Mechanism and Enhanced Yield of Carbon Nanotube Growth on Stainless Steel by Oxygen-Induced Surface Reconstruction

It is well-known that carbon nanotubes (CNTs) can be grown directly on the surface of stainless steel (SS) alloys, because the native composition of SS contains elements that seed CNT growth upon hydrocarbon exposure at elevated temperature. Often such methods use acid immersion or oxidation in air to treat the surface prior to hydrocarbon exposure for CNT growth. However, there lacks a general understanding of how the surface chemistry and morphology influences the nucleation and growth of CNTs. Using environmental transmission electron microscopy, we observe that CNT growth is enabled by surface reconstruction of SS upon oxygen exposure at elevated temperature, followed by further breakup of the surface upon reduction, and subsequent CNT nucleation and growth upon hydrocarbon exposure. Using electron energy loss spectroscopy, we find that catalyst particles consist of both pure iron as well as iron alloys such as Fe–Cr and Fe–Ni. We use these insights to study the synthesis of CNTs on bulk net-shaped porous SS materials and show that annealing of the SS at 1000 °C in air prior to CVD using an ethylene feedstock mixture produces a 70-fold increase in CNT yield. Our findings demonstrate how process conditions can be designed for efficient manufacturing of CNT-enhanced stainless steel materials, and guide improved understanding of CNT growth on other industrially relevant metal substrates.