An alternative process of filling the multiwall nanotubes (MWCNTs) with cobalt metal was developed. Empty core of nanotubes was first filled with CoCl2 by stirring with CoCl2 and alcohol at room temperature for six hours. CoCl2 filling inside MWCNTs was then converted into Co after treating with NaBH4 at room temperature. High resolution transmission electron microscope (HRTEM) studies showed the filling of the CoCl2 and Co inside the nanotubes before and after the treatment. EDX studies show the nonexistence of chlorine after the reduction with NaBH4. Amount of filling was also reduced after the treatment. Paper describes the possible mechanism of filling CoCl2 inside nanotube and its reduction by NaBH4. 1. Introduction Exceptional properties of carbon nanotubes, like high strength, discrete electronic states, and so forth, make them highly suitable for the applications in nanodevices. They are the most appropriate material for using in devices like AFM probes [1], FE transistors [2, 3] display devices [4–6], and so forth. Filling of carbon nanotubes with materials enhances their physical and chemical properties and their potential applications in different areas [7–9]. They show interesting physical and structural properties which are different than their parent materials [7, 10]. Magnetic metal nanoparticles (such as Fe, Co, and Ni) have applications such as high-density magnetic data storage, magnetic separation of biomolecules, and treatment of cancer [11, 12]. However, the poor oxidation resistance of the metal nanoparticles is a great hindrance for their applications. Encapsulation of nanoparticles in carbon nanotubes may be highly useful with the combination of properties of magnetic nanoparticles and carbon nanotubes [13–16]. Ferromagnetic metals filled carbon nanotubes have significant potential in data storage technology [17]. Additionally, the walls of carbon nanotubes provide an effective shelter against oxidation of magnetic nanoparticles and thus ensure a long-term stability of the ferromagnetic core [17]. Various methods are employed for the filling of carbon nanotubes, such as arc discharge [18, 19], high-temperature heat treatment [20], capillary induced [21], ion-beam sputtering [22], and chemical vapor deposition (CVD) [23, 24]. Although arc discharge technique is a better method, low yield is a problem for commercial applications. CVD method is a simple and low cost method and can produce filled carbon nanotubes in large quantities. However, all these methods are the in situ synthesis of metal filled nanotubes. There are very few
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