Morphological control of t-REPO4 (RE = Ho-Lu and Y) via solvothermal reaction and investigation of down-/up-conversion photoluminescence

Doughnut-shaped t-LuPO4 microparticles (∼1.3−1.5 μm in diameter and 1.1–1.3 μm thick) were originally synthesized via lactic acid (LA) assisted solvothermal reaction, and the effects of LA content and PO43−/Lu3+ molar ratio on phase/morphology evolution were thoroughly investigated. It was demonstrated that LA significantly altered the intrinsic crystallization habit of t-LuPO4 and simultaneously promoted 2D growth and 3D self-assembly of the crystallites. The synthesis strategy was also shown to be well extendable to the other t-REPO4 of RE = Ho-Yb and Y. With t-LuPO4 as a representative host, the down-conversion photoluminescence properties (excitation/emission, florescence decay, quantum efficiency) of Sm3+, Dy3+, Tb3+, Eu3+ and Tb3+/Eu3+ pair were thoroughly investigated. For the series of t-(Lu0.98-xTb0.02Eux)PO4 (x = 0−0.12) ternary phosphors, color-tunable emission (from green to red) was obtained through Tb3+ → Eu3+ energy transfer, and the optimal Eu3+ content was determined to be x = 0.08 for the total luminescence of Eu3+ and Tb3+. Under 978 nm laser excitation, t-(Lu0.88Yb0.10Er0.02)PO4 phosphor was found to exhibit strong green (522/547 nm; 2H11/2/4S3/2 → 4I15/2) and red (655/665 nm; 4F9/2 → 4I15/2) up-conversion luminescence via a two-photon process. The merits of the morphology control of this work were also clearly demonstrated by comparing the luminescence performances of Eu3+ in microdoughnut particles and nanoparticles synthesized without LA.