Effect of surfactant concentration on the formation of Fe2O3@SiO2 NIR-reflective red pigments

Abstract

High NIR-reflective red pigments were developed by encapsulating Fe2O3 particles in a transparent SiO2 shell layer to form Fe2O3@SiO2 core-shell structures. The pigments were prepared by a sol-gel process and the effect of CTAB surfactant concentration on structural formation and optical properties was investigated. The reaction between CTAB and silica species was found to increase the yield of SiO2 formation, which had a major role to control the SiO2 shell thickness by ranging from 8 to 62 nm when varying CTAB concentration from 0 to 2 mM. After encapsulation, Fe2O3@SiO2 pigments still had the red-brown hue of hematite and the NIR reflectivity improved to 62.7 %–65.3%, or twice that of the uncoated Fe2O3. The pigments prepared using the highest CTAB concertation changed from the original Fe2O3 color (CIE Lab color space coordinates) by only ΔE 8.2% due to the formation of second phase SiO2 microspheres formed from excess CTAB micelles. The NIR-reflective red Fe2O3@SiO2 pigments showed good chemical stability and light resistance, thus they were suitable for use as cool pigments for coating applications.