Resolving Single-layer Nanosheets as Short-lived Intermediates in the Solution Synthesis of FeS

Short-lived reaction intermediates play a critical role in mediating material synthesis. Such short-lived species often elude characterization because of the mismatch between the time scale of measurements capable of describing them and their lifetimes. Thus, we have limited ability to probe, understand, and control the mechanism for material synthesis. Here we demonstrate a new approach to in situ X-ray pair distribution function (PDF) measurements of dynamic nanomaterials structure that yields an unprecedented combination of reaction time resolution and sensitivity. Reaction time is resolved spatially as a function of position along a flow path. By applying this approach to the well-studied aqueous reaction leading to FeS, mackinawite, we identify a novel metastable intermediate, FeSlayer, that forms in the first second of the reaction and which can be described as individual FeS nanosheets. Recognizing these nanosheets as synthons in the reaction opens up the possibility to deliberately redirect this assembly of the nanosheet toward different phases, including novel heterostructures.