Scientists uncover chemical transformations in cobalt nanoparticles

 The evolution schematics of transition from cobalt to cobalt phosphide nanocrystals.

Understanding the intricacies of how nanoparticles undergo chemical transformations could lead to better ways to tailor their composition, which can lead to advanced material properties.

Using the Cornell High Energy Synchrotron Source, scientists led by Richard Robinson, assistant professor of materials science and engineering, uncovered exactly what happens when cobalt nanoparticles transform into two phases of cobalt phosphides.

Their work, published in the Journal of Materials Chemistry, was featured by the journal as a "Hot Article" earlier this month.

The effect Robinson's team observed in the cobalt phosphide transitions was a nanoparticle hollowing due to asymmetric diffusivities of cations and anions. In other words, the cations move out from the core faster than anions can diffuse in, leading to a hollow particle.

Other groups have reported on this "Kirkendall" effect, but the Robinson team was the first to show that this hollowing is more complex than previously thought and can be studied as a two-step process. Their work could be used to control this process and produce complex particles with properties tailored for use in energy applications. Metal phosphides have a wide range of properties -- ferromagnetism, superconductivity, catalytic activity and magnetoresistance among them.

The work was done in collaboration with scientists led by Richard Hennig, assistant professor of materials science and engineering. It was supported by King Abdullah University of Science and Technology, the Cornell Center for Materials Research and the Energy Materials Center at Cornell.

Provided by Cornell University (news : web)

Taking the guesswork out of understanding complex, multi-molecule transformations

Multicomponent reactions (MCRs) that chemically combine three or more molecules into a brand new product are faster and generate less waste than traditional step-by-step synthetic procedures, making them invaluable in efforts to improve efficiency and sustainability. Since 1921, chemists have used an MCR called the Passerini reaction to produce bioactive, peptide-like chains made from three partners: carboxylic acids, carbonyl compounds, and cyanide-bearing molecules. However, a full understanding of this process has eluded researchers because its multipart workings are difficult to detect experimentally.

Now, Satoshi Maeda and Keiji Morokuma from Kyoto University and Shinsuke Komagawa and Masanobu Uchiyama from the RIKEN Advanced Science Institute in Wako have developed a computerized way to better identify the hidden mechanisms of one-pot, multi-step chemical transformations. Their technique, the artificial force induced reaction (AFIR), systematically squeezes and joins model compounds together in order to rapidly detect signatures of real MCR energy barriers.

According to Maeda, detailed theoretical understandings of MCRs have been scarce because most calculations require excellent estimates of transition state structures—intermediate and often highly strained geometric arrangements between molecules that correspond to the energetic peak of a reaction barrier. “Consequently, a trial-and-error process based only on [chemistry-based] intuition is unavoidable,” he says.

The AFIR method, on the other hand, requires no such presumptions. Maeda explains that when two molecules are pushed into each other with a weak force, they spontaneously relax into ‘dents’ in the potential barrier created by attractive electronic interactions between the reactants. By methodically pressing over all possible orientations, and inducing an artificial reaction from the relaxed positions, AFIR searches can identify every stable reaction pathway in a system.

To the team’s surprise, applying AFIR calculations to the Passerini reaction revealed that four components, not the long-thought three, must be involved (Fig. 1). Since the reaction barriers were so high, the researchers realized that an additional carboxylic acid—a known proton transfer catalyst—had to participate in the transition states leading to the final product. This finding should enable design of Passerini reactions with improved structural selectivity, notes Maeda.

Once perfected, the researchers anticipate their technique will make MCRs even more widespread. “Unlike standard methods, giving only pathways that are easy to find, the AFIR method has a unique ability to find unknown pathways in MCRs systematically,” says Maeda. “Information on reaction pathways is very important even for processes that do not normally occur, because chemists can initiate such reactions by controlling conditions, modifying substituents, and introducing new catalysts.”

More information: Maeda, S., et al. Finding reaction pathways for multicomponent reactions: The Passerini reaction is a four-component reaction. Angewandte Chemie International Edition 50, 644–649 (2011). http://onlinelibra … 336/abstract

Provided by RIKEN (news : web)