Phase Pure Pyrite FeS₂ Nanocubes Synthesized Using Oleylamine as Ligand, Solvent, and Reductant

Pyrite (FeS₂) nanocrystals with a narrow size distribution and optical absorption from 1600 to 380 nm were synthesized in a single stage by reacting ferrous chloride with elemental sulfur in oleylamine (OAm) without an additional ligand. X-ray diffraction, Raman, and scanning electron microscopy showed that pyrite cubes with dimensions of 88 ± 14 nm formed at 200 °C, an S to Fe ratio of 6, and 1 h reaction time in OAm. The time required to form phase pure pyrite depended on the S to Fe ratio. Phase purity was obtained in less than 1 h at a ratio of 6, but times as long as 24 h were necessary at a ratio of 2.75. The rate of pyrite formation increased with higher sulfur concentrations, which shows that molecules containing sulfur are involved in the rate-determining step. Both H₂S and polysulfides of the form S_<i>n</i>^2– are known to form in oleylamine. The slow step is the reaction between FeS and these molecules. Fe²⁺S^2– undergoes nucleophilic attack by H₂S and S_<i>n</i>^2–; S^2– converts to S^– and sulfur is transferred forming pyrite Fe²⁺S₂^2–