Comparative Study of Sol–Gel and Co-Precipitation Synthesized CoAl2O4 Spinels

Cobalt aluminate (CoAl2O4) is a spinel oxide with notable structural stability and optical properties, making it a promising material for applications in pigments, sensors, and optoelectronics. In this study, CoAl2O4 samples were synthesized via two different methods: sol–gel auto-combustion (SGAC) using D‑Glucose (DG) and Aloe vera (AV) as fuel agents, and co-precipitation. The synthesized samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV‑Vis) spectrophotometry, and photoluminescence (PL) measurements to analyze their structural and optical characteristics. Rietveld refinement of the XRD data revealed a cubic spinel structure (space group Fd–3m), with an average crystallite size ranging from 20.65 to 29.88 nm. Sol–gel derived samples exhibited lower microstrain values ((0.45–0.85) × 10–3) compared to co-precipitated samples (0.97 × 10–3), suggesting enhanced crystallinity and reduced lattice distortions. FT-IR spectra indicated characteristic Co–O and Al‒O vibrations. UV-Vis analysis revealed absorption features characteristic of Co2+ d–d transitions, and the optical band gaps, estimated using Tauc plots, ranged from 3.69 to 3.97 eV. The photoluminescence spectra displayed distinct emission peaks near 595 nm, corresponding to the electronic transition from the 4T1(P) excited state to the 4A2(F) ground state of Co2+ ions situated in tetrahedral sites. These findings emphasize the effectiveness of green fuels in modulating nanoscale properties and suggest that sol–gel synthesis with bio-derived fuels offers a sustainable alternative to traditional methods for tailoring functional spinel oxides.