Tunable physical properties of CaWO4 nanocrystals via particle size control

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 A(u) phonon mode. A drastic blue shift of the electronic absorption bands was also observed for the transition of (1)A(1) -> T-1(1) 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.