Recent advances in transition metal dichalcogenide-based heterostructured materials for electrochemical water splitting applications

Abstract

Exploring renewable and sustainable energy materials as substitutes for fossil fuels presents a promising approach for addressing global challenges such as energy scarcity and environmental degradation. Transition metal dichalcogenide (TMD)-based heterostructured materials have emerged as promising electrocatalysts, garnering increasing interest due to their high efficiency in the hydrogen evolution reaction (HER) and/or oxygen evolution reaction (OER). This review provides an outline of the current advancements in using TMD-based heterostructured materials as electrocatalysts for the HER and OER. We begin with a comprehensive introduction to the fundamentals of electrochemical water splitting, followed by an overview of various TMD-based heterostructure combinations, a summary of the different synthesis techniques, and a discussion of the characterization methods employed for these materials. Moreover, special attention is given to structure–performance relationship strategies aimed at enhancing the electrocatalytic activity and durability of TMD-based heterostructured materials for the HER and OER. Finally, we discuss the existing challenges and provide insights into future prospects for TMD-based heterostructured materials as electrocatalysts in water-splitting technologies.

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Angappan Jayanthi

Angappan Jayanthi received her BSc in Chemistry from CNC College, Erode, in 2009, her MSc in Chemistry from CBM College, Coimbatore, in 2011, and her MPhil in Chemistry from Sri Vasavi College, Erode, in 2015. She is currently pursuing her PhD under the supervision of Dr S. Jayabal at KPR Institute of Engineering and Technology, Coimbatore. Her research interests focus on the design of nanocomposite materials and their applications in water splitting.

Subramaniam Jayabal

Subramaniam Jayabal is currently working as an Assistant Professor-III in the Department of Chemistry at KPR Institute of Engineering and Technology, Coimbatore, India. He received his MSc (2007), MPhil (2008), and PhD (2016) degrees in Chemistry from Madurai Kamaraj University, Madurai. He worked as a postdoctoral researcher under the prestigious OCPC-Postdoctoral International Exchange Program at the University of Science and Technology Beijing, Beijing (2017–2020), and as a Senior Research Assistant at the University of Malaya, Kuala Lumpur (2014–2016). His research interests focus on the design of smart materials and their electrocatalytic applications in electrolyzers and fuel cells.

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1. Introduction

Electrochemical water splitting is widely recognized as the cleanest method for producing hydrogen, generating only hydrogen and oxygen as products of the reaction.¹ Electrocatalytic hydrogen generation from water has attracted considerable attention in recent times due to its great efficiency, environmental friendliness, and substitution method of storing electrical energy.^2–4 Water splitting involves two half–cell reactions, namely the hydrogen evolution reaction (HER) occurring at the cathode and the oxygen evolution reaction (OER) taking place at the anode.^3,5–7 Electrochemical water splitting provides a clean and efficient method for producing high-purity hydrogen.⁸ The thermodynamic potential values for the electrochemical water splitting reaction are 0 V (vs. reversible hydrogen electrode (RHE)) for the HER and 1.23 V (vs. RHE) for the OER at 25 °C and 1 atm.⁹ However, due to kinetic barriers, a higher potential is required than these thermodynamic values to drive water electrolysis. This additional potential is the overpotential (η) and arises mainly from the intrinsic activation energy barriers to both the HER and OER.¹⁰ Overpotential is a key parameter in evaluating the performance of electrocatalysts, and the overpotential value at a current density of 10 mA cm⁻² is used as a standard for comparing the electrocatalytic activity of different catalysts.¹¹ Electrocatalytic splitting of water into hydrogen and oxygen has garnered significant interest as an alternative energy source owing to its low cost and environmental friendliness.^1,12 The overall water splitting process comprises two half-cell reactions; the HER occurs at the cathode, while the OER takes place at the anode, as shown in Fig. 1. The energy loss in the OER is much greater than in the HER, as the OER is a sluggish four-electron transfer reaction that needs an additional overpotential.^13,14 Therefore, the OER is considered the bottleneck of water electrolysis.

Fig. 1 Scheme for overall electrocatalytic water splitting.

Noble metals like platinum (Pt) are commonly utilized as catalysts for the HER, whereas iridium (Ir) and ruthenium (Ru)-based oxides are widely used as catalysts for the OER.^15,16 Despite their excellent performance, their high expense and scarcity limit their large-scale application in electrochemical water splitting.^17–19 Thus, the development of alternative, non-Pt-based electrocatalysts that offer robust electrocatalytic performance at a more affordable cost is essential.²⁰ An effective electrocatalyst accelerates the reaction by reducing the activation energy barrier in water splitting.²¹ In recent decades, considerable efforts have been focused on developing cost-effective, efficient, and durable catalysts derived from earth-abundant elements for the HER and OER. Electrocatalysts incorporating 3d, 4d, and 5d transition metals with higher valence states tend to perform better in electrocatalytic water splitting applications.^22,23 Two-dimensional (2D) materials offer a promising choice due to their robust structure and open active sites, which can improve the electrocatalytic performance for both the HER and the OER.^24,25 Numerous 2D materials have been investigated as promising candidates for electrochemical water splitting, including graphene,²⁶ polymers,^27–29 black phosphorus,^30,31 silicene,³² antimonene,^33,34 inorganic perovskites,³⁵ hexagonal boron nitride (h-BN),^36–38 transition metal dichalcogenides (TMDs),^39–41 graphitic carbon nitride (g-C₃N₄),^42–44 transition metal oxides,^45,46 layered double hydroxides (LDHs),^47–49 and MXenes.^50,51

Due to their low cost and low toxicity, earth-abundant transition metal compounds with vacant d orbitals and unpaired d electrons present a clear alternative to Pt-based materials.⁵² To date, the transition metal compounds designed for the HER have included transition metal carbides (TMCs),^53,54 transition metal nitrides (TMNs),^55,56 transition metal oxides (TMOs),^57–59 transition metal dichalcogenides (TMDs),^19,60,61 and transition metal phosphides (TMPs).^62–64 TMDs comprise a transition metal atom (Mo or W) positioned between two chalcogen atoms (S, Se, or Te), resulting in an MX₂ stoichiometry, with their distinctive layered structures, and the weak van der Waals interactions connecting their covalently bonded atomic layers enable them to be exfoliated into few-layer sheets or single sheets, exhibiting properties significantly different from the bulk forms.⁶⁵ The high surface area and layered structure of TMDs enable efficient ion transport and storage, making them highly promising for applications, including electronics,⁶⁶ energy storage and conversion,^67,68 optoelectronics,⁶⁹ sensors,^70,71 spintronics,⁷² thermoelectric devices,⁷³ and quantum technologies.⁷⁴ However, despite their potential, the efficiency of 2D materials is still not as high as that of Pt, Ir, and Ru-based oxide materials.⁷⁵ The low intrinsic conductivity and limited active sites are the primary causes of the poor electrocatalytic performance in the HER and OER.⁷⁶ The electrocatalytic performance of TMDs can be enhanced through various design strategies and the construction of heterostructures. Numerous studies have been explored for TMD-based heterostructures, including combinations such as TMD/transition metal oxides (TMOs),⁷⁷ TMD/carbon materials,⁷⁸ TMD/transition metal phosphides (TMPs),⁷⁹ TMD/transition metal dichalcogenides (TMDs),⁸⁰ and TMD/MXenes⁸¹ (Fig. 2). The excellent electrocatalytic activity and durability of TMD-based heterostructured materials are a result of the synergistic effect, interfacial effects of their components, altered electronic structures, and reduced kinetic energy barriers in the electrochemical processes.⁸²

Fig. 2 Design strategies of TMD-based heterostructured materials for high-efficiency electrochemical water splitting.

Although numerous studies have been reported in recent years on TMDs as catalysts for electrocatalytic water splitting, existing review articles have focused on mechanistic insights, different synthesis methods, properties, and strategies of various TMDs to improve electrocatalytic HER and OER performances.^83–89 While attempts have been made to incorporate the water-splitting performance of TMD-based electrocatalyst materials, previous reviews have presented the role of interface effects/engineering in TMDs as electrocatalysts,^90,91 engineering unsaturated electronic structures in MoS₂ for the HER,⁹² doping engineering of MoS₂ for improving the HER,^93,94 and further investigated defect-engineered TMDs for enhanced HER and OER performances.^95,96 However, within the existing literature, less emphasis has been placed on the role of TMD-based heterostructured materials as electrocatalysts for the HER and OER. Therefore, this review highlights the necessity and emergence of structure–performance correlation studies in TMD-based heterostructured materials for water splitting applications. In this review, we focus on delivering a complete up-to-date analysis of TMD-based heterostructures as electrocatalysts for the HER and OER. We hope that readers will gain a thorough understanding of HER and OER mechanisms, as well as various methods for evaluating electrocatalysts, using TMD-based heterostructured materials as an example. Additionally, we discuss the preparation techniques for these materials, various characterization techniques, and focus on various strategies to enhance their electrocatalytic performance. Moreover, the impact of structure on performance in TMD-based heterostructured materials is explored. Finally, we highlight the challenges at the present stage and potential future perspectives and directions for TMD-based heterostructured materials in water-splitting electrocatalysis.

2. Fundamentals of the HER and OER

Two standard pathways for producing hydrogen from the reduction of protons toward electrocatalytic HER are the Volmer–Tafel and Volmer–Heyrovsky mechanisms (Table 1).^97–99 In an acidic solution, a proton adsorbed onto an accessible catalytic surface site receives an electron from the electrode, forming surface hydrides. This initial dissociation step is referred to as the Volmer reaction (Table 1). Suppose the M–H bond is strong; it is harder to remove hydrogen from the surface, which lowers the overall HER activity. In the Volmer–Tafel process, two neighbouring hydrides interact and combine to release a hydrogen molecule. Conversely, the Volmer–Heyrovsky process involves one hydride accepting an additional electron from the electrode and another proton from the electrolyte, resulting in the release of a hydrogen molecule. In an alkaline solution, the HER is governed by the adsorption of hydroxyl ions (OH_ad) and the dissociation of water (Table 1). The Volmer and Heyrovsky steps emphasize four key factors influencing HER efficiency under alkaline conditions: water adsorption on active sites, the ability to facilitate water dissociation, hydrogen binding energy, and the adsorption strength of hydroxide ions in solution. The initial stage of the HER is marked by weaker water adsorption under alkaline conditions compared to the stronger H₃O⁺ adsorption observed under acidic conditions. Therefore, strengthening the metal–H₂O bond can enhance HER performance.¹⁰⁰ Therefore, a catalyst with a Gibbs free energy of adsorbed hydrogen (ΔG_H*) near zero is optimal, as extreme values, whether too low or too high, can disrupt the effective adsorption of hydrogen ions and the release of hydrogen molecules.^101–103 The water dissociation step under alkaline conditions is lower than under acidic conditions.

Table 1 Mechanism of the HER in acidic and alkaline solutions

	In acidic solution	In alkaline solution
HER	2H^+(aq) + 2e^− → H2(g)	2H2O + 2e^− → H2(g) + 2OH^−(aq)
Volmer step	H^+ + e^− → Hads	2H2O + 2e^− → 2Hads + 2OH^−
Tafel step	2Hads → H2	2Hads → H2
Heyrovsky step	H^+ + Hads + e^− → H2	H2O + Hads + e^− → H2 + OH^−

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In the OER, four electrons are extracted from a water molecule through various intermediates, leading to the formation of molecular oxygen (Table 2).^104,105 This process involves multiple electron–proton oxidation steps, driven by the adsorption and desorption of various oxygen intermediates, collectively known as the adsorbate evolution mechanism (AEM).¹⁰⁶ Identifying these intermediates with precision remains a challenge in elucidating the OER pathway in both acidic and alkaline/neutral solutions. Numerous reaction pathways have been proposed, with the stepwise mechanism being the most widely accepted¹⁰⁷ as shown in Fig. 3(a and b), where * indicates the active site of the catalyst. In the OER mechanism, the catalyst's active sites interact with adsorbed species of oxygen, including *OH, *O, and *OOH intermediates. The electrochemical oxidation process involves the oxidation of water or hydroxyl ions to *OH, which then deprotonates and oxidizes further to generate *O and *OOH, ultimately leading to O₂ production. In acidic media, the reaction begins with the oxidation of a water molecule, resulting in electron loss, whereas under alkaline or neutral conditions, oxidation occurs at a hydroxide ion.¹

Fig. 3 Schematic illustration of AEM pathways for the OER under (a) acidic and (b) alkaline/neutral conditions. Schematic illustration of LOM pathways for the OER on (c) the oxygen site and (d) the metal site. (a–d) Reproduced with permission from ref. 108. Copyright 2023 The Royal Society of Chemistry.

Table 2 Mechanism of the OER in acidic and alkaline solutions

In acidic solution	In alkaline/neutral solution
* + H2O → *OH + H^+ + e^−	* + OH^− → *OH + e^−
*OH → *O + H^+ + e^−	*OH + OH^− → *O + H2O + e^−
*O + H2O → *OOH + H^+ + e^−	*O + OH^− → *OOH + e^−
*OOH → * + O2 + H^+ + e^−	*OOH + OH^− → * + O2 + H2O + e^−

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The lattice oxygen-mediated mechanism (LOM) has recently emerged as an alternative pathway, emphasizing the redox dynamics of surface lattice oxygen.^109,110 Unlike the AEM, where the catalyst surface remains stable and only the valence state of the active site changes, the LOM suggests that the catalyst surface is inherently unstable due to the activation of lattice oxygen, which actively participates in the OER. Two distinct OER pathways have been identified within the LOM framework, each involving different active centers. As illustrated in Fig. 3(c), one pathway involves activated lattice oxygen serving as an active site, facilitating the nucleophilic attack of OH⁻ to generate *OOH species. The subsequent release of O₂ generates an oxygen vacancy, which is then replenished by OH⁻, forming the *OH intermediate. An alternative LOM utilizes the metal sites as active centers, where OH⁻ adsorption leads to a deprotonation reaction, as depicted in Fig. 3(d).^108,111 Through surface reconstruction, the *O species combines with activated lattice oxygen to form the *OOH species that ultimately evolves into molecular O₂ before its release. Consequently, the activation of lattice oxygen is a crucial aspect of both LOM pathways in the OER process.^112–114

3. TMD-based heterostructured materials

Heterostructures are typically defined as hybrids that contain heterojunctions or hetero-interfaces made up of two or more materials.¹¹⁵ The interaction between these distinct materials and their impact on catalytic activity can be attributed to synergistic and interfacial effects. By optimizing their physical and chemical characteristics through surface modification approaches, TMDs can exhibit improved electrocatalytic performance. The electrocatalytic performance is further influenced by interactions between intercalated atoms, which modify their electronic and conducting properties.¹¹⁶ To improve the electrocatalytic activity and durability of relevant electrocatalytic reactions, the performance of the TMDs can be further enhanced through various techniques, including surface functionalization, designing hybrid structures, and introducing defects/doping on the material surface.⁹⁶ TMDs provide an excellent platform for novel, highly adaptable electrocatalysts with exceptional properties owing to their advantages, such as high active surface area,¹¹⁷ efficient mass transfer, and easy access of electrolytes to active sites.¹¹⁸ Among the various TMD materials, stable 2D crystalline structures such as tungsten diselenide (WSe₂),^119–121 molybdenum diselenide (MoSe₂),^122–124 tungsten disulfide (WS₂),^125–127 and molybdenum disulfide (MoS₂)^128–130 stand out as prominent candidates for electrocatalytic applications. TMDs show a variety of interesting characteristics, including semiconducting (WS₂, MoS₂), insulating (HfS₂), metallic (VSe₂, NbS₂), and semi-metallic (TiSe₂, WTe₂). The formula for TMDs is MX₂, where M and X stand for transition metals and chalcogens (S, Se, or Te),^131–133 respectively. The stacking polytypes of bulk TMDs, like W and Mo dichalcogenides, vary, with 1T, 2H, and 3R phases being the most prevalent.¹³⁴ The polymorphs of Mo and W dichalcogenides' 1H and 1T phases have similar layer-to-layer distances of approximately 6-7Å.^89,135 In MoX₂, Mo exists in a +4 oxidation state and has a 4d² valence electron configuration.¹³⁶ The 2H and 3R phases exhibit hexagonal symmetry and rhombohedral symmetry, respectively, while both maintain the same trigonal prismatic coordination. In contrast, the 1T phase is characterized by tetragonal symmetry with octahedral coordination. In the electronic structure of the 1T phase, the valence band corresponds to the t_2g orbital, which is filled by electrons in octahedral coordination (as unpaired in d_xy and d_yz). Electrons are packed into the d_z² orbital in a trigonal prismatic form, naturally semiconducting as they transition to the d_x²_−y² and d_xy orbitals. The semi-metallic appearance is attributed to the partially occupied valence band of the 1T phase.^136–138 Since the energy of the d_z² orbital is slightly smaller than that of the t_2g orbital, the 2H phase has a lower total electron energy than the 1T phase.¹³⁹ Compared to the 2H phase, the 1T phase exhibits greater activity in terms of the electrocatalytic HER performance. The 2H phase of MoS₂ is found naturally.¹⁴⁰ In the 1T phase, Mo coordinates octahedrally and displays paramagnetic behavior. Similarly, MoSe₂ (drysdallite) is a rare mineral, which is present in the Earth's crust.¹³⁶ The cell parameters of 2H–MoSe₂, a = 3.283 Å and c = 12.918 Å, place it in the hP₆ space group. Compared to 2D MoS₂, the electron mobility in 2D MoSe₂ is greater; Mo–Se and Se–Se possess superior activity due to their respective bond lengths of 2.528 Å and nearly 3.293 Å. Due to the high atomic radius of Se atoms, MoSe₂ monolayers have a higher thickness than MoS₂.¹³⁶ MoTe₂ crystallized into two distinct phases: 1T′ and 2H.¹⁴¹ The distinctive quasi-metallic 1T′-phase features a distorted sandwich-like structure composed of one-dimensional zigzag transition-metal chains.^142,143 The 2H phase has unit cell parameters of a = 3.519 Å and c = 13.946 Å, placing it in the hP6, P6₃/mmc space group. In contrast, the 1T phase has unit cell parameters of a = 6.33 Å, b = 3.469 Å, and c = 13.86 Å, and is associated with the P12, P2₂/m space group.¹⁴⁴ Due to the small difference in energy between the 2H and 1T′ phases, temperature-induced phase transitions in MoTe₂ make it difficult to obtain a single pure phase.¹⁴⁵ Simple synthesis techniques must be used to generate pure-phase MoTe₂, since its complicated structure limits its usage and development. Direct band gaps appear in monolayers of other semiconducting TMDs, including MoSe_2, WS₂, and WSe₂, which share the same trigonal prismatic coordination as MoS_2, while bi- or multilayers exhibit indirect band gaps.¹⁴⁶ For instance, each Mo atom in the thermodynamically stable 2H phase MoS₂ is prismatically coordinated to six S atoms, whereas in the metastable 1T phase MoS₂, six S atoms form a distorted octahedron around one Mo atom.¹⁴⁷ Although a weak van der Waals force attracts two separate sheets, the atoms in the same layer are kept together by strong covalent bonds.¹⁴⁸ Bulk TMDs with 2H phase occur as indirect band-gap semiconducting materials with band gaps ranging from 1.1 eV for MoSe₂, 1.2 eV for WSe₂, 1.0–1.29 eV for MoS₂, and 1.31–1.4 eV for WS₂.¹⁴⁹ Direct-band-gap semiconducting behavior can be observed in a single monolayer with comparable band gaps found at 1.8–1.9 eV for MoS₂, 1.5–1.6 eV for MoSe₂, 1.6–1.7 eV for WSe₂, and 1.8–2.1 eV for WS₂.^149,150

3.1. TMD/TMD heterostructures

The design of TMDs and their heterostructures has emerged as a promising way for achieving high electrocatalytic performance, attributed to their distinctive electronic characteristics, layered architectures, and high surface-to-volume ratios.¹⁵¹ Particularly, TMD/TMD heterostructures, created by stacking or combining two distinct TMDs, have attracted considerable interest for their potential to further improve the electrocatalytic performance.¹⁵² These heterostructures facilitate modified band alignments, enhanced charge transfer, and synergistic catalytic activity, rendering them particularly effective in reactions such as the HER and OER.¹⁵³ The interfacial interactions present in TMD/TMD heterostructures provide opportunities for fine-tuning electronic structures and optimizing active sites, thereby presenting a versatile platform for the development of new electrocatalysts. Cheng et al.¹⁵⁴ successfully developed hollow CoSe₂ nanocubes integrated with ultrathin MoSe₂ nanosheets (CoSe₂@MoSe₂). Their research indicates that the incorporation of these ultrathin MoS₂ nanosheets with hollow CoSe₂ nanocubes significantly increases the accessibility of active sites, enhances both electron and mass transfer, promotes the release of bubbles, and optimizes the movement of charge carriers. Additionally, the interactions of surface electrons within the heterostructures generate extra sites for the adsorption of H⁺ and/or OH⁻, which in turn reduces the activation energy necessary for the adsorption and dissociation of water molecules. The CoSe₂@MoSe₂ hollow heterostructures exhibit excellent electrocatalytic performance, with overpotentials of 183 mV for the HER and 309 mV for the OER at a current density of 10 mA cm⁻² in 1.0 M KOH. Muskha et al.¹⁵⁵ synthesized CoSe₂ nanoparticles embedded within WSe₂ nanosheets, creating heterostructured nanohybrids through a hot-injection colloidal synthesis technique to improve their electronic characteristics. Electrochemical analyses showed that the WSe₂/CoSe₂ heterostructured nanohybrids exhibit exceptional electrocatalytic performance for both acidic HER and basic OER, when compared to the individual components of CoSe₂ and WSe₂. The electrocatalytic properties of these nanohybrids are characterized by low overpotentials of 157 mV and 330 mV (η₁₀) for the HER and OER, respectively. Additionally, the Tafel slope values are 79 mV dec⁻¹ for the HER and 76 mV dec⁻¹ for the OER. This improved electrochemical performance is due to enhanced electronic conductivity within the nanoscale heterostructured hybrids, which aids in electron transfer between CoSe₂ and WSe₂, thereby facilitating charge transfer reactions at the surface/interface during electrochemical activities and increasing ion adsorption due to the larger surface area.

3.2. TMD/TMO heterostructures

The improved electrocatalytic performance of TMD and TMO materials for water splitting is well-established. Recent studies emphasize the synergistic benefits obtained by merging the high surface area and conductivity of TMDs with the abundant active sites and adjustable electronic structures of TMOs, resulting in enhanced catalytic activity and stability compared to the individual components.¹⁵⁶ Jian et al.¹⁵⁷ successfully engineered hybrid MoSe₂/MoO₂ nanosheets with rich edge sites and high electrical conductivity by adding a MoO₂ layer onto a Mo foil substrate. Partial conversion of MoO₂ to MoSe₂ further increased the density of edge sites, enhancing catalytic activity. The resulting MoSe₂/MoO₂/Mo hybrid demonstrated superior HER performance compared to pure MoSe₂. Specifically, the hybrid catalyst exhibited a small overpotential of 142 mV at a current density of 10 mA cm⁻² and a low Tafel slope of 48.9 mV dec⁻¹. These enhanced electrocatalytic properties are attributed to the synergistic effects between the abundant active sites on the MoSe₂ surface and the efficient electron transport facilitated by the conductive MoO₂/Mo substrate. Zhang et al.¹⁵⁸ have successfully developed MoS₂ nanosheets on Fe₃O₄ nanospheres utilizing a SiO₂ shell-assisted sacrificial template method. Fe₃O₄ nanospheres act as a support for anchoring edge-exposed MoS₂ nanosheets, significantly enhancing charge transfer efficiency. The use of a SiO₂ sacrificial template in the synthesis process ensures the uniform and stable anchoring of MoS₂ nanosheets onto the surface of the Fe₃O₄ spheres. The successful formation of a heterostructure between MoS₂ and Fe₃O₄ induces a high density of defects and facilitates rapid charge exchange, thereby improving the overall electrocatalytic performance. The polarization curves for MoS₂@Fe₃O₄, pristine MoS₂, and Fe₃O₄ reveal that MoS₂@Fe₃O₄ has a notably lower onset potential of 110 mV, in contrast to 200 mV for MoS₂, while Fe₃O₄ demonstrates negligible catalytic activity. Consequently, the synergistic effects discussed above markedly improved catalytic performance toward the HER of the MoS₂@Fe₃O₄ electrocatalyst.

3.3. TMD/TMP heterostructures

The rational design of TMD/TMP interfaces promotes synergistic interactions that significantly enhance charge transfer kinetics, increase the density of catalytically active sites, and improve electrochemical durability.¹⁵⁹ The formation of such heterointerfaces often induces defect-rich structures and modifies the local electronic structure, thereby optimizing the adsorption energies of key reaction intermediates and facilitating more efficient electrocatalytic processes.^160,161 Pan et al.¹⁶² designed an innovative hybrid catalyst, CoP/MoS₂-CNTs, by the in situ thermal decomposition growth of CoP nanorods (CoP NRs) on the surface of MoS₂ and CNTs. The resulting CoP/MoS₂-CNTs hybrid catalyst reveals exceptional catalytic activity, exhibiting an overpotential nearly equal to zero and a Tafel slope of 42 mV dec⁻¹, making it the most effective among all non-noble metal catalysts. The remarkable catalytic activity is primarily due to the synergistic interactions between CoP and MoS₂, along with the incorporation of CNTs, which increase both the number of active catalytic sites and the electron transfer capabilities. Du et al.¹⁶³ developed a novel strategy to regulate the interface structure between MoS₂ and MoP by introducing S vacancies. This method involves the synthesis of MoS₂ nanotubes enriched with S vacancies (Sv-MoS₂) through a reduction process. Subsequently, the Sv-MoS₂ is phosphated by adding a phosphorus (P) source, leading to the formation of a MoP/Sv-MoS₂ composite. This catalyst exhibited excellent stability and durability for the HER, demonstrating an overpotential of 109 mV and a Tafel slope of 60 mV dec⁻¹ at a current density of 10 mA cm⁻² in an acidic medium. The key contributing factors include the S vacancies induced by NaBH₄ etching, which facilitate P incorporation to activate the inert interfaces. The introduction of P not only increases the density of S vacancies but also exposes additional active sites. Furthermore, the heterogeneous structure composed of coexisting MoP and MoS₂ phases enhances electron transfer efficiency. Simultaneously, P atoms replace S atoms to form Mo–P bonds, contributing to improved catalyst stability. The outstanding HER performance is attributed to the synergistic effect of incorporated P atoms and S vacancies, which collectively reduce the energy barrier of the Volmer step and facilitate the adsorption/desorption of H* intermediates.

3.4. TMD/carbon-based heterostructures

A potential role of carbon-based supports is to expose sufficient active sites, ensure uniform dispersion of active coupling materials, and protect against the agglomeration of these materials under the harsh conditions of electrocatalytic processes.¹⁶⁴ Well-engineered 2D carbon-based heterostructures can effectively increase the density of active sites, enhance electrical conductivity, modulate electronic structures, and optimize the binding energy of reaction intermediates. These combined effects significantly boost the intrinsic catalytic activity of the associated components. Such synergistic interactions between active catalytic materials and carbon-based substrates ensure extended stability and improved adsorption of HER and OER intermediates.¹⁶⁵ Cao et al.¹⁶⁶ developed a novel three-dimensional (3D) array of 1T/2H–MoS₂/Co nanoflowers grown on carbon cloth (1T/2H–MoS₂/Co/CC). The 1T/2H–MoS₂/Co/CC achieves a remarkably low overpotential of 83 mV at a current density of 10 mA cm⁻², along with a small Tafel slope of 39 mV dec⁻¹ for the HER. This outstanding performance of 3D 1T/2H–MoS₂/Co/CC is due to leveraging the combined benefits of its 3D architecture, the incorporation of the 1T phase, defect engineering, and Co doping. Zhang et al.¹⁶⁷ designed 3D hierarchical MoSe₂/NiSe₂ nanowires (NWs) on carbon fibre paper using a simple two-step hydrothermal method. The MoSe₂/NiSe₂ NWs demonstrated excellent HER activity characterized by a low overpotential and a small Tafel slope. The MoSe₂/NiSe₂ composite NWs demonstrated significantly enhanced electrocatalytic activity, primarily due to their 3D hierarchical architecture. This structure effectively prevents the aggregation and restacking of nanosheets, thereby maximizing the exposure of active sites for the HER. Additionally, the incorporation of highly conductive NiSe₂ within the nanosheets promotes efficient electron transport from the electrode to the catalytically active edges of MoSe₂, thereby further boosting the HER performance.

3.5. TMD/MXene heterostructures

MXenes, a group of two-dimensional materials composed of transition metal carbides, nitrides, or carbonitrides with surface groups like –OH, –O, and –F, have garnered attention as promising candidates for electrocatalytic applications due to their unique surface and electronic properties.¹⁶⁸ However, the tendency of MXene layers to restack, driven by hydrogen bonding and van der Waals interactions, significantly limits their practical utility. To overcome this limitation, TMD nanosheets such as MoS₂, WS₂, and MoSe₂ have been uniformly integrated onto the surface of MXenes. This strategy not only effectively inhibits layer restacking but also enhances electrochemical performance, owing to the synergistic interactions between MXenes and TMDs.¹⁶⁹ Wei et al.¹⁷⁰ developed a TMD/MXene heterostructure using a single-step solvothermal synthesis method to produce a ternary hybrid composed of dual-phase MoS₂ (DP-MoS₂), titanium carbide (Ti₃C₂) MXene, and carbon nanotubes (CNTs). The DP-MoS₂ is directly grown on the MXene surface along with CNTs, resulting in a composite that exhibits excellent HER performance with an overpotential of 169 mV at a current density of 10 mA cm⁻², and a low Tafel slope of 51 mV dec⁻¹ was achieved. The presence of edge-rich metallic 1T-MoS₂, combined with the conductive properties of MXene and the crosslinking role of CNTs, significantly enhances the HER performance. Furthermore, the incorporation of 1T-MoS₂ into the MXene matrix increases interlayer spacing and partially inhibits both oxidation and restacking of MXene layers, leading to improved long-term catalytic stability. Lim et al.¹⁷¹ demonstrated a simple and efficient strategy for synthesizing a strongly coupled Mo₂CT_x/2H–MoS₂ nanohybrid via a two-step process. This method involves the controlled in situ growth of 2H–MoS₂ through direct sulfidation of adventitiously oxidized regions on the Mo₂CT_x surface. The presence of oxidized species enables self-limiting growth, facilitating strong epitaxial alignment of 2H–MoS₂ at the Mo₂CT_x interface while avoiding excessive coverage that could hinder catalytic activity. The resulting nanohybrid exhibits outstanding and durable HER performance, outperforming other non-platinum catalysts based on MXenes, Mo₂C, MoS₂, or their composites.

4. Synthesis of TMD-based heterostructured materials

The frequently used methods for synthesizing TMD-based heterostructured materials include hydrothermal reaction, solvothermal reaction, chemical vapor deposition (CVD), microwave-assisted, and intercalation methods (Fig. 4). Among these methods, hydrothermal and solvothermal are the most significant preparation methods for TMD-based heterostructures.

Fig. 4 Schematic design for different preparation methods.

4.1. Hydrothermal method

The hydrothermal method of heterogeneous reactions uses a water-based medium within a sealed steel pressure vessel with a Teflon-lined autoclave to effectively synthesize advanced materials with high yield. The hydrothermal process was conducted within a controlled temperature range of approximately 100–200 °C, with the development of self-generated pressure. Moreover, the moderate temperatures facilitate fast reaction kinetics and reduce processing time.^136,172,173 This method is the most convenient and cost-effective for the preparation of composite materials on a large scale. Wu et al.⁷⁹ synthesized MoP/MoS₂ heterostructures, as shown schematically in Fig. 5(a). The treated carbon cloth functioned as a substrate for the uniform growth of MoS₂ nanosheets through a simple hydrothermal method, utilizing phosphomolybdic acid and thiourea as precursors. These MoS₂ nanosheets then served as templates for MoP formation. The MoP/MoS₂ heterostructure nanosheets were prepared through a controlled substitution of S with P. A phosphorization treatment was applied to enhance the catalyst's performance. The electronic interaction between MoP and MoS₂ phases facilitates electron migration, which is crucial for improving HER performance. Li et al.¹⁷⁴ synthesized MoSe₂/CoP intercalated hybrids (Fig. 5(b)) by ultrasonically treating MoSe₂ nanosheets with cobalt nitrate, urotropine, and ethylene glycol for 12 h, and then annealing at 100 °C for 2 h. Then this material was thermally phosphorized in an Ar atmosphere for 1 h using sodium hypophosphite monohydrate. Owing to its unique sandwich-like architecture, the synthesized MoSe₂/CoP hybrid electrode exhibits HER activity comparable to that of Pt and demonstrates remarkable long-term stability.

Fig. 5 (a) Schematic diagram of MoP/MoS₂ heterostructure nanosheets. (b) Schematic for the fabrication processes of MoSe₂/CoP hybrid nanosheets. (c) Synthesis of MoS₂ in solution with and without a graphene sheet. (d) Schematic illustration of the synthesis process of cryo-mediated liquid phase exfoliation and the formation mechanism of the defect-rich MoS₂ nanosheets. (e) Schematic representation of bare MoS₂ and MoS₂-rGO nanocomposite sheets and corresponding defects. (f) Synthesis of vertically grown MoS₂/WS₂ heterostructures on RGO sheets. (g) Construction of MoS₂-VS₂ heterostructures. (a) Reproduced with permission from ref. 79. Copyright 2019 The American Chemical Society. (b) Reproduced with permission from ref. 174. Copyright 2020 Science Direct. (c) Reproduced with permission from ref. 129. Copyright 2011 The American Chemical Society. (d) Reproduced with permission from ref. 177. Copyright 2019 The American Chemical Society. (e) Reproduced with permission from ref. 182. Copyright 2018 The American Chemical Society. (f) Reproduced with permission from ref. 185. Copyright 2021 The American Chemical Society. (g) Reproduced with permission from ref. 187. Copyright 2018 The American Chemical Society.

4.2. Solvothermal method

The solvothermal method is a process where reactions take place in a sealed vessel using organic or non-aqueous solvents, typically under elevated temperature and pressure.⁴⁹ Li et al.¹²⁹ synthesized a MoS₂/rGO hybrid (Fig. 5(c)) through a one-step solvothermal reaction of ammonium tetrathiomolybdate and hydrazine in an N,N-dimethyl formamide solution containing slightly oxidized graphene oxide at 200 °C. During this reaction, ammonium tetrathiomolybdate was reduced to MoS₂ on the GO surface, while the slightly oxidized GO underwent hydrazine reduction to form rGO. The resulting MoS₂/rGO hybrid demonstrates outstanding HER electrocatalytic activity, characterized by a low overpotential and small Tafel slope. Zhang et al.¹⁷⁵ synthesized an innovative 3D porous nitrogen-doped graphene (NG) derivative that incorporates MoS₂ nanosheets through a solvothermal process utilizing urea as a nitrogen source. The ammonium groups derived from the urea facilitate the introduction of a nitrogen atom into the graphene structure and integrate into the MoS₂ layers, resulting in a mixed 1T–2H phase. By tuning the urea concentration and the porous architecture, the material achieved improved conductivity and a rich supply of catalytically active sites. The MoS₂ nanosheets, characterized by their small size and increased interplanar spacing, were vertically aligned on the NG surface. Due to the synergistic effects, the material shows excellent HER activity and stability in acidic solution, demonstrated by a lower overpotential of 157 mV at 10 mA cm⁻², a lower Tafel slope of 45.8 mV dec⁻¹, and outstanding electrocatalytic stability.

4.3. Liquid phase exfoliation method

The liquid phase exfoliation method is used to achieve the large-scale synthesis of atomic-layer nanosheets from bulk materials.¹⁷⁶ As a result, various techniques have been utilized to produce extremely thin layers of nanosheets. Zhang et al.¹⁷⁷ synthesized water-soluble defect-rich MoS₂ ultrathin nanosheets (d-MoS₂ NSs) (Fig. 5(d)) via a facile cryo-assisted liquid phase exfoliation method employing NaBH₄ as a reducing agent. The resulting d-MoS₂ NSs display improved electrocatalytic HER activity and stability compared to MoS₂ NSs, attributed to the abundant active edge sites and their improved surface hydrophilicity. Nguyen et al.¹⁷⁸ introduced an innovative and effective one-step method that combines plasma-induced doping with the exfoliation of MX₂ bulk materials into nanosheets, specifically targeting MoS₂, MoSe₂, WS₂, and WSe₂. This process is accomplished in a short duration and at low temperatures (ca. 80 °C), enhancing the electron transport characteristics of MoS₂ through the dual effects of exfoliation and nitrogen doping, which significantly improves their catalytic performance. The MoS₂ nanosheets doped with N (5.2 at%) exhibit remarkable catalytic activity for the HER, achieving a low overpotential of 164 mV at a current density of 10 mA cm⁻² and a low Tafel slope of 71 dec mV⁻¹ with long-term stability over 25 h in a 0.5 M H₂SO₄ solution. A noticeable reduction in these values is observed compared to both exfoliated MoS₂ nanosheets (207 mV, 82 dec mV⁻¹) and bulk MoS₂ (602 mV, 198 dec mV⁻¹).

4.4. Intercalation method

Intercalation is a synthesis method for creating composite materials by inserting guest molecules or ions into the interlayer spaces of layered host materials.¹⁷⁹ Through this method, the size and morphology of the host structure can be precisely tailored, leading to the development of new composite materials. The incorporation of guest species can significantly enhance the composite's properties, such as electrical conductivity, thermal stability, and mechanical strength.¹⁸⁰ Ozgur et al.¹⁸¹ successfully synthesized zinc intercalated MoS₂ (Zn@MoS₂) through a lithium exchange method conducted at a controlled temperature. They investigated the electrocatalytic performance of Zn@MoS₂ and its synergistic interaction with graphene in the HER in a 0.5 M H₂SO₄ solution. The electrocatalytic analysis revealed an overpotential of 378 mV and a Tafel slope of 81 mV dec⁻¹. This can be attributed to the intercalation of Zn into the MoS₂ layers, which stabilizes the structure, along with the synergistic interaction between Zn@MoS₂ and graphene that enhances the intrinsic activity and HER performance. Mondal et al.¹⁸² synthesized MoS₂/rGO using a modified Hummers' method, which involves mixing thiourea and ammonium carbonate in an ammonium molybdate tetrahydrate solution, stirring until clear, then spray-drying. The resulting product was calcined at 800 °C for 5 h in a 10% hydrogen in nitrogen atmosphere. Then, a controlled amount of aqueous GO suspension was introduced into the clear solution formed after adding ammonium carbonate. The mixture was vigorously stirred for 30 min, followed by sonication for an additional 30 min. The resultant product MoS₂-rGO composite (Fig. 5(e)) exhibits enhanced electrocatalytic HER activity.

4.5. Microwave-assisted method

Microwave irradiation provides rapid and energy-efficient heating of reactants, promoting better interaction between components, especially in the synthesis of nanocomposites.¹⁸³ This method helps minimize agglomeration during preparation and is more environmentally friendly than conventional heating techniques.⁸⁹ Key advantages include fast heating rates, shorter synthesis times, high energy efficiency, precise process control, and a more sustainable reaction environment.¹⁸⁴ Lee et al.¹⁸⁵ synthesized a vertically aligned MoS₂/WS₂ heterostructure via a simple one-pot microwave-assisted method on conductive rGO sheets, ensuring maximum edge exposure for enhanced HER electrocatalytic performance. The schematic representation in Fig. 5(f) depicts the synthetic approach for MoS₂ and MoS₂-rGO nanocomposite sheets, highlighting key defect formations, interlayer spacing expansion for ammonium ion intercalation, creation of S vacancies, ammonium ions facilitating the phase transformation from 2H to 1T, and formation of step edge structures. Furthermore, the expanded interlayer spacing in the vertical MoS₂/WS₂ heterostructure significantly enhances HER activity. Sahoo et al.¹⁸⁶ reported a simple microwave-assisted synthetic method for designing a nanocomposite composed of graphitic-C₃N₄, rGO, and MoS₂. Importantly, the ternary composite was prepared through rapid microwave irradiation in a short time using a cost-effective synthetic process. The excellent electrocatalytic activity for both the HER and OER of this prepared composite can be attributed to the development of multi-functional heterostructures, where each interface is well-connected. This configuration is beneficial for enhancing the electrochemically active sites and facilitating electron transport during the electrocatalytic process.

4.6. Chemical vapor deposition

In the chemical vapor deposition (CVD) method, various precursors required for material synthesis are introduced in vapor form onto a substrate, typically placed inside a temperature-controlled furnace.⁸⁹ The high temperatures cause the precursors to decompose, resulting in the formation of thermodynamically stable phases directly on the substrate surface.⁸⁹ Leong et al.¹⁸⁷ fabricated the VS₂–MoS₂ heterostructure (Fig. 5(g)) using a CVD method. The two-step CVD process allows specific control over the seamless transition from pre-synthesized VS₂ to the subsequently grown MoS₂ layers. Fig. 5(g) depicts the MoS₂ growth on a SiO₂/Si substrate with pre-deposited VS₂ nanosheets, illustrating two probable growth mechanisms: edge- and surface-mediated, indicated by arrows. The schematic diagram includes: (i and ii) atomic models of 1T-VS₂ and 2H–MoS₂, (iii and iv) the density of states of VS₂ and MoS₂ near their Fermi levels, and (v) a conceptual representation of the MoS₂ growth, either guided by the edge or surface of the pre-grown VS₂ nanosheets. Yu et al.¹⁸⁸ reported the preparation of a MoS₂/VS₂ hybrid through a single-step CVD process, enabling the in situ growth of MoS₂ microflowers on VS₂ flakes for enhanced electrocatalytic HER performance in an acidic solution. The MoS₂/VS₂ hybrid exhibits enhanced HER performance as compared to the individual pristine MoS₂ and VS₂ due to the formation of lightly vanadium-doped MoS₂. The key advantages of fabricating the MoS₂/VS₂ composite via a one-step tube furnace synthesis directly on carbon fiber paper (CFP) as a HER electrocatalyst include enhanced intrinsic conductivity of 2H–MoS₂ through vanadium doping, an open-channel microflower structure that promotes electrolyte penetration, shortens ion diffusion paths, and facilitates H₂ bubble release, and strong electrical contact and mechanical adhesion due to direct integration with CFP.

5. Characterization of TMD-based heterostructured materials

5.1. X-ray diffraction (XRD) and Raman spectroscopy

XRD is a crucial technique for characterizing the structure of crystalline materials. It enables the determination of lattice parameters and atomic arrangements in single crystals and phase analysis in polycrystalline materials and compounds.^189,190 Chen et al.¹⁹¹ presented the crystal phase of WS₂/CNTs hollow microspheres (HMS). The observed diffraction peaks at 14.3° and 28.9° in Fig. 6(a) correspond to (002) and (004) planes of WS₂. Furthermore, the remaining peaks correspond to 2H-WS₂, confirming the formation of tungstate ions and H₂S to form WS₂. Yan et al.¹⁹² observed that the XRD pattern of MoS₂ nanosheets (Fig. 6(b)) displays three distinct diffraction peaks at 2θ values of 14.6°, 39.8°, and 50.1°, corresponding to the (002), (103), and (200) crystal planes of 2H–MoS₂. The high-intensity diffraction peaks suggest the significant re-stacking or re-aggregation of the MoS₂ nanosheets. However, 3D cross-linked ternary structures synthesized using graphene, few-layer MoS₂, and g-C₃N₄ (represented as MoS₂–CN/G), the intensity of the MoS₂ diffraction peaks decreases notably. The emergence of two broad peaks at 26.3° and 28.1° can be attributed to few-layer graphene and g-C₃N₄ nanosheets. This suggests that the creation of a 3D porous network efficiently suppresses the re-aggregation or re-stacking of MoS₂ nanosheets.

Fig. 6 (a) XRD patterns of WS₂/CNTs HMS, and (b) MoS₂–CN/G, g-C₃N₄, MoS₂ nanosheets, and GO. (c) Raman spectra of pristine MoS₂ (MoS₂–P) and 20% rGO incorporating MoS₂ (MoS₂-rGO20) samples, and (d) Raman spectra of MoS₂–CN/G, MoS₂, and graphene. (a) Reproduced with permission from ref. 191. Copyright 2021 Wiley-VCH GmbH. (b and d) Reproduced with permission from ref. 192. Copyright 2020 The American Chemical Society. (c) Reproduced with permission from ref. 182. Copyright 2018 The American Chemical Society.

Mondal et al.¹⁸² demonstrated that the MoS₂ sheets incorporated with rGO represented as MoS₂-rGO (Fig. 6(c)). The Raman spectra of pristine MoS₂ displayed four intense peaks at 403, 375, 280, and 143 cm⁻¹, along with a weaker peak at 330 cm⁻¹. The peaks at 403, 375, and 280 cm⁻¹ correspond to the A_1g, E_2g¹, and E_g¹ modes characteristic of 2H MoS₂. Meanwhile, the peaks at 330 and 143 cm⁻¹ are attributed to the J₃ and J₁ modes associated with the 1T-MoS₂. The Raman spectrum of MoS₂-rGO also exhibited the A_1g, E_2g¹, E_g¹, J₃, and J₁ peaks characteristic of pristine MoS₂. Additionally, a weak peak at 230 cm⁻¹, corresponding to the J₂ mode of the 1T-MoS₂ phase, respectively. Though the positions of the peaks remain similar to those of pristine MoS₂, the peaks in MoS₂-rGO appear broader, suggesting an increased S deficiency. Furthermore, the intensity of the E_g¹ mode is significantly higher compared to pristine MoS₂, indicating an enhanced degree of oxygen incorporation in the MoS₂-rGO material. Apart from these MoS₂-associated peaks, two additional peaks at 1346 cm⁻¹ and 1585 cm⁻¹, corresponding to the D and G bands of rGO, were detected in the Raman spectrum of MoS₂-rGO. These peaks were absent in the pristine MoS₂ sample, confirming the presence of rGO in MoS₂-rGO samples. Raman spectroscopic analysis further confirms the coexistence of both 2H and 1T phases in all materials, as well as the presence of S vacancies and oxygen incorporation. Notably, the extent of S vacancies and oxygen incorporation is more pronounced in rGO-containing samples than in pristine MoS₂, as evidenced by the increased peak broadening and higher E_g¹ peak intensity. However, quantifying the relative fractions of the 2H and 1T phases by Raman spectroscopy remains challenging.

Yan et al.¹⁹² showed the Raman spectrum of MoS₂–CN/G architecture (Fig. 6(d)) displays two distinctive peaks at 377 cm⁻¹ and 396 cm⁻¹, which correspond to the in-plane E¹_2g and out-of-plane A_1g vibrational modes of 2H–MoS₂, respectively. The frequencies of these modes progressively decrease as the number of MoS₂ layers decreases. Notably, the small frequency difference (19 cm⁻¹) among these two peaks suggests the presence of ultrathin MoS₂ nanosheets in the MoS₂–CN/G hybrid and consists of only a few layers. Additionally, two distinct Raman peaks are observed at 1338 cm⁻¹ and 1590 cm⁻¹ corresponding to the D band and G band intensities, respectively, further confirming the presence of the MoS₂–CN/G hybrid structure. The relative intensity ratio of D and G bands (I_D/I_G) is commonly used to assess defect density in carbon lattices.¹⁹² Compared to plain graphene, the I_D/I_G value is increased to approximately 1.24 for MoS₂–CN/G hybrid arises from two key aspects: (i) the formation of a 3D architecture introduces rich defects and structural disorder in graphene, and (ii) the reduction of GO leads to a decrease in the size of sp² domains.

5.2. Scanning electron microscopy (SEM)

SEM is a surface imaging technique that creates detailed images by scanning a sample with a focused electron beam. The interaction between the electrons and the sample produces signals that reveal the surface structure and its atomic composition.^193,194 Sharma et al.¹⁵² identified the morphology, composition, and crystallinity of prepared nanosheets with the help of field emission SEM (FESEM). The FESEM images of MoSe₂ and MoS₂ nanosheets (ns) (Fig. 7(a)) reveal that the “ns” are thicker than the as-obtained thin sheets (ts). Fig. 7(a) presents the FESEM images of MoSe₂-ts@MoS₂-ts, showing the formation of small spherical structures composed of nanoflakes. Due to their extremely small size, accurately measuring the length and width of the flakes is challenging using FESEM images. Mu et al.¹⁹⁵ proposed that the SEM images of MoSe₂/CoSe₂ composites with different Mo/Co ratios (Mo : Co = 1 : 1, 3 : 1, 1 : 3, labelled as MC11, MC31, and MC13) reveal distinct morphologies of MoSe₂ and CoSe₂. By varying the Mo and Co precursor ratios under identical reaction conditions, different composite structures are formed. As shown in Fig. 7(b and c), the resulting products exhibit a hierarchical tube-like nanosheet assembly forming microcages. These microcages exhibit narrow size distributions, with MC31 averaging 1.1 μm (diameter) × 3.1 μm (length) and MC11 at 1.65 μm × 3.37 μm. Notably, increasing the Co ratio in MC13 leads to an irregular yet preserved cage-like morphology. Structurally, the inner surfaces are dense and smooth, contrasting with the outer surfaces composed of multi-fold nanosheets (5–10 nm thick). Wu et al.'s⁷⁹ SEM images of MoP/MoS₂ in Fig. 7(d–f) reveal that the nanosheet-like structures remain largely intact, exhibiting minimal structural collapse. However, the nanosheet surfaces appear rougher compared to those of bare MoS₂ nanosheets. Additionally, EDS mapping of MoP/MoS₂ and bare MoS₂ nanosheets confirms the uniform distribution of Mo, P, and S elements in MoP/MoS₂, further verifying the successful design of MoP/MoS₂ heterostructures.

Fig. 7 (a) FESEM image of MoSe₂-ts@MoS₂-ts. (b and c) SEM images of the as-synthesized pure MoSe₂ and CoSe₂, (d and e) SEM images of MoP/MoS₂, (f) elemental mapping of Mo, S, and P for MoP/MoS₂, (g and h) TEM and HRTEM images of MoP/MoS₂, (i) TEM elemental mapping images of Mo, S, and P for MoP/MoS₂, (j–n) TEM and HR-TEM images of MoS₂–P and (o–u) MoS₂-rGO. (a) Reproduced with permission from ref. 152. Copyright 2020 The American Chemical Society. (b and c) Reproduced with permission from ref. 195. Copyright 2016 The Royal Chemical Society. (d–i) Reproduced with permission from ref. 79. Copyright 2019 The American Chemical Society. (j–u) Reproduced with permission from ref. 182. Copyright 2018 The American Chemical Society.

5.3. Transmission electron microscopy (TEM)

TEM is one of the important techniques that can provide detailed information about the size, shape, crystalline or amorphous structure, and orientation of the sample's internal components at the atomic level.^190,196 The integration of high-resolution TEM (HRTEM), energy-dispersive X-ray spectroscopy (EDS), and scanning transmission electron microscopy (STEM) provides valuable insights into the geometric relationship between TMDs and their support materials. Wu et al.⁷⁹ demonstrated that the TEM images of MoS₂ nanosheets and MoP/MoS₂ at various scales provide further insight into their morphology. The low-magnification TEM images of MoS₂ nanosheets (Fig. 7(g)) confirm their sheet-like structure. In the MoP/MoS₂ sample, the nanosheet surfaces appear rougher, but their overall morphology and thickness remain largely unchanged. The HRTEM image (Fig. 7(h)) clearly reveals an interplanar spacing of 0.64 nm corresponding to the MoS₂ (002) crystal plane, while the lattice fringe with a 0.28 nm interplanar distance matches the MoP (100) crystal plane. These planes form the MoP/MoS₂ heterointerface, facilitating strong interphase contact. This intimate MoP/MoS₂ interaction enhances electron migration, playing a vital role in improving HER performance. Furthermore, due to the thin and uniform nature of the MoP/MoS₂ nanosheets, the TEM elemental mapping images (Fig. 7(i)) confirm the uniform distribution of Mo, P, and S elements across the entire nanosheet structure.

Mondal et al.¹⁸² proposed that the TEM images of prepared pristine MoS₂ (MoS₂–P) and rGO-incorporating MoS₂ (MoS₂-rGO) samples provide insights into their morphology. The TEM images of MoS₂–P (Fig. 7(j)) reveal the presence of individual sheets oriented both laterally and horizontally, while the TEM images of MoS₂-rGO (Fig. 7(o)) depict well-separated MoS₂ sheets distributed on the rGO. The selected area electron diffraction (SAED) mapping (inset of Fig. 7(j and o)) confirms the formation of a highly crystalline MoS₂ nanostructure, with sheet sizes in the nanometer range. Fig. 7(k and l) reveal that the MoS₂ sheets consist of 15–20 stacked layers, with a width of approximately 10 nm. In contrast, the TEM image of MoS₂-rGO (Fig. 7(p)) shows the presence of ‘hairy’ structures at the edge-curled positions of the MoS₂ sheets on the surface of graphene. Magnified TEM images (Fig. 7(q)) further confirm the existence of 5–8 stacked MoS₂ layers of variable sizes (5–15 nm) distributed across the rGO surface. Finally, the HRTEM images of MoS₂–P and MoS₂-rGO at the edges reveal that the interlayer distance is not constant, leading to a wide (002) peak as opposed to discrete ones. This suggests that the fringes at the edges of individual sheets are discontinuous, as highlighted by yellow circles in Fig. 7(l and q). The presence of a nanosized structure along the basal planes and a high degree of edge defects is evident in the MoS₂-rGO samples. The HRTEM image in the basal plane of MoS₂–P (Fig. 7(m)) displays distinct lattice fringes with an interplanar spacing of 0.27 nm at a 60° angle, corresponding to the (100) and (010) planes of 2H–MoS₂, and indicates the exposure of (001) facets. The atomic arrangement of Mo and S, marked by red and yellow circles in their FFT image (Fig. 7(n)), confirms the presence of both 1T and 2H phases of MoS₂. The HRTEM images of MoS₂-rGO (Fig. 7(r)) confirm that the MoS₂ sheets are supported on the rGO surface. Additionally, the presence of step edges and the stacking of four layers is evident, as marked by white lines in Fig. 7(s). The interplanar spacing of 0.27 nm at a 60° angle further confirms 2H–MoS₂ formation. Similar to MoS₂–P, FFT-corrected images of MoS₂-rGO from different regions (Fig. 7(t)) indicate a hexagonal atomic arrangement (octahedral coordination) of Mo and S, confirming 2H–MoS₂ formation. Additionally, the presence of 1T-MoS₂ is verified by the trigonal prismatic coordination of Mo in Fig. 7(u). Atomic dislocations from their regular positions, highlighted by yellow circles in Fig. 7(t), suggested that the strained crystal structures were caused by S vacancies (defects) in the basal plane. TEM images further confirm edge defects, layer discontinuities, and a higher defect density in rGO-containing samples. Additionally, S-deficient sites and the 1T phase are more prevalent in MoS₂-rGO, further highlighting the impact of rGO incorporation on structural characteristics.

Recently, Gonzalez et al.¹⁹⁷ performed in situ TEM experiments to better understand the changes in characterization of Au@MoS₂ nanostructures under various thermal treatments within the reducing environment, using a closed-cell sample holder to ensure controlled conditions. These experiments allowed for real-time observation of the Au@MoS₂ nanostructures at the nanoscale under practical usage scenarios, thereby complementing ex situ TEM measurements that yield statistical data. Initially, the Au@MoS₂ sample was examined under an argon atmosphere at 200 °C to establish its baseline characteristics, which revealed a shell approximately 1.5 nm thick surrounding the Au core. The temperature was then increased to 400 °C in a H₂ atmosphere. After one hour, a reduction in shell thickness to about 0.8 nm was observed, indicating a decrease from 2–3 layers to roughly one layer. This finding illustrates the reduction in layer count when the sample is exposed to a reducing gas at elevated temperatures. Furthermore, the temperature was raised to 600 °C and maintained for an additional hour in a hydrogen environment, at which point the MoS₂ layers enveloping the Au core became undetectable. The sintering behavior of the Au nanoparticles was also tracked at 600 °C, revealing that the metallic phase becomes increasingly unstable once the MoS₂ layers are removed, resulting in the merging of smaller particles into larger ones. In contrast to ex situ experiments, the sintering of Au nanoparticles occurred at lower temperatures during in situ experiments, which can be attributed to the longer exposure time required for TEM observations (20 minutes for ex situ and 60 minutes for in situ). As a result, in situ experiments demonstrated that temperatures below 800 °C, when sustained for extended periods, are adequate to completely delaminate the initial core–shell structure, indicating a thermodynamically favorable process for the structural and catalytic performance of Au@MoS₂ nanostructures.

5.4. X-ray photoelectron spectroscopy (XPS)

XPS is a vital technique for analyzing surface elemental composition, oxidation states, and the electronic properties of metals. A notable shift in binding energy strongly indicates interactions between TMDs and their supporting material.¹⁹⁸ Jayabal et al.²⁰ explored the phase identification of the prepared 1T-MoS₂/CNTs composite using XPS analysis. The XPS survey spectrum (Fig. 8(a)) confirms the existence of Mo, S, C, N, and O elements in the composite. In the Mo 3d region (Fig. 8(b)), the two characteristic peaks at 228.9 eV and 232.1 eV correspond to Mo 3d_5/2 and Mo 3d_3/2, respectively, confirming the presence of the 1T phase of MoS₂. Additionally, the two smaller peaks at 229.9 eV and 233.2 eV, shifted approximately 1 eV higher, indicate a minor presence of the semiconducting 2H phase of MoS₂. The S 2p region (Fig. 8(c)) exhibits two doublets at 161.8 eV and 163.1 eV, corresponding to S 2p_3/2 and 2p_1/2, further confirming the dominant 1T phase. The two additional peaks at 162.8 eV and 164.1 eV indicate the presence of a small fraction of the 2H phase. The Mo 3p region (Fig. 8(d)) features a strong Mo 3p peak at 395.1 eV, alongside two weaker peaks at 398.7 eV and 402.1 eV, corresponding to pyridinic N and NH₄⁺, respectively. The presence of pyridinic N suggests nitrogen doping at defect sites on the CNT surface. Additionally, small amounts of nitrogen and oxygen-containing groups on CNTs may enhance HER performance. Notably, the Mo 3d and S 2p peaks in the 1T-MoS₂/CNTs composite show energy shifts of 0.5 eV and 0.3 eV related to pure 1T-MoS₂. This shows electron transfer takes place partially from CNTs to 1T-MoS₂ thus increasing electron density around MoS₂. The resulting electron-rich catalyst surface enhances electron donation, facilitating hydrogen adsorption and ultimately improving electrocatalytic activity for the HER.

Fig. 8 (a) XPS survey spectrum of the 1T-MoS₂/CNTs composite and (b–d) high-resolution XPS spectra of the Mo 3d, S 2p, and Mo 3p regions of the 1T-MoS₂/CNTs composite. (e and f) XPS data of the molybdenum 3d core level of MoS₂–P and MoS₂-rGO, and (g and h) S 2p core level of MoS₂–P and MoS₂-rGO. (a–d) Reproduced with permission from ref. 20. Copyright 2019 The Royal Society of Chemistry. (e–h) Reproduced with permission from ref. 182. Copyright 2018 The American Chemical Society.

Mondal et al.¹⁸² presented the high-resolution XPS spectra of the Mo 3d and S 2s regions for MoS₂–P and MoS₂-rGO, which are shown in Fig. 8(e and f). In both samples, the Mo 3d region exhibits two intense peaks at 229.3 eV and 232.6 eV, corresponding to Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2, respectively. Additionally, the low-intensity peak at 227.2 eV is attributed to the S 2s peak of MoS₂. The deconvoluted spectra show that the prominent Mo 3d peaks of the 1T phase MoS₂ appear as doublets, while a lower-intensity peak, associated with the 2H phase MoS₂, appears at a higher binding energy (230.2 eV and 233.5 eV), with an energy separation of approximately 0.9 eV. The high-resolution S 2p spectra (Fig. 8g and h) display characteristic peaks at 161.6 eV and 162.8 eV, corresponding to S²⁻ 2p_3/2 and S²⁻ 2p_1/2, with an energy separation of approximately 1.2 eV. A downward shift in the S 2p binding energies related to bulk 2H–MoS₂ further confirms the dominant existence of the 1T phase.

6. Structure–performance correlation of TMD-based materials for water splitting

The structure of TMDs plays a crucial role in enhancing the water splitting performance, as it directly impacts the efficiency of hydrogen and oxygen generation reactions. By carefully optimizing the active sites, electronic configuration, phase structure, defects, and doping, researchers strive to maximize electrocatalytic performance for HER and OER processes.^199,200 Phase engineering can significantly enhance the conductivity of MoS₂ by facilitating the change from the semiconducting 2H phase to the metallic 1T phase.^201,202 Additionally, intrinsic catalytic sites such as edges, vacancies, and grain boundaries have been identified as key contributors to catalytic activity.²⁰³ Compared to crystalline structures, amorphous nanostructures and substrate-supported TMDs can expose more active sites, improving their distribution and optimizing electron flow and mass transfer.²⁰⁴ Phase and structure engineering, along with crystallization modulation and nanostructure fabrication have been applied to enhance the conductivity and activity of the TMD catalyst.¹⁴³ Doping and heterojunction formation have proven to be current strategies for generating a high density of catalytically active sites in TMD materials.²⁰⁵ Similar to graphene, TMDs possess a layered structure, with bulk crystalline forms composed of stacked layers, analogous to graphite. However, the basal plane of bulk 2D TMDs is typically catalytically inert, which significantly limits their overall electrocatalytic activity.⁹⁶ Recent studies have explored numerous tactics, including phase engineering, edge engineering, and defect engineering, which have been investigated to maximize catalytically active sites and activate the inert basal planes of TMDs for superior electrocatalytic performance.²⁰⁶ Among TMDs, MoS₂ is particularly well known for its electrochemical properties due to its graphene-like layered structure. Although the basal planes of semiconducting MoS₂ are largely inert, reducing the material to a few layers significantly increases the number of exposed edge sites, thereby enhancing its electrocatalytic activity.²⁰⁷

6.1. Edge engineering

The edge sites of TMD catalysts demonstrate excellent electrocatalytic activity; their overall performance is often hindered by the limited number of active sites on the largely inactive basal planes.²⁰⁸ To overcome this limitation, numerous approaches have been extensively explored to increase the number of active sites and activate the inert basal surfaces.²⁰⁹ Li et al.²¹⁰ demonstrate the optical images of both continuous monolayer MoS₂ films and isolated monolayer MoS₂ flakes, which were synthesized on sapphire substrates and subsequently transferred onto glassy carbon substrates (Fig. 9(a and b)). The obtained monolayer film exhibits minimal edge sites, whereas the flakes, approximately 1 μm in size, possess well-defined edges. Magnetic measurements reveal a significant ferromagnetic moment in the flakes but not in the film (Fig. 9(c)). The electrocatalytic activity of MoS₂ films is expected to be significantly lower than that of flakes, as only edge sites are catalytically active. However, contrary to this expectancy, the edgeless single-layer film displays superior electrocatalytic activity compared to the single-layer flakes. The polarization curves in Fig. 9(d and e) indicate that both the film and flakes exhibit comparable Tafel slopes of 70 mV dec⁻¹, yet the film achieves a much higher exchange current density (40 μA cm⁻²) compared to the flakes (3.5 μA cm⁻²). This suggests that the monolayer film enhances vertical charge transfer efficiency, outperforming conventional MoS₂ catalysts. Additionally, the monolayer film demonstrates outstanding stability, with minimal loss in catalytic activity even after 10 000 cycles. To further investigate the role of S vacancies, both the film and flakes were immersed in (3-mercaptopropyl) trimethoxysilane (MPS) and subsequently annealed at 300 °C. MPS molecules are believed to adsorb at S vacancies and facilitate S atom transfer through S–C bond dissociation at elevated temperatures (Fig. 9(f)). Post-treatment, the monolayer film photoluminescence (PL) intensity decreases notably and the S : Mo stoichiometric ratio increases, which confirms successful S vacancy repair (Fig. 9(g and h)). On the other hand, the flakes show no significant changes in PL intensity (Fig. 9(i)(inset)) and the S : Mo ratio, indicating that they inherently contain fewer S vacancies. Consequently, the electrocatalytic activity of the flakes remains largely unchanged after treatment (Fig. 9(i)). However, after S vacancy repair, the electrocatalytic activity of the monolayer film significantly declines, with its exchange current density decreasing from 30.1 to 1 μA cm⁻², and its Tafel slope increasing from 70 to 125 mV dec⁻¹. These results suggest that both S vacancies and edge sites contribute significantly to the enhancement in MoS₂ electrocatalytic activity; grain boundaries provide only a slight advantage. These results highlight that engineering S vacancies is a more effective strategy for enhancing catalytic performance than simply increasing edge site density. Consequently, in addition to phase engineering, edge site modification, and improved electron transport, S vacancy engineering has emerged as a promising route for optimizing MoS₂ in the HER.

Fig. 9 (a and b) Optical images of monolayer MoS₂ films and discrete monolayer MoS₂ flakes. (c) Magnetic measurement results of monolayer MoS₂ films (upper) and flakes (lower). (d) Polarization curves of monolayer MoS₂ films (blue) and flakes (red). Inset shows the stability test results of monolayer MoS₂ films. (e) Corresponding Tafel plots. The dashed lines serve to illustrate the Tafel slope and the exchange current density at 0 V overpotential. (f) Schematic illustration for the process of repairing the S vacancies in MoS₂. (g) XPS results of the film before (red) and after (blue) the repair of S vacancies. (h) Polarization curves of the MoS₂ film before (red) and after (blue) the repair of S vacancies. Inset shows the PL of the MoS₂ film before (red) and after (blue) the repair. (i) Polarization curves of the MoS₂ flakes before (red) and after (blue) the repair of S vacancies. (a–i) Reproduced with permission from ref. 210. Copyright 2016 The American Chemical Society.

Sang et al.²¹¹ developed edge configurations in the Mo_1−xW_xSe₂ material through an in situ edge engineering technique. The Mo_0.95W_0.05Se₂ monolayer flake on a microchip was subjected to rapid heating at a rate of 10 °C sec⁻¹ until it reached 500 °C, where it was maintained to facilitate adequate structural evolution and edge reconstruction. The combination of in situ heating and electron beam irradiation proved effective in generating new edges by etching pores in 2D crystals under vacuum conditions. Unlike the as-synthesized edges, which may suffer from damage, contamination, or distortion during sample preparation, the newly formed edges at the pores in a vacuum more accurately reflect the intrinsic edge structure within a specific local chemical environment. These features can be particularly beneficial for investigating the thermodynamics and kinetics associated with chemistry-dependent edge reconstruction. Li et al.²¹² successfully developed a highly dispersed heterojunction catalyst composed of NiS₂/MoS₂/CNTs by employing a facile etching process on nanoflower-like NiS₂/MoS₂ spheres supported on CNTs. This etching treatment effectively increases the lattice spacing of 2H–MoS₂, resulting in the formation of numerous Mo–S edge sites, generating a significant quantity of unsaturated S atoms. Moreover, it enhances the dispersity of NiS₂/MoS₂ nanosheets, thereby significantly modifying the electronic and geometric structures of the NiS₂/MoS₂/CNTs composite. The strong electronic interactions at the MoS₂/NiS₂ interfaces further contribute to the excellent performance of the optimized NiS₂/MoS₂/CNTs catalyst in alkaline water splitting, achieving overpotentials of 149 mV for the HER and 315 mV for the OER at a current density of 10 mA cm⁻², which are reductions of 29 mV and 35 mV, respectively, compared to those of the untreated catalyst (NiS₂/MoS₂/CNTs-p). Furthermore, the electrolyzer equipped with NiS₂/MoS₂/CNTs operates at a voltage of 1.73 V at 10 mA cm⁻² and demonstrates stable performance over a continuous period of 50 h for overall water splitting.

6.2. Defect engineering

The composition, distribution, structure, and properties of atomic-level defect sites play a critical role in fundamental catalysis research.^213,214 Defects, which often form during material synthesis or application, can act as active sites that promote catalytic reactions and the formation of desired catalytic materials.^215,216 Defect engineering is a widely used strategy to tailor the electronic structure, enhance charge transfer rates, and activate inert atoms, ultimately increasing the density of active sites and significantly improving catalytic efficiency.²¹⁷ Li et al.²¹⁸ synthesized MoS₂ nanosheets through a hydrothermal reaction in N,N-dimethylformamide using ammonium thiomolybdate as a dual Mo/S precursor (Fig. 10(a)). To controllably introduce S vacancies into the 2H–MoS₂ lattice, the nanosheets were annealed under H₂ at temperatures ranging from 400 to 800 °C (Fig. 10(a)). Notably, the S-vacancy concentration increased with annealing temperature up to 600 °C, beyond which the vacancy signal sharply declined. This reduction suggests a decrease in Mo–S dangling bonds, likely due to lattice reorganization at higher temperatures. This contradiction in S-vacancy formation with increasing temperature suggests a fundamental change in defect behavior. Point defects are created below 600 °C, resulting in Mo–S dangling bonds. Beyond 600 °C, the S atoms begin to strip, leading to larger defects. The results indicate that the HER mechanism on defective MoS₂ proceeds in two stages: the first involves ‘point’ defects associated with low concentrations of surface S vacancies, while the second arises from undercoordinated Mo regions resulting from extensive S atom stripping at high surface defect concentrations. The highest catalytic performance is achieved in the second stage, where undercoordinated Mo atoms are present, delivering a turnover frequency of approximately 2 s⁻¹ at an overpotential of 160 mV in 0.1 M KOH. Finally, this study suggests deeper insights into the HER mechanism on defect-engineered MoS₂ and provides valuable guidance for the design of high-efficiency TMD-based electrocatalysts.

Fig. 10 (a) Evolution of the different structures of MoS₂. (b and c) Transmission electron microscope images of defective MoS₂. Typical defects observed under high-resolution TEM show the creation of distortions and kinks in the slabs of MoS₂ due to disorder within the basal planes of the nanosheets. (d) High-resolution TEM image of an ultrathin layer of defective MoS₂. The hexagonal symmetry of 2H–MoS₂ can be identified. Examples of S vacancies are highlighted by red cycles. (e and f) Electrocatalysis measurements toward hydrogen evolution from defective MoS₂ nanosheets. Polarization curves of 2H–MoS₂ and defected 2H–MoS₂ after annealing from 400 °C up to 800 °C under H₂. (g) Evolution of the Tafel slopes with the annealing temperatures and measured at pH ≈ 0 and pH ≈ 13. (a–g) Reproduced with permission from ref. 218. Copyright 2019 The American Chemical Society.

Fruehwald et al.²¹⁹ elucidated that structural defects can result in electronic and strain-related phenomena within the lattice. They observed that the MoS₂ monolayer prepared through CVD and ion irradiation predominantly exhibits S-vacancies as the main type of structural imperfection, which has led to the identification of various distinct behaviors. In these, S-vacancy is primarily concerned with efficient catalytic active sites to promote its HER activity. Jiang et al.²²⁰ demonstrated that the creation of S vacancies can be facilitated by the presence of elements, such as Se and Zn. Followed by annealing to eliminate the Se atoms from Se-doped MoS₂, uniformly distributed and isolated S vacancies can be created within the MoS₂ structure. The MoS₂ obtained through the hydrothermal synthesis, followed by the deselenization of Se-doped MoS₂, exhibited a remarkable HER performance, requiring an overpotential of 100 mV to achieve a current density of 10 mA cm⁻², accompanied by a Tafel slope of 49 mV dec⁻¹.²²⁰

Zhao et al.²²¹ carried out in situ growth of spherical MoSe₂/MoS₂ heterojunction nanosheets on N-doped carbon nanotubes/carbon cloth (N-CNTs/CC) frameworks. This design combines the benefits of a porous, conductive N-CNTs/CC network for efficient electron transfer, increased active surface area, and improved ion adsorption. Subsequently, the phosphorization process introduces a significant number of electron-rich defects and vacancies. The MoSe₂/MoS₂ heterojunctions feature abundant interfaces that enable synergistic effects and electronic modulation, while PO₄³⁻ doping expands the interlayer spacing and introduces beneficial crystalline distortions and defects. Based on the designed rational structure, the hierarchical P-MoSe_xS_2−x/N-CNTs/CC composite displays a low overpotential of 108.3 mV at 10 mA cm⁻² and a small Tafel slope of 58.6 mV dec⁻¹, as well as exceptional durability towards the HER.

6.3. Phase engineering

Phase engineering is a widely adopted strategy to alter the atomic arrangement in TMDs, thereby increasing the number of active sites and tuning their electronic structure.²²² TMDs commonly exist in two distinct phases: the semiconducting 2H phase and the metallic 1T phase.²²³ The 2H phase possesses a tunable band gap, with catalytic activity primarily localized at the edge sites. In contrast, the 1T phase is metallic, lacks a band gap, and activates both edge and basal planes, leading to significantly improved electrical conductivity. TMD-based electrocatalysts in the 1T phase exhibit enhanced electron transport and stronger interactions between the electrolyte and active sites, making them particularly effective for water splitting applications.²²⁴ Kwon et al.²²⁵ prepared 2H and 1T′ phase MoSe₂ nanosheets (MoSe_x, where x = 1.8, 2.0, 2.2, 2.3, and 2.4) by the hydrothermal method. Due to Se enrichment, the phase transition takes place from 2H to 1T′. Specifically, this phase conversion takes place when the Se/Mo ratio exceeds 2. The crystal structure and morphology of nonstoichiometric MoSe_x nanosheets are illustrated in Fig. 11(a). 2H remains the predominant phase for x = 1.8 and 2.0, while the 1T′ phase becomes dominant for x = 2.2, 2.3, and 2.4. In the structural representation, Mo, Se, and additional Se atoms are depicted using cyan, orange, and purple spheres, respectively. Fig. 11(b) presents the LSV curves observed at pH 0 (vs. RHE). The overpotentials required to achieve a current density of 10 mA cm⁻² (η_J=10) were 217, 192, 160, 130, and 180 mV for x = 1.8, 2.0, 2.2, 2.3, and 2.4, respectively. In contrast, the commercial 20 wt% Pt/C catalyst required a significantly lower η_J=10 of 266 mV. Fig. 11(c) presents the Tafel slopes of MoSe_x, which were measured as 68, 59, 52, 46, and 57 mV dec⁻¹ for x = 1.8, 2.0, 2.2, 2.3, and 2.4, respectively. Among these, MoSe_2.3, with a Tafel slope of 46 mV dec⁻¹, is the closest to that of Pt/C (30 mV dec⁻¹), highlighting its superior catalytic performance. Fig. 11(d) further demonstrates that MoSe_2.3 exhibits the highest stability among all catalysts, showing only a 1 mA cm⁻² decrease at η_J=10 and η_J=20, highlighting its excellent HER performance. Fig. 11(e) compares the LSV curves before and after 12 h chronoamperometric (CA) studies at η_J=20, confirming stable HER performance of MoSe_2.3 while maintaining the nanosheet morphology, composition, and phase. Fig. 11(f) presents the variations in η_J=10 and Tafel slope values as a function of x, reinforcing that MoSe_2.3 exhibits excellent HER performance. The electrocatalytic activity improves with increasing x from 1.8 to 2.3 but declines at x = 2.4. From Fig. 11(g), the η_J=10 value of MoSe_2.3 closely matches that of earlier reported MoSe₂-based catalysts, further emphasizing its performance in electrocatalytic HER. Moreover, the density functional theory (DFT) calculations for Se-rich 1T′ phase MoSe_x, based on a (4 × 4) supercell, proposed four models: Model A (Mo₃₂Se₇₂, x = 2.25), Model B (Mo₃₀Se₆₆, x = 2.2), Model C (Mo₃₀Se₆₈, x = 2.27), and Model C′ (Mo₃₀Se₆₈, x = 2.27) as shown in Fig. 11(h). Both experimental and computational results suggest that two distinct yet equally stable structures (Models A and C′) exist in the Se-rich 1T′ phase MoSe₂. In Model A, excess Se atoms form bridging bonds that connect MoSe₂ layers. In contrast, Model C′ features Se atoms substituting for Mo atoms, with additional Se atoms positioned near existing Se atoms to form interstitial bonds or interlayer bridges. Furthermore, interstitial Se (Se^Se) atoms typically convert into extra Se atoms that bridge Se atoms from two layers (Se^B), leading to an energy reduction of 0.06 eV, which is comparable to thermal energy at room temperature. When Se^Mo, Se^Se, and Se^B atoms coexist, this configuration is defined as Model C′, with its structural representation revealed in Fig. 11(h(iv)) for Mo₃₀Se₆₈ (x = 2.27). Compared with experimental data, the lattice constant (c) in Models A, B, and C is shorter than that of Model C′. The most supportive intermediate (T_A) of Model A (x = 2.25) and Model C′ (x = 2.27) is illustrated in Fig. 11(i and j). Fig. 11(k) presents the ΔG_H* values for the most supportive HER sites in Model A and Model C′. The ΔG_H* calculations indicate outstanding HER activity experimentally attributed to a reaction pathway engaging Se adatoms in Model C′. As x increases, the concentration of Se adatoms rises, enhancing electrocatalytic performance up to x = 2.3. However, the decline at x = 2.4 is likely due to Se precipitation, which decreases catalytic activity.

Fig. 11 (a) Schematic diagram for the nonstoichiometric MoSe₂ nanosheet samples with x = [Se]/[Mo] = 1.8, 2.0, 2.2, 2.3, and 2.4. (b) HER performance of MoSe_x nanosheets with different [Se]/[Mo] ratios. (c) Tafel plots derived from the LSV curves. (d) CA responses of Pt/C at η_J=10, MoSe₂ at η_J=10, and MoSe_2.3 at η_J=10 and η_J=20 for 12 h. (e) LSV curves of MoSe_2.3 and Pt/C before and after 12 h of CA test. (f) η_J=10 (left y-axis) and Tafel slope (right y-axis) vs. x. (g) Comparison of η_J=10 (x-axis) and Tafel slope (y-axis) values of MoSe_2.3. (h) DFT calculation of Se-rich 1T′ phase MoSe_x derived from the (4 × 4) supercell. (i) Model A (Mo₃₂Se₇₂, x = 2.25), (ii) Model B (Mo₃₀Se₆₆, x = 2.2), (iii) Model C (Mo₃₀Se₆₈, x = 2.27), and (iv) Model C′ (Mo₃₀Se₆₈, x = 2.27). DFT calculation of HER pathways. The most favorable HER intermediate (TA) for (i) Model A (x = 2.25) and (j) Model C′ (x = 2.27) in the slab geometry. (k) Gibbs free energy diagram (ΔG_H*) of the HER at TA sites of the 1T′ phase (Model A and Model C′) and 2H phase MoSe₂ (Mo₃₂Se₆₄). (a–k) Reproduced with permission from ref. 225. Copyright 2020 The American Chemical Society.

Cheng et al.²²⁶ synthesized Co_xMo_1−xSe₂ (x = 0–1) by a one-step hydrothermal process, adjusting the molar ratios of Co(NO₃)₂·6H₂O and Na₂MoO₄ at a temperature of 200 °C. The incorporation of Co facilitates electron transfer from Co to Mo, successfully transforming MoSe₂ from the 2H phase to the 1T phase for values of x between 0.4 and 0.8. The resulting Co_xMo_1−xSe₂ demonstrates significantly increased catalytic activity for the HER compared to both the 2H phase of MoSe₂ and CoSe₂. Notably, Co_0.6Mo₀.₄Se₂ nanoparticles demonstrated outstanding electrocatalytic performance for the HER, achieving low overpotentials of 57 mV and 95 mV at 10 mA cm⁻² in 1 M and 0.1 M KOH electrolytes, respectively.

6.4. Interface engineering

Interface engineering in electrocatalysis involves modifying the interface between different materials in a catalyst to enhance its overall performance.²²⁷ This is achieved through several strategies, such as modulating the electronic structure to facilitate catalytic activity, promoting efficient electron transfer between components, improving mass transport of reactants and products, fine-tuning the binding energy of reaction intermediates to favor desirable reaction pathways, and optimizing the interface area and structure to expose more active sites or create better phase interactions.^227–229 These modifications collectively lead to enhanced catalytic efficiency, selectivity, and stability. Rehman et al.⁷⁸ synthesized the 1T-WS₂/1T-WSe₂ heterostructure via a low-temperature plasma-aided chemical vapor reaction, specifically through plasma-aided sulfurization and plasma-aided selenization processes. The 1T-phase WS₂/WSe₂ heterostructure demonstrates catalytic activity at both the edge sites and basal planes, which leads to improved HER activity compared to that of individual WS₂ and WSe₂. Interfacial electron transfer within the heterostructure promotes hydrogen atom reduction and increases the adsorption capacity of the catalyst surface. The fabricated 1T phase WS₂/WSe₂ heterostructure exhibits outstanding catalytic performance, characterized by a Tafel slope of 57 mV dec⁻¹ and an overpotential of 291 mV at 10 mA cm⁻². The highly interacting interfaces within the heterostructure significantly improve surface electron conductivity, contributing to its outstanding catalytic performance. For comparison, the LSV curves for the WS₂/WSe₂ heterostructure, bare WS₂ and WSe₂, the bare Au-treated silicon substrate, and Pt/C powders are presented in Fig. 12(a). The negligible impact of the Au substrate on catalytic performance confirms that the observed HER activity originated from the synthesized 2D films. Fig. 12(b) provides a comparative analysis of overpotentials at current densities of 2 mA cm⁻² and 10 mA cm⁻². The WS₂/WSe₂ catalyst demonstrates the best performance, achieving the lowest overpotentials of 291 mV at 10 mA cm⁻², respectively. In contrast, bare WS₂ and WSe₂ exhibit significantly higher overpotentials of around 392 mV and 364 mV at 10 mA cm⁻². This enhancement in catalytic performance is attributed to efficient electron–hole separation at the heterointerfaces, which substantially reduces the overpotential. Moreover, the 1T phase displays lower overpotentials than its 2H counterpart due to its metallic nature, which leads to a higher density of active sites. The superior electrocatalytic performance of the 1T-WS₂/1T-WSe₂ catalyst in the HER is likely due to Se atom diffusion into the top layer, replacing S atoms. To further investigate the rate-limiting steps and electrocatalyst behavior, a Tafel plot analysis was performed, as illustrated in Fig. 12(c). The Volmer, Heyrovsky, and Tafel reactions can each serve as the rate-limiting step, corresponding to Tafel slopes of around 120, 40, and 30 mV dec⁻¹, respectively. The calculated Tafel slope for WS₂/WSe₂ is 57 mV dec⁻¹, which is significantly lower than that of WS₂ (139 mV dec⁻¹) and WSe₂ (102 mV dec⁻¹), further demonstrating the improved catalytic efficiency of the heterostructure. These findings suggest that the HER process on the WS₂/WSe₂ heterostructure catalyst proceeds via the Volmer–Tafel and/or Volmer–Heyrovsky reaction pathways. To evaluate the catalyst's stability and durability, polarization curve measurements were conducted before and after 5000 cycles (Fig. 12(d)). The results confirmed minimal degradation in current density, confirming the stability of the WS₂/WSe₂ heterostructure. This exceptional stability is ascribed to its strong compositional and structural integrity. Furthermore, Fig. 12(e) reveals that the current density remains extremely stable even after 24 hours, further reinforcing the long-term reliability of the WS₂/WSe₂ heterostructure as a catalyst. EIS measurements were used to obtain Nyquist plots, indicating efficient charge transfer, fast mass transport, and a kinetically controlled reaction (Fig. 12(f)). The charge transfer resistance (R_ct) of the WS₂/WSe₂ heterostructure is lower than that of WS₂ and WSe₂, demonstrating its superior catalytic performance due to enhanced electron exchange. The heterointerface between WS₂ and WSe₂ modifies the electronic structure, facilitating charge transfer and further enhancing the catalytic activity of the WS₂/WSe₂ heterostructure.

Fig. 12 Electrocatalytic performance of different catalysts for the HER in 0.5 M H₂SO₄ solution: (a) polarization curves, (b) overpotentials at 2 and 10 mA cm⁻²vs. RHE, (c) Tafel plots, (d) polarization data before and after 5000 CV cycles, (e) time-dependent current density (i–t) curve, and (f) Nyquist plots obtained from EIS measurements (Inset: enlarged EIS profiles for WS₂/WSe₂). (a–f) Reproduced with permission from ref. 78. Copyright 2024 The American Chemical Society.

Afshan et al.²³⁰ designed a multilevel Bi₂Se₃@NiSe₂ nanostructure integrated with a carbon nanotube (CNT) framework that effectively diminishes energetic barriers and enhances the kinetics of the OER, while also facilitating more rapid faradaic reactions to improve charge storage capabilities. Furthermore, the electronic interactions established between the core selenides and the thin layers of hydroxide/oxide synergistically accelerate reaction kinetics, evidenced by a reduced overpotential of 199 mV at 20 mA cm⁻², a small Tafel slope of 59.2 mV dec⁻¹, and a high electrochemical surface area of 1460.0 cm² for the OER.

6.5. Doping engineering

Doping with either metal or non-metal involves modifying the electronic and structural properties of catalysts to improve catalytic activity, selectivity, and stability.^222,231,232 Panday et al.²³³ prepared an rGO/MoS₂/Pd heterostructure through a two-step solvothermal process, for effective bifunctional catalytic performance toward water splitting in an alkaline medium. Fig. 13(a) presents the LSV analysis in 1 M KOH, which confirms an exceptionally low OER overpotential of 245 mV for the rGO/MoS₂/Pd heterostructure at a current density of 10 mA cm⁻², compared with other catalysts. In contrast, MoS₂/Pd requires a higher overpotential of 330 mV to achieve the same current density. The superior electrocatalytic activity of rGO/MoS₂/Pd can be ascribed to the synergistic effects of its components. The incorporation of rGO decreases the interlayer stacking of MoS₂, thereby enhancing the density of catalytically exposed edge sites and improving charge transport across the MoS₂ basal plane. Additionally, Pd plays a crucial role in facilitating the phase transition of MoS₂ from the semiconducting 2H phase to the metallic 1T phase while also introducing S vacancies. These S vacancies in 1T-MoS₂ reduce the active energy barrier and thereby enhance OER electroactivity, contributing to the overall superior catalytic performance. The reaction kinetics of these catalysts were calculated using Tafel plots, as displayed in Fig. 13(b). The rGO/MoS₂/Pd catalyst exhibited the smallest Tafel slope (42 mV dec⁻¹) compared to MoS₂ (135 mV dec⁻¹), rGO/MoS₂ (98 mV dec⁻¹), and MoS₂/Pd (68 mV dec⁻¹), indicating improved reaction kinetics and efficient electron transfer for the OER. To further investigate charge transfer properties, EIS was conducted, and the corresponding Nyquist plots are plotted in Fig. 13(c). The rGO/MoS₂/Pd catalyst exhibited the lowest charge transfer resistance compared to MoS₂/Pd, rGO/MoS₂, and MoS₂, suggesting rapid electron transport across the interfacial layers of the ternary heterostructure. The Nyquist plot of MoS₂ demonstrated the highest resistance among the rGO/MoS₂, MoS₂/Pd, and rGO/MoS₂/Pd catalysts, likely due to the accumulation of O₂ bubbles on the electrode surface. To evaluate durability, the rGO/MoS₂/Pd catalyst was subjected to 5000 scanning cycles. As revealed in Fig. 13(d), the polarization curve remains nearly unchanged after 3000 and 5000 cycles, with minimal anodic current density loss, confirming excellent long-term stability. Additionally, the time-dependent current density curve observed at 1.5 V (inset of Fig. 13(d)) demonstrated a stable current density over 12 h, further facilitating the outstanding OER durability of the rGO/MoS₂/Pd catalyst in an alkaline solution. For the evaluation of the bifunctional electrocatalytic activity of the catalyst, the electrocatalytic HER performance of MoS₂, rGO/MoS₂, MoS₂/Pd, and rGO/MoS₂/Pd was investigated in 1 M KOH. The overpotential needed to achieve a cathodic current density of 10 mA cm⁻² was 85 mV for rGO/MoS₂/Pd (Fig. 13(e)), which is lower than that of MoS₂ (154 mV), rGO/MoS₂ (134 mV), and MoS₂/Pd (114 mV), highlighting the superior HER activity of rGO/MoS₂/Pd. Additionally, rGO/MoS₂/Pd exhibited a Tafel slope of 35.9 mV dec⁻¹ (Fig. 13(f)), which is lower than that of MoS₂ (137.2 mV dec⁻¹), rGO/MoS₂ (89.6 mV dec⁻¹), and MoS₂/Pd (45.4 mV dec⁻¹), demonstrating enhanced reaction kinetics and effective electron transfer for the HER. Furthermore, the stability of the catalysts was evaluated through CA analysis at an overpotential of 154 mV (Fig. 13(g)). The cathodic current density for rGO/MoS₂/Pd remained stable at a current density of 40 mA cm⁻² for up to 12 h, demonstrating its excellent stability for the HER. Fig. 13(g)(inset) presents an enlarged view of the time-dependent current density curves for rGO/MoS₂/Pd, where the serrated feature shows the accumulation and release of H₂ gas bubbles on the catalyst surface. Additionally, the electrocatalytic stability of rGO/MoS₂/Pd was further assessed through continuous cycling for 5000 sweeps (Fig. 13(h)), showing no significant loss in cathodic current density, thereby confirming its excellent durability for the HER in an alkaline medium. A key factor in this mechanism is the structural conversion of MoS₂ from the 2H phase to the 1T phase upon Pd incorporation. This phase transition is fundamentally important as it generates 1T-MoS₂ edge sites densely populated with unsaturated S atoms, as shown in the basal plane view of 1T-MoS₂ (Fig. 13(i)). These unsaturated S atoms form weak bonds with H⁺ ions in the electrolyte, facilitating their reduction by electrons and leading to hydrogen evolution (Fig. 13(j and k)).

Fig. 13 (a) Polarization curve, (b) Tafel plot, and (c) Nyquist plot of all catalysts; the inset shows the magnified view of RMoS₂Pd. (d) Polarization curve of RMoS₂Pd initially and after 3000 and 5000 cyclic voltammetry (CV) sweeps in 1 M KOH; the inset shows the time-dependent catalytic current density. (e) Polarization curve, (f) Tafel plot, and (g) time-dependent catalytic current density of all catalysts; the inset shows the magnified view of RMoS₂Pd. (h) Polarization curve of RMoS₂Pd initially and after 1000, 3000, and 5000 CV sweeps in 1 M KOH, (i) structural transformation of MoS₂ from 2H to 1T, (j) basal plane view of 1T MoS₂, (k) schematic of the proposed mechanism for the OER and HER on the RMoS₂Pd catalyst. (a–k) Reproduced with permission from ref. 233. Copyright 2019 The American Chemical Society.

Xu et al.²³⁴ successfully prepared ultrathin S-doped MoSe₂ nanosheets, which exhibited improved HER activity characterized by a low onset overpotential of 90 mV and a Tafel slope of 58 mV dec⁻¹. This enhanced HER catalytic performance is attributed to the increased number of unsaturated active sites in MoSe₂ due to the incorporation of S. During the HER process catalyzed by S-doped MoSe₂, protons (H⁺) first adsorb onto the electrode surface through an electrochemical discharge step. This is followed by the movement of the unstable adsorbed hydrogen atoms () to a more stable configuration, ultimately leading to hydrogen production via electrochemical desorption. Furthermore, the migration of the adsorbed is identified as the rate-limiting step in the overall HER process, which contributes to a Tafel slope of 60 mV dec⁻¹ for the S-doped MoSe₂. Shi et al.²³⁵ demonstrated that Zinc-doped MoS₂ (Zn–MoS₂) shows enhanced electrocatalytic HER performance with an onset potential of −0.13 V vs. RHE and a Tafel slope of 51 mV dec⁻¹. The improved performance of Zn–MoS₂ is due to a synergistic optimization of electronic and morphological factors. Specifically, improved energy level alignment and an increased number of active sites contribute to significant thermodynamic and kinetic enhancements in the HER.

Recently, TMD-based heterostructures have gained attention as effective electrocatalysts for water splitting. The literature summarized in Table 3 highlights various studies on TMD-based heterostructures aimed at this application. The overarching design principles for developing high-performance MoS₂-based electrocatalysts include increasing the density of catalytic sites and enhancing charge transport during the catalytic reaction. Several approaches have been devised to optimize the electrocatalytic performance of MoS₂ nanosheets. As indicated in Table 3, current approaches for enhancing the electrocatalytic activity of MoS₂ nanosheets primarily focus on two key aspects: (i) engineering active sites – achieved through edge exposure, defect creation, phase transformation, and doping strategies; and (ii) improving electrical conductivity and charge transfer – accomplished via interface engineering and integration with conductive substrates. The summarized strategies not only improve current TMD-based heterostructures but also inspire design ideologies for next-generation advanced materials.

Table 3 HER and/or OER electrocatalytic performances of various TMD-based heterostructures in different electrolytes

Materials	Preparation method	Engineering method	Electrolyte	Reaction	η 10 (mV at 10 mA cm^−2)	Mass loading (mg cm^−2)	Stability	Cell voltage	Ref
MoS2/graphene composite	Hydrothermal	Edge	0.5 M H2SO4	HER	142	0.701	2 h	—	236
CuCo(OH)2/CNT/MoS2	Co-precipitation method	Edge	0.5 M H2SO4	HER	211	—	25 h	—	237
			1 M KOH	OER	65	—	40 h
MoS2/G	Hydrothermal	Defect	0.5 M H2SO4	HER	107	0.22	8 h	—	238
MoSe2/CoSe2 microcage	Hydrothermal	Defect	0.5 M H2SO4	HER	110	0.285	1000 cycles	—	195
d-MoS2 nanosheets	Liquid phase exfoliation method	Defect	0.5 M H2SO4	HER	71.5	1.0	20 h	—	177
Oxygen-incorporating defect-rich MoS2 nanosheets	Liquid phase exfoliation	Defect	0.5 M H2SO4	HER	—	0.282	39 h	—	239
MoSe2/CoP	Intercalation	Defect and interface	0.5 M H2SO4	HER	105	0.4	25 h	—	174
MoS2-rGO composite	Hummers' method	Defect and phase	0.5 M H2SO4	HER	168	0.2547	20 h	—	182
Hierarchical P–MoSe0.5S1.5/N-CNTs/carbon cloth	CVD, hydrothermal, phosphidation	Interface and defect	0.5 M H2SO4	HER	108.3	15.5	12 h	—	221
1T/2H–MoS2/Co nanoflowers	Hydrothermal	Phase, defect and doping	0.5 M H2SO4	HER	83	—	3000 cycles	—	166
Mn-1T-MoS2-SV	Hydrothermal method	Interface, defect and doping	0.5 M H2SO4	HER	151	—	12 h	—	240
MoS2/GO composite	Hydrothermal Hummers' method	Phase	0.5 M H2SO4	HER	—	—	4 h	—	241
MoS2/MoO3 nanosheets	Hydrothermal	Phase	0.5 M H2SO4	HER	210	0.283	2000 cycles	—	242
CoxMo1−xSe2	Hydrothermal	Phase	1 M KOH	HER	57 mV at 20 mA cm^−2	1.0	40 h	—	226
			0.1 M KOH	HER	95 mV at 20 mA cm^−2
5%1T/2H Cu–MoSe2	Hydrothermal	Phase	0.5 M H2SO4	HER	113	—	80 h	—	243
Co–Ru–MoS2 hybrid	Solvothermal, hydrothermal	Phase and doping	0.1 M KOH	HER	52	0.39	—	—	244
				OER	308	—	16 h
MoS2/carbon fibre	Hydrothermal	Interface	0.5 M H2SO4	HER	∼125	0.28	16 h	—	245
MoS/carbon quantum dots	Hydrothermal	Interface	1 M KOH	HER	130	—	10 h	—	246
MoP/MoS2 nanosheets	Hydrothermal, phosphorization treatment	Interface	1 M phosphate buffer	HER	96	2.4	24 h	1.51 V at 10 mA cm^−2	79
P–NiCoC-MOF/MoS2	Hydrothermal, chemical vapor deposition	Interface	0.5 M H2SO4	HER	84	—	25 h	—	247
			1 M KOH	OER	184
MoS2/SiC nanowires-carbon fibre composites	Hydrothermal	Interface	0.5 M H2SO4	HER	198	0.283	24 h	—	80
RGO/MoS2/Pd	Hydrothermal	Interface	1 M KOH	HER	86	0.35	12 h	—	233
				OER	245
MoSe2-ts@MoS2-ts	Hydrothermal and microwave method	Interface	0.5 M H2SO4	HER	186	0.63	24 h	—	152
VS2@MoS2 nanocomposite	Hydrothermal method	Interface	0.5 M H2SO4	HER	177	0.285	20 h	—	248
MoS2-WS2 heterostructure	Hydrothermal	Interface	0.5 M H2SO4	HER	129	0.8	20 h	—	249
MoSe2/CoSe2 nanocubes	Hydrothermal	Interface	0.5 M H2SO4	HER	183	0.53	12 h	1.524 V and 1.846 V at 10 and 50 mA cm^−2	154
			1 M KOH	OER	309
MgFeO3/MXene/VS2 hybrid	Hydrothermal	Interface	1 M KOH	HER	35	—	24 h	1.47 V at 10 mA cm^−2	250
			1 M KOH	OER	214		24 h
MoS2@CoSe2-CC hybrid	Hydrothermal	Interface	1 M KOH	HER	101	3	48 h	—	251
MoS2/CNTs	Solvothermal method	Interface	0.5 M H2SO4	HER	—	0.136	2000 cycles	—	252
MoS2–CN/G	Solvothermal	Interface	0.5 M H2SO4	HER	140	0.140	4000 cycles	—	192
MoSe2/rGO	Solvothermal	Interface	1 M KOH	HER	136	0.5	70 h	1.64 V at 100 mA cm^−2	253
Bi2Se3@NiSe2-CNT/CTs	Solvothermal	Interface	1 M KOH	OER	199 mV at 20 mA cm^−2	∼0.40	72 h	—	230
3D MoS2/MoO2	Chemical vapor deposition process	Interface	0.5 M H2SO4	HER	—	∼0.30	1000 cycles	—	77
WSe2/CoSe2 heterostructure	Hot-injection colloidal	Interface	0.5 M H2SO4	HER	157	0.7	11 h	—	155
			1 M KOH	OER	330	0.7	2000 cycles
p-MoS2/Ni3S2/NF	Electrodeposition and solvothermal reaction	Interface	0.5 M H2SO4	HER	99	58.74	48 h	1.50 V, 1.62 V and 1.71 V at 10, 20, and 50 mA cm^−2	254
			1 M KOH	OER	185		35 h
CoP/MoS2-CNTs hybrid	In situ thermal decomposition growth method	Interface	0.5 M H2SO4	HER	200	—	1000 cycles	—	162
MoSe2–NiSe nanohybrids	Colloidal epitaxial growth	Interface	0.5 M H2SO4	HER	210	0.285	—	—	255
MoS2−x–NbSx heterostructure	Solid state reaction method and liquid exfoliation method	Interface	0.5 M H2SO4	HER	159	0.707	24 h	—	256
			0.5 M H2SO4	OER	295	—	12 h	—
NiS2/MoS2/CNTs	Etching method	Interface	1 M KOH	HER	149	0.59	8000 cycles	1.73 V at 10 mA cm^−2	212
				OER	315		8000 cycles
O–MoS2/rGO	Solvent assisted hydrothermal	Doping and interface	0.5 M H2SO4	HER	200	0.13	2000 cycles	—	257
In-situ-Pd doped MoS2	Hydrothermal method	Doping	0.5 M H2SO4	HER	89	0.581	15h	1.98 V at 10 mA cm^−2	258
			1 M KOH	OER	149		5000 cycles	2.03 V at 10 mA cm^−2
1% Pd–MoS2	Hydrothermal method	Doping	0.5 M H2SO4	HER	78	0.222	5000 cycles	—	259
Fe-doped MoS2/nickel foam	Solvothermal	Doping	0.5 M H2SO4	HER	173	—	1000 cycles	1.52 V at 10 mA cm^−2	260
P-doped MoS2	Hydrothermal	Doping	0.5 M H2SO4	HER	133 mV at 20 mA cm^−2	2.8	1h	—	261
V@CoSe2	Hydrothermal	Doping	1 M KOH	HER	212	0.025	40 h	1.96 V at 10 mA cm^−2	262
			1 M KOH	OER	310		16 h
MoS2/CoS2/carbon cloth	Hydrothermal	Doping	0.5 M H2SO4	HER	87	18.6	40 h	—	263
N-doped MoS2 nanosheets	Solvothermal	Doping	0.5 M H2SO4	HER	164	0.214	24 h	—	178
Zn–MoS2	Solvothermal	Doping	0.5 M H2SO4	HER	—	0.1414	—	—	235
MoS2/Co@N-GNWs	Solvothermal	Doping	0.5 M H2SO4	HER	165	0.285	10 h	—	264
3D-N doped MoS2 MNG	Solvothermal	Doping	0.5 M H2SO4	HER	157	0.285	1000 cycles	—	175
MoS2-CNFs	Electrospinning, graphitization	Doping	0.5 M H2SO4	HER	93	0.280	1000 cycles	—	265
S-doped MoSe2 nanosheets	Anion doping or incorporation method	Doping	0.5 M H2SO4	HER	156 mV at 30 mA cm^−2	0.28	1000 cycles	—	234
Ni-doped MoSe2 nanofilms on carbon fibre paper	Vapour-liquid-solid method	Doping	0.5 M H2SO4	HER	30	—	15 000 cycles	—	266
PANI-pf-GCC@Ni–MoS2	Electrochemical polymerization of aniline and photothermal pyrolysis	Doping	1M KOH	HER	175	—	60 h	—	267
Zn@MoS2/graphene	Hummers' and borohydride reduction method	—	0.5 M H2SO4	HER	378	—	—	—	181
CoSx@MoS2 microcubes	Liquid precipitation method	—	0.5 M H2SO4	HER	239	0.285	500 cycles	—	268
			1M KOH	OER	347
MoS2 hollow carbon nanospheres	Sol–gel method, hydrothermal	—	0.5 M H2SO4	HER	126	0.1385	2000 cycles	—	269
MoS2 nanosheets	Ball milling method	—	0.5 M H2SO4	HER	—	0.285	1000 cycles	—	270

---

7. Conclusion and outlook

This review primarily addressed recent advances in TMD-based heterostructure electrocatalysts for water-splitting applications, specifically in HER and OER processes. Typically, TMD-based heterostructures are synthesized through multiple methods ranging from solution-based (hydrothermal, solvothermal, liquid exfoliation) to solid-state (intercalation) and gas-phase (CVD) methods, allowing precise control over structure and properties as outlined. Key characterization techniques, including XRD, RAMAN, SEM, TEM, and XPS, play a critical role in understanding the structural behavior of TMD-based heterostructures. The excellent HER and OER performance of TMD-based materials is largely attributed to electronic interactions between components, which facilitate balanced chemisorption and dissociation of reaction intermediates (H* and OH*). Furthermore, we systematically summarized key optimization strategies, including edge engineering, defect engineering, phase engineering, interface engineering, and doping engineering, and demonstrated their significant effects on the electrocatalytic performance of TMD-based heterostructures. Existing challenges in fabricating TMD-based materials for electrocatalytic water splitting are outlined, and future outlook is presented.

A major challenge in electrocatalytic water splitting lies in achieving durable electrocatalyst stability under corrosive operational conditions. The acidic or alkaline solutions essential for efficient water splitting frequently induce catalyst dissolution and structural degradation, diminishing catalytic activity and operational longevity. The intrinsically slow reaction kinetics of the OER and HER require excessive overpotentials to sustain efficient catalytic activity. Such elevated overpotentials exacerbate energy inefficiencies, undermining the practical viability of hydrogen generation. Addressing this challenge necessitates designing catalysts with precisely tailored active sites and electronic configurations to enhance reaction kinetics. Despite substantial advances in TMD-based heterostructures, which exhibit HER and OER activities comparable to those of noble metal catalysts, critical challenges remain before they can supplant commercial systems. Addressing these limitations requires targeted improvements in the following key areas. Through sustained research and innovation, efforts are underway to develop more efficient and stable TMD-based electrocatalysts. These materials hold the potential to drive sustainable energy conversion, tackle environmental challenges, and advance energy technologies by enabling continuous breakthroughs. A primary focus is on enhancing the activity and stability of TMD-based electrocatalysts for long-term reactions. Tactics such as surface modification and doping have proven effective in improving stability and extending their operational lifespan. Efforts will also focus on engineering TMD-based catalysts capable of concurrent HER and OER activity to enable efficient bifunctional systems.

Another crucial direction is the development of scalable synthesis and cost-effective approaches for TMD-based electrocatalysts. Performance optimization remains challenging but can be achieved by tailoring their structure, composition, and morphology, and constructing heterostructures. Innovative approaches, such as doping engineering, phase engineering, interface engineering, surface modification, plasma treatment, and edge-site formation, have shown significant promise. These strategies enhance catalytic activity and stability by increasing the number of active sites, improving intrinsic conductivity, and generating synergistic effects. Future advancements in 2D-layered TMDs will likely focus on structural and interfacial engineering to achieve superior catalytic activity. However, scaling up production while maintaining performance for industrial applications remains a significant hurdle. TMD-based materials are cost-effective, environmentally friendly, and can be easily combined with conducting carbon-based materials to further improve their stability and performance. Furthermore, the integration of experimental characterization and first-principles calculations can effectively elucidate structure–performance relationships. Among experimental techniques, in situ methods stand out as particularly promising, offering valuable insights into the dynamic changes in the coordination and electronic states of TMD-based materials during electrocatalytic processes.

To optimize electrocatalysts and gain a deeper understanding of structure–activity relationships, a synergistic approach that combines innovative synthesis strategies with in situ and operando characterization techniques proves to be highly effective. Furthermore, coupling these efficient water-splitting systems with renewable solar power will be critical for scalable and sustainable green hydrogen generation. Despite these advantages, challenges persist, including refining preparation methods, investigating interface effects, and integrating TMDs with other functional materials. Despite considerable progress, the path to commercial-scale electrolyzer deployment requires overcoming persistent technical challenges. The primary research directions for the field are: (i) developing next-generation catalysts to boost the efficiency of electrocatalytic water splitting, (ii) designing advanced component materials in electrolytes and membranes to achieve superior chemical stability, performance, and durability, and (iii) optimizing intelligent system design through improved stack design and control strategies to minimize energy loss, enhance reliability, and reduce costs. Future research will aim to uncover the underlying mechanisms of heterostructure electrocatalysts and explore novel synthesis strategies, and in situ characterizations and practical applications can speed up the development of potential TMD-based materials tailored for electrocatalytic reactions.

Conflicts of interest

There are no conflicts to declare.

Data availability

This is a review article, and no new data were generated or analyzed in this study. Therefore, data sharing is not applicable.

Acknowledgements

This work was supported by the KPR Institute of Engineering and Technology, Coimbatore.

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