Unraveling the Surface Termination and Evolution of Topological Surface States for Electrocatalyst PtSn4 in Alkaline HER

Dirac nodal arc semimetal PtSn4 has been experimentally demonstrated as a promising electrocatalyst for hydrogen evolution reaction (HER) under both acidic and alkaline conditions. While two possible mechanisms have been proposed to explain its activity, the role of its topological surface states in HER remains unclear. It is indeed in question if the topological surface of this alloy is catalytically consequential. In this study, we investigate the surface termination that sustains topological surface states on PtSn4, and track their evolution during HER catalysis. We show that a reconstructed surface with a Sn-poor termination reproduces both the topological character and the scanning tunneling microscopy pattern observed in experiments. Through phase diagram and surface structure analysis, we outline the HER profile following the Volmer-Heyrovsky mechanism. As hydrogen atoms adsorb onto the surface, the structure undergoes further reconstruction to an equilibrium phase with a coverage of two hydrides per unit cell. Meanwhile, the surface electronic bands evolve in response to interactions with the adsorbed hydrogen atoms. We propose a hybridization diagram for understanding the surface state evolution based on wavefunction and chemical bonding analyses. While the Pt atoms serve as conventional sites for hydrogen binding, the surface states of PtSn4 are essential for stabilizing the hydrogen antibonding states via in-phase electronic interactions with the Sn components. This stabilization results in frontier surface bands that are responsible for driving the HER catalysis. Our findings provide a detailed description for the direct involvement of surface states on PtSn4 when employed as a topological catalyst for HER.