In the following sections, the major advances in production, separa- tion, structures, electronic/magnetic and solid state properties of endo- hedral metallofullerenes will be discussed in an effort to shed light on this fascinating new class of fullerene-related materials.
2. SYNTHESIS, EXTRACTION FROM SOOT, AND
has been used to produce alkaline metallofullerenes (Campbell et al., 1997; Tellgmann et al., 1996a,b) and nitrogen atom endofullerenes (Knapp et al., 1997; Mauser et al., 1997; Murphy et al., 1996) such as Li@C60 and N@C60, respectively. Also, Hirata et al. (1996) reported the production of K@C60by introducing negatively charged C60 into a low- temperature (ca 0.2 eV) potassium plasma column by a strong axial magnetic field. However, isolation and structural characterization of metallofullerenes prepared by the above methods have not been per- formed yet due to the insufficiency of the materials produced by the implantation and the plasma techniques, although partial high-perfor- mance liquid chromatography (HPLC) separation on Li@C60 has been reported (Krawez et al., 1998). In the following, we review the current status of the laser furnace and arc discharge methods for the synthesis of endohedral metallofullerenes.
In the laser-furnace method (Figure 3), a target composite rod or disc for laser vaporization, which is composed of metal-oxide/graphite with a high-strength pitch binder, is placed in a furnace at 1,200 C (Haufler et al., 1991). A frequency-doubled Nd:YAG laser at 532 nm is focused onto the target rod, which is normally rotating/translating to ensure a fresh surface, in an Ar gas flow (100–200 Torr) condition. Metallofullerenes and empty fullerenes are produced by the laser vaporization and then flow down the tube with the Ar gas carrier, and are finally trapped on the quartz tube wall near the end of the furnace.
To produce fullerenes and metallofullerenes, a temperature above 800C was found to be necessary, and below this critical temperature no fullerenes were produced (Haufler et al., 1991; Suzuki et al., 1997a;
Wakabayashi et al., 1997), suggesting that relatively slow thermal annealing processes are required to form fullerenes and metallofullerenes. The laser- furnace method is suited to the study of growth mechanism of fullerenes and metallofullerenes (Curl and Smalley, 1991; Haufler et al., 1991;
Metal impregnated composite rod Cold trap
Pump Electric furnace
YAG laser 532 nm Ar
FIGURE 3 Schematic diagram of the high-temperature laser-furnace apparatus to produce fullerenes and metallofullerenes by laser vaporization of a rotating
metal-impregnated graphite target in an electric furnace with flowing argon carrier gas.
Smalley, 1992; Wakabayashi and Achiba, 1992; Wakabayashi et al., 1993;
Ying et al., 1994). The laser-furnace method is also known to be an efficient production method for single-wall carbon nanotubes when Ni/Co or Ni/Fe binary metal is stuffed with graphite powder for target composite rods (Thess et al., 1996).
Figure 4 represents a third-generation large-scale DC arc discharge apparatus for the production of metallofullerenes developed and installed at Nagoya (Dennis and Shinohara, 1998; Nakane et al., 1997;
Shinohara, 1998, 2000; Shinohara et al., 1996a). The arc generator consists
Soot collection chamber
Collection gloves
Air
Air Translator
MFC
M
He
Bellows Bellows
Motor Anode(compositerod)
Carbon inner case SV
Micro heaters Automatic brush
Liq. N2 trap
DG PiG
Cathode
Arc chamber
FIGURE 4 A cross-sectional view of the third-generation DC arc discharge apparatus (in Nagoya University) with an anaerobic collection and sampling mechanism. The produced metallofullerene-containing soot is effectively trapped by the liquid N2trap installed in the center of the collection chamber. Typical arc discharge conditions:
40–100 Torr He flow, 300–500 A, and 25–30 V.
of a production chamber and a collection chamber, equipped with an anaerobic sampling and collection mechanism of raw soot containing metallofullerenes (Bandow et al., 1993; Shinohara et al., 1994a). Anaerobic sampling of the soot is preferred to conventional collection under ambient conditions because many of the metallofullerenes in primary soot are air (moisture)-sensitive and may be subjected to degradation during the soot handling.
Metal-oxide/graphite composite rods, e.g., La2O3to prepare La@C82, are normally used as positive electrodes (anodes) after a high-tempera- ture (above ca 1,600 C) heat treatment where the composite rods are cured and carbonized. At such high temperatures, various metal carbides in the phase of MC2are formed in the composite rods (Adachi et al., 1991), which actually is crucial to an efficient production of endohedral metal- lofullerenes: uniformly dispersed metal atoms as metal-carbides in a composite rod provide metallofullerenes in higher yields.
For example, the yield of La@C82is increased by a factor of ten or more when LaC2-enriched composite rods are used for the arc generation of soot instead of using La2O3as a starting material for the composite rods (Bandow et al., 1993). The rods (20 mm diameter500 mm long), are arced in the DC (300–500 A) spark mode under 50–100 Torr He flow conditions (Figure 4). The soot so produced is collected under totally anaerobic conditions to avoid unnecessary degradation of the metalloful- lerenes produced during the soot collection and handling. It was found that the soot collected in the upper chamber contains a substantial amount of metallofullerenes compared to that collected in the arc discharge (lower) chamber. The fullerene smoke (soot) which rises along a convec- tion flow around the evaporation source has the maximum content of metallofullerenes (Saito et al., 1996). Furthermore, the extraction effi- ciency of the anaerobic soot is much higher than that of the ambient soot in the lower chamber.
In general, the yield of a metallofullerene varies sensitively on He buffer gas pressure during the arc synthesis. An optimum He pressure depends on arc conditions such as the size of a composite rod, DC current, and the arc gap of the two electrodes, which is normally close to that of empty higher fullerenes such as C82and C84. It has been reported that the formation of La@C82, for example, is closely related to the evaporation rate of a composite rod; the maximum yield of La@C82 is attained at the highest evaporation rate of the rod (Saito et al., 1996). Mieno (1998) reported that the production of endohedral metallofullerenes can be much enhanced under gravitation-free arc discharge conditions as com- pared with the normal gravitational condition. This is due to the fact that the gravitation-free conditions suppress thermal convection of hot gas in the arc region and thus enable long-duration hot reaction of carbon clusters suited to metallofullerene production.
The production of scandium fullerenes, Scn@C82 and/or ScnC2@C80
(n¼1–4), is especially interesting, because scandium fullerenes exist in a solvent extract as mono-, di-, and tri-scandium fullerenes (Shinohara et al., 1992b; Yannoni et al., 1992), which is quite unique compared to the other rare earth metallofullerenes. Even a tetra-scandium fullerene, Sc4C2@C80, has also been produced and isolated (Kuroki et al., 1999;
Wang et al., 2009). The synthesis of the mono-, di-, tri-and tetra-scandium fullerene was found to be sensitive to the mixing ratio of scandium and carbon atoms in the composite rods; the relative abundance of di-, tri-and tetra-scandium fullerenes increases as the carbon/scandium ratio decreases. For example, with a carbon/scandium (atomic) ratio of 86.2, the formation of mono-and di-scandium fullerenes such as Sc@C82 and Sc2C2@C82 was dominant, and the production of Sc3C2@C80 and Sc4C2@C80was almost negligible. It was observed that the major scandium fullerene produced was Sc2C2@C82 over a wide range of the carbon/
scandium mixing ratios (10–100) (Shinohara et al., 1992a,b).
2.2 Solvent extraction from primary soot containing metallofullerenes
The so-called solvent extraction method by toluene, o-xylene or carbon disulfide is the most common and frequently used extraction method, in which metallofullerenes and hollow fullerenes are preferentially dis- solved in solvents. The so-called Soxhlet extraction (a continuous and hot solvent extraction) or ultrasonic extraction is normally employed to increase the solvent extraction efficiency (Khemani et al., 1992). Insolubles in soot are easily separated from this solution by filtration. However, in many cases, the toluene or CS2extraction is not sufficient, since nearly half of the metallofullerene still remains in the residual soot even after the extensive CS2 extraction. It has been found that metallofullerenes are further extracted from the residual soot by such solvents as pyridine (Inakuma et al., 1995) and 1,2,4-trichlorobenzene (Yamamoto et al., 1994a,b). The metallofullerenes were found to be concentrated in this pyridine or trichlorobenzene extracted fraction. When necessary, the metallofullerene extracts can be stored in carbon disulfide solution for an extended period of time, up to a year.
In a sublimation method (Chai et al., 1991; Diener et al., 1997; Yeretzian et al., 1993), as in the case of empty fullerenes (Abrefah et al., 1992; Averitt et al., 1994; Cox et al., 1991; Kraetschmer et al., 1990b; Pan et al., 1991;
Taylor et al., 1990), the raw soot containing metallofullerenes is heated in He gas or in vacuum up to 400 C, where metallofullerenes such as La@C82and Y@C82start to sublime. The metallofullerenes then condense in a cold trap, leaving the soot and other nonvolatiles behind in the sample holder. However, a complete separation of metallofullerenes
has not been achieved to date by sublimation. Extraction by sublimation has the advantage over solvent extraction for obtaining ‘‘solvent-free’’
extracts, whereas the latter method is suited to large-scale extraction of metallofullerenes.