Cobalt Catalyzed Carbon Nanotube Growth on Graphitic Paper Supports

The catalytic growth of multi-wall carbon nanotubes on carbon paper is reported. The study employed three cobalt carbonyl clusters as catalyst precursors. These were deposited on graphitic paper prior to chemical vapour deposition (CVD) of methane or ethyl- ene. The clusters show differentiated growth behaviour in accordance with precursor size, and with Co2(CO)8 displaying additional activ- ity in the growth of helical nanotube structures. We therefore report an approach for the decoration of graphitic papers with carbon nano- tubes with a view to the production of high area supports. Since their identification by Iijima in 1991 (1), carbon nano- tubes (CNTs) have been the subject of intensive research owing to their unique electronic and mechanical properties (2). While various synthetic strategies for their production have been devised, chemi- cal vapor deposition (CVD) has been established as one of the more commonly used techniques by virtue of its versatility in terms of the effect of experimental parameters (such as flow speed and composi- tion of the gaseous carbon precursors, deposition temperature and time) on the type of CNTs produced. Nevertheless, the development of procedures that are able to achieve sufficient morphological and structural product control remains an ongoing challenge. This is, on the one hand, due to there being an as yet incomplete understanding of the tube formation mechanism and, on the other hand, groups reporting a lack of control over catalyst morphology. The result of these issues is that irregular tube structures are generally obtained. In the case of morphological control, both experimental and compu- tational studies have demonstrated the importance of catalyst parti- cle size in influencing the resulting tube diameters (3). More fun- damentally, particle diameters in the nanometer regime are known to affect the metal melting point through the Gibbs-Thomson effect. In addition, the temperature at which liquefaction of nanoparticulate species occurs (and which can be assumed to influence the agglom- eration behavior), has been shown to depend on saturation of the metal species with carbon (4). These issues pose intrinsic problems in that it is not trivial to maintain narrow particle size distributions for commonly employed nanoparticulate catalysts derived from either physical vapour deposition or wet chemical methods (such as sputtering and metal salt reduction). In seeking to overcome these limitations, researchers have more recently focused on the employ- ment of molecular clusters as catalyst precursors, and on the deposi- tion of these on various supports. Most notably, use of the high pressure carbon monoxide (HiPco) process to produce single- walled carbon nanotubes (SWCNTs) using iron pentacarbonyl has offered control over nanotube yield and morphology by means of controlling the deposition parameters (5). In recent years, considerable effort has been directed towards addressing both the issue of seed size control and the development of a wider range of suitable supports with which to provide CNT- based high surface area systems. In this context, some of the present authors have previously reported CNT syntheses using both nickel formate precursors and colloidal cobalt nanoparticles on supports