Mechanisms of alumina growth via atomic layer deposition on nickel oxide and metallic nickel surfaces

Abstract

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.