A Low‐Temperature Synthetic Route Toward a High‐Entropy 2D Hexernary Transition Metal Dichalcogenide for Hydrogen Evolution Electrocatalysis

Abstract

High‐entropy (HE) metal chalcogenides are a class of materials that have great potential in applications such as thermoelectrics and electrocatalysis. Layered 2D transition‐metal dichalcogenides (TMDCs) are a sub‐class of high entropy metal chalcogenides that have received little attention to date as their preparation currently involves complicated, energy‐intensive, or hazardous synthetic steps. To address this, a low‐temperature (500 °C) and rapid (1 h) single source precursor approach is successfully adopted to synthesize the hexernary high‐entropy metal disulfide (MoWReMnCr)S2. (MoWReMnCr)S2 powders are characterized by powder X‐ray diffraction (pXRD) and Raman spectroscopy, which confirmed that the material is comprised predominantly of a hexagonal phase. The surface oxidation states and elemental compositions are studied by X‐ray photoelectron spectroscopy (XPS) whilst the bulk morphology and elemental stoichiometry with spatial distribution is determined by scanning electron microscopy (SEM) with elemental mapping information acquired from energy‐dispersive X‐ray (EDX) spectroscopy. The bulk, layered material is subsequently exfoliated to ultra‐thin, several‐layer 2D nanosheets by liquid‐phase exfoliation (LPE). The resulting few‐layer HE (MoWReMnCr)S2 nanosheets are found to contain a homogeneous elemental distribution of metals at the nanoscale by high angle annular dark field‐scanning transmission electron microscopy (HAADF‐STEM) with EDX mapping. Finally, (MoWReMnCr)S2 is demonstrated as a hydrogen evolution electrocatalyst and compared to 2H‐MoS2 synthesized using the molecular precursor approach. (MoWReMnCr)S2 with 20% w/w of high‐conductivity carbon black displays a low overpotential of 229 mV in 0.5 M H2SO4 to reach a current density of 10 mA cm−2, which is much lower than the overpotential of 362 mV for MoS2. From density functional theory calculations, it is hypothesised that the enhanced catalytic activity is due to activation of the basal plane upon incorporation of other elements into the 2H‐MoS2 structure, in particular, the first row TMs Cr and Mn.

Synthetic Strategies toward High Entropy Materials: Atoms-to-Lattices for Maximum Disorder

“…For example, the 2D HE metal disulfide (CrMnMoWRe)­S 2 shows significantly enhanced electrocatalytic hydrogen evolution performance compared to the parent MoS 2 (Figure b) . A combination of experimental and computational analysis showed that while the electrochemically active surface area increased by around 2×, the catalytic activity increased by 8×.…”

“…54 The DFT calculations further showed that combinations of elements improved the E ads , even when only two elemental substitutions of Mo for Mn and Cr at the MoS 2 surface were considered, demonstrating the synergistic effect of multiple elements within a local area. 34 ■ TRADITIONAL SOLID-STATE SYNTHETIC ROUTES TO HE MATERIALS Myriad approaches have been used for the synthesis of HE alloys, oxides, and chalcogenides, and have already been reviewed elsewhere in more detail than will be covered here. 8,55−57 The seminal work on HE materials by Cantor and Yeh both employed electrical heating of the constituent elements.…”

“…88 We also used this method to synthesize a 2D HE MoS 2 -structured (CrMnMoWRe)S 2 transition-metal dichalcogenide, which was found to be significantly more catalytically active than the parent MoS 2 due to the presence of first-row transition metals within the 2H MoS 2 lattice structure (Figure 8). 34 ■ SUMMARY AND OUTLOOK High-entropy materials are already of significant interest for a wide range of applications, including energy conversion and storage, catalysis, and thermoelectrics, and we expect these materials to become even more important in the future as they are diversified away from alloys and toward multilattice structures such as chalcogenides, MXenes, perovskites, and spinels. It can be demonstrated with simple mathematical models that the chemical space in which high-entropy materials exist is so huge that it could potentially dwarf the number of all of the known compounds that have been discovered to date, and there will no doubt be new families of high-performance HE materials discovered over the next few years with new applications following.…”

“…48−52 For example, the 2D HE metal disulfide (CrMnMoWRe)S 2 shows significantly enhanced electrocatalytic hydrogen evolution performance compared to the parent MoS 2 (Figure 2b). 34 A combination of experimental and computational analysis showed that while the electrochemically active surface area increased by around 2×, the catalytic activity increased by 8×. Density-functional theory (DFT) calculations determined that the presence of Mn and Cr significantly reduced the adsorption energy E ads for H to the basal plane, activating the previously inactive surface for catalysis.…”

“…In the two decades since, the library of HE materials has been expanded beyond alloys and now includes oxides, , chalcogenides, carbides, flourites, silicides, borides, nitrides, phosphides/phosphates, − perovskites, , and spinels, , among others . Moreover, nanoscale HE materials including 0D nanoparticles, − 1D nanowires, − and 2D nanosheets − have been developed in addition to bulk materials.…”