Tailoring luminescent and photocatalytic properties via oxygen vacancies of Manganese-doped CeO2 thin films

The following document seeks to explain the physicochemical impacts resulting from the stepwise insertion of 1 to 6% of the Mn dopant into cerium oxide Ce O 2 films ( C e 1 − x M n x O 2 ) elaborated by the Nebulizer Spray pyrolyis. X-ray diffraction demonstrates a polycrystalline crystallization in the evident fluorine structure of Ce O 2 at 0% Mn, and as the percentage of Mn increases, a structural change in terms of preferential orientation has been clearly noticed from both (111) and (222) for 0% Mn to only (111) at 6% -Mn. A Raman analysis was then performed twice to better illuminate these structural changes caused by manganese doping, using Laser excitation at 633 and 325 nm The first identified the main mode 463 c m − 1 ( F 2g mode) specific to the fluorine Ce O 2 structure undergoing a mid-height broadening under the effect of doping 0% → 6%-Mn. However, the second excitation revealed structural defects related to oxygen vacancies ( V o ) created by the partial substitution of C e 4 + by M n 2 + / M n 3 + dopant into the crystal lattice. Scanning electron microscopy (SEM) confirmed that the incremental introduction of Mn disrupts the Ce O 2 crystal growth, leading to a morphological evolution characterized by the formation of localized aggregates in Mn-rich regions. The incorporation of Mn into the Ce O 2 lattice was further substantiated by energy-dispersive X-ray spectroscopy (EDS) analysis. Optical characterization revealed a significant narrowing of the bandgap from 3.15 eV to 2.94 eV as doping levels increased, a phenomenon primarily attributed to the proliferation of oxygen vacancies ( V O ) induced by the dopant. The electronic interplay between Mn dopants and these ( V O ) defects was further elucidated by cathodoluminescence (CL) studies; the emission, initially broad, exhibited a decrease in intensity with higher doping concentrations, effectively delaying the recombination of photogenerated charge carriers. The emergence of distinct CL bands signifies the coupling of oxygen vacancies with Cerium ions, resulting in the creation of F + + ( V O • • ), F + ( V O • ), and F o ( V O x ) type color centers through the interaction of C e 3 + and/or C e 4 + with these structural defects. Consequently, the photocatalytic degradation efficiency improved from 10.1% (undoped CeO 2 ) to 27% (6% Mn-doped CeO 2 ), representing 2.7-fold enhancement with corresponding first order rate constants of k app = 0.65 × 10 –3 min -1 vs 1.8 × 10 –3 min -1 , respectively. This improvement arises from the presence of M n 2 + / 3 + ions and their associated oxygen vacancies, which facilitate the generation and separation of electron-hole pairs.