Unveiling the Catalytic Potential of Topological Nodal-Line Semimetal AuSn4 for Hydrogen Evolution and CO2 Reduction

Abstract

In recent years, the correlation between the existence of topological electronic states in materials and their catalytic activity has gained increasing attention, due to the exceptional electron conductivity and charge carrier mobility exhibited by quantum materials. However, the physicochemical mechanisms ruling catalysis with quantum materials are not fully understood. Here, we investigate the chemical reactivity, ambient stability, and catalytic activity of the topological nodal-line semimetal AuSn₄. Our findings reveal that the surface of AuSn₄ is prone to oxidation, resulting in the formation of a nanometric SnO₂ skin. This surface oxidation significantly enhances the material's performance as a catalyst for the hydrogen evolution reaction in acidic environments. We demonstrate that the peculiar atomic structure of oxidized AuSn₄ enables the migration of hydrogen atoms through the Sn-O layer with a minimal energy barrier of only 0.19 eV. Furthermore, the Volmer step becomes exothermic in the presence of Sn vacancies or tin-oxide skin, as opposed to being hindered in the pristine sample, with energy values of -0.62 and -1.66 eV, respectively, compared to the +0.46 eV energy barrier in the pristine sample. Our model also suggests that oxidized AuSn₄ can serve as a catalyst for the hydrogen evolution reaction in alkali media. Additionally, we evaluate the material's suitability for the carbon dioxide reduction reaction, finding that the presence of topologically protected electronic states enhances the migration of hydrogen atoms adsorbed on the catalyst to carbon dioxide.

Figure 1

(a, b) Side and top views of the atomic structure of AuSn₄. Yellow and gray balls denote Au and Sn atoms, respectively. (c) XPS survey spectrum of as-cleaved AuSn₄. (d) XRD spectrum for a single crystal of AuSn₄. The inset shows a photograph of grown single crystals. (e) Bulk electronic structure of AuSn₄ obtained by density functional theory. Bands were obtained by projecting the available k_z within the whole Brillouin zone. (f) The corresponding ARPES data are shown as a sum of in-plane and out-of-plane light polarization, with photons of 48 eV. At this energy, it is possible to access the full Brillouin zone along Γ-X, along the orange line of the inset, and the photoemission matrix elements are favorable. The overall agreement between the calculated and the measured energy-momentum dispersion is evident.

Figure 2

Orbital-resolved band structure (left) and total and partial densities of states of bulk AuSn₄ (right).

Figure 3

Optimized atomic structure of (a) molecular oxygen physisorbed on one Sn vacancy in the surface layer of AuSn_3.88, (b) oxygen molecule decomposed on the surface of AuSn₄, (c) formation of oxidized layer on the surface of AuSn₄. Panels d–f represent the various reactions steps of the HER on the oxidized surface of AuSn₄, namely (d) adsorption of hydrogen, (e) decomposition of water, and (f) first step of desorption of OH^– on the oxidized surface of AuSn₄.

Figure 4

Changes of the charge density after formation of the interface between (a) surface Sn and (b) SnO₂ layers and the subsurface part of the AuSn₄ slab. Panel (c) reports the total densities of states for both structures shown on panels (a) and (b). Panels (d) and (e) report the partial densities of states (pDOS) of 5d orbitals of the Au atoms from subsurface layers for the cases of (d) AuSn₄ and AuSn_3.88 (in the latter case, the pDOS is shown for the Au atom closest to Sn vacancy), and for (e) AuSn₄O and AuSn₄O+H (in the latter case, the pDOS is shown for the Au atom closest to adsorbed H).

Figure 5

Core-level spectra of Au-4f and Sn-3d collected from as-cleaved AuSn₄ (black curve) and from the same surface exposed to O₂ (red curve), H₂O (green curve), and CO (blue curve). The AuSn₄ surface was also kept in air for different times: 10 min (pink curve), 90 min (brown curve), and 12 h (dark blue curve). The photon energy is 900 eV, and the spectra are normalized to the maximum.

Figure 6

Free energy diagram for HER in (a) acidic and (b) alkali media and (c) OER in acidic media over AuSn₄-based substrates and reference compounds. The oxidized surface of AuSn₄ is denoted as AuSn₄O. Dotted lines correspond to the energies of the same processes over similar PdSn₄ substrates. The asterisk denotes the substrate, and “···” corresponds to physisorption.^,