Influence of physicochemical-electronic properties of transition metal ion doped polycrystalline titania on the photocatalytic degradation of Indigo Carmine and 4-nitrophenol under UV/solar light

To understand the role of dopant inside TiO2 matrix, anatase TiO2 was doped with transition metal ions like Mn2+, Fe3+, Ru3+ and Os3+ having unique half filled electronic configuration and their photocatalytic activity was probed in the degradation of Indigo Carmine (IC) and 4-nitrophenol (NP) under UV/solar light. For comparison, TiO2 was also doped with V5+, Ni2+ and Zn2+ metal ions having d0, d8 and d10 electronic configuration respectively. Irrespective of excitation source UV/solar light and nature of the organic pollutant, photocatalytic activities of doped photocatalysts followed the order: Mn2+-TiO2 > Fe3+-TiO2 > Ru3+-TiO2 ≥ Os3+-TiO2 > Zn2+-TiO2 > V5+-TiO2 > Ni2+-TiO2 at an optimum concentration of dopant. Based on the experimental results obtained, it is proposed that the existence of dopant with half filled electronic configuration in TiO2 matrix which is known to enhance the photocatalytic activity is not universal! Rather it is a complex function of several physicochemical-electronic properties of doped titania. Enhanced photocatalytic activity of Mn2+ (0.06 at.%)-TiO2 was attributed to the combined factors of high positive reduction potential of Mn2+/Mn3+ pairs, synergistic effects in the mixed polymorphs of anatase and rutile, smaller crystallite size with high intimate contact between two phases and favorable surface structure of the photocatalyst. Despite the intense research devoted to transition metal ion doped TiO2, it is rather difficult to make unifying conclusion which is highlighted in this study.