Low-temperature CVD synthesis of carbon-encapsulated magnetic Ni nanoparticles with a narrow distribution of diameters

Abstract

Carbon-encapsulated magnetic nanoparticles (CEMNPs) have received considerable attention in recent years due to their expectant applications, including high density magnetic data storage, magnetofluid, magnetic resonance imaging and biomedical techniques. Enclosure of nanometer-sized magnetic particles in carbon capsules is of particular interest since it is beneficial for protection against hostile chemical environments and for avoiding low proximity phenomena. To date, many research works have been concentrated on the synthesis of CEMNPs [1–3]. However, in these works byproducts such as carbon nanotubes/ nanofibers, carbon nanoparticles and free amorphous carbon formed or the diameter distribution of CEMNPs was uneven as well as the CEMNPs were agglomerated by the coalescence of the outer shells, all these disadvantages would limit their potential applications. Herein, we report a different process for synthesizing carbon-encapsulated Ni nanoparticles with a very narrow distribution of diameters and a considerably less amount of byproducts. In the present work, CEMNPs have been formed by CVD over Ni/Al catalyst at 550 C. The preparation processes of the Ni/Al catalyst were described in our previous paper [4]. To encapsulate these nickel particles, an amount of Ni/Al catalyst with a weight ratio of 3:1 was applied as catalyst to decompose methane (60 ml/min) with hydrogen as carrier gas (600 ml/min) at 550 C for 1 h, which is based on the CNT synthesis. These nickel nanoparticles were thus fully encapsulated by graphene layers. The product was adsorbed by magnet and then immersed in NaOH (1 mol/L) solution for 6 h at 60 C to obtain pure CEMNPs. Transmission electron microscopy (TEM) (Philips Technai G F20) was used to characterize the Ni particle distribution, the CEMNPs just produced, and the CEMNPs exposed to the air for 6 months. The magnetic measurements were conducted using a Princeton vibrating sample magnetometer (VSM) at room temperature. The distribution of the catalytic Ni particles, which were obtained by the reduction of the NiO/Al precursor in H2 atmosphere for 2 h [4], is shown in Fig. 1a. It can be seen that uniform and small nickel particles are dispersed. The size of the nickel particles ranges between 4 and 16 nm. According to HRTEM (Fig. 1b), most nickel nanoparticles present quasi-sphere structures. Fig. 2a–c show the TEM images of the CEMNPs just produced. We can see that the sample consists of well-dispersed and homogeneous CEMNPs. The diameters of the CEMNPs range from 5 to 18 nm and the diameter distribution statistics derived by HRTEM analysis further reveals that the CEMNPs just produced have a mean diameter of 15 nm with a standard deviation of 3 nm, as displayed in Fig. 2e. Furthermore, the contrast of the TEM image of individual particles clearly shows that the CEMNP has a core–shell structure, indicating the encapsulating of the metal particles by carbon. The thickness of the coating shells is about 2–5 nm, but varies from one particle to another. Except for the CEMNPs, carbon nanotubes or fibers and carbon nanoparticles without encapsulation of nickel