Photoluminescence properties of ZnWO4:Eu3+ nanocrystals prepared by a hydrothermal method

ZnWO4:Eu3+ nanocrystals were prepared by the hydrothermal method at various temperatures and pH values. Their luminescent properties including excitation and emission processes, luminescent dynamics, and local environments surrounding Eu3+ ions were systemically studied. The results indicate that the particle size of the nanocrystals grows with increasing hydrothermal temperature, while it rarely changes with pH value. The excitation bands for Eu3+, which are contributed by different components, shift considerably to blue with the increasing pH value. The relative intensity of blue-green emission bands caused by the tungstate groups can be greatly modified by changing the pH value, thus the white color phosphors can be obtained. There exist two symmetry sites for the D-5(0)-F-7(2) emissions of Eu3+, inner and surface. The former corresponds to lines with narrower inhomogeneous width and slower decay time, while the latter to the lines with broader width and faster decay time. The emissions for tungstate groups mainly originate from the charge transfer from excited 2p orbits of O2- to the empty orbits of the central W6+ ions. On the other hand, the emissions for Eu3+ ions are contributed by both the charge transfer from O2- to Eu3+ and the energy transfer from W6+ ions to Eu3+ ions. A schematic is proposed to explain the photoluminescence processes in the ZnWO4:Eu3+ nanocrystals.