2D/2D Heterojunction Interfaces of 1T-MoS2 and Ti3C2 MXene: Designing High-Performance Catalyst for the Hydrogen Evolution Reaction

2D/2D Heterojunction Interfaces of 1T-MoS2 and Ti3C2 MXene: Designing High-Performance Catalyst for the Hydrogen Evolution Reaction | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article 2D/2D Heterojunction Interfaces of 1T-MoS 2 and Ti 3 C 2 MXene: Designing High-Performance Catalyst for the Hydrogen Evolution Reaction Sana Akir, bastian schmiedecke, debabrata bagchi, jan plutnar, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7868982/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Designing efficient electrocatalysts for the hydrogen evolution reaction (HER) is crucial for advancing sustainable energy technologies. In this study, a 2D/2D heterostructure composed of 1T-phase molybdenum disulfide (1T-MoS 2 ) and titanium carbide MXene (Ti 3 C 2 T x, denoted as Ti 3 C 2 ) is synthesized using a one-step hydrothermal method. The hybrid catalyst exhibits improved HER kinetics, demonstrated by a low overpotential of 248 mV at 10 mA cm -2 and a Tafel slope of 55 mV dec -1 , indicating fast reaction kinetics and favorable charge transfer. The addition of tetramethylammonium ions (TMA + ) induces interlayer expansion, increasing the 1T phase content to 89%. Incorporating only 1 % Ti 3 C 2 MXene suppresses oxidation and enhances stability. The composite demonstrates a large electrochemical surface area, high turnover frequency (TOF), and retains over 90% of its catalytic activity after extended electrolysis. This scalable approach offers a promising route to developing stable, efficient 2D electrocatalysts for hydrogen production. Hydrogen evolution reaction Electrocatalyst MoS2 Ti3C2 Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Highlights One-step hydrothermal synthesis of 1T-MoS 2 /Ti 3 C 2 MXene heterostructures. TMA⁺ ions expand MoS₂ layers, increasing 1T phase content up to 89%. Low overpotential (248 mV at 10 mA cm -2 ) and a Tafel slope of 55 mV dec -1 . 1% MXene addition suppresses oxidation and improves catalyst stability. Catalyst maintains over 90% activity after extended electrolysis testing. 1. Introduction The global transition to sustainable energy solutions has driven an urgent need for efficient, carbon-free energy sources. Hydrogen, as a clean and renewable energy carrier, has gained significant attention due to its high gravimetric energy content and environmentally benign byproducts 1 . Among various hydrogen production methods, electrochemical water splitting is considered one of the most promising and environmentally friendly approaches. However, the practical deployment of this approach relies on the availability of highly active and durable electrocatalysts for the hydrogen evolution reaction (HER). Platinum (Pt)-based catalysts have long been the benchmark for HER due to their outstanding catalytic activity and stability. However, the scarcity and high cost of Pt have driven the search for alternative materials that are cost-effective, scalable, and capable of delivering comparable performance 2 . Transition metal dichalcogenides (TMDs) have emerged as promising candidates for HER due to their distinctive electronic features and adjustable catalytic behavior 3 – 5 . Among them, molybdenum disulfide (MoS 2 ) is particularly attractive due to its abundance, layered structure, favorable hydrogen adsorption energy (ΔG H * ≈ 0.08 eV at edge sites) 6 and various structural engineering possibilities 5 . The properties of MoS 2 are significantly influenced by its polymorphic phases, especially the hexagonal 2H phase and the octahedral 1T phase. The semiconducting 2H phase has an inherent low electrical conductivity, which restricts the performance of MoS 2 -based electrocatalysts 7 . In contrast, the metallic 1T phase demonstrates improved conductivity and catalytically active basal planes, making it exceptionally suitable for HER applications 8 . To enhance the conductivity of 2H-MoS 2 , significant efforts have been made to induce a phase transition from the 2H phase to the metallic 1T phase. This can be achieved through various techniques, including ion intercalation, doping, and strain engineering 4 , 5 . Among these methods, ion intercalation is particularly appealing due to its practicality and ease of integration into hydrothermal or solvothermal synthesis, eliminating the need for additional post-treatment steps 9 . MXenes, another family of 2D materials, have attracted significant interest due to their exceptional intrinsic properties 10 . With the general formula M n+1 X n T x , where M is an early transition metal, X represents carbon and/or nitrogen, and T x refers to surface functional groups. MXenes are produced by selectively etching the A-element from MAX phases, where A is typically an element from groups IIIA or IVA 11 . MXenes exhibit highly hydrophilic surfaces (functionalized with -O, -OH, and/or -F groups), remarkable metallic conductivity (e.g., Ti 3 C 2 ~ 9880 S cm − 1 ), and a large interlayer distance 12 , 13 . These characteristics make MXenes ideal conductive matrices for anchoring and nucleating other active materials 12 , 14 , 15 . However, due to the high proportion of exposed metal atoms on the surface, MXenes are usually thermodynamically metastable with high surface energy, suffering from poor oxygen resistance in air or oxygen-dissolved solutions even at ambient conditions 4 , 13 , 16 . This oxidation leads to structural degradation, compromising their electrical and electrochemical properties. Therefore, creating effective strategies to reduce oxidation is essential for the practical use of MXenes. Furthermore, oxidation is difficult to prevent using common nanomaterial manufacturing methods, such as hydrothermal synthesis, solvothermal synthesis, calcination, and refluxing. This limitation hinders both the full exploitation of MXenes and a deeper understanding of their chemical nature 17 . To overcome this limitation, encapsulating the surface of MXenes with another material has been explored as a promising solution. Recent studies have focused on fabricating 2D/2D heterostructures that combine MXenes with MoS 2 . These hybrid structures utilize the synergistic effects of both materials to prevent layer restacking and improve catalytic activity 4 . Recently, 2H-MoS 2 has been combined with various MXenes through in-situ sulfidation [19] and microwave-assisted growth techniques [20]. Although these methods have demonstrated enhanced catalytic activity and durability for HER, they often require complex, multi-step processes that need sophisticated equipment, making them expensive and time-consuming. Furthermore, the inherently low electrical conductivity of 2H-MoS 2 remains a significant barrier to achieving optimal catalytic performance in these composites. Hydrothermal synthesis has also been investigated to produce 2H-MoS 2 /MXene composites 18 , but the low electrical conductivity of the 2H-phase MoS 2 remains a significant obstacle, hindering overall catalytic efficiency. In this work, we present a facile one-step hydrothermal method for synthesizing a 1T-MoS 2 /Ti 3 C 2 MXene heterostructure. For simplicity, Ti 3 C 2 T x is hereafter denoted as Ti 3 C 2 throughout the manuscript. Unlike previously reported approaches, which often require multi-step procedures, carbon coating, or sophisticated equipment, our method enables the direct formation of a stable 2D/2D composite without post-treatment or protective barriers. In this synthesis, MoS₂ nanosheets are uniformly intercalated between Ti 3 C 2 layers, forming a tightly integrated heterostructure. A key innovation of our strategy is the use of tetramethylammonium hydroxide (TMAOH), which plays a dual role: it promotes the phase transition of MoS 2 from the 2H to the more conductive 1T phase and simultaneously enables the formation of a stable, oxidation-resistant 2D/2D composite without the need for complex equipment or additional post-treatment. This method not only simplifies the fabrication process but also significantly enhances the electrochemical performance of the resulting composite. The 1T-MoS 2 /Ti 3 C 2 heterostructure exhibits a favorable Tafel slope of 55 mV dec − 1 and an overpotential of 248 mV at -10 mA cm − 2 , despite containing only 1% Ti 3 C 2 MXene. These values are competitive with or superior to previously reported Ti 3 C 2 /MoS 2 -based catalysts, confirming the synergistic effect of the 2D/2D interface, which offers abundant active sites and efficient charge transfer pathways. Additionally, the composite demonstrates exceptional durability and operational stability over 15 hours in acidic electrolyte, highlighting its potential for practical hydrogen evolution applications. Overall, this work provides a cost-effective, scalable, and oxidation-resistant strategy for integrating MXenes with 1T-phase MoS 2 , advancing the development of efficient non-precious metal HER electrocatalysts. 2. Experimental Section 2.1 Sample preparation 2.1.1 Synthesis of few-layer Ti 3 C 2 MXene Few-layer Ti 3 C 2 MXene was synthesized by selectively removing the aluminum component from the Ti 3 AlC 2 powder (≤ 40 µm, Carbon-Ukraine) using a hydrofluoric acid (HF, 48%, ACS reagent), based on a modified protocol from previous studies 19 . As HF is extremely hazardous and highly corrosive, all procedures were carried out in a well-ventilated fume hood with appropriate personal protective equipment, including acid-resistant gloves, face shield, and protective lab coat. Specifically, 1 g of Ti 3 AlC 2 was added slowly to 20 mL of HF under ambient conditions and allowed to react for 24 hours to ensure complete etching of the Al layers. After the reaction, the mixture was subjected to repeated centrifugation (five cycles at 3500 rpm for 5 minutes each) with deionized water until the pH of the supernatant reached around 6. To promote exfoliation, the washed sediment was redispersed in 15 mL of a 25% aqueous solution of tetramethylammonium hydroxide (TMAOH,9 99.9999%, ThermoFisher) and stirred for 24 hours, facilitating the intercalation of TMA⁺ ions between the MXene layers. The resulting dispersion, with a basic pH (~ 10), was washed twice (3500 rpm, 10 minutes each cycle) to neutralize the pH to about 7. After a final centrifugation step (1 hour at 3500 rpm), the free-standing MXene film was collected by vacuum filtration using a Celgard membrane and then dried under vacuum at 70°C overnight. 2.1.2 Preparation of 2H-MoS 2 Nanosheets Ammonium heptamolybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 .4H 2 O, CAS No.12054-85-2, 99.98%) and thiourea (CAS No. 62-56-6, ≥ 99%), both obtained from Sigma Aldrich, were used as received without further purification. For the synthesis, 0.617 g of ammonium heptamolybdate and 1.14 g of thiourea were dissolved in 20 mL of deionized water under vigorous stirring for 30 minutes to obtain a uniform solution. This precursor mixture was then sealed in a 45 mL Teflon-lined stainless-steel autoclave (filled to ~ 45% capacity) and subjected to hydrothermal treatment at 200°C for 18 hours. Once the autoclave cooled naturally to ambient temperature, the resulting black product was collected, thoroughly washed with both deionized water and ethanol using centrifugation at 8000 rpm for 10 minutes per wash cycle, and then dried under vacuum at 60°C overnight. 2.1.3 Preparation of Ti 3 C 2 /1T-MoS 2 heterostructures Hierarchical heterostructures composed of Ti 3 C 2 and 1T-MoS 2 were fabricated via a hydrothermal synthesis approach, as schematically shown in Fig. 1 . First, thiourea and ammonium heptamolybdate tetrahydrate were dissolved in 20 mL of deionized water. Subsequently, 0.3 g of tetramethylammonium hydroxide pentahydrate (TMAOH, ((CH 3 ) 4 N(OH).5H 2 O, CAS No. 10424-65-4, Sigma Aldrich, ≥ 97%) was introduced to the solution. Varying amounts of Ti 3 C 2 (corresponding to 1%, 2%, 5%, and 10% relative to the mass of (NH 4 ) 6 Mo 7 O 24 .4H 2 O) were incorporated to evaluate the effect on HER performance. These resulting composites were designated as TM-1, TM-2, TM-5, and TM-10, respectively. The mixture was stirred vigorously for an additional 30 minutes to ensure homogeneity. The mixtures underwent hydrothermal treatment at 200°C for 18 h, followed by washing and drying as above. 2.1.4 Preparation of 1T-MoS 2 Nanosheets As a reference sample, 1T-MoS2 nanosheets were synthesized using the same procedure described in Section 2.1.3 , but without Ti 3 C 2 addition, using ammonium heptamolybdate (0.617 g), thiourea (1.14 g), and TMAOH (0.3 g). 2.2 Characterization methods The crystalline phases and structural properties of the materials were characterized by X-ray diffraction (XRD) using a Bruker D8 ADVANCE diffractometer equipped with a copper Kα radiation source (λ = 1.5406 Å). Data were collected across a 2θ range of 5° to 70°, with an increment of 0.02° per step. Raman spectroscopy was performed using an i-Raman Plus 532H portable spectrometer, operating with a 532 nm laser source. Thermogravimetric analyses (TGA) were performed on a NETZSCH TG 209F1 Iris instrument under a flow of Ar with a temperature range of 27–800°C. Surface morphology was examined via scanning electron microscopy (SEM) on a ZEISS Gemini SEM 460 system, which features a field emission gun capable of operating in the 0.1–30 keV range. Elemental composition and surface chemistry were analyzed by X-ray photoelectron spectroscopy (XPS) using a SPECS instrument fitted with a monochromatic Al Kα source (energy: 1486.7 eV) and a Phoibos 150 hemispherical analyzer. Survey scans were acquired with a pass energy of 100 eV, while core-level spectra were collected at 50 or 30 eV. The system operated under ultra-high vacuum conditions (≤ 10 − 9 mbar). An electron flood gun was employed to neutralize surface charging during measurement. High-resolution structural and compositional imaging was conducted using a JEOL ARM200F transmission electron microscope (TEM) at 200 kV. This system was outfitted with CEOS aberration correctors for both probe and image formation, and a dual silicon drift detector for energy-dispersive X-ray spectroscopy (EDS). For high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), a convergence semi-angle of approximately 27 mrad and a collection angle of 55 mrad were used. The STEM imaging resolution was around 0.8 Å, and EDS mapping provided elemental distributions at both nanometre and atomic resolutions. 2.4 Electrochemical measurements Electrochemical measurements were performed at room temperature using a conventional three-electrode configuration connected to a CHI760E electrochemical workstation. A 0.5 M H 2 SO 4 solution served as the electrolyte. The working electrode was a glassy carbon (GC) disk with a 2 mm diameter, while an Ag/AgCl electrode (saturated with KCl) and a carbon rod were used as the reference and counter electrodes, respectively. Catalyst ink for HER testing was prepared by dispersing 10 mg of the synthesized material in a solvent mixture containing 500 µL of deionized water, 500 µL of isopropanol, and 8 µL of 5 wt.% Nafion solution (Sigma-Aldrich). The suspension was subjected to ultrasonic agitation for 30 minutes to ensure uniform dispersion. A 1 µL aliquot of the resulting ink was drop-cast onto the GC electrode, achieving an approximate catalyst loading of 0.32 mg cm - 2 , followed by air drying at ambient conditions. For benchmarking purposes, a commercial Pt/C (20 wt.%) ink was prepared and deposited using the same protocol. Before testing, the electrolyte was deaerated by purging with nitrogen gas for 30 minutes. All measured potentials were calibrated against the reversible hydrogen electrode (RHE) using the equation: E RHE = E Ag/AgCl + 0.059 pH + 0.222 V. Linear sweep voltammetry (LSV) was conducted at a scan rate of 5 mV s⁻¹, with 90% iR compensation. The double-layer capacitance (C dl ) was obtained by plotting half the difference in anodic and cathodic current densities (ΔJ/2) at 0.27 V vs. RHE against varying scan rates. The electrochemically active surface area (ECSA) was estimated using the relation: ECSA = C dl​ /C s , where Cs​ is the specific capacitance for standard electrode materials, with a literature value of 0.04 mF cm - 2 used for carbon-based materials 5 . The turnover frequency (TOF) for each catalyst was determined by quantifying the number of active sites involved in the hydrogen evolution reaction. TOF, expressed in s - 1 , was calculated using the equation: TOF (s -1 ) = (J × S) / (2 × F × n) 13 . Where J represents the current density measured at various overpotentials, S (0.0314 cm²) is the geometric surface area of the electrode, and F (96500 C mol - 1 ) is the Faraday constant. The factor of 1/2 accounts for the number of electrons required to produce one hydrogen molecule, and n (in moles) is the number of active sites obtained using the cyclic voltammetry (CV) method, where n = Qs/F. All CV tests were conducted in a solution of 0.5 M H 2 SO 4 at a scan rate of 40 mV s - 1 . The surface charge (Q s ) can be obtained by integrating the CV curve’s charge over the whole potential range; the half value of the charge is the integrated charge over the entire potential range 13 , 20 , 21 . Electrochemical impedance spectroscopy (EIS) was performed across a frequency range of 0.1 Hz to 1 MHz, using a 5 mV AC amplitude and an applied overpotential of 250 mV vs. RHE. The catalyst durability was evaluated by conducting 3000 continuous CV cycles at a scan rate of 100 mV s - 1 . Additionally, the chronopotentiometric stability of the catalyst was evaluated by applying a constant current density of 10 mA cm - 2 for 15 hours in 0.5 M H 2 SO 4 . The working electrode was prepared using an identical catalyst ink as described as previously described, ensuring consistent mass loading. The ink was drop-cast onto a 1 cm × 1 cm carbon paper substrate and allowed to dry under ambient conditions. Carbon paper was chosen due to its electrochemical inertness toward the HER, ensuring that observed activity originated solely from the catalyst. 3. Results and discussion 3.1 Characterization of electrocatalysts The crystallographic structure and phase composition of the synthesized catalyst were investigated using X-ray diffraction (XRD) and Raman spectroscopy. Figure S1a displays the XRD profiles of both Ti 3 C 2 MXene and its precursor Ti 3 AlC 2 MAX phase. The Ti 3 C 2 pattern shows distinct reflections at 6.15°, 18.17°, 27.7°, and 36.5°, which correspond to the (002), (004), (006), and (008) planes, respectively, indicative of layered MXene formation 4 . In contrast, the peak around 38.7°, typically associated with aluminum (Al) in Ti 3 AlC 2 , disappears after etching, confirming successful removal of the Al layers and formation of Ti 3 C 2 22 . Using Bragg’s law, the calculated interlayer spacing for Ti 3 C 2 is approximately 1.42 nm, consistent with literature values 23 , 24 . The XRD spectrum of 2H-MoS₂ exhibits sharp peaks at 14.1°, 33.4°, and 58.8°, corresponding to the (002), (100), and (110) reflections of its hexagonal phase (P6₃/mmc, JCPDS 37-1492) 25 , 26 . A comparison with the 1T-MoS 2 pattern shows a shift of the (002) peak from ~ 14° to around 9°, indicating an increase in interlayer distance. Moreover, the disappearance of the peak at ~ 14° implies the formation of a few-layered MoS 2 . Broadening and shifting of the (100) and (110) peaks to lower angles, along with the emergence of a (004) reflection at ~ 18.2°, suggest distortion and phase transition to the metallic 1T-MoS₂ phase 9 . These observations confirm the crucial role of tetramethylammonium hydroxide (TMAOH) in promoting 1T phase formation. The intercalation of TMA + ions between the MoS 2 layers expands the interlayer spacing and induces distortions in the atomic arrangement. This process promotes the formation of the 1T phase and generates numerous grain boundary defects, which enhance the density of active sites for hydrogen production 7 . This interpretation is further supported by the increase in (002) d-spacing from 0.63 nm (2H phase) to 0.97 nm (1T phase). To directly assess the intercalation of TMA⁺ and its influence on phase transformation, we performed thermogravimetric analysis (TGA) on both 2H-MoS 2 and 1T-MoS 2 after TMAOH treatment. As shown in Figure S1c , the 2H-MoS 2 sample exhibited a total mass loss of approximately 8.0 wt% up to 800°C, primarily due to desorption of surface-bound water and minor structural changes. In contrast, the TMAOH-intercalated 1T-MoS₂ exhibited a significantly larger mass loss of around 22.5 wt%, with a distinct weight loss event observed between 200°C and 400°C. This thermal behaviour indicates the presence and subsequent decomposition of intercalated organic species (TMA⁺), providing strong evidence for successful intercalation and structural modification toward the 1T phase. This interpretation is consistent with previous reports showing that the decomposition of TMA⁺ occurs between 200°C and 400°C, contributing to a similar weight loss in the TMAOH-intercalated MoS 2 sample 27 . Furthermore, in the Ti 3 C 2 /1T-MoS 2 composites (TM-1 through TM-10), XRD peaks indicative of both MXene and 1T-MoS 2 are present. Interestingly, the Ti 3 C 2 peak is absent in TM-1, likely due to its low loading, which suggests good dispersion of MXene within the composite. In samples with higher MXene content (TM-2, TM-5, TM-10), a slight upshift of the (002) MXene peak is observed, possibly resulting from the partial detachment of terminal functional groups during hydrothermal treatment 24 , 28 . Notably, the (002) diffraction peak of Ti 3 C 2 in these composites remains at a low angle (~ 7.15°), indicating that the MXene structure remains delaminated and highly expanded even after hydrothermal processing at 200°C. This suggests that the interlayer spacing is still significantly increased, likely due to the retention of intercalated TMA⁺ ions and 1T-MoS₂ nanosheets between the Ti 3 C 2 layers, consistent with previous reports 19 . Additionally, a minor peak at 25.35° in samples with > 1% MXene ( Figure S1b ) suggests trace formation of TiO 2 , likely due to surface oxidation of Ti under synthesis conditions. This peak is notably absent in TM-1, supporting the conclusion that low MXene content helps preserve structural integrity and promotes efficient charge transport. Another noteworthy feature in TM-10 is the reappearance of a (002) peak at 2θ ≈ 14.4°, indicating the stacking of MoS 2 layers ( Figure S1b ). This may imply partial transformation back to the 2H phase under high MXene loading, which could impact electronic properties. Raman spectroscopy provided further insights into the materials' microstructure ( Figure S1d and Fig. 2 b). The Ti 3 C 2 MXene sample exhibited characteristic peaks at 126, 209, 359, and 723 cm − 1 , in agreement with typical vibrations of delaminated MXenes, corresponding to A 1g (Ti, O, C), E g (O), and A 1g (C) modes, respectively 22 , 29 . Figure 2 b shows Raman spectra for 2H-MoS 2 , 1T-MoS 2, and composites. Considering 2H-MoS 2 , the characteristic peaks appearing at 373 and 401 cm − 1 are assigned to the A 1g and E 1 2g bonds of the 2H phase, respectively 30 . The intensity of the out-of-plane A 1g mode surpasses that of the in-plane E 1 2g mode, suggesting an edge-enriched MoS 2 structure 30 . Upon TMA⁺ treatment, the MoS₂ bands decreased in intensity, and a new band at 145 cm − 1 (J 1 mode) emerged, indicating successful formation of the 1T phase. A further signal at ~ 283 cm − 1 (E 1g ) confirmed the presence of Mo atoms in octahedral coordination, characteristic of 1T-MoS 2 26 . The composite spectra (TM-1 to TM-10) showed similar vibrational features. Notably, TM-1 exhibited the weakest signals related to 2H-MoS 2 , suggesting a more complete phase conversion. In contrast, TM-10 showed increased 2H-character, corroborating the XRD findings. Additionally, the peak intensity of Ti 3 C 2 , observed at approximately 205 cm − 1 across all composites, appears notably flat. This reduced intensity is attributed to the surface of the Ti 3 C 2 MXene being covered by a layer of 1T-MoS 2 , coupled with the intrinsically weak signal characteristics of Ti 3 C 2 . Consequently, the laser cannot penetrate the MoS 2 layer effectively, resulting in limited information about the underlying Ti 3 C 2 MXene 13 . The surface morphology of the synthesized catalysts was examined using scanning electron microscopy (SEM). As illustrated in Figure S1e , Ti 3 C 2 MXene displays exfoliated, layered nanosheets with a distinct and organized structure. The inset further reveals its micrometer-scale dimensions, which offer a suitable platform for the uniform deposition of MoS 2 layers. SEM imaging of 2H-MoS 2 (Fig. 3 a) presents a flower-like morphology with a tendency for sheet aggregation. In contrast, the 1T-MoS₂ sample exhibits a more flake-like texture, indicating structural variation between the two phases (Fig. 3 a). In the composite samples (Figs. 3 c-e), MoS 2 sheets are observed to grow both within the interlayer spaces and across the surface of the Ti 3 C 2 MXene. This layered Ti 3 C 2 /1T-MoS 2 configuration mitigates nanosheet restacking and concurrently shields the oxidation-sensitive Ti 3 C 2 structure 5 , a finding consistent with the XRD results. Interestingly, even though deionized water was used during hydrothermal treatment, the TM-1 composite (with 1% MXene) showed negligible signs of oxidation, while slight oxidation was detected in samples with higher MXene content. This suggests that tetramethylammonium (TMA⁺) ions play a stabilizing role during synthesis, encouraging 1T-MoS 2 phase formation and reducing oxidative degradation of the MXene, as previously observed in related studies 5 , 31 . Closer inspection (inset of Fig. 2 c) reveals that the MoS 2 nanosheets closely overlay the Ti 3 C 2 layers, forming a compact heterostructure. This architecture promotes greater exposure of catalytic sites and enhances diffusion pathways for hydrogen evolution reaction (HER) processes 30 . To confirm the structural integrity of Ti 3 C 2 in the TM-1 composite and assess the potential formation of TiO 2 during hydrothermal treatment, a combination of high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS) was employed. TEM analysis of the Ti 3 C 2 MXene ( Figure S2a ) displayed its characteristic few-layered, sheet-like morphology, confirming its two-dimensional nature. The HRTEM image ( Figure S2b ) showed distinct lattice fringes with a spacing of around 0.52 nm, corresponding to the (004) crystallographic planes, in agreement with interlayer distances measured via XRD. Additionally, selected area electron diffraction (SAED) patterns (inset of Figure S2b ) indicated a hexagonal symmetry typical of Ti 3 C 2 . For 2H-MoS 2 , the TEM images (Figure S2c) revealed aggregated nanosheets, while the observed d-spacing of approximately 0.62 nm aligned with the (002) plane, confirming its layered structure. A structural transition to the 1T phase was evidenced by increased interlayer spacing (~ 0.92 nm) and lattice distortion seen in HRTEM (Figure S2f) , characteristics associated with the metallic 1T form of MoS 2 . Notably, the 1T-MoS 2 exhibited a thin, few-layered architecture, an advantageous feature that promotes efficient ion transport and electron mobility during electrocatalytic processes. In the case of the TM-1 composite, both TEM and HRTEM (Fig. 4 a, b) revealed a homogeneous hybrid architecture, where 1T-MoS 2 nanosheets were well-dispersed across the MXene substrate, forming a cohesive 2D network. HRTEM images (Fig. 4 b) revealed well-defined lattice fringes corresponding to the (002) plane of Ti 3 C 2 (~ 1.1 nm) alongside the (002) plane of 1T-MoS₂ (~ 0.9 nm), confirming that the MXene structure remains intact after synthesis. Notably, no TiO 2 nanoparticles were observed on the surface of the MXene, in contrast to previous reports where hydrothermal treatment led to the formation of TiO 2 nanocrystals 5 . STEM-EDS elemental mapping (Fig. 4 c) further confirmed the homogeneous distribution of Ti, C, Mo, S, and O, with no localized oxygen enrichment indicative of oxide formation. The O/Ti atomic ratio, determined from EDS spectra of Ti 3 C 2 before and after hydrothermal treatment at 200°C, was approximately 0.53, closely matching that of freshly prepared, minimally oxidized Ti 3 C 2 (0.46, Figure S2g ). In contrast, aged or hydrothermally oxidized MXenes typically exhibit significantly higher O/Ti ratios, reflecting extensive TiO 2 formation. Additionally, neither the XRD patterns nor the Raman spectra of TM-1 displayed any signatures of crystalline TiO 2 , further confirming the chemical stability of Ti 3 C 2 under the applied hydrothermal conditions. This exceptional structural preservation is attributed to the presence of TMAOH, used both during the initial delamination of MXene and in small quantities during the in-situ synthesis of the 1T-MoS 2 /Ti 3 C 2 composite. Beyond facilitating delamination through TMA⁺ intercalation, TMAOH acts as a pH buffer and surface passivating agent. Literature reports have shown that ammonium-based hydroxides, such as NH 4 OH and TMAOH, stabilize Ti 3 C 2 in aqueous media by maintaining the dispersion pH around 6–7, a range where MXenes remain colloidally stable for extended periods without significant aggregation or oxidation. For instance, NH 4 OH-treated Ti 3 C 2 has been reported to remain stable for over 40 days at pH ~ 6, whereas oxidation and sedimentation accelerate under acidic conditions or when pH exceeds 12 due to flocculation and hydrolysis 32 , 33 . The mildly basic environment provided by TMAOH during our synthesis likely suppresses hydrolytic degradation and preserves interlayer TMA⁺ cations, both of which contribute to protecting Ti sites from oxidation. SAED analysis of the TM-1 composite (Fig. 4 d) reveals several well-defined diffraction rings, highlighted in blue. These rings are characteristic of polycrystalline 1T-MoS 2 and indicate the presence of numerous randomly oriented nanocrystalline domains, consistent with previous reports on chemically stabilized 1T-MoS 2 33,34 . In addition to the 1T-MoS 2 pattern, distinct diffraction spots corresponding to Ti 3 C 2 MXene and 2H-MoS 2 are also observed, confirming the coexistence of multiple structural phases within the composite. X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the elemental composition and surface chemical states of Ti 3 C 2 and its composites. The full survey scan for pristine Ti 3 C 2 , as shown in Figure S3a , displays distinct signals corresponding to C 1s, Ti 2p, O 1s, and F 1s. Notably, no aluminum peaks were detected, confirming the complete etching of Al layers from the precursor MAX phase. The presence of oxygen- and fluorine-containing groups is consistent with HF treatment, which introduces functional surface terminations. The high-resolution Ti 2p spectrum ( Figure S3b ) reveals four prominent peaks: those at around 455.8 eV and 461.8 eV are assigned to Ti 2p 3/2 and Ti 2p 1/2 in Ti-C bonds, respectively. Peaks at higher binding energies (~ 459.3 eV and ~ 465.1 eV) indicate Ti-O bonding, suggesting the formation of TiO 2 species, likely due to surface oxidation. The C 1s spectrum ( Figure S3c ) includes a main peak at 282.4 eV related to Ti-C bonding, confirming Ti 3 C 2 formation 35 . Additional peaks at 285.1 eV, 286.7 eV, and 289.4 eV are attributed to adventitious carbon, C-O, and C = O functionalities, respectively, likely arising from atmospheric exposure and residual surface groups 36 , 37 . The O 1s spectrum ( Figure S3d ) displays peaks at 529.5 eV and 531.3 eV, which correspond to hydroxylated and oxidized Ti sites (C-Ti-OH and C-Ti-O), further indicating the presence of -OH and -O terminations on the MXene surface due to selective etching 26 . Also, the complete survey XPS spectra of the composites are shown in Figure S3a . Only signals corresponding to S 2p, Mo 3d, C 1s, and O 1s are detected, while no distinct Ti 2p signals are observed. This absence of Ti peaks suggests that the MoS 2 layer fully covers the Ti 3 C 2 MXene surface, effectively shielding it from XPS detection. This behavior is consistent with previous reports in the literature 38 , 39 . The Mo 3d spectrum of all prepared composites (Fig. 5 a) displays prominent peaks at 228.4 eV and 231.5 eV corresponding to the Mo⁴⁺ 3d 5/2 and Mo⁴⁺ 3d 3/2 characteristic peaks of 1T-MoS 2 , respectively. Additional weaker signals located at 232.6 and 229.7 eV correspond to the characteristic 3d 5/2 and 3d 3/2 of Mo 4+ species from the non-metallic 2H phase 13 , both of which were also detected in the 2H-MoS 2 sample ( Figure S3e ). Furthermore, the peak at 235.6 eV illustrates Mo 6+ , suggesting the partial oxidation of MoS 2 26 . A low-intensity peak at 225.6 eV corresponds to the S 2s. Upon deconvolution of the S 2p spectrum (Fig. 5 b), two principal peaks appear at 161.1 eV and 162.3 eV, assigned to S 2p 3/2 and S 2p 1/2 of the 1T-MoS 2 phase, respectively 4 . Minor peaks at 161.7 eV and 163.8 eV reflect the corresponding signals from the 2H phase. These assignments were further validated by comparison with the XPS spectra of individual 1T- and 2H-MoS₂ reference samples ( Figure S3g and S3h ), which exhibit consistent peak positions and spectral profiles. Additionally, the full-width at half-maximum (FWHM) values of the deconvoluted peaks are summarized in Table S1a , further supporting the reliability of the fitting and the accuracy of the 1T/2H peak assignments. The relative 1T phase content was quantified by integrating the areas of the deconvoluted S 2p 3/2 and S 2p 1/2 peaks assigned to 1T- and 2H-MoS 2 , and calculating the ratio of the 1T signal area to the total S 2p area ( Table S1b ). It is worth noting that a high proportion of 1T-MoS 2 is advantageous for HER performance due to its enhanced electronic conductivity and higher density of catalytically active sites. 3.2 Electrocatalytic performance for HER The HER performance of the synthesized materials was assessed using a conventional three-electrode system in 0.5 M H 2 SO 4 , with iR-compensation applied throughout. A commercial Pt/C catalyst (20 wt.%) and a bare glassy carbon (GC) electrode served as performance references. As depicted in Fig. 6 a, the GC electrode showed negligible catalytic activity, confirming that the measured performance originates from the tested materials. Predictably, Pt/C delivered outstanding HER efficiency, requiring just 34 mV to reach a current density of 10 mA cm − 2 (Fig. 6 b), reaffirming its status as a benchmark catalyst. In contrast, pure Ti 3 C 2 MXene displayed relatively poor HER activity, with an overpotential of 770 mV at 10 mA cm − 2 . This suggests that while it is not intrinsically active, Ti 3 C 2 serves as a conductive substrate that facilitates electron transport in hybrid systems. Meanwhile, 1T-MoS 2 significantly outperformed its 2H counterpart, achieving an overpotential of 278 mV compared to 369 mV, respectively, an improvement attributed to the stabilization of the 1T phase via TMA + ion intercalation. To further investigate HER enhancement, 1T-MoS₂ was combined with varying amounts of Ti 3 C 2 (1, 2, 5, and 10 wt.%) to form hybrid composites denoted as TM-1, TM-2, TM-5, and TM-10. Among these, TM-1 exhibited the most promising activity, achieving an overpotential of approximately 248 mV at 10 mA cm − 2 . However, higher MXene loadings led to decreased performance; for example, TM-10 required 309 mV at the same current, implying that excessive Ti 3 C 2 may obscure the active MoS 2 surface, thereby hindering catalytic efficiency. For comparison, a physically blended sample (PM-1) was also prepared by mixing pre-synthesized 1T-MoS 2 with 1 wt.% Ti 3 C 2 in a 1:1 water-isopropanol solution, followed by ultrasonication (4 hours), filtration, and vacuum drying at 70°C. As shown in Fig. 6 a, PM-1 required a much higher overpotential (334 mV) to achieve 10 mA cm − 2 , substantially underperforming relative to TM-1 (~ 248 mV). This result emphasizes the significance of the synthesis strategy; unlike the hydrothermally grown TM-1, the physical mixture demonstrated weak interfacial contact between components, which restricted charge transfer and overall HER activity. 40 To gain further insights into the reaction mechanism and kinetics of HER, Tafel slope analysis was conducted. A lower Tafel slope typically suggests more efficient charge transfer and improved catalytic kinetics. As shown in Fig. 6 c, the extracted Tafel slope values for 2H-MoS 2 ,1T-MoS 2 , TM-1, TM-2, TM-5, TM-10, and Ti 3 C 2 were 137, 81, 55, 70, 83, 91, and 203 mV dec − 1 , respectively. Among these, TM-1 demonstrated the smallest Tafel slope (55 mV dec − 1 ), signifying its enhanced intrinsic catalytic performance relative to the other samples. This result also implies that the HER mechanism on TM-1 primarily follows the Volmer-Heyrovsky pathway 41 . TM-1 catalyst exhibits a Tafel slope of 55 mV dec⁻¹, which is superior to many previously reported systems, as shown in Table S2 and Figure S6 . The proposed mechanism of the 1T-MoS 2 /Ti 3 C 2 MXene is shown in the inset of Fig. 6 b. In this pathway, a proton (H 3 O + ) interacts with an electron (e⁻) at the active catalytic site (*), producing an adsorbed hydrogen atom (H) through the step: H 3 O⁺ + e⁻ + * → H* + H 2 O. Subsequently, the H* species combines with another proton and electron to form molecular hydrogen (H₂), completing the reaction: H 3 O⁺ + e⁻ + H* → H 2 + H 2 O. The hydrogen evolution reaction (HER) performance of an electrocatalyst is closely linked to its electrical conductivity, which affects charge transport efficiency. To investigate the interfacial charge transfer behavior and elucidate HER kinetics, electrochemical impedance spectroscopy (EIS) was conducted. This analytical method provides valuable insights into the electrode-electrolyte interface and helps characterize the electrical response of catalytic systems. As shown in the Nyquist plots (Fig. 6 d), the charge transfer resistance (R ct ) is a critical parameter for evaluating HER activity. Among the tested samples, TM-1 exhibited the lowest R ct (9.3 Ω), indicating more efficient electron movement at the catalyst-electrolyte boundary. This resistance is notably lower than that of pristine 1T-MoS 2 (15.9 Ω), demonstrating that incorporating Ti 3 C 2 MXene substantially enhances electron conductivity and overall HER performance 42 . This improvement is attributed to the strong interfacial interaction between Ti 3 C 2 and MoS 2 , which supports rapid charge transfer and reduces interfacial resistance. Furthermore, the Nyquist plots for MoS 2 -based catalysts reveal two distinct semicircular features: a smaller one at high frequencies related to surface texturing or porosity, and a larger semicircle at lower frequencies corresponding to the faradaic charge transfer process. These observations were modelled using an equivalent circuit ( Figure S5 ) 6 , 43 . The improved HER behavior of the composite is further supported by the synergistic combination of 1T-MoS 2 and Ti 3 C 2 , which enhances charge separation and lowers the energy barrier for hydrogen evolution 26 . Beyond charge transport, the availability of active catalytic sites plays a pivotal role in HER activity and is quantitatively represented by the electrochemically active surface area (ECSA). This metric, derived from non-Faradaic capacitive behavior, provides an independent assessment of active site density and intrinsic site activity 44 . To determine ECSA, cyclic voltammetry (CV) was performed at different scan rates in a potential window that excludes faradaic reactions ( Figure S4 ). From these CV curves, the double-layer capacitance (C dl ) was extracted (Fig. 6 e). As ECSA is directly proportional to C dl 45 , this value offers a proxy for comparing the accessible surface area across samples ( Table S3 ). Ti 3 C 2 MXene exhibited the lowest C dl (0.8 mF cm − 2 ), likely due to nanosheet restacking that restricts active site exposure. Similarly, 2H-MoS₂ showed a modest C dl (1.82 mF cm − 2 ), consistent with the low reactivity of its semiconducting basal planes. In contrast, 1T-MoS₂ and the composite materials demonstrated significantly enhanced C dl values, underscoring the role of TMAOH-assisted phase conversion in increasing catalytic site density. Among all samples, TM-1 had the highest ECSA (350 cm²), corresponding to a C dl of 14 mF cm − 2 . This is attributed to its enriched 1T-phase content (89%), which contributes both more active edge/basal sites and better conductivity. The significant increase in the number of active sites within the composites indicates that the presence of 1T-MoS 2 , both on top of and within the interlayer spacing of Ti 3 C 2 MXene, not only prevents aggregation but also enhances the accessibility of the aqueous electrolyte to the catalytically active surface. To further confirm the intrinsic properties of the electrocatalysts, the turnover frequency (TOF) was evaluated. TOF quantifies electron transfer efficiency and active site availability, which can be determined from cyclic voltammetry (CV) measurements 5 , 26 . TOF values were calculated in the potential range of 0.23 to 0.25 V versus RHE. As shown in Fig. 6 f, TM-1 achieved the highest TOF at 0.81 s − 1 at 250 mV overpotential, indicating superior site accessibility and electrocatalytic activity. This enhanced performance stems from the optimized 1T-MoS 2 /Ti 3 C 2 interface, which improves both surface exposure and charge transport, thereby elevating the overall HER efficiency. To investigate the durability of the TM-1 electrocatalyst, cyclic voltammetry (CV) was conducted in 0.5 M H 2 SO 4 at a scan rate of 100 mV s − 1 (Fig. 7 a). After 3,000 consecutive cycles, the resulting polarization curves showed only slight deviation from the initial measurements, indicating the robust operational stability of TM-1. Stability over prolonged usage is a crucial metric for assessing electrocatalyst performance, especially in the context of industrial-scale hydrogen production. we employed chronopotentiometry to evaluate long-term electrochemical performance. TM-1 and a commercial 20 wt% Pt/C reference were tested under a steady current density of 10 mA cm − 2 . The catalyst ink was uniformly deposited on one side of carbon paper, maintaining a mass loading of 0.32 mg cm − 2 , and dried at 50°C to ensure consistent testing conditions. As illustrated in Fig. 7 b, TM-1 exhibited only a minor potential increase of 23 mV over 15 hours, retaining more than 91% of its initial activity. This performance surpasses that of the Pt/C reference and underscores the exceptional stability of the nanocomposite during HER operation. To further confirm the catalyst’s durability under industrial conditions, we extended the stability test to a much higher current density of 200 mA cm − 2 . As shown in Fig. 7 b, TM-1 maintained excellent electrochemical stability even at this increased current. This confirms that TM-1 is effective not only in laboratory settings but also possesses the durability needed for large-scale hydrogen production systems. Post-stability tests at a current density of 10 mAcm − 2 were conducted using EIS, LSV, SEM, Raman spectroscopy, and XPS to examine any structural or electrical degradation. EIS results revealed that the charge transfer resistance (R ct ) remained largely unchanged (Fig. 7 c), indicating that electrical conductivity was preserved. Furthermore, the LSV curves recorded before and after the 15-hour chronopotentiometry test were nearly superimposable (Fig. 7 d), reinforcing the stability of the catalyst. SEM images of TM-1 after durability testing (Fig. 7 e) confirmed that the nanosheet morphology was retained, with no signs of material degradation. Raman spectroscopy provided additional insight into the structural integrity of the 1T-MoS 2 phase following long-term testing. As shown in Fig. 7 f, the post-test spectrum exhibited the distinctive vibrational modes of the 1T phase, including J 1 (~ 145 cm − 1 ), J 3 (~ 344 cm − 1 ) 46 , and A 1g (~ 402 cm − 1 ). Notably, the absence of the E¹ 2g mode (~ 373 cm − 1 ), typically associated with the 2H phase, suggests that no significant phase transition from 1T to 2H occurred during electrolysis. Figure 7 g–i presents the XPS spectra of the TM-1 catalyst after the HER stability test. The survey spectra confirm the continued presence of Mo, S, O, F, and C elements. The enhanced C signal is attributed to the underlying carbon paper substrate, while the F signal originates from the Nafion used as a bonding agent during sample preparation for the HER test. Notably, the Mo 3d and S 2p peaks characteristic of the 1T-MoS 2 structure remain well preserved after the HER test. However, the relative intensity analysis of the S 2p peak indicates a slight decrease in the 1T-phase content to 79.51%, suggesting that while some phase transformation may occur under prolonged electrochemical operation, a significant proportion of the catalytically active 1T phase remains intact. This highlights the structural robustness and catalytic relevance of the 1T-phase MoS 2 component during HER. 4. Conclusion In this work, we report a scalable and one-step hydrothermal synthesis of a 2D/2D 1T-phase molybdenum disulfide (1T-MoS 2 ) and titanium carbide MXene (Ti 3 C 2 ) heterostructure that demonstrates promising catalytic performance for the hydrogen evolution reaction (HER). Unlike prior studies, which often rely on multi-step procedures, inert atmospheres, or carbon coating to preserve MXene integrity, our approach uniquely incorporates tetramethylammonium hydroxide (TMAOH) to simultaneously induce the 1T phase in MoS 2 and stabilize Ti 3 C 2 against oxidation during synthesis. Our key findings reveal that incorporating just 1 wt.% Ti 3 C 2 into the 1T-MoS2 matrix forms a strong interfacial heterojunction, promoting fast charge transfer, a higher density of active sites, and improved phase stability. The optimized TM-1 composite achieved a low overpotential of 248 mV at 10 mA cm − 2 , a Tafel slope of 55 mV dec − 1 , and remarkable stability over 15 hours. Compared to existing catalysts, our material achieves competitive and efficient HER kinetics with minimal synthesis complexity and material usage. This work paves the way for integrating this catalyst into full-cell electrolyzers and exploring its applicability in alkaline or neutral media, as well as expanding this method to other TMDs/MXene combinations for broader green hydrogen applications. Declarations Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by an ERA Fellowship-Green Hydrogen of the German Academic Exchange Service (DAAD, Funding Program Number 57685698). We gratefully acknowledge the Helmholtz Association's Initiative and Networking Fund (Helmholtz Young Investigator Group VH-NG-1719) for the funding. M.P.B and P.W.M. greatly acknowledge support from the German Federal Ministry of Education and Research in the framework of the project Catlab (03EW0015A/B). S.A. would like to express particular thanks to R. Schwiddessen, M. Tovar, and K. Schwarzburg from the X-Ray and Microscopy and Spectroscopy Corelabs of the Helmholtz Zentrum Berlin for providing access to their facility and training on the equipment. Z.S. was supported by the ERC-CZ program (project LL2101) from the Ministry of Education, Youth and Sports (MEYS) and by the project Advanced Functional Nanorobots (reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). Funding This research received no external funding. The APC was waived by the journal. Data availability All data produced or analyzed in this study are included in this published article and its supplementary information files. Author information Authors and Affiliations Helmholtz Young Investigator Group Electrocatalysis: Synthesis to Devices, Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany. Sana Akir, Bastian Schmiedecke, and Michelle P. Browne Department of Materials Chemistry for Catalysis, Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Str. 15, 12489 Berlin, Germany. Department of Chemistry: Metalorganics and Inorganic Materials, Technische Universität Berlin, Straße des 17 Juni 135, Sekr. C2, 10623 Berlin, Germany. Debabrata Bagchi, Prashanth W. Menezes Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 16628 Prague 6, Czech Republic. Jan Plutnar, Zdenek Sofer Institute of Electrochemical Energy Storage, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany. Institute for Technical and Environmental Chemistry, Friedrich-Schiller-Universität Jena and Helmholtz Institute for Polymers in Energy Applications (HIPOLE Jena), Philosophenweg 7b, 07743 Jena, Germany. 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ACS Catal 2:1916–1923. https://doi.org/10.1021/cs300451q Deng SJ et al (2018) Phase modulation of (1T-2H)-MoSe2/TiC-C shell/core arrays via nitrogen doping for highly efficient hydrogen evolution reaction. Adv Mater 30. https://doi.org/10.1002/adma.201802223 Gupta U et al (2014) Characterization of few-layer 1T-MoSe2 and its superior performance in the visible-light induced hydrogen evolution reaction. APL Mater 2. https://doi.org/10.1063/1.4892976 Additional Declarations The authors declare no competing interests. Supplementary Files SI.SanaAkir.docx GraphicalAbstract.png Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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composites.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/ed7fb6155cec92529126e313.png"},{"id":93691171,"identity":"237255ad-ebcf-4566-a56b-978fba318fd6","added_by":"auto","created_at":"2025-10-16 14:05:43","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":805461,"visible":true,"origin":"","legend":"\u003cp\u003eSEM of (a) 2H-MoS\u003csub\u003e2\u003c/sub\u003e, (b) 1 T-MoS\u003csub\u003e2\u003c/sub\u003e, (c) TM-1, (d) TM-2, (e) TM-5, (f) TM-10\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/37423c218ac0b8c5a5943caf.png"},{"id":93691182,"identity":"62b286c8-e26f-45da-bd00-79bb38b807d3","added_by":"auto","created_at":"2025-10-16 14:05:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":595634,"visible":true,"origin":"","legend":"\u003cp\u003e(a) TEM of TM-1, (b) HRTEM of TM-1, (c) STEM-EDX mapping images of Mo, S, C, O, and Ti of TM-1 heterostructure, and (d) SAED pattern of TM-1 (inset: SAED patterns of 2H, 1T-MoS\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e)\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/21c447d7b361a4b1efe70036.png"},{"id":93691178,"identity":"b65e5238-23c2-4d25-b36d-5e9bbd989100","added_by":"auto","created_at":"2025-10-16 14:05:43","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":208415,"visible":true,"origin":"","legend":"\u003cp\u003eDeconvoluted XPS spectra of (a) Mo 3d and (b) S 2p for TM-1, TM-2, TM-5, and TM-10 composites, and (c) Relative fraction of 1T and 2H phase in 1T-MoS\u003csub\u003e2\u003c/sub\u003e and composites.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/0ef857006b0f47644e3fb90c.png"},{"id":93691185,"identity":"133fc821-8773-4247-be59-8692dcefc6ce","added_by":"auto","created_at":"2025-10-16 14:05:43","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":297779,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical measurements for hydrogen evolution in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e: (a) LSV curves, (b) overpotential at j = 10 mA cm\u003csup\u003e-2\u003c/sup\u003e for different electrocatalysts (inset: schematic representation of the mechanism involving TM-1), (c) corresponding Tafel plots, (d) EIS Nyquist plot of different catalysts (inset: enlarged high-frequency response of all composites), (e) capacitive Δj as a function of scan rate for different catalysts, and (f) TOF versus potential plot (inset: TOF values for composites at 250 mV).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/1b87e39ab0b33328542e17e3.png"},{"id":93691195,"identity":"2ec44fd1-fee5-4e34-a5b1-7f4beaa97a94","added_by":"auto","created_at":"2025-10-16 14:05:43","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":403566,"visible":true,"origin":"","legend":"\u003cp\u003e(a) LSV polarization curves of TM-1 before and after 3,000 cycles, (b) Stability test of TM-1 and Pt / C (20 %) \u0026nbsp;under constant current density of 10 and 200 mA cm\u003csup\u003e-2\u003c/sup\u003e, (c) EIS, (d) LSV, (e) SEM, (f) Raman of TM-1 before and after stability test, (g) XPS survey spectra of TM-1 after stability test, and Deconvoluted XPS spectra of (h) Mo 3d , and (i) S 2p for TM-1 after stability test.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/0418f2e3f4210d92c60fc108.png"},{"id":93694490,"identity":"acc59f65-51e4-4f7a-bdf2-4f8bacb83ae8","added_by":"auto","created_at":"2025-10-16 14:29:44","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3495471,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/d87415a2-984f-4d00-a144-1db5228aab58.pdf"},{"id":93691175,"identity":"35a35e2b-a7ee-4152-8ebc-1892b84eed6a","added_by":"auto","created_at":"2025-10-16 14:05:43","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2333286,"visible":true,"origin":"","legend":"","description":"","filename":"SI.SanaAkir.docx","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/b76ce57f1d8098f3eaf7a549.docx"},{"id":93692582,"identity":"9b43c2f6-621f-4d6b-b609-06264a96d205","added_by":"auto","created_at":"2025-10-16 14:13:43","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":500812,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.png","url":"https://assets-eu.researchsquare.com/files/rs-7868982/v1/3cabe757e9f6e9c36c7d6e06.png"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003e2D/2D Heterojunction Interfaces of 1T-MoS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and Ti\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eC\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e MXene: Designing High-Performance Catalyst for the Hydrogen Evolution Reaction\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Highlights","content":"\u003cul\u003e\n \u003cli\u003eOne-step hydrothermal synthesis of 1T-MoS\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene heterostructures.\u003c/li\u003e\n \u003cli\u003eTMA⁺ ions expand MoS₂ layers, increasing 1T phase content up to 89%.\u003c/li\u003e\n \u003cli\u003eLow overpotential (248 mV at 10 mA cm\u003csup\u003e-2\u003c/sup\u003e) and a Tafel slope of 55 mV dec\u003csup\u003e-1\u003c/sup\u003e.\u003c/li\u003e\n \u003cli\u003e1% MXene addition suppresses oxidation and improves catalyst stability.\u003c/li\u003e\n \u003cli\u003eCatalyst maintains over 90% activity after extended electrolysis testing.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eThe global transition to sustainable energy solutions has driven an urgent need for efficient, carbon-free energy sources. Hydrogen, as a clean and renewable energy carrier, has gained significant attention due to its high gravimetric energy content and environmentally benign byproducts \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Among various hydrogen production methods, electrochemical water splitting is considered one of the most promising and environmentally friendly approaches. However, the practical deployment of this approach relies on the availability of highly active and durable electrocatalysts for the hydrogen evolution reaction (HER). Platinum (Pt)-based catalysts have long been the benchmark for HER due to their outstanding catalytic activity and stability. However, the scarcity and high cost of Pt have driven the search for alternative materials that are cost-effective, scalable, and capable of delivering comparable performance \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Transition metal dichalcogenides (TMDs) have emerged as promising candidates for HER due to their distinctive electronic features and adjustable catalytic behavior \u003csup\u003e\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e–\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Among them, molybdenum disulfide (MoS\u003csub\u003e2\u003c/sub\u003e) is particularly attractive due to its abundance, layered structure, favorable hydrogen adsorption energy (ΔG\u003csub\u003eH\u003c/sub\u003e* ≈ 0.08 eV at edge sites) \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e and various structural engineering possibilities \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. The properties of MoS\u003csub\u003e2\u003c/sub\u003e are significantly influenced by its polymorphic phases, especially the hexagonal 2H phase and the octahedral 1T phase. The semiconducting 2H phase has an inherent low electrical conductivity, which restricts the performance of MoS\u003csub\u003e2\u003c/sub\u003e-based electrocatalysts \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. In contrast, the metallic 1T phase demonstrates improved conductivity and catalytically active basal planes, making it exceptionally suitable for HER applications \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. To enhance the conductivity of 2H-MoS\u003csub\u003e2\u003c/sub\u003e, significant efforts have been made to induce a phase transition from the 2H phase to the metallic 1T phase. This can be achieved through various techniques, including ion intercalation, doping, and strain engineering \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Among these methods, ion intercalation is particularly appealing due to its practicality and ease of integration into hydrothermal or solvothermal synthesis, eliminating the need for additional post-treatment steps \u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eMXenes, another family of 2D materials, have attracted significant interest due to their exceptional intrinsic properties \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. With the general formula M\u003csub\u003en+1\u003c/sub\u003eX\u003csub\u003en\u003c/sub\u003eT\u003csub\u003ex\u003c/sub\u003e, where M is an early transition metal, X represents carbon and/or nitrogen, and T\u003csub\u003ex\u003c/sub\u003e refers to surface functional groups. MXenes are produced by selectively etching the A-element from MAX phases, where A is typically an element from groups IIIA or IVA \u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. MXenes exhibit highly hydrophilic surfaces (functionalized with -O, -OH, and/or -F groups), remarkable metallic conductivity (e.g., Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e ~ 9880 S cm\u003csup\u003e− 1\u003c/sup\u003e), and a large interlayer distance \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. These characteristics make MXenes ideal conductive matrices for anchoring and nucleating other active materials \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. However, due to the high proportion of exposed metal atoms on the surface, MXenes are usually thermodynamically metastable with high surface energy, suffering from poor oxygen resistance in air or oxygen-dissolved solutions even at ambient conditions \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. This oxidation leads to structural degradation, compromising their electrical and electrochemical properties. Therefore, creating effective strategies to reduce oxidation is essential for the practical use of MXenes. Furthermore, oxidation is difficult to prevent using common nanomaterial manufacturing methods, such as hydrothermal synthesis, solvothermal synthesis, calcination, and refluxing. This limitation hinders both the full exploitation of MXenes and a deeper understanding of their chemical nature \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. To overcome this limitation, encapsulating the surface of MXenes with another material has been explored as a promising solution. Recent studies have focused on fabricating 2D/2D heterostructures that combine MXenes with MoS\u003csub\u003e2\u003c/sub\u003e. These hybrid structures utilize the synergistic effects of both materials to prevent layer restacking and improve catalytic activity \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Recently, 2H-MoS\u003csub\u003e2\u003c/sub\u003e has been combined with various MXenes through in-situ sulfidation [19] and microwave-assisted growth techniques [20]. Although these methods have demonstrated enhanced catalytic activity and durability for HER, they often require complex, multi-step processes that need sophisticated equipment, making them expensive and time-consuming. Furthermore, the inherently low electrical conductivity of 2H-MoS\u003csub\u003e2\u003c/sub\u003e remains a significant barrier to achieving optimal catalytic performance in these composites. Hydrothermal synthesis has also been investigated to produce 2H-MoS\u003csub\u003e2\u003c/sub\u003e/MXene composites \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, but the low electrical conductivity of the 2H-phase MoS\u003csub\u003e2\u003c/sub\u003e remains a significant obstacle, hindering overall catalytic efficiency.\u003c/p\u003e\u003cp\u003eIn this work, we present a facile one-step hydrothermal method for synthesizing a 1T-MoS\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene heterostructure. For simplicity, Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003ex\u003c/sub\u003e is hereafter denoted as Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e throughout the manuscript. Unlike previously reported approaches, which often require multi-step procedures, carbon coating, or sophisticated equipment, our method enables the direct formation of a stable 2D/2D composite without post-treatment or protective barriers. In this synthesis, MoS₂ nanosheets are uniformly intercalated between Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e layers, forming a tightly integrated heterostructure. A key innovation of our strategy is the use of tetramethylammonium hydroxide (TMAOH), which plays a dual role: it promotes the phase transition of MoS\u003csub\u003e2\u003c/sub\u003e from the 2H to the more conductive 1T phase and simultaneously enables the formation of a stable, oxidation-resistant 2D/2D composite without the need for complex equipment or additional post-treatment. This method not only simplifies the fabrication process but also significantly enhances the electrochemical performance of the resulting composite. The 1T-MoS\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e heterostructure exhibits a favorable Tafel slope of 55 mV dec\u003csup\u003e− 1\u003c/sup\u003e and an overpotential of 248 mV at -10 mA cm\u003csup\u003e− 2\u003c/sup\u003e, despite containing only 1% Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene. These values are competitive with or superior to previously reported Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e/MoS\u003csub\u003e2\u003c/sub\u003e-based catalysts, confirming the synergistic effect of the 2D/2D interface, which offers abundant active sites and efficient charge transfer pathways. Additionally, the composite demonstrates exceptional durability and operational stability over 15 hours in acidic electrolyte, highlighting its potential for practical hydrogen evolution applications. Overall, this work provides a cost-effective, scalable, and oxidation-resistant strategy for integrating MXenes with 1T-phase MoS\u003csub\u003e2\u003c/sub\u003e, advancing the development of efficient non-precious metal HER electrocatalysts.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"2. Experimental Section","content":"\u003cp\u003e\u003cem\u003e2.1 Sample preparation\u003c/em\u003e\u003c/p\u003e\u003cdiv class=\"Heading\"\u003e2.1.1 Synthesis of few-layer Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene\u003c/div\u003e\u003cp\u003eFew-layer Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene was synthesized by selectively removing the aluminum component from the Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e powder (≤ 40 µm, Carbon-Ukraine) using a hydrofluoric acid (HF, 48%, ACS reagent), based on a modified protocol from previous studies \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. As HF is extremely hazardous and highly corrosive, all procedures were carried out in a well-ventilated fume hood with appropriate personal protective equipment, including acid-resistant gloves, face shield, and protective lab coat. Specifically, 1 g of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e was added slowly to 20 mL of HF under ambient conditions and allowed to react for 24 hours to ensure complete etching of the Al layers. After the reaction, the mixture was subjected to repeated centrifugation (five cycles at 3500 rpm for 5 minutes each) with deionized water until the pH of the supernatant reached around 6. To promote exfoliation, the washed sediment was redispersed in 15 mL of a 25% aqueous solution of tetramethylammonium hydroxide (TMAOH,9 99.9999%, ThermoFisher) and stirred for 24 hours, facilitating the intercalation of TMA⁺ ions between the MXene layers. The resulting dispersion, with a basic pH (~ 10), was washed twice (3500 rpm, 10 minutes each cycle) to neutralize the pH to about 7. After a final centrifugation step (1 hour at 3500 rpm), the free-standing MXene film was collected by vacuum filtration using a Celgard membrane and then dried under vacuum at 70°C overnight.\u003c/p\u003e\u003cdiv class=\"Heading\"\u003e2.1.2 Preparation of 2H-MoS\u003csub\u003e2\u003c/sub\u003e Nanosheets\u003c/div\u003e\u003cp\u003eAmmonium heptamolybdate tetrahydrate ((NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003eMo\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e24\u003c/sub\u003e.4H\u003csub\u003e2\u003c/sub\u003eO, CAS No.12054-85-2, 99.98%) and thiourea (CAS No. 62-56-6, ≥ 99%), both obtained from Sigma Aldrich, were used as received without further purification. For the synthesis, 0.617 g of ammonium heptamolybdate and 1.14 g of thiourea were dissolved in 20 mL of deionized water under vigorous stirring for 30 minutes to obtain a uniform solution. This precursor mixture was then sealed in a 45 mL Teflon-lined stainless-steel autoclave (filled to ~ 45% capacity) and subjected to hydrothermal treatment at 200°C for 18 hours. Once the autoclave cooled naturally to ambient temperature, the resulting black product was collected, thoroughly washed with both deionized water and ethanol using centrifugation at 8000 rpm for 10 minutes per wash cycle, and then dried under vacuum at 60°C overnight.\u003c/p\u003e\u003cdiv class=\"Heading\"\u003e2.1.3 Preparation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e/1T-MoS\u003csub\u003e2\u003c/sub\u003e heterostructures\u003c/div\u003e\u003cp\u003eHierarchical heterostructures composed of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e and 1T-MoS\u003csub\u003e2\u003c/sub\u003e were fabricated via a hydrothermal synthesis approach, as schematically shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. First, thiourea and ammonium heptamolybdate tetrahydrate were dissolved in 20 mL of deionized water. Subsequently, 0.3 g of tetramethylammonium hydroxide pentahydrate (TMAOH, ((CH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003eN(OH).5H\u003csub\u003e2\u003c/sub\u003eO, CAS No. 10424-65-4, Sigma Aldrich, ≥ 97%) was introduced to the solution. Varying amounts of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e (corresponding to 1%, 2%, 5%, and 10% relative to the mass of (NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e6\u003c/sub\u003eMo\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e24\u003c/sub\u003e.4H\u003csub\u003e2\u003c/sub\u003eO) were incorporated to evaluate the effect on HER performance. These resulting composites were designated as TM-1, TM-2, TM-5, and TM-10, respectively. The mixture was stirred vigorously for an additional 30 minutes to ensure homogeneity. The mixtures underwent hydrothermal treatment at 200°C for 18 h, followed by washing and drying as above.\u003c/p\u003e\u003cdiv class=\"Heading\"\u003e2.1.4 Preparation of 1T-MoS\u003csub\u003e2\u003c/sub\u003e Nanosheets\u003c/div\u003e\u003cp\u003eAs a reference sample, 1T-MoS2 nanosheets were synthesized using the same procedure described in Section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e2.1.3\u003c/span\u003e, but without Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e addition, using ammonium heptamolybdate (0.617 g), thiourea (1.14 g), and TMAOH (0.3 g).\u003c/p\u003e\u003ch2\u003e2.2 Characterization methods\u003c/h2\u003e\u003cp\u003eThe crystalline phases and structural properties of the materials were characterized by X-ray diffraction (XRD) using a Bruker D8 ADVANCE diffractometer equipped with a copper Kα radiation source (λ = 1.5406 Å). Data were collected across a 2θ range of 5° to 70°, with an increment of 0.02° per step. Raman spectroscopy was performed using an i-Raman Plus 532H portable spectrometer, operating with a 532 nm laser source. Thermogravimetric analyses (TGA) were performed on a NETZSCH TG 209F1 Iris instrument under a flow of Ar with a temperature range of 27–800°C. Surface morphology was examined via scanning electron microscopy (SEM) on a ZEISS Gemini SEM 460 system, which features a field emission gun capable of operating in the 0.1–30 keV range.\u003c/p\u003e\u003cp\u003eElemental composition and surface chemistry were analyzed by X-ray photoelectron spectroscopy (XPS) using a SPECS instrument fitted with a monochromatic Al Kα source (energy: 1486.7 eV) and a Phoibos 150 hemispherical analyzer. Survey scans were acquired with a pass energy of 100 eV, while core-level spectra were collected at 50 or 30 eV. The system operated under ultra-high vacuum conditions (≤ 10\u003csup\u003e− 9\u003c/sup\u003e mbar). An electron flood gun was employed to neutralize surface charging during measurement.\u003c/p\u003e\u003cp\u003eHigh-resolution structural and compositional imaging was conducted using a JEOL ARM200F transmission electron microscope (TEM) at 200 kV. This system was outfitted with CEOS aberration correctors for both probe and image formation, and a dual silicon drift detector for energy-dispersive X-ray spectroscopy (EDS). For high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), a convergence semi-angle of approximately 27 mrad and a collection angle of 55 mrad were used. The STEM imaging resolution was around 0.8 Å, and EDS mapping provided elemental distributions at both nanometre and atomic resolutions.\u003c/p\u003e\u003ch2\u003e2.4 Electrochemical measurements\u003c/h2\u003e\u003cp\u003eElectrochemical measurements were performed at room temperature using a conventional three-electrode configuration connected to a CHI760E electrochemical workstation. A 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution served as the electrolyte. The working electrode was a glassy carbon (GC) disk with a 2 mm diameter, while an Ag/AgCl electrode (saturated with KCl) and a carbon rod were used as the reference and counter electrodes, respectively. Catalyst ink for HER testing was prepared by dispersing 10 mg of the synthesized material in a solvent mixture containing 500 µL of deionized water, 500 µL of isopropanol, and 8 µL of 5 wt.% Nafion solution (Sigma-Aldrich). The suspension was subjected to ultrasonic agitation for 30 minutes to ensure uniform dispersion. A 1 µL aliquot of the resulting ink was drop-cast onto the GC electrode, achieving an approximate catalyst loading of 0.32 mg cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, followed by air drying at ambient conditions. For benchmarking purposes, a commercial Pt/C (20 wt.%) ink was prepared and deposited using the same protocol. Before testing, the electrolyte was deaerated by purging with nitrogen gas for 30 minutes. All measured potentials were calibrated against the reversible hydrogen electrode (RHE) using the equation: E \u003csub\u003eRHE\u003c/sub\u003e = E \u003csub\u003eAg/AgCl\u003c/sub\u003e + 0.059 pH + 0.222 V. Linear sweep voltammetry (LSV) was conducted at a scan rate of 5 mV s⁻¹, with 90% iR compensation. The double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e) was obtained by plotting half the difference in anodic and cathodic current densities (ΔJ/2) at 0.27 V vs. RHE against varying scan rates. The electrochemically active surface area (ECSA) was estimated using the relation: ECSA = C\u003csub\u003edl​\u003c/sub\u003e/C\u003csub\u003es\u003c/sub\u003e, where Cs​ is the specific capacitance for standard electrode materials, with a literature value of 0.04 mF cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e used for carbon-based materials \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. The turnover frequency (TOF) for each catalyst was determined by quantifying the number of active sites involved in the hydrogen evolution reaction. TOF, expressed in s\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, was calculated using the equation: TOF (s\u003csup\u003e-1\u003c/sup\u003e) = (J × S) / (2 × F × n) \u003csup\u003e13\u003c/sup\u003e. Where J represents the current density measured at various overpotentials, S (0.0314 cm²) is the geometric surface area of the electrode, and F (96500 C mol\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e) is the Faraday constant. The factor of 1/2 accounts for the number of electrons required to produce one hydrogen molecule, and n (in moles) is the number of active sites obtained using the cyclic voltammetry (CV) method, where n = Qs/F. All CV tests were conducted in a solution of 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e at a scan rate of 40 mV s\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. The surface charge (Q\u003csub\u003es\u003c/sub\u003e) can be obtained by integrating the CV curve’s charge over the whole potential range; the half value of the charge is the integrated charge over the entire potential range \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. Electrochemical impedance spectroscopy (EIS) was performed across a frequency range of 0.1 Hz to 1 MHz, using a 5 mV AC amplitude and an applied overpotential of 250 mV vs. RHE. The catalyst durability was evaluated by conducting 3000 continuous CV cycles at a scan rate of 100 mV s\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Additionally, the chronopotentiometric stability of the catalyst was evaluated by applying a constant current density of 10 mA cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e for 15 hours in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. The working electrode was prepared using an identical catalyst ink as described as previously described, ensuring consistent mass loading. The ink was drop-cast onto a 1 cm × 1 cm carbon paper substrate and allowed to dry under ambient conditions. Carbon paper was chosen due to its electrochemical inertness toward the HER, ensuring that observed activity originated solely from the catalyst.\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Characterization of electrocatalysts\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe crystallographic structure and phase composition of the synthesized catalyst were investigated using X-ray diffraction (XRD) and Raman spectroscopy. \u003cb\u003eFigure S1a\u003c/b\u003e displays the XRD profiles of both Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and its precursor Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e MAX phase. The Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e pattern shows distinct reflections at 6.15\u0026deg;, 18.17\u0026deg;, 27.7\u0026deg;, and 36.5\u0026deg;, which correspond to the (002), (004), (006), and (008) planes, respectively, indicative of layered MXene formation \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. In contrast, the peak around 38.7\u0026deg;, typically associated with aluminum (Al) in Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e, disappears after etching, confirming successful removal of the Al layers and formation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e22\u003c/sup\u003e. Using Bragg\u0026rsquo;s law, the calculated interlayer spacing for Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e is approximately 1.42 nm, consistent with literature values \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. The XRD spectrum of 2H-MoS₂ exhibits sharp peaks at 14.1\u0026deg;, 33.4\u0026deg;, and 58.8\u0026deg;, corresponding to the (002), (100), and (110) reflections of its hexagonal phase (P6₃/mmc, JCPDS 37-1492) \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. A comparison with the 1T-MoS\u003csub\u003e2\u003c/sub\u003e pattern shows a shift of the (002) peak from ~\u0026thinsp;14\u0026deg; to around 9\u0026deg;, indicating an increase in interlayer distance. Moreover, the disappearance of the peak at ~\u0026thinsp;14\u0026deg; implies the formation of a few-layered MoS\u003csub\u003e2\u003c/sub\u003e. Broadening and shifting of the (100) and (110) peaks to lower angles, along with the emergence of a (004) reflection at ~\u0026thinsp;18.2\u0026deg;, suggest distortion and phase transition to the metallic 1T-MoS₂ phase \u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. These observations confirm the crucial role of tetramethylammonium hydroxide (TMAOH) in promoting 1T phase formation. The intercalation of TMA\u003csup\u003e+\u003c/sup\u003e ions between the MoS\u003csub\u003e2\u003c/sub\u003e layers expands the interlayer spacing and induces distortions in the atomic arrangement. This process promotes the formation of the 1T phase and generates numerous grain boundary defects, which enhance the density of active sites for hydrogen production \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. This interpretation is further supported by the increase in (002) d-spacing from 0.63 nm (2H phase) to 0.97 nm (1T phase). To directly assess the intercalation of TMA⁺ and its influence on phase transformation, we performed thermogravimetric analysis (TGA) on both 2H-MoS\u003csub\u003e2\u003c/sub\u003e and 1T-MoS\u003csub\u003e2\u003c/sub\u003e after TMAOH treatment. As shown in \u003cb\u003eFigure S1c\u003c/b\u003e, the 2H-MoS\u003csub\u003e2\u003c/sub\u003e sample exhibited a total mass loss of approximately 8.0 wt% up to 800\u0026deg;C, primarily due to desorption of surface-bound water and minor structural changes. In contrast, the TMAOH-intercalated 1T-MoS₂ exhibited a significantly larger mass loss of around 22.5 wt%, with a distinct weight loss event observed between 200\u0026deg;C and 400\u0026deg;C. This thermal behaviour indicates the presence and subsequent decomposition of intercalated organic species (TMA⁺), providing strong evidence for successful intercalation and structural modification toward the 1T phase. This interpretation is consistent with previous reports showing that the decomposition of TMA⁺ occurs between 200\u0026deg;C and 400\u0026deg;C, contributing to a similar weight loss in the TMAOH-intercalated MoS\u003csub\u003e2\u003c/sub\u003e sample \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Furthermore, in the Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e/1T-MoS\u003csub\u003e2\u003c/sub\u003e composites (TM-1 through TM-10), XRD peaks indicative of both MXene and 1T-MoS\u003csub\u003e2\u003c/sub\u003e are present. Interestingly, the Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e peak is absent in TM-1, likely due to its low loading, which suggests good dispersion of MXene within the composite. In samples with higher MXene content (TM-2, TM-5, TM-10), a slight upshift of the (002) MXene peak is observed, possibly resulting from the partial detachment of terminal functional groups during hydrothermal treatment \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Notably, the (002) diffraction peak of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e in these composites remains at a low angle (~\u0026thinsp;7.15\u0026deg;), indicating that the MXene structure remains delaminated and highly expanded even after hydrothermal processing at 200\u0026deg;C. This suggests that the interlayer spacing is still significantly increased, likely due to the retention of intercalated TMA⁺ ions and 1T-MoS₂ nanosheets between the Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e layers, consistent with previous reports \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Additionally, a minor peak at 25.35\u0026deg; in samples with \u0026gt;\u0026thinsp;1% MXene (\u003cb\u003eFigure S1b\u003c/b\u003e) suggests trace formation of TiO\u003csub\u003e2\u003c/sub\u003e, likely due to surface oxidation of Ti under synthesis conditions. This peak is notably absent in TM-1, supporting the conclusion that low MXene content helps preserve structural integrity and promotes efficient charge transport. Another noteworthy feature in TM-10 is the reappearance of a (002) peak at 2θ\u0026thinsp;\u0026asymp;\u0026thinsp;14.4\u0026deg;, indicating the stacking of MoS\u003csub\u003e2\u003c/sub\u003e layers (\u003cb\u003eFigure S1b\u003c/b\u003e). This may imply partial transformation back to the 2H phase under high MXene loading, which could impact electronic properties. Raman spectroscopy provided further insights into the materials' microstructure (\u003cb\u003eFigure S1d\u003c/b\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). The Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene sample exhibited characteristic peaks at 126, 209, 359, and 723 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, in agreement with typical vibrations of delaminated MXenes, corresponding to A\u003csub\u003e1g\u003c/sub\u003e (Ti, O, C), E\u003csub\u003eg\u003c/sub\u003e (O), and A\u003csub\u003e1g\u003c/sub\u003e (C) modes, respectively \u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb shows Raman spectra for 2H-MoS\u003csub\u003e2\u003c/sub\u003e, 1T-MoS\u003csub\u003e2,\u003c/sub\u003e and composites. Considering 2H-MoS\u003csub\u003e2\u003c/sub\u003e, the characteristic peaks appearing at 373 and 401 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are assigned to the A\u003csub\u003e1g\u003c/sub\u003e and E\u003csup\u003e1\u003c/sup\u003e\u003csub\u003e2g\u003c/sub\u003e bonds of the 2H phase, respectively \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. The intensity of the out-of-plane A\u003csub\u003e1g\u003c/sub\u003e mode surpasses that of the in-plane E\u003csup\u003e1\u003c/sup\u003e\u003csub\u003e2g\u003c/sub\u003e mode, suggesting an edge-enriched MoS\u003csub\u003e2\u003c/sub\u003e structure \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. Upon TMA⁺ treatment, the MoS₂ bands decreased in intensity, and a new band at 145 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (J\u003csub\u003e1\u003c/sub\u003e mode) emerged, indicating successful formation of the 1T phase. A further signal at ~\u0026thinsp;283 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (E\u003csub\u003e1g\u003c/sub\u003e) confirmed the presence of Mo atoms in octahedral coordination, characteristic of 1T-MoS\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e26\u003c/sup\u003e. The composite spectra (TM-1 to TM-10) showed similar vibrational features. Notably, TM-1 exhibited the weakest signals related to 2H-MoS\u003csub\u003e2\u003c/sub\u003e, suggesting a more complete phase conversion. In contrast, TM-10 showed increased 2H-character, corroborating the XRD findings. Additionally, the peak intensity of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e, observed at approximately 205 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e across all composites, appears notably flat. This reduced intensity is attributed to the surface of the Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene being covered by a layer of 1T-MoS\u003csub\u003e2\u003c/sub\u003e, coupled with the intrinsically weak signal characteristics of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e. Consequently, the laser cannot penetrate the MoS\u003csub\u003e2\u003c/sub\u003e layer effectively, resulting in limited information about the underlying Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe surface morphology of the synthesized catalysts was examined using scanning electron microscopy (SEM). As illustrated in \u003cb\u003eFigure S1e\u003c/b\u003e, Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene displays exfoliated, layered nanosheets with a distinct and organized structure. The inset further reveals its micrometer-scale dimensions, which offer a suitable platform for the uniform deposition of MoS\u003csub\u003e2\u003c/sub\u003e layers. SEM imaging of 2H-MoS\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) presents a flower-like morphology with a tendency for sheet aggregation. In contrast, the 1T-MoS₂ sample exhibits a more flake-like texture, indicating structural variation between the two phases (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). In the composite samples (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-e), MoS\u003csub\u003e2\u003c/sub\u003e sheets are observed to grow both within the interlayer spaces and across the surface of the Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene. This layered Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e/1T-MoS\u003csub\u003e2\u003c/sub\u003e configuration mitigates nanosheet restacking and concurrently shields the oxidation-sensitive Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e structure \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e, a finding consistent with the XRD results. Interestingly, even though deionized water was used during hydrothermal treatment, the TM-1 composite (with 1% MXene) showed negligible signs of oxidation, while slight oxidation was detected in samples with higher MXene content. This suggests that tetramethylammonium (TMA⁺) ions play a stabilizing role during synthesis, encouraging 1T-MoS\u003csub\u003e2\u003c/sub\u003e phase formation and reducing oxidative degradation of the MXene, as previously observed in related studies \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. Closer inspection (inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec) reveals that the MoS\u003csub\u003e2\u003c/sub\u003e nanosheets closely overlay the Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e layers, forming a compact heterostructure. This architecture promotes greater exposure of catalytic sites and enhances diffusion pathways for hydrogen evolution reaction (HER) processes \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eTo confirm the structural integrity of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e in the TM-1 composite and assess the potential formation of TiO\u003csub\u003e2\u003c/sub\u003e during hydrothermal treatment, a combination of high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS) was employed. TEM analysis of the Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene (\u003cb\u003eFigure S2a\u003c/b\u003e) displayed its characteristic few-layered, sheet-like morphology, confirming its two-dimensional nature. The HRTEM image (\u003cb\u003eFigure S2b\u003c/b\u003e) showed distinct lattice fringes with a spacing of around 0.52 nm, corresponding to the (004) crystallographic planes, in agreement with interlayer distances measured via XRD. Additionally, selected area electron diffraction (SAED) patterns (inset of \u003cb\u003eFigure S2b\u003c/b\u003e) indicated a hexagonal symmetry typical of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e. For 2H-MoS\u003csub\u003e2\u003c/sub\u003e, the TEM images \u003cb\u003e(Figure S2c)\u003c/b\u003e revealed aggregated nanosheets, while the observed d-spacing of approximately 0.62 nm aligned with the (002) plane, confirming its layered structure. A structural transition to the 1T phase was evidenced by increased interlayer spacing (~\u0026thinsp;0.92 nm) and lattice distortion seen in HRTEM \u003cb\u003e(Figure S2f)\u003c/b\u003e, characteristics associated with the metallic 1T form of MoS\u003csub\u003e2\u003c/sub\u003e. Notably, the 1T-MoS\u003csub\u003e2\u003c/sub\u003e exhibited a thin, few-layered architecture, an advantageous feature that promotes efficient ion transport and electron mobility during electrocatalytic processes. In the case of the TM-1 composite, both TEM and HRTEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, b) revealed a homogeneous hybrid architecture, where 1T-MoS\u003csub\u003e2\u003c/sub\u003e nanosheets were well-dispersed across the MXene substrate, forming a cohesive 2D network. HRTEM images (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) revealed well-defined lattice fringes corresponding to the (002) plane of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e (~\u0026thinsp;1.1 nm) alongside the (002) plane of 1T-MoS₂ (~\u0026thinsp;0.9 nm), confirming that the MXene structure remains intact after synthesis. Notably, no TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles were observed on the surface of the MXene, in contrast to previous reports where hydrothermal treatment led to the formation of TiO\u003csub\u003e2\u003c/sub\u003e nanocrystals \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. STEM-EDS elemental mapping (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec) further confirmed the homogeneous distribution of Ti, C, Mo, S, and O, with no localized oxygen enrichment indicative of oxide formation. The O/Ti atomic ratio, determined from EDS spectra of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e before and after hydrothermal treatment at 200\u0026deg;C, was approximately 0.53, closely matching that of freshly prepared, minimally oxidized Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e (0.46, \u003cb\u003eFigure S2g\u003c/b\u003e). In contrast, aged or hydrothermally oxidized MXenes typically exhibit significantly higher O/Ti ratios, reflecting extensive TiO\u003csub\u003e2\u003c/sub\u003e formation. Additionally, neither the XRD patterns nor the Raman spectra of TM-1 displayed any signatures of crystalline TiO\u003csub\u003e2\u003c/sub\u003e, further confirming the chemical stability of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e under the applied hydrothermal conditions. This exceptional structural preservation is attributed to the presence of TMAOH, used both during the initial delamination of MXene and in small quantities during the in-situ synthesis of the 1T-MoS\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composite. Beyond facilitating delamination through TMA⁺ intercalation, TMAOH acts as a pH buffer and surface passivating agent. Literature reports have shown that ammonium-based hydroxides, such as NH\u003csub\u003e4\u003c/sub\u003eOH and TMAOH, stabilize Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e in aqueous media by maintaining the dispersion pH around 6\u0026ndash;7, a range where MXenes remain colloidally stable for extended periods without significant aggregation or oxidation. For instance, NH\u003csub\u003e4\u003c/sub\u003eOH-treated Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e has been reported to remain stable for over 40 days at pH\u0026thinsp;~\u0026thinsp;6, whereas oxidation and sedimentation accelerate under acidic conditions or when pH exceeds 12 due to flocculation and hydrolysis \u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. The mildly basic environment provided by TMAOH during our synthesis likely suppresses hydrolytic degradation and preserves interlayer TMA⁺ cations, both of which contribute to protecting Ti sites from oxidation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSAED analysis of the TM-1 composite (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed) reveals several well-defined diffraction rings, highlighted in blue. These rings are characteristic of polycrystalline 1T-MoS\u003csub\u003e2\u003c/sub\u003e and indicate the presence of numerous randomly oriented nanocrystalline domains, consistent with previous reports on chemically stabilized 1T-MoS\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e33,34\u003c/sup\u003e. In addition to the 1T-MoS\u003csub\u003e2\u003c/sub\u003e pattern, distinct diffraction spots corresponding to Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and 2H-MoS\u003csub\u003e2\u003c/sub\u003e are also observed, confirming the coexistence of multiple structural phases within the composite.\u003c/p\u003e\u003cp\u003eX-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the elemental composition and surface chemical states of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e and its composites. The full survey scan for pristine Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e, as shown in \u003cb\u003eFigure S3a\u003c/b\u003e, displays distinct signals corresponding to C 1s, Ti 2p, O 1s, and F 1s. Notably, no aluminum peaks were detected, confirming the complete etching of Al layers from the precursor MAX phase. The presence of oxygen- and fluorine-containing groups is consistent with HF treatment, which introduces functional surface terminations. The high-resolution Ti 2p spectrum (\u003cb\u003eFigure S3b\u003c/b\u003e) reveals four prominent peaks: those at around 455.8 eV and 461.8 eV are assigned to Ti 2p\u003csub\u003e3/2\u003c/sub\u003e and Ti 2p\u003csub\u003e1/2\u003c/sub\u003e in Ti-C bonds, respectively. Peaks at higher binding energies (~\u0026thinsp;459.3 eV and ~\u0026thinsp;465.1 eV) indicate Ti-O bonding, suggesting the formation of TiO\u003csub\u003e2\u003c/sub\u003e species, likely due to surface oxidation. The C 1s spectrum (\u003cb\u003eFigure S3c\u003c/b\u003e) includes a main peak at 282.4 eV related to Ti-C bonding, confirming Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e formation \u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Additional peaks at 285.1 eV, 286.7 eV, and 289.4 eV are attributed to adventitious carbon, C-O, and C\u0026thinsp;=\u0026thinsp;O functionalities, respectively, likely arising from atmospheric exposure and residual surface groups \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. The O 1s spectrum (\u003cb\u003eFigure S3d\u003c/b\u003e) displays peaks at 529.5 eV and 531.3 eV, which correspond to hydroxylated and oxidized Ti sites (C-Ti-OH and C-Ti-O), further indicating the presence of -OH and -O terminations on the MXene surface due to selective etching \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Also, the complete survey XPS spectra of the composites are shown in \u003cb\u003eFigure S3a\u003c/b\u003e. Only signals corresponding to S 2p, Mo 3d, C 1s, and O 1s are detected, while no distinct Ti 2p signals are observed. This absence of Ti peaks suggests that the MoS\u003csub\u003e2\u003c/sub\u003e layer fully covers the Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene surface, effectively shielding it from XPS detection. This behavior is consistent with previous reports in the literature\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. The Mo 3d spectrum of all prepared composites (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea) displays prominent peaks at 228.4 eV and 231.5 eV corresponding to the Mo⁴⁺ 3d\u003csub\u003e5/2\u003c/sub\u003e and Mo⁴⁺ 3d\u003csub\u003e3/2\u003c/sub\u003e characteristic peaks of 1T-MoS\u003csub\u003e2\u003c/sub\u003e, respectively. Additional weaker signals located at 232.6 and 229.7 eV correspond to the characteristic 3d\u003csub\u003e5/2\u003c/sub\u003e and 3d\u003csub\u003e3/2\u003c/sub\u003e of Mo\u003csup\u003e4+\u003c/sup\u003e species from the non-metallic 2H phase \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, both of which were also detected in the 2H-MoS\u003csub\u003e2\u003c/sub\u003e sample (\u003cb\u003eFigure S3e\u003c/b\u003e). Furthermore, the peak at 235.6 eV illustrates Mo\u003csup\u003e6+\u003c/sup\u003e, suggesting the partial oxidation of MoS\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e26\u003c/sup\u003e. A low-intensity peak at 225.6 eV corresponds to the S 2s. Upon deconvolution of the S 2p spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), two principal peaks appear at 161.1 eV and 162.3 eV, assigned to S 2p\u003csub\u003e3/2\u003c/sub\u003e and S 2p\u003csub\u003e1/2\u003c/sub\u003e of the 1T-MoS\u003csub\u003e2\u003c/sub\u003e phase, respectively \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Minor peaks at 161.7 eV and 163.8 eV reflect the corresponding signals from the 2H phase. These assignments were further validated by comparison with the XPS spectra of individual 1T- and 2H-MoS₂ reference samples (\u003cb\u003eFigure S3g\u003c/b\u003e and \u003cb\u003eS3h\u003c/b\u003e), which exhibit consistent peak positions and spectral profiles. Additionally, the full-width at half-maximum (FWHM) values of the deconvoluted peaks are summarized in \u003cb\u003eTable S1a\u003c/b\u003e, further supporting the reliability of the fitting and the accuracy of the 1T/2H peak assignments. The relative 1T phase content was quantified by integrating the areas of the deconvoluted S 2p\u003csub\u003e3/2\u003c/sub\u003e and S 2p\u003csub\u003e1/2\u003c/sub\u003e peaks assigned to 1T- and 2H-MoS\u003csub\u003e2\u003c/sub\u003e, and calculating the ratio of the 1T signal area to the total S 2p area (\u003cb\u003eTable S1b\u003c/b\u003e). It is worth noting that a high proportion of 1T-MoS\u003csub\u003e2\u003c/sub\u003e is advantageous for HER performance due to its enhanced electronic conductivity and higher density of catalytically active sites.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Electrocatalytic performance for HER\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe HER performance of the synthesized materials was assessed using a conventional three-electrode system in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, with iR-compensation applied throughout. A commercial Pt/C catalyst (20 wt.%) and a bare glassy carbon (GC) electrode served as performance references. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, the GC electrode showed negligible catalytic activity, confirming that the measured performance originates from the tested materials. Predictably, Pt/C delivered outstanding HER efficiency, requiring just 34 mV to reach a current density of 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb), reaffirming its status as a benchmark catalyst. In contrast, pure Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene displayed relatively poor HER activity, with an overpotential of 770 mV at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. This suggests that while it is not intrinsically active, Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e serves as a conductive substrate that facilitates electron transport in hybrid systems. Meanwhile, 1T-MoS\u003csub\u003e2\u003c/sub\u003e significantly outperformed its 2H counterpart, achieving an overpotential of 278 mV compared to 369 mV, respectively, an improvement attributed to the stabilization of the 1T phase via TMA\u003csup\u003e+\u003c/sup\u003e ion intercalation. To further investigate HER enhancement, 1T-MoS₂ was combined with varying amounts of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e (1, 2, 5, and 10 wt.%) to form hybrid composites denoted as TM-1, TM-2, TM-5, and TM-10. Among these, TM-1 exhibited the most promising activity, achieving an overpotential of approximately 248 mV at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. However, higher MXene loadings led to decreased performance; for example, TM-10 required 309 mV at the same current, implying that excessive Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e may obscure the active MoS\u003csub\u003e2\u003c/sub\u003e surface, thereby hindering catalytic efficiency. For comparison, a physically blended sample (PM-1) was also prepared by mixing pre-synthesized 1T-MoS\u003csub\u003e2\u003c/sub\u003e with 1 wt.% Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e in a 1:1 water-isopropanol solution, followed by ultrasonication (4 hours), filtration, and vacuum drying at 70\u0026deg;C. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, PM-1 required a much higher overpotential (334 mV) to achieve 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, substantially underperforming relative to TM-1 (~\u0026thinsp;248 mV). This result emphasizes the significance of the synthesis strategy; unlike the hydrothermally grown TM-1, the physical mixture demonstrated weak interfacial contact between components, which restricted charge transfer and overall HER activity.\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eTo gain further insights into the reaction mechanism and kinetics of HER, Tafel slope analysis was conducted. A lower Tafel slope typically suggests more efficient charge transfer and improved catalytic kinetics. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec, the extracted Tafel slope values for 2H-MoS\u003csub\u003e2\u003c/sub\u003e,1T-MoS\u003csub\u003e2\u003c/sub\u003e, TM-1, TM-2, TM-5, TM-10, and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e were 137, 81, 55, 70, 83, 91, and 203 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. Among these, TM-1 demonstrated the smallest Tafel slope (55 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), signifying its enhanced intrinsic catalytic performance relative to the other samples. This result also implies that the HER mechanism on TM-1 primarily follows the Volmer-Heyrovsky pathway \u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. TM-1 catalyst exhibits a Tafel slope of 55 mV dec⁻\u0026sup1;, which is superior to many previously reported systems, as shown in \u003cb\u003eTable S2\u003c/b\u003e and \u003cb\u003eFigure S6\u003c/b\u003e. The proposed mechanism of the 1T-MoS\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene is shown in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb. In this pathway, a proton (H\u003csub\u003e3\u003c/sub\u003eO\u003csup\u003e+\u003c/sup\u003e) interacts with an electron (e⁻) at the active catalytic site (*), producing an adsorbed hydrogen atom (H) through the step: H\u003csub\u003e3\u003c/sub\u003eO⁺ + e⁻ + * \u0026rarr; H* + H\u003csub\u003e2\u003c/sub\u003eO. Subsequently, the H* species combines with another proton and electron to form molecular hydrogen (H₂), completing the reaction: H\u003csub\u003e3\u003c/sub\u003eO⁺ + e⁻ + H* \u0026rarr; H\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e\u003cp\u003eThe hydrogen evolution reaction (HER) performance of an electrocatalyst is closely linked to its electrical conductivity, which affects charge transport efficiency. To investigate the interfacial charge transfer behavior and elucidate HER kinetics, electrochemical impedance spectroscopy (EIS) was conducted. This analytical method provides valuable insights into the electrode-electrolyte interface and helps characterize the electrical response of catalytic systems. As shown in the Nyquist plots (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed), the charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e) is a critical parameter for evaluating HER activity. Among the tested samples, TM-1 exhibited the lowest R\u003csub\u003ect\u003c/sub\u003e (9.3 Ω), indicating more efficient electron movement at the catalyst-electrolyte boundary. This resistance is notably lower than that of pristine 1T-MoS\u003csub\u003e2\u003c/sub\u003e (15.9 Ω), demonstrating that incorporating Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene substantially enhances electron conductivity and overall HER performance \u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. This improvement is attributed to the strong interfacial interaction between Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e and MoS\u003csub\u003e2\u003c/sub\u003e, which supports rapid charge transfer and reduces interfacial resistance. Furthermore, the Nyquist plots for MoS\u003csub\u003e2\u003c/sub\u003e-based catalysts reveal two distinct semicircular features: a smaller one at high frequencies related to surface texturing or porosity, and a larger semicircle at lower frequencies corresponding to the faradaic charge transfer process. These observations were modelled using an equivalent circuit (\u003cb\u003eFigure S5\u003c/b\u003e) \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. The improved HER behavior of the composite is further supported by the synergistic combination of 1T-MoS\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e, which enhances charge separation and lowers the energy barrier for hydrogen evolution \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Beyond charge transport, the availability of active catalytic sites plays a pivotal role in HER activity and is quantitatively represented by the electrochemically active surface area (ECSA). This metric, derived from non-Faradaic capacitive behavior, provides an independent assessment of active site density and intrinsic site activity \u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. To determine ECSA, cyclic voltammetry (CV) was performed at different scan rates in a potential window that excludes faradaic reactions (\u003cb\u003eFigure S4\u003c/b\u003e). From these CV curves, the double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e) was extracted (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee). As ECSA is directly proportional to C\u003csub\u003edl\u003c/sub\u003e \u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e, this value offers a proxy for comparing the accessible surface area across samples (\u003cb\u003eTable S3\u003c/b\u003e). Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene exhibited the lowest C\u003csub\u003edl\u003c/sub\u003e (0.8 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), likely due to nanosheet restacking that restricts active site exposure. Similarly, 2H-MoS₂ showed a modest C\u003csub\u003edl\u003c/sub\u003e (1.82 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), consistent with the low reactivity of its semiconducting basal planes. In contrast, 1T-MoS₂ and the composite materials demonstrated significantly enhanced C\u003csub\u003edl\u003c/sub\u003e values, underscoring the role of TMAOH-assisted phase conversion in increasing catalytic site density. Among all samples, TM-1 had the highest ECSA (350 cm\u0026sup2;), corresponding to a C\u003csub\u003edl\u003c/sub\u003e of 14 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. This is attributed to its enriched 1T-phase content (89%), which contributes both more active edge/basal sites and better conductivity. The significant increase in the number of active sites within the composites indicates that the presence of 1T-MoS\u003csub\u003e2\u003c/sub\u003e, both on top of and within the interlayer spacing of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, not only prevents aggregation but also enhances the accessibility of the aqueous electrolyte to the catalytically active surface. To further confirm the intrinsic properties of the electrocatalysts, the turnover frequency (TOF) was evaluated. TOF quantifies electron transfer efficiency and active site availability, which can be determined from cyclic voltammetry (CV) measurements \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. TOF values were calculated in the potential range of 0.23 to 0.25 V versus RHE. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef, TM-1 achieved the highest TOF at 0.81 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 250 mV overpotential, indicating superior site accessibility and electrocatalytic activity. This enhanced performance stems from the optimized 1T-MoS\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e interface, which improves both surface exposure and charge transport, thereby elevating the overall HER efficiency.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo investigate the durability of the TM-1 electrocatalyst, cyclic voltammetry (CV) was conducted in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e at a scan rate of 100 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). After 3,000 consecutive cycles, the resulting polarization curves showed only slight deviation from the initial measurements, indicating the robust operational stability of TM-1. Stability over prolonged usage is a crucial metric for assessing electrocatalyst performance, especially in the context of industrial-scale hydrogen production. we employed chronopotentiometry to evaluate long-term electrochemical performance. TM-1 and a commercial 20 wt% Pt/C reference were tested under a steady current density of 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. The catalyst ink was uniformly deposited on one side of carbon paper, maintaining a mass loading of 0.32 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, and dried at 50\u0026deg;C to ensure consistent testing conditions. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb, TM-1 exhibited only a minor potential increase of 23 mV over 15 hours, retaining more than 91% of its initial activity. This performance surpasses that of the Pt/C reference and underscores the exceptional stability of the nanocomposite during HER operation. To further confirm the catalyst\u0026rsquo;s durability under industrial conditions, we extended the stability test to a much higher current density of 200 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb, TM-1 maintained excellent electrochemical stability even at this increased current. This confirms that TM-1 is effective not only in laboratory settings but also possesses the durability needed for large-scale hydrogen production systems.\u003c/p\u003e\u003cp\u003ePost-stability tests at a current density of 10 mAcm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e were conducted using EIS, LSV, SEM, Raman spectroscopy, and XPS to examine any structural or electrical degradation. EIS results revealed that the charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e) remained largely unchanged (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec), indicating that electrical conductivity was preserved. Furthermore, the LSV curves recorded before and after the 15-hour chronopotentiometry test were nearly superimposable (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed), reinforcing the stability of the catalyst. SEM images of TM-1 after durability testing (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ee) confirmed that the nanosheet morphology was retained, with no signs of material degradation. Raman spectroscopy provided additional insight into the structural integrity of the 1T-MoS\u003csub\u003e2\u003c/sub\u003e phase following long-term testing. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ef, the post-test spectrum exhibited the distinctive vibrational modes of the 1T phase, including J\u003csub\u003e1\u003c/sub\u003e (~\u0026thinsp;145 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), J\u003csub\u003e3\u003c/sub\u003e (~\u0026thinsp;344 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) \u003csup\u003e46\u003c/sup\u003e, and A\u003csub\u003e1g\u003c/sub\u003e (~\u0026thinsp;402 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Notably, the absence of the E\u0026sup1;\u003csub\u003e2g\u003c/sub\u003e mode (~\u0026thinsp;373 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), typically associated with the 2H phase, suggests that no significant phase transition from 1T to 2H occurred during electrolysis. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eg\u0026ndash;i presents the XPS spectra of the TM-1 catalyst after the HER stability test. The survey spectra confirm the continued presence of Mo, S, O, F, and C elements. The enhanced C signal is attributed to the underlying carbon paper substrate, while the F signal originates from the Nafion used as a bonding agent during sample preparation for the HER test. Notably, the Mo 3d and S 2p peaks characteristic of the 1T-MoS\u003csub\u003e2\u003c/sub\u003e structure remain well preserved after the HER test. However, the relative intensity analysis of the S 2p peak indicates a slight decrease in the 1T-phase content to 79.51%, suggesting that while some phase transformation may occur under prolonged electrochemical operation, a significant proportion of the catalytically active 1T phase remains intact. This highlights the structural robustness and catalytic relevance of the 1T-phase MoS\u003csub\u003e2\u003c/sub\u003e component during HER.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this work, we report a scalable and one-step hydrothermal synthesis of a 2D/2D 1T-phase molybdenum disulfide (1T-MoS\u003csub\u003e2\u003c/sub\u003e) and titanium carbide MXene (Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e) heterostructure that demonstrates promising catalytic performance for the hydrogen evolution reaction (HER). Unlike prior studies, which often rely on multi-step procedures, inert atmospheres, or carbon coating to preserve MXene integrity, our approach uniquely incorporates tetramethylammonium hydroxide (TMAOH) to simultaneously induce the 1T phase in MoS\u003csub\u003e2\u003c/sub\u003e and stabilize Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e against oxidation during synthesis. Our key findings reveal that incorporating just 1 wt.% Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e into the 1T-MoS2 matrix forms a strong interfacial heterojunction, promoting fast charge transfer, a higher density of active sites, and improved phase stability. The optimized TM-1 composite achieved a low overpotential of 248 mV at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, a Tafel slope of 55 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and remarkable stability over 15 hours. Compared to existing catalysts, our material achieves competitive and efficient HER kinetics with minimal synthesis complexity and material usage. This work paves the way for integrating this catalyst into full-cell electrolyzers and exploring its applicability in alkaline or neutral media, as well as expanding this method to other TMDs/MXene combinations for broader green hydrogen applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by an ERA Fellowship-Green Hydrogen of the German Academic Exchange Service (DAAD, Funding Program Number 57685698). We gratefully acknowledge the Helmholtz Association's Initiative and Networking Fund (Helmholtz Young Investigator Group VH-NG-1719) for the funding. M.P.B and P.W.M. greatly acknowledge support from the German Federal Ministry of Education and Research in the framework of the project Catlab (03EW0015A/B). S.A. would like to express particular thanks to R. Schwiddessen, M. Tovar, and K. Schwarzburg from the X-Ray and Microscopy and Spectroscopy Corelabs of the Helmholtz Zentrum Berlin for providing access to their facility and training on the equipment. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eZ.S. was supported by the ERC-CZ program (project LL2101) from the Ministry of Education, Youth and Sports (MEYS) and by the project Advanced Functional Nanorobots (reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research received no external funding. The APC was waived by the journal.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data produced or analyzed in this study are included in this published article and its supplementary information files.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAuthors and Affiliations\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eHelmholtz Young Investigator Group Electrocatalysis: Synthesis to Devices, Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eSana Akir, Bastian Schmiedecke, and Michelle P. Browne\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eDepartment of Materials Chemistry for Catalysis, Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Str. 15, 12489 Berlin, Germany.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eDepartment of Chemistry: Metalorganics and Inorganic Materials, Technische Universität Berlin, Straße des 17 Juni 135, Sekr. C2, 10623 Berlin, Germany.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eDebabrata Bagchi, Prashanth W. Menezes\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eDepartment of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 16628 Prague 6, Czech Republic.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eJan Plutnar, Zdenek Sofer\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eInstitute of Electrochemical Energy Storage, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eInstitute for Technical and Environmental Chemistry, Friedrich-Schiller-Universität Jena and Helmholtz Institute for Polymers in Energy Applications (HIPOLE Jena), Philosophenweg 7b, 07743 Jena, Germany.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Yael Rodriguez-Ayllon, Yan Lu\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eS.A. was responsible for the synthesis and characterization of the materials, as well as for writing the manuscript.\u003c/p\u003e\n\u003cp\u003eB.S. performed the SEM measurement\u003c/p\u003e\n\u003cp\u003eD.B. aided in the synthesis of the initial materials.\u003c/p\u003e\n\u003cp\u003eJ.P. conducted and assessed the XPS results, as well as the TEM and EDS analyses.\u003c/p\u003e\n\u003cp\u003eP.W.M. provided the precursors and laboratory facilities necessary to initiate the hydrothermal synthesis.\u003c/p\u003e\n\u003cp\u003eY.R. and Y.L. carried out the TGA measurements.\u003c/p\u003e\n\u003cp\u003eZ.S. revised the manuscript\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eM.P.B. supervised the project, validated the results, and revised the manuscript.\u003c/p\u003e\n\u003cp\u003eAll authors reviewed the paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAkpasi SO et al (2025) Hydrogen as a clean energy carrier: advancements, challenges, and its role in a sustainable energy future. 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APL Mater 2. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1063/1.4892976\u003c/span\u003e\u003cspan address=\"10.1063/1.4892976\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Hydrogen evolution reaction, Electrocatalyst, MoS2, Ti3C2","lastPublishedDoi":"10.21203/rs.3.rs-7868982/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7868982/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDesigning efficient electrocatalysts for the hydrogen evolution reaction (HER) is crucial for advancing sustainable energy technologies. In this study, a 2D/2D heterostructure composed of 1T-phase molybdenum disulfide (1T-MoS\u003csub\u003e2\u003c/sub\u003e) and titanium carbide MXene (Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003ex,\u003c/sub\u003e denoted as Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e) is synthesized using a one-step hydrothermal method. The hybrid catalyst exhibits improved HER kinetics, demonstrated by a low overpotential of 248 mV at 10 mA cm\u003csup\u003e-2\u003c/sup\u003e and a Tafel slope of 55 mV dec\u003csup\u003e-1\u003c/sup\u003e, indicating fast reaction kinetics and favorable charge transfer. The addition of tetramethylammonium ions (TMA\u003csup\u003e+\u003c/sup\u003e) induces interlayer expansion, increasing the 1T phase content to 89%. Incorporating only 1 % Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2 \u003c/sub\u003eMXene suppresses oxidation and enhances stability. The composite demonstrates a large electrochemical surface area, high turnover frequency (TOF), and retains over 90% of its catalytic activity after extended electrolysis. This scalable approach offers a promising route to developing stable, efficient 2D electrocatalysts for hydrogen production.\u003c/p\u003e","manuscriptTitle":"2D/2D Heterojunction Interfaces of 1T-MoS2 and Ti3C2 MXene: Designing High-Performance Catalyst for the Hydrogen Evolution Reaction","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-16 14:05:38","doi":"10.21203/rs.3.rs-7868982/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ac9400af-3d2a-439e-b75b-bc51950cec54","owner":[],"postedDate":"October 16th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-10-16T14:05:38+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-16 14:05:38","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7868982","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7868982","identity":"rs-7868982","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}