Figure 4: Photocatalytic degradation with time of Congo red in the presence of   [ R 2 O 3 ] π‘₯ β‹… [ T i O 2 ] 1 βˆ’ π‘₯ composites; (a) Y3+-doped samples, (b) Yb3+-doped samples, and (c) Gd3+-doped samples with the results of experiments without any catalyst (+), and in the absence of light irradiation (×). The degradation with commercial TiO2 is also shown for comparison. The lines represent the best-fit curves.