The XPS observation that Mn can exist in different oxidation states in the TiO2 matrix is also consistent with the ionic diameters of Mn ions similar to that of Ti in that all Mn ion diameters apart from Mn2+ (0.80 Å) are slightly smaller than that for Ti4+ (0.68 Å) (ion diameters for Mn3+ and Mn4+ are 0.60 Å and 0.66 Å, respectively) [24].