Disordered Atoms Organized into Rows and Columns Enable Sustainable Catalysis

Scientists have created an unusually ordered version of a chemically complex alloy that could help reveal how advanced electrocatalysts work atom by atom. The material combines iridium, palladium, platinum, rhodium, and ruthenium—five elements widely associated with catalytic activity—yet remains highly mixed at the chemical level. By growing the alloy as an epitaxial film rather than as a conventional collection of disordered nanocrystals, researchers have produced a model system that can be examined with exceptional precision.

The study, led by researchers associated with Ruhr University Bochum and collaborating institutions in Germany, addresses a central problem in the development of high-performance materials. Compositionally complex alloys can contain many elements and enormous numbers of possible atomic arrangements. These variations create interfaces, defects, and local chemical environments that may improve performance, particularly in catalytic reactions. At the same time, that complexity makes it difficult to determine which atomic structures are responsible for a specific property.

The researchers normally produce such materials using combinatorial co-sputtering. In this process, atoms from five separate elemental sources are fired toward a wafer from different directions. The atoms land randomly, forming chemically disordered nanocrystals containing countless internal boundaries. This approach allows scientists to explore a broad range of compositions on a single substrate, but the resulting nanoscale disorder can obscure the relationship between structure and function when researchers attempt to study the material at atomic resolution.

To bring greater structural order to the same chemically complex mixture, the team used a sapphire wafer as a single-crystal foundation. Before depositing the alloy, they coated the sapphire with an ultrathin platinum layer that acted as an intermediate or “mediator” between the substrate and the final film. Under suitable conditions, incoming atoms align themselves with the crystal lattice beneath them, extending its organization across micrometer-scale regions. This process, known as epitaxial growth, creates a smooth, highly defined crystalline surface while preserving the alloy’s multielement composition.

The growth conditions were critical. The sputtering process had to be performed at temperatures between approximately 400 and 600 degrees Celsius, while the arrival rate of atoms had to be carefully controlled. If deposition occurred too rapidly or too slowly, the atoms would not arrange themselves into the desired crystal structure. The choice of the intermediate layer was equally important because it had to connect the sapphire substrate with the complex alloy and promote the correct atomic alignment.

Using the iridium-palladium-platinum-rhodium-ruthenium system as a test case, the researchers found that the five elements remained strongly intermixed even though the film displayed a high degree of structural order. This combination is significant: chemical randomness and crystallographic organization, often treated as opposing characteristics, were present in the same material. Such a structure could allow scientists to investigate how local chemical composition influences catalytic behavior without the added complication of a completely irregular crystal framework.

Microscopic examinations revealed that the film developed two distinct crystallographic orientations. Interfaces formed where these orientations met, but they were substantially fewer and more clearly defined than the dense network of boundaries found in conventional nanocrystalline materials. To make it possible to revisit exactly the same regions during different experiments, the researchers used a tiny diamond tip to inscribe reference marks into the wafer. These markers allowed them to locate individual areas, compare them across multiple microscopes, and connect structural observations with measurements of material performance.

That correlative approach produced an early indication that the two orientations are not functionally identical. Nanoelectrochemical analyses carried out by researchers at the University of Duisburg-Essen showed that one orientation performed better electrochemically than the other. The finding suggests that crystallographic orientation may influence how the surface participates in chemical reactions, potentially affecting the adsorption of reactants, the release of products, and the energy required for individual reaction steps.

Additional transmission electron microscopy studies uncovered another layer of complexity. In some regions, the alloy crystals continued to grow in direct agreement with the sapphire substrate. In other areas, the atomic stacking changed, producing structures that appeared as transverse stripes under the electron microscope. These defects may alter strain, atomic spacing, charge distribution, or the accessibility of active sites. Their exact influence on catalytic performance remains unknown, but the new epitaxial platform gives researchers a controlled way to investigate it.

The team believes that this strategy could transform the study of compositionally complex electrocatalysts. Instead of examining a chemically and structurally chaotic powder, scientists can now map composition, crystal orientation, defects, and electrochemical activity across defined regions of a thin film. The resulting structure-activity maps could help identify which combinations of elements and atomic arrangements produce useful catalytic behavior. In the longer term, the method may support the targeted design of catalysts for energy conversion and other technologies, turning complex alloys from materials that are difficult to understand into systems that can be engineered with increasing precision.

Web References: https://doi.org/10.1039/D6MH00852F

References: Materials Horizons, DOI: 10.1039/D6MH00852F

Keywords

compositionally complex alloys, electrocatalysts, epitaxial films, co-sputtering, nanocrystals, crystallographic orientation, atomic defects, nanoelectrochemistry, catalytic materials, materials science