Spectrally Selective Smart Window with High Near-Infrared Light Shielding and Controllable Visible Light Transmittance

Mie scattering theory applied to light scattering of large nonhomogeneous colloidal spheres

Colloidal suspensions made of smart core-shell structures are of current interest in many fields. Their properties come from the possibility of varying the core and shell materials for modifying the composite particles' chemical, biological, and optical properties. These particles are formed with a material with a constant refractive index core and a shell with a refractive index decaying until it matches the solvent refractive index. Poly(N-IsoPropyl AcrylaMide) (PNIPAM) is a typical example of materials forming shells. In this report, we present how to apply Mie scattering theory to predict and understand the static light scattering of large nonhomogeneous colloidal particles with spherical symmetry whose size is comparable with or larger than the light wavelength used for developing scattering experiments, where the Rayleigh-Gans-Debye approximation is not valid. Here, the refractive index decay was approximated by a Gaussian RI profile numerically evaluated through a multilayer sphere. We calculated the form factor functions of suspensions of PNIPAM microgels previously reported and core-shell suspensions made of polystyrene/PNIPAM at 20 and 40 °C synthesized by us. In all the cases, our method succeeded in providing the scattering intensity as a function of the angle. The software for using the numerical method is fairly straightforward and is accessible as an open-source code. The results can not only help predict and understand the photonic properties of microgels with large core-shell structures but also for any particle with a refractive index distribution with spherical symmetry, as in the case of microgels with super chaotropic agents, hollow microgels, or microparticles.

Thermal-Responsive Gel-Based Overheat Limiter Enabled Intelligent Photothermal Therapy

Uncontrolled and excessive photothermal heating in photothermal therapy (PTT) inevitably causes thermal damage to surrounding normal tissues, severely limiting the universality and safety of PTT. To address this issue, an intelligent cooling thermal-responsive (ICTR) gel containing poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAM-AM))microgel is applied onto the skin to realize intelligent PTT, which can avoid excessive heating and accidental injury. The high near-infrared (NIR) light transmittance (> 95%) of the ICTR gel ensures effective light delivery at low temperatures, while the refractive index of the P(NIPAM-AM) microgel increases remarkably when the temperature exceeds a predetermined threshold, resulting in progressively enhanced light scattering and weakened photothermal conversion. In animal studies, the negative feedback regulation of ICTR gel on light transmittance and photothermal heating allows the photothermal temperature in the lesion site to be stabilized within the effective therapeutic range (45 °C) while ensuring that the skin surface temperature does not exceed 35 °C. Compared with the severe skin thermal damage found in the histological staining of mice skin receiving conventional PTT, the mice skin receiving the ICTR gel-enabled intelligent PTT remains in good condition. This study establishes an intelligent and universal paradigm for PTT thermal regulation, which is of great significance for achieving safe and effective PTT.

Adaptive Thermal Management Radiative Cooling Smart Window with Perfect Near-Infrared Shielding

The architectural window with spectrally selective features and radiative cooling is an effective way to save building energy consumption. However, architectural windows that combine both functions are currently based on micro-nano photonic structures, which undoubtedly hinder their commercial application due to the complexity of manufacture. Herein, a novel tunable visible light transmittance radiative cooling smart window (TTRC smart window) with perfect near-infrared (NIR) shielding ability is manufactured via a mass-producible scraping method. TTRC smart window presents high luminous transmittance (Tlum = 56.8%), perfect NIR shielding (TNIR = 3.4%), bidirectional transparency adjustment ability unavailable in other transparent radiative coolers based on photonic structures (ΔTlum = 54.2%), and high emittance in the atmospheric window (over 94%). Outdoor measurements confirm that smart window can reduce 8.2 and 6.6 °C, respectively, compared to ordinary glass and indium tin oxide (ITO) glass. Moreover, TTRC smart window can save over 20% of annual energy in the tropics compared to ITO and ordinary glass. The simple preparation method employed in this work and the superior optical properties of the smart window have significantly broadened the scope of application of architectural windows and advanced the commercialization of architectural windows.

Machine Learning-Guided Performance Evaluation of an All-Liquid Electrochromic Device

Electrochromic devices, capable of modulating light transmittance under the influence of an electric field, have garnered significant interest in the field of smart windows and car rearview mirrors. However, the development of high-performance electrochromic devices via large-scale explorations under miscellaneous experimental settings remains challenging and is still an urgent problem to be solved. In this study, we employed a two-step machine learning approach, combining machine learning algorithms such as KNN and XGBoost with the reality of electrochromic devices, to construct a comprehensive evaluation system for electrochromic materials. Utilizing our predictive evaluation system, we successfully screened the preparation conditions for the best-performing device, which was experimentally verified to have a high transmittance modulation amplitude (62.6%) and fast response time (5.7 s/7.1 s) at 70 A/m2. To test its stability, experiments over a long cycle time (1000 cycles) are performed. In this study, we develop an innovative framework for assessing the performance of electrochromic material devices. Our approach effectively filters experimental samples based on their distinct properties, substantially minimizing the expenditure of human and material resources in electrochromic research. Our approach to a mathematical machine learning evaluation framework for device performance has effectively propelled and informed research in electrochromic devices.

Synthesis of 2D PtSe2 Nanolayers on Glass Substrates and Their Integration in Near-Infrared Light Shutters

PtSe2 has asserted its key role among the emerging 2D transition metal dichalcogenides, however, its simplified growth process with controlled number of layers, high crystalline quality, and on inexpensive substrates is still challenging. Here, we report the synthesis details of PtSe2 layers on soda lime glass substrates by selenization of predeposited Pt layers using the thermally assisted conversion method at atmospheric pressure. PtSe2 syntheses are confirmed by X-ray photoelectron spectroscopy and Raman analysis. The layers were further investigated with transmission electron microscopy and optical ellipsometry, revealing the thickness and its dependence on the metal precursor sputtering time. Finally, the integration of PtSe2 as transparent conductive layers in polymer-dispersed liquid crystal structures operating as near-infrared light shutters is demonstrated and device performance is discussed. The proposed simple and inexpensive synthesis approach opens up new directions toward PtSe2 potential technological applications, including ITO-free optoelectronics.

Thermosensitive Scattering Hydrogels Based on Triblock Poly-Ethers: A Novel Approach to Solar Radiation Regulation

Energy conservation in buildings is paramount, especially considering that glass accounts for 50% of energy consumption. The solar heat gain coefficient (SHGC) of glass is a critical energy-saving index for transparent structures. However, the fixed SHGC of ordinary glass makes it difficult to provide both summer shading and winter heating. In this study, we synthesized a hydrogel with a thermosensitive scattering (TS) property using triblock polyether and acrylamide. This hydrogel can realize the transition of clearness and atomization based on the temperature. When sealed within a glass cavity, it exhibits a high SHGC of 0.682 in its transparent state and a low SHGC of less than 0.31 when atomized. The lower critical solution temperature (LCST) of the TS glass can be adjusted from 0 to 70 °C to suit different regions. The photothermal properties of the material remained stable after 200 hot and cold cycles and 200 h of ultraviolet irradiation. This glass can prevent solar radiation from entering the room in summer, thereby reducing air conditioning usage and power consumption. In winter, it allows solar heat radiation to enter the room, minimizing the need for artificial heating. Its adaptable temperature design makes it an excellent solution for designers to create energy-efficient building exteriors.

Freezing-resistant poly(N-isopropylacrylamide)-based hydrogel for thermochromic smart window with solar and thermal radiation regulation

Adaptive regulation of solar and thermal radiation by windows in diverse (hot and cold) climates is essential to reduce building energy consumption. However, conventional hydrogel-based thermochromic smart windows lack thermal radiation regulation, and have difficulty to combine high solar regulation with excellent freezing resistance. It is challenging to integrate the above performance into one hydrogel-based thermochromic window. Here, we firstly prepared poly(N-isopropylacrylamide-co-N, N-dimethylacrylamide)/ethylene glycol (PNDE) hydrogels with tunable and excellent freezing resistance (below -100 °C) by adding the anti-freezing agent ethylene glycol, and assembled PNDE hydrogels, polyvinylidene fluoride and polymethyl methacrylate-silver nanowires panels into a freezing-resistant smart window with solar and thermal radiation regulation (STR). PNDE hydrogels had an excellent thermochromic performance with luminous transmittance (Tlum) of 89.3 %, solar regulation performance (ΔTsol) of 80.7 % and tunable phase change temperature (τc, 22-44 °C). The assembled STR window showed high Tlum of 68.2 %, high ΔTsol of 62.6 %, suitable τc of ∼30 °C and freezing resistance to low temperature of -27 °C. Moreover, the different thermal emissivity (0.94 and 0.68) of the two sides of the STR window gave it the ability of radiative cooling in hot climates and warm-keeping in cold climates. Compared to the conventional thermochromic windows, the STR window promotes heat dissipation in hot conditions while reduces heat loss in cold conditions and is applicable to diverse climates, which is a promising energy-saving device for reducing building energy consumption.

Facile Preparation of a Low-Cost Liquid Interlayer Material with Intelligent UV-NIR-Shielding Function for Smart Windows

High-performance interlayer materials have garnered considerable interest owing to their low manufacturing costs and applicability in smart windows. In this study, a novel smart-window interlayer material capable of selective shielding against both near-infrared (NIR) and ultraviolet (UV) radiation is developed based on the light transmittance control mechanism. An excellent thermoresponsive liquid, denoted as CDs@TRL (viz., carbon quantum dots at thermal-responsive liquid), is synthesized by compositing biomass-based fluorescent carbon quantum dots (CDs) and poly(N-isopropylacrylamide) (pNIPAM) at natural ambient temperature and in an aqueous phase. Due to the characteristics of CDs and synergistic effect of hydrogen bonds, CDs@TRL exhibits a high specific heat capacity (4.41 kJ kg-1 K-1), large thermal storage capacity (264.6 kJ kg-1), and better UV-NIR-blocking properties, compared to pure pNIPAM, as well as improves the sensitivity of thermal response. When injected into a window as a liquid interlayer, CDs@TRL can intelligently adjust the light transmittance according to ambient light intensity to achieve an intelligent response. The shielding rate of a 10 mm-thick CDs@TRL composite liquid against UV radiation (200-400 nm) was more than 95% in an overcast environment with insufficient light and close to 100% in a well-lighted environment. In addition, CDs@TRL is a cost-effective material that can be prepared from a wide range of raw material sources using a simple preparation process and exhibits excellent mobility and recyclability. Because of these features, it is considered to be a promising candidate for developing energy-saving and climate-adapted smart windows.

Picoampere Dark Current and Electro-Opto-Coupled Sub-to-Super-linear Response from Mott-Transition Enabled Infrared Photodetector for Near-Sensor Vision Processing

Light-intensity selective superlinear photodetectors with ultralow dark current can provide an essential breakthrough for the development of high-performing near-sensor vision processing. However, the development of near-sensor vision processing is not only conceptually important for device operation (given that sensors naturally exhibit linear/sublinear responses), but also essential to get rid of the massive amount of data generated during object sensing and classification with noisy inputs. Therefore, achieving the giant superlinear photoresponse while maintaining the picoampere leakage current, irrespective of the measurement bias, is one of the most challenging tasks. Here, Mott material (vanadium dioxide) and silicon-based integrated infrared photodetectors are developed that show giant superlinear photoresponse (exponent >18) and ultralow dark current of 4.46 pA. Specifically, the device demonstrates an electro-opto-coupled insulator-to-metal transition, which leads to outstanding photocurrent on/off ratio (>10⁶ ), a high responsivity (>1 mA W^-1 ), and excellent detectivity (>10¹²  Jones), while maintaining response speed (τ_r  = 6 µs and τ_f  = 10 µs). Further, intensity-selective near-sensor processing is demonstrated and night vision pattern reorganization even with noisy inputs is exhibited. This research will pave the way for the creation of high-performance photodetectors with potential uses, such as in night vision, pattern recognition, and neuromorphic processing.

A Facile yet Versatile Strategy to Construct Liquid Hybrid Energy-Saving Windows for Strong Solar Modulation

Smart windows with light management and indoor solar heating modulation capacities are of paramount importance for building energy conservation. Thermochromic poly(N-isopropylacrylamide) (PNIPAm) hydrogel smart windows exhibit advantages of the relatively suitable transition temperature of 32 °C, high cost-effective and automatic passive sunlight regulation, but sustain slow response rate and unsatisfactory solar modulation efficiency. Herein, a strategy of one-step copolymerization of NIPAm and different olefine acids (OA) using reverse atom transfer radical polymerization method is developed to fabricate various chain/microparticle hybrids (CMH) for liquid energy-saving windows. Synergetic mechanisms of thermal-induced dissolution and aggregation of linear polymer chains integrated with water capture and release of microgel particles contribute to tunable light-scattering behaviors and adaptive solar modulation. Without any post-treatment, the as-prepared poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAm-co-AA))-based CMH suspension is injected into sandwich glass to construct energy-saving windows, which exhibits appreciated near-room-temperature transition (26.7 °C), rapid response (5 s), extraordinary luminous transmittance (91.5%), and solar modulation efficiency (85.8%), resulting in a substantial decline of indoor temperature of 24.5 °C in simulation experiment. Combining the versatile strategy with flexible adjustment on transition temperature, multifarious P(NIPAm-co-OA)-based CMH windows with eminent light management capacity are obtained. This work will powerfully promote the development and renovation of energy-efficient windows.

Synthesis of temperature- and humidity-induced dual stimulation film PU-PNIPAm n and its independent film formation as a smart window application

Dynamic windows, which switch between transparent and opaque states as the temperature changes, can be applied in buildings to reduce building energy consumption. Poly(N-isopropylacrylamide) (PNIPAm) is the most studied thermochromic hydrogel for climate-resilient smart window applications. In addition to its poor mechanical properties and low reaction rate, the PNIPAm hydrogel must be sandwiched between two pieces of glass to form an interlayer in practical applications. Here, durable PU-PNIPAm _n copolymers for smart windows were synthesized by reacting the synthesized poly-NIPAm diols with isocyanate (-NCO) monomer, which greatly improved the mechanical properties of the hydrogel and it was able to form a film alone. These temperature-sensitive films can switch between transparent (>80% transmittance) and opaque (<5% transmittance) states in less than 10 minutes, with no degradation in optical contrast, switching speed, or uniformity after at least 100 switching cycles.

Phase-Changing Polymer Film for Smart Windows with Highly Adaptive Solar Modulation

Adjusting sunlight and thermal radiation from windows is important in efficient energy-saving applications. A high solar modulation (ΔT_sol) capability as well as a high luminous transmittance (T_lum) are the ultimate aim of smart windows. In this study, hydroxypropyl cellulose (HPC), W-doped VO₂, and poly-N-iso-propylacrylamide (PNIPAm) composite hydrogel films were produced. The sample was prepared between two glasses to constitute a smart window with a sandwich construction structure, which exhibits a high T_lum of 87.16%, a ΔT_sol of 65.71%, and a lower critical solution temperature (LCST) of 29 °C. The practical applications of conventional PNIPAm hydrogels are limited by the volume contraction of phase transition. Here, this challenge is addressed by the simple method of combining with HPC. The PNIPAm-2.5 wt % HPC hydrogels possess thermo-responsive contractility with a volume shrinkage rate of 8.5%. Even after 100 high- and low-temperature cyclic durability tests, the smart windows still exhibit a high solar modulation capability.

Thermo-Responsive Poly(N-isopropylacrylamide)/Hydroxypropylmethyl Cellulose Hydrogel with High Luminous Transmittance and Solar Modulation for Smart Windows

Thermochromic smart windows are considered to be promising energy-saving devices for reducing energy consumption in buildings. The ideal materials for thermochromic smart windows should have high transmittance, high solar modulation, low phase-transition temperature, and excellent high-temperature thermal stability, which are difficult to achieve simultaneously. This work reports a simple one-step low-temperature polymerization method to prepare a thermo-responsive poly(N-isopropylacrylamide)/hydroxypropylmethyl cellulose (PNIPAM/HPMC) hydrogel achieving the above performances simultaneously. The low-temperature polymerization environment endowed the hydrogel with a high luminous transmittance (T_lum) of 90.82%. HPMC as a functional material effectively enhanced the mechanical properties and thermal stability of the hydrogel. Meanwhile, the PNIPAM/HPMC hydrogel showed a low phase-transition temperature (∼32 °C) and high solar modulation (ΔT_sol = 81.52%), which proved that it is an ideal material for thermochromic smart windows. Moreover, a PNIPAM/HPMC smart window exhibited high light transmittance (T_380-760 = 86.27%), excellent light modulation (ΔT₃₆₅ = 74.27%, ΔT_380-760 = 86.17%, and ΔT₉₄₀ = 63.93%), good indoor temperature regulation ability and stability, which indicated that it was an attractive candidate for application in reducing energy consumption in buildings. This work also provides an option and direction for modifying PNIPAM-based thermochromic smart windows.

Smart thermally responsive perovskite materials: Thermo-chromic application and density function theory calculation

With the continuous improvement of human's requirements for temperature control suitable for living, the energy consumption of electrical appliances such as air conditioners has become a major challenge in traditional architectural design. Generally, most of the solar energy passes through the glass to enter and exit the building, but the traditional glass can hardly control the light and heat energy, causing the indoor temperature to change dramatically with the environment. Therefore, it is more urgent to develop green and efficient smart windows. Perovskite is a temperature-adaptive material, which has the ability of phase transition and can adjust its band gap for thermochromic applications. In this work, we study the perovskite-based thermochromic smart window. As a new application of perovskite, a number of experiments have been carried out. However, there is still a lack of theoretical analysis on phase transition mechanisms and crystal structure prediction. Density functional theory (DFT) calculation is the most useful tool in optoelectronics, especially for perovskite crystal. Here, we extracted typical cases from published literature for analysis and comparison and summarized the crystal structure, electronic structure stability, interface engineering, and thermal characteristics employing DFT calculation We believe this work will pave the way for DFT application for the study of thermochromic perovskite.

Dual-Responsive Hydrogels with Three-Stage Optical Modulation for Smart Windows

Since room temperature management consumes a large amount of building energy, thermochromic smart windows have been extensively used for temperature regulation and energy management. However, the development of the smart window is still limited by its simple thermochromic performance, unreasonable thermochromic temperature, and the lack of additional stimulation conditions. In this work, a dual-responsive hydrogel was developed by introducing sodium dodecyl sulfate (SDS) and sodium chloride into the cross-linking network of poly(N-isopropylacrylamide) (PNIPAM) and polyacrylamide (PAM) for energy-saving and privacy protection. By controlling the temperature from low (<15 °C) to medium (15-28 °C) to high (>28 °C), the dual-responsive hydrogel achieved a reversible three-stage transition of opaque-transparent-translucent. The hydrogel exhibited a satisfactory solar modulation ability (T_lum = 80.3%, ΔT_sol,15-18°C = 72.9%, ΔT_sol,18-35°C = 42.7%) and effective IR and UV shielding at high (or low) temperatures. Moreover, compared with traditional windows, smart windows made of dual-responsive hydrogels could offer better thermal insulation and heat preservation. The electrochromic properties of the dual-responsive hydrogel presented a facile strategy to meet the needs of different situations. The dual-responsive hydrogel features energy-saving, privacy protection, three-stage optical modulation, and multistimulus responsiveness, making it an ideal smart window candidate.

Review of Window Performance in A Hot and Humid Climate

Incorrect implementation of window parameters, such as configuration, position, and size, cause an unpleasant indoor environment. The authors reviewed window performance in a hot and humid climate in this paper. Articles were screened in detail to determine eligibility, compiled, and organised according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) requirements. The articles included in this review concerned natural ventilation and window performance in a hot and humid climate. Keywords or topics were reviewed and focused on indoor environment comfort. The results demonstrated that sliding windows were unfavourable openings that were nevertheless in demand. This review was performed to guide consumers, designers, and the market of the built environment industry.

Thermochromic Smart Windows Assisted by Photothermal Nanomaterials

Thermochromic smart windows are optical devices that can regulate their optical properties actively in response to external temperature changes. Due to their simple structures and as they do not require other additional energy supply devices, they have great potential in building energy-saving. However, conventional thermochromic smart windows generally have problems with high response temperatures and low response rates. Owing to their great effect in photothermal conversion, photothermal materials are often used in smart windows to assist phase transition so that they can quickly achieve the dual regulation of light and heat at room temperature. Based on this, research progress on the phase transition of photothermal material-assisted thermochromic smart windows is summarized. In this paper, the phase transition mechanisms of several thermochromic materials (VO₂, liquid crystals, and hydrogels) commonly used in the field of smart windows are introduced. Additionally, the applications of carbon-based nanomaterials, noble metal nanoparticles, and semiconductor (metal oxygen/sulfide) nanomaterials in thermochromic smart windows are summarized. The current challenges and solutions are further indicated and future research directions are also proposed.

Microstructural Control of CaO-Al2O3-SiO2 Glass Ceramics by Oxidation and Mixing with Nucleation Agents

Microstructural control of CaO-Al₂O₃-SiO₂ (CAS) glass ceramics (GCs) was achieved by oxidation and mixing with nucleation agents. CAS GCs were precipitated with hexagonal platy particles of metastable CaAl₂Si₂O₈ layered crystals (CAS GC-H), which are typically prepared under a reductive atmosphere that forms metallic Mo or W particles as nucleation agents. The average particle size of crystals decreased significantly from 50 to 11 μm when the CAS GC-H containing metallic W particles was prepared under an oxidative atmosphere. Compared to this CAS-GC-H, the crystal particle size increased from 8-20 to 10-30 μm when the CAS GC-H was prepared by mixing glass cullet containing metallic Mo and that containing metallic W particles. These results indicate that one microstructure of CAS GC-H is controlled on the micrometer scale from a parent glass with one composition by varying the experimental conditions related to the glass melting state.

Research Progress of Cholesteric Liquid Crystals with Broadband Reflection

Cholesteric liquid crystal (ChLC) materials with broadband reflection are witnessing a significant surge in interest due to their unique ability to self-organize into a helical supra-molecular architecture and their excellent selective reflection of light based on the Bragg relationship. Nowadays, by the virtue of building self-organized nanostructures with pitch gradient or non-uniform pitch distribution, extensive work has already been performed to obtain ChLC films with a broad reflection band. This critical review systematically summarizes the optical background of the ChLCs with broadband reflection characteristics, methods to obtain broadband reflection of ChLCs, as well as the application in this area. Combined with the research status and the advantages in the field, the challenges and opportunities of applied scientific problems in the research direction are also introduced.

Micelle Dynamic Reconstruction to Effectively Modulate the Transmission of Smart Windows

Micelles are extremely dynamic equilibrium aggregates. The size and shape of micelles are subject to appreciable structural fluctuations with the introduction of foreign ions, temperature, etc. The highly dynamic character has for a long time hugely attracted the interest of researchers to investigate the mechanism of micellar structure change and the dependence of their optical properties on the structure change. Herein, taking the most common sodium dodecyl sulfate (SDS) as an example, the aggregation behavior of SDS with excess K⁺ and the effect of temperature on the K⁺/SDS mixed system were detailed and systematically investigated by combining with molecular dynamics simulations and experiments. The addition of K⁺ leads to a reconfiguration of the original micelle structure, resulting in a significant change in micelle size from the nanoscale up to the microscale. And simultaneously, temperature can induce a dynamic process of conjugation/deconjugation of K⁺/SDS micelles in the mixed solution, which is manifested macroscopically by the change of transmittance. Finally, a temperature-responsive smart gel was prepared by introducing K⁺/SDS into a polyacrylamide (PAM) gel, which showed an excellent tunable performance in transmittance (ΔT_550 nm = 60.1%, ΔT_808 nm = 42.72%). The designed smart window shows potential applications in room temperature control (Δt = 4.1 °C) and excellent stability over the course of 50 cycles.

Near-Infrared-Activated Thermochromic Perovskite Smart Windows

Perovskite-based thermochromic smart windows that can change color have attracted much interest. However, the high transition temperature (>45 °C in air) hinders their practical application. Herein, a near-infrared (NIR) activated thermochromic perovskite window that enables reversible transition cycles at room temperature is proposed. Under natural sunlight (>700 W m-2 ), it efficiently harvests 78% NIR light to trigger the thermochromism of perovskites, blocking the heat gain from both the visible and NIR light. Meanwhile, it also exhibits a low mid-infrared emissivity of <0.3, suppressing thermal radiation to the indoor environment. A field test demonstrates that this smart window can reduce the indoor temperature by 8 °C compared to a normal glass window at noon. The near-room-temperature color change, multispectral thermal management, outstanding energy-saving ability, and climate adaptability, and solution-based process of this window make it unique and promising for real applications.

Application of New Energy Thermochromic Composite Thermosensitive Materials of Smart Windows in Recent Years

Thermochromic smart windows technology can intelligently regulate indoor solar radiation by changing indoor light transmittance in response to thermal stimulation, thus reducing energy consumption of the building. In recent years, with the development of new energy-saving materials and the combination with practical technology, energy-saving smart windows technology has received more and more attention from scientific research. Based on the summary of thermochromic smart windows by Yi Long research groups, this review described the applications of thermal responsive organic materials in smart windows, including poly(N-isopropylacrylamide) (PNIPAm) hydrogels, hydroxypropyl cellulose (HPC) hydrogels, ionic liquids and liquid crystals. Besides, the mechanism of various organic materials and the properties of functional materials were also introduced. Finally, opportunities and challenges relating to thermochromic smart windows and prospects for future development are discussed.

Transparent photothermal hydrogels for wound visualization and accelerated healing

Utilizing photothermal hydrogels as a wound dressing is a promising strategy to accelerate wound healing. Usually, a photothermal hydrogel has a strong light-absorbing capability, and hence its transparency can be largely sacrificed, which is unbeneficial for the visual monitoring of wound states. It remains challenging to balance the trade-off between the photothermal conversion and wound visualization for the photothermal hydrogel dressing. Herein, a composite photothermal hydrogel film with high transparency is presented for the visual monitor of the wound, which is constructed by incorporating CsxWO3 nanorods into the networks of polyacrylamide hydrogels. The composite photothermal hydrogel film exhibits high light absorption in the near-infrared region and high transmittance in the visible light region. Under 980 nm laser irradiation, the composite hydrogel can be heated up to 45 °C. In vivo animal experiment on mouse skin wound model shows that the composite hydrogel film can locally heat the skin wound to accelerate healing while maintaining more than 70% transparency to realize real-time observation of the wound. This study provides the first attempt to solve the problem of opacity in photothermal hydrogel dressings, promoting the possibility of its clinical applications.

Dual-functional NIR/UV-shielding poly(lactic acid) nanocomposite films through CWO@PDA core–shell nanoparticles

Fabrication of dual-functional NIR/UV-shielding PLA nanocomposite films by constructing interfacial stereocomplex crystallites.

Research Progress of Photo-/Electro-Driven Thermochromic Smart Windows

Thermochromic smart windows can automatically control solar radiation according to the ambient temperature. Compared with photochromic and electrochromic smart windows, they have a stronger applicability and lower energy consumption, and have a wide range of application prospects in the field of building energy efficiency. At present, aiming at the challenge of the high transition temperature of thermochromic smart windows, a large amount of innovative research has been carried out via the principle that thermochromic materials can be driven to change their optical performance by photothermal or electrothermal effects at room temperature. Based on this, the research progress of photo- and electro-driven thermochromic smart windows is summarized from VO₂-based composites, hydrogels and liquid crystals, and it is pointed out that there are two main development trends of photo-/electro-driven thermochromic smart windows. One is exploring the diversified combination methods of photothermal materials and thermochromic materials, and the other is developing low-cost large-area heating electrodes.

Humidity-Driven Switch in the Transparency of a Nanofiber Film for a Smart Window

Traditional smart windows use electrical signals to transform transparency. However, this electric transmission mode greatly limits their uses. Here, we have prepared a transparent PVA-co-PE/CA composite film, which can realize the reversible transformation of transparency under the stimulation of humidity. The preparation method of the composite film included simple immersion and a thermal curing process, showing high optical transparency (96.61%) and an excellent tensile strain at break of 536.34%. Once exposed to moisture stimulation, the rapid hygroscopic swelling of the composite film led to the increase in the difference in the refractive index between the citric acid filling phase and the nanofibers, which directly led to the sharp decrease in the composite film's transparency. Moreover, the composite film can be arbitrarily attached to the surface of the transparent substrate and designed as some special visualization devices or smart windows, which have a promising future in information encryption and intelligent homes.

Rapid Photothermal Responsive Conductive MXene Nanocomposite Hydrogels for Soft Manipulators and Sensitive Strain Sensors

Stimulus-responsive hydrogels are of great significance in soft robotics, wearable electronic devices, and sensors. Near-infrared (NIR) light is considered an ideal stimulus as it can trigger the response behavior remotely and precisely. In this work, a smart flexible stimuli-responsive hydrogel with excellent photothermal property and decent conductivity are prepared by incorporating MXene nanosheets into the physically cross-linked poly(N-isopropyl acrylamide) hydrogel matrix. Because of outstanding photothermal effect and dispersion of MXene, the composite hydrogel exhibits rapid photothermal responsiveness and excellent photothermal stability under the NIR irradiation. Furthermore, the anisotropic bilayer hydrogel actuator shows fast and controllable light-driven bending behavior, which can be used as a light-controlled soft manipulator. Meanwhile, the hydrogel sensor exhibits cycling stability and good durability in detecting various deformation and real-time human activities. Therefore, the present study involving the fabrication of MXene nanocomposite hydrogels for potential applications in remotely controlled actuator and wearable electronic device provides a new method for the development of photothermal responsive conductive hydrogels.

A Systematic Review of the Most Recent Concepts in Smart Windows Technologies with a Focus on Electrochromics

In the context of sustainability and in the face of ambitious goals towards the reduction of CO2 emission, the modification of transparency in architecture becomes an important tool of energy flow management into the building. Windows that dim to stop the energy transfer reduce the cooling load in the building. Recently, however, the latest achievements in the development of electrochromic materials allowed us to integrate some additional—previously unknown—functionalities into EC devices. The purpose of this paper is to provide a systematic review of recent technological innovations in the field of smart windows and present the possibilities of recently established functionalities. This review article outlines recent general progress in electrochromic but concentrates on multicolour and neutral black electrochromism, spectrally selective systems, electrochromic energy storage windows, hybrid EC/TC systems, OLED lighting integrated with the EC device, and EC devices powered by solar cells. The review was based on the most recent publication from the years 2015–2020 recorded in the databases WoS and Scopus.

Modifying Precursor Solutions to Obtain Screen-Printable Inks for Tungsten Oxides Electrochromic Film Preparation

Tungsten trioxide (WO3) is used to prepare the important electrochromic layer of the electrochromic device as a wide bandgap semiconductor material. In this study, WO3 electrochromic film was successfully prepared by screen printing. To modify the thixotropy and wettability of the ink, polyvinyl alcohol (PVA) and 2-perfluoroalkyl ethanol (FSO) were added in the ammonium meta-tungstate (AMT) solution. We found that the PVA additive could improve the dynamic viscosity of the solution and modify the uniformity of the film. 2-Perfluoroalkyl ethanol (FSO) could lower the surface tension and increase the wettability of the AMT solution on the substrate. By observing the morphology of the printed films, the ink formulas for screen printing were selected. We found the annealing process could help remove PVA. Through characterization of electrochromic performance, it was found that the best performing device had 42.57% modulation and 93.25 cm2·C−1 coloration efficiency (CE) for 600 nm light. This study showed great potential in the preparation of WO3 electrochromic devices by a low-cost screen-printing method.

Radiochromic Hydrogen-Bonded Organic Frameworks for X-ray Detection

Porous materials have been investigated as efficient photochromic platforms for detecting hazardous radiation, while the utilization of hydrogen bonded organic frameworks (HOFs) in this field has remained intact. Herein, two HOFs were synthesized through self-assembly of tetratopic viologen ligand and formic acid (PFC-25, PFC-26), as a new class of "all-organic" radiochromic smart material, opening a gate for HOFs in this field. PFC-26 is active upon both X-ray and UV irradiation, while PFC-25 is only active upon X-ray irradiation. The same building block yet different radiochromic behaviors of PFC-25 and PFC-26 allow us to gain a deep mechanistic understanding of the factors that control the detection specificity. Theoretical and experimental studies reveal that the degree of π-conjugation of viologen ligand is highly related to the threshold energy of triggering a charge transfer, therefore being a vital factor for the particularity of radiochromic materials. Thanks to its convenient processibility, nanoparticle size, and UV silence, PFC-25 can be further fabricated into a portable naked-eye sensor for X-ray detection, which shows obvious color change with the merits of high transmittance contrast, good sensitivity (reproducible dose threshold of 3.5 Gy), and excellent stability. The work exhibits the promising practical potentials of HOF materials in photochromic technology.

Flexible core-shell Cs x WO3-based films with high UV/NIR filtration efficiency and stability

Cesium-doped tungsten bronze Cs x WO3 (CWO) is an ideal near infrared (NIR) shielding material for solar filters. However, the NIR shielding ability of CWO-dispersed films easily deteriorates in hot humid environments, which severely hinders the commercial application of CWO. In this paper, UV/NIR shielding nanocomposite films were prepared by dispersing core-shell structured CWO@polydopamine (CWO@PDA) in a poly(vinyl alcohol) matrix. Because of the strong ultraviolet light absorption ability of PDA, it can shield ultraviolet light, which is generally detrimental to our health. The prepared nanocomposite films can efficiently shield 88.3% UV and 85.5% NIR radiation even though they show relatively high transparency in the visible range. Importantly, the good protection of the continuous PDA shells played an important role in enhancing the stability of CWO nanoparticles. The nanocomposite films also exhibit excellent stability in hot humid environments. Therefore, core-shell structured CWO@PDA nanoparticles have great potential as a novel UV/NIR shielding material for the development of efficient energy-saving windows.

Tailoring Broad-Band-Absorbed Thermoplasmonic 1D Nanochains for Smart Windows with Adaptive Solar Modulation

Controlling solar transmission through windows promises to reduce building energy consumption. A new smart window for adaptive solar modulation is presented in this work proposing the combination of the photothermal one-dimensional (1D) Au nanochains and thermochromic hydrogel. In this adaptive solar modulation system, the Au nanochains act as photoresponsive nanoheaters to stimulate the optical switching of the thermochromic hydrogel. By carefully adjusting the electrostatic interactions between nanoparticles, different chain morphologies and plateau-like broad-band absorption in the NIR region are achieved. Such broad-band-absorbed 1D nanochains possess excellent thermoplasmonic effect and enable the solar modulation with compelling features of improved NIR light shielding, high initial visible transmittance, and fast response speed. The designed smart window based on 1D Au nanochains is capable of shielding 94.1% of the solar irradiation from 300 to 2500 nm and permitting 71.2% of visible light before the optical switching for indoor visual comfort. In addition, outdoor cooling tests in model house under continuous natural solar irradiation reveal the remarkable passive cooling performance up to ∼7.8 °C for the smart window based on 1D Au nanochains, showing its potential in the practical application of building energy saving.

Highly Efficient Photochromic Tungsten Oxide@PNIPAM Composite Spheres with a Fast Response

Coloration efficiency and a fast response are important in developing materials for optical switching. A novel, highly efficient photochromic tungsten oxide@poly(N-isopropylacrylamide) (PNIPAM) hybrid sphere is reported, whose colors can be rapidly converted between yellow and blue under different lights. The color change can be seen clearly even if the tungsten oxide content in the hybrid sphere is very low, exhibiting outstanding coloration efficiency of tungsten oxide. A photochromic mechanism is proposed in which the amide group in PNIPAM spheres participates in electron injection and the transition of valence states between W⁵⁺ and W⁶⁺ in the photochromic process. The interaction between tungsten oxide and PNIPAM plays a key role in enhancing the coloration efficiency of tungsten oxide and accelerating the switchable speed of color transformation, which is very useful in developing new photochromic materials. These hybrid spheres can be used in rewritable record displays and have wide potential applications in controlling energy transmittance in smart windows or in detecting UV light in optical sensors.

Spectrally Selective Absorbers/Emitters for Solar Steam Generation and Radiative Cooling-Enabled Atmospheric Water Harvesting

Renewable energy harvesting from the sun and outer space have aroused significant interest over the past decades due to their great potential in addressing the energy crisis. Furthermore, the harvested renewable energy has benefited another global challenge, water scarcity. Both solar steam generation and passive radiative cooling-enabled atmospheric water harvesting are promising technologies that produce freshwater in green and sustainable ways. Spectral control is extremely important to achieve high efficiency in the two complementary systems based on absorbing/emitting light in a specific wavelength range. For this reason, a broad variety of solar absorbers and IR emitters with great spectral selectivity have been developed. Although operating in different spectral regions, solar selective absorbers and IR selective emitters share similar design strategies. At this stage, it is urgent and necessary to review their progress and figure out their common optical characteristics. Herein, the fundamental mechanisms and recent progress in solar selective absorbers and IR selective emitters are summarized, and their applications in water production are reported. This review aims to identify the importance of selective absorbers/emitters and inspire more research works on selective absorbers/emitters through the summary of advances and the establishment of the connection between solar absorbers and IR emitters.

Transmittance Tunable Smart Window Based on Magnetically Responsive 1D Nanochains

Smart optical materials are drawing more and more attention because of their wide application in energy conservation, wearable sensors, optical tuning, and medical devices. However, current smart optical materials, including electroresponsive, thermoresponsive, and mechanoresponsive materials, are greatly restricted in practical applications because of their long response time, complicated preparation, and high cost. This study develops a novel, magnetically tunable, smart optical material with swift and high-contrast optical switching based on one-dimensional (1D) Fe₃O₄@SiO₂ nanochains (NCs), which have the large shape anisotropy of the 1D structure and the superparamagnetic properties of Fe₃O₄ particles. The material exhibited a clear transparent state when NCs were arranged parallel to the viewing direction under an applied magnetic field, whereas it showed good shielding effect when the NCs were randomly oriented upon removal of the field. The light transmittance could be dynamically adjusted over the wide range of 20-80% through a small applied magnetic field of 50-100 Oe, which is superior to most of the currently reported systems. This swift, sensitive, and reversible response is attributed to the good responsivity of magnetic NCs. Also, an effective model was proposed to explain the transmittance modulation scheme and forecast its optical potential. The large tunable range and the low triggered field make Fe₃O₄@SiO₂ NCs an advantageous candidate for application in smart windows, optical switchers, and other fields.

Fast Switching of Bright Whiteness in Channeled Hydrogel Networks

Beside pigment absorption and reflection by periodic photonic structures, natural species often use light scattering to achieve whiteness. Synthetic hydrogels offer opportunities in stimuli-responsive materials and devices; however, they are not conventionally considered as ideal materials to achieve high whiteness by scattering due to the ill-defined porosities and the low refractive index contrast between the polymer and water. Herein, a poly(N-isopropylacrylamide) hydrogel network with percolated empty channels (ch-PNIPAm) is demonstrated to possess switchable bright whiteness upon temperature changes, obtained by removing the physical agarose gel in a semi-interpenetrating network of agarose and PNIPAm. The hydrogel is highly transparent at room temperature and becomes brightly white above 35 °C. Compared to conventional PNIPAm, the ch-PNIPAm hydrogel exhibits 80% higher reflectance at 800 nm and 18 times faster phase transition kinetics. The nanoscopic channels in the ch-PNIPAm facilitate water diffusion upon phase transition, thus enabling the formation of smaller pores and enhanced whiteness in the gel. Furthermore, fast photothermally triggered response down to tens of milliseconds can be achieved. This unique property of the ch-PNIPAm hydrogel to efficiently scatter visible light can be potentially used for, e.g., smart windows, optical switches, and, as demonstrated in this report, thermoresponsive color displays.

Switchable Cavitation in Silicone Coatings for Energy-Saving Cooling and Heating

Space cooling and heating currently result in huge amounts of energy consumption and various environmental problems. Herein, a switching strategy is described for efficient energy-saving cooling and heating based on the dynamic cavitation of silicone coatings that can be reversibly and continuously tuned from a highly porous state to a transparent solid. In the porous state, the coatings can achieve efficient solar reflection (93%) and long-wave infrared emission (94%) to induce a subambient temperature drop of about 5 °C in hot weather (≈35 °C). In the transparent solid state, the coatings allow active sunlight permeation (95%) to induce solar heating to raise the ambient temperature from 10 to 28 °C in cold weather. The coatings are made from commercially available, cheap materials via a facile, environmentally friendly method, and are durable, reversible, and patternable. They can be applied immediately to various existed objects including rigid substrates.

A new application of Krafft point concept: an ultraviolet-shielded surfactant switchable window

We report thermo-responsive gels of polyacrylamide (PAAm) with ionic surfactants in mixed water and ethylene glycol (W-EG) solvents as smart windows to solubilize ultraviolet absorbents for UV shielding materials. The Krafft point (T_K) of the ionic surfactants determines the phase transition that switches the transparency and opacity of the gels. The T_K can be flexibly tuned within a wide temperature range (6-75 °C) with high predictability by changing the surfactant or adjusting the ratio of W-EG solvents.

Hydrochromic Smart Windows to Remove Harmful Substances by Mimicking Medieval European Stained Glasses

Medieval European stained glass windows are known to display various splendid colors and remove harmful airborne substances. At present, the functions of medieval stained glass windows are imperative, from the environment, health, and energy perspectives, to develop multi-functional windows that report/control environmental conditions and remove harmful substances by utilizing visible-near-infrared light sources. Here, we suggest a strategy to mimic medieval European stained glasses for devising plasmonic-based multi-functional smart stained glass windows. The stained glass windows are prepared from the deposition of gold nanoparticles on a glass that is preliminarily coated with a responsive colloidal nanosheet. The temperature responsiveness of the nanosheet enables the effective control the gold nanoparticle density of the stained glasses. Therefore, the windows can display blue, violet, and cranberry colors. The colors become iridescent by introducing a photonic crystal monolayer. The stained glass windows are hydrochromic: they switch the colors (blue ↔ cranberry) and modulate light transmittance depending on humidity conditions. Moreover, they can remove formaldehyde under the illumination of a low-power indoor light. These functions provide a new platform for the futuristic smart windows that clean indoor air for the human health and save energy.

Solvothermal Synthesis and Near-Infrared Shielding Properties of Cs0.3WO3/WO3 Composites

The Cs[Formula: see text]WO3/WO3 composite with near-infrared shielding properties was synthesized by the solvothermal method using tungstic acid and cesium salt as raw materials. The as-prepared composites were tested by X-ray powder diffraction, scanning electron microscopy, energy spectrum analysis, transmission electron microscopy, electron energy loss spectroscopy, and ultraviolet-visible near-infrared spectroscopy. The effects of different reaction conditions on the structure and near-infrared shielding properties of the synthesized composites were investigated. The best near-infrared light transmittance of as-prepared composites can reach up to 9%, which provides a feasible solution for the near-infrared shielding material. The new homogeneous composites of cesium tungsten bronze and tungsten oxide are good candidates for solar filters.

Photoresponsive Shape Memory Hydrogels for Complex Deformation and Solvent-Driven Actuation

A new design for photoresponsive shape memory hydrogels and their possible applications are demonstrated in the present study. We show that the photodissociable Fe³⁺-carboxylate coordination can be utilized as a molecular switch to realize photocontrol of shape memory on both macroscopic and microscopic scales and enable a number of functions. Indeed, Fe³⁺-carboxylate coordination can fix a large tensile strain (up to 680%) of the sodium alginate/polyacrylamide hydrogel through cross-linking of sodium alginate chains, and subsequent UV irradiation allows strain energy release in spatially selected regions through reduction of Fe³⁺ to Fe²⁺. By manipulating light irradiation, complex 3D structures are obtained from 2D hydrogel sheets, and they exhibit complex solvent-driven actuation behaviors due to a light-changeable modulus and cross-linking density in the hydrogel. Based on the same approach, micropatterns can be inscribed on the hydrogel surface using mask-assisted irradiation, and they exhibit chain orientation-mediated anisotropic topography change upon solvent exchange. Moreover, light-controlled strain energy release also enables changing hydrogel surface wettability by solvent replacement. The demonstrated mechanism for photoresponsive hydrogels is highly efficient and applicable to many systems, which offers new perspectives in developing hydrogels with multiple photoresponsive functions.

Responsive Smart Windows Enabled by the Azobenzene Copolymer Brush with Photothermal Effect

An azobenzene side chain liquid crystalline copolymer (MAzo-co-GMA) is successfully synthesized through copolymerizing the monomer 6-(4-((4-butylphenyl)diazenyl)phenoxy)hexyl methacrylate (MAzo) with glycidyl methacrylate (GMA). The obtained MAzo-co-GMA copolymer can form stabilized polymer brush on the surface after thermal annealing. The obtained polymer brush not only induces the alignment of liquid crystals but also shows a photothermal effect under UV light irradiation due to the azobenzene side group. On basis of these results, the LC cell with this polymer brush as the substrate is further used to fabricate the polymer-stabilized liquid crystal (PSLC) smart window. The resultant PSLC smart window shows the transparent state because the homeotropic alignment in the SmA* phase of PSLC is induced by the polymer brush on the surface of the LC cell. The opaque state can be achieved in the scattering N* phase by UV light irradiation or heating. The response time of the PSLC smart window can be regulated by adjusting the concentration of MAzo-co-GMA copolymer brush and the intensity of UV light. This kind of PSLC smart window with both thermal and UV response shows good reversibility and stability, which endows enormous promising applications in energy-saving devices.