Synthesis, characterization and luminescence properties of Eu3+-doped hydroxyapatite nanocrystal and the thermal treatment effects

In this work, we present the synthesis, characterization and the luminescence properties of Ca-10(PO4)(6)(OH)(2) (hydroxyapatite/HAp) nanocrystals doped with europium trivalent ions. The most important processes that lead to europium emissions in the visible region were identified. Eu:HAp nanopowder excited at 394 nm (or 460 nm) exhibits several emissions: (i) weak emissions at 579 nm, 592 nm and 616 nm due to the D-5(0) -> F-7(0), D-5(0) -> F-7(1) and D-5(0) -> F-7(2) transitions, respectively, with europium ion occupying site I in hydroxyapatite structure and (ii) strong emissions due to the D-5(0) 7F0 (574 nm), D-5(0) -> F-7(1) (602 nm) and D-5(0) -> F-7(2) (610-630 nm) transitions, when Eu3+ is occupying site II. The emission spectrum and the time-resolved luminescence analysis showed that the HAp nanocrystals (nanopowder) thermally treated at temperature (T) between 500 and 800 degrees C have a change in the initial Eu3+ site distribution of 100 % of Eu3+ at site I to a more stable one where the majority of europium ions are at site II: 30% remains at site I and 70% migrates to site II. In addition, an enhancement of the Eu3+. emission intensity is observed due to the increasing crystallite size. A time-resolved luminescence investigation using a short pulse laser excitation at 460 nm was employed to measure the luminescence decays and to determine the most important mechanisms involved in the deexcitation process of D-5(0) excited state of Eu3+, where it is seen a fast (2.9 mu s) energy transfer from Eu3+- site I (donor) to Eu3+- site II (acceptor) in the thermally treated nanopowders with T > 500 degrees C. The initial presence of 100% of Eu3+ at site I in the synthesized nanocrystals is gradually modified by the thermal treatments with temperatures above 500 degrees C by thermal activation of Ca2+ vacancy (the charge compensator) diffusion through the HAp lattice, which propitiates the Ca2+ vacancies and Eu3+ ions to exchange positions in the lattice. By this thermal activated mechanism, Eu3+ ion migrates through the lattice until get the final distribution of 30% at site land 70% at site II. As a result, the complete description of the Eu3+ (D-5(0)) decay and the energy transfer process from Eu3+ (site I) -> Eu3+ (site II) were proposed. (C) 2015 Elsevier B.V. All rights reserved.