Influence of surface coating on structural and photoluminescent properties of CaMoO₄:Pr nanoparticles.

CaMoO4:Pr(core), CaMoO4:Pr@CaMoO4 (core/shell) and CaMoO4:Pr@CaMoO4@SiO2 (core/shell/shell) nanoparticles were synthesized using polyol method. X-ray diffraction (XRD), thermogravimatric analysis (TGA), UV-vis absorption, optical band gap energy analysis, Fourier transform infrared (FTIR), FT-Raman and photoluminescence (PL) spectroscopy were employed to investigate the structural and optical properties of the synthesized core and core/shell nanoparticles. The results of the XRD indicate that the obtained core, core/shell and core/shell/shell nanoparticles crystallized well at ~150 °C in ethylene glycol (EG) under urea hydrolysis. The growth of the CaMoO4 and SiO2 shell (~12 nm) around the CaMoO4:Pr core nanoparticles resulted in an increase of the average size of the nanopaticles as well as in a broadening of their size distribution. These nanoparticles can be well-dispersed in distilled water to form clear colloidal solutions. The photoluminescence spectra of core, core/shell and core/shell/shell nanoparticles show the characteristic charge transfer emission band of MoO4 (2-) (533 nm) and Pr(3+) 4f(2) → 4f(2), with multiple strong (3)H4 → (3)P2, (1)D2 → (3)H4 and (3)P0 → (3) F2 transitions located at ~490, 605 and 652 nm, respectively. The emission intensity of the CaMoO4:Pr@CaMoO4 core/shell and CaMoO4:Pr@CaMoO4@SiO2 core/shell/shell nanoparticles increased ~4.5 and 1.7 times,respectively, with respect to those of CaMoO4:Pr core nanoparticles. This indicates that a significant amount of nonradiative centers existing on the surface of CaMoO4:Pr@CaMoO4 core/shell nanoparticles can be eliminated by the shielding effect of CaMoO4 shells.