Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer

Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers

Direct and co-catalytic oxidative aromatization of 1,4-dihydropyridines and related substrates using gold nanoparticles supported on carbon nanotubes

International audienceA heterogeneous catalyst was assembled by stabilization of gold nanoparticles on carbon nanotubes. The resulting nanohybrid was used in the catalytic aerobic oxidation of 1,4-dihydropyridines. The system proved very efficient on the investigated substrates either directly or in the presence of a quinone co-catalyst. Pyridines have found applications in various domains such as in the synthesis of drugs, 1 herbicides, 2 or insecticides. 3 In addition, the pyridine scaffold plays a central role in living systems since nicotinamide adenine dinucleotide (NAD + / NADH) and nicotinamide adenine dinucleotide phosphate (NADP + /NADPH) are key pyridine-incorporating co-factors involved in oxido-reduction processes. 4 Among the various methods developed for the synthesis of pyridine derivatives, 5 the oxidation of Hantzsch 1,4-dihydropyridines is likely one of the most straightforward approaches. The Hantzsch pyridine synthesis is a multi-component process which involves an aldehyde, two equivalents of a β-keto ester, and a nitrogen atom source. The initial condensation affords a dihydro-pyridine intermediate which can be oxidized, in a second step, into the corresponding pyridine. This reaction was reported for the first time in 1881 by Arthur Hantzsch. 6 Numerous reagents have been reported to promote the second step-oxidative aromatization reaction and include, for example , copper bromide, 7 ferric chloride, 8 palladium on carbon, 9 manganese dioxide, 10 and tert-butylhydroperoxide. 11 However, the oxidation of dihydropyridines is not always easy when substrates bear sensitive functional groups and there is still the quest for mild and general protocols. Also, some of the reported procedures lead to the formation of by-products which can be difficult to remove from the reaction mixture. Therefore, the development of milder, sustainable, and more effective methods for the oxidative aromatization of 1,4-dihydropyridines is sound. In the present article, we report the assembly and use of a recyclable CNT-supported gold catalyst for the selective and mild aerobic oxidation of 1,4-dihydropyridines (DHPs) and related substrates (Fig. 1). The CNT-gold catalyst was assembled using a layer-by-layer approach according to a previously reported procedure Fig. 1 Overview of the catalytic assembly and catalyzed oxidation process studied in the present work

Growth Kinetics and Characterization of Chromia Scales Formed on Ni–30Cr Alloy in Impure Argon at 700 °C

The oxidation of a Ni–30Cr alloy at 700 °C in impure argon was studied in order to provide new elements of understanding on chromia scale growth in low oxygen partial pressure atmosphere (10−5 atm). Oxidation tests were carried out during 30 min to 50 h in a thermogravimetric analysis system using a symmetrical balance with in situ monitoring of the oxygen partial pressure. The oxidation kinetics were determined as parabolic with an estimated stationary parabolic constant value of 10−15 cm2 s−1, after a transient stage of about 3 h. The oxide scale was identified as a pure chromia layer by TEM and XPS characterisations. After 50 h at 700 °C, the scale thickness estimated by TEM cross section observation was about 100 nm. A slightly thicker and more porous oxide scale was observed above the alloy’s grain boundaries. The metal/oxide interface also exhibited a deeper recession towards the substrate above the alloy’s grain boundaries. The orientation of chromia grains was determined by TKD (transmission Kikuchi diffraction). A strong preference was noted for the orientation perpendicular to the surface, along the direction of the corundum structure. Finally, the type of semiconduction was determined for the scales formed after 7 h and 50 h of oxidation. For the shorter oxidation time, the chromia scale exhibited an n-type semiconduction, whereas for the longer exposure, both n-type and insulating semiconduction were identified

Radio-oxidation of electric cabled models: ageing evaluation at the atomic scale

International audienceElectric cables being safety components of plants, their functionality has to be ensured. In Nuclear Power Plants when they are in the reactor building, these cables can suffer γ irradiation even in normal operation conditions. Their ageing behavior has to be well understood to be able to determine a precise end-of-life criterion. As polymers are the most radiosensitive material of the cables, this paper focus on the ageing of this kind of material and more specifically on the ageing of silane-crosslinked polyethylenes (XLPE). XLPE are now one of the most employed polymers to manufacture cables. We performed irradiation under oxidative conditions of few model silane-crosslinked polyethylenes with different additives and filler: at three different doses (0, 67, 220 and 374 kGy) for one dose rate (78 Gy.h$^{-1}$) and at one dose (67 kGy) for three dose rates (8.5, 78 and 400 Gy.h$^{-1}$). Modifications in the organic materials were followed by X-ray photoelectron spectroscopy. This analytical technique allows to follow the evolution of the different chemical products formed under irradiation. A better understanding at the atomic scale of the effect of additives on the degradation of polymers will be proposed, as a function of the ageing conditions

XPS study of Ni-base alloys oxide films formed in primary conditions of pressurized water reactor

International audienceThe oxide scales formed on alloy 690 during its exposure to pressurized water reactor primary simulated media at 325 °C for different periods, ranging between 24 and 858 h have been characterized through XPS analysis. These results have been compared with the data obtained on Cr2O3, NiCr2O4, NiFe2O4, and Ni(OH)2 reference compounds. This comparison leads to the following conclusions about the scale formed on alloy 690: (i) the deconvolution of Cr-2p3/2 core level spectra seems to show that the internal oxide scale is made of a mix of chromite and chromia; (ii) the evolution of the height ratio between satellite and principal peaks, on the Ni-2p3/2 core level, has highlighted the presence of nickel hydroxide for all corrosion durations; and (iii) XPS analysis can be used to study the kinetics of nickel ferrite precipitation. In order to obtain a more complete description of such oxide scales based upon XPS measurements, reference data, obtained on nickel ferrites and chromites in which Fe is partially substituted to Ni, are needed. The comparison of published data and reference spectra performed in this work also leads to conclude that core level fits are dependent on the energy resolution, which can be achieved with specific spectrometer and analysis procedures. Therefore, reference compound spectra need to be recorded in the very identical analytical configuration before dealing with results obtained on corroded specimen

Bimetallic ruthenium–rhodium particles supported on carbon nanotubes for the hydrophosphinylation of alkenes and alkynes

International audienceRuRh bimetallic particles supported on carbon nanotubes were applied to the hydrophosphinylation of alkenes and alkynes. This recyclable system operates under sustainable conditions: low catalyst loading, room temperature and atmospheric pressure

Continuous Flow Photocatalytic Hydrogen Production from Water Synergistically Activated by TiO₂, Gold Nanoparticles, and Carbon Nanotubes

Titanium dioxide nanoparticles were combined with carbon nanotubes and gold to develop improved photocatalysts for the production of hydrogen from water. The entangled nature of the nanotubes allowed for the integration of the photoactive hybrid catalyst, as a packed-bed, in a microfluidic photoreactor, and the chips were studied in the photocatalyzed continuous flow production of hydrogen. The combination of titanium dioxide with carbon nanotubes and gold significantly improved hydrogen production due to a synergistic effect between the multi-component system and the stabilization of the active catalytic species. The titanium dioxide/carbon nanotubes/gold system permitted a 2.5-fold increase in hydrogen production, compared to that of titanium dioxide/carbon nanotubes, and a 20-fold increase, compared to that of titanium dioxide

Crystal field effects on the photoemission spectra in Cr2O3 thin films: from multiplet splitting features to the local structure

International audienceChanges in the shape of X-ray photoemission (XPS) spectra can be related to changes in the local structure of a transi-tion metal. By combining Crystal Field Multiplet calculations and well-controlled molecular beam epitaxy growth of alpha-Cr2O3(0001) thin films on alpha-Al2O3(0001) substrates, we prove that it possible to link the features of Cr 2p XPS spectra with local distortions of CrO6 octahedra and d-orbitals reorganization. Hence, we show that the splitting of the Cr 2p3/2 envelope is related to the degeneracy of the t2g orbital triplet, which corresponds to a fully relaxed structure. Conversely, the broad unstructured Cr 2p3/2 envelope relies on splitting of t2g orbitals and it is the fingerprint of large trigonal distor-tions of CrO6 octahedra. Then, using the Cr 2p XPS as a structural tool for -Cr2O3, we show that the Cr2O3 protective layer formed by oxidation of polycrystalline Ni30Cr alloy exhibits in-plane strains at early oxidation stages and grows preferentially along the c-axis

Aqueous 1,3-dipolar cycloadditions promoted by copper nanoparticles in polydiacetylene micelles

International audienceA novel colloidal catalyst was developed through the encapsulation of cuprous oxide nanoparticles in polydiacetylene micelles. The Cu-based catalyst was applied to the Huisgen cycloaddition reaction in fully aqueous medium. The process neither requires heating nor controlled atmosphere and the catalyst can be recycled in subsequent reactions without any loss of activity