Synthesis and characterization of ultrafine Co2AlO4 pigment by freeze–drying

The preparation of cobalt blue pigment (CoAl2O4) using freeze–drying technology was investigated. Cobalt blue nanopowder was prepared using cobalt acetate [Co(CH3COO)2·4H2O] and aluminum sulfate [Al2(SO4)3·18H2O], with liquid nitrogen as the cryogenic medium. Cobalt blue nanopowder samples with Co/Al = 1:2 and 1:2.4 were obtained by using processes including pre-freezing spraying, vacuum drying, and calcination. X-ray diffraction, transmission electron microscopy and ultraviolet spectroscopy were used to analyze the freeze-dried precursors and calcined products. The freeze-dried precursor was amorphous. Co2AlO4 appeared at the calcination temperature of 750 °C, while CoAl2O4 was converted from Co2AlO4 at 900 °C, and the particle size was about 25 nm. The higher the calcination temperature, the greater the degree of crystallization. The cobalt blue pigment with Co/Al = 1:2 was a lighter but more saturated color than the 1:2.4 sample. Increasing the calcination temperature and lengthening the time could optimize the optical performance of the pigment.