Quantitative Assessment of Band Gap Reduction with Respect to Molar Content and Crystallinity in CeO

This study presents a quantitative analysis of band-gap reduction in CeO2 nanoparticles by correlating dopant molar concentration with crystallinity, crystallite size, and defect formation. Data compiled from Eu3+, Zn2+, Ag+, Cu2+, and Co2+ doped CeO2 systems synthesized via green solution combustion routes reveal a consistent structure-property trend. For Eu3+ doping (1-11 mol%), crystallite size decreases from 23.5 to 14.3 nm, accompanied by band-gap narrowing from 3.06 to 3.00 eV. Zn2+ doping lowers the band gap from 3.00 to 2.83 eV, driven by enhanced lattice microstrain and oxygen-vacancy formation. Ag+ substitution yields the strongest effect, where crystallite size drops from 23.7 to 11.3 nm and the band gap contracts from 2.92 to 2.20 eV. These reductions arise from dopant-induced lattice distortion, which introduces defect states (Vo, Ce3+) within the forbidden gap, promoting band-tail formation and narrowing the effective optical transition energy. Undoped green-synthesized CeO2 exhibits a 10 nm crystallite size with a band gap of 2.87-2.92 eV, confirming its inherently defect-rich nature. In contrast, Cu2+ and Co2+ dopants cause slight band-gap widening due to increased crystal-field splitting and partial suppression of oxygen-vacancy density. Overall, the analysis establishes that dopant concentration controls crystallinity-driven disorder, which governs band-gap modulation through quantum confinement relaxation, defect-state proliferation, and bond-length distortions within the fluorite lattice.