Nitric oxide oxidation of a Ta encapsulating Si cage nanocluster superatom (Ta@Si₁₆) deposited on an organic substrate; A Si cage collapse indicator

The chemical reaction kinetics of an alkali-like superatom comprising a tantalum encapsulating Si₁₆ cage nanocluster (Ta@Si₁₆) deposited on an n-type organic substrate composed of overlayered C₆₀ fullerene upon exposure to nitric oxide (NO) as a reactive gas are investigated. Core level X-ray photoelectron spectroscopy reveals that Ta@Si₁₆ oxidation with NO proceeds stepwise from the outer Si₁₆ cage to the central Ta atom; during the initial stage, NO is dissociatively chemisorbed by the cage surface of Ta@Si₁₆ without penetrating the cage, while under extreme reaction conditions, the collapse of the Si₁₆ cage leads to NO oxidation of the central Ta atom. In particular, molecular NO adsorption is associated with Ta oxidation only after the collapse of the Si₁₆ cage of Ta@Si₁₆. The reaction kinetics of M@Si₁₆ with NO in the earlier stages of oxidation are discussed in conjunction with density functional theory calculations. Due to the superatomic nature of the shell closure with valence electrons coupled with metal encapsulation, surface oxidation of the caged Si in Ta@Si₁₆ takes place gently compared to that of a naked Si surface, with molecularly physisorbed NO functioning as an indicator of Si cage collapse.