Band gap engineering, electronic state and local atomic structure of Ni doped CeO₂ nanoparticles

In cerium dioxide (CeO₂) as semiconductor compound, the tuning of band gap energy is a pivotal feature for visible light applications. In this report, nanoparticles (NPs) of Ce_1 − xNi_xO₂ (0.0 ≤ x ≤ 0.10) were synthesized through co-precipitation route. The structural, electronic state and optical band gap were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, Raman scattering and UV–Vis spectroscopy. XRD results revealed the single-phase nanocrystalline behavior of CeO₂ with cubic fluorite structure. The electronic configuration of the samples, probed via NEXAFS spectra at Ce M_{5, 4} edges, demonstrated that the cerium exists in Ce⁴⁺ and Ce³⁺ mixed valence states. The formation of Ce³⁺ ions in the vicinity of oxygen vacancies (Ov) were assessed also via Raman scattering and XANES spectra at Ce L₃ edge. The estimated concentrations of Ce³⁺ were increased from 6 to 13% as the Ni content increase from 0 to 10% respectively. Here, the inclusion of Ni²⁺ ions into CeO₂ network induced additional Ov and simultaneously reduced the optical band gap from 3.9 eV for pure CeO₂ to 2.6 eV for 10% Ni doped CeO₂ NPs. Therefore, the oxygen loss population seem to be responsible for the band gap reduction. The role of Ov for creating profound donor band near the conduction band and narrowing the band gap energy were discussed.