Synthesis and characterization of lanthanide hydroxide single-crystal nanowires.

Abstract

Nanowires represent a class of quasi-one-dimensional materials, in which carrier motion is restricted in two directions so that they usually exhibit significant photochemical, photophysical, and electron-transport properties which differ from that of bulk or nanoparticle materials.[1±11] In the last decade, many important materials have been prepared in the form of nanorods or nanowires to generate some unexpected properties[2±5] based on which, many new potential applications have been explored. For example, wellcontrolled and monodisperse CdSe nanorods can be used in solar cells,[2] and can form liquid-crystal phases with orientational order and positional disorder in organic solvents;[3] single-crystalline, well-defined ZnO nanowires can serve as natural resonance cavities, and well-aligned ZnO nanowire arrays may serve as room-temperature ultraviolet nanolasers.[4] Along with chemical composition and crystal structure, shape and dimensionality are now regarded as particularly important factors that influence the chemical and/or physical properties of materials. As a consequence of their unique electronic structures and the numerous transition modes involving the 4f shell of their ions, lanthanide compounds usually have outstanding optical, electrical, and magnetic properties,[12±22] and have been widely used as high-quality phosphors,[12] up-conversion materials,[13] catalysts,[14] and time-resolved fluorescence (TRF) labels for biological detection.[15] There have been extensive studies regarding lanthanide chemistry at bulk or atomic levels,[16±19] and the synthesis of lanthanide monothiooxides,[20] lanthanide-doped oxides or sulfides, organolanthanide compounds, the complexation behavior of lanthanide ions or atoms,[16,17] and the emission phenomena from lanthanides when bound to biomolecules (including calcium-binding proteins and nucleic acids).[18,19] Very recently, the synthesis of lanthanide oxide[21] and fluoride[22] nanoparticles with enhanced luminescence and photomagnetic properties has also been reported. However, to the best of our knowledge, few studies have focused on the synthesis of nanowires or nanorods of lanthaniderelated compounds. Such materials would be of great significance because of the possible novel properties induced by their reduced dimensionality. Herein, we report the synthesis of lanthanide hydroxide nanowires (La(OH)3, Pr(OH)3, Nd(OH)3, Sm(OH)3, Eu(OH)3, Gd(OH)3, Dy(OH)3, Tb(OH)3, Ho(OH)3, Tm(OH)3, and YbOOH) through a facile solution-based hydrothermal synthetic pathway. The synthesis of Ln(OH)3 nanowires was based on the preparation of colloidal Ln(OH)3 at room temperature, with subsequent hydrothermal treatment at 180 8C for about 12 h. Figure 1A shows typical XRD patterns for La(OH)3 nanowires. All of the reflections could be readily indexed to the hexagonal phase (space group P63/m (no. 176)) of La(OH)3