Effects of silver loading in zinc oxide on the photodegradation of methyl orange and methylene blue

Silver-supported zinc oxide catalysts were tested for the photodegradation of methyl orange (MO) and methylene blue (MB) dyes under UV irradiation at 25°C. These Ag/ZnO catalysts, which vari-able silver loadings (1, 2 and 3 wt.%) were prepared by impregnation and characterized by atomic absorption spectroscopy, N₂ adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy. The XRD, high-resolution transmission electron microscopy and XPS results revealed that ZnO was wurtzite phase, and metallic silver (Ag⁰) was the support in ZnO nanorods; the crystal sizes were 72 and 18.4 nm in ZnO and Ag⁰, respectively. The experimental results showed that the xAg/ZnO catalysts were highly active in MO and MB degradation compared with the zinc oxide supports because of the presence of metallic silver, which acted as an electron trap, thus inhibiting the electron-hole recombination. The 2Ag/ZnO catalyst exhibited greater degradation activity in both dyes, reaching 100% degradation after 30 and 90 min in MB and MO, respectively, because of the greater dispersion of the silver and the higher concentration of hydroxyl groups on the surface compared with the other catalysts. The effects of different scavengers of the main species involved, such as hydroxyl radicals, superoxide anions radicals and positive holes, which were studied to determine the photodegradation mechanisms of the dyes on these systems. The superoxide radical was the main reactive species during the photocat-alytic degradation of MB and MO. Moreover, 2Ag/ZnO catalyst exhibited excellent photocatalytic stability and activity after five cycles.