Toward the Effective Exploitation of Topological Phases of Matter in Catalysis: Chemical Reactions at the Surfaces of NbAs and TaAs Weyl Semimetals

Abstract

By means of density functional theory and experiments, surface chemical reactivity of single crystals of NbAs and TaAs Weyl semimetals is studied.Weyl semimetals exhibit outstanding reactivity toward simple molecules (oxygen, carbon monoxide, and water), with several active sites available for surface chemical reactions (adsorption, decomposition, formation of reaction products, recombination of decomposition fragments). When different chemical species are adsorbed on Weyl semimetals, strong lateral interactions between coadsorbed species occur, evidenced by CO-promoted water decomposition at room temperature. The resulting OH groups react with CO to form HCOO, which is an intermediate species in watergas shift reaction. These findings unambiguously demonstrate that Weyl semimetals could be effectively used in catalysis, whereas their employment in nanoelectronics or plasmonics is complicated by the poor ambient stability, due to the rapid surface oxidation, inevitably occurring unless protective capping layers are used.

Surface Reconstruction, Oxidation Mechanism, and Stability of Cd3As2

“…Technological applications based on Cd 3 As 2 should work in ambient atmosphere and, thus, it is crucial to assess the stability of the surface in atmospheric conditions. Previously, we found that for NbAs and TaAs Weyl semimetals, when exposed to oxygen, arsenic oxide immediately forms with the subsequent removal of topological bands, so that the oxidized systems are topologically trivial. The missing robustness of topological bands in Weyl semimetals in ambient atmosphere strongly reduces their potential applications in technology.…”

“…We attribute it to As–O–Cd stretching, according to the vibrational mode characteristic of type‐I oxidation depicted in Figure A (see also corresponding videos in the Supporting Information). For the sake of completeness, we report that the O‐As stretching, expected to occur in early‐step oxidation of As‐terminated Cd 3 As 2 , would have a higher energy (102–107 meV).…”

Strong and Tunable Electrical Anisotropy in Type‐II Weyl Semimetal Candidate WP2 with Broken Inversion Symmetry

“…A Weyl semimetal (WSM), which is a type of quantum matters with pairs of Weyl cones in the bulk and topologically protected Fermi arcs on the surface, offers an excellent platform for the realization of exotic phenomena, such as the chiral magnetic effect, high efficient catalysis, and quantum anomalous Hall effect . Two types of WSMs can be achieved by breaking spatial inversion symmetry or time reversal symmetry .…”

Tunable surface plasmons in Weyl semimetals TaAs and NbAs

“…Being As-terminated, TWS samples are not inert and interact strongly with oxygen [50]. The resulting loss spectrum is dominated by a band centered at 100 meV [51], with the main contribution from the As-O stretching mode and minor spectral contribution from Nb(Ta)-O stretching [50]. The plasmon frequency is redshifted compared to the pristine crystal to 58 meV in both NbAs and TaAs (solid gray line).…”

“…1) as surface rather than bulk excitations. Especially, interaction with oxygen has been recently demonstrated only to affect Fermi-arc states [50], while the O-induced modification of the other electronic bands is insignificant.…”