Structure and photocatalytic properties of the composite of Al-doped porous ZnO hollow micro-spheres and Ag nanoparticles

Al-doped porous ZnO hollow micro-spheres were prepared using a sol-gel method with a template of agglomerates of polystyrene (PS) nanospheres, and the composite of these ZnO hollow micro-spheres and Ag nanoparticles was prepared with further photo-deposition. The effects of Al doped on the structure and the optical properties of the specimen were investigated by means of x-ray diffractometer, SEM, TEM, an UV-Vis diffuse reflectance analyzer, and photoluminescence (PL) spectroscopy. Different doping concentrations of Al affect not only the existing form of Al atoms in the crystal but also the concentration of the intrinsic defects of ZnO, thus influencing the morphology of the hollow micro-spheres. The appropriate amount of Al doping provides the hollow spheres with a porous, loose morphology, showing the largest specific surface area, which significantly improves the absorbance of light and the number of reaction sites in photocatalysis. For the composite specimen, the introduction of Ag nanoparticles improves the absorbance of light in the visible region because of the surface plasmon resonance: in the PL measurements, because many electrons in the ZnO crystal flow to the surface of Ag nanoparticles due to the difference of energy levels between Ag and ZnO crystals, the intensity of the intrinsic and defect-related emissions of ZnO is reduced. The photocatalytic properties of the specimens were evaluated with the degradation of a methyl orange dye solution under simulated solar irradiation at room temperature. The specimen of porous ZnO hollow micro-spheres with 3% Al dopant concentration demonstrates excellent photocatalytic performance due to its large specific surface area. However, with the designed energy structure, the composite of porous ZnO hollow micro-spheres and Ag nanoparticles exhibits the best photocatalytic performance among the specimens tested by separating electrons and holes in the ZnO crystal.