Investigation of 2D Boridene from First Principles and Experiments

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 (Mo2/3Y1/3)2AlB2 and (Mo2/3Sc1/3)2AlB2. The chemical formula of the boridene was suggested to be Mo4/3B2-xTz, where Tz denotes surface terminations. Here, the termination composition and material properties of Mo4/3B2-xTz 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. Mo4/3B2-xTz 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 Mo1.33B1.9O0.3(OH)1.5F0.7 from X-ray photoelectron spectroscopy (XPS) analysis which, within error margins, is consistent with the theoretical results. Finally, metallic and additive-free Mo4/3B2-xTz shows high catalytic performance for the hydrogen evolution reaction, with an onset potential of 0.15 V versus the reversible hydrogen electrode.Funding agencies: The Knut and Alice Wallenberg Foundation (KAW 2020.0033), The Swedish Foundation for Strategic Research (EM16-0004 and ARC19-0026), The Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No 2009 00971), The Swedish Research Council (no. 2018-03927 and 2019-04233). The calculations were carried out using supercomputer resources provided by the Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre (NSC) and the PDC Center for high-performance computing partially funded by the Swedish Research Council through grant agreement no. 2018-05973.