Surface functionalization of two-dimensional boridene family: Enhanced stability, tunable electronic property, and high catalytic activity

Recently, a two-dimensional (2D) boridene sheet, in the form of molybdenum boride with ordered metal vacancies (Mo4/3B2), has been experimentally realized (Science 2021, 373, 801). The synthesized boridene layer has active surfaces, which offer great feasibility to employ functional groups to modulate the structural, physical, and chemical properties of the materials. Herein, we conduct systematical first-principles calculations to study the structural, mechanical, electronic, and catalytic properties of functionalized boridene sheets M4/3B2Tz (M = Mo, W, Ta, Nb; Tz = O, OH, F). We demonstrate that the surface functional groups can effectively improve the dynamical stabilities of the materials and nine stable configurations were screened out. Among them, Mo4/3B2O2 and W4/3B2O2 sheets are semiconducting with indirect band gaps of 1.07 and 0.81 eV, respectively. While other structures are metallic, some of which possess Dirac points close to the Fermi level. Furthermore, we found a mixture of O and OH terminations can activate the HER performance at the basal plane for all structures. Remarkably, the calculated Gibbs free energy for W4/3B2Tz and Ta4/3B2Tz exhibit near-zero values, which are ideal as HER catalysts. Our work provides theoretical insights of surface functionalization of 2D boridenes for novel electronic and catalytic applications.