Effect of variable oxidation states of Mn+n ion impregnated TiO2 nanocomposites for superior adsorption and photoactivity under visible light

Abstract

This paper deals with the importance of oxidation state and ionic size of Mn(II) and Mn(VII) co-catalyst impregnated TiO2 for the observed electrokinetic, adsorption and photocatalytic properties of Mn+n-TiO2 nanocomposites. Due to difference in net electronic charge and ionic size of Mn(II) and Mn(VII) ions, the zeta potential, streaming potential and surface charge demand of Mn(II)–TiO2 and Mn(VII)–TiO2 nanocomposites are greatly varied. The absorption edge of Mn+n-TiO2 nanocatalysts revealed the red shift (515–550 nm) with increased oxidation state (from II to VII) of Mn+n due to 6A1g → 4T1g transition. The photoluminescence (404–530 nm) of Mn(II)–TiO2 is highly quenched relative to Mn(VII)–TiO2 during 340 nm excitation. Further, the HRTEM analysis confirmed the presence of Mn(VII) (∼20–45 nm) and Mn(II) (∼40–60 nm) nanodeposits over TiO2 surface. Binding energies at 639.1 and 640.3 eV showed the presence of Mn in (0) and (II) oxidation state for Mn(II)–TiO2 and 646 eV alongwith a satellite peak for Mn(VII)–TiO2 co-catalyst. As a result, the adsorption and photocatalytic degradation of cationic (Fuchsin blue and methylene blue) and anionic (Salicylic acid and Salicylaldehyde) pollutants by Mn+n-TiO2 nanocomposites are notably improved relative to bare TiO2 depending on their respective oxidation state and surface morphological features.