Rümmeli et al. Nanoscale Research Letters 2011, 6:303 http://www.nanoscalereslett.com/content/6/1/303 Ceramic and semiconductor catalysts Of the non-metallic catalysts for CNT, SiC is the most widely used and historically one of the first to be exploited. The early investigations involved the high temperature annealing (>1500°C) of SiC and was first demonstrated by Kusunoki et al. [18]. An example of the CNT is provided in Figure 3. Kusunoki and co- workers showed that in low vacuum conditions the SiC decomposes through the following oxidation route: SiC (s) + CO g → SiO g + 2C (s) (1) The controlled oxidation process depletes Si at the surface, enabling the construction of CNTs. However, the formation of the initial caps at the nucleation stage has yet to be clarified [19]. Some argue a transformation process of surface graphene layers [20,21] or amorphous carbon [22] forms nucleation caps. Others argue the for- mation of convex structures on the surface enable initial cap formation [23-25]. Single-walled carbon nanotubes (SWNTs) can also be grown from SiC nanoparticles in CVD as was shown by Takagi [26]. Botti et al. [27,28] demonstrated laser annealing of SiC nanoparticles as a technique to obtain CNT. The potential of semiconducting catalyst particles was first demonstrated by Uchino et al. [29,30] in which car- bon-doped SiGe islands on Si were used to grow CNT after chemical oxidation and annealing treatments. Growth of the CNT was argued to occur from Ge clusters. This is due to the greater thermodynamic tendency of Si to be oxidized as compared to Ge. Thus, the oxida- tion treatment results in the formation of SiO2 and the segregation of Ge clusters. Takagi et al. [26] also showed that SWNT could be grown directly from Ge particles as well as from Si nanoparticles. Numerous investigators have shown oxides are well suited for CNT growth. An early example was the use of MgO as the catalysts for SWNT formation via the laser evaporation route [11]. More recently, Liu et al. [31] showed Al2O3 nanoparticles could be used to grow SWNT using an alcohol CVD route. Steiner et al. [32] showed both multi- and single-walled carbon nanotubes could be grown from zirconia. The use of magnesium borates can yield B-doped CNT (Figure 4) as was first demonstrated by Bystrzejewski et al. [33,34]. In 2009, two groups showed SWNT formation using SiO2 nanoparticles [35,36]. A little later Bachmatiuk et al. [37,38] showed stacked cup CNT could be grown from amorphous SiO2 nano-particles. However, trans- mission electron microscopy (TEM), infrared (IR) and Raman spectroscopic studies showed the nano-particles at the root of the CNT to be SiC. Their data points to Page 3 of 9 Figure 3 Transmission electron micrograph of the interface between the graphite constructing a carbon nanotube and b- SiC on the surface of (111) b-SiC. Lower panel: Schematic of the orientation relationship between one [111] SiC plane, on which carbon nanotubes are standing perpendicularly, and the other [111] SiC planes. Reprinted with permission from Kusunoki et al. [18]. the carbo-thermal reduction of SiO2. This result is in contrast to X-ray photoemission studies (XPS) by Huang et al. [36] which did not show any carbide for- mation and hence they argued growth occurred from the SiO2 particles. Steiner et al. [32] also conducted XPS studies and also found no evidence for carbide