Imaging the strain evolution of a platinum nanoparticle under electrochemical control.

Surface strain is widely employed in gas phase catalysis and electrocatalysis to control the binding energies of adsorbates on active sites. However, in situ or operando strain measurements are experimentally challenging, especially on nanomaterials. Here we exploit coherent diffraction at the new fourth-generation Extremely Brilliant Source of the European Synchrotron Radiation Facility to map and quantify strain within individual Pt catalyst nanoparticles under electrochemical control. Three-dimensional nanoresolution strain microscopy, together with density functional theory and atomistic simulations, show evidence of heterogeneous and potential-dependent strain distribution between highly coordinated ({100} and {111} facets) and undercoordinated atoms (edges and corners), as well as evidence of strain propagation from the surface to the bulk of the nanoparticle. These dynamic structural relationships directly inform the design of strain-engineered nanocatalysts for energy storage and conversion applications.

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.