Renewable energy harvesting from the sun and outer space have aroused significant interest over the past decades due to their great potential in addressing the energy crisis. Furthermore, the harvested renewable energy has benefited another global challenge, water scarcity. Both solar steam generation and passive radiative cooling‐enabled atmospheric water harvesting are promising technologies that produce freshwater in green and sustainable ways. Spectral control is extremely important to achieve high efficiency in the two complementary systems based on absorbing/emitting light in a specific wavelength range. For this reason, a broad variety of solar absorbers and IR emitters with great spectral selectivity have been developed. Although operating in different spectral regions, solar selective absorbers and IR selective emitters share similar design strategies. At this stage, it is urgent and necessary to review their progress and figure out their common optical characteristics. Herein, the fundamental mechanisms and recent progress in solar selective absorbers and IR selective emitters are summarized, and their applications in water production are reported. This review aims to identify the importance of selective absorbers/emitters and inspire more research works on selective absorbers/emitters through the summary of advances and the establishment of the connection between solar absorbers and IR emitters.

centuries, it was only recently that noontime cooling was first demonstrated using a multilayer photonic structure. [2] In tropical and subtropical areas, the demand for cooling is particularly huge due to high ambient temperatures. However, it is proved that daytime cooling is difficult to achieve in such areas with stronger solar irradiation and higher humidity than mid-latitude regions. [3][4][5][6][7] With the increase in humidity, the transmittances within the atmospheric windows remarkably decrease, while most secondary windows such as the 16-25 µm window may even disappear in humid climates (Figure S1, Supporting Information). This significantly increases the atmospheric radiation and limits the cooling capability of radiative coolers, especially those with strong IR absorption/emission beyond the major atmospheric window. Although broadband IR emitters may provide ≈10% higher cooling power potential than selective ones in less humid climates due to the existence of secondary windows, in humid areas with high precipitable water vapor (PWV), this advantage vanishes and their cooling temperatures are notably limited by broadband IR absorption. [8] To maintain applicability in different climates, an ideal daytime radiative cooler should show high emittance only within the main Daytime radiative cooling provides an eco-friendly solution to space cooling with zero energy consumption. Despite significant advances, most state-ofthe-art radiative coolers show broadband infrared emission with low spectral selectivity, which limits their cooling temperatures, especially in hot humid regions. Here, an all-inorganic narrowband emitter comprising a solutionderived SiO x N y layer sandwiched between a reflective substrate and a selfassembly monolayer of SiO 2 microspheres is reported. It shows a high and diffusive solar reflectance (96.4%) and strong infrared-selective emittance (94.6%) with superior spectral selectivity (1.46). Remarkable subambient cooling of up to 5 °C in autumn and 2.5 °C in summer are achieved under high humidity without any solar shading or convection cover at noontime in a subtropical coastal city, Hong Kong. Owing to the all-inorganic hydrophobic structure, the emitter shows outstanding resistance to ultraviolet and water in long-term durability tests. The scalable-solution-based fabrication renders this stable high-performance emitter promising for large-scale deployment in various climates.

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