Enhancing moist-heat resistance of cesium-doped tungsten bronze nanoparticles via ultrathin silica coating: A sol-gel approach for optimized near-infrared absorption

Abstract

Near-infrared (NIR) absorbing materials are widely used to enhance energy efficiency in window applications. Cesium-doped tungsten bronze (CsWO) nanoparticles, known for their strong NIR absorption, comprise metal oxides with tungsten having a reduced oxidation state (W⁵⁺) and intercalated Cs⁺. However, in humid environments, CsWO nanoparticles are susceptible to degradation due to the oxidation of W⁵⁺ and elution of Cs⁺, leading to decolorization and diminished NIR absorption. This study presents a simple and effective method for enhancing the stability of CsWO nanoparticles by coating them with ultrathin silica layers (CsWO/SiO₂). The silica coating, applied via a sol-gel process, is optimized by controlling the concentrations of silicon alkoxide, base catalyst, water, and CsWO nanoparticles. The thickness of the silica layer, which can be precisely tuned at the nanoscale, depends on the reaction temperature. Notably, the single-nanometer-scale silica shell minimally affects the optical absorption properties of CsWO/SiO₂ while mitigating light scattering associated with increased particle size. A 5 nm-thick silica shell significantly improves the resistance of CsWO nanoparticles to moist-heat conditions compared to uncoated CsWO. Furthermore, the degradation mechanism of CsWO/SiO₂ under humid conditions is elucidated, highlighting the role of Cs⁺ elution and silica dissolution in the stability of coated nanoparticles.