Understanding short-lived reaction species is essential for new materials synthesis, including for sustainability applications. However, probing the structure of these short-lived species, which are often nanoscale, particularly in solution, is challenging because the high-quality measurements needed to resolve structure features often take longer than the lifetime of the species being probed. Here, we present a new approach, the mixing-flow reactor, which decouples reaction time from measurement time by flowing the reaction through space. Precursor solutions are mixed at an initiation point then continue flowing through a reaction capillary, such that distance from the mixing point corresponds to reaction time. Therefore, measurements can be taken for as long as is needed without compromising the time resolution of the measurement. The mixing-flow reactor was used to identify a novel metastable intermediate in the aqueous formation of mackinawite (FeS) using <i>in situ</i> pair distribution function (PDF) measurements. This intermediate, composed of individual iron sulfide nanosheets, is present in the first second of the reaction.

Time-resolved measurements in space to resolve the structure of iron sulfide nanosheets

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ACS Fall 2022

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