Weyl Semimetals as Hydrogen Evolution Catalysts

Abstract

The search for highly efficient and low-cost catalysts is one of the main driving forces in catalytic chemistry. Current strategies for the catalyst design focus on increasing the number and activity of local catalytic sites, such as the edge sites of molybdenum disulfides in the hydrogen evolution reaction (HER). Here, the study proposes and demonstrates a different principle that goes beyond local site optimization by utilizing topological electronic states to spur catalytic activity. For HER, excellent catalysts have been found among the transition-metal monopnictides-NbP, TaP, NbAs, and TaAs-which are recently discovered to be topological Weyl semimetals. Here the study shows that the combination of robust topological surface states and large room temperature carrier mobility, both of which originate from bulk Dirac bands of the Weyl semimetal, is a recipe for high activity HER catalysts. This approach has the potential to go beyond graphene based composite photocatalysts where graphene simply provides a high mobility medium without any active catalytic sites that have been found in these topological materials. Thus, the work provides a guiding principle for the discovery of novel catalysts from the emerging field of topological materials.

Weyl and Dirac semimetals in three-dimensional solids

“…For instance, it has been recently proposed that one may utilize topological electronic states to enhance catalytic activity. In this regard, it was shown that the combination of topological surface states and large room temperature carrier mobility (both of which originate from bulk bands of the WSM and DSM) may be a recipe for high activity hydrogen evolution reaction catalysts and may be used in solar energy harvesting to produce hydrogen from water (Rajamathi et al, 2016). In the device domain, it has been proposed that WSMs in thin film form can be used to build a spin-filter transistor with a controllable spin polarized current.…”

Phase‐Engineered Weyl Semi‐Metallic MoxW1‐xTe2 Nanosheets as a Highly Efficient Electrocatalyst for Dye‐Sensitized Solar Cells

“…This unique phenomenon allows TWS carrying charges much faster than ordinary semiconductors; furthermore, this unique transport property of TWS gives rise to ultrahigh charge mobility, large mangetoresistance, and electrical superconductivity, enabling TWS to be an ideal material for future technological applications, such as magnetic storage, quantum computation, and spintronics aplications . A recent report reveals the potential of employing TWS in catalysis, where the key concept behind this catalysis mechanism arises from the topologically protected surface states of TWS to serve as an effective charge transport platform. These topological surface states provide an alternative catalytic mechanism, in contrast to the traditional approach of increasing active edge sites or vacancies .…”

“…A recent report reveals the potential of employing TWS in catalysis, where the key concept behind this catalysis mechanism arises from the topologically protected surface states of TWS to serve as an effective charge transport platform. These topological surface states provide an alternative catalytic mechanism, in contrast to the traditional approach of increasing active edge sites or vacancies . In particular, the type‐II TWS have an inverted band structure with broken inversion symmetry, leading to the existence of topological surface states, which are stable against defects, impurities, and other surface modifications .…”

“…Moreover, the lattice distortion with spontaneous surface charge density wave on the basal plane could provide an additional advantage to allow a rapid charge exchange in electrochemical reactions, thus improving the catalytic activities . Ultimately, resolving the surface contamination in a corrosive electrolytic solution is one of the major concerns in catalysis; propitiously, this issue can be effectively addressed by using TWS as a catalyst, because the topologically protected surface states are robust and stable against contamination …”

“…Mo x W 1‐x Te 2 is a layered crystal, in which a single layer of W/Mo is sandwiched between two Te layers and stacked along the z‐axis via van der Waals interactions (Figure a). It is interesting to note that the crystal phases of Mo x W 1‐x Te 2 can vary from the semiconducting 2H phase (hexagonal, the P6 3 /mmc space group) to the semi‐metallic 1T′ phase (monoclinic, the P2 1 /m space group) or T d phase (orthorhombic, Pmn 21 space group), where each of the polymorphisms of Mo x W 1‐x Te 2 possesses distinctive properties due to the differences in intrinsic charge density and lattice symmetry . For instance, a semi‐metallic T d ‐Mo x W 1‐x Te 2 with the lattice constants of a = 6.282 Å, b = 3.496 Å, and c = 14.07 Å is the most stable phase at room temperature and possesses broken inversion symmetry, which is a prerequisite to hold the type‐II TWS surface states .…”