Surface termination of MgB2 unveiled by a combination of adsorption experiments and theoretical calculations

Improving the Catalytic Performance of the Hydrogen Evolution Reaction of α-MoB2 via Rational Doping by Transition Metal Elements

Abundant transition metal borides are emerging as promising electrochemical hydrogen evolution reaction (HER) catalysts which have a potential to substitute noble metals. Those containing graphene-like (flat) boron layers, such as α-MoB2, are particularly promising and their performance can be further enhanced via doping by the second metal. In order to understand intrinsic effect of doping and rationalize selection of dopants, we employ density functional theory (DFT) calculations to study substitutional doping of α-MoB₂ by transition metals as a route towards systematic improvement of intrinsic catalytic activity towards HER. We calculated thermodynamic stability of various transition metal elements to select metals which form a stable ternary phase with α-MoB₂ . We inspected surface stability of dopants and assessed catalytic activity of doped surface through hydrogen binding free energy at various hydrogen coverages. We calculated the reaction barriers and pathways for the Tafel step of HER for the most promising dopants. The results highlight iron as the best dopant, simultaneously lowering the reaction barrier of the Tafel step while having suitable thermodynamic stability within MoB₂ lattice.

Spontaneous dynamical disordering of borophenes in MgB2 and related metal borides

Layered boron compounds have attracted significant interest in applications from energy storage to electronic materials to device applications, owing in part to a diversity of surface properties tied to specific arrangements of boron atoms. Here we report the energy landscape for surface atomic configurations of MgB₂ by combining first-principles calculations, global optimization, material synthesis and characterization. We demonstrate that contrary to previous assumptions, multiple disordered reconstructions are thermodynamically preferred and kinetically accessible within exposed B surfaces in MgB₂ and other layered metal diborides at low boron chemical potentials. Such a dynamic environment and intrinsic disordering of the B surface atoms present new opportunities to realize a diverse set of 2D boron structures. We validated the predicted surface disorder by characterizing exfoliated boron-terminated MgB₂ nanosheets. We further discuss application-relevant implications, with a particular view towards understanding the impact of boron surface heterogeneity on hydrogen storage performance.

Layered boron compounds attract enormous interest in applications. This work reports first-principles calculations coupled with global optimization to show that the outer boron surface in MgB₂ nanosheets undergo disordering and clustering, which is experimentally confirmed in synthesized MgB₂ nanosheets.