shell introduces an additional electronic level of energy
between the O2 valence band and Ce4+ conduction band,
and a reduced band gap of 1.88 eV is observed. The CeO2 band
gap falls in indigo region of visible wavelengths, and a
complementary light yellow color is observed. By contrast,
Pr4+–CeO2 absorbs in the wavelength region below 600 nm
producing a red color