Exploring the Influence of Synthesis Parameters on the Optical Properties for Various CeO2 NPs

Abstract

Cerium oxide (CeO₂) nanoparticles were synthesized with a chemical precipitation method in different experimental conditions using cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) as a precursor, modifying the solution pH, the reaction time, and Co atoms as dopants, in order to tune the band gap energy values of the prepared samples. The physical characteristics of the synthesized ceria nanoparticles were evaluated by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis analyses and photoluminescence measurements. XRD data revealed a pure cubic fluorite structure of CeO₂ NPs, the estimation of crystallite sizes by Scherrer's formula indicates the formation of crystals with dimensions between 11.24 and 21.65 nm. All samples contain nearly spherical CeO₂ nanoparticles, as well as cubic, rhomboidal, triangular, or polyhedral nanoparticles that can be identified by TEM images. The optical investigation of CeO₂ samples revealed that the band gap energy values are between 3.18 eV and 2.85 eV, and, after doping with Co atoms, the E_g of samples decreased to about 2.0 eV. In this study, we managed to obtain CeO₂ NPs with E_g under 3.0 eV by only modifying the synthesis parameters. In addition, by doping with Co ions, the band gap energy value was lowered to 2.0 eV. This aspect leads to promising results that provide an encouraging approach for future photocatalytic investigations.

Keywords: CeO2 nanoparticles; Co-doping; band gap tuning; optical properties; pH variation.

Figure 1

FTIR spectra of CeO₂ nanoparticles prepared using various synthesis methods.

Figure 2

XRD patterns of CeO₂/Co-doped CeO₂ nanoparticles.

Figure 3

SEM images of V2 (a) and V2-Co (b) samples and EDX elemental analysis spectra of V2 (c) and V2-Co (d).

Figure 4

TEM images of CeO₂ nanoparticles and the corresponding particle size distribution. Scale bar is 50 nm.

Figure 5

TEM images of Co-doped CeO₂ nanoparticles and the corresponding particle size distribution. Scale bar is 50 nm.

Figure 6

UV–Vis absorption spectra of pristine CeO₂ NPs: V1–V6 samples.

Figure 7

UV–Vis absorption spectra of V2 samples, pristine and Co-doped (a), and of V5 samples, pristine and Co-doped (b).

Figure 8

Reflectance spectra of pristine CeO₂ NPs (V1–V6) (a) and of Co-doped CeO₂ NPs (V2-Co, V5-Co) comparative to pristine V2 and V5 (b) (Inset: photo of V2 and V2-Co samples).

Figure 9

The Kubelka–Munk plots of different CeO₂ NPs samples: V1–V6 (a), V2-Co comparative to V2 (b), and V5-Co relative to V5 (c).

Figure 10

Fluorescence spectra of pristine CeO₂ NPs (V1–V6) at λ_ex = 270 nm (a) and λ_ex = 325 nm (b).

Figure 11

Fluorescence spectra of Co-doped CeO₂ NPs (V2-Co, V5-Co) comparative to pristine V2 and V5 pristine CeO₂ NPs at λ_ex = 270 nm (a) and λ_ex = 325 nm (b).