Conversion of Colloidal ZnO−WO₃ Heteroaggregates into Strongly Blue Luminescing ZnWO₄ Xerogels and Films

Abstract

The colloidal synthesis of ZnO−WO3 heteroaggregates composed of 3−5 nm 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 °C, and their mean crystal size increases with temperature to 35 nm at 400 °C and 120 nm at 600 °C. At the same time, the fluorescence intensity increases by 3 orders of magnitude within the aforementioned temperature regime, no matter whether 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 aggregate samples at sizes between 10 and 30 nm, the slow millisecond process dominates the recombination of the charge carriers whereas in 50−275 nm crystals and above, the faster 2.5 μs 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 μs recombination process takes place only in the perfectly developed Sanmartinite phase.