Selective Colloidal Synthesis of Hexagonal and Monoclinic Substoichiometric Tungsten Oxide Nanocrystals as Dual-Band Electrochromic Materials

Selective dual-band modulation of visible and near-infrared (NIR) absorption in tungsten oxide materials can be synthetically achieved either via heteroatom dopants or via intrinsic formation of oxygen vacancies. Such advanced optical features are of strong interest for creating novel energy-efficient electrochromic smart windows, allowing for independent control over solar luminosity and heat fluxes entering buildings. Tungsten oxide nanostructures are expected to crystallize in different phases depending on the doping strategy employed, mainly as hexagonal with heteroatoms and as monoclinic in the case of “self-doping”. Herein, we report a single colloidal method to yield substoichiometric tungsten oxide nanocrystals (NCs) that, depending on the synthetic conditions, selectively adopt either the monoclinic phase, being reported as the most stable, or the hexagonal phase, being much more complex and challenging to isolate. While both exhibit plasmonic absorption and selective modulation of NIR wavelengths, the distinct morphologies and ratios of substoichiometry obtained for each phase lead to a significant difference in both optical and electrochemical responses. As compared to diversely shaped hexagonal (h-WO3–y) NCs, monoclinic (m-WO3–x) NCs show an isometric nanorod morphology, and therefore demonstrate more intense NIR selectivity as well as increased capacitive current densities and ionic diffusion rates. Such differences highlight the influence and relevance of size, shape, and structure of the synthesized NCs, while further positioning h-WO3–y NCs─whose present selective processing from m-WO3–x counterparts is of high originality─as strong innovative alternatives also bearing efficient dual-band absorption capacities toward electrochromic applications.