Dance of the molecules

Hamburg Scientists investigate nanoscale interfaces for tailor-made materials systems

A Hamburg research team is paving the way for new methods of producing tailor-made materials: for the first time, the scientists from the Hamburg University of Technology (TUHH) and DESY NanoLab have deciphered the collective arrangement of organic acid molecules on an iron oxide surface at the atomic level. The formic acid molecules they studied perform a kind of dance in groups of three on magnetite (Fe3O4), as the team led by Gregor Vonbun-Feldbauer from TUHH and Andreas Stierle from DESY is reporting in The Journal of Physical Chemistry Letters. The work is part of the TUHH’s Collaborative Research Centre 986 “Tailor-Made Multi-Scale Materials Systems (M3)”, which has just been prolonged for another three and a half years.

The scientists are trying to understand the mechanical properties of composite materials that are inspired by nature. In this case hard inorganic cores are embedded in organic molecules. In order to tailor their mechanical properties, it is essential to understand the processes at the interface between those two very different classes of materials. However, that knowledge has mostly been lacking so far.

“When oleic acid is combined with nanoparticles, the resulting materials already display many fantastic properties,” says Marcus Creutzburg from DESY NanoLab. “Such composites have a particularly high strength, for example.” However, the exact details of how this comes about have not yet been determined. “We know that the acid fills the gaps between the nanoparticles, but until now we did not know what exactly happens there at the molecular level,” says Creutzburg.

The team therefore studied how formic acid behaves on a magnetite surface, as a comparatively simple model system, using a scanning tunnelling microscope, infrared spectroscopy, surface sensitive X-ray diffraction and quantum-mechanical simulations. “Formic acid is the simplest organic acid,” explains Kai Sellschopp from the TUHH, who ran the computer simulations. “Oleic acid molecules are much more complex, but the end group which anchors the acid molecules to the iron oxide looks exactly like that of formic acid.”

The magnetite sample was first treated to create surface defects in the iron oxide. These make it easier for the acid molecules to attach to the surface, leading to a greater stability of the interface. The experiments and calculations show that the formic acid molecules predominantly attach themselves to the iron oxide in groups of three. Each of these groups of three are about a millionth of a millimetre (one nanometre) apart and perform a kind of swaying dance. Because the molecules are directly coupled and thus influence each other, preferred vibrations (“eigen-modes”) of the molecules are particularly easily excited.

The scientists hope to use this knowledge, together with further research, to establish a basis for systematically designing the properties of composite materials. It is also important in terms of understanding how magnetite operates as a catalyst in energy conversion processes. 


Heterogeneous Adsorption and Local Ordering of Formate on a Magnetite Surface; Marcus Creutzburg, Kai Sellschopp, Steffen Tober, Elin Grånäs, Vedran Vonk, Wernfried Mayr-Schmölzer, Stefan Müller, Heshmat Noei, Gregor B. Vonbun-Feldbauer, and Andreas Stierle; The Journal of Physical Chemistry Letters, 2021, 12, 3847-3852; DOI: 10.1021/acs.jpclett.1c00209

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