## 3 Results and Discussion  
To confirm the phases and structures of the as-prepared samples, the corresponding XRD pattern of Fe\({}_{3}\)O\({}_{4}\)/ZnO composites is shown in Figure 2. As for Fe\({}_{3}\)O\({}_{4}\)/ZnO composites, the existence of major diffraction peaks corresponding to the (220), (311), (400), (422), (511), and (440) planes can be observed. These planes can be readily indexed to standard cards of JCPDS No.88-0866, revealing that the crystallinity of Fe\({}_{3}\)O\({}_{4}\) remains unchanged after coating. Six diffraction peaks were assigned to the (100), (101), (102), (110), (103), and (112) planes, which is consistent with ZnO (JCPDS No.36-1451). Therefore, the XRD patterns confirmed the coexistence of Fe\({}_{3}\)O\({}_{4}\) and ZnO. The surface elemental states of Fe\({}_{3}\)O\({}_{4}\)/ZnO nanocomposites were further analyzed by XPS, and the results are presented in Figure 3. From the typical survey spectrum, the existence of Fe, Zn, C and O elements can be found. Figure 1: The coaxial waveguide instrumentation (**a**) and the toroidal–shaped sample (**b**). In Figure 3b, the high-resolution spectrum of Fe is given; two peaks appeared at 710.9 and 724.3 eV, corresponding to the band energies of Fe 2p\({}_{3/2}\) and Fe 2p\({}_{1/2}\), respectively [28]. This indicates the generation of oxide of Fe(II) and Fe(III), which is in good agreement with the literature and is consistent with Fe\({}_{3}\)O\({}_{4}\)[29]. The existence of the Fe element indicates that the shell of ZnO may be in porous condition. Figure 3c displays the high-resolution spectrum of Zn. The peaks at 1021.8 and 1044.8 eV correspond to Zn 2p\({}_{3/2}\) and Zn 2p\({}_{1/2}\), respectively. Hence, the composites are composed of Fe\({}_{3}\)O\({}_{4}\) and ZnO. The scanning electron microscopy (SEM) image of Fe\({}_{3}\)O\({}_{4}\) NPs is shown in Figure 4a. It can be seen that Fe\({}_{3}\)O\({}_{4}\) NPs have a relatively uniform spherical shape, and the Fe\({}_{3}\)O\({}_{4}\) NPs with smooth surfaces have diameters in the range of 250-300 nm. Figure 4b,c shows the as-synthesized Fe\({}_{3}\)O\({}_{4}\)/ZnO nanocomposites under different experimental conditions. Sample A is shown in Figure 4b; it is visible that the products had a disorderly composition comprising a few scattered Fe\({}_{3}\)O\({}_{4}\) NPs and short ZnO nanorods. The presence of disordered nanorods and NPs suggests that ZnO particles failed to generate chemical bonds with the Fe\({}_{3}\)O\({}_{4}\) NPs and grew into short rod shapes alone with the introduction of a larger amount of ammonium hydroxide (3 mL). When the amount of ammonium hydroxide was reduced to 2 mL (sample B) in the mix solution, the product exhibited a spherical shape (Figure 4c), and the diameters were a bit larger than those of the Fe\({}_{3}\)O\({}_{4}\) NPs in Figure 4a. We deduced that Fe\({}_{3}\)O\({}_{4}\) NPs were uniformly covered by the ZnO shells in a spherical shape. The magnetic NPs are utilized as a seed-mediated growth mechanism to grow a layer of ZnO on their surfaces, thus making the surface much rougher than the naked Fe\({}_{3}\)O\({}_{4}\) NPs. The morphology and distribution of the Fe\({}_{3}\)O\({}_{4}\)/ZnO core-shell structured nanocomposites are clearly recognizable from the low-magnification TEM image in Figure 4d. It can be discerned that the nanocomposites are nearly spherical in shape, with a diameter distribution of 280-330 nm, which is consistent with the SEM image in Figure 4c. The high-resolution TEM image of one typical core-shell structured Fe\({}_{3}\)O\({}_{4}\)/ZnO composite is demonstrated in Figure 4e. Figure 3: XPS spectra of Fe\({}_{3}\)O\({}_{4}\)/ZnO nanocomposites: (**a**) survey spectrum; (**b**) Fe 2p binding energy spectrum; and (**c**) Zn 2p binding energy spectrum. Figure 2: XRD patterns of and Fe\({}_{3}\)O\({}_{4}\)/ZnO composites. The distinction between the transparent boundary and the dark core confirms the growth of ZnO on the Fe\({}_{3}\)O\({}_{4}\) NP.