Highly Reflective Nanostructured Titania Shell: A Sustainable Pigment for Cool Coatings

Core–shell materials are designed to bring synergy of two materials. We describe a greener approach for fabrication of fly ash (FA)-based cool pigment having core–shell morphology, viz., FA as core and multilayers of nano-TiO2 as shell—introducing high reflectivity in the near infrared region. Core–shell structure was prepared in an aqueous medium under sonication without use of organic solvent or surfactant. Dispersion of FA in aqueous medium is achieved by activating the surface of FA while increasing its surface roughness. High zeta potential (−42.7 mV) of FA, coupled with ultrasonication, helps in achieving better suspension of FA. Well-separated FA particles act as nuclei for the seeding of nano-TiO2, resulting in uniform shell formation as seen in scanning electron microscopy and transmission electron microscopy studies. A significant improvement in reflectivity is evident by its high solar reflective index of 94%. Thermal diffusivity of a 123 ± 5 μm thick cool coating indicate a temperature difference of 10 ± 0.5 °C on the front and back side of a mild steel panel. Significant use of a waste material in making a value-added product by adopting a green approach coupled with enhancing weight content of pigment in coating formulation is an attempt toward achieving sustainable material for energy conservation.