Blue CoAlO 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 CoAlO nanoparticles on α-AlO microparticles. Nanodispersion of CoO nanoparticles on the surface of α-AlO 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. CoAlO nanocrystals are controlled in size by the frequency site of the previously dispersed CoO nanoparticles. 2D particles form to fill the distance between CoO 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.

The authors express their thanks to the MINECO (Spain) project MAT2013-48009-C4-1-P for their financial support. C.M.D. is also indebted to MINECO for a Predoctoral Grant (BES-2014-069779), which is co-financed with FEDER funds.