Application of KZnF3 as a Single Source Precursor for the Synthesis of Nanocrystals of ZnO2:F and ZnO:F; Synthesis, Characterization, Optical, and Photocatalytic Properties

摘要

Mixed metal fluoride, KZnF3, possessing a cubic perovskite structure has successfully been employed as a single source precursor for the synthesis of fluoride-doped ZnO2 nanocrystals by a simple low-temperature oxidation procedure. Utilizing the fact that ZnO2 is a precursor for ZnO, F~--doped wurtzite ZnO was readily obtained by a straightforward decomposition procedure. The structure, optical, and photocatalytic properties of doped ZnO2:F and ZnO:F were studied and compared with the undoped ones. The preservation of the cubic pyrite structure of ZnO2 by the inclusion of F~--ions was revealed by the powder X-ray diffraction pattern. Uniform cube morphology of the nanocrystals of ZnO2:F was observed in both the scanning electron microscopy and transmission electron microscopy images with the crystallite size of 20 nm. The IR and Raman spectroscopy analysis implied the absence of any Zn-F direct bonding in ZnO2:F. The high-resolution core level X-ray photoelectron spectroscopy (XPS) spectrum of F 1s observed at 687.9 eV confirmed the presence of fluoride ions in the ZnO2 lattice. By fitting the core level F Is spectrum, the concentration of the F~- ion was found to be 8.6%. A red shift in the excitonic absorption was observed on F~ doping in ZnO2. A similar trend was also observed in the band edge emission from the photoluminescence spectrum recorded at 300 K. The intensity of the violet emission in the pure ZnO2 (with a decay time of 18 ns) decreased on F~- doping (with a decay time of 13 ns). While ZnO2 nanocrystals efficiently degraded methylene blue (MB) solution under UV radiation and moderately under visible radiation, F~--doped samples showed lesser efficiency for the photo degradation of the MB solution. F~--doped ZnO was obtained by decomposing the ZnO2:F in air at 450 ℃ for 3 h. The symmetry remained hexagonal on F~--doping as revealed by the powder X-ray diffraction pattern. The intensity of the Raman bands of fluoride-doped ZnO nano powders were in general less as compared to the undoped ZnO, except the one observed at 582 cm~(-1), which indicated the presence of higher oxygen vacancies in ZnO:F. Core level XPS measurements provided conclusive evidence for the doping of fluorine (6.1%) in ZnO. The band gap value of ZnO:F, estimated from the diffuse reflectance spectrum, was 3.0 eV, and it showed broad visible emission. As a consequence of higher oxygen vacancies, ZnO:F exhibited efficient photocatalytic activity under visible irradiation for the degradation of aqueous MB dye solution.