Mapping surface morphology and phase evolution of iron sulfide nanoparticles

Iron sulfide nanoparticles play an essential role in the global biogeochemistry sulfur cycle and the origin of life. Controlling phase and morphology is essential but a great challenge for nanomaterials. Herein, we have investigated surface energy, morphology, thermodynamic stability, and phase evolution of FexSy nanoparticles through density functional theory calculations and Wulff construction. The sulfur-rich surfaces become more stable under higher sulfur chemical potential with low temperature and high partial H2S pressure, while the trends of sulfur-poor surfaces are reversed. Thermodynamic equilibrium phase diagrams imply that the intrinsic iron is sulfurated to mackinawite (FeS) and further to the polymorphs of FeS2 when the particle size is larger than 5 nm. When the size is decreased to 2 nm, the mackinawite (FeS) could be sulfurated to pyrrhotite (Fe7S8) and greigite (Fe3S4) and finally to the pyrite (FeS2) phase.