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1.
Nanoscale ; 12(42): 21923-21931, 2020 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-33112348

RESUMO

The structure and phase transformation of a cobalt (Co) catalyst, during single walled carbon nanotube (SWCNT) growth, is elucidated for inactive, active and deactivated nanoparticles by in situ imaging using an environmental transmission electron microscope. During nanotube growth, the structure was analyzed using Miller indices to determine the types of planes that favor anchoring or liftoff of nanotubes from the Co catalyst. Density functional theory was further applied to model the catalyst interactions to compare the work of adhesion of the catalyst's faceted planes to understand the interactions of different Miller planes with the graphene structure. Through in-depth studies of multiple distinct Co nanoparticles, we established a dominant nanoparticle phase for SWCNT growth. In addition, we identified the preferred lattice planes and a threshold for work of adhesion to allow the anchoring and liftoff of SWCNTs.

2.
Phys Chem Chem Phys ; 21(44): 24543-24553, 2019 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-31663578

RESUMO

We aim at elucidating the mechanism of the trimethyl aluminum (TMA) decomposition on oxidized nickel (NiO) and metallic nickel (Ni) facets in the absence of a source of hydroxyl groups. This TMA decomposition mechanism constitutes the earliest stage of growth of Al2O3 coatings with the atomic layer decomposition (ALD) method, which stabilizes nickel catalysts in energy-intensive processes such as the dry reforming of methane. Our first-principles calculations suggest thermodynamic favorability for the TMA decomposition on metallic nickel compared to oxidized nickel. Moreover, the decomposition of TMA on metallic nickel showed almost no differences in terms of energy barriers between flat and stepped surfaces. Regarding the impact of the CH3 radicals formed after TMA decomposition, we calculated stronger adsorption on metallic nickel facets than on oxidized nickel, and these adsorption energies are comparable to the adsorption energies calculated in earlier works on Al2O3 ALD growth on palladium surfaces. These results lead us to believe in the growth of porous Al2O3 coatings triggered by CH3 contamination rather than due to preferential TMA decomposition on stepped and/or defective facets. The CH3 radicals are likely to be thermally stable at temperatures used during Al2O3 ALD processes, partially passivating the surface towards further TMA decomposition.

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