Environmental blue CoAl2O4 pigment co-doped by Zn2+ and Mg2+: synthesis, structure and optical properties

Nano-sized blue solid solutions ZnxMg0.5−xCo0.5Al2O4 (x = 0–0.5) have been synthesised by the Pechini method. Single-phase ZnxMg0.5−xCo0.5Al2O4 with crystallite size of ∼40 nm was identified by XRD measurement. The TG-DSC results indicated that the phase formation temperature of ZnxMg0.5−xCo0.5Al2O4 increased with the substitution of Zn2+/Mg2+ → Co2+ proceeding. The UV–vis spectra illustrated that the Zn0.3Mg0.2Co0.5Al2O4 pigment displayed the most intensive blue colour with the strongest absorbance appearing within the visible region. The FT-IR spectra suggested that the inversion degree of ZnxMg0.5−xCo0.5Al2O4 pigment reduces with the increase of Zn2+ rather than Mg2+, enabling to control the pigment colour by tuning the Zn2+ content. The FE-SEM images showed an irregular shaped morphology of ZnxMg0.5−xCo0.5Al2O4 crystal, different from the cubic-like morphology of CoAl2O4 crystal. The XPS results illustrated that the inversion of pure CoAl2O4 pigment is larger than that of Zn0.3Mg0.2Co0.5Al2O4. Both Zn0.2Mg0.3Co0.5Al2O4 and Zn0.3Mg0.2Co0.5Al2O4 show commercial potential in pigments application.