Rümmeli et al. Nanoscale Research Letters 2011, 6:303 http://www.nanoscalereslett.com/content/6/1/303 Page 7 of 9 Figure 7 Schematic representation of the carbothermal reduction of silica to silicon carbide and carbon nanostructure formation: (a) SiO2 is reduced to SiC via a carbothermal reaction, (b) SiC nanoparticles coalesce, (c) carbon caps form on the surface of the SiC particles through precipitation and/or SiC decomposition. Reproduced with permission from Bachmatiuk et al. [37]. circumstances during growth. On the back of this some argue in situ measurements as the only way forward. However, these routes present key limitations such as the need to work at very low pressures, well beyond any conventional or commercial route would use, as is the case for TEM and XPS in situ studies. Moreover, in in situ TEM only tiny sample sizes are examined and in the case of XPS in situ examinations, as already dis- cussed above, the technique is surface sensitive and hence provides limited information on the catalyst dur- ing growth. Another area to investigate is how nature produces carbon nanotubes. Surprisingly, there is little evidence on planet Earth for their formation with only a few examples of MWNT and none for SWNT [59]. However, CNT may form more readily in outer space. Graphite whiskers have been found in high-temperature components of meteorites [60]. In addition, it has been proposed they can form in protostellar nebulae via Fischer-Tropsch-type catalytic reactions [61,62]. Recent experiments by the same group investigating the poten- tial of Fischer-Tropsch and Haber-Bosch type reactions appear to support this hypothesis [63]. Thus, it is the collective data from both ex situ and in situ examina- tions that are important; however, the limitations of each implemented technique, and the specifics of the synthesis route in question must be considered as there is no single universal growth mode. Summary There remains a fair amount of controversy in explaining carbon nanotube growth; this in part is due to the sheer number of possible synthesis routes and the fact that there is no single universal growth mode. Even so, tremendous advances have been made. This includes the development of new catalyst systems and even catalyst-free systems. Nonetheless the successful integration of CNT into appli- cations and large-scale production processes remains limited and is dependant on the understanding of several fundamental issues. Some of these issues are highlighted by the disparate catalyst and catalyst free options available which raise new questions on nucleation and growth as well as the role of supports in supported catalysts. In some sense the rapid development of graphene may render CNT less important, for example, in the integration of carbon nanotubes in integrated circuit manufacturing, however, many of the questions raised in understanding carbon nanotube growth are directly relevant to graphene also. Abbreviations CNT: carbon nanotubes; CVD: chemical vapour deposition; FRC: free radical condensate; IR: infrared; MWNTs: multi-walled carbon nanotubes; SWNTs: single-walled carbon nanotubes; TEM: transmission electron microscopy; VLS: vapour-liquid-solid; XPS: X-ray photoemission studies. Acknowledgements MHR thanks the EU (ECEMP) and the Freistaat Sachsen, AB and FS the Alexander von Humboldt Foundation and the BMBF, FB the DFG (RU 1540/ 8-1), II the DAAD (A/07/80841) and CC the EU (CARBIO, Contract MRTN-CT- 2006-035616). GC acknowledges support from the South Korean Ministry of Education, Science, and Technology Program, Project WCU ITCE No. R31- 2008-000-10100-0. Author details 1IFW Dresden, P.O. Box 270116, 01069 Dresden, Germany 2Technische Universität Dresden, 01062 Dresden, Germany 3University of Oxford, Parks Road, Oxford, OX1 3PH, UK 4West Pomeranian University of Technology, ul. Pulaskiego 10, 70-322 Szczecin, Poland 5Nanotechnology Center, VSB Technical University of Ostrava, 17. listopadu 15, 70833 Ostrava-Poruba, Czech Republic 6National Center for Nanomaterials Technology, POSTECH, Pohang 790-784, Republic of Korea Authors’ contributions MHR designed the manuscript layout. MHR, AB, FB, FS, II, KC, GS-M, DP, EB-P, GC and BB participated in some of the studies and participated in the drafting of the manuscript. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 14 October 2010 Accepted: 7 April 2011 Published: 7 April 2011