Operando Plasma TEM Reveals Nanoscale Mechanisms of Iron Oxide Reduction

Hummingbird Scientific is leading and advancing gas and liquid cell microscopy for more than 20 years. Recently, we supported and collaborated Jae Hyun Nam, K. Andre Mkhoyan and Peter J. Bruggeman at the University of Minnesota on their recent publication in Nature Communications.

In collaboration with Hummingbird Scientific, the research team developed an operando plasma transmission electron microscopy (TEM) technique to directly observe the reduction of magnetite (Fe₃O₄) nanoparticles by non-thermal hydrogen plasma. Using a custom-built atmospheric pressure plasma cell integrated into a TEM holder, they achieved real-time imaging with ~1 nm resolution, revealing particle shrinkage and crack formation within ~10 seconds of plasma exposure.

Plasma-enabled reduction of magnetite nanoparticles. a Operando sequential images of nanoparticles treated by He + 0.5 % H2 plasma (56 mW). For imaging, a relatively low electron dose rate is used (13 e− Å−1 s−1). b EDX-mapped images and corresponding 1-D profiles (see inset) of net signals of untreated and treated nanoparticles by He + 0.5 % H2 plasma. c High-resolution TEM images of untreated and treated nanoparticles by He + 0.5 % H2 plasma with magnified insets. d Statistics of integrated net intensity ratios of untreated and treated nanoparticles measured from at least 10 particles for each case. The degree of reduction is calculated based on the average intensity ratio change. e Statistics of particle sizes of untreated and treated nanoparticles from the main experiment in a and from SI Additional operando experiments. The particle sizes just before crack formation are measured for treated samples. Note that some nanoparticles in a are already cracked due to the pre-treatment outside TEM ( ~ 10 s) to optically locate the constricted plasma (see Method-Operando plasma experiment). Scale bars denote 50 nm. Copyright © 2025 The Author(s). Nature Communications published by Springer Nature.

The study demonstrates that hydrogen radicals (H^•), generated via He* dissociation of H₂, are the primary agents driving the reduction, even at temperatures below 700 K, well below the threshold for thermal reduction. The shrinking-core model best describes the kinetics, with apparent rate constants up to two orders of magnitude higher than thermal reduction rates. These insights into plasma-enabled reduction mechanisms at the nanoscale are critical for advancing low-carbon steelmaking technologies and can be extended to other plasma-material interactions such as catalysis and nanomaterial synthesis.

Jae Hyun Nam, K. Andre Mkhoyan, Daan Hein Alsem, Peter J. Bruggeman. Nat. Commun. 2025, 16, 7537. DOI: 10.1038/s41467-025-62639-4

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