Synergistic Cooling Effect of Metal-Doped Red-Orange Pigments: One-Step Synthesis for Enhanced Near-Infrared Reflectance and Selective Infrared Emission

In response to the escalating challenges of climate change and the effects of urban heat island, the development of energy-efficient cooling solutions has become increasingly crucial. This study addresses this pressing need by introducing a novel class of metal-doped Li2MnO3 pigments that exhibit synergistic cooling effects through enhanced near-infrared (NIR) reflectance and selective infrared emission. We present a one-step synthesis method for three innovative cooling pigment systems: single-phase Li2Mn1–xTixO3 (0 ≤ x ≤ 0.8) and mixed-phase Mn1–xZrx and Mn1–xHfx, all synthesized via a solid-state reaction. Ti doping in Li2Mn1–xTixO3 significantly enhanced NIR reflectance up to 0.890 (x = 0.4), a substantial improvement over undoped Li2MnO3 (0.486). The mixed-phase Mn1–xZrx and Mn1–xHfx pigments demonstrated similarly high NIR reflectances of 0.883 and 0.911, respectively. Crucially, all pigments maintained high infrared emissivity (>0.830) in the atmospheric window (8–14 μm), contributing to their radiative cooling properties. The metal doping not only improved cooling performance but also modulated the pigment color from deep red to orange-red, enabling the production of aesthetically pleasing cooling materials. Coatings formulated with these pigments exhibited improved NIR reflectance while maintaining high emissivity. Pigments in composite coatings demonstrated superior cooling performance compared to single-phase and commercial pigments. Through simulating sunlight exposure experiments, the doped pigments all showed better cooling effects compared to commercially available iron oxide pigments and undoped pigments. Furthermore, outdoor experiments conducted in Shenzhen, Nanjing, and other locations indicated that the pigments also exhibited excellent cooling effects under direct sunlight. It was observed that the temperature was lower than outdoor facilities such as tracks and seats with similar colors. These findings represent a significant advancement in the development of efficient, colored cooling materials for architectural and industrial applications, addressing the urgent need for sustainable thermal management solutions in the face of global warming and urbanization.