Tracking ultrafast redox reactions at the nanoscale

Thermite reactions – the highly energetic redox processes between a metal and an oxide – play a central role in welding, propulsion systems, fireworks and the design of advanced materials. When these reactions are scaled down to the nanoscale, their energy release becomes even more intense and efficient. However, the ultrafast dynamics governing the first instants of these reactions have remained elusive.

A study recently published in Nature Communications sheds new light on this experimentally challenging research subject. The research team investigated the reaction between nanometric self-standing bilayers of aluminium and hematite (a common iron oxide) with femtosecond time resolution, employing the unique capabilities of the FERMI free-electron laser at Elettra-Sincrotrone Trieste. Experiments were carried out at the TIMEX beamline, where the combination of femtosecond optical lasers and seeded free-electron laser pulses enables unique pump-probe studies of matter under extreme conditions. By using time-resolved extreme ultraviolet absorption spectroscopy, the scientists were able to track the motion of electrons at the very onset of the reaction. These element-specific measurements revealed the formation of small polarons in hematite (localized charge carriers that play a decisive role in the reaction pathway) and demonstrated ultrafast electron transfer from aluminium to hematite.

The work provides the first femtosecond-resolved observation of a nanoscale thermite redox reaction. It shows that the ignition of the process involves not only the melting of aluminium but also the generation and trapping of electrons that modify the electronic structure of hematite, see Figure 1. These findings pave the way for a deeper understanding of how energy is released and transferred in highly reactive systems, with potential implications for energy conversion technologies and the development of advanced energetic materials.

Figure 1: (a) Ultrafast absorption dynamics at selected FEL photon energies across the M_2,3-edge of iron in hematite (a) and L_2,3-edge of aluminium measured on a α-Fe₂O₃/Al sample (b). The red points indicate photon energies just below the absorption edge, the blue points are for photon energies just above the absorption edge. Adapted from E. Paltanin et al., Nat. Commun. 16, 7282 (2025), licensed under CC BY 4.0.

Beyond its fundamental significance, this work opens new perspectives. Ultrafast spectroscopic methods such as those demonstrated here could be applied to a wide variety of energetic and catalytic systems. By observing charge flow and bond rearrangements in real time, researchers can design materials with tailored reactivity, improved safety, or higher efficiency. In the long term, these approaches may contribute to the development of sustainable energy conversion strategies, advanced manufacturing methods, or innovative propulsion systems.

Ettore Paltanin^1,2, Jacopo S. Pelli Cresi¹, Emiliano Principi¹, Wonseok Lee³, Filippo Bencivenga¹, Dario De Angelis^1,4, Laura Foglia¹, David Garzella¹, Gabor Kurdi¹, Michele Manfredda¹, Denys Naumenko^1,5, Alberto Simoncig¹, Scott K. Cushing³, Riccardo Mincigrucci¹ and Claudio Masciovecchio¹

¹ Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
² Dipartimento di Fisica, Università degli Studi di Trieste, Trieste, Italy
³ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, USA
⁴ CNR - Istituto Officina dei Materiali (IOM), AREA Science Park, Trieste, Italy
⁵ Infineon Technologies, Villach, Austria

Contact person email: emiliano.principi@elettra.eu