Improved photochromic stability in less deficient cesium tungsten bronze nanoparticles

Hexagonal cesium tungsten bronze (Cs_0.32WO₃) nanoparticles are well known as near-infrared (NIR) shielding materials. However, one of the critical issues for industrial applications involves a photochromic instability, i.e., a blue color change of this material under ultraviolet (UV) irradiation. This color change is attributed to the formation of H_xWO₃ phase through doping of H⁺ ions into Cs-deficient sites present at the surface of Cs_0.32WO₃ nanoparticles. Therefore, a new approach was adopted to prevent the color change by synthesizing a Cs_0.32WO₃ nanoparticle with less Cs-deficient sites using spray pyrolysis followed by heat treatment and short-time milling. The Cs_0.32WO₃ nanoparticles of spray pyrolysis route exhibited an improved photochromic stability under UV irradiation compared to those synthesized through a solid-state-reaction route. Detailed analysis using atomic resolution transmission electron microscopy revealed that the Cs_0.32WO₃ nanoparticles of spray pyrolysis route retain Cs ions up to a depth of less than 1 nm from the surface. This work demonstrates that the less-Cs-deficient Cs_0.32WO₃ nanoparticles are beneficial to improve the photochromic stability.