A novel photoresponsive ZnO-flyash nanocomposite for environmental and energy applications

Abstract

ZnO nanoparticles were in situ synthesized on lignite fly ash (ZnO-FA) characterized and tested for the degradation of methyl orange (MO) and water vapor adsorption toward evaporative cooling of hydrophilic surfaces. Surface area (BET) measurements of the supported catalysts revealed type IV adsorption isotherms with typical mesopores of 2–4nm in their pore size distribution. The pore volume of the 0.25M ZnO-FA was increased to 0.133cm3g−1 from 0.009cm3g−1 of the raw FA and 0.012cm3g−1 of 0.25M ZnO. In scanning electron microscopy (SEM) images the non-shaped ZnO aggregates existed either on the surface of the FA spheres or heterogeneously dispersed over the whole catalyst matrix. X-ray photoelectron spectroscopy (XPS) confirmed the buildup of ZnO nanoparticles while the form of Zn2+ oxidation state on the catalyst surface was demonstrated by the sharp Zn 2p3/2 peaks. The absorption edges in the UV–vis diffuse reflectance spectra (UV–vis DRS) of all supported catalysts were found to be slightly shifted to longer wavelength regions due to the existence of FA in the composite structure. Moreover, the mesoporous ZnO-FA was hydrophilic with capillary condensation in the water vapor adsorption isotherm. Water vapor sorption was increased to 0.2gg−1 in the presence of 0.25M ZnO-FA from 0.037gg−1 in the hydrophobic FA and 93% of relative humidity. In dark adsorption experiments, drastic decrements in the MO concentration in aqueous solutions were acquired by the implication of the ZnO-FA catalysts. This action was further enhanced under UV illumination with almost total MO photodegradation in less than 10min with 0.5M ZnO-FA. Thus, transformation of FA structure into a mesoporous adsorber with photocatalytic properties after the in situ ZnO growth is observed for the first time and widens the energy and environmental application prospects of coal residues.