Direct growth of MWCNTs on 316 stainless steel by chemical vapor deposition: Effect of surface nano-features on CNT growth and structure

Multi-walled carbon nanotubes were directly grown by chemical vapor deposition on as-received or pretreated 316 SS without application of an external catalyst. A detailed study of the size distribution of surface features formed by different steps of the synthesis process showed that the heating cycle and any complementary pretreatment may produce significant changes of the surface topography, thus suggesting that the influence of any primary characteristics of the original surface, as well as those caused by a pretreatment, should be assessed in conjunction with the effects of heating. Average lateral size of nano-features less than 60 nm (after heating) were shown to favor mainly the carbon nanotube growth while a larger features size was associated predominantly to the carbon nanofiber synthesis. Scanning and transmission electron microscopy observations suggest two different mechanisms for nanotube/nanofiber growth: (1) base growth mode caused by nanosized hills on the surface catalyzing the nanotube/nanofiber synthesis, (2) tip growth mode requiring substrate surface break-up as a preliminary step to form catalytic particles, with similarities to the "metal dusting" mechanisms. While untreated steel showed the best results concerning carbon nanotube coverage and homogeneity, oxidized-reduced samples showed an almost exclusive growth of carbon nanofibers with a full coverage.