Size-Dependent Oxidative Stability of Silicon Nanoclusters Mixed with a Tantalum Atom

Metal-encapsulating silicon cage (M@Si₁₆) nanoclusters (NCs) are promising superatoms (SAs) as function-tunable nanomaterials, which exhibit a superior chemical stability owing to electronic and geometric closures. Here, we examine how the superatomic nature of an alkali-like Ta@Si₁₆SA is staggered by the variation of the number of Si atoms in TaSi_nNCs (n = 6, 8, 12, 15, 17, and 18) and subsequent immobilization on a C₆₀fullerene substrate. Using X-ray photoelectron spectroscopy, the size dependence of chemical robustness of TaSi_nNCs on C₆₀against O₂exposures is quantitatively evaluated: In addition to the most outstanding stability of a Ta@Si₁₆SA as compared to TaSi₁₅, some enhanced stability is also observed at Ta@Si₁₇, which couples in a Si-adatom structure of (Ta@Si₁₆)-Si to the support. While oxidative reactivities of TaSi_nare gradually suppressed from small NCs (n = 6) to larger ones (n = 18) with increasing number of Si atoms, the results show that (1) an enclosing Si cage around a Ta atom is completed at Ta@Si₁₆on C₆₀and (2) Ta@Si₁₇NCs are robust against O₂oxidation with assistance from the stability of Ta@Si₁₆SA despite the Si-adatom.