From the single-atom limit to the mixed-metal phase: finding the optimum condition for activating the basal plane of a FePSe 3 monolayer towards HER.

Layered ternary transition metal tri-chalcogenides are some of the most promising candidates for hydrogen evolution reaction (HER) because of their ease of synthesis and affordability. However, majority of the materials in this category have HER active sites only at their edges, rendering a large portion of the catalyst useless. In this work, ways for activating the basal planes of one of these materials, namely, FePSe 3 , are explored. The effects of substitutional transition metal doping and external biaxial tensile strain on the HER activity of the basal plane of a FePSe 3 monolayer are studied via first principles electronic structure calculations based on density functional theory. This study reveals that although the basal plane of the pristine material is inactive towards HER (value of H adsorption free energy, Δ G H* = 1.41 eV), 25% Zr, Mo, and Tc doping makes it more active (Δ G H* = 0.25, 0.22 and 0.13 eV, respectively). The effect of reducing the doping concentration, moving to the single-atom limit, on the catalytic activity is studied for Sc, Y, Zr, Mo, Tc and Rh dopants. For Tc, the mixed-metal phase FeTcP 2 Se 6 is also studied. Among the unstrained materials, 25% Tc-doped FePSe 3 gives the best result. Significant tunability of HER catalytic activity in the 6.25% Sc doped FePSe 3 monolayer via strain engineering is also discovered. An external tensile strain of 5% reduces Δ G H* to ∼0 eV from 1.08 eV in the unstrained material, making this an attractive candidate for HER catalysis. The Volmer-Heyrovsky and Volmer-Tafel pathways are examined for some of the systems. A fascinating correlation between the electronic density of states and HER activity is also observed in most materials.