Sulfur Line Vacancies in MoS₂ for Catalytic Hydrogen Evolution Reaction

Abstract

Defects in transition metal dichalcogenides play important roles in the field of the catalytic hydrogen evolution reaction (HER). However, the use of defective MoS₂ as HER catalysts remains controversial because the types of defects are various, including zero-dimensional point defects, one-dimensional linear defects, and two-dimensional plane defects. Recently, novel structures of linear defects have drawn more and more attention, and it is necessary to explore their unique properties. This review focuses on the formation mechanism, fabrication method, accurate atomic structure, and catalytic hydrogen evolution mechanism of sulfur line vacancies in MoS₂ as electrocatalysts. The structure–activity relationship between line defects and catalytic performance is discussed in detail. This will provide a route for the design of excellent catalysts by engineering line defects.

1. Introduction

The development and utilization of new energy is the key to realizing the double carbon strategy, the core of which is the design and preparation of catalytic materials with high performance [1,2,3,4,5]. Hydrogen energy, as a form of high-efficiency, zero-emission, and renewable clean energy, is a new option for optimizing the energy structure and ensuring the national energy supply, and the electrocatalyst is the core of hydrogen for hydrogen production in our country [6,7,8]. The unique structural features give two-dimensional materials excellent physical and chemical properties and rich scientific connotations.

There are various defect types in monolayer MoS₂, including point defects (vacancy and substitution), dislocation and grain boundaries, etc. [9,10,11,12,13,14,15]. These defects will change the electrical properties of monolayer MoS₂ and may induce novel physical phenomena, such as ferromagnetism. However, in practical applications, the existence of defects will lead to the electrical performance of single-layer MoS₂ (reduced mobility, etc.), and the coexistence of different types of defects may make single-layer MoS₂ become a compensation semiconductor [16,17,18,19,20]. Therefore, it is of great significance to accurately identify the different defect types of single-layer MoS₂, control the defect concentration, and then regulate the band structure and electrical properties of single-layer MoS₂ for relevant application research. Chalcogen vacancy lines, another frequently observed 1D line defect in 2D TMD materials, can provide additional metallic channels and thus promote catalytic performance, such as the hydrogen evolution reaction (HER) activity of the host materials [21,22,23,24,25,26,27].

Sulfur line vacancy engineering triggered by an electron beam (e-beam), plasma, chemical treatment, and so forth is comprehensively reviewed. Firstly, e-beam irradiation-induced defect evolution, structural transformation, and novel structure fabrication are introduced [28,29,30,31,32,33,34]. With the assistance of state-of-the-art characterization methods, in situ observation of sulfur line vacancy engineering could be realized. Thus, the catalytic HER mechanism of MoS₂ could be ascertained. The challenges and outlooks of sulfur line vacancy engineering in promoting the development of 2D materials are discussed. Through this review, we aim to build a correlation between the sulfur line vacancies and HER catalytic properties of 2D materials to support the design and enhancement of high catalytic HER performance.

2. Understanding Linear Defects

Understanding the atomic structure and dynamic evolution of defects is of great significance for the improvement and performance of two-dimensional material functional devices [35,36,37,38]. Spherical aberration correction transmission electron microscopy not only has subangstrom spatial resolution but also has many experimental functions [35,36,37,38,39,40]. It is a very effective method for studying the structure–activity relationship of materials by simultaneously studying the crystal structure of materials and the corresponding electronic structure characteristics at the atomic scale so as to understand the correlation between the microscopic crystal structure and the properties of samples. Therefore, it has a very wide range of applications in physics, materials science, and chemistry.

The defect structures of two-dimensional materials are closely related to their physical and chemical properties, but systematic research on them is still relatively rare. To date, high-resolution images of monolayer MoS₂ have been obtained using high-resolution scanning transmission electron microscopy (STEM) to identify defect types such as vacancies, substitutions, and grain boundaries [40,41,42,43,44,45,46]. However, in order to achieve STEM representation, MoS₂ needs to be transferred from the growing substrate to the STEM microgate for imaging, and high-energy electron beam bombardment will also induce the generation of defects. In contrast, scanning tunneling microscopy/tunneling spectroscopy (STM/STS) in an ultra-high vacuum system can not only realize the in situ atomic-scale morphology study but also clarify the evolution of electronic structures induced by different defect types, which provides the most intuitive experimental basis for the subsequent application research of materials [47,48,49,50,51,52,53,54]. Based on this, Liu et al. reported the generation mechanism of different types of sulfur vacancies in monolayer MoS₂ and studied the regulation effect of different defect types on the band structure of monolayer MoS₂ by STM/STS technology. It was found that with the increase in the sample annealing temperature (400 to 900 K), the density of S vacancies gradually increases, and S vacancies will aggregate to form a chain structure (S2, S3, S4, etc.) [43]. The STS measurements show that the formation of S vacancies induces the formation of new electronic states at the bottom of the conduction band and leads to a smaller band gap (2.2 eV after annealing at 400 K; 1.8 eV after annealing at 900 K), and an N-type doping effect on monolayer MoS₂ is produced. Combined with data statistics and theoretical calculations, it was found that the chemical potential of S in single S vacancy and chain S vacancy structures is almost the same (−6.87 eV~−6.5 eV), indicating that the concentration of different types of defects strictly complies with the thermodynamic statistical law, and temperature is the only inducing factor for the formation of defects. This work provides the most intuitive experimental basis and theoretical analysis for understanding the generation and regulation mechanism of defects in monolayer MoS₂, as well as its regulatory effect on the local electronic structure, and also provides a reference for the future application of semiconducting monolayer disulfide compounds in electronic devices. An atomic model of the 1D vacancy line consisting of 2S missing atomic rows is shown in Figure 1a. This type of 1D line defect in MoS₂ and WS₂ was well studied by 4D STEM, as reported by Warner et al. Electron beam irradiation can create defects in 2D materials due to either a “knock-on” effect, ionization, or beam-induced chemical etching, facilitating the sculpting of 2D membranes with high spatial accuracy and flexible pattern design. The removal of a single S atom causes little perturbation to the surrounding MoS₂ lattice, while the loss of two S atoms from the same atomic column results in measurable local shrinkage. S vacancies aggregate into linear line defects along a zigzag path, leading to greater lattice compression, which is more pronounced with increasing line defect length. Figure 1b,c show two different types of S atom reconstructions with different amounts of lattice compression. AC-TEM images in Figure 1d,e clearly show the aggregation of S vacancies into extended line defects along a zigzag path with different lengths and widths. The directional transport of Mo atoms (or Mo cluster) along the sulfur vacancy lines can induce the formation of Mo chains, as shown in Figure 1f. Tarak K. Patra et al. comprehensively studied the dynamic behavior and spatial distribution of defects in 2D transition metal disulfide using machine learning, molecular dynamics simulation, and high-resolution electron microscopy, as shown in Figure 1h. This study provides a good basis for the subsequent rational design and large-scale application of 2D transition metal disulfide-containing defects.

3. Application of Linear Defects for Catalytic HER

As a typical 2D transition metal disulfide compound, the MoS₂ edge structure has moderate hydrogen adsorption strength and exhibits good HER activity [55,56,57,58,59,60,61,62]. Therefore, molybdenum disulfide is widely considered a potential non-precious metal HER catalyst to replace precious metal Pt. However, a large number of sulfur atoms on the molybdenum disulfide surface are inert to electrocatalytic hydrogen evolution, and the number of active edge sites is very limited [63,64,65,66,67,68,69,70]. Therefore, it is of great significance to develop effective methods to stimulate and optimize the S activity on the molybdenum sulfide surface to increase the number of catalytic active sites of MoS₂ and improve its catalytic HER activity [71,72,73,74,75,76]. The one-dimensional monoatomic molybdenum chain was successfully prepared by regulating the MoS₂ electrocatalyst with sodium ions. The S atoms on the substrate were eliminated linearly. The one-dimensional monoatomic molybdenum chain has a large potential gradient, which makes it easy to accumulate electrons here, storing them like a “reservoir”, and has a higher hydrogen atom coverage, as shown in Figure 2a, which makes catalytic hydrogen evolution easier to proceed. As shown in Figure 2b, multilayer MoS₂ samples rich in sulfur vacancies were prepared by high-temperature hydrogen etching technology, and the role and effect of surface sulfur vacancies on HER catalytic performance were analyzed in detail. Multilayer MoS₂ samples with an ultra-high concentration (90%) of surface sulfur vacancies were prepared to achieve the highest HER activity under alkaline conditions (Figure 2c,d), and their stability could be maintained.

4. Conclusions and Outlook

In summary, we have witnessed the rapid development of sulfur line vacancies in MoS₂ in terms of both their fabrication strategies and application fields in electrochemical hydrogen evolution. Firstly, although many advanced methods have been proven to be a powerful way of preparing sulfur line vacancies, the development of novel synthesis strategies for the “precise control” of sulfur line vacancies is still challenging but highly required. Secondly, sulfur line vacancies provide a suitable platform for investigating the structure–activity relationships and the potential catalytic mechanisms of hydrogen evolution reactions. Thirdly, theoretical prediction based on computational modeling has proved to be a useful tool for discovering sulfur line vacancies for hydrogen evolution reactions. Strategies based on DFT studies for the rational design of model defects with a precise atomic structure are still highly desired [77,78,79,80,81].

In conclusion, despite the exciting progress achieved in defects for electrochemical energy conversion, both opportunities and challenges remain. Thus, defects with a well-controlled coordination environment and electronic state, combined with advanced in situ/operando techniques and DFT theoretical studies, will pave the way for both fundamental research and future industrial applications of sulfur line vacancies for various fields related to electrochemical energy conversion.

(1) The development of in situ dynamic characterization technology and real-time monitoring of the catalytic reaction process is of great significance for the in-depth understanding of reaction mechanisms and the design of efficient catalysts. In situ characterization instruments, especially under operational conditions, can realize the study of the structure and chemical composition of materials under working conditions to a certain extent, which is conducive to understanding the causes of the observed phenomena. Therefore, it is very important and necessary to develop working conditions, sample surface structure, and chemical properties in a growing environment.

(2) The wide application of DFT has produced a large amount of data, but the reliability and intrinsic self-consistency of these data still need extensive attention. In addition, in recent years, research on accelerating traditional quantum chemical computation based on database and machine learning has rapidly emerged, but there is still a lack of original innovation in the underlying methods (neural networks, etc.), and it is urgent to develop “intelligent” tools that can be used for “intelligent” mining catalytic theory and real theoretical research.