Synthesis of two-dimensional CeO_2-δ-GQD composites and their structural and optical properties

The present study reports on the synthesis of two-dimensional (2D) ceria and graphene quantum dots (CeO_2-δ-GQD) composites and their functional properties. Composites were prepared by a simple liquid phase deposition (LPD) method followed by a thermal treatment in air at 250 and 400 °C. The structural and optical properties of composites were investigated by X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), High-Resolution Transmission Electron Microscopy (HR-TEM), Raman spectroscopy, Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), UV-Vis and photoluminescence (PL). The composites consist of crystalline CeO_2-δ regions of about 1.8–2.9 nm in size, coexisting with highly disordered GQDs areas. The CeO_2-δ-GQD composites experience gradual dehydration and oxidation of carbon processes during thermal treatment, influencing the CeO₂ crystal size, Ce^3+/Ce⁴⁺ ratio, and O vacancies presence. These effects allow for the control of some of the composite functional properties. Specifically, the CeO₂-GQD₂₅₀ composites exhibit a low recombination rate of charge carriers due to the synergistic interaction between the components and low presence of Ce³⁺. In contrast, the CeO₂-GQD₄₀₀ composites show a high E_g of 3.8 eV (E_g ∼3.2 eV for CeO₂NPs) associated with a CeO₂ quantum size effect and extremely low fraction of GQDs.