MXenes are a new, and growing, family of 2D materials with very promising properties for a wide variety of applications. Obtained from the etching of MAX phases, numerous properties can be targeted thanks to the chemical richness of the precursors. Herein, we highlight how etching agents govern surface chemistries of Ti 3 C 2 T x , the most widely studied MXene to date. By combining characterization tools such as X-ray diffraction, X-ray photoelectron, Raman and electron energy loss spectroscopies, scanning and transmission electron microscopies and a surface sensitive electrochemical reactionthe hydrogen evolution reaction, HERwe clearly demonstrate that the etching agent (HF, LiF/HCl or FeF 3 /HCl) strongly modifies the nature of surface terminal groups (F, OH and/or O), oxidation sensitivity, delamination ability, nature of the inserted species, interstratification, concentration of defects and size of flakes. Beyond showing how using these different characterization tools to analyze MXenes, this work highlights that the MXene synthesis routes can influence targeted applications.

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The reduction degree of graphene oxide substrate governs the activity and stability of Co 3 O 4 /rGO nanocomposites toward oxygen reduction reaction and oxygen evolution reaction. In this work, Co 3 O 4 nanoparticles with narrow size distribution were uniformly deposited onto graphene oxide materials with different reduction degrees, by using a microwave-assisted hydrothermal method. The physicochemical characterization of these nanocomposites indicates that oxygenated groups grafted onto a reduced graphene oxide surface allow creating strong interactions between the carbon-based substrate and Co 3 O 4 nanocrystals. The obtained results denote that the electrocatalytic activity and stability of these nanocomposites toward the ORR and OER depend on the entanglement between the strength of the carbon/oxide interaction and the electronic conductivity of the substrate.

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The surface functionalization of 2D transition metal carbides or nitrides, so-called MXenes, is one of the fundamental levers allowing to deeply modify their physicochemical properties. Beyond new approaches to control this pivotal parameter, the ability to unambiguously assess their surface chemistry is thus key to expand the application fields of this large class of 2D materials. Using a combination of experiments and state of the art density functional theory calculations, we show that the NMR signal of the carbon�the element common to all MXene carbides and corresponding MAX phase precursors�is extremely sensitive to the MXene functionalization, although carbon atoms are not directly bonded to the surface groups. The simulations include the orbital part to the NMR shielding and the contribution from the Knight shift, which is crucial to achieve good correlation with the experimental data, as demonstrated on a set of reference MXene precursors. Starting with the Ti 3 C 2 T x MXene benchmark system, we confirm the high sensitivity of the 13 C NMR shift to the exfoliation process. Developing a theoretical protocol to straightforwardly simulate different surface chemistries, we show that the 13 C NMR shift variations can be quantitatively related to different surface compositions and number of surface chemistry variants induced by the different etching agents. In addition, we propose that the etching agent affects not only the nature of the surface groups but also their spatial distribution. The direct correlation between surface chemistry and 13 C NMR shift is further confirmed on the V 2 CT x , Mo 2 CT x , and Nb 2 CT x MXenes.

A polyol-assisted solvothermal route is used to synthesize NixFey nanoalloys supported on a highly electron conductive 2D transition metal Mo2CTx MXene. Structural, morphological and chemical characteristics of the materials are determined using several physicochemical techniques. The MXene support allows not only the formation of a nanostructured metallic NixFey nanoalloys, but also favors the interfacial charge transfer for the OER. The NixFey@Mo2CTx material with a Ni/Fe ratio of 2.66 leads to the outstanding activity for the OER with an amazingly low Tafel slope value of 34 mV dec-1 and a current density of 10 mA.cm-2 at a potential of only 1.50 V vs. RHE. In situ Raman experiments show that β-NiOOH formed by oxidation of the nanoalloys under positive scan, likely containing a very small amount of Fe, is the active phase for the OER. This material exhibits also an excellent stability over 168 h in a 5 M KOH electrolyte. TEM-EELS analyses after 100 voltammetric cycles between 0.2 to 1.55 V vs. RHE evidence for the first time that the MXene support is not fully oxidized in the first cycle. Also, oxyhydroxide layer formed in the OER potential region at the surface of the NixFey nanoparticles can be reversibly reduced.

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This work reports on the facile synthesis and characterisation of a non-precious-metal bifunctional catalyst for oxygen reduction and evolution reactions (ORR and OER). A few-layer reduced graphene oxide-supported NiCo O catalyst is prepared using a rapid and easy two-step method of synthesis. It consists of the solvothermal poyl(vinylpyrrolidone)-assisted assembly of metal complexes onto few-layer graphene followed by a calcination step aiming at converting metal complexes into the spinel phase. Using this synthesis approach, the most active material demonstrates an outstanding activity towards the OER and ORR, making it one of the best bifunctional catalysts of these reactions ever reported. This composite catalyst exhibits improved bifunctional behaviour with a low reversibility criterion of 746 mV. The ORR process follows a four-electron pathway and the hydroxyl selectivity is higher than those with pure reduced graphene oxide or NiCo O materials, showing the synergistic effect between the two phases. Moreover, the high activity of this composite catalyst is confirmed by comparing its performance with those obtained on other cobaltite catalysts prepared using a different synthesis method, or those obtained using a different graphene-based support.