Hydrothermal Formation of Tungsten Trioxide Nanowire Networks on Seed-Free Substrates and Their Properties in Electrochromic Device

Abstract

Facile and cost-efficient hydrothermally grown 3-D tungsten trioxide $({\rm WO}_{3})$ nanowires (NWs) network film on seed-free fluorine-doped tin oxide (FTO) substrates without using additive or capping agent is reported. The NW networks are hexagonal phase and the morphologies are changed by adjusting the pH of precursor solutions. Growth mechanism of ${\rm WO}_{3}$ NWs is also extensively discussed. The designed porous film exhibits remarkable enhancement of the electrochromic (EC) properties. In particular, a significant optical modulation (57% at 632 nm), fast color switching speed (bleaching: 7 s and coloration: 21 s), high coloration efficiency ( $120.3~{\rm cm}^{2}~{\rm C}^{-1}$ at 632 nm), high ${\rm Li}^{+}$ diffusion coefficient $(2.14\times{10}^{-9}{\rm cm}^{2}~{\rm s}^{-1})$ , and excellent cycling stability (87% after 1000 cycles) are achieved for the 3-D NWs film. The improved EC properties are mainly attributed to the highly porous film, which makes the ${\rm Li}^{+}$ diffusion becomes easier and provides larger specific surface area for charge-transfer reactions. Furthermore, we also reported an EC device (50 mm $\times\,$ 50 mm) with a simple two-electrode configuration showing high optical contrast. The 3-D ${\rm WO}_{3}$ NWs network film acts as an excellent EC material.