Formation mechanisms of pyrite (FeS 2 ) nano-crystals synthesized by colloidal route in sulfur abundant environment

Pyrite (FeS2) nano-crystals (NCs) were synthesized in an excess sulfur environment using a colloidal hot-injection method, and the phase change behavior of the iron sulfide compounds was investigated. As the growth time increased, the phase of the iron sulfide NCs transformed from mackinawite (FeS) via greigite (Fe3S4) to pyrite (FeS2). Thus, Fe3S4 phases was considered as intermediate precursors on the pathway of FeS2 phases in the reaction between FeS phases and excess sulfur. The elemental ratio of [S/Fe] increased from 1.1 to 2.1 during the phase change, and the shape of the NCs changed from a hexagonal nano-sheet (Fe3S4), via cubic (FeS2), to a cubic-hedral structure (FeS2). Strong absorption peaks in the UV–Vis spectra were observed in the FeS2 phase, and its optical band gap was estimated to be ∼0.9 eV, indicating the semiconducting nature of pyrite. Consequently, the synthesis of FeS2 in sulfur abundant environment was suitable method to acquire a pure semiconducting FeS2 phases. The reason was thought that the depletion of Fe-element after the formation of FeS2 phases led to the decrease of intermediate phases and the gradual changes from intermediate phases to FeS2 resulted in pure phases.