Catalytically grown carbon nanotubes: from synthesis to toxicity

Catalytic chemical vapour deposition (CCVD) is currently the most promising technique to produce carbon nanotubes (CNTs) on a specific site as well as on a large-scale. Here, we review our recent experimental studies on CNTs grown by CCVD. The yield as well as the structural characteristics of CNTs can strongly be affected by the choice of the catalyst as well as the catalyst support. In particular, CaCO3 is found to be an excellent support material which actively contributes to the CNT growth. Our systematic study of the elastic measurements of multi-walled CNTs grown by CCVD indicates that Young's modulus is generally low, independent from their precise growth conditions. This behaviour is attributed to the high density of structural defects typically present in CCVD grown CNTs, which cannot be healed by additional heat treatment. However, Young's modulus of about 1 TPa is found for double-walled CNTs indicating that CCVD grown CNTs can also have a high strength as arc-discharge tubes as long as their diameter is small. Furthermore, CNTs are mechanically attached to scanning probe tips and tested in contact as well as in tapping mode. The scans reveal that the contact between the probe and the CNT is the major problem. Finally, we compare the cytotoxicity of CNTs with that of carbon nanoparticles as well as nanofibres. Our results indicate that toxicity strongly depends on the number of chemically active sites on the surface.