Preparation and Characterization of Mn-Doped Titanates with a Bicrystalline Framework: Correlation of the Crystallite Size with the Synergistic Effect on the Photocatalytic Activity

Mn 2+ ion was doped into the sol−gel anatase TiO 2 in the concentration range of 0.02−0.1%. Powder X-ray diffraction results revealed that the phase transformation from anatase to rutile proceeds at a slower rate up to an intermediate dopant concentration of 0.06% and accelerates for a higher dopant concentration of 0.1%. In comparison with the previous studies, the phase transition temperature was reduced by 100 °C for the intermediate dopant concentration. The photocatalytic activity of Mn 2+ (0.06%)−TiO 2 for the degradation of an oxo-fused polycyclic aromatic dye, Indanthrane BR Violet, was almost equal to that of Degussa P-25 under UV light, but under solar light its efficiency was nearly 4 times higher. This enhanced activity can be attributed to the (i) synergistic effect in the bicrystalline framework of anatase and rutile, (ii) small crystallite size, and (iii) high intimate contact between both the phases. The correlation of the crystallite size with the synergistic effect on the photocatalytic activity is studied in detail.