## 3 Results  
### Morphology and Structure  
The SEM and TEM images of as-prepared samples are represented in Figure 1 and Figure S1 (Supplementary Materials). When the rGO was decorated by silane coupling reagents, there were more Fe\({}_{3}\)O\({}_{4}\) nanoparticles (NPs) loaded on the surface of Si-modified rGO (Figure 1a,d). The reason is that the existence of -SiOH groups derives from the hydrolysis of -SiOCH\({}_{3}\) group within the silane coupling reagent [36], which are preferable to coordinate with Fe\({}^{3+}\). Then the Fe\({}_{3}\)O\({}_{4}\) NPs were prepared by in situ reduction. Meanwhile, the spherical Fe\({}_{3}\)O\({}_{4}\) NPs on the Si-modified rGO sheets are featured with rough surface and non-uniform diameters (Figure 1a,b,d,e). As demonstrated in Figure S1 (Supplementary Materials), the composite Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\) still possesses abundant wrinkles as same as that of rGO@Fe\({}_{3}\)O\({}_{4}\) composite, which will benefit for improving the multiple-reflection of EMW [37; 38; 39]. Furthermore, the SEM images of the Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\)/PVDF-_co_-HFP films illustrate that the Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\) are well dispersive in PVDF-_co_-HFP (Figure 1c,f). The analysis of FT-IR and XRD is first devoted to investigating the composition of the Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\) nanocomposites. In most cases, the bands at 1722 cm\({}^{-1}\), 1617 cm\({}^{-1}\), 1223 cm\({}^{-1}\), 1052 cm\({}^{-1}\), and 3356 cm\({}^{-1}\) are attributed to C=O, C=C, C-O-C, C-OH, and -OH groups in the pure GO, respectively (Figure S2, Supplementary Materials). After modification with silane groups, the stretching vibration of -OH groups decrease obviously and the new peaks at 2918 cm\({}^{-1}\) and 2840 cm\({}^{-1}\) are observed, corresponding to the vibration of -CH\({}_{2}\), -CH\({}_{3}\) bond in silane groups. The peaks at about 1037 cm\({}^{-1}\) is connected to the Si-O-C (Si-O-Si) bonds (Figure S2, Supplementary Materials), verifying the successful synthesis of Si-modified GO. After in situ hydrothermal reduction, the characteristic peak at 1543 cm\({}^{-1}\) belongs to the vibration of C=C bond, suggesting that the Si-modified GO has been successfully  
Figure 1: SEM images of (**a,b**) Si(1)-rGO@Fe\({}_{3}\)O\({}_{4}\), (**c**) the film of S1 sample, (**d,e**) Si(2)-rGO@Fe\({}_{3}\)O\({}_{4}\), (**f**) the film of S2 sample. reduced to the Si-modified rGO (Figure 2a). Moreover, a new peak at 556 cm\({}^{-1}\) is characterized to the stretching vibration of -Fe-O bond in Fe\({}_{3}\)O\({}_{4}\) NPs (Figure 2a). In order to investigate the crystalline structures of as-achieved Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\), the XRD patterns of all the samples are shown in Figure 2b. The broad peak at 2\(\theta\) = 21.3\({}^{\circ}\) is attributed to the (002) facet of rGO [34]. After various silane groups are grafted on the surface of rGO, the diffraction peaks shift to a higher degree around 2\(\theta\) = 25.1\({}^{\circ}\) [Si(1)] and 22.5\({}^{\circ}\) [Si(2)], suggesting that the insertion of the silane group into GO can result in shrinking the \(d\)-spacing of the rGO sheet according to the Bragg equation [34]. At the same time, the seven obvious peaks at 18.1\({}^{\circ}\), 30.1\({}^{\circ}\), 35.4\({}^{\circ}\), 43.1\({}^{\circ}\), 53.5\({}^{\circ}\), 56.8\({}^{\circ}\), and 62.5\({}^{\circ}\) are assigned to (111), (220), (311), (400), (422), (511), and (440) planes of Fe\({}_{3}\)O\({}_{4}\) NPs (JCPDS file no.19-0629), verifying that the composites contain the phase of Fe\({}_{3}\)O\({}_{4}\) NPs. To further confirm whether the Fe\({}_{3}\)O\({}_{4}\) NPs is the unique magnetic phase in Si-modified rGO@Fe\({}_{3}\)O\({}_{4}\) nanocomposites, XPS analysis of Fe 2p spectrum is illustrated in Figure 3d. Remarkably, two peaks at 711.5 eV and 725.1 eV is characterized to the Fe 2p 1/2 and Fe 2p 3/2, respectively, and no satellite peak at 719.0 eV indicates no \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) phase in composites [40; 41].