Smart Window Applications Research Articles

WO3/TiO2 Hybrids and Their Applications for the Photocatalytic Degradation of P-Substituted Phenols

Tungsten trioxide (WO3) has received a great attention in last decade because of its commercial applications in smart windows, display devices, mobile phones, photocatalysis, solar cells and hydrogen generation. Due to a bandgap around 2.8 eV, WO3 is a semiconductor suitable for photocatalytic and photovoltaic applications using sunlight1, in contrast to most popular TiO2 of the bandgap of about 3.2 eV.2 However, a combination of WO3 with TiO2 leads to significantly enhancement of photoactivity of the system mainly due to increased separation efficiency of photogenerated electron-hole pairs3. When these charge carriers reach the semiconductor/solution interface, they may be involved in the surface reactions leading to decomposition of pollutants. These reactions occur via formation of highly reactive intermediates, such as hydroxyl radicals and superoxide radicals that break down organic pollutants into safe, smaller compounds such as carbon dioxide, water, and short-chain acids.5 Photocatalysis is important in the degradation of phenolic compounds, for example 4-chlorophenol that are products of industrial waste.4 This method is both eco-friendly and inexpensive. Tungsten trioxide and its composites were synthesized on indium-doped tin oxide (ITO) plates using sol-gel, electrochemical and hydrothermal methods. The morphology and composition of the synthesized WO3 and WO3/TiO2 hybrid were characterized using SEM, XRD, UV-Vis and an optical microscope. Electrochemical studies including cyclic voltammetry and chronoamperometry were performed to test their photoactivity. Lastly, the samples were used in the photocatalytic degradation of 4-chlorophenol monitored by UV-vis spectroscopy. Chronoamperometry studies showed that the WO3 by itself had a rapid electron-hole recombination rate. After combination with TiO2, the charge recombination rate decreased and the system was rapidly stabilized under illumination. The photoactivity of WO3 and WO3/TiO2 hybrid was measured with the use of cyclic voltammetry. HPLC analysis of the degradation products of 4-chlorophenol over time using the TiO2/WO3 composite indicated that the hybrid was able to degrade the pollutant more effectively, in a shorter time than WO3 alone. The addition of TiO2 to WO3 deposited on ITO resulted in increase of photoactivity of the system due to slowing down the electron-hole recombination process. Secondly, the degradation of 4-chlorophenol was more efficient using the WO3/TiO2 system. Future experiments will be focused on explanation of differences in the degradation mechanisms of 4-chlorophenol with the use of WO3 alone and WO3/TiO2 hybrid. The effect of the ratio of TiO2 and WO3 in the hybrid on photoactivity and photocatalytic degradation will also be explored. Tada et. al, Langmuir. 20, 4665-4670 (2004)Dette et. al, Nano Lett. 14, 6533-6538 (2014)DohLevil-Mitrovil et. al, Materials Research Bulletin. 83, 217-224 (2016)Meng et. al, Materials Science and Engineering: B. 180, 20-26 (2014)Linsebigler et. al, Chem. Rev. 95, 735-758 (1995)

X-ray and UV Dual Photochromism, Thermochromism, Electrochromism, and Amine-Selective Chemochromism in an Anderson-like Zn7 Cluster-Based 7-Fold Interpenetrated Framework.

Smart materials are highly desirable over the recent decade due to the growing demand of complicated nature. Stable stimuli-responsive smart materials exhibit widespread potential for applications in smart windows, sensors, separators, chemical valves, and release platforms but are rare. Despite being good candidates, viologen-based multifunctional smart materials are still a challenging task for chemists. To obtain such materials, the judicious strategy is to introduce polynuclear metal-carboxylate clusters as electron donors into a stable framework to increase chromic sensitivity. Toward this endeavor, we have synthesized a novel viologen-based polymer with a unique Anderson-like metal-carboxylate cluster, [Zn7(bpybc)3(o-BDC)6]·2NO3·6H2O (bpybc = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium, o-BDC = o-benzenedicarboylic acid) (1), which is a particular 7-fold interpenetrated framework with a 3D pcu network in which bpybc ligand as the linker and Zn7O30C12 as the second building unit (Zn7 SBU) were used as 6-connected nodes. More importantly, it shows excellent chromic behavior in response to multiple external stimuli especially soft X-ray and UV dual light, temperature, electricity, and organic amines, which stand out in the viologen-based polymers. Interestingly, the coloration process of 1 from "core" to "edge" is observed upon heating at the appropriate temperature, which has not yet been found in other reported thermochromic materials. Of particular interest for 1 is the couple of quaternary stimuli-sensitive abilities because it simultaneously meets the following conditions: (i) the capability of withstanding high light, higher temperature, extreme pH, and other harsh conditions; and (ii) the high sensitivity to external stimuli keeping away from photodegradation, thermal relaxation, side reactions, and so on. To be noted, 1 has high thermal stability and chemical stability, which are excellent advantages as smart materials. To further develop possible practical utilization, 1 has been doped into the polymer matrixes to construct a hybrid film, which not only keeps the response to external stimuli but also significantly improves the repeatability of the photochromic process, indicating that a new smart device with multi-stimuli-responsive functions will emerge successively in the future.

Self-Rechargeable-Battery-Driven Device for Simultaneous Electrochromic Windows, ROS Biosensing, and Energy Storage.

A self-powered electrochromic device (ECD) powered by a self-rechargeable battery is easily fabricated to achieve electrochromic window design, quantitative reactive oxygen species (ROS) sensing, and energy storage. The special design of the battery was composed of Prussian blue (PB) and magnesium metal as the cathode and anode, respectively, which exhibits fast self-charging and high power-density output for continuous and stable energy supply. Benefitting from the fast electrochromic response of PB, it was not only used for structuring self-rechargeable batteries but also used as an electrochromic display for highly sensitive self-powered ROS sensing and visual analysis. We believe that this work provides a solution to self-powered ECDs limited to a single application and could combine the applications in smart windows, ROS sensing, and other fields together, and in the meantime provide a solution for energy supply problems.

Lead-free thermochromic perovskites with tunable transition temperatures for smart window applications

The structural flexibility of hybrid perovskite materials allows for phase transition and consequently thermochromic properties. Here we investigate the thermochromic performance in a series of copper-based layered perovskites with organic cations having different alky chain lengths. Their transition temperature is found to be dependent on the organic cations due to molecular motion and hydrogen bond interaction, providing possibilities to prepare thermochromic semiconductors near room temperature for smart window applications.

Dual-response and Li+-insertion induced phase transition of VO2-based smart windows for selective visible and near-infrared light transmittance modulation

Chromogenic materials are widely used in the field of smart window with controllable throughput of solar energy and visible light. However, single modulation mode is often difficult to meet practical applications. Herein, we construct a novel multilayer device by effectively integrating thermochromic (TC) and electrochromic (EC) materials to selectively modulate visible and near-infrared (NIR) transmittance. The all-solid-state VO2-based multilayer device consists of VO2/LiTaO3/WO3 sandwich structure, with transparent conductor FTO and ITO as bottom electrode and top electrode, respectively. Therefore, this five-layer structure realized four reversible optical states, that is, TC layer passively responds to environmental temperature, while EC layer actively responds to external voltage simultaneously. Moreover, reversible migration of Li+ within the lattice of VO2 is achieved with a low voltage (just 3 V bias), which has an obvious effect (over 10 °C) on its phase change temperature (PCT). Finally, this novel dual-response VO2-based multilayer device provide a new strategy for VO2-based smart window applications.

Fully automatic modulation for broadband sunlight based on YOxHy/VO2 hybrid structures

Vanadium dioxides (VO2) thermochromic materials could exhibit reversible change in optical transmittance in response to external temperature. However, traditional VO2-based smart window is limited to an insufficient solar modulation ability as well as excessive transition temperature. Herein, we designed an all-solid form of YOxHy/VO2 multilayer film structure on quartz glass substrate, which can separately regulate visible and NIR light transmittance responding to environmental light and temperature, respectively. The hybrid film exhibits transparency under normal condition of room temperature and weak illumination, and solar energy transmittance can be dynamically regulated from 43% to 28%, 16% and finally 14% by controlling the conditions of temperature and illumination. Also, the optical and electrical transition temperature of m-VO2 shows an obvious decline. This hybrid film features high solar modulation ability and great conditional responsiveness with zero-energy consumption and low transition temperature. Our current results would supply new opportunities for smart-window applications, and also provide a new strategy for solar transmittance modulation.

Simultaneous achievement of high visible transmission and near-infrared heat shielding in flexible liquid crystal-based smart windows via electrode design

Light and heat management are the two vital requirements for smart window towards ideal solar control. However, as a representative polymer dispersed liquid crystal (PDLC) device, little attention has been paid to heat management. Herein, a TiO2/Ag(Cu)/TiO2 (TCAT) electrode was designed and optimized to achieve the tunability of transmission in visible range and near-infrared heat shielding simultaneously. The continuous growth of ultrathin Ag films with low percolation threshold was achieved on TiO2 underlayer by doping a certain amount of Cu. Then the trade-off of PDLC devices’ optoelectrical properties was realized. The average ON state specular transmittance of TCAT-based PDLC devices reached 81.9%, 84.9%, 81.9% and 79.9% for Ag(Cu) thicknesses of 7 nm, 9 nm, 11 nm and 13 nm, respectively. In addition, TCAT electrodes exhibited satisfactory ratio of light to solar gain by taking visible transmittance and solar heat gain coefficient into comprehensive consideration. Compared with ITO-based PDLC counterpart, a notable temperature-decreases of 4.6 °C was observed at ON state of TCAT-based device, indicating an excellent heat shielding performance. The proposed TCAT electrode design is promising in smart window application especially for building energy conservation in hot areas or the summer season.

Optical properties of periodic micropatterned VO2 thermochromic films prepared by thermal and intense pulsed light sintering

Vanadium dioxide (VO2) has garnered considerable attention in recent years as a promising material for thermochromic window applications because of its reversible metal-insulator transition (MIT) near room temperature and corresponding significant changes in its optical properties. However, the trade-off between improving luminous transmittance (Tlum) and maintaining the near-infrared modulation ability (ΔTNIR) limits the commercialization of VO2-based smart windows. To overcome these restrictions, we manufactured nanoinks based on VO2 nanoparticles and fabricated films via spin-coating these inks on quartz substrates. In this study, a simple photolithography method is employed to produce micropatterned VO2 films with different periodicities. The dried VO2 films were sintered by thermal and intense pulsed light (IPL) sintering processes. These micropatterned structures could favorably transmit visible light without sacrificing high ΔTNIR, and the patterned films showed improved Tlum compared to continuous films. The patterned VO2 films thermally sintered at 400 °C have Tlum of 51.5%–58.8%, ΔTNIR of 19.5%–27.6%, and ΔTsol of 5.7%–8.0%. The patterned VO2 films IPL-sintered at 2000 V have Tlum of 42.7%–50.7%, ΔTNIR of 21.7%–26.9%, and ΔTsol of 6.1%–7.6%. This facile fabrication method offers a new and promising direction for the use of VO2 thermochromic materials in smart window applications.

Preparation of TiO2@W-VO2 thermochromic thin film for the application of energy efficient smart windows and energy modeling studies of the produced glass

Buildings consume huge amounts of energy in order to maintain thermal comfort for users. Among different buildings components, windows are considered as one of the most inefficient parts; so studies on energy-efficient windows have recently become crucial due to the energy crisis in the world. Thermochromic (TC) glazing is a new generation of energy-efficient smart glazing that passively changes its transmittance rate in response to temperature. Vanadium dioxide (VO2) is the most widely used TC material in thermochromic windows. To increase the transparency of glazing, augment the thermochromic properties of VO2, and set the transition temperature near the comfort zone, many dopants have been investigated and many fabrication methods have been proposed.In this research, by using the sol–gel method, we doped tungsten ions (W6+) on a vanadium dioxide film. Titanium dioxide (TiO2) was also added to the solution to produce a nanoparticle structure with both thermochromic and photocatalytic properties. To quantify the energy saving achieved by the produced TC glazing compared to standard products, a simple model of a room in a residential building was created with EnergyPlus computer software. The simulation results showed that the fabricated TC glazing could bring about a significant reduction in energy demand of buildings compared to current approaches.

Fabrication of thermo-responsive polymer nanocomposites for smart window applications

Thermo-responsive polymers have attracted much attention in recent years because they can provide a variety of applications for smart devices, but the lack of sufficient mechanical properties and late response rate of the polymers limit their applications. In the current work, we fabricated polymer nanocomposites made from poly(vinyl methyl ether) and nanofillers with photothermal conversion property such as graphene oxide, antimony tin oxide nanoparticles, and silver nanoplates. The prepared nanocomposites showed rapidly reversible and reliable transparent-opaque transitions during the repeated heating and cooling processes, suggesting that they can be applied to smart window applications.

Smart glass electrochromic device fabrication of uniform tungsten oxide films from its powder synthesized by solution combustion method

Recently, there has been renewed interest in tungsten oxide based electrochromic (EC) materials. A variety of studies related to these materials have been reported, however, no serious studies so far have been reported on the electrochromic effect of tungsten oxide that is synthesized by solution combustion method. The major advantage of this process is the ease and simplicity with preparing nanosize oxides. Herein, the principal objective of this project was to produce uniform and adhesive tungsten oxide thin films from as-synthesized powder by solution combustion route for EC device. Although extensive research has been carried out on EC properties of tungsten oxide, limited study exists in e-beam evaporated tungsten oxide thin film studies for smart window application. Therefore, tungsten oxide thin films were deposited from our own produced powder by e-beam evaporation. To present the effect of new powder on the properties of the thin films, their physical and morphological properties were compared with thin film deposited by commercial tungsten oxide powder. Finally, electrochemical and optical characteristics of an all-solid-state EC device with a structure of glass/ITO/SCS-WO3/Nafion/ITO/glass with high optical transmittance modulation (63% at 550 nm) was fabricated. This study has been one of the first attempts to thoroughly examine in detail of tungsten oxide powder synthesized by solution combustion method for EC active layer. These findings contribute in several ways to our understanding of solution combustion synthesized powder to be used for production of EC active layer of thin film and provide a basis for developing and designing energy-efficient EC active materials and devices.

(Invited) Nanohybrids for Manipulating Solar Energy

With unique physical and chemical properties, and high potential for many important applications, nanomaterials have attracted extensive attention in the past two decades. For instance, due to their unique, size- and shape-tunable surface plasmon resonance, plasmonic nanostructures have recently been explored for increasing the efficiency of solar cells and photocatalysis. Essentially they enhance and/or broaden solar photon harvesting via improved light scattering, strong near field effect and/or hot electron injection. Combination of different nanomaterials into a single architecture leads to greatly enhanced properties, or even better, promising, multifunctional nanomaterials. In this talk, I will present our recent work on the synthesis of hybrid nanomaterials (the assemblies of different types of nanoscale materials) as well as their applications in smart windows, solar cells, solar fuel, photocatalysis, etc. [1-8]. Rational design of hybrid nanomaterials, which is the key to maximize the benefits from respective nano-components, is highlighted.

Electrophoretic deposition of reduced graphene oxide thin films for reduction of cross-sectional heat diffusion in glass windows

Abstract Effective management of heat transfer, such as conduction and radiation, through glass windows is one of the most challenging issues in smart window technology. In this work, reduced Graphene Oxide (rGO) thin films of varying thicknesses are fabricated onto Fluorine-doped Tin Oxide (FTO) glass via electrophoretic deposition technique. The sample thicknesses increase with increasing number of deposition cycles (5, 10, 20 cycles). It is hypothesized that such rGO thin films, which are well-known for their high thermal conductivities, can conduct heat away laterally towards heat sinks and reduce near-infrared (NIR) transmittance through them, thus effectively slowing down the temperature increment indoors. The performance of rGO/FTO in reducing indoor temperatures is investigated with a solar simulator and a UV-Vis-NIR spectrophotometer. The 20-cycles rGO thin films showed 30% more NIR blocked at 1000 nm as compared to clean FTO, as well as the least temperature increment of 0.57 °C following 30 min of solar irradiation. Furthermore, the visible transmittance of the as-fabricated rGO films remain on par with commercial solar films, enabling up to 60% of visible light transmittance for optimal balance of transparency and heat reduction. These results suggest that the rGO thin films have great potential in blocking heat transfer and are highly recommended for smart window applications.

The effects of surface plasmon resonance coupling effect on the solar energy modulation under high transmittance of VO2 nanostructure

Improving solar energy modulation ability while maintaining high visible-light transmittance has been a challenge of VO2 application in smart window. The improvement of solar modulation ability will often be accompanied by the decrease of visible light transmittance. In the present work, we propose a solution to improve the solar energy modulation ability and maintain high visible-light transmittance through etching the sputtered VO2 thin film by dilute sulfuric acid, which is based on the strong resonance coupling effect. We prepared planar VO2 thin film and acid-treated film with the similar transmittance. Their transmittances (Tlum) are about 57%. However, the solar energy modulation rate (ΔTsol) of acid-treated VO2 particles is increased by 277.8% compared with planar VO2 film. FDTD method is used to calculate the optical properties of acid-treated and planar VO2 film, the simulation results are consistent with experimental results. The increment of ΔTsol is related to the surface plasmon resonance and its coupled electromagnetic field of metallic state VO2. These results demonstrate that surface plasmon resonance and its coupling effect can be used to further enhance the solar energy modulation while maintaining the visible light transmittance. We hope these results will contribute to the area of smart window and other fields.

Single-Stimulus-Induced Modulation of Multiple Optical Properties.

Stimuli-responsive smart optical materials hold great promise for applications in active optics, display, sensing, energy conversion, military camouflage, and artificial intelligence. However, their applications are greatly restricted by the difficulty of tuning different optical properties within the same material, especially by a single stimulus. Here, magnetic modulations of multiple optical properties are demonstrated in a crystalline colloidal array (CCA) of magnetic nanorods. Small-angle X-ray scattering studies reveal that these nanorods form an unusual monoclinic crystal in concentrated suspensions. The CCA exhibits optical anisotropy in the form of a photonic bandgap and birefringence, thus enabling magnetic tuning of the structural color and transmittance at a rate of 50 Hz. As a proof-of-concept, it is further demonstrated that the fabrication of a multifunctional device for display, anticounterfeiting, and smart-window applications based on this multiple magneto-optical effect. The study not only provides a new model system for understanding colloidal assembly, but also opens up opportunities for new applications of smart optical materials for various purposes.

A novel method to improve phase transition of VO2 thermochromic films by Cs0.32WO3 nanoparticles

The VO2/Cs0.32WO3 composite thermochromic films were designed to promote the practical application of the VO2-based smart windows. The addition of 0.9 wt% Cs0.32WO3 can significantly shorten the phase transition response time and improve the solar heat shielding capacity of VO2, while maintaining high visible light transmittance (49.6%) and good solar modulation ability (10.4%). Furthermore, the phase transition of the composite film can occur at indoor temperature of 30 °C, and the indoor temperature can be greatly reduced by 7.7 °C under the simulated solar irradiation. These excellent properties make the VO2/Cs0.32WO3 films promising as smart energy-saving windows.

Dual functionality of K0.3WO3/Ag2O nanocomposites for smart window: Energy saving and visible photocatalytic self-cleaning performance

Series of novel K0.3WO3/Ag2O nanocomposites were successfully prepared by simple two steps method. An exciting fact that near-infrared light shielding and visible photocatalytic self-cleaning effects are integrated into the K0.3WO3/Ag2O film (weight ratio of K0.3WO3 to Ag2O = 30) could offer significant potential applications in smart windows. Furthermore, the optical properties and photocatalytic activity of the composites could be tuned by adjusting the weight ratio of K0.3WO3 to Ag2O. The obtained K0.3WO3/Ag2O films presented enhanced photocatalytic self-cleaning performance compared with pure K0.3WO3 under visible light irradiation. The improved self-cleaning performance of the K0.3WO3/Ag2O composites was ascribed to a combined contribution of the matched energy band structures and boosted separation efficiency of photo-generated carriers. An approximately 2.3°C-3.4 °C decrease in inner temperature was obtained for the heat box covered with K0.3WO3/Ag2O coated glass. Therefore, the proposed K0.3WO3/Ag2O (weight ratio of K0.3WO3 to Ag2O = 30) smart coating is a promising material for self-cleaning energy-saving windows.

Enhancing the electrical conduction in sodium borosilicate titanate glass doped with Nd or Gd ions to increase its optical absorption for smart windows applications

There are many glass systems that have applications due to their advantages like high optical absorption, high ionic and electronic conductivities. In this work two groups of borosilicate titanate glass doped with one of rare earth oxides with composition in mol% 40 Na2B4O7-40 SiO2-(20–x) TiO2-x M2O3, where M is (Nd or Gd) and x=(0, 0.5, 1 or 1.5) were prepared by melt quenching technique at 1373 K. The prepared glasses were characterized using XRD, FTIR, UV- Vis spectroscopy and dc, ac conductivities. The dc electrical conductivity was investigated for all glass samples in the temperature range (298–523) K. The ac electrical conductivity was measured in the frequency range (50 Hz to 2 MHz) and temperature range (298–523) K. The XRD results confirmed the amorphous nature of the studied glasses. UV–vis measurements revealed that the titanium ions exist in two valance states. The FTIR analysis revealed the presence of non-bridging oxygen, trigonal and tetrahedral borate groups. The highest electrical conductivity reached 3.54 × 10−3 S cm−1. The electron hopping due to the presence of two titanium valances is responsible for electronic conduction while the presence of sodium ions, tetrahedral borate groups and non-bridging oxygen are responsible for ionic conduction, the samples that have optical absorption and the highest conductivity at relatively low temperature close to the room temperature are the samples (H and K) which are promising for smart windows application.

Novel triazine-centered viologen analogues for dual-band electrochromic devices

Dual-band electrochromic device (Db-ECD) can tune visible and near-infrared (NIR) radiation independently. The Db-ECDs have inspired considerable interest for their potential applications in smart windows and camouflage. Organic small molecules and polymers were examined carefully though, exploring organics with dual-band electrochromism is still challenging. Herein, the triazine-centered viologen analogues, 2,4,6-tri(pyridyl-4-propyl)-1,3,5-triazine bromide (TPPT) and 2,4,6-tri(pyridyl-4-benzyl)-1,3,5-triazine bromide (TPBT) show excellent dual-band electrochromic behaviors with high dynamic contrasts (66.63% at 887 nm and 50.71% at 600 nm for TPPT, and 59.79% at 900 nm and 66.90% at 600 nm for TPBT, respectively). The Db-ECDs fabricated via gel electrolyte mixed with TPPT or TPBT can work at a low driving voltage down to ∼0.6 V, rendering a high coloration efficiency (up to 230.94 cm2 C-1) in the NIR region, the absorbance difference at 1037 nm reaching up to 2.58 for TPPT. This work sets the stage for novel organic materials of dual-band electrochromic gadgets.

Theoretical study of the electronic and optical properties of rare-earth (RE = La, Ce, Pr, Nd, Eu, Gd, Tb)-doped VO2 nanoparticles

Vanadium dioxide (VO2) is an important thermochromic material due to its reversible metal-insulator transition at 340 K. However, the high phase transition temperature of VO2 prevents its practical application in smart windows. Here, we use first-principles calculations to study the effect of rare-earth (RE = La, Ce, Pr, Nd, Eu, Gd, Tb) dopants on the variation of the local octahedral volumes, lattice distortion of VO2 and dimerization of the V-V chain in VO2. Interestingly, Tb and La dopants with a concentration of 3.1 at.% decreased the phase transition temperature of VO2 to 206 K and 225 K, respectively. Moreover, three factors, including the V-V chain change, distortion of the REO6 octahedron, and formation of impurity states contributed by RE-f states, assist the phase transition of VO2. Additionally, the incorporation of an RE dopant scaled the band gap of VO2 and further tuned its near-IR absorption. These findings provide novel insights and guidance on chemically tailoring the phase transition of VO2.