Structural Changes of 2D FexMn1−xO2 Nanosheets for Low‐Temperature Growth of Carbon Nanotubes

The synthesis of carbon nanotubes (CNTs) is usually done by metallic catalysts with a gaseous carbon precursor at high temperature. Yet, mild synthesis conditions can broaden the application of CNTs and their composites. In the present work, it is unraveled why partially substituted Fe ions in 2D MnO2 nanosheets lead to the growth of CNTs at low temperatures of 400−500 °C. The local formation of Fe3C by carbon precursor explains the unusually high catalytic activity of 2D FexMn1−xO2 nanosheets for preparing CNTs. Finally, Fe3C is oxidized to Fe3C/FeOx yolk/shell morphology in ambient atmosphere after the CNT formation reaction. These results shed light on the development of novel catalyst materials that allow for efficiently prepare CNTs under mild conditions for their wider use in energy‐harvesting applications.


Introduction
Carbon nanotubes (CNTs) are of great interest because of their superior mechanical, optical, thermal, electrical, magnetic, and chemical properties, not only as single material but also as composites with other functional materials. [1] More than several thousand tons of CNTs are being produced per year and are used in bicycle frames, battery electrodes, water filters, and so on. [2] For preparation of CNTs, chemical vapor deposition is regarded as the most facile and efficient method. Metals and metal carbides act as active catalysts to produce CNTs from gas phase carbon sources (e.g., methane and acetylene). [3,4] Using metal oxides as catalysts for CNTs are also reported, but in most cases, they were reduced to zero valent metals under reducing atmosphere. Determining the exact structure of the active catalyst is still one of the key questions, which is crucial for understanding and developing CNT-based materials. [5][6][7] 1s → 3d transitions and also the main peak (denoted as C) related to the dipole-allowed 1s → 4p transition. [15][16][17][18] The increase of Fe content decreases the intensity of the resonance peak C for both Mn K-edge and Fe K-edge in the XANES data. Since the intensity of this peak reflects the local ordering around Mn/Fe ion, the observed lower intensity of this peak upon Fe substitution clearly demonstrates an increase of local structural disordering around Mn/Fe ion. [15] This finding can be ascribed to the diversification of local atomic arrangement around metal ions by enhancing the mixed distribution of MnO 6 and FeO 6 octahedra in the FMO lattices. Such an increase of structural disorder implies the lowering of lattice stabilization energy upon Fe substitution, which might facilitate the phase transformation of FMO during CNT growth.
The effect of the annealing treatment in C 2 H 2 atmosphere as well as the Fe content on the crystal structure of FMOs were investigated with powder X-ray diffraction (XRD) (Figure 1a). It is important to mention that the FMO before annealing in C 2 H 2 has an MnO 2 crystal structure. [14] Most FMO_0 peaks in the XRD pattern are assigned to an orthorhombic Mn 3 O 4 phase. FMO_0 shows additional strong peaks which correspond to the (111), (200), and (220) reflections of a cubic MnO phase. The phase transformation from the initial MnO 2 nanosheet to Mn 3 O 4 /MnO is related to the reduction of Mn ions during the reaction with C 2 H 2 . All the FMO_1−4 samples display similar XRD peaks, which can be assigned to both the Mn 3 O 4 and MnFe 2 O 4 phases. The formation of MnFe 2 O 4 phases is due to the presence of Fe ions in the materials. There are no strong peaks of the MnO phase in these materials, strongly suggesting a limited crystal growth and depressed reduction of MnO x by the Fe substitution. [14] Broad peaks of CNTs, that is, (001) and (002) are found in FMO_3 and FMO_4, highlighting the low crystalline nature of CNTs formed in FMOs with higher Fe contents.

XANES Spectroscopic Analysis
The oxidation state and local symmetry of Fe and Mn species in FMOs were analyzed with XANES. The overall shape of several main peaks (denoted as A, B, and C) and edge position of Fe K-edge XANES spectra are similar for FMOs and reference Fe 2 O 3 , indicating the trivalent Fe 3+ oxidation state of FMOs ( Figure 1b). This finding clearly demonstrates that FMOs contain Fe 2 O 3 and/or the MnFe 2 O 4 with Fe 3+ oxidation state. A small pre-edge peak (denoted P) is found in Fe K-edge XANES spectra of FMOs, presenting the 1s to 3d transition. This transition is forbidden by an electronic dipolar selection rule but can be partially allowed in the case of a quadrupole-allowed transition or a mixing of 4p and 3d states in the tetrahedral symmetry. [15][16][17][18] A weak pre-edge peak in FMOs provides strong evidence for the octahedral symmetry of Fe species in the structures. The Mn K-edge spectra of FMOs display similar shape and edge position to those of Mn 3 O 4 reference (Figure 1c), reflecting mixed Mn 2+ and Mn 3+ oxidation states of FMOs. A weak pre-edge peak in Mn K-edge XANES spectra of FMOs also confirms the octahedral symmetry of Mn ion in FMOs. [19]

Electron Microscopy Analysis
To verify the structure of FMOs with the evolution of CNTs, SEM, and transmission electron microscopy (TEM) images of Adv. Funct. Mater. 2020, 30,2003849  FMOs were acquired. As presented in Figure 2a, the FMO_0 consists of bigger and smaller crystalline particles coated with thin carbon layers. The lattice distance from the small particles is assigned to (110) of the Mn 3 O 4 in accordance with XRD result (inset in Figure 2a). Both FMO_1 and FMO_2 show smaller interconnected particles in 2D assemblies due to the presence of Fe ions preventing crystal growth [14] of MnO x matrix (Figure 2b,c). The carbon-coated small particles of FMO_1 and FMO_2 are assigned to the (110) and (111) planes of the Mn 3 O 4 , respectively. The growth of CNTs is observed for FMO_3 and FMO_4. FMO_4 displays the formation of longer and thicker CNTs than FMO_3, underscoring the important role of Fe substituent ion in CNT growth (Figure 2d,e). The 2D assemblies are still attached to CNTs, strongly suggesting the growth of CNTs from these 2D assemblies ( Figure S3, Supporting Information).
Crystal particles detached from 2D assemblies are still present at the head part of CNTs, demonstrating a tip-growth mechanism of CNTs (Figure 2d,e). [20] The crystal planes in FMO_3 can be assigned not only to (400) of MnFe 2 O 4 but also to (211) of an orthorhombic Fe 3 C. In FMO_4 lattice, distances corresponding to (112) of Mn 3 O 4 , (222) of MnFe 2 O 4 , and (200) of Fe 3 C are found. Using the presented synthetic method, CNTs can be obtained within a very short reaction time of 30 min but not without C 2 H 2 ( Figure S4a,b, Supporting Information). Longer CNTs are prepared by employing a higher temperature of 500 °C and longer reaction time of 20 h ( Figure S4c, Supporting Information). These results emphasize that the Fe substitution as well as the control of reaction time and reaction temperature are essential for the efficient growth of CNTs from FMOs.
For comparison, Sn-substituted MnO 2 nanosheets (SMOs) were also prepared to examine the importance of Fe for CNT growth at low temperature. [14] SMOs were annealed in C 2 H 2 gas at 400 °C for 2 h. The products were denoted as SMO_0−2, which have an average Sn content of 0 (SMO_0), ≈9 at% Sn (SMO_1), and ≈39 at% Sn (SMO_2). The elements are homogeneously distributed at a microscopic scale ( Figure S5, Supporting Information). Similar to Fe, Sn induces a phase transformation of nanosheets to SnMn 2 O 4 phases suppressing the formation of MnO x crystals ( Figure S6, Supporting Information). However, all SMOs do not catalyze the formation of CNTs, strongly confirming the unique role of Fe for CNT growth at low temperature ( Figure S7, Supporting Information).

Local X-Ray Dispersive Spectroscopy Analysis
To unravel the structure of active catalyst which promotes CNT growth, local investigation of the corresponding FMO particles is indispensable. First, the changes in chemical composition of the FMOs were analyzed using high angle annular dark field (HAADF) and EDS in scanning TEM (STEM) mode ( Figure 3 and Table 1). FMOs are composed of interconnected particles in 2D assemblies coated with carbon. Particles in the tip of short CNTs (<≈1 µm) and long CNTs (>≈1 µm) are detached from the assembled region. The 2D assemblies have a homogeneous distribution of Fe, Mn, O, and C elements (Figure 3a). The particles on the top of short CNTs show a higher Fe content than the 2D assemblies and a higher Fe than Mn content (Figure 3b). For particles attached to long CNTs, Fe signal appears only in the core part whereas O signal is discernible for entire parts of particles, strongly suggesting a core/shell morphology without the presence of Mn (Figure 3c). The increase of the total Fe content in FMO_0 to FMO_4 leads to the elongation of CNT, confirming the major role of Fe ion and negligible role of Mn ion for catalyzing the growth of CNT.

Electron Energy Loss Spectroscopy Analysis
Electron energy loss spectroscopy (EELS) was conducted in STEM mode to further investigate the chemical composition of  FMOs. Figure 4a displays the core loss EELS data in the energy loss regime from 600 to 740 eV for 2D assemblies, short CNT, and long CNT, respectively. In this energy loss regime, the Mn L 2,3 -and Fe L 2,3 -edges occur and the energy loss near-edge structure (ELNES) of each of these element specific edges shows the typical white lines of transition metals in accordance with the literature. [21,22] The intensity of the Mn L 2,3 -edge is highest in the 2D assemblies, low in the short CNT, and not detected above the noise level in the long CNT. The relative amount of Mn and Fe can be obtained from the edge intensity in the core loss EELS data combined with the low loss spectra to remove plural scattering contributions. The calculated ratio of Mn:Fe is 58:42, 39: 61, and 0:100 for 2D assemblies, short CNT, and long CNT, respectively, confirming the key role of Fe for CNT growth. The intensity of the Fe L 2,3 -and Mn L 2,3 -edges were also used to visualize the elemental distribution within these morphologies. The maps are shown in Figure 4b-d together with the corresponding ADF-STEM image. Both Fe and Mn elemental maps reveal a similar, homogeneous distribution of the elements in the entire monitored area of the 2D assemblies (Figure 4b). While Fe is homogeneously distributed in the catalyst particle of the short CNT, Mn is concentrated only in the core of the particle (Figure 4c). This is due to the higher reactivity of Fe species with carbon source than Mn species. Figure 4d shows that Mn is not present in the catalyst particles of the long CNT besides their core/shell morphology. This is further discussed later.

Detailed EELS Analysis
ELNES was analyzed in detail to further investigate the structural changes of FMOs. All the regions display π* and σ* peaks at C K-edge (Figure 5a). Because the π* peak corresponds to sp 2 bonding, the weak π* peak in the 2D assemblies represents amorphous carbon layers whereas pronounced π* peaks in both short CNT and long CNT regions verify graphitic CNT. [23] ELNES spectra of Fe L 2,3 -and Mn L 2,3 -edges exhibit two white lines whose ratio provides a sensitive probe to the oxidation state of the transition metal ion (Figure 5b-d and Table 1). The higher intensity ratio of I(L 3 )/I(L 2 ) reflects the lower oxidation state of Fe-and Mn-based oxides. [21,22] Both 2D assemblies and short CNT regions display similar I(L 3 )/I(L 2 ) Fe values of ≈5.2, which is larger than that of long CNT region (≈4.4). The number of Fe 3+ ions from the entire Fe species can be estimated using the following equation.
The O K-edges ELNES spectra of all the regions show characteristic pre-edge peaks at ≈529 eV corresponding to transitions from 1s state to unoccupied hybrid states of oxygen 2p and transition metal 3d orbitals (Figure 5e). [24] This result demonstrates the existence of Fe-and Mn-based oxides in all the regions under investigation.
ADF-STEM image of particles on the head of long CNTs exhibits a core/shell morphology (Figure 6a). The core has a brighter intensity than shell which is composed of tiny particles. There is an empty space between core and shell, clarifying the yolk/shell morphology of these particles. ELNES maps were taken to obtain elemental distributions around the yolk/shell (Figure 6a). Carbon is present in yolk/shell as well as CNT region, as evidenced by C K-map. Fe L-and O K-edge ELNES maps show a more intense Fe signal from the core while the highest O signal stems from the shell, indicating that the shell is FeO x but the core is in metallic state. C K-edge ELNES spectrum from CNT area presents strong π* peak related to sp 2 bonding of graphitic carbon (Figure 6b). The π* peak from shell area is weaker due to the initial amorphous carbon shell covering the particles in this area. The strong π* peak from core area is interpreted as the characteristic of carbon atoms in iron carbides. [23,25] The asymmetric Fe L 2,3 -edges ELNES spectral feature from core area also suggests the existence of iron carbide phases (Figure 6c). [23,25] These iron carbides can be formed by the reaction with C 2 H 2 gas under reduction condition. The core has a lower Fe 3+ /ΣFe value than does the shell, confirming the presence of Fe species as iron carbide phase ( Figure 6c and Table 1). The iron carbide core is supposed to be Fe 3 C, because it is the most thermodynamically favorable phase among many iron carbide phases. However, even for Fe 3 C, the formation of this carbide phase is unfavorable in the range of 400 and 450 °C (ΔG° = 22 500 − 18.4T(J)). [26] Also, a recent in situ TEM study reported that formation of CNT using Fe 3 C needed a higher onset temperature (≈800 °C). [5] This fact implies that highly exposed Fe ions on the thin layer of 2D nanosheets employed in this study play a crucial role in providing a faster mobility and a higher reactivity toward the Fe 3 C formation than other metal oxide nanostructures. Additionally, the increased structural disorder of FMOs upon Fe substitution results in the lowering of lattice stabilization energy, which is effective in promoting the phase transformation into the Fe 3 C phase. The oxide shell is formed in ambient atmosphere after completing the growth reaction of CNT, because Fe 3 C is unstable especially below 750 °C. [27] The formation of Fe 3 C/FeO x yolk/shell structure can be understood in terms of different diffusion rates of the elements, that is, Kirkendall effect. [28] The phase segregation into Mn-free Fe 3 C can be explained by the fact that Mn 3 C is thermodynamically less stable than Fe 3 C (especially below 850 °C) and this phase can exist only in the temperature range of 950-1050 °C. [29,30]

3D Electron Tomography
The full 3D morphology of the crystalline particles attached to the head of long CNTs was investigated using electron    tomography. First low magnification tomogram shows a bright particle at the head of CNT (Figure 7A). The morphology was reconstructed using a tilt series of HAADF-STEM images obtained in the range of ±60° in 5° steps (Figure 7Aa-c). The particle (yellow) in reconstructed volume is entirely covered by carbon (purple) grown as 1D CNT (Figure 7Ad-f and Movies S1 and S2, Supporting Information). This result supports that the mechanism responsible for the CNT formation begins with a carbon layer formation around FMO crystal grains expelling the Mn species, and then the CNT growth follows. To visualize 3D morphology of the yolk/shell morphology of the head particles, another reconstruction was performed where the contrast setting was changed for visualization (Figure 7Ba-c). A yellow core particle is surrounded by gray empty area, separating the core and outer shell (Figure 7Bd and Movie S3, Supporting Information), verifying the yolk/shell morphology of Fe 3 C/FeO x .
The reaction mechanism of FMOs with C 2 H 2 is illustrated in Figure 8. With elevating reaction temperature under C 2 H 2 flow, 2D nanosheets start to get reduced to form small crystal grains coated with amorphous carbon layers. Homogeneously dispersed Fe ions in MnO 2 are migrated to form Fe 3 C phase. As evidenced by Fe K-edge XANES analysis, enhancement of structural disorder around Fe ion with the increase of Fe content promotes the formation of Fe 3 C phase during the reaction with C 2 H 2 . High carbon solubility and catalytic activity of Fe 3 C are responsible for the CNT formation via the tip-growth mechanism, [20] where the catalyst particles are detached from the main 2D assemblies during CNT growth. After completing the reaction, the outer shell of Fe 3 C grains is oxidized to Fe 3 C/FeO x yolk/ shell morphology due to the different diffusion rates of elements.

Conclusion
This work has shed light on the structural change of partially Fesubstituted 2D Fe x Mn 1−x O 2 nanosheets for CNT growth at low temperature. The effects of Fe content, reaction temperature, and reaction time on the efficiency of CNT growth were investigated. The reaction with gaseous carbon precursor induces the formation of Fe 3 C particles from Fe x Mn 1−x O 2 nanosheets, which have an unusually high catalytic activity for the formation of CNT. After the reaction, the Fe 3 C is oxidized to form Fe 3 C/FeO x yolk/shell morphology. Since the catalytic properties for CNT growth are strongly dependent on the structure of the catalyst, Figure 7. HAADF-STEM images from a) 0, b) −60°, and c) +60° tilt angle of a particle in a long CNT. d) Reconstructed volume and e) side-and f) topviewed images. Row A and row B differ in the settings of the contrast to visualize (A) the whole CNT or (B) the yolk/shell particle more clearly.