Anchoring effect of Ni²⁺ in stabilizing reduced metallic particles for growing single-walled carbon nanotubes

The suitability of the Ni[sbnd]MgO catalyst as a catalyst in chiral-selective growth of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition has been assessed. It reveals that catalyst calcination temperature plays an important role in affecting the catalyst performances. Using CO as the carbon precursor and a chemical vapor deposition reaction temperature of 600 °C, Ni[sbnd]MgO pre-calcined at 600 °C demonstrates the best performances in catalyzing the growth of SWNTs with predominant (6, 5) species. Systematic characterizations on catalysts calcinated at different temperatures indicate that Ni²⁺ ions diffuse towards the interior of MgO matrix upon annealing. DFT-based calculations reveal that the binding energy between Ni²⁺ and adjacent Ni(0) is larger than that between Mg²⁺ and Ni (0), while Ni²⁺ situated deep inside MgO has weak interactions with surface Ni atoms. This work highlights the importance of subsurface Ni²⁺ in anchoring reduced surface Ni atom, which inhibits the aggregation of Ni particles and therefore, facilitates the growth of SWNTs with a narrow chirality distribution.