Scaled-Up Production of VО2 Nanoparticles

V02 coatings are a potential energy-efficient option for buildings, and their production needs to be scaled up scientifically and technologically. Almost all the synthesis strategies/methods discussed in this book, either gas based (chemical vapor deposition [CVD] and physical vapor deposition [PVD]) or solution based (hydrothermal and sol-gel process), are currently limited to the lab scale. Among them, the CVD method developed so far is one of the most suitable ones for industrial processing, similar to coating silver or tin oxide on float glass. However, the cost issue in operation and small-scale production and the difficulty in controlling the pristine V02(M) phase as well as the elemental-doped ones while using gas-based methods have limited their large-scale production.

In the ongoing and future development of thermochromic V02 smart coatings, development of facile but efficient scale-up strategies is critical. This requires the integration of a few of the technologies into the glass production lines, contrary to the direct deposition of V02 on substrates using the CVD or PVD method. The combination of chemical and physical methods may achieve dramatic effects. For instance, a two-step method has been recently developed for generating glass coatings for smart windows, which may be a feasible solution for carrying out industrial-scale productions. By this method, V02 NPs with a uniform size distribution could be first prepared by the hydrothermal method (chemical method) in acid (oxalic acid) or base (ammonia) systems and then dispersed in an aqueous solution as a nanoink for the preparation of film coatings by physical techniques, such as spraying, rolling, casting or screen printing, which could significantly reduce costs and readily achieve thermochromic coatings to satisfy the huge market needs. In such a strategy, the preparation of uniform-sized V02 NPs (ranging from 30 nm to 100 nm) is the most crucial step, while the pertinent parameters (temperature, pressure, reaction time, concentration, pH, drying conditions, etc.) need to be carefully optimized.

Process

The harsh preparation requirements of V02 and the complicated fabrication procedure of devices are a great challenge in current approaches, such as solution-based methods, CVD, and PVD, which have a limited ability to produce V02-based devices in mild conditions with the simple methods summarized in this book. Development of manufacturing technologies like roll-to-roll coating of polymer foil may open paths toward the production of cheap products. However, the ultimate goal to come up with a simple, moderate, and highly productive technique is one of the important research directions to bring the V02-based smart device out from the laboratory and into our daily lives.

Illustration of the four optical modulation modes, the multifunctional technologies, and other potential technologies. Reproduced from [13], Copyright © 2019 John Wiley and Sons

Figure 12.6 Illustration of the four optical modulation modes, the multifunctional technologies, and other potential technologies. Reproduced from [13], Copyright © 2019 John Wiley and Sons.

Combination of Thermochromic VО2 with Other Energy-Saving System and Other Functionalities

To fully utilize the large surface areas available in the urban windows, in one recent review paper published by Long et al. [13], future developments were suggested to continue to break the boundaries among different modes of stimuli to integrate functionalities like energy production and photocatalytic selfcleaning (Fig. 12.6). One of the recent trends is to rationally combine the different stimuli to further enhance chromic window performance. For example, the combination of thermochromic VO2 with mechanical kirigami structures renders a much-enhanced performance by passively modulating the indoor solar energy in response to the environmental temperature together with actively controlling the kirigami structure (Fig. 12.7a) [1, 14]. The adaptive thermal-responsive system is applied in a house demo to demonstrate the potential for the thermochromic smart window (Fig. 12.7b). It was observed the film changes from a noncompact (strain of 100%) to a compact (strain of 0%) state through a gradual close up of kirigami cuts with increasing temperature, accompanied by a less clear image of the indoor clamp (Fig. 12.7c).

Smart, energy-efficient windows,

Figure 12.7 Smart, energy-efficient windows, (a) Illustration of an energysaving smart window that adaptively reduces the indoor solar energy irradiation under elevated temperatures, (b) The photograph of a house model and a schematic of the thermal-responsive system for the smart window demo, (c) The configuration transition of the smart window model under increasing temperatures. The fully thermal-responsive system is applied, (d) Transmittance spectra of the film at (state 1) 90r C with a compact configuration, (state 2) 20' C with a compact configuration, and (state 3) 20 C with a noncompact configuration (strain of 100%). (e) Cycling test of the film switching between states 1 and 3. (f) Calculated ATsot, Tium, and ATi260nm of the film in states 1-3 and the reference film based on planar VO2 and PDMS, respectively. Reproduced from [15], Copyright ©2019 Elsevier.

In Fig. 12.7d, the respective contributions of the active local surface plasmonic resonance (LSPR) induced by V02 particles and the kirigami structure are identified by comparing three states: state 1 (0% strain @ 95°C); state 2 (0% strain @ 20 C); and state 3 (100% strain @ 20°C). More specifically, the contributions of active LSPR, kirigami, and their synergistic effect are calculated on the basis of the contrasts of states l-to-2, 2-to-3, and l-to-3 in Fig. 12.7d, respectively. The results of ATS0, average visible transmittance (7'ium)< and the transmittance contrast at 1260 nm (ДГибОпт) are plotted in Fig. 12.7f. The sole active LSPR and kirigami structure effects contribute to ATso of 9.7% and 28%; however, the combined effect of the LSPR and kirigami structure gives a record-high ATS0 (up to 37.7%), as well as a significantly enhanced Tlum (from 17.6% [plasmonic sample] to 35.2% ["plasmonic-kirigami sample"]) as shown in Fig. 12.7f. Meanwhile, the introduction of active LSPR largely enhances the NIR controllability and the contrast at 1260 nm (Д T1260) increases from 16.1% on the kirigami sample to 49.3% on the plasmonic kirigami sample (Fig. 12.7f). Both Д Tsol and 7]um were simultaneously enhanced by this simple approach.

Energy harvesting, superhydrophobicity for self-cleaning windows [16, 17] energy storage [18], water harvesting from the environment and its storage [19, 20], and anti-icing [21-23] are highly desired functionalities as these will make such windows more competitive in the market and very economical in terms of the energy bill. The inevitable challenge lies in drawing a balance between the luminous transmission, transparency, energy saving, device performance, and durability.

 
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