Dr. Satyakam Kar (left) and Dr. Alejandro Esteban Perez Mendoza (University of Duisburg-Essen).
Photo Credit: Courtesy of Ruhr-Universität Bochum
Scientific Frontline: Extended "At a Glance" Summary: Epitaxial Solid Solutions for Sustainable Catalysis
The Core Concept: Epitaxial solid solutions are compositionally complex materials that maintain a randomly mixed chemical state across multiple elements while achieving a highly ordered, single-crystal-like structural arrangement.
Key Distinction/Mechanism: Traditional multielement co-sputtering deposits mixtures as chemically disordered nanocrystals with unpredictable interfaces. The epitaxial method differs by utilizing a sapphire single-crystal substrate and a very thin platinum mediator layer, forcing the incident atoms to mimic the underlying structural order while retaining chemical chaos.
Major Frameworks/Components:
- Combinatorial Co-Sputtering: An advanced deposition process that targets a substrate with distinct atomic streams from five different directional sources.
- Epitaxial Layer Growth: The use of a precisely heated environment (400 to 600 °C) combined with a sapphire and platinum base to dictate crystal structural alignment across micrometer-scale areas.
- Complex Elemental Composition: A meticulously designed mixture of iridium, palladium, platinum, rhodium, and ruthenium.
- Correlative Multiscale Characterization: The integration of nanoelectrochemical analysis and transmission electron microscopy to map structure-activity relationships, revealing specific stacking defect structures and two distinct crystallographic orientations.
Branch of Science: Materials Science, Electrochemistry, and Nanotechnology.
Future Application: The targeted design and deployment of highly efficient, sustainable electrocatalysts necessary for future energy infrastructures, specifically for the scalable generation of green hydrogen.
Why It Matters: By successfully unifying structural order with chemical complexity, this experimental platform enables scientists to resolve and analyze catalytic surface reactions at the atomic scale, thereby accelerating the deliberate engineering of next-generation multielement catalysts.
Can order and disorder be unified? This is possible with epitaxial solid solutions, which serve as a platform for understanding new electrocatalysts.
The energy system of the future will require sustainable catalysts that, for example, enable the efficient production of green hydrogen. Materials consisting of mixtures of five chemical elements show great promise for enabling ideal catalysts in the future. Researchers from the German Research Foundation (DFG) Collaborative Research Center CRC 1625, “Atomic-Scale Understanding and Design of Multifunctional Compositionally Complex Solid Solution Surfaces,” in Germany, have succeeded in growing so-called complex solid solutions in a nearly single-crystal state on a sapphire wafer and examining them in great detail using four different microscopy methods.
Atoms from Five Directions To produce complex solid solutions, the researchers use a combinatorial coating system (co-sputtering) that fires atoms of the five selected source elements onto a wafer from five different directions. “There, they deposit randomly and form chemically disordered nanocrystals with countless interfaces,” explains Professor Alfred Ludwig of Ruhr University Bochum, spokesperson for CRC 1625. “While this is beneficial for applications, it also poses a challenge when it comes to studying these new materials in basic research at resolutions as high as the atomic level. However, achieving this is important for their future targeted design.”
To maintain the mixture of the five atomic components while introducing more structural order into the materials, Dr. Satyam Kar from the Bochum research group chose a sapphire single-crystal wafer as a base, which was coated with an extremely thin layer of platinum as a mediator. “As a result, the incident atoms deposit themselves in such a way that they extend the crystal structure of the substrate over micrometer-scale areas,” Ludwig explains, describing the process known as epitaxial layer growth. This results in a very well-defined crystalline surface that is extremely smooth, thereby enabling high-resolution analyses. “For this to work, the sputter deposition process must take place at high temperatures ranging from 400 to 600 °C; the rate of incoming atoms must be neither too high nor too low; and the correct intermediate layer between the single-crystal substrate and the actual layer must be selected,” explains Kar, who now works as an assistant professor at the Indian Institute of Technology Gandhinagar.
Experimental Platform under Microscopes The research team tested the method on a mixture of iridium, palladium, platinum, rhodium, and ruthenium. They used the resulting layers as an experimental platform: through microscopic analysis, they observed that, despite the high degree of structural order, the five starting elements had deposited in a highly chemically mixed state. “What we can also see is that, during the film growth, two different orientations form, between which there are also interfaces—but far fewer and better defined than in the case of nanocrystals,” says Ludwig. Using a tiny diamond tip, the researchers made markings on the wafer to use as reference points during further microscopic examinations, locate the various areas, and compare them with one another. Nanoelectrochemical analyses by Professor Corina Andronescu’s group at the University of Duisburg-Essen showed that one of the orientations is electrochemically advantageous.
Further investigations by Professor Christina Scheu’s group at the Max Planck Institute for Sustainable Materials showed that the epitaxial films feature an interesting defect structure. In some areas, the crystals continue to grow perfectly in accordance with the sapphire substrate. In other areas, however, stacking structures form that are visible under a transmission electron microscope as transverse stripes. “Future work will show what significance these defects have for the desired properties of catalysts,” says Alfred Ludwig.
Funding: This research was funded by the German Research Foundation (DFG) within the collaborative research center CRC 1625.
Published in journal: Materials Horizons
Note: At the time of publishing the journals server is having issues. We've located the preprint on arXiv
Authors: Satyakam Kar, Alejandro E. Perez-Mendoza, Huixin Xiu, Miran Joo, Kirill V. Yusenko, Ulrich Hagemann, Christoph Somsen, Janine Pfetzing-Micklich, Christina Scheu, Corina Andronescu, and Alfred Ludwig
Source/Credit: Ruhr-Universität Bochum | Meike Drießen
Edited by: Scientific Frontline
Reference Number: ms073126_01
