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DOI: 10.1039/D4TA01767F (Review Article) J. Mater. Chem. A, 2024, 12, 12960-12982

Multifunctional thermochromic smart windows for building energy saving

Dingkun Wang† , Guoqi Chen† and Jun Fu *
Key Laboratory of Polymeric Composite and Functional Materials of Ministry of Education, Guangdong Engineering Technology Research Centre for Functional Biomaterials, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-mail: fujun8@mail.sysu.edu.cn

Received 15th March 2024 , Accepted 25th April 2024

First published on 25th April 2024

Abstract

Windows are an important part of buildings and transmit light between indoors and outdoors. Frequent heat exchange through windows increases building energy consumption. Smart windows can change optical properties and modulate solar radiation, which are recognized as frontrunners in building energy saving. Among various smart windows, thermochromic windows usually passively regulate light transmittance in response to environmental temperature and have showed great potential for practical applications. Thermochromic materials are key to constructing thermochromic smart windows. Usually, a reversible phase transition takes place for thermoresponsive materials near the critical transition temperature, leading to changes in transmittance over different spectrum bands. Representative thermochromic materials include metal oxides, hydrogels, perovskites, ionic liquids, liquid crystals, etc. The intrinsic phase transition temperature, luminous transmittance, and solar modulation ability are among the critical parameters defining the performance of smart windows. New strategies have been developed to modulate the performance of thermochromic materials and smart windows to meet demands from different environments and climates. Such endeavors have boosted smart windows to modulate full-spectrum solar regulation and to achieve efficient all-climate building energy saving. Next generation smart windows will not only modulate solar transmission, but also convert and store solar energy through new power technologies such as thermoelectricity conversion and solar cells. Challenges and future prospects of smart windows are discussed to inspire future building energy saving.

image file: d4ta01767f-p1.tif

Dingkun Wang

Dingkun Wang obtained his bachelor's degree in chemical engineering of forestry from Northwest A&F University in 2019 and a Master’s degree in chemical engineering of forestry from the Chinese Academy of Forestry in 2023. He is currently pursuing a PhD degree supervised by Prof. Jun Fu at the School of Materials Science and Engineering, Sun Yat-sen University. His research interests include functional hydrogels for flexible electronics.

image file: d4ta01767f-p2.tif

Guoqi Chen

Guoqi Chen obtained his bachelor's (2018) and Master's (2021) degrees from the College of Chemical Engineering, Fuzhou University. He is currently pursuing a PhD degree supervised by Prof. Jun Fu at the School of Materials Science and Engineering, Sun Yat-sen University. His research interests include functional hydrogels for smart windows and flexible electronic devices.

image file: d4ta01767f-p3.tif

Jun Fu

Dr. Jun Fu is a Professor at the School of Materials Science and Engineering at Sun Yat-sen University (SYSU). He obtained his BSc in applied chemistry at Wuhan University and PhD in polymer chemistry and physics from the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CAS). He was a visiting scholar at the Max Planck Institute for Polymer Research and a research fellow at Massachusetts General Hospital/Harvard Medical School. He was appointed as a professor at the Ningbo Institute of Materials Technology and Engineering, CAS, and then moved to SYSU. His research focuses on high performance and functional hydrogels for smart windows, flexible wearables, and implantable devices.

1. Introduction

Rapid development of the economy and technology boosts energy consumption in modern society. Building energy consumption accounts for 30–40% of the total in developed countries.1,2 Common building energy services comprise heating, ventilation, and air conditioning.3 Windows are considered the least energy efficient part of a building and account for about 50% of building energy consumption.4 Therefore, smart windows are needed to regulate heat exchange to reduce energy consumption in buildings, which eventually facilitates a low-carbon economy.

Smart windows have been developed to intelligently regulate indoor solar radiation in response to external stimuli and consequently reduce heat exchange to conserve energy,5,6 including electrochromic,7 mechanochromic,8 magnetochromic,9 photochromic,10,11 and thermochromic smart windows.12,13 The former three types are known as active smart windows that need extra energy input to trigger and maintain the “ON” or “OFF” state. The latter two types are categorized as passive smart windows that spontaneously respond to environmental conditions without extra power input.

Thermochromic windows can modulate indoor solar irradiation by reversibly regulating the transmittance or reflectance of ultraviolet (UV, 250–380 nm), visible (380–780 nm) and near-infrared (NIR, 780–2500 nm) solar radiation.14,15 Thermoresponsive materials that undergo phase transitions and reversible transparent-to-opaque transitions upon temperature changes have been utilized for thermochromic smart windows for solar modulation.16,17 To date, many inorganic and organic thermochromic materials have been used for smart windows, including metal oxides13,18,19 and hydrogels.20–22 VO2, for example, experiences a phase transition at 68 °C, above which the IR transmittance is decreased, while the transmittance in the visible band barely changes.12,13,15 Besides, the solubility and/or hydrophilicity/hydrophobicity of polymer chains in thermochromic hydrogels can change at different temperatures, which causes a phase transition, chain conformation change, or precipitation of polymer chains.20,23 Such transitions can cause variations in color or transparency to modulate solar modulation.24,25 Other thermochromic materials, including ionic liquids, perovskites and liquid crystals, have also been used for thermochromic smart windows.26

The performances of smart windows are defined by several parameters including phase transition temperature (τc), luminous transmittance (Tlum), and solar modulation ability (ΔTsol). τc determines the optimal working conditions and potential additional methods needed to trigger its transition. A transition temperature close to the target application conditions is desired. Tlum is important for daily use. High Tlum is desired when smart windows are not triggered to block solar radiation. ΔTsol is defined as the solar transmittance change before and after the phase transition, which characterizes the ability to block solar energy. For light-harvesting smart windows, the power conversion efficiency (η) is a vital indicator that reflects the electrical output capability by converting the incident energy from the sun. The values of these parameters can be calculated using the following equations:

 
image file: d4ta01767f-t1.tif(1)
 
image file: d4ta01767f-t2.tif(2)
 
ΔTsol = Tsol(T < τc) − Tsol(T > τc)(3)
 
image file: d4ta01767f-t3.tif(4)
where φlum(λ) represents the standard luminous efficiency function for visible light (380–780 nm), T(λ) is the transmittance at wavelength λ, Tsol is the solar energy transmittance, JSC is the short-circuit current density, VOC is the open-circuit voltage, FF is the fill factor, and Pin is the incident light density.

Next generation smart windows are expected to harness solar energy instead of simply blocking and wasting it through scattering.27,28 New concepts to integrate thermoelectricity conversion and solar cells in smart windows can harness solar energy and convert it into electricity, thus maximizing building energy exploitation.28 Therefore, the next generation design principle of thermochromic smart windows requires intelligent management of incident sunlight according to temperature and rational utilization of solar light for power generation through photovoltaic technologies.

There are several review articles on thermochromic smart windows. Long's group has reviewed progress on thermochromic smart windows based on VO2,13 hydrogels,20 and some other thermochromic materials.12 Jin's group discussed thermochromic VO2-based smart windows for practical applications.29 Those review articles mainly focussed on single thermochromic materials for smart windows. Systematic and comparative investigations on smart windows based on different materials are needed to further discuss the influence of chemical structures, nanostructures, thermochromic properties, and device design on the thermochromic performance of smart windows, which will inspire novel strategies to develop high performance and functional smart windows to match application demands. In this review, we present a comprehensive review of high performance smart windows based on thermochromic materials. The intrinsic properties and response mechanisms of representative thermochromic materials, including VO2, hydrogels, ionic liquids, perovskites, and liquid crystals, and their applications to smart windows are discussed in detail (Fig. 1). Novel strategies ranging from molecule engineering to hybridizing, copolymerization, compositing, and nano-/micro-structure engineering to develop high performance thermochromic materials to match application demands under diverse conditions and climates are introduced. In each section, current challenges and potential development directions are analyzed. Next generation smart windows by utilizing thermoelectricity conversion and solar cell technology to harness reflected solar energy for power generation are discussed (Fig. 1). Future development of thermochromic smart windows in building energy saving is discussed. This review aims to inspire research interest and innovations to promote the development of smart windows for building energy saving.


image file: d4ta01767f-f1.tif
Fig. 1 Schematic outline of three components of smart windows in building energy saving: thermochromic materials, thermal exchange, and power generation. Thermochromic materials include VO2, hydrogels, ionic liquids, perovskites, and liquid crystals. Power generation techniques involve thermoelectricity conversion and solar cell integration.

2. Thermochromic smart windows for solar modulation

Most thermochromic smart windows are based on thermoresponsive materials that undergo a phase transition at specific temperatures.15 Such a phase transition induces either changes in crystal structures or phase separation that result in a transition from transparent to opaque status.12 A variety of thermochromic materials have been used to fabricate smart windows.8,14,20 The performances of smart windows highly rely on the transition temperature and transmittance in the UV-vis-infrared spectrum.13,30 Great efforts have been made to modulate the transition temperature and transmittance to achieve autonomous transition and maximize solar modulation under mild conditions.23,31

2.1 Vanadium dioxide

2.1.1 Thermochromic mechanism. Vanadium is a transition element with a valence electron layer structure of 3d34s2. It can form a variety of oxides, including VO, V2O3, VO2, and V2O5. Only VO2 undergoes a metal-to-insulator transition (MIT) at a critical temperature of 68 °C.13,32 VO2 is a polycrystalline metal oxide with a metastable phase, monoclinic phase (M), rutile phase (R), triclinic phase (T), etc.33 At temperatures above 68 °C, VO2 adopts the metallic rutile phase.34,35 Therein, V4+ occupies the eight apexes and center of the unit cell (Fig. 2a). Besides, the V–V distance is equal to about 2.85 Å along the c axis. Below critical temperature, the VO2 structure changes from R to M. V4+ migrates from the octahedron center to the outer surface. The V–V distances become 3.12 Å and 2.65 Å. The M phase VO2 is insulating along the c axis.13,36
image file: d4ta01767f-f2.tif
Fig. 2 (a) Schematic of the atomic structure for the metallic VO2 R phase and the insulating VO2 M phase. The V–V distances in each crystal structure are highlighted. Reproduced with permission from Whittaker et al., J. Phys. Chem. Lett. 2, 745 (2011). Copyright 2011 American Chemical Society.34 (b) Band structures of the metallic R and insulating M phases of VO2 depicted by molecular orbital diagrams. Reproduced with permission from Cui et al., Joule 2, 1707 (2018). Copyright 2018 Elsevier, Inc.13

Currently, diverse mechanisms have been proposed for the phase transition of VO2.37–40 The electron–phonon interaction mechanism deems that the appearance of metallic rutile phase bandgap (R) comes from the distortion of the monoclinic phase (M).41 The electron correlation mechanism suggests that the strong electron–electron correlation produces an insulating band gap.42–44 The reversible phase transition of VO2 (M/R) is described by using crystal field and molecular orbital theory (Fig. 2b). Briefly, in the rutile VO2 (R), the 2p orbital of O2− and 3d orbital of V4+ are hybridized to generate a narrow anti-bonding orbital π* and a wide bonding π. Besides, V4+ offers an anti-bonding orbital d// parallel to the c axis. The semi-filled d// and π* bands partially overlap at the Fermi level, showing metallic characteristics. But in the monoclinic VO2 (M), the d// and π* orbitals separate. The d// band splits into a filled orbital and an empty orbital with a band gap of 0.7 eV. Therefore, the electrons are trapped in the low energy d// orbital and VO2 (M) becomes insulating.

2.1.2 High performance VO2-based smart windows. VO2 has always been the research hotspot in the field of smart windows due to its reversible phase transition temperature with prominent optical changes to modulate solar incident light. However, the high phase transition temperature,13 limited luminous transmittance,45 and weak solar modulation ability46 have restricted the development of VO2-based smart windows. Many strategies have been proposed to tackle these problems, including element doping,46,47 multilayer films,48,49 nanocomposite films,50,51 and nano-/microstructural engineering.52,53 For example, W can donate extra electrons to VO2 and lower the thermal driving force for electronic movement. Thus, a W-doped VO2 film has a lower τc of around 39 °C.54 Multilayer VO2 film containing functional layers, such as antireflection coating TiO2,55 can enhance Tlum to 45.0% and improve ΔTsol to 12.1%. The VO2/Si–Al gel nanocomposite film shows enhanced optical properties (Tlum of 59.0% and ΔTsol of 12.0%) due to the relatively low refractive index of the Si–Al gel matrix and good dispersion of VO2 particles.56 A karst landform-like VO2 film exhibits a higher Tlum of 61.3% and ΔTsol of 11.9% than a continuous VO2 film because of the localized surface plasmon resonance (LSPR) effect induced by the formation of isolated VO2 nanoparticles.57

Thermochromic properties of representative VO2-based materials for smart windows are summarized in Table 1. Elemental doping can lower the τc of VO2 to about 20–50 °C, depending on the elements used. Meanwhile, the ΔTsol of the VO2 film can be improved to around 10–20% by constructing multilayer films. The Tlum of the VO2 film can be increased to about 40–85% through fabricating nanocomposite films or nano-/microstructures. These results represent significant progress toward practical applications of VO2-based smart windows.

Table 1 Thermochromic properties of VO2 based on different modification strategies
Category Materials/structures T lum (%) ΔTsol (%) τ c (°C) Ref.
Element doping W-doped VO2 coating 45.0 10.0 22.0 58
W-doped VO2 film 54.4 10.7 39.0 54
Zr-doped VO2 film 61.4 10.3 — 59
Mg-doped VO2 film 59.4 9.5 — 59
Si-doped VO2 film 54.7 13.9 — 46
H-doped VO2 — — 30.0 60
W/Zr co-doped VO2 film 60.7 10.6 46.9 61
SiO2/W co-doped VO2 film 60.0 — — 62
W/Mg co-doped VO2 film 46.2 10.8 36.9 63
Hf/W co-doped VO2 film 41.1 13.1 38.9 64
SiO2/VO2 double-layered film 55.0 — 70.0 65
VO2/TiO2 double-layered film 61.5 15.1 — 66
VO2/TiO2 double-layered structure 49.0 7.0 — 67
Si–Al/VO2 double-layered coating 44.0 18.9 — 68
Multilayer films TiO2/VO2/TiO2/VO2/TiO2 multilayered film 45.0 12.1 60.0 55
SiNx/NiCr/NiCrOx/VOx/NiCrOx/NiCr/SiNx multilayered film 40.5 18.4 54.0 48
SiNx/VO2/SiNx multilayered film 40.4 14.5 — 49
SiO2/VO2/SiO2/polymer multilayered coating 54.0 16.4 — 69
WO3/VO2/WO3 multilayered structure 55.4 — 52.0 70
Nanocomposite films VO2 nanoparticles in a Ni-based thermochromic system 73.4 18.2 — 71
VO2 nanoparticles and Sb-doped SnO2 nanoparticles 60.1 20.0 66.3 72
VO2 nanoparticles and Sb-doped SnO2 nanoparticles 84.4 11.6 84.5 72
HfO2/VOx nanocomposite film 51.6 15.4 60.6 73
VO2 nanoparticles with SiO2 aerogel particles 41.2 18.4 — 74
VO2 nanoparticles in a polyurethane matrix (single layer) 54.0 14.5 — 50
VO2 nanoparticles in a polyurethane matrix (three layers) 46.8 20.0 — 50
VO2 nanoparticles in polyvinyl butyral 43.4 17.3 — 51
Core–shell VO2@SnO2 nanoparticles 47.5 25.0 ∼65.0 75
VO2 nanoparticles in a polyvinyl pyrrolidone matrix 57.3 13.8 — 76
W-doped VO2 nanoparticle film 50.0 10.0 32 77
Double-layered nanoparticle array of VO2 46.1 13.2 — 78
Nanostructure/microstructure Three-dimensional ordered microporous VO2 71.1 10.8 — 52
Nanoporous VO2 film 78.0 14.1 — 79
VO2 nanoparticles film with VO2 clusters 46.3 11.2 — 80
VO2 mesh film 86.0 — — 81
Micro-patterned VO2 film 43.3 14.9 — 53


2.1.2.1 Elemental doping. Elemental doping is used to reduce the critical phase transition temperature of VO2.13,15 Doping elements into VO2 increases its electronic concentration and decreases the orbital splitting gap, which lowers the thermal driving force for electronic movement.82 On the other hand, the doping elements partly occupy the positions of V or O and thus destroy the V–V chain in the monoclinic structure.83 As a result, τc decreases. Many elements, including H,60 W,58 Zr,61 Mg,63 Hf,64 Ta,47etc., are used to dope VO2. For example, H doping leads to the formation of a typical H–O covalent bond, which distorts the V–V bonds (Fig. 3a).60 Thus, the H-doped VO2 is prone to a phase transition at relatively lower temperatures (τc) as reduced by 38 K/at% H. Similarly, τc can be lowered by doping the W element. A sputtered V0.982W0.018O2 layer deposited on an ultrathin flexible glass substrate shows an ultralow τc of 22 °C.58 A W-doped VO2 layer sandwiched by two SnO2 layers deposited on glass substrates through pulsed laser deposition exhibits a τc of 39 °C. Interestingly, the ΔTsol and Tlum of this film are increased from 2.6% to 10.7% and from 45.0% to 54.4%, respectively.54 Zr has lower electronegativity (1.33) than V (1.63) and therefore can serve as a donor in VO2. A W/Zr co-doped VO2 film prepared by the hydrothermal reaction and spin-coating shows a τc of 46.9 °C. W doping weakens the transition temperature, and Zr widens the energy band gap. This method simultaneously shifts the absorption edge and enhances ΔTsol and Tlum.61 Similarly, combined doping with W and Mg decreases τc to 36.9 °C and enhances ΔTsol to 10.8% and Tlum to 46.2%.63 Similar Hf–W co-doping reduces the τc of the VO2 film to 38.9 °C, together with a ΔTsol of 13.1% and Tlum of 41.1%.64 Hf–W co-doping shortens the V–V bond because of the higher ion valence of W6+ than V4+ (Fig. 3b). W can donate extra electrons to VO2 and thus reduce τc. Specifically, HfO2 has a wider optical bandgap (5.7 eV) than pure VO2 (2.5 eV). Therefore, the Hf4+-doped VO2 film has a widened thermoresponsive optical bandgap, which is attributed to a high Tlum. Element doping is feasible to lower the transition temperature, regulate the transmittance, and improve solar modulation of VO2.
image file: d4ta01767f-f3.tif
Fig. 3 (a) The V–V chains along the c axis in pure and H-doped VO2 (R). Reproduced with permission from Cui et al., Phys. Chem. Chem. Phys. 17, 20998 (2015). Copyright 2015 Royal Society of Chemistry.60 (b) The calculated super-cell structures of pure V32O64 and Hf4W1V27O64. Reproduced with permission from Wang et al., Appl. Phys. Lett. 118, 192102, (2021). Copyright 2021 American Institute of Physics.64

2.1.2.2 Multilayer films. Moreover, designing multilayer films is an important way to improve the optical properties of a VO2 film.13,15,84 To start with, a single functional layer is commonly introduced to improve the luminous transmittance of a pure VO2 film. For example, a thin SiO2 layer (about 100–300 nm) on top of a VO2 film increases Tlum from 42% to 55% at 650 nm.65 TiO2 with a high refractive index85 of 2.2 serves as an antireflection coating to increase the Tlum (from 40.3% to 61.5%).66 The film thickness is important. An optimal VO2 (50 nm)/TiO2 (40 nm) structure is reported with a high luminous transmittance of 49%.67 Anti-reflection can be achieved by micro-patterning the VO2 film to simultaneously modulate ΔTsol and Tlum. The refractive index bilayer film is responsive to temperature variation,68 which presents a higher solar transmittance at lower temperature with enhanced Tlum (from 39% to 44%) (Fig. 4a).
image file: d4ta01767f-f4.tif
Fig. 4 (a) Schematic illustration of the refractive index-tunable bilayer film with nano-VO2. Reproduced with permission from Liu et al., J. Alloys Compd. 731, 1197 (2018). Copyright 2018 Elsevier, Inc.68 (b) XRD pattern and cross-section SEM image of a multilayer SiNx/VO2/SiNx film. Reproduced with permission from Long et al., Sol. Energy Mater Sol. Cells 189, 138 (2019). Copyright 2019 Elsevier, Inc.49 (c) Three-layered Ag/VO2/Pt film and Marilyn Monroe's image composed of different SiO2 thicknesses at different temperatures. Reproduced with permission from Kim et al., Appl. Surf. Sci. 565, 150610 (2021). Copyright 2021 Elsevier, Inc.86

Multilayer structures balance the trade-off between ΔTsol and Tlum and further promote the solar modulation ability. A SiNx/NiCr/NiCrOx/VOx/NiCrOx/NiCr/SiNx multilayer film deposited on a glass substrate shows a solar modulation of 18.4% and luminous transmittance of 40.5%.48 The multilayer SiNx/VO2/SiNx film consisting of a ∼80 nm VO2 layer and two SiNx layers has enhanced solar modulation from 10.8% to 14.5% and luminous transmittance from 36.1% to 40.4% (Fig. 4b).49 Moreover, a dip-coated four-layered SiO2/VO2/SiO2/polymer film exhibits a solar modulation of 16.4% and luminous transmittance of 54.0%.69 Furthermore, a three-layer Ag/VO2/Pt film is fabricated as a metal–dielectric–metal structure (Fig. 4c).86 The trilayer film shows a VO2 thickness-dependent reflective color. As the insulating VO2 layer undergoes a phase transition, the reflective color changes in response to temperature changes, leading to a unique solar modulation ability (Fig. 4c).


2.1.2.3 Nanocomposite films. Furthermore, compositing VO2 nanoparticles with other functional materials can improve the optical properties.71,79,87 For example, a nanocomposite film of VO2 and a Ni-based ligand exchange thermochromic system (NLETS) presents a 127% increase from 8.02% to 18.19% in ΔTsol at a high Tlum (73.36% at 20 °C and 68.71% at 90 °C).71 The VO2 nanoparticles are uniformly distributed in the film. The NLETS has a macroscopic color change (from transparent to light blue) as temperature is increased, which thereby enhances the solar modulation ability of the nanocomposite film. A nanocomposite film containing VO2 nanoparticles and Sb-doped SnO2 (ATO) nanoparticles increases Tlum from 39% to 84% at a ΔTsol of 11.57%.72 ATO nanoparticles are distributed on the surfaces and interstices of VO2 nanoparticles, which widens the optical band gap of VO2 nanoparticles and thus improves the luminous transmittance of the film.

SiO2 is often used to modify the optical performances of pure VO2 films because of its outstanding visible transparency and low-cost coating process.13 VO2/SiO2 composite films are constructed through a facial mechanical ball-milling technology (Fig. 5).74 The SiO2 aerogel particles are coated on VO2 particles to prevent aggregation, which enhances the solar modulation (ΔTsol = 18.4%) and luminous transmittance (Tlum = 41.2%). Similarly, VO2 is composited in polyurethane with good dispersion and exhibits enhanced solar modulation from 6.6% to 14.5% and a luminous transmittance of 54%.50 However, the abovementioned research focuses on enhancing luminous transmittance and solar modulation ability of VO2. The high phase transition temperature of VO2 is rarely addressed. New VO2-based thermoresponsive materials with both moderate transition temperatures and excellent dispersion could be anticipated by compositing element-doped VO2 nanoparticles into matrices.


image file: d4ta01767f-f5.tif
Fig. 5 The mechanism of solar transmission increment of a VO2/SiO2 composite film. Reproduced with permission from Kang et al., Appl. Surf. Sci. 573, 151507 (2022). Copyright 2022 Elsevier, Inc.74

2.1.2.4 Nano-/microstructure engineering. Nano-/microstructure engineering is another effective strategy to modulate the thermochromic performances of VO2.15 Ordered or periodical nano-/microscale distribution of VO2 has superior thermochromic properties compared with that of randomly distributed VO2.29,88 For example, two-dimensional ordered VO2 nanostructures are fabricated by infiltrating VO2 precursor solution into colloidal crystal templates.78 By adjusting the fluidity of the VO2 precursor and separation distance of template spheres, VO2 nanoparticles, nanodomes and nanonets are fabricated on substrates.89 The two-dimensional ordered VO2 nanoparticle array presents a luminous transmittance of 46.1% and solar modulation ability of 13.2%. The nanoparticle array has a porosity-induced luminous transmittance strengthening and a temperature-response LSPR effect, which is helpful to enhance the solar modulation ability because the LSPR intensity is increased at high temperatures, leading to higher absorption and lower transmittance.

Apart from ordered VO2 structures, random VO2 nanoporous structures with the LSPR effect can also improve luminous transmittance and solar modulation ability. VO2 films with spontaneous random nanoporous structures are prepared to enhance the optical performances (ΔTsol = 14.1% and Tlum = 78%) (Fig. 6a).79 The VO2 film with random teeth-like nanoparticles and nanopores enhances absorption of LSPR in the NIR range and thus increases the Tlum and ΔTsol. The Tlum of VO2 films at different incident angles is enhanced by structure design. Similarly, a VO2 film with random large pores and isolated optical interaction particles is obtained by intense pulsed light (IPL) sintering to regulate light transmittance performance (Fig. 6b).80 The film sintered at 2000 V presents a visible light transmittance of 63.6% at 550 nm and luminous transmittance over 46.3%.


image file: d4ta01767f-f6.tif
Fig. 6 (a) Schematic diagram of fabrication route for nanoporous VO2 and optical properties of nanoporous VO2 films. Reproduced with permission from Long et al., ACS Appl. Mater. Interfaces 11, 22692 (2019), Copyright 2019 American Chemical Society.79 (b) Schematic illustration of VO2 nanoparticle films prepared by intense pulsed light and spin coating. Reproduced with permission from Kim et al., Mater. Des. 176, 107838 (2019). Copyright 2019 Elsevier, Inc.80

2.2 Hydrogels

2.2.1 Thermochromic mechanism. Hydrogels comprise a three-dimensional crosslinked polymer network with infiltrated water, behaving like both liquids and solids,90–92 and usually possess mechanical flexibility, softness, permeability, and conductivity. Thermochromic hydrogels undergo phase separation and transparent-to-translucent transitions upon temperature changes, which have been utilized for smart windows.93–98

Hydrogels may have the upper critical solution temperature (UCST) and the lower critical solution temperature (LCST). Above LCST or below UCST, polymer chains in the hydrogel network are less soluble and adopt collapsed conformation or phase separated microdomains that scatter incident light. As a result, hydrogels appear opaque or translucent. At temperatures below LCST or above UCST, the polymer chains become soluble and swollen and the hydrogels become transparent.

LCST hydrogels at temperatures below τc are transparent and allow incident light to easily pass through thermochromic hydrogels (Fig. 7).23 However, at temperatures above τc, the hydrophobic polymer chains contract and aggregate to resist hydrophilic water molecules, and then phase separation occurs inside the hydrogels to form collapsed polymer clusters that can effectively scatter incident light to impair the visible transmittance of hydrogels. This is fundamental for solar modulation of thermochromic hydrogels.


image file: d4ta01767f-f7.tif
Fig. 7 Schematic illustration of thermochromic mechanism of thermochromic hydrogels. Reproduced with permission from Chen et al., Adv. Mater. 35, 2211716 (2023). Copyright 2023 Wiley-VCH GmbH.23
2.2.2 LCST hydrogels. Poly(N-isopropylacrylamide) (PNIPAM) hydrogels are a typical LCST hydrogel that undergoes a phase transition from hydrophilic to hydrophobic at 32 °C.16 Above 32 °C, PNIPAM chains become hydrophobic and less soluble and adopt a collapsed conformation and form globules that scatter solar light.98,99 This is the basis for PNIPAM hydrogels to serve as smart window materials to modulate solar incidence.

Zhou et al. reported the first PNIPAM hydrogel-based smart windows with a τc of around 32 °C.100 The luminous transmittance of hydrogel films decreases with an increase in thickness and shows an optimal Tlum of 70.7% at 52 μm thickness. The PNIPAM hydrogel-based smart window becomes opaque above τc and shows a ΔTsol of 25.5%. It is needed to modulate the intrinsic phase separation behavior of PNIPAM to further improve both the Tlum and ΔTsol for high performance thermochromic smart windows.

One solution is to modulate the scattering behavior of PNIPAM-based hydrogels. Fang et al. synthesized microparticles of a poly(N-isopropylacrylamide)-2-aminoethylmethacrylate hydrochloride (PNIPAM-AEMA) copolymer with large diameters and uniform crosslinks.16 The large particle size (∼1388 nm) and homogeneous internal structures with a very low light scattering contrast provide a high Tlum of 87.2% at 25 °C (Fig. 8a). In contrast, the particles shrink to about 546 nm at 35 °C, which scatter light over a broad spectrum, and thus show a high ΔTsol of 81.3% (Fig. 8b).


image file: d4ta01767f-f8.tif
Fig. 8 (a) The phase transition of the PNIPAM-AEMA hydrogel-based smart window triggered by the hand. (b) Thermochromic performances of PNIPAM-AEMA hydrogel and other reported materials. Reproduced with permission from Li et al., Joule 3, 290 (2019). Copyright 2019 Elsevier, Inc.16

A homogeneous hydrophilic network, and thus high Tlum, can be further achieved by introducing hydrophilic chains through free radical polymerization at low temperatures to avoid unwanted phase separation induced by reaction heating. Hydrophilic hydroxypropylmethyl cellulose (HPMC) is mixed with N-isopropylacrylamide for free radical polymerization at 0 °C.97 The heat released during the polymerization process could be dispersed in time. No phase separation takes place as that observed during high temperature-induced free radical polymerization of N-isopropylacrylamide. Here, the abundant hydroxyl groups of HPMC could form extensive hydrogen bonds with PNIPAM chains and water. The PNIPAM and HPMC chains form a uniform distribution, leading to a high transparency Tlum of 90.8% at low temperature.

A hydroxypropyl cellulose (HPC) hydrogel is another important LCST hydrogel.101–103 It possesses both hydrophobic epoxy ether groups and hydrophilic hydroxyl groups and has a miscible–immiscible transition at 42 °C.104,105 The random coil chains of HPC transform into collapsed spheres above 42 °C, resulting in an opaque phase that scatters solar radiation. The high LCST of HPC presents solar modulation at temperature much higher than the comfortable room temperature.

Zhang et al. constructed smart windows by using a hydroxypropyl cellulose-polyacrylic acid (HPC-PAA) hydrogel (Fig. 9a). The LCST of the HPC-PAA hydrogel is sensitive to solution pH and can be tuned down to 26.5 °C at pH 2.5.106 The hydrogel has massively interlaced pores (∼13 μm) because of the incorporation of PAA. The enlarged pores with a low scattering contrast increase Tlum to 90.1% at 18 °C. The pore diameter is reduced due to phase separation at 40 °C, which enhances light scattering, and thus the hydrogel shows a ΔTsol of 47.5%. Afterwards, photothermal cesium tungsten oxide (Cs0.32WO3) nanoparticles are further introduced to the HPC-PAA hydrogel to enhance NIR shielding performance (Fig. 9b).107 Cs0.32WO3 can transform the absorbed NIR light into heat through local surface plasmon resonance. Thus, the composite hydrogel reduces the NIR transmittance from 81.7% to 41.6% at 22 °C.


image file: d4ta01767f-f9.tif
Fig. 9 (a) The transmittance modulation mechanism of HPC-PAA hydrogel. Reproduced with permission from Zhang et al., ACS Appl. Energy Mater. 4, 9783 (2021), Copyright 2021 American Chemical Society.106 (b) The schematic illustration of the phase separation of photothermal nanoparticles-embedded HPC-PAA hydrogel. Reproduced with permission from Zhang et al., Ceram. Int. 48, 37122 (2022). Copyright 2022 Elsevier, Inc.107

Combining the temperature-responsive hydrogel mechanism with other mechanisms can endow smart windows with richer modulation abilities. Gao et al. constructed a smart window of HPC-PAA hydrogel doped with H+ and Li+.108 The phase transition temperature is adjusted through changing the hydrogen bond between molecular chains by ion doping. By integrating the ion doped thermochromic HPC-PAA hydrogel layer with the electrochromic WO3-indium tin oxide (ITO) layer, the smart window possesses four reversibly switch modes: transparent state, thermochromic state, electrochromic state, and electrothermal dual response state. Thus, the HPC-PAA hydrogel-based smart window shows a electric-/thermal-dual response.

2.2.3 UCST hydrogels. Polyampholyte (PAH) hydrogels are a typical UCST material.22,109 Above UCST, the ionic bonds are broken and the polymer chains become soluble. The polymer chains aggregate and separate from water below UCST. Thus, PAH exhibits a reversible optical transition from transparency to opaqueness.110

Chung et al. reported a highly flexible smart window based on PAH hydrogels that allows visible light to transmit at high temperature and blocks mid-infrared radiation at low temperature.109 Such smart windows not only protect personal privacy but also prevent heat loss through blackbody radiation. A hydrophobic monomer (3-(methacryloylamino)propyl)trimethylammonium chloride (MPTC) is introduced to modulate the phase separation behavior of PAH. The PAH-MPTC hydrogel presents a phase transition temperature of 43.4 °C. The particle size (∼1 μm) and homogeneous pore structures at 55 °C provide a transmittance of 82% at 800 nm wavelength. The hydrogel with bubble-like porous structures (∼40 μm) at 20 °C represents macroscopic phase separation, leading to a good mid-infrared modulation ability. By combining a printed electrical heater with the PAH-MPTC hydrogel, this PAH hydrogel-based smart window could be actively controlled. It is necessary to induce the phase separation of PAH without electrical energy input to achieve active control of smart windows.

Salt can be used to trigger PAH hydrogel-based smart windows.22 Since the phase separation of PAH is dominated by ionic bonds and dense entanglement to scatter natural light (Fig. 10), when PAH is immersed in sodium chloride solution [NaCl(aq)], the ionic bonds in PAH are disassociated. Thus, the polymer chain associates are partially ruptured, and the entanglement density decreases to pass through incident light, leading to a Tlum of 92%. Subsequently, the PAH hydrogel automatically recovers its opaqueness within 20 minutes when transferred from NaCl(aq) to H2O. The reversible transparency-shifting endows the PAH hydrogel with a ΔTsol of 88%.


image file: d4ta01767f-f10.tif
Fig. 10 Pictures of PAH hydrogel in opaque and transparent states when immersed in H2O and NaCl(aq), respectively, and the proposed dynamic nature of ionic bonds. Reproduced with permission from Guo et al., Mater. Horizons 9, 3039 (2022). Copyright 2022 Royal Society of Chemistry.22
2.2.4 High performance hydrogel-based smart windows. Thermochromic hydrogels are considered the most promising and competitive materials for smart windows and have attracted increasing research interest in the field of building energy saving.15,20,111,112 However, current challenges of hydrogel-based smart windows include fixed transition temperature,23 limited solar modulation ability,12 poor mechanical strength,15 and low-temperature freezing.95 The τc of most thermochromic hydrogels is fixed and does not satisfy intricate weather conditions. The solar modulation of hydrogels in the NIR region is lower than that of VO2. Pure hydrogels are mechanically fragile and usually freeze at temperatures below the freezing point of water. Methods have been proposed to tackle these problems15,113 by introducing functional materials,114,115 regulating structure configuration,116,117 and developing liquid smart windows.21 For example, the phase separation of HPC is susceptible to inorganic salts because of the weakened hydrogen bonds between polymer chains and water.102 Thus, the τc of HPC could be reduced from 42 °C to 30 °C by introducing NaCl. A supramolecular alcogel containing host-cellulose and guest-PAAM shows an excellent tensile strength of 13.6 MPa.116 The addition of glycerol breaks the hydrogen bond between water molecules and inhibits the crystallization of water, resulting in smart windows with freezing tolerance at −18 °C.118

Thermochromic properties and characteristics of these thermosensitive hydrogels are shown in Table 2. The performances of hydrogels and related smart windows can be flexibly modulated over a broad range, including adjustable τc (16.0–43.5 °C), strong solar modulation ability (33.4–89.9%), good mechanical strength (89.5 kPa–5.58 MPa tensile strength), and anti-freezing capability (−18 to −100 °C freezing point). Unlike metal oxides, hydrogels are soft materials with excellent processibility and adaptiveness to match specific application demands, even at temperature far below the freezing point.

Table 2 Thermochromic properties and characteristics of various thermosensitive hydrogels
Materials Characteristic T lum (%) ΔTsol (%) τ c (°C) Ref.
PNIPAM Decent visible transmittance 70.7 25.5 32.0 100
PNIPAM/AEMA Low-cost and scalable fabrication process 87.2 81.3 32.0 16
PNIPAM/HPMC Good solar modulation capability 90.8 81.5 32.0 97
HPC First smart window based on HPC hydrogel 67.4 25.7 30.0 102
HPC/PAA Reduced LCST by modulating pH 90.1 47.5 26.5 106
HPC/PAA Reduced LCST by modulating pH, and outstanding cycling stability 91.8 73.7 25.8 107
HPC/PAA Reduced LCST by modulating pH 90.1 80.0 35.2 119
HPC/PAA Electric-/thermal-dual-responsive 89.0 59.0 — 108
PAAM/SDS Adjustable LCST in the range of 16–34 °C 85.0 — 16.0 120
PAAM/C18/SDS Adjustable LCST in the range of 20–50 °C 99.0 33.4 20.0 121
PNIPAM/NaCl Adjustable LCST in the range of 31.7–24.5 °C 95.1 89.9 27.2 122
PNIPAM/DMMA Adjustable LCST in the range of 32.5–43.5 °C 91.3 88.8 32.5 23
PNIPAM/Ethanol Enhanced solar modulation ability and solar shielding 89.9 71.8 28.0 123
PNIPAM/HA Enhanced visible transmittance and solar modulation ability 85.9 68.6 32.0 114
W-VO2/PNIPAM/HPC Enhanced solar modulation ability 87.2 65.7 29 124
PNIPAM particles/water Revolutionarily liquid smart window for enhancing optical properties 90 68.1 32.5 21
PNIPAM/PAA chain/microparticle hybrids suspension Liquid smart window for enhancing solar modulation ability 91.5 85.8 26.7 112
SPU/binary ionic liquid Enhanced mechanical strength by hydrogen bonds 95.1 83.0 — 115
PNIPAM/ACMO Enhanced mechanical strength by 3D printing technology 85.8 79.3 — 99
PNIPAM/PDMAA/EG Excellent anti-freezing ability and adjustable LCST 89.3 80.7 ∼30.0 125
PNIPAM/glycerol Good anti-freezing ability and adjustable LCST 90.0 60.8 ∼26.0 118
PNIPAM/HPC Good anti-freezing ability 80.7 64.5 ∼29.0 95
PNIPAM microgel High luminous transmittance and fast response time 91.2 81.8 — 126
HPC/KCl/polyaniline Thermo/electrochromic dual response 73.0 57.5 30.0 127
PNIPAM/upconverting nanoparticles Converting infrared light into visible light 82.8 79.8 40.0 128
PNIPAM/Si/Al Printable hydrogel 80.1 72.7 35.0 129


2.2.4.1 Modulation of τc. The critical temperatures of thermochromic hydrogels need to be tunable to meet the requirement of different climates.130 Some effective strategies, for example, introducing inorganic salts122 and hydrophilic monomers,23 have been demonstrated to modulate τc through suppressing phase separation of thermochromic hydrogels.

Inorganic salts are introduced into hydrogel networks to regulate phase transition temperature of thermochromic hydrogels. A thermochromic hydrogel-based smart window with good optical properties (Tlum of 99.1% and a ΔTsol of 33.4%) is developed by copolymerizing hydrophilic acrylamide (AAM), hydrophobic stearyl methacrylate (C18), and sodium dodecyl sulfate (SDS) micelles.121 SDS micelles serve as surfactants to modulate the self-assembled particle size of the hydrophobic C18 phase that is covalently linked to PAAM chains, which influence the light transmittance and the phase transition temperature. The metal ions interact with the SDS headgroup (–SO4−) and reduce the electrostatic repulsion, leading to a lower critical micelle concentration (CMC) of SDS. The salt concentration and cation radius also affect the CMC of SDS. Thus, the thermochromic hydrogel presents a tunable phase transition temperature from 20 °C to 50 °C. Similarly, Tian et al. developed a PNIPAM hydrogel-based smart window with optical properties (Tlum of 92.7% and a ΔTsol of 93.6%).122 Adjustable phase transition temperature within 24.5–31.7 °C is achieved by changing the sodium chloride (NaCl) concentration. Na+ interacts with the polymer surface and weakens the hydrogen bonds between amide groups of polymer chains and water. Thus, the phase separation of PNIPAM is more susceptible to the salt concentration, resulting in a tunable τc.


2.2.4.2 Solar modulation. Many thermochromic hydrogels have weak solar regulation capability. For example, pure PNIPAM hydrogels only possess a ΔTsol of 25.5% at 52 μm thickness,100 which limits the overall solar modulation performance of PNIPAM-based smart windows.15,131 Many endeavors to improve light scattering performance have been made to enhance the modulation by introducing organic solvents,123 incorporating inorganic nanoparticles,124,132,133 developing chain/microparticle hybrid liquid smart windows,112etc.

Ding et al. constructed a PNIPAM hydrogel-based smart window through introducing ethanol to enhance solar modulation to 71.8%.123 Ethanol decreases the size of scattering centers and increases the scattering center number. Meanwhile, it weakens interactions between water and polymer chains and discharges more water from the PNIPAM network during the phase transition, resulting in a high refractive index difference between PNIPAM and water. Thus, the hydrogel has highlight scattering behavior and presents an enhanced solar modulation ability.

W-doped VO2 nanoparticles are introduced into PNIPAM/HPC hydrogels to enhance ΔTsol.124 W-doped VO2 nanoparticles are attached to the surface and interior of PNIPAM/HPC hydrogels. As a result, the hydrogel presents a thicker pore wall and higher porous density, which scatters light above its τc, leading to a higher solar modulation ability of 65.7%.

A liquid hydrogel-based smart window is recently developed by copolymerizing N-isopropylacrylamide (NIPAM) and acrylic acid (AA).112 The liquid hydrogel includes linear PNIPAM-PAA chains and Cu3Cit2@PNIPAM-PAA core–shell microgel particles. Above τc, the chains are dissolved and aggregate to form new compact microgel core–shell particles, which undergo a phase transition and shrink. The optical contrast between microgel particles and water results in strong light scattering performance, leading to an enhanced solar modulation ability. The synergetic effect of polymer chains-microgel particle conversion and core–shell particle shrinkage strengthens ΔTsol to 85.5%.


2.2.4.3 Mechanical strength. Good mechanical properties of thermochromic materials are conducive to improving the service life of smart windows. Conventional thermochromic hydrogels usually have poor mechanical strength due to shrinkage and rupture of networks during the phase transition.134–137 Many strategies based on ionic interactions,117 double networks,138 supramolecular assembly,116 hydrogen bonding,115etc., have been developed to improve the strength, toughness, and self-healing of thermochromic hydrogels.

Dai et al. developed a physically crosslinked ionogel by photopolymerizing hydroxyethyl acrylate (HEA) and butyl acrylate (BA) within ionic liquid [BMIm]+[BF4]− (Fig. 11a).117 HEA and BA show excellent solubility in [BMIm]+[BF4]−. The ionogel with three-dimensional porous structures has high-density molecular entanglements and cross-linking. The maximum fracture strain increases with the [BMIm]+[BF4]− content. Thus, the ionogel presents a tensile strength of 89.5 kPa at 670% fracture strain.


image file: d4ta01767f-f11.tif
Fig. 11 (a) Schematic diagram of the ionogel network structure. Reproduced with permission from Dai et al., ACS Appl. Polym. Mater. 5, 3398 (2023). Copyright 2023 American Chemical Society.117 (b) Schematics of the PNIPAM-PAA/PAAM hydrogel with a crack under tension. Reproduced with permission from Zheng et al., Compos. Commun. 42, 101684 (2023). Copyright 2023 Elsevier, Inc.138 (c) Impact resistance testing of ordinary glass, photochromic glass, armored glass, and smart glass. Reproduced with permission from Chen et al., Adv. Funct. Mater. 33, 2214382 (2023). Copyright 2023 Wiley-VCH GmbH.116

Zheng et al. reported a double-network hydrogel by incorporating PNIPAM-PAA microgels into a PAAM matrix for thermochromic smart windows, forming interpenetrating networks.138 PNIPAM-PAA microgels prevent crack propagation, and the PAAM matrix serves as a sacrificial network to dissipate energy (Fig. 11b). As a result, the hydrogel with only 2 wt% microgels shows a fracture strength of around 70 kPa and breaking elongation of 420%.

An alcogel with supramolecular configuration is constructed to improve mechanical properties of smart windows.116 The alcogel with 68.7 wt% ethanol content has host-cellulose chains wrapping guest-polyacrylamide (PAAM) chains through extensive hydrogen bonds. The alcogel shows compact supramolecular structures between cellulose and PAAM and ethanol-induced curling of PAAM chains. It shows a tensile strength of 13.6 MPa and a higher impact resistance of 42.8 kJ m−2 than ordinary glass (4.1 kJ m−2) (Fig. 11c).

Li et al. fabricated a poly(propylene glycol) (PPG)-based ionogel by synthesizing hydroxyl-terminated PPG, isophorone diisocyanate (IPDI), and adipic acid dihydrazide (ADH) for thermochromic smart windows.115 The hydrazide groups of ADH react with isocyanate groups to produce acylsemicarbazide (ASCZ) moieties. Meanwhile, multiple hydrogen bonds form between the ASCZ moieties, which increase the crosslinking density of the polymer network. Thus, the ionogel presents a high tensile strength of 5.58 MPa and a tensile strain of 3210%.


2.2.4.4 Antifreezing. Thermochromic anti-freezing hydrogels are required for smart windows in cold regions.93,139 Recently, several strategies based on hydrogen bonding competition between polymer chains and water have been proposed to improve the anti-freezing ability of thermochromic hydrogels, including the introduction of organic solvents118,125 and modulation of polymer structures.95

Wang et al. prepared an anti-freezing smart window with a Tlum of 89.3% and ΔTsol of 80.7% by using thermochromic poly(N-isopropylacrylamide)-poly(N,N-dimethylacrylamide)/ethylene glycol hydrogel (PNIPAM-PDMAA/EG) that is resistant to low temperatures.125 The introduction of EG is conducive to the formation more hydrogen bonds between polymer chains and water, which effectively hampers the freezing of the hydrogel. The hydrogel does not freeze at temperatures down to −100 °C. Similarly, an anti-freezing smart window with a Tlum of 90% and ΔTsol of 60.8% is fabricated based on the noncovalent crosslinked PNIPAM hydrogel.118 The PNIPAM hydrogel with a freezing point of around −18 °C is synthesized in a glycerol–water (GW) binary solvent system. The hydrogen bonds between glycerol and water are more stable than that between water and water, thereby resulting in excellent low-temperature resistance.

Feng et al. developed an anti-freezing hydrogel for smart windows with a Tlum of 80.7% and ΔTsol of 64.5% by simply entangling PNIPAM and HPC together without adding other organic solvents.95 The HPC fibers are liable to form hydrogen bonds with water, efficiently restricting water crystallization. As a result, the PNIPAM/HPC hydrogel has a good low-temperature resistant ability at −13 °C.

2.2.5 Thermochromic hydrogel smart windows for all climate solar modulation. Most thermochromic smart windows are triggered by high temperature in hot climates to block excessive solar radiation into buildings, which are usually applicable in low latitude areas in the summer. In order to extend the application spectrum of smart windows and improve their energy saving capability, it is desirable to develop smart windows suitable for different climates, particularly in cold weather in the winter. The capability to block heat radiation from buildings to outdoors is of special interest to keep buildings warm and save heating energy in winter. To achieve this, it is critical to develop thermochromic hydrogels with different transition temperatures over a broad range that adapt to different climates.

Dai et al. developed a thermochromic hydrogel with a reversible three-stage transition of opaque-transparent-translucent by introducing SDS and NaCl into the cross-linking network of PNIPAM-PAAM.140 The solubility of SDS increases as the temperature increases and shows a clear transition point (Krafft point, Tk). At the temperature below Tk, SDS molecules separate from the solvent and precipitate inside the hydrogel network, making the hydrogel opaque. At the temperature between Tk and τc, SDS micelles are formed and the hydrogel becomes transparent, which results in a Tlum of 80.3%. At temperature above τc, PNIPAM chains aggregate and form phase separated microdomains to scatter incident light. Thus, the hydrogel becomes opaque to inhibit indoor heat dissipation through the window to the outside at low temperature and demonstrates a ΔTsol of 72.9%. Meanwhile, it becomes translucent to scatter sunlight at high temperature, showing a ΔTsol of 42.7%. A model styrofoam house with a hydrogel smart window is used for the solar energy shielding test. The model house with normal glass is set as a control. The indoor temperature of the model house with a smart window is reduced by 3.6 °C after 30 minutes of infrared irradiation (55 °C), and the interior temperature of the model house with normal glass is 1.3 °C lower than that of the model house with a smart window after being placed in a cold closet for 40 minutes.

Recently, our group developed thermochromic hydrogel smart windows for all climate applications to modulate solar radiation and heat radiation.23 The smart windows are equipped with LCST type thermoresponsive hydrogels with different τc. Transparent thermochromic hydrogels are prepared by copolymerizing hydrophilic N,N-dimethylacrylamide (DMAA) and NIPAM monomers at room temperature with UV initiation. The introduction of a hydrophilic DMAA monomer enhances hydrophilicity and then slightly suppresses phase separation at high temperatures. As a result, the LCST or τc of hydrogels gradually increases from 32.5 °C to 43.5 °C with increasing DMAA content in the network. Here, UV-initiated free radical polymerization at low temperature is used to avoid potential phase separation caused by reaction heat. The obtained hydrogels are highly transparent, with a luminous transmittance of 91.3%. In August, in Guangzhou, as an example, after 1 h of sunlight exposure at 14:00 p.m., the hydrogel smart window becomes opaque (Fig. 12a).


image file: d4ta01767f-f12.tif
Fig. 12 Smart windows for all-climate applications. (a) Photos of a hydrogel smart window in the sun in Guangzhou. (b) Schematic illustration of a two-compartment simulation test setup. (c) Temperature distribution in the setup in the sun in Guangzhou, left: control and right: test group. (d) Field testing of hydrogel smart windows in Beijing, Xi'an, Shanghai, Fuzhou, Guangzhou and Dalian. Monthly temperature regulation and energy saving of smart windows in (e) Guangzhou and (f) Dalian, and temperature differences between test and control rooms, where T0, T1, and T2 are temperatures in the shadow of the test group and the control group. (g) Thermal infrared images of hydrogel smart windows with different grid patterns. Reproduced with permission from Chen et al., Adv. Mater. 35, 2211716 (2023). Copyright 2023 Wiley-VCH GmbH.23

Two-compartment thermal insulation boxes are assembled to test the solar regulation ability of the smart windows in various climates. The test room is equipped with a thermochromic hydrogel smart window, and the control room is equipped with an ordinary double layer glass window (Fig. 12b). Here, the smart window comprises a thermochromic hydrogel sandwiched between two glasses. Since the refractive constant of the hydrogel is close to that of glass, the transmittance of smart windows is augmented from 79.4% to 85.5%. At high temperatures, the smart windows become opaque and prevent local heating. Therefore, the temperature of the test room is more uniform than that of the control room (Fig. 12c). The temperature increase of the test room (40.6–44.0 °C) is less than that of the control room (45.8–51.6 °C).

The smart windows show outstanding solar and heat modulation in different climates. Simulation tests are carried out in six cities (Beijing, Dalian, Xi'an, Shanghai, Fuzhou and Guangzhou) with different geographical locations and climates from December 2021 to August 2022 (Fig. 12d). In Guangzhou, a tropical southern city, the temperature in direct sunlight areas in the summer can reach 50–60 °C. The hydrogel smart window with a τc of 40.3 °C becomes opaque and scatters sunlight, showing a ΔTsol of 88.8%. As a result, the air temperature in the test room (T1) is lower than that in the control room (T2). The indoor temperature is reduced by 4 °C in March, and an energy saving of 5.14 kJ m−3 is achieved (Fig. 12e). In December, in Dalian, the northern city, the ambient temperature in winter is below the freezing point and central heating temperature is used. As the room temperature is raised to 35 °C as a proof-of-concept test, the transparent hydrogel smart window with a τc of 33 °C becomes opaque and prevents indoor heat dissipation through the window to the outside. The temperature outside the test room (T1) is 4.7 °C lower than that of the control room (T2), and an energy saving of 6.05 kJ m−3 is achieved (Fig. 12f). These results demonstrate the capability of such smart windows to keep rooms “warm in winter and cool in summer”.

On the other hand, the energy saving performance of smart windows is investigated by theoretical calculations based on theoretical models.141 For example, Long et al. reported a thermochromic smart window using a HPC entangled PNIPAM hydrogel with a fiber structure.95 At 20 °C, the fibers are thin and elongated to allow light to pass through, leading to a Tlum of 80.7%. In contrast, the fibers aggregate to scatter light at 40 °C, and thus the smart window shows a ΔTsol of 64.5%. The energy consumption is calculated by using an actual-size house model (8 m in length, 6 m in width, and 2.7 m in height) with four windows (3 m in width and 2 m in height) and the weather conditions in Singapore. The calculation results show an energy saving of ∼6 MJ m−2 in July. Moreover, the same group estimated the energy saving of another thermochromic smart window with a Tlum of 90% and ΔTsol of 68.1% based on PNIPAM particle dispersion in water.21 Such a smart window combines solar heat absorption by water with large specific heat capacity (4.2 kJ kg−1 K−1) and the solar modulation of PNIPAM particles at 32 °C, which results in an energy saving of ∼7 MJ m−2 in July. These results indicate that thermochromic smart windows are promising for building energy saving.

Such thermochromic hydrogel smart windows can be processed into well-designed structures to balance visibility and solar regulation. Gridded thermochromic hydrogel smart windows are prepared by 3D printing. Upon heating, the hydrogel grids become opaque, while the rest of the domains remain transparent. By adjusting the mesh size and density, both good visibility and solar modulation ability (59.6%) are achieved (Fig. 12g).

2.3 Other thermochromic materials

2.3.1 Ionic liquids. Ionic liquids (ILs) are regarded as non-volatile liquids and salts that consist of organic cations and anions.142,143 ILs have no leakage or combustion problems and have been widely investigated and applied.144–146 Meanwhile, ionic liquids have intrinsic thermochromic response behavior and are considered competitive candidates for smart windows (Fig. 13a).147 When the temperature is below critical temperature, thermochromic ionic liquids adopt an octahedral configuration, which can change into tetrahedra as temperature exceeds the critical transition temperature, resulting in a significant light transmittance variation.12
image file: d4ta01767f-f13.tif
Fig. 13 (a) The temperature-dependent crystal structure change of ionic liquids. Reproduced with permission from Zhu et al., ACS Appl. Mater. Interfaces 8, 29742 (2016), Copyright 2016 American Chemical Society.147 (b) Schematic illustration of the change process of perovskite materials: with increasing temperature, the perovskite crystals nucleate and grow. Reproduced with permission from Ke et al., Adv. Funct. Mater. 28, 1800113 (2018). Copyright 2018 Wiley-VCH GmbH.12 (c) Schematic diagram of the structures in a hybrid perovskite compound. Reproduced with permission from Teri et al., J. Mater. Chem. C 11, 8903 (2023). Copyright 2023 Royal Society of Chemistry.160 (d) Schematic illustration of the phase transition between the aligned phase and focal conic phase. Reproduced with permission from Shen et al., Laser Photonics Rev. 17, 2200207 (2023). Copyright 2023 Wiley-VCH GmbH.167

Ionic liquids can be used to prepare thermochromic ionogels for use in smart windows. For example, Wu et al. reported a thermochromic ionic liquid-based gel electrolyte with a Tlum of 90.2% and ΔTsol of 82.3% for thermos-/electro-dual-responsive smart windows.148 With the addition of an ionic liquid, hydrogen bonds form between the polymer network and ionic liquid, which result in better swelling and movement of polymer segments, leading to an adjustable τc (47–55 °C).

Combining thermochromic ionic liquid-based gels with other mechanisms can impart multiple adjustabilities to smart windows. A thermochromic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI) microemulsion-based gel is prepared by polymerizing an acrylamide monomer in an ionic liquid-based microemulsion.149 Tween 20 is distributed at the oil–water interface to stabilize the droplets in the ionic liquid microemulsion. At 25 °C, the droplets are stable and difficult to precipitate. Thus, the thermochromic gel shows a Tlum of 99.3%. When temperature is above 40 °C, the microemulsion droplets aggregate, which highly scatter the incident light, leading to a high ΔTsol of 92.6%. Besides, [EMIM]TFSI endows the thermochromic gel with ionic conductivity. Thus, the gel is also utilized as an electrolyte layer for electrochromic smart windows. Integrating the conductive gel with a WO3 electrochromic layer achieves electric-/thermal-dual modulation of smart windows. Similarly, Deng et al. constructed a thermochromic ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4)-based gel electrolyte by polymerizing N-isopropylacrylamide.150 At temperature below LCST, the PNIPAM chains are dissolved in BMIMBF4 and water. When temperature is above LCST, the polymer chains aggregate into globules to scatter incident light, leading to a good thermochromic performance. Moreover, BMIMBF4 endows the gel with ionic conductivity up to 15.4 mS cm−1. Combining the thermochromic PNIPAM layer with an electrochromic TiO2 layer, an electric-/thermal-dual smart window is developed.

2.3.2 Perovskites. Perovskites are a class of materials with significant optoelectronic performances and scalable manufacturing processibility.151–153 Thermochromic perovskites have a reversible color switch in response to temperature changes without additional energy input and are thus recognized as ideal candidate materials for smart windows.154,155 Thermochromic perovskites have thousands of complex constructures, mainly including three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), zero-dimensional (0D), double perovskite forms, etc. Among them, 3D perovskites are the focus of research. Metal halide perovskites with structure ABX3 [A and B are the larger and smaller cations respectively and X represents the anion] are widely used in smart windows.156–158

The thermochromic mechanism of perovskites is based on its abnormal crystallization phenomenon that its solubility gradually decreases with an increase in temperature. At low temperature, the solubility of thermochromic perovskites is high, and the hydrated perovskite is colorless to allow sunlight to pass through. As the temperature increases, the solubility of the perovskite decreases, and the colorless perovskite hydrate phase transforms into a colored photovoltaic perovskite phase to block sunlight (Fig. 13b).12,159

Thermochromic perovskites show a red shift in absorbance with increasing temperature. Teri et al. developed a copper-based hybrid perovskite (MBA)2CuCl4 (MBA is 4-methoxybenzylamine) with a reversible phase transition temperature of 430.2 K.160 The dielectric states are different because the organic aminium cations transition between partial order and disorder states (Fig. 13c). The coordinate geometry of Cu2+ is relevant with the position of the absorption maximum for the d–d transition. The structure of Cu-based perovskite changes from tetrahedral coordination to an octahedron with increasing temperature, which causes a shift in the electronic absorption band. Thus, the red shift of hydride perovskite occurs in the absorbance spectrum, resulting in a color change from yellow to brown.

One challenge of perovskite-based smart windows is how to reduce the high critical transition temperature to around ambient temperature. Liu et al. fabricated a novel smart window based on the thermochromic perovskite H-MAPbI3−Clx with a ΔTsol of 23.7%.161 The hydration and dehydration processes of thermochromic perovskite H-MAPbI3−xClx during the phase transition demonstrate that the ambient humidity is important to the τc. When the relative humidity decreases to 50%, the τc is around 40 °C, which is lower than that of 80% relative humidity (51.4 °C). The τc can be reduced from 51.4 °C to 29.4 °C with a decrease in relative humidity (from 80% to 25%).

Developing alternative strategies to trigger the thermochromic effect of perovskite-based smart windows at ambient temperatures is necessary for applications. Liu et al. proposed a NIR activated perovskite-based smart window at room temperature.154 The smart window is fabricated by integrating cesium-doped tungsten trioxide (CWO) with T-perovskite H-MAPbI3−xClx. CWO has high visible transmittance of 82% and strong NIR absorption of 90%. As a result, the perovskite-based smart window shows a Tlum of 65.7%. At 23.5 °C, the strong NIR absorption of the CWO layer produces thermal energy and heats the window to the phase transition temperature of thermochromic perovskite. Then the thermochromic T-perovskite dissociates water from the MAPbI3−xClx layer. Thus, the smart window changes from a transparent state to a colored state and shows a ΔTsol of 17.5%.

2.3.3 Liquid crystals. Liquid crystals (LCs) are neutral organic compounds and regarded as the fourth state of matter. LCs typically present rodlike or disclike phases, which are anisotropic in optical, electrical, and magnetic properties.162 The arrangement of mesogenic molecules can be regulated in response to an electrical field,163 external temperature,164 a magnetic field,165etc. Although liquid crystal smart windows driven by an electric field have been widely investigated and commercialized,166 it is needed to develop next-generation liquid crystal smart windows with high modulation capability and free of energy input (e.g., an electric field).167 Thermochromic liquid crystals have been recently reported to change molecular orientation upon temperature changes.168–170 Specifically, thermochromic liquid crystals show perpendicularly aligned smectic A (SmA) phases at low temperatures and will undergo phase transitions to chiral nematic (N*) phases with focal-conic texture when the temperature is raised above the phase transition temperature. The phase transition temperature can be modulated in the range of 10–40 °C by adjusting the concentration of monomers.171,172 The temperature-induced phase transition is accompanied by changes in molecular orientation and optical states (Fig. 13d).167 Thermochromic liquid crystals are composited with a polymer matrix to prepare thin film smart windows.173,174 Liquid crystals are embedded in crosslinked polymer networks as droplets. The composites with liquid crystal molecules anchored in the polymer network usually experience reversible changes in the liquid crystal orientations, matching/mismatching in refractive indices,170 and thus optical properties upon an external stimulus.175

Liquid crystal-based thermochromic smart windows have excellent solar modulation capability because their strong control in both visible and near-infrared regions. For example, Liang et al. developed a thermochromic liquid crystal-based film with a Tlum of 67%.172 The composite film is composed of liquid crystal, poly(vinylpyrrolidone) (PVP), and tungsten bronze (CsxWO3) nanorods. The phase of liquid crystal changes from a smectic state to a chiral nematic state with an increase in temperature, which provides a change from transparency to opaqueness. CsxWO3 has strong and broadband LSPR absorption of NIR irradiation. Thus, the liquid crystal-based smart window shows an ultrahigh near-infrared irradiation shielding of 95% from 800 nm to 2500 nm.

Combining the thermochromic mechanism of liquid crystals with other mechanisms can give the smart windows multiple adjustment capabilities. Meng et al. constructed a photothermal dual-driven liquid crystal-based smart window.176 When the ambient temperature is 26 °C, the liquid crystal presents a smectic phase, allowing incident light to pass through, resulting in a Tlum of 70%. When the ambient temperature is above 41 °C, the liquid crystal changes from the smectic phase to a cholesteric phase. Then the refractive index of the liquid crystal is changed in the thickness direction. Therefore, the liquid crystal-based smart window scatters light, leading to 50% reduction in transmittance. Moreover, a photothermal molecule, isobutyl-substituted diimmonium borate (IDI), is doped into the liquid crystal-based system. IDI has strong absorption in the NIR range, and the temperature of the liquid crystal-based system with IDI increases from 24 °C to about 31.7 °C under sunlight for 160 seconds. IDI can convert light irradiation to heat and accelerate the phase change of the liquid crystal, leading to a photothermal dual-driven smart window. Similarly, an electrical/thermal dual-control film with 57.8% Tlum and 34.6% ΔTsol is prepared by combining liquid crystal/polyacrylate with W-doped VO2.177 The composite materials are sandwiched between two plastic conductive substrates. At low environmental temperature and no voltage, the composite materials present a homeotropically aligned liquid-crystalline polymer network. Both the liquid crystal and polymer possess similar refractive indices so that the film appears transparent. When an electric field is applied, the liquid crystal domains are aligned parallel across the direction of the electric field, yielding an anisotropic distribution of refractive index values. The difference between the liquid crystal and polymer results in light scattering. Meanwhile, the thermochromic W-doped VO2 exhibits passively thermochromic behavior during the phase transition at 43.2 °C. Thus, the liquid crystal-based composite materials are promising for smart windows that can be activated by temperature changes and/or using an electric field.

3. Thermoelectricity conversion and solar cells

The current philosophy of thermochromic smart windows discards reflected solar energy.27,178,179 To further improve the energy saving efficiency, it is necessary to harness blocked solar energy. Many studies have been reported to achieve simultaneous modulation and utilization of solar radiation with smart windows. For example, solar thermoelectricity materials and devices can produce electrical potential during temperature changes.180–182 Moreover, integrating solar cells with thermochromic materials can boost the generation of new smart windows that can not only modulate solar radiation, but also generate electricity through direct solar-electrical conversion.183,184

3.1 Thermoelectricity conversion

Thermoelectricity conversion is considered a clean photovoltaic technology based on the Seebeck effect,185,186 which usually converts temperature variations induced by the solar-thermal conversion effect into electricity by using a solar thermoelectric generator.187 For example, Zhang et al. prepared a phase change composite with polybenzobisoxazole fibers through a molding process.179 The fibrous crystals in fibers offer thermal pathways during the charging/discharging processes (Fig. 14a). The unique actinomorphic structure facilitates a controllable thermo-conductive performance and promises a good match with the charging rate to convert solar thermal energy into electricity. As a result, the solar thermoelectric generator shows an output voltage of 3.41 V and a power density of 198.7 W m−2.
image file: d4ta01767f-f14.tif
Fig. 14 (a) Schematic for multi-directional heat conduction in the phase change composite. Reproduced with permission from Zhang et al., Adv. Funct. Mater. 32, 2109255 (2022). Copyright 2022 Wiley-VCH GmbH.179 (b) Schematic illustration of the proposed power-generating smart window architecture and working process. Reproduced with permission from Zhang et al., Adv. Energy Mater. 11, 2101213 (2021). Copyright 2021 Wiley-VCH GmbH.28

Although the solar thermal-electric conversion ability has been improved, it is still rare to design thermal-electric conversion smart windows due to high operation temperature and low visible transmittance. Zhang et al. constructed a Cs0.33WO3/resin/glass smart window with a high luminous transmittance of 88% and outstanding solar absorption (Fig. 14b).28 The absorbed near-infrared light and ultraviolet light can be effectively converted into heat. The thermoelectric system attached to the edges of the film collects the heat and transforms it into electricity. The smart window shows an output voltage of around 4 V. Besides, under a sunlight intensity of 100 mW cm−2, the open-circuit voltage and maximum power density reach up to 3.8 V and 0.4 mW cm−2 respectively. This work not only realizes efficient energy saving and power generation in the field of smart windows, but also provides significant insights and promising pathways for building energy saving.

3.2 Integration with solar cells

Integrating thermochromic materials with solar cells is an effective way to achieve thermochromic modulation and power generation.188,189 It has been demonstrated that combining thermochromic VO2-based materials with solar cells can simultaneously realize light harvesting and modulation. For example, Guo et al. constructed a photovoltaic smart window through combing a thermochromic VO2 layer and an organic solar cell.188 The VO2 layer can intelligently modulate near-infrared light according to the environmental temperature. The organic solar cell can convert visible light into electricity. Therefore, the VO2-based smart window shows a luminous transmittance of 28.2%, a solar modulation ability of 7.5%, and a power generation efficiency of 3.1%. This work pioneers a research direction for designing electrical-generating smart windows, but the thermochromic properties and electrical generation efficiency of smart windows are limited. Meng et al. prepared a photovoltaic smart window by integrating W-doped VO2 nanoparticles and organic perovskites.190 As shown in Fig. 15a, the smart window presents not only high visible transmittance at both high and low temperatures, but also a strong solar modulation capability of 10.7% due to the phase transition behavior of thermochromic W-doped VO2. A low-E coating is placed on the surface of glass to enhance the thermal insulation. The electron transport layer between thermochromic W-doped VO2 and perovskite promotes extraction and transport of photogenerated charges. As a result, the smart device exhibits a high-power conversion efficiency of 15.4% at 25 °C and 16.1% at 45 °C.
image file: d4ta01767f-f15.tif
Fig. 15 (a) Illustration of the photovoltaic smart window work on cold and warm days. Reproduced with permission from Meng et al., Nano Energy 91, 106632 (2022). Copyright 2022 Elsevier, Inc.190 (b) The schematic diagram of the energy saving and energy storage integrated smart window in different environments. Reproduced with permission from Niu et al., Adv. Sci. 9, 2105184 (2022). Copyright 2022 Wiley-VCH GmbH.191

Thermochromic hydrogels have also been widely combined with solar cells to achieve both energy saving and energy generation. For example, Niu et al. constructed a multi-layer louver smart window containing a thermochromic host–guest hydrogel (HGT hydrogel) and Si-based solar cell (Fig. 15b).191 The HGT hydrogel is prepared by introducing HPC microparticles into the transparent PAM-PAA hydrogel matrix. Solar light can pass through the smart window at low temperatures, but the incident light is blocked due to the microstructure change of the HGT hydrogel at high temperatures. Therefore, the louver smart window has an ultrahigh luminous transmittance of around 90% and a strong solar modulation capability of about 54%. The unique louver structure of solar cell endows the smart window with an outstanding energy generation capability of 18.2%. Besides, the louver smart window exhibits a lower temperature of 13.3 °C than the normal one after 1 h radiation of simulated sunlight, showing good energy-saving performance. Similarly, Meng et al. developed a photovoltaic smart window by integrating a thermochromic HPC hydrogel with a perovskite solar cell.192 Because of the structural transition of the HPC hydrogel, the photovoltaic smart window has a luminous transmittance of 27.3% at 20 °C and 10.4% at above 40 °C, as well as a solar modulation ability of 15.7%. Indium tin oxide is used as the electrode of the solar cell. The smart window presents a high peak conversion efficiency of 17.5%. An excellent combination of solar modulation and power generation is achieved in this smart window, contributing to sustainable building energy saving.

4. Conclusion and outlook

Thermochromic smart windows have received increasing attention for applications in building energy saving. In this review, we have summarized the recent progress of high performance smart windows based on thermochromic materials from response mechanisms and improved strategies for energy-saving applications. Emerging thermochromic smart windows can convert reflected solar energy into electricity through thermoelectricity conversion or solar cell technology.

Despite many excellent properties of thermochromic smart windows, there are still many challenges and unmet demands that stimulate further studies in this field. More novel thermoresponsive materials should be explored to satisfy the expansive application range of smart windows. Besides, the performances of current thermochromic materials are unsatisfactory and need to be improved. For example, the undesired color, biotoxicity and instability of VO2 severely restrict its development. And it is difficult to achieve a perfect balance of τc, ΔTsol and Tlum in thermochromic VO2. Thermochromic hydrogels have an outstanding solar modulation ability in smart windows based on their reversible hydrophilic/hydrophobic phase transition. However, the problems of solvent volatilization, freezing and mechanical weakness of hydrogels cannot be ignored. Then the high cost and intricate fabrication process of ionic liquids, perovskites and liquid crystals also need to be solved. Additionally, structural modification that elevates thermochromic performances have been successfully realized in VO2 and hydrogel systems, and different thermosensitive materials typically possess different modulation ranges of solar light. Thus, more efforts should be devoted to structure modification and composite application of thermochromic materials.

Thermoelectricity conversion based on the Seebeck effect for thermochromic smart windows is rarely investigated. More investigations are needed to develop new materials and mechanisms to promote the development of building energy generation. For solar cell integration, the low light transmittance and undesired color state should be addressed. Solar cells in smart windows have a lower power conversion efficiency than traditional perovskite cells due to the limitation of the phase transition of thermochromic materials; hence more fundamental research on the photovoltaic properties of solar cells should be conducted to enhance the electricity generation of thermochromic smart windows.

There are still some challenges with thermochromic smart windows to be addressed to meet commercial use. Common thermochromic smart windows are fabricated by sandwiching thermosensitive materials between two pieces of normal glass. The additional expenditure for thermosensitive materials and encapsulation is huge. Besides, the thermochromic properties of thermosensitive materials weaken with time, and therefore smart windows need to be regularly maintained and repaired. The long-term service life of thermochromic smart windows is important in practical applications of building. To this end, thermosensitive materials should have stable structures and optical properties. Traditional thermochromic materials show slow response times due to the tardy phase transition process caused by outdoor temperature changes. Thus, photo-thermal nanoparticles or additional applied voltage should be introduced to the thermochromic intelligent system to accelerate the response process. To meet the practical requirements of active control or privacy protection, thermal response smart windows should be combined with other stimuli to realize dual-response or even multi-response smart windows. Moreover, the promising trend is to integrate other functions into smart windows, such as self-cleaning, water harvesting, energy storage, super hydrophobicity, anti-drying, anti-freezing, etc. On the one hand, it is necessary to optimize the solar modulation of smart windows while maintaining good luminous transmittance. On the other hand, combining smart windows with solar cells to collect solar radiation and convert it into electricity will further enhance building energy saving.

In the future, smart windows will become a meaningful platform. Artificial intelligence and 5G society emerge one after another with increasing sophistication, so that smart windows may be more intelligent with other amazing features to serve people, for example, danger warning, sound insulation, intelligent ventilation, film projection, weather forecasts, and so on. We expect that more research communities will devote efforts together to promote the development of smart windows because the smart window system is a highly interdisciplinary technique. Throughout future developments, thermochromic smart windows will face tremendous tasks and challenges, but in parallel, they will also offer the exciting prospect of building energy saving.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are indebted to financial support from the National Natural Science Foundation of China (22375225, J. F.) and Postgraduate Education Foundation of Sun Yat-sen University (29000-11230011, G. C.).

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Footnote

† These two authors contributed equally.
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