Conversion of colloidal ZnO-WO3 heteroaggregates into strongly blue luminescing ZnWO4 xerogels and films
The colloidal synthesis of ZnO-WO3 heteroaggregates composed of 3-5nm particles is presented. These sols can be used to prepare nanocrystalline ZnWO4 xerogels, powders, and thin films with strong blue fluorescence (quantum yield between 25 and 50% at room temperature). The conversion of the precursor aggregates into nano- and microcrystalline monoclinic ZnWO4 Sanmartinite has been followed by XRD, DTA-TG-MS, SEM, FTIR, optical absorption, and fluorescence methods. The ZnWO4 crystallites start to develop at 350 deg C, and their mean crystal size increases with temperature to 35nm at 400 deg C and 120nm at 600 deg C. At the same time, the fluorescence intensity increases by 3 order of magnitude within the aforementioned temperature regime, no matter wheter X-rays or UV photons are used to excite the samples. In complementary time-resolved X-ray fluorescence investigations, two recombination processes in the micro- and millisecond range were detected. In weakly fluorescing ZnO-WO3 aggrega te samples at sizes between 10 and 30nm, the slow millisecond process dominates the recombination of the charge carriers whereas in 50-275nm crystals and above, the faster 2.5 mu s process dominates the decay kinetics. Accordingly, the slow afterglow process is attributed to a recombination between electrons and holes deeply trapped at the surface of the heteroaggregates. The faster 2.5 mu s recombination process takes place only in the perfectly developed Sanmartinite phase.