Highly blue emission of ZnO nanoparticles embedded in a SiO₂ matrix

The main drawbacks of fluorescent ZnO nanoparticles in practical optoelectronic applications are fast particle growth and agglomeration in water, causing a periodic decrease of light emission intensity. In this work, ZnO nanoparticles were successfully embedded within a silica matrix to prevent their agglomeration. The ZnO NPs were synthesized by the polyol method, using ethylene glycol and glycerol in sizes ranging from 5–6 nm. The as-obtained ZnO particles were dispersed in an amorphous silica matrix under three different ZnO:SiO₂ molar ratios: 20:80, 15:85 and 10:90. With the aim of enhancing the luminescent properties of the ZnO@SiO₂ composites, various annealing treatments in the range of 150–210 °C were carried out. The chemical analyses of the bare ZnO and the composites were carried out by Fourier transform infrared spectroscopy, which confirmed the presence of functional groups attached to the ZnO surface. By means of thermogravimetric analysis, it was determined that the evaporation of solvents shifted to lower temperatures (180 °C) when silica was added to the ZnO colloidal solution. By means of photoluminescence, a bright blue emission, centered at 440 nm, was observed in all of the samples, even at high temperatures (i.e., 210 °C); only a red shift of 20 nm was observed. Interestingly, the emission intensity of the sample 20:80, when annealed at 170 °C for 4 h, was enhanced 2.6 times, compared to the bare ZnO.