Nanocrystalline titania as photocatalyst has attracted considerable attention for its potential use in environmental cleaning.Recently,lanthanide ions doped titania samples have been shown to increase the photocatalytic efficiency of selected reactions.In this work,TiO2 nanoparticles doped with Er3+ were prepared via an ultrasonic assisted sol-gel method.The optical properties of the samples were determined by photoacoustic(PA) spectroscopy.It was found that the absorption edge shifted to lower wavelength when the particle size fell into nano-meter range,which is the evidence of a quantum size effect.The nature of ligand bonding to Er3+ and the structural properties were investigated with the PA absorptions of the f-f transitions of Er3+,which are sensitive to the local environment.The results showed that TiO2 gel heated at 70 oC still contained abundant trapped water and ethanol,and the environment around Er3+ was similar to the one in the aqueous ion.The "degree of covalency" for Er3+ bonding increased continuously during the gel-to-anatase transition.For the sample calcined at 1100 °C,however,the f-f transitions of Er3+ showed blue shifts and the intensity of the hypersensitive transition decreased,indicating an increase of ionicity in the Er3+ bonding.This can be attributed to the segregation of Er3+ ions to the external surface,forming Er2Ti2O7 during the ana-tase-to-rutile transition,which was confirmed with XRD and TEM measurements.
