Synthesis And Optical Properties Of ZnO Micro-/Nanomaterials

ZnO porous microspheres, hexagonal prisms, apertured architectures, and nanospheres have been synthesized via simple and facile methods. The structures and optical properties of as-prepared products were thoroughly characterized using X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), Brunner-Emmett-Teller (BET) method, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM), UV-Vis spectra, electron paramagnetic resonance (EPR), and photoluminescence (PL), respectively. The results are summarized as follows.S-doped ZnO porous microspheres were obtained by directly annealing of commercial ZnS powders at 700°C in air. The average diameter of as-prepared microspheres is about 3μm, with many nanoparticles (100-300 nm) around their surface. The as-obtained ZnO microspheres show good UV absorption properties in the range of 250 to 400 nm, exhibiting strong green emission centered at 496 nm under the excitation wavelength of 365 nm.ZnO hexagonal prisms and apertured architectures were prepared by annealing of the mixtures of ZnS and KCl and NH4Cl in air, respectively. Compared with those of the ZnO microspheres, remarkable improvement of crystallization and perfect growth of the ZnO crystal lattice on the particles surface are observed. Moreover, the UV absorption edge shows a clear red shift of～40 nm as well as the maximum excitation peak shifts from 365 nm to 380 nm, and than the green PL intensity is enhanced more than 4 times.Sulfur and halogen co-doped ZnO nanospheres with average diameter about 60 nm were fabricated by annealing of the precursor contained ZnS and KX (X=Cl, Br, I) prepared via sol-gel route in air. A new UV absorption peak at～385 nm and red shift of 30 nm are found due to the incorporation of halogen ions into ZnO lattice. A clear shrinkage in the optical band gap is also observed, which depends on the properties of the incorporated halogen ions and the annealing temperature. Furthermore, the excitation band at～3.20 eV is highly enhanced as the halogen ions were doped into ZnO. An impurity energy level is proposed, and the possible mechanisms are also suggested, which is used to explain the enhancement of green emission. The correlation of the doping concentration of halogen ions and the green PL intensity of the ZnO products is also discussed.