With the aim of obtaining a highly stable and active catalyst for oxygen evolution reaction (OER), a core–shell-like IrO2@RuO2 material was synthesized by using a surface modification/precipitation method in ethanol medium. The comparison of this catalyst with pure RuO2 and pure IrO2 showed that the obtained mixed oxide catalyst displayed the highest amount of active sites as well as a good accessibility for water. Moreover, this catalyst was shown to be highly stable toward repetitive redox cycling. Polarization curves of the three catalysts showed that the IrO2@RuO2 was the most active for the OER due to the large number and high accessibility of active sites. These catalytic benefic effects are attributed to an intimate contact between the two oxides in the IrO2-covered RuO2 nanocatalyst that combines the RuO2 intrinsic activity and the IrO2 stability. The present study contributes therefore to the rational design of efficient and stable electrocatalysts for water splitting in acidic media.

Since their discovery in 2011, the 2D transition metal carbide, nitrides, and carbonitridesdubbed MXenes -have garnered a lot of worldwide interest. Given their 2D structure, surface, or termination, chemistries play a vital role in most applications. X-ray photoelectron spectroscopy, XPS, is one of the most common characterization tools for quantifying surface terminations and overall chemistry. Herein we critically review the XPS fitting models proposed for Ti 3 C 2 T z MXene in the literature and make the case that they are at best incomplete and at worst contradictory. We propose a new fitting algorithm based on all the data obtained from previously published studies. In our approach, we assign the Ti 2p peak at 455.0 eV, to the C-Ti-O\O\O and C-Ti-C octahedra. The peaks at 456.0, 457.0, 457.9, and 459.6 eV are assigned to C-Ti-O\O\F, C-Ti-O\F\F, C-Ti-F\F\F, and TiO 2-x F 2x , respectively. The first four represent possible Ti atom terminations; the last is an oxyfluoride. In our proposed model we do not distinguish between O and OH terminations in the Ti 2p spectra; we only do so in the O spectra. Lastly, we propose and recommend a method for quantifying the surface terminations in Ti 3 C 2 T z .

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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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This work reports the synthesis of Co3O4 particles that can be used as effective electrode materials for both the oxygen reduction (ORR) and evolution (OER) reactions. The development of such catalysts, free from precious group metals and capable of decreasing overpotentials in fuel cells, metal–air batteries, and water electrolyzers, also requires stable supporting and conducting substrates in order to deposit low metal oxide loadings. This challenging approach led us to prepare Co3O4 materials on graphene-based composites more stable than common used Vulcan carbon. Co3O4 particles synthesized from a solvothermal method were thereby deposited onto reduced graphene oxide (RGO) and N-doped reduced graphene oxide (NRGO) prepared from the Hummers method. The structural properties and surface composition of the different materials characterized by X-ray diffraction, transmission electron microscopy, and X-ray induced photoelectron spectroscopy measurements were combined to cyclic voltammetry experiments for revealing the charge transfer from cobalt to nitrogen, which greatly affects the charge acceptance of the surface Co atoms. Electrochemical measurements provided sound evidence of active and stable Co3O4 catalysts toward the ORR and OER.

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