One-Step Synthesis and Optical Properties of Benzoate- and Biphenolate-Capped ZrO2 Nanoparticles

A simple one-pot approach based on the benzyl alcohol route is used for the preparation of benzoate- and biphenolate-capped zirconia and, benzoate-capped Eu-doped zirconia nanoparticles. Powder X-ray diffraction studies and high- resolution transmission electron microscopy (HR-TEM) showed that the nanoparticles present high crystallinity and uniform particle sizes ranging from 3 to 4 nm. FT-IR and solid state NMR (SS-NMR) studies revealed that the nanoparticles are coated with a large amount of organic species when the reaction temperature is above 300 degrees C. It was found that the alcohol used as solvent is oxidized at the surface of the nanoparticles to the respective carboxylic acid which acts as a stabilizer, controlling the nanoparticles growth. The optical properties of these hybrid nanoparticles were studied by room and low (12K) temperature photoluminescence spectroscopy, time-resolved spectroscopy and absolute emission quantum yield. The as-synthesized benzoate- and biphenolate-capped nanoparticles exhibit interesting emission properties in the UV and blue spectral regions together with values of emission quantum yields much higher than those reported for zirconia nanoparticles of similar size. The photoluminescent properties were attributed to a cooperative effect of the capping ligands and the defects associated to the ZrO2 nanoparticles. Due to the overlapping of the various emission components involved (i.e., the emission of europium(III) intra-4f6 transitions, defects in the zirconia and capping ligands) a tunable emission color ranging from purplish-pink to greenish-blue could be obtained for the europium-doped zirconia nanoparticles by simply selecting different excitation wavelengths.