Synthesis of scheelite-type nanocolloidal particles by pulsed laser ablation in liquid and their size distribution analysis

A novel pulsed laser ablation process in liquid was investigated to prepare scheelite-type ceramic [calcium tungstate ($CaWO_4$) and calcium molybdate ($CaMoO_4$)] nanocolloidal particles. The crystalline phase, particle morphology, particle size distribution, absorbance and optical band-gap were investigated. Stable colloidal suspensions consisting of well-dispersed $CaWO_4$ and $CaMoO_4$ nanoparticles with narrow size distribution could be obtained without any surfactant. Particle tracking analysis using optical microscope combined with image analysis was applied for a fast determination of particle size distribution in the prepared nanocolloidal suspensions. The mean nanoparticle size of $CaWO_4$ and $CaMoO_4$ colloidal nanoparticles were 16 nm and 30 nm, with the standard deviations of 2.1 and 5.2 nm, respectively. The optical absorption edges showed blue-shifted values about 60~70 nm than those of reported in bulk crystals. And also, the estimated optical energy band-gaps of $CaWO_4$ and $CaMoO_4$ colloidal particles were 5.2 and 4.7 eV. The observed band-gap widening and blue-shift of the optical absorbance could be ascribed to the quantum confinement effect due to the very small size of the $CaWO_4$ and $CaMoO_4$ nanocolloidal particles prepared by pulsed laser ablation in liquid.