Role of Fe/NaCl/NH4F Compositions for the Catalytic Chemical Vapor Deposition Synthesis of Multiwalled Carbon Nanotubes

Multi-wall carbon nanotubes (MWNTs) have been intensely studied, and promising applications have been demonstrated, because of their outstanding mechanical, electronic, and chemical properties [1]. Although SWNTs have been synthesized by several different processes, the most versatile is chemical vapor deposition (CVD), in which preformed nanometer-scale catalyst particles supported on surfaces are heated to 700-1000 °C in an atmosphere containing either a hydrocarbon [2] or carbon monoxide [3]. Two potential SWNT application areas indicate the need for SWNT synthesis at reduced temperatures. The first is the growth of SWNTs directly into device architectures. Electrodes have been applied to surface-bound SWNTs with nanolithography to create devices such as nanometer-scale field effect transistors [4] and chemical sensors [5]. A more scalable procedure is to grow SWNTs directly into preformed electrode patterns; however, the high growth temperatures are usually destructive. Although devices have been formed with refractory molybdenum interconnects for greater stability, [6] a more productive route would be to reduce the SWNT synthesis temperature to make it compatible with a wider range of substrates and nanofabrication technology. Among the various parameters found to affect the DWNT growth in the CVD synthesis, the nature of the support material has been studied in less detail. Iijima and co-workers have reported the production of a mixture of DWNTs and SWNTs by the decomposition of methane on mesoporous silica, whereas the zeolite support material is reported to yield only SWNTs [7]. MgO is also used as a support material for DWNT growth [7]. Ren and co-workers have reported that high surface area, mesoporous Nantek MgO with mesopores of 3 nm selectively produces DWNTs, whereas nonporous Nanomyte MgO produces large-diameter tubes [7]. The selectivity of DWNTs observed in the above-mentioned case is due to the mesopores and high surface area of MgO. Flahaut and co-workers have reported that the preparation conditions of MgO have drastically affected the selectivity of DWNT over MWNTs [8]. Here we aim to understand the role of NaCl/NH4F support materials in the production of MWNTs using C2H2 as the carbon source.