We report a new route of akaganéite (β-FeOOH) formation and maghemite (γ-Fe2O3) formation. Akaganéite can be produced by stirring Fe2+ at room temperature for a day under mild conditions. We used FeCl2 ·4H2O as the precursor and mixed it with the Na-rich particle from the oxidation debris solution. The role of the concentration ratio between graphene oxide (GO) and NaOH was addressed to generate oxidation debris (OD) on the surface. In particular, the characterization of OD by transmission electron microscope (TEM) imaging provides clear evidence for the crystal formation of Na-rich particle under electron beam irradiation. For the base treatment process, increasing the concentration of a NaOH in Na-rich solution contributed primarily to the formation of γ-Fe2O3. The characterization by scanning electron microscope (SEM) and TEM showed that the morphology was changed from needle-like to small-oval form. In addition, β-FeOOH can be effectively produced directly using GO combined with FeCl2 ·4H2O at room temperature. More specifically, the role of parent material (Hummer's GO and Brodie's GO) was discussed, and the crystal transformation was identified. Our results concluded that β-FeOOH can be formed in basic and acidic conditions.

Silica is used in shell materials to minimize oxidation and aggregation of nanoparticles. Particularly, porous silica has gained attention because of its performance in adsorption, catalysis, and medical applications. In this study, to investigate the effect of the density of the silica coating layer on the color of the pigment, we arbitrarily change the structure of a silica layer using an etchant. We use NaOH or NH4OH to etch the silica coating layer. First, we synthesize α-FeOOH for a length of 400 nm and coat it with TEOS to fabricate particles with a 50 nm coating layer. The coating thickness is then adjusted to 30–40 nm by etching the silica layer for 5 h. Four different shapes of α-FeOOH with different colors are measured using UV–vis light. From the color changes of the four different shapes of α-FeOOH features during coating or etching, the L* value is observed to increase and brighten the overall color, and the b* value increases to impart a clear yellow color to the pigment. The brightest yellow color was that coated with silica; if the sample is etched with NaOH or NH4OH, the b* value can be controlled to study the yellow colors.

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).

Abstract This manuscript reports on compared color evolution about phase transformation of α-FeOOH@SiO₂ and β-FeOOH@SiO2 pigments. Prepared α-FeOOH and β-FeOOH were coated with silica for enhancing thermal properties and coloration of both samples. To study phase and color of α-FeOOH and β-FeOOH, we prepared nano sized iron oxide hydroxide pigments which were coated with SiO2 using tetraethylorthosilicate and cetyltrimethyl-ammonium bro- mide as a surface modifier. The silica-coated both samples were calcined at high temperatures (300, 700 and 1000°C) and characterized by scanning electron microscopy, CIE L*a*b* color parameter measurements, transmission electron microscopy and UV-vis spectroscopy. The yellow α-FeOOH and β-FeOOH was transformed to α-Fe2O3 with red, brown at 300, 700°C, respectively.

This manuscript reports on compared color evolution about phase transformation of α-FeOOH@SiO2 and β-FeOOH@SiO2 pigments. Prepared α-FeOOH and β-FeOOH were coated with silica for enhancing thermal properties and coloration of both samples. To study phase and color of α-FeOOH and β-FeOOH, we prepared nano sized iron oxide hydroxide pigments which were coated with SiO2 using tetraethylorthosilicate and cetyltrimethyl-ammonium bro- mide as a surface modifier. The silica-coated both samples were calcined at high temperatures (300, 700 and 1000°C) and characterized by scanning electron microscopy, CIE L*a*b* color parameter measurements, transmission electron microscopy and UV-vis spectroscopy. The yellow α-FeOOH and β-FeOOH was transformed to α-Fe2O3 with red, brown at 300, 700°C, respectively.