Effects of Zn<SUP>2+</SUP> and Cr<SUP>3+</SUP> Doping on Nano-sized CoAl<SUB>2</SUB>O<SUB>4</SUB> Spinel Pigments by Hydrothermal Processing

Abstract

Nanosized spinel type cobalt blue pigments of CoAl2O4(AB2O4) were hydrothermally synthesized using CoCl2·6H2O, ZnCl2, AlCl3 and CrCl3·6H2O as raw materials. Colorimetric analysis, X-ray diffraction (XRD), transmission electron microscope (TEM) and High resolution transmission electron microscope(HRTEM) were employed to investigate the effects of various amounts of Zn doping at A site and Cr doping at B site and various hydrothermal temperatures on the coloration, crystal phase compositions, crystal size and interplanar spacing of the CoAl2O4 spinels. The results indicated that Zn and Cr doping cause incomplete crystal growth and result in smaller particles than that of undoped spinels obtained at the same hydrothermal conditions. Zn doping at A site lowered the synthesis temperature of the spinel crystalline colorants. Co0.95Zn0.05Al2O4 pigments with good coloration were obtained at 230°C. The hues of the as-prepared pigments changed from blue to green with increased amount of Zn doping, and the coloration of samples became lighter. Similarly, the hues of the pigments also changed from blue to green with increasing the amount of Cr, but the coloration became deeper. Zn doped spinels are of better coloration than that of Cr doped.