Synthesis, thermal behaviour and luminescence properties of rare earth-doped titania nanofibers

Abstract

Undoped and rare earth (RE=La, Eu and Er)-doped titania nanofibers have been fabricated by electrospinning technique, starting from polyvinylpyrrolidone, titanium tetraisopropoxide (Ti(OiPr)4) and Eu(NO3)3, La(NO3)3 and Er(NO3)3. A systematic microstructural and spectroscopic characterisation of RE-doped TiO2 nanofibers is presented by means of scanning electron microscopy (SEM), thermal analysis (TG–DTA), X-ray diffraction (XRD and HT-XRD) and Raman spectroscopy. Optical properties were investigated both by means of luminescence spectroscopy and by a FEG-SEM, equipped with a cathodoluminescence device. All electrospun materials consisted of randomly oriented nanofibers of fairly uniform diameter. The average fiber size was 40nm and 40–79nm for undoped and RE-doped TiO2 calcined at 500°C, respectively. The progressive lowering of the thermal decomposition temperatures of the PVP matrices upon admixing TiO2 precursor and RE ion sources might be correlated to catalytic properties of both metal centres.The presence of RE elements shifted toward higher values the anatase to rutile phase transition temperature (up to 900–1000°C), with the simultaneous formation of the Ln2Ti2O7 phase (Ln=Eu, Er). The Raman spectroscopy analysis revealed the typical anatase phase vibrational modes at 500°C and the rutile phase ones at 1000°C, accordingly to the XRD phase evaluations. In the case of La-doped sample, luminescence spectra were comparable to those registered for undoped TiO2, suggesting that La3+ doping did not cause a new luminescent phenomenon, but affected the response range and intensity. The luminescence spectra of Eu- and Er-doped samples clearly testified the presence of lanthanide ions in the TiO2 host lattice, showing the characteristic transitions of Ln3+ ions. The emission spectra were characterised by inhomogeneously broadened bands, suggesting a relevant disorder around the Ln3+ sites, typical of Eu3+ and Er3+ doped systems.