State of Transition Metal Catalysts During Carbon Nanotube Growth

Abstract

We study catalyst-support and catalyst-carbon interactions during the chemical vapor deposition of singlewalled carbon nanotubes by combining environmental transmission microscopy and in situ, time-resolved X-ray photoelectron spectroscopy. We present direct evidence of what constitutes catalyst functionality by comparing the behavior of Ni, Fe, Pd, and Au model catalyst films on SiO 2 during preannealing in O 2 and NH 3 and during C 2 H 2 decomposition. The catalyst metal surface supplies sites to dissociate the hydrocarbon precursor and then guides the formation of a carbon lattice and the liftoff of a carbon cap. The catalysts are sharply distinguished by their reactivity toward activation of the hydrocarbon precursor, following trends known from heterogeneous catalysis. For Fe and Ni, the active state of the catalyst is a crystalline metallic nanoparticle. Graphitic networks do not form on oxidized Fe. Pd forms a silicide on SiO 2 under our reducing conditions. Pd (silicides) and Au nanocrystals are catalytically less efficient in terms of precursor dissociation, while the low adhesion of C on Au surfaces impedes nanotube nucleation.

X-ray photoelectron spectroscopy study on Fe and Co catalysts during the first stages of ethanol chemical vapor deposition for single-walled carbon nanotube growth

“…This strongly suggests that Fe is only active in CNT growth in its metallic phase. This is consistent with previous results, 5,6 however, the metal oxidation state is not the only variable influencing CNT growth. Surprisingly, the oxidized Fe on Al 2 O 3 exposed to ethanol with filament on did not give a SWCNT forest, while the Fe 0 peak is relatively strong, while metallic Fe on SiO 2 exposed to ethanol with the filament on has a smaller Fe 0 signal, but does grow a small forest.…”

“…This reduces the yield of CNT growth, since only metallic Fe is active in CNT growth. 5,6 Ethanol has to be decomposed (yielding a partly reducing atmosphere, with activated carbon species) by either a hot filament (as in our case) or by another technique such as a plasma 3,14 or hot wall CVD 4,13 in order to be a active carbon containing gas for SWCNT forest growth using iron as a catalyst. Co is not oxidized in ethanol at CNT growth temperatures.…”

“…[5][6][7][8][9][10] It has been claimed that Fe/Al 2 O 3 interfacial bonding restricts Fe surface mobility and hence allows for high density SWCNT forest growth. 5,6 In addition, it has been suggested 7,8 that Al 2 O 3 , which is a well known catalyst for hydrocarbon formation, 11 aids in decomposing the carbon precursor and therefore enhances SWCNT forest growth from the catalyst. This suggestion, however, has been rejected by Mattevi et al 6 In addition, the morphology of the substrate has been proven to play an important role in the behavior of the catalyst, 9,10 and therefore determines whether or not a SWCNT forest is grown.…”

“…In situ x-ray photoelectron spectroscopy (XPS) studies 5,6 have revealed that the catalyst has to be in its metallic state in order to be active in carbon nanotube growth. However, little is known about the influence of the carbon containing gas on the oxidation state of the catalyst.…”

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

“…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.…”