Formation of carbon capsules from an amorphous carbon film by Ga and Ni/Co catalysts in a transmission electron microsco
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Chuan-Pu Liua) Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701; and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan 701
C.B. Boothroyd Institute of Materials Research and Engineering, 3 Research Link, Singapore, 117602 (Received 28 June 2008; accepted 24 November 2008)
Direct conversion of an amorphous carbon (C) film to capsules by gallium (Ga), and nickel and cobalt (NiCo) alloy particles upon heating is investigated in situ by transmission electron microscopy (TEM). Capsules are catalyzed in an NH3 atmosphere when the temperature is raised to 1050 C. High resolution TEM reveals that graphene flakes initially nucleate at the surface of the catalysts, then segregate and transform into faceted multi-shell capsules upon continued heating. The solubility of carbon in the NiCo alloy particles can be differentiated from the solubility of carbon in Ga particles by the thickness of the walls. The C/Ga binary phase in nanoparticles is discussed regarding the formation of thin-walled carbon capsules.
I. INTRODUCTION
Carbon nanocapsules are zero dimensional structures consisting of carbon shells wrapping solid cores. When the shells wrap transition metal crystals of nanometer size, the core-shell structures offer promising potential in high-density data storage1 and in vitro applications.2 One of the advantages of carbon shells is that they protect core crystals, typically iron-group metals, from oxidizing and etching. Carbon capsules have been synthesized by chemical vapor deposition (CVD),2 arc discharge,3,4 and radio frequency plasma torch.5 The carbon products synthesized by arc discharge include not only nanocapsules, but also nanotubes,6,7 nanoparticles,8,9 and nanocages.10 Many metals have been used as catalysts when using arc discharge and laser vaporization to grow core-shell structures, including transition metals of Sc,11 Y,12 Fe,13 and the lanthanide metal La.14 Seraphin et al.15 systematically categorized elements into the four groups commonly used for manufacturing core-shell nanostructures by arc discharge. In the first group, the core is in the form of carbides and consists of B, V, Mn, Y, Zr, Nb, Mo, and La. In the second group, the core, which cannot be encapsulated but tolerates the formation of carbon cages, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0166
1388
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 4, Apr 2009 Downloaded: 26 Jan 2015
consists of Cu, Zn, Pd, Ag, and Pt. In the third group, the core has a strong potential that forms stable carbides and competes with the formation of cages, and consists of Al, Si, Ti, Sn, and W. The last group consists of the iron group transition metals, Fe, Co, and Ni, which can stimulate the formation of filled and/or unfilled tubules, beads, and cages. In addition to arc discharge, carbon nanocapsules have been synthesized by using copper phthalocyanine (CuPc) as the precursor in the CVD.1
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