2D Transition Metal Carbo‐Chalcogenides: An Emerging Family of Layered Materials

Abstract

Transition metal carbo‐chalcogenides (TMCCs) represent an emerging class of 2D materials that combine the structural and functional features of 2D transition‐metal carbides/nitrides (MXenes) and transition‐metal dichalcogenides (TMDs). TMCCs exhibit remarkable potential in functional applications such as energy storage and catalysis and are expected to become a new research focus within the family of 2D materials. At present, research on TMCCs is still in its infancy. Their compositional diversity remains to be further expanded, and their functional properties have yet to be fully explored. The relatively high synthesis difficulty is one of the key factors limiting the development of this class of materials. This work provides a systematic overview of the development of TMCC materials, summarizing recent advances in their structural characteristics, synthesis methods, property investigations, and potential applications. In the future, breakthroughs in the fabrication of high‐quality, large‐area samples are expected to offer new opportunities for the further development of TMCCs.

Transition metal carbo‐chalcogenides (TMCCs) integrate MXene‐like conductive frameworks and transition‐metal dichalcogenides (TMD)‐like chalcogen terminations, offering great potential for energy storage, electrocatalysis, EMI shielding, and solid lubrication. This review systematically summarizes recent advances in their synthesis, structure, and multifunctional applications.

1. Introduction

Since the discovery of graphene, 2D materials have rapidly attracted widespread attention and become a major frontier in materials science. When a bulk material is exfoliated into a single layer, its electronic structure, mechanical properties, and surface chemistry can undergo significant changes, offering new opportunities for fundamental physics as well as the design of advanced functional materials [1, 2, 3, 4]. As shown in Figure 1, following graphene, a variety of 2D materials with distinct chemical bonding characteristics and crystal structures have been discovered. Representative examples include layered transition metal dichalcogenides (TMDs) and layered transition metal carbides/nitrides (MXenes). TMDs possess rich chemical compositions (e.g., MoS₂, WS₂) and exhibit outstanding performance in optoelectronics, photocatalysis and electrocatalysis (such as the hydrogen evolution reaction and photoelectrocatalytic processes), as well as in tribology [10, 11, 12, 13, 14, 15, 16]. MXenes, on the other hand, feature metallic conductivity and tunable surface chemistry, enabling broad applications in electrochemical energy storage, sensing, electromagnetic shielding/absorption, and catalysis [17, 18, 19, 20, 21, 22]. In addition, other 2D materials such as hexagonal boron nitride and phosphorene have also been discovered and extensively investigated [23, 24, 25, 26]. The structure of transition metal carbo–chalcogenides (TMCCs) can be regarded as an integration of an MXene‐like transition metal–carbon framework and TMD‐like sulfur/selenium terminal features, endowing them with both intrinsic electrical conductivity and enhanced structural stability, thereby achieving a synergistic integration of electron transport and structural integrity within a single system.

FIGURE 1.

Key milestones in the development of 2D materials [5, 6, 7, 8, 9]. Reproduced with permission [7]. Copyright 2004, American Association for the Advancement of Science. Reproduced with permission [8]. Copyright 2011, Wiley. Reproduced with permission [9]. Copyright 2022, Wiley. Reproduced with permission [23]. Copyright 2014, Springer Nature.

TMCCs discussed in this work can be traced back to 1970, when H. Boller et al. [5], during their investigation of structural similarities between alloy phases and nonmetallic compounds, reported a carbide with a layered crystal structure, Ta₂S₂C. Structurally, this compound can be regarded as a stacked composite of TaS₂ and TaC layers, exhibiting similarities to the earlier discovered H‐phases (e.g., Nb₂SC, Ti₂AlC). In 1977, J. Rouxel et al. [27] found that alkali metals could be intercalated into the van der Waals gaps of Ta₂S₂C to form stable compounds M _x Ta₂S₂C (M = Li, Na, K, Rb, Cs; 0 < x < 1), demonstrating the chemical tunability of the interlayer region. Between 1992 and 1994, H. Boller and coworkers [28] further expanded the elemental diversity of this family by synthesizing Nb₂S₂C and Ta₂Se₂C. In 2022, Michael Naguib et al. [9] formally unified this class of transition metal carbo‐chalcogenides under the name “Transition Metal Carbo‐Chalcogenides” (TMCC). They conducted electrochemical Li⁺ intercalation followed by ultrasonic exfoliation to obtain 2D Nb₂S₂C and Ta₂S₂C nanosheets. Subsequent investigation of their physical and chemical properties revealed that 2D Nb₂S₂C exhibits a superconducting transition at 7.55 K, which attracted increasing scientific interest in these materials. Tao Hu et al. [29] systematically studied the stability of TMCCs using density functional theory and identified six stable compositions (Ti₂S₂C, V₂S₂C, Zr₂S₂C, Nb₂S₂C, Hf₂S₂C, Ta₂S₂C) from thirty possible combinations. Among them, Ti₂S₂C and Zr₂S₂C were predicted to be promising candidates for lithium/sodium‐ion battery anodes and supercapacitors. Notably, not all theoretically predicted stable TMCCs have yet been experimentally synthesized. To date, only Ta₂S₂C, Nb₂S₂C, and the selenium analogs Ta₂Se₂C and Nb₂Se₂C have been successfully produced. These developments indicate that TMCCs, as a new class of 2D materials combining characteristics of both MXenes and TMDs, hold great potential for diverse functional applications. Meanwhile, their compositional space remains far from fully explored, warranting further investigation.

2. Structure and Properties of TMCCs

2.1. Structure of TMCCs

As illustrated in Figure 2, the general molecular formula of TMCCs is TM₂C₂C, where TM denotes a transition‐metal element, the first C represents a chalcogen element, and the second C corresponds to carbon. TMCCs crystallize in a hexagonal lattice with space groups P3m1 or R‐3m, adopting a characteristic five‐layer stacking sequence of S–TM–C–TM–S as the basic structural unit [30, 31, 32]. To date, only TMCCs containing Nb and Ta (e.g., Ta₂S₂C, Nb₂Se₂C) have been experimentally synthesized. Although compounds such as Zr₂S₂C and Hf₂S₂C have been theoretically predicted to be stable, their synthesis has not yet been achieved. In addition, several studies have indicated that replacing the carbon atom with other elements can also yield analogous layered structures. For example, Yeoh et al. [33] theoretically predicted the structural stability of Ta₂S₂Si, Ta₂Se₂Si, and Nb₂Se₂Si via first‐principles calculations in 2024. Philipp et al. [34, 35, 36] further reported the synthesis of Zr₂Se₂P, Zr₂Te₂P, and Ti₂Te₂P, in which the carbon site is replaced by phosphorus. If these structurally analogous compounds are classified together with TMCCs, the family of TMCC‐related materials can be significantly expanded.

FIGURE 2.

Atomic structure schematic and elemental distribution of TMCC (the molecular formula is TM₂C₂C): The structure consists of a transition metal–carbon (TM–C) framework sandwiched by chalcogen (S/Se/Te) atomic layers, forming a layered 2D configuration. Transition metal atoms occupy the octahedral coordination sites, strongly bonded with carbon atoms to construct a robust metallic backbone, while chalcogen atoms are located at the outer surfaces, serving as terminal layers. Among these, the blue represents transition metal elements, gray‐green represents carbon and nitrogen group elements, and orange represents chalcogen elements. The shaded elements indicate those that have been experimentally synthesized.

It is worth noting that the TMCC structure shown in Figure 2 exhibits certain similarities to MXenes derived from 211‐type MAX phases, such as Ti₂CCl₂. Consequently, some researchers classify TMCCs as MXenes, referring to Nb₂Se₂C, for example, as a Se‐terminated MXene. However, we argue that the distinctions between MXenes and TMCCs are sufficient to categorize them as two separate classes of materials. From a structural perspective, MXenes are produced by selectively etching the A‐layer from MAX phases, thereby inheriting the twinned stacking configuration of the carbide layers in the parent MAX structure. As a result, the smallest structural periodicity of MXenes consists of two mirror‐symmetric non‐van der Waals sublayers. In contrast, each non‐van der Waals sublayer in TMCCs displays an identical atomic arrangement without twinning. Evidently, atomic stacking configurations have a profound impact on the intrinsic properties of layered materials [22, 37, 38, 39, 40]. The distinct stacking sequences of TMCCs and MXenes are therefore expected to lead to differences in their physical properties—an aspect that has not yet been systematically investigated and warrants further study. From an energetic standpoint, TMCCs are synthesized via high‐temperature thermal equilibrium reactions (e.g., the synthesis of Ta₂S₂C requires temperatures up to 1200 °C), indicating that they are thermodynamically stable phases with good structural robustness. In contrast, MXenes are typically obtained by chemically etching the A‐site elements from MAX phases [20], making them thermodynamically metastable. This metastability results in poor environmental stability: MXenes are prone to oxidation at room temperature and often undergo terminal‐group desorption under elevated temperatures.

2.2. Properties of TMCCs

Current investigations on the intrinsic physical properties of TMCC materials are predominantly based on theoretical calculations. Tao Hu et al. [29] demonstrated that all structurally stable TMCCs (Ti₂S₂C, Zr₂S₂C, Nb₂S₂C, Hf₂S₂C, and Ta₂S₂C) exhibit metallic behavior. The electronic states near the Fermi level mainly originate from the strong bonding interactions between the d orbitals of the transition metal atoms and the p orbitals of C/S atoms. Band‐structure calculations further reveal pronounced anisotropy in electronic transport, with in‐plane carrier mobility significantly higher than that along the interlayer direction [41]. Prarena Jamwal [42], Masatsugu Suzuki [43], and Ahmad Majed et al. [9] also showed, through theoretical and experimental investigations, that Ta₂S₂C, Zr₂S₂C, and Nb₂S₂C display low‐temperature diamagnetism, suggesting the possible presence of superconductivity.

Benefiting from their van der Waals layered architecture [44, 45], the interlayer spacing of TMCCs can host a variety of intercalated ions to tailor their physical properties. Taking Ta₂S₂C as an example [46], a series of intercalation compounds can be formed, such as Fe_0.33Ta₂S₂C, Co_0.33Ta₂S₂C, Ni_0.25Ta₂S₂C, and Cu_0.60Ta₂S₂C. Among them, Fe_0.33Ta₂S₂C undergoes an antiferromagnetic transition at 117 K and exhibits characteristics of a quasi‐2D XY‐type antiferromagnet on a triangular lattice; Co_0.33Ta₂S₂C experiences an antiferromagnetic transition at 18.6 K and behaves as a quasi‐2D Ising‐type antiferromagnet; both Ni_0.25Ta₂S₂C and Cu_0.60Ta₂S₂C are superconductors, with T_cu = 8.7 K for Ni and 6.4 K for Cu, while all systems share a common superconducting transition temperature of T_Cl = 4.6 K.

In terms of mechanical properties, TMCCs—similar to other layered crystalline materials—exhibit strong anisotropy: their elastic response is weakest along the c axis and strongest within the ab plane, with relatively low shear moduli. Theoretical calculations by Hu Tao et al. [29] indicate that the bulk moduli of Ti₂S₂C, Zr₂S₂C, Nb₂S₂C, and Ta₂S₂C range from 71–95 GPa, Young’s moduli from 95–160 GPa, and shear moduli only 37–‍66 GPa, reflecting weak interlayer bonding and ease of exfoliation. According to the Pugh ratio, all materials except V₂S₂C (1.45, brittle) exceed 1.75, classifying them as ductile. The combination of intralayer rigidity and interlayer softness not only facilitates mechanical exfoliation and flexible‐device applications but also helps mitigate volume expansion and maintain structural integrity during electrochemical cycling [47, 48].

3. Synthesis and Exfoliation of TMCCs

The synthesis of TMCC materials is typically achieved through high‐temperature solid‐state reactions, representing a thermodynamically controlled synthetic route. In 1970, H. Boller et al. obtained highly pure and stable Ta₂S₂C and Ta₂Se₂C phases by mixing TaS₂/TaSe₂ with carbon powder in stoichiometric ratios and heating the mixture to 1150 °C [5, 28, 49]. In 1989, Robin P. et al. reported that Ta₂S₂C cannot be directly synthesized from elemental powders, as the dominant intermediates formed during the bottom‐up synthesis of TMCCs are transition‐metal sulfides and carbides. These phases are strongly bonded ionic solids, making it difficult for them to transform directly into the TMCC structure [50]. They further discovered that introducing trace amounts (~0.01 wt%) of C₆Cl₆, CCl₄, C₂Br₆, or I₂ enabled successful synthesis of Ta₂S₂C at 1200 °C. This is because halogens, as strong oxidizing agents, promote the formation of crucial transition‐metal halide intermediates (e.g., TaCl₅ and TaBr₅), which stabilize the metal in high oxidation states and thus facilitate electron transfer within the reaction system. Xin Pang et al. [51] synthesized Nb₂Se₂C at 1200 °C by adding a small amount of C₆Cl₆, further confirming the essential role of halogens in the synthesis of TMCCs. Collectively, these studies demonstrate that generating high‐valence transition‐metal intermediates during the reaction is key to the successful synthesis of TMCC materials.

Due to the volatilization of sulfur or selenium during the synthesis of TMCCs from elemental powders, strict control of the reaction environment is required. Consequently, alternative synthetic routes have also been explored. Ahmad Majed et al. [9] proposed a strategy in which sulfur is first stabilized using specific metal chalcogenides, followed by high‐temperature synthesis. For example, Fe_0.33Ta₂S₂C was obtained by reacting FeS, Ta, and TaC at 1550 °C, while Fe _x Ta₂Se₂C was synthesized from FeSe, Ta, and TaC at 1200 °C. Subsequent acid etching with sulfuric acid effectively removed Fe from these intercalated products, yielding the corresponding TMCC materials [52]. Notably, this approach also significantly shortens the reaction time.

Given the structural similarity between TMCCs and MAX phases (H‐phases), TMCCs can also be synthesized via transformation from MAX precursors. H. Boller et al. [28] reported that covalent MAX phase Nb₂SC reacts with CuS at 1200 °C to yield Cu_0.6Nb₂S₂C. Subsequent acid etching to remove Cu produced Nb₂S₂C. Notably, their work indicated that Nb₂S₂C is a thermodynamically metastable phase that cannot be obtained through direct synthesis, demonstrating that such a topochemical approach provides a viable pathway for accessing metastable TMCC materials. Ziqian Li et al. [36] used the MAX phase Zr₂SeB as a precursor and first obtained the unconventional MAX phase Zr₂SeP through atomic substitution. They then reacted CuSe with Zr₂SeP to form CuZr₂Se₂P, and removal of the intercalated Cu ultimately yielded Zr₂Se₂P (Figure 3). Although referred to as TMXC (where X denotes a pnictogen), Zr₂Se₂P is structurally identical to TMCCs, with the only difference being the substitution of C by P. Thus, Zr₂Se₂P can be regarded as a new member of the TMCC family. It is also noteworthy that after the reaction between Zr₂SeP and CuSe, the twin structure characteristic of MAX phases disappears, and the atomic arrangement no longer exhibits mirror‐symmetric stacking. This observation further supports the feasibility of synthesizing TMCC materials via structural transformation from MAX‐like precursors.

FIGURE 3.

Structural illustration showing the transformation from MAX phases to TMXC [36]. (a) Schematic illustration showing the transformation of Zr₂SeB—Zr₂SeP—CuZr₂Se₂P—Zr₂Se₂P. (b) The STEM image showing atomic arrangements of Zr₂SeP viewed along the [110] zone axis. (c) The STEM image showing atomic arrangements of CuZr₂Se₂P viewed along the [110] zone axis. (d) The STEM image showing atomic arrangements of Zr₂Se₂P viewed along the [110] zone axis. Reproduced with permission [36]. Copyright 2025, Springer Nature.

When a van der Waals solid is exfoliated into its 2D form, its physicochemical properties (such as electrical behavior [9] and catalytic activity [52], as illustrated in Figure 4) undergo pronounced changes. This makes the exfoliation of TMCCs particularly significant. Common exfoliation techniques include mechanical exfoliation [2], liquid‐phase exfoliation [53], chemical intercalation–assisted exfoliation [9, 52], and more recently developed solid‐state physicochemical exfoliation methods [54]. Ahmad Majed and coworkers [9, 52] demonstrated that TMCC powders (e.g., Nb₂S₂C, Ta₂S₂C, Ta₂Se₂C) undergo a 100%–114% expansion in interlayer spacing after electrochemical Li intercalation. Subsequent hydration‐assisted ultrasonication enables effective exfoliation, yielding stable dispersions of 2D nanosheets [9, 52]. It can be observed that the exfoliated Nb₂S₂C exhibits superconducting behavior (Figure 4a). During rate capability measurements at current densities ranging from 20 to 1000 mA·g⁻¹, the exfoliated Nb₂S₂C consistently delivers higher specific capacities and superior rate stability compared with multilayer Nb₂S₂C (Figure 4b), indicating faster charge–transfer kinetics and a more stable energy‐storage structure. Across multiple test current densities, the nanosheets exhibit capacities approximately three times higher than those of the bulk powder samples. Ziqian Li et al. [36] transformed the synthesized CuZr₂Se₂C into LiZr₂Se₂P and KZr₂Se₂P, followed by exfoliation into Zr₂Se₂P nanosheets through soaking in 0.05 mol/L HCl combined with ultrasonication. Nevertheless, the exfoliation efficiency of TMCCs remains limited (typically below 50%), and the resulting products often display a broad thickness distribution, with some compositions difficult to fully isolate as monolayers [9, 36]. Therefore, developing more efficient and controllable exfoliation strategies is of great scientific significance and practical value for achieving well‐defined 2D TMCCs and further optimizing their performance.

FIGURE 4.

Performance enhancement induced by exfoliation [9, 52]. (a) Temperature‐dependent resistivity for the cold‐pressed m‐Nb₂S₂C [9]. Reproduced with permission [9]. Copyright 2022, Wiley. (b) Rate performance of multilayer (m) and few‐layer (d) Nb₂S₂C under different current densities [9]. Reproduced with permission [9]. Copyright 2022, Wiley. (c,d) Comparison of hydrogen evolution overpotentials and Tafel polarization curves between multilayer (m) and few‐layer (2D) Ta₂Se₂C [52]. Reproduced with permission [52]. Copyright 2025, American Chemical Society.

4. Applications of TMCC Materials

TMCCs structurally combine the key features of both TMDs and MXenes. Their crystal framework contains the easily sheared chalcogenide layers characteristic of TMDs, while also incorporating the metallic carbide backbone typical of MXenes, thereby endowing TMCCs with high electrical conductivity, excellent chemical stability, and tunable interlayer chemistry. Consequently, the application landscape of TMCCs resembles that of TMDs and MXenes, spanning a broad range of fields, including energy storage [55, 56], electrocatalysis [34], gas sensing [57, 58], tribology [59, 60], and optoelectronic devices [61].

In the field of energy storage, Yu et al. [55] theoretically demonstrated that monolayer Nb₂S₂C exhibits excellent metallicity and structural stability, together with strong adsorption capability toward Li, Na, K, and Mg ions, yielding theoretical capacities of 194.36, 348.20, 157.60, and 690.52 mAh·g⁻¹, respectively. The ion diffusion barriers are remarkably low (Li: 0.23 eV; Na: 0.11 eV; K: 0.07 eV). Similarly, Maokun Wu et al. [56] calculated that monolayer Ta₂S₂C possesses an ultralow Na⁺ diffusion barrier of only 0.09 eV and a theoretical capacity of 367.23 mAh·g⁻¹, suggesting its distinct advantages for sodium‐ion batteries. Nb₂Se₂C synthesized via a one‐step gas‐phase reaction by Xin Pang et al. [62] exhibited high electrical conductivity (7.14 × 10⁴ S·m⁻¹) and carrier concentration (1.85 × 10²¹ cm⁻³). When applied as an anode material for lithium‐ion batteries, Nb₂Se₂C delivers outstanding fast‐charging performance, with a high rate capacity of 270 mAh·g⁻¹ at 10 C and an ultralong cycle life exceeding 6500 cycles. Extending this work, Xin Pang et al. further developed Nb₂Se₂C‐based flexible electrodes. As shown in Figure 5a, compared with Nb₂CT _x MXene, Nb₂Se₂C exhibits a lower reduction peak at 1.75 V, which facilitates enhanced lithiation capacity. Figure 5b demonstrates that the Nb₂Se₂C electrode maintains a reversible capacity of 80 mAh·g⁻¹ after 2000 cycles at 1 A·g⁻¹. Moreover, the flexible electrode retains stable conductivity after 10,000 bending cycles, highlighting its potential for use in flexible electronic devices [51]. Recently, in situ/operando characterization techniques have been widely applied to investigate electrochemical reaction processes, providing deeper insights into the energy storage mechanisms of TMCC materials [62]. For instance, cyclic voltammetry (CV) measurements can be employed to analyze the redox behavior and electrochemical reversibility of electrode materials (Figure 5c), while in situ Raman and in situ XRD enable dynamic monitoring of lattice vibration and interlayer spacing evolution during Li⁺ intercalation and deintercalation processes (Figure 5d,e). Combined with high‐resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED), the structural stability after cycling can be further verified. Related studies demonstrated that Nb₂Se₂C mainly undergoes a reversible interlayer intercalation process during lithiation or delithiation [60], accompanied by reversible interlayer expansion while maintaining its overall crystal framework, indicating a solid‐solution reaction mechanism rather than a phase transition. Such mild structural evolution effectively alleviates volume variation during cycling, thereby resulting in excellent cycling stability and rate capability. It is noteworthy that TMCCs possess high electrical conductivity and strong affinity toward sulfur species, which effectively suppresses polysulfide shuttle and enables high‐efficiency sulfur redox reactions. These features make TMCCs promising hosts for Li–S and Na–S batteries. Qi Zhang et al. [64], based on density functional theory, reported that Ti₂S₂C exhibits strong binding to Li₂S (adsorption energy of 4.28 eV) due to the presence of Li–S and S–S interactions. Furthermore, its uniform surface charge distribution promotes rapid Li⁺ diffusion. Therefore, employing TMCCs as cathode hosts is expected to significantly advance the development of high‐performance lithium–sulfur batteries.

FIGURE 5.

Applications of transition metal carbo‐sulfides in batteries [51, 62]. (a) The CV of the Nb₂Se₂C flexible electrode [51]. Reproduced with permission [51]. Copyright 2024, Wiley. (b) Cycling stability of the Nb₂Se₂C flexible battery [51]. Reproduced with permission [51]. Copyright 2024, Wiley. (c) CV curves of Nb₂CSe₂ in the first three cycles [62]. Reproduced with permission [62]. Copyright 2023 Elsevier. (d) In situ Raman spectra of Nb₂CSe₂ in the first cycle [62]. Reproduced with permission [62]. Copyright 2023 Elsevier. (e) In situ XRD patterns of Nb₂CSe₂ in the first cycle [62]. Reproduced with permission [62]. Copyright 2023 Elsevier.

In the field of electrocatalysis, Xin Pang et al. [32] reported that Nb₂Se₂C exhibits an overpotential of only 321 mV for the oxygen evolution reaction (OER), outperforming most TMDs, MXenes, and commercial catalysts (Figure 6a,b). This superior performance is attributed to the abundant active sites provided by Se and the high conductivity of the Nb–C layers. Figure 6c,d illustrate the potential of TMCC materials for the hydrogen evolution reaction (HER). Elham Loni et al. [52] found that Ta₂Se₂C shows excellent electrochemical activity in water‐splitting for hydrogen production, and the performance of exfoliated 2D Ta₂Se₂C is further enhanced, with the HER overpotential decreasing from 678 mV for the multilayer state to 264 mV and the Tafel slope reduced to 91 mV·dec⁻¹, indicating significant kinetic advantages. Shrish Nath Upadhyay et al. [65, 66], based on first‐principles calculations, investigated the adsorption and conversion behavior of lithium polysulfides (LiPS) on Ta₂S₂C and Nb₂Se₂C. For Ta₂S₂C, the adsorption energy is –2.02 eV, and the energy barrier for the sulfur reduction reaction (SSR) from Li₂S₂ to Li₂S is 0.59 eV, demonstrating moderate catalytic activity. Nb₂Se₂C exhibits adsorption energies for polysulfides ranging from –0.71 to –1.51 eV, and the Gibbs free energy (ΔG) for the SRR from S₈ to Li₂S₈ is –2.55 eV, making it a suitable high‐performance catalyst for suppressing the shuttle effect.

FIGURE 6.

Applications of transition metal carbo‐chalcogenides in the field of electrocatalysis [32, 52]. (a) Polarization curves of the oxygen evolution reaction (OER) for Nb₂Se₂C [32]. Reproduced with permission [32]. Copyright 2020, Royal Society of Chemistry. (b) Overpotentials required to achieve a current density of 10 mA cm⁻² for various OER catalysts [32]. Reproduced with permission [32]. Copyright 2020. Royal Society of Chemistry. (c) Side view of H adsorption on different positions of a TMCC edge [52]. Reproduced with permission [52]. Copyright 2025, American Chemical Society. (d) ΔG _H* of H(5) on different TMCC edges [52]. Reproduced with permission [52]. Copyright 2025, American Chemical Society.

In the field of gas sensing, transition‐metal‐doped TMCCs also demonstrate significant application potential [57, 58]. This is attributed to the hybridization between the d orbitals of the transition metal atoms and the p orbitals of surface S atoms in TMCC, which generates surface active sites, thereby enhancing the adsorption of specific gas molecules and facilitating charge transfer. Seetha Lakshmy et al. [58], using density functional theory simulations, investigated the sensing performance of Nb₂S₂C doped with transition metals (Pd, Ag, Cu, Sc) toward nitrobenzene. Among them, the Cu‐doped system exhibited rapid response and a short recovery time of 12 s, demonstrating both high sensitivity and practical applicability. Similarly, Unnati Jethawa et al. [57] studied the effects of Ti, Co, Cu, Pd, and Ag doping on Nb₂S₂C for H₂S sensing. Their results showed that Ti‐ and Co‐doped systems exhibited strong chemisorption (–1.09 eV) toward the toxic gas H₂S and excellent reversibility with a recovery time of 6–8 s, providing a high‐sensitivity, low‐energy‐consumption material platform for environmental monitoring and industrial safety sensors. Table 1 summarizes the gas adsorption performance of various transition‐metal‐doped TMCC‐Nb₂S₂C materials.

TABLE 1.

Gas adsorption properties of TMCC‐Nb₂S₂C materials modified with different transition metals [57, 58].

Element	Target molecule	Adsorption energy, eV	Binding energy, eV	Charge transfer (e)	Recovery time (350 K)	Reference
Sc	NB	−2.40	−5.50	2.13	Extremely slow	[58]
Cu	NB	−0.91	−2.66	0.58	12S	[58]
Ti	H2S	−1.08	−5.44	1.95	6S	[57]
Co	H2S	−1.09	−4.91	0.60	8.5S	[57]
Pd	NB/H2S	−0.64/−0.91	−2.77/−3.17	0.33/0.31	>10S	[57, 58]
Ag	NB/H2S	−0.72/−0.94	−1.43/−1.96	0.51/0.52	>15S	[57, 58]

In the field of tribology, TMCCs combine the low shear characteristics of TMDs with the high hardness and chemical stability of carbides, enabling them to reduce friction while providing excellent wear resistance and oxidation protection at contact interfaces. This makes TMCCs promising candidates for next‐generation high‐performance solid lubricants [59, 60, 63]. Carsten Gachot et al. [60] found that TMCCs can form protective tribofilms during friction, effectively reducing wear. The friction coefficient was measured below 0.2, outperforming WS₂ and Ti₃C₂T _x MXene powders (Figure 7a). Yana Xiao et al. [59] incorporated Nb₂S₂C and Ta₂S₂C into polydimethylsiloxane (PDMS) as coatings for triboelectric nanogenerators. At a doping concentration of 3 wt%, the electrical performance of PDMS was significantly enhanced while maintaining optimal frictional properties. For example, the composite exhibited a low friction coefficient of 0.43 under a 1 N load, with an average wear depth of 4.078 µm after 1 h of friction (compared to 9.244 µm for pristine PDMS). Under a 20 N load, shear action further reduced friction, demonstrating that TMCC incorporation effectively minimizes energy loss during sliding (Figure 7b). Calculating the potential energy surfaces (PES) of Nb₂S₂C and Ta₂S₂C under normal loads ranging from 0 to 5 GPa provides insight into the energy evolution during interlayer sliding in bilayer TMCC structures (Figure 7c) [60]. The TMCC materials possess a hexagonal crystal structure with energetically stable stacking configurations (denoted as min 1 and min 2). The minimum energy path (MEP), indicated by the yellow dashed line, exhibits ultralow sliding energy barriers that remain below 0.05 J m⁻² even under a high normal load of 5 GPa. Such low energy barriers suggest that the weak interlayer coupling and favorable stacking characteristics enable stable interlayer sliding under low shear stress, resulting in low friction coefficients consistent with the experimental observations. These findings further demonstrate the strong potential of TMCC materials as high‐performance 2D solid lubricants.

FIGURE 7.

Applications of TMCC materials in the field of tribology [59, 60]. (a) Friction coefficients of different materials on alumina [60]. Reproduced with permission [60]. Copyright 2025, Elsevier. (b) Average COF after data stabilization (150s after the start of the experiment approximately) at the load of 1, 5,10, 20, and 30 N, respectively [59]. Reproduced with permission [59]. Copyright 2024, Wiley. (c) Schematic of friction behavior for Nb₂S₂C and Ta₂S₂C [60]. Reproduced with permission [60]. Copyright 2025, Elsevier.

5. Summary and Outlook

2D transition metal carbon sulfides (TMCCs) represent an emerging class of two‐dimensional materials, exhibiting significant research value and application potential in the field of materials science. TMCCs possess a layered structure and surface terminations similar to those of MXenes and thus demonstrate excellent performance in the advantageous domains of MXenes, such as energy storage, catalysis, and gas sensing. Meanwhile, due to their fundamental differences in chemical composition, TMCCs generally exhibit superior environmental stability, making them promising candidates for harsh conditions where MXenes may not perform well, such as high‐temperature, aqueous, or oxygen‐containing environments prone to oxidation.

Currently, research on TMCC is still in its infancy, with the primary limitation to their development stemming from the difficulties in material synthesis. TMCC synthesis mainly relies on high‐temperature solid‐state reactions and halogen‐assisted methods, which involve complex processes and are limited to a narrow range of elemental systems, essentially confined to transition metal elements such as Nb and Ta. Furthermore, current synthesis methods also struggle to achieve the preparation of high‐quality, large‐size samples, thereby hindering the study of their properties and further functional applications. These challenges originate from the intrinsically limited synthetic routes and restricted compositional space, which collectively impede the scalable and controllable synthesis of TMCC materials, ultimately constraining their systematic development.

Looking forward to the future development of TMCC materials, the following research directions deserve attention. (1) Developing more efficient and versatile synthetic methods to expand the elemental composition of TMCCs and enable the preparation of high‐quality, large‐size, and even single‐crystal samples, which is crucial for the long‐term advancement of TMCC materials. (2) Deepening the study of the intrinsic properties of TMCCs, as the differences in the stacking arrangement of single atomic layers between TMCCs (single‐layer stacking with a period of 1, e.g., Nb₂S₂C) and MXenes (double‐layer stacking with a period of 2, e.g., Nb₂CS₂) with the same composition deserve in‐depth investigation and may open new research avenues in 2D materials, Such variations in stacking arrangement may significantly influence their electronic structures, interlayer interactions, and emergent physical properties, warranting systematic experimental and theoretical exploration to uncover structure–property relationships in 2D materials. (3) Further enhancing the performance of TMCCs in electrochemical, catalytic, and tribological applications through approaches such as elemental doping, interface engineering, and heterostructure construction. (4) Exploring untapped application fields, such as electromagnetic interference shielding, wave absorption, and optoelectronic devices.

Funding

This study was supported by the National Natural Science Foundation of China (U23A2093, 12435017, and 52172254).

Conflicts of Interest

The authors declare no conflicts of interest.