Variations of topological surface states of nodal line semimetal AlB₂ after adsorption in aqueous environment

Abstract

Topological semimetals have been attracting great research interests due to their intrinsic topological physics and potential devices applications. A key feature for all topological materials is the so-called bulk-boundary correspondence, which means if there is non-trivial band topology in the bulk, then we can expect unique topologically protected conducting states in the edges, i.e., the topological surface state (TSS). Previously the studies for the surface states of topological materials mainly focus on the pristine surfaces, while the topological nodal line semimetal surface states with adsorbates are rarely systematically studied. In this paper, the topological properties of the topological semimetal AlB<sub>2</sub> are studied by first-principles calculations, and the TSS position is calculated by constructing the Al- and B-terminated slab models. Observing the topological surface state, it is found that the drumhead-like TSS connects two Dirac nodes with no energy gaps on the node line, and the TSS of the Al end-terminated slab has a smaller energy dispersion than that of the B-terminated slab. The adsorption characteristics of AlB<sub>2</sub> (010) surface were studied, and it was found that the Gibbs free energy (∆<i>G</i><sub>H<sup>*</sup></sub>) for hydrogen adsorption on the surface of Al was only -0.031 eV, demonstrating excellent hydrogen evolution reaction (HER) performance. The changes of TSS after surface adsorption of H, OH and H<sub>2</sub>O on the surface of AlB<sub>2</sub> in aqueous solution environment were observed, as shown in FIG. 1. The TSS change was the most significant when H was adsorbed, followed by OH adsorption. And the influence of H<sub>2</sub>O on TSS due to its electrical neutrality and weak surface adsorption was very weak, Before and after adsorption, because the topology protection TSS still exists, only the energy changes, which confirms its robust in the environment. The results of this work provide a systematic understanding of the effects of different adsorbents on the TSS of AlB<sub>2</sub>, pave the way for future theoretical and experimental research in related fields, and also provide theoretical support for the practical application of topological materials.