Engineering electronic structures of Nb₆I superatomic clusters by metal atom incorporation: a first-principles study

Akin to the conventional building blocks of solids, the elements in the periodic table, the iodine capped Nb₆I (x = 12-18) octahedron cluster is a well-known "superatomic building block" for designing not only cluster assemblies like Prussian blue analogues and hybrid perovskites but also for new catalysts. Recent achievements within the realm of atomic clusters using experimental and theoretical approaches have shown synthesis and precise tuning of magnetic and electronic properties of small octahedron clusters via atom-by-atom substitution. In this work, state-of-the-art density functional theory (DFT) calculations highlight the feasibility of tuning the valence electron concentration (VEC) by endohedral doping of 3d-block transition metals (M = Sc-Zn) inside the core of the homo-iodide Nb₆ octahedron. The energetics of various structural isomers imply that all other M atoms can be doped within the Nb₆I cluster with high I atom concentrations, except Sc and Ti. In-depth analysis shows the cohesive energy per atom and the formation energy (FE) of the clusters correlate with various structural and electronic parameters of these clusters exhibiting electronic shell closures with 18 and 24 VEC, with a substantial HOMO-LUMO gap within the range of 0.72 to 1.76 eV using HSE06 hybrid functionals. Interestingly, the FE calculations demonstrate that the MNb₆I₁₈ cluster can be formed even under M-rich conditions, having robust formation stability. We further explain how the Gibbs free energy (ΔG_H) of adsorbed hydrogen correlates with different d-band centres of Nb and M atoms to highlight the impact of electronic structure on the catalytic activity towards the hydrogen evolution reaction (HER). Thus, it is possible to achieve potential HER catalysts with very small ΔG_H for VNb₆I₁₈ to CoNb₆I₁₈ comparable to the Pt(111) surface. This work provides the atomic structure, stability, and electronic properties of endohedral doped Nb₆I clusters that can pave the way for various potential applications in catalysis, molecular electronics and spintronics, and for further experimental research.