## 3 Results
### Morphology and Structure
Besides, the XPS analysis also provides an avenue to better understand the chemical valence state of Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\) nanocomposites. For Si-modified GO, there have three elements corresponding to the C, O, and Si elements in the wide span spectra of Si-modified GO (Figure S3a, Supplementary Materials). The four peaks at 284.4 eV, 285.2 eV, 287.2 eV, and 289.3 eV are attributed to  
Figure 3: XPS spectra of Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\): High-resolution C1s spectra of (**a**) Si(1)-rGO@Fe\({}_{3}\)O\({}_{4}\), (**b**)Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\), (**c**) high resolution Si2p spectrum of Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\), and (**d**) Fe2p spectrum of Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\). (**e**) Raman spectra of all the samples. Figure 2: FT-IR spectra (**a**) and XRD patterns (**b**) of Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\) nanocomposites. the C-C, C-OH, C=O, and C(O)O bonds of GO, respectively (Figure S3b, Supplementary Materials). After modification with silane groups, the intensity of C-C peak of Si(1)-GO obviously increases, whereas its C-OH (285.6 eV) and C=O (287.3 eV) peaks decrease remarkably (Figure S3c, Supplementary Materials). Compared with Si(1)-GO, the intensity of C=O peak of Si(2)-GO is greater than that of Si(1)-GO (Figure S3d, Supplementary Materials), suggesting that the Si(1) is more prone to coupling reaction with -COOH groups in GO. From the Si 2p spectrum of Si(2)-GO (Figure S3e, Supplementary Materials), the peaks at 102.6 eV and 103.5 eV stand for the Si-O-C and Si-O-Si bond, respectively, verifying that most Si(2) reagents have been grafted on the surface of GO and only a part of Si-O-Si groups were obtained by the self-condensation of Si(2) silane coupling reagent [36]. After hydrothermal reduction, there are several oxygen-containing groups still existed in the Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\) composite (Figure 3a,b). Notably, the intensity of C-OH peak of Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\) is less than that of Si(1)-rGO@Fe\({}_{3}\)O\({}_{4}\). This result reveals that the Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\) possesses higher reduction degree, which is beneficial for improving the imaginary permittivity of the microwave absorption materials [15]. As shown in Figure 3c, the Si 2p spectrum of Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\) exhibits the presence of residual -Si-O-C bonds and -Si-O-Si bonds, confirming that several Si(2) silane groups have been existed in Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\) composite. It is well known that the lattice defects in the graphene structure are beneficial for enhancing the EMA performance [19]. Here, the influence of various silane groups on rGO defects was investigated by Raman spectroscopy (Figure 3e). Generally, the D-band (1330 cm\({}^{-1}\)) and G-band (1595 cm\({}^{-1}\)) represent the lattice defects and the radical C-C stretching vibration of carbon sp\({}^{2}\) hybrid in the graphene structure, respectively. The graphitization degree is typically determined by the intensity ratio \(I_{D}\)/\(I_{G}\)[42]. The higher the \(I_{D}\)/\(I_{G}\) value, the greater the lattice defects in graphene structure. As revealed in Figure 3e, the ratio \(I_{D}\)/\(I_{G}\) of rGO@Fe\({}_{3}\)O\({}_{4}\) (1.25) is greater than that of rGO (1.11) due to the introduction of the Fe\({}_{3}\)O\({}_{4}\) NPs. Furthermore, the \(I_{D}\)/\(I_{G}\) values of Si(1)-rGO@Fe\({}_{3}\)O\({}_{4}\) and Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\) are 1.47 and 1.62, respectively, implying that the silane group Si(2) grafted on the rGO surface contributes to generating abundant lattice defects in graphene structure.