From capacitance-controlled to diffusion-controlled electrochromism in onedimensional shape-tailored tungsten oxide nanocrystals

The engineering of electrochemically active films based on structurally and geometrically controlled transitionmetal oxide nanocrystals holds promise for the development of a new generation of energy-efficient dynamic windows that may enable a spectrally selective control of sunlight transmission over the near-infrared regime. Herein, the different spectro-electrochemical signatures of two sets of engineered nanotextured electrodes made of distinct anisotropic-shaped tungsten oxide building blocks are comparatively investigated. The electrodes were fabricated starting from corresponding one-dimensional colloidal nanocrystals, namely solid and longitudinally carved nanorods, respectively, which featured identical crystal phase and lattice orientation, but exposed two distinct space-filled volume structures with subtly different lattice parameters and nonequivalent types of accessible surfaces. The shape of nanocrystalline building blocks greatly impacted on the fundamental electrochemical charge-storage mechanisms and, hence on the electrochromic response of these electrodes, due to concomitant bulk and surface-structure effects that could not be entirely traced to mere differences in surfaceto- volume ratio. Electrodes made of carved nanorods accommodated more than 80% of the total charge through surface-capacitance mechanisms. This unique prerogative was ultimately demonstrated to enable an outstanding spectral selectivity as well as an extremely efficient dynamic modulation of the optical transmittance at nearinfrared frequencies (~ 80% in the range 700-1600 nm).