Hydrothermal synthesis of ZnO nanopigments with high UV absorption and vis/NIR reflectance

ZnO nanopigments with five different morphologies were synthesized by a simple low temperature hydrothermal process by using zinc nitrate hexahydrate without using additives. ZnO nanopowders were characterized by X-ray diffraction and field-emission scanning electron microscopy. Optical properties of the ZnO nanostructures were investigated by UV/vis/NIR spectrophotometer and room-temperature photoluminescence spectroscopy. The obtained results indicated that ZnO nanopigments had high UV absorption and vis/NIR reflectance. Sizes of ZnO powders were strongly affected by hydrothermal conditions and increased gradually with the increase of the reaction temperature. The UV absorption increased with the reduction of particle size and nanoparticles showed higher UV absorption at 332 nm. The vis/NIR reflectance had the highest amount at sample 4 (with particle size distribution of 250–350 nm). The photoluminescence spectra from the ZnO nanostructures revealed a strong UV emission peak at 380 nm and no green emission band at ~530 nm. The photoluminescence spectra of the other samples exhibited a broad emission peak at ~420 nm and a weak visible green emission at 530 nm.