Colored and paintable bilayer coatings with high solar-infrared reflectance for efficient cooling

Hierarchically Micro- and Nanostructured Polymer via Crystallinity Alteration for Sustainable Environmental Cooling

Cooling environments are a pervasive need in our society, with conventional air conditioners being the most popular approach. However, air conditioners rely heavily on electricity and Freon, a chemical that depletes ozone and contributes to greenhouse gas effects. To address this issue, passive daytime radiative coolers (PDRCs) have been proposed to achieve cooling by simultaneously reflecting sunlight and allowing internal heat to escape without electricity. Despite their potential, most high-performance PDRCs are composed of thick polymer films, which increases material costs during PDRC preparation and limits thermal transport. In this work, we introduced an economical and scalable solvent evaporation-based method to prepare a relatively thin hierarchically micro- and nanostructured poly(vinylidene fluoride-trifluoroethylene) via crystallinity alteration. Particularly, we find that the key to generating nanosized pores is to remove the water residual within the film without sample annealing, which significantly enhances the scattering efficiency across the solar spectrum. With our design, we demonstrate effective cooling of the outdoor environment, achieving a cooling temperature of Δ2.5 °C, with a film thickness of only 215 μm. Furthermore, our model suggested that applying this material could lead to annual energy savings of up to ∼39% in warmer climates across the country and up to 715 GJ nationwide. Developing effective PDRCs with reduced material thickness, such as the one discussed here, is imperative for implementing sustainable cooling solutions and reducing our carbon footprint.

Switchable and Tunable Radiative Cooling: Mechanisms, Applications, and Perspectives

The cost of annual energy consumption in buildings in the United States exceeds 430 billion dollars ( Science 2019, 364 (6442), 760-763), of which about 48% is used for space thermal management (https://www.iea.org/reports/global-status-report-for-buildings-and-construction-2019), revealing the urgent need for efficient thermal management of buildings and dwellings. Radiative cooling technologies, combined with the booming photonic and microfabrication technologies ( Nature 2014, 515 (7528), 540-544), enable energy-free cooling by radiative heat transfer to outer space through the atmospheric transparent window ( Nat. Commun. 2024, 15 (1), 815). To pursue all-season energy savings in climates with large temperature variations, switchable and tunable radiative coolers (STRC) have emerged in recent years and quickly gained broad attention. This Perspective introduces the existing STRC technologies and analyzes their benefits and challenges in future large-scale applications, suggesting ways for the development of future STRCs.

Passive Self-Sustained Thermoelectric Devices Powering the 24 h Wireless Transmission via Radiation-Cooling and Selective Photothermal Conversion

The rapid development of the Internet of Things has triggered a huge demand for self-sustained technology that can provide a continuous electricity supply for low-power electronics. Here, a self-sustained power supply solution is demonstrated that can produce a 24 h continuous and unipolar electricity output based on thermoelectric devices by harvesting the environmental temperature difference, which is ingeniously established utilizing radiation cooling and selective photothermal conversion. The developed prototype system can stably maintain a large temperature difference of about 1.8 K for a full day despite the real-time changes in environmental temperature and solar radiation, thereby driving continuous electricity output using the built-in thermoelectric device. Specifically, the large output voltage of >102 mV and the power density of >4.4 mW m-2 could be achieved for a full day, which are outstanding among the 24 h self-sustained thermoelectric devices and far higher than the start-up values of the wireless temperature sensor and also the light-emitting diode, enabling the 24 h remote data transmission and lighting, respectively. This work highlights the application prospects of self-sustained thermoelectric devices for low-power electronics.

Anti-UV Passive Radiative Cooling Chiral Nematic Liquid Crystal Films for Thermal Management

Passive radiative cooling (PRC) can spontaneously dissipate heat to outer space through atmospheric transparent windows, providing a promising path to meet sustainable development goals. However, achieving simultaneously high transparency, color-customizable, and thermal management of PRC anti ultraviolet (anti-UV) films remains a challenge. Herein, a simple strategy is proposed to utilize liquid crystalline polymer, with high mid-infrared emissive, forming customizable structural color film by molecular self-assembly and polymerization-induced pitch gradient, which guarantees the balance of transparency in visible spectrum and sunlight reflection, rendering anti-UV colored window for thermal management. By performing tests, temperature fall of 5.4 and 7.9 °C are demonstrated at noon with solar intensity of 717 W m-2 and night, respectively. Vivid red-, green-, blue-structured colors, and colorless films are designed and implemented to suppress the solar input and control the effective visible light transmissivity considering the efficiency function of human vision. In addition, temperature rise of 11.1 °C is achieved by applying an alternating current field on the PRC film. This study provides a new perspective on the thermal management and aesthetic functionalities of smart windows and wearables.

Bioinspired electrically stable, optically tunable thermal management electronic skin via interfacial self-assembly

The skin is the largest organ in the human body and serves vital functions such as sensation, thermal management, and protection. While electronic skin (E-skin) has made significant progress in sensory functions, achieving adaptive thermal management akin to human skin has remained a challenge. Drawing inspiration from squid skin, we have developed a hybrid electronic-photonic skin (hEP-skin) using an elastomer semi-embedded with aligned silver nanowires through interfacial self-assembly. With mechanically adjustable optical properties, the hEP-skin demonstrates adaptive thermal management abilities, warming in the range of +3.5°C for heat preservation and cooling in the range of -4.2°C for passive cooling. Furthermore, it exhibits an ultra-stable high electrical conductivity of ∼4.5×104 S/cm, even under stretching, bending or torsional deformations over 10,000 cycles. As a proof of demonstration, the hEP-skin successfully integrates stretchable light-emitting electronic skin with adaptive thermal management photonic skin.

Materials in Radiative Cooling Technologies

Radiative cooling (RC) is a carbon-neutral cooling technology that utilizes thermal radiation to dissipate heat from the Earth's surface to the cold outer space. Research in the field of RC has garnered increasing interest from both academia and industry due to its potential to drive sustainable economic and environmental benefits to human society by reducing energy consumption and greenhouse gas emissions from conventional cooling systems. Materials innovation is the key to fully exploit the potential of RC. This review aims to elucidate the materials development with a focus on the design strategy including their intrinsic properties, structural formations, and performance improvement. The main types of RC materials, i.e., static-homogeneous, static-composite, dynamic, and multifunctional materials, are systematically overviewed. Future trends, possible challenges, and potential solutions are presented with perspectives in the concluding part, aiming to provide a roadmap for the future development of advanced RC materials.

Anti-Environmental Aging Passive Daytime Radiative Cooling

Passive daytime radiative cooling technology presents a sustainable solution for combating global warming and accompanying extreme weather, with great potential for diverse applications. The key characteristics of this cooling technology are the ability to reflect most sunlight and radiate heat through the atmospheric transparency window. However, the required high solar reflectance is easily affected by environmental aging, rendering the cooling ineffective. In recent years, significant advancements have been made in understanding the failure mechanisms, design strategies, and manufacturing technologies of daytime radiative cooling. Herein, a critical review on anti-environmental aging passive daytime radiative cooling with the goal of advancing their commercial applications is presented. It is first introduced the optical mechanisms and optimization principles of radiative cooling, which serve as a basis for further endowing environmental durability. Then the environmental aging conditions of passive daytime radiative cooling, mainly focusing on UV exposure, thermal aging, surface contamination and chemical corrosion are discussed. Furthermore, the developments of anti-environmental aging passive daytime radiative cooling materials, including design strategies, fabrication techniques, structures, and performances, are reviewed and classified for the first time. Last but not the least, the remaining open challenges and the insights are presented for the further promotion of the commercialization progress.

A dual-selective thermal emitter with enhanced subambient radiative cooling performance

Radiative cooling is a zero-energy technology that enables subambient cooling by emitting heat into outer space (~3 K) through the atmospheric transparent windows. However, existing designs typically focus only on the main atmospheric transparent window (8-13 μm) and ignore another window (16-25 μm), under-exploiting their cooling potential. Here, we show a dual-selective radiative cooling design based on a scalable thermal emitter, which exhibits selective emission in both atmospheric transparent windows and reflection in the remaining mid-infrared and solar wavebands. As a result, the dual-selective thermal emitter exhibits an ultrahigh subambient cooling capacity (~9 °C) under strong sunlight, surpassing existing typical thermal emitters (≥3 °C cooler) and commercial counterparts (as building materials). Furthermore, the dual-selective sample also exhibits high weather resistance and color compatibility, indicating a high practicality. This work provides a scalable and practical radiative cooling design for sustainable thermal management.

Ultrahigh Passive Cooling Power in Hydrogel with Rationally Designed Optofluidic Properties

The cooling power of a radiative cooler is more than halved in the tropics, e.g., Singapore, because of its harsh weather conditions including high humidity (84% on average), strong downward atmospheric radiation (∼40% higher than elsewhere), abundant rainfall, and intense solar radiation (up to 1200 W/m2 with ∼58% higher UV irradiation). So far, there has been no report of daytime radiative cooling that well achieves effective subambient cooling. Herein, through integrated passive cooling strategies in a hydrogel with desirable optofluidic properties, we demonstrate stable subambient (4-8 °C) cooling even under the strongest solar radiation in Singapore. The integrated passive cooler achieves an ultrahigh cooling power of ∼350 W/m2, 6-10 times higher than a radiative cooler in a tropical climate. An in situ study of radiative cooling with various hydration levels and ambient humidity is conducted to understand the interaction between radiation and evaporative cooling. This work provides insights for the design of an integrated cooler for various climates.

Tandem Electro-Thermo-Catalysis for the Oxidative Aminocarbonylation of Arylboronic Acids to Amides from CO2 and Water

Direct CO2 electroreduction to valuable chemicals is critical for carbon neutrality, while its main products are limited to simple C1 /C2 compounds, and traditionally, the anodic O2 byproduct is not utilized. We herein report a tandem electrothermo-catalytic system that fully utilizes both cathodic (i.e., CO) and anodic (i.e., O2 ) products during overall CO2 electrolysis to produce valuable organic amides from arylboronic acids and amines in a separate chemical reactor, following the Pd(II)-catalyzed oxidative aminocarbonylation mechanism. Hexamethylenetetramine (HMT)-incorporated silver and nickel hydroxide carbonate electrocatalysts were prepared for efficient coproduction of CO and O2 with Faradaic efficiencies of 99.3 % and 100 %, respectively. Systematic experiments, operando attenuated total reflection surface-enhanced Fourier transform infrared spectroscopy characterizations and theoretical studies reveal that HMT promotes *CO2 hydrogenation/*CO desorption for accelerated CO2 -to-CO conversion, and O2 inhibits reductive deactivation of the Pd(II) catalyst for enhanced oxidative aminocarbonylation, collectively leading to efficient synthesis of 10 organic amides with high yields of above 81 %. This work demonstrates the effectiveness of a tandem electrothermo-catalytic strategy for economically attractive CO2 conversion and amide synthesis, representing a new avenue to explore the full potential of CO2 utilization.

Quantifying colors at micrometer scale by colorimetric microscopy (C-Microscopy) approach

The color is the primal property of the objects around us and is direct manifestation of light-matter interactions. The color information is used in many different fields of science, technology and industry to investigate material properties or for identification of concentrations of substances. Usually the color information is used as a global parameter in a macro scale. To quantitatively measure color information in micro scale one needs to use dedicated microscope spectrophotometers or specialized micro-reflectance setups. Here, the Colorimetric Microscopy (C-Microscopy) approach based on digital optical microscopy and a free software is presented. The C-Microscopy approach uses color calibrated image and colorimetric calculations to obtain physically meaningful quantities i.e., dominant wavelength and excitation purity maps at micro level scale. This allows for the discovery of the local color details of samples surfaces. Later, to fully characterize the optical properties, the hyperspectral reflectance data at micro scale (reflectance as a function of wavelength for a each point) are colorimetrically recovered. The C-Microscopy approach was successfully applied to various types of samples i.e., two metamorphic rocks unakite and lapis lazuli, which are mixtures of different minerals; and to the surface of gold 99.999 % pellet, which exhibits different types of surface features. The C-Microscopy approach could be used to quantify the local optical properties changes of various materials at microscale in an accessible way. The approach is freely available as a set of python jupyter notebooks.

Radiative Cooling for Energy Sustainability: From Fundamentals to Fabrication Methods Toward Commercialization

Radiative cooling, a technology that lowers the temperature of terrestrial objects by dissipating heat into outer space, presents a promising ecologically-benign solution for sustainable cooling. Recent years witness substantial progress in radiative cooling technologies, bringing them closer to commercialization. This comprehensive review provides a structured overview of radiative cooling technologies, encompassing essential principles, fabrication techniques, and practical applications, with the goal of guiding researchers toward successful commercialization. The review begins by introducing the fundamentals of radiative cooling and the associated design strategies to achieve it. Then, various fabrication methods utilized for the realization of radiative cooling devices are thoroughly discussed. This discussion includes detailed assessments of scalability, fabrication costs, and performance considerations, encompassing both structural designs and fabrication techniques. Building upon these insights, potential fabrication approaches suitable for practical applications and commercialization are proposed. Further, the recent efforts made toward the practical applications of radiative cooling technology, including its visual appearance, switching capability, and compatibility are examined. By encompassing a broad range of topics, from fundamental principles to fabrication and applications, this review aims to bridge the gap between theoretical research and real-world implementation, fostering the advancement and widespread adoption of radiative cooling technology.

Vivid Colored Cooling Structure Managing Full Solar Spectrum via Near-Infrared Reflection and Photoluminescence

Colored radiative cooling (CRC) offers an attractive alternative for surface and space cooling, while preserving the aesthetics of an object. However, there has been no study on the CRC using phosphors in regard to vivid coloration, sophisticated performance investigation, retention of properties, functionality, and structural flexibility all at once. Thus, to manage the entire solar spectrum, a colored cooling structure comprising a near-infrared (NIR)-reflective bottom layer and a top colored layer with a phosphor-embedded polymer matrix is proposed. The structure is paintable, vividly colored, hydrophobic, and ultraviolet (UV) and water resistant. In the daytime outdoor measurement, the structure with red, orange, and yellow colors exhibited lower temperature than a control group using commercial white paint by 4.7 °C, 7.2 °C, and 7.4 °C, respectively. After precise theoretical and experimental time-tracing temperature validation, the CRC performance enhancement from NIR reflection and photoluminescence effects was thoroughly analyzed, and a temperature reduction of up to 16.1 °C was achieved for the orange-colored structure. Furthermore, experiments of hydrophobicity infusion and exposure to UV and deionized water verified the durability of the colored cooling structure. In addition, flexible-film-type colored cooling structures were demonstrated using different bottom reflective layers, such as a silver thin film and porous aluminum oxide particle-embedded poly(vinylidene fluoride-co-hexafluoropropylene), suggesting the potential applicability of these colored cooling structures for vivid-colored, functional, and durable CRC.

Structural Color from Cellulose Nanocrystals or Chitin Nanocrystals: Self-Assembly, Optics, and Applications

Widespread concerns over the impact of human activity on the environment have resulted in a desire to replace artificial functional materials with naturally derived alternatives. As such, polysaccharides are drawing increasing attention due to offering a renewable, biodegradable, and biocompatible feedstock for functional nanomaterials. In particular, nanocrystals of cellulose and chitin have emerged as versatile and sustainable building blocks for diverse applications, ranging from mechanical reinforcement to structural coloration. Much of this interest arises from the tendency of these colloidally stable nanoparticles to self-organize in water into a lyotropic cholesteric liquid crystal, which can be readily manipulated in terms of its periodicity, structure, and geometry. Importantly, this helicoidal ordering can be retained into the solid-state, offering an accessible route to complex nanostructured films, coatings, and particles. In this review, the process of forming iridescent, structurally colored films from suspensions of cellulose nanocrystals (CNCs) is summarized and the mechanisms underlying the chemical and physical phenomena at each stage in the process explored. Analogy is then drawn with chitin nanocrystals (ChNCs), allowing for key differences to be critically assessed and strategies toward structural coloration to be presented. Importantly, the progress toward translating this technology from academia to industry is summarized, with unresolved scientific and technical questions put forward as challenges to the community.

A Hyperspectral Camouflage Colorant Inspired by Natural Leaves

The unmet spectral mimicry of foliar green in camouflage materials is hampered by the lack of colorants with similar spectral properties to chlorophyll, resulting in substantial risks of exposure from hyperspectral target detection. By drawing inspiration from leaf chromogenesis, a microcapsule colorant with a chloroplast-like structure and chlorophyll-like absorption is developed, and a generic bilayer coating is designed to provide high spectral similarity to leaves with different growth stages, seasons, and species. Specifically, the microcapsule colorant preserves the monomeric absorption of the internal phthalocyanine and features the manufacturability of conventional pigments, such as amenability to painting and patterning, and compatibility to different substrates. The pigmented artificial leaves successfully deceive the hyperspectral classification algorithm in a foliar background, and outperforming the state-of-art spectral simulation materials. This coloration strategy expands the knowledge base of the spectral fine tuning of composite colorants, which are essential for their application in spectral-resolved optical materials.

Artificial Skin with Patterned Stripes for Color Camouflage and Thermoregulation

Chameleons are famous for their quick color changing abilities, and it is commonly assumed that they do this for camouflage. However, recent reports revealed that chameleons also change color for body temperature regulation. Inspired by the structure of the panther chameleon's skin, a stripe-patterned poly(N-isopropylacrylamide) (PNIPAM) and polyacrylamide (PAM) hydrogel film with a laminated structure is fabricated in this work; thus, both camouflage and thermoregulation can be achieved through controlling Vis and NIR light effectively. For the PNIPAM stripe, the upper layer is the native PNIPAM hydrogel and the lower layer is the carbon nanotube-composited PNIPAM hydrogel. Thus, the PNIPAM stripe is capable of reaching 28 °C at a low environmental temperature (12 °C) and a low radiation intensity (20 mW cm-2), while preventing the body temperature from rising by changing to white under a strong radiation intensity (100 mW cm-2). For the PAM stripe, the upper layer combines colloidal photonic crystals and displays a tunable structural color by stretching, and the lower layer is mixed with PNIPAM microgels for thermal regulation. Through the fabrication of multifunctional patterns, the film can achieve both dynamic structural color and thermoregulation by precisely controlling solar radiation absorption, scattering, and reflection. More importantly, in the stripe-patterned system, the shrinkage of the PNIPAM stripes can effectively trigger the elongation of the PAM stripe, which endows the structural color changing process to be self-powered completely. The performances show that the stripe-patterned film may have potential applications in intelligent coatings, especially in areas with large temperature differences during the day such as high plains.

Tailor-Made White Photothermal Fabrics: A Bridge between Pragmatism and Aesthetic

Maintaining human thermal comfort in the cold outdoors is crucial for diverse outdoor activities, e.g., sports and recreation, healthcare, and special occupations. To date, advanced clothes are employed to collect solar energy as a heat source to stand cold climates, while their dull dark photothermal coating may hinder pragmatism in outdoor environments and visual sense considering fashion. Herein, tailor-made white webs with strong photothermal effect are proposed. With the embedding of cesium-tungsten bronze (Csx WO3 ) nanoparticles (NPs) as additive inside nylon nanofibers, these webs are capable of drawing both near-infrared (NIR) and ultraviolet (UV) light in sunlight for heating. Their exceptional photothermal conversion capability enables 2.5-10.5 °C greater warmth than that of a commercial sweatshirt of six times greater thickness under different climates. Remarkably, this smart fabric can increase its photothermal conversion efficiency in a wet state. It is optimal for fast sweat or water evaporation at human comfort temperature (38.5 °C) under sunlight, and its role in thermoregulation is equally important to avoid excess heat loss in wilderness survival. Obviously, this smart web with considerable merits of shape retention, softness, safety, breathability, washability, and on-demand coloration provides a revolutionary solution to realize energy-saving outdoor thermoregulation and simultaneously satisfy the needs of fashion and aesthetics.

Scalable Bacterial Cellulose-Based Radiative Cooling Materials with Switchable Transparency for Thermal Management and Enhanced Solar Energy Harvesting

Radiative cooling materials that can dynamically control solar transmittance and emit thermal radiation into cold outer space are critical for smart thermal management and sustainable energy-efficient buildings. This work reports the judicious design and scalable fabrication of biosynthetic bacterial cellulose (BC)-based radiative cooling (Bio-RC) materials with switchable solar transmittance, which are developed by entangling silica microspheres with continuously secreted cellulose nanofibers during in situ cultivation. Theresulting film shows a high solar reflection (95.3%) that can be facilely switched between an opaque state and a transparent state upon wetting. Interestingly, the Bio-RC film exhibits a high mid-infrared emissivity (93.4%) and an average sub-ambient temperature drop of ≈3.7 °C at noon. When integrating with a commercially available semi-transparent solar cell, the switchable solar transmittance of Bio-RC film enables an enhancement of solar power conversion efficiency (opaque state: 0.92%, transparent state: 0.57%, bare solar cell: 0.33%). As a proof-of-concept illustration, an energy-efficient model house with its roof built with Bio-RC-integrated semi-transparent solar cell is demonstrated. This research can shine new light on the design and emerging applications of advanced radiative cooling materials.

Colorful low-emissivity paints for space heating and cooling energy savings

Space heating and cooling consume ~13% of global energy every year. The development of advanced materials that promote energy savings in heating and cooling is gaining increasing attention. To thermally isolate the space of concern and minimize the heat exchange with the outside environment has been recognized as one effective solution. To this end, here, we develop a universal category of colorful low-emissivity paints to form bilayer coatings consisting of an infrared (IR)-reflective bottom layer and an IR-transparent top layer in colors. The colorful visual appearance ensures the aesthetical effect comparable to conventional paints. High mid-infrared reflectance (up to ~80%) is achieved, which is more than 10 times as conventional paints in the same colors, efficiently reducing both heat gain and loss from/to the outside environment. The high near-IR reflectance also benefits reducing solar heat gain in hot days. The advantageous features of these paints strike a balance between energy savings and penalties for heating and cooling throughout the year, providing a comprehensive year-round energy-saving solution adaptable to a wide variety of climatic zones. Taking a typical midrise apartment building as an example, the application of our colorful low-emissivity paints can realize positive heating, ventilation, and air conditioning energy saving, up to 27.24 MJ/m2/y (corresponding to the 7.4% saving ratio). Moreover, the versatility of the paint, along with its applicability to diverse surfaces of various shapes and materials, makes the paints extensively useful in a range of scenarios, including building envelopes, transportation, and storage.

Thermal Differences in the Plantar Surface Skin of the Foot after Using Three Different Lining Materials for Plantar Orthotics

The lining materials of plantar orthoses are chosen for their hardness, breathability, and moisture absorption, but without there being any clear scientific criterion. Thermographic analysis would provide information about the thermal response of the sole of the foot, and would thereby allow the choice to be adapted in accordance with this criterion. The objective of this study was to evaluate plantar temperatures after the use of three materials with different characteristics. Plantar temperatures were analyzed by using a FLIR E60BX thermographic camera on 36 participants (15 men and 21 women, 24.6 ± 8.2 years old, 67.1 ± 13.6 kg, and 1.7 ± 0.09 m). Measurements were made before and after (3 h) the use of three lining materials for plantar orthoses (Material 1: PE copolymer; Material 2: EVA; Material 3: PE-EVA copolymer) on different days. For Material 1 (PE), the temperature under the heel was significantly higher after exercise, increasing from 30.8 ± 2.9 °C to 31.9 ± 2.8 °C (p = 0.008), and negative correlations were found between room temperature and the pre/post temperature difference for the big toe (r = -0.342, p = 0.041) and the 1st metatarsal head (r = -0.334, p = 0.046). No significant pre/post temperature differences were found with the other materials. The three materials thermoregulated the plantar surface efficiently by maintaining the skin temperature at levels similar to those evaluated before exercise. If PE is used as a lining material, it should be avoided for the heel area in patients with hyperhidrosis or those with a tendency to suffer from skin pathologies due to excess moisture.

Universal Color Retrofit to Polymer-Based Radiative Cooling Materials

Polymers with broad infrared emission and negligible solar absorption have been identified as promising radiative cooling materials to offer a sustainable and energy-saving venue. Although practical applications desire color for visual appearance, the current coloration strategies of polymer-based radiative cooling materials are constrained by material, cost, and scalability. Here, we demonstrate a universally applicable coloration strategy for polymer-based radiative cooling materials by nanoimprinting. By modulating light interference with periodic structures on polymer surfaces, specular colors can be induced while maintaining the hemispheric optical responses of radiative cooling polymers. The retrofit strategy is exemplified by four different polymer films with a minimum impact on optical responses compared to the pristine films. Polymer films feature low solar absorption of 1.7-3.7%, and daytime sub-ambient cooling is exemplified in the field test. The durability of radiative cooling and color are further validated by dynamic spectral analysis. Finally, the potential roll-to-roll manufacturing empowers a scalable, low-cost, and easy-retrofitting solution for colored radiative cooling films.

Emerging Materials and Strategies for Passive Daytime Radiative Cooling

In recent decades, the growing demands for energy saving and accompanying heat mitigation concerns, together with the vital goal for carbon neutrality, have drawn human attention to the zero-energy-consumption cooling technique. Recent breakthroughs in passive daytime radiative cooling (PDRC) might be a potent approach to combat the energy crisis and environmental challenges by directly dissipating ambient heat from the Earth to the cold outer space instead of only moving the heat across the Earth's surface. Despite significant progress in cooling mechanisms, materials design, and application exploration, PDRC faces potential functionalization, durability, and commercialization challenges. Herein, emerging materials and rational strategies for PDRC devices are reviewed. First, the fundamental physics and thermodynamic concepts of PDRC are examined, followed by a discussion on several categories of PDRC devices developed to date according to their implementation mechanism and material properties. Emerging strategies for performance enhancement and specific functions of PDRC are discussed in detail. Potential applications and possible directions for designing next-generation high-efficiency PDRC are also discussed. It is hoped that this review will contribute to exciting advances in PDRC and aid its potential applications in various fields.

Scalable Colored Subambient Radiative Coolers Based on a Polymer-Tamm Photonic Structure

Daytime radiative coolers cool objects below the air temperature without any electricity input, while most of them are limited by a silvery or whitish appearance. Colored daytime radiative coolers (CDRCs) with diverse colors, scalable manufacture, and subambient cooling have not been achieved. We introduce a polymer-Tamm photonic structure to enable a high infrared emittance and an engineered absorbed solar irradiance, governed by the quality factor (Q-factor). We theoretically determine the theoretical thresholds for subambient cooling through yellow, magenta, and cyan CDRCs. We experimentally fabricate and observe a temperature drop of 2.6-8.8 °C on average during the daytime and 4.0-4.4 °C during the nighttime. Furthermore, we demonstrate a scalable-manufactured magenta CDRC with a width of 60 cm and a length of 500 cm by a roll-to-roll deposition technique. This work provides guidelines for large-scale CDRCs and offers unprecedented opportunities for potential applications with energy-saving, aesthetic, and visual comfort demands.

Radiative cooling for passive thermal management towards sustainable carbon neutrality

Photonic structures at the wavelength scale offer innovative energy solutions for a wide range of applications, from high-efficiency photovoltaics to passive cooling, thus reshaping the global energy landscape. Radiative cooling based on structural and material design presents new opportunities for sustainable carbon neutrality as a zero-energy, ecologically friendly cooling strategy. In this review, in addition to introducing the fundamentals of the basic theory of radiative cooling technology, typical radiative cooling materials alongside their cooling effects over recent years are summarized and the current research status of radiative cooling materials is outlined and discussed. Furthermore, technical challenges and potential advancements for radiative cooling are forecast with an outline of future application scenarios and development trends. In the future, radiative cooling is expected to make a significant contribution to global energy saving and emission reduction.

Dual-Encapsulated Nanocomposite for Efficient Thermal Buffering in Heat-Generating Radiative Cooling

Radiative cooling has been considered an innovative passive method to resolve the problem of overheating of electronic devices. However, it is inefficient for cooling huge heat generation components. Herein, we report a dual-encapsulated nanocomposite (DEN) by integrating radiative cooling and phase-change materials (PCMs) for thermal buffering in heat-generating radiative cooling. The leak of PCMs is avoided by a simple dual-encapsulated structure with a three-dimensional (3D) interconnected cellular-like network structure and radiative cooling layer on the surface, 75% superior to the state-of-the-art single encapsulation designs. Additionally, our DEN not only shows outstanding optical properties with strong solar reflection (R̅_solar = 0.96) and IR-selective emission (ε̅_8-13 μm = 0.94 and η_ε = 1.15) but also exhibits high phase-change enthalpy (ΔH_m = 192.2 J/g, ΔH_c = 175.7 J/g), enabling remarkable radiative cooling capability and desirable thermal energy peak shaving and valley filling effect. Outdoor experiments demonstrate that DEN achieves a temperature drop up to 23 °C compared to the control group without DEN coverage when electronics generate heat. This dual-encapsulated nanocomposite provides a novel strategy and solution for outdoor passive thermal management.

Daytime Sub-Ambient Radiative Cooling with Vivid Structural Colors Mediated by Coupled Nanocavities

Daytime radiative cooling is a promising passive cooling technology for combating global warming. Existing daytime radiative coolers usually show whitish colors due to their high broadband solar reflectivity, which is not suitable for aesthetic demands and effective display. It is challenging to produce high-cooling performance materials with vivid colors because colors are often produced by the absorption of visible light, decreasing net cooling power. In this work, we design a series of colorful multilayered radiative coolers (CMRCs) consisting of an optimized selective emitter for cooling and coupled nanocavities for structural coloration, which can successfully delicately balance the trade-off between the chromaticity and cooling performance. By judiciously designing the geometric parameters and manipulating the coupling effect inside the coupled nanocavities, our coolers show sub-ambient cooling performance and a larger color gamut (occupying 17.7% sRGB area) than reported ones. We further fabricate CMRCs and demonstrate that they have temperature drops of 3.4-4.4 °C on average based on outdoor experiments. These CMRCs are promising in thermal management of electronic/optoelectronic devices and outdoor facilities.

Superhydrophobic Porous Coating of Polymer Composite for Scalable and Durable Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) technology provides an eco-friendly cooling strategy by reflecting sunlight reaching the surface and radiating heat underneath to the outer space through the atmospheric transparency window. However, PDRC materials face challenges in cooling performance degradation caused by outdoor contamination and requirements of easy fabrication approaches for scale-up and high cooling efficiency. Herein, a polymer composite coating of polystyrene, polydimethylsiloxane and poly(ethyl cyanoacrylate) (PS/PDMS/PECA) with superhydrophobicity and radiative cooling performance was fabricated and demonstrated to have sustained radiative cooling capability, utilizing the superhydrophobic self-cleaning property to maintain the optical properties of the coating surface. The prepared coating is hierarchically porous which exhibits an average solar reflectance of 96% with an average emissivity of 95% and superhydrophobicity with a contact angle of 160°. The coating realized a subambient radiative cooling of 12.9 °C in sealed air and 7.5 °C in open air. The self-cleaning property of the PS/PDMS/PECA coating helped sustain the cooling capacity for long-term outdoor applications. Moreover, the coating exhibited chemical resistance, UV resistance, and mechanical durability, which has promising applications in wider fields.

Structurally Colored Radiative Cooling Cellulosic Films

Daytime radiative cooling (DRC) materials offer a sustainable approach to thermal management by exploiting net positive heat transfer to deep space. While such materials typically have a white or mirror-like appearance to maximize solar reflection, extending the palette of available colors is required to promote their real-world utilization. However, the incorporation of conventional absorption-based colorants inevitably leads to solar heating, which counteracts any radiative cooling effect. In this work, efficient sub-ambient DRC (Day: -4 °C, Night: -11 °C) from a vibrant, structurally colored film prepared from naturally derived cellulose nanocrystals (CNCs), is instead demonstrated. Arising from the underlying photonic nanostructure, the film selectively reflects visible light resulting in intense, fade-resistant coloration, while maintaining a low solar absorption (≈3%). Additionally, a high emission within the mid-infrared atmospheric window (>90%) allows for significant radiative heat loss. By coating such CNC films onto a highly scattering, porous ethylcellulose (EC) base layer, any sunlight that penetrates the CNC layer is backscattered by the EC layer below, achieving broadband solar reflection and vibrant structural color simultaneously. Finally, scalable manufacturing using a commercially relevant roll-to-roll process validates the potential to produce such colored radiative cooling materials at a large scale from a low-cost and sustainable feedstock.

Structurally Colored Cellulose Nanocrystal Films as Transreflective Radiative Coolers

Radiative cooling forms an emerging direction in which objects are passively cooled via thermal radiation to cold space. Cooling materials should provide high thermal emissivity (infrared absorptance) and low solar absorptance, making cellulose an ideal and sustainable candidate. Broadband solar-reflective or transparent coolers are not the only systems of interest, but also more pleasingly looking colored systems. However, solutions based on wavelength-selective absorption generate not only color but also heat and thereby counteract the cooling function. Intended as coatings for solar cells, we demonstrate a transreflective cellulose material with minimal solar absorption that generates color by wavelength-selective reflection, while it transmits other parts of the solar spectrum. Our solution takes advantage of the ability of cellulose nanocrystals to self-assemble into helical periodic structures, providing nonabsorptive films with structurally colored reflection. Application of violet-blue, green, and red cellulose films on silicon substrates reduced the temperature by up to 9 °C under solar illumination, as result of a combination of radiative cooling and reduced solar absorption due to the wavelength-selective reflection by the colored coating. The present work establishes self-assembled cellulose nanocrystal photonic films as a scalable photonic platform for colored radiative cooling.

Highly solar reflectance and infrared transparent porous coating for non-contact heat dissipations

Passive daytime radiative cooling (PDRC) can dissipate heat to outer space with high solar reflectance (R¯solar) and thermal emittance (ε¯LWIR) in the atmospheric transmission window. However, for the non-contact heat dissipation, besides the high R¯solar, a high infrared transmittance (τ¯LWIR) is needed to directly emit thermal radiation through the IR-transparent coating to outer space. In this work, An IR-transparent porous PE (P-PE) coating with R¯solar= 0.96 and τ¯LWIR= 0.88 was prepared for non-contact heat dissipations. Under the direct sunlight of 860 W m⁻², the IR-transparent coating obtained a 4°C lower heater temperature than the normal PDRC coating under the same condition. In addition, the spectral reflectance of the P-PE coating after immersing in air or water changed little, which showed excellent durability for long-term outdoor applications. These results indicate the P-PE coating can be a potential IR-transparent coating for non-contact heat dissipations under direct sunlight.

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Infrared transparent coating was used for non-contact heat dissipations

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High solar reflectance R¯solar and IR-transmittance τ¯LWIR can be 0.96 and 0.88

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IR transparent coating obtained a 4°C lower heater temperature than normal coating

Photonic-Structure Colored Radiative Coolers for Daytime Subambient Cooling

Daytime subambient radiative cooling provides a powerful strategy for realizing sustainable thermal management without any external energy consumption. However, in practical situations a dazzling white or silver appearance is undesirable for aesthetic and functional reasons. Therefore, developing colored radiative cooling materials is greatly significant for more potential applications but remains a big challenge so far. Here, we reported a flexible colored radiative cooler based on interferometric retroreflection-induced structural color, which resolves the conflict between a colorful appearance for aesthetics and high solar reflection for cooling. All colored radiative coolers achieve subambient cooling of 4 K even under sunshine stronger than 1000 W/m², while the same color commercial paints are 9-27 K higher than the ambient. Such a flexible, scalable, and low cost colored radiative cooler is expected to replace commercial paint in a practical scenario with aesthetic and cooling requirements, enabling substantial reduction in carbon emission and energy consumption.

Color-preserving passive radiative cooling for an actively temperature-regulated enclosure

Active temperature control devices are widely used for the thermal management of enclosures, including vehicles and buildings. Passive radiative cooling has been extensively studied; however, its integration with existing actively temperature regulated and decorative enclosures has slipped out of the research at status quo. Here, we present a photonic-engineered dual-side thermal management strategy for reducing the active power consumption of the existing temperature-regulated enclosure without sacrificing its aesthetics. By coating the exterior and interior of the enclosure roof with two visible-transparent films with distinctive wavelength-selectivity, simultaneous control over the energy exchange among the enclosure with the hot sun, the cold outer space, the atmosphere, and the active cooler can be implemented. A power-saving of up to 63% for active coolers of the enclosure is experimentally demonstrated by measuring the heat flux compared to the ordinary enclosure when the set temperature is around 26°C. This photonic-engineered dual-side thermal management strategy offers facile integration with the existing enclosures and represents a new paradigm toward carbon neutrality.

Recent Progress in Daytime Radiative Cooling: Advanced Material Designs and Applications

Passive daytime radiative cooling (PDRC) is emerging as a promising cooling technology. Owing to the high, broadband solar reflectivity and high mid-infrared emissivity, daytime radiative cooling materials can achieve passive net cooling power under direct sunlight. The zero-energy-consumption characteristic enables PDRC to reduce negative environmental issues compared with conventional cooling systems. In this review, the development of advanced daytime radiative cooling designs is summarized, recent progress is highlighted, and potential correlated applications, such as building cooling, photovoltaic cooling, and electricity generation, are introduced. The remaining challenges and opportunities of PDRCs are also indicated. It is expected that this review provides an overall picture of recent PDRC progress and inspires future research regarding the fundamental understanding and practical applications of PDRC.

Cool Surface Strategies with an Emphasis on the Materials Dimension: A Review

The need to tackle the urban heat island effect demands the implementation of cool surfaces as a mitigation strategy. This study comprehensively reviews the evolution of this research field from a materials perspective. It provides a bibliometric analysis of the relevant literature using the SciMAT software processing of bibliographic records from 1995 to 2020, for the evolution of cool surfaces. The results obtained show an increased interest in the field from 2011 to 2020, particularly for roof applications, and present the scientific evolution of reflective materials. According to the materials dimension adopted by the development of the research field, the study is refined from a bibliometric analysis of 982 selected records for the analysis of five themes: (i) Pigments; (ii) Phase change materials; (iii) Retroreflective materials; (iv) Ceramic materials; and (v) Glass. These materials present promising results in terms of their solar reflectance performances in the mitigation of the urban heat island phenomenon. At the end of this review, recommendations for future studies are provided for the creation of economic and environmentally friendly materials based on waste glass recycling. This study represents a valuable contribution that provides a scientific background with regard to cool surfaces from a materials perspective for future investigations.

Superinsulating BNNS/PVA Composite Aerogels with High Solar Reflectance for Energy-Efficient Buildings

With the mandate of worldwide carbon neutralization, pursuing comfortable living environment while consuming less energy is an enticing and unavoidable choice. Novel composite aerogels with super thermal insulation and high sunlight reflection are developed for energy-efficient buildings. A solvent-assisted freeze-casting strategy is used to produce boron nitride nanosheet/polyvinyl alcohol (BNNS/PVA) composite aerogels with a tailored alignment channel structure. The effects of acetone and BNNS fillers on microstructures and multifunctional properties of aerogels are investigated. The acetone in the PVA suspension enlarges the cell walls to suppress the shrinkage, giving rise to a lower density and a higher porosity, accompanied with much diminished heat conduction throughout the whole product. The addition of BNNS fillers creates whiskers in place of disconnected transverse ligaments between adjacent cell walls, further ameliorating the thermal insulation transverse to the cell wall direction. The resultant BNNS/PVA aerogel delivers an ultralow thermal conductivity of 23.5 mW m^-1 K^-1 in the transverse direction. The superinsulating aerogel presents both an infrared stealthy capability and a high solar reflectance of 93.8% over the whole sunlight wavelength, far outperforming commercial expanded polystyrene foams with reflective coatings. The anisotropic BNNS/PVA composite aerogel presents great potential for application in energy-saving buildings.

Interfacial Solar Steam/Vapor Generation for Heating and Cooling

Interfacial solar steam/vapor technology uses abundant and clean solar energy and water to achieve heating and cooling, a promising technology to alleviate environmental and energy issues. To obtain higher conversion and utilization efficiency, designing and optimizing materials, structures, and devices of interfacial solar steam/vapor technologies attract the attention of the research community. Given the significant progress made in the past 5 years, it is valuable to systematically summarize and discuss recent developments and future trends in this new multidisciplinary direction. This review aims to introduce interfacial solar steam/vapor principles to realize heating and cooling and the recent progress in materials, structures, devices, and applications. Meanwhile, some unsolved scientific and technical problems with outlook will also be discussed, hoping to promote further the rapid development and application of interfacial solar steam/vapor technology in heating and cooling to alleviate energy and environmental problems.

Full-Color Solar-Heat-Resistant Films Based on Nanometer Optical Coatings

Nanometer optical coatings with absorbing materials allow the tuning of structured absorption spectra, thus developing ultrathin color devices. However, these coatings are limited by the narrow bandwidth and tunability of wavelength that restrict the chroma and hue characteristics of colors, respectively, apart from imposing adverse thermal problems under sunlight exposure. Here, we demonstrate that inversely designed TiN/ZnS/Ag coatings attain a wide color gamut in the trilayer configuration and efficiently dissipate heat through thermal radiation when transfer-printed on high-emissivity polymers. Daytime experiments reveal that fabricated optical films yield an almost color-independent heat dissipation rate against solar heating. Moreover, they outperform commercial paints of the same color when applied to three-dimensional miniature houses. All magenta, green, cyan, and yellow optical films lower the roof temperature by 10, 6, 8, and 2 °C below one sun irradiance, respectively, compared to their paint counterparts; the temperature gradient increases directly with the level of sunlight.

Scalable thermochromic smart windows with passive radiative cooling regulation

[Figure: see text].

One-step preparation of Cr2O3-based inks with long-term dispersion stability for inkjet applications

Inkjet printing of functional materials has shown a wide range of applications in advertising, OLED display, printed electronics and other specialized utilities that require high-precision, mask-free, direct-writing deposition techniques. Nevertheless, the sedimentation risk of the refractory functional materials dispensed in inks hinders their further implementation. Herein, we present a bottom-up ink preparation strategy based on Cr₂O₃ by a one-step solvothermal method. The obtained ink remained stable under an equivalent natural sediment test for 2.5 years. The chemical composition of the solvothermal product was characterized, and the mechanism of the superior dispersion stability of Cr₂O₃ particles was analysed. These amorphous Cr₂O₃ particles were capped by ligands generated via low-temperature solvothermal reactions. Ethanol and acetylacetone covering the particle surfaces play an essential role in enhancing the solubility of Cr₂O₃ particles in the solvent forming the ultrastable colloidal ink. Moreover, this ink was successfully printed using a direct-write inkjet system JetLab®II on nylon fabrics, and the printed area of the fabrics shows a spectral correlation coefficient of 0.9043 to green leaves. Finally, we believe that the one-step bottom-up fabrication method of Cr₂O₃-based pigment inks may provide a general approach for preparing metal oxide-based pigment inks with long-term dispersion stability.

Highly Sunlight Reflective and Infrared Semi-Transparent Nanomesh Textiles

Radiative cooling in textiles is one of the important factors enabling cooling of the human body for thermal comfort. In particular, under an intense sunlight environment such as that experienced with outdoor exercise and sports activities, high near-infrared (NIR) reflectance to block sunlight energy influx along with high IR transmittance in textiles for substantial thermal emission from the human body would be highly desirable. This investigation demonstrates that a nanoscale geometric control of textile structure alone, instead of complicated introduction of specialty polymer materials and composites, can enable such desirable NIR and IR optical properties in textiles. A diameter-dependent Mie scattering event in fibers and associated optical and thermal behavior were simulated in relation to a nonwoven, nanomesh textile. As an example, a nanomesh structure made of PVDF (polyvinylidene fluoride) electrospun fibers with ∼600 nm average diameter was examined, which exhibited a significant radiative cooling performance with over 90% solar and NIR reflectance to profoundly block the sunlight energy influx as well as ∼50% IR transmittance for human body radiative heat dissipation. An extraordinary cooling effect, as much as 12 °C, was obtained on a simulated skin compared to the normal textile fabric materials. Such a powerful radiative cooling performance together with IR transmitting capability by the nanomesh textile offers a way to efficiently manage sunlight radiation energy to make persons, devices, and transport vehicles cooler and help to save energy in an outdoor sunlight environment as well as indoor conditions.

High-Temperature Carbonized Ceria Thermophotovoltaic Emitter beyond Tungsten

Thermophotovoltaics (TPVs) require emitters with a regulated radiation spectrum tailored to the spectral response of integrated photovoltaic cells. Such spectrally engineered emitters developed thus far are structurally too complicated to be scalable, are thermally unstable, or lack reliability in terms of temperature cycling. Herein, we report wafer-scale, thermal-stress-free, and wavelength-selective emitters that operate for high-temperature TPVs equipped with GaSb photovoltaic cells. One inch crystalline ceria wafers were prepared by sequentially pressing and annealing the pellets of ceria nanoparticles. The direct pyrolysis of citric acid mixed with ceria nanoparticles created agglomerated, pyrolytic carbon and ceria microscale dots, thus forming a carbonized film strongly adhering to a wafer surface. Depending on the thickness of the carbonized film that was readily tuned based on the amount of citric acid used in the reaction, the carbonized ceria emitter behaved as a tungsten-like emitter, a graphite-like emitter, or their hybrid in terms of the absorptivity spectrum. A properly synthesized carbonized ceria emitter produced a power density of 0.63 W/cm² from the TPV system working at 900 °C, providing 13 and 9% enhancements compared to tungsten and graphite emitters, respectively. Furthermore, only the carbonized ceria emitter preserved its pristine absorptivity spectrum after a 48 h heating test at 1000 °C. The scalable and facile fabrication of thermostable emitters with a structured spectrum will prompt the emergence of thermal emission-harnessed energy devices.

Superhydrophobic and Recyclable Cellulose-Fiber-Based Composites for High-Efficiency Passive Radiative Cooling

Passive daytime radiative cooling (PDRC) involves cooling down an object by simultaneously reflecting sunlight and thermally radiating heat to the cold outer space through the Earth's atmospheric window. However, for practical applications, current PDRC materials are facing unprecedented challenges such as complicated and expensive fabrication approaches and performance degradation arising from surface contamination. Herein, we develop scalable cellulose-fiber-based composites with excellent self-cleaning and self-cooling capabilities, through air-spraying ethanolic poly(tetrafluoroethylene) (PTFE) microparticle suspensions embedded partially within the microsized pores of the cellulose fiber to form a dual-layered structure with PTFE particles atop the paper. The formed superhydrophobic PTFE coating not only protects the cellulose-fiber-based paper from water wetting and dust contamination for real-life applications but also reinforces its solar reflectivity by sunlight backscattering. It results in a subambient cooling performance of 5 °C under a solar irradiance of 834 W/m² and a radiative cooling power of 104 W/m² under a solar intensity of 671 W/m². The self-cleaning surface of composites maintains their good cooling performance for outdoor applications, and the recyclability of the composites extends their life span after one life cycle. Additionally, dyed cellulose-fiber-based paper can absorb appropriate visible wavelengths to display specific colors and effectively reflect near-infrared lights to reduce solar heating, which synchronously achieves effective radiative cooling and esthetic varieties.

Spectrally Selective Nanoparticle Mixture Coating for Passive Daytime Radiative Cooling

Passive daytime radiative cooling, which is a process that removes excess heat to cold space as an infinite heat sink, is an emerging technology for applications that require thermal control. Among the different structures of radiative coolers, multilayer- and photonic-structured radiative coolers that are composed of inorganic layers still need to be simple to fabricate. Herein, we describe the fabrication of a nanoparticle-mixture-based radiative cooler that exhibits highly selective infrared emission and low solar absorption. Al₂O₃, SiO₂, and Si₃N₄ nanoparticles exhibit intrinsic absorption in parts of the atmospheric transparency window; facile one-step spin coating of a mixture of these nanoparticles generates a surface with selective infrared emission, which can provide a more powerful cooling effect compared to broadband emitters. The nanoparticle-based radiative cooler exhibits an extremely low solar absorption of 4% and a highly selective emissivity of 88.7% within the atmospheric transparency window owing to the synergy of the optical properties of the material. The nanoparticle mixture radiative cooler produces subambient cooling of 2.8 °C for surface cooling and 1.0 °C for space cooling, whereas the Ag film exhibits an above-ambient cooling of 1.1 °C for surface cooling and 3.4 °C for space cooling under direct sunlight.

Outdoor Personal Thermal Management with Simultaneous Electricity Generation

Outdoor personal thermal comfort is of substantial significance to ameliorate the health conditions of pedestrian and outdoor laborer. However, the uncontrollable sunlight, substantial radiative loss, and intense temperature fluctuations in the outdoor environment present majestic challenges to outdoor personal thermal management. Here, we report an eco-friendly passive nanostructured textile which harvests energy from the sun and the outer space for optional localized heating and cooling. Compared to conventional heating/cooling textiles like black/white cotton, its heating/cooling mode enables a skin simulator temperature increase/decrease of 8.1 °C/6 °C, respectively, under sunlight exposure. Meanwhile, the temperature gradient created between the textile and human skin allows a continuous electricity generation with thermoelectric modules. Owing to the exceptional outdoor thermoregulation ability, this Janus textile is promising to help maintain a comfortable microclimate for individuals in outdoor environment and provide a platform for pervasive power generation.

Outdoor-Useable, Wireless/Battery-Free Patch-Type Tissue Oximeter with Radiative Cooling

For wearable electronics/optoelectronics, thermal management should be provided for accurate signal acquisition as well as thermal comfort. However, outdoor solar energy gain has restricted the efficiency of some wearable devices like oximeters. Herein, wireless/battery-free and thermally regulated patch-type tissue oximeter (PTO) with radiative cooling structures are presented, which can measure tissue oxygenation under sunlight in reliable manner and will benefit athlete training. To maximize the radiative cooling performance, a nano/microvoids polymer (NMVP) is introduced by combining two perforated polymers to both reduce sunlight absorption and maximize thermal radiation. The optimized NMVP exhibits sub-ambient cooling of 6 °C in daytime under various conditions such as scattered/overcast clouds, high humidity, and clear weather. The NMVP-integrated PTO enables maintaining temperature within ≈1 °C on the skin under sunlight relative to indoor measurement, whereas the normally used, black encapsulated PTO shows over 40 °C owing to solar absorption. The heated PTO exhibits an inaccurate tissue oxygen saturation (StO2) value of ≈67% compared with StO2 in a normal state (i.e., ≈80%). However, the thermally protected PTO presents reliable StO2 of ≈80%. This successful demonstration provides a feasible strategy of thermal management in wearable devices for outdoor applications.

Flexible Daytime Radiative Cooling Enhanced by Enabling Three-Phase Composites with Scattering Interfaces between Silica Microspheres and Hierarchical Porous Coatings

Daytime radiative cooling has attracted considerable attention recently due to its tremendous potential for passively exploiting the coldness of the universe as clean and renewable energy. Many advanced materials with novel photonic micro/nanostructures have already been developed to enable highly efficient daytime radiative coolers, among which the flexible hierarchical porous coatings (HPCs) are a more distinguished category. However, it is still hard to precisely control the size distribution of the randomized pores within the HPCs, usually resulting in a deficient solar reflection at the near-infrared optical regime under diverse fabrication conditions of the coatings. We report here a three-phase (i.e., air pore-phase, microsphere-phase, and polymer-phase) self-assembled hybrid porous composite coating, which dramatically increases the average solar reflectance and yields remarkable temperature drops of ∼10 and ∼ 30 °C compared to the ambient circumstance and black paint, respectively, according to the rooftop measurements. Mie theory and Monte Carlo simulations reveal the origin of the low reflectivity of as-prepared two-phase porous HPCs, and the optical cooling improvement of the three-phase porous composite coatings is attributed to the newly generated interfaces possessing the high scattering efficiency between the hierarchical pores and silica microspheres hybridized with appropriate mass fractions. As a result, the hybrid porous composite approach enhances the whole performance of the coatings, which provides a promising alternative to the flexible daytime radiative cooler.

Beyond the Visible: Bioinspired Infrared Adaptive Materials

Infrared (IR) adaptation phenomena are ubiquitous in nature and biological systems. Taking inspiration from natural creatures, researchers have devoted extensive efforts for developing advanced IR adaptive materials and exploring their applications in areas of smart camouflage, thermal energy management, biomedical science, and many other IR-related technological fields. Herein, an up-to-date review is provided on the recent advancements of bioinspired IR adaptive materials and their promising applications. First an overview of IR adaptation in nature and advanced artificial IR technologies is presented. Recent endeavors are then introduced toward developing bioinspired adaptive materials for IR camouflage and IR radiative cooling. According to the Stefan-Boltzmann law, IR camouflage can be realized by either emissivity engineering or thermal cloaks. IR radiative cooling can maximize the thermal radiation of an object through an IR atmospheric transparency window, and thus holds great potential for use in energy-efficient green buildings and smart personal thermal management systems. Recent advances in bioinspired adaptive materials for emerging near-IR (NIR) applications are also discussed, including NIR-triggered biological technologies, NIR light-fueled soft robotics, and NIR light-driven supramolecular nanosystems. This review concludes with a perspective on the challenges and opportunities for the future development of bioinspired IR adaptive materials.

Designing Mesoporous Photonic Structures for High-Performance Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) has drawn significant attention recently for electricity-free cooling. Porous polymers are attractive for PDRC since they have excellent performance and scalability. A fundamental question remaining is how PDRC performance depends on pore properties (e.g., radius, porosity), which is critical to guiding future structure designs. In this work, optical simulations are carried out to answer this question, and effects of pore size, porosity, and thickness are studied. We find that mixed nanopores (e.g., radii of 100 and 200 nm) have a much higher solar reflectance R̅_solar (0.951) than the single-sized pores (0.811) at a thickness of 300 μm. With an Al substrate underneath, R̅_solar, thermal emittance ε̅_LWIR, and net cooling power P_cool reach 0.980, 0.984, and 72 W/m², respectively, under a semihumid atmospheric condition. These simulation results provide a guide for designing high-performance porous coating for PDRC applications.

Three-Dimensional Printable Nanoporous Polymer Matrix Composites for Daytime Radiative Cooling

Daytime radiative cooling presents an exciting new strategy for combating global warming, because it can passively cool buildings by reflecting sunlight and utilizing the infrared atmospheric window to eject heat into outer space. Recent progress with novel material designs showed promising subambient cooling performance under direct sunlight. However, large-scale implementation of radiative cooling technologies is still limited by the high-cost and complex fabrication. Here, we develop a nanoporous polymer matrix composite (PMC) to enable rapid production and cost reduction using commercially available polymer processing techniques, such as molding, extrusion, and 3D printing. With a high solar reflectance of 96.2% and infrared emissivity > 90%, the nanoporous PMC achieved a subambient temperature drop of 6.1 °C and cooling power of 85 W/m² under direct sunlight, which are comparable to the state-of-the-art. This work offers great promise to make radiative cooling technologies more viable for saving energy and reducing emissions in building cooling applications.