Tungsten Suboxide Nanoneedles as an Effective Thermal Shield through Near-Infrared Reflection and Absorption

Internal cooling is a major factor that exacerbates the already scarce energy supply; thermal shielding material therefore becomes a prospective solution to prevent unwanted heat passage, hence reducing energy consumption by appliances such as air-conditioners. We report the fabrication of effective, transparent, and direct substrate bonding tungsten suboxide (W17O47) thermal shield through a combination of magnetron sputtering and chemical vapor deposition (sputtering-CVD). The shielding property arises from the samples’ ability to reflect and absorb near-infrared (NIR) light while permitting transmission of visible spectrum. Out of the two morphologies, W17O47 nanoneedles and flat tungsten oxide layers, we found that the needle structure performed better. It demonstrates a visible transmission of 60% and NIR reflectance of 40%, resulting in a 5 °C overall passive cooling. Our result shows that reflection plays a major role in thermal shielding in addition to absorption, which is often cited in the literature as the main contributing factor. Transmission electron microscopy (TEM) and selective area electron diffraction (SAED) studies confirm the W17O47 stoichiometry, a composition not so frequently found in the literature. Additionally, density functional theory (DFT) testifies the high conductivity of W17O47, which is related to good NIR scattering according to the Hagen–Rubens relation. Finally, high-aspect-ratio nanoneedles form a dense, “forest-like” morphology, which is shown to be conducive to the overall better thermal shielding property of these W17O47 nanoneedles.