Rümmeli et al. Nanoscale Research Letters 2011, 6:303 http://www.nanoscalereslett.com/content/6/1/303 studies are required to better understand which oxide systems are stable and which are reducible. Previous studies of ours in which nano-crystalline oxides were subjected CVD reactions showed many oxides are stable, whilst others are not. These studies confirmed oxides are capable of graphitising carbon [40]. Hybrid metal/metal-oxide catalyst systems Many of the oxides described above as catalytic nano- particles for CNT growth are often used as supports in supported catalyst CVD. Commonly used oxide supports are Al2O3, SiO2, TiO2 and MgO. All these oxides have been shown to grow CNT. Their role is primarily to sta- bilize the metal catalysts, viz. prevent coalescence. How- ever, in oxide-supported metal catalysis it is well known that small clusters can have enhanced catalytic activity. A well-known example is Au, which is a bulk material is rather inert, but finely dispersed and deposited on oxi- des as small nano-clusters Au exhibits high catalytic ability (e.g. Haruta [41]). This enhanced catalytic activity is generally accepted to occur at the circumference of the nano-cluster/support interface. It is then natural to query if oxides and the catalyst/ support interface play a role in the case of CNT grown from oxide-supported metal catalyst clusters. To this end, we conducted various studies on CNT grown from Fe and Co clusters supported on alumina. Whilst the studies showed a good correlation between the initial catalyst size and the CNT outer diameter, after synthesis the catalyst particles are found to lie within the core of the CNT and are elongated [42]. In addition, the roots of the graphitic walls do not terminate on the metal par- ticle but rather on the oxide support as shown in Figure 5 [43]. This highlights the diversity with which carbon nanotubes can grow, in that some base growth modes show the CNT is rooted at the metal catalyst particle [44] much like tip growth grown CNT [45] or in other cases from the oxide support [42,43]. Page 5 of 9 Another hybrid metal/metal-oxide example is the hydrocarbon dissociation over supported less active metal catalysts like Au and Cu, where it is argued that electron donation to the support creates d-vacancies for hydrocarbon dissociation [46]. All carbon systems The formation of CNT on the cathode in the arc-dis- charge route can occur without catalyst addition as shown by the work of Bacon in 1957 [47] and more recently by Iijima [1]. Despite the huge impact of Iiji- ma’s 1991 Nature paper, the fact that no catalyst was required was largely ignored or forgotten. More recently, a broad array of growth routes using pure carbon sys- tems without any catalyst particle addition have emerged. Takagi et al. [48] have shown that SWNT can be grown in CVD using nano-diamond particles as cata- lysts. Moreover, nano-diamond particles do not suffer from coalescence and sintering difficulties. Exciting stra- tegies to open fullerenes and use them as nucleation caps for SWNT have also been demonstrated. Once the fullerenes have been opened they are subjected to a CVD process and grow tubes [49,50]. The proposed growth mechanism is given in Figure 6. In a similar vein, the direct cloning of SWNT was shown by Liu and co-workers [51]. The formation of CNT on graphitic surfaces has also been demonstrated in various works by Lin et al. [52,53]. In these studies by Lin et al., it was shown that the early formation of amorphous nano- humps apparently serve as seed sites for the self-assem- bly of CNT. Growth Mechanisms Whilst significant strides have been made in under- standing CNT synthesis, the mechanisms behind growth remain a highly debated issue. In part this is due to some mechanisms being presented as universal. The brief variety of synthesis strategies presented in this Figure 5 TEM micrographs showing cross section view of a CNT root at the support surface. The (Co) catalyst particle resides in the core of the tube. The fringes at the base of the particle correspond to the (200) lattice fringes of cubic Co. The outer walls of the CNT align themselves with the lattice fringes of the a-alumina nanoplatelet. The middle micrograph is a magnification of the boxed region from the left micrograph. The right micrograph is a copy of the middle image with lines added to highlight the alignment of the graphitic planes with the rhombohedral (110) lattice fringes of the corundum support. Reprinted from Rümmeli et al. [43].