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

Abstract

The colloidal synthesis of ZnO-WO 3 heteroaggregates composed of 3-5 nm particles is presented. These sols can be used to prepare nanocrystalline ZnWO 4 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 ZnWO 4 Sanmartinite has been followed by XRD, DTA-TG-MS, SEM, FTIR, optical absorption, and fluorescence methods. The ZnWO 4 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-WO 3 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.

General Synthesis of Single‐Crystal Tungstate Nanorods/Nanowires: A Facile, Low‐Temperature Solution Approach

“…comparable with that of the previously reported ZnWO 4 particles [32] and ZnWO 4 thin films [53] that were annealed at high temperature.…”

“…[32] The soft methods include the block copolymer-assisted synthesis of thin one-dimensional and two-dimensional CdWO 4 nanoparticles, [15] and the reverse micelle soft-template method for the synthesis of BaWO 4 nanorods. [14d] The hydrothermal synthesis of CdWO 4 nanorods [33] and ZnWO 4 microcrystals [34] has been reported.…”

Selective Synthesis and Characterization of Single‐Crystal Silver Molybdate/Tungstate Nanowires by a Hydrothermal Process

“…[30] Most previous approaches for preparation of these families of molybdates and tungstates need high temperature and hard reaction conditions such as the solid state metathesis reaction at 1000 8C, [31] and sol±gel method. [32] Few reports are related with the synthesis of metal molybdates through solution methods. [33] Silver tungstates such as Ag 2 Mo 4 O 13 , Ag 2 Mo 2 O 7, and Ag 2 MoO 4 were traditionally synthesized by straight and harsh reaction in MoO 3 /Ag 2 O system.…”

Fe2(MoO4)3 microspheres as a potential anode material for lithium ion batteries

“…Most previous approaches to obtain molybdates or tungstates need high temperature and harsh reaction conditions [1][2][3] , such as solid-state reaction [4] , sol-gel method [5] , hydrothermal synthesis, self-templated methods, microwave-assisted solution synthesis, etc. For example, FeMoO 4 hollow microspheres with good photocatalytic activity via a template-free hydrothermal method [6] .…”