Microwave-assisted synthesis of blue-green NiAl2O4 nanoparticle pigments with high near-infrared reflectance for indoor cooling

Abstract Non-toxic, blue-green near-infrared (NIR) reflecting inorganic NiAl2O4 nanoparticle pigments were synthesized through a new microwave-assisted solvothermal method as an alternative to existing chromium and cobalt-based blue and green colourants. The as-prepared powder was annealed up to 1200 °C to optimize the temperature for highest NIR reflectivity. Annealing changed the colour of the sample from greenish-blue to sky blue resulting from the crystal field splitting of the Ni2+ d-orbitals in an octahedral symmetry. The pigments were characterized by X-Ray diffraction, high-resolution transmission electron microscopy, and UV–Vis–NIR reflectance spectroscopy. The sample annealed at 1000 °C showed the highest near-infrared reflectance and a significant near-infrared solar reflectance of almost 60%. The ability of the pigments to transfer colour and NIR reflectance was demonstrated by coating on a galvanized-steel block.