Study On Preparation Of Novel Rare Earth Upconversion Luminescent Nanocrystal/Semiconductor Nanocomposite Photocatalysts And Their Properties

TiO₂ and CeO₂ are typical semiconductor photocatalysts, which have a similar band gap（3.2e V） and performance. Both of them has attracted much attention and has been widely used in photocatalysis due to their unique properties such as non-toxicity, chemical and irradiation inertness, low cost, and relatively high catalytic activity. However, their photocatalytic efficiencies are still not desirable for practical application using solar light due to their low light absorption efficiency and high recombination rate of photogenerated electron-hole pairs. It is the most important to broaden their absorption arrange on solar light and restrain the recombination of photogenerated electron-hole pairs.In this study, we fabricated novel TiO₂（or CeO₂） /upconversion nanocrystals（UCNCs） based nanocomposites. The photocatalytic activities of TiO₂（or CeO₂）were improved significantly through a synergetic effect with the nanocomposites.The UCNCs absorb NIR light and transfer energy to TiO₂（or CeO₂）, which extend the light responsive range of TiO₂（or CeO₂） to NIR region and make the TiO₂（or CeO₂） producing highly energetic electron-hole pairs. Meanwhile, introduction of noble metal to restrain the recombination of photogenerated electron-hole pairs, or doping metal ions to red-shift the absorption edge, or combining with graphene to restrain the recombination of photogenerated electron-hole pairs and enhance adsorption for pollutant. Through this work, desired results were obtained that provides some valuable information for the study of semiconductors catalysts.The main contents and results of this study are listed below:（1）. A UCNCs of NaLuF₄:Gd,Yb,Tm was successfully synthesized by a facile thermal decomposition method firstly. Then a core/shell nanoparticles of NaLuF₄:Gd,Yb,Tm@SiO₂ was prepared by a reverse microemulsion method. The morphologies, crystalline phases, compositions, grain sizes and upconversion luminescence spectra were characterized by XRD 、 SEM 、 HRTEM 、 EDX and fluorescence spectrometer, respectively. The results showed that all of the UCNCs were uniform in morphology and size, and of high monodispersity. The crystalline phase UCNCs（NaLuF₄:Gd,Yb,Tm） was pure hexagonal phase, and the diameters of the products were 45 nm and 93 nm for NaLuF₄:Gd,Yb,Tm and NaLuF₄:Gd,Yb,Tm@SiO₂, respectively. The typical UCL spectra of Tm³⁺ were observed in both of the prepared products.（2）. A novel nanocomposite photocatalyst NaLuF₄:Gd,Yb,Tm@SiO₂@Ag @TiO₂ was developed for the first time. This composite material has a sandwich structure,including a core of UCNCs of NaLuF₄:Gd,Yb,Tm, a media shell of amorphous SiO₂ decorated with Ag nanoparticles, and an outer shell of anatase TiO₂. The designed new structure takes advantage of synergetic effect from UCNCs, Ag nanoparticles and TiO₂. The results of photocatalytic degradation of Rh B under simulated solar light irradiation 210 min showed that Rh B was almost completely degraded（99.46%） in the presence of NaLuF₄:Gd,Yb,Tm@SiO₂@Ag@TiO₂, which was 3.8 and 3.2 folds higher than that of pure TiO₂ and commercial P25, respectively. These results demonstrate that the photocatalytic activity of TiO₂ can be effectively improved through preparing the composite, and the as-prepared product can be applied in the photocatalytic degradation of dyes and other organic pollutants using solar light.（3） A novel nanocomposite photocatalyst of NaLuF₄:Gd,Yb,Tm@SiO₂@CeO₂:T m/graphene has been developed for the first time. Firstly, NaLuF₄:Gd,Yb,Tm@SiO₂@CeO₂ was prepared by a homogeneous precipitation method. Then the T m3+ ions were doped in the shell of CeO₂ by a hydrothermal method, and the resultant product was immobilized on the supporter of graphene through a hydr othermal process. The photocatalytic activity of CeO₂ was improved significantl y through a synergetic effect with the presence of UCNCs, graphene and Tm³⁺-doping in the composite synchronously. The results of photocatalytic degradati on of Rh B under simulated solar light irradiation 210 min showed that Rh B was almost completely degraded（95.42%） in the presence of NaLuF₄:Gd,Yb,T m@SiO₂@CeO₂:Tm/graphene, which was 2.1-folds higher than that of pure CeO₂. These results demonstrate that the designed approach is an effective metho d to improve the photocatalytic activity of CeO₂, and the as-prepared product i s potential to be used in photocatalysis using solar light.