Thermal decomposition of bismuth oxysulfide from photoelectric Bi2O2S to superconducting Bi4O4S3.

With the addition of oxygen into the chain-like bismuth sulfide of Bi2S3, there are two interesting functional compounds of Bi2O2S (photoelectric) and Bi4O4S3 (superconducting) containing the PbO-like [Bi2O2] layers. Nanoscale Bi2O2S crystals with an indirect band gap of 1.12 eV are synthesized via a facile hydrothermal method. This semiconductor shows excellent photoelectric response under the irradiation of visible light lamp at room temperature. Theoretical calculations and packing factor model both indicate that the loosely packed Bi2O2S is an excellent photoelectric material. When the Bi2O2S phase was annealed at 500 °C in an evacuated quartz tube, nanocrystals of Bi4O4S3 were obtained. The powder X-ray diffraction and electron microscope analyses (SEM, TEM, EDX) confirmed the thermal decomposition from orthorhombic Bi2O2S to tetragonal Bi4O4S3. The superconducting transition temperature of Bi4O4S3 was observed to be 4.6 K from the temperature-dependence measurements of electrical resistivity and magnetic susceptibility. Our results also provide a new method utilizing thermal decomposition to prepare a new phase without high temperature reaction.