Amorphous MoS_x Nanosheets with Abundant Interlayer Dislocations for Enhanced Photolytic Hydrogen Evolution Reaction

Abstract

Transition metal dichalcogenides (TMSs), exemplified by molybdenum disulfide (MoS₂), exhibit significant potential as alternatives to noble metals (e.g., Pt) for the hydrogen evolution reaction (HER). However, conventional synthesis methods of MoS_x often suffer from active site loss, harsh reaction conditions, or undesirable oxidation, limiting their practical applicability. The development of MoS_x with high-density active sites remains a formidable challenge. Herein, we propose a novel strategy employing [Mo₃S₁₃]²⁻ clusters as precursors to construct three-dimensional amorphous MoS_x nanosheets through optimized hydrothermal and solvent evaporation-induced self-assembly approaches. Comprehensive characterization confirms the material’s unique amorphous lamellar structure, featuring preserved [Mo₃S₁₃]²⁻ units and engineered interlayer dislocations that facilitate enhanced electron transfer and active site exposure. This work not only establishes [Mo₃S₁₃]²⁻ clusters as effective building blocks for high-performance MoS_x catalysts, but also provides a scalable and environmentally benign synthesis route for the large-scale production of such nanostructured a-MoS_x. Our findings facilitate the rational design of non-noble HER catalysts via structural engineering, with broad implications for energy conversion technologies.

1. Introduction

Since the end of the 70s of the last centuries, thiomolybdate clusters have received widespread attention from the scientific research community. In particular, in 1979, Müller et al. used experimental methods to isolate [Mo₃S₁₃]²⁻ nanoclusters from ammonium thiomolybdate crystals [(NH₄)₂(Mo₃S₁₃)·nH₂O] for the first time. This milestone discovery laid a solid foundation for subsequent in-depth study of molybdenum thiolate clusters [1]. Since then, numerous scientists have successively conducted systematic research on the structural characteristics, physicochemical properties, and application potential of such clusters. The [Mo₃S₁₃]²⁻ cluster, with its relatively well-defined molecular configuration, precise stoichiometric composition, and unique geometric structure, has demonstrated broad prospects in various fields, including catalysis, materials science, and electrochemistry. MoS_x-based electrocatalysts have emerged as promising candidates to replace noble metal Pt catalysts due to their exceptional electrochemical stability and catalytic activity, particularly in the hydrogen evolution reaction (HER). However, despite significant advances in the synthesis techniques of MoS_x materials in recent years including thermal decomposition [2], wet-chemical methods [3], electrodeposition [4], and solvothermal approaches [5]. These methods have different advantages but also have certain limitations. For instance, materials prepared via thermal decomposition often struggle to retain catalytically active unsaturated sulfur sites, resulting in lower catalytic performance in practical applications. While wet-chemical and electrodeposition methods can produce highly active MoS_x, they frequently require toxic chemical reagents or harsh reaction conditions, hindering their scalability for industrial applications. Moreover, certain synthesis processes may induce partial oxidation of Mo into other valence states (e.g., Mo⁵⁺ or Mo⁶⁺), thereby diminishing the electrocatalytic activity of the material. Given these challenges, developing a novel synthetic strategy that simultaneously ensures the retention of highly active sites, environmental friendliness, and industrial applicability has become a critical task in current MoS_x research.

Amorphous MoS_x materials exhibit significant research value in the field of HER catalysis due to their short-range ordered but long-range disordered structural characteristics and abundant defect sites. Yu et al. [6] demonstrated that the structural disorder in MoS_x facilitates the formation of numerous active sites on the catalyst surface, substantially reducing the hydrogen adsorption free energy (ΔG_H) and enabling efficient HER at low overpotentials. Density functional theory (DFT) calculations further revealed [7] that these defects and unsaturated sulfur sites optimize electron distribution, promoting rapid electron-proton interactions on the catalyst surface and providing a low-energy-barrier pathway for H₂ formation. Experimental studies have confirmed that MoS_x-based catalysts exhibit lower overpotentials in acidic media compared to conventional crystalline MoS₂ (c-MoS₂), highlighting their superior catalytic activity [8]. The amorphous structure, with its abundant dislocations and localized strain, significantly enhances charge transfer efficiency, thereby improving the reaction kinetics of HER [9]. Additionally, Zong et al. [10] employed photoluminescence spectroscopy and transient absorption spectroscopy to investigate charge carrier separation and transport in MoS_x under photocatalytic conditions. Their findings indicate that MoS_x effectively separates photogenerated electrons and holes, minimizing recombination and supplying more reactive electrons for HER. Owing to its unique disordered structure and rich active sites, MoS_x demonstrates remarkable potential in HER catalysis.

This study focuses on the synthesis of a-MoS_x using [Mo₃S₁₃]²⁻ clusters as precursors, with a systematic investigation of its structural characteristics and photocatalytic potential. The article presents a novel hydrothermal method for fabricating a-MoS_x, followed by comprehensive characterization of its morphological features, elemental composition, and chemical states, which collectively confirm the non-crystalline nature of the obtained MoS_x nanosheets. Furthermore, the feasibility and advantages of a-MoS_x as a structural material are discussed, with particular emphasis on its contribution to enhanced catalytic activity. This innovative approach not only provides a practical pathway for synthesizing high-performance MoS_x nanomaterials but also opens new opportunities for precisely tailoring cluster superstructures to achieve superior catalytic performance. The findings of this research establish a theoretical and technical foundation for the broad application of MoS_x-based catalysts in energy conversion and environmental remediation.

2. Results and Discussion

2.1. Morphological Characterization

The highly disordered structure of the a-MoSₓ material is not accidental but is a direct consequence of the carefully optimized synthesis pathway (Figure S1), designed to favor kinetic trapping over thermodynamic equilibrium. The process can be deconstructed into two key stages. First, the hydrothermal step promotes the rapid nucleation of metastable thiomolybdate clusters under a high sulfur chemical potential. The moderate temperature and controlled duration are crucial; they provide sufficient energy for nanoparticle formation but are deliberately chosen to be insufficient for complete crystallization and Ostwald ripening, thus preserving intrinsic defects within the primary nanosheets [11]. Second, the solvent evaporation-induced self-assembly stage is critical for engineering the interlayer disorder. The microscopic morphology of the MoS_x nanosheets was characterized via SEM. Figure 1a presents SEM image of the as-prepared MoS_x nanosheets. The image reveals that the MoS_x material exhibits a nanosheet morphology. These nanosheets do not exist as isolated entities but rather interconnect through stacking, forming irregular nanoflower-like architectures. The red dashed area serves as a representative example, clearly demonstrating the stacking pattern of these nanosheets. Such nanoflower structures originate from inter-nanosheet interactions, where intermolecular forces and surface energy drive spontaneous nanosheet stacking during the synthesis process, ultimately yielding this complex microstructure. The SEM image confirms that the synthesized MoS_x material possesses nanosheets as fundamental structural units, which further assemble into well-defined nanoflower architectures through specific stacking configurations. This structural characterization provides essential microstructural evidence for subsequent investigations of the material’s properties. Figure 1b clearly shows the macroscopic aggregation morphology of MoS_x nanosheets, exhibiting irregular cluster-like structures. Figure 1c presents a HRTEM image. Notably, even at this high resolution, no distinct atomic lattice fringes are directly observable in the image. This result provides direct microscopic evidence that the prepared MoS_x nanosheets possess an amorphous structure, which theoretically confirms the amorphous nature of the synthesized catalytic material. The amorphous properties of the prepared MoS_x nanosheets were further verified by Fast Fourier Transform (FFT) analysis. As shown in Figure 1d, the FFT image of all selected regions in the corresponding FFT images do not show obvious diffraction peaks, but show typical diffuse ring characteristics. This phenomenon fully proves that the base surface of the prepared MoS_x nanosheets is amorphous. To further investigate the microstructure of the a-MoS_x nanosheet, this study conducted detailed elemental analysis using HAADF-STEM and EDS. The HAADF-STEM image (Figure 1e) and corresponding EDS elemental mapping (Figure 1f) demonstrate the homogeneous distribution of Mo, S, and O throughout the sample. The EDS spectrum of the a-MoS_x nanosheet (Figure 1g) further confirms its composition primarily consisting of Mo and S elements. Additionally, the presence of O element may result from partial oxidation during synthesis or storage processes.

2.2. Analysis of Interlayer Dislocations

The interlayer dislocations in a-MoS_x nanosheets significantly influence the local structure of the material by inducing the formation of strain fields. FFT processing was performed on different regions to further investigate all types of defect structures present in the sample. Figure 2 displays atomic-scale images of various interlayer dislocation types in the a-MoS_x nanosheet. Through HRTEM observations, lattice distortion regions with different morphologies were identified and categorized into three major defect types (I, II, III), each further subdivided into distinct subclasses (i–vii). Similarly, dislocations in the images were marked with “T”.

Type I defects exhibit discontinuous extension and misalignment between layers, commonly found in regions of atomic rearrangement driven by localized stress. These typically involve asymmetric interlayer overlap or stress-induced atomic slippage. Type II defects display severe interlayer interleaving, warping, and overlapping, with significant lattice orientation deviation. They usually arise from the mutual proximity, coupling, and evolution of multiple Type I defects in localized regions, reflecting the combinability and cooperative reconstruction tendency among defect structures. Type III defects demonstrate relatively regular bending or slip morphologies, resembling continuous steps or spiral-twisted layers. FFT analysis reveals periodic strain modulation in these regions, generally resulting from microscale shear inhomogeneity. The coexistence of these three defect types in the sample reflects the highly tunable stacking process and complex dislocation reconstruction mechanisms in MoS_x. These interlayer defects not only enhance material stability but also provide a foundation for the formation of catalytic sites and electronic structure modulation, thereby endowing the material with superior catalytic potential.

2.3. Analysis of Structure and Elemental Composition

To further determine the crystal structure of the synthesized a-MoS_x nanosheet, XRD analysis was performed on the a-MoS_x sample. As shown in Figure 3a, no distinct diffraction peaks were observed in the 2θ range of 10° to 50°, indicating the amorphous nature of the prepared sample, which is consistent with typical characteristics of amorphous materials. However, a prominent diffraction peak appeared at 2θ = 13.8°, corresponding to the (002) crystal plane. The emergence of this characteristic peak suggests the existence of locally ordered structural features, which can be explained by the multilayer stacking structure. Although the sample exhibits an overall amorphous structure, local ordered lamellar structures can form as MoS_x nanosheets grow and stack along the c-axis direction. This local layered stacking results in the observed characteristic diffraction peak. As shown in Figure 3b, Raman spectroscopy was employed to investigate the phase structure of the a-MoS_x nanosheet. The peaks in the range of 280–380 cm⁻¹ are attributed to the ν(Mo-S) vibrational modes [11,12]. The characteristic peaks in this range cover the E_2g and A_1g vibrational modes of Mo-S bonds, indicating the presence of typical MoS₂ layered structures, which is consistent with the XRD results discussed above. Additionally, the two peaks observed at 772 cm⁻¹ and 815 cm⁻¹ are associated with the asymmetric and symmetric stretching vibrational modes of Mo-O-Mo bonds, respectively. These oxide-related signals typically arise from surface oxidation or slight oxidation under experimental conditions, suggesting partial oxidation after exposure to air. The peak at 965 cm⁻¹ is assigned to the asymmetric stretching vibration of Mo⁵⁺ = O [13], further confirming surface oxidation. Overall, the Raman spectral features of the a-MoS_x nanosheet indicate that its internal structure mainly consists of Mo-S bonds, while exhibiting a certain degree of surface oxidation. XPS analysis was further conducted to investigate the elemental composition and valence states in the a-MoS_x nanosheet. Detailed compositional analysis of XPS reveals that the atomic Mo:S ratio is 1:3.22 and the atom% of oxygen is 5.40%, indicating a sulfur-rich composition compared to stoichiometric a-MoS_x. The Mo 3d spectrum (Figure 3c) reveals three sets of fitted peaks. The peaks located at 229.9 eV and 233.0 eV correspond to Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2 [14], respectively, indicating the predominant existence of Mo⁴⁺ in MoS_x. The peaks at 231.7 eV and 234.1 eV are assigned to Mo⁵⁺ 3d_5/2 and Mo⁵⁺ 3d_3/2 [15], which may be associated with defects or unsaturated coordination in the amorphous structure. Meanwhile, the peaks at 233.9 eV and 235.7 eV are attributed to Mo⁶⁺ 3d_5/2 and Mo⁶⁺ 3d_3/2 [16], suggesting the presence of minor oxide impurities. Additionally, the S 2s peak around 226 eV further confirms the existence of Mo-S bonds. Figure 3d presents the S 2p spectrum. The lower binding energy peaks at 161.7 eV and 163.0 eV correspond to terminal S₂²⁻, while the higher binding energy peaks at 163.7 eV and 164.6 eV are assigned to apical S²⁻/bridging S₂²⁻ [17], demonstrating diverse bonding configurations of sulfur at different sites.

2.4. Photocatalytic Hydrogen Evolution Performance Test and Mechanisms

To enhance photocatalytic performance, this study incorporated dye sensitizers. Dye sensitizers can effectively extend the light absorption range of wide-bandgap semiconductors, thereby improve visible light utilization efficiency and promote photocatalytic reactions. Given the relative difficulty of achieving complete photocatalytic water splitting, this experiment employed electron donor/acceptor-assisted reactions. The introduction of sacrificial agents effectively consumes photogenerated holes, reducing electron-hole recombination and consequently significantly improving the photocatalytic HER rate. Specifically, Eosin Y (EY) was selected as the dye sensitizer, while triethanolamine (TEOA) served as the sacrificial agent. Coupled with a xenon lamp as the simulated light source, a MoS_x nanosheet photocatalytic system was established, and its photocatalytic HER performance was systematically evaluated.

In this experiment, TEOA served as the electron sacrificial agent and EY as the photosensitizer, with a xenon lamp employed to simulate actual sunlight. Within this system, the a-MoS_x nanosheet sample was introduced as a catalyst into the EY-containing solution to investigate its photocatalytic hydrogen evolution performance. Catalyst concentration significantly influenced the reaction performance. At lower concentrations, the hydrogen evolution rate exhibited a positive correlation with catalyst loading. However, beyond a certain threshold, the performance plateaued, and further increases in catalyst concentration could even lead to diminished activity. This phenomenon is attributed to light shielding effects at high catalyst loadings, which compromise efficient light penetration. Temperature control was critical for reproducible hydrogen evolution measurements. Through systematic optimization, the catalyst loading was determined to be 10 mg per 100 mL of reaction solution for maximum performance. A comparison of the hydrogen evolution rates among different catalyst samples is presented (Figure 4a). The a-MoS_x nanosheet catalyst demonstrated superior photocatalytic HER activity (4.53 mmol g⁻¹ h⁻¹) compared to the reference S-O-MoS₂ material [18]. Table S1 summarizes the photocatalytic performance of our a-MoS_x nanosheet system alongside literature values for other photocatalytic materials.

This study investigates the influence of interlayer dislocations in MoS_x materials on their photocatalytic HER performance. Owing to their abundant interlayer structures and defect characteristics, MoS_x materials exhibit superior catalytic properties, particularly demonstrating high activity in HER. Compared with conventional MoS₂, MoS_x possesses unique advantages in photocatalytic reactions, where the presence of interlayer dislocations and defects provides abundant active sites for catalytic processes. These dislocations not only facilitate electron transport but also effectively reduce the energy barrier of reactions, thereby enhancing reaction kinetics. Under illumination, MoS_x materials significantly improve photocatalytic hydrogen evolution efficiency through synergistic interactions with photosensitizers (e.g., EY) and sacrificial agents (e.g., TEOA). Upon light absorption, EY molecules transition from the ground state to an excited state, subsequently reaching a triplet excited state via intersystem crossing. The EY^3* species then undergoes electron loss to form the reduced state (EY⁻), while simultaneously oxidizing TEOA to TEOA⁺ through electron transfer. This process not only maintains the stability of the reaction system but also promotes electron transfer. However, in the absence of catalysts, the activity of EY molecules gradually decays due to self-redox reactions, indicating that EY alone cannot sustain high catalytic performance. The introduction of MoS_x catalysts significantly enhances catalytic activity by enabling more efficient electron transfer from EY⁻ to the catalyst. The interlayer dislocations and defects in MoS_x provide additional active sites, allowing the catalyst to participate more effectively in the reaction.

The interlayer dislocations in MoS_x materials constitute one of the key factors contributing to their exceptional catalytic performance. These dislocations disrupt the regular crystal structure, creating additional pathways for electron transfer and proton adsorption. The catalytic properties of MoS_x can be further optimized through precise modulation of these interlayer dislocations. Mechanistically, the dislocations in MoS_x may directly correlate with H* adsorption sites during catalytic reactions, thereby facilitating hydrogen generation. In the HER process, these structural defects fulfill dual functions: (1) enhancing material conductivity by promoting electron transport, and (2) providing abundant reactive sites that endow MoS_x with superior catalytic activity compared to conventional MoS₂. This dislocation-mediated catalytic mechanism establishes a new paradigm for HER activity, offering both theoretical foundations and experimental support for developing high-efficiency photocatalytic hydrogen evolution catalysts. Further investigation into the characteristics of interlayer dislocations in MoS_x materials may yield novel design principles for next-generation photocatalysts.

3. Materials and Methods

3.1. Materials Syntheses

A-MoS_x samples were synthesized through a modified Müller method. The key steps in the preparation process are as follows (Figure S1): First, 4 g of ammonium molybdate tetrahydrate [(NH₄)₆Mo₇O₂₄·4H₂O] was precisely weighed and introduced into a conical flask containing 20 mL of deionized water, followed by thorough stirring to ensure complete dissolution. Add 120 mL (NH₄)₂S (8 wt%) aqueous solution to the above solution, and cover the mouth of the bottle with a glass plate after the operation is completed. The reaction flask was placed in an oil bath, and the temperature was ramped from room temperature to 90 °C at a rate of 5 °C/min. Then place the conical flask in a constant temperature oil bath pot and heat it in an oil bath at 90 °C for 5 days, without stirring during the heating process. After natural cooling to room temperature, the brownish-red MoS_x products were collected by centrifugation filtration with 8000 rpm for 15 min, and washed with water and ethanol three times, respectively, to remove the residual substances, and the MoS_x was dried in a vacuum environment of 60 °C after washing. The solvent-evaporated assembly process happens during the drying stage after the washing step. More specifically, it is the act of removing the solvent under controlled conditions (60 °C in a vacuum oven) that drives the assembly. The S-O-MoS₂ samples were synthesized through the methods published by our group [18].

3.2. Materials Characterization

SEM and TEM images were measured using a Hitachi S-4800 and Hitachi HT7700 (Tokyo, Japan). More detailed morphology and atomic structure were characterized by FEI Talos F200X (acceleration voltage of 200 kV, Hillsboro, OR, USA). XRD dates were recorded by using Bruker AXS D8 Advance A25 (2θ = 5.0–80.0°, Billerica, MA, USA). XPS test was conducted using Thermo Fisher Scientific K-Alpha with an X-ray photoelectron spectrometer (Waltham, MA, USA).

3.3. Photocatalytic Hydrogen Evolution Performance Test

This study employed an online photocatalytic analysis system provided by Beijing Perfectlight Technology Co., Ltd. (Beijing, China) for photocatalytic hydrogen evolution performance testing. The equipment is equipped with a PLS SXE300+/UV xenon lamp (Beijing, China) that covers spectral ranges from ultraviolet to visible and near-infrared, with a color temperature up to 6000 K. The photocatalytic reactions were illuminated using a 300 W Xe lamp with an AM 1.5G filter to simulate the solar spectrum. The light intensity was adjusted to 100 mW/cm² (1 sun). For quantitative analysis of photocatalytic products, the experiment utilized a GC9790II gas chromatograph provided by Zhejiang Fuli Instrument Co., Ltd. (Wenling, China), equipped with a 5A molecular sieve column suitable for hydrogen detection. The reaction was conducted in a 100 mL reactor, maintaining a fixed distance of 10 cm between the light source and the liquid surface. The photosensitizer EY was used at a concentration of 20 mg per 100 mL of reaction solution, consistent with commonly reported configurations in the literature. The sacrificial agent TEOA was added at 15 mL per 100 mL of reaction solution, an optimized ratio determined through experimental studies to achieve optimal performance. A circulating cooling system maintained constant reaction temperature throughout the experiments. Hydrogen production was quantified every 15 min using an online gas chromatograph system equipped with a six-port sampling valve.

4. Conclusions

In this study, we successfully constructed a three-dimensional a-MoS_x nanosheet using [Mo₃S₁₃]²⁻ clusters as precursors through a modified Müller method. A comprehensive characterization approach incorporating SEM, TEM, HRTEM, XRD, and XPS systematically revealed the material’s microstructural and compositional features. The results demonstrate that the synthesized MoS_x material exhibits an amorphous layered stacking structure, wherein the fundamental structural units of [Mo₃S₁₃]²⁻ clusters are partially preserved, providing abundant potential catalytic active sites. These interlayer defects facilitate enhanced electron transfer between layers, thereby optimizing HER performance. Within a visible-light-driven photocatalytic system employing Eosin Y solution as the photosensitizer, we conducted preliminary evaluations of the hydrogen evolution performance of the MoS_x nanosheet. Experimental results revealed significant photocatalytic activity under visible light irradiation, with a hydrogen evolution rate reaching 4.53 mmol·g⁻¹·h⁻¹. This validates the potential of [Mo₃S₁₃]²⁻-derived a-MoS_x nanostructures for photocatalytic HER applications. Our findings indicate that [Mo₃S₁₃]²⁻ clusters serve not only as effective building blocks for amorphous molybdenum-sulfur materials but also provide a valuable strategic reference for developing novel non-precious metal photocatalysts.