Study of erbium (III) doped titanium dioxide nanoparticles by photoacoustic spectroscopy

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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, TiO 2 nanoparticles doped with Er 3+ 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 nanometer range, which is the evidence of a quantum size effect. The nature of ligand bonding to Er 3+ and the structural properties were investigated with the PA absorptions of the f-f transitions of Er 3+, which are sensitive to the local environment. The results showed that TiO 2 gel heated at 70 °C still contained abundant trapped water and ethanol, and the environment around Er 3+ was similar to the one in the aqueous ion. The “degree of covalency” for Er 3+ bonding increased continuously during the gel-to-anatase transition. For the sample calcined at 1100 °C, however, the f-f transitions of Er 3+ showed blue shifts and the intensity of the hypersensitive transition decreased, indicating an increase of ionicity in the Er 3+ bonding. This can be attributed to the segregation of Er 3+ ions to the external surface, forming Er 2Ti 2O 7 during the anatase-to-rutile transition, which was confirmed with XRD and TEM measurements.