Tailoring the structural and optical properties of CaO nanoparticles through green-assisted Gd3+/Al3+ Co-doping

In this work, we synthesized gadolinium (Gd 3+ ) and aluminum (Al 3+ ) co-doped calcium oxide (CaO) nanoparticles using a green sol-gel route with guava leaf extract as the capping agent. This synergistic co-doping strategy suppressed thermal-induced grain growth and improved the structural stability of pristine CaO nanoparticles. X-ray diffraction (XRD) analysis confirmed that the samples maintained a single-phase cubic rock-salt framework. However, significant lattice strain was induced due to the ionic size mismatch between the dopant ions and Ca 2+ host lattice. This mismatch compressed the average crystallite size from 25.40 nm to 13.46 nm, while increasing both lattice microstrain and X-ray density. Scanning electron microscopy (SEM) revealed a morphological transformation from large spherical particles to clustered flake-like nanostructures with reduced particle size. Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of Gd and Al into the CaO lattice, the Ca 2+ oxidation state, and the presence of lattice oxygen and oxygen-vacancy-related surface species. The optical results showed a progressive decrease in the optical band gap from 3.66 eV for pristine CaO to 3.46 eV for the sample containing 3 mol% Gd 3+ /Al 3+ . This band-gap narrowing is attributed to dopant-induced modifications of the electronic structure and defect-related states associated with incorporating Gd 3+ and Al 3+ into the CaO matrix. Thermogravimetric and differential thermal analyses (TGA/DTA) showed a clear three-stage mass loss. The residual plant organics burned away completely below 350°C and intermediate carbonate phases decomposed between 600°C and 750°C , leaving a phase- stable profile above 800°C. These composition-dependent structural, optical, and thermal modifications demonstrate the potential of the co-doped CaO nanoparticles for applications requiring tunable physicochemical properties.