Advancing All‐Day Passive Radiative Cooling Performance of 3D Anodized Aluminum Oxide Nanostructures on Aluminium

Abstract Passive radiative cooling, which uses the cold sink of outer space for sub‐ambient cooling, offers a sustainable alternative to energy‐intensive conventional cooling methods. This study investigates the impact of transversal channel distances within three dimensional‐ 3D‐ anodic aluminum oxide (AAO) dielectric and inorganic nanostructures on their morphology, emissivity, and passive radiative cooling performance. 3D‐AAO nanostructures with varying alumina thicknesses and transverse channel spacings are fabricated. 3D‐AAO metamaterials on Aluminium ‐ Al‐ bulk with larger transverse channel distances exhibit high solar reflectance and high mid‐IR emissivity, since they act as Bragg reflectors. Their porosity is significantly higher than the more common 1D‐AAO due to the periodically ordered transverse channels. Cooling power density calculations showed that greater transverse channel distances improve daytime cooling performance. Outdoor characterization under direct sunlight confirmed these findings, with a 3D‐AAO nanostructure on Al bulk achieving a maximum temperature reduction of 10.2 °C. This research establishes the critical role of transverse nanochannels in tailoring the radiative cooling properties of durable, inorganic, all‐dielectric 3D‐AAO nanostructures and highlights their potential for multiple applications, including energy‐efficient building cooling.