Photocatalytic and photosensitization reactions of surface modified W⁶⁺ and N^3‒ doped TiO₂ with curcumin/potassium curcuminate molecules

Curcumin and potassium curcuminate molecules were anchored on W⁶⁺ and N^3‒ doped TiO₂ catalyst (WNT) surface by wet impregnation method and were designated as Cur-WNT and K₃Cur-WNT. The XPS and FTIR techniques confirm the surface anchoring of curcumin and potassium curcuminate molecules on the surface of WNT through enolate linkage. K₃Cur-WNT catalyst possesses multiple charge trapping states involving dopant energy levels, inherently created defect levels/surface states. Hence the mean free path of the photogenerated electrons is decreased in this catalyst by trapping and detrapping events. The energy band positions of valence band (E_VB), conduction band (E_CB) and Fermi energy (E_F) were determined from the electronegativity values of the atoms present in the catalyst sample and also from the valence band X-ray photoelectron spectroscopic (VBXPS) studies. Energetically the position of conduction band (CB) and W 5d dopant energy levels were found to be located below the lowest unoccupied molecular orbital (LUMO) of curcumin and potassium curcuminate molecules in Cur-WNT and K₃Cur-WNT samples. This suitable excited singlet energy band position (S₁) of sensitizer molecule facilitates the electron transfer process to the CB/W 5d energy levels of the WNT catalyst. The accumulated electrons in CB/W⁶⁺ dopant energy level influence the position of E_F within the band gap. The observed higher quantum efficiency of K₃Cur-WNT sample compared to all the other catalysts can be accounted to the narrowed band gap, generated shallow traps, reduced particle size, enhanced photosensitivity and its higher capability in generating reactive singlet oxygen species.