Controlled synthesis of blue spherical CoAl2O4 pigment powder in Pickering emulsion assisted with a hydrothermal process

Abstract A spherical blue CoAl 2 O 4 pigment powder commonly used for coloring ceramic products has successfully been synthesized by co-precipitation of Co 2+ and Al 3+ ions in Pickering emulsion assisted with a hydrothermal process, and the formation mechanism of the as-prepared powder has briefly been discussed. The effects of pH condition on the crystalline phase and color tone of the pigment powder were investigated at 280 °C by varying the pH value of the reaction system. Then, under an optimized pH condition to produce blue, the pigment powder was synthesized still hydrothermally at different temperatures (210, 240, 270 and 300 °C) to assess the effect of hydrothermal reaction temperature on the pigment particle morphology. The resulting pigment powders were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), CIELAB colorimetric analysis (CIE-L ∗ a ∗ b ∗ ), laser particle size analysis (LPS), and N 2 -adsorption measurement (BET). FE-SEM and TEM observations indicated that the pigment powders of spherical shape obviously were constructed by a core–shell double-layer structure, the radii of which all were ∼150 nm or so with a shell of ∼25 nm in thickness. XRD results demonstrated that the pigment powders that were obtained from the aforementioned method under a wide range of pH values invariably contained basically crystalline CoAl 2 O 4 of high purity. CIE-L ∗ a ∗ b ∗ data suggested that the pigment powders at 280 °C and pH ≤ 11.2 appeared different chromas of bluish colors compared with that displaying black which was obtained at pH = 12.3. LPS and BET results showed that the CoAl 2 O 4 pigment powder prepared under the urea concentration of 3.6040 g for 24 h at the hydrothermal temperatures of 240 °C exhibited a narrow particle size distribution, the specific surface area of which was evaluated to be  ∼56.4 m 2 /g.