(29b) Oxygen-Deficient Monoclinic Tungsten Oxide Nanowires for Spectrally Selective Electrochromic Smart Windows

Dual-band electrochromic smart windows where the near-infrared (NIR) and visible (VIS) light transmittance can be dynamically and independently varied can significantly reduce the building energy consumption on heating, ventilation, air conditioning (HVAC) and lighting.¹ Most of the dual-band electrochromic materials reported to date are composites^1-2 formed by the deliberate and delicate integration of NIR-selective and VIS-modulating components which operate in different potential windows. The use of single-component materials with a good dual-band performance would have been preferred but few have been identified.

We discovered that that the dynamic and independent tuning of NIR and VIS transmission is possible with oxygen-deficient monoclinic tungsten oxide nanowires (m-WO_3-x NWs) - a single-component material.³ Its NIR transmittance can be modulated by localized surface plasmon resonance (LSPR) in the 4-2.8 V window; and by LSPR and phase-transition (from dielectric to metallic) in the 2.8-2.6 V window. On the other hand, its VIS light transmittance is modulated by bandgap transitions (intraband and interband transitions) in the lower 2.6-2 V window. Consequently cast films of the m-WO_3-x NWs can operate in three distinct modes: the bright mode (4 V, NIR and VIS transparent), the cool mode (2.8-2.6 V, mostly NIR blocking) and the dark mode (2 V, NIR and VIS opaque). In particular a m-WO_3-x NW film in the cool mode (2.6V) can block 92.2% of the NIR to reduce the building solar heat gain, while maintaining a high VIS transmittance of 64.6% for daylighting. The cast films of m-WO_3-x NWs also provide a high optical modulation of the full solar spectrum (91.7%, 92.7%, 94.6%, 87.3% at 633, 800, 1200, 1600 nm respectively), a high coloration efficiency (101.7 and 184.3 cm² C^-1 at 633 and 1200 nm respectively) and good cycle stability (8.3% of optical loss at 633 nm after 1000 cycles).

References

[1] A. Llordés, G. Garcia, J. Gazquez and D. J. Milliron, Nature, 500, 323 (2013).

[2] A. Llordes, Y. Wang, A. Fernandez-Martinez, P. Xiao, T. Lee, A. Poulain, O. Zandi, C. A. Saez Cabezas, G. Henkelman and D. J. Milliron, Nat. Mater., 15, 1267 (2016).

[3] S. Zhang, S. Cao, T. Zhang, Q. Yao, A. Fisher, J. Y. Lee, Mater. Horiz., 5, 291 (2018)