Microencapsulation of n-dodecane into zirconia shell doped with rare earth: Design and synthesis of bifunctional microcapsules for photoluminescence enhancement and thermal energy storage

We have designed a new type of bifunctional microcapsules composed of an n-dodecane PCM (phase change material) core and a rare-earth-doped zirconia shell for photoluminescence enhancement and thermal energy storage and have synthesized a series of microcapsule samples through in-situ polycondensation in a non-aqueous emulsion templating system. The scanning electron microscopic investigation indicated that the resultant microcapsules presented the regular spheres with a smooth surface and a uniform size distribution, while the transmission electron microscopy showed a well-defined core-shell structure for these microcapsules. The Fourier-transform infrared spectroscopy characterized the chemical compositions of the resultant microcapsules, and the rare-earth elements doped within the zirconia shell were confirmed by energy-dispersive X-ray patterns and X-ray photoelectron spectroscopy. The resultant microcapsules exhibited a good thermal regulation capability with high encapsulation and energy-storage efficiencies. Most of all, the photoluminescence characterization indicated that these rare-earth-doped microcapsules achieved a significant enhancement in emission intensity of the cyan-colored up-conversion and purple-colored down-conversion fluorescence after excited by radiation at wavelengths of 850 and 280 nm, respectively. The enhancement level of emission intensity varied with different rare-earth species and was determined by the bond length of rare-earth ion to oxygen in the solid solution of zirconia host.