In situ X-ray Synchrotron Studies Reveal the Nucleation and Topotactic Transformation of Iron Sulfide Nanosheets.

Iron sulfides (FexSy), including greigite (Fe3S4), are key materials in geological processes and technological applications. However, in the context of colloidal synthesis, the mechanism by which these nanoparticles form remains unexplored. Here, we employ in situ X-ray diffraction and photon-in photon-out spectroscopic studies to elucidate the reaction pathway of Fe(acac)3 and thioacetamide (TAA) in benzyl alcohol (BA), which yields crumpled Fe3S4 nanosheets. Using powder X-ray diffraction (PXRD), we identify FeS (mackinawite) as a crystalline intermediate whose anisotropic growth, driven by its layered crystal structure, governs the crumpled nanosheet-like morphology of Fe3S4 (greigite) through a topotactic transition. By performing high-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy, we show that the formation of Fe3S4 proceeds through a multistep mechanism involving two intermediates. Supported by density functional theory (DFT), we find that Fe(acac)3 is initially reduced in the presence of TAA in BA, forming a molecular intermediate [Fe(acac)2(BA)2], which subsequently transforms into FeS and ultimately into Fe3S4. Complementary valence-to-core X-ray emission spectroscopy (vtc-XES) reveals the evolution of the coordination environment from Fe-O to Fe-S throughout the reaction. Our work provides a comprehensive understanding of the formation mechanism of Fe3S4 nanosheets in solution, shedding light on how crystal growth dynamics and electronic structure evolution dictate their unique crumpled nanosheet morphology.