Filling the carbon nanocages

Twenty elements were codeposited with carbon in an arc discharge between graphite electrodes. The majority of them were evaporated from composite anodes that contained the elements or their oxides stuffed into central bores in the graphite rods. The deposits, found in the soot at the reactor walls or as slag at the cathode, were characterized using scanning and transmission electron microscopy, electron energy loss spectroscopy, and x-ray diffraction. The products fall into four categories: (1) elements that can be encapsulated in the form of their carbides (B, V, Cr, Mn, Y, Zr, Nb, Mo); (2) elements that are not encapsulated but tolerate the formation of graphitic carbon cages (Cu, Zn, Pd, Ag, Pt); (3) elements that form stable carbides, competing with and pre-empting the carbon supply for the graphitic cage formation (Al, Si, Ti, W); and (4) the iron-group metals (Fe, Co, Ni) that stimulate the formation of single-walled tubes and strings of nanobeads in the conventional arc discharge condition, and produce the nanometer-size carbon-coated ferromagnetic particles in a modified arc discharge in which metals are in molten form in graphite crucible anodes exposed to a helium jet stream. The criterion determining the formation according to one of the four categories is discussed on the basis of this extended study. It is apparent that the physical properties such as vapor pressure, melting and boiling points, the completeness of the electronic shells of the elements, or their heat of carbide formation are not sufficient to explain the selectivity of the encapsulation without exceptions. A hypothesis is advanced that emphasizes the existence of the carbide, interfacial compatibility with the graphitic network, as well as the transport and supply parameters in the reaction space.