<i>In situ</i> X-ray Synchrotron Studies Reveal the Nucleation and Topotactic Transformation of Iron Sulfide Nanosheets

Iron sulfides (Fe_<i>x</i>S_<i>y</i>), 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 <i>in situ</i> 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.