hcp metal nanoclusters with hexagonalA−Abilayer stacking stabilized by enhanced covalent bonding

First-principles total energy calculations within density functional theory have been performed to study the geometric and electronic structures of ${\text{Ru}}_{n}$ nanoclusters of varying size $n$ $(14\ensuremath{\le}n\ensuremath{\le}42)$. Strikingly, for the size range of $n=14$ to 38, the clusters always prefer a hexagonal bilayer structure with $A\text{\ensuremath{-}}A$ stacking, rather than some of the more closely packed forms, or bilayer with $A\text{\ensuremath{-}}B$ stacking. Such an intriguing ``molecular double-wheel'' form is stabilized by substantially enhanced interlayer covalent bonding associated with strong $s\text{\ensuremath{-}}d$ hybridization. Similar $A\text{\ensuremath{-}}A$ stacking is also observed in the ground states or low-lying isomers of the clusters composed of other hcp elements, such as Os, Tc, Re, and Co. Note that these ``molecular double-wheels'' show enhanced chemical activity toward ${\text{H}}_{2}\text{O}$ splitting relative to their bulk counterpart, implying its potential applications as nanocatalysts.