Functional pigments from chromium(III) oxide nanoparticles

Pigments with properties such as weather stability, thermal resistance and having functional properties such as NIR reflectance are desirable. In this, the inorganic pigments serve better than the organic pigments. However, the release of metal ions such as Pb and Co on exposure to the environment is raising alarms. One of the approaches considered ideal is to use naturally occurring oxides or their synthetic analogs as pigments after suitable modifications. One such classic example is that of chromium(III) oxide. We report a predominant solid state route, wherein a chromium–urea complex prepared under solvent free conditions was calcined at high temperature to obtain chromium(III) oxide nanoparticles. Introducing rare earth metal ions such as La and Pr, which have in recent years been reported to be environmentally benign and available in sufficient quantities resulted in nano-oxides with enhanced near-infrared reflectance, thus qualifying them as cool colorants.

► A green approach to nanopigments from Cr2O3.
► High NIR reflectance obtained through rare earth doping into Cr2O3 lattices.
► Combination of size and reflectance effect for generating cool green pigments.