Ambient spark generation to synthesize carbon-encapsulated metal nanoparticles in continuous aerosol manner Jeong Hoon Byeon, a Jae Hong Park, b Ki Young Yoon b and Jungho Hwang * bc Received 6th May 2009, Accepted 13th October 2009 First published as an Advance Article on the web 3rd November 2009 DOI: 10.1039/b9nr00058e We report the use of spark generation in an inert gas atmosphere to synthesize carbon-encapsulated metal nanoparticles (CEMNs) in a continuous aerosol manner using a metal (nickel, cobalt, iron)– graphite carbon electrode configuration without the use of a vacuum. The spark-generated particles consisted of CEMNs and carbona- ceous aggregated debris. The outer layer of the CEMNs showed parallel fringes (ordered graphitic nanostructures) while the debris consisted of disordered nanostructures. Electron and X-ray diffraction showed that both metal and graphite in the CEMNs were the pure elements except for iron–carbon, which contained a carbide phase. Based on the order of the activation energies for carbon diffusion into a metal: iron–carbon (10.5–16.5 kcal mol 1 ) < cobalt– carbon (34.7 kcal mol 1 )  nickel–carbon (33.0–34.8 kcal mol 1 ), it was concluded that carbide particles form more easily from elemental iron than nickel or cobalt. The metal-to-carbon mass fractions of the spark-generated particles from nickel (anode)– carbon (cathode), cobalt–carbon, and iron–carbon spark configu- rations were 18.7, 28.3, and 11.2%, respectively, while the mass fractions for the configurations of metal (cathode)–carbon (anode) were 6.4, 9.1, and 4.3%, respectively. Similarly, the yield of CEMNs from the metal (anode)–carbon (cathode) electrodes was higher (54, 61, and 53%) than that of metal (cathode)–carbon (anode) elec- trodes (18, 30, and 18%). 1. Introduction Nanocrystalline metal particles are prone to rapid environmental degradation on account of their high surface-area-to-volume ratio and reactivity. This limits their practical applications, even making scientific evaluations of the nanocrystalline properties difficult. 1–4 The encapsulation of nanocrystalline metal particles with a chemically stable species, such as graphite, has been a recent breakthrough in this regard. 1,2,5–9 Carbon-encapsulated metal nanoparticles (CEMNs) may have applications in ferrofluids, sensor devices, hydrogen storage, xerography, micro-machinery, and recording media, as well as a variety of biomedical applications. 2,10–18 Among the many types of CEMNs with various core materials, those with iron group metals (nickel, cobalt, and iron) are of particular interest, not only because of their ferromagnetic properties, but also because these metals have a unique catalyzing ability to transform carbon into graphite. 5,11,19–26 The arc-synthesis of CEMNs, which involves the evaporation of a metal-inserted graphite anode in an inert atmosphere, requires high power, exceeding 1 kW, and expensive vacuum systems to generate the plasma. 2–7,9,10,12–14,19,23,24,27,28 Moreover, the arc method produces unwanted byproducts due to the harsh synthesis conditions arising from the high energy generated during the process. Synthesis tech- nologies of CEMNs including the arc method are not mature, and require further research and optimization. 18 Spark generation has been used to generate monometallic and bimetallic particles of a variety conducting materials with particle sizes ranging from several nanometers to 100 nm in the aerosol state at ambient temperatures and pressures. 29,30 These metallic aerosol nanoparticles have been used as initiators for the electroless deposition of metals. 31–35 This paper reports the feasibility of spark generation in an inert gas atmosphere to synthesize CEMNs in a continuous aerosol manner using a metal (nickel, cobalt, iron)–graphite carbon electrode configuration without the use of a vacuum. The morphology and structure of the synthesized particles were examined by transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Raman spectroscopy. 2. Experimental In the spark generation apparatus, a spark was generated between a metal (nickel, cobalt, or iron) and a graphite carbon rod (each; 3 mm diameter, 100 mm length, Nilaco, Japan) in a chamber under a pure nitrogen (<10 4 impurities) atmosphere at standard temper- ature and pressure (STP). The following conditions were used to generate the spark: a current of 2 mA; a voltage drop of 2.4 kV with a frequency of 667 Hz between the electrodes, which was kept stable by continuously translating the carbon rod to the metal rod to maintain a constant distance of 1 mm. When a spark was generated between the metal and carbon rods inside a reactor, the gas temperature inside the spark channel was increased beyond a critical value, which was sufficient to sublimate parts of the electrodes. 29 The duration of each spark was very short (1.5 ms) and the vapors cooled rapidly downstream of the spark. This formed resulted in supersaturation and particle formation through nucleation/conden- sation. A nitrogen gas flow (0.5 L min 1 ) carried the spark-generated particles as they exited the spark chamber. The chamber was cleaned periodically with compressed dry particle-free air to eliminate any residual particles. The non-purified spark-generated particles were analyzed using a variety of methods. A TEM (JEM-3010, JEOL, Japan) was oper- ated at 300 kV with an EDX attachment (Oxford). SAED patterns of a LCD Division, Samsung Electronics Co., Ltd., Yongin 446-711, Republic of Korea b School of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea. E-mail: hwangjh@yonsei.ac.kr; Fax: +82 2 312 2821; Tel: +82 2 2123 2821 c Yonsei Center for Clean Technology, Yonsei University, Seoul 120-749, Republic of Korea This journal is ª The Royal Society of Chemistry 2009 Nanoscale, 2009, 1, 339–343 | 339 COMMUNICATION www.rsc.org/nanoscale | Nanoscale