(521aj) Thermodynamic and Electronic Properties of MoSi2N4 Edges

The past two decades have seen extensive investigation of two-dimensional materials for catalysis. A novel two-dimensional nitride, MoSi₂N₄, was fabricated in 2020. Analogous to previous studies of MoS₂, we hypothesize that edges of MoSi₂N₄ could display interesting catalytic properties. However, there has not yet been a systematic characterization of the thermodynamic stability of various edges of MoSi₂N₄ and their chemical behavior.

In this work, we first identified a range of chemical potentials that quantify the thermodynamic favorability of formation of MoSi₂N₄. We then modeled several armchair and zigzag edges and calculated their edge energies to quantify their thermodynamic stability. Once the favorable edges were determined, we characterized them via electronic structure calculations (band structures and electronic density of states). The metallic Mo sites on the lowest energy edge were selected for further studies of catalytic activity towards the hydrogen evolution reaction. Vienna ab initio Simulation Package was used to perform Density Functional Theory calculations.

Results and Discussions:

To understand the activity of edges towards the hydrogen evolution reaction (HER) catalyst, we studied the adsorption of a proton on various active edge sites of the lowest energy edge (armchair edge). Figure 1b depicts the Gibbs free-energy for sequential adsorption of H at the Mo sites. Initially, the Mo site adsorbs H very strongly with an adsorption energy of ~0.8 eV per H atom (edge+2H* in Figure 1b) and these H atoms are not expected to participate in HER. However, the Mo site is still sufficiently undercoordinated such that a third H atom can be adsorbed, and this step is thermoneutral (~0 eV). Thus, we predict that the studied armchair edge will initially adsorb H strongly (spectator species) but, beyond a critical coverage, will catalyze HER effectively.