At material interfaces, even the smallest changes can determine how substances interact. A research team at Leibniz IPHT has now demonstrated that cyanide bonds in a specific class of materials can undergo structural rearrangement – controlled by the oxidation state of cobalt and their interactions with the ­underlying surface.

Like a switch that flips with a single impulse, a subtle adjustment at the surface of a material can trigger a profound structural transformation. Researchers at Leibniz IPHT have visualized this mechanism experimentally for the first time: In Prussian blue analogues (PBAs) – materials used in batteries and catalysts, among other applications – cyanide bonds realign themselves when the oxidation state of cobalt changes.

“Interfaces are more than just transitions between materials,” says first author Ratnadip De. During his Ph.D. at Leibniz IPHT, which he completed in 2024, he studied how chemical processes unfold at these interfaces. “This is where the electronic properties of a material can undergo fundamental changes.”

The study was conducted in collaboration with Prof. Dr. Ferdi Karadas of Bilkent University in Ankara. Karadas, an expert in photocatalytic materials, spent a year in Jena as a Humboldt Fellow, working with De and the team led by Prof. Dr. Benjamin Dietzek-Ivanši´c, head of the Functional Interfaces research department. Together, they studied PBA layers on zinc oxide surfaces and analyzed how the chemical structure of these materials changes under certain conditions.

Using vibrational sum frequency generation (VSFG) spectroscopy, the team made the bond rearrangement visible. Their measurements showed that as the cobalt shifts between oxidation states at the interface, the cyanide linker flips its connection between the cobalt and the adjacent iron atom. “It’s like flipping a switch – a small change triggers a structural reorganization,” explains De.

Further analysis, including X-ray photoelectron spectroscopy (XPS) performed in collaboration with researchers from the University of Jena, revealed a link between this effect and defects in the material structure. “Interfaces are not static transitions,” says Dietzek-Ivanši´c. “They are active sites for chemical processes. If we succeed in controlling the making, breaking or reorganization of chemical bonds at these interfaces, we can tailor materials more effectively for applications such as energy storage and catalysis.”