Growth of multi-wall and single-wall carbon nanotubes with in situ high vacuum catalyst deposition

We describe an efficient procedure for the synthesis of multi- and/or single-wall carbon nanotubes. The method is based on CVD process using merely acetylene as precursor gas and starting from a catalyst film deposited under high vacuum (HV) conditions (base pressure:~10e-8 mbar) just before the CVD process. The use of HV is not favorable regarding the economics in a production process, however no hydrogen, ammonia or any other time-consuming pre-treatment of the catalyst film are needed. This reduces the total time (i.e. catalyst film preparation and CVD process) for CNTs synthesis to 15-40 min (principally depending on catalyst thickness, see below), which has to be compared to the typical time needed in the other methods based on a supported catalyst film (i.e. several hours [3]). Actually, in several CVD processes, the pre-treatment of the catalyst film with hydrogen and/or ammonia is crucial [15], and the CNTs grow if hydrogen and/or ammonia are used in well defined relative fluxes together with the hydrocarbon gas [16]. These pre-treatments act to reduce the thin oxide film of the catalyst to form metallic nanoparticles. For a deeper understanding of the role of the catalyst and of its oxidization state, and for a deeper control of the CNTs growth, we have compared the CVD growth of carbon nanotubes on metallic and oxidized Fe and Ni films. When the catalyst is oxidized, only an amorphous carbon film grows, consisting of carbon-coated particles with some embedded tubes. If the film is not oxidized, it is possible to grow the nanotubes also when the catalyst thickness is as low as 1 nm and in this case the grown CNTs are isolated SWCNTs.