Influence of Alumina Type on the Evolution and Activity of Alumina-Supported Fe Catalysts in Single-Walled Carbon Nanotube Carpet Growth

Abstract

We have studied the lifetime, activity, and evolution of Fe catalysts supported on different types of alumina: (a) sputter deposited alumina films (sputtered/Fe), (b) electron-beam deposited alumina films (e-beam/Fe), (c) annealed e-beam deposited alumina films (annealed e-beam/Fe), (d) alumina films deposited by atomic layer deposition (ALD/Fe), and (e) c-cut sapphire (sapphire/Fe). We show that the catalytic behavior, Ostwald ripening, and subsurface diffusion rates of Fe catalyst supported on alumina during water-assisted growth or "supergrowth" of single-walled carbon nanotube (SWNT) carpets are strongly influenced by the porosity of the alumina support. The catalytic activity increases in the following order: sapphire/Fe < annealed e-beam/Fe < ALD/Fe < e-beam/Fe < sputtered/Fe. With a combination of microscopic and spectroscopic characterization, we further show that the Ostwald ripening rates of the catalysts and the porosity of the alumina support correlate with the lifetime and activity of the catalysts. Specifically, our results reveal that SWNT carpet growth is maximized by very low Ostwald ripening rates, mild subsurface diffusion rates, and high porosity, which is best achieved in the sputtered/Fe catalyst. These results not only emphasize the connection between catalytic activity and particle stability during growth, but guide current efforts aimed at rational design of catalysts for enhanced and controlled SWNT carpet growth.

Data-driven understanding of collective carbon nanotube growth by in situ characterization and nanoscale metrology

“…Moreover, for the same buffer layer material aluminum oxide, different processing routes and film properties were shown to significantly influence CNT growth. 15 Over the years, many approaches were explored for modifying the catalyst to improve growth. For example, growth was prolonged by tuning the stoichiometry of bimetallic catalyst particles, such as Ni-Fe, 28 and Co-Mo.…”

“…33 In cases of strong interactions between the active catalyst nanoparticles and the underlying support layer, such as between iron and aluminum oxide, base-growth is the dominant mechanism. 15,34 Moreover, the growth mechanism was shown to shift between tip growth and base growth based on the diameter of the catalyst nanoparticle. 35 Previously, backscatter-electron imaging was used to show that there are no traces of metal nanoparticles on the top of grown CNT forests, 36 to confirm the base-growth process for Fe nanoparticle catalysts on aluminum oxide support layers.…”

“…Film restructuring of the iron/aluminum oxide system was studied previously by a number of surface characterization techniques, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), electron diffraction (ED), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and in situ grazing incidence small angle X-ray scattering (GI-SAXS). 7,15,27,[37][38][39][40][41][42][43] Many imaging and chemical characterization techniques such as electron microscopy and XPS are typically difficult to employ in situ owing to the stringent requirements of low pressure, and limitations on the gases that can be introduced into the measurement chamber. On the other hand, scattering-based methods such as GI-XRD and GI-SAXS provided unique information on the morphology and chemical state of the catalyst during the preparation steps at higher pressures.…”

“…[5][6][7][8] These variations arise from the dynamics of growth, which are dominated by time-varying kinetics of chemical decomposition, catalytic activation, catalyst poisoning/deactivation, and atomic diffusion. [9][10][11][12][13][14][15][16] Importantly, the non-uniform morphology of CNT forests influences their properties, such as in mechanical compression for example, which was shown to be affected by density gradients. 17 Hence, revealing the process-structure and structure-property relationships requires a comprehensive understanding of the atomic scale physicochemical processes underlying the bottom-up synthesis and selforganization of aligned CNTs.…”

Investigation of carbon nanotube growth termination mechanism by in-situ transmission electron microscopy approaches

“…Along with lots of attempts to optimize CNT growth parameters, there has also been a great deal of effort to understand the mechanisms of growth termination. Even though several plausible mechanisms have been proposed (carbide formation of catalyst particles [11], mechanical stresses exerted on the growing array [12], and others), a recent review paper [13] introduced two mechanisms considered the most probable for the termination of CNT growth: carbon over-layer formation [14,15] and dynamic morphological evolution of catalyst particles [16][17][18][19]. For the growth termination mechanism by carbon over-layer formation, as CNT growth proceeds, amorphous carbon or a carbon over-layer starts to accumulate on the active catalyst surfaces.…”