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

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.