A superhydrophobic wood aerogel for radiative cooling and sound absorption

Abstract

Passive daytime radiative cooling, as an environmentally friendly and sustainable cooling approach, can effectuate efficient temperature reduction without energy input. Nevertheless, the majority of radiative cooling materials were obtained by making use of unsustainable materials and have limited functionality. To overcome these limitations, we fabricated a radiative cooling wood aerogel (WA) with sound absorption ability. The as-fabricated WA reflects 92% of solar light and remits 98% of infrared, achieving an average temperature reduction of 10 °C under direct sunlight. Importantly, the WA possesses a high sound absorption coefficient of 0.4 within the frequency range of 500–3000 Hz owing to its high porosity of 95%. Superhydrophobization of the WA empowers the aerogel to be stable against outdoor rain rinsing and weathering. This work provides a new strategy for multifunctional radiative cooling materials for diverse applications using naturally available resources.

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Jun Cheng

Jun Cheng, a researcher at Northwest Institute for Non-ferrous Metal Research, Xi'an high-level talents-local leading talents (Class I), is the deputy director of Shaanxi Key Laboratory of Biomedical Metal Materials. His research direction is metallic functional materials. He has presided over more than 20 scientific research projects such as the National Natural Science Foundation of China, published more than 140 research papers, and authorized more than 20 national patents. He participated in the revision of two national standards and one enterprise standard. He is a reviewer of more than 20 journals such as Rare Metals, Tribology International, Materials Characterization, Materials Science & Engineering A, and International Journal of Hydrogen Energy and so on.

Introduction

Passive daytime radiative cooling, as a green and energy-free cooling approach, has become highly attractive in reducing energy consumption.^1–3 It can emit heat to the cold universe through the transparent atmospheric window (8–13 μm) in an energy-free manner, thereby allowing the cooling of objects without any input of energy.^4–6 Over the past few decades, radiative cooling technology has witnessed rapid advancements, particularly in the design and fabrication of various radiative cooling materials including porous coatings,^7,8 super ceramics,^9,10 polymer composite materials,^6,11 leather-like materials,¹²etc. These materials have achieved highly efficient cooling performance after further optimized design. Nevertheless, preparing these radiative cooling materials often demands meticulous design and complex fabrication processes. Moreover, some materials usually rely on non-renewable resources and, at the end of their service life, face disposal treatment such as landfill and incineration, giving rise to new environmental issues.^13,14 Therefore, the utilization of renewable resources and the fabrication of radiative cooling materials through simple preparation processes hold significant importance for alleviating environmental pressure.

Wood, as a natural and renewable structural material, has been extensively employed in the field of radiative cooling due to its economic and environmentally friendly characteristics.¹⁵ Researchers have developed integrated multi-functional radiative cooling wood with heat insulation,¹⁶ durability,¹⁷ fog water collection,¹⁸ or fire prevention.¹⁹ Despite the remarkable progress achieved in the research of multi-functional radiative cooling wood, it still fails to meet people's demands in terms of application scenarios, particularly in the domain of residential buildings.^20,21 Buildings involved in social activities are inevitably confronted with noise issues such as vehicle traffic, construction, and those generated in daily life, which may affect the health and well-being of residents.^22,23 However, there are scarce reports regarding radiative cooling materials with sound absorption for buildings.

Herein, we developed a wood aerogel (WA) that integrates radiative cooling and sound absorption. First, we carried out a high-temperature chemical delignification of wood to reduce the lignin content to improve the solar reflectance. Then the delignified wood was freeze-dried to obtain hierarchical pores inside (Fig. 1a). The as-obtained porous wood is light as an aerogel with a solar reflectivity of 92%, an infrared emissivity of 98%, and a very low thermal conductivity, enabling radiative cooling of 10 °C under direct sunlight and a thermal insulation temperature increase of 2 °C at night. The wood aerogel achieved a 36.1 dB sound reduction and could be easily turned superhydrophobic by simple hydrophobization. These processes make a facile strategy to fabricate thermally insulated radiative cooling wood materials with noise-reducing and anti-staining properties.

Fig. 1 (a) The fabrication process of the WA. (b) The design and working principle of the multifunctional WA. (c and d) SEM images of the cross-section of the pristine balsa wood (c) and WA (d). (e and f) SEM images of the growth direction of the pristine natural wood (e) and WA (f). (g) Thermal conductivities of the pristine balsa wood, delignified wood, and WA. (h) Reflectance and emittance of the pristine balsa wood and WA. (i) The sound absorption coefficient of the pristine balsa wood and WA with frequency.

Experimental section

Materials and chemicals

Balsa wood was sourced from Xi'an in Shaanxi province. Sodium hypochlorite (NaClO, 80%), ethyl acetate (C₄H₈O₂), perfluorooctyltriethoxysilane (C₁₄H₁₉F₁₃O₃Si), and glacial acetic acid (CH₃COOH, 99.5%) were obtained from Shanghai Macklin Biochemical Technology Co., Ltd.

Delignification process

The balsa wood was immersed in a 1 L solution containing 2 wt% sodium hypochlorite. To enhance the removal of lignin and hemicellulose, glacial acetic acid was added to adjust the pH of the solution to 4.6. The resulting mixture was heated in an oil bath at a temperature of 110 °C for 8 hours. Following this treatment, the wood block underwent washing with deionized water until the pH reached neutral. Finally, freeze-drying was performed on the wood for 12 hours to produce a wood aerogel (WA).

Preparation of a superhydrophobic wood aerogel

A solution containing 23.4 wt% perfluorooctyltriethoxysilane was formulated using ethyl acetate as the solvent. This solution was then applied to the surface of the WA through spraying and allowed to air dry, resulting in the formation of a superhydrophobic wood aerogel (SWA).

Measurements and characterization

The morphology and structure of natural wood and WA were characterized using a scanning electron microscope (Hitachi S-4800). The solar reflectance in the wavelength range of 2.5 to 15 μm was measured using a Fourier transform infrared spectrometer (Bruker Vertex 80) equipped with a diffuse reflectance gold integrating sphere. Compression tests were performed using an Instron 3367 Universal Test System with a load capacity of 30 kN at a crosshead speed of 5 mm min⁻¹. Temperature distribution was measured using a FLIR E8 infrared camera. The surface roughness of the wood sample (20 mm × 20 mm × 10 mm) was determined using an optical profilometer (Contour GTK 3D, Bruker, Germany).

Wood composition analysis

The cellulose, hemicellulose, and lignin composition of natural wood and WA was measured following GB/T 36057-2018. Each measurement was performed three times independently.

Optical properties

A UV-visible-near-infrared spectrophotometer (Cary 5000, Agilent) was used to measure the reflectance of the aerogels in the sunlight band (0.3–2.5 μm) with BaSO₄ as the benchmark. The average solar reflectance of the aerogel was calculated from eqn (1)where R(λ) is the reflectance of the WA in the wavelength range of 0.3 to 2.5 μm, and I_solar(λ) is the solar spectral irradiance according to ASTM G173 Global Solar.

Thermal conductivity

The thermal conductivity of the WA was measured using a TPS2200 Thermal Conductivity Meter from Hot Disk in Sweden using the ISO22007-2.2 test standard.

Sound-absorption measurement

The absorption test was executed via an ACUPRO impedance tube (provided by TFAcoustics, LLC), which conforms to ISO10534-2 and ASTM E1050 for the quantification of the sound absorption coefficient as a function of frequency. The specimens under measurement were fabricated in the form of cylinders with a radius of 35 mm and a thickness of 10 mm. A speaker positioned at one end of the impedance tube emitted broadband white noise. The incident wave and the echo reflected from the specimen at the other end of the tube were gauged using two microphones. Based on these measurements, the sound absorption coefficient of each specimen was precisely computed within the frequency range of 500 to 3000 Hz at intervals of 6.25 Hz. The measurements were calibrated by employing materials with known dissipation characteristics, and each measurement was repeated thrice to enhance the accuracy.

Outdoor radiative cooling performance

To appraise the outdoor cooling performance of the WA, we fabricated a device composed of polystyrene foam with four rectangular cavities at the top as a supportive framework. These cavities were subsequently covered with 2 cm × 2 cm × 1 cm-sized specimens consisting of WA and natural wood. To warrant the accuracy and stability of the experiments, the foam was enveloped in aluminum foil and covered with a transparent polyethylene film. Eventually, the test configuration was placed on the rooftop of a building under sunny circumstances at noon on a cloudless day.

Results and discussion

Design, morphology, and properties of the wood aerogel

The design and working principle of the multifunctional WA, as depicted in Fig. 1b, are based on balsa wood and features radiative cooling, heat insulation, and sound absorption properties. During the daytime when exposed to direct sunlight, the strong solar reflectance effectively inhibits heat input, while the high emissivity enables the outward emission of WA's heat.²⁴ Thus, a highly effective cooling effect is achieved. During cold nights, the low thermal conductivity can maximize the prevention of internal and external heat exchange, minimize heat loss²⁵ and maintain the interior warmth, thereby alleviating the issue of excessive cooling. Additionally, the high sound absorption coefficient enables the WA to absorb sound waves and reduce the harm caused by noise to individuals.²⁶ The synergy of radiative cooling, heat insulation, and sound absorption enables the WA to adapt to different environmental conditions.

The as-obtained WA is very light (Fig. S1†) with a density of 0.06 g cm⁻³, which is merely half that of the natural balsa wood (Fig. S2†). Notably, the radial compressive strength of the WA amounts to as high as 2.2 MPa, and it only reduces by 0.25 MPa in radial compressive strength after 100 compression cycles, indicating extraordinary mechanical stability (Fig. S3†). Furthermore, with the removal of lignin and hemicellulose, the toughness of the wood was enhanced from 187 kJ m⁻³ (Fig. S4†) to 700 kJ m⁻³. It should be noted that the pristine natural balsa wood presents a honeycomb-like cellular architecture with cell walls shaping the vertically aligned tubules (30–60 μm) and large channels (100–200 μm) (Fig. S5†). The cell wall is predominantly constituted of lignin, hemicellulose, and cellulose (Fig. S6†). Under the effect of lignin and hemicellulose, the cells within the pristine balsa natural wood are densely arrayed, making it arduous to form distinguishable gaps (Fig. 1c). After delignification and freeze-drying, the resultant WA sustained the vertically oriented tubular configuration (Fig. 1d), yet the intercellularity underwent a marked transformation when compared with the pristine natural one (Fig. S7†). This was primarily ascribed to the depletion of lignin/hemicellulose, which gave rise to numerous minute voids between the cell walls of the adjacent wood, thereby inducing an increase in structural porosity.²⁷ Furthermore, the average diameter of the pores on the surface of WA is 50 μm and they are uniformly distributed, which is beneficial for enhancing the probability of solar light reflection and multiple scattering.^28–30

Furthermore, the pristine natural wood exhibits a laminar arrangement in the growth direction (Fig. 1e), while the WA retains the laminar structure, which offers potential for sound absorption (Fig. 1f). Analysis of the Fourier transform infrared spectra of the pristine natural wood and WA affirmed that the lignin and hemicellulose have been mostly eliminated (Fig. S8†). After the chemical treatment, the conspicuous lignin peaks at 1,595, 1,500, and 1460 cm⁻¹ within natural wood dissipate, signifying that the lignin has been excised.³¹ Simultaneously, the abatement of the peak at 1735 cm⁻¹ indicates that hemicellulose was also eliminated during the delignification process.³² Additionally, chemical analysis divulged that the lignin content of the natural wood declined from 24.1% to 3.3% following the delignification treatment, whilst the hemicellulose content plummeted from 23.8% to 1.15% (Fig. S9†). This constitutes supplementary evidence that lignin and hemicellulose can be efficiently removed from natural wood via the delignification treatment. Owing to the ramifications of delignification, its porosity ascends from approximately 75% to 95% (Fig. S10†). It is worthy of emphasis that the porosity is appraised based on the mass and volume of the pristine natural wood and WA. The high porosity confers a low thermal conductivity to the WA, which is 0.043 W m⁻¹ K⁻¹ (Fig. 1g), much lower than that of the pristine wood, and enables alleviating over-cooling at night by preventing heat from dissipating outward.³³ Moreover, after the delignification treatment, there are abundant hydroxyl groups in the WA. The molecular vibration and elongation of the hydroxyl groups lead to a strong emission of the WA in the infrared region (Fig. S11†).³⁴ Furthermore, the WA is rich in cellulose. Cellulose has nearly no absorption within the visible light range, in combination with the porous structure which increases the reflectivity of the WA.^18,34 As expected, the emissivity of the WA was determined to be 98%, and the reflectivity was 92% (Fig. 1h), indicating that the WA will have outstanding radiative cooling performance. Fig. 1i demonstrates that the WA has a high sound absorption coefficient, which benefits from the retention of the hierarchical porous structure of natural wood after freeze-drying. These excellent results suggest that the WA integrates radiative cooling, heat insulation, and sound absorption and can be regarded as a novel multifunctional aerogel with broad application prospects.

Outdoor radiative cooling performance of the WA

Following delignification and lyophilization, the WA encompasses multi-scale cellulose nanofibers that are oriented in the identical direction as the growth trajectory of the tree (Fig. S12†). Even more significantly, these fibers do not absorb light within the visible spectrum, offering an optical foundation for outdoor cooling.³⁵ Furthermore, the WA preserves the natural conduits of the wood (Fig. S13†), and these multi-scale conduits can serve as a stochastic and disordered scattering entity with robust broadband reflection within the visible spectrum.³⁶ The disordered arrangement of cellulose renders the WA indistinctive, and the indistinctive surface can efficaciously disseminate the incident light into a hemispherical stereoscopic angle, circumventing the visual discomfort engendered by intense specular reflection (Fig. S14†), thereby satisfying people's expectations.³⁷

We evaluated the daytime outdoor cooling performance (Fig. 2a) of the WA by conducting outdoor tests in Xi'an with a self-developed device. Specifically, the self-developed device was positioned at the rooftop area directly exposed to the sun, and the entire top of the device was covered with transparent PE to simulate an environment without wind. The temperature of the air in the cavity covered by the sample was continuously monitored for six hours via a thermocouple, and this was employed to depict the temperature of the sample (Fig. 2b).^38–40 It is notable that this testing approach might lead to a test temperature higher than that disclosed by the meteorological bureau.^12,28,41 The outcomes indicated that the temperature of the pristine natural wood remained consistently higher than that of the air during the successive six hours of testing, whilst the WA manifested a lower temperature than that of the air. This is primarily attributed to the fact that lignin was eliminated from the WA, which possesses absorptive characteristics in the solar wavelength band, thereby substantially reducing the absorption of sunlight on the surface.⁴² Even more importantly, at midday, with a relative humidity of 10% and a maximum light intensity of 800 W m⁻², the WA is capable of attaining a maximum temperature drop of 13 °C and an average of 10 °C, demonstrating remarkable outdoor cooling efficacy (Fig. 2c).

Fig. 2 (a and b) Photo (a) and schematic (b) of outdoor radiative cooling test apparatus. (c) The temperatures and temperature differences (ΔT) of the natural wood, WA, and air with solar radiation intensity (orange area) were tracked continuously for six hours (Xi'an, China, June 27th, 2024). The ΔT is obtained by subtracting the air temperature. (d) Theoretically calculated cooling power of WA at different non-radiative heat transfer coefficients. (e and f) Digital (e) and thermal infrared images (f) of natural wood and the WA under direct sunlight, where the surface temperature of WA is merely 36.3 °C when the maximum temperature is 50.5 °C. (g) Continuous 3 hour temperature monitoring and temperature difference (ΔT) during the night. The ΔT is obtained by subtracting the air temperature (Xi'an, China, October 20th, 2024). (h) Schematic diagram of the principles of daytime cooling and nighttime heat preservation.

Furthermore, taking into account the existence of non-radiative heat transfer in the natural setting, we theoretically ascertained the net cooling power (Fig. 2d) of the WA at various non-radiative heat transfer coefficients (q). Two pivotal aspects merit attention. Firstly, when T_a − T_c = 0, it implies that the cooling power is exclusively derived from thermal radiation, which directly reflects the cooling capacity of the material.⁴³ Secondly, when the cooling power is 0, it indicates the maximum achievable cooling temperature of the material, which reflects the optimal cooling performance of the material.⁴⁴ For the WA, when T_a − T_c = 0, it possesses a cooling power of up to 65 W m⁻², signifying outstanding cooling performance. And when the cooling power is 0 W m⁻², the WA attains a cooling range of 6–15 °C. Specifically, if there is no non-radiative heat transfer in the environment (q = 0 W m⁻² K⁻¹), then the WA induces a cooling of up to 15 °C, and even at q = 8 W m⁻² K⁻¹, it can achieve a 6 °C cooling during the daytime, demonstrating exceptional cooling performance.

To visually demonstrate the outdoor cooling efficacy of the WA, it was exposed to direct sunlight using the pristine natural wood as a comparison (Fig. 2e). Advantageously, after 5 minutes, the surface temperature of the WA was merely 36.3 °C, significantly lower than the maximum temperature of 50.5 °C and the pristine natural wood temperature of 48.1 °C (Fig. 2f), showcasing the vigorous sub-ambient radiative cooling capacity. Furthermore, a continuous 3-hour temperature monitoring of the WA during the cold night was also conducted (Fig. 2g). It was found that the temperature of the WA could achieve a maximum increase of 2.8 °C with an average increase of 1.5 °C, demonstrating an insulating effect. This can effectively mitigate the problem of radiative over-cooling at night. This is primarily attributed to the complex multi-layer micro-nanoporous structure of the WA, which results in low thermal conductivity. During the daytime, the reflectivity and emissivity of the upper part of the WA can achieve excellent cooling effects. At the same time, because the external temperature is higher during the day, heat will spontaneously conduct to the lower-temperature interior. However, due to the low thermal conductivity of the WA, it impedes the diffusion of heat to the interior, thereby achieving a heat insulation effect and guaranteeing that the internal temperature is lower than the external temperature.¹⁸ While in the cold night, as the emissivity of the WA is fixed, heat is inevitably emitted to the outside. However, the WA has a certain thickness, coupled with the complex porous structure and good insulation performance, which weakens the heat conduction between the interior and the exterior, thereby reducing heat loss (Fig. 2h). Overall, the reduced heat loss can compensate for the heat dissipation caused by emissivity.⁴⁵ Therefore, the WA has a heat preservation effect during the cold night and can alleviate the problem of excessive cooling.

The thermal insulation performance of the WA

The thermal insulation performance of the WA was assessed by gauging the temperature response of the WA during the heating phase and the temperature disparity between the heating table and the WA. Specifically, a WA with dimensions of 5 cm × 5 cm × 0.1 cm, along with samples of non-woven material, expanded polystyrene (EPS), cotton, and the pristine natural wood with identical magnitudes, was placed on a heating table heated from 20 °C to 60 °C (Fig. S15†). The surface temperature of the samples was recorded by utilizing thermocouples, while an infrared camera was employed to monitor the distribution of the surface temperature of the samples. Fig. 3a depicts the infrared image of each sample at 60 °C on the hot stage. In contrast to the bright yellow color distribution of the pristine natural wood, the surface of the WA exhibits a blue temperature distribution, suggesting a significantly cooling surface. Furthermore, the WA demonstrates superior thermal insulation when compared to EPS and cotton fabrics. Even more significantly, the WA has a sluggish temperature response during the heating of the hot stage. When the temperature of the hot stage attains 60 °C (Fig. 3b), the temperature of the WA merely amounts to 35 °C, resulting in a temperature difference of 25 °C (Fig. 3c), marginally exceeding that of cotton fabrics (21 °C) and conspicuously surpassing that of the pristine natural wood (4 °C). This is primarily attributed to the fact that the thermal conductivity of wood is predominantly governed by the thermal conductivity of air molecules within the cell walls and pores.⁴⁶ The cell walls are mainly constituted of cellulose, hemicellulose, and lignin, and their thermal conductivity is significantly higher than that of air.⁴⁷ Accordingly, augmenting the number of air molecules in the wood, that is, increasing the porosity of the wood, is key to reducing the thermal conductivity. However, the pristine natural wood demonstrates a relatively elevated thermal conductivity due to its compact cellularity and high solid content (approximately 20%), whereas delignification eliminates lignin and hemicellulose, which have a high thermal conductivity within the cell wall. Consequently, it reduces the solid content of the wood while enhancing the porosity of the wood, giving rise to a lower thermal conductivity of the WA and reducing the propensity of heat transfer through the heat conduction process.

Fig. 3 (a) Infrared images of samples of the non-woven, EPS, cotton, natural wood, and WA on a heating stage of 60 °C. (b) The temperature variation curve of the sample surface during the heating process with the temperature of the heating stage. (c) The temperature difference (ΔT) between each sample and the heating stage. (d) The pore distribution of natural wood and the WA. (e) The thermal conductivity of the WA. (f and g) Under low-temperature conditions, the temperature curves (f) and temperature differences (ΔT) of natural wood and the WA, among which ΔT is acquired by subtracting the air temperature (g). (h and i) At 15 minutes, the infrared photos of the surface (h) and the backside (i) of the WA. (j) The schematic diagram of heat transfer of the WA.

Fig. 3d discloses that in contradiction to the pristine natural wood, the pore diameters of the WA are mainly distributed within the range of 0–5 μm, which markedly enhances the likelihood of phonon scattering at the solid/air interface, thereby hampering the heat transfer of the solid.⁴⁸ The outcomes of the thermal conductivity test manifest (Fig. 3e) that the thermal conductivity of the WA is merely 0.043 W m⁻¹ K⁻¹ as opposed to 0.132 W m⁻¹ K⁻¹ of the pristine natural wood, which is conspicuously much lower. Furthermore, the high porosity constitutes another pivotal factor accountable for the low thermal conductivity of the WA.⁴⁹

To undertake a more comprehensive and meticulous verification of the insulating performance of the WA, the device illustrated in Fig. 2b was positioned within a −20 °C cold trap for 15 minutes (Fig. S16†). Subsequently, it was translocated to the natural environment until the temperature reverted to normal. The temperature within the cavity encompassed by the WA was recorded in real-time, and infrared images were captured employing an infrared camera. The findings suggest that within the 15-minute duration, the WA manifests a more sluggish temperature alteration tendency and ultimately attains a temperature equilibrium of 5 °C. During this process, the average temperature of the WA was 10 °C, with an average temperature difference of 30 °C. However, the average temperature of the pristine natural wood was merely 3 °C. These results confirm that the WA demonstrated superior thermal insulation performance (Fig. 3f and g). Additionally, at the 15-minute juncture, infrared images of the surface and rear of the WA and the natural wood (Fig. S17†) showed that the surface temperature of the WA is 3.9 °C (Fig. 3h) with the rear temperature of 10.8 °C (Fig. 3i), while the surface temperature of the natural wood is −1.0 °C with a rear temperature of 6.7 °C. Furthermore, when the device was transferred to the natural environment, the WA exhibited a certain degree of latency in its response to temperature in contrast to the natural wood. Specifically, the natural wood reached the air temperature in 20 minutes, while the WA required 25 minutes. Meanwhile, in contrast to commercial materials and untreated wood, experiments indicate that the WA still demonstrates superior thermal insulation performance (Fig. S18†). These results indicate that the WA demonstrates a slower temperature variation both in cold and natural surroundings and possesses exceptional insulating properties. Fig. 3j presents the heat transfer process of the WA, in which the stratified porous structure of the WA plays a crucial role in heat insulation.⁵⁰ The entire heat transfer process encompasses heat convection, heat conduction, and heat radiation.⁵¹ As the pore size is far below 1 mm (the starting size for natural convection),⁵² the influence of heat convection need not be considered. Furthermore, due to the relatively low temperature, the impact of heat radiation on the entire heat transfer process is minor and can be neglected.⁵³ Hence, in the entire heat transfer process, heat conduction plays a leading role. Since the WA has a relatively low thermal conductivity, it weakens the occurrence of heat transfer, thereby demonstrating outstanding heat insulation performance.

Sound absorption performance of the WA

The acoustic characteristics of wood are dictated by two predominant factors: the surface configuration and the internal pore attributes,⁵⁴ including pore dimension and pore distribution. For natural wood, the interior encompasses a substantial number of pore structures. Nevertheless, due to lignin and hemicellulose, the cell walls are compactly arrayed to form rigid and dense pores, which will engender two consequences. On the one hand, the sleek, rigid, and compact pore size confers smoothness upon the surfaces of natural wood; on the other hand, the inter-connected cell walls engender a low porosity, which is detrimental to the sound absorption of natural wood. As a result, the sound absorption capacity of natural wood is deficient. In contrast, the wood undergoing delignification treatment, owing to the removal of hemicellulose and lignin, induces cell wall dissociation and gives rise to numerous cracks (Fig. S19†). This not only elevates the roughness (Fig. 4a) of the wood surface but also augments the porosity, thereby incurring greater friction and air viscous loss during the transmission of sound and fortifying the sound absorption performance of the WA in the high-frequency region.⁵⁵ More significantly, the tubular pore structure in the WA is exposed (Fig. S20†). These micron and nanoscale pores enable more sound waves to penetrate the material and act as sound-absorbing pores. Furthermore, the pore sizes of the tube pores and pits are smaller than those formed between the cellulose. Following Helmholtz resonance theory, this cascade resonance structure featuring different pore sizes promotes the conversion of acoustic waves into either mechanical or thermal energy.⁵⁶ Determination of the sound absorption coefficients of the WA and the pristine natural wood was conducted using the two-microphone transfer function method, where sound is transmitted along a direction perpendicular to the growth direction of the wood (Fig. S21†). The results demonstrate that the sound absorption coefficient of a 10 mm-thick WA is markedly higher than that of natural wood of the same thickness within the 500–3000 Hz range. The average sound absorption coefficient of the WA reaches up to 0.4, which is significantly higher than that of natural wood at 0.084 (Fig. 4b). Notably, the first sudden change in frequency of the sound absorption coefficient of WA occurs at 500 Hz. At this frequency, the sound absorption coefficient of the WA is significantly higher than that of natural wood, reaching 0.29, which is approximately four times that of natural wood. The augmentation of the sound absorption performance at low frequencies could potentially be ascribed to the fact that the porosity of the WA increases subsequent to delignification treatment. This will heighten the resonance frequencies among gases at diverse positions, thereby intensifying sound loss and enhancing the sound absorption performance in the low-frequency domain.^57–59 More interestingly, the sound absorption coefficient of the pristine natural wood exhibits a linear relationship between the sound absorption coefficient and frequency, and the sound absorption coefficient is nearly independent of frequency. In contrast, the sound absorption coefficient of the WA shows a nonlinear increase with frequency. When the frequency increases to 2500 Hz, the sound absorption coefficient of the WA increases sharply to 0.86, demonstrating superior high-frequency sound absorption performance (Fig. 4c). This could potentially be ascribed to the fact that the surface of natural wood is smooth and dense, thereby manifesting inferior high-frequency sound absorption performance. After delignification treatment, the surface roughness of the WA increases, and concurrently, delignification gives rise to micropores and cracks. This will engender greater friction and air viscosity loss, which will augment the sound absorption performance of the WA, particularly in the high-frequency domain.^54,58,60 Hence, in comparison with the pristine natural wood, the WA exhibits superior sound absorption performance throughout the full band range. It also reveals the significance of delignification treatment in enhancing the sound absorption coefficient of natural wood.

Fig. 4 (a) Surface roughness of wood samples with R_a denoting the arithmetic average roughness and R_b indicating the root mean square roughness. (b) The average sound absorption coefficients of the WA and natural wood are within the frequency range of 500–2500 Hz. (c) The sound absorption coefficients of the WA and natural wood at frequencies of 500 Hz and 2500 Hz. (d) Schematic diagram of sound absorption of the WA. (e) The diagram and schematic of the sound absorption test device. (f) The curves of sound pressure level (SPL) variations of natural wood, fiber felt, and the WA for 15 consecutive minutes. (g) The sound decibel differences of natural wood, fiber cushion, and the WA. (h) A radar graph presenting the characteristic comparison of the WA with other materials.

Fig. 4d presents the three conversions that occur when sound interacts with the WA, namely, reflection, absorption, and transmission.^61,62 Owing to the rough surface of the WA, a portion of the sound is reflected.⁶³ Furthermore, when sound propagates into the interior of the WA, the propagation of sound waves in the pores induces vibration and frictional interaction among the air molecules within the pores, converting the sound energy into thermal energy.⁵⁷ Additionally, due to the viscous resistance of air,⁶⁴ air also undergoes friction with the pore walls, further converting the sound energy into thermal energy and being absorbed. Thanks to the abundant hierarchical porous structure within the WA, the absorption of sound can be significantly enhanced, thereby reducing the transmission of sound.

To further verify the sound absorption performance of the WA, we examined the actual sound absorption performance of the WA in a room-temperature environment. As shown in Fig. 4e, two test points were positioned, one at the noise source and the other beneath the test specimen. The decibel values at the two test points were monitored, and the difference between the two was employed to evaluate the sound absorption performance of the specimen. Commercially available mainstream fiber mats with a thickness of 10 mm were also utilized in the test setup for comparison. The results indicate that in an environment with an average noise of 103.1 dB (Fig. S22†), the maximum attenuation of white noise for the WA amounts to 36.6 dB with an average attenuation of 36.1 dB (Fig. 4f), both of which outperform that of the natural wood. Surprisingly, in contrast to the WA, the average noise attenuation of the fiber mats is merely 14 dB with a maximum attenuation of 15.1 dB, which highlights the outstanding noise attenuation performance of the WA (Fig. 4g). To assess the overall performance of the WA, we evaluated the superiority of the WA as a house building material in respect of five crucial indices: density, strength, cooling performance, thermal insulation performance, and noise reduction performance. As depicted in Fig. 4h, although synthetic fibers and foam plastics exhibit favorable noise reduction properties, their solar absorption rate is elevated, and their radiative cooling performance is subpar. Glass, as a prevalently utilized building material, possesses good strength; nevertheless, its mass is relatively large and its density is relatively high. Additionally, hollow bricks, despite having outstanding thermal insulation and sound absorption properties, have a significant mass and poor radiative cooling performance (Table S1†). In contrast to these commonly employed materials, the WA demonstrates superior comprehensive performance in five aspects: ultra-lightweight, cooling, thermal insulation, sound absorption, and compressibility, revealing significant application potential in residential building domains.

Application and weathering properties of the WA

To further assess the performance of the WA in practical house applications, houses were constructed respectively using natural wood and the WA and placed in diverse environments for testing, including high-temperature, low-temperature, and noisy environments. Fig. 5a demonstrates the capacity of the WA to manage heat and sound when utilized as a building material. In a high-temperature environment, owing to the high reflectivity and emissivity of the WA, it will reflect the majority of sunlight and emit its energy to maintain appropriate temperature inside the house.⁶⁵ Meanwhile, in a low-temperature environment, compared with traditional radiant coolers, the WA can diminish heat exchange with the outside by taking advantage of its low thermal conductivity properties and achieving thermal insulation.⁶⁶ Additionally, in a noisy environment, its excellent sound-absorbing characteristics weaken the sound spread into the house, reducing the harm of noise to the human body.⁶⁷ Hence, this multi-functional WA has superior energy-saving potential.

Fig. 5 (a) Schematic illustration of the principles of cooling, heat insulation, and sound absorption when the WA is employed as a building material. (b) Infrared photographs of the houses constructed with the WA and natural wood at 50 °C. (c) Continuous 5-hour temperature detection within the house (Xi'an, China, August 8th, 2024). (d) Continuous 4-hour temperature detection within the houses (Xi'an, China, October 10th, 2024). (e) The average decibel values of various samples near the construction site with a decibel value of 68.4 dB. (f) Illustration of spray-coating of fluorosilane to obtain the superhydrophobic wood aerogel (SWA). (g and h) The contact angles (g) and emissivity (h) of WA and SWA. (i) Continuous 4-hour outdoor temperature monitoring of the WA and SWA (Xi'an, China, September 15th, 2024). (j) The ratios of the reflectivity, thermal conductivity and sound absorption coefficient of the samples to the original WA after all tests.

The small houses constructed by WA were positioned in a hot outdoor environment (Fig. S23†). The findings demonstrate that under direct solar irradiation, the WA manifests a temperature of merely 29 °C, whilst the surface temperature of the natural wood ascends to as high as 45 °C (Fig. 5b). It is captivatingly remarkable to discern that the top, side, and front expanse of the WA abode manifest varied temperature distribution paradigms. The temperature on the side of the abode surpasses that on the top by 7 °C, whilst the temperature on the front expanse exceeds that on the top by 6 °C (Fig. S24 and S25†). The possible cause contributing to this phenomenon might be that during the assembly procedure, the top of the abode constitutes an angle of approximately 45° with the horizontal plane, which differs from the vertical dispositions of the sides and the front expanse, enabling the incident light to undergo polarization, thereby engendering a higher reflectivity of WA and attaining preeminent radiative cooling efficacies.⁶⁸ Moreover, the findings from monitoring the internal temperature of the house for five consecutive hours disclose that the internal temperature of the WA house is perpetually lower than that of the air, and an average drop of 10 °C is accomplished in contrast to the air (Fig. 5c), manifesting extraordinary cooling performance. Notably, traditional radiative cooling materials frequently encounter excessive cooling issues at low temperatures due to the fixed emissivity of such materials.⁸ Although the WA also has the problem of fixed emissivity, dissimilar to traditional radiative cooling materials, the low thermal conductivity of the WA endows it with superior insulation performance. This empowers the WA to curtail the heat transfer from the interior of the house to the exterior under low-temperature conditions, countervailing or compensating for the temperature descent caused by the fixed emissivity,¹⁸ thereby facilitating the establishment of a congenial indoor environment. To further validate the actual thermal insulation capability of the WA, the house was situated in a low-temperature environment (Fig. S26†), and the internal temperature of the house was continuously measured for 4 hours. Fascinatingly, in an environment with an average temperature of −3 °C, the internal temperature of the house fabricated with the WA was higher than the air temperature (Fig. 5d), which attests that the WA boasts exceptional thermal insulation performance and also indicates that enhancing the thermal insulation capacity of radiative cooling materials is an efficacious means to alleviate the issue of excessive cooling. To evaluate the bona fide sound absorption performance of the WA when utilized as a house material, we sited the house proximate to the building site and measured the sound absorption performance by the variance in sound decibels between the interior and exterior of the house. The outcomes manifest that within an average noise magnitude of 68.4 dB, the interior of the house assembled using the WA merely attains 44.4 dB, which is lower than that of the house assembled using natural wood (66.6 dB), thereby evincing superior sound-absorption capability (Fig. 5e).

Moreover, common radiative cooling materials are prone to being affected by rain and dust when utilized outdoors.^69,70 Taking this issue into account, we treated the WA with fluorosilanes (Fig. 5f) to fabricate the superhydrophobic wood aerogel (SWA). The results indicate that the water contact angle of the SWA reaches 152°, presenting superhydrophobic characteristics (Fig. 5g). This will further enhance the weather resistance of the WA and safeguard the WA from the influence of rainwater. After the superhydrophobic treatment, the emissivity of the SWA scarcely changed compared with the WA (Fig. 5h). It is notable that the SWA and WA possess similar mechanical properties (Fig. S27†), which might be attributed to the fact that the fluorination treatment on the surface does not influence the internal structure of the WA.^71,72 Moreover, the outdoor test results reveal that the SWA exhibits a cooling effect similar to that of the WA (Fig. 5i). This makes the WA more appealing as a structural material and is an advantage that distinguishes the WA from other radiative cooling materials.^28,73 Furthermore, the WA was subjected to ultraviolet radiation (Fig. S28†), high-temperature (Fig. S29†), scratch (Fig. S30†), impact (Fig. S31†), and bending tests (Fig. S32†). The test outcomes demonstrated that the various performances of the WA remained stable and it exhibited good durability. Moreover, taking into account the durability requirements during the long-term outdoor application process, the SWA was placed in the outdoor environment for exposure for 30 days (Fig. S33†). The results revealed that the SWA displayed outstanding environmental durability (Fig. 5j and Table S2†). This also implies that the superhydrophobic treatment of the WA is highly necessary for outdoor applications, effectively preventing the scouring of rainwater and the pollution of dust. It significantly broadens the application scenarios of the WA.

Conclusions

In conclusion, a multifunctional wood aerogel with both radiative cooling and insulation properties was fabricated based on natural wood through chemical delignification and freeze-drying techniques. This aerogel realizes a temperature drop of 13 °C during the day and an insulation that is 2.8 °C higher than the ambient temperature at night. The houses constructed with this aerogel exhibit outstanding thermal management capabilities, achieving a temperature reduction of 10 °C in high-temperature environments and an insulation that is 3 °C higher than the ambient temperature in low-temperature environments. Moreover, this aerogel possesses sound absorption capacity and demonstrates weather resistance. Significantly, after undergoing superhydrophobic treatment, the outdoor stability of the aerogel can be further enhanced. This aerogel features environmental friendliness and energy conservation and holds great promise in the field of house energy conservation.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

A-Jun Chang: conceptualization, methodology, formal analysis, investigation, writing – original draft preparation, visualization. Chao-Hua Xue: methodology, formal analysis, writing – original draft preparation, visualization, supervision, funding acquisition. Jiao-Jiao Sun: resources, writing – review & editing. Chao-Qun Ma: validation, writing – review & editing. Meng-Chen Huang: validation, writing – review & editing. Bing-Ying Liu: methodology, formal analysis. Hui-Di Wang: methodology, formal analysis. Xiao-Jing Guo: validation, writing – review & editing. Jun Cheng: validation, data curation, resources, supervision, writing – review & editing, funding acquisition. Li Wan: methodology, formal analysis. Yong-Gang Wu: methodology, formal analysis. Yan-Yan Yan: methodology, formal analysis.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52103263 and 52271249), Key Project of International Science & Technology Cooperation of Shaanxi Province (2023-GHZD-09), Key Project of Science Foundation of Education Department of Shaanxi Province (22JY011), Key Project of Scientific Research and Development of Shaanxi Province (2023GXLH-070), Qinchuangyuan “Scientist + Engineer” Team of Shaanxi Province (2023KXJ-069), Key Research and Development Program of Shaanxi (2023-YBGY-488), State Key Laboratory of Solidification Processing in NPU (SKLSP202415), Xi'an Talent Plan (XAYC240016), and Sci-tech Innovation Team of Shaanxi Province (2024RS-CXTD-46).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta08817d

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