Three-Layered Hollow Nanospheres Based Coatings with Ultrahigh-Performance of Energy-Saving, Antireflection, and Self-Cleaning for Smart Windows

LaVO4: Eu3+ nano-islands onto silica for achieving fluorescence enhancement and their detection of Fe3+ ions and anti-counterfeiting applications

Rare earth (RE) composite fluorescent materials are favored by researchers in the field of anti-counterfeiting and ion sensing due to their fascinating optical properties. Ultra-small RE fluorescent nanoparticles are anchored on inorganic carriers by a simple preparation method to improve luminous intensity and hydrophilicity, which has not been explored yet. Herein, LaVO4: Eu3+ nano-islands anchored on silica with high fluorescence intensity and easy formation of stable colloidal solution is designed. Through a simple and mild hydrothermal approach, ultra-small LaVO4: Eu3+ nano-islands are highly dispersed on the surface of hierarchical hollow silica sphere (HHSS) to expose more luminescent centers. Remarkably, the stable HHSS@LaVO4: Eu3+ colloidal solution displayed highly sensitive and selective sensor for Fe3+ ions. The "island-sea synergy" structure formed by the LaVO4: Eu3+ nano-islands and the surrounding silica surface makes HHSS@LaVO4: Eu3+ to be an outstanding sensor for the effective detection of iron ions in water. In addition, HHSS@LaVO4: Eu3+ phosphor exhibit unique properties for anti-counterfeiting and encryption applications. These findings provide a promising strategy for the carrierisation of RE luminescent materials to improve optical properties and enable broader applications.

Enhanced Thermochromic Performance of VO2 Nanoparticles by Quenching Process

Vanadium dioxide (VO2) has been a promising energy-saving material due to its reversible metal-insulator transition (MIT) performance. However, the application of VO2 films has been seriously restricted due to the intrinsic low solar-energy modulation ability (ΔTsol) and low luminous transmittance (Tlum) of VO2. In order to solve the problems, the surface structure of VO2 particles was regulated by the quenching process and the VO2 dispersed films were fabricated by spin coating. Characterizations showed that the VO2 particles quenched in deionized water or ethanolreserved VO2(M) phase structure and they were accompanied by surface lattice distortion compared to the pristine VO2. Such distortion structure contributed to less aggregation and highly individual dispersion of the quenched particles in nanocomposite films. The corresponding film of VO2 quenched in water exhibited much higher ΔTsol with an increment of 42.5% from 8.8% of the original VO2 film, because of the significant localized surface plasmon resonance (LSPR) effect. The film fabricated from the VO2 quenched in ethanol presented enhanced thermochromic properties with 15.2% of ΔTsol and 62.5% of Tlum. It was found that the excellent Tlum resulted from the highly uniform dispersion state of the quenched VO2 nanoparticles. In summary, the study provided a facile way to fabricate well-dispersed VO2 nanocomposite films and to facilitate the industrialization development of VO2 thermochromic films in the smart window field.

Spectrally selective antireflection of nanoimprint lithography-formed 3D spherical structures on film coated with a silver layer

We fabricate moth-eye antireflection (AR) coatings using high-resolution and low-cost UV nanoimprint lithography with polyethylene terphthalate (PET) molds. Several various thicknesses of silver films placed on the moth-eye structure were analyzed for reflectance and transmission. On PET, the conical nanostructured surface arrays had a spatial period length of approximately 250 nm, a diameter of approximately 200 nm, and a height of approximately 160 nm. After them, a silver (Ag) layer of 18 nm is deposited satisfactorily on the PET substrate surface. The never-ending moth-eye formations of the imprinted mold were fabricated by Ni mold electroplating, interference lithography, and replication. We found that an Ag layer of suitable thickness on AR film in the spectrum range that can be seen has high transmittance (Highest value is 72%) while in the infrared spectrum it has high reflectance (At least 60%). For an optical film with a silver coating has been placed on an anti-reflection subwavelength-structured (ASS) surface, such properties, including heat insulation, have obvious applications in windows for homes and vehicles.

λ/4–λ/4 Double-Layer Broadband Antireflective Coatings with Constant High Transmittance

Antireflective (AR) coatings can suppress the undesired interfacial Fresnel reflections, and they are widely used in optical devices and energy-related instruments. Conventional single-layer AR coatings, which only work at a single wavelength, encounter serious limitations in some practical applications because of their inherent properties. In this paper, λ/4–λ/4 double-layer antireflective (AR) coatings with constant high transmittance in a pre-determined wavelength range was prepared by the sol–gel method via acid-catalyzed and base-catalyzed SiO2 thin films. A double-layer antireflective coating with an almost constant transmittance value of 99.8% in the range of 550–700 nm was obtained, and the transmittance of this coating was higher than 99% in a wider range of 450–850 nm with a fluctuation of less than 1%. The coatings had good environmental stability and maintained constant high transmittance after two weeks of exposure in 50% humidity. The broadband AR coatings may have important applications in fields such as electroluminescent display.

Controllable Design of Bifunctional VO2 Coatings with Superhydrophobic and Thermochromic Performances

The structure and functions of natural organisms provide great inspirational sources for designing and manufacturing bionic coatings, which hold a distinguished scientific promise to tackle challenges facing humans. In this work, we report a facile and controllable approach to prepare various hexagonal periodic array VO₂ thin films by simply manipulating the speed of the dip-coating operation. The hexagonal cellular-structured VO₂ surface delivered the best thermochromic performance with a T_lum of 79.34% and a ΔT_sol of 9.87%. Impressively, superhydrophobic and thermochromic properties could be integrated into hexagonal semi-dome thin films (with a T_lum of 70.9%, a ΔT_sol of 9.3%, and a water contact angle of 150°) without any post-treatment by low-surface-energy chemicals, which hold considerable potential for application in multifunctional smart windows. Moreover, based on the Cassie-Baxter mode and finite-difference time-domain calculations, the dependence of the thermochromic and wettability performances on the VO₂ structure has been investigated in this study.

High-Contrast Optical Modulation from Strain-Induced Nanogaps at 3D Heterogeneous Interfaces

The realization of high-contrast modulation in optically transparent media is of great significance for emerging mechano-responsive smart windows. However, no study has provided fundamental strategies for maximizing light scattering during mechanical deformations. Here, a new type of 3D nanocomposite film consisting of an ultrathin (≈60 nm) Al2O3 nanoshell inserted between the elastomers in a periodic 3D nanonetwork is proposed. Regardless of the stretching direction, numerous light-scattering nanogaps (corresponding to the porosity of up to ≈37.4 vol%) form at the interfaces of Al2O3 and the elastomers under stretching. This results in the gradual modulation of transmission from ≈90% to 16% at visible wavelengths and does not degrade with repeated stretching/releasing over more than 10 000 cycles. The underlying physics is precisely predicted by finite element analysis of the unit cells. As a proof of concept, a mobile-app-enabled smart window device for Internet of Things applications is realized using the proposed 3D nanocomposite with successful expansion to the 3 × 3 in. scale.

Optimization of Shapes and Sizes of Moth-Eye-Inspired Structures for the Enhancement of Their Antireflective Properties

Novel antireflective (AR) structures have attracted tremendous attention and been used in various applications such as solar cells, displays, wearable devices, and others. They have also stimulated the development of several other methods, including moth-eye-inspired technologies. However, the analyses of the shapes and sizes of nanostructures remain a critical issue and need to be considered in the design of effective AR surfaces. Herein, moth-eye and inverse-moth-eye patterned polyurethane-acrylate (PUA) structures (MPS and IMPS) with three different sizes are analyzed and compared to optimize the designed nanostructures to achieve the best optical properties pertaining to maximum transmittance and minimum reflectance. We fabricated moth-eye-inspired conical structures with three different sizes using a simple and robust fabrication method. Furthermore, the fabricated surfaces of the MPS and IMPS structures were analyzed based on the experimental and theoretical variation influences of their optical properties according to their sizes and shapes. As a result of these analyses, we herein propose a standard methodology based on the optimal structure of IMPS structure with a 300 nm diameter.

Raspberry-Shaped Thermochromic Energy Storage Nanocapsule with Tunable Sunlight Absorption Based on Color Change for Temperature Regulation

A novel raspberry-shaped thermochromic energy storage nanocapsule (RTESN) is successfully designed and fabricated with switchable sunlight absorption capacity based on color change for temperature regulation. The RTESN is developed by grafting amino-modified silica shell thermochromic nanoparticles (amino-TLD@SiO₂ ) on the surface of epoxy-functionalized energy storage nanocapsules (paraffin@PSG), with a total particle size about 450 nm. RTESN exhibits a deep color under low temperatures, which can absorb sunlight for heating. During the continuous thermal energy supply, paraffin@PSG is capable of storing thermal energy owing to its large latent heat capacity of 118.7 J g^-1 , thereby maintaining the slow temperature increase. When the temperature is higher than the phase change temperature of paraffin@PSG, the color of amino-TLD@SiO₂ turns to white with more reflection of sunlight so that it reduces the absorption of thermal energy and prevents the further increase of temperature. The thermal regulation behavior is confirmed by setting up a wooden house with the surface covered with RTESN. Compared with the blank wooden house, the RTESN covered wooden house (RTESN-H) displays thermal insulation performances during heating and cooling with a maximum temperature difference of 7 °C.

Bifunctional Template-Induced VO2@SiO2 Dual-Shelled Hollow Nanosphere-Based Coatings for Smart Windows

Thermochromic vanadium dioxide (VO₂) as one of the most promising candidates for smart windows has attracted widespread attention in recent years. Excellent optical performances (luminous transmittance, T_lum, and solar modulation efficiency, Δ T_sol) of VO₂-based coatings are usually pursued as crucial issues. In the current work, we report an ingenious approach for the synthesis of VO₂@SiO₂ dual-shell hollow nanospheres (DSHNs) and the preparation of DSHNs thermochromic coatings. A sequential bifunctional template-induced mechanism for the formation of DSHNs was proposed. Because of the unique hollow-core and dual-shell structure, the as-prepared VO₂@SiO₂ DSHNs coatings exhibited appealing optical performances with enhanced luminous transmittance of 61.8% and solar modulation efficiency of 12.6%, compared with continuous and dense VO₂ coatings. It has been proved that the improvement of visible transmittance could be ascribed to the effective reduction of refractive index (from 2.6 to 1.6 at 630 nm). In addition, its excellent thermochromic performance has been confirmed by the model cubes measurements, expressing a great potential as energy-efficient smart windows in high-rise buildings. The bifunctional template-induced synthetic strategy may inspire more facile, efficient and inexpensive processes for development of well-defined multishelled hollow nanostructures for varied applications.

Supramolecular strategy for smart windows

Supramolecular strategy-based materials are outlined and their applications for fabricating smart windows are summarized for future exploration of ideal smart windows.

Self-assembled nanoparticle antireflection coatings on geometrically complex optical surfaces

Here we report a simple and scalable electrostatics-assisted colloidal self-assembly technology for fabricating monolayer nanoparticle antireflection coatings on geometrically complex optical surfaces. By using a surface-modified glass volumetric flask with a long neck as a proof-of-concept demonstration, negatively charged silica nanoparticles with 110 nm diameter are electrostatically adsorbed on both the interior and exterior surfaces of the flask possessing positive surface charges. The self-assembled monolayer nanoparticle antireflection coatings can significantly improve light transmission through different regions of the flask with varied curvatures, as revealed by optical transmission measurements and numerical simulations using a simplified thin-film multilayer model.