Mechanism of formation of Co-Ru nanoalloys: the key role of Ru in the reduction pathway of Co.

The chemical synthesis of alloy nanoparticles requires adequate conditions to enable co-reduction instead of separate reduction of the two metal cations. The mechanism of formation of bimetallic cobalt-ruthenium nanoalloys by reducing metal salts in an alcohol medium was explored to draw general rules to extrapolate to other systems. The relative kinetics of the reduction of both metal cations were studied by UV-visible and in situ Quick-X-ray absorption spectroscopies as well as H2 evolution. The addition of Co(II) ions does not influence the reduction kinetics of Ru(III) but adding Ru(III) to a Co(II) solution promotes the reduction of cobalt cations. Indeed, while CoO is formed when reaching the boiling temperature of the solvent for the monometallic system, a direct reduction of Co is observed at this temperature without formation of the oxide for the bimetallic one. The co-reduction of the metal cations results in the formation of bimetallic nanoplatelets, the size of which can be tuned by changing the Ru content.

Green synthesis of water splitting electrocatalysts: IrO2 nanocages via Pearson's chemistry.

Highly porous iridium oxide structures are particularly well-suited for the preparation of porous catalyst layers needed in proton exchange membrane water electrolyzers. Herein, we report the formation of iridium oxide nanostructured cages, via a water-based process performed at room temperature, using cheap Cu2O cubes as the template. In this synthetic approach, based on Pearson's hard and soft acid-base theory, the replacement of the Cu2O core by an iridium shell is permitted by the difference in hardness/softness of cations and anions of the two reactants Cu2O and IrCl3. Calcination followed by acid leaching allow the removal of residual copper oxide cores and leave IrO2 hierarchical porous structures with outstanding activity toward the oxygen evolution reaction. Fundamental understanding of the reaction steps and identification of the intermediates are permitted by coupling a set of ex situ and in situ techniques including operando time-resolved X-ray absorption spectroscopy during the synthesis.

The origin of the high electrochemical activity of pseudo-amorphous iridium oxides.

Combining high activity and stability, iridium oxide remains the gold standard material for the oxygen evolution reaction in acidic medium for green hydrogen production. The reasons for the higher electroactivity of amorphous iridium oxides compared to their crystalline counterpart is still the matter of an intense debate in the literature and, a comprehensive understanding is needed to optimize its use and allow for the development of water electrolysis. By producing iridium-based mixed oxides using aerosol, we are able to decouple the electronic processes from the structural transformation, i.e. Ir oxidation from IrO2 crystallization, occurring upon calcination. Full characterization using in situ and ex situ X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy allows to unambiguously attribute their high electrochemical activity to structural features and rules out the iridium oxidation state as a critical parameter. This study indicates that short-range ordering, corresponding to sub-2nm crystal size for our samples, drives the activity independently of the initial oxidation state and composition of the calcined iridium oxides.

On the importance of the crystalline surface structure on the catalytic activity and stability of tailored unsupported cobalt nanoparticles for the solvent-free acceptor-less alcohol dehydrogenation.

Unsupported nanoparticles are now recognized as model catalysts to evaluate the intrinsic activity of metal particles, irrespectively of that of the support. Co nanoparticles with different morphologies, rods, diabolos and cubes have been prepared by the polyol process and tested for the acceptorless catalytic dehydrogenation of alcohols under solvent-free conditions. Rods crystallize with the pure hcp structure, diabolos with a mixture of hcp and fcc phases, while the cubes crystallize in a complex mixture of hcp, fcc and ε-Co phases. All the cobalt particles are found to be highly selective towards the oxidation of a model secondary alcohol, octan-2-ol, into the corresponding ketone while no significant activity is found with octan-1-ol. Our results show the strong influence of particle shape on the activity and catalytic stability of the catalysts: Co nanorods display the highest conversion (85%), selectivity (95%) and recyclability compared to Co diabolos and Co cubes. We correlate the nanorods excellent stability with a strong binding of carboxylate ligands on their {1 1 2¯ 0} facets, preserving their crystalline superficial structure, as evidenced by phase modulation infrared reflection absorption spectroscopy.

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation.

The one-step seed-mediated synthesis is widely used for the preparation of ferromagnetic metal nanoparticles (NPs) since it offers a good control of particle morphology. Nevertheless, this approach suffers from a lack of mechanistic studies because of the difficulties of following in real time the heterogeneous nucleation and predicting structure effects with seeds that are generated in situ. Here, we propose a complete scheme of the heteronucleation process involved in one-pot seed-mediated syntheses of cobalt nanoparticles in liquid polyols, relying on geometrical phase analysis (GPA) of high-resolution high-angle annular dark field (HAADF)-STEM images and in situ measurements of the molecular hydrogen evolution. Cobalt particles of different shapes (rods, platelets, or hourglass-like particles) were grown by reducing cobalt carboxylate in liquid polyols in the presence of iridium or ruthenium chloride as the nucleating agent. A reaction scheme was established by monitoring the H2 evolution resulting from the decomposition of metal hydrides, formed in situ by ß-elimination of metal alkoxides, and from the polyol dehydrogenation, catalytically activated by the metal particles. This is a very good probe for both the noble metal nucleation and the heterogeneous nucleation of cobalt, showing a good separation of these two steps. Ir and Ru seeds with a size in the range 1-2 nm were found exactly in the center of the cobalt particles, whatever the cobalt particle shape, and high-resolution images revealed an epitaxial growth of the hcp Co on fcc Ir or hcp Ru seeds. The microstructure analysis around the seeds made evident two different ways of relaxing the lattice mismatch between the seeds and the cobalt, with the presence of dislocations around the Ir seeds and compression zones of the cobalt lattice near the Ru seeds. The relationship between the nature of the nucleating agent, the reaction steps, and the microstructure is discussed.

Nanodiffusion in electrocatalytic films.

In the active interest aroused by electrochemical reactions' catalysis, related to modern energy challenges, films deposited on electrodes are often preferred to homogeneous catalysts. A particularly promising variety of such films, in terms of efficiency and selectivity, is offered by sprinkling catalytic nanoparticles onto a conductive network. Coupled with the catalytic reaction, the competitive occurrence of various modes of substrate diffusion-diffusion toward nanoparticles ('nanodiffusion') against film linear diffusion and solution linear diffusion-is analysed theoretically. It is governed by a dimensionless parameter that contains all the experimental factors, thus allowing one to single out the conditions in which nanodiffusion is the dominant mode of mass transport. These theoretical predictions are illustrated experimentally by proton reduction on a mixture of platinum nanoparticles and carbon dispersed in a Nafion film deposited on a glassy carbon electrode. The density of nanoparticles and the scan rate are used as experimental variables to test the theory.

On-farm evaluation of inundative biological control of Ostrinia nubilalis (Lepidoptera: Crambidae) by Trichogramma brassicae (Hymenoptera: Trichogrammatidae) in three European maize-producing regions.

BACKGROUND: A 2 year study was conducted to evaluate the efficacy of biological control with optimally timed Trichogramma brassicae releases as an integrated pest management tool against the European corn borer (ECB), Ostrinia nubilalis (Hübner), in on-farm experiments (i.e. real field conditions) in three European regions with dissimilar geoclimatic conditions and ECB pressure and conventional management (i.e. insecticide treated and untreated). RESULTS: Biological control with Trichogramma (1) provided ECB protection comparable with conventional management, (2) in all cases maintained mycotoxin levels below the EU threshold for maize raw materials destined for food products, (3) was economically sustainable in southern France and northern Italy, but not in Slovenia where it resulted in a significant decrease in gross margin, mainly owing to the cost of Trichogramma product, and (4) enabled avoidance of detrimental environmental effects of lambda-cyhalothrin use in northern Italy. CONCLUSION: Optimally timed mass release of T. brassicae could be considered a sustainable tool for IPM programmes against ECB in southern France and northern Italy. Better involvement of regional advisory services is needed for the successful dissemination and implementation of biological control. Subsidy schemes could also motivate farmers to adopt this IPM tool and compensate for high costs of Trichogramma product.

Control of the anisotropic shape of cobalt nanorods in the liquid phase: from experiment to theory and back.

The polyol process is one of the few methods allowing the preparation of metal nanoparticles in solution. Hexagonal close packed monocrystalline Co nanorods are easily obtained in basic 1,2-butanediol at 448 K after a few minutes using a Co(II) dicarboxylate precursor. By using a combined experimental and theoretical approach, this study aims at a better understanding of the growth of anisotropic cobalt ferromagnetic nanoparticles by the polyol process. The growth of Co nanorods along the c axis of the hexagonal system was clearly evidenced by transmission electron microscopy, while the mean diameter was found to be almost constant at about 15 nm. Powder X-ray diffraction data showed that metallic cobalt was generated at the expense of a non-reduced solid lamellar intermediate phase which can be considered as a carboxylate ligand reservoir. Density functional theory calculations combined with a thermodynamic approach unambiguously showed that the main parameter governing the shape of the objects is the chemical potential of the carboxylate ligand: the crystal habit was deeply modified from rods to platelets when increasing the concentration of the ligand, i.e. its chemical potential. The approach presented in this study could be extended to a large number of particle types and growth conditions, where ligands play a key role in determining the particle shape.

Gadolinium(III) complexes of 1,4,7-triazacyclononane based picolinate ligands: simultaneous optimization of water exchange kinetics and electronic relaxation.

The two new tripodal picolinate H(3)ebpatcn (1-carboxyethyl-4,7-bis((6-carboxypyridin-2-yl)methyl)-1,4,7-triazacyclononane) and H(4)pbpatcn (1-methylphosphonic-acid-4,7-bis((6-carboxypyridin-2-yl)methyl)-1,4,7-triazacyclononane) ligands based on the 1,4,7-triazacyclononane anchor were prepared and their lanthanide complexes were characterized by NMR, fluorescence and potentiometric studies. The [Gd(ebpatcn)(H(2)O)] complex displays a relaxivity of r(1) = 4.68 mM(-1) s(-1) at 45 MHz and 298 K, whereas r(1) = 4.55 mM(-1) s(-1) was measured for [Gd(Hpbpatcn)(H(2)O)] under the same conditions. The modified scaffold of the ligands with respect to the previously reported H(3)bpatcn (1-(carboxymethyl)-4,7-bis[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane) leads to an optimization of the properties of these gadolinium complexes. The replacement of an acetate binding group of the H(3)bpatcn ligand with a propionate group (H(3)ebpatcn) or a phosphonate group (H(4)pbpatcn) leads to a faster exchange rate of the coordinated water molecule in both mono-aquo gadolinium complexes. The resulting water exchange rate is optimized for the future design of high relaxivity macromolecular gadolinium based contrast agents with a value measured by O(17) NMRD of k(ex) = 34 x 10(6) s(-1) for [Gd(Hpbpatcn)(H(2)O)] falling in the range of optimum values of (30 to 50) x 10(6) s(-1) predicted by the SBM theory. The water exchange rate k(ex)(298) = 86 x 10(6) s(-1) of the complex [Gd(ebpatcn)(H(2)O)] is the fastest reported in the literature for a neutral complex with only one inner-sphere water molecule. The relatively high stability of these modified gadolinium complexes (pGd = 14.1 for Gd(pbpatcn) and 13.1 for Gd(ebpatcn)) is similar to that of the [Gd(bpatcn)(H(2)O)] complex (pGd = 13.6). The high luminescence efficiency is also retained for the terbium complex. However, whereas the longitudinal electronic spin relaxation time keeps a value for [Gd(ebpatcn)(H(2)O)], which is long enough not to affect the relaxivity in macromolecular complexes (transient ZFS amplitude Delta(2) [10(20) rad(2) s(-2)] = 0.39), the O(17) relaxation and the (1)H NMRD indicate a rather fast electron spin relaxation for the phosphonate containing complex (Delta(2) [10(20) rad(2) s(-2)] = 1.3).

Structural and photophysical studies of highly stable lanthanide complexes of tripodal 8-hydroxyquinolinate ligands based on 1,4,7-triazacyclononane.

The tripodal H(3)thqtcn ligand allows the synthesis of well-defined neutral monomeric syn-tris(hydroxyquinolinate) complexes of lanthanides. Pure [Ln(thqtcn)] complexes (Ln = Nd, 1; Er, 2; Yb, 3) of the triply deprotonated ligand thqtcn(3-) were prepared. Crystallographic characterization was carried out for complexes 1 and 3, showing that the ligand is flexible enough to wrap around Ln(III) of different size with a tricapped trigonal-prism coordination geometry. The partially protonated H(1.5)thqtcn(1.5-) ligand also binds strongly to Ln(III) ions in methanol and water (at pH approximately 5). The X-ray diffraction study shows that protonated complexes crystallize as chiral dimers of formula [Ln(H(1.5)thqtcn)](2)(OTf)(3) x 3 MeOH (Ln = Nd, 4; Yb, 5) in which two equivalent monomeric complexes of the partially protonated H(1.5)thqtcn(1.5-) are bridged by very strong hydrogen bonds between the phenol oxygen atoms. The ligand thqtcn(3-) sensitizes efficiently the near-infrared emission of Er, Nd (0.10% Qy), and Yb (0.60% Qy). For the first time, the effect of ligand protonation on the efficiency of the solid-state luminescence emission of lanthanides complexes is demonstrated by the decrease of the luminescence quantum yield observed for [Yb(H(1.5)thqtcn)](2)(OTf)(3) (0.26%) with respect to [Yb(thqtcn)] (0.60%). The water-soluble H(3)thqtcn-SO(3) analogue of H(3)thqtcn and its lanthanide complexes has been prepared. The solution quantum yields of the thqtcn-SO(3)(3-) complexes were measured in water at pH 7.4 (0.016% for Nd(III) and 0.14% for Yb(III)) and in deuterated water (Nd, 0.047%; Yb, 0.55%), and they are among the highest reported in the literature for Yb(III) in aqueous solutions. The high thermodynamic and kinetic stability in water at physiological pH of the gadolinium complex of thqtcn-SO(3)(3-) indicate that the lanthanide complexes of thqtcn(3-) and thqtcn-SO(3)(3-) are highly resistant to hydrolysis and therefore are well suited for the development of luminescent devices and for application as probes in biomedical imaging.

Determination of the rate of a fast exchanging coordinated molecule in a lanthanide(III) complex by proton NMR.

An elementary procedure is proposed and applied to study the exchange rate of a solvent or solute molecule bound to a complexed paramagnetic Ln(III) ion, other than Gd(III), from the measured longitudinal and transverse relaxation rates and paramagnetic resonance frequency shift at a given temperature.

Efficient sensitization of lanthanide luminescence by tetrazole-based polydentate ligands.

Tetrazolate groups have been included by a convenient synthetic route in diverse ligand topologies, which have allowed the incorporation of lanthanide ions into highly luminescent double- and triple-helical complexes, demonstrating their potential for the expansion of lanthanide chemistry and the development of lanthanide-based applications.