Rare earth tungstate and molybdate compounds – from 0D to 3D architectures

Abstract

This tutorial review focuses on the recent development of diverse synthetic approaches and possibilities for chemical tuning of the size, shape, and morphology, as well as properties such as luminescence, of rare earth tungstate and molybdate materials. The use of rare earth tungstate and molybdate nano- and micromaterials as single materials for the generation of white light is reviewed. Additionally, the use of these materials as red phosphors in phosphor-converted white light emitting diodes (pc-WLEDs) when employing GaN or InGaN chips as the primary light source is explored.

Facile hydrothermal crystallization of NaLn(WO4)2 (Ln=La-Lu, and Y), phase/morphology evolution, and photoluminescence

“…A handful of methodologies have been successfully employed for Ln tungstate synthesis [6,7], but some important issues still need to be addressed. For example, the conventional solid reaction method usually requires a high temperature of up to 1300 °C to promote mass transfer between the Ln 2 O 3 and tungstate sources, leading to overgrown particles of significant aggregation [8,9].…”

“…For example, the conventional solid reaction method usually requires a high temperature of up to 1300 °C to promote mass transfer between the Ln 2 O 3 and tungstate sources, leading to overgrown particles of significant aggregation [8,9]. The sol-gel technique, with Ln nitrate and ammonium tungstate as the frequent reactants, may lower the crystallization temperature of the aimed phase through better reactant mixing, but usually involves the complicated stages of sol formation, gelation, evaporation, and calcination [6]. Durairajan et al [1] prepared NaGd(WO 4 ) 2 powder with a solgel procedure that comprises heating under stirring of the reaction system at 110 °C for 5 hours, pre-firing at 250 °C, and then a final firing at 800 °C.…”

“…The product, nonetheless, is composed of aggregated particles. Ln tungstates have also been prepared via the molten salt technique, which employs Ln and tungsten oxides as the reactants and a salt with a relatively low melting point as the fluxing medium (for example, chloride or sulfate) [6]. With NaCl and KCl as flux, Lei et al [10] obtained Gd 2 WO 6 via molten salt reaction at 950 °C for 6 hours, but the particles are also aggregated, though the extent of aggregation is less than that of solid reaction.…”

“…With NaCl and KCl as flux, Lei et al [10] obtained Gd 2 WO 6 via molten salt reaction at 950 °C for 6 hours, but the particles are also aggregated, though the extent of aggregation is less than that of solid reaction. The hydrothermal route generally shows substantial advantages over the aforementioned techniques in that it allows better control of the crystallite size, size distribution, particle morphology, and composition of the product, and is thus one of the most commonly used techniques for Ln tungstate preparation [6,11]. In addition, various chelates and surfactants (such as citric acid, ethylene diamine tetraacetic acid (EDTA), cetyltrimethyl ammonium bromide (CTAB), polyvinylpyrrolidone (PVP), ethylene glycol (EG), and oleic acid) have been tested to regulate the morphology of the hydrothermal product [2,5,6].…”

“…The hydrothermal route generally shows substantial advantages over the aforementioned techniques in that it allows better control of the crystallite size, size distribution, particle morphology, and composition of the product, and is thus one of the most commonly used techniques for Ln tungstate preparation [6,11]. In addition, various chelates and surfactants (such as citric acid, ethylene diamine tetraacetic acid (EDTA), cetyltrimethyl ammonium bromide (CTAB), polyvinylpyrrolidone (PVP), ethylene glycol (EG), and oleic acid) have been tested to regulate the morphology of the hydrothermal product [2,5,6]. Through hydrothermal reaction at 200 °C for 24 hours in the presence of a small amount of EDTA-2Na, Liu et al [12] obtained a product that can be transformed into NaLa(WO 4 ) 2 microspindles by annealing at 600 °C for 2 hours.…”