Jean-Yves Piquemal

High temperature structural and magnetic properties of cobalt nanorods

Abstract We present in this paper the structural and magnetic properties of high aspect ratio Co nanoparticles (∼10) at high temperatures (up to 623xa0K) using in-situ X ray diffraction (XRD) and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500xa0K. The coercivity can be modelled by μ 0 H C =2( K MC + K shape )/ M S with K MC the magnetocrystalline anisotropy constant, K shape the shape anisotropy constant and M S the saturation magnetization. H C decreases linearly when the temperature is increased due to the loss of the Co magnetocrystalline anisotropy contribution. At 500xa0K, 50% of the room temperature coercivity is preserved corresponding to the shape anisotropy contribution only. We show that the coercivity drop is reversible in the range 300–500xa0K in good agreement with the absence of particle alteration. Above 525xa0K, the magnetic properties are irreversibly altered either by sintering or by oxidation.

Nanoparticles of metal and metal oxides: some peculiar synthesis methods, size and shape control, application to catalysts preparation

One step (direct) and multisteps synthesis methods of metal and oxide nanoparticles are presented including both bottom-up and top-down procedures. Chemical methods involving the polyol process allows to get either isotropic or anisotropic nanoparticles. Nanowires, nanorods and dumbells can be generated either by heterogeneous nucleation or by using a template (mesoporous silica). Top-down procedures are illustrated by laser irradiation of immersed bulk targets (Ag, Au) which generates self-organized nanostructures or colloidal solutions. Laser irradiation of these colloidal solutions modifies the size distribution and the shape of nanoparticles. Bimetallic nanoparticles are generated from mixtures of monometallic colloidal solutions. Surface-mediated methods involving a host support and an invited phase are also presented. They lead to the formation of metal or oxide clusters and nanoparticles. The speciation of these entities is correlated with the observed catalytic performances.

Low dipolar interactions in dense aggregates of aligned magnetic nanowires

Magnetic interactions in aggregates of polyol-synthesized cobalt nanowires are studied by δM plots. Negative δM values are obtained as expected for magnetostatic coupling between physically isolated nanowires, without exchange interactions. Very weak interactions (down to δMu2009=u2009−0.05) are achieved by alignment and dispersion into polymeric matrices. We attribute this to the fact that during the fabrication of the composite materials, the nanowires reorganize so as to minimize the dipolar interactions. Since the δM-plots are sensitive to the nature of the demagnetized state, it is shown by micromagnetic simulations that the ac demagnetized state provides the best choice of a starting point since it ensures that macroscopic configurations consisting of single domain states of each wire are compared. The low interactions observed in these materials suggest that these composite materials are candidates for rare-earth free permanent magnets since demagnetization phenomena are minimized.

Control of the crystal habit and magnetic properties of Co nanoparticles through the stirring rate

Ferromagnetic hcp cobalt nanoparticles (NPs) are prepared by the polyol process using long-chain carboxylates as shape-directing agents, applying different stirring rates during the synthesis. Particles prepared with a stirring rate lower than ca. 150 rpm are very well-defined anisotropic crystals with smooth lateral facets and low stacking fault densities. The growth proceeds along the c axis of the hcp structure which corresponds to the long axis of the anisotropic particles. Increasing the stirring speed leads to a lowering of the mean aspect ratio, an increase in structural disorder as well as to a strong modification of the particle habit. The resulting Co NPs then display a marked surface roughness. We propose here a mechanism to explain the different morphologies observed. A high stacking fault density triggers the local disorganisation of the adsorbed capping agent layer which in turn favours the diffusion of Co monomers through the organic layer, resulting in the formation of excrescences perpendicular to the long axis of the particles. The magnetic properties are strongly related to the morphological and micro-structural properties of the particles. The stirring rate appears, so far, as the driving force to control the magnetic properties of the NPs from hard magnetic behaviour at low stirring rates to an almost soft magnetic one under high stirring conditions.

Unsupported shaped cobalt nanoparticles as efficient and recyclable catalysts for the solvent-free acceptorless dehydrogenation of alcohols

Oxidation of alcohols is a key-reaction for the valorization of biomass compounds, and green processes are preferred to avoid the use or production of toxic compounds. In this context, unsupported nanometer-sized catalysts have emerged as very promising materials for heterogeneous catalysis. In this paper we explore the catalytic activity of unsupported cobalt nanoparticles towards the dehydrogenation of aliphatic primary and secondary alcohols under solvent-free conditions. The unsupported particles are found to be highly active for the conversion of secondary alcohol to the corresponding ketone vs. the primary alcohol. The oxidation process is following an acceptorless dehydrogenation mechanism, where the only by-product of the reaction is the highly valuable H2 molecule. DFT calculations evidence that the chemoselectivity of secondary vs. primary alcohols originates from a more favorable desorption of the ketone reaction product compared to the aldehyde. It is also found that the morphology of the particles has a strong influence on the catalyst efficiency and stability: Co nanorods can be recycled at least three times without a loss in catalytic performances.