Synthesis of Uniform Rare Earth Fluoride (NaMF4) Nanotubes by In Situ Ion Exchange from Their Hydroxide [M(OH)3] Parents

In this article, we demonstrate the production of uniform hexagonal sodium rare earth fluoride (beta-NaMF4) nanotubes through a hydrothermal in situ ion-exchange reaction by using rare earth hydroxides [M(OH)3] as a parent. The trivalent rare earth hydroxides were hydrothermally prepared at 120 degrees C and possessed a quasi-layered structure, which could be formed to be nanotubal morphology through a rolling up process from 2-D sheets. Moreover, the hexagonal structure of rare earth hydroxides [M(OH)3] displays a noticeable similarity with beta-NaMF4. This similarity makes the formation of beta-NaMF4 with nonlayered structure possible through in situ chemical transformation from M(OH)(3) with a layered structure. The single-crystal beta-NaMF4 nanotubes were synthesized with well-controlled diameter (80-500 nm), aspect ratio (6-30), wall thickness (25-80 nm), and contents (such as M = Pr, Sm, Gd, Tb, Dy, Er, as well as lanthanide-doped rare earth NaMF4). The multicolor upconversion fluorescence has also been successfully realized in the Yb3+/Er3+ (green) and Yb3+/Tm3+ (blue) co-doped beta-NaMF4 nanotubes by UC excitation in the NIR region. The various UC emission ratios of the samples were investigated as a function of hydrothermal reaction time to research the UC properties of the products and to further demonstrate the hydrothermal in situ ion-exchange process.