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).