Tunable Physical Properties of CaWO₄ Nanocrystals via Particle Size Control

Abstract

This work reports on a combinational methodology to CaWO4 nanocrystals of controllable particle sizes for comprehension of particle size dependences of physical properties. Citric acid as a capping molecule restricted the growth of CaWO4 nanocrystals and gave a small particle size of 3.6 nm at room temperature at pH 8, while large particle sizes or the absence of CaWO4 nanocrystals were always observed when the pH value was higher or lower than 8. Hydrothermal treatments over 3.6 nm CaWO4 at high temperatures led to a systematic increase in particle sizes. X-ray diffraction, infrared spectroscopy, transmission electron microscopy, and UV−vis spectroscopy were employed to study the particle size dependences of properties including lattice dimensions, vibrational variations, and transition energies. It is found that, with particle size reduction, CaWO4 nanocrystals showed a lattice expansion, an enhanced structural symmetry, and a red shift in the Au phonon mode. A drastic blue shift of the electronic absorption bands was also observed for the transition of 1A1 → 1T1 in CaWO4 nanocrystals at small sizes, which was explained by taking into account the quantum confinement effect and the enhanced ionicity of W−O bonds.

Understanding the defect chemistry of oxide nanoparticles for creating new functionalities: A critical review

“…To verify this conclusion and to uncover the relevant physics, Errandonea and his co-workers [27] theoretically analyzed the volume dependence of the structural parameters in CaWO 4 nanoparticles that has been experimentally determined by Li and his group [23]. With the help of ab initio total-energy calculations, they concluded that the enhanced symmetry in CaWO 4 nanoparticles with particle size reduction is a consequence of the negative pressure exerted on the nanoparticles.…”

“…Inspired by these observations, Li and his co-workers continued to study the lattice variations in many other simple oxides [21,22] like ZnO and NiO, and found the similar lattice expansion -a seemly general law for oxide nanopartiles. To make sure the validity of this general law, Li and his research group also extended their findings to the complex oxide nanoparticles like MWO 4 (M = Ca, Cd, Mn) [23][24][25][26]. It was found that the lattice expansion is followed by systematic variations in lattice symmetry.…”

Effect of vacancy and activator concentrations on the luminescence of Sr1−1.5xTbx□0.5xWO4 and Tb3+/Li+ co-doped phosphors

“…[13,14] Recently, there are increasing efforts devoted to the synthetic investigation of inorganic nano-particles. Various synthetic methods have been employed to prepare alkaline-earth tungstates, including solid-state reaction, [13] polymeric precursor, [18] Pechini, [19] hydrothermal, [14,20] sol-gel, [21] molten salt, [22] co-precipitation, [23] solvothermal, [24] combustion reaction [25] et al Various nano/micro-crystals with beautiful morphologies have been successfully fabricated, such as nanowires, nanotubes, [26] nanoparticles, [24,27] nanorods, [28] microspheres, [8] and hollow spheres. [29] However, in contrast to the extensive studies on shape-controlled syntheses, the structure characterization on alkaline-earth tungstates after RE-doping is rare.…”

Síntese pelo método da coprecipitação e caracterização estrutural do tungstato de cálcio com estrutura tipo scheelita

“…O estudo e o desenvolvimento de determinados materiais semicondutores, com propriedades ópticas ativas como, por exemplo, propriedades fotoluminescentes, propriedades fotocatalíticas, eletroluminescência, propriedades ópticas não lineares vem sendo estudado diariamente pela comunidade científica [1,2]. Os molibdatos (MoO 4 2-) e os tungstatos (WO 4 2-), em especial o tungstato de cálcio (CaWO 4 ), são exemplos desses materiais.…”