Chemisorption Can Reverse Defect-Defect Interaction on Heterogeneous Catalyst Surfaces

Electron Release via Internal Polarization Fields for Optimal S-H Bonding States

Transition metal dichalcogenides (TMDs) have received considerable attention as promising electrocatalysts for the hydrogen evolution reaction (HER), yet their potential is often constrained by the inertness of the basal planes arising from their poor hydrogen adsorption ability. Here, the relationship between the electronic structure of the WS2 basal plane and HER activity is systemically analyzed to establish a clear insight. The valance state of the sulfur atoms on the basal plane has been tuned to enhance hydrogen adsorption through sequential engineering processes, including direct phase transition and heterostructure that induces work function-difference-induced unidirectional electron transfer. Additionally, an innovative synthetic approach, harnessing the built-in internal polarization field at the W-graphene heterointerface, triggers the in-situ formation of sulfur vacancies in the bottom WSx (x < 2) layers. The resultant modulation of the valance state of the sulfur atom stabilizes the W-S bond, while destabilizing the S-H bond. The electronic structural changes are further amplified by the release and transfer of surplus electrons via sulfur vacancies, filling the valance state of W and S atoms. Consequently, this work provides a comprehensive understanding of the interplay between the electronic structure of the WS2 basal plane and the HER activity, focusing on optimizing S-H bonding state.

Insights into the Dynamic Evolution of Defects in Electrocatalysts

This review focuses on the formation and preparation of defects, the dynamic evolution process of defects, and the influence of defect dynamic evolution on catalytic reactions. The summary of the current advances in the dynamic evolution process of defects in oxygen evolution reaction, hydrogen evolution reaction, nitrogen reduction reaction, oxygen reduction reaction, and carbon dioxide reduction reaction, and the given perspectives are expected to provide a more comprehensive understanding of defective electrocatalysts on the structural evolution process during electrocatalysis and the reaction mechanisms, especially for the defect dynamic evolution on the performance in catalytic reactions.

Vacancy Defects in 2D Transition Metal Dichalcogenide Electrocatalysts: From Aggregated to Atomic Configuration

Vacancy defect engineering has been well leveraged to flexibly shape comprehensive physicochemical properties of diverse catalysts. In particular, growing research effort has been devoted to engineering chalcogen anionic vacancies (S/Se/Te) of 2D transition metal dichalcogenides (2D TMDs) toward the ultimate performance limit of electrocatalytic hydrogen evolution reaction (HER). In spite of remarkable progress achieved in the past decade, systematic and in-depth insights into the state-of-the-art vacancy engineering for 2D-TMDs-based electrocatalysis are still lacking. Herein, this review delivers a full picture of vacancy engineering evolving from aggregated to atomic configurations covering their development background, controllable manufacturing, thorough characterization, and representative HER application. Of particular interest, the deep-seated correlations between specific vacancy regulation routes and resulting catalytic performance improvement are logically clarified in terms of atomic rearrangement, charge redistribution, energy band variation, intermediate adsorption-desorption optimization, and charge/mass transfer facilitation. Beyond that, a broader vision is cast into the cutting-edge research fields of vacancy-engineering-based single-atom catalysis and dynamic structure-performance correlations across catalyst service lifetime. Together with critical discussion on residual challenges and future prospects, this review sheds new light on the rational design of advanced defect catalysts and navigates their broader application in high-efficiency energy conversion and storage fields.

Understanding the air stability of defective MoS2and the oxidation effect on the surface HER activity

The defective single layer MoS₂(SL-MoS₂) with high defect concentrations has shown promising electrocatalytic potential, but it is also highly reactive with gas molecules. The study of electro-chemical activity on gas doped defective SL-MoS₂is of importance yet still scarcely discussed. Herein, we performed density functional theory calculations to study the adsorption and chemical activity of four major air molecules on the defective SL-MoS₂under different defect concentrations, and evaluated the influence on the hydrogen evolution reaction activity. The N₂and CO₂molecules are in physisorption states, H₂O molecule is in molecular chemisorption state, while O₂can be strongly captured and dissociated into atomic O^*, which repair the S-vacancy and form O-doped structure. Further study showed that compared to the inert S surface of pure MoS₂, the O incorporation greatly enhance the surface reactivity. Using H adsorption as the test probe, the adsorption of H becomes stronger with the increasing oxygen concentration. We further unravel the electronic origins underlying the catalytic activity. The lowest unoccupied electronic states are shown to correlate linearly with the activity, and thus can be used as an electronic descriptor to characterize the electrocatalytic activity.

First-principles study of sulfur vacancy concentration effect on the electronic structures and hydrogen evolution reaction of MoS2

Defect engineering has been widely used in experiments to modulate the electrocatalytic properties of molybdenum disulfide (MoS₂). However, the effect of vacancy concentration on the vacancy distribution, electronic properties, and hydrogen evolution reaction (HER) activity remains elusive. Herein, we perform density functional theory (DFT) studies to investigate defective MoS₂ with different numbers of sulfur vacancies. In the case of low S-vacancy concentration, the vacancies prefer to agglomerate rather than being dispersed, while at the higher-vacancy concentration, the combination of local point defect and clustered vacancy chain is preferred. The coupling between S-vacancies leads to decreased band gap and increased Mo-H adsorption strength with increasing vacancy concentration. The optimal HER activity is identified to occur below vacancy concentration of 12.50%. Our work provides an atomic-level understanding about the role of S-vacancies in the HER performance of MoS₂, and offers useful guidelines for the design of defective MoS₂ and other TMDs electrocatalysts.

Oxidation Notably Accelerates Nonradiative Electron-Hole Recombination in MoS2 by Different Mechanisms: Time-Domain Ab Initio Analysis

Two-dimensional transition metal dichalcogenides (TMDs) experience degradation in optoelectronic properties under ambient conditions. By performing nonadiabatic (NA) molecular dynamics simulations, we demonstrate that the MoS₂ monolayer containing substitutional oxygen and oxygen adatom accelerates nonradiative electron-hole recombination by a factor of about 1.5 compared to perfect film but operates by different mechanisms. The substitutional oxygen creates no midgap states while enhancing NA coupling by increasing the overlap between electron and hole wave functions, accelerating electron-hole recombination. In contrast, electrons significantly populate the deep trap state created by the oxygen adatom because the trap is modestly delocalized and coupled strongly to free charges. The trap mediated instead of the direct pathway dominates the electron-hole recombination. The generated insights uncover the mechanisms for different types of defects on influencing charge dynamics in TMDs and suggest that the oxygen defects should be avoided for the design of high-performance optoelectronic devices.