Colloidal inorganic nano- and microparticles for passive daytime radiative cooling

Abstract

Compared to traditional cooling systems, radiative cooling (RC) is a promising cooling strategy in terms of reducing energy consumption enormously and avoiding severe environmental issues. Radiative cooling materials (RCMs) reduce the temperature of objects without using an external energy supply by dissipating thermal energy via infrared (IR) radiation into the cold outer space through the atmospheric window. Therefore, RC has a great potential for various applications, such as energy-saving buildings, vehicles, water harvesting, solar cells, and personal thermal management. Herein, we review the recent progress in the applications of inorganic nanoparticles (NPs) and microparticles (MPs) as RCMs and provide insights for further development of RC technology. Particle-based RCMs have tremendous potential owing to the ease of engineering their optical and physical properties, as well as processibility for facile, inexpensive, and large area deposition. The optical and physical properties of inorganic NPs and MPs can be tuned easily by changing their size, shape, composition, and crystals structures. This feature allows particle-based RCMs to fulfill requirements pertaining to passive daytime radiative cooling (PDRC), which requires high reflectivity in the solar spectrum and high emissivity within the atmospheric window. By adjusting the structures and compositions of colloidal inorganic particles, they can be utilized to design a thermal radiator with a selective emission spectrum at wavelengths of 8-13 μm, which is preferable for PDRC. In addition, colloidal particles can exhibit high reflectivity in the solar spectrum through Mie-scattering, which can be further engineered by modifying the compositions and structures of colloidal particles. Recent advances in PDRC that utilize inorganic NPs and MPs are summarized and discussed together with various materials, structural designs, and optical properties. Subsequently, we discuss the integration of functional NPs to achieve functional RCMs. We describe various approaches to the design of colored RCMs including structural colors, plasmonics, and luminescent wavelength conversion. In addition, we further describe experimental approaches to realize self-adaptive RC by incorporating phase-change materials and to fabricate multifunctional RC devices by using a combination of functional NPs and MPs.

Keywords: Colloid; Microparticle; Multifunctionality; Nanoparticle; Radiative cooling; Thermal management.

Fig. 1

PDRC films consisting of selective thermal emitters and a metal reflective layer. a Schematic of an RCM comprising SiO₂- and SiC-doped PE films. b Measured reflectance spectra of RCMs at thermal radiation wavelengths. Reproduced with permission from [64]. Copyright 2010, American Chemical Society. c Schematic of SiO₂–TPX hybrid metamaterial. d Photograph of 300-mm-wide hybrid metamaterial film. e Emissivity/absorptivity and f outdoor temperature measurement results of the hybrid metamaterial. Reproduced with permission from [44]. Copyright 2017, Science

Fig. 2

a PDRC films without a metal reflective layer. Schematic of a PDRC film composed of a particle-embedded double-layer coating. Reproduced with permission from [69]. Copyright 2017, Elsevier. b Conceptual description of particle-based PDRC films. c Outdoor temperature profiles of SiO₂ MP films and a commercial paint relative to ambient temperature. Reproduced with permission from [56]. Copyright 2018, American Chemical Society. d Schematic of nanoporous PE film doped with SiO₂ NPs for PDRC, and e its temperature profile under solar irradiation. Reproduced with permission from [62]. Copyright 2021, American Chemical Society. f Top and g cross-sectional SEM images, and h photograph of RC paint on glass substrate. i Temperature profiles of commercial CW paint, RC paint, and ambient temperature in the daytime. Reproduced with permission from [52]. Copyright 2021, Elsevier

Fig. 3

a Schematics of PDRC film composed of NPs and MPs acting as solar reflectors, fluorescent particles, and IR emitters in polymer binders. b Absorption, fluorescent excitation, and emission spectra of fluorescent MPs to minimize UV absorption of PDRC films. Reproduced with permission from [57]. Copyright 2020, Wiley. c Photograph and SEM image of CaCO₃-acrylic paint along with commercial white paint and d outdoor temperature measurement results of CaCO₃-acrylic paint. Reproduced with permission from [54]. Copyright 2020, Cell Press. e Photograph and SEM image of BaSO₄-based PDRC films and f outdoor RC test results of BaSO₄-acrylic paint compared with the ambient temperature. Reproduced with permission from [51]. Copyright 2021, American Chemical Society. g Fabrication process of ZnO-SiO₂ core-shell particles, and h schematics of the light scattering from core-shell structure. Reproduced with permission from [58]. Copyright 2021, Elsevier

Fig. 4

a Schematic of ZnO NP-embedded nanoporous PE textile. b AM1.5 solar irradiation spectra and thermal radiation of human body simulated using Planck’s law at the skin temperature of 34 ℃. c Photograph of ZnO-PE fabric. Reproduced with permission from [50]. Copyright 2018, Wiley. d SEM image of hybrid membrane radiator composed of PVDF/TEOS fibers and SiO₂ MPs. e Photograph of scalable flexible hybrid membrane radiator. Reproduced with permission from [59]. Copyright 2020, Wiley. f Fabrication process of SiO₂-attached fabrics. g Spectral reflectivity and emissivity of SiO₂-attached fabrics and raw materials. Reproduced with permission from [60]. Copyright 2021, American Chemical Society

Fig. 5

a Design principle of colored RCMs using photonic opal crystals. Bright-field optical microscopy (top) and SEM (bottom) images of b reddish, c greenish, and d bluish opals. e Experimentally measured absorptivity/emissivity of colored opals in the mid-IR regions. Reproduced with permission from [78]. Copyright 2020, American Chemical Society. f Colored coating composed of SiO₂-Ag core-shell NPs embedded in silica. Reproduced with permission from [79]. Copyright 2020, American Chemical Society. g Schematic of bilayer colored PDRC coating consisting of a layer composed of plasmonic spheres and another layer composed of randomly dispersed dielectric spheres. h Spectral reflectance of white SiO₂ and colored coatings. Reproduced with permission from [80]. Copyright 2022, Elsevier

Fig. 6

a Photograph of yellow PDRC film coating on a flexible substrate. b Normalized PL spectra and photographs of CuGaS₂@ZnS, CuInS₂@ZnS, and CuInSe₂@ZnS core-shell QD solutions. c Temperature profiles of white, yellow, red, and brown PDRC films compared to the ambient temperature. Reproduced with permission from [74]. Copyright 2021, Elsevier. d Schematic of colored RCMs with silica-embedded perovskite NCs/PMMA + ZnO/PET/Ag films. e Photos of perovskite NCs in hexane and colored RCM films. f Daytime temperature profiles of colored RCMs. Reproduced with permission from [81]. Copyright 2021, Elsevier. g Schematic illustration and h photographs of colored RC coatings with a bilayer structure. i Outdoor temperature profiles of colored RC coatings compared to the ambient temperature. Reproduced with permission from [92]. Copyright 2022, Elsevier

Fig. 7

a Schematic structure and b spectral transmissivity and emissivity of VO₂ smart window. Reproduced with permission from [110]. Copyright 2021, Science. c Schematic diagram, d SEM image, and e thermal cycling performance of SiO₂-PCMs MPs. Reproduced with permission from [114]. Copyright 2022, Cell Press. f Photos of various RC devices under indoor light (upper) and UV lamp (below). g Daytime outdoor temperature and difference between ambient temperature and device temperatures. h Artificial houses equipped with a light-emitting cooling roof and conventional roof. Reproduced with permission from [116]. Copyright 2020, American Chemical Society