In this work, we synthesize brilliant yellow color α-FeOOH by controlling the rod length and core-shell structure. The characteristics of α-FeOOH nanorods are controlled by the reaction conditions. In particular, the length of the α-FeOOH rods depends on the concentration of the raw materials, such as the alkali solution. The length of the nanorods is adjusted from 68 nm to 1435 nm. Their yellowness gradually increases, with the highest b^* value of 57 based on the International Commission on Illumination (CIE) Lab system, by controlling the nanorod length. A high quality yellow color is obtained after formation of a silica coating on the α-FeOOH structure. The morphology and the coloration of the nal products are investigated in detail by X-ray diffraction, scanning electron microscopy, UV-vis spectroscopy, and the CIE Lab color parameter measurements.

Fe-based pigments have attracted much interest owing to their eco-friendliness. In particular, the color of nanosized pigments can be tuned by controlling their size and morphology. This study reports on the effect of length on the coloration of β-FeOOH pigments prepared using an NH₄OH solution. First, rod-type β-FeOOH is prepared by the hydrolysis of FeCl₃·6H₂O and NH₄OH. When the amount of NH₄OH is increased, the length of the rods decreases. Thus, the length of the nanorods can be adjusted from 10 nm to 300 nm. The color of β-FeOOH changes from orangered to yellow depending on the length of β-FeOOH. The color and phase structure of β-FeOOH is characterized by UVvis spectroscopy, CIE Lab color parameter measurements, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and powder X-ray diffraction (XRD).

This work describes the coloration, chemical stability of SiO₂ and SnO₂-coated blue CoAl₂O₄ pigment. The CoAl₂O₄, raw materials, were synthesized by a co-precipitation method and coated with silica (SiO₂) and tin oxide (SnO₂) using sol-gel method, respectively. To study phase and coloration of CoAl₂O₄, we prepared nano sized CoAl₂O₄ pigments which were coated SiO₂ and SnO₂ using tetraethylorthosilicate, Na₂SiO₃ and Na₂SnO₃ as a coating material. To determine the stability of the coated samples and their colloidal solutions under acidic and basic conditions, colloidal nanoparticle solutions with various pH values were prepared and monitored over time. Blue CoAl₂O₄ solutions were tuned yellow color under all acidic/basic conditions. On the other hand, the chemical stability of SiO₂ and SnO₂-coated CoAl₂O₄ solution were improved when all samples pH values, respectively. Phase stability under acidic/basic condition of the core-shell type CoAl₂O₄ powders were characterized by transmission electron microscope, X-ray diffraction, CIE L*a*b* color parameter measurements.