Investigation of 2D Boridene from First Principles and Experiments

Abstract Recently, a 2D metal boride – boridene – has been experimentally realized in the form of single‐layer molybdenum boride sheets with ordered metal vacancies, through selective etching of the nanolaminated 3D parent borides (Mo 2/3 Y 1/3 ) 2 AlB 2 and (Mo 2/3 Sc 1/3 ) 2 AlB 2 . The chemical formula of the boridene was suggested to be Mo 4/3 B 2‐ x T z , where T z denotes surface terminations. Here, the termination composition and material properties of Mo 4/3 B 2‐ x T z from both theoretical and experimental perspectives are investigated. Termination sites are considered theoretically for termination species T = O, OH, and F, and the energetically favored termination configuration is identified at z = 2 for both single species terminations and binary termination mixes of different stoichiometries in ordered and disordered configurations. Mo 4/3 B 2‐ x T z is shown to be dynamically stable for multiple termination stoichiometries, displaying semiconducting, semimetallic, or metallic behavior depending on how different terminations are combined. The approximate chemical formula of a freestanding film of boridene is attained as Mo 1.33 B 1.9 O 0.3 (OH) 1.5 F 0.7 from X‐ray photoelectron spectroscopy (XPS) analysis which, within error margins, is consistent with the theoretical results. Finally, metallic and additive‐free Mo 4/3 B 2‐ x T z shows high catalytic performance for the hydrogen evolution reaction, with an onset potential of 0.15 V versus the reversible hydrogen electrode.