Composite oxide cooling pigments mitigate the impact of urban heat islands

Abstract

The escalating threat of ever-increasing urban heat islands presents a significant global challenge regarding energy usage. Hence, the passive daytime solar radiative cooling technique relying on cooling materials is considered an innovative strategy to mitigate this issue without the utilization of any external energy. However, typical solar reflective cooling materials tend to have a bright white appearance, hence prompting an interest in aesthetic-colored reflective coolers with the requisite properties. However, achieving a balance between properties and appearance remains challenging. The present work establishes the synthesis of La₂W₂O₉ and La₂W_1.86M_0.14O₉ (M = Co, Cu, Zn, and Fe) radiative cooling pigments showcasing high near-infrared reflectance via a solution combustion route. Doping with different ions results in a tunable hue, enabling the preparation of coated surfaces with variable colors. The nano-pigments exhibited a pure triclinic phase of LaW₂O₉ with the P1 space group. Doped transition metal chromophores were successfully substituted into the LaW₂O₉ lattice without altering its initial structure. The best performing La₂W_1.86Fe_0.14O₉ cooling nano-pigment exhibits a relatively high near-infrared reflectance of around 97.8% with International Commission on Illumination chroma color coordinates L* = 62.77, a* = 19.34, and b* = 19.79. Interestingly, the thermal conductivity of the prepared pigments was found to be 0.07–0.08 W m⁻¹ K⁻¹, which is relatively smaller than conventional roofing materials, implying their advantage in cooling systems. Thanks to the high reflectance and low thermal conductivity of the synthesized pigments, a decrease in the interior temperature was recorded, ranging from 7 to 10 °C under infrared-light illumination for up to 60 minutes. Furthermore, building energy simulation results indicate that 17.54 kW h m⁻² of electricity can be saved annually if the colored La₂W_1.86Fe_0.14O₉ nano-pigment is employed. The aforementioned results demonstrated the efficacy of the prepared La₂W_1.86M_0.14O₉ (M = Co, Cu, Zn, and Fe) cooling nano-pigments as passive daytime solar radiative cooling materials to mitigate urban heat islands and achieve energy sustainability.