Photoacoustic Study on the Structural Variation of Titania Nanomaterials Using the Pr (III) Ion as a Spectral Probe

Abstract

In this work, $\hbox {Pr}^{3+}$-doped titania nanomaterials were prepared by a sol–gel method. The structural variations of the samples during the phase transitions were studied by using the $\hbox {Pr}^{3+}$ ion as a photoacoustic spectral probe. The result shows that for the gel sample heated at $80\,^{\circ }\hbox {C}$, the coordination environment of $\hbox {Pr}^{3+}$ is similar to that of its aqueous ion. The f–f transitions of $\hbox {Pr}^{3+}$ exhibit a continuous red shift along with the gel-to-anatase transition, indicating an increase of the ‘degree of covalency’ for the $\hbox {Pr}^{3+}$ bonding. For the sample calcined at $1100\,^{\circ }\hbox {C}$, however, the f–f transitions of $\hbox {Pr}^{3+}$ show obvious blue shift. This can be attributed to the segregation of $\hbox {Pr}^{3+}$ ions to the external surface during the anatase-to-rutile transition, forming $\hbox {Pr}_{4}\hbox {Ti}_{9}\hbox {O}_{24}$. The stabilization effect of the doped $\hbox {Pr}^{3+ }$ions on the anatase phase of the samples is also discussed.