Controllable Synthesis And Luminescence Properties Of Rare Earth Doped Titanium Dioxide Nano And Micro Crystals

In the past decades, lanthanide ions doped materials have drawn much attentionbecause of their potential applications in the fields of light phosphor powder,advanced flat panel displays, biological and chemical labels, and so on. As one of themost important types of semiconductors, titanium oxide (TiO2) has long been studiedas a rare earth doped host material because of its particular crystal structure, highrefractive index, low phonon energy, and adequate thermal and environmentalstability. It is of significance to control the size, dimensionality and structure offunctional nano-and micro-meter TiO2materials because their chemical and physicalproperties are fundamentally related to the morphologies.In this work, rare earth doped nano-and micro-meter TiO2materials have beensuccessfully synthesized through hydrothermal and solvothermal method. X-raydiffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electronmicroscopy (SEM), transmission electron microscopy (TEM), and selected-areaelectron diffraction (SAED) were utilized to characterize the samples. Furthermore,the factors influencing on the luminescence properties and the luminescencemechanisms of the materials were studied by photoluminescence spectra (PL). Themain results of the research work are as follows:1. Highly crystalline and uniform TiO2:Eu3+spindle-like nanorods weresynthesized directly without further sintering treatment through a facile hydrothermalroute using citric acid (H3Cit) and ethanediamine (En) as the assistant agents for thefirst time. The results show that the dosage of ethanediamine (En) and citric acid(H3Cit) are important parameters that affecting the morphology and size of the finalproducts. The as-obtained samples are uniform spindle-like nanostructures withdiameters of60nm at the center and lengths of ca.460nm under optimal conditions.Under UV light excitation, the TiO2:Eu3+spindle-like nanorods exhibit strong redemission corresponding to the5D0-7F2transition of the Eu3+ions without anypostcalcination treatment. The luminescence intensity of the TiO2:Eu3+spindle-like nanorods is higher than that of the TiO2:Eu3+nanoparticles. The difference inemission intensity should be mainly associated with the defects which come from thesurface states of nanoparticles. Furthermore, the site-selective spectroscopy showsthat Eu3+is located at a distorted site which might be close to the surface.2.3D flower-like TiO2:Eu3+microspheres were prepared by a simplehydrothermal method and crystallized by subsequent heat treatments at elevatedtemperatures. The as-prepared samples are loose and porous with flower-like structure,the subunits are irregularly shaped nanosheets with smooth surfaces, and the thicknessis about10nm. The results show that the synergistic effect of citric acid (H3Cit),ethanediamine (En) and hydrofluoric acid (HF) is of crucial importance for thecontrollable fabrication of TiO2:Eu3+flower-like hierarchical structures. Under UVlight excitation, the calcined flower-like TiO2:Eu3+hierarchical microspheres exhibitstrong red emission corresponding to the5D0-7F2transition of the Eu3+ions.3. TiO2:Sm3+hollow spheres were fabricated by a simple solvothermal methodusing carbon spheres as hard template. Experimental results indicat that the TiO2:Sm3+hollow spheres are anatase phase after calcination at500°C for2h. The innerdiameter and the shell thickness of Sm-doped titania hollow spheres is55-75and10-14nm, respectively. Under UV light excitation, the hollow TiO2:Sm3+spheresexhibit orange-red emission corresponding to the4G5/2-6H7/2transition of the Sm3+ions. Furthermore, it is found that the PL intensity of Sm-doped titania hollow spheresphosphor is relevant to the Sm3+ion concentration and the optimal Sm-dopedconcentration is2mol%.4. Three dimensional (3D) TiO2core-shell microspheres were successfullysynthesized by a one-step hydrothermal method using ethylenediaminetetraacetic acid(EDTA) as the assistant agent. Experimental results indicate that the TiO2core-shellmicrospheres are crystalline without further sintering treatment. The as-formedsamples consist of a large quantity of irregular core-shell microspheres with diametersof0.5-1.5μm. The BET specific surface area of the TiO2core-shell microspheres isabout134.32m2/g, which is much larger than that of the Degussa P25(50±15m2/g).It is well-known that the surface area of nanoparticle increases with the decrease in its size. The larger surface area of nanoparticle, the smaller radius and more surfacedefects it has. Thus, the TiO2:Eu3+core-shell microspheres are almost not luminous,which is due to the electron-hole pairs undergo nonradiative recombination in thesamples. However, the core-shell microspheres exhibit higher photocatalytic activitythan that of commercial TiO2(Degussa P25) for oxidation of MO under UV lightirradiation. The enhanced photocatalytic activity is due to the high BET surface areaof the sample, which might possess more surface reaction sites and adsorb andtransport more dye molecules. The results indicate that the proposed photocatalystshows promising potential for the application in organic dye degradation.