ 34], implies that the α-Fe 2O 3 layer suffers from interfacial tensile stress exerted by the Fe 3O 4 layer [ 35]. As elaborated in our previous study [ 15], this leads to the grown α-Fe 2O 3 NWs being tensile-strained, for which the (3 $$\overline{3}$$0)-related ordered structure, identified to be periodic (3 $$\overline{3}$$0)-related interplanar gap expansions induced by oxygen vacancy accumulations, is a thermodynamically preferred route for strain accommodation. Thus, assuming that the formation of the [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs is based on [001]-oriented matrix grains containing the (3 $$\overline{3}$$0)-related ordered structure (Fig.  3b), their emergence is reasonable. Associated with lattice expansion, the (3 $$\overline{3}$$0)-related ordered structure eliminates the anticipated energy barrier of developing an [ $$\overline{1}$$11]-oriented epitaxial grain on an [001]-oriented matrix grain. From another perspective, the wider matching spacing of the (1 $$\overline{1}$$2) plane set in the (1 $$\overline{1}$$2)–(3 $$\overline{3}$$0) coupling (Fig.  3a) signifies that the [ $$\overline{1}$$11]-oriented epitaxial grain only inherits a reduced strain degree from the [001]-oriented matrix grain. Therefore, the formation of the [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs can be considered to be originating from strain relaxation. In light of our deduction, as illustrated in Fig.  3c, the (3 $$\overline{3}$$0)-related ordered structure in [001]-oriented grains of the [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs must result in periodic (1 $$\overline{1}$$2)-related lattice expansions in [ $$\overline{1}$$ 11]-oriented grains of the NWs, which we believe is the microstructural characteristic of the (1 $$\overline{1}$$2)-related ordered structure. Referring to prior studies, we have noticed that the periodicity of the (1 $$\overline{1}$$2)-related ordered structure (14.7 Å) is substantively the same as the most observed periodicity of the (3 $$\overline{3}$$0)-related ordered structure (14.5 Å). This coincidence provides a vital demonstration of the correlation between these two ordered structures.
Figure  4a shows a TEM image of an α-Fe 2O 3 NW segment. Verified by site-specific microstructure identifications (Fig.  4b and c), the segment bottom simultaneously contains regions dominated by the [ $$\overline{1}$$11] ∥ [001] bicrystal and [001]-oriented grain. That is, it has a partly overlapped bicrystalline structure. Note that the [001]-oriented grain comprising the (3 $$\overline{3}$$0)-related ordered structure can be recognized from Fig.  4c. Conducting close microstructural examination of the segment’s right edge, the bottom of which is dominated by the [001]-oriented grain, we observed that the FFT patterns of HR-TEM images acquired at upper areas along the NW growth direction (Fig.  4d and e) moreover exhibited evident spot feature associated with the [ $$\overline{1}$$11]-oriented grain. This finding supports the deduction that [ $$\overline{1}$$11]-oriented grains can develop on [001]-oriented grains containing the (3 $$\overline{3}$$0)-related ordered structure. Also, in Fig.  4e, where the spots of Bragg diffraction along the [ $$\overline{1}$$11] zone axis are rather prominent, there is another set of brighter extra spots present, just like the situation in Fig.  1c. This evolution reflects that along with the bicrystal development getting mature, the condition to excite sensible double diffraction can be met. In a way, the consecutive site-specific microstructure identifications shown in Fig.  4c to e confirm the aforesaid elaboration on the FFT pattern in Fig.  1c.
The composition evolution of the [ $$\overline{1}$$11]-oriented grain from the area corresponding to Fig.  4c to that corresponding to Fig.  4e was further probed by inspecting the variations of the intensity ratio of the ( $$\overline{1}$$2 $$\overline{3}$$) and ( $$\overline{1}$$20) spots, respectively connected with the [ $$\overline{1}$$11]- and [001]-based Bragg diffractions. (See supplementary information for details.) Since these two spots are proximal to each other, their intensity ratio is not significantly influenced by the objective lens defocus condition, and thus, this practice is considered to be a plausible approach. As the result shown in Fig.  4f, a trend that the composition weight of the [ $$\overline{1}$$11]-oriented grain increases along the NW growth direction was found, suggesting that [ $$\overline{1}$$11]-oriented grains are favored in the crystal development of the [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs. We principally attribute this to their being relatively less strained.
For a purpose of ascertaining the microstructural characteristic of the (1 $$\overline{1}$$2)-related ordered structure, an enlarged image of the area enclosed within the red box in Fig.  1a is shown in Fig.  5a, wherein the superstructure modulation is indicated by red arrows. A simulated HR-TEM image of pristine α-Fe 2O 3 along the [ $$\overline{1}$$11] zone axis (see supplementary information for the adopted supercell), calculated with a thickness value of 50.5 nm and an underfocus of 11 nm, is displayed in Fig.  5b. It can be seen that the experimental image and simulated image closely resemble one another, indicating that the projected potential of the [ $$\overline{1}$$11]-oriented grain dominated in the HR-TEM observation of the bicrystalline α-Fe 2O 3 NW. Therefore, an approximated insight into the microstructural characteristic of the (1 $$\overline{1}$$2)-related ordered structure should be obtained by analyzing the superstructure modulation evident in Fig.  5a.
The simulated HR-TEM image in Fig.  5b includes three types of image spots, designated as types A, B, and C; their corresponding crystallographic positions are illustrated by the superimposed lattice model. Comparing the experimental HR-TEM image with the simulated image, we noticed that half of the type-B spots at the superstructure modulation were distorted into a streak-like shape and that the type-A spots next to the distorted type-B spots are slightly shifted along the [110] direction (Fig.  5c). This represents the presence of atomic column displacements around the superstructure modulation. In addition, measuring a series of interval values between adjacent type-A and type-C spots (Fig.  5d), each of which corresponds to the interplanar spacing of the (1 $$\overline{1}$$2) planes, we found that spots flanking the superstructure modulation had wider interval values, signifying that the atomic column displacements are due to tensile strain. Overall, the HR-TEM observation suggests that the (1 $$\overline{1}$$2)-related ordered structure is periodic atomic column displacements that serve as tensile strain accommodation.
Based on the HR-TEM image spot variations, we deduce that the atomic column displacement scenario of the (1 $$\overline{1}$$2)-related ordered structure is the splitting of neighboring atomic columns to accommodate tensile strain. A specific illustration of this is shown in Fig.  5e, where a row of atomic column splits is introduced into the supercell to simulate the HR-TEM image shown in Fig.  5b. With the same calculation parameters, a simulated HR-TEM image based on this locally distorted supercell is shown in Fig.  5f, wherein the experimentally observed type-B spot distortion and type-A spot shift are reproduced at positions corresponding to the atomic column splits. Although the simulation does not perfectly reproduce the experimental image details of the (1 $$\overline{1}$$2)-related ordered structure, understood to be mainly attributed to a local expansion that is not really introduced into the supercell and the use of a single-crystal approximation, it verifies the plausibility of the deduced atomic column displacement scenario.
Being exempt from the multiplet effects [ 36], O K-edge core excitation spectra of α-Fe 2O 3 effectively reflect its structural characteristic. Therefore, by comparing the O K-edge EELS spectrum fine structure of the [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs with that of pristine α-Fe 2O 3, microchemistry-based evidence that supports the existence of tensile strain in the NWs should be obtained. Figure  6a shows monochromated O K-edge near-edge EELS spectra of the bicrystalline α-Fe 2O 3 NW displayed in Fig.  1 and a stoichiometric α-Fe 2O 3 reference sample (α-Fe 2O 3 powder from Sigma-Aldrich Inc., which was further atmospherically annealed at 500 °C for 20 h). The spectra have been normalized to the maximum of peak A1, and their onsets were determined using the intensity of the first inflection in the reference sample spectrum. Peaks A1 and A2 are known to be pre-peaks [ 37], which correspond to the electronic transitions from O 1s orbitals to the portion of O 2p orbitals hybridizing with Fe 3d orbitals. We noticed that the pre-peak interval in the NW spectrum is ~0.05 eV smaller than that in the reference sample spectrum. This ligand field splitting reduction may imply that the NW has a lower Fe 3d–O 2p hybridization degree than the reference sample [ 37], i.e., the NW possesses a longer average Fe–O interatomic distance. Peak B can be regarded as the near-edge main peak; its formation, in the light of the real-space multiple scattering theory, is substantially attributed to the first-oxygen-shell scattering resonance of the excited 1s core electrons [ 38]. That is, for each of the acquired EELS spectra, the energy interval between the spectrum onset and peak B (ΔE) is connected with the average radius of the first oxygen shell surrounding an excited oxygen anion (R) via the relation in Eq.  1 [ 38]:
$$\Delta E \times R^{2} = {\text{constant}}.$$
(1)
R is essentially equivalent to the average interatomic distance between two adjacent oxygen anions ( R O–O). The ΔE values in the spectra of the NW and reference sample were found to be 11.9 and 12.0 eV, respectively, meaning that R O–O in the α-Fe 2O 3 NW is 0.4% longer than that in pristine α-Fe 2O 3. Since the lattice framework of α-Fe 2O 3 is constructed by close-packed oxygen anions, this reconfirms the HR-TEM observation result that the α-Fe 2O 3 NW has tensile strain.
The slightly expanded lattice of the α-Fe 2O 3 NW is also reflected by the onset shift in the EELS spectra. Through fitting an experimentally acquired monochromated Fe L 3-edge EELS spectrum of the reference sample with the charge transfer multiplet calculation [ 39, 40], we evaluated the average Fe–O bonding covalency in α-Fe 2O 3 to be only approximately 21.9% (see supplementary information for details), indicating a high ionicity of this material. Therefore, for α-Fe 2O 3, the binding energy of O 1s core electrons is considerably affected by the Madelung potential at oxygen anions. To this extent, when the lattice of α-Fe 2O 3 is tensile-strained, the increased average Fe–O interatomic distance will reduce the binding energy of the O 1s core electrons [ 41] and will thus result in a lower O K-edge onset energy. This argument tells that the relative left shift of the NW spectrum’s onset in Fig.  6a is a sign of an expanded average Fe–O interatomic distance in the α-Fe 2O 3 NW. Displayed in Fig.  6b, O K-edge near-edge EELS spectra of α-Fe 2O 3 with and without 0.4% lattice parameter expansion were calculated (see supplementary information for details); the onset shift in the simulated spectra affords definite theoretical support to the connection between lattice expansion and O K-edge downward shift for α-Fe 2O 3.
For transition metal oxides, oxygen vacancy formation can be facilitated by introducing tensile strain [ 4244]. Since the microstructural characteristic of the (1 $$\overline{1}$$2)-related ordered structure has been verified to be periodic local lattice expansions, this ordered structure must be oxygen deficient. Although definite lattice positions of the oxygen vacancies still require further investigations, our study results elucidate the root cause of the long-asserted connection between the (1 $$\overline{1}$$2)-related ordered structure and oxygen vacancy ordering. Per the explanation given above, the difference in strain degree between the (1 $$\overline{1}$$2)- and (3 $$\overline{3}$$0)-related ordered structures should cause the former to have lighter oxygen deficiency than the latter. To verify this, wide-range EELS spectra of two [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs with differing crystal constitutions were acquired (Fig.  7a and b). By extracting the O K-edge and Fe L-edge near-edge signals from the spectra (the adopted signal window was 24 eV, determined on the basis of the L 2,3-edge energy width) and normalizing them on the O K-edge signal area, we conducted an EELS-based quantitative comparison between the two α-Fe 2O 3 NWs. The result is shown in Fig.  7c, wherein the explicit difference in the Fe L 3-edge signal intensity reflects that the α-Fe 2O 3 NW with the higher [ $$\overline{1}$$11]-oriented grain proportion has a lower Fe/O atomic ratio, i.e., it is less oxygen deficient. Therefore, in comparison with the (3 $$\overline{3}$$0)-related ordered structure, the (1 $$\overline{1}$$2)-related ordered structure indeed contributes less oxygen deficiency to the [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs. This further reinforces the fact that the presence of oxygen vacancy ordering in α-Fe 2O 3 NWs synthesized via oxidation of iron-based substrates is attributed to tensile strain.

## Conclusions

In summary, for oxidation-synthesized α-Fe 2O 3 NWs, the known (1 $$\overline{1}$$2)-related ordered structure was observed to actually occur in [ $$\overline{1}$$11]-oriented grains of [ $$\overline{1}$$11] ∥ [001] bicrystalline α-Fe 2O 3 NWs and originate in the (3 $$\overline{3}$$0)-related ordered structure of [001]-oriented grains of the NWs. Interpreting the C S-corrected HR-TEM observation results, we further revealed that the microstructural characteristic of the (1 $$\overline{1}$$2)-related ordered structure was periodic atomic column displacements serving for accommodating tensile strain. This HR-TEM finding is supported by the monochromated O K-edge EELS analysis, which indicates that, in comparison with pristine α-Fe 2O 3, α-Fe 2O 3 NWs hosting the (1 $$\overline{1}$$2)-related ordered structure have larger average O–O and Fe–O interatomic distances. The association of the (1 $$\overline{1}$$2)-related ordered structure with local lattice expansion explicates its long-asserted relevance to oxygen vacancy ordering. The difference in oxygen deficiency degree between the (1 $$\overline{1}$$2)- and (3 $$\overline{3}$$0)-related ordered structures, verified by the EELS-based quantitative comparison, also suggests that the oxygen vacancy ordering in oxidation-synthesized α-Fe 2O 3 NWs is strain-induced.

## Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research (Grant Number: 17H02739) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

## Compliance with ethical standards

### Conflict of interest

The authors declare that there are no conflicts of interest.
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