Role of different oxidation states of Crn+-TiO2 nanocomposites for the degradation of drugs under solar irradiation

Abstract

The present work focuses on the effect of different oxidation states of impregnated Crn + ionic species over TiO2 surface. The resulting Crn + -TiO2 nanocomposites have been revealed to have improved optical response, charge carrier relaxation dynamics, adsorption and photocatalytic behaviour which is correlated to variable oxidation states of these composite materials. The optical absorption spectra (400–500 nm and 583–800 nm) covered the entire visible region after Crn + incorporation to have maximum photoresponse. These bands have been assigned to the 4A2g → 4T1g and 4A2g → 4T2g transitions, respectively. A remarkable quenching of photoluminescence peaks and further successive enhanced carrier relaxations from 3.62 ns (bare TiO2) to 3.73 and 4.09 ns for Cr3+ and Cr6+ loaded TiO2 catalysts were observed. The XPS analysis confirmed the presence of Cr3+ and Cr6+ oxidation states while a reduction in particle sizes with increased oxidation state (Cr3+; 16.9 nm and Cr6+; 8.6 nm) was witnessed by HRTEM images. The electrokinetic parameters such as surface charge demand, zeta and streaming potentials were also found to vary as a function of the oxidation state of the nanocatalysts. These Crn + -TiO2 catalysts were then analyzed for the photooxidation of aspirin and paracetamol drugs with some interesting behaviour. For aspirin, Cr6+-TiO2 with rate constant, k = 18.815 × 10−3 min−1 while in case of paracetamol, Cr3+-TiO2 catalyst with k = 8.052 × 10−1 h−1 showed maximum activity under solar irradiation. The observed photoactivity has been correlated to the different chemical nature of substrate molecules, catalyst oxidation state effects and catalyst-substrate associations.