International audienceThe stability of carbon-supported electrocatalysts has been largely investigated in acidic electrolytes, but the literature is much scarcer regarding similar stability studies in alkaline medium. Herein, the degradation of Vulcan XC-72-supported platinum nanoparticles (noted Pt/C), a state-of-the-art proton exchange membrane fuel cell electrocatalyst, is investigated in alkaline medium by combining electrochemical measurements and identical location transmission electron microscopy; electrochemical surface area (ECSA) losses were bridged to electrocatalyst morphological changes. The results demonstrate that the degradation in 0.1 M NaOH at 25 degrees C is severe (60% of ECSA loss after only 150 cycles between 0.1 and 1.23 V vs RHE), which is about 3 times worse than in acidic media for this soft accelerated stress test. Severe carbon corrosion has been ruled out according to Raman spectroscopy and X-ray photoelectron spectroscopy measurements, and it seems that the chemistry of the carbon support (in particular, the interface (chemical bounding)) between the Pt nanoparticles and their carbon substrate does play a significant role in the observed degradations

International audienceMetal oxide (SnO₂, TiO₂, In₂O₃, ZnO) sols are prepared by various sol-gel processes in such a way as to hinder the condensation reactions. The obtained sols are injected at 160 °C into a solution of tetradecene and dodecylamine, and kept under heating for different periods of time. Depending on the starting sol, variously crystallized oxide nanoparticles are obtained, whose phase compositions and chemical structure have been studied by X-ray diffraction (XRD) and Fourier transform IR spectroscopy. The elimination of the organic residuals has been carried out by thermal treatment, and the thermal evolution of the nanoparticles has been studied by thermal analyses and Raman spectroscopy. High-resolution transmission electron microscopy studies coupled with XRD measurements show that the thermal treatment does not markedly affect the particle size, which remains in the nanometer-sized regime (from 3.5 to 8.5 nm, depending on the system), except in the case of ZnO. The thermally purified and stabilized powders, drop-coated onto alumina substrates with pre-deposited electrical contacts, have been tested as gas-sensing devices, displaying outstanding sensing properties even at room temperature

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A study of the adsorption of NO 2 on a nanocrystalline tin(IV) oxide powder was carried out in situ by Raman spectroscopy. Heat treatment up to 300°C of the sample in NO 2 revealed the complexity of the surface reactions. Thus, numerous NO x adsorbed species were evidenced such as weakly adsorbed nitrates, NO 2 dimers, nitrites, bridging and various bidentate nitrates. The stability of these last species has been related to the N O bond strength. It has been proposed that the nitrite species are responsible for the SnO 2 response in the presence of NO 2 traces.

5 O 26 were investigated in the range 80-1000 K and 80-623 K, respectively. Tentative assignments of bands to stretching and bending modes of SiO 4 tetrahedra and to M'O vibrations are proposed. Except for the two new bands, which appear around 700 cm −1 when Al is replaced by Fe, only some band broadenings and relative intensity changes are observed as a function of the rate of O5 or La vacancies. Most of the bands soften and broaden continuously when raising the temperature. This is an indication that the Al-and Fe-substituted apatites do not undergo any structural change up to 1000 K. Above 1000 K, the broad and weak shoulder observed at 850 cm −1 for La 10 Si 4 Fe 2 O 26 is replaced by a strong band at 868 cm −1 , suggesting that SiO 4 tetrahedra undergo a structural modification. All compounds show the same residual band broadening at 80 K. This suggests that there is a small rate of static disorder preferentially related to the solubility of Al and Fe in the Si sublattice rather than to other defects. Moreover, the observation of FeO modes indicates that the dynamics of the solid solution obeys the so-called two-mode behavior. The occurrence of FeO stretching vibrations 150 cm −1 lower than for those of SiO suggests that the coordination number of iron could be larger than 4, particularly for the Fe 4+ species.

We report the design of a H2/O2 (fuel) cell for in situ μ‐Raman spectroscopic measurements. The horizontal orientation of the cell is conceived to allow the penetration of the laser beam along the z‐axis from one electrode to the other. We show that during in‐depth analyses the Raman signal is not significantly lost and the axial resolution is sufficiently good to allow quantitative and qualitative spectral interpretation. We report “proof of principle” tests performed on Nafion membranes swelled with protic ionic liquids to demonstrate the validity of the design and the potentiality of the method. Our results confirm that this experimental setup can efficiently be used to follow structural changes and concentration gradients in the electrolyte of a fuel cell operando. In particular, we have been able to resolve both in time and in space the hydration state of the membrane as well as spatial variations for the $ {\rm SO}_3^- $ coordination shell during a compositional transient state.

Amorphous CdSe nanoparticles were prepared by a base-catalyzed room-temperature reaction between cadmium nitrate and selenourea, with dodecanethiol as a capping ligand. The nanoparticle size could be controlled from 1.9 to 3.6 nm by increasing the water concentration in the reaction. When the nanoparticles were heated in a pyridine suspension, excitonic peaks appeared in the initially featureless optical absorption spectra. By changing the suspension solvent and the capping ligand and its concentration, it was shown that the dynamic surface exchange between the ligand and pyridine controls the crystallization process. This phenomenon was interpreted as a surface rigidity effect imposed by the ligand, whose importance was separately evidenced on the dried nanoparticles by the evolution of X-ray diffraction patterns and Raman spectra. In particular, both techniques showed that a threshold temperature is needed before crystallization occurs, and such a threshold was related to ligand desorption. The surface effect was directly visualized by high-resolution transmission electron microscopy observations of the amorphous particles, where crystallization under the electron beam was observed to start by the formation of a crystalline nucleus in the nanoparticle interior and then to extend to the whole structure.

We report on experimental characterizations of nanocrystalline oxides by Raman spectroscopy, which has proven to be a reference technique when the scale of the coherent domains becomes too small for X-ray diffraction. Crystallite-size modification of ZrO 2 and WO 3 phase diagrams along with surface modes, surface reactions and adsorbed species characterization on WO 3 and SnO 2 will be reviewed.