Study On Preparation And Photoluminescence Properties Of TiO₂ Nanocrystals Sol

Semiconductor crystallites with colloidal dimension, display intriguing quantum size effect in their photoluminescence (PL) properties and exhibit strong and tunable luminescence, which have been investigated because of their use in molecular biochemistry, biological labels etc. Titania (TiO₂), an important semiconductor material, has previously been researched for photo-assisted degradation of a variety of toxic chemicals and as a promising electrode material in dye-sensitized solar cells. As it is an indirect-bandgap semiconductor, band-edge luminescence from TiO₂ nanocrystals is very difficult to observe. As a result, to study and apply of PL characteristics of TiO₂ has not been simultaneously satisfied, and in the past three decades there is neither a general conclusion about its luminescence nature nor an agreement about its spectral position.In this paper, highly dispersible anatase TiO₂ nanocrystals were prepared via a facile alcohothermal method using tetra-n-butyl titanate as a precursor, including subsequent thermal treatment. The products were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The effects of experimental and environmental conditions on PL properties of TiO₂ nanocrystals have been investigated, and the further discussion has been carried out to understand the mechanism of PL. Furthermore, the luminescence quantum yield and the content of TiO₂ nanocrystals in sol were determined respectively. As a potential spectral probe we have obtained the fluorescence micrograph of such TiO₂ nanocrystals encapsulated in liposomes.The main work includes the following aspects:A typical procedure for synthesis of TiO₂ nanocrystals is described as follows: a solution containing absolute ethanol, concentrated HNO3 and purified water was stirred heavily in a purified nitrogen flushed system at room temperature for 30 min. After that,9n)4 was added slowly via a dropping pipette under vigorous stirring during another 30 min. The final reaction mixture was sealed in a Teflon-lined stainless steel 30 mL autoclave, which was employed to obtain a high temperature and pressure for the nucleation and growth of nanocrystals. As shown in the TEM images, TiC>2 nanocrystals with a relatively uniform particle size distribution of <10 nm are present in our transparent sol. The products present a process of amorphous-crystalline transition tendency during thermal treatment according to AFM images.A strong and stable PL emission with a maximum at 450 nm has been observed in our study. A serious of experimental conditions, such as time and temperature of thermal treatment, solvent, pH value and concentration of TiC>2 nanocrystals etc., have been investigated as factors influencing upon the fluorescence characteristics of TiC>2 nanocrystals. During the investigation, the results clearly indicate that thermal treatment, pH value and concentration of TiC>2 are the two main factors influencing upon its fluorescence characteristics. On the other hand, TiC>2 nanocrystals prepared with different parental alcohol demonstrate the shifts of the luminescence spectra. The results demonstrate that concentration effect has led to the changes of not only intensity but also position of emission. When the initial TiC^sol was diluted by different multiples, a series of blue-shift in emission position was found. The assumption about the influencing mechanism of all effects have been proposed in our paper.Using quinine bisulphate as a reference, luminescence quantum yield of T1O2 nanocrystals was measured to be about 0.21, which was much higher than the values reported in previous works. The concentration of TiC>2 in sol was determined by EDTA complex formation titration, and the results are consistent with the experimental conclusion.In the end, we have obtained the fluorescence micrograph of such TiC>2 nanocrystals encapsulated in liposomes. The result shows that a potential spectral probe with emission wavelength-controlled will be achieved in the near future.In conclusion, T1O2 nanocrystals sol with particle size distribution of <10nm was prepared in this paper. We pay attention to the PL behaviors of TiC>2 nanocrystal intransparent sol for the first time in the present work. The unique characteristics lead them to have distinct advantages over current organic fluorescence markers: high luminescence yield, colloidal stability, repeatability and sensitivity to changes in its surroundings, which make it possible to be applied in biological labels.