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

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 WO3 nanorod (h-WO3NR) thin films. We synthesized two h-WO3NR 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.