The environmentally benign synthesis of nanosized CoxZn1−xAl2O4 blue pigments

Nanosized CoxZn1−xAl2O4 (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1) blue pigments were prepared using a novel, starch-based synthetic route, in which the carbohydrate acts as chelating, template and gelation agent. The (Zn,Al,Co)-starch gel precursors were characterized using FTIR and thermal analysis, while their corresponding oxides were characterized using FTIR, NIR-UV–Vis spectroscopy, XRD and SEM. The FTIR spectra of the pink-reddish metals-starch gel precursors, dried at 80 °C, indicated the formation of a complex (Zn,Al,Co)-starch assembly as well as the first signs of a spinelic phase. Thermal analysis of the precursors showed that mass loss was accomplished at ∼ 430 °C; for CoxZn1−xAl2O4 (0 ≤ x ≤ 0.6) metal-oxide samples the spinel crystallization phase transformation occurred in the temperature range ∼700–830 °C. The presence of a pure spinel lattice after calcination of the metal-starch gel precursors at 800 °C was confirmed; particles were homogeneous in shape and size, with a pronounced tendency to form agglomerates and aggregatess. NIR-UV–Vis spectra of the blue oxides were characteristic of Co(II) metal ions located in tetrahedral sites and also showed disorder dependence on cobalt(II) cation-content.