Tb³⁺- and Eu³⁺-Doped Lanthanum Oxysulfide Nanocrystals. Gelatin-Templated Synthesis and Luminescence Properties

Tb 3+ - and Eu 3+ -doped lanthanum oxysulfide nanocrystals (NCs) with homogeneous grain size have been prepared by a gel-network coprecipitation method using gelatin as the template and ammonium sulfate as the sulfurizing agent and a following reduction process at relatively low temperatures (750 °C) in a H 2 atmosphere. The products were characterized using thermogravimetry−differential thermal analysis (TG−DTA), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). The gelatin template can act as a “nanoreactor”, which is beneficial to the dispersion and size control of the coprecipitation product. The high decomposition temperature (about 482 °C) of the gelatin network is advantageous to prevent aggregation of the newly formed (LaO) 2 SO 4 NCs during calcination of the coprecipitation product. Single hexagonal phase La 2 O 2 S NCs were obtained by reduction of the (LaO) 2 SO 4 powder, and the NCs are equiaxial in shape with particle sizes ranging from 40 to 50 nm. The photoluminescence spectra and time-resolved spectra of the La 2 O 2 S:Tb(Eu) in phosphate buffer were investigated. The results show that the La 2 O 2 S:Tb NCs have a strong luminescence at 544 nm, whose intensity decreases along with decreasing particle size. Concentration quenching occurs when the Tb concentration reaches 8 mol %. The fluorescence color of the La 2 O 2 S:Eu NCs depends on the concentration of Eu 3+ ions. The red emission (at 625 nm) from 5 D 0 transition has a much longer decay time than the green emission from 5 D 1 transition of the La 2 O 2 S:Eu NCs. The Tb 3+ - and the Eu 3+ -doped La 2 O 2 S NCs have respective fluorescence lifetimes of 1.10 and 0.41 ms in phosphate buffer, allowing their potential applications in biological labeling.