Screening of Nb-to-W Ratios in Bimetallic Niobium Tungsten Oxide Nanostructures as Advanced Dual-Band Electrochromic Materials

The next generation of electrochromic smart windows must more effectively filter heat and light fluxes from solar radiation, owing to an improved ability for selective and independent dual-band modulation of visible (VIS) and near-infrared (NIR) wavelengths. Bimetallic niobium tungsten oxides, (Nb,W)O x, are a recent addition to the family of dual-band modulation-capable materials, showing remarkable structural features and consecutive optical and electronic properties, owing to a synergistic effect between Nb and W active sites. In this study, (Nb,W)O x nanocrystals are synthesized with varying Nb-to-W mol ratios under sol-hydrothermal conditions, an acknowledged and robust wet chemical synthetic approach to obtain the stable tetragonal tungsten bronze (TTB) crystal phase. The results show that Nb 18 W 16 O 93 nanostructures with the targeted TTB crystal structure can be synthesized, which is advantageous for consecutive electrochemical and electrochromic performances. Furthermore, the study identifies a trend with increasing Nb content that leads to shape distortion in nanocrystal morphology, inducing diminished electrochemical capacities, and therefore, restricting the dual-band electrochromic modulation capabilities. Crucially, three (Nb,W)O x formulations with Nb:W mol ratios of 0.78, 1.00, and 1.49, among which the first two are original, are identified as delivering excellent optical modulation and switching kinetics metrics, together with verified VIS/NIR selectivity. This study highlights the significant fundamental potential of such formulations for the further applied design of unconventional electrochromic systems as energy-efficient, solar-control glazing devices.