85-1436), which may be due to the lack of oxygen in the tube furnace for prolonged annealing process [24]. The average grain diameters can be estimated by the Scherrer formula. They are 9.1, 15.7, 18.0, and 20.9 nm for the as-synthesized, 2-h annealed, 4-h annealed, and 6-h annealed samples, respectively. It indicates that the grain size grows up with increasing T

A . However, for 8-h annealed sample, the concentration of Fe is too low so that the grain size can hardly be estimated. Figure 1 X-ray diffraction patterns of the as-synthesized and annealed samples. Figure 2 shows the TEM bright field images of the samples before and after annealing. see more In Figure 2a,b, it shows that the as-synthesized sample is one-dimensional sphere-chain-like nanowire. The average diameter of the nanowire is approximately 70 nm, while the length is over 1 μm.

Besides, the TEM image in Figure 2b reveals the contrast between the gray edge and the dark center, suggesting the core-shell structure of the nanowires. The diameter of the core is more than 50 nm, while the thickness of the shell is less than 10 nm. Considering the facts that the metallic Fe is unstable in air and according to the XRD patterns shown in Figure 1, it can be inferred that the shell should be a thin layer of α-Fe2O3. Figure 2c,d shows the images of the nanowires after 4-h annealing. The annealed nanowires are also in core-shell structure with the diameter of core between 50 and 100 nm, which is not very uniform. Compared with the as-synthesized Milciclib cost nanowires, the thickness of the shell is substantially increased after annealing. Moreover, it is interesting to find Liothyronine Sodium that after the 4-h annealing process, some novel fluffy-like phases germinate and grow on the surface of the oxidation layer as shown in Figure 2d. The morphology of the fluffy-like phases obtained here is similar to the urchin-like

α-Fe2O3 reported in the literature [24], which were prepared via the oxidation of Fe spheres in air at the temperatures between 250°C and 400°C. It should be noticed that since the nanowires are oxidized in air and they are only composed of Fe and α-Fe2O3 phases as XRD patterns shown, we can infer that the fluffy-like phase here is the α-Fe2O3. Figure 2 TEM bright field images of Fe@Fe 2 O 3 core-shell nanowires. Panels (a) and (b) indicate the as-synthesized sample. Panels (c) and (d) indicate the 4-h annealed sample. Figure 3 shows the hysteresis loops (MH) of the as-synthesized samples measured at 5 and 300 K. The 5 K Oligomycin A saturation magnetization (M s ) is approximately 116 emu/g, which is lower than that of the bulk Fe (218 emu/g) [25]. The decrease of M s may be due to the existence of the AFM α-Fe2O3 at the surface of the nanowire as shown in the TEM image in Figure 2. It may also be caused by the defects and disorders in the nanostructure [26].