Tunable Physical Properties of CaWO4 Nanocrystals via Particle Size Control

This work reports on a combinational methodology to CaWO 4 nanocrystals of controllable particle sizes for comprehension of particle size dependences of physical properties. Citric acid as a capping molecule restricted the growth of CaWO 4 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 CaWO 4 nanocrystals were always observed when the pH value was higher or lower than 8. Hydrothermal treatments over 3.6 nm CaWO 4 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, CaWO 4 nanocrystals showed a lattice expansion, an enhanced structural symmetry, and a red shift in the A u phonon mode. A drastic blue shift of the electronic absorption bands was also observed for the transition of 1 A 1 → 1 T 1 in CaWO 4 nanocrystals at small sizes, which was explained by taking into account the quantum confinement effect and the enhanced ionicity of W−O bonds.