2D particles forming a nanostructured shell: A step forward cool NIR reflectivity for CoAl2O4 pigments

Blue CoAl2O4 are inorganic pigments with impressive optical effects and a high chemical stability. However, this spinel type structure has two main drawbacks: cobalt is scarce and expensive, and the dark pigment absorbs the solar spectra resulting in undesirable heating. In this work a new type of cool NIR blue pigment is attained by in-situ forming of a shell of 2D CoAl2O4 nanoparticles on α-Al2O3 microparticles. Nanodispersion of Co3O4 nanoparticles on the surface of α-Al2O3 microparticles has been carried out by means of a low energy dry mixing method and further in-situ thermal treatment at 1200 °C forms a nanostructured shell. CoAl2O4 nanocrystals are controlled in size by the frequency site of the previously dispersed Co3O4 nanoparticles. 2D particles form to fill the distance between Co3O4 nanoparticles on the alumina surface. The new shell structure makes it possible to obtain a high efficient NIR pigment with a solar reflectance SR>70% and a large cobalt content reduction. A sample containing only 10 wt% of cobalt shows similar chromatic coordinates to standard bulk pigments, which means a significant saving of raw materials.