Colour tuning of core–shell fluorescent materials

Generally, two or more phosphors are mixed to achieve multiplex colour in the fluorescence industry. However, such a simple mixture of fluorescent materials usually leads to colour discrepancy and may even affect the colour uniformity due to the distinct physicochemical properties of the different components. Fabrication of a core–shell structure is a novel strategy to prepare colour-tuned fluorescent materials. Here we report a core–shell structure made up from two phosphors, Y2O3:Eu and LnPO4, which can emit red and green light respectively. It is important to control the homogeneous and relatively low supersaturation of LnPO4 (Ln = La, Ce, and Tb) in the solution so that the precipitates of LnPO4 can deposit onto the Y2O3:Eu particles uniformly. This could result in an LnPO4 shell around the Y2O3:Eu core to form micron-sized complex particles. In the preparation of the core–shell structure, the slow hydrolysis of tripolyphosphate to release free phosphate ions is a key factor. It is well known that dissociation of tripolyphosphate is temperature sensitive so that this reaction can be controlled by heating the solution. Under UV excitation, both the core (Y2O3:Eu) and the newly formed shell (LnPO4) can emit their characteristic light; it is interesting that each individual core–shell complex can provide the multiplex colour homogeneously at the micron scale. By adjusting the proportion of core and shell in the complex, the fluorescence colours of the micron-sized phosphor can be tuned conveniently.