Evolution of Dirac Nodal Arc Surface States as the Electronic Engine of Alkaline HER on Oxide-Free PtSn4 Surface

Dirac nodal arc semimetal PtSn4 has been experimentally shown to be a promising catalyst for the hydrogen evolution reaction (HER) owing to its reduced consumption of the precious metal and low overpotential. The catalytic activity of PtSn4 has been hypothetically attributed to the topologically protected surface states when the surface remains unoxidized. However, the specific involvement of its topological surface states in the HER process remains to be fully elucidated, and it is indeed in question if the topological nature of this alloy is catalytically consequential. This study stands upon the viewpoint of topological catalysis and reveals the evolution of the PtSn4 surface states within an alkaline HER cycle from first principles. We show that Sn vacancies on the pristine Sn-terminated (010) surface result in a reconstructed surface exposing both the Pt and Sn atoms. Such a reconstruction is supported by the excellent agreement between the simulated and the experimental scanning tunneling microscopy images. Hydrides then form on this surface as the resting state of the catalyst. As hydrogen atoms bind to the surface, the topologically protected Dirac node surface states respond to further reconstructions, splitting into two surface bands near the Fermi level. Through band structure and wavefunction analysis, we highlight three key surface states: initial bonding state, reserved bonding state, and antibonding state. They are responsible for driving the HER cycle through interacting with the incoming hydrogen in various stages. Despite the major perturbation created by the reaction conditions, the surface bands form a stable configuration, serving consistently as the frontier states. Thus, the topological protection allows the Dirac node surface states to be an active and evolving driver of HER catalysis on PtSn4. Our findings provide detailed evidences for the direct participation of nontrivial surface states in HER on PtSn4 when utilized as a topological catalyst.