In this paper, as-synthesized and oleic acid (OA)-surface-treated zinc oxide (ZnO) nanocrystals were successfully synthesized and investigated for cool-nanopigment applications. ZnO nanocrystals were synthesized using the thermal decomposition method. The OA-surface-treated ZnO sample was obtained with an OA:ZnO ratio of 1:1. The structural, optical and morphological properties of the samples were characterized via X-ray diffraction (XRD), Fourier transform infrared (FTIR), UV-VIS-NIR diffused reflectance spectroscopy (DRS) and field-emission scanning electron microscopy (FE-SEM) techniques. ZnO nanocrystals possess a well-known zincate phase of ZnO, as confirmed for the as-synthesized sample with a reduction in the integrity of the ZnO crystalline structure upon the application of the OA coating. XRD peaks broadening and decreasing in crystallite size were obtained upon the surface modification of the ZnO by OA. The average crystallite size decreased from 31.5 to 17.4 nm, and the surface area changed from 23.37 to 7.48 m2/g upon OA treatment. These changes were attributed to the well-capping of the ZnO nanoparticles by OA, and, furthermore, OA increased the dispersion of the nanoparticles. The optical band gap of the investigated samples demonstrated a blue shift from 3.06 eV to 3.22 eV upon treatment. Both samples showed high NIR reflectance (RNIR∗) values, which makes them well qualified for use as cool nanopigments. Additionally, the as-synthesized sample showed an RNIR∗ value higher than that of the treated sample.
Keywords: NIR reflectance; ZnO nanocrystals; cool-nanopigment applications; nanopigments; oleic acid; surface modification; thermal decomposition.