Synthesis and luminescence properties of Er-doped and Er/Yb-codoped CoAl2O4 pigments

The development of methods to obtain ceramic pigments with fluorescence characteristics or hyper-domain performance has attracted research interest. In this study, Er-doped and Er/Yb-codoped CoAl2O4 pigments were synthesized by co-precipitation. The influence of Er doping, Er/Yb codoping, and the doping contents on the colour, composition, and luminescence properties of CoAl2O4 pigments were investigated. Er doping did not significantly affect the phase composition and colour performance of the CoAl2O4 pigment. The upconversion effect was induced by excitation with a 950 nm laser. Er doping could enhance the upconversion luminescence intensity of the CoAl2O4 pigment. At a 1 mol.% Er doping content, all luminous intensities reached maximum values. The downconversion effect was observed after excitation with a 350 nm laser. The luminescence was blue, and slight differences in the luminescence intensity with different Er doping contents were observed. Yb2O3 was formed after Er/Yb was codoped at > 6 mol.% Yb doping content, thereby affecting the phase composition and colour performance of the CoAl2O4 pigment. The luminous intensity of red light (739 nm) was the strongest in all cases. The luminous intensities of all of the colours decreased to the minimum value at 9 mol.% Yb doping content compared with those at other doping contents. At 3 mol.% Yb doping content, the luminous intensities of all colours decreased to very low values (close to 0), and the quenching effect of Yb3+ was achieved using a 350 nm laser. Er/Yb-codoped CoAl2O4 pigments exhibited the upconversion luminescence effect under an infrared laser and inhibited visible-light reflection under an UV laser. This ceramic pigment can be developed further to obtain ceramic pigments with anti-counterfeit function.