Tuning of oxygen vacancies to control color and electron dynamics in alkali metal substituted bismuth layered oxides for energy-efficient coatings†

Cities grapple with the urban heat island (UHI) effect, as dense urban landscapes trap solar heat. To combat this growing issue, reducing building energy consumption is crucial. The utilization of cool pigments in passive radiative cooling systems enables surfaces to efficiently reflect sunlight and release heat, thereby decreasing energy usage in buildings and mitigating the urban heat island effect. This work highlights the successful production of vibrant yellow pigments exhibiting exceptional infrared reflection through a simple solid-state reaction method having a distinctive orthorhombic Auruvillius-type structure. The particle sizes were tailored to a size range of 1 to 24 μm for various applications. Through the manipulation of alkali metals, a range of pigment colors spanning from bright yellow to softer yellow tones were achieved. The presence of oxygen vacancies contributes to the color of the pigments by capturing excited electrons by oxygen vacancy defects. Particularly noteworthy is K_0.5Bi_2.5Nb₂O₉, which exhibits an outstanding near-infrared (NIR) reflectance of 96% within 1100 nm, effectively lowering the treated concrete's surface temperature by 20 °C under IR lamp exposure. The robust stability of the pigment suggests its viability for application in a high-temperature environment, potentially resulting in reduced energy consumption in air-conditioning systems and fostering a more sustainable environment.