Lanthanide absorption spectral probe in titania nanoparticles synthesized by ultrasonic assistant sol-gel method

Titania nanoparticles were obtained by an ultrasonic assistant sol-gel method. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis. The optical absorption of the samples has been measured by photoacoustic (PA) spectroscopy, which is powerful for detecting small amount of strongly scattering materials. The structural variations of the sample during the phase transitions were firstly studied using Nd³⁺ as an absorption spectral probe. The PA results show that the TiO₂ gel heated at 50 °C is basically amorphous and still contains abundant trapped water and ethanol, which makes the environment around Nd³⁺ similar with that of its aqueous ion. For the sample calcined at higher temperature, the f-f transitions of Nd³⁺ exhibit a continuous red shift along with the gel-to-anatase transformation, indicating an increase of the “degree of covalency” for Nd³⁺ bonding. For the sample calcined at 1100 °C, however, the f-f transitions of Nd³⁺ show blue shifts and the hypersensitive transition intensities of Nd³⁺ decrease, indicating an increase of ionicity for Nd³⁺ bonding. This can be attributed to the segregation of Nd³⁺ ions to the external surface, forming Nd₄Ti₉O₂₄ during the anatase-to-rutile transition.