Structure and luminescent properties of Co²⁺/Cr³⁺ co-doped ZnGa₂O₄ nanoparticles

Co²⁺/Cr³⁺ co-doped ZnGa₂O₄ nanoparticles were prepared by a citrate sol–gel method. Their structure and luminescent properties were studied by using X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and fluorescence spectrophotometer. The nanoparticles with the size of 18–50 nm were obtained when the precursor was annealed at 500 °C or above. Ga³⁺ ions occupied the octahedral sites as well as the tetrahedral sites, while Cr³⁺ ions only entered the octahedral sites by replacing Ga³⁺. The as-synthesized samples had partially inverted spinel structure and the inversion degree slightly decreased with increasing temperature. The Cr³⁺:ZnGa₂O₄ sample exhibited an intense emission at 695 nm, corresponding to the ²E→⁴A₂ transition of octahedral Cr³⁺. The emission intensity of Cr³⁺ was largely reduced when the sample was co-doped with Co²⁺. However, the emission intensity of Co²⁺ in 2% Co/1% Cr co-doped ZnGa₂O₄ is improved approximately 2 times when compared with 2% co-doped sample. This enhanced emission could be attributed to an efficient energy transfer from Cr³⁺ to Co²⁺. The energy transfer mechanism is also discussed. - Highlights: • Co²⁺/Cr³⁺: ZnGa₂O₄ nanoparticles have been synthesized by a citrate sol–gel method. • The chemical environments of Ga³⁺ and Cr³⁺ as a function of annealing temperature were studied. • Ga³⁺ ions occupy both the tetrahedral and octahedral sites of spinel structure in the samples. • The emission intensity of Co²⁺ is improved by Cr³⁺ codoping in ZnGa₂O₄ with 550 nm excitation. • The possible energy transfer process was discussed