A Superhydrophobic Dual-Mode Film for Energy-Free Radiative Cooling and Solar Heating.

Traditional electric cooling in summer and coal heating in winter consume a huge amount of energy and lead to a greenhouse effect. Herein, we developed an energy-free dual-mode superhydrophobic film, which consists of a white side with porous coating of styrene-ethylene-butylene-styrene/SiO2 for radiative cooling and a black side with nanocomposite coating of carbon nanotubes/polydimethylsiloxane for solar heating. In the cooling mode with the white side, the film achieved a high sunlight reflection of 94% and a strong long-wave infrared emission of 92% in the range of 8-13 μm to contribute to a temperature drop of ∼11 °C. In the heating mode with the black side, the film achieved a high solar absorption of 98% to induce heating to raise the air temperature beneath by ΔT of ∼35.6 °C. Importantly, both sides of the film are superhydrophobic with a contact angle over 165° and a sliding angle near 0°, showing typical self-cleaning effects, which defend the surfaces from outdoor contamination, thus conducive to long-term cooling and heating. This dual-mode film shows great potential in outdoor applications as coverings for both cooling in hot summer and heating in winter without an energy input.

Introduction

Passive daytime radiative cooling (PDRC) has emerged as a highly appealing technique that offered a sustainable way without an additional energy input to cool objects.[1,2] It could simultaneously reflect solar energy in the ultraviolet visible–near infrared (UV–Vis–NIR) wavelength range (0.3–2.5 μm) and send heat into the cold universe through the atmospheric long-wave infrared transparency window (8–13 μm).[3,4] At present, the radiative cooling works reported have basically achieved a good effect that can effectively cool the objects in hot summer.[5−10] However, most of the radiative cooling materials reflect and radiate heat all year round, which saves energy in summer but increases energy consumption in winter.

Therefore, it is significant to realize adaptive heating and cooling in one system to achieve energy saving all year round, including smart windows[11−15] and Janus membranes.[16,17] Kang et al.’s group[18] demonstrated a smart window combining the lower critical solution temperature (LCST) of thermal-responsive polymer hydrogels and the electrical actuation of graphene-based heaters, which modulated thermal energy by transferring from the incident sunlight. When the power was cut off, graphene was cooled and the polymer failed to reach its LCST, where it behaved transparently, allowing light to pass through to heat the object. When powered, graphene was heated and the polymer reached its LCST, where it appeared white and reflected sunlight to cool the object. Zhao et al.[19] proposed a switching strategy based on the dynamic cavitation of silicone coatings that could be tuned from a highly porous state to a transparent solid by stretching and compression. The coating in porous state could reflect light to achieve cooling, while that in transparent solid could allow light to pass through to achieve heating. Hsu et al.[20] demonstrated a dual-mode textile composed of a bilayer emitter embedded inside an infrared-transparent nanoporous polyethylene layer, in which the bilayer emitter was made up of a carbon layer and an emitting layer. It achieved heating by facing the low-emissive carbon layer outside and cooling by facing the high-emissive emitting layer outside. These reports have great potentials to save the limited energy reserves and mitigate an urban heat island effect. However, it should be noted that there are only a few works about such materials, which need improvement in heating and cooling performance, and existing materials are limited in their practical application, which require external electricity and extra force to achieve a heat-regulated switch. In addition, these materials are so easily affected by moisture and dust that their reflection and photothermal ability would decrease as time goes on and dust accumulates.[21−29] Therefore, it is necessary to develop an efficient cooling and heating material with a self-cleaning function to remove dust pollutants for long-term outdoor applications.

Here, a superhydrophobic dual-mode film was developed by a nonsolvent-induced phase separation and one-step spray method (Figure ). The film consisted of a white side with porous coating of styrene-ethylene-butylene-styrene and SiO2 nanoparticles (SEBSH/YH/SiO2) for radiative cooling and a black side with nanocomposite coating of carbon nanotube/polydimethylsiloxane (CNT/PDMS) for solar heating. Owing to the disordered micro/nanoporous structures of the SEBSH/YH/SiO2 coating in the white side, the film showed a high solar reflectance of 94% and a long-wave infrared emissivity of 92% to contribute to a temperature drop of ∼11 °C. Meanwhile, a high solar absorption of 98% was achieved by the black side of the film due to the photothermal conversion capacity and full absorption of the UV–Vis–NIR spectrum of the CNT/PDMS coating, which induced solar heating to raise the air temperature beneath by ΔT of ∼35.6 °C. Additionally, the micro/nanorough structures and the low surface energy of the cooling and heating surfaces endowed the dual-mode film superhydrophobic self-cleaning properties, which prevented the material from contamination, which thus helped maintain the cooling and heating performance for long-term outdoor applications. Moreover, this dual-mode film was made from commercially available materials via a facile, fast method; therefore, it shows great potentials in many applications.

Figure 1

Schematic illustration of the fabrication procedures of the dual-mode film.

Experimental Section

Materials

The copolymer of styrene-ethylene-butylene-styrene (SEBSH, H1062, S/EB = 18:82) was purchased from Japan Asahi Kasei Co., Ltd. The copolymer of SEBS (SEBSYH, YH602, S/EB = 35:65) was obtained from China Petrochemical Group Co., Ltd. Hydrophobic SiO2 (16 nm) was purchased from Yuanjiang Chemical Co., Ltd. (Shanghai, China). Multiwalled carbon nanotubes (CNTs; purity, 95%; 10–30 nm in diameter) were acquired from Chengdu Organic Chemistry Co., Ltd. The PDMS (Sylgard 184) precursor principal agent and its curing agent were purchased from Dow Corning, USA. Other reagents and solvents like isopropyl alcohol, chloroform, and tetrahydrofuran (THF) were bought from Sinopharm Chemical Reagent Co., Ltd., and used as received.

Fabrication of the SEBSH/YH/SiO2 Film

SEBSH and SEBSYH were first dissolved in 30 g of chloroform at a mass ratio of 6:4 to prepare SEBSH/YH solution. Then, 0.02 g of hydrophobic SiO2 was dispersed in the SEBSH/YH solution under stirring. Next, 8 g of isopropyl alcohol as a nonsolvent was slowly added to the prepared SEBSH/YH/SiO2 dispersion by a peristaltic pump under stirring, followed by sonication for 10 min to form a SEBSH/YH/SiO2 gel. The resulted gel was poured into a culture dish and dried under ambient conditions to complete the phase separation with the surrounding humidity controlled at about 65%. Finally, a SEBSH/YH/SiO2 film with micro/nanopores and microspheres was obtained. For comparison, SEBSH, SEBSYH, and SEBSH/YH films without adding SiO2 were also fabricated using the same phase separation method to serve as control samples.

Fabrication of the CNT/PDMS-SEBSH/YH/SiO2 Film

A PDMS solution was prepared by dissolving PDMS principal agent and curing agent (mass ratio of 10:1) into THF with PDMS, accounting for 6 wt % THF, and ultrasonicated for 10 min. Afterward, a given amount of CNTs was added to the PDMS solution and ultrasonicated for 20 min with the mass ratio of PDMS to CNT controlled at 2:1 to form a CNT/PDMS solution. Then, the CNT/PDMS solution was spray-coated on the above-fabricated SEBSH/YH/SiO2 film through a stencil mask using an airbrush (G222; Master Airbrush). The spraying process was carried out under ambient conditions with a nozzle to the substrate distance of ∼15 cm and airbrush pressure of ∼20 psi. Finally, the CNT/PDMS-coated SEBSH/YH/SiO2 film was dried at 60 °C for 30 min in air to obtain a CNT/PDMS-SEBSH/YH/SiO2 film.

Characterization

Morphology and Wetting States

The surface and cross-sectional morphologies of the films were measured using field emission scanning electron microscopy (SEM; Hitachi S-4800). Energy dispersive spectroscopy (EDS) was used to measure the distribution of the elements in the film. Water contact angles (CAs) and sliding angles (SAs) of the sample were measured using a video optical contact angle system (OCA 20, Data Physics, Germany) with 5 and 10 μL of water droplets, respectively. All the values of CA and SA were determined by averaging values measured at five points on each sample.

Optical Characterization

The reflectance of the films was measured separately in the ultraviolet, visible-to-near-infrared (0.3–2.5 μm), and mid-infrared ranges (2.5–25 μm). The first range measurement was taken using an ultraviolet–visible–near-infrared (UV–Vis–NIR) spectrophotometer (PE Lambda 750) with an integrating sphere and BaSO4 as the baseline reference. A Fourier transform infrared spectrometer (Nicolet IS50, Thermo Fisher Scientific) with a gold integrating sphere was used to measure the second range with an optical glass as the baseline reference. The average reflectivity (R̅sun) in the solar spectral range was calculated using eq . The average transmission (E̅TIR) of the sample was calculated using eq .where λ is the wavelength, R(λ) is the sample surface’s reflectance spectra, and Isun (λ) is the AM 1.5 solar spectral radiation defined by ISO standard 9845-1 (1992).where RTIR (λ) is the film surface’s reflectance in the thermal infrared spectrum. The emissivity spectrum was obtained by measuring the reflectivity (RTIR), which was calculated using eq , in which transmittance can be ignored (it follows Kirchhoff’s law of thermal radiation states). IB (λ,T) is the blackbody radiation at temperature (T) according to Planck’s law (eq ), where c is the speed of light, λ is the wavelength, h is Planck’s constant, and KB is the Boltzmann constant.

Cooling and Heating Performance Measurement

A self-made device was designed to measure the cooling and heating performance of the obtained dual-mode film. The film was placed, covering a 5 × 5 × 1 cm air cavity in a polystyrene foam wrapped with a layer of aluminum foil for reflecting sunlight to reduce solar heating. The apparatus was placed on a 1 m-high desk to reduce the influence of heat radiation from the rooftop heated by the sun. Furthermore, an infrared transparency polyethylene film (PE, 0.04 mm in thickness) was used to seal an air space above the setup to shield against convection and conduction with the environment. A four-channel K thermocouple (Yili Technology Co., Ltd., China) was used to detect the real-time temperature. The ambient air temperature around the sample was also tested. In addition, the solar irradiance and relative humidity were recorded by a data logging solar power meter (TES-132) and hygrothermograph (TH10R) around the device at the same time. The infrared images were taken by a thermal infrared camera (E6390). The indoor test device is smaller than that for outdoors, which has the same design concept and test method. A xenon lamp (Zhongjiao Jinyuan HXF300) with a high power that matches well with the solar spectrum was used to simulate solar radiation.

Results and Discussion

Morphologies of the CNT/PDMS-SEBSH/YH/SiO2 Film

The structures both inside and on the surface of the film are very important to obtain cooling and superhydrophobic properties. In the preparation process, a phase separation method[30−32] was adopted to adjust the microstructure of the film. It was found that the ratio of S to EB in SEBS influenced greatly the morphology of the SEBS film, although they have the same chemical structures (Figure S1). The SEBSH film consists of spheres of about 2 μm on the surface, while SEBSYH has pores of about 5 μm with smooth walls (Figure a,b). When SEBSH and SEBSYH were mixed together, the obtained SEBSH/YH film consists of microscale pores with microscale particles decorated by spheres of about 2 μm, making the surface rough and hydrophobic with a CA of 135° (Figure c). When hydrophobic nanoparticles of SiO2 were further incorporated, the as-obtained SEBSH/YH/SiO2 film possessed microspheres decorated with nanoparticles of SiO2, forming a typical micro/nanostructured surface and making the film show a CA of 162° (Figure d and Figure S2). The SiO2 roughened surface of the SEBSH/YH/SiO2 film might help enhance the scattering of solar light.

Figure 2

SEM images of (a) SEBSH film, (b) SEBSYH film, (c) SEBSH/YH film, (d) and SEBSH/YH/SiO2 film; (e) magnification of SEBSH/YH/SiO2 in the cross section of the CNT/PDMS-SEBSH/YH/SiO2 film; (f) cross section of the CNT/PDMS-SEBSH/YH/SiO2 film; (g) magnification of CNT/PDMS at the cross section of the CNT/PDMS-SEBSH/YH/SiO2 film.

To obtain a dual-mode film, CNT/PDMS coating was made on one side of the SEBSH/YH/SiO2 film. The cross-sectional SEM image of the CNT/PDMS-SEBSH/YH/SiO2 film clearly depicted that the thickness of the film was about 1510 μm (Figure f), in which the CNT/PDMS coating was about 10 μm. The SEBSH/YH/SiO2 side was porous in its interior with abundant microscale spheres and SiO2 aggregation (Figure e), which provides space for air to stay inside the film, favoring the enhancement of emissivity. The CNT/PDMS side was full of entangled nanowires of CNTs, roughening the coating surface (Figure g). The rough nanostructured surface in combination with the low surface energy substance of PDMS made the film at a mass ratio of CNT:PDMS = 1:2 superhydrophobic with a CA of 165° (Figure S3).

Superhydrophobic and Optical Properties of the Dual-Mode Film

The superhydrophobic surface usually has a water droplet contact angle of greater than 150° and the rolling angle of less than 10°. Water droplets are easy to roll on such a surface and take away the stains during the rolling process. The micro/nanorough structure and low surface energy property are two necessary conditions for constructing a superhydrophobic surface. The CNT/PDMS-SEBSH/YH/SiO2 film has two sides with contrast colors (Figure a), on both of which the stained water droplets exhibited spherical shapes, showing excellent liquid repellency (Figure b,c). When the film was dipped into water, a bright plastron formed on the surface because of the trapped air layer reflecting incident light (Figure d). When water droplets were put on the film surface, they rolled off easily (Movie 1). The black color of the other side was due to the full absorption of the visible spectrum of CNT in the CNT/PDMS coating, which only had an average reflectance of 2% in the 0.3–2.5 μm wavelength range. The white side consists of SEBSH/YH/SiO2, which is porous inside and rough on the surface, causing strong scattering of visible light and giving an ultrawhite appearance, which is ascribed to the mismatch of the refractive index between the air inside the pores, SiO2 particles, and the SEBSH/YH copolymers. Calculation showed that the porous side of SEBSH/YH/SiO2 on a 1021 μm-thick film at a mass ratio of SEBSH:SEBSYH = 6:4 (Figure S4) possesses a reflectance of 94% in the solar spectral region and an emissivity of 92% (Figure e). It was because of the finding that the film consists of rich micro/nanopores and microspheres in the 0.2–20 μm range (Figure f) with a percent porosity of 98% (Figure i), favoring scattering sunlight and emitting heat.[33−35]

Figure 3

Optical and superhydrophobic properties of the CNT/PDMS-SEBSH/YH/SiO2 film. (a) Photograph of the film; (b, c) photograph of stained water droplets on the black side and white side of the film; (d) photograph of the film stuck to a glass slide in water; (e) reflectance spectra of the film with a thickness of 1020 μm at the AM1.5 spectra (yellow-shaded area) and an atmospheric transparency window (blue-shaded area) with the red color curve for the white side and the blue color curve for the black side; (f) statistical distribution of the pore diameters of the SEBSH/YH/SiO2 film. (g) Reflectance spectra of SEBSH, SEBSYH, SEBSH/YH, and SEBSH/YH/SiO2 films; (h) emissivity spectra of SEBSH, SEBSYH, SEBSH/YH, and SEBSH/YH/SiO2 films; (i) porosity of the SEBSH/YH/SiO2 film.

For comparison, the emissivity and reflectance of the films of SEBSH, SEBSYH, SEBSH/YH, and SEBSH/YH/SiO2 were also investigated. Calculations from Figure g showed that the SEBSYH film presents a lower average solar reflectance of 88% in the UV–Vis–NIR wavelengths because it consists only of large micropores in the range of 5–10 μm (Figure b), whereas the SEBSH, SEBSH/YH, and SEBSH/YH/SiO2 films exhibit high reflectance values of 91, 91, and 94%, respectively, because they consist of not only microscale pores but also microscale spheres, which scatter sunlight effectively.[36,37] Additionally, it was found that addition of SiO2 made a positive effect on improving the reflectivity (Figure g). The emission spectra show that the SEBS-based film displays high emissivity. From Figure h, the average emissivity values of the films of SEBSH, SEBSYH, SEBSH/YH, and SEBSH/YH/SiO2 were calculated to be 68, 82, 87, and 92%, respectively, in which the SEBSH/YH/SiO2 film shows the highest emissivity in the LWIR transparency window. It was obviously demonstrated that not only incorporation of the micro/nanoporous structure enhanced the emissivity[38,39] but also adding SiO2 due to the existence of a Si–O bond.[40,41]

Cooling and Heating Performance of the Dual-Mode Film

The radiative cooling performances of the films with a thickness of ∼1050 μm and a dimension of 9 cm × 9 cm were measured on the rooftop of a building under a clear and breeze sky in Xi’an (108.97°E, 34.38°N, Shaanxi, China) using a self-made setup (Figure a,b). It was found that both the SEBSH film and the SEBSYH film show a cooling ability, obtaining lower temperatures than the PE-shielded air (Figure d), indicating that SEBS is suitable to be a candidate for radiative materials. When SEBSH and SEBSYH were mixed together, the temperature obtained by SEBSH/YH was further decreased, which might be caused by the strengthened reflectance and the enhanced emittance due to the micro/nanostructure in the SEBSH/YH film as mentioned in Figure d. Additionally, when SiO2 nanoparticles were incorporated, the obtained SEBSH/YH/SiO2 film displayed a further decreased temperature, which might be resulted from the enhanced emissivity by a Si–O bond in SiO2. Figure e shows that, compared with the temperature around the film under a PE cover in the daytime with an average solar intensity (Isolar) of 900 W/m2, the SEBSH, SEBSYH, SEBSH/YH, and SEBSH/YH/SiO2 films got temperature drops ΔT of 6.7, 5.5, 8.3, and 11.3 °C, respectively, showing obviously increased cooling performance, which is in accordance with their reflectivity and emissivity mentioned above.

Figure 4

Cooling and heating performance of the dual-mode film. (a) Picture of the device on the rooftop; (b) schematic of the device for testing the cooling and heating performance; (c) diagram of the indoor-simulated heating device; (d) temperature tracking of air and the films in summer; (e) solar intensity and the temperature drop (ΔT) of panel (d); (f) temperature tracking of the air and dual-mode film in winter; (g) outdoor heating temperature profiles of the dual-mode film and ambient air; (h) solar intensity and the temperature rise (ΔT) in panel (g); (i) temperature tracking of the air and dual-mode film under indoor-simulated heating.

To fabricate a dual-mode film, the SEBSH/YH/SiO2 film was coated with CNT/PDMS. Excitedly, the obtained CNT/PDMS-SEBSH/YH/SiO2 film with the white side facing outside maintained excellent cooling ability with an average temperature drop ΔT of 11 °C similar to that of SEBSH/YH/SiO2, which indicates that coating of CNT/PDMS affected slightly the cooling ability of the SEBSH/YH/SiO2 film. The nocturnal radiation capacity measured by the same setup displayed an average temperature drop ΔT of 3 °C (Figure S5), showing cooling ability in nighttime. When the CNT/PDMS-SEBSH/YH/SiO2 film was directly exposed with the white side facing the ambient air without any convection shielding in daytime, it got a temperature drop ΔT of 7 °C (Figure S6). When using a xenon lamp as a standard solar light with irradiation of Isolar ≈ 1000 W/m2 indoor, the dual-mode film can also achieve a temperature drop ΔT of 10 °C (Figure S7). These results demonstrated that the CNT/PDMS-SEBSH/YH/SiO2 film with the white side facing out works in the cooling mode, which could be applied in hot weather such as summer. Additionally, to better express the cooling effect of the film, we calculated the theory radiative cooling power of the dual-mode film (Figure S8), the power of average solar radiation is set to approximately 837.59 W/m2, and the ambient temperature Tamb is assumed to be 312.15 K. The calculated maximum net cooling power is 99.8 W/m2 at thermal equilibrium, which intuitively reflects excellent abilities of the dual-mode film for energy saving in summer.

However, when the CNT/PDMS-SEBSH/YH/SiO2 film was applied in winter with the white side facing out, it still achieved a temperature drop ΔT over 6 °C (Figure f) under Isolar ≈ 700 W/m2 (Figure S9). Such cooling in cold weather goes against energy saving because it might cause extra energy consumption in heating for warming. In fact, outdoor application with the black coating of CNT/PDMS facing outside in cold weather was the design intention of the film, which works in heating mode. When the film was applied in heating mode in a simulated cold environment (−20 °C) under a xenon lamp with irradiation of Isolar ≈ 1000 W/m2 (Figure c), its temperature was much higher than that of the surrounding air with a temperature rise ΔT of ∼40 °C (Figure i). Practical application in winter under an average Isolar of 700 W/m2 (Figure h) showed that the temperature of the air covered by the film is much higher than that of the PE-shielded air with a temperature rise ΔT of 35.6 °C (Figure g), demonstrating excellent heating performance in cold weather. The integration of CNT/PDMS coating with SEBSH/YH/SiO2 into one film, namely, forming CNT/PDMS-SEBSH/YH/SiO2, obtained a material with two modes, which cools in summer and heats in winter.

To further evaluate and analyze the effects of the environment on the radiative cooling performance of the dual-mode film, measurements were conducted about the temperature changes under different light intensities with different humidities. On a sunny day with a light intensity of up to ∼900 W/m2 (Figure a), when the setup was placed on the rooftop with a surrounding humidity of ∼20.4% (Figure b), the temperature drop was 9.7 °C on average (Figure c); when the setup was placed on the lawn with a surrounding humidity of ∼44.3%, the temperature drop was 9.0 °C on average (Figure d–f). On a cloudy day with a light intensity of up to ∼700 W/m2 (Figure g), when the setup was placed on the rooftop with a surrounding humidity of 53.6% (Figure h), the average temperature drop was 7.2 °C (Figure i). It is obvious that the cooling capacity of the film depends on the air humidity and the supply of solar light, which is in accordance with a previous report.[42]

Figure 5

Cooling performance in various humidities and light intensities. (a–c) Solar irradiance, relative humidity, and temperatures of the ambient air and dual-mode film on a sunny day. (d–f) Solar irradiance, relative humidity, and temperatures of the ambient air and dual-mode film on a sunny day. (g–i) Solar irradiance, relative humidity, and temperatures of the ambient air and dual-mode film on a cloudy day.

To further investigate the heating performance of the dual-mode film, the setup was placed inside a refrigerator with different temperatures under different irradiation intensities. Figure a shows that irradiation of light increased quickly the temperature of the cavity covered by the film to a steady state in 5 min when the inside temperature of the refrigerator was set at 22 °C. Also, the steady temperature of the film increased with the increase in light intensity. Under a light intensity of 1 kW/m2, the temperature underneath the film reached up to 75 °C. When the refrigerator was set at −20 °C, irradiation of the film obtained a temperature rise ΔT of about 40 °C (Figure b). When the refrigerator was set at −10 or 10 °C, the temperature rise was about 30 °C (Figure c,d). Infrared measurements under the simulated solar irradiance also confirmed the great heating capability of the dual-mode film (insets in Figure b–d). It was obviously shown that this dual-mode film could work for heating in winter.

Figure 6

Solar heating performance of the dual-mode film in various cold environments provided by a refrigerator. (a) Temperature profiles of the film with irradiation time under different solar intensities when the refrigerator was set at 22 °C. Temperature profiles of the film and air close to the film when the refrigerator was set at (b) −20 °C, (c) −10 °C, and (d) 10 °C under irradiation of 1 kW/m2.

Practical Applications of the Dual-Mode Film for the Shutter and Model Car

To demonstrate both the radiative cooling capability and solar heating capability of the dual-mode film on real objects, the film was applied in a self-made shutter as the blades. Infrared pictures were used to show the temperature difference between the film and the ambient air when the film was rotated clockwise to different angles. It was found that, when the blades were put at angles of 0°, 30°, 90°, 120°, and 180°, respectively (Figure a–e), the temperature of the blades increased correspondingly (Figure a1–e1). When the blades were set at 0° with the white side facing up (Figure a), the temperature under the blades was much lower than that of the ambient air (Figure f). The film worked in the typical cooling mode, obtaining a temperature drop of about 15 °C (Figure g). When the blades were rotated 30°, the temperature was gradually increased but still below the ambient temperature, indicating the cooling effect of the film. When the blades were rotated 120°, the temperature was increased gradually over the ambient temperature, showing a heating effect. At the angle of 180°, all the blades covered the whole surface above the shutter with the black side facing up and the temperature was much higher than that of the ambient air.

Figure 7

Practical applications of the dual-mode film. (a–e) Picture of the dual-mode film applied to a shutter and pictures of the blade rotated 0°, 30°, 90°, 120°, and 180°, respectively. (a1–e1) Infrared images of the shutter with the dual-mode film corresponding to panels (a)–(e). (f) Temperature profiles of 120 min inside the shutter with the blades at 0°, 30°, 120°, and 180°. (g) Temperature difference corresponding to panel (f). (h–j) Pictures, infrared images, and temperature profiles of the dual-mode film and commercial cooling material used for car model cooling.

The film worked in the typical heating mode, obtaining a temperature rise of about 20 °C (Figure g). In addition, three identical car models were placed on the floor with one naked, one covered with a dual-mode film, and another covered by a commercial cooling material, which consisted of a thin plastic layer covered with aluminum foil (Figure h). The infrared images showed that the dual-mode film temperature was cooler than that of the commercial material and much cooler than that of the naked car model (Figure i). Figure j shows that the average temperatures on the surface of the bare car, underneath the commercial material, and underneath the dual-mode film were 49.3, 42.3, and 36.9 °C, respectively, indicating that the dual-mode film possesses greater cooling ability than the commercial material.

Self-Cleaning and Durability Performance of the Dual-Mode Film

As a material for outdoor applications, its surface inevitably suffers from the rain flushing and dust settling, which might affect the optical property and weaken the cooling performance as well as the heating ability. Inspiringly, the dual-mode film possesses unique superhydrophobicity on both sides, which makes the stains on the film easily taken away by water droplets (Figure a,b and Movie 2). Further, the cooling abilities of the film were evaluated before staining and after self-cleaning of the stains. The result shows that the self-cleaning effect of the white surface made the stains easily removed away and maintained the cooling function with a temperature drop ΔT of 10 °C, which is close to the original film with a drop ΔT of 11 °C (Figure c). Similarly, the superhydrophobic self-cleaning defends the black side from stain contamination and maintains the heating ability with a temperature rise by 35 °C, which is close to the original surface with a rise by 35.6 °C (Figure d). This shows the importance of making the film have superhydrophobicity, which favors long-time outdoor applications of the dual-mode film.

Figure 8

Self-cleaning performance of the dual-mode film. (a) Self-cleaning test for the white side. (b) Self-cleaning test for the black side. (c) Temperature profiles of the white side of the film before and after the self-cleaning test and the PE-shielded air. (d) Temperature profiles of the black side of the film before and after the self-cleaning test and the PE-shielded air.

Additionally, the durability of self-cleaning was evaluated by immersion of the samples in solutions with different pH values for 5 days (Figure S10). It was found that the CAs were more than 150° and SAs were less than 10° on both sides of the dual-mode film after immersion. Also, the cooling and heating abilities changed slightly. The thermal stability of the film was tested by thermogravimetric analysis (Figure S11). It was found that the dual mode film can sustain the temperature of about 350 °C, exhibiting good thermal stability. This provides an effective guarantee for the long-term outdoor application of the dual-mode film.

Conclusions

A superhydrophobic dual-mode film was fabricated by phase separation of SEBSH/YH/SiO2 dispersion followed by CNT/PDMS coating. The film consists of a porous white side of SEBSH/YH/SiO2 with strong solar reflectivity and heat emissivity and a black side of CNT/PDMS with high solar absorptivity. The white side enables the film to cool objects in hot summer and the black side enables the film to heat objects in cold winter. The superhydrophobicity of the film protected the surface from contamination and endowed the film with sustainable cooling and heating performance. Also, the two modes of the film have been successfully demonstrated to be easily switched by a smart shutter through deliberate design, allowing a wide range of radiative cooling and solar heating systems to be constructed with cooling and heating abilities. We proposed that the integration design provides a way to achieve synergistic multifunction and will unleash the immense potential for modern intelligent home in energy saving and sustainable cooling and heating indoor.