Arc Discharge Plasma

The formation mechanism of nanoparticles/nanocapsules in plasma is that matter clusters with high activity exist in the plasma, which could exchange rapidly the energy with the reacted matter clusters, beneficial to the reaction between them. When the reacted matter clusters leave the high temperature in the flame tail region of the plasma, the rapid decrease of the temperature lets the clusters be in the saturation state in dynamic equalibrium; thus they are dissociated. The rapid cooling/quenching leads to the nucleation of crystallites and the formation of nanoparticles/nanocapsules. According to the method of production, the plasma can be of two kinds: direct current arc plasma and high frequency plasma. Direct current arc plasma uses the direct current arc in inert/active gases to ionize the gases, generate the high temperature plasma, and melt the materials. The cooling, reaction, and condensation of the evaporating matters lead to the formation of nanoparticles and nanocapsules. The high frequency plasma is produced by induction coils outside the quartz tube, which can generate the magnetic field with MHz frequency. The advantages of the high frequency plasma method are: less contamination because no cathode is used, the use of reaction gases, and enough space for plasma so that the reaction and heating are sufficient for the plasma matter. The direct current arc plasma and the high frequency plasma methods can be combined in equipment for industrial purposes.

There has been great interest in the incorporation of foreign materials into fullerene structures (C60, nanotubes, nanoparticles, onions), which has been driven by the potential applications of the filled fullerenes, which lie in diverse areas such as optics, electronics, magnetic recording materials, and nuclear medicine. In particular, the onion structures of extreme strength may offer excellent protection of their encapsulated nanomaterials for applications. The arc vaporization/discharge method has been widely used for the formation of fullerenes and related materials [102]. One of the remarkable properties of fullerenes and related materials is that they have hollow structures with a cavity of nanometer-scale diameter; outer cages are made up of single or concentric multilayers of graphene sheets. When foreign materials, such as metal atom(s) or nanocrystallites, are trapped in the inner cavity, unusual physical and chemical properties are brought about.

The structures of carbon polyhedral particles stuffed with YC2, synthesized by arc discharge of carbon rods containing yttrium, were studied with a TEM [103]. The YC2 crystals with typical sizes of a few tens of nanometers were wrapped by multilayered graphitic sheets and were protected against hydrolysis. It was demonstrated that a finely focused electron beam opened the graphite cage. A growth model of the carbon nanocapsules stuffed with metal carbides was proposed. Then the encapsulation of metals in multilayered graphitic capsules was studied for all the rare-earth elements (Sc, Y, and R = La, Ce,..., Lu) excluding Pm by using electric arc discharge [104]. Most rare-earth metals (Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu) were encapsulated in the form of carbides, but the others, Sm, Eu, and Yb, were not. The metals in the former group that were encapsulated had vapor pressures definitely lower than those in the latter group. In the case of thulium (Tm), whose vapor pressure was intermediate between the two groups, only a trace amount of encapsulated carbide was formed. The volatility of metal played an important role in the metal encapsulation process.

Crystallites of scandium carbides, nesting in multilayered polyhedral graphitic cages, were produced by evaporating a scandium-graphite composite rod in helium gas [105]. The encapsulated scandium carbide was identified as Sc15C19, instead of dicarbide RC2, the form of carbide commonly found for other rare-earth-based nanocapsules. The size of the capsules ranged from about 10 to 100 nm. Morphological features of the outer graphitic carbon, multilayered and polyhedral, were quite similar to those for the capsules protecting RC2 (R = Y, La, Ce,..., Lu) [104].

Experiments aimed at the encapsulation of foreign materials within hollow graphitic cages were carried for iron group metals (Fe, Co, Ni) [106-114]. For iron group metals, particles of both a carbide phase (M3C, M = Fe, Co, Ni) and also a metallic phase [a-Fe, y-Fe, hexagonal close-packed (hcp) Co, face-centered cubic (fcc) Co, fcc Ni] were encapsulated in graphitic carbon [106, 107]. Sometimes, metallic nanocrystals of Fe, Co, and Ni were in the fcc phase, and no trace (or a trace amount) of the bulk equilibrium phases of bcc Fe and hcp Co was found [108, 109]. Especially for Ni, exotic carbon materials with hollow structures, bamboo-shaped tubes, and nanochains as well as single-layered nano-tubes were discovered [110]. Figure 2 shows high resolution TEM (HRTEM) images of the shell/core structure of Ni(C) nanocapsules [109]. The particles were nominally spherical in shape and typically 10-100 nm in diameter. The core Co metal was protected against oxidation and coalescence.

Evaporation of Fe, Ni, or Co with graphite in a hydrogen atmosphere resulted in graphite encapsulated nanoparticles. Similar experiments in helium led to nanoparticles embedded in an amorphous carbon/fullerene matrix. Comparing the experimental results in helium and hydrogen, a mechanism for the formation of encapsulated nanoparticles was proposed [113, 114]. The hydrogen arc produced polycyclic aromatic hydrocarbon molecules, which can act as a precursor to the graphitic layers around the nanoparticles. Direct evidence for this mechanism was given by using methane (CH4) [113] or pyrene (C16H10) [114], a polycyclic aromatic hydrocarbon molecule, as the only carbon source to form encapsulated nanoparticles. Pyrolysis of hydrocarbon

Figure 2. HRTEM images showing the shell/core structure of Ni(C) nanocapsules. After [109], V. P. Dravid et al., Nature 374, 602 (1995). © 1995, Macmillan Magazines Ltd.

compounds on evaporated metal-based catalysts was used to produce carbon shells, hollow fibers, and other hollow carbon forms [115]. A kinetic model for their formation and the mechanisms for obtaining different types of hollow carbons were proposed [115], in which two competing processes, the linear growth rate of the carbon and the catalyst outlet rate relative to the growing carbon, determined the type of hollow carbon.

A process of arc discharge was developed to fabricate carbon-encapsulated magnetic nanocapsules in methane (CH4) atmosphere, where a carbon rod was used as the cathode and a metal or alloy block was used as the anode [113]. y-Fe(C), a-Fe(C) and Fe3C [113, 116], Fe-Ni(C) [117, 118], and Fe-Co(C) [119-121] nanocapsules were synthesized successfully by arc discharge in methane. Figure 3 presents a HRTEM image showing the shell/core structure of Fe-Co(C) nanocapsules with a graphite shell of about 3.4 nm thickness and a crystalline Fe-Co core [121]. The shell is characterized by curved lattice fringes of interplanar spacing 0.34 nm corresponding to the (0002) lattice plane of graphite carbon [121]. This process was employed to produce nanocapsules with different types of shells and cores only by changing the atmosphere. Fe(B) and Co(B) nanocapsules sheathed with boron oxide were prepared in dibo-rane (B2H6) atmosphere [122-124]. Figure 4a shows a TEM photograph of Co(B) nanocapsules which arrange as a chain due to the magnetostatic energy, while Figure 4b gives a HRTEM image showing the shell/core structure of Co(B) nanocapsules with an amorphous boron oxide shell of about 4.0 nm thickness and a crystalline CoB core [124]. Amorphous boron nanoparticles and BN encapsulating boron nanocapsules were prepared by arc decomposing diborane and nitriding [125].

Twenty elements were co-deposited with carbon in an arc discharge between graphite electrodes [126]. 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

Figure 3. HRTEM image showing the shell/core structure of Fe-Co(C) nanocapsules with a graphite shell of about 3.4 nm thick and crystalline Fe-Co core. The shell is characterized by curved lattice fringes of inter-planar spacing 0.34 nm corresponding to the (0002) lattice plane of graphite carbon. After [121], Z. D. Zhang et al., J. Phys. Cond. Matter 13, 1921 (2001). © 2001, IOP Publishing.

Figure 3. HRTEM image showing the shell/core structure of Fe-Co(C) nanocapsules with a graphite shell of about 3.4 nm thick and crystalline Fe-Co core. The shell is characterized by curved lattice fringes of inter-planar spacing 0.34 nm corresponding to the (0002) lattice plane of graphite carbon. After [121], Z. D. Zhang et al., J. Phys. Cond. Matter 13, 1921 (2001). © 2001, IOP Publishing.

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Figure 4. (a) TEM photographs of Co(B) nanocapsules which arrange as a chain due to the magnetostatic energy; (b) HRTEM image showing the shell/core structure of Co(B) nanocapsules with an amorphous boron oxide shell of about 4.0 nm thick and a crystalline CoB core with d = 0.219 nm corresponding to the (111) lattice plane of CoB phase with a = 3.948 A, b = 5.243 A, and c = 3.037 A. After [124], I. Skorvanek et al., submitted for publication. Image courtesy of J. G. Zheng, Northwestern University, U.S.A.

Figure 4. (a) TEM photographs of Co(B) nanocapsules which arrange as a chain due to the magnetostatic energy; (b) HRTEM image showing the shell/core structure of Co(B) nanocapsules with an amorphous boron oxide shell of about 4.0 nm thick and a crystalline CoB core with d = 0.219 nm corresponding to the (111) lattice plane of CoB phase with a = 3.948 A, b = 5.243 A, and c = 3.037 A. After [124], I. Skorvanek et al., submitted for publication. Image courtesy of J. G. Zheng, Northwestern University, U.S.A.

reactor walls or as slag at the cathode, were characterized. 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 preempting 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 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 was discussed, according to one of the four categories. It was apparent that the physical properties, such as vapor pressure, melting and boiling points, the completeness of the electronic shells of the elements, and the heat of carbide formation, were not sufficient to explain the selectivity of the encapsulation without exceptions. A hypothesis was 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.

Encapsulation of platinum-group metals (Ru, Ph, Pd, Os, Ir, Pt) within carbon nanocapsules and synthesis of single-layered carbon nanotubes by arc evaporation of metal/carbon composites were studied [127]. All the platinum-group metals, forming small particles (10-200 nm in diameter), were encapsulated within multilayered graphitic cages. Particles trapped in the cages were single-domain crystallites in normal metallic phases. Ph, Pd, and Pt showed catalytic activity for growing single-layered carbon tubes, but the other metals did not.

Fine crystallites of titanium and hafnium carbides encapsulated within graphite cages were formed by arc discharge between a metal/carbon composite anode and a graphite cathode [128]. Encapsulated TiC crystallites ranging from 30 to 150 nm in size were found in carbonaceous materials formed around a cathode. Short, single-walled carbon nanotubes were also observed, extruding from carbon layers with complicated structures surrounding TiC crystallites. For hafnium whose vapor pressure was lower than that of carbon at high temperatures, the formation of HfC crystallites (20 to 80 nm in diameter) was limited within a slaglike deposit, which was also encapsulated in graphite cages.

Encapsulation of Cr, Mo, and W in multilayered graphitic cages by arc evaporation of metal/carbon composites under different pressures (100, 600, and 1500 Torr) of helium gas was studied [129]. The encapsulated crystallites were carbides, that is, Cr7C3 and Cr3C2 for Cr; Mo2C, 5-MoC1-x (NaCl-type), and y'-MoC (AsTi-type) for Mo; W2C and (NaCl-type) for W. The effect of helium gas pressure on the formation of the nanocapsules was found for tungsten: Encapsulated tungsten carbides were formed at the highest pressure, but not at the lower pressures.

A dc arc discharge was generated between graphite and molybdenum electrodes at 25 kPa of He gas ambient, in order to reveal the relation between nanotube growth and arc discharge phenomena [130]. Numerous multiwall carbon nanotubes and nanocapsules were observed at the cathode spot area of the C cathode. An explanation was presented for the growth of soft-core containing nanotubes and a hard shell in the usual arc with a C-cathode and C-anode electrode system.

A form of graphitic cage in nanometer scale, rectangular parallelopiped (or cube), was produced by arc evaporation of a carbon electrode containing calcium or strontium [131]. Both empty and filled rectangular nanocapsules averaging 20-100 nm in size were formed, though the empty ones were dominant. The rectangular cages were made up of multiwalled (5-20 layers) graphitic carbon. The encapsulated materials were ^-CaC2 (tetragonal), y-CaC2 (cubic), and S-CaC2 (monoclinic) for C/Ca evaporation, and ;S-SrC2 (tetragonal) and metallic Sr for C/S revaporation.

The growth phenomena of different metals encapsulated into carbon cages were studied in emphasizing the effect of carbon and metal supply on the size of particles [132, 133]. Postdeposition annealing was introduced as a process that induces structural rearrangements and thus enables changes in morphologies. Particles made under the same experimental conditions of the arc discharge process were of roughly the same size. The average diameter of the particles produced by using a larger diameter of the metal pool was bigger than those of the particles produced from the smaller metal pool. The annealing provided additional thermal energy making structural rearrangement possible long after the initial deposition process was terminated.

The effects of the carbon content, chamber pressure, arc current, and blowing gas velocity on the encapsulation of nickel in graphite layers were observed by systematically varying each of these variables in a tungsten-arc encapsulation setup [134]. The properties of the arc translated into changes in the encapsulated product. A larger and hotter arc resulted in more encapsulation in the final sample. These findings, along with evidence of graphite layers forming on precrystallized particles, indicated that the graphite layers might form by two sequential formation steps. The first step was the simple phase segregation of carbon from a cooling liquid particle, resulting in surface graphite. The second step was the growth of carbon on a crystallized nickel particle, regardless of the temperature at which this occurs.

Gold, iron oxide, and germanium nanocapsules encapsulated in the boron nitride sheets were produced by arc melting in a nitrogen gas atmosphere using a tungsten electrode and a boron-based mixture powder [135, 136]. Nanoscale materials created from boron nitride by arc discharge between ZrB2 electrodes in N2 atmosphere were investigated [137]. Boron nitride nanotubes were formed together with Zr-compound nanoparticles encapsulated in boron nitride cages. Concerning the helical structures of the boron nitride tubes, a variety of chiral angles, including the zigzag and the armchair types, were observed. Weak peaks of photoluminescence spectrum were observed from the nanocapsules with germanium nanoparticles [136].

The spatial distribution of the chemical species (B, C, N, and Hf) present in multielement nanoparticles and nano-tubes, which were produced by arc discharging a hafnium diboride rod with a graphite rod in a nitrogen atmosphere [138], was investigated by means of electron energy loss spectroscopy (EELS). For the hafnium-boride metallic particles coated by C/BN envelopes, a model was proposed based on the solidification from the outside to the inside of isolated liquidlike droplets: the carbon phase solidifies first according to theoretical phase diagrams and forms the outer shells.

Various boron nitride and carbon nanocage fullerene materials (clusters, metallofullerenes, onion, nanotubes, and nanocapsules) were synthesized by arc melting, electron beam irradiation, chemical reaction, and self-organization [139-141]. Boron nitride nanotubes, nanocapsules, and nano-cages were fabricated by arc melting LaB6 and boron/LaB6 powder compacts in a nitrogen/argon gas atmosphere [141]. A guideline for designing the boron nitride and carbon fullerene materials was summarized. B-C-N nanotubes prepared by a flush evaporation method using a dc arc plasma were characterized by TEM and EELS [140]. The nano-tubes obtained were divided into three types, such as carbon, boron nitride, or carbon nanotubes surrounded with boron nitride nanotubes. These types of nanotubes were obtained only at temperatures higher than approximately 3000 K. On the other hand, nanocapsules were formed at all the temperature regions, but the nanocapsules obtained at lower temperatures were smaller. The addition of nickel produced a bundle of single-walled carbon nanotubes and boron nitride nanocapsules surrounding nickel particles.

Ge and SiC nanoparticles and nanowires encapsulated in carbon nanocapsules and nanotubes were produced by direct current and radio frequency hybrid arc discharge of

C, Ge, and Si elements [142, 143]. HRTEM images showed the formation of Ge and SiC nanoparticles and nanowires encapsulated in carbon nanocapsules and nanotubes. The growth direction of the Ge nanowires was found to be (111) of Ge, and a structure model for Ge/C interface was proposed. Silicon-fullerene compounds [email protected] (n = 74, 86, etc.) and carbon nanocapsules filled with SiC were produced in a modified fullerene generator, where a direct current or radio frequency discharge was superimposed in the periphery region of an arc-discharge plasma [144].

HRTEM images of C3N3 6-4 5Ox 2H41-4 2 showed nano-cage- and nanotube-like structures, which have cage and tube sizes in the range of 10-500 nm [145]. Electron diffraction of the cage structures indicated a disordered CNX structure, and carbon and nitrogen were detected. Nanocrys-talline grains encapsulated in CNX nanotubes and nano-capsules were also observed, and carbon, oxygen, and a little nitrogen were detected.

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