Perovskite-carbon composites synthesized through in situ autocombustion for the oxygen reduction reaction: the carbon effect

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How the surface state of nickel/gadolinium-doped ceria cathodes influences the electrochemical performance in direct CO 2 electrolysis

International audienceThe effect of the surface state on the electrochemical performance of nickel gadolinium-doped ceria (NiGDC) cermet electrodes in direct CO 2 electrolysis was studied by operando near ambient pressure X-ray photoelectron spectroscopy combined with on-line gas phase and electrical measurements. The CO 2 electrolysis was limited at overpotentials below the carbon deposition threshold to avoid irreversible cathode deactivation. The results revealed the dynamic evolution of the NiGDC electrode surface and disclose the side reactions associated to electrode activation in CO 2 electrolysis. Comparison of reduced and oxidized electrodes shows that metallic Ni is a prerequisite for CO 2 electrolysis, at least at low potentials, suggesting that CO 2 electoreduction occurs primarily at the three phase boundaries between gas, metallic nickel and partially reduced ceria. We also provide evidences of in situ reduction of NiO upon polarization in CO 2 , implying that addition of reductive gases to CO 2 is not indispensable to maintaining the cermet electrode in the reduced state. Inspired by this observation, we use a conventional button cell setup to demonstrate improved i-V characteristics of NiGDC electrodes in direct CO 2 electrolysis as compared to CO 2 /H 2 fuel conditions and we rationalize this behavior based on NAP-XPS results

Examination of C12A7 electride work function and surface composition by means of XPS, UPS and thermionic emission measurements

International audienceThe work functions of LaB6 and C12A7:e-samples have been estimated by thermionic emission measurements and compared with ultraviolet photoelectron spectroscopy (UPS) data. In addition to this, the surface composition of the samples is characterized by X-ray photoelectron spectroscopy (XPS). Consistent work function values of 3.2-3.3 eV have been measured for clean LaB6 samples. A wider range of values has been measured with the C12A7:e-sample, where a work function of 2.4-2.8 eV and 3.2 eV is estimated when using a setup for thermionic emission and UPS, respectively

Monitoring the gravitational reflex of the ectoparasitic mite Varroa destructor: A novel bioassay for assessing toxic effects of acaricides

We investigated the effect of several acaricides on Varroa destructor by monitoring the rhythmic expansion of the sternum, followed by a strong flexion of the legs, initiated when the mite was placed in a dorsal side-down position, as an indication of a mite's vitality. The pulses generated by the force of the rhythmic expansions had an average duration of 3.11 s, force (amplitude) of 73 mu N, and frequency of 0.228 Hz. These parameters remained constant for the first 10 h of recording, whilst significant changes occurred after 15 h. The rhythmic sternal expansion is an indication of a Varroa mite's gravitational reflex, or attempt to return to an upright position, this reflex is observed in all invertebrates and vertebrates. The sternal expansion can be recorded for over 20 h, or for as long as the Varroa mite remains alive, and the expansion stops as soon as the mite is placed in a normal, upright position. Proper function of the chain of proprioceptors, interneurons, motorneurons, neuromuscular junctions, and muscles of Varroa is required for the initiation and maintenance of such a behavioural motor pattern. Any deleterious effect of synthetic chemicals or natural compounds (acaricides) may have a direct effect on one or more of these links, thereby disturbing or even inhibiting the reflex. Topical application of 1.81 x 10(-3) mg/mite of amitraz completely inhibited the gravitational reflex within 60-70 min for all mites tested. The volatile acaricides formic acid (13.83 mg), thymol crystals (250 mg), and Apiguard (R) (1000 mg) eliminated the reflex within 10-35 min. This bioassay, based on the gravitational reflex, could be a useful tool for accurate assessment of the acaricidal action of numerous compounds under laboratory conditions, saving money and time necessary to conduct field trials. (C) 2011 Elsevier Inc. All rights reserved

Activation of solid grinding-derived Au/TiO2 photocatalysts for solar H2 production from water-methanol mixtures with low alcohol content

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1,2,4-Triazole-Based Approach to Noble-Metal-Free Visible-Light Driven Water Splitting over Carbon Nitrides

MoS₂/Co₂O₃/poly­(heptazine imide) composite photocatalysts which are active in both noble-metal-free water reduction and water oxidation half reactions upon irradiation with visible light were synthesized by a one-step procedure. Here, the LiCl/KCl eutectic melt was used as a high-temperature solvent; 3-amino-1,2,4-triazole-5-thiol simultaneously acts as a carbon nitride precursor, sulfur source, and reducing agent for Mo⁵⁺, while MoCl₅ was added as a metal source for MoS₂ nanoparticles being active centers for the water reduction reaction. Water oxidation centers could be created using the rich complexation chemistry of 1,2,4-triazoles. Namely, cobalt species were introduced into carbon nitride network using Co₃[3,5-diamino-1,2,4-triazole]₆ complex as a dopant, prepared in a separate step. The developed method enables one to control the cocatalysts’ loading and tune the dimensions of MoS₂ NPs. The materials reported here show 10% of the efficiency of a reference Pt/mesoporous graphitic carbon nitride composite in hydrogen evolution, and half of the performance of the reference Co₃O₄/S-doped carbon nitride material, prepared by multistep synthesis, in oxygen evolution, however, in one and the same system

Few-Layer Graphene from Mechanical Exfoliation of Graphite-Based Materials: Structure-Dependent Characteristics

We present a high-scale method to produce few-layer graphene (FLG) based on the mechanical exfoliation of graphite and compare the obtained FLG with the one reported earlier arising from pencil lead ablation. Several elements were modified and improved in the new approach. The purification and the ablation set-up were simplified, and the morphology of the FLG was modified and improved in view of some applications. The morphology-dependent properties of FLGs, lead-FLG, and graphite-FLG as conductive layers and in nanocomposites were investigated. The newly obtained FLG had a higher aspect ratio (high lateral size vs thickness/higher 2D aspect), which is reflected by enhanced transparency–conductivity features of the layer (film) and elongation-at-break behavior of the polymer composites. On the contrary, the nanocomposite containing lead-FLG showed, for instance, excellent gas barrier properties due to the multi-step structure of the lead-FLG flakes. Such structure exhibited less 2D and more 3D character, which can be highly suitable for applications where the presence of active/reactive edges is beneficial, e.g., in catalysis or supercapacitors’ electrodes. Nuclear reaction analysis was employed to investigate the morphology of graphite-FLG film

Influence of Nitridation Conditions on the Doping Sites and Photocatalytic Visible Light Activity of Nb,N-Codoped TiO₂

International audienceThe photocatalytic performance of Nb,N-codoped TiO 2 nanoparticles obtained via the sol-gel method was compared to that of N-doped TiO 2 . The study focused on investigating the effects of nitridation conditions on nitrogen insertion with a highlight on the nature of the doping sites in the photocatalyst depending on the initial presence of niobium in the TiO 2 . The photodegradation of methylene blue in solution under UV, visible, and simulated solar light was used to evaluate the photocatalytic activity of TiO 2 , Nb-or N-doped TiO 2 , and Nb,N-codoped TiO 2 nanoparticles. Codoped TiO 2 produced by mild thermal nitridation exhibits the best photocatalytic activity, with a strong contribution from visible light. On the contrary, the codoped TiO 2 produced by more intense thermal nitridation presents lower photocatalytic performances than TiO 2 despite a small improvement of activity in the visible range. In addition to material characterization (X-ray diffraction, UV-vis spectroscopy, and X-ray photoelectron spectroscopy), electron paramagnetic resonance and reversed double-beam photoacoustic spectroscopy measurements were used to identify the respective doping sites and ultimately propose the electronic band structure for each sample of Nb:TiO 2 , N:TiO 2 , and Nb,N:TiO 2 . Proper thermal nitridation conditions improve the charge compensation between Nb 5+ and N 3-, thereby enhancing the photocatalytic activity. However, too intense nitridation conditions led to the generation of oxygen vacancies and a large amount of Ti 3+ acting as charge recombination centers, resulting in significant deterioration of the photocatalytic performances. This study highlights the importance of understanding the intricate charge compensation process in codoped (M,N) TiO 2 materials, as the photocatalytic performance cannot be elucidated solely by the cation/anion ratio but also by considering the nature of the doping sites generated during synthesis

Synchrotron Radiation X‑ray Photoelectron Spectroscopy as a Tool To Resolve the Dimensions of Spherical Core/Shell Nanoparticles

In this work we demonstrate the potential of synchrotron X-ray photoelectron spectroscopy (XPS) to provide quantitative information on the intrinsic dimensions of core–shell nanoparticles. The methodology is based on the simulation of depth profiling curves, using simplified quantitative models earlier proposed in the literature. Three model systems consisting of X@Fe₂O₃ (with X = Au, Pt, and Rh) metal–iron oxide core–shell nanoparticles, formed via oxidation of size-selected 5 nm bimetallic FeX nanoparticles inside the spectrometer, were measured in situ by near ambient pressure XPS. We show that when the shell layer is composed of a unique component, the experimental depth profiling curve can be simulated by the quantitative calculations and reveal the core and the shell thickness of the nanoparticles. On the contrary, a significant offset between the experimental and the theoretical depth profiling curves implies intermixing between the core and the shell layers. In this case the theoretical model has been modified to represent the more complex particle morphology. Transmission electron microscopy results are in good agreement with the XPS findings, confirming the validity of the model to predict the nanoparticle dimensions

Synchrotron Radiation X‑ray Photoelectron Spectroscopy as a Tool To Resolve the Dimensions of Spherical Core/Shell Nanoparticles

In this work we demonstrate the potential of synchrotron X-ray photoelectron spectroscopy (XPS) to provide quantitative information on the intrinsic dimensions of core–shell nanoparticles. The methodology is based on the simulation of depth profiling curves, using simplified quantitative models earlier proposed in the literature. Three model systems consisting of X@Fe₂O₃ (with X = Au, Pt, and Rh) metal–iron oxide core–shell nanoparticles, formed via oxidation of size-selected 5 nm bimetallic FeX nanoparticles inside the spectrometer, were measured in situ by near ambient pressure XPS. We show that when the shell layer is composed of a unique component, the experimental depth profiling curve can be simulated by the quantitative calculations and reveal the core and the shell thickness of the nanoparticles. On the contrary, a significant offset between the experimental and the theoretical depth profiling curves implies intermixing between the core and the shell layers. In this case the theoretical model has been modified to represent the more complex particle morphology. Transmission electron microscopy results are in good agreement with the XPS findings, confirming the validity of the model to predict the nanoparticle dimensions