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

Highlights: • Cr^{n +} -TiO₂ nanocomposites with different oxidation state effects. • Cr⁶⁺ ions imparted improved charge carrier dynamics of Cr⁶⁺-TiO₂ nanocomposites. • Oxidation state dependent surface charge density and zeta/streaming potentials. • New catalyst-substrate associations as a result of different structure of substrate. • Higher photodegradation of aspirin and paracetamol drugs under solar irradiation. The present work focuses on the effect of different oxidation states of impregnated Cr^{n +} ionic species over TiO₂ surface. The resulting Cr^{n +} -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 Cr^{n +} incorporation to have maximum photoresponse. These bands have been assigned to the ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g 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 Cr^{n +} -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.