The sphere-enhanced emitter and its spectrum. The emitter has a triple-layer structure, comprising a monolayer of closely packed silica microspheres, a silicon oxynitride (SiOxNy) layer derived from perhydropolysilazane (PHPS) and a silver layer on a substrate (Figs. 1b-c). This simple all-inorganic design can generate highly selective but strong IR emission (Fig. 1a) by rationally exploiting the optical behaviors of the SiOxNy layer and silica microspheres.
The emitter was fabricated using facile solution processes (Fig. 1e). PHPS, an inorganic precursor containing Si-N skeleton, was used to generate a dense SiOxNy layer on a silver-coated substrate through natural solidification in the air (Supplementary Fig. S2-4). The SiOxNy/Ag structure was mirror-like and showed highly selective IR emission (ε8−13µm ≈ 80% and ƞε > 1.4), as shown in Fig. 1a. Another emissive peak caused by Si-H bonds23 was observed at 4.63 µm, which matches the 4.5-5 µm atmospheric window. To enhance its IR emissivity while maintaining the spectral selectivity, a monolayer of SiO2 microspheres with diameters of ~ 2 µm was coated on top (Supplementary Fig. S5) using the Langmuir-Blodgett (LB) self-assembly process24, which has been widely used for roll-to-roll microparticle deposition. After immobilizing the silica microspheres with a thin PHPS-derived bonding layer, a white paper-like inorganic emitter was attained. Figure 1d shows a 20 cm×20 cm sample fabricated on a silver-coated aluminum film.
The optical function of each component of the emitter is schematically shown in Fig. 2a. The SiOxNy layer is transparent to sunlight but it generates intrinsic selective emission within the 8–13 µm atmospheric window. The monolayer of close-packed SiO2 microspheres above the SiOxNy layer further selectively enhances the emissivity at the wavelengths near 9 µm and 12 µm. The normalized emissive power of each part is shown in Supplementary Fig. S6. The bottom silver layer can elongate the optical path to improve the emittance within 8–13 µm and produce high reflection beyond the window. The coupling of these three layers leads to strong narrowband IR emission in the atmospheric window (ε8−13µm = 94.6%) and excellent spectral effectiveness (ƞε = 1.44). Also, strong scatterings of visible light by the microspheres enable the transition from specular to diffusive reflection on the emitter surface (Fig. 1d). It is important in practical applications to avoid glare appearance and light pollution problems.
The refractive index of the PHPS-derived SiOxNy is shown in Fig. 2d (upper panel). Owing to the Si-N and Si-O bonds23, its extinction coefficient k is quite large between 8 µm and 13 µm but rather small in other regions, resulting in strong emission mainly in the 8–13µm window. Figure 2e (upper panel) illustrates the emissivities of SiOxNy layers with different thicknesses on a reflective surface. Within the atmospheric window, its ε8−13µm rapidly rises to around 80% when the thickness increases to 2 µm, and then only slightly improves with the increasing thicknesses. Beyond the atmospheric window, the average emissivity keeps increasing with the increasing layer thickness because of the non-zero k value. The highest effectiveness (ƞε = 2.2) is achieved at a thickness of ~ 0.6 µm but the corresponding ε8−13µm is small; further increasing the thickness will gradually reduce the effectiveness to ~ 1.1 (Supplementary Fig. S7). Thus, if the layer thickness is controlled between 1.5 and 4 µm, the PHPS-derived emitter can offer both a high emissivity (ε8−13µm > 80%) and high effectiveness (ƞε > 1.4). This large thickness tolerance provides great opportunities for cost reduction in large-scale fabrication, compared to previous photonic or plasmonic structures with nanometer-level tolerance.
The ε8−13µm of a single SiOxNy layer can reach ~ 80%, but it is difficult to be further improved without sacrificing the effectiveness ƞε through increasing the thickness, because of the reflection peaks at around 9 and 12 µm caused by the Reststrahlen bands of bulk SiOxNy. Figure 2e (upper panel) clearly shows weaker emission near 9 and 12 µm due to the Reststrahlen bands for almost all the thicknesses. Different from its bulk counterpart, SiO2 microspheres show resonance-enhanced light-matter interactions especially at wavelengths of around 9 and 12 µm and therefore high absorption/emission19, which is perfectly complementary to the SiOxNy layer (Fig. 2e (lower panel)). The resonance with strong near-field confinement was identified as surface phonon polaritons (SPhP). Further, the high-intensity near-field effect can be outcoupled into far-field through diffraction by a periodic surface structure and induce emission enhancement25. A close-packed monolayer can generate 20% more IR emission than that with randomly distributed spheres of the same size26. Compared with non-close-packed SiO2 microsphere monolayers of different periodicities, a close-packed monolayer shows the strongest resonance absorption (Supplementary Fig. S8). Meanwhile, increasing the number of microsphere layers rapidly broadens the emission spectrum. Therefore, a close-packed monolayer of SiO2 microspheres was deposited on top of SiOxNy to selectively enhance ε8−13µm.
The IR selectivity of SiO2 microspheres strongly relies on their size. Figure 2e (lower panel) shows the emissive spectra of a monolayer of SiO2 microspheres with different diameters on a reflective surface. When the diameter is larger than 1.6 µm, two strong narrowband emission peaks appear at desired wavelengths (9 µm and 12 µm). When the size exceeds 2.6 µm, higher-order FrÖhlich resonances will be excited beyond the Reststrahlen bands17, deteriorating the infrared selectivity. Hence, SiO2 spheres with sizes of 1.6–2.6 µm are selected to provide complementary emission while keeping the IR selectivity. This is different from previous works using 8-µm SiO2 microspheres to realize broadband IR emssion17, 26. Figures 2b-c show that the electric field at λ = 9 µm is enhanced by seven-fold while that at λ = 12 µm becomes 5-fold stronger. With the complementary coupling of SPhP with the selective emission spectrum of the SiOxNy layer, the microsphere-enhanced emitter shows a much-improved IR emissivity (ε8−13µm = 94.6%) while maintaining a high ƞε of 1.44 (Fig. 1a). The measured angular emissivity of the microsphere-enhanced emitter can maintain above 90% up to 60° emissive angle (Supplementary Fig. S9).
When PHPS is annealed at low temperatures (< 300 ºC), the film is slightly hydrophobic due to low dispersive and polar forces23. The contact angle of the PHPS-derived SiOxNy layer near room temperature is close to 90° (Fig. 2f). However, after depositing a monolayer of hydrophobic microspheres, the surface became more hydrophobic and the water contact angle reached ~ 130° (Fig. 2g). Such hydrophobic surfaces endow the emitters with water-repelling behaviours and self-cleaning capability, which may benefit their operation and maintenance in harsh outdoor environments.
The outdoor experiment in a subtropical coastal city (Hong Kong). The field test was conducted for 4 consecutive days (4–7 November 2019) on the roof of a seaside building in Hong Kong (Supplementary Fig. S10), where the daily averaged relative humidity ranges from 60–80% all over the year27. The experimental setup is shown in Figs. 3a-b. A 4-inch emitter was directly mounted on the top of a cubic high-density expanded polystyrene (EPS) foam without adopting any solar shading or convection cover (the so-called roof cooling mode). The EPS foam shows a spectrum very similar to those of commercial heat-reflective paints (R = 92%, Supplementary Fig. S11) and its thermal conductivity is only 0.035 W/ m·k28, which can minimize the absorbed heat transferring from the foam to the emitter. Type-T thermocouples were attached to the backside of the emitter via a small tunnel penetrating through the foam. To accurately measure the ambient air temperature, the heating from the concrete ground and the direct exposure of the thermometer under sunlight should be avoided, which might cause overestimation of the ambient air temperature and in turn the cooling temperature by at least 2–3°C according to our tests. Here we used a well-calibrated commercial weather station standing beside with a shutter box kept at the same height level as the emitter (about 1 meter from the floor) to accurately measure the ambient air temperature, relative humidity, solar irradiation and wind speed simultaneously
During the test, the sky was clear in the first 1.5 days but became cloudy thereafter (the cloud coverages were 47% and 32% respectively in the last two days according to the Hong Kong Observatory). The measured temperatures of ambient air and the emitters (including PHPS-derived and sphere-enhanced emitters) together with the solar irradiation are shown in Fig. 3d while the measured wind speed and relative humidity are shown in Fig. 3e. The daytime cooling effects on clear days were obviously larger than on cloudy days while the emitter with microspheres consistently showed lower temperature than that without microspheres. For the sphere-enhanced emitter, about 5°C temperature drop below the ambient temperature was observed at noontime under 800 W/m2 solar irradiation and 50% RH. This emitter maintained a cooling temperature of 3–5°C during the daytime even when the wind speed reached up to 2 m/s. When the cloud coverage increased to ~ 50%, the cooling temperature decreased to 1–2°C at noontime. In the nighttime, the relative humidity increased rapidly to 80–90%, but the temperature drop could still be maintained at 3–6°C with clear skies and 1–4°C with cloudy skies. Around 50 W/m2 of cooling power was recorded on a clear night with a humidity level of around 70% (Fig. 3c). The corresponding nonradiative heat transfer coefficient was calculated to be ~ 10 W/m2K, agreeing well with the empirical formula attained by Zhao et al29.
Comparative study of roof cooling was also conducted using two wooden model houses (60×70×85 cm) with a standard thermal insulating roof and an emitter-assembled roof, respectively. Figure 3f shows the IR image of the houses at noontime (the ambient temperature was ~ 31°C). Due to the variation in surface orientation and side wall heating by sunlight, the emitter roof temperature varied with position and the lowest temperature could reach 24°C. In contrast, the temperature of the standard roof ranged from 55°C to 67°C. The average air temperature inside the house with the cooling roof was ~ 8°C and 2°C lower than that of the reference house at daytime and nighttime, respectively (Supplementary Fig. S12). Besides, a 20×20 cm emitter was mounted on top of a 200-ml water container to cool water. Subambient water coolings of up to 7°C and 3°C were observed at nighttime and noontime, respectively (Supplementary Fig. S13). These tests show the great potential of this emitter for space cooling.
Cooling performance and durability evaluation of the emitters. Most emitters in the literature show high solar reflectances (> 94%), but their infrared emissive spectra are quite different. Two indexes have been proposed to quantify the selectivity and cooling capability of a radiative surface29. One is the weighted emissivity ε8−13µm over the 8–13 µm window at Tamb = 300 K and the other is the spectral effectiveness ƞε. Since the secondary atmospheric windows only slightly contribute to the cooling performance, ε8−13µm can indicate the emission power of an emitter and ƞε indicates the ratio of the surface emission to the absorption from the ambient, i.e., the capability in achieving a high cooling temperature in different climates9, 30.
The ε8−13µm and ƞε of various emitters for daytime cooling3, 16–18, 20, 21, 31–38 were calculated based on their spectra and plotted in Fig. 4a. Most emitters with a ε8−13µm above 90% show a relatively low effectiveness ƞε (< 1.1), indicating their broadband emitter nature. So far, the highest ƞε (1.48) was achieved by the photonic emitter developed by Raman et. al3, which showed a ε8−13µm of 65%. Our emitter exhibits strong solar reflection (96%), high IR emission (ε8−13µm = 94.6%) and outstanding selectivity (ƞε = 1.44), showing a performance closest to an ideal selective emitter compared to state-of-the-art emitters (Fig. 4a). The cooling performance of the emitter in six typical climates from MODTRAN 6 was evaluated and compared with those of an ideal selective emitter and ideal IR-broadband emitter (Supplementary Fig. S14). Our emitter’s behavior is close to that of the ideal selective emitter and can achieve a much larger cooling temperature than the ideal broadband emitter in different climates, illustrating its excellent cooling performance and climate applicability
The durability of daytime radiative emitters is crucial for long-term operation. UV exposure by sunlight and water immersion by rain are two main challenges for maintaining emitters’ cooling performances in practical applications. UV can break some chemical bonds in organic materials and in turn modify the spectrum, causing degradation and aging problems22. Moisture and oxygen can attack the reflective layer through pinholes and induce matrix variation, while some materials with pores will absorb water and affect their optical properties29. The dense all-inorganic structure of our emitter inherently endows it with excellent resistance to UV and water. The Si-O bond energy is larger than the energy of the shortest UV in sunlight (~ 300 nm) and water cannot pass through or accumulate in the dense hydrophobic SiOxNy/silica microsphere structure. To confirm this, we conducted durability tests with the emitter directly exposed to UV and water. A sphere-enhanced emitter was exposed in solar UV for up to 3 equivalent months (720 hours, 8 hours/day) and immersed in water continuously for up to 3 weeks. The spectra were measured before and after the tests for comparison and the results are shown in Fig. 4b. The spectra of the emitter were almost identical after these tests, illustrating its long-term reliability and high resistance to UV irradiation and water. For comparison, we also conducted the same durability tests for a 100-µm PDMS film, which has been widely used in passive radiative coolers. After 400-hour UV exposure, a noticeable increase in the solar absorption of the PDMS film was observed (Supplementary Fig. S15).