Mechanisms for Chemical Vapor Deposition Carbon Nanotube Growth by Surface Modification of 316L Stainless Steel

Carbon nanotubes (CNT) and carbon nanofibers (CNF) can be grown on a variety of substrates by coating the surface with a nanoparticle catalyst layer, usually iron. A hydrocarbon gas is then flowed over the catalyst layer at high temperatures. The catalyst will break down the hydrocarbons, resulting in CNT growth. Stainless steel stands out as a unique choice of substrate because it may not require an additional catalyst to grow CNTs [1]. Instead, the stainless steel surface must be modified in some manner to encourage CNT growth. One such method is an oxidative heat treatment, which builds up a faceted oxide layer on the steel [2]. Current models predict that iron nanoparticles reduce out of the oxide when it is exposed to the hydrocarbon gas, acting as catalysts for the nanotubes [3]. In our experiment, we used polished chips of 316L stainless steel as our CNT substrate. The substrate is brought up to temperature (700 or 800 °C) and held at that temperature while being exposed to a series of gasses. The sample is first exposed to air for a short amount of time (4 minutes), serving as an oxidative heat treatment. Next, the samples were exposed to a mix of ethylene and argon gas, initiating CNT growth. Some samples received an additional step between the air and ethylene flows, during which hydrogen and argon were passed over the oxidized steel to reduce the oxides. Several runs were made to study the parameter space, such as variations in furnace temperature, reduction time, and growth time, in order to help us draw comparisons as to how each parameter affects the substrate. Our samples were initially studied using a FEI Helios Nanolab 600 dual-beam scanning electron microscope/focused ion beam (SEM/FIB). A portion of each sample was lightly scraped to remove the CNTs and expose the modified steel surface. Under the SEM we gathered data regarding tube diameter, forest height, and details on surface modification. Using the FIB in the same instrument, cross-sections of the surface were milled and inspected under the SEM to study the modified surface of the substrate. Additionally, thinned lamella cross-sections were extracted out of several samples to further study the interface between the CNT growth attachment points and the modified steel surface. These lamellae were imaged using an FEI Tecnai F20 S/TEM. Energy dispersive x-ray spectroscopy (EDS) was used to compare the elemental composition of the steel surface against the bulk material. Our collected images and EDS data support the model that iron nanoparticles reduce out of the oxidized surface as it is exposed to high temperature ethylene. Iron particles can be seen in the nanotubes themselves as well as within the oxide layer. The oxide layer, as generated by the initial heat treatment in air, begins rich in both iron and chromium, but it appears that it becomes more chromium rich after growth. This suggests that iron left the oxide to form these nanoparticles. The nanotubes were seen to grow from holes in the oxide layer rather than the surface. Reducing the oxides with hydrogen before exposing the sample to ethylene was observed to inhibit CNT growth. This is likely due to the iron nanoparticles growing too large to support CNT growth. The nanotubes found on our sample ranged from 10–100 nm in diameter, and the growth was of too low density to support aligned growth [4]. Carbon nanotubes grown on a steel sample. Note the brighter iron particles that can be seen in the nanotube tips and cores. Oxidized steel surface after removal of CNTs by scraping. Note the holes that were left behind by the CNTs. Cross-section of the nanotubes and steel surface made using a FIB. Nanotube-steel interface can be seen at the boundary. CNTs are coated in a protective layer of alumina. Dark Field STEM image of a lamella cross-section taken from the sample. CNTs are seen interacting with the oxide. Iron particles can be seen inside the CNTs.