Facile microwave-assisted synthesis of Cr2O3 nanoparticles with high near-infrared reflection for roof-top cooling applications

Microwave-assisted solution synthesis was employed for the preparation of Cr2O3 nanoparticles using Cr(acac)(3) and ethylene glycol. The optimum conditions for the completion of the reaction forming a green precipitate (yield-95%) were: irradiation for 40-60 min at 220 degrees C solution temperature. The as-prepared sample was barely crystalline; annealing in air at above 300 degrees C formed nanocrystalline Cr2O3, as shown by X-ray diffraction (XRD). High-resolution transmission electron microscopy (HRTEM) of the "700 degrees C sample" confirmed the well-crystallised Cr2O3 formation with similar to 37 nm average size. The asprepared powder (bluish-green) showed high near-infrared (NIR) reflectance, whereas the annealed samples showed bright-green reflectance, whose International Commission on Illumination L*a*b* (CIE L*a*b*) values shifted towards the green region. The higher D-q/B value (3.76), suggested that Cr3+ lies in a strong crystal field. Stainless steel blocks covered with the as-prepared (intermediate) material reduced heating under sunlight by 5 degrees C, indicating its suitability as an easily prepared tile coating for roof-top applications. The annealed samples are found suitable as an anti-dazzling coating for automobile windscreens and rear-view mirrors. (C) 2019 Elsevier B.V. All rights reserved.