Synthesis and Characterization of Magnetic Metal-encapsulated Multi-walled Carbon Nanobeads
© to the authors 2008
Received: 28 August 2007
Accepted: 9 January 2008
Published: 26 January 2008
A novel, cost-effective, easy and single-step process for the synthesis of large quantities of magnetic metal-encapsulated multi-walled carbon nanobeads (MWNB) and multi-walled carbon nanotubes (MWNT) using catalytic chemical vapour deposition of methane over Mischmetal-based AB3alloy hydride catalyst is presented. The growth mechanism of metal-encapsulated MWNB and MWNT has been discussed based on the catalytically controlled root-growth mode. These carbon nanostructures have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM and HRTEM), energy dispersive analysis of X-ray (EDAX) and thermogravimetric analysis (TGA). Magnetic properties of metal-filled nanobeads have been studied using PAR vibrating sample magnetometer up to a magnetic field of 10 kOe, and the results have been compared with those of metal-filled MWNT.
Carbon nanotubes (CNTs) [1–3], including single-walled and multi-walled carbon nanotubes (SWNT and MWNT), have attracted tremendous interest both from fundamental and technological perspectives due to their unique physical and chemical properties . They promise a wide range of practical applications such as catalyst supports in heterogeneous catalysis, electronic devices, field emitters, sensors, gas-storage media and molecular wires for next-generation electronic devices. Further, MWNT filled with magnetic materials have attracted a great attention due to their fundamental interest and potential applications. Electrical, thermal and mechanical properties of CNTs as well as magnetic properties of metals may be altered significantly with the introduction of ferromagnetic metals into CNTs. CNTs filled with Ni nanowires have been produced by CVD using LaNi2catalysts . MWNT filled with Co and Fe have been obtained by a three-step process and their magnetic properties have been studied [6–9]. In these new types of metal-encapsulated nanostructures, the carbon shells provide an effective barrier against oxidation of metals. Therefore, these materials can be used in high-temperature magnetic applications apart from their use as electronic devices, catalysts, magnetic storage devices and sensors [10–12]. On the other hand, carbon nanotubes today represent a class of emerging materials that are capable of intracellular delivery of biologically functional peptides, proteins, nucleic acids and small molecules covalently or non-covalently attached on their surface [13–15]. The application of functionalized CNT as a new method for drug delivery was apparent immediately after the first demonstration of the capacity of this material to penetrate into cells. It is important that defects should be produced on the surface of the CNTs for the attachment of various functional materials, and therefore nanotubes are subjected to various treatments which create the defects in nanotube structure. On the other hand it may be possible to facilitate the targeted delivery of drugs in the lymphatic tissue more effectively by using a magnetic carbon nanotube . Hence a defective carbon nanotube with magnetic nature would be a proper candidate for biological applications. Metal-encapsulated carbon nanobeads, a new kind of carbon nanotubes having both defects (by its structure) and magnetic nature (due to the encapsulated metal), can be considered as one of the promising materials for drug delivery and other biological applications.
Many research groups have concentrated on the synthesis of SWNT, MWNT and metal-filled MWNT, addressing various key issues such as scale up, reproducibility and low cost. Several methods of filling of CNTs by foreign metal nanowires have been reported using capillary action, wet chemical method, arc-discharge technique, catalysed hydrocarbon pyrolysis and condensed phase electrolysis [16–19]. But many of these techniques suffer from various drawbacks such as low yield and controlled shape and size of metal-filled CNTs . Hence it is important to develop a new process which will provide easy and large-scale production of CNTs and metal-filled CNTs. We have already developed a single-step technique for the synthesis of single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and magnetic metal-filled MWNTs by fixed bed thermal chemical vapour deposition technique [21, 22]. In this paper we present a novel, cost-effective, easy and single-step process for the synthesis of metal-encapsulated multi-walled carbon nanobeads (MWNB) in large quantity which have these defects present already. Catalytic chemical vapour deposition (CCVD) technique using a single-reaction zone facility has been used to grow these nanostructures in the temperature range of 850–950 °C using Mischmetal (Bharat Rare Earths Metals, India; composition—Ce 50%, La 35%, Pr 8%, Nd 5%, Fe 0.5% and other rare earth elements 1.5%)-based AB3 (B = Ni/Fe/Co) alloy hydride catalyst, obtained through a hydrogen-decrepitation technique. The as-grown and purified samples have been characterized using TGA, SEM, TEM, HRTEM and EDAX. Magnetic properties of metal-filled MWNB have been studied using a PAR vibrating sample magnetometer up to a magnetic field of 10 kOe, and the results have been discussed by comparing with those of metal-filled MWNT. In addition, using the catalytically controlled root-growth mode, the growth mechanism of metal-encapsulated MWNB and MWNT has been discussed.
Mm-based AB3(B = Ni/Fe/Co) alloys were prepared by arc melting the constituent elements in a stoichiometric ratio under argon atmosphere. The alloy buttons were re-melted six times by turning them upside down after each solidification to ensure homogeneity. Single-phase formation of alloys was confirmed by powder X-ray diffraction. Each of these alloys was then hydrogenated to their maximum storage capacity of about 1.5 wt% using a high-pressure Seivert’s apparatus. Fine powders of alloys, with fresh surfaces, were obtained with several cycles of hydrogen absorption and desorption.
Synthesis, Characterization and Magnetization Measurements of Metal-Encapsulated MWNB
The growth of carbon nanostructures has been carried out using a single-stage furnace with precisely controlled temperatures in the range 850–950 °C. Fine powders of alloy obtained after several cycles of hydrogen absorption/desorption were directly placed in a quartz boat and kept at the centre of a quartz tube, which was placed inside a tubular furnace. Hydrogen (50 sccm) was introduced into the quartz tube for 1 h at 500 °C, to remove the presence of any oxygen on the surface of the alloy hydride catalysts. Hydrogen flow was then stopped, and the furnace was heated up to the desired growth temperature followed by the introduction of methane at a flow rate of 100 sccm. Pyrolysis was carried out for 30 min and thereafter furnace was cooled to room temperature. Argon flow was maintained throughout the experiment. The carbon soot obtained in the quartz boat was purified by air oxidation and acid treatment  and was analysed by SEM, TEM, HRTEM, EDAX and TGA. Magnetization measurements of metal-encapsulated MWNB and MWNT were carried out using a PAR vibrating sample magnetometer at 30 °C up to a magnetic field of 10 kOe.
Results and Discussion
Mm-based AB3alloys, after several hydrogen absorption/desorption cycles, were found to be finely powdered to about 5–10 μm due to the plastic deformation of these alloys upon hydrogenation/dehydrogenation cycles. These hydride catalysts prepared using the hydrogen decrepitation technique provide fresh surfaces with a large surface area, free from oxidation for the growth of CNTs. High hydrogen absorption, large decrepitation and low cost make these hydrides better catalysts for large-scale production of CNTs. Experiments have been carried out using Mm-based AB3alloy hydride catalyst with Ni at the B site at growth temperatures of 850 and 950 °C, keeping all other parameters same. Interestingly, different types of carbon nanostructures have been observed at these two different growth temperatures. At 850 °C, Ni-encapsulated MWNB were obtained, while at 950 °C, Ni-encapsulated MWNT could be observed. These carbon nanostructures have been characterized using SEM, TEM and HRTEM.
A root-growth mechanism  could be responsible for the growth of metal-filled MWNB and MWNT. In the present study, as the size of the alloy hydride catalyst particles is seen to be in the range of 5–10 μm, we propose that each alloy hydride particle would be composed of a number of catalytic centres, which could act as nucleation sites for the growth of CNTs. There could be a further reduction in the catalyst particle size during the hydrogen treatment before the carbon deposition. Further, nickel, iron or cobalt particles are well interspersed in the alloy, allowing better dispersion of the active catalytic sites. This would further result in lesser sintering of the particles. Here, the possible growth mechanism could be through the precipitation of carbon in the form of MWNT from the molten catalytic particles. The melting temperatures of the alloy-C system are lower than those of the metal-C system. Further, the reduction in particle size lowers the melting point, which in turn affects the diffusion rate of carbon and thus changes the growth rate of CNTs [25, 26]. According to two widely accepted ‘tip-growth’ and ‘root-growth’ mechanisms, the hydrocarbon gas decomposes on the metal surfaces of the metal particle to release carbon, which dissolves in these metal particles. The dissolved carbon diffuses through the particle and gets precipitated to form the body of the filament. The saturated metal carbides have lower melting points. Hence they are fluid-like during the growth process resulting in their easy encapsulation due to the capillary action of the nanotube process. The encapsulated fluid results in solid metal nanowire. Thus the growth process is by the vapour–liquid–solid (VLS) mechanism, which is catalytically controlled with the capillary action of nanotubes. Figures 1 and 2 show encapsulated MWNB and MWNT, grown at 850 and 950 °C, respectively, over Mm-based AB3 alloy hydride catalysts containing Ni at the B site. These structures have the uniform and complete encapsulation of Ni in the form of nanobeads and nanowires.
A single-step process for the synthesis of good quality and large quantities of metal-encapsulated MWNB and MWNT by a thermal CVD technique using alloy hydride catalysts has been developed. The growth mechanism of metal-encapsulated MWNB and MWNT has been discussed based on the catalytically controlled root-growth mode. Saturation magnetization of Ni-encapsulated MWNB shows lower value compared to that of Ni-encapsulated MWNT due to the discontinuous filling of Ni during the growth process.
We thank IITM and DST, Govt. of India, for the support of this work.
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