A high NIR-reflective black pigment is developed by Mn doping of Fe₂O₃. The pigment powders are prepared by spray pyrolysis, and the effect of the Mn concentration on the blackness and optical properties is investigated. Mn doping into the crystal lattice of α-Fe₂O₃ is found to effectively change the powder color from red to black, lowering the NIR reflectance compared to that of pure Fe₂O₃. The pigment doped with 10% Mn, i.e., Fe_1.8Mn_0.2O₃, exhibits a black color with an optical bandgap of 1.3 eV and a Chroma value of 1.14. The NIR reflectance of the prepared Fe_1.8Mn_0.2O₃ black pigment is 2.2 times higher than that of commercially available carbon black, and this material is proven to effectively work as a cool pigment in a temperature rise experiment under near-infrared illumination.

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.

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• Trend of Ceramic Nano Pigments
  Ri Yu, YooJin Kim
  Ceramist.2019; 22(3): 256.     CrossRef

Nanosized Gd₂O₃:Eu³⁺ red phosphor is prepared using a template method from metal salt impregnated into a crystalline cellulose and is dispersed using a bead mill wet process. The driving force of the surface coating between Gd₂O₃:Eu³⁺ and mica is induced by the Coulomb force. The red phosphor nanosol is effectively coated on mica flakes by the electrostatic interaction between positively charged Gd₂O₃:Eu³⁺ and negatively charged mica above pH 6. To prepare Gd₂O₃:Eu³⁺-coated mica (Gd₂O₃:Eu/mica), the coating conditions are optimized, including the stirring temperature, pH, calcination temperature, and coating amount (wt%) of Gd₂O₃:Eu³⁺. In spite of the low luminescence of the Gd₂O₃:Eu/mica, the luminescent property is recovered after calcination above 600°C and is enhanced by increasing the Gd₂O₃:Eu³⁺ coating amount. The Gd₂O₃:Eu/mica is characterized using X-ray diffraction, field emission scanning electron microscopy, zeta potential measurements, and fluorescence spectrometer analysis.

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• Optimization of dispersed LaPO4:Tb nanosol and their photoluminescence properties
  Mahboob Ullah, Se-Min Ban, Dae-Sung Kim
  Optical Materials.2019; 97: 109366.     CrossRef

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).