Surface-Facet-Dependent Electrochromic Properties of WO₃ Nanorod Thin Films: Implications for Smart Windows

Abstract

The influence of nanoparticle surface facets on electrochromic properties remains largely unexplored in nanostructured "smart" materials. Here, we explore how surface facets influence the coloration efficiency (CE) and long-term optical density (OD) stability of hexagonal WO 3 nanorod (h-WO 3 NR) thin films. We synthesized two h-WO 3 NR samples with distinct surface facet orientations and studied how the electrochemical, electrochromic, electrical, and surface chemistry properties change after long-term cycling. The sample with unique {120} facets exhibited reversible optical switching after 500 cycles and negligible variation in interfacial charge transfer resistance. The (120) surface features an open network of square window channels that may enable reversible ion transport and reduced ion trapping, enhancing the optical switching stability. However, the {120}-dominant sample exhibited lower CE than the {100}-dominant sample. The reduced optical density changes in the {120}-dominant sample could be due to a greater fraction of optically inactive trigonal cavity sites on the {001} end-caps. The results indicate surface facet and particle morphology engineering are viable strategies to enhance the CE and long-term stability/lifetime in electrochromic thin films for smart window applications.

Site-Selective Ion Intercalation Controls Spectral Response in Electrochromic Hexagonal Tungsten Oxide Nanocrystals

“…Sodium ions were inhibited from occupying the small trigonal sites, which contribute only weakly to the optical response, improving the coloration efficiency. Similar results have been shown for hexagonal tungsten oxide nanorods, which showed variations in the amount of achievable coloration based on different surface faceting of the particles, varying the amount of accessible sites for lithium intercalation . Here, in Cs:WO 3 NCs we reveal that site-selective occupation of interstitial sites based on differential cation size can not only tune the magnitude of the coloration but also give rise to dramatically different spectral signatures.…”

Controlling the Shape Anisotropy of Monoclinic Nb12O29 Nanocrystals Enables Tunable Electrochromic Spectral Range

“…13,17,18 Moreover, it was shown that engineering the surface facets of WO 3−x nanocrystals by structural control can be used as a method to optimize several electrochromic properties. 18 Nevertheless, no significant change in the electrochromic spectral range was observed by engineering the Li insertion sites in these materials where a polaronic mechanism dominates. By contrast, in plasmonic transition metal oxide nanocrystals, nanocrystal shape has significant impact on the spectral range of electrochromic response in the nanocrystals.…”

“…By applying a colloidal approach, several structural features of the nanocrystals can be precisely controlled, including crystal phase, particle size, morphology, aliovalent dopant concentration, etc. At an atomic level, it was reported for WO 3– x bulk films and nanocrystals that the local structure of the Li insertion sites has an influence on their coloration efficiency and stability. ,, Moreover, it was shown that engineering the surface facets of WO 3– x nanocrystals by structural control can be used as a method to optimize several electrochromic properties . Nevertheless, no significant change in the electrochromic spectral range was observed by engineering the Li insertion sites in these materials where a polaronic mechanism dominates.…”

Site-selective ion intercalation controls spectral response in electrochromic hexagonal tungsten oxide nanocrystals

“…Similar results have been shown for hexagonal tungsten oxide nanorods, which showed variations in the amount of achievable coloration based on different surface faceting of the particles which varied the amount of accessible sites for lithium intercalation. 14 Here, in Cs:WO3 NCs we reveal that site-selective occupation of interstitial sites based on differential cation size can tune not only the magnitude of the coloration, but can also give rise to dramatically different spectral signatures. We assign these distinct spectral responses to either plasmonic or polaronic absorption, based on extent of charge localization.…”