MXenes, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides, are synthesized by selectively etching the A-layer from MAX phases—layered ternary compounds where M is an early transition metal, A is a group 12–16 element, and X is carbon and/or nitrogen. The resulting materials retain a layered structure and are typically functionalized with surface groups such as O, OH, or F, depending on the synthesis route and subsequent treatments.

These surface terminations critically influence both the stability and catalytic properties of MXenes. While high-temperature hydrogen treatment can strip away these groups, producing bare MXene surfaces, such configurations often exhibit excessive reactivity for practical applications. Instead, stable terminated surfaces dominate under realistic conditions and play a central role in dictating catalytic behavior. Additionally, alternative stacking arrangements further contribute to the diversity of MXene surface chemistry.

In this work, we use dispersion-corrected density functional theory (DFT) to study how different terminations and stacking geometries impact the catalytic potential of selected MXenes. Focusing on the (reverse) water-gas shift reaction as a model system, we find that certain surface groups degrade upon gas interaction, while others allow for effective modulation of reactivity. Our results highlight the tunability of MXene catalytic behavior through targeted surface engineering, offering a strategic approach to the development of next-generation catalytic materials.

Inorganic molten salts are non-volatile liquids stable at high temperatures, often up to 1000 °C. Performing chemical reactions in inorganic molten salts is a way to trigger reactivity in liquids at temperatures that usually pertain to solid-state reactions. This enables syntheses of unprecedented materials, including new compounds like oxychalcogenides[1] and oxyhalides,^[2] but also original nano-objects, from III-V quantum dots[3] to bidimensional carbides[4] and strongly covalent materials,[5] like borides[6,7] or silicides.[8–10] Such opportunities have been demonstrated for MXenes in the last years. Indeed, the adequate choice of molten salts enables selective etching and replacement of A elements (A=Al, Si, Ga) in MAX phases, thus delivering carbide and nitride MXenes[4,11–13] with new terminations and properties. The involved reactions remain however poorly understood, which hinders their precise control. Especially reaction kinetics, and the applicability range for surface modification and the recovery of new compounds are still to be discovered.

In this talk, we will discuss the reactivity of layered materials into molten salts, first by studying in situ their synthesis, the etching and replacement mechanisms, second by considering post-modifications of MXenes directly in molten salts. We will introduce the sample environment we have developed to enable synchrotron-based in situ X-ray diffraction and X-ray absorption spectroscopy experiments in molten salts. We will then discuss how galvanic etching, replacement and delamination reactions occur in MAX phases depending on the composition of the MAX phase and of the molten salts.[14] We will then introduce the synthesis and reactivity of layered metal borides (so-called ‘MAB’ phases[15,16]) phases in molten salts. Finally, we will discuss how to trigger modifications of the composition of MXenes through molten salt-mediated reactions for the design of electrocatalysts for water splitting.

Förster Resonance Energy Transfer (FRET) is the gold-standard optical ruler for tracking nanometre-scale structural changes in biomolecules, sensitive to distance changes in the ~3-10 nm range. Yet every FRET experiment demands two covalently attached dyes whose photophysics, mutual orientation, and chemical stability can complicate data interpretation. Solid-state quenchers such as graphene or gold films remove the need for an acceptor dye and extend the working range far beyond 10 nm. Even though graphene has found elegant applications in single molecule fluorescence [1], their hydrophobicity, layer-dependent quenching and working distance range limit direct biological compatibility (beyond DNA), method robustness and certain applications (ultrathin assemblies), respectively.

Here we introduce titanium-carbide MXenes (Ti₃C₂Tₓ) as hydrophilic and robust surface-based quenchers that operate within an ultrashort distance window and possess superior biocompatibility.

We first characterised MXene-DNA interactions using ensemble fluorescence spectroscopy and molecular dynamic simulations [2]. DNA adsorption was found to occur through hydrated ion bridges, avoiding the strong hydrophobic and π–π stacking forces typical of graphene and thereby preserving nucleic-acid structure. In real-time surface hybridisation assays, a mere 0.3 nm increase in dye-MXene separation produced a clear fluorescence rise, demonstrating sub-nanometre axial sensitivity.

To map the distance law precisely, we employed dye-labeled DNA origami nanostructures that position fluorophores 1-8 nm above MXene-coated glass and determined the distance dependent energy transfer by single molecule fluorescence lifetime measurements [3]. We found that single-flake Ti₃C₂Tₓ extinguishes >95 % of donor emission at 1 nm and recovers to baseline by 8 nm, following a cubic distance dependence that is nearly insensitive to MXene thickness. With this calibration, we investigated 5-nm supported lipid bilayers, which are minimal cell-membrane mimics. MXenes enabled leaflet-specific, single-lipid read-out without the hydrophilic spacers required for graphene, underscoring both their compatibility with soft matter and their unique, steep sensitivity. 

Ti₃C₂Tₓ MXenes thus offer a powerful new tool for structural studies of ultrathin biological assemblies such as lipid membranes, protein monolayers, and DNA nanostructures, at the single molecule level.  

MXenes are commonly synthesized through selective etching of the corresponding MAX phase, which has the formula M-A-X, where "M" is the early transition metal, "A" is a main group metal, and "X" is carbon or nitrogen. Over the past decade, they have gained attention for their applications, particularly as electrocatalysts, becoming one of the most active materials in this field【4】

The research concept to be presented explores the potential of MXenes as highly efficient and stable thermal catalysts for organic reactions. The active sites in MXenes include M–O and M–OH groups, which are analogous to those found on the surfaces of transition metal oxides. Additionally, MXenes feature unique surface terminations resembling those in molecular complexes, further broadening their catalytic versatility.

Results and Discussion

The presentation will highlight the potential of MXenes as materials with intrinsic catalytic activity for a wide range of organic reactions. These include aerobic oxidations, oxidative dehydrogenation of hydrocarbons and other functional groups, hydrogenation of unsaturated C-C multiple bonds, aldol condensations, hydroamination of C≡C triple bonds, guanylation of amines, among others. The study focuses on a series of MXenes, including Ti₃C₂, Nb₂C, and V₂C, synthesized from commercially available MAX phase precursors.

The characterization of Brønsted and Lewis acid/base sites using NH₃-TPD, CO₂-TPD, and pyridine adsorption/desorption monitored by IR spectroscopy reveals a low density of active sites. These sites are likely associated with structural defects, atomic vacancies, and surface terminations introduced during the harsh etching process. Despite their low abundance, these defects are primarily responsible for the observed catalytic activity. Additionally, the morphology of the MXene samples—whether multilayered accordion-like, expanded layered, or exfoliated—significantly influences catalytic performance. Post-synthesis surface functionalization further modifies catalytic activity, with certain modifications positioning MXenes among the most active solid catalysts in terms of turnover frequency.

Beyond their intrinsic catalytic properties, MXenes are particularly effective as supports for single-atom catalysis. Single atoms can be incorporated into MXenes directly during their synthesis via the molten salt method. These single atoms occupy vacant sites in the metal layer, enabling catalytic activity in specific reactions, such as hydrogenations. This dual functionality—intrinsic activity and support for single-atom catalysis—illustrates the versatility and potential of MXenes in advanced catalytic applications.

Significance

The catalytic activity of metal oxides, carbides, and related compounds is well established. In this context, MXenes offer distinct advantages as catalysts due to their unique 2D morphology, which provides highly accessible active sites and improved atom utilization. The tunable chemical composition of MXenes opens up an expansive chemical space, with over 70 materials reported to date. Moreover, their catalytic activity can be further optimized through precise control of surface termination groups, enhancing their performance. These features make MXenes highly promising catalysts for a wide range of organic reactions, combining structural efficiency with chemical versatility.

The urgent transition to sustainable energy demands versatile, low-cost electrocatalysts capable of driving both water splitting (HER/OER) and CO₂ reduction (CO₂RR). However, most high-performance systems rely on scarce noble metals or require separate catalysts for each reaction, limiting their scalability and economic viability. Two-dimensional MXenes offer metallic conductivity, tuneable surface terminations (–O, –OH, –F), and a high density of accessible active sites, making them an ideal platform to host earth-abundant catalytic species. In this work, we develop and compare several functionalization MXenes with non-precious metals and oxides. Each hybrid catalyst is synthesized using standard laboratory equipment and optimized to achieve intimate metal–MXene interfaces. Comprehensive structural (XRD, TEM/SEM), surface (XPS, Raman), and in‑situ spectroscopic analyses elucidate the nature of active species under operating conditions. Electrochemical evaluation reveals low overpotentials (<200 mV at 10 mA cm⁻² for HER/OER), rapid kinetics (Tafel slopes <60 mV dec⁻¹) and sustained durability (>24 h at 100 mA cm⁻²), while CO₂RR tests in KHCO₃ electrolytes demonstrate high Faradaic efficiencies toward CO, C₂ products or formate, depending on the metal composition. By bridging the gap between earth-abundant materials and multifunctional performance, this study establishes a modular MXene–metal hybrid platform for scalable, bifunctional energy conversion, paving the way for integrated electrolyzer designs and circular carbon technologies.

The excessive exploitation of fossil fuels has led to escalating energy shortages and CO₂ emissions, posing severe threats to sustainable development. Against the backdrop of carbon neutrality goals, green CO₂ conversion technologies have garnered significant attention. Among them, photocatalytic CO₂ reduction offers a sustainable route to convert CO₂ into value-added solar fuels using abundant sunlight. However, current systems face critical challenges such as rapid recombination of photogenerated charge carriers and insufficient redox capability, which limit overall efficiency. To address these bottlenecks: (1) A SnO₂/CDs Ohmic heterojunction was constructed by incorporating carbon quantum dots (CDs) into SnO₂ nanofibers. Under illumination, photogenerated electrons in SnO₂ are driven to the CDs surface by band bending and the built-in electric field, where they are synergistically excited with the intrinsic free electrons of CDs via localized surface plasmon resonance (LSPR), forming a stable carrier cycling mechanism that prolongs carrier lifetime. Enhanced light absorption and efficient CO₂ chemisorption by CDs further boost the photocatalytic performance. (2) An In₂O₃/Nb₂O₅ S-scheme heterojunction was fabricated via one-step electrospinning, achieving tight interfacial contact and ultrafast charge transfer (<10 ps). The photogenerated electrons and holes accumulate in the conduction band of Nb₂O₅ and the valence band of In₂O₃, respectively, benefiting from extended carrier lifetimes and strong redox potential. Moreover, the strong CO₂ adsorption and activation capability of Nb₂O₅ contributes to the improved catalytic activity. (3) To further overcome charge recombination and reaction kinetics mismatch, a spatially engineered Nb₂C/Nb₂O₅/ZnO ternary heterostructure is developed by anchoring ZnO quantum dots (QDs) onto Nb₂O5 nanorods grown in situ from Nb₂C MXene. This architecture integrates an Nb₂O₅/ZnO S-scheme heterojunction and an Nb₂C/Nb₂O₅ Schottky junction, both sharing Nb₂O₅ as a central mediator, thereby establishing bidirectional interfacial electric fields (IEFs) that direct photogenerated electrons toward ZnO and holes toward Nb₂C. This spatial charge separation effectively suppresses Coulombic recombination and prolongs carrier lifetimes. Additionally, the intrinsic photothermal effect of Nb₂C MXene enhances CO₂ chemisorption and activation at defective ZnO QDs. These synergistic effects collectively enable high-efficiency CO₂ photoreduction without molecular cocatalysts or sacrificial agents, providing a mechanistically distinct and scalable approach for artificial photosynthesis.

MXenes are novel 2D nanomaterials composed of alternating layers of an early transition metal and a carbide or nitride.[1] The top and bottom external surfaces of MXenes consist of metallic sheets covered by surface functional groups, whose nature depends on the preparation method.[2] Multilayered MXenes are formed in the early stages of synthesis by etching away the “A layers” (e.g., aluminum) from the parent MAX phase. Structural defects generated during etching create active sites that can serve as catalytic centers.[3] In this study, Ti₃C₂ samples prepared from Ti₃AlC₂ via NH₄F-HCl etching and post-treated with DMSO expansion, ultrasound exfoliation, thermal annealing (500°C), and surface modifications were used to catalyze the guanylation of symmetrical carbodiimides with amines, forming guanidines and dialkylureas. The initial reaction rates between N,N’-diisopropylcarbodiimide and p-toluidine correlated with the density of weak acid sites on Ti₃C₂, identifying these sites as active centers. The most active sample exhibited turnover numbers and frequencies of 101 and 114 h⁻¹, ranking Ti₃C₂ among the most efficient noble metal-free guanylation catalysts. Upon reuse, Ti₃C₂ gradually deactivates while maintaining crystallinity. Deactivation results from product deposition and increased oxygenated surface groups but can be partially reversed through thermal desorption. These findings underscore the potential of Ti MXenes as solid catalysts for organic reactions,[4] particularly those involving amine activation.

MXenes are a novel family of two-dimensional (2D) transition metal carbides, carbonitrides and nitrides, directly prepared from selective etching of the A-site element in MAX phase precursors. Their general chemical formula Mn+1XnTx describes comprising n+1 layers (n=1–4) of early transition metals (‘M’), interleaved by n layers of carbon and/or nitrogen atoms (‘X’), with Tx representing surface terminations bonded to the outer M layers (i.e. –F, –O, –OH). Thus, their unique morphology mechanical, electronic, chemical, and optical properties makes them promising materials for advanced applications, such as in the consolidated role in the field of photocatalysis [1].

The development of new photo-thermal catalysts for the efficient valorisation of CO2 and N2 into high value chemical is of great interest.[2] Herein we deal with multi-layered titanium nitride (Ti2N) MXene as promising photothermal catalysts, see figure 1a. Selectivity of Ti2N can be easily controlled modifying its chemical surface partially oxidating the superficial layers of Ti2N MXene during the purification process, (figure 1b). Results show that when multi-layered titanium nitride (Ti2N) MXene is not oxidized the activity towards CO2 photothermal reduction is less favoured, whereas shows selectivity in the N2 fixation to NH3. Terminal nitrogen on the multi-layered Ti2N MXene surface offered an active role in the activation of N2 molecules by occupying the nitrogen vacancies produced during photothermal-catalytic reaction and efficiency shows sensitivity to the persulfate (PS) precursor that is used in the purification see figure 1c. In the counterpart, partially oxidation of T2N surfaces induces a kind of passivation by the formation of a thin layer of TiO2 avoiding the N2 molecules to occupy nitrogen vacances. Moreover, partial oxidation induced the formation of heterojunction between TiO2 and Ti2N, switching selectivity and improving overall catalytic efficiency of original Ti2N MXene. Passivation of Ti2N creates new active sites (TiO2) on top of titanium nitride layers that are able now to easily activate CO2 molecules to make possible the reduction of CO2 to CO and CH4. Formed TiO2 layer, also favours the formation of a third-party junction composites with perovskite nanoparticles enhancing light harvesting, improving photoactivity of the partially oxidized MXene (POM), see figure 1d.

MXenes are gaining increasing attention as heterogeneous catalysts[1, 2]. For these applications, surface defects – typically present at very low densities, often at sub-nanometric populations per gram – are believed to play a key role. However, due to this extremely low density, very few experimental techniques are sensitive enough to detect and monitor these types of sites.

In this presentation, I will show that pyrene photoemission, using borylated and amino-functionalized pyrene derivatives[3], can report on subtle surface differences in Ti₃C₂ MXenes prepared by different synthetic methods.

Specifically, Ti₃C₂ obtained by etching Ti₃AlC₂ with NH₄F, which introduces –F, –OH, and –O surface terminations, interacts strongly with boronic acid–functionalized pyrene. The binding of this molecule indicates the presence of surface –OH groups with a spatial arrangement similar to vicinal diols in organic molecules.

In contrast, 1-aminopyrene reveals unique properties when tested on Ti₃C₂Br₂ prepared by molten salt etching of Ti₃AlC₂ in a eutectic LiBr/KBr mixture. In this case, the characteristic excimer emission of 1-aminopyrene disappears upon binding to the surface, and only monomer fluorescence is observed. This indicates that NH₂ groups strongly attach to specific sites on the Br-terminated MXene, which are sufficiently isolated to prevent π–π interactions between neighboring pyrene molecules (Fig. 1).

These results demonstrate the potential of photophysical probes to detect and monitor defect sites on MXene surfaces, even at sub-nanometric concentrations.