Cobalt doping ZnO nanoparticles: from substitutional doping to interstitial expansion for optoelectronic applications

The influence of cobalt (Co) doping on the structural and optical properties of ZnO nanoparticles (NPs) synthesized via the sol-gel method was investigated. The obtained nanopowder was calcined at 400 °C for 2 h in a muffle furnace. X-ray diffraction (XRD) patterns were analyzed using the Rietveld refinement method, confirming a hexagonal wurtzite ZnO structure (space group P6₃ mc) with no secondary phases. With increasing nominal Co content, the XRD peaks shifted progressively toward higher diffraction angles, indicating the successful substitution of Co ions at Zn sites within the ZnO lattice. As the Co concentration increased up to 3%, both lattice parameters (a and c) decreased, whereas at 5% Co, an opposite trend occurred and a slight lattice expansion was observed, which suggests additional interstitial incorporation of Co²⁺ ions. The crystallite size and lattice strain were evaluated using the Williamson-Hall method coupled with Rietveld refinement. These structural observations were corroborated by scanning electron microscopy (SEM) analyses, which revealed the morphology and grain homogeneity of the samples. Fourier-transform infrared (FTIR) spectra confirmed the formation of a pure wurtzite ZnO phase, showing the characteristic Zn-O stretching vibrations. Optical characterization revealed a non-monotonic evolution of the optical band gap with Co content (decrease up to 3%, followed by a partial increase at 5%), consistent with the photoluminescence (PL) measurements pattern. The electron paramagnetic resonance (EPR) spectra reveal signals attributed to intrinsic point defects together with paramagnetic Co²⁺ in tetrahedral environments and Co³⁺ in interstitial octahedral environments, with evidence suggestive of exchange coupling (to be confirmed by direct magnetic measurements).