Influence of crystalline and shape anisotropy on electrochromic modulation in doped semiconductor nanocrystals

Localized surface plasmon resonance (LSPR) modulation appearing in the near-infrared range in doped semiconductor nanocrystals improves electrochromic performance. Although crystalline and shape anisotropies influence LSPR spectra, studies of their impact on electrochromic modulation are lacking. Here, we study how crystalline anisotropy in hexagonal cesium-doped tungsten oxide nanorods and nanoplatelets affects essential metrics of electrochromic modulation - coloration efficiency (CE) and volumetric capacity - using electrolyte cations of different sizes (tetrabutylammonium, sodium, and lithium) as structurally sensitive electrochemical probes. The CE of nanorod films is higher than that of nanoplatelets in all of the electrolytes owing to the low effective mass along the crystalline c-axis. When using sodium cations, which diffuse through one-dimensional hexagonal tunnels, the electrochemical capacity is significantly greater for platelets than for nanorods. This difference is explained by the hexagonal tunnel sites being more accessible in platelets than in nanorods. Our work sheds light on the role of shape and crystalline anisotropy in charge capacity and CE, both of which contribute to overall modulation.