An advanced passive radiative cooling emitter with ultrahigh sub-ambient cooling performance

Abstract

Passive radiative cooling technologies are crucial for energy conservation and emission reduction, yet their development encounters substantial challenges. In this study, we fabricate an advanced passive radiative cooling emitter by coating BaSO₄@SiP/H-SiO₂@PAN on an aluminum substrate. This design highlights a remarkable average temperature reduction of 20.1 °C and an ultrahigh cooling power of 121.0 W m⁻² under strong solar radiation, showcasing exceptional sub-ambient cooling performance. Through meticulous design optimization, the emitter exhibits an efficient reflectance of 95.5% in the visible spectrum and a superior mid-infrared emissivity of 97.9%. Additionally, the emitter's superhydrophobic surface, characterized by a water contact angle (WCA) of 155°, ensures excellent self-cleaning properties. Furthermore, its performance remains stable even after prolonged UV exposure, demonstrating its outstanding durability. This innovative emitter provides a critical solution for energy-efficient cooling in buildings and outdoor equipment, significantly contributing to reduced energy consumption and carbon emissions.

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1 Introduction

Recently, there has been an urgent quest for innovative and sustainable cooling solutions due to both the escalating global energy demand and the imperative to reduce greenhouse gas emissions.^1–3 Traditional cooling methods, predominantly reliant on vapor-compression cycles, consume substantial electrical energy and contribute significantly to carbon footprints.^4–6 As an alternative solution, passive radiative cooling has garnered considerable attention because of its electricity- and cryogen-free advantages.^7–11 In this technology, materials or devices are designed to emit more efficiently in the atmospheric window (8–13 μm), where the atmosphere is more transparent, to achieve temperatures below the ambient without the need for external power inputs.^12–15 By minimizing reliance on conventional cooling methods, passive radiative cooling not only saves energy but also mitigates the environmental impact associated with both energy production and use. With continuous development, passive radiative cooling could become a cornerstone in the advancement of energy-efficient and environmentally friendly technologies across various domains. It has the potential to revolutionize traditional thermal management, particularly in the fields of architecture, fabrics, electronics, and the automotive industry.^16–19

In the domain of passive radiative cooling, BaSO₄ is lauded for its superior solar reflectance and infrared emissivity, which are crucial for radiative cooling technology.^20–22 These properties have been instrumental in the development of cooling coatings that can be conveniently applied to a variety of surfaces, achieving impressive sub-ambient temperatures under sunlight. Meanwhile, silicon polymer (SiP), known for its durability and weather resistance, has emerged as a promising material for radiative cooling applications.²³ As a component in radiative cooling coatings and membranes, it offers several key advantages, including flexibility, scalability, high solar reflectance, infrared emissivity, and environmental stability.²⁴ Research on BaSO₄ and SiP for radiative cooling is progressing, with a focus on improving their performance and practicality. The proposed innovative solutions hold the promise of establishing radiative cooling as a sustainable alternative to traditional cooling methods, paving the way for more energy-efficient technologies in the future.

For instance, Li et al. unveiled a groundbreaking radiative cooling technology leveraging BaSO₄ nanoparticle films and coatings,²⁵ presenting an efficient and economically viable solution for passive daytime cooling. The wide band gap of BaSO₄ facilitating superior solar reflectance and phonon resonance enables the material to reflect incoming sunlight and emit thermal radiation. Comprehensive field trials substantiate that the BaSO₄ film consistently maintains temperatures 4.5 °C below ambient conditions, achieving a commendable cooling power of 117 W m⁻². The BaSO₄-acrylic paint variant, meticulously optimized for enhanced outdoor resilience, reflects an impressive 98.1% of solar radiation and boasts an emissivity of 95%. In the study of Gao et al.,²⁶ a novel radiative cooling emitter was developed by utilizing a textured polydimethylsiloxane (PDMS) film, which significantly enhances its transparency and cooling efficiency by 2.1% and 2.7%, respectively. When integrated with a polished Al mirror, this advanced emitter achieves an impressive temperature difference of 12.8 °C and a cooling power of 103 W m⁻². As a radiator for solar cells, it markedly boosts radiative heat dissipation by 67 W m⁻², resulting in a reduction of cell temperature by over 17 °C and improving photoelectric conversion efficiency by 1.02%.

Hence, passive radiative cooling technologies provide a promising strategy for global energy savings that is both electricity- and cryogen-free.^27–29 However, optimizing cooling efficiency necessitates rigorous designs that selectively emit thermal radiation into outer space while maximizing solar reflectance. This dual-function strategy ensures effective dissipation of excess heat without counteracting absorbed solar energy. By carefully engineering surfaces and materials to reflect a significant portion of incoming sunlight while efficiently radiating infrared heat, we can substantially improve the overall cooling performance. These designs are critical for applications spanning from building roofs to wearable technology, where temperature management is vital for energy conservation and comfort.^30–33 Meanwhile, the accumulation of dust on surfaces significantly reduces their efficiency by attenuating long-wave infrared radiation transmission.^34–36 Additionally, this accumulation perturbs the spectral emissivity of the cooling layer, compromising its ability to achieve the intended sub-ambient temperatures.^37–39

In this work, we develop a novel passive radiative cooling emitter with superior properties by coating BaSO₄@SiP/H-SiO₂@PAN on an aluminum substrate. The emitter comprises three distinct layers: BaSO₄@SiP, H-SiO₂@PAN, and an aluminum substrate, which are meticulously engineered for a reflectance of 95.5% in the visible spectrum, an emissivity of 97.9% in the mid-infrared range, and the superhydrophobicity with a water contact angle (WCA) of 155°. Significantly, it demonstrates a remarkable average temperature reduction of 20.1 °C and an ultrahigh cooling power of 121.0 W m⁻² under intense solar irradiation, representing ultrahigh cooling performance. Additionally, it maintains stable properties after prolonged UV exposure, indicating its excellent durability and environmental stability, which makes it a highly promising candidate for outdoor thermal management applications. This work not only provides an effective solution to reduce reliance on traditional cooling methods, but also underscores the importance of innovative design in promoting energy efficiency and environmental sustainability in the field of passive radiative cooling.

2 Materials and methods

2.1 Materials

Silicone and the curing agent were obtained from Hubei Yichang Xingfa Group. Ethyl acetate was bought from Tianjin Fuyu Fine Chemical Co. BaSO₄ with an average particle size of 10 μm was obtained from Sinopharm Chemical Reagent Co., Ltd. Polyacrylonitrile (PAN) was purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Hydrophobic nano-SiO₂ (H-SiO₂, 7–40 nm) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. N,N-Dimethylformamide (DMF) was also acquired from the same supplier. All materials were utilized without the need for further purification.

2.2 Fabrication of the emitter

The typical fabrication process of the passive radiative emitter is as follows: a homogeneous mixture was obtained by dispersing BaSO₄ particles in ethyl acetate using ultrasonication for 10 minutes at 25 °C. Subsequently, silicone and the curing agent in a mass ratio of 20 : 1 were dissolved in the mixture and stirred magnetically for 2 hours. This suspension was then drop-coated on an aluminum sheet and allowed to dry under ambient conditions to form the BaSO₄@SiP coating (the ratio of silicone to BaSO₄ was 1 : 10). Next, PAN was dissolved in DMF with rapid agitation, followed by the addition of an equivalent amount of H-SiO₂ nanoparticles for mixing. Afterwards, the solution was stirred magnetically for 1 h until even dispersion. And then, the resulting mixture was spin-coated onto the pre-dried BaSO₄@SiP coating. Finally, the BaSO₄@SiP/H-SiO₂@PAN complex coating on the aluminum substrate was cured at 80 °C for 30 minutes to ensure optimal bonding and stability.

2.3 Measurements

The surface morphologies and elemental distributions of the samples were investigated by using Field Emission Scanning Electron Microscopy (FESEM, JSM-7500F) and Energy-Dispersive X-ray Spectroscopy (EDS), respectively. The surface roughness analysis was conducted using an atomic force microscopy (AFM) instrument (Bruker Dimension Icon). The reflectance of the samples across the UV/visible/near-infrared (UV/vis/NIR) spectrum was measured using a spectrophotometer (UV-3600, Japan). Additionally, the mid-infrared region's reflectance and transmittance (2.5–25.0 μm) were assessed using a PerkinElmer Fourier transform infrared spectrometer in conjunction with a diffuse gold integrating sphere. Material emissivity can be calculated from Kirchhoff's law: emissivity = 1 − reflectance − transmittance. The device for testing the cooling performance is a polystyrene foam cube with a cavity for the sample and an aluminum foil covering, which effectively isolate the sample from environmental heat conduction and minimize additional solar absorption. A polyethylene (PE) film may or may not be used to cover the cavity's top for different measurements. The location is Yichang, Hubei Province, China (30°71′N, 111°29′E). The net radiative cooling powers were calculated through simulation according to a reported method.³⁰ Briefly, the net radiative cooling power P_net(T) is given by the equation P_net(T) = P_rad(T) − P_sun − P_atm(T_amb) − P_{cond + conv}. P_rad(T) is the total radiation power emitted by the emitter. P_sun is the solar power absorbed by the emitter. P_atm(T_atm) is the power loss owing to the atmospheric radiation of the emitter. P_{cond + conv.} is the power loss due to conduction and convection. The cooling temperature ΔT = T − T_amb was calculated by the extraction of the cooling temperature under the condition P_net(T) = 0.

The water contact angles (WCAs) and slide angles (SAs) were measured with a 5 μL water droplet on an optical contact angle measuring instrument (Sci 3000F, Beijing Zhongchen Digital Technology Instrument Co., Ltd.). The results were determined as the average of five measurements at different points on the sample. The self-cleaning ability was evaluated through dropping water onto the surface that had been contaminated with soil. The endurance against aging of the emitter was evaluated in a UV-accelerated chamber at an irradiation intensity of 1 W m⁻² for a prolonged duration of 200 hours.

3 Results and discussion

3.1 Structures and characterization

The state-of-the-art passive radiative cooling emitter is meticulously engineered with a multi-layered coating on an aluminum substrate, which serves as a robust and thermally conductive base, as shown in Fig. 1(a). The emitter consists of three distinct layers, including 298 μm BaSO₄@SiP , H-35 μm SiO₂@PAN, and a 970 μm aluminum substrate. At the core lies the BaSO₄@SiP layer, strategically harnessing the exceptional thermal radiative properties of SiP. These materials are renowned for their prowess in efficiently emitting heat within the atmospheric transparency window while exhibiting minimal solar absorption. This layer is instrumental in ensuring that the emitter maintains a low heat absorption profile, thereby significantly contributing to the overall cooling effect. Moreover, the integration of BaSO₄ into this layer leverages its high solar reflectance capabilities. By reflecting a substantial portion of incident solar energy, BaSO₄ complements SiP by further minimizing heat absorption. The synergistic combination of BaSO₄ and SiP within a single layer optimizes both solar reflectance and mid-infrared emissivity, establishing a cornerstone for achieving superior cooling performance.

Fig. 1 (a) Illustration of the structure of the emitter; (b) reflectance in the solar spectrum (0.3–2.5 μm) and emissivity in the mid-infrared region (2.5–25 μm) of the emitter; (c) reflectance comparison and (d) emissivity comparison between the BaSO₄@SiP layer on Al substrate and the emitter.

The superhydrophobic layer, composed of PAN and H-SiO₂, not only provides high transmittance in the atmospheric window, thanks to PAN's transparency,¹⁷ but also augments surface roughness for water repellency. An appropriate surface texture can also reduce solar absorption through scattering effects. In addition, the wide bandgap nature of BaSO₄, paired with the significant polarizability of H-SiO₂, maximizes both the reflectance and emissivity across a broad spectrum, reinforcing the material's efficacy in rejecting solar heat while facilitating efficient thermal radiation into space. Moreover, the incorporation of an aluminum substrate with its excellent thermal conductivity ensures rapid and efficient heat dissipation from the cooling surface. This strategic combination not only amplifies the radiative cooling performance but also enhances the stability and durability of the system under diverse environmental conditions, marking a significant advancement in the realm of sustainable and energy-efficient cooling technologies. This integrated design reflects most of the visible light and effectively emits heat in the 8–13 μm atmospheric window, achieving exceptional radiative cooling performance and enduring outdoor stability.

The emitter demonstrates exceptional optical properties, which are crucial for effective radiative cooling applications. As shown in Fig. 1(b), it reveals that the emitter, i.e., the BaSO₄@SiP/H-SiO₂@PAN coating on the Al substrate, achieves an excellent average reflectance of 95.5% in the solar spectrum (0.3–2.5 μm) and an ultrahigh average emissivity of 97.9% in the mid-infrared region (8–13 μm). These high values enable efficient reflection of sunlight and emission of thermal radiation, thereby promoting substantial cooling effects even under intense solar exposure. The effectiveness of the coating stems from its designed structures and the unique properties of its constituent materials.

Fig. 1(c) and (d) show the comparisons of reflectance and emissivity between the BaSO₄/SiP layer on the Al substrate and the emitter. It indicates that the values of reflectance decrease slightly only within a narrow wavelength range, having almost no effect on the overall average, as shown in Fig. 1(c). Although an outer layer of H-SiO₂@PAN was added to achieve superhydrophobicity, it has virtually no impact on the reflectance of the complex coating. Meanwhile, Fig. 1(d) shows that the average emissivity of the emitter has been enhanced from 97.0% to 97.9%, compared with the BaSO₄@SiP layer on the Al substrate. It fully demonstrates that the innovative design is meticulously crafted to be rational and effective. The incorporation of an outer layer of H-SiO₂@PAN not only confers superhydrophobic properties, but also remarkably increases the emissivity, while showing a negligible impact on its reflectance. This highlights the design's exceptional finesse, adeptly balancing the coating's water-repelling capabilities with its preservation of optical excellence.

The morphologies of both layers are investigated by SEM. As shown in Fig. 2(a–d), the SEM images of the BaSO₄@SiP layer indicate that SiP forms the matrix and BaSO₄ appears densely packed and evenly embedded within this matrix. Elemental mapping for Si and Ba of the BaSO₄@SiP layer demonstrates that the BaSO₄ particles are evenly distributed in the cooling layer (Fig. 2(e and f)), which is significant for achieving high passive radiative cooling efficiency. However, the surface of the H-SiO₂@PAN layer exhibits a completely different morphology, as shown in Fig. 2(g–j). It indicates that H-SiO₂ nanoparticles have a certain degree of aggregation within specific domains, forming a characteristic rough micro-nano-structure. Elemental mapping for C and O of the coating also demonstrates that the H-SiO₂ nanoparticles are distributed throughout the PAN matrix and show a tendency for agglomeration (Fig. 2(k and l)), providing the outer layer with both low surface energy and the necessary rough structure.

Fig. 2 (a–d) SEM images with different magnification and (e and f) elemental mapping for Si and Ba of the BaSO₄@SiP layer; (g–j) SEM images with different magnification and (k and l) elemental mapping for C and O of the H-SiO₂@PAN layer; AFM images of (m and n) the BaSO₄@SiP layer and (o and p) the H-SiO₂@PAN layer.

AFM analysis is employed to meticulously examine the surface roughness of two distinct layers: H-SiO₂@PAN and BaSO₄@SiP, as shown in Fig. 2(m–p). The results reveal a notably higher roughness for the H-SiO₂@PAN layer, measuring 108.0 nm, in contrast to the BaSO₄@SiP layer, which exhibited a comparatively lower roughness of 81.9 nm. The observed increase in surface roughness correlates with a reduction in reflectance, in concordance with the data in Fig. 1(c). This phenomenon arises because the irregularities on the surface cause reflected light to scatter away from the specular direction, thereby reducing the intensity of light that is directly reflected back. Concurrently, this enhanced roughness imparts superior hydrophobic properties to the surfaces. Due to the hydrophobic nature of these nanoparticles and the resultant roughness, the surface demonstrates excellent superhydrophobicity, which will be discussed in the subsequent sections.

3.2 Outdoor cooling performances

For controlling environmental factors for the precision measurements, the outdoor cooling performance of the emitter was explored by using a custom-designed device with a PE film, as shown in Fig. 3(a). The results demonstrate that the emitter exhibits a significant temperature reduction in the sub-ambient environment during the peak daylight hours from 10 : 00 to 13 : 00 under an average solar radiation of 810 W m⁻² (Fig. 3(b)). Remarkably, the average temperature reduction in the sub-ambient environment is 20.1 °C with a peak reduction of 23.8 °C, when comparing the emitter with the air in the device. These findings highlight the exceptional cooling capacity of the emitter, demonstrating its huge potential in thermal management applications. Furthermore, the relationship between the radiative cooling power and temperature difference under different heat transfer coefficients has been simulated with a solar irradiation of 810 W m⁻² and an average sub-ambient temperature of 335 K, as shown in Fig. 3(c). Under thermal equilibrium between the emitter and the sub-ambient environment, the maximum net cooling power of the coating can reach as high as 121 W m⁻².

Fig. 3 Radiative cooling capacity of the emitter in the device with the PE film (humidity is about 34% and wind speed is 0–3 m s⁻¹): (a) photograph and schematic diagram of the device; (b) the temperatures and the temperature difference of the emitter and the air; (c) simulation of radiative cooling power with the temperature difference for different heat transfer coefficients; (d) year-round cooling energy consumption of building models in different cities; (e) year-round energy savings in different regions in China.

To evaluate the energy conservation performance of the emitter, we undertook building energy simulations by utilizing EnergyPlus. Using a representative building model depicted in Fig. 3(d) (on the top), the simulations have meticulously quantified the energy conservation potential of the emitter in reducing annual cooling energy consumption. The results reveal a substantial reduction in energy consumption across various cities, with an average decrease of 21.9% when the emitter is applied as the top-layer material. Notably, Beijing, Shanghai, and Hangzhou exhibited reductions of 19.9%, 21.9%, and 21.3%, respectively. Further analysis highlights that the emitter's potential is particularly pronounced in tropical and subtropical climates, as evidenced by the energy savings map in Fig. 3(e). The emitter's capacity to achieve cooling energy savings exceeding 40 MJ m⁻² in certain tropical and subtropical regions underscores its superior cooling capabilities. This performance is crucial in the global effort to reduce energy consumption and mitigate carbon emissions, which are central challenges for sustainable development. Integrating the emitter into building design presents a cutting-edge solution for enhancing energy efficiency and advancing sustainable building practices. It aligns with the growing recognition of the importance of low-carbon materials in construction and the need to adopt strategies that support net-zero targets in the building sector. Therefore, our findings contribute significantly to the broader discourse on energy conservation and sustainable building design, emphasizing the role of innovative materials in achieving these goals.

Moreover, a comparison of various radiative cooling materials in terms of their reflectivity, emissivity, and cooling power has been provided in Table 1. It indicates that the BaSO₄@SiP/H-SiO₂@PAN coating on the Al substrate demonstrates an efficient reflectance of 95.5% in the visible spectrum and a superior mid-infrared emissivity of 97.9%, surpassing most materials. Additionally, the emitter shows a remarkable cooling power of 121.0 W m⁻², surpassing the most values documented and highlighting its exceptional efficacy in passive cooling applications. It provides a sustainable and energy-efficient solution for managing thermal loads in various industries from construction to electronics, highlighting the pivotal role in advancing the field of passive radiative cooling technologies.

Table 1 The comparison of various radiative cooling materials with this work

Materials	Reflectance	Emissivity	Cooling power (W m^−2)	Ref.
P(VDF-HFP)/SiO2	93.0%	98.0%	103.4	7
Al2O3/SiO2 aerogels	95.0%	93.0%	133.1	27
Porous PVDF film	96.7%	96.1%	107.5	32
Al2O3@PDMS	96.0%	95.0%	—	34
Al2O3/PDMS-TiO2/PDMS	96.0%	97.0%	55.1	36
HGM-FHA	93.0%	94.0%	81.76	39
PS/PDMS/PECA	92.8%	95.4%	43.2	40
Multilayer silk textile	96.5%	97.1%	108.05	41
Poplar catkin derived film	94.5%	84.4%	75.3	42
BaSO4/SiO2/PVDF	95.00%	96.00%	85.3	43
Al(H2PO4)3/Metakaolin/BaSO4/SiO2	94.71%	96.34%	—	44
Acrylic acid/BaSO4/CaCO3/SiO2	97.60%	89.00%	94.3	45
BaSO4-acrylic paint	98.1%	95.0%	117	46
MOFs	91.00%	96.80%	26	47
POM/PTFE	95.40%	83.20%	—	48
PE/PEO	93.60%	93.90%	104.285	49
Glass/Al2O3	96.00%	95.00%	60	50
BaSO4@SiP/H-SiO2@PAN coating on an Al substrate	95.5%	97.9%	121.0	This work

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The outdoor cooling performance of the emitter was also measured by using a custom-designed device without the PE film (Fig. 4(a and b)) under an average solar irradiance intensity of 721 W m⁻² from 10 : 00 to 13 : 00, as illustrated in Fig. 4(c). Notably, the temperature disparity between the emitter and the air averages about 5.6 °C, as shown in Fig. 4(d) and (e). In addition, the relationship between the radiative cooling power and temperature difference under different heat transfer coefficients has been simulated with a solar irradiation of 721 W m⁻² and an ambient temperature of 303 K, as shown in Fig. 4(f). For simulating the cooling power under actual environmental conditions with a heat transfer coefficient of 10 W m⁻² K⁻¹, the maximum net cooling power can reach 78 W m⁻² under thermal equilibrium between the emitter and environment.

Fig. 4 Radiative cooling capacity of the emitter in the device without the PE film (humidity is about 19% and wind speed is 0–4.7 m s⁻¹): (a) photograph and (b) schematic diagram of the device; (c) solar irradiation; (d) temperature and (e) temperature difference of the emitter and the air; (f) simulation of radiative cooling power with the temperature difference for different heat transfer coefficients; (g) comparative analysis of infrared thermography for the emitter (right) and pure silicone coating (left).

To further elucidate the superior cooling efficacy of the emitter, we conducted a comparative analysis of infrared thermography between it and the pure silicone coating on the Al substrate under identical outdoor environmental conditions (Fig. 4(g)). Initially, both samples exhibit comparable temperatures. However, as time progresses, a significant divergence in their cooling performance becomes evident. The temperature difference between them expands significantly to 4.1 °C within just one hour, highlighting the notably superior thermal management capabilities of the emitter.

Overall, the cutting-edge radiative cooling emitter achieves an outstanding sub-ambient temperature reduction of 20.1 °C and a notable 5.6 °C reduction, respectively, in the different devices with or without the PE film. This breakthrough is attributed to the emitter's multi-layered design and the strategic integration of BaSO₄, H-SiO₂, and the Al substrate, which together form a high-performance cooling system. The mechanism can be interpreted as follows: the wide bandgap of BaSO₄ plays a crucial role in achieving a solar reflectance of 95.5%, significantly reducing heat absorption. This is seamlessly complemented by the role of H-SiO₂ in increasing surface roughness, reducing solar absorption, and enhancing emissivity. Facilitated by PAN's exceptional transmittance in the atmospheric window, the emissivity within this window can reach 97.9%, which is critical for efficient thermal radiation. Additionally, the ultra-white coating with high diffuse reflectance minimizes solar energy uptake across the visible spectrum, while the synergistic interplay between the BaSO₄@SiP and H-SiO₂@PAN layers optimizes thermal management. The Al substrate ensures efficient heat dissipation, reinforcing the cooling effect. This meticulously engineered emitter transcends conventional cooling methods, offering substantial temperature reduction with high performance and efficiency, thereby creating a superior solution to enhance passive radiative cooling technology.

Beyond building materials, the emitter has an impact on other sectors such as electronics cooling and the automotive industry. In the field of electronics, it can potentially reduce the thermal burden, leading to improved performance and reliability by incorporating the emitter into electronic components. In the automotive industry, the application of the emitter could enhance the efficiency of cooling systems, particularly for electric vehicles where thermal management is crucial for battery performance and longevity, which aligns with the growing global focus on sustainability and energy conservation.

3.3 Wetting properties and self-cleaning function

Fig. 5(a) and (b) show that the surface of the BaSO₄@SiP layer exhibits a WCA of 120° and a SA higher than 20°. This suggests that the layer does not reach the threshold for superhydrophobicity and thus cannot provide the necessary conditions for a robust self-cleaning effect. In contrast, the surface of the H-SiO₂@PAN layer, as depicted in Fig. 5(c) and (d), demonstrates a WCA of 155° and a SA of only 5°, which are definitive characteristics of superhydrophobic surfaces. This exceptional water-repellent behavior is primarily attributed to the presence of hydrophobic silica nanoparticles in the layer, which confer a low surface energy. Additionally, the micro- and nano-scale roughness of the surface plays a critical role in enhancing the hydrophobic effect. The combination of these features creates a surface that not only repels water but also allows water droplets to roll off easily, which is significant for the self-cleaning function. Fig. 5(e) illustrates the distinct behavior of different liquids on the surface of the emitter, i.e., the H-SiO₂@PAN layer. The spherical shape maintained by juice, milk, coffee, and tea droplets is a testament to the surface's exceptional water-repellent properties.

Fig. 5 Wetting properties and self-cleaning functions of the BaSO₄@SiP layer and H-SiO₂@PAN layer: (a) WCA and (b) SA of BaSO₄@SiP; (c) WCA and (d) SA of H-SiO₂@PAN; (e) various liquid droplets on H-SiO₂@PAN; the comparison of the self-cleaning tests for (f–h) BaSO₄@SiP and (i–k) H-SiO₂@PAN.

Fig. 5(g, h and i–k) depict the comparative self-cleaning processes of the BaSO₄@SiP layer and the H-SiO₂@PAN layer, respectively. It is evident that the BaSO₄@SiP layer lacks enough self-cleaning capabilities over the observed period (Fig. 5(g and h)). In contrast, Fig. 5(i–k) demonstrate the superior self-cleaning performance of the H-SiO₂@PAN layer. The progression observed from 0 to 10 seconds clearly demonstrates that the layer has exceptional contaminant removal capabilities, exhibiting its critical role in providing self-cleaning functionality to the emitter. This attribute is particularly valuable for outdoor applications, where the emitter is exposed to environmental elements that could compromise its performance. However, it should be clarified that while the superhydrophobic surface enhances efficiency and longevity, its self-cleaning ability is primarily effective against water-based contaminants. Non-water contaminants may not be as effectively repelled, which is a limitation to be addressed under diverse outdoor conditions.

3.4 Durability assessment

To evaluate the emitter's endurance against aging, it underwent rigorous testing in a UV-accelerated chamber at an irradiation intensity of 1 W m⁻² for a prolonged duration of 200 hours. Despite the intense exposure, the outer surface preserved its pristine ultra-white appearance (Fig. 6(a)). Furthermore, the WCA of the coating, a crucial indicator of its superhydrophobic nature, sustained a value above 150°, with a SA less than 5° after aging, as shown in Fig. 6(b). This enduring superhydrophobicity, coupled with its UV resistance, substantiates the surface's robust self-cleaning and longevity, making it an exemplary candidate for sustainable outdoor applications. Furthermore, the emitter demonstrates exceptional durability through its high solar reflectance and mid-infrared emissivity, both of which exhibit negligible degradation and consistently remain above 95% and 97% for the average values, respectively, despite being subjected to intensive UV exposure for 200 hours, as shown in Fig. 6(c) and (d). It ensures the consistent energy-saving benefits and thermal management capabilities of the emitter in outdoor environments.

Fig. 6 (a) Emitter appearance before and after UV exposure; (b) WCA and SA variation under UV irradiation (WCAs: blue bars and SAs: red bars); (c) reflectivity and (d) emissivity before and after UV exposure.

4 Conclusion

In summary, this novel passive radiative cooling emitter, realized through the strategic application of BaSO₄@SiP/H-SiO₂@PAN coatings on an Al substrate, marks a significant advancement in the field of passive cooling technology. This emitter achieves a superior average temperature reduction of 20.1 °C and an ultrahigh cooling power of 121.0 W m⁻² under intense solar irradiation. The emitter's optimal combination of an excellent 95.5% reflectance in the visible spectrum and an ultrahigh 97.9% emissivity in the mid-infrared range minimizes solar heat absorption while maximizing the emission of thermal radiation, thereby enhancing the radiative cooling effect. Furthermore, the superhydrophobic surface, with a WCA of 155° and a SA of 5°, ensures remarkable self-cleaning capabilities of the emitter, crucial for sustaining the cooling efficiency under outdoor conditions. The emitter's performance remains stable after extensive UV exposure, highlighting its environmental stability. This work is pivotal for energy conservation in various outdoor applications, reducing energy consumption and advancing sustainable radiative cooling solutions. This breakthrough emitter is poised to advance the field of radiative cooling technology with its enhanced efficiency and innovative design.

Data availability

Data for this article are available at the Science Data Bank at https://www.scidb.cn/s/MnIBzq.

Author contributions

Jiawei Huang and Weifeng Chen contributed to the writing and performed the synthesis and analysis; Qiyan Kuang contributed to the simulation; Ting Xiao and Lihua Jiang contributed to the investigation (http://159.203.176.220/contributor-roles/investigation/) and visualization; Xinyu Tan and Yizhu Lei contributed to the conceptualization and project supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the support of The National Natural Science Foundation of China (No. 22162017), the High-Level Innovative Talents of Guizhou Province (No. GCC[2023]049), Fund of Liupanshui Normal University (LPSSY2023KJZDPY01), the Natural Science Foundation of Hubei Province (No. 2022CFD036), the 111 Project of China (No. D20015), and the High-Level Talent Initiation Foundation of Liupanshui Normal University (LPSSYKYJJ202403).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta05869k

‡ The authors contributed equally.

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