Hydrogen has been deemed as an ideal substitute fuel to fossil energy because of its renewability and the highest energy density among all chemical fuels One of the most economical, ecofriendly, and high-performance ways of hydrogen production is electrochemical water splitting Recently, 2D transition metal dichalcogenides (also known as 2D TMDs) showed their utilization potentiality as cost-effective hydrogen evolution reaction (HER) catalysts in water electrolysis Herein, recent representative research efforts and systematic progress made in 2D TMDs are reviewed, and future opportunities and challenges are discussed Furthermore, general methods of synthesizing 2D TMDs materials are introduced in detail and the advantages and disadvantages for some specific methods are provided This explanation includes several important regulation strategies of creating more active sites, heteroatoms doping, phase engineering, construction of heterostructures, and synergistic modulation which are capable of optimizing the electrical conductivity, exposure to the catalytic active sites, and reaction energy barrier of the electrode material to boost the HER kinetics In the last section, the current obstacles and future chances for the development of 2D TMDs electrocatalysts are proposed to provide insight into and valuable guidelines for fabricating effective HER electrocatalysts

2D Transition Metal Dichalcogenides: Design, Modulation, and Challenges in Electrocatalysis.

Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS2-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS2 nanosheets synthesized by a liquid-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS2 nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS2 nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS2. A comparative study revealed that the phase serves as the key role in determining the HER performance, as 1T phase MoS2 always outperforms the corresponding 2H phase MoS2 samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS2 samples. Then, using combined defect characterization techniques of electron spin resonance spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.

(Invited) Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets

Hierarchical Co and Nb dual-doped MoS2 nanosheets shelled micro-TiO2 hollow spheres as effective multifunctional electrocatalysts for HER, OER, and ORR

The discovery of moire superlattices (MSLs) opened an era in the research of 'twistronics'. Engineering MSLs and realizing unique emergent properties are key challenges. Herein, we demonstrate an effective synthetic strategy to fabricate MSLs based on mechanical flexibility of WS2 nanobelts by a facile one-step hydrothermal method. Unlike previous MSLs typically created through stacking monolayers together with complicated method, WS2 MSLs reported here could be obtained directly during synthesis of nanobelts driven by the mechanical instability. Emergent properties are found including superior conductivity, special superaerophobicity and superhydrophilicity, and strongly enhanced electro-catalytic activity when we apply 'twistronics' to the field of catalytic hydrogen production. Theoretical calculations show that such excellent catalytic performance could be attributed to a closer to thermoneutral hydrogen adsorption free energy value of twisted bilayers active sites. Our findings provide an exciting opportunity to design advanced WS2 catalysts through moire superlattice engineering based on mechanical flexibility.

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WS2 moiré superlattices derived from mechanical flexibility for hydrogen evolution reaction.

Numerous efforts in improving the hydrogen evolution reaction (HER) performance of transition metal dichalcogenides mostly focus on active sites exposing, vacancy engineering, and phase engineering...

Magnetic Enhancement for Hydrogen Evolution Reaction on Ferromagnetic MoS2 Catalyst.

A MoS2 moire superlattice with a twisted angle of θ ≈ 7.3° via a facile method instead of a conventional mechanical stacking method is successfully fabricated. With reduced interlayer potential bar...

MoS2 Moiré Superlattice for Hydrogen Evolution Reaction

As one of the promising low-cost and high-efficiency catalysts for the electrochemical hydrogen evolution reaction (HER), it is well-known that there are both tiny exposed catalytic active edge sit...

Wafer-Scale Sulfur Vacancy-Rich Monolayer MoS2 for Massive Hydrogen Production.

Laminated bilayer MoS2 structures are prepared with MoS2 nanoparticles trapped between two individual MoS2 layers which can prevent the formation of a true stacking structure held together by van der Waals interaction. The laminated bilayer MoS2 clearly indicates a weak interlayer coupling with reduced van der Waals interaction between adjacent layers. As the interlayer coupling is insufficient to modify the band structure of MoS2, the laminated bilayer MoS2 can retain the direct bandgap structure of an isolated monolayer. Furthermore, by controlling the size of the MoS2 nanoparticles trapped in between, the interlayer distance and interlayer coupling of bilayer MoS2 structures can be engineered in a wide range, resulting in different bandgap behaviors. This finding is extremely important as it provides an effective approach to fabricate bandgap engineered bilayer MoS2 structures, which is a crucial step forward to making multi-layer MoS2-based p–n junctions and homo/hetero-structures, and thus advanced electronic devices, especially optoelectronic devices. This approach is applied to not only bilayer MoS2 structures, but also other layer structured two-dimensional materials.

Laminated bilayer MoS2 with weak interlayer coupling

Nanoclusters are ideal electrocatalysts due to their high surface activity. However, their high activities also lead to serious agglomeration and performance attenuation during the catalytic process. Here, highly dispersed Ni nanoclusters (∼3 nm) confined in an amorphous carbon matrix are successfully fabricated by pulsed laser deposition, followed by rapid temperature annealing treatment. Then, the Ni nanoclusters are further doped with nitrogen element through a clean N2 radio frequency plasma technology. It is found that the nitrogen-doped Ni nanoclusters obtained under optimized conditions showed superior OER performance with a very low overpotential of 240 mV at a current density of 10 mA/cm2, together with good stability. The excellent OER performance of the nanoclusters can be attributed to the unique confined structure and nitrogen doping, which not only provide more active sites but also improve the conductivity. Our work provides a controllable method for the construction of a novel confined structure with controllable nitrogen doping, which can be used as a high-efficiency OER electrocatalyst.

Wenda Zhou

Papers

Magnetic Enhancement for Hydrogen Evolution Reaction on Ferromagnetic MoS2 Catalyst.

MoS2 Moiré Superlattice for Hydrogen Evolution Reaction

Wafer-Scale Sulfur Vacancy-Rich Monolayer MoS2 for Massive Hydrogen Production.

Laminated bilayer MoS2 with weak interlayer coupling

Boosting the OER Performance of Nitrogen-Doped Ni Nanoclusters Confined in an Amorphous Carbon Matrix.