The valorization of biomass into high-value chemicals via heterogeneous catalysis is a critical step toward sustainable chemical production. While acid catalysis has been extensively explored, basic catalysis remains underdeveloped due to challenges such as catalyst instability and energy-intensive activation. Here, we report the first use of nanostructured magnesium hydroxide methoxide Mg(OH)(x)(OCH3)(2-x) as a highly active, low-temperature basic catalyst for the aldol condensation of furfural with acetone, a key reaction for biofuel synthesis. The nanosized materials, synthesized via a scalable sol-gel route, exhibit exceptional specific surface areas (up to 825 m(2) g(-1)) with mean nanosheets lateral size of approximate to 10 nm. They outperform conventional MgO and Mg(OH)(2) catalysts (the conversion is multiplied by at least a factor of 5 under equivalent reaction conditions) under mild conditions (50 degrees C, atmospheric pressure) in a batch reactor, achieving 100% furfural conversion after only 30 min, with >70% selectivity toward the desired C-13 product (F2Ac). The methoxy groups (-OCH3) are identified as the primary active sites, enabling high catalytic activity without thermal activation, unlike MgO. Systematic investigations reveal that material preparation parameters (H2O/Mg ratio, solvent, drying conditions) and storage atmosphere critically influence the material structure, stability and performance. Vacuum-dried samples retain superior activity due to minimized carbonate poisoning, while air-dried materials remain practical for industrial applications, balancing performance and cost. This work demonstrates that Mg(OH)(x)(OCH3)(2-x) is a promising nanocatalyst for sustainable biomass upgrading, offering a low-energy, scalable alternative to traditional basic catalysts.

The magnetoconductance of Ti 3 C 2 T x MXene thin films under both perpendicular and parallel magnetic fields is systematically investigated to elucidate how finite thickness impacts weak localization (WL). In thicker films ( ∼ 700 nm ), the magnetoconductance is nearly orientation independent, indicating a three-dimensional (3D) regime. In contrast, thinner films ( ≲ 70 nm ) exhibit pronounced angle-dependent WL, consistent with two-dimensional (2D) behavior. Notably, in some of these thinner films, a small negative magnetoconductance is also detected at low temperatures in parallel fields—a hallmark of weak antilocalization arising from spin-orbit coupling. By fitting the 2D data in both orientations, the key transport parameters are extracted, including an effective thickness of around 12 nm (smaller than the physical thickness) and dephasing lengths. These findings deepen the understanding of quantum interference in MXenes and lay the groundwork for harnessing their tunable dimensionality and spin-orbit effects in emerging electronic and spintronic applications.

Two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides known as MXenes form a class of inherently functionalized layers. The large variety of surface terminations plays a pivotal role in MXene properties and governs the interactions with their environment. In particular, numerical simulations suggest that these functional groups could be key players in gas sensing applications (toward, e.g., humidity, volatile organic compounds─VOCs─or NH3) for which MXenes have been identified as highly promising. Focusing on the benchmark Ti3C2Tx MXene (T being OH, O, F, or Cl), we here use electron energy-loss spectroscopy (EELS) in environmental transmission electron microscopy (ETEM) to characterize in situ, and on the nanometer scale, the selective interactions of different standard surface terminations with two model gases: ethanol (a typical VOC) and water vapor. The quantitative analysis of the core-edge fine structure, supported by density functional theory simulations, demonstrates the much higher affinity of chlorine terminations toward ethanol than water vapor and their superior response for ethanol as compared to oxygen terminations. In addition, the analysis of the carbon K-edge brings evidence of the different modifications of the Ti3C2 conducting core electronic structure upon ethanol or water vapor adsorption, bringing fundamental elements for the understanding of the different sensing mechanisms in Ti3C2Tx layers. Our results highlight the benefits of MXene surface engineering for their rational design as gas sensors, as well as the high relevance of EELS in gas-phase TEM to reveal the intrinsic mechanisms at play in gas adsorption on nanomaterials.

Abstract MXenes, a large family of two-dimensional transition-metal carbides and nitrides, have attracted considerable interest owing to their unique physicochemical properties. Structural defects play a key role in tuning these properties, yet their characterization remains highly challenging. Herein, controlled Ne2+ ion implantation of Ti3AlC2 MAX phases is employed to generate well-defined defect densities, followed by exfoliation through a mild LiF/HCl route to obtain defect-engineered Ti3C2Tx MXenes. Raman spectroscopy reveals a high sensitivity of A1g vibration at 206 cm–1 to structural disorder in the Ti3C2Tx MXene skeleton. In particular, the full width at half-maximum (FWHM) of this mode increases linearly with implantation fluence, providing a direct semiquantitative descriptor of structural disorder. The relevance of the A1g mode as a disorder marker is further confirmed using a second defect-generation route based on increasingly harsh HF etching conditions. Finally, the Raman results are corroborated by complementary XPS/HAXPES and TEM-EELS analyses, which reveal defect-induced modifications of the MXene structure while preserving its overall crystallographic framework. Together, these findings establish Raman spectroscopy as a straightforward, nondestructive, and widely accessible tool for the assessment of structural disorder in Ti3C2Tx MXenes, with strong relevance for the development of defect engineering strategies in this large class of 2D materials.

MXenes, a large family of two-dimensional transition metal carbides and nitrides, have attracted considerable interest owing to their physical-chemical properties. It is now well established that structural defects play a key role in these properties with possible significant improvement but also detrimental impact when uncontrolled. This issue is particularly important for MXenes due to the potentially harsh chemical etching conditions used for their exfoliation from the MAX phase precursors. Despite their important role, characterization defects remains highly challenging. Herein, we employ Raman spectroscopy as the primary probe to systematically investigate defect formation in carefully defect-engineered Ti3C2Tx MXenes derived from ion-implanted Ti3AlC2 MAX phases and exfoliated via a mild LiF/HCl route. Controlled ion implantation in the MAX precursor is used to generate well-defined defect densities, enabling direct correlation between implantation dose and Raman response in the resulting MXenes, together with improved exfoliation efficiency. In particular, we demonstrate the high sensitivity of the A1g mode at 206 cm—1, characteristic of Ti-C vibrational modes, to structural disorder with a peak broadening linearly scaling with defect density. This sensitivity is also confirmed for samples obtained with increasingly harsh etching conditions. Raman characterizations were corroborated by complementary spectroscopic analysis at various scales based on XPS/HAXPES and TEM-EELS. Together, these results demonstrate that Raman spectroscopy is a robust, accessible and reliable technique for mapping defects in MXenes, and provide a practical spectroscopic workflow for correlating vibrational, chemical and electronic signatures of disorder in Ti3C2Tx layers.

Nowadays, the development of efficient electrochemical energy storage and conversion systems has become a crucial research topic. For systems such as electrolyzers, the highest challenge is designing a sustainable, active and stable electrocatalysts for the reactions involved, particularly the oxygen evolution reaction (OER). Efficient proton exchange membrane water electrolyzers have been developed with the use of noble metals. Nevertheless, their high cost and scarcity hinder the widespread use of such systems. An interesting alternative approach is the anion exchange membrane water electrolyzer since it allows using non-noble metal-based catalysts that may be thermodynamically stable in alkaline electrolyte at high electrode potentials. Among these, sulfides are increasingly studied as catalysts for the OER 1 . Whereas the sulfide core is preserved, an (oxy)hydroxide shell is formed after the surface reconstruction occurring at potentials required for the OER, and constitutes the active phase 2 . However, the use of unsupported sulfides has two major drawbacks: (i) even if their conductivity is higher than that of oxides, it remains relatively low and ii) the lack of dispersion of the active phase. To address these issues, the use of a conductive support is often considered to improve both the conductivity and the dispersion of the active phase. A possible support is carbon but it is subject to corrosion at potentials required for the OER, resulting in a strong loss of activity over time. It is then essential to find alternative and stable supports in alkaline conditions at high potential. Among them, M n+1 X n T z MXenes, a two-dimensional family materials constituted of a transition metal (M), a p-block element (X) usually carbon or nitrogen and T terminal groups (T = OH, F, O), are promising alternatives 3 . In this context, non-noble metal-based sulfides supported on a Ti 3 C 2 T z titanium carbide MXene were studied in this work. Under high potential, the surface of this support in contact with the electrolyte is passivated by a titanium dioxide layer but the MXene core is preserved, thus ensuring a high electronic conductivity in the catalytic layer. To obtain the composite, the Ti 3 C 2 MXene was first synthetized using a LiF/HCl route 4 . Then, Ni x Co y S z nanoparticles with various Ni/Co atomic ratio were synthesized using a microwave-assisted polyol route in the presence of the MXene at different loadings to promote the heterogeneous nucleation of active phase onto the MXene surface. The obtained materials were characterized using various physico-chemical techniques (XRD, ICP-OES, SEM) in order to gain insight into their structure, composition and dispersion over the MXene surface. It was observed that the MXene structure is preserved after the deposition of the active phase. The latter consists of two cubic phases, namely Co 3-x Ni x S 4 and Co 9-x Ni x S 8 , and two hexagonal phases NiS (figure 1.a), their corresponding mass fraction depending on both the Co/Ni ratio and the content of MXene. ICP-OES results show that experimental Co/Ni and active phase/MXene ratios are in good agreement with nominal ones. Interestingly, the addition of Co to Ni-rich sulfides and the increase of the MXene content reduce the size of particles, as highlighted by the evolution of the width at half maximum of the XRD peaks. Finally, electrochemical experiments were carried out to assess the activity of catalysts toward OER in an alkaline electrolyte (purified KOH 1 mol L -1 ) (figure 1.b). The presentation will aim at correlating the apparent activity of the different catalysts with their physico-chemical properties and compositions. References: (1) Wang, M.; Zhang, L.; He, Y.; Zhu, H. Recent Advances in Transition-Metal-Sulfide-Based Bifunctional Electrocatalysts for Overall Water Splitting. J. Mater. Chem. A 2021 , 9 (9), 5320–5363. https://doi.org/10.1039/D0TA12152E. (2) El-Refaei, S. M.; Rauret, D. L.; Manjón, A. G.; Spanos, I.; Zeradjanin, A.; Dieckhöfer, S.; Arbiol, J.; Schuhmann, W.; Masa, J. Ni-Xides (B, S, and P) for Alkaline OER: Shedding Light on Reconstruction Processes and Interplay with Incidental Fe Impurities as Synergistic Activity Drivers. ACS Appl. Energy Mater. 2024 , 7 (4), 1369–1381. https://doi.org/10.1021/acsaem.3c03114. (3) Anne, B. R.; Kundu, J.; Kabiraz, M. K.; Kim, J.; Cho, D.; Choi, S.-I. A Review on MXene as Promising Support Materials for Oxygen Evolution Reaction Catalysts. Advanced Functional Materials 2023 , 33 (51), 2306100. https://doi.org/10.1002/adfm.202306100. (4) Benchakar, M.; Loupias, L.; Garnero, C.; Bilyk, T.; Morais, C.; Canaff, C.; Guignard, N.; Morisset, S.; Pazniak, H.; Hurand, S.; Chartier, P.; Pacaud, J.; Mauchamp, V.; Barsoum, M. W.; Habrioux, A.; Célérier, S. One MAX Phase, Different MXenes: A Guideline to Understand the Crucial Role of Etching Conditions on Ti3C2Tx Surface Chemistry. Applied Surface Science 2020 , 530 , 147209. https://doi.org/10.1016/j.apsusc.2020.147209. Figure 1

Currently, MXenes represent probably the widest family of 2D materials. They are obtained from the exfoliation of the A layer of bulk three dimensional nanolamellar M n+1 AX n MAX phases (where A is an element mainly from column 13 or 14 from the periodic table) [1]. Their formula is M n+1 X n T z where M is a transition metal and X is C and/or N, and their structure consists in M n+1 X n octahedra layers where n is an integer varying from 1 to 4, covered with terminal groups T (with T = OH, O, F and/or Cl), formed during the etching of MAX precursors [2], [3]. The wide range of possible substitutions in the M, X, and T sites leads to materials with tunable properties and applications (sensors, bio-medical applications, optoelectronic systems, energy storage, heterogeneous catalysis, ...) [4], [5]. For the last decade, MXenes have been especially used in electrocatalysis as (i) substrates due to their high electronic conductivity, hydrophilicity and the presence of terminal groups acting as anchoring points for the deposition of active phases or (ii) directly as active phases especially for hydrogen evolution reaction (HER) [6], the most efficient MXenes towards HER being Mo-based [7]. Though their reactivity has been extensively studied, their surface redox properties are still not fully understood which brings the need for an in-depth investigation. In this study, Mo 2 CT x MXenes have been synthetized from the chemical etching of Ga in Mo 2 Ga 2 C MAX-like phase employing hydrofluoric acid. In some cases, delamination of multilayers MXenes was performed using TBAOH as delaminating agent. According to the etching and delamination conditions, MXenes with various Mo/C atomic ratios were obtained ( i.e. , Mo vacancies formation). The physicochemical properties of the different synthesized materials were investigated using various techniques such as XRD, SEM, Raman spectroscopy, ICP-OES and XPS. The surface redox properties of these MXenes have been investigated in alkaline electrolyte by performing cyclic voltammetry experiments. In order to get insights on the dynamics of electrode/electrolyte interface, diverse electrochemical measurements were conducted, with several parameters ( e.g. , scan rate, potential limits, number of cycles, polarization time) being systematically varied. Through mathematical models, the surface redox properties of these MXenes have been first inspected and correlated to the physicochemical properties of the synthesized materials. It was further demonstrated that after intensive cycling, a modification of the redox properties occurs (Fig.1). This implies that the material presents a surface reconstruction, not documented in the literature so far. Outcomes of this work will contribute to a deeper electrochemical understanding of Mo-based MXenes, in particular regarding their stability and reactivity, while guiding the rational synthesis engineering of MXenes. References [1] Y. Gogotsi, « The Future of MXenes », Chem. Mater. , vol. 35, n o 21, p. 8767-8770, nov. 2023, doi: 10.1021/acs.chemmater.3c02491. [2] A. VahidMohammadi, J. Rosen, et Y. Gogotsi, « The world of two-dimensional carbides and nitrides (MXenes) », Science , vol. 372, n o 6547, p. eabf1581, juin 2021, doi: 10.1126/science.abf1581. [3] M. Benchakar et al. , « One MAX phase, different MXenes: A guideline to understand the crucial role of etching conditions on Ti3C2Tx surface chemistry », Appl. Surf. Sci. , vol. 530, p. 147209, nov. 2020, doi: 10.1016/j.apsusc.2020.147209. [4] Y. Gogotsi et B. Anasori, « The Rise of MXenes », ACS Nano , vol. 13, n o 8, p. 8491-8494, août 2019, doi: 10.1021/acsnano.9b06394. [5] M. Naguib, « MXenes: A rising star in the constellation of two-dimensional materials », Curr. Opin. Solid State Mater. Sci. , vol. 24, n o 1, p. 100809, févr. 2020, doi: 10.1016/j.cossms.2020.100809. [6] Z. W. Seh et al. , « Two-Dimensional Molybdenum Carbide (MXene) as an Efficient Electrocatalyst for Hydrogen Evolution », ACS Energy Lett. , vol. 1, n o 3, Art. n o 3, sept. 2016, doi: 10.1021/acsenergylett.6b00247. [7] L. Loupias et al. , « Guideline for synthesis and surface chemistry characterization of 2D Mo/Ti solid solutions based MXene. Application to hydrogen evolution reaction in alkaline media », FlatChem , vol. 43, p. 100596, janv. 2024, doi: 10.1016/j.flatc.2023.100596. Figure 1

MXenes are a large family of two-dimensional transition metal carbides and/or nitrides combining hydrophilicity with metallic conductivity, thereby leading to a plethora of potential applications. This study expands the current possibilities for the structural engineering of MXenes by demonstrating medium energy range ion irradiation as a controllable and flexible strategy to deeply modify Ti3C2Tz thin films. By adjusting the fluence of a 180 kV He+ ion beam, we evidence the gradual modification of the different structural elements of typical MXene multilayers, inducing well-defined impacts on properties. Low fluences allow modifying the interlayer spacing, inhibiting the long-term rehydration capacity of the thin films with expected major benefits on MXene aging issues. In addition to this, irradiation allows affecting the layers functionalization with major impact on the normalized optical transmission profile, suppressing the absorption valley at IR-visible limit and expanding their transparency in the UV. These effects, combined with improved electrical contact between the Ti3C2Tz layers, are highly desirable for transparent conductive electrodes applications. Finally, higher fluence irradiations induce preferential sputtering of titanium atoms. Transition metal vacancies being known to be highly relevant to deeply modify properties beyond those investigated here, these results show the large benefits of ion irradiation for MXene design.

2D MXenes have gained an ever-increasing attention in various application fields owing to the combination of their layered structure with their excellent physico-chemical properties. MXene properties can be strongly tuned by modifying the M element in the Mn+1XnTx structure. Among them, Mo-based MXenes are beginning to be successfully explored in many areas. However, few studies dealt with the synthesis and characterization of the (Mo,Ti)n+1CnTx solid solutions. The aim is to understand their complex chemistry in terms of structure, microstructure and surface chemical composition and to compare them with those of mono-metallic Mo2CTx and Ti3C2Tx MXenes as well as parent MAX phases. Then, the potential of these materials as HER (hydrogen evolution reaction) catalysts is determined in alkaline medium, never done so far, and their activity is correlated with their surface chemistry. It is particularly shown that Mo2Ti2C3Tx MXenes are a credible alternative to Mo2CTx MXenes since the surface properties of both MXenes are similar while their composition is quite different. Indeed, the (Mo,Ti)n+1CnTx MXenes require lower temperatures and shorter time for the synthesis than for Mo2CTx MXenes, a great advantage from an industrial point of view. Finally, this study aims at providing a roadmap to carry out the synthesis and characterization of (Mo,Ti)n+1CnTx MXenes.

Mo2Ti2C3Tx MXene materials, bare or loaded with strongly anchored single Pt atoms, were investigated using various methods, including STEM, XPS, XAS and DFT calculations. Upon Pt impregnation, the delaminated Mo-rich MXene surface undergoes partial oxidation, which is reversed by an H2 thermal treatment at 400 °C. The optimized MXene shows high catalytic activity for CO2 hydrogenation to CO and smaller amounts of methane and methanol. Around and above the pretreatment temperature of 400 °C, the MXene is gradually defunctionalized from O- and F-containing groups and depleted in carbidic carbon, leading to deactivation. Single Pt atoms are cationic after impregnation, and reduce upon H2 treatment, filling surface Mo vacancies. Pt addition increases the MXene activity, in particular by facilitating H2 dissociation, but has little effect on the single-atom catalyst selectivity and on the rate dependence upon reactant partial pressures. The lowest Pt loading leads to the highest turnover frequency, indicating that the MXene surface sites are key to CO2 activation.

MXenes are prototypes of surface tunable 2D materials with vast potential for properties tuning. Accurately characterizing their surface functionalization and its role in electronic structure is crucial, X-ray photoelectron spectroscopy (XPS) being among the go-to methods to do so. Despite extensive use, XPS analysis remains however intricate. Focusing on the benchmark MXene Ti3C2Tz, Density Functional Theory (DFT) calculations of core-level binding energy shifts (BE.s.) are combined with experiments in order to provide a quantitative interpretation of XPS spectra. This approach demonstrates that BE.s. are driven by the complex interplay between chemical, structural, and subtle electronic structure effects preventing analysis from intuitive arguments or comparison with reference materials. In particular, it is shown that O terminations induce the largest BE.s. at Ti 2p levels despite lower electronegativity than F. Additionally, F 1s levels show weak sensitivity to the F local environment, explaining the single contribution in the spectrum, whereas O 1s states are significantly affected by the local surface chemistry. Finally, clear indicators of surface group vacancies are given at Ti 2p and O 1s levels. These results demonstrate the combination of calculations with experiments as a method of the highest value for MXenes XPS spectra analysis, providing guidelines for otherwise complex interpretations.

A still unexplored class of heterostructured bifunctional catalysts for oxygen evolution (OER) and reduction (ORR) reactions is investigated: Ni-Fe sulfides deposited onto a N,S-co-doped zeolite-templated carbon (ZTC) substrate achieved upon the impregnation of a ZTC with thiourea and Ni and Fe precursors (Ni/Fe atomic ratio of 1). By heat-treating the impregnated ZTC substrate at 700 degrees C under N2 atmosphere the efficient bifunctional catalyst is generated. A difference of only 0.76 V is measured between the potential required to drive an OER current density of 10 mA cm-2 and the ORR half-wave potential. Using electrochemical measurements and physico-chemical characterizations, it is evidenced that OER activity results from the presence of a sulfide containing both Fe and Ni whereas the ORR activity originates from the N,S-doped ZTC substrate. The selectivity of the ORR process is improved through the presence of sulfide phases. Compared to more conventional carbon substrate such as N,S-co-doped reduced graphene oxide, high specific surface area ZTC favors high ORR performances. The functional ZTC material was further successfully implemented as bifunctional air electrode in a coin-type Zn-air battery.

This study investigated (Ru-)Mo2C (MXene) materials for ammonia thermo-catalytic synthesis under atmospheric or moderate pressure. Under a H2-N2 (3:1) flow, the Mo2C MXene phase showed limited activity in ammonia synthesis at atmospheric pressure at 400 °C (0.01mmolh⁻¹ g⁻¹), which increased significantly with temperature and pressure, reaching 2.07mmolh⁻¹ g⁻¹ at 500°C and under 5bar. Interestingly, Mo-based MXene was able to generate an appreciable quantity of ammonia without promoters such as ruthenium, the most active metal for the ammonia synthesis reaction. Unexpectedly, the addition of Ru to Mo2C did not enhance its activity. The nitriding of the MXene under NH3 or N2 was then performed and characterized. A thermal treatment under NH3 (600 °C, 5bar) was efficient and, interestingly, nitriding also occurred in a lower extent under N2 (600 °C, 5bar) for the sample containing ruthenium. The N-containing MXene produced ammonia under pure H2 flow from temperatures lower than 250 °C. Consecutive nitriding treatments and ammonia production under pure H2 were successfully achieved and demonstrated for 5 cycles. This result is promising for chemical looping ammonia production process. This work highlights essential aspects that should be explored for future advances to consider using Mo2CTx MXene for the efficient thermal production of ammonia.

MXenes stand out from other 2D materials because they combine very good electrical conductivity with hydrophilicity, allowing cost-effective processing as thin films. Therefore, there is a high fundamental interest in unraveling the electronic transport mechanisms at stake in multilayers of the most conducting MXene, Ti3C2Tx. Although weak localization (WL) has been proposed as the dominating low-temperature (LT) transport mechanism in Ti3C2Tx thin films, there have been few attempts to model it quantitatively. In this work, the role of important structural parameters - thickness, interflake coupling, defects - on the dimensionality of the LT transport mechanisms in spin-coated Ti3C2Tx thin films is investigated through LT and magnetic field dependent resistivity measurements. A dimensional crossover from 2D to 3D WL is clearly evidenced when the film thickness exceeds the dephasing length lϕ, estimated here in the 50-100 nm range. 2D WL can be restored by weakening the coupling between adjacent flakes, the intrinsic thickness of which is lower than lϕ, hence acting as parallel 2D conductors. Alternatively, lϕ can be reduced down to the 10 nm range by defects. These results clearly emphasize the ability of WL quantitative study to give deep insights in the physics of electron transport in MXene thin films.

Galvanic replacement in molten salts has recently been uncovered as a pathway to design new compositions of layered transition-metal carbides related to the family of MAX phases, Mn+1AlXn , where M is a transition metal, X is carbon for carbide phases, and A is mainly an element of the IIIA and IVA groups of the periodic table. These compounds can be further involved in etching processes by the removal of A elements to yield new 2D transition-metal carbides, Mn+1XnTz , the so-called MXenes, where T stands for surface groups. Although promising, the chemical modification of MAX-related materials in molten salts has been reported only for a few compounds, and little is known about the corresponding reaction mechanisms. In this work, we question the versatility of galvanic reactions in molten salts for MAX-related phases by combining for the first time in situ X-ray diffraction and in situ X-ray absorption spectroscopy during reactions in molten salts for two compounds, Ti3AlC2 and Mo2Ga(2)C. The first one shows minute-scale transformation into molten ZnCl2 toward Ti3ZnC2, followed by evolution into Ti3C2Cl2. On the contrary, we do not observe replacement but etching of Mo2Ga2C into Mo2GaC and then orthorhombic Mo2C, with a loss of the layered structure. We highlight the role of molten salt chemistry in this process and discuss the different behaviors of these two MAX-related phases versus galvanic reactivity in molten salts to open the way to new MAX compositions.