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+/VO; VO: 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+/VO). 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.