Effects of Zn2+ and Cr3+ Doping on Nano-sized CoAl2O4 Spinel Pigments by Hydrothermal Processing

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 Zn2+ doping at A site and Cr3+ 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 Zn2+ and Cr3+ doping cause incomplete crystal growth and result in smaller particles than that of undoped spinels obtained at the same hydrothermal conditions. Zn2+ doping at A site lowered the synthesis temperature of the spinel crystalline colorants. Co0.95Zn0.05Al2O4 pigments with good coloration were obtained at 230 The hues of the as-prepared pigments changed from blue to green with increased amount of Zn2+ 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 Cr3+, but the coloration became deeper. Zn2+ doped spinels are of better coloration than that of Cr3+ doped.