Synthesis of Ultrathin Alloy (Mo, V)-Tungsten-Oxide Nanowires: Implications for Electrochromic and Supercapacitor Applications

Doped and alloyed transition metal-oxides (TMOs) attract vast attention owing to their tunable electronic properties (e.g., conductivity, band gap, and optical absorption), making them appealing for many (photo)electronic, chromic, and green energy applications. Dual-functional materials combining electrochromic (EC) and energy storage (e.g., supercapacitor, SC) applications are of interest as they can store energy while shading the light transmission through a window or give off a visual signal of their current energy storage state by a color change. Pure tungsten-oxides exhibit distinctive EC properties but attain low energy density compared to other TMOs (e.g., MoO₃ and V₂O₅). The coloration efficiency and energy density can be enhanced by controlling the morphology, size, and composition of the nanoscale TMOs that constitute the active EC film. Thus far, most EC-SC works showed a trade-off between increased areal capacitance and a decrease in the coloration efficiency or transmittance; the improved EC-SC properties for doped or alloyed metal-oxides were related mostly to the small grain size or to structural distortion caused by the added cation, exhibiting more active sites. Herein, we demonstrate a straightforward and facile synthesis of crystalline Mo/V-alloyed tungsten-oxide ultrathin nanowires (uNWs). We investigated the growth mechanism and succeeded in preserving the crystallinity up to 25% (atomic) alloying. The additional properties (compared to unmodified tungsten-oxide) of the alloyed uNWs, such as absorbance peaks, lead to improved specific capacitance while preserving the high coloration efficiency of uNW W–O, and in the case of W–Mo–O, a better coloration efficiency is measured.