Synthesis and Characterization of Solvothermal Processed Calcium Tungstate Nanomaterials from Alkoxide Precursors

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Abstract

An evaluation of calcium tungsten oxide (CaWO4) nanoparticles’ properties was conducted using the powders generated from an all-alkoxide solvothermal (SOLVO) route. The reaction involved a toluene/pyridine mixture of tungsten(V) ethoxide ([W(OEt)5]) with calcium bis(trimethyl silyl) amide ([Ca(N(Si(CH3)3)2]) modified in situ by a series of alcohols (H-OR) including neo-pentanol (H-OCH2C(CH3)3 or H-ONep) or sterically varied aryl alcohols (H-OC6H3R2-2,6 where R = CH3 (H-DMP), CH(CH3)2 (H-DIP), C(CH3)3 (DBP))]. Attempts to identify the intermediates generated from this series of reactions led to the crystallographic identification of [(OEt)4W(μ-OEt)2Ca(DBP)2] (1). Each different SOLVO generated “initial” powder was found by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) to be nanomaterials roughly assigned as the scheelite phase (PDF 00-041-1431); however, these initial powders displayed no luminescent behavior as determined by photoluminescence (PL) measurements. Thermal processing of these powders at 450, 650, and 750 °C yielded progressively larger and more crystalline scheelite nanoparticles. Both PL and cathodoluminescent (CL) emission (422–425 and 429 nm, respectively) were observed for the nanomaterials processed at 750 °C. Ion beam induced luminescence (IBIL, 478 nm) appeared to be in agreement with these PL and CL measurements. Further processing of the materials at 1000 °C, led to a coalescence of the particles and significant improvement in the observed PL (445 nm) and CL measurements; however, the IBIL spectrum of this material was significantly altered upon exposure. These data suggest that the smaller nanoparticles were more stable to radiation effects possibly due to the lack of energy deposits based on the short track length; whereas the larger particles appear to suffer from radiation induced structural defects.

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