Towards helical and Y-shaped carbon nanotubes: the role of sulfur in CVD processes

Sulfur is introduced as an additive in CVD processes for the production of carbon nanotubes over Co–Mo/MgO sol–gel catalysts employing two different approaches: for the first time as sulfonated Co–Mo/MgO sol–gel catalysts and, alternatively, as thiophene vapour during the CVD reaction process. On one hand, the use of sulfur compounds during the sol–gel catalyst preparation process generally leads to a significant modification of the matrix composition and the matrix–catalyst interaction. When employing a catalyst here containing sulfur compounds the induced matrix modifications yield enhanced growth of helical carbon nanotubes (HCNTs). On the other hand, adding sulfur in the form of thiophene vapour over non-sulfonated sol–gel catalysts, the growth of Y-shaped carbon nanotubes (YCNTs) is favoured depending on the flow conditions of the hydrocarbon source employed. The obtained results underline the general importance of the nature of the sol–gel matrix for the formation of HCNTs as well as of the local fluctuations in the gas phase for the growth of YCNTs. The findings are of importance for the fabrication of nanotube-based electronic devices.Support by the EU project HYMOSSES (ENK6-CT-2002-00653), the Spanish MCYT project NANOSIN (MAT 2002-04540-C05-04) and GDRE No. 2756 ‘Science and applications of nanotubes—NANO-E’.Peer reviewe