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.