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

Metal-encapsulating silicon cage (M@Si-16) 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@Si-16 SA is staggered by the variation of the number of Si atoms in TaSin NCs (n = 6, 8, 12, 15, 17, and 18) and subsequent immobilization on a C-60 fullerene substrate. Using X-ray photoelectron spectroscopy, the size dependence of chemical robustness of TaSin NCs on C-60 against O-2 exposures is quantitatively evaluated: In addition to the most outstanding stability of a Ta@Si-16 SA as compared to TaSi15, some enhanced stability is also observed at Ta@Si-17, which couples in a Si-adatom structure of (Ta@Si-16)-Si to the support. While oxidative reactivities of TaSin are 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@Si-16 on C-60 and (2) Ta@Si-17 NCs are robust against O-2 oxidation with assistance from the stability of Ta@Si-16 SA despite the Si-adatom.