Bandgap Modulation of CeO2 Nanoparticles by Codoping of Y and Co Impurities

Interplay of trivalent and divalent dopants in (Y, Co) codoped CeO2 nanoparticles with dominantly tetravalent host cations has been investigated using a variety of structural and optical techniques. The nanoparticle samples were prepared by a Polyol method. As revealed by the x-ray diffraction (XRD) data, all nanocrystal samples under investigation have similar average particle size. The concentration of O vacancies in the samples was found to increase with Y doping level as indicated by the Raman spectroscopy, extended x-ray absorption fine structure (EXAFS), and x-ray absorption near edge structure (XANES) data. As determined from diffuse reflectance spectra, the bandgap of the sample appears to decrease with increasing Y concentration. However, a series of Co-free samples measured for comparison show no dependence of bandgap on Y concentration. We have proposed a theoretical model and performed numerical simulation using the Vienna ab initio simulation package (VASP) to explain such bandgap modulation effect.