## 3 Results and Discussion  
### Characterization  
Figure 1a shows the SEM micrograph of the as-grown SWCNTs. The image reveals that the SWCNTs, synthesized with the aid of an iron catalyst, possess a loose arrangement. The bundles of CNTs are observed to be entangled with each other, and they are coated with large amounts of amorphous carbon. The SEM image at a high magnification (Figure 1b) more clearly reveals the prepared SWCNT bundle loaded with iron nanoparticles. From the energy dispersive spectra (EDS) finding (Figure 1c), it can be seen that the region comprised 51.79 wt% C element, 6.41 wt% O element, and 41.62 wt% Fe element, indicating that the iron nanoparticles were successfully attached to the surface of the CNTs. Figure 2a displays the XRD spectra of the as-grown SWCNTs and the air-heat-treated SWCNTs. It shows that the diffraction peaks of C in the as-grown SWCNTs diminish after the air heat treatment, indicating the effective removal of amorphous carbon from the composite, thereby enhancing its adsorption capacity. Simultaneously, the Fe phase is transformed into the Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{4}\) phases after heat treatment. Figure 2b depicts the Raman spectra obtained under 633 nm laser excitation for the as-grown SWCNTs and SWCNTs/Fe\({}_{3}\)O\({}_{y}\). Within the range of 100-400 cm\({}^{-1}\), resonant breathing mode (RBM) peaks of the SWCNTs are evident, allowing for the calculation of the diameter of the SWCNTs using the frequency (\(\omega_{\rm RBM}\)) of the RBM peak. For the SWCNTs, the calculation formula is \(\omega_{\rm RBM}\) (cm\({}^{-1}\)) = 234/d (nm) + 10, and the diameter of the SWCNTs in the original sample ranges between 0.89 and 1.78 nm. The weak Raman peak at 1350 cm\({}^{-1}\) is the D peak, which arises from defects like pentagon and heptagon rings within the CNTs, reflecting the defect density directly. The prominent Raman peak near 1590 cm\({}^{-1}\) is the G peak, which is associated with in-plane tangential vibrations of carbon atoms. The intensity ratio of the D peak to the G peak (I\({}_{\rm D}\)/I\({}_{\rm G}\)) is typically utilized to characterize the defect content in SWCNTs; a lower ratio signals fewer defects and higher crystallinity. The I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio of the SWCNTs/Fe\({}_{\rm x}\)O\({}_{y}\) and as-grown SWCNTs was 0.06 and 0.2196, respectively. The findings demonstrate that the crystallinity of the sample was enhanced subsequent to the heat treatment and the D peak was significantly decreased after the air heat treatment. Figure 3 displays the XPS spectra of SWCNTs/Fe\({}_{\rm x}\)O\({}_{y}\) (Figure 3a-c) and as-grown SWCNTs (Figure 3d-f), including the regions for Fe 2p, O 1s, and C 1s. The analysis shows that the main valence state of iron in the as-grown SWCNTs prepared by the arc discharge method exists in the form of Fe\({}^{0}\), although a small amount of Fe\({}_{3}\)C phase is formed during the arc evaporation process. After air oxidation treatment, the iron valence states in SWCNTs/Fe\({}_{\rm x}\)O\({}_{y}\) are mainly Fe\({}^{2+}\) and Fe\({}^{3+}\), while a small amount of Fe\({}^{0}\) phase still exists, which may be due to the incomplete oxidation of Fe nanoparticles in the internal  
Figure 1: Scanning electron micrographs of (**a**) low-magnification as-grown SWCNTs and (**b**) high-magnification as-grown SWCNTs; (**c**) EDS image. region. It is worth noting that although the XRD analysis failed to detect the Fe\({}^{0}\) signal, this may be due to its low content, which is below the detection limit of X-rays.