Effects of Zn~(2+) and Cr~(3+) Doping on Nano-sized CoAl_2O_4 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.