Synthesis And Properties Of La₂O₂S, Gd₂O₃ And Gd₂O₂S-Coated CoFe₂O₄ Luminescent Particles

Eu/Tb doped rare earth oxide and oxysulfide nanoparticles have broad potential applications in high resolution flat panel display and biomedical field owing to their high luminescence intensity, narrow line-shaped emission bands, excellent inherent photostability, long-lived luminescence as well as their high chemical stability and low toxicity. The synthesis and application of new, high quality and multifunctional rare earth oxysulfide and oxide nanoparticles are the present research hotspot. CoFe204 nanoparticles of reverse spinel structure are promisingly used in the field of high density storage, magnetic drug delivery, and magnetic fluid hyperthermic treatment due to their large magnetic anisotropy, high coercivity, moderate saturation magnetization and structural chemical stability at high temperatures. Bifunctional magnetic-luminescent nanocomposites obtained by coating the CoFe2O4 nanoparticles with the Eu/Tb doped rare earth oxysulfide are of well luminescent and magnetic properties and of promising applications in various fields, especially in biomedical field. In this work, Eu or Tb doped La2O2S nanoparticles were synthesized via a reduction method under Gelatin-Templated control and the luminescence properties of the nanoparticles were investigated in the water environment. CoFe204 nanoparticles were prepared via a homogeneous coprecipitation method using hexamethylenetetramine (HMT) as precipitant. The as-synthesized nanoparticles were then wrapped with a SiO2 layer to improve the stability in acid solutions. The Eu/Tb doped Gd2OS bifunctional magnetic-luminescent nanocomposites were prepared by a coprecipitation-reduction method using the SiO2 coated CoFe2O4 as the heterogeneous nucleation center. Gd2O3:Eu nanoparticles were also synthesized by homogeneous coprecipitation method using HMT as precipitant and the luminescence properties of the nanoparticles were investigated.La2O2S:Tb and La2O2S:Eu nanoparticles were synthesized by a H2 reducing method under gelatin-templated control at relatively low temperatures (about 750℃) using low-cost ammonium sulphate as the vulcanizing agent. 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℃) of the gelatin network is advantageous to restrain the growth and aggregation of the newly formed (LaO)2SO4 during calcination of the coprecipitation product. The product is of near spherical shape, uniform particle size (30-50 nm), and well dispersed. La2O2S:Tb nanoparticles have a strong green luminescence, whose intensity increases along with the increase of Tb doping concentration and reduction temperature. Concentration quenching occurs when the Tb doping ratio in mol reaches 8%. La2O2S:Tb nanoparticles have a fluorescence lifetime of 1.10 ms in phosphate buffer. The fluorescence color of the La2O2S:Eu nanoparticles depends on the concentration of Eu3+ ions. The relative intensity of red emission (at 625 nm) from 5D0 transition increases and that of green emission from 5D1 transition decreases with increasing Eu3+ concentration. The Eu3+doped La2O2S nanoparticles have a fluorescence lifetime of 0.41 ms for the red emission in phosphate buffer.CoFe2O4 nanoparticles were prepared via a homogeneous coprecipitation method using HMT as the precipitant and FeCl2 and Co(NO3)2 as the raw materials. The as-synthesized nanoparticles, near spherical in shape and uniform in particle size (average particle size about 30 nm), show higher saturation magnetization and moderate coercivity compared with the materials synthesized by the traditional coprecipitation method. The particle size and saturation magnetization increase with the increase of coprecipitation reaction temperature, concentration of coprecipitation ions and oxidization time in air. The magnetic properties are poor when the as-synthesized CoFe2O4 nanoparticles are heated at 200℃. The saturation magnetization increases with increasing temperature, and coercivity increases first with temperature increase till 800℃and then decreases with the increase of heat treatment temperature.CoFe2O4 @SiO2 core-shell nanoparticles were prepared by the Stober method, the Dense-Liquid method, and a Two-Step method, respectively. At the same SiO2 content, i.e. m(SiO2):m(CoFe2O4)=1:4, the SiO2 layer from the Two-Step method is dense and uniform, the one from the Stober method is uniform but porous, and the one from the Dense-Liquid method is dense but the coating is incomplete. For both the Two-Step method and the Stober method, isoelectric point of the produced core-shell nanoparticles is close to that of SiO2, beneficial to the dispersion stability of the nanoparticles in neutral solution. The SiO2 shell established by the Two-Step method restrains the dissolution of Fe3+ in acid solutions. With the increase of thickness of SiO2 shell, the saturation magnetization of the CoFe2O4 @SiO2 core-shell nanoparticles decreases and coercivity increases.Gd2O2S:Tb nanoparticles were synthesized by reducing in H2 the precursor obtained via the homogeneous coprecipitation method using ammonium sulphate as vulcanizing agent and HMT as the precipitant. The products are spherical in shape and 50-100 nm in size. Gd2O2S coated CoFe2O4@SiO2 composite particles were synthesized by introducing the as heterogeneous nucleation center during the coprecipitation stage. The composite particles, relatively coarse in size (～200) and wide in size distribution, have higher coercivity and magnetization, lower intensity of luminescence, and similar fluorescence lifetime compared with the Gd2O2S:Tb(Eu) particles. The degree of reduction of the magnetic core by H2 can be adjusted by the reduction temperature. The magnetization increases and coercivity decreases with the increase of the degree of magnetic core reduction.Gd2O3:Eu nanoparticles were synthesized via a homogeneous coprecipitation method using HMT as the precipitant. The as-synthesized nanoparticles, near spherical in shape and uniform in particle size (average particle size about 100 nm) show strong red emission. The emission intensity increases with the increase of calcination temperature and Eu3+ doping concentrations. Concentration quenching occurs when the Eu3+ doping ratio reaches 10 mol%.