A Large-Scale Preparation Approach for Daytime Radiative Cooling Using SiO₂ Hollow Microsphere Composite Film

Abstract

Radiative cooling is a passive cooling strategy that dissipates heat externally through the atmospheric window (8–13 μm). This study presents a radiative cooling film with a simple and cost-effective fabrication process. The film was fabricated by mixing SiO₂ hollow microspheres with a UV-curable resin, employing a photopolymerization-induced phase separation method. The resulting gradient refractive index structure enhanced thermal radiation emissivity. At an optimal silica-to-resin mass ratio of 1:1.5 and a film thickness of 1.1 mm, the film achieved a solar reflectivity of 85% and an emissivity of 91% within the atmospheric window. Outdoor experiments conducted in both summer and winter demonstrated stable cooling performance. Under a solar irradiance of 796.9 W/m² (summer), the film reduced surface temperature by 10 °C compared to ambient air and 20 °C compared to an uncoated glass substrate, achieving a radiative cooling power of 76.7 W/m². In winter (solar irradiance of 588.8 W/m²), the film maintained a significant cooling effect, though with reduced efficiency due to lower solar exposure. Furthermore, long-term stability tests over six months showed that the film retained high solar reflectivity and infrared emissivity, indicating good durability. Overall, the developed radiative cooling films demonstrate excellent optical properties, structural stability, and cooling efficiency, making it a promising candidate for real-world radiative cooling applications. Further studies on environmental resilience and optimization under diverse climatic conditions are necessary for broader deployment.

1. Introduction

Radiative cooling is an innovative passive cooling technology that uses thermal radiation to dissipate heat externally, and is regarded as a natural heat sink [1,2,3,4]. Unlike the traditional cooling methods, which depend on mechanics and electricity, radiative cooling works without external energy, offering significant energy saving and environmental benefits. The process relies on emitting infrared radiation, particularly within the atmospheric windows with a wavelength range of 8–13 μm, where the earth’s atmosphere exhibits a high transparency, minimizing the absorption by gases like water vapor, carbon dioxide, etc. [5,6,7,8]. Materials designed with a high emissivity in this range enable efficient heat transfer, providing cooling without conventional energy use. Radiative cooling holds vast potential in applications such as reducing building energy consumption, enhancing solar cell efficiency, and developing passive cooling clothing. It can mitigate urban heat island effects and contribute to sustainable energy solutions. With advancements in material science and surface engineering, radiative cooling is poised to play an important role in addressing global energy challenges.

In recent years, research on radiative cooling has gained widespread attention [9,10]. Raman et al. developed a multilayer structure comprising seven alternating layers of HfO₂ and SiO₂ with different thicknesses [11]. The design featured the bottom four layers of HfO₂ and SiO₂ to enhance solar reflection, while the top three layers were optimized for infrared radiation through the atmospheric window. By leveraging the combined effects of material properties and interference, these layers yielded high solar reflectivity and robust thermal radiation. The structure boasted a solar reflectivity of up to 97% and could cool to a temperature of 4.9 °C below the ambient temperature under direct sunlight with an intensity exceeding 850 W/m². Lu et al. proposed a novel radiative cooling material based on a composite of aluminum oxide and silver [12]. By optimizing the structure and composition of the material, they achieved a high reflectivity within the solar spectrum and a low emissivity in the thermal infrared range. Mandal et al. [13] utilized a chemical phase separation method to fabricate a polymer known as poly (vinylidene fluoride-co-hexafluoropropene) (P (VdF-HFP) HP) into a multi-scale porous coating. The emissivity curve of this coating closely approximated the ideal curve for radiative cooling at ambient temperatures. It showed a reflectivity of up to 96% within the solar spectrum and an emissivity of 97% in the long-wave infrared (LWIR) range.

Nanomaterials and polymer coatings have demonstrated excellent performance in radiative cooling applications. For instance, Rephaeli et al. [14] developed a radiative cooler based on SiC and AlN, and designed a two-dimensional circular hole array photonic crystal structure on these materials to enhance the optical absorption in the atmospheric window, which markedly improved light absorption within the atmospheric window; SiO₂ has also been extensively studied due to its favorable optical properties [15]. Zhao et al. [16] fabricated a silica-based mirror emitter by depositing a silver layer onto a SiO₂ substrate, achieving significant cooling performance. Similarly, Zhai et al. [17] designed a polymeric metamaterial incorporating randomly distributed SiO₂ microspheres, which exhibited a high solar reflectivity of 96% and a mid-infrared emissivity of 0.93. However, despite their outstanding optical properties, these nanomaterial-based approaches often require precise deposition techniques or intricate nanostructure fabrication, making them complex and cost-intensive to manufacture.

On the other hand, polymer coatings have gained attention due to their low cost and large-scale applicability. Wang et al. [18] developed a superhydrophobic polymer composite coating (PS/PDMS/PECA) with a hierarchical porous structure, achieving an average solar reflectivity of 96%; Li et al. [19] employed an electrospinning process to fabricate a hierarchically designed polymer nanofiber membrane, which demonstrated selective mid-infrared emission, efficient solar reflection, and excellent all-day radiative cooling performance. Although polymer-based coatings show promising cooling performance by reducing solar absorption and enhancing infrared radiation, they may suffer from aging and weathering issues, potentially limiting their long-term stability.

Despite the promising results of the aforementioned radiative cooling materials, the practical applications still encounter many challenges [20,21,22]. Therefore, based on these studies, this study introduces a radiative cooling strategy based on a SiO₂ hollow microspheres and ultraviolet-curable adhesive that achieves high solar reflectivity and mid-infrared emissivity while optimizing the fabrication process for simplicity and large-scale application. Moreover, the proposed film demonstrates superior environmental stability, ensuring sustained cooling performance over extended periods. This work provides a promising approach for advancing passive radiative cooling technologies toward practical implementation.

2. Materials and Methods

2.1. Theoretical Calculations

Thermal radiation is a mode of heat transfer that relies on the emission and propagation of electromagnetic waves [23]. Any object at a temperature releases heat to the environment in the form of radiation. The ability to release this heat is related to the object temperature, surface characteristics (emissivity), and wavelength. Its core principle is based on the Stefan–Boltzmann law. For an ideal black body, which is a perfect emitter and absorber of radiation, the radiant power is proportional to the fourth power of its absolute temperature, expressed by the following:

$$P=\sigma A\left(T^4-T_0^4\right),$$

(1)

where $P$ is the thermal radiation power, $A$ is the surface area of object, $\sigma$ is the Stefan–Boltzmann constant, $T$ is the temperature of physical surface, and $T_0$ is the temperature of environment.

In thermodynamic equilibrium, the spectral radiation of a black body can be described by Planck’s law, which is as follows:

$$I_{\mathrm{B}\mathrm{B}}={\displaystyle \frac{2\mathrm{h}{\mathrm{c}}^2}{{\lambda }^5}}{\displaystyle \frac{1}{{\mathrm{e}}^{\frac{\mathrm{h}\mathrm{c}}{\left(\lambda {\mathrm{k}}_{\mathrm{B}}T\right)}}-1}}.$$

(2)

An ideal black body is a perfect absorber and emitter of radiation across all wavelengths, with an emissivity of 1. In contrast, the emissivity of real materials is less than 1, and their radiation characteristics might depend on both wavelength and temperature. Objects on the earth dissipate heat through thermal radiation ($P_{\mathrm{r}\mathrm{a}\mathrm{d}}$), while also absorbing it from the atmosphere ($P_{\mathrm{a}\mathrm{t}\mathrm{m}}$) and solar irradiance ($P_{\mathrm{s}\mathrm{u}\mathrm{n}}$) during the day. Additionally, they exchange heat with the environment through non-radiative processes, such as convection and conduction. Consequently, the net heat flux associated with radiative cooling ($P_{\mathrm{c}\mathrm{o}\mathrm{o}\mathrm{l}}$) can be expressed as follows:

$$P_{\mathrm{c}\mathrm{o}\mathrm{o}\mathrm{l}}=P_{\mathrm{r}\mathrm{a}\mathrm{d}}-P_{\mathrm{a}\mathrm{t}\mathrm{m}}-P_{\mathrm{s}\mathrm{u}\mathrm{n}}-P_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}+\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}},$$

(3)

$$P_{\mathrm{r}\mathrm{a}\mathrm{d}}=A\int \mathrm{d}\Omega \mathrm{c}\mathrm{o}\mathrm{s}\theta {\int }_0^{\infty }\mathrm{d}\lambda I_{\mathrm{B}\mathrm{B}}\left(T,\lambda \right)\epsilon \left(\lambda ,\theta \right),$$

(4)

$$P_{\mathrm{a}\mathrm{t}\mathrm{m}}=A\int \mathrm{d}\Omega \mathrm{c}\mathrm{o}\mathrm{s}\theta {\int }_0^{\infty }\mathrm{d}\lambda I_{\mathrm{B}\mathrm{B}}\left(T,\lambda \right)\epsilon \left(\lambda ,\theta \right){\epsilon }_{\mathrm{a}\mathrm{t}\mathrm{m}}\left(\lambda ,\theta \right),$$

(5)

$$\begin{array}{c}{P}_{\mathrm{s}\mathrm{u}\mathrm{n}}=A{\int }_0^{\infty }\mathrm{d}\lambda \epsilon \left(\lambda ,{\theta }_{\mathrm{s}\mathrm{u}\mathrm{n}}\right)I_{\mathrm{A}\mathrm{M}1.5}\left(\lambda \right),\end{array}$$

(6)

$$P_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}+\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}}=A{h}_{\mathrm{c}}\left(T_{\mathrm{a}\mathrm{m}\mathrm{b}}-T\right),$$

(7)

where $I_{\mathrm{A}\mathrm{M}1.5}\left(\lambda \right)$ is the solar irradiance, $\epsilon \left(\lambda ,\theta \right)$ is the emissivity, ${\epsilon }_{\mathrm{a}\mathrm{t}\mathrm{m}}\left(\lambda ,\theta \right)$ is the spectral emissivity of the atmosphere, determined by wavelength $\lambda$ and angle $\theta$, and $h_{\mathrm{c}}$ is the sum of $h_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}}$ and $h_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}}$, representing the combined non-radiative heat transfer coefficient, indicating the collective effect of conduction and convection heating due to the contact of the radiative cooler with the external surface and nearby radiative coolers in the air. $I_{\mathrm{B}\mathrm{B}}$ is the radiant intensity of black body radiation at temperature $T$ for wavelength $\lambda$. $\mathrm{h}$ is Planck’s constant, ${\mathrm{k}}_{\mathrm{B}}$ is the Boltzmann constant, $T$ is the temperature of the black body, $\mathrm{c}$ is the speed of light, and $\lambda$ is the wavelength.

2.2. Fabrication

Figure 1a schematically depicts the experimental procedure. In the first step, SiO₂ hollow microspheres [24] were carefully mixed with an ultraviolet-curable adhesive, which was placed in a clean glass container [25,26]. This mixture was then subjected to ultrasonication for 1 h to ensure the homogeneous dispersion of the microspheres in the adhesive matrix. Ultrasonication was performed at a frequency and power level optimized to prevent agglomeration of the microspheres, ensuring a well-dispersed solution.

Following sonication, the resulting dispersion was applied onto a clean glass substrate using the scrape coating method, a well-established technique for achieving a uniform and controlled coating thickness. The coated substrate was then exposed to ultraviolet (UV) light, initiating the polymerization of the adhesive and effectively curing the film. The UV exposure parameters, including intensity and duration, were meticulously controlled to ensure thorough curing while preserving the structural integrity of the microspheres which are critical for the radiative cooling functionality.

Figure 1b shows a photograph of the final sample where the cooling film is visible. The film exhibited a smooth, homogenous surface and a distinctive white color, characteristic of the SiO₂ microspheres’ high reflectivity. This color indicates the film’s capacity for effective radiative cooling as the microspheres are designed to reflect a broad spectrum of sunlight and efficiently radiate heat. The white appearance of the film is not only a visual indicator of its cooling potential but also underscores the importance of the microsphere–adhesive composite in enhancing the overall performance of the radiative cooling layer. On the basis of our cost estimation, the material cost can be as low as ~$2/m² without compromising performance, making it more economically feasible for large-scale production compared to traditional lithography or vacuum deposition techniques.

Throughout the process, a series of samples were prepared by varying the mass ratio of SiO₂ to ultraviolet-curable adhesive and adjusting the film thickness. These samples were then subjected to optical testing [27,28]. Reflectivity and emissivity measurements were performed on the fabricated cooling films to evaluate their performance.

2.3. Characterization

Optical performance measurements [29,30] of the fabricated films were conducted using a UV-visible-near-infrared spectrophotometer [31] and a Fourier-transform infrared spectrometer. The UV-visible-near-infrared spectrophotometer used was the UV 3600 Plus, (Shimadzu, Kyoto, Japan), with a barium sulfate integrating sphere (model ISR-603) applied to measure the reflectivity/transmissivity in a wavelength range of 0.2–2.5 µm. Background correction with barium sulfate was performed. The Fourier-transform infrared spectrometer used was the Nicolet iS50, Thermo Fisher, (Waltham, MA, USA), equipped with a Pike gold-coated integrating sphere and an MCT liquid nitrogen detector for the reflectivity/transmissivity in a wavelength range of 2.5–15.3 µm. Background correction was performed with gold 128 scans and a resolution of 4.0.

Scanning electron microscopy (SEM) is a powerful imaging technique that utilizes a focused electron beam to interact with the surface of a sample. This interaction generates signals such as secondary electrons, backscattered electrons, and characteristic X-rays. These signals provide detailed information on the topography, composition, and other surface properties of sample.

In addition, the emissivity measurements were conducted with an IR-2 dual-band emissivity meter [32,33], developed by the Shanghai Institute of Technical Physics. This instrument was configured with a specialized temperature control heating device. The normal reflectivity of the sample surface was measured using an active blackbody radiation source, which was then used to determine the normal emissivity in specific infrared bands.

To assess the practical cooling performance of the fabricated radiative cooling films, we constructed a custom solar radiation temperature measurement setup [34], as Figure 2 shows. A polystyrene box with its surface entirely covered by aluminum foil [35] was used. Inside this box, four smaller polystyrene chambers were placed. Each chamber was covered with either the radiative cooling film or a non-coated substrate, making isolated cooling environments [36,37]. Thermocouples were used to measure temperatures in different regions, with sensors inserted through the bottom of each small chamber and connected to each radiative cooler. An additional thermocouple was placed in the larger foam box to monitor temperature changes. In addition, to minimize the influence of air flow and prevent non-radiative heat exchange during measurements the large foam box was covered with a layer of polyethylene film and the thermocouple insertion points were sealed. A solar radiometer was positioned next to the measurement box to continuously record solar irradiance. All data were logged every second using a laptop connected to the measurement equipment.

3. Results and Discussion

3.1. SEM Morphology Analysis

Figure 3 presents the SEM images of the sample, where images (a–d) show the surface morphology of the fabricated film at different magnifications. However, due to the curing process of the UV adhesive, a smooth barrier was formed on the film surface, resulting in blurred images. In contrast, the cross-sectional SEM images of the film in images (e–h) display a clear and uniform morphology. These images indicate that the SiO₂ microspheres and UV-curable adhesive were well mixed, and it is evident that the SiO₂ microspheres had an approximate diameter of 8.6 µm and were hollow. Also, the diameter of its internal cavities was about 10 nm, and they were randomly distributed. Due to the different refractive indices of the two materials, a structure with a gradient refractive index was formed from the substrate to the film material (with UV adhesive silica and a vacuum inside) and then to the air, which effectively reduced the radiation loss of light when it passed through the interface [27]. This greatly increased the thermal emissivity of the film material, which indicates that the film is very suitable for radiation cooling and refrigeration applications in terms of its microstructure.

3.2. Optical Property Analysis

It is well established that effective daytime radiative cooling materials must satisfy two critical criteria [38,39]: high reflectivity in the solar spectrum and high emissivity in the mid-infrared range. To optimize the performance of such materials, we conducted a series of comparative experiments by varying the mass ratio of hollow silica microspheres to UV-curable adhesive and adjusting the film thickness. These variations allowed us to investigate their combined effects on the material’s radiative cooling performance and identify the optimal formulation for maximum efficiency.

Initially, samples with a uniform thickness of 0.4 mm were designed based on mass ratios of 1:1.5, 1:2, 1:2.5, and 1:3, and these were compared with a control sample consisting solely of the UV-curable adhesive. Figure 4a presents the results of optical testing, which clearly demonstrates that the UV-curable adhesive alone exhibits a low solar reflectivity of approximately 10%. In contrast, the incorporation of hollow silica microspheres substantially improved the solar reflectivity. When the thickness of the cooling film was constant, significant variations in solar reflectivity were observed across the 300–2500 nm wavelength range, depending on the different mass ratios of silica to adhesive. The highest solar reflectivity, approximately 85%, was achieved at a mass ratio of 1:1.5.

In the mid-infrared range (wavelength 8–13 µm), as Figure 4b shows, the emissivity of all samples remained relatively stable, with values around 92%, including the control sample composed solely of the UV-curable adhesive which exhibited an emissivity of approximately 90%. Additionally, the infrared emissivity of a sample with a thickness of 0.4 mm and a mass ratio of 1:2 was measured using an IR-2 dual-band infrared radiation power meter, yielding an emissivity of 91% in the infrared range, as

Table 1. Emissivity measured with IR-2 dual-band infrared radiation power detector.

Emissivity Test 8–14 μm
Test Number	Emissivity
1	0.912
2	0.915
3	0.911
Instrument: Infrared radiation power detector
Model specification: IR-2, China

lists. These results indicate that the designed samples possessed high infrared emissivity, making them well-suited for radiative cooling applications. The data also suggest that variations in the mass ratio or the addition of hollow silica microspheres had minimal impact on the infrared emissivity. However, the incorporation of silica microspheres had a marked effect on enhancing the solar reflectivity, underscoring their importance in optimizing the performance of radiative cooling materials.

Furthermore, to assess the influence of film thickness on the emission spectrum, we fixed the mass ratio of silica microspheres to UV adhesive at 1:1.5 and prepared samples with varying thicknesses of 0.05, 0.15, 0.25, 0.4, 0.8, and 1.1 mm. We then analyzed the changes in both solar reflectivity and infrared emissivity. Figure 4c presents the spectra of solar reflectivity as a function of thickness. The results demonstrate that thickness had a substantial effect on the solar reflectivity of the films. At a thickness of 1.1 mm, the reflectivity reached approximately 94%, whereas at 0.05 mm, the reflectivity was only around 40%. This indicates that thicker films exhibited higher solar reflectivity. As Figure 4d shows, the emissivity of the samples in the mid-infrared range remained consistently around 90%, suggesting that variations in film thickness did not significantly influence the infrared emissivity. These findings highlight the potential of this material for radiative cooling applications, demonstrating its strong suitability for practical use.

3.3. Cooling Performance Analysis

To evaluate the practical cooling performance of the radiative cooling films, an experiment was conducted using three film samples prepared with different parameters, where both thickness and size variables were controlled. These samples were applied to glass substrates and tested by using the outdoor temperature measurement setup previously described. For comparison, the temperatures of the uncoated glass substrate and ambient air were also recorded.

Table 2. Parameters of different samples used for the cooling test.

Number	Substrate Status	Thickness (mm)	Size (mm × mm)
1	Coated	0.5	50 × 50
2	Coated	1.1	50 × 50
3	Coated	1.1	50 × 100
4	Uncoated	/	/

provides the specific parameters of each test sample along with the corresponding probe connections.

The tests were conducted in Shanghai (latitude 31°14′ N and longitude 121°29′ E) from 11:00 a.m. to 3:00 p.m. on 1 September 2024. Figure 5 illustrates the radiative cooling effects observed during this period. The average solar irradiance during the test was approximately 796.9 W/m². Throughout the test, the cooling device was continuously exposed to direct sunlight, leading to heat absorption by the glass substrate and resulting in the highest measured temperature, which even exceeded the ambient air temperature [40]. At around 12:20 p.m., the glass substrate reached a peak temperature of 76.5 °C, while the ambient air temperature was 66 °C. In contrast, the substrates (1, 2, and 3) coated with the cooling films, registered temperatures of 62.5 °C, 56.5 °C, and 57 °C, respectively, all significantly lower than the ambient air temperature. The maximum cooling effect observed was a temperature reduction of up to 10 °C relative to the surrounding air temperature, and up to 20 °C compared to the uncoated glass substrate. The radiative cooling power of our prepared film was calculated to be 76.7 W/m², demonstrating the material’s substantial cooling capability.

Additionally, around 12:40 p.m., a brief period of cloud cover caused a sudden drop in all measured temperatures due to the lack of direct sunlight. However, the temperatures of the substrates coated with the cooling films remained consistently lower than the ambient air temperature, further validating the material’s effectiveness for daytime radiative cooling applications.

To investigate the influence of size and thickness on the cooling performance, a comparison between materials 1 and 2 was conducted. The film with a thickness of 1.1 mm demonstrated an average temperature reduction of approximately 6 °C compared to the 0.05 mm thick film. This indicates that, for a given surface area, thicker films provide superior cooling performance. In contrast, when comparing the cooling performance of materials 2 and 3, which had the same thickness but different areas, the measured temperatures were found to be very similar with only minimal variation. This suggests that changes in film area have a negligible impact on cooling performance, while film thickness plays a more significant role in enhancing the cooling effect.

3.4. Stability Analysis

The long-term stability of radiative cooling films is a critical factor influencing their practical applications. To maintain effective cooling performance over extended periods, the film must retain its optimal optical properties, particularly high solar reflectivity and infrared emissivity. However, various environmental factors, including prolonged ultraviolet (UV) exposure, weathering, fluctuations in temperature and humidity, and potential physical damage, may lead to the degradation of the film’s performance, thereby compromising its cooling effectiveness. To assess the stability of the radiative cooling film, this study evaluates the optical properties of samples after six months. The samples tested, which had a mass ratio of 1:1.5 and a film thickness of 0.04 mm, were compared with their initial measurements, in order to analyze variations in solar reflectivity and infrared emissivity. As Figure 6 shows, both the solar reflectivity and the emissivity in the atmospheric window experienced a slight reduction after six months. However, despite these minor decreases, the reflectivity and emissivity values remained at relatively high levels. This suggests that the cooling performance of the film was not significantly impacted by the observed changes in optical properties. The stability of the film is therefore considered to be robust, and the minimal variation in cooling performance over time further supports this assertion.

Furthermore, to investigate the impact of environmental conditions on the cooling performance of the fabricated film, we conducted temperature measurements in different seasons while keeping all other experimental conditions constant. Cooling performance tests were performed on the same set of samples at the same location (Shanghai) during two distinct periods: summer (1 September 2024) and winter (1 March 2025). The temperature variations in the samples under different environmental conditions were recorded and analyzed to evaluate their seasonal adaptability. Figure 5 and Figure 7 show the measurement results for summer and winter, respectively.

To evaluate the cooling performance of the fabricated film under realistic outdoor conditions, the experimental setup was continuously exposed to sunlight. A significant seasonal variation in solar irradiance was observed, with the average solar irradiance measured in winter being 588.8 W/m², which is considerably lower than the 796.9 W/m² recorded in summer. This reduction is primarily attributed to seasonal factors, such as variations in the solar zenith angle and atmospheric transmittance. Additionally, differences in ambient temperature between summer and winter may also influence the cooling performance. During peak solar exposure (12:00 p.m.–13:00 p.m.), the uncoated glass substrate absorbed substantial solar radiation, reaching a maximum surface temperature of approximately 62 °C, while the ambient air temperature was around 53 °C. In contrast, the maximum temperatures of the coated glass samples (Sample 1, Sample 2, and Sample 3) were recorded at 48 °C, 42 °C, and 45 °C, respectively, all significantly lower than the uncoated substrate. The coated samples exhibited a maximum temperature reduction of approximately 10 °C compared to the surrounding air and up to 20 °C compared to the uncoated substrate, demonstrating the film’s excellent cooling capability.

Beyond seasonal variations, cloud cover also influenced the cooling performance. After 13:00 p.m., the appearance of cumulus clouds intermittently blocked direct solar radiation, leading to fluctuations in solar irradiance. This resulted in a general decrease in the temperature of all test samples, as reflected in the irradiance curve. However, even under cloud cover, the coated samples consistently maintained lower temperatures than both the uncoated substrate and the ambient air, indicating that despite reduced direct solar radiation the cooling film remained effective. Furthermore, as the solar altitude angle decreased more rapidly in winter ambient temperatures began to drop significantly after 15:00 p.m., leading to an overall cooling trend for all samples. Notably, the coated samples consistently exhibited lower temperatures than the uncoated substrate, further confirming the film’s stable cooling performance across different seasonal conditions.

In addition to short-term seasonal effects, the long-term stability of the cooling film was assessed by examining its performance after six months. While the cooling effect observed in winter was slightly reduced compared to summer—primarily due to lower ambient temperatures and decreased solar irradiance—the coated samples still demonstrated effective cooling performance. This suggests that even after prolonged environmental exposure, the cooling film maintains high efficiency and remains durable over extended periods. The results highlight the film’s potential for long-term applications in passive cooling technologies across various climatic conditions. And the performance of the radiative cooling films may vary under different climatic conditions due to environmental factors, such as in a Desert environment where high solar irradiance and low humidity enhance radiative cooling efficiency. However, prolonged exposure to extreme aridity may affect the material’s long-term stability and thermal resistance. In a high-humidity environment, water vapor can interfere with thermal radiation, potentially reducing the cooling effect. In a cold climate, lower solar irradiance may limit the radiative cooling efficiency, while the overall cooling demand is also reduced. Despite these challenges, the film’s design allows for optimization to enhance its performance under varying conditions. The surface structure and material composition can be adjusted to improve cooling efficiency in humid or cold environments, while enhanced thermal stability and durability can improve performance in extreme heat. These modifications ensure the film’s adaptability across diverse climatic conditions.

4. Conclusions

This study developed a radiative cooling film using a simple fabrication process based on photopolymerization-induced phase separation. By incorporating hollow silica spheres into a UV-curable resin, a refractive index gradient structure was formed, enhancing thermal radiation. The optimal composition in this work, with a silica-to-resin mass ratio of 1:1.5 and a film thickness of 1.1 mm, achieved a solar reflectivity of 85% and an infrared emissivity of 91% within the atmospheric window (8–14 μm). Outdoor experiments conducted in Shanghai during the summer (1 September 2024) and winter (1 March 2025) demonstrated effective cooling performance. In the summer, under 796.9 W/m² solar irradiance, the film reduced the surface temperature by 10 °C compared to ambient air and 20 °C compared to an uncoated glass substrate, achieving a cooling power of 76.7 W/m². In the winter, with 588.8 W/m² solar irradiance, the film maintained a significant cooling effect, though with reduced efficiency due to lower solar exposure. Increasing the film thickness from 0.05 mm to 1.1 mm enhanced cooling by 6 °C, while surface area changes had minimal impact. Further research is needed to assess the long-term stability under extreme conditions and to optimize the film for diverse environments. Additionally, challenges may arise in scaling up the film for commercial production, requiring further specialized studies. Expanded field studies will be essential for broader application and validation.