Visible-light-active chlorophyll/flavonoid-sensitized ZnO nanoparticles: preparation and optical and photocatalytic studies

We describe preparation of chlorophyll/flavonoid-sensitized ZnO nanoparticles with visible-light-induced photocatalytic activity. ZnO catalysts were sensitized by an incipient wetness impregnation in which chlorophyll and flavonoid natural pigments were extracted from parsley leaves and Curcuma longa roots, respectively. Structural studies were conducted using characterization techniques including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). Diffuse reflectance spectra (DRS) were employed to analyze optical characteristics indicating a decrease in bandgap energies of as-sensitized samples towards visible light absorption compared with nonsensitized ZnO catalysts. The photocatalytic property of chlorophyll/flavonoid-sensitized ZnO nanoparticles was evaluated by measuring photodegradation of methylene blue (MB) as a function of involved operating parameters including irradiation time, photocatalyst concentration, pH, and initial MB concentration in aqueous media. Chlorophyll-sensitized catalysts showed higher photodegradation efficiency in comparison with the flavonoid-sensitized sample relevant to a larger optical absorption edge. The photocatalytic results also indicated less optimum irradiation time in the presence of flavonoid-sensitized catalysts compared with that in the presence of the chlorophyll-sensitized sample due to stronger absorption ability of photons in a region around the optical absorption edge. The kinetic expressions were studied using the Langmuir-Hinshelwood theory, and an appropriate model was presented for the MB photodegradation process.