Phase-Controlled Colloidal Syntheses of Iron Sulfide Nanocrystals via Sulfur Precursor Reactivity and Direct Pyrite Precipitation

In the colloidal synthesis of iron sulfides, a series of dialkyl disulfides, alkyl thiols, and dialkyl disulfides (allyl, benzyl, <i>tert</i>-butyl, and phenyl) were employed as sulfur sources. Their reactivity was found to tune the phase between pyrite (FeS₂), greigite (Fe₃S₄), and pyrrhotite (Fe₇S₈). DFT was used to show that sulfur-rich phases were favored when the C–S bond strength was low in the organosulfurs, yet temperature dependent studies and other observations indicated the reasons for phase selectivity were more nuanced; the different precursors decomposed through different reaction mechanisms, some involving the oleylamine solvent. The formation of pyrite from diallyl disulfide was carefully studied as it was the only precursor to yield FeS₂. Raman spectroscopy indicated that FeS₂ forms directly without an FeS intermediate, unlike most synthetic procedures to pyrite. Diallyl disulfide releases persulfide (S–S)^2– due to the lower C–S bond strength relative to the S–S bond strength, as well as facile decomposition in the presence of amines through S_N2′ mechanisms at elevated temperatures