The Assemblies of NiC and NiO Particle Systems

Powder X-ray diffraction profiles of Fig. 1A clearly show the presence of a majority of fcc Ni, with a small amount of hexagonal Ni3C present from carbon-encapsulated Ni nanoparticles. Additionally, that the hexagonal Ni3C phase is detectable using XRD suggests that the hexagonal Ni3C most likely is one single phase along with majority of fcc Ni phase in these assemblies of Ni(C) nanoparticles. However, the main reflection peaks of XRD (Fig. 1B) for pure Ni nanoparticles coated with NiO layers are similar to those of the pure fcc Ni phase, except that the peaks are broader. One can infer from such a XRD study that the concentration of NiO oxide on the surface of Ni(O) nanoparticles should be less than a few percent. This is quite similar to the results reported before.[13] The average grain size estimated from the XRD

Nanoparticles Pxrd

Fig. 2 (A) HRTEM image and corresponding SAED patterns of Ni(C) nanoparticles. Note that arrows indicate the carbon layers; G symbol is carbon phase; C symbol is Ni3C phase. (B) HRTEM image and corresponding SEAD patterns of Ni(O) nanoparticles. The arrows indicate NiO oxide layers. Source: From Ref.[24].

Fig. 2 (A) HRTEM image and corresponding SAED patterns of Ni(C) nanoparticles. Note that arrows indicate the carbon layers; G symbol is carbon phase; C symbol is Ni3C phase. (B) HRTEM image and corresponding SEAD patterns of Ni(O) nanoparticles. The arrows indicate NiO oxide layers. Source: From Ref.[24].

patterns using the Scherrer formula shows that these two kinds of Ni nanoparticles have an average grain size of 10-15 nm, which is consistent with the corresponding HRTEM images (Fig. 2).

High-resolution transmission electron microscopy images and SAED patterns (Fig. 2A and B) also indicate that most of particles are spherical in shape and do not show any well-developed facets. It is worth noting from Fig. 2A that several carbon layers encapsulating the Ni lattice planes are quite visible. The contiguous carbon fringes around the Ni nanocrystal are good evidence for completed encapsulation by carbon layers. Both fcc-Ni (111) and hexagonal Ni3C (101) phases are clearly observed from SAED patterns of Ni(C) particles. In addition, the (002) reflection is also present from those encapsulated carbon layers. However, only fcc-Ni and NiO phases are verified from SAED patterns of Ni(O) particles (Fig. 2B). Nano-energy dispersive spectroscopy (EDS) analysis can further confirm the presence of Ni, O, and carbon phases in these two-kind Ni particles. Those results are in good agreement with the XRD analysis and HRTEM observations.

One significant morphological characteristic to note, from the HRTEM images (Fig. 2A), is that neither gaps nor intermediate phases are observed between the outer carbon layer and the core materials in the Ni(C) carbon cages. These characteristics typically reflect a growth history of these nanoencapsu-lated Ni particles, in which carbon atoms dissolve into a molten or solid carbon-metal alloy and then graphite precipitate at the surface during carbon-arc discharge plasma process.[19] However, Fig. 2B shows some polycrystalline discontinuities in the layers of the Ni(O) nanoparticles.

Fig. 3 shows the magnetization vs. field plots (M vs. H hysteresis loops) at 300 K (Fig. 3A) and 2 K (Fig. 3B) for the assemblies of Ni(C) nanoparticles. Figure 3A indicates a rapid increase with increasing applied magnetic field without saturation because of the superpar-amagnetic relaxation and the noncollinear moment of the surface spins in the smaller particle assembly.[20] Hysteresis is absent with a little remanence and coer-civity (HC), which suggests the presence of a long-range magnetic dipole-dipole interaction among the assemblies of superparamagnetic Ni(C) particles. With decreasing temperature, the magnetization of the samples increases and exhibits a symmetric hysteresis loop under both ZFC and FC at 2K, indicating a transition from superparamagnetic to ferromagnetic behavior. Specifically, the temperature dependence of

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