A cluster-assembled B26 monolayer and its inverse-sandwiched derivative B24P4: stable 2D boron-based materials with topological properties, superconductivity, and electrocatalytic activity

Abstract

The rich bonding versatility of boron enables the design of exotic two-dimensional (2D) monolayers with unconventional properties. Previous studies have demonstrated that stable boron sheets can be constructed from combinations of triangles and hexagons, while inverse sandwich nanostructures exhibit remarkable stability and multifunctionality. In this work, we propose a novel B₂₆ monolayer which consists of six B₄ rhombuses and two central B atoms connected in a zigzag pattern, forming a planar structure with 28 triangles and 6 hexagons per unit cell. The new B₂₆ monolayer has a cohesive energy very close to that of experimentally prepared β₁₃ and χ₃, and it displays nontrivial topological properties, including the nontrivial Berry phase of π and the presence of topologically protected edge states. Meanwhile, it exhibits superconductivity with a transition temperature T_c of ∼1.46 K. Furthermore, by vertically substituting two central boron atoms located in the fused tri-hexagon units in B₂₆ with P₂ dimers, we construct a stable inverse-sandwich B₂₄P₄ monolayer. This material not only shows enhanced superconductivity (T_c ∼20.10 K), but also acts as a promising metal-free electrocatalyst for the hydrogen evolution reaction (HER), surpassing B₂₆, owing to more favourable multi-hydrogen adsorption energetics and closer alignment with ideal thermodynamic conditions for hydrogen evolution. Our findings offer a new strategy for designing boron-based monolayers with exceptional structural stability and multifaceted properties and functionalities.