Systematic growth of non-epitaxial ZnO shell over Ln³⁺-Li⁺ co-doped Y₂O₃ phosphor core

Here, Ln³⁺-Li⁺ co-doped Y₂O₃@ZnO core-shell heterostructures were synthesized by three different techniques - intermediate layer conversion method, a hydrothermal method, and an interlayer mediated hydrothermal method. The synthesis procedure is optimized based on the thickness and compactness of the developed shell. The growth kinetics and synthesis mechanism of each adopted method have been explained in detail using XRD, FESEM, TEM, SAED, and EDX characterization techniques. The interlayer-mediated hydrothermal method proved to be the best technique for developing a thin, compact layer of ZnO shell on the upconversion microsphere, but only up to a critical thickness. Optimization of the shell thickness was done to understand the role of the magnitude and sign of lattice strain in the core-shell design. Conformal ZnO shell growth on the upconversion core was possible only for a small volume of the precursor solution, as anisotropic structures started to appear in the solution for larger volumes.