Glass has been prepared that selectively absorbs visible and near-infrared light when an electrochemical voltage is applied. This opens the way to 'smart' windows that block heat on demand, with or without optical transparency. See Letter p.323
Residential and commercial buildings account for about 40% of energy use and about 30% of energy-related carbon emissions in the United States1. To decrease this energy demand, materials are needed that help to regulate the heating and lighting requirements of buildings by responding to environmental changes. In particular, electrochromic window materials, which change colour and/or transparency when subjected to an electric field, could significantly reduce energy consumption in buildings2. On page 323 of this issue, Llordés et al.3 report a great advance in the development of such materials. They have made a composite in which nanometre-scale crystals of indium tin oxide are embedded in a niobium oxide glass, with high control of nanocrystal loading and dispersion. The electrochromic performance of the composite is much better than expected from a simple sum of the optical absorption of its two separate components, because of both the nanostructure of the material and synergistic interactions that occur at the interface between the components.
Inorganic nanocrystals are typically synthesized chemically with organic capping groups attached to aid the crystals' dispersibility in solvents and to prevent aggregation or undesired particle growth. Unfortunately for many applications, the organic groups do not have useful electrical or optical properties. There has thus been much effort to replace the organic groups with inorganic groups that either add to the capabilities of the crystals or can be converted into an electrically or optically active material. This approach has been used to make nanocrystal assemblies with greatly improved electrical properties4 and to convert nanocrystals capped with inorganic complexes into a more useful photovoltaic material5 (a material that converts light into electricity).
Llordés et al. have used this strategy to create their nanoparticle-in-glass materials. The authors first stripped indium tin oxide (ITO) nanocrystals of their organic caps and replaced them with niobium-containing polyatomic ions known as polyoxometalate (POM) clusters. These clusters attach covalently to the ITO surface to create a shell around the nanocrystal. The researchers then condensed the modified nanocrystals into a film, simply by evaporating the solvent from a dispersion of the crystals. Finally, they converted the POM between the densely packed ITO nanocrystals into a niobium oxide (NbOx) glass matrix by heating the film to 400 °C. Compared with previously reported synthetic routes for making nanoparticle-in-glass materials, in which inorganic crystals are grown within a glass6, Llordés and co-workers' method provides rigorous control over the nanocrystals' size distribution and volume fraction. And, by adding more POM to the dispersion of POM-stabilized ITO nanocrystals, the authors could increase the volume fraction of the NbOx glass matrix.
One of the key features of the ITO nanocrystal–NbOx glass material is that the glass is covalently bonded to the nanocrystals. This restricts the molecular orientations available to the octahedral NbO6 units found in the glass, and leads to remarkable structural ordering that differs from that of pure NbOx. It turns out that this ordering improves the electrochromic properties of the glass matrix: NbOx in the composite is five times darker than the bulk material when a similar voltage is applied.
ITO nanocrystals are also electrochromic, but in a different wavelength region from NbOx: they undergo reversible electrochemical redox reactions and absorb near-infrared light in the reduced state, but are transparent to this part of the spectrum when oxidized7. The combination of ITO nanocrystals with a NbOx glass matrix therefore yields a material in which both visible and near-infrared light absorption can be electrochemically modulated. This material could thus be used in smart windows, to control the amount of both heat (near-infrared) and light passing through them (Fig. 1). What's more, the optical transparency can be tuned independently of the near-infrared transparency.
Llordés and colleagues' approach for making composite materials of inorganic nanocrystals in glass opens the way to a range of new material properties and applications, not just in electrochromics. The challenge for each application is to identify the best combinations of nanocrystal composition and modifiable inorganic capping groups.
More specifically, several issues must still be addressed before the material can be used in windows. The authors used lithium metal as a counter electrode to test the performance of their material, but this is not acceptable for commercial applications because of safety concerns. A suitable counter electrode must be identified. Additionally, the researchers performed their photoelectrochemical tests using a liquid electrolyte as a charge-carrying material, whereas a solid electrolyte is probably more appropriate for buildings applications. The materials needed to build an electrochromic window will be more expensive than conventional window materials, so the extra expense will need to be balanced by the energy and cost savings that can be achieved through their use. Ideally, no power input will be needed to maintain transparency or opacity, but this ability remains to be explored.
Nevertheless, Llordés and co-workers' results are promising. With appropriate counter electrodes and a solid-state electrolyte, and if long-term stability of the composite can be demonstrated, windows that have multispectral band transparency may be just around the corner, potentially enabling buildings that offer unprecedented energy efficiency and comfort.
Richter, B. et al. Rev. Mod. Phys. 80, S1–S109 (2008).
Li, S.-Y., Niklasson, G. A. & Granqvist, C. G. J. Appl. Phys. 108, 063525 (2010).
Llordés, A., Garcia, G., Gazquez, J. & Milliron, D. J. Nature 500, 323–326 (2013).
Panthani, M. G. & Korgel, B. A. Annu. Rev. Chem. Biomol. Eng. 3, 287–311 (2012).
Jiang, C., Lee, J.-S. & Talapin, D. V. J. Am. Chem. Soc. 134, 5010–5013 (2012).
Sakamoto, A., Yamamoto, S. Int. J. Appl. Glass Sci. 1, 237–247 (2010).
Garcia, G. et al. Nano Lett. 11, 4415–4420 (2011).
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