Phase-pure iron pyrite (FeS₂) micro- and nano-sized crystals synthesized by simple one-step microwave-assisted hydrothermal method

Iron pyrite micro- and nano-sized crystals are desirable as active materials in lithium ion batteries and photovoltaics, and are particularly suitable for nanocrystal inks for roll-to-roll deposited or ink-jet printed solar cells. In this paper we report the synthesis of iron pyrite micro- and nano-sized crystals via simple and convenient green one-step microwave-assisted hydrothermal (M − H) process at relatively low growth temperatures, using commonly used precursors such as FeCl₃, Na₂S and S₈ and. The structural, morphological and optical properties of the resulting nanostructured materials have been thoroughly investigated for two typical M − H growth temperatures. X-ray diffraction (XRD) pattern and Raman data revealed good crystalline quality for the as synthesized pyrite NCs. Typical XRD patterns show the dominant peaks which can be indexed as a pure cubic phase of FeS₂, with lattice constant values close to the lattice parameters reported for FeS₂, and in agreement with a stoichiometric pyrite phase. XRD and Raman analysis also confirm that no other impurities phases such as hexagonal FeS, marcasite, pyrrhotite, greigite, S or Fe–O compounds were detected, confirming the high purity of the synthesized iron pyrite nanocrystals. Various shapes of pyrite like quasi-cubic, cubic and flower-like FeS₂ nanocrystals have been observed, which can be modulated by using different synthetic conditions. The sizes of the pyrite micro- and nanostructures were in the range of 150 nm to 1 μm as obtained. The present study indicates that the M − H method is a facile one-step way to obtain phase pure iron pyrite micro- and nano-sized crystals. Optical characterization confirms direct bandgap transitions (values of 2.61 eV and 2.55 eV for iron pyrite NCs samples hydrothermal grown at 130 °C and 160 °C, respectively), and indirect bandgap transitions (values of 1.19 eV and 1.52 eV for samples hydrothermal grown at 130 °C and 160 °C, respectively). Optical studies show high absorbance in the entire UV–Vis wavelength range making the as-synthesized pyrite nanocrystals potential candidate as absorber in nanoscale photovoltaic solar cells.