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Energy-saving windows made from common glass and cheap nanocrystals

(Nanowerk Spotlight) Buildings and other man-made structures consume as much as 30-40% of the primary energy in the world, mainly for heating, cooling, ventilation, and lighting. 'Smart' windows are expected to play a significant role in reducing this energy consumption in two ways: by generating energy themselves, and by providing better insulation by allowing light in and keeping the heat out (in hot summers) or in (in cold winters).
Infrared (IR)-blocking windows contribute to the energy efficiency of a building since they are transparent to visible light but block the radiation that transfers heat but does not contribute to illumination. These windows are passive systems that help reduce energy consumption (from active cooling methods, such as air conditioning) and are already available in the market.
One of the main challenges for energy efficient technologies is to lower their cost by making cheap energy-efficient materials and devices by preferably using green manufacturing technologies. For example, commercial infrared-blocking windows, both passive and active, are simply too expensive (most of these IR-blocking windows contain an expensive silver coating) and they are not used in the majority of our homes.
By lowering the cost of heat-blocking window technology, for instance by replacing silver with cheaper metals, they could see wider acceptance, ultimately translating into an overall greater energy-efficiency of buildings.
irradiance spectra
Two irradiance spectra important for designing windows: the solar spectrum and the spectrum of thermal radiation from a hot street in a hot climate. (Image: Govorov Lab, Ohio University)
There are a range of alternatives for the fabrication of IR-blocking glasses using nanotechnology. Some of them even show the very interesting capability of being actively tunable with voltage, so that one can control their state between mostly transparent to mostly opaque. Of course, passive windows would be typically much cheaper.
One approach to this problem involves creating passive infrared-blocking glasses using plasmonic nanocrystals. Researchers have shown that mixtures of specially shaped plasmonic nanocrystals made of noble (Ag and Au) and alternative materials (TiN, Al, and Cu) can efficiently block IR solar radiation. In particular, nanocrystals of relatively inexpensive plasmonic materials (Ag, Cu, Al, and TiN) show an overall good performance as IR-blocking elements.
These findings have been reported in Nano Letters ("Plasmonic Glasses and Films Based on Alternative Inexpensive Materials for Blocking Infrared Radiation").
"The fundamental motivation of our approach was to explore the creation of a purely passive metamaterial built with fabrication processes that could scale well enough to suite the needs of an industrial enterprise and, crucially, using relatively cheap materials and technologies that could bring the cost of this technology down," Alexander O. Govorov, Distinguished Professor at Ohio University and senior author of the paper, tells Nanowerk.
infrared-blocking window
The scheme of the double-pane argon window with the embedded nanocrystals inside the outer pane. The nanocrystals strongly absorb and reflect the infrared light while being transparent for the visible light. (Image: Govorov Lab, Ohio University) (click on image to enlarge)
The design approach that the team developed uses ensembles of metal nanoparticles dispersed in a transparent medium, as opposed to thin silver layers coating one of its sides, which is an industry standard.
"We explored different particle geometries and materials, with the dual purpose of seeking cheaper material alternatives for the fabrication of high-performance energy-saving windows and of illustrating a design approach relying on a dispersed collection of nanoparticles," notes Lucas Vazquez Besteiro, the paper's first author.
The team's work uses well-understood phenomena, fundamentally related to the interaction of light with metallic nanoparticles through the excitation of plasmonic resonances.
Plasmons are collective excitation of the free electrons in the metallic nanoparticles. Importantly, these collective oscillations interact strongly with light, much more than typical single-electron light absorption, which allows for the use of relatively low amounts of material to achieve useful optical properties.
In research labs, all steps of fabrication of nanocrystals of the required shapes (nanoshells or nanocups) already have been demonstrated. In addition, there are effective processes for embedding of nanocrystals into a transparent film or glass.
So, overall, the team's proposal for the IR-blocking medium is based on well-developed technologies. However, given that this work was geared towards the application of this type of designs in industrial production, it is key to address the challenges that will certainly arise when implementing real-life systems.
Although the researchers do not anticipate intrinsic difficulties with the plasmonic glass approach, all the steps required for preparing, refining and scaling up the fabrication process make them significant projects.
"What we bring to the table is a way to use relatively simple principles of nanophotonics in the development of technologies that address our global energy challenges," says Govorov. "We expect that our theoretical work and design guidelines excite the interest of applied research labs and companies, so that they further investigate the specific details of its implementation. Ultimately, our hope is that this technique proves itself competitive with the current state of the art, not only in terms of performance, but also with regard to cost."
The main application of this research would be the straightforward integration of the plasmonic glass approach to existing energy-efficient window designs.
Furthermore, the proposed method is flexible in terms of what regions of the electromagnetic spectrum it filters, so that other light-filtering metamaterials can be fashioned in a similar way to respond to very different needs requiring other spectral profiles.
"From a metamaterial design perspective, one can still explore different particle ensembles and different materials, to increase overall efficiency or lower the potential cost of the glasses," Besteiro points out. "However, at this point we believe that the most fruitful direction is to bring these designs to wide-spread commercial reality. Implementing these systems in an efficient, scalable and affordable way will present a range of specific challenges that are not explored yet."
"From my experience, it is relatively easy to design nanostructured or metastructured materials with certain superior properties, but, very often, such materials would be very expensive," Govorov concludes. "I guess, the challenge is to find physical principles and available materials, and fabricated with available technologies, that are both cheap and efficient."
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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