Resolving Overlapping Optical Transitions in CeO2 by Derivative Analysis of UV–Vis Spectra: Application to Au/CeO2

Abstract

The effect of Au nanoparticles on the energy transition and associated band gap of CeO₂ has been studied by UV–vis absorbance then analyzed by the Tauc, first derivative (dA/dλ), and second derivative (d²A/dλ²) methods; A: absorbance and λ: wavelength. CeO₂ mean particles size is ca. 12 nm while those of Au were 4–7 nm for the investigated series (with Au wt % = 1, 2 and 4). For CeO₂, two clear transitions are observed and are attributed to O 2p to Ce 4f⁰ (Ce⁴⁺) and O 2p to 4f⁰/4f¹ (Ce⁴⁺ close to Ce³⁺/V_O; V_O: oxygen vacancy) with the former having a higher energy than the latter. The presence of Au largely suppressed the weaker transition associated with defects states (Ce³⁺/V_O). This was observed in the first derivative plots where a blue-shift of 22 nm is seen and a narrowing of the Full Width at Half Maximum of peaks by up to 35 nm when compared to CeO₂ alone. The second derivative plots showed a more pronounced shift at the low energy side with an extracted band gap energy of 2.9 eV that shifted to 3.2 eV for 4 wt % Au/CeO₂, while the high energy side (at ca. 3.6 eV) was largely unchanged. These changes, collectively attributed to Au particles, might be linked to decreased defects centers in Au/CeO₂ catalysts when compared to CeO₂ alone. Therefore, the first derivative gives qualitative assessment of the role of Au in CeO₂ for light absorption while the second derivative gives a more quantitative information on the optical transitions. On the contrary, the Tauc plots, while identified both transitions, were insensitive.