Engineered molybdenum disulfide-based nanomaterials for capacitive deionization applications

Abstract

Capacitive deionization (CDI) is a highly promising technique used for the removal of ions from water, showing great potential in the desalination of salty water and wastewater remediation. CDI has the advantages of energy efficiency, simple operation, excellent reversibility, long-term stability, and high feasibility for coupling with other techniques. Similar to supercapacitors, the electrode materials play a crucial role in determining the CDI performance, such as operating voltage, desalination capacity, and lifecycle of CDI cells. Molybdenum disulfide (MoS₂), a typical two-dimensional (2D) metal sulfide, has gained tremendous attention in the CDI technique due to its exceptional mechanical, electrical, and optical properties. Herein, we critically discuss the inventory and recent progress in the rational design of MoS₂-based electrodes for CDI cells. Initially, we present a brief introduction on the foundation knowledge of CDI systems and the structure of MoS₂. For a comprehensive review, we summarize the common techniques employed to prepare MoS₂-based nanomaterials, ranging from various exfoliation processes to chemical vapor deposition, colloidal synthesis, hydrothermal/solvothermal synthesis, and molten salt synthesis. Significantly, the recent progress in MoS₂-based electrodes for application in CDI is summarized in detail. These systems are divided into pristine MoS₂ and various MoS₂-based composites with other species, such as carbon, conducting polymers, metal oxides, MXenes, and C₃N₄. Finally, to aid in the further development of MoS₂ electrodes for efficient and long-term stable CDI, some challenges and possible solutions are outlined.

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Maiyong Zhu

Maiyong Zhu received his PhD degree from Yangzhou University (China) in 2011. In 2012, he started independent research work at school of Materials Science & Engineering, Jiangsu University (China), as an assistant professor. In 2015, he was promoted to associate professor. In 2020, he worked as a visiting professor at Kyoto University (Japan) under the support of the China Scholarship Council. Currently, the research of his group covers a wide range, including green strategies for synthesizing advanced functional materials for energy/environment related applications, valorization and recycling of solid wastes, and so on.

Xinyue Xiang

Xinyue obtained a bachelor's degree in Inorganic Non-metallic Materials Engineering from the School of Urban Construction, Anhui Jianzhu University in June 2024. Currently, she is pursuing a master's degree at the School of Materials Science and Engineering, Jiangsu University, under the guidance of Dr. Maiyong Zhu. Her research focuses on the application of two-dimensional layered materials in capacitive deionization.

1 Introduction

Owing to the rapid industrialization and growing population, the past decades have witnessed a dramatic decrease in groundwater levels and overconsumption of fresh water, resulting in the shortage of fresh water. It is estimated that more than 4 billion people worldwide are affected by severe freshwater shortages. Thus, considering sustainable development, there are at two efficient strategies to address the issues of freshwater shortage and water resource pollution. On the one hand, water reuse and wastewater recovery demonstrate great promise in saving water resources. On the other hand, seawater or brackish water, accounting for 97% of the Earth's water volume, can be converted into fresh water through appropriate technology. Thus, considering these goals, one of the most critical tasks is to decrease the ion contents in water.¹

Capacitive deionization (CDI) is a technique that removes cations and anions from water at the anode and cathode with the assistance of an external voltage, respectively. In the few past decades, as a significant process, CDI has been widely applied in wastewater remediation, resource extraction, water softening, and production of fresh water from seawater.^2,3 Compared to commercial deionization techniques, such as reverse osmose, adsorption, and precipitation, CDI delivers superior advantages in terms of energy efficiency, capability to remove ions with ultralow concentration, and operation simplicity. Similar to supercapacitors, electrodes play an important role in determining the performance of CDI devices. In recent years, transition metal dichalcogenides (TMDs), with the common formula of MX₂ (M represents a transition metal and X represents S, Se, or Te), have demonstrated great promise in CDI owing to their unique semiconductor features, such as narrow band gap and outstanding carrier mobility.^4–6 Based on their structure, there are three representative crystal phases for MoX₂, including 1T, 2H, and 3R phases. As presented in Fig. 1, MX₂ typically possesses a layer structure. The interlayer interaction has been evidenced to be van der Waals not covalent bonds, whereas the intralayer interaction is M–X bond, either covalent or ionic, as determined by the electronegative difference between M and X. Among these structures, the 1T phase is thermodynamically metastable, exhibiting metallic behavior. The 2H phase, which is comprised of two layers of X/Mo/X sheets in a Bernal stacking arrangement, is thermodynamically stable, possessing semiconductor characteristics. The 3R phase is the lowest energy metastable phase and is described by a rhombohedral unit cell. It is common to describe and visualize the unit cell with hexagonal axes and these phases can be transformed among each other under certain conditions. This diversity of structural features endows MX₂ with many fascinating properties, resulting in its wide application in many areas.^7–12

Fig. 1 Different phases of MX₂: 1T (tetragonal symmetry, one layer per repeating unit), 2H (hexagonal symmetry, two layers per repeating unit), and 3R (rhombohedral symmetry, three layers per repeating unit). Reproduced from ref. 13 with permission. Copyright 2024, published by the American Chemical Society under a CC-BY license.

Molybdenum disulfide (MoS₂), as a typical TMD, has gained increasing attention due to its ease preparation and low cost. During the past few years, a great deal of research on bare, layered MoS₂ CDI materials has been conducted due to their layered nature. However, considering the intrinsically poor electrical conductivity, low removal efficiency (capacity/rate), and poor selectivity toward target ions of MoS₂, scientists and engineers made many attempts to improve its CDI performance, including expanding its interlayer spacing and boosting its electrochemical activity. Some strategies, including introducing defects, intercalating guest species, doping with foreign atoms, forming heterostructures, and hybridization with other materials, have been used to improve the performance of MoS₂ CDI cells. Fig. 2 provides the timeline of MoS₂-based materials applied in CDI, and also outlines the key progress achieved in the past years in the development of MoS₂-based CDI devices.

Fig. 2 Timeline illustrating the key progress in MoS₂-based CDI devices achieved during the past few years. Reproduced from ref. 14 with permission. Copyright 2017, Elsevier. Reproduced from ref. 15 with permission. Copyright 2018, Elsevier. Reproduced from ref. 16 with permission. Copyright 2020, Elsevier. Reproduced from ref. 17 with permission. Copyright 2022, Elsevier. Reproduced from ref. 18 with permission. Copyright 2023, Elsevier. Reproduced from ref. 19 with permission. Copyright 2025, Elsevier.

To date, some excellent reviews have been reported, either highlighting the specific synthetic methods or application of MoS₂. For example, Aggarwal et al.²⁰ provided a comprehensive review focusing on few-layer exfoliated MoS₂ for application in various fields, including biosensing, gas sensing, catalysis, photodetectors, energy storage devices, light-emitting diodes (LEDs), and adsorption. Jia et al.²¹ summarized the recent advances in doping strategies to improve the electrocatalytic performance of MoS₂ toward the hydrogen evolution reaction. Dong et al.²² highlighted the synthetic strategies for MoS₂, focusing on their application in agricultural contaminant control. Yu et al.²³ provided a review on the applications of MoS₂ for modulating reactive oxygen species, highlighting its catalytic activity, photoluminescent behavior, and piezoelectric properties. Wang et al.²⁴ summarized the application of MoS₂ nanosheets in environmental remediation, such as adsorptive/photocatalytic elimination of contaminants, membrane isolation, sensing, and disinfection. Singh et al.²⁵ summarized the different strategies to generate large-area MoS₂ atomically thin layers for flexible electronic applications. Li et al.²⁶ summarized the methods for the preparation of low-dimensional MoS₂ nanostructures toward microwave applications.

However, despite these great achievements, a timely comprehensive review on MoS₂-based materials for CDI application is obviously lacking. Thus, based on this context, herein we provide the necessary review. Initially, the theory of CDI and structural characteristics of MoS₂ are briefly discussed, followed by a systematic overview on the common methods to prepare MoS₂-based nanomaterials. For convenience, these methods are classified according to their operation principles, including exfoliation, chemical vapor deposition, colloidal synthesis, hydrothermal/solvothermal, and salt-assisted synthesis. More importantly, the application of MoS₂-based CDI materials is summarized including pristine MoS₂ and various composites. Finally, we outline the challenges and bottlenecks in the future development of MoS₂-based CDI devices. It can be found that MoS₂ nanostructures have illuminated boundless roadmaps for further advancement as high-performance CDI materials.

2 Synthesis methods

Generally, there are two ways to prepare molybdenum disulfide nanosheets, i.e., bottom-up and top-down methods. The former method refers to various exfoliation processes using bulk MoS₂ as the precursor. To date, great achievement has been gained in exfoliating bulk MoS₂. The latter method refers to the various preparation processes in which the precursors of molybdenum and sulfur undergo reactions to generate MoS₂ nanomaterials. The common top-down methods include chemical vapor deposition, colloidal synthesis, hydrothermal/solvothermal synthesis, and salt-assisted synthesis. In this section, we systematically summarize these synthetic methods. It should be noted that many cases in this section may focus on thin MoS₂ (either monolayer or few-layer) nanosheets owing to their wide applications given that they possess a bandgap of 1.8 eV, which is much wider than that of bulk MoS₂.

2.1 Exfoliation

Similar to many other transition metal dichalcogenides, natural MoS₂ exists in bulk. As a result, it is necessary to exfoliate it into single or few-layered nanosheets for optimizing its application performances. The conversion of bulk MoS₂ to mono- and few-layer MoS₂ can be done via the process of exfoliation in organic solvents and aqueous media. In this section, we briefly describe the process for the exfoliation of MoS₂. To date, many exfoliation techniques have been developed to obtain thickness-controlled MoS₂ nanosheets. Among the current reported approaches, liquid-phase exfoliation under sonication irradiation has been demonstrated to be the most versatile given that it produces distinct advantages, such as ease of processing, simplicity, and versatility. Nguyen et al.²⁷ developed a general exfoliation process to prepare TMD nanosheets with a size of approximately a few hundred nanometers. As presented in Fig. 3a, the exfoliation is carried out in an ultrasonicator. This sonication process has been demonstrated to be feasible for preparing a variety of TMDs from the corresponding TMDs, such as MoS₂, WS₂, TaS₂, and TiS₂. Fig. 3b–e show the SEM images of the obtained TMDs, which are approximately 100–500 nm in size. AFM observation revealed that the thickness of these TMDs is about 3–4 nm. Owing to this small size, many edge sites can be obtained, endowing these TMD nanosheets with an excellent hydrogen evolution performance. Considering the long time required for the sonication process, Mohiuddin et al.²⁸ demonstrated that the introduction of an electric field can offer new opportunities for fast and efficient exfoliation processes. The efficiency improvement can be ascribed to the combination of shear force and an electric field, making the exfoliation process much more efficient. In the sonication exfoliation process, the properties of the utilized solvent have a significant impact on the resulting 2D material, such as boiling point, surface tension, and energy, as well as solubility parameters. Nguyen et al.²⁹ developed a two-solvent grinding-assisted sonication exfoliation process to prepare MoS₂ nanosheets with single- and few-layer thick flakes. They found that the quality of the resulting MoS₂ nanosheets and flake dimensions were dependent on the grinding solvent. Gupta et al.³⁰ reported that the addition of trace water to NMP is beneficial for stabilizing MoS₂ nanosheets in NMP dispersions. Their controlled investigation revealed that the MoS₂ nanosheets are fragmented and chemically unstable in the absence of water. Chavalekvirat et al.³¹ reported an exfoliation process using water and NMP to produce MoS₂ nanosheets. They found that the utilization of a mixed solvent resulted in a remarkable exfoliation yield. Under the optimal conditions, the yield of MoS₂ nanosheets reached up to ca. 1.26 mg mL⁻¹. Nevertheless, higher contents of water are not always a better case given that the system may generate MoO₃ when the content of water exceeds 66.7% v/v.

Fig. 3 (a) Schematic of the formation of TMD nanosheets via liquid exfoliation with ultrasonication. SEM images of the obtained (b) MoS₂, (c) WS₂, (d) TaS₂, and (e) TiS₂ nanosheets. Reproduced from ref. 27 with permission. Copyright 2016, the American Chemical Society. (f) Nafion-assisted exfoliation of MoS₂. (g) Digital images of N-MoS₂ dispersion with chemical structures of MoS₂ and Nafion, respectively. (h and i) N_x-MoS₂ (x = volume of Nafion in mL) dispersion in water before (h) and after (i) centrifugation. (j) Observation of the Tyndall effect from dispersed MoS₂ with varying concentrations of Nafion. Reproduced from ref. 39 with permission. Copyright 2018, the American Chemical Society.

It was noted that the MoS₂ nanosheets directly exfoliated in aqueous solution readily aggregated owing to the high surface tension of water. Thus, to solve this problem, surfactants or polymers are usually applied, which are inserted between the layers of MoS₂ nanosheets.³² For example, Bang et al.³³ obtained single-layer MoS₂ sheets in NMP with NaOH assistant. They found that the addition of NaOH improved the exfoliation efficiency and dispersibility of the MoS₂ nanosheets. Similarly, Li et al.³⁴ reported that the concentration of MoS₂ nanosheets can reach 1.44 mg mL⁻¹ with the highest yield of 4.8% by adding sodium citrate to NMP. This concentration and yield are much higher than that obtained by exfoliation carried out in pure NMP under the optimized conditions, i.e., 0.96 mg mL⁻¹ and 3.2%, respectively. Yu et al.³⁵ also improved the exfoliation efficiency and restacking of nanosheets by using sonopolymer surfactant in 1,2-dichlorobenzene as the solvent. The improvement can be ascribed to the functionalization effect of the used sonopolymer. Owing to the expanded spacing provided by the sonopolymer, the attraction between layers dramatically decreased compared with the bare MoS₂, enabling their facile redispersion, which was verified by UV–vis spectroscopy. Varrla et al.³⁶ demonstrate the shear-exfoliation of MoS₂ nanosheets in aqueous surfactant solution using a kitchen blender. Their result indicated that the mixing parameters, including the MoS₂ concentration, C_i; mixing time, t; liquid volume, V; and rotor speed, N, have a significant impact on the nanosheet concentration and production rates. Under the optimal conditions, a high nanosheet concentration and production rate were achieved, reaching up to 0.4 mg mL⁻¹ and 1.3 mg min⁻¹, respectively. With the assistance of the cationic surfactant cetyltrimethylammonium bromide (CTAB), Gupta et al.³⁷ prepared MoS₂ nanosheets via sonication exfoliation in aqueous solution, demonstrating excellent stability when dispersed in aqueous solution. The excellent stability can be ascribed to the interaction between MoS₂ nanosheets and the CTAB surfactant chains. The same group³⁸ also investigated the surface chemistry and the role of surfactant bonding in sonication-exfoliated MoS₂ nanosheets by controlling the sign of the charge on the MoS₂ nanosheets, either positive or negative. The solution NMR investigation indicated that the surfactant chains on the surface of MoS₂ undergo rapid exchange given that the bonding interaction between the surfactant and MoS₂ are weak, which was also evidenced by in situ nuclear Overhauser effect spectroscopy.

Considering that the covalent/noncovalent interaction between MoS₂ and surfactant deteriorates the properties of the starting materials and the rinsing with surfactant may worsen the exfoliated flakes, leading to restacking, Oh et al.³⁹ proposed the Nafion-assisted exfoliation of MoS₂ (Fig. 3f). The exfoliation driving force can be ascribed to the hydrophobic interaction between the Nafion molecules and MoS₂ layer, as presented in Fig. 3g. Owing to simultaneous presence of both hydrophilic and hydrophobic moieties, the Nafion-assisted exfoliated MoS₂ showed good dispersibility. As shown in Fig. 3h and i, the dispersibility of the exfoliated MoS₂ demonstrated little dependence on the variation in the amount of Nafion, evidencing the positive influence of Nafion surfactant on the dispersion efficiency. Furthermore, the improved dispersibility of all the Nafion-assisted exfoliated MoS₂ was also confirmed by the Tyndall effect, as presented in Fig. 3j. Karunakaran et al.⁴⁰ employed thiol-containing surfactant-assisted exfoliation to generate functionalized 2H-MoS₂. The resulting functionalized 2H-MoS₂ exhibited highly enhanced antibacterial efficiency. Melaku et al.⁴¹ developed a programmed exfoliation process to generate water-dispersible exfoliated MoS₂ nanosheets, possessing well-tunable structural characteristics using an environmentally friendly water-soluble cytosine-functionalized supramolecular polymer (Cy-PPG). In their work, dozen-layered MoS₂ nanosheets could be obtained by exfoliating MoS₂ under ultrasound irradiation.

In addition, several biomacromolecules (such as polysaccharides, pullulan, cyclodextrin, wool keratin, natural polyphenols, and amino acid) have also been reported to assist in the sonication exfoliation of bulk MoS₂ and dispersing exfoliated MoS₂ nanosheets in water.^42–47 For example, Sonker et al.⁴⁸ presented a one-step exfoliation process to obtain water-stable MoS₂ nanosheet suspensions using sulfated cellulose nanocrystals (CNC) as surfactants and exfoliating agents. A high extraction efficiency of 34 wt% was achieved, which is much higher than that using CNF as the exfoliating agent (21 wt%).⁴⁹ Yadav et al.⁵⁰ developed an ultrasonication process in a bio-based polymer-solvent to prepare quasi 2D-TMDs (including MoS₂, TiS₂, WS₂, NbS₂ and MoSe₂) with 1–5 layers. As shown in Fig. 4, the whole exfoliation includes three steps. Initially, bulk TMD and bio-based polymer are mixed homogeneously, which is achieved by grinding. Afterward, solvent mixing and intercalation will occur by adding the mixture of TMD/polymer to the desired solvent. Finally, ultrasonication is applied to induce the exfoliation of TMD. A series of solvent-polymer systems was screened in their work to optimize the yield. The results indicated that MoS₂/HPC/BuOH (I), MoS₂/CMC/BuOH (II), MoS₂/HPMC/BuOH (III), MoS₂/HPC/IBA (IV) and MoS₂/HPC/EG (V) delivered the highest yield of the corresponding 2D TMD nanosheets, respectively.

Fig. 4 Bio-based polymer-assisted ultrasonicated exfoliation of TMD in organic solvent. Reproduced from ref. 50 with permission. Copyright 2022, Elsevier.

Recently, ionic liquids (ILs) have attracted extensive attention owing to their superior advantages, such as negligible vapor pressure, versatile combability with many precursors, and wide redox potential.^51–57 ILs can serve as the reaction medium, electrolyte, catalyst, structure directing agent, etc. Tian et al.⁵⁸ successfully exfoliated bulk MoS₂ into MoS₂ ultrathin nanosheets via sonication in water with the assistance of IL crystal (ILC), which served as both the exfoliation and dispersing agent (Fig. 5a). The driving force for the exfoliation can be ascribed to the cation–π interaction between the imidazolium in ILC and MoS₂. Meanwhile, this interaction can prevent the re-aggregation of the exfoliated MoS₂ nanosheets. As presented in Fig. 5b, the resulting MoS₂ nanosheets showed poor dispersibility in water before adding ILCs. Once a small amount of ILCs was introduced, they could be homogeneously dispersed. Furthermore, high aggregation was observed in the case of the bulk MoS₂ powder before adding ILCs, which is ascribed to their strong hydrophobicity given that they possess a large contact angle (125°) (Fig. 5c). This value is much larger than that of the ILC₃₀₀–MoS₂ nanohybrid (7.6°). In addition, the ILC-assisted ultrasonic treatment also resulted in an increase in the concentration of ILC–MoS₂ dispersion collected by centrifugation, which was determined to be approximately 7.32 mg mL⁻¹ (Fig. 5d). The resulting ILC–MoS₂ dispersion could yield a macroscopic aerogel of MoS₂ after freeze-drying. The mass of the macroscopic aerogel reached about 0.43 mg. This result demonstrates the high concentration and dispersibility of the as-prepared ILC–MoS₂ dispersions (Fig. 5e). Fig. 5f and g display the SEM/TEM images of the bulk MoS₂ and exfoliated MoS₂ nanosheets, whose lateral size were determined to be in the range of several micrometers and 80–320 nm, respectively. Guo et al.⁵⁹ demonstrated that IL can serve as a gate to inject H⁺ into layered MoS₂ to manipulate its carrier concentration. Interestingly, the injected H⁺ was found to be located in the interlayer of MoS₂ but not the crystal lattice. Consequently, the injection of H⁺ had an influence on the multilayer MoS₂. The injection concentration was found to be as high as ∼1.08 × 10¹³ cm⁻². Wang et al.⁶⁰ also demonstrated the IL gating of suspended few-layer MoS₂ transistors. In their work, the accumulation of ions could occur on both exposed surfaces. Upon applying IL, a significant improvement in conductance was obtained for all the free-standing samples, whereas this was not the case for the substrate-supported devices. Impressively, a transition from metal to insulator could be achieved in the suspended MoS₂ devices by modulating the IL gate voltage. By measuring the charge density of states in IL-gated MoS₂, Lee et al.⁶¹ concluded the coexistence of monolayer MoS₂ nanosheets with multilayer MoS₂ in the channel area of IL-gated MoS₂ transistors.

Fig. 5 (a) Procedure for the exfoliation of bulk MoS₂ into nanosheets by ILC-assisted exfoliation. (b) ILC-dependent dispersibility of bulk MoS₂ in water. (c) Water contact angle of bulk MoS₂ and ILC–MoS₂. Photographs of ILC–MoS₂ (d) dispersion and generated (e) aerogel. (f) SEM image of bulk MoS₂. (g) TEM image of ILC–MoS₂. Reproduced from ref. 58 with permission. Copyright 2022, the American Chemical Society.

As analogues of ILs, deep eutectic solvents (DESs), which are comprised of hydrogen bond donors and hydrogen bond acceptors, have also attracted significant attention owing to their easy preparation, excellent electronic conductivity, and negligible vapor pressure. DESs are regarded as reaction media or modification agents to generate functional materials or extract functional species from biomass.^62–67 Owing to their adjustable composition and acidity, DESs show great capability to dissolve a wide range of species, showing great promising in pretreating biomass and extracting valuable species from biomass or sludge.^68–73 Inspired by their excellent dissolubility, DESs can also be utilized to synthesize inorganic materials. For example, Mohammadpour et al.⁷⁴ introduced a mechanical exfoliation approach to generate two-dimensional nanosheets from bulk MoS₂ sugar-based natural deep eutectic solvents, which played dual roles, i.e., the exfoliation medium and intercalating solvent. A high yield of 44% was achieved using this approach. Interestingly, the thickness, length, concentration of nanosheets, and phase ratio of 2H to 1T could be conveniently tuned by adjusting the experimental parameters. Under the optimized conditions, the average thickness and length of the exfoliated nanosheets were determined to be 4 and 150 nm, respectively, as evidenced by their UV–vis absorption spectrum. The resulting nanosheets were confirmed to be comprised of 2H and 1T phases with the ratio of 2H/1T = 1.4.

Considering the liquid nature of ILs, recently poly(ionic liquid)s (PILs) have attracted extensive attention as alternatives to ILs to modify materials, generating new desired functions. Biswas et al.⁷⁵ demonstrated a quick sonication process, which showed high efficiency for the preparation of single or few-layer MoS₂ nanosheet dispersions in the presence of cationic PILs.⁷⁶ Owing to the versatile solubility of PILs, this exfoliation can be carried out in both aqueous and organic media. Owing to the adsorption of the PIL onto the surface of the MoS₂ nanosheets, the resulting PIL-stabilized nanosheet dispersions demonstrated excellent stability in both water-soluble poly(vinyl alcohol) and nonaqueous-soluble poly(methyl methacrylate) matrices. Interestingly, the PIL made the resulting MoS₂ nanosheets responsive toward ions and temperature in aqueous medium. Wang et al.⁷⁷ reported that thermoresponsive PILs can be used as versatile surfactants to assist the exfoliation of various 2D materials, such as MoS₂, graphite, and hexagonal boron nitride, through consecutive sonication. They also demonstrated that the reliable interaction between 2D materials and the thermoresponsive PIL would facilitate exfoliation, achieving the noncovalent functionalization of the exfoliated nanosheets. Meanwhile, the resulting exfoliated nanosheet suspensions were stable, which could be reversibly tuned by temperature owing to the thermoresponsive phase transition behavior of the thermoresponsive PIL.

Gopalakrishnan et al.⁷⁸ reported a liquid exfoliation technique to deliver a highly dispersed suspension of MoS₂ quantum dots interspersed in few-layered MoS₂ nanosheets. As presented in Fig. 6, the process involved a combination of bath sonication and ultrasound probe sonication of MoS₂ flakes. The role of bath sonication is to induce the transformation of MoS₂ flakes into MoS₂ quantum dots. As shown in Fig. 6a, the size of the MoS₂ quantum dots was about 2 nm. The driving force for this conversion can be ascribed to the hydrodynamic forces caused by the increased pressure and temperature, which resulted in the breakdown of the bulk MoS₂ into quantum dots. Fig. 6b displays the TEM image of MoS₂ quantum dots interspersed in few-layered MoS₂ nanosheets. Benefiting from this unique structure, the resulting MoS₂ nanostructures possessed a larger concentration of active edges, leading to excellent activity for the hydrogen evolution reaction. As presented in Fig. 6c and d, the onset potential and Tafel slope of the resultant MoS₂ electrode were both determined to be very low, respectively. By simply controlling the duration of ultrasonication, Dai et al.⁷⁹ developed a sulfuric acid-assisted ultrasonication route to synthesize various MoS₂ structures including monolayer MoS₂ flakes, nanoporous MoS₂, and MoS₂ quantum dots. In another study, Xu et al.⁸⁰ reported a process of sonication followed by hydrothermal treatment to prepare MoS₂/WS₂ quantum dots (Fig. 6e). To improve the stability of the MoS₂ nanosheets obtained by sonication-assisted exfoliation, chemical modification demonstrates versatility in modifying/adding new properties to help realize this.⁸¹ Accordingly, Vera-Hidalgo et al.⁸² presented a sonication-assisted exfoliation process to modify 2H-MoS₂ with maleimide derivatives. In their work, the modification was achieved by a thiol–ene “click” reaction. Li et al.⁸³ reported a sonication–hydrothermal method to prepare a CD@2D MoS₂ heterostructure, regulating the energy level configuration and visible light absorption performance.

Fig. 6 Schematic of the procedure for the synthesis of MoS₂ quantum dots interspersed in MoS₂ nanosheets. TEM images of (a) MoS₂ quantum dots and (b) MoS₂ quantum dots interspersed in the exfoliated MoS₂ nanosheets. Reproduced from ref. 78 with permission. Copyright 2014, the American Chemical Society. (e) Sonication-assisted exfoliation process to synthesize MoS₂/WS₂ quantum dots. Reproduced from ref. 80 with permission. Copyright 2015, Wiley-VCH.

It has been noted that exfoliation under sonication conditions usually gives MoS₂ with few- and several-layer thick nanosheets, making it difficult to obtain single-layer MoS₂. Meanwhile, it is difficult to control the degree of exfoliation. In some cases, excessive exfoliation may lead to the formation of quantum dots. In addition, the yield of MoS₂ nanosheets through this technique is not high. Thus, to overcome these issues, introducing intercalation species may be an alternative. Knirsch et al.⁸⁴ demonstrated a functionalization route to bond organic groups to the surface of MoS₂. As presented in Fig. 7a, n-butyllithium (n-BuLi) is initially used to react with bulk MoS₂, achieving 2D nanomaterials. Owing to this intercalation, the exfoliated MoS₂ will be negatively charged. Subsequently, a mild bath-type sonication is applied to the negatively charged MoS₂, resulting in efficient exfoliation into individual sheets. Upon the addition of 4-methoxyphenyldiazonium tetrafluoroborate, the negative charges on MoS₂ can be quenched, thus yielding functionalized MoS₂ (f-MoS₂). The interaction between MoS₂ and 4-methoxyphenyldiazonium tetrafluoroborate was evidenced to be C–S covalent bonds. Interestingly, excessive sonication exposure may lead to the defunctionalization of f-MoS₂, generating df-MoS₂. Fig. 7b–d show the HRTEM images of CE-MoS₂, f-MoS₂, and df-MoS₂, respectively. The lattice is intact over wide regions, indicating that neither functionalization nor defunctionalization caused severe structural damage to MoS₂. The interlayer spacing, which could be adjusted by inserting guest species, had a significant impact on the properties of MoS₂. The redox potential of organolithium is significant for the intercalation and exfoliation of bulk MoS₂. Owing to the pyrophoric nature of n-BuLi, the intercalation and exfoliation process using n-LiBu is usually carried out under oxygen-free and water-free conditions to avoid a serious hazard for safe operation. Thus, to circumvent this problem, Loh et al.⁸⁵ developed an alkaline metal (Li, Na, and K) naphthalenide salt-assisted expansion and intercalation process to generate MoS₂. In this way, high-quality single-layer MoS₂ sheets could be obtained with an unprecedentedly large flake size (up to 400 μm²). Zhu et al.⁸⁶ compared several lithium reagents, including n-butyllithium (n-BuLi), naphthalenide lithium (Nap-Li) and pyrene lithium (Py-Li). Their results indicated that the redox potential of the lithium-containing compounds has significant influence. A too high potential (above 1.13 V vs. Li⁺/Li) may lead to the decomposition of bulk MoS₂. In contrast, a too low potential (below 0.55 V vs. Li⁺/Li) is not capable of achieving intercalation. Fortunately, the redox potential of Py-Li was determined to be 0.86 V (vs. Li⁺/Li), which is suitable for precise Li⁺ intercalation. Interestingly, the intercalation led to the formation of the LiMoS₂ lamellar compound without undesirable structural damage. Pondick et al.⁸⁷ observed a 2H-1T′ phase transition induced by lithium intercalation in MoS₂ nanosheets. Zhang et al.⁸⁸ developed a surfactant intercalation technique to yield single-layer TMDs. As shown in Fig. 7e, C₁₅H₃₄ClN molecules are intercalated into the interlayer of bulk TMDs, which is a thermodynamically controlled process. This is based on the expansion of the long-chain molecule and weak van der Waals interaction between the stacking layers. By intercalating C₁₅H₃₄ClN molecules, the interlayer spacing demonstrated a dramatic enlargement. Owing to the generated shear force as well as the extended free spacing, TMDs can be further exfoliated to generate stable TMD suspensions in NMP with mild ultrasound. Fan et al.⁸⁹ developed a fast sonication-assisted lithium intercalation route to yield exfoliated 2H MoS₂. As presented in Fig. 7f, an intermediate, stoichiometric LiMoS₂, is generated during the exfoliation process through the reaction of n-butyllithium (n-BuLi) for the complete intercalation of MoS₂. Furthermore, hexane as the solvent serves to accelerate the intercalation. Interestingly, the resulting LiMoS₂ could be further converted into 1T MoS₂ by further sonication. Meanwhile, a phase transition from 2H to 1T could be achieved during the intercalation process, which was reversed by IR laser irradiation using a DVD optical drive. The same group⁹⁰ later obtained MoS₂ nanosheets by exfoliating crystalline MoS₂, with the retention of the semiconducting 2H phase. The controlled reaction of MoS₂ with substoichiometric amounts of n-BuLi led to intercalation of the edges of the crystals. Upon further exfoliation in a 45 vol% ethanol–water solution, trilayer MoS₂ nanosheets were obtained, which demonstrated excellent suspension behavior.

Fig. 7 (a) Schematic of the intercalation exfoliation of MoS₂ using an organic lithium agent. HRTEM images of MoS₂ obtained by different processes: (b) chemical exfoliation, (c) functionalized MoS₂, and (d) defunctionalized MoS₂. Reproduced from ref. 84 with permission. Copyright 2015, the American Chemical Society. (e) Schematic of the intercalation and exfoliation of TMDs using dodecyl trimethyl ammonium chloride (C₁₅H₃₄ClN). Reproduced from ref. 88 with permission. Copyright 2022, the American Chemical Society. (f) Reaction scheme for the preparation of exfoliated 2H MoS₂. Reproduced from ref. 89 with permission. Copyright 2015, the American Chemical Society.

Liu et al.⁹¹ proposed an electrochemical exfoliation process to prepare monolayer and few-layer MoS₂ nanosheets. As depicted in Fig. 8a, the bulk MoS₂ was used as the cathode and Pt wire served as the anode in the system. The electrolyte was Na₂SO₄ solution. Under direct current assistance, the bulk MoS₂ crystals could be exfoliated into monolayer or single-layer MoS₂ nanosheets. Owing to its hydrophilicity, the resulting MoS₂ nanosheet showed poor wettability toward water and polar organic solvents. As a result, the MoS₂ nanosheet could not be well dispersed in Na₂SO₄ solution and NMP solvent, as shown in Fig. 8c and d, respectively. Fig. 8e illustrates the exfoliation mechanism, which can be divided into two steps. Initially, ˙OH and ˙O radicals are generated by the oxidation of water upon the application of a positive bias to the working electrode. The generated radicals as well as SO₄²⁻ anions are easily assembled around the bulk MoS₂ crystal by inserting themselves between the MoS₂ layers. Consequently, the van der Waals interactions between the layers will be weakened. Afterward, the reactivity of the radicals and/or anions results in the release of O₂ and/or SO₂, expanding the interlayer spacing of the MoS₂ sheets. Finally, with the assistance of the erupting gas, the MoS₂ flakes are detached from the bulk MoS₂ crystal, which are then suspended in the solution. It should be noted that during the electrochemical exfoliation process, oxidation reactions should occur on the surface of the bulk MoS₂ crystal, which is crucial in determining the quality and degree of oxidation of the exfoliated MoS₂ nanosheets. Fig. 8f–i display the TEM/HRTEM images of the resulting thin MoS₂ nanosheet, which possessed a lateral size of 10 μm and a folded edge of a monolayer MoS₂ nanosheet.

Fig. 8 (a) Schematic of the exfoliation of bulk MoS₂ crystal via electrochemical process. (b) Photograph of a bulk MoS₂ crystal. Exfoliated MoS₂ flakes suspended in Na₂SO₄ solution (c) and dispersed in NMP solution (d). (e) Schematic for mechanism of electrochemical exfoliation of bulk MoS₂ crystal. (f–i) TEM and HRTEM images of the resulting MoS₂ nanosheets. Reproduced from ref. 91 with permission. Copyright 2014, the American Chemical Society.

To improve the stability and oxidation resistance of MoS₂, García-Dalí et al.⁹² proposed a biomolecule-assisted electrolytic exfoliation method to prepare MoS₂ in aqueous solution. Benefiting from the introduction of biomolecules, the resulting MoS₂ nanosheets exhibited colloidal dispersion with water, oxide-free, and phase-preservation. The mechanism study indicated that the exfoliation is driven by the penetration of hydrated cations from the electrolyte. Considering the inert characteristics of 2H-MoS₂, making its further functionalization difficult, the same group⁹³ later reported an electrolytic approach to achieve 2H-MoS₂ nanosheets functionalized with organic iodide-derived molecular moieties. This electrolytic exfoliation/functionalization triggered the expansion of the MoS₂ crystal in an accordion-like fashion. In another work, Zhuo et al.⁹⁴ demonstrated an electrochemical exfoliation process, in which the chemical functionalization of MoS₂ layers was simultaneously realized. In detail, aryl diazonium salts were applied in their system, which played two roles. On the one hand, it can be covalently grafted onto 2H-MoS₂, endowing 2H-MoS₂ with more functions. On the other hand, its intercalation effect aids the exfoliation process, enlarging the interlayer spacing and preventing restacking.

To date, three main exfoliation mechanisms have been reported, i.e., exfoliation induced by ultrasound or shear forces, triggered by lithium intercalation making use of organolithium compounds (e.g., n-butyllithium), and electrochemical exfoliation. Ultrasound- and shear force-induced exfoliation is preferred to provide few- and several-layer-thick nanosheets of high structural quality. However, a limitation of this technique is its low exfoliation yield. By contrast, lithium intercalation approaches, which are usually conducted in organic solvents, are capable of affording single-layer nanosheets in high yields (>50 wt%). Nevertheless, their stringent conditions (i.e., in the absence of oxygen and humidity) constitute their main drawback. Furthermore, lithium intercalation often leads to a phase transition (from 2H to 1T), generating numerous defects and imperfections in the final product. In the case of electrochemical exfoliation, the redox potential has a significant impact on the exfoliation and functionalization process. Caution should be taken during the electrochemical exfoliation process given that the anodes (such as platinum) can undergo unintentional doping, which is not desirable in most cases. In addition, owing to the lack of a stable reference electrode, the applied potential is not a reliable value. It should be noted that there are some other exfoliation techniques besides the above-discussed three techniques, such as femtosecond laser exfoliation,⁹⁵ microwave-assisted exfoliation,⁹⁶ plasma-induced exfoliation,⁹⁷ light-induced exfoliation,⁹⁸ and redox-based exfoliation.^99,100

2.2 Chemical vapor deposition

Chemical vapor deposition (CVD) has been demonstrated to be a successful approach for synthesizing various 2D materials for many important technological applications, such as microelectronics, superconductors, hard coatings, and smart windows.^101,102 This method has several advantages, for example low-cost and powerful method and capability to tune the nucleation density, enabling the preparation both monolayer 2D materials and interesting structures such as superlattices and super-twisted spirals. In the study by Bai et al.,¹⁰³ a flow of nonreactive gas (such as N₂) was introduced to carry the sulfur vapor, allowing it to react with the molybdenum precursor located downstream of the furnace (Fig. 9a–d). Utilizing a similar procedure, Wang et al.¹⁰⁴ reported the growth of monolayer MoS₂ on SiO₂, where the growth substrate SiO₂ was placed upside down on a quartz boat directly above the Na₂MoO₄·2H₂O solution precursor. Tarasov et al.¹⁰⁵ synthesized MoS₂ with a large area through the direct sulfurization of Mo thin films, which were generated by evaporation and deposited on SiO₂.

Fig. 9 (a) Schematic of the solid–gas process to prepare MoS₂ deposited on DWCNT. (b) SEM and (c) high-magnification SEM images of MoS₂ flakes, and (d) enlarged image of the marked area. Reproduced from ref. 103 with permission. Copyright 2022, published by Wiley-VCH under a CC-BY license. (e–g) Schematic of the fabrication process of MoS₂ nanoribbons from MoO₃ nanoribbon precursors: (e) formation of MoO₃ precursor, (f) reaction to generate hybrid MoS₂/MoO₂ nanoribbons and (g) conversion to MoS₂ nanoribbons. (h–j) SEM images of the obtained MoO₃, MoS₂/MoO₂ hybrid and MoS₂ nanoribbons. Inset in (f) MoS₂ nanoribbons synthesized in one batch. Reproduced from ref. 109 with permission. Copyright 2020, Wiley-VCH.

Recently, Mouloua et al.¹⁰⁶ reported a chemical vapor deposition process to deposit MoS₂ nanostructures on a quartz substrate, exhibiting exceptional crystalline quality and a triangular-like morphology. By placing S and Se powder upstream and MoO₃ downstream, Li et al.¹⁰⁷ prepared alloy MoS_2xSe_2(1−x) triangular nanosheets with complete composition tunability. The resulting MoS_2xSe_2(1−x) triangular nanosheets exhibited tunable optical properties, which is in good agreement with the composition of the alloy nanosheets. By controlling the growth parameters, the same group also realized the continuous lateral growth of the MoS_2(1−x)Se_2x alloy with a graded bilayer composition. Specially, the value of x gradually increased from x = 0 (pure MoS₂) to x = 0.68 from the center to the edge of the nanosheet.¹⁰⁸ In another work, Huang et al.¹⁰⁹ developed a chemical vapor deposition process to synthesize MoS₂. As illustrated in Fig. 9e–g, their synthesis involves three steps. The first step is the formation of the MoO₃ beam precursor, which is achieved through a conventional hydrothermal procedure, briefly heating a solution containing (NH₄)₂MoO₄·4H₂O and nitric acid at 180 °C for 20 h. Owing to its easy operation, this step generates MoO₃ nanoribbons in large quantities.¹¹⁰ The second step refers to the formation of the intermediate of MoS₂/MoO₂. This procedure is carried out in a two-zone furnace with the sulfur powder placed upstream to generate sulfur vapor. In this step, the temperature is crucial for the formation of the MoS₂/MoO₂ hybrid. A high temperature may change the morphology of the nanoribbons. For example, the temperature of 650 °C may cause the collapse of the MoO₃ nanoribbons given that the melting temperature of bulk MoO₃ powder is about 800 °C.¹¹¹ Alternatively, a low temperature cannot generate sulfur vapor efficiently. Considering these factors, in their system the temperature of 500 °C was found to be suitable. The third step is the formation of the final MoS₂ nanoribbons, which can be considered further sulfurization. Thus, it is not necessary to isolate the intermediate MoS₂/MoO₃ hybrid. To achieve complete sulfurization within a short duration, this step can be carried out at a high temperature (900 °C) given that the melting temperature of MoS₂ and MoO₂ is as high as 2375 °C and 1100 °C, respectively.

To reveal the formation mechanism of MoS₂ in solid–gas synthesis, Dong et al.¹¹² performed a systematic theoretical study. Their results indicated that the edge reconstruction has a significant impact on the growth kinetics of MoS₂. The reconstruction is dramatically determined by the concentrations of Mo and S in the growth environment (Fig. 10a). The substrate applied to accommodate MoS₂ in the solid–gas synthesis system also has a significant impact on the morphology of the resultant MoS₂. For example, Chowdhury et al.¹¹³ demonstrated a gas-phase synthesis of dimensional TMD crystals without lithography. In their work, two substrates were employed, SiO₂ and Si–P_x (PH₃-treated Si substrates). When SiO₂ was used as the substrate, triangular 2D MoS₂ crystals were generated (Fig. 10b and c), which is the most common phenomenon. This was not the case for the Si–P_x substrate. The grown MoS₂ on Si–P_x produced crystalline nanoribbons, as presented in Fig. 10d and e. This different directional growth can be explained by the preferential growth from one of the edges of a nanoscale 2D crystal seed, leading to the formation of 1D MoS₂ nanoribbons. Yang et al.¹¹⁴ used high-Miller-index Au facets as templates to synthesize monolayer TMD ribbons (e.g., MoS₂, WS₂, MoSe₂, WSe₂, and MoS_xSe_2−x). To unlock the morphological evolution from triangular domains to patterned ribbons, they further investigated the growth kinetics of specific edges. Furthermore, the growth process conformed well to the one-dimensional edge epitaxial growth mode, leading to the formation uniformly aligned TMD ribbons merging into single-crystal films.

Fig. 10 (a) Schematic showing the strategy of exploring the growth mechanism of MoS₂. Reproduced from ref. 112 with permission. Copyright 2023, the American Chemical Society. Scheme depicting the low-pressure synthesis of 2D MoS₂ crystals on SiO₂ substrate (b) and Si–P_x substrate (d). Typical 2D MoS₂ crystal omni-directional lateral growth mode on SiO₂ substrate (c) and Si–P_x substrate (e). Reproduced from ref. 113 with permission. Copyright 2019, the authors published by Springer Nature.

Kwon et al.¹¹⁵ developed an improved chemical vapor deposition process to prepare MeS₂ (Me = Mo or W) layers with a polycrystalline structure. The diagram of the growth setup is presented in Fig. 11a and b, where the system integrates CVD and spin-coating and contains three heating zones. The chemical principle of forming MeS₂ is based on the thermolysis of (NH₄)₂MeS₄ (Me = Mo or W) under an N₂ atmosphere. Fig. 11c–e display the TEM images and corresponding EDS and EDX information of the obtained MoS₂ layers. These results indicate that the resultant MoS₂ layers produce mainly single-crystalline characteristics.¹¹⁶ Nevertheless, the HRTEM images and ring motifs in the SAED patterns indicated the presence of polycrystalline areas, where the atomic arrangement is nonperiodic. The mechanism investigation revealed that the conversion of (NH₄)₂MeS₄ into MeS₂ is a two-step process, which is dependent on the applied temperature. When the temperature is 500 °C, the (NH₄)₂MeS₄ precursor is converted into MoS_x thin layers, which will be further converted into MeS₂ thin films at a temperature of 900 °C. It was noted that both steps occur in the presence of H₂, the role of which can be regarded as a reducing agent, as described by the following equations:

(NH₄)₂MeS₄ + H₂ → 2NH₃ + H₂S + MeS₃ below 500 °C (1)

MeS₃ + H₂ → MeS₂ + H₂S above 900 °C (2)

Fig. 11 (a) Fabrication of (NH₄)₂MeS₄ precursors. (b) Thermal CVD process to generate MoS₂ and WS₂. Microscopic analysis of synthesized MoS₂ and WS₂ layers. (c–e) Characterization of MoS₂. Reproduced from ref. 115 with permission. Copyright 2015, the American Chemical Society. (f) Diagram of MOCVD setup for growth of MoS₂. (g) Growth time-dependent optical images of MoS₂. Reproduced from ref. 118 with permission. Copyright 2015, Springer Nature.

Wree et al.¹¹⁷ prepared hexagonal MoS₂ layers through a metalorganic CVD (MOCVD) process, in which elemental sulfur was utilized as the co-reactant and the Mo source was a molybdenum-organic compound, 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(VI) [Mo(N^tBu)₂(^tBu₂DAD)]. Owing to the combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center, a monomeric could be obtained, possessing excellent thermal characteristics relevant for vapor phase deposition applications. The structure and composition characterization revealed that the resulting MoS₂ films were crystalline and stoichiometric. Benefiting from the moderate process conditions, scalability and controlled targeted composition, this MOCVD process is highly promising. In another work, Kang et al.¹¹⁸ reported an alternative MOCVD process to generate high-mobility 4-inch wafer-scale films of monolayer molybdenum disulphide (MoS₂). Fig. 11f displays a diagram of the MOCVD growth setup. In this system, all the precursors are in gaseous form, including (C₂H₅)₂S, W(CO)₆, Mo(CO)₆, and H₂, which are diluted in argon as a carrier gas. The concentration of each reactant can be precisely controlled during the entire growth time by regulating the partial pressure (P_X) of each reactant, X. Fig. 11g shows the optical images of the resulting MoS₂ at different growth times. The results indicate no nucleation of a second layer during the formation of the first layer.

The CVD process has advantages for the large-scale production of MoS₂ nanosheets at low cost. In many cases, solid MoO₃ and sulfur powder are used as the precursors. In this process, the greatest disadvantage is the spatial nonuniformity, which can be ascribed to the fact that these two precursors exhibit different vapor pressures and melting temperatures. Meanwhile, the growth process should be further controlled to improve the reproducibility. The emergence of organometallic precursors provides opportunities to address these problems. Consequently, CVD will still occupy a great position in synthesizing MoS₂ nanosheets. Unfortunately, these metal–organic compounds are toxic, having a serious environmental impact and corrosion to instruments. Thus, in the future, more attention should be focused on addressing issues such as the low growth rate, small domain size, high production cost, and search for environmentally friendly organometallic precursors.

2.3 Colloidal synthesis

Colloidal synthesis is another popular bottom-down strategy, which has attracted significant interest in materials synthesis because of their shape-dependent physical and chemical properties. In the synthesis of 2D materials, such as nanoflakes and nanosheets, it is possible to tune their lateral sizes and stacking confinement down to the atomic scale through this method. The most commonly applied colloidal synthesis method for the preparation of MoS₂ nanoflakes is based on the reaction of molybdenum compounds (such as [Mo(CO)₆], [Mo(CH₃COO)₂]₂, and MoCl₅) with various sulfur sources (such as thioacetamide (TAA) and hexamethyldisilazane (HMDS)) in the presence of a surfactant (such as oleyamine or stearic acid). For example, Altavilla et al.¹¹⁹ reported a colloidal process to synthesize few-layer MoS₂ nanosheets, wherein the single-source precursor (NH₄)₂MoS₄ decomposed in oleylamine solvent at 360 °C. The resulting MoS₂ nanosheets were covered by a dynamic protective coating of oleylamine. Consequently, the MoS₂ suspension exhibited excellent stability, avoiding aggregation and oxidation phenomena. Zechel et al.²³⁹ investigated the effect of the sulfur source in the synthesis of MoS₂ nanoflakes via colloidal synthesis. In their work, [Mo(CH₃COO)₂]₂ served as the Mo source, whereas a series of sulfur-precursors was screened, including thioacetamide (TAA), 3-mercaptopropionic acid (3-MPA), L-cysteine (L-CYS), mercaptosuccinic acid (MSA), 11-mercaptoundecanoic acid (MUA), 1-dodecanethiol (DDTH), and di-tert-butyl disulfide (DTBD). They concluded that TAA, the most commonly used S-precursor, is a known carcinogen, delivering higher toxicity than the other investigated S-precursors. Sanikop et al.¹²⁰ prepared MoS₂ nanosheets with control of the average number of layers (N) and defect concentration via colloidal synthesis. Frauendorf et al.¹²¹ prepared MoS₂ nanosheets and nanoparticles using MoCl₅ as the precursor reacting with elemental sulfur and hexamethyldisilazane in oleic acid (OA) and oleylamine (OlAm) at 320 °C. Many properties of nanomaterials are dependent on their morphology/size/shape, which can be controlled by adjusting their synthetic parameters. For example, Lambora et al.¹²² investigated the evolution of the morphology of colloidal MoS₂ nanostructures, which were synthesized using (NH₄)₂MoS₄ as both the Mo and S source, whereas octadecene (ODE) served as the solvent. The oleic acid and oleylamine served as the surfactant to direct the morphology of the product (Fig. 12a). Similar to other wet chemical synthetic systems, the formation of these MoS₂ nanostructures also involves nucleation, growth and assembly into the final product (Fig. 12b). Different from the common cases where the morphologies are usually controlled by varying the concentration/dosage of oleic acid and oleylamine, here the reaction temperature is the determining parameter. Specially, it was noted that no precipitation occurred when the reaction temperature was below 90 °C, which can be ascribed to the high pyrolysis temperature of (NH₄)₂MoS₄ (≈155 °C). When the reaction temperature was above 160 °C, a non-emissive black precipitate was formed, which can be attributed to the ultrafast reaction occurring at high temperature. Recently, the same group¹²³ further prepared MoS₂ quantum dots using the same reaction system at 120 °C. Interestingly, by varying the OAc/OAm (mL mL⁻¹) concentration, the optical characteristics of MoS₂ quantum dots could be easily adjusted, which can be ascribed to the different microstructures of the resulting MoS₂ quantum dots. Fig. 12c–h display the HRTEM images of the MoS₂ quantum dots capped with different concentration combinations of OAc and OAm. All the OAc/OAm pairs produced spherical dots. In the absence of OAc or OAm, the quantum dots with an average diameter of ≈13 nm were produced, which were larger than that of the other samples. In the case of volume fractions of 3 mL/0 mL, smaller dots with an average diameter of ≈3 nm were generated. Regarding size distribution, the volume fraction of OAc = 3 produced a narrow size distribution. Liu et al.¹²⁴ prepared a series of nanosized 1T′-TMD monolayers with high phase purity via the general scalable colloidal synthetic technique. These 1T′-TMD monolayers include 1T′-MoS₂, 1T′-MoSe₂, 1T′-WS₂, and 1T′-WSe₂. Moreover, the generation mechanism investigation (combination of systematic experiments and theoretical calculations) indicated that the surfactant prevented the stacking of the layers. Particularly, the resulting 1T′-MoS₂ nano-monolayers demonstrated a significant HER performance with an onset overpotential of −117 mV and a current density of 10 mA cm⁻² at 149 mV (vs. the RHE). Meerbach et al.¹²⁵ also reported a general colloidal synthesis strategy to prepare a series of TMDs, where in the metallic chloride and carbon disulfide were applied as the metal and sulfur source, respectively. Besides a single sulfur source, a combination of sulfur sources has also been reported for the synthesis of MoS₂ nanostructures. Luo et al.¹²⁶ reported the synthesis of chiral MoS₂ nanomaterials through a colloidal synthesis method. In this synthesis process, chiral cysteine enantiomers and NaHS both acted as the sulfur source, whereas MoO₃ served as the molybdenum source. To ensure the formation of MoS₂, ascorbic acid was introduced as a reducing agent. Moreover, the addition of cysteine enantiomers was responsible for the preferential folding of the MoS₂ planes, leading to the formation of chiral MoS₂ nanomaterials.

Fig. 12 (a) Colloidal synthesis of MoS₂ nanostructures using a one-pot heating method and (b) possible mechanism of nanostructure formation at different reaction temperatures. Reproduced from ref. 122 with permission. Copyright 2023, published by the American Chemical Society under a CC-BY license. (c–h) HRTEM images of MoS₂ quantum dots capped with different concentration combinations of OAc and OAm. Reproduced from ref. 123 with permission. Copyright 2024, the American Chemical Society.

A series of Ag@MoS₂ core–shell heterostructures was prepared via a two-stepped colloidal synthetic process (Fig. 13a).¹²⁷ Initially, core@shell structured Ag₂S@MoS₂ heterostructures were formed via a one-pot procedure, in which the silver and molybdenum precursors were sequentially injected into the S powder containing oleylamine, respectively. The size of Ag₂S@MoS₂ can be adjusted by controlling the reaction time. Afterward, the core@shell Ag₂S@MoS₂ will be transferred to Ag@MoS₂ via a facile heat treatment process in the presence of trioctylphosphine. Fig. 13b–e present the typical TEM images of the resulting samples. The Ag@MoS₂ core–shell heterostructures demonstrate ideal platforms for real-time SERS application due to the strong electromagnetic field generated in the plasmonic Ag core. By simultaneously adding S and Se powder, Gong et al.¹²⁸ prepared ultrathin MoS_2(1−x)Se_2x alloy nanoflakes. The chemical composition of the MoS_2(1−x)Se_2x alloy nanoflakes could be completely tuned. As presented in Fig. 13f–o, all the samples possessed a nanoflake morphology with monolayer or few-layer thickness and abundant edges. The dark-contrast lines represent the standing edges. An increase in the selenium content resulted in an increase in the lateral dimensions of the individual nanoflakes, growing from 10–15 nm (for x = 0 and 1/3) to nearly >20 nm (for x = 2/3 and 1). Furthermore, the nanoflake thickness also increased with an increase in the Se content, from mostly monolayers to predominantly few-layers. This result is ascribed to the stronger interlayer interaction at a high selenium content. Similarly, Li et al.¹²⁹ presented a room temperature general colloidal method for the synthesis of TMD nanoparticles (including MoS₂, RuS₂, ReS₂, MoSe₂, RuSe₂, and ReSe₂). In their synthetic systems, the metallic chloride served as the metallic source and the combination of Na₂S and NaHSe as the sulfide and selenide source, respectively.

Fig. 13 (a) Synthesis of Ag₂S@MoS₂ core–shell heterostructure. (b) TEM and (c) HRTEM images of the synthesized Ag₂S@MoS₂ with a size of 25.8 ± 3.4 nm. (d) TEM and (e) HRTEM images of synthesized Ag@MoS₂ with a size of 23.4 ± 3.8 nm. Reproduced from ref. 127 with permission. Copyright 2020, the American Chemical Society. TEM images of MoS_2(1−x)Se_2x nanoflakes with (f and g) x = 0, (h and i) x = 1/3, (j and k) x = 1/2, (l and m) x = 2/3, and (n and o) x = 1. Reproduced from ref. 128 with permission. Copyright 2015, the American Chemical Society.

The colloidal synthetic approach, overcoming the limitations associated with conventional strategies, presents a highly promising strategy for the large-scale production of MoS₂, particularly nanoflakes. The main advantage of colloidal synthesis is the possibility of the tuning of the lateral sizes and stacking confinement of nanoflakes down to the atomic scale. However, its greatest disadvantage is the use of toxic precursors, such as CS₂, TAA, and [Mo(CO)₆]. Besides, these compounds are unstable in the presence of air or moisture in certain cases. Thus, in the future, less toxic compounds are necessary for this method. In addition, owing to the diversity of transition metals in crystalline phases, more attention should be paid to the phase control in colloidal synthesis in the future.¹³⁰

2.4 Hydrothermal/solvothermal method

Hydrothermal synthesis is a conventional bottom-up approach, which is widely applied in synthetic chemistry. The hydrothermal method offers advantages of precious control of the crystal form to give a well-defined morphology. To date, a variety of functional materials has been successfully synthesized using this method, such as oxides, sulfides, nitride, and inorganic–organic hybrids. Geng et al.¹³¹ synthesized stable 2D metallic MoS₂ (1T phase) nanosheets by mixing MoO₃, thioacetamide (TAA) and urea in an autoclave and performing the reaction under hydrothermal conditions (200 °C). In this system, TAA serves as the S source given that its decomposition is capable of generating S²⁻, whereas urea plays the role of reducing agent to convert Mo(VI) into Mo(IV). The stacking of the layered structure in the resulting metallic MoS₂ is dramatically decreased owing to the intercalation effect of water molecules. As a result, the resulting metallic MoS₂ exhibits excellent stability in water. Kumar et al.¹³² prepared MoS₂ micro/nanostructures, including microrods, microspheres, and microrods, using sodium diethyldithiocarbamate trihydrate as the S source. The resulting MoS₂ microstructures possessed an increased interlayer spacing and excellent adsorption behavior for the removal of Pb(II) from aquatic systems. Saias et al.¹³³ fabricated MoS₂-supported alumina via a hydrothermal process and demonstrated the effect of the growth temperature on the crystallinity of MoS₂ (Fig. 14a). At the screened temperatures, all the MoS₂ samples possessed a flower-like morphology, which further assembled into ball-like microstructures (Fig. 14b–d). The growth temperature demonstrated a significant influence on the quality of the MoS₂ flower spheres. Specially, a high temperature is favorable for forming spheres with a large diameter. This result is ascribed to the additional kinetic energy present during growth, which induces the formation of lower-energy thermodynamic products, exhibiting obvious boundaries formed on the top of the surface. To control the interlayer spacing in MoS₂, Li et al.¹³⁴ developed a hydrothermal process to prepare MoS₂ submicrospheres assembled by tangled MoS₂ nanosheets with the assistance of foreign species with varying sizes. As presented in Fig. 14e, three foreign molecules were examined, including L-cysteine, thiourea, and glucose. Fig. 14f–h display the corresponding HRTEM images of the samples, indicating their interlayer gaps are 0.62 (L-cysteine), 0.87 (thiourea), and 1.02 nm (glucose), respectively. Usually, foreign species with a larger size are favorable for producing a larger interlayer spacing. Obviously, glucose possesses the largest size among the three organic compounds, and thus the sample obtained with the introduction of glucose produces the largest interlayer gaps. L-cysteine is a small-sized molecule, producing interlayer gaps similar to that in pristine MoS₂. Meanwhile, in this case, L-cysteine also serves as sulfur precursor to react with Na₂MoO₄, generating MoS₂. In the case of thiourea, another small-sized molecule also acting as the sulfur precursor, it demonstrates a larger interlayer gap of 0.87 nm, which is increased by 0.25 nm compared to that of pristine MoS₂. This result can be ascribed to the generation of NH₃ molecules by the decomposition of thiourea, whose diameter is approximately 0.31 nm. Similarly, in the case of introducing glucose, the interlayer distance increased by 0.5 nm compared to that of pristine MoS₂. This result demonstrates good agreement with the size of the monosaccharide, which is about 0.6–0.8 nm. In another work, Kaushik et al.¹³⁵ utilized a similar process to prepare MoS₂ with different morphologies, including nanoflower, interconnected nanoplates, and nanosheets (Fig. 14i–k), respectively. In their work, thiourea and ammonium molybdenum were used as the sulfur and molybdenum precursors, respectively, and morphology control was achieved by adjusting the reaction time, producing nanoflower (18 h in water), interconnected nanoplates (36 h in water), and nanosheets (48 h in MDF/water mixture). Yu et al.¹³⁶ prepared rose-shaped MoS₂ nanoflowers with TAA and Na₂MoO₄ as the precursors, demonstrating an efficient SER substrate for detecting R6G. Later, they further demonstrated that the rose-shaped MoS₂ nanoflowers also are capable of serving as a support to deposit Au nanomaterials for the detection of crystal violet molecules.¹³⁷ Panchu et al.¹³⁸ applied the hydrothermal process to synthesize pristine MoS₂ and an Ni-MoS₂ composite. In their system, the formation of MoS₂ is based on the reaction taking place between ammonium molybdate and thiourea. The interconnected sheet morphology of MoS₂ is attributed to the unreacted NH₄⁺ ions, which prevented the stacking of the nanosheets. Huang et al.¹³⁹ reported a hydrothermal process, wherein ammonium molybdate, thiourea, and N-acetyl-L-cysteine (NAC) were used as the precursors and the capping reagent to obtain monolayer MoS₂ quantum dots, which were water-soluble with a uniform lateral size of ≈2.1 nm.

Fig. 14 (a) Procedures for the synthesis of MoS₂ supported on an alumina substrate. (b–d) SEM images of MoS₂ obtained at different temperatures. Reproduced from ref. 133 with permission. Copyright 2022, the American Chemical Society. (e) Synthesis of MoS₂ microspheres with different interlayered spacings. (f–h) HRTEM images of MoS₂ microspheres with different interlayered spacings. Reproduced from ref. 134 with permission. Copyright 2020, the American Chemical Society. SEM images of MoS₂ with different morphologies: (i) nanoflower, (j) interconnected nanoplates, and (k) nanosheets. Reproduced from ref. 135 with permission. Copyright 2023, the American Chemical. Society.

Gao et al.¹⁴⁰ developed a hydrothermal process to grow high-purity MoS₂ square nanotubes. As presented in Fig. 15a, both Na₂MoO₄ and MnCl₂ were employed in the system. During the initial stage, square-column MnMoO₄ nanorods were formed via the reaction between Na₂MoO₄ and MnCl₂. Owing to the presence of thiourea, the MnMoO₄ nanorods could be converted into MnCO₃ and MoS₂, generating a core@shell-structured composite. Interestingly, the resulting MoS₂ possessed a nanosheet morphology, covering the outside the composite. As the reaction proceeded, MnMoO₄ gradually disappeared, generating the core@shell MoS₂@MnCO₃. Finally, the core MnCO₃ was removed by washing with HCl, and thus MoS₂ nanotubes were obtained. In this system, the formation of MnCO₃ and MoS₂ nanosheet can be ascribed to the decomposition of urea, generating CO₃²⁻ and S²⁻ under hydrothermal conditions. Fig. 13b–f show the SEM and TEM images of the resulting square MoS₂ nanotube, which demonstrated the typical length of 7.13 μm and width of ∼634 nm. The interesting phenomenon in this system is the formation of MnCO₃ but not MnS given that the former possesses a larger solubility product (K_sp (MnCO₃) = 2.24 × 10⁻¹¹) than that of MnS (K_sp (MnS) = 4.65 × 10⁻¹⁴).^141,142 This phenomenon can be ascribed to the fact that the process is controlled by kinetics but not thermodynamics.

Fig. 15 (a) Procedure for the formation of MoS₂ nanotube. (b and c) SEM images of the resulting MoS₂ nanotube at different magnifications. (d–f) Element mapping of square MoS₂ nanotube. Reproduced from ref. 140 with permission. Copyright 2023, the American Chemical Society.

Similar to solid–gas synthesis, the hydrothermal process is also capable of yielding MoS_2−xSe_x when both S and Se sources are employed in the system. For example, Taufik et al.¹⁴³ prepared MoS_2−xSe_x solid-solution nanoparticles using a hydrothermal process. The whole preparation process is presented in Fig. 16a. Initially, MoO₄²⁻ is formed by dissolving Na₂MoO₄·2H₂O in water, which then is reduced by hydrazine to generate Mo⁴⁺. Afterwards, the system is transferred to a hydrothermal vessel. Owing to the relatively high-temperature conditions, the Mo⁴⁺ ions will react with the sulfur and selenium powder, leading to the formation MoS_2−xSe_x. The composition of the MoS_2−xSe_x solid-solution nanoparticles can be preciously controlled by adjusting the selenium addition ratio. Fig. 16b–f display the TEM images of the MoS_2−xSe_x samples. The size of these samples showed a decreasing trend with an increase in the selenium addition ratio. The as-x = 0 sample possessed a flower-like shape with a diameter of about 500 nm. The particle size of the as- x = 2 sample decreased to below 100 nm. In another work, Zhang et al.¹⁴⁴ developed a two-step process to synthesize an MoS_2−xSe_x/G composite alloy. As presented in Fig. 16g, MoS₂ with an interlayer spacing of 0.62 nm was initially prepared via a hydrothermal procedure in the presence of graphene oxide. Afterward, a thermal selenization treatment is applied to convert MoS₂/G to MoS_2−xSe_x/G. Owing to the larger size of Se than S, MoS_2−xSe_x produces a larger interlayer spacing (0.66 nm). The composition of MoS_2−xSe_x in the final product can be conveniently controlled by adjusting the ratio of Se powder to MoS₂/G composite.

Fig. 16 (a) Hydrothermal process to synthesize MoS_2−xSe_x. TEM images of (b) as-x = 0, (c) as-x = 0.2, (d) as-x = 1, (e) as-x = 1.8, and (f) as-x = 2. Reproduced from ref. 143 with permission. Copyright 2021, the American Chemical Society. (g) Schematic of two-step process to synthesize MoS_2−xSe_x/G. Reproduced from ref. 144 with permission. Copyright 2019, the American Chemical Society.

Besides synthesizing pristine MoS₂, composites containing MoS₂ can also be obtained via the hydrothermal process. For example, Zheng et al.¹⁴⁵ prepared an MoS₂/CdS composite via a one-pot hydrothermal process. The resulting MoS₂/CdS composite was rich in sulfur vacancies, which was verified by electron paramagnetic resonance (EPR) and UV–vis diffuse-reflectance spectroscopy (UV-DRS). When evaluated as a photocatalyst for the N₂ reduction reaction, it delivered a high production of NH₃ (249.7 mg L⁻¹ g⁻¹), which was 5.4- and 3.9-times higher than that of pure MoS₂ (45.9 mg L⁻¹ g⁻¹) and pristine CdS (64.5 mg L⁻¹ g⁻¹), respectively. Similarly, Chen et al.¹⁴⁶ prepared Co-doped MoS₂ and CoS₂ composites with a coupling interface via a one-pot hydrothermal process, which enabled the chemoselective formation of p-chloroaniline via the hydrogenation of p-chloronitrobenzene. By reacting pre-synthesized few-layer MoS₂ with the Zn_xCd_1−xS/ZnS(en)_1/2 heterostructure under solvothermal conditions with ethylenediamine as the solvent, Dong et al.¹⁴⁷ prepared MoS₂/ZnCdS/ZnS dual heterostructures, delivering ZnS nanosheets decorated with ZnCdS nanorods and few-layer MoS₂. This advantageous heterostructure endowed the resulting sample with an excellent photocatalytic performance, offering an H₂ evolution rate of up to 79.3 mmol g⁻¹ h⁻¹ under visible light irradiation. Wang et al.¹⁴⁸ developed a two-step hydrothermal process to prepare a CuS/MoS₂ composite. As presented in Fig. 17a, the synthesis involved two hydrothermal steps. It was noted that the composition of CuS/MoS₂ could be adjusted in the second hydrothermal stage by controlling the amount of CuS. Fig. 17b–d present the morphology characterization of the sample obtained by using 0.75 mmol CuS, whose diameter is around 2 μm. Furthermore, the distance separating adjacent lattice fringes is 0.62 nm, which is in accordance with the (002) crystal plane of 2H-MoS₂. In addition, the other observed lattice spacing of 0.28 nm is ascribed to the (103) crystal plane of CuS. Hu et al.¹⁴⁹ prepared an MoS₂/PANI composite via a hydrothermal process. Specially, they initially prepared PANI via chemical oxidation polymerization, which was further utilized as the substrate to deposit MoS₂. For the formation of MoS₂, MoO₃ and KSCN were employed as the Mo and S sources, respectively. The resulting MoS₂/PANI could be further converted to an MoS₂/C composite. Both the MoS₂/PANI and MoS₂/C composites exhibited an excellent performance in lithium ion batteries.

Fig. 17 (a) Hydrothermal synthesis of CuS/MoS₂ composite. (b and c) TEM images and (d) high-resolution transmission electron microscopy (HRTEM) images of CuS/MoS₂ composite. Reproduced from ref. 148 with permission. Copyright 2024, the American Chemical Society.

The hydrothermal process, using hot compressed water to convert reactants to the desired products, is emerging technology that contributes to the sustainable production of nanomaterials. Similar to colloidal synthesis, the hydrothermal process is also classed as a liquid synthetic system. Compared to other conventional processes, the reactions in the hydrothermal process are carried out in a closed system, leading to several advantages such as easy operation, energy efficiency, cost saving, better nucleation control, and less pollutant emission. Besides, the hydrothermal technique is capable of accelerating interactions between solid and fluid species, yielding phase-pure and homogeneous materials. Compared to CVD or milling, the hydrothermal process usually involves slightly a longer reaction time.

2.5 Salt-assisted synthesis

In the past few years, the salt-assisted synthetic technique has attracted wide attention for preparing many functional materials. Salts may play several roles in synthetic systems, such as the reaction medium, template for generating target materials with special shapes/porosity, and precursor.^150,151 Cevallos et al.¹⁵² presented the growth of single crystals of MoS₂ through a liquid salt flux made from a low-melting mixture of NaCl and CsCl. The crystal characterization revealed that the resulting MoS₂ crystals were comprised of a dominant 2H-MoS₂ phase with a very small percentage (about 3%) of 3R intergrowths. Suleman et al.¹⁵³ report that NaCl is capable of promoting the formation of MoS₂ via CVD, which is ascribed to the confinement effect of NaCl. Furthermore, the morphology of the grown MoS₂ can be well controlled by adjusting the amount of NaCl, growth temperature, and Mo/S precursor ratio. Utilizing a similar NaCl-assisted CVD process, Singh et al.¹⁵⁴ unlocked the role played by the reactant concentration in the boundary layer. Fig. 18a displays the diagram of the setup used for growing 3L-MoS₂, whose critical parameters such as s, h, and d, are schematically shown in Fig. 18b. Here, s refers to the separation between the precursors and growing face of the SiO₂/Si substrate, h refers to the height of the boat, and d is the distance between the growing face of the Si substrate and central axis of the tube/boundary layer. The gaseous precursors are input with the carrier gas (Ar) from a concentration boundary layer, as shown in Fig. 18c. To collect the formed MoS₂, the SiO₂/Si substrate was positioned on the crucible in a face-down manner. As a result, there was a space between the sidewalls of the crucible (Fig. 18d). The solid precursor of MoO₃ was mixed with NaCl in the middle of the setup. Upon heating, the reactions could be confined in the space created by NaCl. The quality of the resulting MoS₂, including layer number, growth area coverage, and nucleation density could be easily regulated by controlling the concentration boundary layer formation, which can be achieved by tuning the separation between the precursors and growing face of the substrate. Khan et al.¹⁵⁵ demonstrated that the NaCl-assisted CVD process decreased the growth temperature from 850 °C to 650 °C and high-quality few-layer MoS₂ could be prepared.

Fig. 18 (a) Schematic of NaCl-assisted CVD process for 3L-MoS₂ growth. (b) Schematic of definition of s, h, and d. (c) Reactant concentration variation in the synthesis system and its boundary layer formation in the CVD tube. (d) Schematic of A, B, and C points on the substrate. Reproduced from ref. 154 with permission. Copyright 2021, the American Chemical Society.

To reveal the mechanism of yielding MoS₂ monolayers in the NaCl-assisted CVD system, Lei et al.¹⁵⁶ employ theoretical tools, i.e., first-principles calculations and ab initio molecular dynamics (AIMD) simulations, to examine the growth of MoS₂. According to the binary phase diagram of NaCl-MoO₃ (Fig. 19a), there is a eutectic point at x_(NaCl) = 0.59 and the system delivers a low melting point of 648 °C. This temperature is ∼150 °C lower than that of the pure compounds. Interestingly, the system can further be reduced to form MoO₂Cl₂ (dashed line in Fig. 19a). The formation of MoO₂Cl₂ can be described by the following equation:

3MoO₃ (s) + 2NaCl (s) → MoO₂Cl₂ (g) + Na₂Mo₂O₇ (l) (3)

Fig. 19 (a) MoO₃-NaCl binary phase diagram as well as chemical structure of MoO₂Cl₂. (b) Three plausible reaction pathways for MoO₂Cl₂ sulfurization toward MoS₆. (c) Energy profiles of MoS₂ monolayer growing process: salt-assisted (red line) and conventional (black line) CVD. Reproduced from ref. 156 with permission. Copyright 2022, the American Chemical Society.

Owing to its volatile feature, MoO₂Cl₂ reaches a significant vapor pressure (∼800 mbar) sufficient for experimental MoS₂ growth at a temperature as low as 150 °C. In the presence of S₂ gas (dominant sulfur species at the growth temperature), MoO₂Cl₂ can be sulfurized into MoS_n (n = 4, 5, and 6) molecules, with MoS₆ being the dominant one. Fig. 19b depicts the three plausible pathways. A comparison of the energy profiles in the salt-assisted synthesis and the conventional CVD process is provided in Fig. 19c. The former is ∼2 to 3 orders of magnitude faster at the growth temperature. Besides chloride, other halide salts such as F, Br, and I also deliver similar behavior in assisting the CVD growth of MoS₂.

Besides acting as the reaction medium, some salts may serve as precursors to generate the target materials, which is dependent on the chemical properties of the utilized salts. Li et al.¹⁵⁷ reported a KSCN molten salt strategy to synthesize MoS₂ and MoS_xSe_2−x at a modest temperature of 320 °C. As presented in Fig. 20a and b, for both cases, the decomposition of KSCN is crucial, in which S²⁻ is generated. For the formation of MoS₂, an intermediate, MoS₄²⁻, is formed by reaction between MoO₄²⁻ and S²⁻. Afterward, the formed MoS₄²⁻ intermediate is pyrolyzed into MoS₂ via heat treatment (Fig. 20a). For the formation of MoS_xSe_2−x, the Se powder may react with S²⁻ released by the decomposition of KSCN to form the Se_xS²⁻ polyanion, which further reacts with MoO₄²⁻ upon heating, yielding MoS_xSe_2−x (Fig. 20b). Interestingly, the decomposition of KSCN is verified to be promoted by MoO₄²⁻ to S²⁻, which then activates the Se powder, culminating in the formation of MoS_xSe_2−x. Furthermore, structural characterization revealed that the resulting MoS_xSe_2−x is characterized by abundant anion vacancies, which can be ascribed to the introduction of Se heteroatoms, causing lattice distortion. Compared to S²⁻, the large size of Se_xS²⁻ offers substantial steric hindrance, resulting in the slow/uncompleted growth of the MoS_xSe_2−x crystal. Theoretical analyses indicated that the electronic structure of MoS_xSe_2−x can be well adjusted by Se atoms and anion vacancies. As a result, the optimized MoS_1.5Se_0.5 possesses a minimized band gap of 0.88 eV and an almost zero ΔG_H* of 0.09 eV. Fig. 20c and d depict the IFFT patterns of MoS₂ and MoS_1.5Se_0.5. The results indicated that MoS₂ delivers well-defined lattice fringes, indicating high crystallinity (Fig. 20c). In the case of MoS_1.5Se_0.5, mainly an amorphous region was produced, exhibiting low crystallinity (Fig. 20d). Considering the high toxicity of KSCN, Jin et al.¹⁵⁸ introduced Na₂SO₄ as the sulfur precursor to prepare MoS₂. In their work, Na₂SO₄ also plays roles in adjusting the diffusion of the source precursors and balancing their mass flux. However, the system needs H₂ to generate H₂S, which reacts with the molybdenum source. Furthermore, the generation rate of H₂S was demonstrated instantaneously. Meanwhile, the decomposition temperature of Na₂SO₄ is much higher (above 690 °C) than that of KSCN (below 350 °C).

Fig. 20 Schematic of molten KSCN0assisted synthesis of MoS₂ (a) and MoS_xSe_2−x (b). IFFT patterns of (c) MoS₂ and (d) MoS_1.5Se_0.5. Reproduced from ref. 157 with permission. Copyright 2024, the American Chemical Society.

Compared to solid-state synthetic processes, salt-assisted synthesis shows advantages in terms of operation, demonstrating high reactivity for the preparation of various inorganic nanomaterials. In the case of molten salt systems with relatively low melting points, the ionic pool results in the high dispersion of the reactants, ensuring fast mass transfer. As a result, chemical reactions can be completed swiftly, showing high efficiency in generating products. In addition, molten salt systems have the advantages of low cost, high yield, time efficiency, and environmental friendliness. Furthermore, some particular salts were recently demonstrated to act as templates in the fabrication of inorganic nanomaterials with special morphologies and sizes, which have been applied to replace some of the traditional structure-directing agents or pore forming agents. These characteristics endow salt synthesis systems with a safer synthesis process, easy movability, and the possibility of scalable production. It should be noted that to date, the salt-assisted synthetic techniques are still in their infancy, which are subject to further investigation by emphasizing their general mechanisms in the future.

In this section, we discussed several common strategies. A comparison of these established techniques is presented in Table 1. It should be noted that many other methods are available to synthesize MoS₂, such as atomic layer deposition,^159–161 electron beam irradiation,¹⁶² and phase transition.^163,164

Table 1 Advantages and disadvantages of methods for synthesizing MoS₂

Method	Advantages	Disadvantages
Exfoliation	Low pollution	Complex technology
	Low energy consumption	Limited reproducibility
	Wide applicability	Large structural polydispersity
Chemical vapor deposition	Large treatment capacity	High equipment input
	Wide applicability	Complex technology
	Flexible scale	High energy consumption
		Potential secondary pollution
Colloidal synthesis	Capable of large-scale production	Toxic precursors involved
	Mild reaction conditions	Poor crystalline control
		Generates polluted liquid
Solvothermal	Versatile precursors available	Generates polluted liquid
	Mild reaction conditions	Relatively long reaction time
	Energy saving
Salt assisted synthesis	High efficiency of electric heat conversion	The technology is not mature
	Low pollutant emissions
	Recovery of reusable materials

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3 CDI application

The search for high-performance electrodes constitutes one of the critical issues to realize CDI. To date, various materials have been demonstrated to be efficient CDI electrode candidates, such as carbon, MXenes, and PBA. Table 2 lists the advantages and disadvantages of these materials. Benefiting from their huge specific adsorption area, good conductivity and good chemical and thermal stability, carbon-based materials have been confirmed to be excellent CDI electrode candidates, such as activated carbon, carbon fibers, carbon aerogel, carbon cloth and graphene. However, a disadvantage of these carbon-based materials is their electric double-layer capacitance (EDLC) mechanism, delivering a low desalination capacity and poor selectivity. Metallic oxides mainly exhibit pseudocapacitive behavior, showing superior desalination capacity over carbon. However, the stability of metallic oxides constitutes the main barrier to their widespread application, which is very similar in capacitive energy storage systems. LDH, MXenes, and MoS₂ belong to the class of 2D layered materials, which all suffer from easy restacking. Generally, the phase diversity of MoS₂ offers more opportunities to adjust its phase crystal structure and form hybrids with other species to boost their CDI performance. In this section, we discuss the application of MoS₂-based materials in CDI in detail.

Table 2 Advantages and disadvantages of some commonly available CDI electrodes

Electrode candidate	Advantages	Disadvantages
Carbon	High electronic conductivity	Limited by EDLC mechanism
	Good stability	Not all surfaces are fully utilized
	High specific surface area
Metallic oxides	Variable oxidation state	Poor cyclic stability
	Pseudocapacitive mechanism
	High desalination capacity
LDH	Abundant active sites	Poor electronic conductivity
	High structural stability	Ease of restacking
	High desalination capacity	Low desalination rate
MXene	Excellent metallic conductivity	Interlayer restacking
	High hydrophilicity	Easily oxidized
	Ease of functionalization	Hazardous agents involved in synthesis
MoS2	Adjustable phase to tune the conductivity	Poor cyclic stability
	Easy modification	Ease of restacking

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3.1 Pristine MoS₂

Molybdenum disulfide has three different phase structures, and different phase types have different physical and chemical properties. The 1T phase is a special ultra-thin structure, with a large layer spacing, high conductivity, excellent electrochemical performance (>2H), high desalination performance, more active sites and higher ion migration rate than the 2H phase, but the thermal stability of the 1T phase is poor.¹⁶⁵ Also, the 1T phase easily transforms into the 2H phase in the cycling process, and thus its synthesis process is complex. The 2H phase is a semiconductor with poor conductivity and strong van der Waals force between layers, resulting in its easy restacking. However, the 2H phase has high thermal stability and is suitable for long-term recycling. The 3R phase is a three-way stacked structure with small layer spacing, which is not conducive to ion diffusion, and its electrochemical performance has not been widely explored.¹⁶⁶

3.1.1 Intrinsic MoS₂. To date, both 1T and 2H phase MoS₂ have been investigated for CDI application. In the case of the 1T phase, it usually demonstrates higher electronic conductivity (10–100 S cm⁻¹) with a lower charge transfer resistance in electrochemical applications, leading to a superior desalination capacity and rate. This advantageous performance can be ascribed to the distorted tetragonal structure with band inversion and spin–orbit interaction. The greatest disadvantage of 1T MoS₂ is its instability, tending to restack into the 2H phase and causing poor cycling stability. By contrast, 2H phase MoS₂ is the most common and stable crystal structure, demonstrating semiconductor characteristics. When evaluated as an electrode in CDI cells, it delivered poor charge transport, suffering from rapid capacity decay and poor rate performance. Xing et al.¹⁴ reported that chemically exfoliated MoS₂ (ce-MoS₂) nanosheets demonstrate a comparable CDI performance to that of carbon materials and other common materials. The ion quality removal capacity reached 8.81 mg g⁻¹, and the ion volume removal capacity reached 16.51 mg cm⁻³ for ce-MoS₂. They further ascribed this excellent CDI performance of ce-MoS₂ to the particular 2D layered thin sheet structure of the 1T phase, exhibiting excellent electrical conductivity and offering abundant space to accommodate ions. Meanwhile, the large interlayer spacing has also been demonstrated to be beneficial for the intercalation and transportation of ions. Wang et al.¹⁶⁷ developed a flexible MoS₂-based electrode with no binders by depositing MoS₂ onto the framework of stainless-steel mesh (Fig. 21a). It was found that the surface treatment is beneficial for providing active nucleation sites for the growth of MoS₂. The morphology observation revealed that MoS₂ possessed Auricularia-like architectures. When evaluated as electrodes in a CDI cell, the binder-free MoS₂-based electrodes delivered a better performance than that of pristine MoS₂. The desalination capacity of the binder-free MoS₂-based electrode was high as 28.76 mg g⁻¹ (Fig. 21b and c). Considering that MoS₂ exists in either the 1T or 2H phase, Ying et al.¹⁶⁸ prepared a 1T-MoS₂ electrode with a compact architecture via the restacking of exfoliated nanosheets, which exhibited a superior electrochemical performance to 2H-MoS₂. The resulting 1T-MoS₂ electrode demonstrated a high capacitance of up to ≈277.5 F cm⁻³ at an exceptional scan rate of 1000 mV s⁻¹. For desalination application, the 1T-MoS₂ electrode demonstrated an ultrahigh volumetric desalination capacity of 65.1 mg NaCl cm⁻³ in CDI experiments. Pang et al.¹⁶⁹ prepared 1T-phase MoS₂ with an expanded interlayer spacing of 9.8 Å, delivering an elevated capacitive contribution percentage (81%). When assembled into a CDI device, a high desalination capacity of 47.1 mg g⁻¹ was achieved in a 200 mg L⁻¹ NaCl solution with a fast desalination rate of 2.4 mg g⁻¹ min⁻¹. Zhang et al.¹⁵ investigated the removal of Pb(II) with a low concentration using an MoS₂ electrode. They found that the removal mechanism involves two steps, electrosorption and chemisorption (CEC). Experimentally, they achieved an ultrahigh adsorption capacity at a voltage of 1.2 V, reaching 86.26 mg m⁻². The mechanism investigation indicated that the enhanced adsorption capacity is mainly ascribed to two factors (Fig. 21d). On the one hand, the positively charged Pb(II) may undergo physical adsorption, which is achieved by van der Waals force given that Pb(II) can be attached and concentrated on the surface of molybdenite, which is achieved by EDL interaction through electrosorption. On the other hand, the exposed S atoms of molybdenite are capable of binding with Pb(II), leading to chemical adsorption. A high Pb(II)concentration in EDL is expected to promote the chemisorption of Pb(II) on the molybdenite electrode. Benefiting from this unique adsorption mechanism, the MoS₂ electrode delivered a superior performance over graphite paper adsorbent, as evidenced in Fig. 21e. The maximum adsorption capacity reached 86.26 mg m⁻². This value is nearly five-times that of single chemisorption without applying a voltage. Excitingly, the resulting molybdenite electrode also demonstrated potential for the CEC removal of other heavy metals. Taking Cd(II) as an example, CEC delivered a higher adsorption capacity (12.84 mg m⁻²) than chemisorption (3.87 mg m⁻²) (Fig. 21f).

Fig. 21 (a) Procedure for synthesizing MoS₂@SSM. (b) Schematic of the MoS₂@SSM-based CDI cell. (c) CDI performance of MoS₂@SSMB and MoS₂@SSM. Reproduced from ref. 167 with permission. Copyright 2024, Elsevier. (d) Schematic for electroadsorption of Pb(II) on molybdenite electrode. (e) Comparison of isotherms of Pb(II) adsorption on graphite paper and molybdenite electrode. (f) Isotherms of CEC (with application of a voltage) and chemosorption (without applying a voltage) of Cd(II) on molybdenite electrode. Reproduced from ref. 15 with permission. Copyright 2018, Elsevier.

3.1.2 Elemental-doped MoS₂. Heteroatom doping is an effective strategy to adjust the electronic structure of semiconductor nanomaterials, leading to improved optical or electrochemical performances.^170–172 Li et al.¹⁷³ developed a hydrothermal process to synthesize manganese-doped MoS₂ nanosheets (Fig. 22a), which demonstrated an excellent CDI performance. Interestingly, manganese doping was demonstrated to enlarge the interlamellar spacing of MoS₂, enriching its mesoporous structure. In the designed hybrid CDI cell, the salt removal capacity reached 24.5 mg g⁻¹ in a 500 mg L⁻¹ NaCl solution. Notably, the desalination process using Mn-MoS₂ exhibited the characteristic of fast kinetics. The salt removal could reach the equilibrium state within min, and the removal rate was determined to be 3.06 mg g⁻¹ min⁻¹. Furthermore, no efficiency decay was observed after 30 cycles, demonstrating superior stability (Fig. 22b). Besides metallic cation doping, the incorporation of oxygen has also been reported as an effective approach to adjust the properties of MoS₂. Sun et al.¹⁷⁴ investigated the effects of incorporating oxygen in MoS₂ nanosheets and concluded that it could dramatically boost the CDI performance of the MoS₂ nanosheets. As presented in Fig. 22c, the optimized MoS₂ electrode was determined to contain a moderate degree of oxygen (2.91 at%), exhibiting the highest desalination capacity of 28.85 mg g⁻¹, which is almost 5.5-times that of MoS₂ without oxygen. The mechanism investigation revealed that this enhancement can be ascribed to two factors. On one hand, owing to the high electronegativity of the incorporated oxygen atoms, the intrinsic conductivity can be dramatically improved. Simultaneously, more active sites will be exposed. Consequently, a higher specific capacitance and lower inner resistance may be generated when used as an electrode. On the other hand, the incorporated oxygen provides chances to achieve a balance between structure and active sites, endowing MoS₂ with the highest salt adsorption capacity. The same group¹⁷⁵ further demonstrated that O-doped MoS₂ is capable of selectively recovering rare earth metals from a low concentration solution with a nearly 100% recovery efficiency. This high recovery efficiency is ascribed to the enhanced chemisorption and electrosorption process.

Fig. 22 (a) Synthesis of Mn-MoS₂. (b) CDI performance of Mn-MoS₂. Reproduced from ref. 173 with permission. Copyright 2024, Elsevier. (c) Oxygen incorporation process and its effect on desalination capacity. Reproduced from ref. 174 with permission. Copyright 2020, Elsevier.

3.1.3 Defect engineering. The introduction of defects into nanomaterials demonstrates another versatile tool to boost their optical and electrical performances. Jia et al.¹⁷⁶ developed defect-rich MoS₂ sheets through thermal treatment. Owing to the abundant defects, MoS₂ demonstrated an excellent desalination capacity. In particular, the desalination capacity reached 35 mg g⁻¹, which is nearly three-times that of MoS₂ without defects (12.8 mg g⁻¹). This enhancement can be ascribed to several factors. Firstly, the defects on the surface create abundant negative charges, high specific capacitance and low inner resistance, which promote the electrostatic attraction and electrosorption of Na⁺ on the electrode surfaces. Secondly, defects can be incorporated on the surface of MoS₂, which are beneficial for a superb CDI electrode performance. In another work, Zhan et al.¹⁷⁷ reported the use of defective MoS₂ as an electrode to capture Pd(II) through the CEC process. Fig. 23a–m depict the atomic configuration of defective MoS₂ with variable vacancies (one or two). By combining theoretical and experimental investigation, they systematically examined the distinct adsorption behaviors of Pb(II) on the defective MoS₂ electrode. Experimentally, a wide range of Pb(II) concentrations was examined (1–100 mg L⁻¹). For sole chemosorption, an increasing trend was obtained in adsorption capacity versus the initial Pb(II) concentration. For example, it increased from 64.99 (at 8.37 mg L⁻¹) to 159.98 mg g⁻¹ (at 18.71 mg L⁻¹). By applying a voltage, both the adsorption and removal efficiency showed a dramatically increase. Compared to the case of no applied voltage, the adsorption capacity and removal efficiency were boosted by almost 8 times, which can be attributed to the strengthened migration of ions, electrostatic interaction and electric double layer. Besides, for recycling use, the desorption of Pb(II) was also investigated, which could be completed within 60 min. After 10 cycles, the defective MoS₂ electrode still maintained a removal of higher than 95%.

Fig. 23 (a–m) Atomic configurations of defective MoS₂ with one or two vacancies. Removal kinetics of Pb(II) on defective MoS₂: (n) adsorption capacity and (o) removal. Reproduced from ref. 177 with permission. Copyright 2024, Elsevier.

The superior CDI performance of MoS₂ can be ascribed to its excellent capacitive property, which originates from two aspects. Firstly, the good EDL capacitance delivered by MoS₂ is owing to its sandwich-type S–Mo–S layered structure, which is capable of offering a high specific surface area. Simultaneously, the layered structure makes the intercalation of electrolyte ions in MoS₂ easy, which is also beneficial for delivering an excellent capacitive property. Secondly, the oxidation state of the Mo atom can dramatically vary from +2 to +6, resulting in a good pseudocapacitive property. Elemental doping and introduction of defects can further vary the surface and electrochemical properties, leading to an enhancement in CDI performance. In particular, defect-rich MoS₂ is capable of delivering abundant negative charges, high specific capacitance and low charge-transfer resistance, promoting the formation of a strong EDL in the CDI process and the electrosorption of Na⁺ on the surface of MoS₂. Unfortunately, the restacking of the MoS₂ sheets as well as the poor electrical conductivity of MoS₂ hinder its application as a CDI electrode material. Thus, by compositing it with other species, the above-mentioned issues may be partly addressed. In the following sections, we discuss various MoS₂-based composites for CDI application in detail.

3.2 Composites with carbon

In daily life, carbon materials are ubiquitous, which are usually characterized by rich porous structures and high specific surface areas. Currently, the application of carbon materials covers numerous fields, such as detection,^178–181 adsorption,^182–184 catalysis,^185,186 energy and the environment.^187–191 Similar to energy storage/conversion systems, the high conductivity, high surface area per unit mass, and intrinsic porosity make nanostructured carbon materials suitable candidates as CDI electrodes, such as activated carbon, carbon nanotubes, graphene, and carbon nanofibers. Also, the combination of carbon with inorganic pseudocapacitive candidates can dramatically boost its specific capacitance, and consequently the cycling stability of its inorganic counterpart will be improved. To date, numerous excellent works have been reported on the design of MoS₂/carbon composites for CDI application. In this section, we provide a comprehensive summary on this sub-topic.

3.2.1 Activated carbon. Generally, activated carbon exhibits a particularly narrow pore size distribution, which has proven to be beneficial for increasing its salt adsorption capacity. However, the high internal resistance and physical pore blockage during extensive cycling result in a decay in its specific capacitance. Consequently, research has been dedicated to the incorporation of inorganic materials on the surface of carbon materials to enhance its specific capacitance through the combination of fast faradaic reactions and double-layer charging. Yin et al.¹⁹² reported the use of a carbon skeleton/molybdenum disulfide composite material (MoS₂/AC) for the capacitive removal of Cd²⁺, achieving a removal capacity of 22.15 mg g⁻¹. Utilizing the high affinity of PDA to inorganic nanomaterials, Hao et al.¹⁹³ deposited few-layer MoS₂ (FL-MoS₂) on PDA spheres, which were prepared via the self-polymerization of DA under alkalic conditions. The obtained FL-MoS₂/PDA spheres could be further converted into FL-MoS₂/NCS (N-doped carbon spheres), as presented in Fig. 24a. In the composite, MoS₂ possessed a hexagonal crystal structure. The presence of NCS prevented the growth of MoS₂ along the (002) crystal plane, leading to a slight expansion of the interlayer distance. Besides, the introduction of NCS also improved the water wettability of the composite, as demonstrated by the lower water contact angle of FL-MoS₂/NCS (62°) than that of the MoS₂ electrode (96°). Benefiting from these features, the resulting FL-MoS₂/NCS exhibited an excellent CDI performance for the removal of ions from water. Their results revealed that the FL-MoS₂/NCS electrode possessed a high saturated Cu²⁺ adsorption capacity (1199.63 mg g⁻¹) even at a low concentration (23.63 mg L⁻¹). In particular, the competitive experiment (Cu²⁺/Na⁺/Fe³⁺) investigation revealed its good selectivity toward capturing Cu²⁺. In the case of the ternary system, the adsorption capacity for Cu²⁺ was 1071.6 mg g⁻¹, which is much higher than that of Na⁺ (199.2 mg g⁻¹) and Fe³⁺ (262.8 mg g⁻¹). Several factors are responsible for the high selective adsorption of Cu²⁺. Firstly, the electrode/electrolyte wetting performance is improved by the excellent hydrophilicity of FL-MoS₂/NCS, resulting in an improvement in its electrosorption performance. Secondly, the migration of ions can be dramatically accelerated owing to the enlarged interlayer spacing, abundant defects and mesoporous structure, ensuring fast adsorption kinetics. Furthermore, according to the hard soft acid–base theory, the adsorption of Cu²⁺ can be facilitated by the abundant S, N active sites (soft base) matching Cu²⁺ (soft acid). Applying XPS and competitive adsorption tests, they further revealed the Cu²⁺ adsorption mechanism. As presented in Fig. 24b, the FL-MoS₂/NCS electrode provides two pathways to selectively capture Cu²⁺, electrical double layer (EDL) and complexation of Cu²⁺ with FL-MoS₂/NCS. Besides, the Cu²⁺ adsorption data conforms to the pseudo-second-order kinetic model (Fig. 24c). Fig. 24d depicts the reusability results for the FL-MoS₂/NCS electrode. A high capacity retention (94.5%) was retained after 5 cycles. Wu et al.¹⁹⁴ reported the preparation of a hollow bowl-type carbon material loaded with MoS₂ (HBC-MoS₂), which also showed good selectivity for the removal of Cu²⁺ using the CDI technique. Similarly, its excellent performance is also ascribed to the synergistic effect of EDL and complexation between MoS₂ and Cu²⁺. Nguyen et al.¹⁹⁵ prepared N-doped carbon spheres using dopamine as the carbon precursor, which could be further utilized as a support to load flower-like MoS₂ nanosheets. The obtained MoS₂@NCS possessed a large specific surface area of 82 m² g⁻¹ and interconnected meso-macroporous channels, which were beneficial for rapid ion and electron transfer. The electrochemical investigation indicated that MoS₂@NCS delivered a specific capacitance of 340 F g⁻¹, demonstrating great potential in electrosorption applications. Further CDI measurement revealed that under the optimal conditions, MoS₂@NCS delivered a superior specific electrosorption capacity of 59.9 mg g⁻¹, in which the experimental data fits well with the modified Donnan model. The durability test indicated that the MoS₂@NCS electrode showed no significant decline in desalination percentage even after 100 electrosorption-desorption cycles. Polyaniline (PANI) has similar properties to PDA, which can also be converted into N-doped carbon. Consequently, Liu et al.¹⁹⁶ integrated MoS₂ with carbonized PANI (MoS₂/CP). Surprisingly, the interlayer spacing of MoS₂ was dramatically enlarged due to the disordered entanglement between MoS₂ and CP nanosheets, resulting in improved pore structure and surface area. These features are capable of offering multiple charge transfer routes and endowing more embedding sites for storing Na⁺. When evaluated as the electrode in a CDI cell, a remarkable desalination capacity (29.14 mg g⁻¹) was delivered with a rapid desalination rate (2.9 mg g⁻¹ min⁻¹) and favorable cyclic durability. The mechanism investigation further revealed that the capacitance-controlled contribution accounted for 85.8%, whereas the diffusion-controlled contribution was 14.2%.

Fig. 24 (a) Synthesis of FL-MoS₂/NCS. (b) Schematic for the electrosorption of ions over FL-MoS₂/NCS. (c) Time-dependent ASC and (d) cycling performance of FL-MoS₂/NCS. Reproduced from ref. 193 with permission. Copyright 2020, Elsevier.

In another work, Tian et al.¹⁹⁷ reported the preparation of hollow nano-flowered MoS₂/N-doped hollow carbon spheres (MoS₂/NHCS), which showed high selectivity for the removal of Pb²⁺ from wastewater using the CDI technique. As presented in Fig. 25a, the process for the preparation of MoS₂/NCHS involves several steps. Initially, a core@shell SiO₂@PDA composite is prepared via one-pot self-assembly, accompanied by polymerization and hydrolysis. This integrated process was also reported elsewhere.^198,199 Afterward, PDA is carbonized into NC, followed removal of the SiO₂ core via chemical etching. Consequently, NHCS is obtained, which can be further utilized as a support to deposit MoS₂. Fig. 25b and c present the TEM and HRTEM images of the resulting MoS₂/NHCS composite, which possessed a hollow structure and spherical morphology. Its hollow spherical edges are coated with lamellar structures. In addition, several lattice distances of 0.265, 0.226, and 0.153 nm were observed, which are attributed to the (101), (103) and (008) crystalline plane of the MoS₂ crystal, respectively. When evaluated as a CDI electrode for the elimination of Pb²⁺, the resulting MoS₂/NHCS demonstrated an excellent removal efficiency of 96.9% under the optimal conditions. Excitingly, excellent regeneration ability was achieved in 10 ppm Pb²⁺ solution, delivering 97.7% after 20 cycles, particularly in mixed solution to selectively remove Pb²⁺. This efficient selective elimination of Pb²⁺ can be ascribed to at least two aspects. Firstly, the formation of PbS precipitation promotes the preference of removing Pb²⁺ owing to the low solubility product of PbS, which was verified by in situ Raman spectroscopy and DFT calculations. Secondly, upon receiving electrons, some S ions can be transformed into S₂²⁻, leading to the generation of PbS₂. Resorcinol formaldehyde resin can also be formed simultaneously with the hydrolysis of TEOS to generate SiO₂@RF, which can be further treated similar to SiO₂@PDA to generate hollow carbonaceous materials.^200,201 As a result, Zhang et al.²⁰² fabricated 1T-MoS₂/C hybrid microspheres. When evaluated as a CDI electrode, the resulting 1T-MoS₂/C hybrid microspheres delivered an excellent performance. The desalination capacity reached up to 48.1 mg g⁻¹ at 1.2 V. In terms of kinetics, an outstanding desalination rate of 9.3 mg g⁻¹ min⁻¹ was achieved, which is much higher than that of the 2H-MoS₂/C hybrid electrode (3.2 mg g⁻¹ min⁻¹). Besides, the energy consumption evaluation indicated not only a lower energy consumption (0.61 kW h kg⁻¹) but also the higher charge efficiency (0.91) of the 1T-MoS₂/C hybrid electrode. Furthermore, an exceptional stability performance was also obtained, preserving a high salt adsorption capacity of 41.5 mg g⁻¹ after 20 cycles. This high CDI performance can be ascribed to the superior structure of the hollow carbon microspheres, offering the confinement effect. On one hand, the presence of hollow carbon microspheres restricts the overgrowth and agglomeration of MoS₂ nanosheets. In this way, the active sites of MoS₂ can be fully exposed, leading to a high removal capacity and rate. On the other hand, the stability of the whole electrode material can be dramatically improved.

Fig. 25 (a) Synthesis of MoS₂/NHCS. (b) TEM image and (c) HRTEM images of MoS₂/NHCS. (d) Pb²⁺ removal efficiency of MoS₂/NHCS at different concentrations of Pb(NO₃)₂. (e) Removal efficiency of heavy metal ions with MoS₂/NHCS in a multicomponent solution. Reproduced from ref. 197 with permission. Copyright 2024, Elsevier. (f) Procedure for synthesizing MoS₂/NMOC composite. (g) Water contact angles of MoS₂, NMOC, and MoS₂/NMOC. (h) CDI performances of MoS₂, NMOC, and MoS₂/NMOC. Reproduced from ref. 203 with permission. Copyright 2021, Elsevier.

In another work, Tian et al.²⁰³ utilized a similar principle to prepare an MoS₂/NOMC (nitrogen-doped highly ordered mesoporous carbon) composite, as schematically shown in Fig. 25f. Owing to the introduction of NMOC, the presence of oxygen-containing groups improved the water wettability, which was evidenced by the water contact angle (Fig. 25g). The contact angles of the pristine MoS₂ and NMOC are 96.2° and 85.1°, respectively. Unexpectedly, MoS₂/NMOC produced a contact angle of 65.1°. This dramatically enhanced hydrophilicity can be ascribed to the generation of unpaired electrons through nitrogen doping, which induced the formation of some defects and disorder on the MoS₂/NOMC composite surface.²⁰⁴ The electronic conductivity investigation revealed that the introduction of NMOC also improved the conductivity of the MoS₂/NMOC composite. Owing to the above-mentioned features, the diffusion of salt ions into the MoS₂/NOMC electrode surface was dramatically improved, leading to an enhanced desalination performance. As presented in Fig. 25h, the electrosorption capacity of the MoS₂/NOMC electrodes reached 28.82 mg g⁻¹ at 1.6 V, which is superior to that of NMOC and pure MoS₂. The mechanism investigation indicated that the desalination process is mainly controlled by the capacitor process (with the contribution of 73.17%), whereas the diffusion-controlled process only contributed 26.83%.

Considering the semiconductive feature of MoS₂, it is feasible to couple CDI with photocatalysis to remove some organic pollutants from water. Li et al.²⁰⁵ designed a system for eliminating tetracycline (TC), in which MoS₂/C plays dual functions, i.e., photocatalyst and electrode, for CDI (Fig. 26a). In their work, the MoS₂/C composite was prepared via the one-pot hydrothermal treatment of fructose, Mo source and S source. Owing to the introduction of fructose-derived carbon, the conductivity of the composite was improved compared with that of pure MoS₂, and the recombination of photo-generated electron–hole pairs will also be efficiently prevented, which is beneficial for the photocatalytic degradation of TC, as presented in Fig. 26b. Experimentally, the TC removal capacity reached up to 1302.5 mg g⁻¹ and the removal efficiency was 84.8%. The kinetic investigation indicated that the system offers fast removal kinetics. Moreover, no activity loss was observed even after six cycles of photocatalytic degradation and electrosorption (Fig. 26c–e).

Fig. 26 (a) Schematic of MoS₂/C-based system coupling with CDI and photocatalysis. (b) Photocatalytic mechanism for the degradation of TC over MoS₂. (c) Removal kinetics for TC in the coupled system. (d) Initial concentration-dependent removal capacity for TC over MoS₂/C_0.5 electrode. (e) Cyclic stability of MoS₂/C_0.5 electrode for the removal of TC after six cycles. Reproduced from ref. 205 with permission. Copyright 2024, Elsevier.

3.2.2 Carbon nanotubes. Carbon nanotubes (CNTs) possess the typical 1D structure with diameters of a few nanometers. Structurally, CNTs consist of helical microtubules of graphitic carbon. The sp² hybridization endows CNTs with fascinating thermal and electrical properties, showing great promise in electrochemical applications.^206–209 The intrinsic tube cavity in CNTs offers paths for ion transfer, demonstrating a superior average salt adsorption rate. When compositing with MoS₂, the extended axial ratio and 1D morphology of CNTs increased the contact area with MoS₂ sheets. As a result, the interfacial resistance between the two may be reduced, increasing the active sites for ion adsorption. Srimuk et al.²¹⁰ fabricated MoS₂/CNT binder-free electrodes, enabling a stable desalination performance over 25 cycles in various molar concentrations, with salt adsorption capacities of 10, 13, 18, and 25 mg g⁻¹ in 5, 25, 100, and 500 mM NaCl aqueous solutions, respectively. By assembling β-CD and micelles (consisting of the nonionic surfactant P123 and sodium oleate) in a dispersion of CNTs, Cai et al.²¹¹ obtained a CNT-β-CD complex, in which a 3D architecture was constructed by the carbon nanotubes (CNT) and β-CD. The CNT-β-CD complex was further used as a substrate to load MoS₂, forming the MoS₂@CNT-β-CD composite. After carbonization treatment, the β-CD could be converted into carbon spheres (CS). Consequently, 2D MoS₂ connected with 3D carbon spheres (CS) (MoS₂@CNT-CS) was obtained, demonstrating an excellent CDI performance (Fig. 27a). Owing to the highly dispersive characteristics of MoS₂@CNT-CS, it provided sufficient effective faradaic sites for ion storage and transfer, delivering an enhanced SAC (25.35 mg g⁻¹), fast SAR (3.9 mg g⁻¹ min⁻¹), and satisfactory cycling stability. Particularly, the specific ion selectivity evaluation of the MoS₂@CNT-CS electrode revealed a selectivity order of Ca²⁺ (0.43 mmol g⁻¹) > Na⁺ (0.36 mmol g⁻¹) > Mg²⁺ (0.28 mmol g⁻¹) > K⁺ (0.19 mmol g⁻¹), as evidenced in Fig. 27b. Unexpectedly, the MoS₂@CNT-CS electrodes showed no structure change after the electrosorption of metal ions, as demonstrated by XRD (Fig. 27c). The molecular dynamic (MD) calculations further provided evidence for the selectivity order of the screened ions. As presented in Fig. 27d–g, the bond parameters indicate that the intercalation energy follows the order of Ca²⁺ (−2.55 eV) < Na (−1.56 eV) < Mg (−1.37 eV) < K (−1.00 eV).

Fig. 27 (a) Synthesis of MoS₂@CNT-CS and CDI profile. SAC (b) of CNT-CS//MoS₂@CNT-CS cell. Postmortem XRD curves (c) of MoS₂@CNT-CS electrode after the saturated electrosorption of different cations. Atomic configuration of Na atoms (d), K atoms (e), Ca atoms (f) and Mg atoms (g) between MoS₂ interlayers. Reproduced from ref. 211 with permission. Copyright 2021, Elsevier.

Due to the high electrical conductivity and excellent mechanical strength of carbon nanotubes, when they are combined with MoS₂, thy will form a three-dimensional conductive network, and at the same time, as a substrate to support MoS₂, inhibit its self-aggregation. Consequently, it enhances the electrical conductivity and mechanical properties of the composite material, and then improves the electrochemical performance of the material. However, the synthesis of CNTs is complicated, which is attributed to the high mechanical strength of CNTs, making their dispersion in solution difficult, and thus their preparation is complex and costly.

3.2.3 Graphene. Graphene is a two-dimensional sheet material with a 1-atom thickness. Owing to its conjugated structure, graphene exhibits excellent ionic conductivity, and thus has found wide application in the CDI desalination process. However, the π–π interaction between graphene sheets can cause reaggregation, reducing the available surface for ion adsorption, which becomes the major obstacle for its use in the field of CDI. Recently, the assembly of graphene onto other species, such as metal oxides and sulfides, heterogeneous atom-doped carbon, and surface charged carbon nanostructures, has been demonstrated to be an optimal strategy to improve the performance of CDI electrodes. Meanwhile, incorporating graphene into MoS₂ is capable of expanding the distance between the stacked adjacent MoS₂ layers, improving the mass transport and electron transfer. Han et al.²¹² reported the fabrication of an MoS₂–graphene hybrid electrode for CDI applications (Fig. 28a and b). In this MoS₂–graphene hybrid, graphene was employed as the substrate to support numerous MoS₂ pleats. This configuration is beneficial for increasing the specific surface area and exposing more active sites. As presented in the TEM (Fig. 28e–f) and high-resolution transmission electron microscopy (HRTEM) (Fig. 28g) images, the surface of the graphene was homogeneously covered by MoS₂ nanosheets. Owing to the relatively separate states between graphene and the MoS₂ nanosheets, the interlayer spacing in the MoS₂ nanosheets showed no obvious expansion, which is 0.68 nm (Fig. 28g), in good agreement with the (002) lattice plane of the hexagonal MoS₂. Benefiting from the above-mentioned characteristics together with conductive networks formed by graphene, the MoS₂–graphene hybrid electrode delivered a high volumetric NaCl adsorption capacity (VAC_NaCl) and high gravimetric NaCl adsorption capacity (GAC_NaCl) as a result of the high electrical conductivity of graphene and the rapid ion transport of MoS₂. In another work, Gao et al.²¹³ reported the preparation of an MoS₂/rGO composite, which can be used as intercalation electrode for CDI. Experimentally, a remarkable salt capacity of 34.20 mg g⁻¹ was delivered with a high charge efficiency of up to 97% in 300 mg L⁻¹ NaCl aqueous solution using MoS₂/rGO as the electrode (Fig. 28h–k). They also confirmed the role of the incorporated rGO in the composite. On one hand, rGO serves as a conductive support, ensuring fast electron transfer. On the other hand, the structural characterization indicated that the interlayer spacing of MoS₂ was expanded to 0.73 nm after incorporating rGO, which was only 0.62 nm without rGO. Owing to the widened MoS₂ interlayer, the diffusion of cations could be dramatically accelerated, while the internal strain decreased. In addition, rGO created a large number of available active adsorption sites and space to accommodate cations from the electrolyte.

Fig. 28 (a) Schematic of CDI device; (b) schematic of preparation of MoS₂–graphene hybrids; (c and d) SEM images of MG-1.6; (e and f) TEM images of MG-1.6; and (g) HRTEM image of MG-1.6. Reproduced from ref. 212 with permission. Copyright 2019, the American Chemical Society. (h) Plots of SAC vs desalination time in 300 mg L⁻¹ NaCl solution at 1.4 V with a flow rate of 12 mL min⁻¹. (i) Corresponding Ragone Plots. (j) SAC at different voltages in 300 mg L⁻¹ NaCl solution of the MoS₂//AC and MoS₂/rGO//AC HCDI systems. (k) SAC and charge efficiencies of MoS₂/rGO//AC at a voltage of 1.2 V with different initial NaCl concentrations. Reproduced from ref. 213 with permission. Copyright 2020, the American Chemical Society.

Liu et al.²¹⁴ developed an MoS₂/graphene oxide heterojunction (MoS₂/GO-H), demonstrating an effective electrode for CDI. As presented in Fig. 29a, a single hydrothermal process was utilized to synthesize the MoS₂/GO-H composite. The resulting MoS₂/GO-H played dual roles in the removal of UiO₂²⁺, i.e., electrosorption and electrocatalysis (Fig. 29b). The morphology observation indicated that the surface roughness of MoS₂/GO-H significantly increased compared to the original materials given that the MoS₂ layers were seamlessly woven into the GO sheets. Fig. 29c presents the HRTEM image of MoS₂/GO-H, revealing the crystalline spacing of ≈0.62 nm, which corresponds to the (002) crystal plane of MoS₂. Compared to pristine MoS₂, the introduction of GO-H resulted in a large increase in specific surface area (from 10.64 to 16.52 m² g⁻¹) and pore volume (from 0.025 to 0.124 cm³ g⁻¹), as evidenced by N₂ adsorption–desorption measurement (Fig. 29d). When evaluated as a CDI electrode for removing UiO₂²⁺ from water, MoS₂/GO-H delivered an excellent performance in terms of removal capacity, adsorption kinetics, selectivity and recycling durability (Fig. 29e–j). Under the optimal conditions, the adsorption capacity reached 805.57 mg g⁻¹ within ∼240 min with over 90% removal efficiency in the presence of numerous competing ions. Notably, MoS₂/GO-H delivered a high desorption rate of 74% after five CDI cycles. Furthermore, its adsorption capacity retention was about ≈70%, indicating its excellent recycling performance.

Fig. 29 (a) Synthesis of MoS₂/GO-H. (b) In situ electrolytic deposition with complexation strategy for UO₂²⁺. (c) HRTEM image and (d) N₂ adsorption–desorption curve of MoS₂/GO-H. (e–j) CDI performance of MoS₂/GO-H in the removal of UO₂²⁺. Reproduced from ref. 214 with permission. Copyright 2024, Wiley-VCH.

Zheng et al.²¹⁵ reported the synthesis of an rGO@PEI/MoS₂ composite rich in heterostructures. The interaction between rGO and MoS₂ to form the rich heterostructures was verified to be van der Waals forces. Owing to the rich heterostructures, the rGO@PEI/MoS₂ composite delivered a maximum specific capacitance of 212.53 F g⁻¹, which was nearly a 45% improvement compared to that of rGO/MoS₂. When evaluated as an electrode in a CDI cell, owing to the synergistic effect generated by the rich heterostructure, the rGO@PEI/MoS₂ electrode delivered a desalination capacity of about 24.13 mg g⁻¹. Peng et al.²¹⁶ prepared 3D flower-like MoS₂/rGO composites using a water/ethanol mixture as the solvent via a one-step hydrothermal method. With a water/ethanol ratio of 2 : 1 (v/v), the resulting MoS₂/rGO composite possessed a big BET surface area (29.7 m² g⁻¹), which is beneficial for its CDI performance. The desalination capacity in 200 mg L⁻¹ NaCl solution was determined to be 16.82 mg g⁻¹ at 1.0 V. Taking advantage of chemical etching, the same group²¹⁷ further fabricated MoS₂/reduced graphene oxide (rGO) composites in which MoS₂ contained defects. As presented in Fig. 30a, chemical etching was achieved based on NaBH₄ reducing MoS₂. The MoS₂ defect had a significant effect on the structure of the composite and its electrochemical and desalination performances. Taking advantage of the fewest amount of (002) interplanar layers, the defect-containing MoS₂/rGO composite (dMSG) could be obtained by introducing NaBH₄ as a reductant. Meanwhile, the number of defects was dependent on the amount of NaBH₄. In their work, the optimal amount of NaBH₄ was found to be 5.0 g, and the corresponding sample was marked as dMSG-5, which delivered the smallest water contact angle and zeta potential, the largest specific capacity (305.6 F g⁻¹ at 1.0 A g⁻¹), and the lowest interfacial charge transfer resistance. When used for the CDI cell, dMSG-5 showed the largest desalination capacity, which was determined to be 25.47 mg g⁻¹ at the voltage of 0.8 V in 200 mg L⁻¹ NaCl solution. The mechanism survey indicated that the greater number of adsorption sites for Na⁺ insertion/extraction and ion diffusion channels was responsible for the improved ion accessibility and transport efficiency, leading to an enhanced CDI performance. Besides, regeneration of the dMSG-5 electrode could be easily achieved, and its conductivity could rapidly recover, indicating the excellent performance of electrosorption-desorption in the processes of regeneration (Fig. 30b). Excitingly, the electrosorption capability of Na⁺ showed no significant loss compared to the first electrosorption (from 25.47 mg g⁻¹ to 24.79 mg g⁻¹) after the 5th cycle (Fig. 30c).

Fig. 30 (a) Schematic for the chemical etching of MSG with various dosage of NaBH₄ and the electrosorption of Na⁺ in the EDL of the dMSG composite electrodes. (b) Electrosorption-desorption cycle curves and (c) cyclic electrosorptive capacity of Na⁺ on dMSG-5 electrode. Reproduced from ref. 217 with permission. Copyright 2021, Elsevier.

The combination of graphene and MoS₂ demonstrates several benefits in boosting the CDI performance of MoS₂. On one hand, the conductivity will be improved, which is ascribed to the high conductivity of carbon materials. On the other hand, the surface functional groups of carbon materials will improve the hydrophilicity of MoS₂, resulting in an enhancement in the ion migration kinetics. In addition, the flexible adjustment of the porous structure of carbon offers further opportunities to control the ion adsorption behavior. However, a disadvantage is its poor selective adsorption capacity, which may constitute a future research direction in this area.

3.2.4 Carbon nanofibers. In the past few decades, carbon nanofibers, as a type of amorphous carbon with high porosity, high specific surface area, and excellent electric and thermal conductivity, have been regarded as the most promising conducting 1D material. Furthermore, the exposed stacking arrangements of graphene sheets and edge plane defects allow efficient electron transfer on the outer wall of CNFs. The advantages of excellent structural stability and high electrical conductivity endow carbon nanofibers with an excellent performance in supercapacitors, capacitive deionization, and oil/water separation. They can be used as conductive scaffolds and as self-supporting electrode materials. By incorporating carbon nanofibers into inorganic materials, they can act as conductive scaffolds and as self-supporting electrode materials, creating electron transfer pathways. As a result, the electrochemical performance of intercalation materials can be improved to a great extent due to the synergistic interaction between two species. Liu et al.²¹⁸ developed MoS₂ nanoflake-coated carbon nanofibers (CNFs@MoS₂) through electrospinning and subsequent hydrothermal reaction (Fig. 31a). By changing the ratio of CNFs in the composite, the interlayer spacing of the (002) and (100) crystal planes of MoS₂ could be adjusted. As presented in Fig. 31b–d, the layer spacing for CNFs@MoS₂-1, CNFs@MoS₂-2, CNFs@MoS₂-3 was measured to be 0.50/0.21, 0.51/0.24, and 0.48/0.22 nm, respectively. Owing to its dual-mode capacitive behavior, CNFs@MoS₂ delivered excellent desalination performances with a desalination capacity of 53.03 mg g⁻¹ and desalination rate of 0.157 mg g⁻¹ s⁻¹ as well as good long-term stability (11.2% reduction in 30 cycles) (Fig. 31e–g).

Fig. 31 (a) Procedure for preparing CNFs@MoS₂ composite. HRTEM images of CNFs@MoS₂-1 (b), CNFs@MoS₂-2 (c), and CNFs@MoS₂-1 (d). (e) Desalination capacities/charge efficiencies of the CNFs@MoS₂-2-based RCDI at various currents. (f) Desalination capacity of the CNFs@MoS₂-2-based RCDI system at various initial concentrations. (g) Cycling performance and charge efficiency of the CNFs@MoS₂-2-based RCDI system. Reproduced from ref. 218 with permission. Copyright 2021 Elsevier.

This section mainly introduces hybrid materials composed of molybdenum disulfide and four common carbon-based materials. It is understood that the properties of different carbon-based materials vary greatly after forming composites. AC has a high specific surface area, high porosity, and can provide abundant ion transport channels, with low cost and a wide range of raw materials. However, its poor conductivity and uneven pore size distribution result in a poor and unstable electrochemical performances by composite materials. CNTs have high electrical conductivity and mechanical strength, but they are prone to aggregation and have high synthesis costs. CNFs also have excellent conductivity, controllable structure, high mechanical strength, but low specific surface area and complex synthesis process. Graphene has ultra-high conductivity, and using GO as a substrate can prevent the suppression of MoS₂ self-stacking, expand the interlayer spacing, and expose more active sites. However, it is prone to aggregation, complex to prepare, costly, and has weak interface bonding between the two, resulting in average material stability.

3.3 Composite with conducting polymer

Besides compositing with inorganics, conducting polymers have also been reported to boost the CDI performance of MoS₂. For example, Wang et al.²¹⁹ demonstrated that the incorporation of PDA may improve the water wettability of MoS₂/PDA, leading to lower inner resistances, higher specific capacitance, enhanced electrosorption rate and desalination capacity. Chen et al.²²⁰ developed a core/shell structured PPy@MoS₂ composite, exhibiting an excellent CDI performance. As presented in Fig. 32a, the synthesis of PPy@MoS₂ started from the formation of MoO₃ nanorods under hydrothermal conditions with the assistance of HNO₃, followed by the oxidative in situ polymerization of Py to generate the MoO₃@PPy composite. It was noted that PPy was coated on the surface of MoO₃ in the MoO₃@PPy composite. Finally, to convert MoO₃@PPy into PPy@MoS₂, a further sulfurization treatment was necessary, which was carried out under hydrothermal conditions using thiourea as the sulfur source. Interestingly, MoS₂ was grown on the surface of PPy. This phenomenon was also observed in the thermal treatment of FeOOH@PPy to generate the Fe₃O₄@NC composite.²²¹ The crystalline phase investigation revealed that MoS₂ existed as a hybrid phase of both 1T and 2H phases. Fig. 32b and c present the SEM and TEM images of the resulting one-dimensional PPy@MoS₂, respectively. Obviously, the MoO₃ microrods were replaced by PPy microtubes, which further served as the substrate for growing MoS₂ nanosheets, leading to MoS₂ being tightly adhered on the PPy microtubes. Benefiting from these features, the resulting PPy@MoS₂ is capable of offering more active sites, boosting the desalination performance by reducing the surface energy. Fig. 32d and e display the CDI performances of PPy, MoS₂, and PPy@MoS₂, from which it can be concluded that PPy@MoS₂ delivered a superior performance to pristine PPy and MoS₂. In particular, a high desalination capacity of 159.6 mg g⁻¹ was achieved by PPy@MoS₂ with a high rate of 3.2 mg g⁻¹ min⁻¹. This outstanding performance can be attributed to the core–shell structure and hybrid phases, which ensure excellent charge efficiency, cycling stability, and low energy consumption (0.22 kW h kg⁻¹), demonstrating its potential for practical desalination applications.

Fig. 32 (a) Procedures for the synthesis of PPy@MoS₂. (b) SEM and (c) TEM images of PPy@MoS₂. Comparison of the desalination capacity of PPy, MoS₂, and PPy@MoS₂ at different current densities (d) and different potentials (e). Reproduced from ref. 220 with permission. Copyright 2024, Elsevier.

Shehzad et al.²²² developed orderly mesoporous composite materials consisting of carbon, PPy and MoS₂. Owing to the introduction of porous activated biocarbon (BCL) and PPy, the resulting composite exhibited the improved diffusion of hexavalent uranium ions owing to the highly conducive network, promoting electron transfer during the CDI process. Under the optimal conditions, an inspiring sorption capacity of 417.90 mg g⁻¹ was obtained at pH = 4.5 and applied voltage of −0.9 V, which is better than that of many other materials used in CDI applications having a similar geometry and morphology. N,P-co-doped templated biochar (BBC) was obtained via the pyrolysis of Bambusa vulgaris Culm, and a BBC/2D-MoS₂/PPy hybrid CDI system was assembled, which delivered a sorption capacity of 402.13 mg g⁻¹ for the selective removal of U(VI).²²³ The same group²²⁴ also reported the fabrication of an MoS₂/CNT/PPy-based composite electrode for U(VI) electrosorption, obtaining an experimental sorption capacity of 274.50 mg g⁻¹ at − 0.90 V. Fang et al.²²⁵ developed a ternary composite comprised of DBS⁻ ion-doped polypyrrole (PPy-DBS)-coated MoS₂ nanosheets, which were tightly attached to hollow carbon spheres (HCS). In this composite, the dispersion of MoS₂ nanosheets could be dramatically improved by HCS owing to the increased layer spacing. Consequently, abundant active sites were provided for ion intercalation and transfer. In addition, PPy-DBS may serve as an electron transport path, ensuring fast on migration kinetics and excellent electrical conductivity. Owing to these advantages, the specific capacitance of the resulting PPy-DBS-MoS₂@HCS reached 168.77 F g⁻¹, which is much higher than that of MoS₂ (72.9 F g⁻¹). Coupling with HCS, an asymmetric CDI cell, HCS//PPy-DBS-MoS₂@HCS, was further constructed to evaluate the CDI performance of PPy-DBS-MoS₂@HCS. An extremely high desalting capacity (53.4 mg g⁻¹) was obtained at 1.2 V in 500 mg L⁻¹ NaCl solution. This salt adsorption capacity is 2.9- and 2.4-times higher than that of HCS//MoS₂ (18.3 mg g⁻¹) and HCS//MoS₂@HCS (22.3 mg g⁻¹), respectively. Liu et al.²²⁶ developed a CNT/PPy/MoS₂ ternary composite, achieving a maximum desalination capacity of 24.8 mg g⁻¹, ultra-high deionization rate of 5.24 mg g⁻¹ min⁻¹, and superior capacity retention rate of 92.7% over 25 cycles. Zargar et al.²²⁷ reported the preparation of a Ti₃C₂T_x/1T-MoS₂/CNT (TMC) heterostructure with an interconnected network, delivering an excellent CDI performance. As presented in Fig. 33a, they initially prepared Ti₃C₂T_x/CNTs via self-assembly with the assistance of ultrasonic irradiation and stirring. Owing to the negative charge of the MXene surface, the CNTs could be easily dispersed in the solution well. Afterward, a one-pot hydrothermal procedure was carried out to deposit MoS₂ onto Ti₃C₂T_x/CNTs, forming the TMC heterostructure. The composition and structural characterization indicated that the Ti₃C₂T_x MXene possessed an accordion-like morphology and the MoS₂ material possessed a nanoflower-like morphology with 1T phase. The introduction of CNTs allowed the fabrication of a well-defined 3D-interconnected network by connecting MoS₂ petals and MXene isles. Owing to these characteristics, the TMC heterostructure was capable of constructing multiple routes for efficient electron transfer, elevating the overall electrical conductivity. When used as the electrode in a hybrid CDI cell coupled with activated carbon, a remarkable salt desalination performance was achieved, including a high salt removal capacity (25.82 mg g⁻¹), outstanding high removal rate (5.25 mg g⁻¹ min⁻¹), and superior cyclic stability (98.1% desalination capacity retention rate). It was noted that these parameters are all superior to that of single MXene and MoS₂ as well as Ti₃C₂T_x/MoS₂ (TM)-based materials, as evidenced in Fig. 33b–e.

Fig. 33 (a) Schematic of the fabrication of MXene/MoS₂/CNT composites. (b) Conductivity and (c) desalination capacity versus time, (d) CDI Ragone plots of the prepared electrodes, (e) adsorption capacities of TMC composite at different voltages (0.8, 1, and 1.2 V) and different initial NaCl concentrations. Reproduced from ref. 227 with permission. Copyright 2024, Elsevier.

In summary, integrating conducting polymers into MoS₂ nanosheets presents a promising solution for boosting their CDI performance, providing crucial benefits such as hydrophilicity, adjustable interlayer spacing, and abundant active sites. These characteristics enable the direct uptake of ionic pollutants in water and synergistic adsorption of ions from water. The enhancement mechanism of combining conducting polymers with MoS₂ can be ascribed to several factors. Initially, the conducting polymer may act as a supportive framework to inhibit the agglomeration of MoS₂ nanosheets, allowing full contact with salt solutions. This solution wettability improvement is also partly attributed to the abundant functional groups in conducting polymers. Secondly, the introduction of a conducting polymer may diminish the charge transfer resistance, which can be ascribed to the good electronic conductivity of conducting polymers. Furthermore, conducting polymers are expected to supply additional electrochemical active sites, boosting the electrosorption capacity. In addition, the interspacing of MoS₂ can be enlarged owing to the presence of a conducting polymer, which is evidenced to be beneficial for improving its CDI performance in terms of mass/charge transfer kinetics.

3.4 Composites with oxides

Many metal oxides, including hydroxides, possess a large specific surface area, good chemical stability, and huge specific capacitance value, making them promising electrode candidates in CDI systems. To date, some reports have demonstrated the potential applications of metal oxides with good adsorption properties in the area of CDI. In particular, the combination of metal oxides and 2D layer compounds is a versatile approach to enhance the electrical conductivity of electrode materials, leading to improved electrochemical and CDI performances. Yao et al.²²⁸ reported a two-step hydrothermal process to fabricate MoS₂/MnO₂ composites (Fig. 34a), which leveraged the atomic-level “pump-driving” effect. Owing to the charge redistribution catalyzing IEF, the electron/ion transfer capabilities within the MoS₂/MnO₂ heterostructure were dramatically enhanced during its application in HCDI. Consequently, a huge SRC of 33.21 mg g⁻¹ and SRR of 1.50 mg g⁻¹ min⁻¹ were obtained at an operating voltage of 1.2 V in a 500 mg L⁻¹ NaCl solution (Fig. 34b–g).

Fig. 34 (a) Synthesis of MoS₂/MnO₂ composite. (b) Variation in solution conductivity vs. time for MoS₂/MnO₂, MnO₂ and MoS₂; (c) SRC vs. time of MoS₂/MnO₂, MnO₂ and MoS₂; (d) SRR vs. time of MoS₂/MnO₂, MnO₂ and MoS₂; (e) SRC and SRR of MoS₂/MnO₂ and MnO₂ at different potentials; (f) variation in current vs. time and charge efficiency (inset: charge efficiency of MoS₂/MnO₂ and MnO₂); and (g) Kim-Yoon plots for MoS₂/MnO₂, MnO₂ and MoS₂. Reproduced from ref. 228 with permission. Copyright 2024, Elsevier.

Besides oxides, hydroxides are also versatile electrode candidates for capacitive applications. Several typical works have been reported on the combination of MoS₂ with hydroxides to boost its CDI performance. For example, Zhao et al.²²⁹ constructed an FeOOH/Pd/MoS₂ hybrid through an ingenious interfacial redox route, which delivered an outstanding desalination capacity and admirable durability. The whole preparation process for the FeOOH/Pd/MoS₂ hybrid is presented in Fig. 35a, which includes two-step spontaneous interfacial redox reactions. Initially, MoS₂ microflowers are synthesized via a conventional hydrothermal procedure, followed by anchoring Pd nanoparticles through a wet chemical spontaneous interfacial redox reaction route. The driving force for the formation of Pd nanoparticles is based on the reducing property of the DMF solvent. Finally, FeOOH is further deposited onto Pd/MoS₂, wherein FeCl₂ is utilized as the precursor. The presence of dissolved oxygen caused the oxidation of Fe(III), ensuring the formation of FeOOH. The features of the FeOOH/Pd/MoS₂ hybrid resulted in enhanced electrochemical conductivity, and superior capacitive contribution. To evaluate its CDI performance, an HCDI device was assembled using the FeOOH/Pd/MoS₂ hybrid as the anode and activated carbon as the cathode (Fig. 35b). As shown in Fig. 35c, the solution conductivity showed a decreasing trend with the working time, indicating efficient CDI capacity. Specially, the desalination capacity of the samples followed the order of MoS₂ (26.8 mg g⁻¹) < Pd/MoS₂ (33.7 mg g⁻¹) < FeOOH/Pd/MoS₂ (41.1 mg g⁻¹). Several factors affecting the performance of the FeOOH/Pd/MoS₂//AC cell were further investigated, including initial ion concentration and applied voltage. The result showed that the outlet solution conductivity produced a decreasing trend with a voltage in the range of 0.8 to 1.2 V (Fig. 35d). These phenomena can be ascribed to the high applied voltage causing a stronger electrostatic force, leading to enhanced ion accumulation and intercalation. Significantly, admirable cycling stability and reversibility were also delivered by the FeOOH/Pd/MoS₂//AC cell, as shown in Fig. 35e. In addition to single metallic hydroxides, bimetallic layered double hydroxides are also effective to form composites with MoS₂, delivering a good CDI performance. Yang et al.¹⁸ designed a composite comprised of nickel–ferric-LDH (NiFe-LDH) and MoS₂ (NiFe/MoS₂), which showed an excellent performance in CDI application for removing high concentrations of Cr(VI). A high deionization capacity of 49.71 mg g⁻¹ was achieved for 100 mg L⁻¹ Cr(VI) and the corresponding removal efficiency was determined to be 99.42%. Significantly, an increased ion selectivity for Cr(VI) in high concentrations of Cl⁻ and SO₄²⁻ was also experimentally verified by the removal mechanism investigation, which involved electrostatic attraction, reduction to low-toxicity Cr(III), and surface complexation. Unfortunately, the experimental removal capacity is still far away from the theoretical value (106.2 mg g⁻¹) and the reusability of LDH-based electrodes in CDI systems needs to be further improved.

Fig. 35 (a) Preparation of FeOOH/Pd/MoS₂ composite. (b) Schematic of the HCDI device of FeOOH/Pd/MoS₂//AC. Dynamic outlet solution conductivity/GAC_NaCl vs working time plots (c) and working time plots of FeOOH/Pd/MoS₂ (d). (e) Cycling performance. Reproduced from ref. 229 with permission. Copyright 2023, Elsevier.

To improve the CDI performance, some ternary composites have also been developed in recent years.²³⁰ For example, Huang et al.²³¹ introduced graphene felt (GF) into an MoS₂/TiO₂-based electrode. In this way, both the wettability and electrochemical characteristics demonstrated an obvious enhancement, leading to a high CDI efficiency and long-term stability. Fig. 36a shows the procedures involved in the preparation of the MoS₂/TiO₂/GF electrodes. An H₃BO₃ solution was used to fully penetrate the GF, followed by the deposition of TiO₂. Interestingly, the water contact angle measurement indicated a dramatic improvement in the wettability. The pristine GF produced a contact angle of 139.8°, which was much larger than that of TiO₂/GF (0°). Finally, MoS₂ was further deposited onto TiO₂/GF. In this way, the aggregation of MoS₂ could be effectively prevented, leading to excellent cycling stability for electrochemical applications. Experimentally, a specific capacitance value of 257.9 F g⁻¹ was obtained for the MoS₂/TiO₂/GF electrode. In the CDI test, it delivered a desalination efficiency of 34.3% and desalination capacity of 40.96 mg g⁻¹ (Fig. 36b and c). After 50 repeated desalination tests, a desalination retention rate of 92.7% was obtained (Fig. 36d).

Fig. 36 (a) Synthesis of MoS₂/TiO₂/GF composite. Na⁺ concentration as a function of time for MoS₂/TiO₂/GF electrode in CDI system with (b) different applied voltages and (c) different NaCl concentrations. (d) SAC and change in conductivity of the synthesized MoS₂/TiO₂/GF electrodes. Reproduced from ref. 231 with permission. Copyright 2024, Elsevier.

3.5 Composites with other materials

In addition to the above-discussed MoS₂-based composites, there are also other functional materials capable of forming composites with MoS₂ to deliver boosted CDI performances. As a two-dimensional layered material, transition-metal carbides/nitrides (MXene) are the most promising. MXenes also possess a layered structure and their interlayer interactions are mainly non-covalent bonds, providing great opportunities to modify their properties.^232–234 Cai et al.²³⁵ designed a 3D MoS₂@MXene composite, delivering an excellent CDI performance. As shown in Fig. 37a, the process for the preparation of the MoS₂@MXene composite involves two steps. The Ti₃C₂T_x MXene is obtained by traditional LiF + HCl etching, followed by sonication delamination. After that, MoS₂ is deposited onto the surface or the interlayer of MXene to generate the 3D MoS₂@MXene composite, which is achieved through a hydrothermal procedure. This 3D MoS₂@MXene composite exhibited several benefits. On one hand, the morphology observation and structural identification indicated that a hierarchical 3D structure is formed by MoS₂ nanosheets vertically and uniformly anchored on the MXene matrix. This structure is capable of effectively preventing the self-aggregation of MXene flakes and MoS₂ nanosheets. On the other hand, more intercalation sites and good electrical conductivity can be offered by the few-layered MXene flakes, ensuring an excellent electrochemical performance. In addition, the water contact angle of MoS₂ was determined to be 98.5°, indicating a slightly hydrophobic characteristic. By forming a composite with MXene, the contact angle decreased to 74.4°, showing a hydrophilic feature. Benefiting from these advantages, the heterostructure MoS₂@MXene electrode delivered a superior desalination efficiency and charge efficiency compared to the pristine MXene and MoS₂ (Fig. 37b). In another work, Chen et al.²³⁶ reported a similar process for the synthesis of an MoS₂/MXene composite, which demonstrated a superior CDI performance over many reported similar MoS₂ based materials.

Fig. 37 (a) Procedures for synthesizing MoS₂@MXene heterostructure. (b) Desalination capacities and charge efficiencies in different initial concentrations of feed solution. Reproduced from ref. 235 with permission. Copyright 2022, the American Chemical Society.

Considering that both MoS₂ and MXene have good CDI desalination performances and strong photocatalysis degradation capacity, it is possible to construct a system integrating CDI and photocatalysis for simultaneously removing ions and organic pollutants from water. Qie et al.²³⁷ developed a coupled device, wherein MoS₂/Ti₃C₂T_x was loaded on a carbon fiber surface (CF/MoS₂/Ti₃C₂T_x) to serve as a penetrating electrode (Fig. 38a). Owing to the good combability of layered MoS₂ and MXene, the interfacial charge of the lamellar Ti₃C₂T_x could be well modulated. By introducing a photocatalytic component, this system was capable of simultaneously removing ions and degrading organic pollutants in water. Fig. 38b presents the practical operation process in wastewater treatment, which can be divided into several stages. In stage I, a significant decrease in the conductivity (from 32317.6 to 1212.35 μS cm⁻¹ within 10 s) was observed, indicating an efficient desalination process. In the initial stage, owing to barely all inorganic ions passing through the CDI device, an extremely strong capability of eliminating salt in water will be achieved. Then, with the desalination processing, an increasing trend in the conductivity was observed, which can be attributed to the decrease in DC (stage II). By further prolonging the process, the conductivity of the effluent was observed to maintain almost the same as that of the inflow (stage III). This result can be ascribed to the saturation of DC. To realize the desorption of ions for use in a second cycle, the discharging process is necessary, which was achieved by applying a reverse potential (−0.6 V). In this way, the electrode could completely release all the intercalated ions (stage IV). This release step lasted until the system reached the final equilibrium state (stage V). For practical application, the ability of the coupled system was evaluated to treat wastewater, achieving a yield of 55%. This efficiency was slightly lower than that of the membrane separation technique (near 60%). With an increase in the treatment cycles, the single CF/MoS₂/Ti₃C₂T_x delivered a decreased ion removal capacitance from 49.64 mg g⁻¹ (the first cycle) to 43.73 mg g⁻¹ (the 144th cycle). Furthermore, the corresponding charge efficiency also showed a gradual decreasing trend, from 90.32% (first cycle) to 81.92% (144th cycle) (Fig. 38c). Compared to the simulated experimental results, both the ion removal capacitance and the charge efficiency slightly decreased, which can be attributed to the competition from coexisting ions. Excitingly, this system also enabled the excellent photocatalytic degradation of organic pollutants, which was evidenced by the dramatic decrease in COD, as shown in Fig. 38d. Furthermore, they evidenced that that the hydration radius of ions has a stronger effect on the desalination performance than the ionic valence state. The mutual interference in the CDI desalination and photocatalysis degradation processes was weakened, broadening the application scope of CDI technology.

Fig. 38 (a) Schematic of the CF/MoS₂/Ti₃C₂T_x-based four-cell PMCDI device. (b) Conductivity and potential variations in the system during the 72nd cycle. (c) Variations in the ion removal capacity and the charge efficiency during 144 cycles (36 h operation). (d) Comparison of the water quality indexes between the original leachate and treated solution. The removal efficiencies are shown in the inset table. Reproduced from ref. 237 with permission. Copyright 2022, the American Chemical Society.

C₃N₄ is another representative 2D nanomaterial, showing great promise in energy and environment applications. Similar to MXenes, C₃N₄ is also capable of loading MoS₂ to improve the CDI performance. Tian et al.²³⁸ fabricated an MoS₂/g-C₃N₄ composite via a sequential process. As presented in Fig. 39a, they initially prepared C₃N₄ via the conventional thermal treatment of urea. Afterward, C₃N₄ was utilized as the substrate to grow MoS₂, which was achieved through a hydrothermal process, wherein thiourea served as the sulfur source. The resulting MoS₂/g-C₃N₄ composite exhibited a looser microstructure, in which g-C₃N₄ nanosheets were coated by MoS₂ nanoflowers without obvious agglomeration. Furthermore, the N₂ adsorption–desorption measurement revealed the porous structure of the MoS₂/g-C₃N₄ composite. Consequently, numerous channels will be provided for electrolyte transport. The electrochemical investigation indicated that a large specific capacitance of 118.3 F g⁻¹ was delivered by the MoS₂/g-C₃N₄ composite electrode at 1 A g⁻¹. When the current density increased to 10 A g⁻¹, the capacitance remained at 61.7 F g⁻¹, demonstrating a remarkable rate capability. Benefiting from these characteristics, an excellent CDI efficiency could be obtained when using the MoS₂/g-C₃N₄-based electrode. As presented in Fig. 39b, the conductivity of the MoS₂/g-C₃N₄ composite-based CDI cell showed a dramatic decreasing trend from the initial to saturation state, ranging from 500 to 372.67 μS cm⁻¹. This result indicates that the MoS₂/g-C₃N₄ composite-based electrode delivered an excellent CDI performance. By contrast, the pristine g-C₃N₄ and MoS₂-based electrodes showed a slight decline in conductivity, ranging from 500 to 467.15 and 421.46 μS cm⁻¹, respectively. According to the decrease in conductivity, the SAC values were calculated to be 6.25, 14.91, and 24.18 mg g⁻¹ for the g-C₃N₄, MoS₂, and MoS₂/g-C₃N₄ composite-based electrodes, respectively. The enhanced CDI performance of the MoS₂/g-C₃N₄ composite-based electrode can be attributed to the intimate contact between MoS₂ and the 2D g-C₃N₄ nanosheets and its controllable porous nanostructure. The stability tests of the MoS₂/g-C₃N₄ composite electrode demonstrated its good stability in consecutive electrosorption–desorption cycling (Fig. 39c).

Fig. 39 (a) Procedure for synthesizing MoS₂/g-C₃N₄ composite. (b) CDI performance comparison of g-C₃N₄, MoS₂, and MoS₂/g-C₃N₄-based electrodes. (c) Cycling stability of MoS₂/g-C₃N₄-based electrode. Reproduced from ref. 238 with permission. Copyright 2020, Elsevier.

The improvement in CDI performance by the combination of MoS₂ and g-C₃N₄ can be ascribed to several factors. On one hand, g-C₃N₄ possesses a rich nitrogen content, unique electronic structure, chemical and thermal stability, and environmentally acceptable character, capable of providing multifunctional properties in catalysis and energy conversion. However, its low intrinsic electronic conductivity is the main limitation in its electrochemical application. Thus, the integration of MoS₂ and g-C₃N₄ may bring several benefits. On one hand, their electrochemical properties can be significantly improved, originating from the synergistic effects between these two components. On the other hand, the re-stacking of MoS₂ can be dramatically prevented, improving its stability.

4 Conclusion and perspectives

The intrinsic features such as straightforward operation, low energy consumption, and no secondary pollution of CDI makes it a promising method for decreasing the ionic concentration of salty water and heavy metal wastewater treatment. MoS₂ is a fascinating 2D layered material exhibiting various physicochemical properties, making it suitable for designing high-performance electrochemical devices, particularly CDI devices. In this review, we summarized and discussed different techniques for the synthesis of MoS₂, focusing on the research developments for CDI application. Many MoS₂-based CDI devices have been investigated for brackish water desalination and wastewater deionization but issues still exist, which can be outlined as follows:

(1) Safe and efficient methods need to be developed for the large-scale preparation of MoS₂. Currently, the most used top-down method for the preparation of MoS₂ involves various exfoliation processes, which suffer from low efficiency and poor control of the thickness and number of layers of MoS₂ nanoflakes (or nanosheets). In the case of bottom-up methods (such as chemical vapor deposition, colloidal synthesis, hydrothermal/solvothermal synthesis, and salt-assisted synthesis), they involve the use of high-toxicity precursors, such as KSCN and metalorganic compounds, or generate harmful waste, such as H₂S and sulfur oxides. In addition, some of them consume plenty of energy or solvents. Consequently, other processes using less hazardous etchants that are environmentally friendly and energy efficient should be developed.

(2) More advanced electrode materials are urgently needed. According to the studies discussed in Section 3, one can easily find that most of the current works are focused on developing MoS₂/carbon composites for gaining high-performance CDI cells, in addition to adjusting the structures/defects in pristine MoS₂. In comparison, there are much less works on composite comprised of MoS₂ and other species (conducting polymers, other inorganic materials, MXenes, etc.).

(3) Ion selectivity should be considered. Considering sustainable development, both brackish water and ion-containing wastewater have environmental/resource impacts, and thus scientists are actively developing advanced CDI electrode materials for capturing target ions. In the case of MoS₂-based CDI cells, more effort should be devoted to modifying MoS₂-based CDI electrodes, through designing particular micro/nanostructures, functionalizing specific groups or compositing with other species to improve the CDI selectivity towards the target ions and boost their efficiency in water treatment applications.

(4) To realize the above-mentioned goals, the incorporation of current advanced technologies such as artificial intelligence (AI), big data analysis and machine learning into CDI systems to design and guide the preparation of MoS₂-based electrodes and fully realize the selective elimination of ions from water. AI-driven CDI is expected to make positive contributions to addressing the global fresh water shortage and pollution challenges by dynamically optimizing the ion selectivity in real-time and adapting to changing ion concentrations and water compositions.

For future specific studies, there are at least three directions, as follows:

(1) Phase engineering. More attention should be paid to controlling the phase structure of MoS₂ and tuning its properties. It is well known that the 1T phase exhibits superior electronic conductivity but inferior stability compared to the 2H phase. Thus, it is necessary to balance these two aspects, which requires scientists to devote more effort to the mechanism of synthesis processes.

(2) Electrode–electrolyte interfaces. The interfaces between the electrode and electrolyte play a significant role in determining the overall performance of electrochemical devices. However, to date, direct techniques to reveal the processes occurring at the interfaces are still lacking. The accumulation of ions at the electrode/electrolyte interfaces may be affected by many factors, such as the wettability of the electrode and the ionic migration characteristics in the system. Thus, to achieve this goal, more in situ characterization techniques should be developed in the future.

(3) The combination of CDI and other techniques is necessary. It is well known that in real wastewater and seawater, various species are simultaneously present besides charged ions, such as organic pollutants. Thus, to obtain high-quality fresh water, simultaneously removing these contaminants by incorporating photocatalytic function into CDI cells is preferred. Expectedly, more and more functions will be integrated into MXene-based CDI systems in the future.

In conclusion, benefiting from the advantages of MoS₂ including unique layered structures and ease of modification, more and more MoS₂-based advanced materials with unique structures and multifunction can be developed. Therefore, it is expected that that these materials will dramatically promote the development of CDI devices.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Conflicts of interest

The authors declare no competing interest.

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